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| <?xml version="1.0" encoding="UTF-8"?><!DOCTYPE article PUBLIC "-//NLM//DTD JATS (Z39.96) Journal Archiving and Interchange DTD v1.1d1 20130915//EN" "JATS-archivearticle1.dtd"><article article-type="research-article" dtd-version="1.1d1" xmlns:xlink="http://www.w3.org/1999/xlink"><front><journal-meta><journal-id journal-id-type="nlm-ta">elife</journal-id><journal-id journal-id-type="hwp">eLife</journal-id><journal-id journal-id-type="publisher-id">eLife</journal-id><journal-title-group><journal-title>eLife</journal-title></journal-title-group><issn publication-format="electronic">2050-084X</issn><publisher><publisher-name>eLife Sciences Publications, Ltd</publisher-name></publisher></journal-meta><article-meta><article-id pub-id-type="publisher-id">02208</article-id><article-id pub-id-type="doi">10.7554/eLife.02208</article-id><article-categories><subj-group subj-group-type="display-channel"><subject>Research article</subject></subj-group><subj-group subj-group-type="heading"><subject>Biochemistry</subject></subj-group><subj-group subj-group-type="heading"><subject>Cell biology</subject></subj-group></article-categories><title-group><article-title>Cell-cycle dependent phosphorylation of yeast pericentrin regulates γ-TuSC-mediated microtubule nucleation</article-title></title-group><contrib-group><contrib contrib-type="author" id="author-10364"><name><surname>Lin</surname><given-names>Tien-chen</given-names></name><xref ref-type="aff" rid="aff1"/><xref ref-type="aff" rid="aff2"/><xref ref-type="other" rid="par-1"/><xref ref-type="fn" rid="con1"/><xref ref-type="fn" rid="conf1"/></contrib><contrib contrib-type="author" id="author-10365"><name><surname>Neuner</surname><given-names>Annett</given-names></name><xref ref-type="aff" rid="aff1"/><xref ref-type="other" rid="par-1"/><xref ref-type="fn" rid="con2"/><xref ref-type="fn" rid="conf1"/></contrib><contrib contrib-type="author" id="author-10366"><name><surname>Schlosser</surname><given-names>Yvonne T</given-names></name><xref ref-type="aff" rid="aff1"/><xref ref-type="other" rid="par-1"/><xref ref-type="fn" rid="con3"/><xref ref-type="fn" rid="conf1"/></contrib><contrib contrib-type="author" id="author-13294"><name><surname>Scharf</surname><given-names>Annette ND</given-names></name><xref ref-type="aff" rid="aff1"/><xref ref-type="other" rid="par-2"/><xref ref-type="fn" rid="con4"/><xref ref-type="fn" rid="conf1"/></contrib><contrib contrib-type="author" id="author-13295"><name><surname>Weber</surname><given-names>Lisa</given-names></name><xref ref-type="aff" rid="aff1"/><xref ref-type="other" rid="par-2"/><xref ref-type="fn" rid="con5"/><xref ref-type="fn" rid="conf1"/></contrib><contrib contrib-type="author" corresp="yes" id="author-10298"><name><surname>Schiebel</surname><given-names>Elmar</given-names></name><xref ref-type="aff" rid="aff1"/><xref ref-type="corresp" rid="cor1">*</xref><xref ref-type="other" rid="par-1"/><xref ref-type="fn" rid="con6"/><xref ref-type="fn" rid="conf1"/></contrib><aff id="aff1"><institution content-type="dept">Zentrum für Molekulare Biologie (ZMBH)</institution>, <institution>Universität Heidelberg</institution>, <addr-line><named-content content-type="city">Heidelberg</named-content></addr-line>, <country>Germany</country></aff><aff id="aff2"><institution content-type="dept">The Hartmut Hoffmann-Berling International Graduate School</institution>, <institution>University of Heidelberg</institution>, <addr-line><named-content content-type="city">Heidelberg</named-content></addr-line>, <country>Germany</country></aff></contrib-group><contrib-group content-type="section"><contrib contrib-type="editor"><name><surname>Pines</surname><given-names>Jon</given-names></name><role>Reviewing editor</role><aff><institution>The Gurdon Institute</institution>, <country>United Kingdom</country></aff></contrib></contrib-group><author-notes><corresp id="cor1"><label>*</label>For correspondence: <email>e.schiebel@zmbh.uni-heidelberg.de</email></corresp></author-notes><pub-date date-type="pub" publication-format="electronic"><day>30</day><month>04</month><year>2014</year></pub-date><pub-date pub-type="collection"><year>2014</year></pub-date><volume>3</volume><elocation-id>e02208</elocation-id><history><date date-type="received"><day>08</day><month>01</month><year>2014</year></date><date date-type="accepted"><day>26</day><month>04</month><year>2014</year></date></history><permissions><copyright-statement>© 2014, Lin et al</copyright-statement><copyright-year>2014</copyright-year><copyright-holder>Lin et al</copyright-holder><license xlink:href="http://creativecommons.org/licenses/by/3.0/"><license-p>This article is distributed under the terms of the <ext-link ext-link-type="uri" xlink:href="http://creativecommons.org/licenses/by/3.0/">Creative Commons Attribution License</ext-link>, which permits unrestricted use and redistribution provided that the original author and source are credited.</license-p></license></permissions><self-uri content-type="pdf" xlink:href="elife02208.pdf"/><abstract><object-id pub-id-type="doi">10.7554/eLife.02208.001</object-id><p>Budding yeast Spc110, a member of γ-tubulin complex receptor family (γ-TuCR), recruits γ-tubulin complexes to microtubule (MT) organizing centers (MTOCs). Biochemical studies suggest that Spc110 facilitates higher-order γ-tubulin complex assembly (<xref ref-type="bibr" rid="bib45">Kollman et al., 2010</xref>). Nevertheless the molecular basis for this activity and the regulation are unclear. Here we show that Spc110 phosphorylated by Mps1 and Cdk1 activates γ-TuSC oligomerization and MT nucleation in a cell cycle dependent manner. Interaction between the N-terminus of the γ-TuSC subunit Spc98 and Spc110 is important for this activity. Besides the conserved CM1 motif in γ-TuCRs (<xref ref-type="bibr" rid="bib65">Sawin et al., 2004</xref>), a second motif that we named Spc110/Pcp1 motif (SPM) is also important for MT nucleation. The activating Mps1 and Cdk1 sites lie between SPM and CM1 motifs. Most organisms have both SPM-CM1 (Spc110/Pcp1/PCNT) and CM1-only (Spc72/Mto1/Cnn/CDK5RAP2/myomegalin) types of γ-TuCRs. The two types of γ-TuCRs contain distinct but conserved C-terminal MTOC targeting domains.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.001">http://dx.doi.org/10.7554/eLife.02208.001</ext-link></p></abstract><abstract abstract-type="executive-summary"><object-id pub-id-type="doi">10.7554/eLife.02208.002</object-id><title>eLife digest</title><p>Microtubules are hollow structures made of proteins that have a central role in cell division and a variety of other important processes within cells. For a cell to divide successfully, the chromosomes containing the genetic information of the cell must be duplicated and then separated so that one copy of each chromosome ends up in each daughter cell. To separate the chromosomes, microtubules extend out from two structures called spindle pole bodies, which are found at either end of the cell, and pull one copy of each chromosome to opposite sides of the cell.</p><p>Although the individual proteins that make up a microtubule can self-assemble into tubes, this occurs very slowly, so cells employ other molecules to speed up this process. In yeast cells, a protein called gamma-tubulin is recruited to the spindle pole body by the protein Spc110, where it combines with two other proteins to form a complex called the gamma-tubulin small complex. Several of these complexes then join together to form a ring, which probably acts as the platform that microtubules grow from. Recent observations suggested that Spc110 may help to construct this ring, but without revealing how.</p><p>Now, Lin et al. reveal that Spc110 can regulate microtubule formation by controlling how several gamma-tubulin small complexes bind together, and have identified the exact section of Spc110 that interacts with the complexes. However, the Spc110 must become active before it can perform this role, and it is only activated during certain stages of cell division, through phosphorylation. The structures in Spc110 that bind to the gamma-tubulin small complex in yeast are also found in gamma-tubulin binding receptor proteins in human cells. The work of Lin et al. demonstrates that proteins that are assumed to have passive roles within cells, such as Spc110, often play more active roles instead.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.002">http://dx.doi.org/10.7554/eLife.02208.002</ext-link></p></abstract><kwd-group kwd-group-type="author-keywords"><title>Author keywords</title><kwd>pericentrin</kwd><kwd>spindle pole body</kwd><kwd>gamma-tubulin complex</kwd><kwd>phosphorylation</kwd><kwd>mitosis</kwd><kwd>microtubule nucleation</kwd></kwd-group><kwd-group kwd-group-type="research-organism"><title>Research organism</title><kwd><italic>S. cerevisiae</italic></kwd></kwd-group><funding-group><award-group id="par-1"><funding-source><institution-wrap><institution-id institution-id-type="FundRef">http://dx.doi.org/10.13039/501100001659</institution-id><institution>Deutsche Forschungsgemeinschaft</institution></institution-wrap></funding-source><award-id>Schi295-3-2</award-id><principal-award-recipient><name><surname>Lin</surname><given-names>Tien-chen</given-names></name><name><surname>Neuner</surname><given-names>Annett</given-names></name><name><surname>Schlosser</surname><given-names>Yvonne T</given-names></name><name><surname>Schiebel</surname><given-names>Elmar</given-names></name></principal-award-recipient></award-group><award-group id="par-2"><funding-source><institution-wrap><institution-id institution-id-type="FundRef">http://dx.doi.org/10.13039/501100001659</institution-id><institution>Deutsche Forschungsgemeinschaft (DFG)</institution></institution-wrap></funding-source><award-id>SFB1036</award-id><principal-award-recipient><name><surname>Scharf</surname><given-names>Annette ND</given-names></name><name><surname>Weber</surname><given-names>Lisa</given-names></name></principal-award-recipient></award-group><funding-statement>The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.</funding-statement></funding-group><custom-meta-group><custom-meta><meta-name>elife-xml-version</meta-name><meta-value>2</meta-value></custom-meta><custom-meta specific-use="meta-only"><meta-name>Author impact statement</meta-name><meta-value>Phosphorylation of Spc110 N-terminal domain encompassing conserved motifs and its interaction with conserved GCP3 N-terminal domain regulate the oligomerization of gamma-tubulin small complexes (γ-TuSCs).</meta-value></custom-meta></custom-meta-group></article-meta></front><body><sec id="s1" sec-type="intro"><title>Introduction</title><p>The budding yeast spindle consists of ∼40 microtubules (MTs) that extend between the two opposed spindle pole bodies (SPBs). Because of the closed mitosis in yeast, the SPBs remain embedded in the nuclear membrane throughout mitosis. Cell-cycle regulated spindle assembly begins in S-phase with nucleation of MTs onto the surface of the newly assembled SPB. As soon as MTs emanate from both SPBs, they interdigitate (pole to pole MTs) or attach to the kinetochores (pole to kinetochore MTs) at the end of S phase, forming the bipolar spindle (<xref ref-type="bibr" rid="bib58">O'Toole et al., 1999</xref>).</p><p>The γ-tubulin complex is the core player in MT nucleation. In budding yeast <italic>Saccharomyces cerevisiae</italic>, two molecules of γ-tubulin (Tub4) assemble together with one molecule of Spc97 (ortholog of human GCP2) and Spc98 (ortholog of human GCP3) into a tetrameric γ-tubulin small complex (γ-TuSC), which is conserved in all eukaryotes (<xref ref-type="bibr" rid="bib28">Geissler et al., 1996</xref>; <xref ref-type="bibr" rid="bib41">Knop et al., 1997</xref>; <xref ref-type="bibr" rid="bib31">Guillet et al., 2011</xref>). The purified γ-TuSC of budding yeast is able to self-oligomerize into symmetric ring-like structures under low salt buffer conditions. The diameter and the pitch of the γ-TuSC ring resemble that of MT cylinder, suggesting that the γ-TuSC ring functions as a MT nucleation template. However, the in vitro nucleation activity of the γ-TuSC assemblies remained poor, presumably because of the suboptimal spacing of every second Tub4 within the γ-TuSC ring that blocks interaction with tubulin in the MT cylinder (<xref ref-type="bibr" rid="bib46">Kollman et al., 2008</xref>, <xref ref-type="bibr" rid="bib45">2010</xref>; <xref ref-type="bibr" rid="bib10">Choy et al., 2009</xref>). The concept of γ-TuSC oligomerization is further supported by in vivo measurements of budding yeast γ-tubulin complex components on detached single MT nucleation sites. The γ-tubulin:Spc97:Spc98 ratio was 2.4:1.0:1.3 with a total of ∼17 γ-tubulin molecules per nucleation site (<xref ref-type="bibr" rid="bib21">Erlemann et al., 2012</xref>), suggesting a slight excess of γ-tubulin and Spc98 molecules over Spc97 in the assembled MT nucleation site.</p><p>In most eukaryotic organisms, such as fission yeast, <italic>Drosophila</italic>, <italic>Xenopus</italic>, and human, multiple γ-TuSCs further assemble with additional GCP family members (GCP4-6) into the larger γ-tubulin ring complex (γ-TuRC) (<xref ref-type="bibr" rid="bib32">Gunawardane et al., 2000</xref>; <xref ref-type="bibr" rid="bib90">Zhang et al., 2000</xref>; <xref ref-type="bibr" rid="bib55">Murphy et al., 2001</xref>; <xref ref-type="bibr" rid="bib4">Anders et al., 2006</xref>). However, these additional GCP proteins are not encoded in the budding yeast genome.</p><p>Various proteins are involved in the recruitment of γ-tubulin complexes to microtubule organizing centers (MTOCs) such as centrosomes and SPBs. The small protein Mozart1 is encoded in most eukaryotic genomes except the one of budding yeast (<xref ref-type="bibr" rid="bib77">Teixido-Travesa et al., 2012</xref>). In <italic>Schizosaccharomyces pombe</italic> and <italic>Arabidopsis thaliana</italic> Mozart1 interacts with the GCP3 subunit of γ-tubulin complexes (<xref ref-type="bibr" rid="bib35">Janski et al., 2012</xref>; <xref ref-type="bibr" rid="bib56">Nakamura et al., 2012</xref>; <xref ref-type="bibr" rid="bib6">Batzenschlager et al., 2013</xref>; <xref ref-type="bibr" rid="bib16">Dhani et al., 2013</xref>; <xref ref-type="bibr" rid="bib52">Masuda et al., 2013</xref>). In <italic>S. pombe</italic> Mozart1 appears important for the γ-TuSC recruitment to SPBs (<xref ref-type="bibr" rid="bib16">Dhani et al., 2013</xref>; <xref ref-type="bibr" rid="bib52">Masuda et al., 2013</xref>).</p><p>Besides Mozart1, a group of conserved proteins called γ-tubulin complex receptors (γ-TuCRs) are required for targeting γ-tubulin complexes to MTOCs. Most of them carry a highly conserved centrosomin motif 1 (CM1) that interacts with GCP subunits of γ-tubulin complexes (<xref ref-type="bibr" rid="bib65">Sawin et al., 2004</xref>). How Mozart1 and γ-TuCRs cooperate is not understood. However, in budding yeast cells that lack a Mozart1 gene, γ-TuCRs are the sole factors responsible for γ-TuSC recruitment to SPBs. Spc110 is the budding yeast homolog of pericentrin (PCNT) and functions as γ-TuCRs at the nuclear side of the SPB (<xref ref-type="bibr" rid="bib42">Knop and Schiebel, 1997</xref>; <xref ref-type="bibr" rid="bib73">Sundberg and Davis, 1997</xref>). The N-terminal Spc110 encompasses the CM1 that interacts with the Spc98 subunit of γ-TuSC (<xref ref-type="bibr" rid="bib42">Knop and Schiebel, 1997</xref>; <xref ref-type="bibr" rid="bib57">Nguyen et al., 1998</xref>; <xref ref-type="bibr" rid="bib81">Vinh et al., 2002</xref>; <xref ref-type="bibr" rid="bib65">Sawin et al., 2004</xref>; <xref ref-type="bibr" rid="bib89">Zhang and Megraw, 2007</xref>; <xref ref-type="bibr" rid="bib25">Fong et al., 2008</xref>; <xref ref-type="bibr" rid="bib63">Samejima et al., 2008</xref>). In addition, the N-terminal region of Spc110 is phosphorylated in a cell-cycle dependent manner. Phospho-Spc110 appears as cells progress from S phase, continues accumulating during mitosis, and vanishes at the anaphase onset (<xref ref-type="bibr" rid="bib27">Friedman et al., 1996</xref>; <xref ref-type="bibr" rid="bib70">Stirling and Stark, 1996</xref>). Spc110 phosphorylation accounts for the impact of Cdk1 and Mps1 kinases on spindle dynamics (<xref ref-type="bibr" rid="bib26">Friedman et al., 2001</xref>; <xref ref-type="bibr" rid="bib33">Huisman et al., 2007</xref>; <xref ref-type="bibr" rid="bib49">Liang et al., 2013</xref>). However, a clear understanding behind this observation is lacking. Interestingly, when γ-TuSC and an N-terminal fragment of Spc110 (amino acids 1–220 of Spc110; Spc110<sup>1–220</sup>) were co-expressed in insect cells, a filament-like γ-TuSC-Spc110<sup>1–220</sup> complex formed. The nucleation capacity of this purified γ-TuSC-Spc110<sup>1–220</sup> complex exceeded that of the γ-TuSC alone (<xref ref-type="bibr" rid="bib45">Kollman et al., 2010</xref>). Thus, Spc110<sup>1–220</sup> influences γ-TuSC properties with yet unclear mechanism.</p><p>Here we have tested the possibility that phosphorylation of the γ-TuCR Spc110 regulates MT nucleation by inducing γ-TuSC oligomerization. Single particle analysis of γ-TuSC incubated with phosphomimetic Spc110 mutant proteins showed that Mps1 and Cdk1 promoted MT nucleation through Spc110 phosphorylation. Phosphorylated Spc110 and the interaction with the N-terminal domain of Spc98 induce γ-TuSC oligomerization. In addition, bioinformatic analysis revealed a conserved region around T18, that we named Spc110/Pcp1 motif (SPM). SPM and CM1 motifs are both important for γ-TuSC binding and oligomerization. A comparison of γ-TuCRs for the presence of SPM and CM1 identified SPM-CM1 (Spc110, Pcp1, PCNT) and CM1-only types of γ-TuCRs (Spc72, Mto1, Cnn, CDK5RAP2, myomegalin) in most organisms. While the SPM-CM1 type of γ-TuCRs carries the PACT domain and is targeted only to the centrosome or the nuclear side of the SPB, the CM1-only type of γ-TuCRs has either a MASC (Mto1 and Spc72 C-terminus) (<xref ref-type="bibr" rid="bib64">Samejima et al., 2010</xref>) or a CM2 motif and is recruited to, centrosomes, the cytoplasmic side of the SPB or acentrosomal MTOCs.</p></sec><sec id="s2" sec-type="results"><title>Results</title><sec id="s2-1"><title>Phosphorylation of N-Spc110 at Mps1 and Cdk1 sites is required for efficient interaction with γ-TuSC</title><p>To test whether Spc110<sup>1–220</sup> phosphorylation promoted γ-TuSC ring formation, we purified GST-Spc110<sup>1–220</sup> (named Spc110<sup>1–220</sup>) from both <italic>E. coli</italic> and the baculovirus expression system. Spc110<sup>1–220</sup> encompasses Cdk1 and Mps1 phosphorylation sites and the conserved CM1 motif (<xref ref-type="fig" rid="fig1">Figure 1A</xref>). Because of the post-translational modification machinery, Spc110<sup>1–220</sup> purified from insect cells harboured phosphorylations on S60/T68 and S36/S91 (<xref ref-type="fig" rid="fig1s1">Figure 1—figure supplement 1A–D</xref>), that correspond to established Mps1 and Cdk1 sites, respectively (<xref ref-type="fig" rid="fig1">Figure 1A</xref>; <xref ref-type="bibr" rid="bib26">Friedman et al., 2001</xref>; <xref ref-type="bibr" rid="bib33">Huisman et al., 2007</xref>). In contrast, Spc110<sup>1–220</sup> was not phosphorylated when purified from <italic>E. coli</italic>.<fig-group><fig id="fig1" position="float"><object-id pub-id-type="doi">10.7554/eLife.02208.003</object-id><label>Figure 1.</label><caption><title>Phosphorylation of N-Spc110 is required for the γ-TuSC oligomerization.</title><p>(<bold>A</bold>) Diagram of Spc110's functional domain organization and the position of phospho-sites investigated in this study. These sites are located at the N-terminal domain of Spc110, which directly interacts with γ-TuSC (<xref ref-type="bibr" rid="bib81">Vinh et al., 2002</xref>). The conserved centrosomin motif 1 (CM1) (<xref ref-type="bibr" rid="bib65">Sawin et al., 2004</xref>; <xref ref-type="bibr" rid="bib89">Zhang and Megraw, 2007</xref>) is also within the N-terminal domain. The C-terminal domain of Spc110 is involved in the interaction with SPB central plaque components Spc29 and Spc42 (<xref ref-type="bibr" rid="bib1">Adams and Kilmartin, 1999</xref>; <xref ref-type="bibr" rid="bib20">Elliott et al., 1999</xref>). CBD: calmodulin binding domain. (<bold>B</bold>) Post-translational modifications of Spc110<sup>1–220</sup> are required for promoting γ-TuSC oligomerization. Spc110<sup>1–220</sup> was expressed and purified from insect cells (middle panel) or from <italic>E. coli</italic> (right panel). Only Spc110<sup>1–220</sup> from insect cells carried post-translational modifications (<xref ref-type="fig" rid="fig1s1">Figure 1—figure supplement 1</xref>). Recombinant γ-TuSC was incubated with Spc110<sup>1–220</sup> or TB150 buffer only on ice. Oligomerization of γ-TuSC-Spc110<sup>1–220</sup> was tested by gel filtration chromatography using a Superdex 200 10/300 column. Peak fractions of the chromatograms were analysed by SDS-PAGE and silver staining.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.003">http://dx.doi.org/10.7554/eLife.02208.003</ext-link></p></caption><graphic xlink:href="elife02208f001"/></fig><fig id="fig1s1" position="float" specific-use="child-fig"><object-id pub-id-type="doi">10.7554/eLife.02208.004</object-id><label>Figure 1—figure supplement 1.</label><caption><title>Phosphorylation of Spc110<sup>1–220</sup> from insect cells.</title><p>(<bold>A</bold>) Table of mass-spectrometry identified phosphopeptides of Spc110<sup>1–220</sup> purified from baculovirus-insect cell expression system. (<bold>B</bold>) Mass spectra of identified Mps1 sites (S60 and T68). (<bold>C</bold>) Mass spectra of identified Cdk1 sites (S36 and S91). (<bold>D</bold>) Comparison of investigated phospho-sites of Spc110 N-terminal domain with published data (<xref ref-type="bibr" rid="bib26">Friedman et al., 2001</xref>; <xref ref-type="bibr" rid="bib33">Huisman et al., 2007</xref>; <xref ref-type="bibr" rid="bib3">Albuquerque et al., 2008</xref>; <xref ref-type="bibr" rid="bib39">Keck et al., 2011</xref>; <xref ref-type="bibr" rid="bib50">Lin et al., 2011</xref>).</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.004">http://dx.doi.org/10.7554/eLife.02208.004</ext-link></p></caption><graphic xlink:href="elife02208fs001"/></fig><fig id="fig1s2" position="float" specific-use="child-fig"><object-id pub-id-type="doi">10.7554/eLife.02208.005</object-id><label>Figure 1—figure supplement 2.</label><caption><title>γ-TuSC does not oligomerize in TB150 buffer.</title><p>(<bold>A</bold>–<bold>C</bold>) Purified γ-TuSC (<bold>A</bold>) spontaneously oligomerizes in BRB80 buffer (<bold>B</bold>) but not in TB150 buffer (<bold>C</bold>). Proteins were analyzed by Superose 6 10/300 and subsequently gel filtration fractions were subjected to SDS-PAGE and silver staining. Samples in the peak fraction were subjected to negative staining and protein complexes were analyzed by electron microscopy. Corresponding scale bars are shown on the images.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.005">http://dx.doi.org/10.7554/eLife.02208.005</ext-link></p></caption><graphic xlink:href="elife02208fs002"/></fig></fig-group></p><p>We incubated purified γ-TuSC with either the phosphorylated Spc110<sup>1–220</sup> from insect cells (Spc110<sup>1–220-P</sup>) or the non-phosphorylated Spc110<sup>1–220</sup> from <italic>E. coli</italic> and then analyzed the protein complexes by gel filtration. Spc110 dimerizes via the coiled-coli region in the centre of the protein (<xref ref-type="bibr" rid="bib40">Kilmartin et al., 1993</xref>; <xref ref-type="bibr" rid="bib54">Muller et al., 2005</xref>). To substitute for the lack of this region, we performed the assays with GST-Spc110<sup>1–220</sup> that dimerizes via GST–GST interactions. We used TB150 buffer in our assay instead of the BRB80 buffer used by <xref ref-type="bibr" rid="bib45">Kollman et al. (2010)</xref>. BRB80 induces oligomerization of γ-TuSC without the need for addition of Spc110 (<xref ref-type="fig" rid="fig1s2">Figure 1—figure supplement 2A,B</xref>). In contrast, in TB150 buffer the majority of γ-TuSC was monomeric (<xref ref-type="fig" rid="fig1s2">Figure 1—figure supplement 2C</xref>). TB150 therefore allowed us to monitor the impact of Spc110<sup>1–220</sup> on γ-TuSC oligomerization by gel filtration. Addition of Spc110<sup>1–220-P</sup> shifted γ-TuSC into fractions that eluted earlier than when γ-TuSC was run on the columns on its own (<xref ref-type="fig" rid="fig1">Figure 1B</xref>, shift from fraction 10 to fraction 8). The peak at the void-volume (fraction 8) likely represented γ-TuSC oligomers. In contrast, non-phosphorylated Spc110<sup>1–220</sup> expressed in <italic>E. coli</italic> did not change the γ-TuSC elution profile. The γ-TuSC pool eluted as monomeric γ-TuSC (<xref ref-type="fig" rid="fig1">Figure 1B</xref>, fractions 10–11). This result suggests that Mps1 and Cdk1 kinases regulate the interaction between Spc110 and γ-TuSC through phosphorylation of Spc110 N-terminal domain.</p></sec><sec id="s2-2"><title>Phosphorylation of N-Spc110 regulates the γ-TuSC oligomerization promoting activity</title><p>To further confirm that phosphorylation of Spc110<sup>1–220</sup> alters the interaction with γ-TuSC, phosphomimetic and non-phosphorylatable Spc110<sup>1–220</sup> mutant proteins were purified from <italic>E. coli</italic> (<xref ref-type="fig" rid="fig2s1">Figure 2—figure supplement 1A</xref>). <italic>spc110</italic><sup><italic>2A</italic></sup> (Cdk1 sites: S36A, S91A) and <italic>spc110</italic><sup><italic>3A</italic></sup> (Mps1 sites: S60A, T64A, T68A) have been studied in vivo before and shown to behave as non-phosphorylatable mutants (<xref ref-type="bibr" rid="bib26">Friedman et al., 2001</xref>; <xref ref-type="bibr" rid="bib33">Huisman et al., 2007</xref>; <xref ref-type="bibr" rid="bib49">Liang et al., 2013</xref>). In addition, we analyzed the phosphomimetic Spc110<sup>1–220-2D</sup> (Cdk1 sites: S36D and S91D), Spc110<sup>1–220-3D</sup> (Mps1 sites: S60D, T64D, T68D), and Spc110<sup>1–220-5D</sup> (Cdk1 and Mps1 sites: S36D, S60D, T64D, T68D, and S91D). Migration of all purified Spc110<sup>1–220</sup> mutant proteins was comparable upon gel filtration, indicating that the mutations did not alter the overall structure of the protein (<xref ref-type="fig" rid="fig2s1">Figure 2—figure supplement 1B,C</xref>).</p><p>We used gel filtration chromatography to address whether Spc110<sup>1–220</sup> mutant proteins induce higher-order γ-TuSC oligomers. The phosphomimetic Spc110<sup>1–220-2D</sup> (Cdk1 sites), Spc110<sup>1–220-3D</sup> (Mps1 sites), and Spc110<sup>1–220-5D</sup> (Cdk1 and Mps1 sites) induced an apparent shift of γ-TuSC towards the void-volume compared to the γ-TuSC only run or γ-TuSC plus non-phosphorylated Spc110<sup>1–220-WT</sup> from <italic>E. coli</italic> (<xref ref-type="fig" rid="fig2">Figure 2A–C</xref>, <xref ref-type="fig" rid="fig2s2">Figure 2—figure supplement 2A,B</xref>). The behavior of the phosphomimetic Spc110 mutant proteins reflected that of in vivo phosphorylated Spc110<sup>1–220-P</sup> purified from insect cells (<xref ref-type="fig" rid="fig1">Figure 1B</xref>), indicating that the mutations to acidic residues did mimic phosphorylation. In contrast, the γ-TuSC shift was not seen upon inclusion of non-phosphorylatable Spc110<sup>1–220-5A</sup> (Cdk1 and Mps1 sites) with the γ-TuSC (<xref ref-type="fig" rid="fig2s2">Figure 2—figure supplement 2A,B</xref>). Thus, Spc110<sup>1–220-5A</sup> behaved as the non-phosphorylated Spc110<sup>1–220</sup> purified from <italic>E. coli</italic>, suggesting that the non-phosphorylatable mutations did not impair the protein. This conclusion was further supported by the observation that Spc110<sup>1–220-2D</sup> or Spc110<sup>1–220-3D</sup> still possessed oligomerization-promoting activity when combined with non-phosphorylatable mutations (Spc110<sup>1–220-2D3A</sup> and Spc110<sup>1–220-2A3D</sup>, <xref ref-type="fig" rid="fig2s3">Figure 2—figure supplement 3</xref>). Taken together, the gel filtration experiment suggests that phosphorylated Spc110 induces oligomerization of γ-TuSC.<fig-group><fig id="fig2" position="float"><object-id pub-id-type="doi">10.7554/eLife.02208.006</object-id><label>Figure 2.</label><caption><title>Mps1 and Cdk1 phosphorylation of N-Spc110 stimulates γ-TuSC oligomerization.</title><p>(<bold>A</bold>) Summary of combination of non-phosphorylatable and phospho-mimicking mutations in Spc110 used in this study. The indicated Spc110<sup>1–220</sup> variants were expressed, purified from <italic>E. coli</italic>, and then tested for γ-TuSC oligomerization. WT: wild-type; 2A/D: S36A/D, S91A/D; 3A/D: S60A/D, T64A/D, T68A/D; 5A/D: S36A/D, S91A/D, S60A/D, T64A/D, T68A/D (see <xref ref-type="fig" rid="fig2s1">Figure 2—figure supplement 1A</xref> for SDS-PAGE of purified Spc110<sup>1–220</sup> proteins and <xref ref-type="fig" rid="fig2s1">Figure 2—figure supplement 1C</xref> for gel filtration chromatograms). (<bold>B</bold>) Spc110<sup>1–220</sup> phospho-mimicking proteins induced γ-TuSC oligomerization. Spc110<sup>1–220</sup> proteins were incubated with γ-TuSC in TB150 buffer. The reconstituted complexes were separated according to size by gel filtration using a Superose 6 column. (<bold>C</bold>) Bar graph of area ratio of void-volume peak to total area of chromatogram of (<bold>B</bold>). ** marks statistical significance at p<italic><</italic>0.01. Error bars represent SEM. N = 3 to 9 for the number of experiments performed. (<bold>D</bold>) Void-volume peak fractions of (<bold>B</bold>) were subjected to negative staining and protein complexes were analyzed with electron microscopy. Representative ring-like structures of γ-TuSC-Spc110<sup>1–220-2D</sup>, γ-TuSC-Spc110<sup>1–220-3D</sup>, and γ-TuSC-Spc110<sup>1–220-5D</sup>. See <xref ref-type="fig" rid="fig2s4">Figure 2—figure supplement 4A</xref> for additional EM pictures. Scale bar: 50 nm. (<bold>E</bold>) Quantification of (<bold>D</bold>). Shown is the particle number per field. Particles were categorized based on the morphology. N is indicated on the figure for the number of fields analysed. (<bold>F</bold>) Quantification of void-volume fractions of γ-TuSC chromatograms. γ-TuSC was incubated with Spc110<sup>1–220-WT</sup>, Spc110<sup>1–220-5D</sup>, Spc110<sup>1–220-T18A</sup>, Spc110<sup>1–220-T18D</sup>, and Spc110<sup>1–220-5D−T18D</sup> as described in (<bold>B</bold>). Bar graph of area ratio of void-volume peak to total area of chromatogram was calculated as in (<bold>C</bold>). ** marks statistical significance at p<italic><</italic>0.01. Error bars represent SEM. N = 3 to 9 for the number of experiments performed. (<bold>G</bold>) Quantification of EM. Void-volume peak fractions of (<bold>F</bold>) were subjected to negative staining and protein complexes were analyzed with electron microscopy. Particles were categorized based on the morphology. Shown is the particle number per field. Note, the Spc110<sup>1–220−WT</sup> and Spc110<sup>1–220-5D</sup> graphs are the same as in (<bold>E</bold>). N is indicated on the figure for the number of fields analysed. (<bold>H</bold>) Multiple sequence alignment of SPM element of γ-complex receptors from yeast to human. Residues are marked according to the ClustalX colour scheme. The occurrence of each amino acid in each position of CM1 motif is presented with Weblogo 2.0. (<bold>I</bold>) The indicated Spc110<sup>1–220</sup> proteins (Spc110<sup>1–220-5D</sup>, Spc110<sup>1–220-5D-CM1-QA</sup>, and Spc110<sup>1–220-5D-<italic>Δ</italic>SPM</sup>) were incubated with γ-TuSC in TB150 buffer. The reconstituted complexes were separated according to size by gel filtration using a Superose 6 column.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.006">http://dx.doi.org/10.7554/eLife.02208.006</ext-link></p></caption><graphic xlink:href="elife02208f002"/></fig><fig id="fig2s1" position="float" specific-use="child-fig"><object-id pub-id-type="doi">10.7554/eLife.02208.007</object-id><label>Figure 2—figure supplement 1.</label><caption><title>Purification of Spc110<sup>1–200</sup> variants.</title><p>(<bold>A</bold>) Purified GST-Spc110<sup>1–220</sup> variants. ∼1 μg of protein was loaded to each lane. The SDS-PAGE was stained with Coomassie Blue. (<bold>B</bold>) Calibration of Superose 6 10/300 gel filtration column. Following protein markers were used: thyroglobulin (667 kDa), ferritin (440 kDa), aldolase (158 kDa), conalbumin (75 kDa), ovalbumin (44 kDa). Partition coefficient (K<sub>av</sub>) was calculated using the equation: (V<sub>e</sub>–V<sub>0</sub>)/(V<sub>c</sub>–V<sub>0</sub>), where V<sub>0</sub> = column void volume, V<sub>e</sub> = elution volume, and V<sub>c</sub> = geometric column volume. The calibration curve (bottom) of K<sub>av</sub> vs log molecular weight was plotted in semi-logarithmic scale. (<bold>C</bold>) Gel filtration chromatograms of purified GST-Spc110<sup>1–220</sup> variants. Proteins were analyzed in TB150 buffer by Superose 6 10/300 chromatography.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.007">http://dx.doi.org/10.7554/eLife.02208.007</ext-link></p></caption><graphic xlink:href="elife02208fs003"/></fig><fig id="fig2s2" position="float" specific-use="child-fig"><object-id pub-id-type="doi">10.7554/eLife.02208.008</object-id><label>Figure 2—figure supplement 2.</label><caption><title>Phosphomimetic but not SPM defective N-Spc110 proteins induce oligomerization of γ-TuSC.</title><p>(<bold>A</bold>) Overlapped gel filtration chromatograms with molecular weight markers. The peak of the void-volume (V<sub>0</sub>, boxed area) corresponds to molecular weight fractions higher than 5000 kDa. Note that the concentrations of γ-TuSC and Spc110<sup>1–220</sup> variants in (<bold>A</bold>) were double as high compared to (<bold>B</bold>) and (<bold>C</bold>). (<bold>B</bold>) Gel filtration chromatograms of γ-TuSC incubated with phosphomimetic or non-phosphorylatable Spc110<sup>1–220</sup> mutant proteins. γ-TuSC was incubated in TB150 buffer with Spc110<sup>1–220</sup> proteins from <xref ref-type="fig" rid="fig2s1">Figure 2—figure supplement 1A</xref>. Complexes were analyzed by Superose 6 10/300 chromatography. Fractions corresponding to the peak were analyzed by SDS-PAGE and silver staining. (<bold>C</bold>) As (<bold>B</bold>) but with Spc110<sup>1–220</sup> variants that have defective SPM due to T18D or T18A.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.008">http://dx.doi.org/10.7554/eLife.02208.008</ext-link></p></caption><graphic xlink:href="elife02208fs004"/></fig><fig id="fig2s3" position="float" specific-use="child-fig"><object-id pub-id-type="doi">10.7554/eLife.02208.009</object-id><label>Figure 2—figure supplement 3.</label><caption><title>Non-phosphorylatable mutations in Mps1 or Cdk1 sites of Spc110 are neutral to <bold>γ</bold>-TuSC oligomerization induced by phosphomimetic mutations.</title><p>Gel filtration chromatograms of γ-TuSC incubated with indicated Spc110<sup>1–220</sup> variants in TB150 buffer. Non-phosphorylatable mutations in addition to phosphomimetic mutations were introduced on Spc110<sup>1–220</sup> to examine the possibility that non-phosphorylatable mutations alter overall structure and thus disrupt the activity to induce γ-TuSC oligomerization. Complexes were analyzed by Superose 6 10/300 chromatography.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.009">http://dx.doi.org/10.7554/eLife.02208.009</ext-link></p></caption><graphic xlink:href="elife02208fs005"/></fig><fig id="fig2s4" position="float" specific-use="child-fig"><object-id pub-id-type="doi">10.7554/eLife.02208.010</object-id><label>Figure 2—figure supplement 4.</label><caption><title>EM single particle analysis of oligomerized γ-TuSC.</title><p>(<bold>A</bold>) Additional examples of ring-like and filament-like γ-TuSC-Spc110<sup>1–220</sup> complexes. Scale bar: 50 nm. (<bold>B</bold>) Summary of particle numbers of γ-TuSC oligomerized by Spc110<sup>1–220</sup> phospho-mimicking variants. Both the absolute particle number and the percentage of each particle category are shown. (<bold>C</bold>) Spc110<sup>1–220-2D</sup>, Spc110<sup>1–220-3D</sup>, and Spc110<sup>1–220-5D</sup> induced similar level of ring-like γ-TuSC-Spc110<sup>1–220</sup> complexes. * marks statistical significance at p<0.05. Error bars represent SEM. N = 15–25 for the number of field analyzed. Correlated to <xref ref-type="fig" rid="fig2">Figure 2E,G</xref>. (<bold>D</bold>) The diameter of the ring-like γ-TuSC-Spc110<sup>1–220</sup> complexes showed no difference among Spc110<sup>1–220</sup> phosphomimetic variants. Error bars represent SEM. N = 9, 12, and 18 for the number of ring-like complexes measured in 2D, 3D, and 5D, respectively.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.010">http://dx.doi.org/10.7554/eLife.02208.010</ext-link></p></caption><graphic xlink:href="elife02208fs006"/></fig><fig id="fig2s5" position="float" specific-use="child-fig"><object-id pub-id-type="doi">10.7554/eLife.02208.011</object-id><label>Figure 2—figure supplement 5.</label><caption><title>Mutations of T18 abolish the <bold>γ</bold>-TuSC oligomerization promoting activity by inactivating the SPM motif.</title><p>(<bold>A</bold>) Gel filtration chromatograms of γ-TuSC incubated with phosphomimetic Spc110<sup>1–220-5D</sup> with additional T18 mutations. Complexes were analyzed by Superose 6 10/300 chromatography in TB150 buffer. (<bold>B</bold>) Growth of ten-fold serial dilutions of <italic>SPC110</italic> shuffle strains carrying the empty integration vector (null) or <italic>SPC110</italic> (WT) or <italic>SPC110</italic> mutant alleles on the integration vector. Wild type and SAC deficient <italic>mad2Δ</italic> cells were analyzed<italic>.</italic> Growth was tested either on synthetic complete (SC) plates containing 5-FOA or SC dropout plates.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.011">http://dx.doi.org/10.7554/eLife.02208.011</ext-link></p></caption><graphic xlink:href="elife02208fs007"/></fig><fig id="fig2s6" position="float" specific-use="child-fig"><object-id pub-id-type="doi">10.7554/eLife.02208.012</object-id><label>Figure 2—figure supplement 6.</label><caption><title>Multiple sequence alignment of CM1 motif-containing proteins.</title><p>Multiple sequence alignment of selected γ-TuSC receptor family members containing CM1 motifs. Two of the most conserved residues within CM1 motif are marked with asterisks and mutated to disrupt CM1 function in the <italic>spc110</italic><sup><italic>CM1-QA</italic></sup> mutant. The occurrence of each amino acid in each position of CM1 motif is presented with Weblogo 2.0.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.012">http://dx.doi.org/10.7554/eLife.02208.012</ext-link></p></caption><graphic xlink:href="elife02208fs008"/></fig></fig-group></p><p>Because the upper limit of the separation range of the Superose 6 gel filtration column is ∼5000 kDa, the γ-TuSC oligomers in the void-volume contain at least 14 copies of γ-TuSC (14-mers). This indicates the presence of ring-like complexes or even higher-order oligomeric structures in the void-volume. To confirm the existence of structurally organized oligomers, fractions corresponding to the void-volume peak (<xref ref-type="fig" rid="fig2s2">Figure 2—figure supplement 2A</xref>, fraction 7) were subjected to EM analysis (<xref ref-type="fig" rid="fig2">Figure 2D,E</xref>, <xref ref-type="fig" rid="fig2s4">Figure 2—figure supplement 4A</xref>). Consistent with the low A<sub>280</sub> absorbance, hardly any oligomeric γ-TuSC species were found in the void fraction of γ-TuSC only or γ-TuSC incubated with the non-phosphorylated Spc110<sup>1–220-WT</sup> (<xref ref-type="fig" rid="fig2">Figure 2E</xref>, <xref ref-type="fig" rid="fig2s4">Figure 2—figure supplement 4B</xref>). In contrast, oligomeric γ-TuSC ring-like assemblies were observed when Spc110<sup>1–220-2D</sup>, Spc110<sup>1–220-3D</sup>, and Spc110<sup>1–220-5D</sup> (<xref ref-type="fig" rid="fig2">Figure 2D,E</xref>, <xref ref-type="fig" rid="fig2s4">Figure 2—figure supplement 4A,B</xref>) had been added to the γ-TuSC preparation. Interestingly, the percentage of γ-TuSC spirals increased steadily with increasing numbers of phosphomimetic mutations from Spc110<sup>1–220-2D</sup>, Spc110<sup>1–220-3D</sup> to Spc110<sup>1–220-5D</sup> (<xref ref-type="fig" rid="fig2">Figure 2E</xref>, <xref ref-type="fig" rid="fig2s4">Figure 2—figure supplement 4B</xref>), while the number of rings per field was relatively constant (<xref ref-type="fig" rid="fig2s4">Figure 2—figure supplement 4C</xref>). The outer diameter of the ring-like assemblies was on average around 40 nm (<xref ref-type="fig" rid="fig2s4">Figure 2—figure supplement 4D</xref>). Thus, combined phosphorylation of Spc110 by Cdk1 and Mps1 kinases enhances the oligomerization activity of Spc110 better than phosphorylation by either kinase alone.</p></sec><sec id="s2-3"><title>The SPM motif is important for Spc110 oligomerization promoting activity</title><p>An additional phosphorylation of N-Spc110 at T18 has been identified in phosphoproteomic studies (<xref ref-type="bibr" rid="bib3">Albuquerque et al., 2008</xref>; <xref ref-type="bibr" rid="bib39">Keck et al., 2011</xref>; <xref ref-type="bibr" rid="bib50">Lin et al., 2011</xref>). However, its precise function remained unclear (<xref ref-type="bibr" rid="bib49">Liang et al., 2013</xref>). To elucidate the role of Spc110<sup>T18</sup> phosphorylation, Spc110<sup>1–220-T18D</sup>, Spc110<sup>1–220-2D-T18D</sup> (Cdk1 plus T18), Spc110<sup>1–220-3D-T18D</sup> (Mps1 plus T18), Spc110<sup>1–220-5D-T18D</sup> (T18D in addition to Mps1 and Cdk1 sites), and Spc110<sup>1–220-5D-T18A</sup> proteins were purified (<xref ref-type="fig" rid="fig2s1">Figure 2—figure supplement 1A</xref>) and tested for their γ-TuSC oligomerization promoting activity. Spc110<sup>1–220</sup> species with T18D or T18A failed to induce γ-TuSC oligomerization even when combined with the activating 5D mutations (<xref ref-type="fig" rid="fig2">Figure 2F</xref>, <xref ref-type="fig" rid="fig2s2">Figure 2—figure supplement 2C</xref>). Similar data were obtained with Spc110<sup>1–220-5D-T18E</sup> and Spc110<sup>1–220-5D-T18V</sup> (<xref ref-type="fig" rid="fig2s5">Figure 2—figure supplement 5A</xref>). Moreover, <italic>spc110</italic><sup><italic>T18D</italic></sup>, <italic>spc110</italic><sup><italic>T18E</italic></sup>, <italic>spc110</italic><sup><italic>T18A</italic></sup>, and <italic>spc110</italic><sup><italic>T18V</italic></sup> showed identical growth defects at 37°C (<xref ref-type="fig" rid="fig2s5">Figure 2—figure supplement 5B</xref>). Since the effects of T18D/E and T18A/V mutations were indistinguishable, we cannot attribute these mutations as phosphomimetic or non-phosphorylatable. However, our results strongly suggest that these mutations affect the structure of an important yet unappreciated element around T18 that is important for γ-TuSC oligomerization. Consistently, with EM we observed hardly any oligomeric structures when γ-TuSC was incubated with Spc110<sup>1–220-5D-T18D</sup> (<xref ref-type="fig" rid="fig2">Figure 2G</xref>).</p><p>With the sequence alignments of Spc110 orthologs from yeast to human, we observed a conserved motif upstream of CM1, which we designated as Spc110/Pcp1 motif (SPM) (<xref ref-type="fig" rid="fig2">Figure 2H</xref>). Spc110<sup>T18</sup> sits within this motif. To evaluate the relative importance of CM1 and SPM, we constructed the Spc110<sup>1–220-CM1-QA</sup> mutant protein with mutations (KE to QA) in two highly conserved residues of the CM1 (<xref ref-type="fig" rid="fig2s6">Figure 2—figure supplement 6</xref>) and the Spc110<sup>1–220-ΔSPM</sup> lacking the SPM motif (deletion of amino acids 1–20). Spc110<sup>1–220</sup> proteins were incubated with γ-TuSC in TB150 buffer to examine their oligomerization capacity. Spc110<sup>1–220-5D-CM1-QA</sup> and Spc110<sup>1–220-5D-ΔSPM</sup> mutant proteins failed to activate γ-TuSC oligomerization even when the five activating Cdk1 and Mps1 mutations were present (<xref ref-type="fig" rid="fig2">Figure 2I</xref>). Thus, Spc110<sup>1–220-5D-ΔSPM</sup> behaved as Spc110<sup>1–220-5D-T18D</sup> (compare <xref ref-type="fig" rid="fig2">Figure 2I</xref> with <xref ref-type="fig" rid="fig2s2">Figure 2—figure supplement 2C</xref>), further emphasizing that T18D inactivates the SPM. Taken together, SPM and CM1 of Spc110 are both required for γ-TuSC oligomerization.</p></sec><sec id="s2-4"><title>The SPM and CM1 motifs and the Cdk1 and Mps1 phosphorylations regulate the affinity of Spc110 to γ-TuSC</title><p>Phosphorylation of Spc110 may regulate the γ-TuSC oligomerization by changing its binding affinity. Therefore we performed GST-pulldown assay to measure the binding of the Spc110<sup>1–220</sup> variants to γ-TuSC. A constant amount of γ-TuSC was incubated with increasing concentrations of Spc110<sup>1–220</sup> variants (0–300 nM). Consistent with the γ-TuSC oligomerization assay (<xref ref-type="fig" rid="fig2">Figure 2</xref>), Spc110<sup>1–220-5D</sup> showed stronger (p<italic>=</italic>0.0017 for Spc97/Spc98 and p<italic>=</italic>0.0006 for Tub4) γ-TuSC binding than Spc110<sup>1–220-WT</sup> (<xref ref-type="fig" rid="fig3">Figure 3A,B</xref>). Interestingly, while Spc110<sup>1–220-2D</sup> and Spc110<sup>1–220-3D</sup> induced comparable levels of γ-TuSC oligomerization (<xref ref-type="fig" rid="fig2">Figure 2B</xref>), Spc110<sup>1–220-2D</sup> showed less γ-TuSC binding than Spc110<sup>1–220-3D</sup>. A likely explanation is the 30-fold reduction in concentration that was used in the GST-pulldown assay compared to γ-TuSC oligomerization assays. These results suggested that phosphorylation of Spc110 on S36, S60, T64, T68, and S91 promotes γ-TuSC binding to Spc110.<fig id="fig3" position="float"><object-id pub-id-type="doi">10.7554/eLife.02208.013</object-id><label>Figure 3.</label><caption><title>Phosphorylation of N-Spc110 regulates the affinity to γ-TuSC.</title><p>(<bold>A</bold>) GST pull-down assays were performed between γ-TuSC (containing His-tagged Spc97-6His and Spc98-6His) and the GST-tagged Spc110<sup>1–220</sup> proteins. The bound proteins were eluted with sample buffer and separated on SDS-PAGE and analyzed by immunoblotting with anti-His, anti-GST, and anti-Tub4 antibodies. Infrared-dye-labelled secondary antibodies were applied and detected with Li-cor imaging system. (<bold>B</bold>) Quantification for bound Spc97, Spc98, and Tub4 from (<bold>A</bold>) at 300 nM Spc110<sup>1–220</sup>. ** marks statistical significance at p<italic><</italic>0.01. Error bars represent SEM. N = 3 for the number of experiments performed. (<bold>C</bold>) Mutations in SPM or CM1 affect γ-TuSC binding. As (<bold>A</bold>) but performed with the indicated SPM and CM1 mutant Spc110<sup>1–220</sup> constructs. (<bold>D</bold>) Quantification of Spc110<sup>1–220</sup> variants and bound Spc97, Spc98, and Tub4 from (<bold>C</bold>) at 300 nM Spc110<sup>1–220</sup>. ** marks statistical significance at p<italic><</italic>0.01 and *** at p<italic><</italic>0.001. Error bars represent SEM. N = 3 for the number of experiments performed.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.013">http://dx.doi.org/10.7554/eLife.02208.013</ext-link></p></caption><graphic xlink:href="elife02208f003"/></fig></p><p>We also analyzed binding of SPM (T18D and ΔSPM) and CM1 (CM1-QA) deficient Spc110<sup>1–220</sup> mutant proteins to γ-TuSC. Both Spc110<sup>1–220-5D-T18D</sup> and Spc110<sup>1–220-5D-CM1-QA</sup> exhibited reduced γ-TuSC binding relative to Spc110<sup>1–220-5D</sup> (p<0.01 for Spc97/Spc98 and p<0.05 for Tub4) (<xref ref-type="fig" rid="fig3">Figure 3C,D</xref>). The γ-TuSC binding capacity of Spc110<sup>1–220-5D-T18D-CM1-QA</sup> was further reduced (p<0.01, <xref ref-type="fig" rid="fig3">Figure 3C,D</xref>), suggesting both CM1 and SPM are important for γ-TuSC binding. Interestingly, while Spc110<sup>1–220-5D-T18D</sup>, Spc110<sup>1–220-5D-CM1-QA</sup>, and Spc110<sup>1–220-ΔSPM</sup> all failed to oligomerize γ-TuSC (<xref ref-type="fig" rid="fig2">Figure 2F,I</xref>), they showed different levels of γ-TuSC binding capacity (<xref ref-type="fig" rid="fig3">Figure 3D</xref>). The decoupling of γ-TuSC binding capacity from γ-TuSC oligomerization suggests that both processes are not inevitably linked. Taken together, we conclude that both CM1 and SPM motifs contribute to γ-TuSC binding and are required for promoting γ-TuSC oligomerization.</p></sec><sec id="s2-5"><title>SPM and the phosphorylation of N-Spc110 control MT nucleation activity in vitro</title><p>In vitro MT nucleation assays were performed to test the effect of Spc110 variants on γ-TuSC-mediated MT nucleation. Since phosphorylation of Spc110 and the presence of a functional SPM regulate γ-TuSC oligomerization into template rings, we expected to see differences in MT nucleation depending on these parameters (<xref ref-type="fig" rid="fig2 fig3">Figures 2 and 3</xref>). Compared to the buffer control and γ-TuSC alone, a ∼threefold increase (p<0.05) in MT nucleation activity was observed for Spc110<sup>1–220-2D</sup>, Spc110<sup>1–220-3D</sup>, or Spc110<sup>1–220-5D</sup> upon incubation with γ-TuSC (<xref ref-type="fig" rid="fig4">Figure 4A,B</xref>). In contrast, Spc110<sup>1–220-WT</sup> showed levels of MT nucleation that were comparable to the buffer control and γ-TuSC alone. Consistent with the proposed role of SPM, the MT nucleation level was reduced to buffer control levels when Spc110<sup>1–220-5D-T18D</sup> was incubated with γ-TuSC (p=0.99). These results are in accordance with EM particle quantification assays, as Spc110<sup>1–220-2D</sup>, Spc110<sup>1–220-3D</sup>, or Spc110<sup>1–220-5D</sup> induced similar and significantly more γ-TuSC ring assemblies than Spc110<sup>1–220-WT</sup> and Spc110<sup>1–220-5D-T18D</sup> (p<0.05, <xref ref-type="fig" rid="fig2">Figure 2E</xref>, <xref ref-type="fig" rid="fig2s4">Figure 2—figure supplement 4C</xref>). In summary, these data support the model that Cdk1 and Mps1 phosphorylations of Spc110 regulate γ-TuSC-mediated MT nucleation by controlling template assembly.<fig id="fig4" position="float"><object-id pub-id-type="doi">10.7554/eLife.02208.014</object-id><label>Figure 4.</label><caption><title>Spc110<sup>1–220</sup> phosphorylation enhances MT nucleation activity in vitro.</title><p>(<bold>A</bold>) Enhancement of MT nucleation activity of Spc110 by Mps1 and Cdk1 phosphorylation. Representative image fields of Alexa546-labeled microtubules from the nucleation assay polymerized in the presence of buffer, γ-TuSC and Spc110<sup>1–220</sup> variants (<bold>B</bold>). Scale bar: 10 µm. (<bold>B</bold>) Quantification of the MT nucleation assay. For each experiment, the number of MTs was counted from 20 fields. The MTs/field was normalized with MTs/field obtained by the MT nucleation reaction in the presence of buffer only. N = 6 to 9 for the number of independent experiments performed. * marks statistical significance at p<0.05 and ** at p<italic><</italic>0.01. Error bars represent SEM.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.014">http://dx.doi.org/10.7554/eLife.02208.014</ext-link></p></caption><graphic xlink:href="elife02208f004"/></fig></p></sec><sec id="s2-6"><title>Phosphorylation of Spc110 is cell cycle dependent</title><p>To analyze cell cycle dependent phosphorylation of Spc110, we generated one phospho-specific antibody against the two Cdk1 sites (Spc110<sup>S36</sup> and Spc110<sup>S91</sup>) (<xref ref-type="fig" rid="fig5">Figure 5A</xref>) and another against the Mps1 sites on Spc110 (Spc110<sup>pS60-pT64-pT68</sup>) (<xref ref-type="fig" rid="fig5">Figure 5B</xref>). Both anti-Spc110<sup>pS36-pS91</sup> and anti-Spc110<sup>pS60-pT64-pT68</sup> antibodies gave specific signals in vitro and in vivo (<xref ref-type="fig" rid="fig5">Figure 5A–C</xref>).<fig-group><fig id="fig5" position="float"><object-id pub-id-type="doi">10.7554/eLife.02208.015</object-id><label>Figure 5.</label><caption><title>Cell cycle dependent phosphorylation of Spc110.</title><p>(<bold>A</bold> and <bold>B</bold>) Two phospho-specific antibodies were generated from guinea pigs to recognize phosphorylation of Cdk1 sites (pS36-pS91) (<bold>A</bold>) and Mps1 sites (p60-p64-pT68) (<bold>B</bold>). In vitro kinase assays were performed in the presence of recombinant Mps1 or Cdk1<sup>as1</sup> and Spc110<sup>1–220</sup>, either wild type (WT) or non-phosphorylatable variants as indicated. 3A (Cdk1) indicates T18A-S36A-S91A and 3A (Mps1) S60A-T64A-T68A. In vitro phosphorylated Spc110<sup>1–220</sup> was subjected to SDS-PAGE and immunoblot with the corresponding phospho-specific antibodies. The specific kinase activity of Cdk1<sup>as1</sup>-Clb2 and Cdk1<sup>as1</sup>-Clb5 was compared using human histone H1 as substrate (<bold>A</bold>, bottom). The numbers in the histone H1 experiment represent the relative kinase activity. As negative control of kinase activity, Mps1 was inactivated with chemical inhibitor SP600125 (<bold>B</bold>), while Cdk1<sup>as1</sup>-Clb2 and Cdk1<sup>as1</sup>-Clb5 overexpressed and purified from budding yeast were inactivated with 1NM-PP1 (<bold>A</bold>). (<bold>C</bold>) Phosphorylation of Spc110 and Spc110<sup>5A</sup> in vivo. <italic>SPC110-GFP</italic> wild type cells (WT) and <italic>spc110</italic><sup><italic>5A</italic></sup><italic>-GFP</italic> cells were arrested in mitosis with nocodazole. Spc110-GFP was enriched using GFP binder conjugated to beads, and the bound proteins were subject to immunoblotting with the indicated antibodies. (<bold>D</bold>) <italic>SPC110-GFP</italic> wild type cells were arrested in G1, S phase, and mitosis as indicated. Spc110-GFP was enriched with GFP-binder and analyzed by immunoblotting with the indicated P-specific antibodies. A non-specific band was used as loading control.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.015">http://dx.doi.org/10.7554/eLife.02208.015</ext-link></p></caption><graphic xlink:href="elife02208f005"/></fig><fig id="fig5s1" position="float" specific-use="child-fig"><object-id pub-id-type="doi">10.7554/eLife.02208.016</object-id><label>Figure 5—figure supplement 1.</label><caption><title>Phosphorylation of T18 most likely affects <bold>γ</bold>-TuSC oligomerization promoting activity of Spc110 in a negative manner.</title><p>(<bold>A</bold>) Phospho-specific antibody against phospho-T18 (pT18) was generated from guinea pigs. In vitro kinase assays were performed in the presence of Cdk1<sup>as1</sup> and Spc110<sup>1–220</sup>, either wild type (WT) or non-phosphorylatable Spc110<sup>1–220-3A</sup> <sup>(Cdk1)</sup> (T18A-S36A-S91A). In vitro phosphorylated Spc110<sup>1–220</sup> was subjected to SDS-PAGE and immunoblot with the phospho-specific antibody. As negative control of kinase activity, Cdk1<sup>as1</sup>-Clb2 and Cdk1<sup>as1</sup>-Clb5 overexpressed and purified from budding yeast were inactivated with 1NM-PP1. Note that the same Spc110<sup>1–220</sup> samples were analyzed in <xref ref-type="fig" rid="fig5">Figure 5A</xref> with the anti-pS36-pS91 antibody. The Spc110<sup>1–220</sup> blot on the bottom is the same as in <xref ref-type="fig" rid="fig5">Figure 5A</xref>. (<bold>B</bold>) Quantification of tryptic phospho-T18 peptide (NMEFpTPVGFIK). Data were acquired in a data independent fashion in a pseudo MRM experiment in the Orbitrap mass spectrometer fragmenting peptides with m/z = 689.81. Extracted ion chromatogram of phosphorylated NMEFTPVGFIK was quantified. Expected retention time of the tryptic phospho-T18 peptide is 24.7 min. (<bold>C</bold>) Mass spectrum of fragmented phospho-T18 peptide. The graph summarizes the identified fragmented ions. (<bold>D</bold>) Gel filtration chromatograms of γ-TuSC. Spc110<sup>1–220-5D</sup> was incubated with Cdk1<sup>as1</sup>-Clb2 in the presence (+1NM-PP1) and absence of the Cdk1<sup>as1</sup> inhibitor 1NM-PP1 and ATP. This allowed Spc110<sup>T18</sup> phosphorylation in the absence but not in the presence of 1NM-PP1. Adding 1NM-PP1 stopped all Cdk1<sup>as1</sup> kinase reactions. The so modified Spc110<sup>1–220-5D-pT18</sup> was then incubated with γ-TuSC. γ-TuSC was analysed by gel filtration using a Superose 6 column for oligomerization. The gel filtration fractions were analyzed by immunoblotting for Spc97/Spc98, Tub4, Spc110<sup>1–220</sup>, and Spc110<sup>1–220−pT18</sup> distribution. The anti-pT18 blot on the bottom is a 25-times of contrast enhancement of the blot on top. Fractions covering the void-volume peak were marked with a red rectangle outline. (<bold>E</bold>) Quantification of void-volume fractions shown in (<bold>D</bold>). Spc110<sup>1–220-5D-T18D</sup> was included as control. Indicated protein band intensities of fractions covering the void-volume peak (red rectangle outline in (<bold>D</bold>)) were summed and divided by the total intensity of fractions. * marks statistical significance at p<italic><</italic>0.05, ** at p<italic><</italic>0.01, and *** at p<italic><</italic>0.001. Error bars represent SEM. N = 4 for the number of experiments performed.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.016">http://dx.doi.org/10.7554/eLife.02208.016</ext-link></p></caption><graphic xlink:href="elife02208fs009"/></fig></fig-group></p><p>To understand by which Cdk1 kinase complex Spc110<sup>S36</sup> and Spc110<sup>S91</sup> were phosphorylated, we co-immunoprecipitated Cdk1<sup>as1</sup> either in complex with Clb5-TAP (S phase cyclin) or Clb2-TAP (mitotic cyclin) from yeast lysates and performed in vitro kinase assays using Spc110<sup>1–220-WT</sup> that had been purified from <italic>E. coli</italic>. Spc110<sup>S36-S91</sup> was phosphorylated equally by Cdk1-Clb2 and Cdk1-Clb5 as detected by Spc110<sup>pS36-pS91</sup> antibodies (<xref ref-type="fig" rid="fig5">Figure 5A</xref>). It is important to note that both Cdk1 kinase complexes had equal kinase activities toward histone H3 (<xref ref-type="fig" rid="fig5">Figure 5A</xref>, bottom).</p><p>To confirm the cell-cycle dependent phosphorylation of Spc110 in vivo, we immuno-precipitated (IP) Spc110-GFP with GFP-binder from cells arrested either in G1 phase with α-factor, S phase with hydroxyurea (HU) or prometaphase with nocodazole, and then blotted with phospho-specific antibodies. At G1 phase, no signal was observed with any of the two phospho-specific antibodies (<xref ref-type="fig" rid="fig5">Figure 5D</xref>). The phosphorylation of Mps1 was detected at S phase and maintained in mitosis. The Spc110<sup>pS36-pS91</sup> signal also appeared at S phase, and continuously increased in mitosis.</p><p>T18 within the SPM motif has been found phosphorylated in vivo and matches the minimal Cdk1 consensus sequence S/T-P (<xref ref-type="bibr" rid="bib39">Keck et al., 2011</xref>). To elucidate the function of T18 phosphorylation, we raised phospho-specific antibodies against pT18. In vitro phosphorylation assay showed that T18 was only phosphorylated by Cdk1-Clb2 but not by Cdk1-Clb5 (<xref ref-type="fig" rid="fig5s1">Figure 5—figure supplement 1A</xref>). Due to the low sensitivity of anti-Spc110<sup>pT18</sup>, we used mass spectrometry for the analysis of T18 phosphorylation in vivo. Total yeast cell lysates were fractionated into supernatant and pellet that contained SPBs. The pellet fraction was solubilized with high salt buffer and then used for Spc110 enrichment. pT18 was predominately detected in the SPB-containing pellet fraction of prometaphase cells but not in the corresponding fraction of G1 or S phase arrested cells (<xref ref-type="fig" rid="fig5s1">Figure 5—figure supplement 1B,C</xref>). These data suggest that T18 of Spc110 becomes phosphorylated at SPBs in M phase but not S phase, which is consistent with published data (<xref ref-type="bibr" rid="bib39">Keck et al., 2011</xref>).</p><p>Finally, we addressed the effects of T18 phosphorylation on γ-TuSC oligomerization. Because the T18A/V/D/E mutations inactivated the SPM function, we turned to an in vitro phosphorylation approach. Spc110<sup>1–220-5D</sup> was incubated with Cdk1<sup>as1</sup>-Clb2 in the presence or absence of the Cdk1<sup>as1</sup> inhibitor 1NM-PP1. Only in the absence of 1NM-PP1, Spc110<sup>1–220-5D</sup> was phosphorylated on T18 as shown by the phospho-specific anti-T18 antibodies. We incubated Spc110<sup>1–220-5D</sup> and Spc110<sup>1–220-5D-pT18</sup> with γ-TuSC and analyzed γ-TuSC by gel filtration. The activity of Spc110<sup>1–220-5D-pT18</sup> to promote γ-TuSC oligomerization was significantly reduced in comparison with Spc110<sup>1–220-5D</sup> (p<0.05) (<xref ref-type="fig" rid="fig5s1">Figure 5—figure supplement 1D,E</xref>). This suggests that in vitro phosphorylation of T18 by Cdk1-Clb2 inhibits γ-TuSC oligomerization.</p><p>Taken together, our results are in agreement with previous reports about the timing of Spc110 phospho-dependent mobility shift in SDS-PAGE (<xref ref-type="bibr" rid="bib27">Friedman et al., 1996</xref>; <xref ref-type="bibr" rid="bib70">Stirling and Stark, 1996</xref>) and suggest that the Cdk1 (S36, S91) and Mps1 sites become phosphorylated in S and in mitosis. In addition, phosphorylation of T18 within SPM occurs in mitosis and may negatively regulate SPM function.</p></sec><sec id="s2-7"><title>Cells with <italic>spc110</italic> phospho-, SPM-, and CM1-mutant alleles have SPBs with impaired MT nucleation activity</title><p>To reveal the function of phosphorylation of Spc110 in vivo, we replaced <italic>SPC110</italic> with phosphomimetic and non-phosphorylatable mutant alleles. There was no significant difference in the expression levels of the Spc110 variants (<xref ref-type="fig" rid="fig6s1">Figure 6—figure supplement 1A</xref>). <italic>spc110</italic><sup><italic>2A/2D</italic></sup>, <italic>spc110</italic><sup><italic>3A/3D</italic></sup>, <italic>spc110</italic><sup><italic>5A/5D</italic></sup>, the SPM deficient <italic>spc110</italic><sup><italic>T18D</italic></sup>, <italic>spc110</italic><sup><italic>ΔSPM</italic></sup>, and the CM1 deficient <italic>spc110</italic><sup><italic>CM1-QA</italic></sup> cells grew at 23°C comparable to wild-type <italic>SPC110</italic> (WT) cells. However, <italic>spc110</italic><sup><italic>T18D</italic></sup> and <italic>spc110</italic><sup><italic>ΔSPM</italic></sup> cells displayed a conditional lethal growth defect at 37°C and the SPM/CM1 defective <italic>spc110</italic><sup><italic>T18D-CM1-QA</italic></sup> double mutant was non-viable at 23°C and 37°C (<xref ref-type="fig" rid="fig6">Figure 6A</xref>, left panel). Combining T18D with 2D (Cdk1 sites), 3D (Mps1 sites) and 5D mutations slightly reduced growth at 37°C compared to the 2D, 3D, and 5D mutations (<xref ref-type="fig" rid="fig6">Figure 6A</xref>). This shows that, consistent with our in vitro γ-TuSC oligomerization results (<xref ref-type="fig" rid="fig2s2">Figure 2—figure supplement 2C</xref>), the SPM mutation T18D is dominant over the activating Cdk1 and Mps1 phosphomimetic mutations.<fig-group><fig id="fig6" position="float"><object-id pub-id-type="doi">10.7554/eLife.02208.017</object-id><label>Figure 6.</label><caption><title>Cells with <italic>spc110</italic> phospho-, SPM-, or CM1-mutant alleles have defects in spindle formation.</title><p>(<bold>A</bold>) Growth of 10-fold serial dilutions of <italic>SPC110</italic> shuffle strains with integration vector encoding <italic>SPC110</italic> (WT) or <italic>SPC110</italic> mutants with and without the SAC gene <italic>MAD2</italic>. Growth was tested either on synthetic complete (SC) plates containing 5-FOA or SC dropout plates. (<bold>B</bold>) The indicated <italic>SPC110</italic> wild type cells and <italic>spc110</italic> mutant cells carrying <italic>GFP-TUB1 SPC42-mCherry</italic> were incubated at 37°C, the restrictive temperature of some of the <italic>spc110</italic> mutants (<xref ref-type="fig" rid="fig6">Figure 6A</xref>). The fluorescent signal of GFP-Tub1 at SPBs was quantified in G1 phase cells (no buds), early S phase cells (small bud with unsplit SPBs) and M phase cells (large bud with split SPBs and spindle length within 2 μm). 50 cells were analyzed per cell cycle phase and strain. Error bars represent SEM. * marks statistical significance at p<italic><</italic>0.05, ** at p<italic><</italic>0.01, and *** at p<italic><</italic>0.001. N = 2 or 3 for the number of experiments performed. (<bold>C</bold>) The representative metaphase cells from (<bold>B</bold>) were scanned for the distribution of the GFP-Tub1 and the Spc42-mCherry signal along the spindle axis. Scale bar: 4.5 µm. (<bold>D</bold>) In vivo MT re-nucleation assay. Wild-type <italic>SPC110</italic> cells (WT) and the indicated <italic>spc110</italic> cells with <italic>GFP-TUB1</italic> were treated as indicated in the outline. The GFP-Tub1 signal at SPBs was measured at 0, 30, and 60 min after nocodazole washout. N = 40 for the number of cells analyzed per time point and strain. Error bars represent SEM. (<bold>E</bold>) Data of (<bold>D</bold>) after 30 and 60 min. Error bars represent SEM. *** marks statistical significance at p<italic><</italic>0.001.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.017">http://dx.doi.org/10.7554/eLife.02208.017</ext-link></p></caption><graphic xlink:href="elife02208f006"/></fig><fig id="fig6s1" position="float" specific-use="child-fig"><object-id pub-id-type="doi">10.7554/eLife.02208.018</object-id><label>Figure 6—figure supplement 1.</label><caption><title>Phenotype of <italic>SPC110</italic> mutants.</title><p>(<bold>A</bold>) There was no significant difference in the protein levels of Spc110 variants. Asynchronous cells grown to OD<sub>600</sub> 0.5 were harvested and subjected to TCA extraction for whole cell protein lysates. SDS-PAGE and immunoblotting with anti-Spc110 were performed. Tub2 was loading control. (<bold>B</bold>) The representative images of wild type <italic>SPC110</italic> (WT) and <italic>spc110</italic> cells in G1 (no bud) and S phase (small bud). Microtubules were marked with GFP-Tub1 and SPBs were marked with Spc42-mCherry. See <xref ref-type="fig" rid="fig6">Figure 6B</xref> for experimental set-up. BF: bright field. Scale bar: 4.5 µm. (<bold>C</bold>) The indicated wild type <italic>SPC110</italic> (WT) and <italic>spc110</italic> cells were grown at 37°C. The cell cycle stages were determined according to cell morphology and spindle length. Unfixed cells were analyzed for Spc97-GFP signal at SPBs. N = 30 for the number of cells analyzed for each category. * marks statistical significance at p<0.05 and ** at p<0.01. Error bars represent SEM. (<bold>D</bold>) <italic>SPC110</italic> and <italic>spc110</italic><sup><italic>T18D</italic></sup> cells with <italic>SPC97-GFP</italic> were analyzed by time lapse analysis. The fluorescent intensity was recorded and measured over time (top). Error bars represent SEM. N = 17 for the number of cells analysed for each strains. The Spc97-GFP signals of selected cells are shown on the bottom. Scale bar: 5 μm. (<bold>E</bold>) The indicated wild type <italic>SPC110</italic> (WT) and <italic>spc110</italic> cells were grown at 37°C. The cell cycle stages were determined according to cell morphology and spindle length. Unfixed cells were analyzed for Spc110-GFP signal at SPBs. Error bars represent SEM. N = 30 for the number of cells analyzed for each category. (<bold>F</bold>) Similar experiment as performed in <xref ref-type="fig" rid="fig6">Figure 6B</xref>. The indicated <italic>SPC110</italic> wild type and <italic>spc110</italic><sup><italic>ΔSPM</italic></sup> carrying <italic>GFP-TUB1 SPC42-mCherry</italic> were incubated at 37°C. The fluorescent signal of GFP-Tub1 at SPBs was quantified in G1 phase cells (no buds), early S phase cells (small bud with unsplit SPBs) and M phase cells (large bud with split SPBs and spindle length within 2 μm). 100 cells were analyzed per cell cycle phase and strain. Error bars represent SEM. ** marks statistical significance at p<italic><</italic>0.01, and *** at p<italic><</italic>0.001. (<bold>G</bold>) Similar experiment as performed in (<bold>D</bold>) with <italic>SPC110</italic> and <italic>spc110</italic><sup><italic>ΔSPM</italic></sup> cells. <italic>SPC110</italic> and <italic>spc110</italic><sup><italic>ΔSPM</italic></sup> cells with <italic>SPC97-GFP</italic> were analyzed by time lapse analysis. Error bars represent SEM. N = 17 for the number of cells analysed for each strain. (<bold>H</bold>) Similar experiment as performed in (<bold>E</bold>). The wild type <italic>SPC110</italic> (WT) and <italic>spc110</italic><sup><italic>ΔSPM</italic></sup> cells were grown at 37°C. The cell cycle stages were determined according to cell morphology and spindle length. Unfixed cells were analyzed for Spc110-GFP signal at SPBs. Error bars represent SEM. N = 40 for the number of cells analyzed for each category.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.018">http://dx.doi.org/10.7554/eLife.02208.018</ext-link></p></caption><graphic xlink:href="elife02208fs010"/></fig></fig-group></p><p>The defect of some mutant alleles involved in spindle assembly becomes apparent only after SAC function has been impaired (<xref ref-type="bibr" rid="bib82">Wang and Burke, 1995</xref>; <xref ref-type="bibr" rid="bib14">Daum et al., 2000</xref>). In the absence of the SAC gene <italic>MAD2</italic> (<xref ref-type="bibr" rid="bib48">Li and Murray, 1991</xref>), slow growth was observed in <italic>spc110</italic><sup><italic>2A</italic></sup> and <italic>spc110</italic><sup><italic>2D</italic></sup> cells at 37°C. This growth defect was even more pronounced in <italic>spc110</italic><sup><italic>5A</italic></sup> <italic>mad2Δ</italic> cells at 37°C (<xref ref-type="fig" rid="fig6">Figure 6A</xref>, right panel). All <italic>spc110 mutant</italic> alleles with SPM or CM1 inactivating mutations showed reduced growth in the <italic>mad2Δ</italic> background compared to <italic>MAD2</italic> SAC proficient cells (<xref ref-type="fig" rid="fig6">Figure 6A</xref>, bottom panel). Together, these growth tests suggest that the absence of phospho-regulation of Spc110 or the lack of SPM or CM1 results in defects that are compensated by a SAC induced delay in cell cycle progression.</p><p>Based on our model built from in vitro experiments, phosphorylation of N-Spc110 by Cdk1 (S36, S91) and Mps1 in S phase promotes the binding and oligomerization of γ-TuSC to form the template required for MT nucleation. In addition, the SPM and the CM1 motifs of Spc110 are important for γ-TuSC oligomerization and MT nucleation (<xref ref-type="fig" rid="fig2">Figure 2</xref>). Thus, mutations in <italic>SPC110</italic> that affect any one of these sites/motifs should display a reduction in their SPB-associated tubulin signal. To test this hypothesis, we expressed <italic>GFP-TUB1</italic> in cells and measured the intensity of the tubulin signal at the SPB at different cell cycle phases. Cells were synchronized with α-factor in G1 and then released into prewarmed media at 37°C, the restrictive temperature of some of the mutant cells. In SPM defective <italic>spc110</italic><sup><italic>T18A</italic></sup> and <italic>spc110</italic><sup><italic>T18D</italic></sup> cells, the reduction of GFP-tubulin signal at SPBs was already apparent in G1 (<xref ref-type="fig" rid="fig6">Figure 6B</xref>, <xref ref-type="fig" rid="fig6s1">Figure 6—figure supplement 1B</xref>, p<0.001). In S phase, the stage where newly synthesized SPBs mature and MT nucleation is initiated, all <italic>spc110</italic> mutants except for <italic>spc110</italic><sup><italic>5D</italic></sup> cells exhibited a marked reduction in MT nucleation at SPBs when compared with WT cells (<xref ref-type="fig" rid="fig6">Figure 6B</xref>; for <italic>spc110</italic><sup><italic>5A</italic></sup>, p<0.05; for <italic>spc110</italic><sup><italic>T18D</italic></sup>, <italic>spc110</italic><sup><italic>T18A</italic></sup>, p<0.001; <xref ref-type="fig" rid="fig6s1">Figure 6—figure supplement 1B</xref>). The reduction in tubulin signal was especially pronounced in <italic>spc110</italic><sup><italic>T18D</italic></sup> and <italic>spc110</italic><sup><italic>T18A</italic></sup> cells, indicating that SPM mutations blocked MT formation in vivo. <italic>spc110</italic><sup><italic>ΔSPM</italic></sup> cells also showed MT defects at 37°C (<xref ref-type="fig" rid="fig6s1">Figure 6—figure supplement 1F</xref>).</p><p>The situation in metaphase was similar to that of S phase although the overall relative differences between WT and <italic>spc110</italic> mutants were less pronounced than in S phase (<xref ref-type="fig" rid="fig6">Figure 6B</xref>, p<0.001). Consistently, the overall spindle MT signal was reduced in <italic>spc110</italic><sup><italic>5A</italic></sup>, <italic>spc110</italic><sup><italic>T18A</italic></sup>, <italic>spc110</italic><sup><italic>T18D</italic></sup>, and <italic>spc110</italic><sup><italic>5D-T18D</italic></sup> cells (<xref ref-type="fig" rid="fig6">Figure 6B,C</xref>). Taken together, the in vivo analysis of the tubulin signal at SPBs in <italic>spc110</italic> cells is consistent with the in vitro data on γ-TuSC oligomerization and MT nucleation (<xref ref-type="fig" rid="fig2 fig4">Figures 2 and 4</xref>).</p><p>Reduced MT signal intensity at SPBs would be indicative of impaired γ-TuSC activity. In the simplest interpretation, γ-TuSC recruitment to SPBs is impaired. We found that the SPB signal from the γ-TuSC marker Spc97-GFP was reduced in <italic>spc110</italic><sup><italic>5A</italic></sup>, <italic>spc110</italic><sup><italic>T18D</italic></sup>, <italic>spc110</italic><sup><italic>5D-T18D</italic></sup>, and <italic>spc110</italic><sup><italic>ΔSPM</italic></sup> cells in G1 and S phase compared to <italic>SPC110</italic><sup><italic>WT</italic></sup> and <italic>spc110</italic><sup><italic>5D</italic></sup> cells (<xref ref-type="fig" rid="fig6s1">Figure 6—figure supplement 1C,G</xref>). However, this reduction was not observed in mitotic cells. Analysis of Spc97-GFP signal intensity at SPBs in <italic>SPC110</italic> wild type, <italic>spc110</italic><sup><italic>T18D</italic></sup>, and <italic>spc110</italic><sup><italic>ΔSPM</italic></sup> cells revealed that the SPM mutation allowed Spc97 SPB binding but with a delay in cell cycle timing (<xref ref-type="fig" rid="fig6s1">Figure 6—figure supplement 1D,G</xref>). After SPB separation in late S phase wild type, <italic>spc110</italic><sup><italic>T18D</italic></sup> and <italic>spc110</italic><sup><italic>ΔSPM</italic></sup> cells eventually carried similar amounts of Spc97-GFP at SPBs.</p><p>Analysis of the GFP signal at SPBs in <italic>SPC110-GFP</italic>, <italic>spc110</italic><sup><italic>5A</italic></sup><italic>-GFP spc110</italic><sup><italic>T18D</italic></sup><italic>–GFP</italic>, and <italic>spc110</italic><sup><italic>5D-T18D</italic></sup><italic>-GFP</italic> cells did not reveal any differences between cell types (<xref ref-type="fig" rid="fig6s1">Figure 6—figure supplement 1E</xref>). Thus, the reduction of the Spc97-GFP signal in G1 and S phase most likely reflects a reduction of γ-TuSC binding to the <italic>SPC110</italic> mutant molecules at SPBs. The observation that all <italic>spc110</italic> cells had similar γ-TuSC signals at mitotic SPBs while the MT signal was only reduced in <italic>spc110</italic><sup><italic>T18D</italic></sup> and <italic>spc110</italic><sup><italic>5A</italic></sup> cells (<xref ref-type="fig" rid="fig6s1">Figure 6—figure supplement 1C</xref> vs <xref ref-type="fig" rid="fig6">Figure 6B</xref>), suggests that the SPB-associated γ-TuSC is not fully active in MT organization in these cells. The functions of Spc110 probably extend beyond γ-TuSC binding, most likely to include γ-TuSC oligomerization (‘Discussion’). However, compared with <italic>spc110</italic><sup><italic>T18D</italic></sup> or <italic>SPC110</italic> cells, Spc110-GFP at the SPB was reduced in <italic>spc110</italic><sup><italic>ΔSPM</italic></sup> cells (<xref ref-type="fig" rid="fig6s1">Figure 6—figure supplement 1H</xref>), suggesting the <italic>SPM</italic> deletion affects incorporation of Spc110 into the SPB. This may contribute to the <italic>spc110</italic><sup><italic>ΔSPM</italic></sup> phenotype and may explain why the growth defect of <italic>spc110</italic><sup><italic>ΔSPM</italic></sup> cells was more pronounced than that of <italic>spc110</italic><sup><italic>T18D</italic></sup> cells (<xref ref-type="fig" rid="fig6">Figure 6A</xref>).</p><p>To further examine the MT nucleation defect in <italic>spc110</italic> phospho-mutant cells, we performed an in vivo MT nucleation assay. Cells were first arrested in S phase with HU, then MTs were depolymerized with nocodazole, followed by nocodazole wash out to trigger MT nucleation by the SPB bound γ-TuSC (<xref ref-type="bibr" rid="bib21">Erlemann et al., 2012</xref>). To prevent cell cycle progression into anaphase we retained the cells in the HU block (<xref ref-type="fig" rid="fig6">Figure 6D</xref>). The re-growth of MTs was measured as a change in GFP-Tub1 intensity at SPBs. In comparison to WT and <italic>spc110</italic><sup><italic>5D</italic></sup> cells, MT nucleation activity was reduced in the <italic>spc110</italic><sup><italic>5A</italic></sup>, <italic>spc110</italic><sup><italic>T18A</italic></sup>, <italic>spc110</italic><sup><italic>T18D</italic></sup> <italic>spc110</italic><sup><italic>5D-T18D</italic></sup> cells (<xref ref-type="fig" rid="fig6">Figure 6E</xref>). Again, these data are consistent with the in vitro data (<xref ref-type="fig" rid="fig1 fig2 fig3 fig4">Figures 1–4</xref>) and suggest that Spc110's ability to stimulate MT nucleation is regulated through stimulatory phosphorylation and requires the SPM motif.</p></sec><sec id="s2-8"><title>Spc110 interaction and γ-TuSC oligomerization are facilitated by the N-terminal region of Spc98</title><p>The N-terminal region of Spc98/GCP3 orthologs is conserved. Spc98/GCP3 family members contain five predicted helices followed by an unstructured linker region (<xref ref-type="fig" rid="fig7">Figure 7A</xref>, <xref ref-type="fig" rid="fig7s1">Figure 7—figure supplement 1A</xref>). The conserved GRIP1 and 2 motifs that are common to all GCP proteins follow this N-terminal region (<xref ref-type="bibr" rid="bib86">Wiese and Zheng, 2006</xref>). Analysis of γ-TuSC by electron microscopy has shown that the N-terminus of Spc98 is in close proximity to the N-Spc110 binding site (<xref ref-type="bibr" rid="bib45">Kollman et al., 2010</xref>). This topological arrangement raises the possibility that N-Spc98 is involved in binding to N-Spc110 and oligomerization of γ-TuSC. To test this idea, we constructed truncations of N-Spc98 with which to assess the function and biochemical behaviour of γ-TuSC<sup>Spc98ΔN</sup>. All N-terminal truncations of <italic>SPC98</italic> supported cell growth at 23°C (<xref ref-type="fig" rid="fig7">Figure 7B</xref>, <xref ref-type="fig" rid="fig7s1">Figure 7—figure supplement 1B</xref>). However, Δ1–156, Δ1–177, and ΔLinker deletions of <italic>SPC98</italic> showed reduced or no growth in <italic>mad2Δ</italic> cells at 37°C.<fig-group><fig id="fig7" position="float"><object-id pub-id-type="doi">10.7554/eLife.02208.019</object-id><label>Figure 7.</label><caption><title>The N-terminus of Spc98 mediates binding to N-Spc110.</title><p>(<bold>A</bold>) Alignment of the amino acid sequence of GCP3 homologues from yeast to human. Shown are the putative α-helical regions H1–H5. Residues are marked according to the ClustalX colour scheme. (<bold>B</bold>) Growth test of <italic>spc98</italic><sup><italic>Δ1–156</italic></sup> cells with and without the SAC gene <italic>MAD2</italic> at 23°C and 37°C. “<italic>SPC98</italic>” indicates the <italic>spc98</italic><italic>Δ</italic><italic>SPC98</italic> cells while YPH499 is the unmodified wild type strain. (<bold>C</bold>) GFP-Tub1 signal at SPBs in <italic>SPC98</italic> wild type or <italic>spc98</italic><sup><italic>Δ1–156</italic></sup> cells. The experiment was performed as in <xref ref-type="fig" rid="fig6">Figure 6B</xref>. ** marks statistical significance at p<italic><</italic>0.01. N = 100 cells analysed for each strain. (<bold>D</bold>) <italic>SPC98</italic> and <italic>spc98</italic><sup><italic>Δ1–156</italic></sup> cells with <italic>SPC42-mCherry</italic>, <italic>SPC97-GFP</italic>, or <italic>SPC110-GFP</italic> were analyzed by time lapse analysis. The fluorescent intensity was measured over time (right). t=0 for the time point of SPB separation. Error bars represent SEM. N = 14 and 19 for the number of cells analysed in <italic>SPC97-GFP</italic> and <italic>SPC-110-GFP</italic> strains respectively. The Spc97-GFP and Spc110-GFP signals of selected cells are shown on the left panel. Scale bar: 5 μm. (<bold>E</bold>) Spc110<sup>5D</sup> combined with γ-TuSC<sup>Spc98Δ1–156</sup> fails to induce oligomerization in TB150 buffer. The experiment was performed as in <xref ref-type="fig" rid="fig2">Figure 2B</xref>. The purified γ-TuSC<sup>Spc98Δ1–156</sup> after SDS-PAGE and Coomassie Blue staining is shown with relative intensity of the protein bands. The protein distribution in the Superdex 200 10/300 chromatogram was analyzed with SDS-PAGE and silver staining. (<bold>F</bold>) Binding of γ-TuSC and γ-TuSC<sup>Spc98Δ1–156</sup> to Spc110<sup>1–220</sup> mutant proteins. The binding reaction was performed and analyzed by immunoblotting as described in <xref ref-type="fig" rid="fig3">Figure 3A</xref>. (<bold>G</bold>) Quantification of <xref ref-type="fig" rid="fig7">Figure 7F</xref>. Pull-downed Spc97, Spc98, and Tub4 proteins at 300 nM Spc110<sup>1–220-5D</sup>. Error bars represent SEM. N = 3 for the number of experiments performed. *** marks statistical significance at p<italic><</italic>0.001.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.019">http://dx.doi.org/10.7554/eLife.02208.019</ext-link></p></caption><graphic xlink:href="elife02208f007"/></fig><fig id="fig7s1" position="float" specific-use="child-fig"><object-id pub-id-type="doi">10.7554/eLife.02208.020</object-id><label>Figure 7—figure supplement 1.</label><caption><title>The importance of the N-terminal region of Spc98.</title><p>(<bold>A</bold>) Structure and functional domain organization of budding yeast Spc98 and human GCP3. Both Spc98 and GCP3 share the conserved N-terminal domain (NTD), the GCP core body and the divergent, unstructured linker connecting NTD and GCP core body. The GRIP1 region is within the region interacting with Spc97, while the GRIP2 covers the region interacting with Tub4 (<xref ref-type="bibr" rid="bib41">Knop et al., 1997</xref>; <xref ref-type="bibr" rid="bib59">Pereira et al., 1998</xref>; <xref ref-type="bibr" rid="bib86">Wiese and Zheng, 2006</xref>; <xref ref-type="bibr" rid="bib45">Kollman et al., 2010</xref>). (<bold>B</bold>) The growth test of <italic>MAD2</italic> or <italic>mad2Δ</italic> cells with <italic>spc98</italic> N-terminal truncated mutant alleles. Serial diluted cells were spotted on YPAD plates and incubated at indicated temperatures for two or three days. <italic>SPC98</italic> indicates the <italic>spc98Δ SPC98</italic> cells while WT is the unmodified wild type strain. The design of each truncated variant is shown in the cartoon below. (<bold>C</bold>) The representative images of <italic>spc98</italic><sup><italic>Δ1–156</italic></sup> cells in M phase (large budded, bipolar spindle within 2 μm long). <italic>SPC98</italic> wild type and <italic>spc98</italic><sup><italic>Δ1–156</italic></sup> cells with <italic>SPC42-mCherry GFP-TUB1</italic> were grown at 37°C for 2 hr. Metaphase-like cells were analyzed for the presence of bipolar spindles. BF: bright field. Scale bar: 4 μm. (<bold>D</bold>) Quantification of MT phenotype of metaphase cells of (<bold>C</bold>). One hundred <italic>SPC98</italic> and <italic>spc98</italic><sup><italic>Δ1–156</italic></sup> cells with a large bud and a SPB distance of ∼2 µm were categorized into ‘normal spindle’ or ‘immature/defective spindle’.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.020">http://dx.doi.org/10.7554/eLife.02208.020</ext-link></p></caption><graphic xlink:href="elife02208fs011"/></fig><fig id="fig7s2" position="float" specific-use="child-fig"><object-id pub-id-type="doi">10.7554/eLife.02208.021</object-id><label>Figure 7—figure supplement 2.</label><caption><title>γ-TuSC<sup>Spc98Δ1–156</sup> maintains self-oligomerization capability in BRB80 buffer.</title><p>(<bold>A</bold>) γ-TuSC<sup>Spc98Δ1–177</sup> was purified from insect cells, incubated in TB150 buffer, and then analyzed by Superose 6 10/300. The left panel presents the Coomassie Blue stained SDS-gel of purified <bold>γ</bold>-TuSC<sup>Spc98Δ1–177</sup>. (<bold>B</bold>) γ-TuSC<sup>Spc98Δ1–177</sup> in fraction 7 from (<bold>A</bold>) were subjected to negative staining and analyzed with electron microscopy. The particles showed irregular morphology, suggesting the formation of protein aggregates. Corresponding scale bars are shown in the images. (<bold>C</bold>) Similar experiment as performed in (<bold>A</bold>). Purified γ-TuSC<sup>Spc98Δ1–156</sup> was incubated in TB150 or BRB80 buffer and then analyzed by Superose 6 10/300. γ-TuSC<sup>Spc98Δ1–156</sup> oligomerized in BRB80 but not in TB150 buffer. (<bold>D</bold> and <bold>E</bold>) Similar experiment as performed in (<bold>B</bold>). After the gel filtration shown in (<bold>C</bold>), the peak fraction of γ-TuSC<sup>Spc98Δ1–156</sup> (fraction 7 in BRB80 and fraction 13 in TB150) was subjected to negative staining and analyzed with electron microscopy. Corresponding scale bars are shown on the images.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.021">http://dx.doi.org/10.7554/eLife.02208.021</ext-link></p></caption><graphic xlink:href="elife02208fs012"/></fig></fig-group></p><p>We analyzed the phenotype of <italic>spc98ΔN</italic> mutants at their restrictive temperature, 37°C. Because γ-TuSC<sup>Spc98Δ1–177</sup> formed aggregates already in TB150 buffer and was therefore unsuitable for oligomerization assay (<xref ref-type="fig" rid="fig7s2">Figure 7—figure supplement 2A,B</xref>), we focus our analysis on <italic>spc98</italic><sup><italic>Δ1–156</italic></sup>. Like the wild-type γ-TuSC, the purified recombinant γ-TuSC<sup>Spc98Δ1–156</sup> was monomeric in TB150 (<xref ref-type="fig" rid="fig7s2">Figure 7—figure supplement 2C,D</xref>). <italic>spc98</italic><sup><italic>Δ1–156</italic></sup> cells incubated at 37°C had weaker GFP-Tub1 signal at their SPBs than wild type <italic>SPC98</italic> cells (WT, <xref ref-type="fig" rid="fig7">Figure 7C</xref>). This is consistent with the observed mild MT nucleation defect of <italic>spc98</italic><sup><italic>Δ1–156</italic></sup> cells at 37°C (<xref ref-type="fig" rid="fig7s1">Figure 7—figure supplement 1C,D</xref>). Moreover<italic>,</italic> time-lapse analysis of α-factor synchronized <italic>SPC98 SPC42-Cherry</italic> and <italic>spc98</italic><sup><italic>Δ1–156</italic></sup> <italic>SPC42-mCherry</italic> cells with <italic>SPC97-GFP</italic> or <italic>SPC110-GFP</italic> revealed a delay in Spc97-GFP recruitment to SPBs at the time of SPB duplication (<xref ref-type="fig" rid="fig7">Figure 7D</xref>). Interestingly, slightly more Spc110-GFP was found at SPBs in <italic>spc98</italic><sup><italic>Δ1–156</italic></sup> cells, which may help to compensate the <italic>spc98</italic><sup><italic>Δ1–156</italic></sup> phenotype. Thus, the N-terminal region of Spc98 is important for optimal MT organization and timely γ-TuSC recruitment to SPBs.</p><p>We next asked whether γ-TuSC<sup>Spc98Δ1–156</sup> can be oligomerized by incubation with Spc110<sup>1–220-5D</sup>. Interestingly, Spc110<sup>1–220-5D</sup> failed to shift γ-TuSC<sup>Spc98Δ1–156</sup> to the void volume of the Superose 6 column, as was the case for γ-TuSC (<xref ref-type="fig" rid="fig7">Figure 7E</xref>). In agreement with the oligomerization experiment, γ-TuSC<sup>Spc98Δ1–156</sup> showed reduced binding to Spc110<sup>1–220-5D</sup> in the pull down assay (<xref ref-type="fig" rid="fig7">Figure 7F,G</xref>). Thus, the N-terminus of Spc98 is involved in binding to Spc110. In spite of not changing our conclusion, we noticed that Spc98<sup>Δ1–156</sup>-6His was detected less strongly than Spc98-6His by the anti-His antibodies, although in Coomassie Blue stained gels both proteins were present in 1:1 ratio (<xref ref-type="fig" rid="fig7">Figure 7E</xref>, upper panel and F). Thus, it is likely that Spc98<sup>Δ1–156</sup>-6His transferred less efficiently to the blotting membrane than Spc97-6His, leading to an underestimation of the Spc98<sup>Δ1–156</sup>-6His signal relative to full-length Spc98-6His.</p><p>Finally, we tested γ-TuSC<sup>Spc98Δ1–156</sup> oligomerization in the BRB80 buffer that supports oligomerization without the aid of Spc110. γ-TuSC and γ-TuSC<sup>Spc98Δ1–156</sup> shifted to the void volume of the Superose 6 column with equal efficiency (<xref ref-type="fig" rid="fig1s2">Figure 1—figure supplement 2B</xref>, <xref ref-type="fig" rid="fig7s2">Figure 7—figure supplement 2C</xref>). Analysis of the void fraction by EM identified oligomeric γ-TuSC<sup>Spc98Δ1–156</sup> rings (<xref ref-type="fig" rid="fig7s2">Figure 7—figure supplement 2E</xref>). Therefore, the N-terminus of Spc98 is important for Spc110-mediated oligomerization but not for Spc110-independent γ-TuSC oligomerization.</p></sec><sec id="s2-9"><title>Bioinformatic analysis of the C-terminal SPB/centrosomal binding domain of SPM-CM1 and CM1-only γ-TuCRs</title><p>Our biochemical and cell biology analyses have identified the novel motif SPM and CM1 (<xref ref-type="bibr" rid="bib65">Sawin et al., 2004</xref>) as important motifs in Spc110 for γ-TuSC binding and oligomerization. Using bioinformatics approaches, we investigated the presence of SPM and CM1 motifs in γ-TuCRs from various species (<xref ref-type="fig" rid="fig8">Figure 8A,B</xref>). A yet unappreciated CM1 motif was identified in another budding yeast γ-TuCR, Spc72 (<xref ref-type="bibr" rid="bib43">Knop and Schiebel, 1998</xref>; <xref ref-type="fig" rid="fig8">Figure 8A</xref>, <xref ref-type="fig" rid="fig8s1">Figure 8—figure supplement 1C</xref>).<fig-group><fig id="fig8" position="float"><object-id pub-id-type="doi">10.7554/eLife.02208.022</object-id><label>Figure 8.</label><caption><title>Two types of γ-TuCRs defined by the N-terminal γ-TuSC binding motifs: SPM-CM1 and CM1-only.</title><p>(<bold>A</bold>) Graphical representations of the patterns of CM1 motif within the multiple sequence alignment of γ-TuCR protein sequences. The CM1 motif sequence logos were shown for γ-TuCR protein sequences retrieved from Pfam database (Microtub_assoc, Pfam id: PF07989), Spc110s and Spc72s from subphylum Saccharomycotina. The Spc72 sequences used to generate the short CM1 logo are shown in <xref ref-type="fig" rid="fig8s1">Figure 8—figure supplement 1C</xref>. The sequence logos were generated with Weblogo 3.0 server. The conserved residues shared by CM1 defined by Pfam, Spc110s, and Spc72s are marked with asterisks. (<bold>B</bold>) SPM and CM1 in PCNT, Spc110, and Pcp1 homologues (SPM-CM1 type) and in CDK5RAP2/Cnn/Spc72/Mto1 (CM1 type). The occurrences of the motif in the training set sequences were calculated and shown as coloured blocks on a line with MEME motif scanning analysis (<xref ref-type="bibr" rid="bib5">Bailey et al., 2009</xref>). Weblogo motif diagram is shown for each sequence showing the sites contributing to that motif in that sequence. The best p-value for the sequence/motif combination is listed. The p-value of an occurrence is the probability of a single random subsequence with the length of the motif, generated according to the zero-order background model, having a score at least as high as the score of the occurrence.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.022">http://dx.doi.org/10.7554/eLife.02208.022</ext-link></p></caption><graphic xlink:href="elife02208f008"/></fig><fig id="fig8s1" position="float" specific-use="child-fig"><object-id pub-id-type="doi">10.7554/eLife.02208.023</object-id><label>Figure 8—figure supplement 1.</label><caption><title>Raw sequence input for the sequence logo of CM2, Spc110 PACT, and Spc72 CM1 motifs.</title><p>(<bold>A</bold>) Multiple sequence alignment of selected γ-TuSC receptor family members containing CM2 motif. Residues are marked according to the ClustalX colour scheme. The occurrence of each amino acid in each position of CM2 motif is presented with Weblogo 3.0. The CM2 motif is present in human CDK5RAP2, myomegalin, and Drosophila centrosomin (Cnn). (<bold>B</bold>) Multiple sequence alignment of PACT motif sequences of Spc110s from the subphylum Saccharomycotina. The graphic presentation was generated as in (<bold>A</bold>). (<bold>C</bold>) Multiple sequence alignment of CM1 motif sequences of Spc72s from the subphylum Saccharomycotina. The graphic presentation was generated as in (<bold>A</bold>).</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.023">http://dx.doi.org/10.7554/eLife.02208.023</ext-link></p></caption><graphic xlink:href="elife02208fs013"/></fig></fig-group></p><p>Based on the presence of SPM and CM1 motifs, γ-TuCRs can be divided into the SPM-CM1 and the CM1-only types. Spc110 and fission yeast Pcp1 fall into the SPM-CM1 class. Our analysis also predicts SPM and CM1 motifs, albeit with moderate sequence deviation, in mammalian PCNT (<xref ref-type="fig" rid="fig8">Figure 8B</xref>). Another group of γ-TuCRs have only a CM1 motif but no SPM. Human CDK5RAP2 and myomegalin, <italic>Drosophila</italic> Cnn, budding yeast Spc72, and fission yeast Mto1 fall into this class (<xref ref-type="fig" rid="fig8">Figure 8B</xref>).</p><p>A further level of category resolution was achieved by the classification based on the C-terminal MTOC targeting domains (<xref ref-type="fig" rid="fig8s1">Figure 8—figure supplement 1A,B</xref>). It was shown that yeast Spc110 (<xref ref-type="bibr" rid="bib74">Sundberg et al., 1996</xref>; <xref ref-type="bibr" rid="bib71">Stirling and Stark, 2000</xref>), Pcp1 (<xref ref-type="bibr" rid="bib24">Fong et al., 2010</xref>), <italic>Drosophila</italic> D-PLP (Cp309) (<xref ref-type="bibr" rid="bib38">Kawaguchi and Zheng, 2004</xref>), and mammalian PCNT (<xref ref-type="bibr" rid="bib29">Gillingham and Munro, 2000</xref>; <xref ref-type="bibr" rid="bib75">Takahashi et al., 2002</xref>) are targeted to nuclear side of SPBs or centrosomal MTOCs via the conserved PACT domain (<xref ref-type="fig" rid="fig9">Figure 9A</xref>). In case of SPBs that are embedded in the nuclear envelope and therefore have a cytoplasmic and a nuclear side, SPM-CM1 γ-TuCRs only function in organizing the nuclear MTs that separate the sister chromatids in mitosis (<xref ref-type="fig" rid="fig9">Figure 9B</xref>). In contrast, CM1-only γ-TuCRs of fungi, such as Spc72 in budding yeast (<xref ref-type="bibr" rid="bib43">Knop and Schiebel, 1998</xref>), Mto1 in fission yeast (<xref ref-type="bibr" rid="bib64">Samejima et al., 2010</xref>), and ApsB in <italic>Aspergillus nidulans</italic> (<xref ref-type="bibr" rid="bib88">Zekert et al., 2010</xref>), have a MASC motif and function at the cytoplasmic side of the SPB or in the cytoplasm. In case of metazoa, CM1-only type of γ-TuCRs such as CDK5RAP2 (<xref ref-type="bibr" rid="bib83">Wang et al., 2010</xref>) possess a CM2 domain (<xref ref-type="fig" rid="fig9">Figure 9B</xref>, <xref ref-type="fig" rid="fig8s1">Figure 8—figure supplement 1A</xref>), targeting γ-TuCRs to both centrosomal and acentrosomal MTOCs.<fig id="fig9" position="float"><object-id pub-id-type="doi">10.7554/eLife.02208.024</object-id><label>Figure 9.</label><caption><title>γ-TuCRs are classified into three subgroups based on N-terminal γ-TuSC binding motifs and C-terminal MTOC targeting motifs.</title><p>(<bold>A</bold>) Graphical representations of the patterns of C-terminal MTOC targeting motifs within the multiple sequence alignment of γ-TuCR protein sequences. The MASC and PACT motif sequence logos were shown for γ-TuCR protein sequences retrieved from Pfam database (Mto2_bdg, Pfam id: PF12808; PACT_coil_coil, Pfam id: PF10495) and Spc110s from subphylum Saccharomycotina. The γ-TuCR and Spc110 protein sequences used to generate the CM2 and PACT motif logo are shown in <xref ref-type="fig" rid="fig8s1">Figure 8—figure supplement 1A,B</xref>. The sequence logos were generated with Weblogo 3.0 server. The conserved residues shared by PACT motif defined by Pfam and Spc110s are marked with asterisks. (<bold>B</bold>) Summary table of categorization of γ-TuCR protein family. Based on the presence of N-terminal γ-TuSC interacting motifs (CM1 and SPM) and C-terminal MTOC targeting motifs (MASC, CM2, and PACT), γ-TuCR protein family can be divided into three subgroups: N-terminal CM1 motif only with either C-terminal MASC or CM2 motifs, and N-terminal CM1 and SPM motifs combined with C-terminal PACT domain. Some PACT domain proteins (AKAP9 or D-PLP) have yet undefined γ-TuSC binding motifs.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.024">http://dx.doi.org/10.7554/eLife.02208.024</ext-link></p></caption><graphic xlink:href="elife02208f009"/></fig></p><p>In summary, γ-TuCR family proteins can be classified regarding the γ-TuSC interaction motifs and the MTOC targeting domain into three groups.</p></sec></sec><sec id="s3" sec-type="discussion"><title>Discussion</title><sec id="s3-1"><title>Spc110 is a γ-TuSC-mediated nucleation regulator</title><p>In this study we show that the γ-TuCR Spc110 actively participates in MT formation through the SPM and CM1 elements and Cdk1 and Mps1 phosphorylation sites. Binding of γ-TuSC to Spc110 is a prerequisite for γ-TuSC oligomerization into the ring-like template that initiates MT assembly. Phosphorylation of Spc110 by Cdk1 and Mps1 regulates γ-TuSC binding and thereby MT nucleation. Thus, Spc110 is a MT nucleation regulator. We thereby provide a molecular understanding of the previously observed spindle length phenotypes (the two Cdk1-Clb5 sites) and genetic interaction with <italic>SPC97</italic> (three Mps1 sites combined with S36) of <italic>SPC110</italic> phosphorylation site mutants (<xref ref-type="bibr" rid="bib26">Friedman et al., 2001</xref>; <xref ref-type="bibr" rid="bib33">Huisman et al., 2007</xref>). Our data also explain why an N-terminal fragment of Spc110 influences oligomerization of γ-TuSC when co-expressed in insect cells (<xref ref-type="bibr" rid="bib45">Kollman et al., 2010</xref>).</p><p>SPBs duplicate in G1/S phase of the cell cycle in a conservative manner (<xref ref-type="bibr" rid="bib8">Byers and Goetsch, 1975</xref>; <xref ref-type="bibr" rid="bib2">Adams and Kilmartin, 2000</xref>; <xref ref-type="bibr" rid="bib60">Pereira et al., 2001</xref>; <xref ref-type="fig" rid="fig10">Figure 10A</xref>). In late G1, a SPB precursor, named the duplication plaque, assembles on the cytoplasmic side of the nuclear envelope at a specialized substructure of the mother SPB, named bridge (<xref ref-type="bibr" rid="bib8">Byers and Goetsch, 1975</xref>; <xref ref-type="bibr" rid="bib36">Jaspersen and Winey, 2004</xref>). In G1/S the duplication plaque then becomes inserted into the nuclear envelope. This allows binding of the Spc110-calmodulin-Spc29 complex from the nuclear side to the central Spc29-Spc42 layer of the embedded duplication plaque (<xref ref-type="bibr" rid="bib7">Bullitt et al., 1997</xref>; <xref ref-type="bibr" rid="bib20">Elliott et al., 1999</xref>). In this cell cycle phase Mps1 and Cdk1-Clb5 phosphorylate the N-terminus of Spc110 at five sites to promote γ-TuSC binding to Spc110 (<xref ref-type="fig" rid="fig10">Figure 10B</xref>). Consistently, Cdk1-Clb5 associates with SPBs throughout this time window (<xref ref-type="bibr" rid="bib33">Huisman et al., 2007</xref>). These phosphorylations increase γ-TuSC affinity for Spc110-5P and induce γ-TuSC oligomerization into ring-like complexes that promote MT nucleation (<xref ref-type="fig" rid="fig10">Figure 10B</xref>). Phospho-regulation of Spc110 is not absolutely essential for viability as indicated by the growth of <italic>spc110</italic><sup><italic>5A</italic></sup> cells. However, <italic>spc110</italic><sup><italic>5A</italic></sup> cells have growth defects in the absence of the SAC gene <italic>MAD2</italic> and fail to organize the full set of MTs in S phase and mitosis (<xref ref-type="fig" rid="fig6">Figure 6</xref>). We therefore suggest that Mps1 and Cdk1-Clb5 coordinate the timing of MT nucleation through Spc110 phosphorylation with SPB duplication. A SAC induced cell cycle delay can temper any defects in this phospho-regulation as indicated by the genetic interaction between <italic>spc110</italic><sup><italic>2A</italic></sup> and <italic>spc110</italic><sup><italic>5A</italic></sup> with <italic>mad2Δ</italic> (<xref ref-type="fig" rid="fig6">Figure 6A</xref>).<fig id="fig10" position="float"><object-id pub-id-type="doi">10.7554/eLife.02208.025</object-id><label>Figure 10.</label><caption><title>Role of Spc110 phosphorylation during SPB duplication.</title><p>(<bold>A</bold>) MT nucleation by the SPB during the cell cycle. See ‘Discussion’ for details. (<bold>B</bold>) Cell cycle dependent, stimulatory phosphorylations of Spc110 by Mps1 and Cdk1-Clb5 (early S phase to early M). (<bold>C</bold>) Model for the interaction of γ-TuSC with Spc110. The Spc110 dimer interacts via the SPM and CM1 motifs with the N-terminus of Spc98 (GCP3) and possibly also with other regions of γ-TuSC.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.025">http://dx.doi.org/10.7554/eLife.02208.025</ext-link></p></caption><graphic xlink:href="elife02208f010"/></fig></p><p>Spc110<sup>T18</sup> is an additional Cdk1 in vivo site (<xref ref-type="bibr" rid="bib3">Albuquerque et al., 2008</xref>; <xref ref-type="bibr" rid="bib39">Keck et al., 2011</xref>; <xref ref-type="bibr" rid="bib50">Lin et al., 2011</xref>; <xref ref-type="fig" rid="fig5s1">Figure 5—figure supplement 1</xref>). Analysis of T18A, T18V, T18E, and T18D mutations resulted in similar in vitro and in vivo phenotypes. We have to assume that all these mutations inactivate the important SPM motif in which T18 resides. The fact that T18 of Spc110 is phosphorylated only by Cdk1-Clb2 and not by Cdk1-Clb5 and the in vivo phosphorylation of SPB-associated Spc110<sup>T18</sup> in mitosis but not in S phase (<xref ref-type="bibr" rid="bib39">Keck et al., 2011</xref>; <xref ref-type="fig" rid="fig5s1">Figure 5—figure supplement 1A–C</xref>) indicates that this Cdk1 site is different compared to the two S phase Cdk1 sites S36 and S91 in Spc110. At least in vitro, Cdk1-Clb2 phosphorylation of Spc110<sup>T18</sup> inactivates in a dominant manner the γ-TuSC oligomerization activity (<xref ref-type="fig" rid="fig5s1">Figure 5—figure supplement 1D,E</xref>). Thus, it is well possible that Spc110<sup>T18</sup> phosphorylation by Cdk1-Clb2 limits the MT nucleation activity of the SPB associated γ-TuSC. Most likely only the surplus of γ-TuSC that is not engaged in MT organization is phosphorylated and affected by Spc110<sup>pT18</sup>. This model fits with the observation that the yeast SPB of haploid cells organizes a constant number of 20 nuclear MTs during mitosis (<xref ref-type="bibr" rid="bib87">Winey et al., 1995</xref>).</p><p>Co-overexpression of <italic>SPC97</italic>, <italic>SPC98</italic>, and <italic>TUB4</italic> does not induce MT nucleation despite of γ-TuSC assembly (<xref ref-type="bibr" rid="bib59">Pereira et al., 1998</xref>). Our and other studies have shown that the missing factor promoting γ-TuSC ring assembly is Spc110 (<xref ref-type="bibr" rid="bib81">Vinh et al., 2002</xref>; <xref ref-type="bibr" rid="bib45">Kollman et al., 2010</xref>). In vivo data indicate that the binding of γ-TuSC to Spc110, although a prerequisite, is insufficient for MT formation. For example, although the γ-TuSC component Spc97 bound to mitotic SPBs of <italic>spc110</italic><sup><italic>T18D</italic></sup> or <italic>spc110</italic><sup><italic>5D-T18D</italic></sup> cells with similar efficiency to wild type cells, the γ-TuSC in the two mutants was insufficient in full MT organization (<xref ref-type="fig" rid="fig6">Figure 6B</xref>, <xref ref-type="fig" rid="fig6s1">Figure 6—figure supplement 1C</xref>). Consistent with this model, the addition of Spc110<sup>1–220-5D-T18D</sup> to recombinant γ-TuSC induced the formation of γ-TuSC dimers at ∼600 kDa (<xref ref-type="fig" rid="fig2s2">Figure 2—figure supplement 2A</xref>; note that the protein concentration in this experiment was two-times higher than in <xref ref-type="fig" rid="fig2s2">Figure 2—figure supplement 2B</xref>) without the formation of γ-TuSC oligomers. We therefore suggest that γ-TuSC binding to Spc110 and TuSC oligomerization are mechanistically distinct steps (<xref ref-type="fig" rid="fig10">Figure 10C</xref>).</p></sec><sec id="s3-2"><title>The N-terminus of Spc98 mediates interaction with N-Spc110 and oligomerization</title><p>The N-terminal region of Spc98 exhibits homology to other GCP3 family members including human GCP3 (hGCP3, <xref ref-type="fig" rid="fig7">Figure 7A</xref>, <xref ref-type="fig" rid="fig7s1">Figure 7—figure supplement 1A</xref>). All homologues contain five predicted helical regions that are followed by an unstructured linker region before the two GRIP domains that are common to all GCP proteins (<xref ref-type="bibr" rid="bib31">Guillet et al., 2011</xref>). Analysis of the structure of yeast γ-TuSC by electron microscopy localised N-Spc98 at of the base of the Y shaped structure, away from the C-Spc98 that interacts with γ-tubulin. N-Spc98 is close to the N-terminus of Spc97 and N-Spc110 (<xref ref-type="bibr" rid="bib10">Choy et al., 2009</xref>; <xref ref-type="bibr" rid="bib45">Kollman et al., 2010</xref>). This is consistent with yeast two-hybrid studies that identified N-Spc98 as an interactor for Spc110 (<xref ref-type="bibr" rid="bib42">Knop and Schiebel, 1997</xref>; <xref ref-type="bibr" rid="bib57">Nguyen et al., 1998</xref>). We have analyzed the importance of N-Spc98 by deleting different portions of this domain including the helices, the linker region, and both the helices and the linker. Surprisingly, the N-Spc98 fragment was not essential for the viability at 23°C (<xref ref-type="fig" rid="fig7">Figure 7B</xref>, <xref ref-type="fig" rid="fig7s1">Figure 7—figure supplement 1B</xref>). The important function of this region, however, was revealed at elevated temperatures and when SAC function was impaired (<xref ref-type="fig" rid="fig7">Figure 7B</xref>).</p><p>Biochemical analysis of γ-TuSC<sup>Spc98Δ1–156</sup> showed that its binding to Spc110<sup>1–220-5D</sup> and oligomerization capability by Spc110<sup>1–220-5D</sup> were reduced (<xref ref-type="fig" rid="fig7">Figure 7E–G</xref>), although in M phase it bound with the same affinity for SPBs as WT γ-TuSC (<xref ref-type="fig" rid="fig7">Figure 7D</xref>). We suggest that the special SPB arrangement of Spc110 as hexameric units (<xref ref-type="bibr" rid="bib54">Muller et al., 2005</xref>) compensates in part for the reduced in vitro binding of γ-TuSC<sup>Spc98Δ1–156</sup> to Spc110<sup>1–220</sup>. This compensation may arise from cooperative interactions of γ-TuSC with N-Spc110. Whatever the mechanism, regions of γ-TuSC in addition to the N-terminal 177 amino acids of Spc98 (<xref ref-type="fig" rid="fig7s1">Figure 7—figure supplement 1B</xref>) have to interact with Spc110. Either Spc110 also interacts with the Spc98/GCP3 core or it binds to the N-terminus of Spc97/GCP2.</p><p>γ-TuSC<sup>Spc98Δ1–156</sup> was still able to oligomerize under special buffer conditions (<xref ref-type="fig" rid="fig7s2">Figure 7—figure supplement 2</xref>), which is in agreement with the observation that γ-TuSC<sup>Spc98Δ1–156</sup> was functional in vivo and promoted MT nucleation, although defects were seen at elevated temperature and in <italic>mad2Δ</italic> cells. Thus, the regions that are essential for γ-TuSC oligomerization await identification.</p></sec><sec id="s3-3"><title>γ-tubulin complex receptors fall into three groups: the SPM-CM1-PACT, CM1-MASC, and CM1-CM2 types</title><p>γ-TuCRs from yeast to <italic>Drosophila</italic> to human carry a conserved CM1 motif. This CM1 motif was first identified in fission yeast Mto1 (<xref ref-type="bibr" rid="bib65">Sawin et al., 2004</xref>). Subsequently the function of CM1 in γ-TuSC binding was confirmed for Mto1, centrosomin (<italic>Drosophila</italic>), and CDK5RAP2 (human) (<xref ref-type="bibr" rid="bib89">Zhang and Megraw, 2007</xref>; <xref ref-type="bibr" rid="bib25">Fong et al., 2008</xref>; <xref ref-type="bibr" rid="bib63">Samejima et al., 2008</xref>; <xref ref-type="bibr" rid="bib9">Choi et al., 2010</xref>). The γ-TuCRs Pcp1 (fission yeast) and Spc110 (budding yeast) also contain putative CM1 motifs (<xref ref-type="bibr" rid="bib17">Dictenberg et al., 1998</xref>; <xref ref-type="bibr" rid="bib75">Takahashi et al., 2002</xref>; <xref ref-type="bibr" rid="bib91">Zimmerman et al., 2004</xref>; <xref ref-type="bibr" rid="bib24">Fong et al., 2010</xref>; <xref ref-type="fig" rid="fig8">Figure 8B</xref>).</p><p>In addition, we identified a yet unappreciated CM1 motif in the yeast γ-TuCR Spc72 (<xref ref-type="bibr" rid="bib43">Knop and Schiebel, 1998</xref>; <xref ref-type="fig" rid="fig8">Figure 8A</xref>). Although the putative CM1 motif in Spc72 appears degenerated compared with the full CM1 of other γ-TuCRs, the conserved residues shared by Spc72-CM1 and full CM1 suggest that Spc72 contains a functional CM1. Consistently, Spc72 carries a C-terminal MASC, as is the case for other fungal CM1-only γ-TuCRs (<xref ref-type="bibr" rid="bib64">Samejima et al., 2010</xref>). Moreover, the predicted CM1 resides in the N-terminal region of Spc72 that is essential for γ-TuSC binding (<xref ref-type="bibr" rid="bib43">Knop and Schiebel, 1998</xref>; <xref ref-type="bibr" rid="bib79">Usui et al., 2003</xref>).</p><p>Human PCNT falls in terms of structure and function into the γ-TuCR class (<xref ref-type="bibr" rid="bib19">Doxsey et al., 1994</xref>; <xref ref-type="bibr" rid="bib17">Dictenberg et al., 1998</xref>; <xref ref-type="bibr" rid="bib29">Gillingham and Munro, 2000</xref>). Analysis of the PCNT sequence for CM1 in combination with the newly identified SPM identified a potential CM1 in the N-terminus of PCNT (<xref ref-type="fig" rid="fig8">Figure 8B</xref>). This is consistent with the γ-tubulin binding ability of human PCNT, which lies within the region that contains the SPM-CM1 (<xref ref-type="bibr" rid="bib75">Takahashi et al., 2002</xref>). In addition, super-resolution analysis of human centrosomes has mapped the localization of γ-tubulin towards the N-terminus of PCNT (<xref ref-type="bibr" rid="bib47">Lawo et al., 2012</xref>; <xref ref-type="bibr" rid="bib53">Mennella et al., 2012</xref>; <xref ref-type="bibr" rid="bib67">Sonnen et al., 2012</xref>). Experimental evidences are awaited to validate the CM1 motif in Spc72 and PCNT.</p><p>Our study has identified SPM as a second conserved motif amongst a subgroup of γ-tubulin complex receptors (<xref ref-type="fig" rid="fig2">Figure 2H</xref>), namely the yeast Spc110 homologues, fission yeast Pcp1, and human PCNT (<xref ref-type="fig" rid="fig10">Figure 10</xref>; <xref ref-type="bibr" rid="bib19">Doxsey et al., 1994</xref>; <xref ref-type="bibr" rid="bib17">Dictenberg et al., 1998</xref>; <xref ref-type="bibr" rid="bib23">Flory et al., 2002</xref>). The distance between SPM and CM1 is around 90–100 amino acids in the Spc110 and Pcp1 type of receptors. In the mammalian PCNT subfamily it is however only around 70 amino acids. Whether these differences are important for regulation or are indicative of co-adaptation with alterations in the γ-TuSC binding sites remains to be seen.</p><p>The SPM is important for Spc110 binding to γ-TuSC and induced oligomerization of γ-TuSC (<xref ref-type="fig" rid="fig3 fig4">Figures 3 and 4</xref>). Based on these results we propose that SPM co-operates with CM1 to form a bipartite binding element for the γ-TuSC (<xref ref-type="fig" rid="fig10">Figure 10C</xref>). Interestingly, however, other γ-tubulin complex receptors such as Spc72, fission yeast Mto1, <italic>Drosophila</italic> Cnn, and human CDK5RAP2 carry only the CM1 element but no SPM motif (<xref ref-type="fig" rid="fig8">Figure 8</xref>). Strikingly, SPM-CM1 type of γ-TuCRs carry a C-terminal PACT domain that targets these proteins to MTOCs (<xref ref-type="fig" rid="fig8">Figure 8B</xref>). The CM1-only type of γ-TuCRs either contain a MASC (fungi) or a CM2 C-terminal MTOC targeting sequence (metazoa) (<xref ref-type="fig" rid="fig9">Figure 9B</xref>). We also noticed that in some metazoa and the fungi phylum Basidiomycota the PACT domain-containing γ-TuCRs (such as AKAP9 in human and D-PLP in <italic>Drosophila</italic>) have N-terminal regions distinctive from SPM and CM1 motifs, probably reflecting different binding partners in γ-TuRC. Altogether, we can classify γ-TuCRs in at least three classes: CM1-MASC, CM1-CM2, and SPM-CM1-PACT.</p><p>In budding and fission yeast, the CM1-MASC types of γ-TuCRs (Spc72 and Mto1) have specific functions in cytoplasmic MT organization, while Spc110 and Pcp1 (SPM-CM1-PACT) organize only nuclear MTs. This functional division is accompanied by the physical separation of the receptors by the nuclear envelope, which remains intact during the closed mitosis of both organisms. Human CDK5RAP2, myomegalin (CM1–CM2 types), and pericentrin (SPM-CM1-PACT type) are associated with the centrosome (<xref ref-type="bibr" rid="bib19">Doxsey et al., 1994</xref>; <xref ref-type="bibr" rid="bib25">Fong et al., 2008</xref>). However, CDK5RAP2 and myomegalin also have functions in acentrosomal MT nucleation for example MT nucleation by the Golgi (<xref ref-type="bibr" rid="bib9">Choi et al., 2010</xref>; <xref ref-type="bibr" rid="bib62">Roubin et al., 2013</xref>). Thus, it may be the acentrosomal/cytoplasmic function that explains the lack of SPM element in Mto1/CDK5RAP2/myomegalin.</p><p>Budding yeast organizes MTs only with γ-TuSC and γ-TuCRs. Other organisms have additional building blocks that contribute to MT assembly, such as Mozart1 and GCP4-6 (<xref ref-type="bibr" rid="bib16">Dhani et al., 2013</xref>; <xref ref-type="bibr" rid="bib52">Masuda et al., 2013</xref>). However, GCP4 to 6 are not essential for MT nucleation in fission yeast and <italic>Drosophila</italic>, while GCP2/Alp4, GCP3/Alp6, and Mozart1 are essential for viability (<xref ref-type="bibr" rid="bib4">Anders et al., 2006</xref>; <xref ref-type="bibr" rid="bib80">Verollet et al., 2006</xref>). Puzzlingly, fission yeast Mozart1 is essential for γ-TuSC recruitment to SPBs (<xref ref-type="bibr" rid="bib52">Masuda et al., 2013</xref>). This raises the question how budding yeast compensates for Mozart1's function. GCP3 and γ-TuCRs of the SPM-CM1 type are conserved between organisms with or without Mozart1. It is likely that small adaptive changes in GCP3 or Spc110 have evolved to compensate for the lack of Mozart1 in budding yeast.</p><p>What could be the function of γ-TuCR in organisms that are able to assemble the more stable γ-TuRC? γ-TuCRs may recruit and activate the already assembled γ-TuRC to MTOCs via GCP interactions. In addition, γ-TuCRs could induce γ-TuSC assembly into rings that then either function without the help of additional GCPs or subsequently become stabilized by GCP4-6. In any case, considering the conservation between the SPM and CM1 binding elements of γ-TuRCs and the N-terminal region of Spc98/GCP3, we suggest that the basic principals of MT nucleation are conserved from yeast to human.</p></sec></sec><sec id="s4" sec-type="materials|methods"><title>Materials and methods</title><sec id="s4-1"><title>Plasmid and strain constructions</title><p>A detailed list of DNA constructs and yeast strains is described in <xref ref-type="supplementary-material" rid="SD1-data">Supplementary file 1</xref> and <xref ref-type="supplementary-material" rid="SD2-data">Supplementary file 2</xref>. <italic>SPC110</italic> alleles were subcloned into the integration vector pRS304, and <italic>SPC98</italic> alleles were subcloned into pRS305K (<xref ref-type="bibr" rid="bib66">Sikorski and Hieter, 1989</xref>; <xref ref-type="bibr" rid="bib11">Christianson et al., 1992</xref>; <xref ref-type="bibr" rid="bib76">Taxis and Knop, 2006</xref>). Point mutations in genes were introduced by PCR-directed mutagenesis and confirmed by DNA sequencing. GST-Spc110<sup>1–220</sup> was cloned into pGEX-5X-1 vector (GE Healthcare, UK) and His-tagged Mps1 was cloned into pET28b for expression in BL21 CodonPlus <italic>E. coli</italic> and into pFastBac1 vector for overexpression in baculovirus-insect cell system. All yeast strains are derivatives of S288c. Gene deletion and epitope tagging of genes at their endogenous loci performed using standard techniques (<xref ref-type="bibr" rid="bib44">Knop et al., 1999</xref>; <xref ref-type="bibr" rid="bib34">Janke et al., 2004</xref>). The red fluorescent mCherry was used to mark SPBs through a fusion with <italic>SPC42</italic> (<xref ref-type="bibr" rid="bib18">Donaldson and Kilmartin, 1996</xref>). For <italic>SPC97-yeGFP</italic> and <italic>SPC110-yeGFP</italic> strains, the endogenous <italic>SPC97</italic> and the integrated <italic>SPC110</italic> alleles were tagged with green fluorescent yeGFP. <italic>GFP-TUB1</italic> strains were constructed using an integration plasmid (<xref ref-type="bibr" rid="bib72">Straight et al., 1997</xref>). To remove <italic>URA3</italic>-based plasmids, transformants were tested for growth on 5-fluoroorotic acid (5-FOA) plates that select against <italic>URA3</italic>-based plasmids.</p></sec><sec id="s4-2"><title>Antibodies</title><p>Affinity-purified anti-Tub4 (1:1000) and anti-Spc110 (1:1000) antibodies were described previously (<xref ref-type="bibr" rid="bib28">Geissler et al., 1996</xref>; <xref ref-type="bibr" rid="bib68">Spang et al., 1996a</xref>, <xref ref-type="bibr" rid="bib69">1996b</xref>). Anti-His-tag (1:1000) and anti-GST (1:1000) antibodies were used to detect Spc97 and Spc98 of recombinant γ-TuSC. Secondary antibodies used in semi-quantitative blotting were IRDye800- or Alexa680-conjugated anti-goat, anti-rabbit, anti-guinea pig, and anti-mouse IgGs (1:50,000; Rockland Immunochemicals Inc., Gilbertsville, PA). Phospho-specific anti-Spc110<sup>pT18</sup>, anti-Spc110<sup>pS36-pS91</sup>, and anti-Spc110<sup>pS60-pT64-pT68</sup> antibodies were raised in guinea pigs and purified with immuno-affinity chromatography (1:200; Peptide Specialty Laboratories GmbH, Germany).</p></sec><sec id="s4-3"><title>Protein purification</title><p>Subunits of γ-TuSC and γ-TuSC<sup>ΔN-Spc98</sup> were expressed with the baculovirus-insect cell expression system. Purification was performed as described (<xref ref-type="bibr" rid="bib30">Gombos et al., 2013</xref>). Spc110<sup>1-220</sup> variants with an amino-terminal GST tag were purified with Glutathione Sepharose 4B (GE Healthcare, UK) as described (<xref ref-type="bibr" rid="bib81">Vinh et al., 2002</xref>). Proteins were eluted with 5 mM glutathione, concentrated, and run over a Superose 6 column equilibrated in HB100 to remove glutathione. Recombinant Mps1 protein was purified in lysis buffer (50 mM NaH<sub>2</sub>PO<sub>4</sub> pH 7.5, 300 mM NaCl, 10% glycerol, 1 mM DTT, 1 mM PMSF, 1X Complete EDTA-free protease inhibitor cocktail (Roche, Canada)) with column packed with Ni-NTA Sepharose (GE Healthcare, UK). After wash with lysis buffer, protein was eluted with lysis buffer containing 200 mM imidazole. Small aliquots for kinase assay were snap frozen with liquid nitrogen and stored at −80°C. Cdk1<sup>as1</sup>-Clb2 and Cdk1<sup>as1</sup>-Clb5 complexes were purified from yeast strains kindly given by David Morgan (see <xref ref-type="supplementary-material" rid="SD2-data">Supplementary file 2</xref> for detail) (<xref ref-type="bibr" rid="bib51">Loog and Morgan, 2005</xref>). <italic>CLB2-TAP</italic> and <italic>CLB5-TAP</italic> were encoded on 2-micron plasmid and driven by Gal1 promoter. After overexpression with 2% galactose, cells were harvested and lysed for TAP-tag purification as previously described (<xref ref-type="bibr" rid="bib78">Ubersax et al., 2003</xref>).</p></sec><sec id="s4-4"><title>γ-TuSC oligomerization assay</title><p>Purified γ-TuSC and γ-TuSC<sup>Spc98ΔN</sup> were mixed with or without Spc110<sup>1–220</sup> in 1:4 molar ratio (2.3:9.3) in TB150 (50 mM Tris–HCl, pH7.0 with 150 mM KCl) or BRB80 (80 mM PIPES pH6.9, 1 mM EGTA, 1 mM MgCl<sub>2</sub>) buffer as indicated in figure legends. After incubating for 1 hr at 4°C, the protein mixture was applied to a gel filtration column (Superdex 200 10/300 or Superose 6 10/300 [GE Healthcare, UK]). Elution profiles were recorded as absorbance at 280 nm. For each fraction 2% of sample volume was analyzed by SDS-PAGE. Proteins were detected by silver staining or immunoblotting.</p></sec><sec id="s4-5"><title>In vitro binding assay</title><p>For the measurement of the binding of γ-TuSC to Spc110<sup>1–220</sup>, recombinant GST (300 nM) or GST-Spc110<sup>1–220</sup> proteins (from 0 to 300 nM or fixed 300 nM) were incubated with γ-TuSC (150 nM) in TB150 buffer in a total reaction volume of 30 μl on a rocking platform for 0.5 hr at 4°C. Glutathione-Sepharose 4B bead slurry (20 μl; GE Healthcare, UK) was then added to each reaction, followed by rocking for an additional 1 hr at 4°C. Beads were washed five times with TB150 buffer, 0.1% NP40 followed by heating in 20 μl SDS sample buffer. Input and bound proteins were analysed by immunoblotting. The signal intensities of protein bands on immunoblots were quantified with ImageJ (NIH). Signal intensities were corrected against the membrane background.</p></sec><sec id="s4-6"><title>In vitro MT nucleation assay</title><p>Microtubule nucleation assay was performed as described (<xref ref-type="bibr" rid="bib30">Gombos et al., 2013</xref>) with some modifications. 3 μM γ-TuSC was pre-incubated with 15 μM GST-Spc110<sup>1–220</sup> in HB100 buffer plus 12.5% glycerol for 30 min on ice. An equal volume of 20 μM bovine brain tubulin containing 4% Alexa568-labelled tubulin in BRB80 buffer with 25% glycerol was added and samples were further incubated for 30 min on ice before being transferred to 37°C for 15 min for MT polymerization. Samples were fixed with 1% formaldehyde and diluted with 1 ml cold BRB80 buffer. 50 μl aliquot of reaction mixture was sedimented onto poly-lysine-coated coverslips with a HB-4 rotor at 25,600 g for 1 hr. Samples were post-fixed with pre-cooled methanol, mounted on slides with CitiFluor mounting media, and imaged with a fluorescence microscope as described. Microtubules were counted in 20 random fields.</p></sec><sec id="s4-7"><title>In vitro kinase assay</title><p>For the in vitro phosphorylation of Spc110<sup>1–220</sup> variants, 0.5 μg Spc110<sup>1–220</sup> substrate was incubated in kinase buffer (50 mM Tris–HCl pH 7.5, 10 mM MgCl<sub>2</sub>, 2 mM EGTA) with purified Mps1 kinase or Cdk1<sup>as1</sup> kinase co-immunoprecipitated with Clb2-TAP or Clb5-TAP. For autoradiography, 5 µCi [γ-<sup>32</sup>P]-ATP was added to the reaction mixture and the kinase reaction was performed for 30 min at 30°C. The reaction was stopped by the addition of SDS sample buffer and analysed by SDS–PAGE, Simply Safe Blue staining (Invitrogen, Carlsbad, CA), and autoradiography. For phospho-specific antibody detection, 0.5 mM unlabeled ATP was used in the kinase reaction. For the γ-TuSC oligomerization assay by gel filtration, the phosphorylation reaction was stopped by addition of 15 μM 1NM-PP1 to inhibit Cdk1<sup>as1</sup> or 10 μM SP600125 (Sigma-Aldrich, St Louis, MO) to inhibit Mps1 kinase.</p></sec><sec id="s4-8"><title>Electron microscopic analysis of γ-TuSC oligomers</title><p>To visualize γ-TuSC monomers and oligomers, proteins were stained with uranyl acetate and analyzed by electron microscopy. Briefly, we evaporated 300 or 400 mesh copper/palladium grids with a carbon layer on the copper side. To enhance the hydrophilic affinity of the carbon layer, grids were glow discharged for 30–45 s directly before the preparation started. The protein solution was incubated for 30 s on the carbon mesh grids at room temperature. The grids were washed and incubated in 2% uranyl acetate for 4 min and blotted on Whatman 50 paper. The images were taken in low dose modus with an under-focus between 0.8 and 1.5 μm. Particles were viewed with a CM120 electron microscope (Philips Electronics NV, Eindhoven, Netherlands), which was operated at 120 kV. Images were captured with a CCD camera (Keen View, Soft Imaging systems, Germany) and viewed with Digital Micrograph Software.</p></sec><sec id="s4-9"><title>Phosphopeptide enrichment and mass spectrometry</title><p>Phosphopeptides were identified according to Villen et al. (<xref ref-type="bibr" rid="bib15">Dephoure et al., 2008</xref>). In brief, SDS-PAGE separated Spc110 was cut out and subjected to trypsin digestion. After digest, phosphopeptides were enriched by IMAC, starting with 25 µl of PHOS-Select beads (Sigma, Germany). Enriched phosphopeptides were eluted and desalted by C18 columns (Stage tips). Peptides were analyzed by LC-MS/MS (Orbitrap Elite, Thermo Fisher Scientific, Waltham, MA) and data were processed using Thermo proteome discoverer software (1.4). Phosphorylation site localization was performed on the Mascot results using PhosphoRS.</p></sec><sec id="s4-10"><title>Growth assay</title><p>Yeast cells in the early log phase were adjusted to an OD<sub>600</sub> of 1 with PBS. 10-fold serial dilutions of cells were spotted onto the indicated plates and incubated as indicated in the figure legends.</p></sec><sec id="s4-11"><title>Quantification of the Spc97-GFP, Spc110-GFP, and GFP-Tub1 signal at SPBs</title><p>Asynchronous cells were grown in filter-sterilized YPD with additional 0.1 mg/l adenine (YPAD) to an OD<sub>600</sub> of 0.3 at 23°C for 3 hr and then shifted to 37°C for 1 hr. Cells were directly sampled for image acquisition. For the image acquisition, z-stack images with 21 0.3 μm steps (2 × 2 binning) were acquired at 37°C with a DeltaVision RT system (Applied Precision, UK) equipped with FITC (fluorescein isothiocyanate), TRITC (tetramethyl rhodamine isothiocyanate), and Cy5 filters (Chroma Technology, Bellows Falls, VT), a 100x NA 1.4 plan Apo oil immersion objective (IX70; Olympus, Japan), and a CCD camera (CoolSNAP HQ; Roper Scientific, Tucson, AZ). Images were processed and analyzed in ImageJ (NIH).</p><p>The quantification of the mean background intensity and mean fluorescence intensity of Spc97-GFP and Spc110-GFP signals at SPBs was performed on planes having SPBs in focus. Spc97-GFP or Spc110-GFP intensity within the 3 × 3 pixel-area covering a single SPB or two unsplit SPBs was measured. For GFP-Tub1 at SPBs, GFP-intensity within a 3 × 3 pixel-area surrounding the SPB was measured. Unsplit SPBs were measured together, whereas split SPBs were measured separately. The standard error (SEM) for each data set (n = 50) and level of significance were determined by one-way ANOVA with Turkey's multiple comparisons' test.</p></sec><sec id="s4-12"><title>Time-lapsed live cell imaging</title><p>Cells were synchronized with α-factor for 1.5 cell cycles and then immobilized onto glass-bottom dishes. Dishes were prepared by incubating them with 100 μl concanavalin A solution (6% concanavalin A, 100 mM Tris-Cl, pH 7.0, and 100 mM MnCl<sub>2</sub>) for 5 min and subsequently washed with 300 μl of distilled water. Yeast cells were allowed to attach to the dishes for 5–15 min at 30°C. The α-factor blocked cells were then released by two washes with 2 ml of prewarmed SC medium. Image acquisition was started 10–15 min after release from α-factor. Conditions for imaging were as follows: 15 stacks in the FITC channel, 0.1-s exposure, 0.3-μm stack distance, one reference image in bright field channel with a 0.05-s exposure, and 61 frames in total every 3 min. Images were quantified by measuring the integrated density of the sum of projected videos for the region of interest (ROI) around the SPBs and a background region selected from the periphery of the analyzed regions. The mean intensity of the background was subtracted from the ROI. To correct for acquisition, bleaching signal intensities were divided by a bleaching factor. The bleaching factor was determined from the mean of three very short videos that had been generated with the same image acquisition conditions. The data points of bleaching factor were fitted with non-linear one-phase decay equation.</p></sec><sec id="s4-13"><title>Nocodazole washout assay (MT regrow assay)</title><p>Cells (5 × 10<sup>6</sup> cells/ml) were pre-grown at 23°C in filter-sterilized YPAD. Cells were arrested in G1 by treatment with 10 μg/ml α-factor (Sigma-Aldrich, Germany) for 3 hr at 23°C until >95% of cells showed a mating projection. G1 arrested cells were released into media with hydroxyurea to arrest cells in S-phase for 2 hr. The culture was then shifted to 37°C and nocodazole was added. After 1 hr, nocodazole was removed by exchanging media with YPAD containing hydroxyurea. After removal of nocodazole, cells were sampled at 0, 30, and 60 min and fixed with 4% paraformaldehyde. Image acquisition and GFP-Tub1 quantification were performed as described in previous section.</p></sec><sec id="s4-14"><title>Trichloroacetic acid (TCA) extraction of yeast cells</title><p>To measure the expression level of Spc110 variants and GFP-Tub1 in vivo, whole cell lysates were prepared for SDS-PAGE and immunoblotting (<xref ref-type="bibr" rid="bib44">Knop et al., 1999</xref>; <xref ref-type="bibr" rid="bib34">Janke et al., 2004</xref>). 2–3 OD<sub>600</sub> of late-log phase liquid culture were resuspended in 0.2 M NaOH and incubated on ice for 10 min. 150 μl 55% (wt/vol) TCA was added and the solutions were mixed and incubated for 10 min on ice. After centrifugation the supernatant was removed. The protein pellet was resuspended in high urea (HU) buffer (8 M urea, 5% SDS, 200 mM Tris–HCl pH 6.8, 0.1 mM EDTA, 100 mM DTT, bromophenol blue) and heated at 65°C for 10 min. One-fifth of total sample amount was loaded for SDS-PAGE and western blotting.</p></sec><sec id="s4-15"><title>Pull-down of recombinant Spc110 protein</title><p>For in vivo phospho-Spc110 detection, strains with integrated <italic>SPC110</italic>-yeGFP or SPC110-TAP alleles were lysed in binding buffer (25 mM HEPES, pH 7.4, 150 mM NaCl, 1 mM EDTA, 1 mM DTT, 50 mM NaF, 80 mM β-glycerophosphate, 0.2 mM Na<sub>3</sub>VO<sub>4</sub>, and protease inhibitors) lysed with acid-washed glass beads (Sigma-Aldrich, Germany) in a FastPrep FP120 Cell Disrupter (Thermo Scientific, Germany). Cell lysates were incubated with 0.1% Triton X-100 for 15 min and then clarified by centrifugation at 10,000×<italic>g</italic> for 10 min. To further solubilize the pellet with insoluble SPBs, binding buffer containing 0.5 M NaCl and 1% Triton X-100 was added to resuspend the pellet and incubated for 40 min. GFP-binder protein (<xref ref-type="bibr" rid="bib61">Rothbauer et al., 2008</xref>) conjugated Sepharose 4B slurry or IgG-conjugated Dynabeads was added into supernatants and rocked at 4°C for 2 hr, followed by five times of washing with binding buffer. Pull-downed Spc110-GFP or Spc110-TAP was eluted by heating with SDS sample buffer and analyzed by SDS-PAGE and immunoblotting or mass spectrometry.</p></sec><sec id="s4-16"><title>Bioinformatic analysis</title><p>Protein sequences of Spc110 and Spc98 and their homologues in selected organisms were aligned with MAFFT algorism built in Jalview software (<xref ref-type="bibr" rid="bib85">Waterhouse et al., 2009</xref>; <xref ref-type="bibr" rid="bib37">Katoh and Standley, 2013</xref>; <xref ref-type="fig" rid="fig2 fig7">Figures 2H and 7A</xref>, <xref ref-type="fig" rid="fig2s6">Figure 2—figure supplement 6</xref>, <xref ref-type="fig" rid="fig8s1">Figure 8—figure supplement 1</xref>). The threshold of conservation was set as 20% and highlighted. The relative frequency of each amino acid in each position of SPM and CM1 motifs was visualized using the WebLogo 2.0 (<xref ref-type="bibr" rid="bib13">Crooks et al., 2004</xref>; <xref ref-type="fig" rid="fig2">Figure 2H</xref>, <xref ref-type="fig" rid="fig8">Figure 8</xref>, <xref ref-type="fig" rid="fig9">Figure 9</xref>, <xref ref-type="fig" rid="fig2s6">Figure 2—figure supplement 6</xref>, <xref ref-type="fig" rid="fig8s1">Figure 8—figure supplement 1</xref>). To demonstrate the occurrence of SPM and CM1 motifs in selected members of γ-TuSC receptor family, 47 protein sequences covering the putative CM1 motifs were subjected to MEME motif scanning analysis (<xref ref-type="bibr" rid="bib5">Bailey et al., 2009</xref>; <xref ref-type="fig" rid="fig8">Figure 8B</xref>).</p><p>Jpred 3 and Disopred predicted the secondary structure of the N-terminal domain of Spc98 and human GCP3 (<xref ref-type="bibr" rid="bib84">Ward et al., 2004</xref>; <xref ref-type="bibr" rid="bib12">Cole et al., 2008</xref>). Domain positions and protein interacting regions of Spc110 and Spc98 (<xref ref-type="fig" rid="fig1">Figure 1A</xref>, <xref ref-type="fig" rid="fig7s1">Figure 7—figure supplement 1A</xref>) were illustrated according to γ-TuSC binding studies of Spc72- and Spc110-truncated forms and to the yeast-two-hybrid studies (<xref ref-type="bibr" rid="bib73">Sundberg and Davis, 1997</xref>; <xref ref-type="bibr" rid="bib43">Knop and Schiebel, 1998</xref>; <xref ref-type="bibr" rid="bib79">Usui et al., 2003</xref>).</p><p>For the identification of full CM1 motif on Spc110s and degenerative CM1 on Spc72, CM1 motif containing γ-TuCR protein sequences were retrieved from Pfam database (Microtub_assoc, Pfam id: PF07989) and multiple-aligned with Spc110s and Spc72s from species of subphylum Saccharomycotina by MAFFT multiple sequence alignment algorism. To define the CM2 motif pattern, CM2 sequences from human CDK5RAP2 and <italic>Drosophila</italic> Cnn were used to retrieve protein sequences containing putative CM2 with HMMER server (<xref ref-type="bibr" rid="bib22">Finn et al., 2011</xref>). The retrieved protein sequences were multiple-aligned with CDK5RAP2 and Cnn by MAFFT algorism and the multiple-aligned CM2 sequences were subjected to Weblogo 3.0 server to visualize the pattern.</p></sec><sec id="s4-17"><title>Statistical analysis</title><p>Statistical analysis of fluorescent intensities, immunoblotting intensities, and in vitro microtubule numbers was performed with GraphPad Prism 6.1. One-way ANOVA with Turkey's multiple comparisons' test was used to compare samples and to obtain adjusted p values. The number of repeated experiments and sample size are indicated in figure legends. No statistical method was used to predetermine sample size. The experiments were not randomized. The investigators were not blinded to allocation during the experiments and outcome assessment. In general the data showed normal distribution.</p></sec></sec></body><back><ack id="ack"><title>Acknowledgements</title><p>This work was supported by a grant of the Deutsche Forschungsgemeinschaft (Schi295-3-2). Dr David Morgen is acknowledged for yeast strains. We thank U Jäkle and S Heinze for excellent technical support. TL is a member of the international graduate school HBIGS and was supported by a fellowship of the Graduiertenkolleg Regulation of Cell Division.</p></ack><sec sec-type="additional-information"><title>Additional information</title><fn-group content-type="competing-interest"><title>Competing interests</title><fn fn-type="conflict" id="conf1"><p>The authors declare that no competing interests exist.</p></fn></fn-group><fn-group content-type="author-contribution"><title>Author contributions</title><fn fn-type="con" id="con1"><p>TL, Conception and design, Acquisition of data, Analysis and interpretation of data, Drafting or revising the article</p></fn><fn fn-type="con" id="con2"><p>AN, Single particle analysis with electronic microscopy, Acquisition of data, Analysis and interpretation of data</p></fn><fn fn-type="con" id="con3"><p>YTS, Characterization of phenotypes of <italic>spc98</italic> mutant cells, Acquisition of data, Analysis and interpretation of data</p></fn><fn fn-type="con" id="con4"><p>ANDS, MS analysis of phosphorylated peptides, Acquisition of data, Analysis and interpretation of data</p></fn><fn fn-type="con" id="con5"><p>LW, MS analysis of phosphorylated peptides, Acquisition of data, Analysis and interpretation of data</p></fn><fn fn-type="con" id="con6"><p>ES, Conception and design, Drafting or revising the article</p></fn></fn-group></sec><sec sec-type="supplementary-material"><title>Additional files</title><supplementary-material id="SD1-data"><object-id pub-id-type="doi">10.7554/eLife.02208.026</object-id><label>Supplementary file 1.</label><caption><p>Plasmids used in this study.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.026">http://dx.doi.org/10.7554/eLife.02208.026</ext-link></p></caption><media mime-subtype="xlsx" mimetype="application" xlink:href="elife02208s001.xlsx"/></supplementary-material><supplementary-material id="SD2-data"><object-id pub-id-type="doi">10.7554/eLife.02208.027</object-id><label>Supplementary file 2.</label><caption><p>Strains used in this study.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.02208.027">http://dx.doi.org/10.7554/eLife.02208.027</ext-link></p></caption><media mime-subtype="xlsx" mimetype="application" xlink:href="elife02208s002.xlsx"/></supplementary-material></sec><ref-list><title>References</title><ref id="bib1"><element-citation publication-type="journal"><person-group person-group-type="author"><name><surname>Adams</surname><given-names>IR</given-names></name><name><surname>Kilmartin</surname><given-names>JV</given-names></name></person-group><year>1999</year><article-title>Localization of core spindle pole body (SPB) components during SPB duplication in <italic>Saccharomyces cerevisiae</italic></article-title><source>The Journal of Cell Biology</source><volume>145</volume><fpage>809</fpage><lpage>823</lpage><pub-id 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pub-id-type="doi">10.7554/eLife.02208.028</article-id><title-group><article-title>Decision letter</article-title></title-group><contrib-group content-type="section"><contrib contrib-type="editor"><name><surname>Pines</surname><given-names>Jon</given-names></name><role>Reviewing editor</role><aff><institution>The Gurdon Institute</institution>, <country>United Kingdom</country></aff></contrib></contrib-group></front-stub><body><boxed-text><p>eLife posts the editorial decision letter and author response on a selection of the published articles (subject to the approval of the authors). An edited version of the letter sent to the authors after peer review is shown, indicating the substantive concerns or comments; minor concerns are not usually shown. Reviewers have the opportunity to discuss the decision before the letter is sent (see <ext-link ext-link-type="uri" xlink:href="http://elifesciences.org/review-process">review process</ext-link>). Similarly, the author response typically shows only responses to the major concerns raised by the reviewers.</p></boxed-text><p>Thank you for sending your work entitled “Cell-cycle dependent phosphorylation of yeast pericentrin regulates γ-TuSC-mediated microtubule nucleation” for consideration at <italic>eLife</italic>. Your article has been favorably evaluated by a Senior editor, a Reviewing editor, and 2 reviewers.</p><p>The Reviewing editor and the reviewers discussed their comments before we reached this decision, and the Reviewing editor has assembled the following comments to help you prepare a revised submission.</p><p>1) The primary concern is with T18. It is likely that this site is phosphorylated in vivo, but Spc110 mutant proteins containing T18D or T18A have the same rather than opposing properties. Therefore, the evidence that that phosphorylation of T18 is really inhibitory is lacking. (It is likely that T18D did not act as a phosphomimetic.) The strong effect of T18Ala is a concern. It is difficult to see why this should perturb the structure, as the authors claim. The authors could try a valine substitution instead.</p><p>2) It is puzzling that mitotic recruitment of Spc97 to the SPB of T18D mutants remains unchanged (<xref ref-type="fig" rid="fig7s1">Figure 7–figure supplement 1D</xref>), although there is an effect on microtubule nucleation. This also contrasts with the effect of T18 phosphorylation on the oligomerization of gamma-TuSCs seen in vitro. It is important to resolve this discrepancy.</p><p>3) It would further strengthen this manuscript if the authors could demonstrate in a more refined way that the pT18 epitope peaks late in mitosis and is lost upon mitotic exit (e.g., by immunoblotting of synchronized cells with anti pT18 in a time course experiment). The anti-phospho-T18 blots in <xref ref-type="fig" rid="fig6">Figure 6A</xref> are not convincing.</p><p>4) The biological significance of T18 phosphorylation to inhibit microtubule nucleation is unclear. The dynamics of spindle microtubules are high during this mid-mitosis, thus to support this property it is likely that nucleation activities from the SPB should be high as well.</p></body></sub-article><sub-article article-type="reply" id="SA2"><front-stub><article-id pub-id-type="doi">10.7554/eLife.02208.029</article-id><title-group><article-title>Author response</article-title></title-group></front-stub><body><p><italic>1) The primary concern is with T18. It is likely that this site is phosphorylated in vivo, but Spc110 mutant proteins containing T18D or T18A have the same rather than opposing properties. Therefore, the evidence that that phosphorylation of T18 is really inhibitory is lacking. (It is likely that T18D did not act as a phosphomimetic.) The strong effect of T18Ala is a concern. It is difficult to see why this should perturb the structure, as the authors claim. The authors could try a valine substitution instead</italic>.</p><p>As suggested by the reviewers, we have constructed <italic>spc110-T18V</italic> and in addition, <italic>spc110-T18E</italic> mutants. We have analyzed both mutants using biochemistry (<xref ref-type="fig" rid="fig2s5">Figure 2–figure supplement 5A</xref>) and cell growth (<xref ref-type="fig" rid="fig2s5">Figure 2–figure supplement 5B</xref>). The bottom line of both experiments is that <italic>spc110-T18A</italic> behaves as <italic>spc110-T18V</italic> and <italic>spc110-T18D</italic> behaves as <italic>spc110-T18E</italic>.</p><p>In addition, we have constructed a destruction box lacking <italic>ΔDB-CLB2-SPC110</italic> gene fusion following a strategy that was originally introduced by D. Morgan. We hoped that this gene fusion will target Cdk1 permanently to the SPB and phosphorylate T18 in a cell cycle independent manner. This should give us the <italic>spc110-pT18</italic> phenotype. However, the <italic>CLB2-SPC110</italic> gene fusion did not provide <italic>SPC110</italic> function (<xref ref-type="fig" rid="fig11">Author response image 1</xref>.)<fig id="fig11" position="float"><label>Author response image 1.</label><graphic xlink:href="elife02208f011"/></fig></p><p>To this end, we cannot say without ambiguities that T18D/E are phosphomimetic and T18A/V are phosphoinhibitory. However, it is clear that T18 lies within a conserved region, that we named SPM. Thus, we have changed the T18 phosphorylation part in this direction. In <xref ref-type="fig" rid="fig5s1">Figure 5 – figure supplement 1B</xref> we now show that T18 of Spc110 is phosphorylated in mitosis and we also show the <italic>in vitro</italic> data on Spc110<sup>1-220-5D-pT18</sup> on γ-TuSC oligomerization (<xref ref-type="fig" rid="fig5s1">Figure 5– figure supplement 1D, E</xref>). These data are consistent with an inhibitory role of pT18. This role is now discussed in the Discussion.</p><p><italic>2) It is puzzling that mitotic recruitment of Spc97 to the SPB of T18D mutants remains unchanged (</italic><xref ref-type="fig" rid="fig7s1"><italic>Figure. 7–figure supplement 1D</italic></xref><italic>), although there is an effect on microtubule nucleation. This also contrasts with the effect of T18 phosphorylation on the oligomerization of gamma-TuSCs seen in vitro. It is important to resolve this discrepancy</italic>.</p><p>In response to this comment, we have performed the experiment shown in <xref ref-type="fig" rid="fig6s1">Figure 6–figure supplement 1D</xref>. It shows that Spc97-GFP is recruited to SPBs of <italic>spc110-T18D</italic> cells with a time delay after SPB duplication. However, despite equal Spc97 amounts at SPBs in mitosis, <italic>spc110-T18D</italic> cells have a MT organization defect (<xref ref-type="fig" rid="fig6">Figure 6B</xref>). Indeed, <xref ref-type="fig" rid="fig2s2">Figure 2–figure supplement 2A</xref> shows that Spc110<sup>1-220-T18D</sup> and Spc110<sup>1-220-5D-T18D</sup> form putative dimers with γ-TuSC (shoulder at 600 kDa) without inducing γ-TuSC oligomerization (Vo). This indicates that γ-TuSC recruitment to SPBs via Spc110 and γ-TuSC oligomerization into a nucleation platform are mechanistically two distinct steps. We discuss this scenario in the Discussion.</p><p><italic>3) It would further strengthen this manuscript if the authors could demonstrate in a more refined way that the pT18 epitope peaks late in mitosis and is lost upon mitotic exit (e.g., by immunoblotting of synchronized cells with anti pT18 in a time course experiment). The anti-phospho-T18 blots in</italic> <xref ref-type="fig" rid="fig6"><italic>Figure 6A</italic></xref> <italic>are not convincing</italic>.</p><p>We tried to detect pT18 of Spc110 in IB with the P-specific anti-pT18 antibodies. However, this antibody only works with <italic>in vitro</italic> phosphorylated Spc110-pT18 (<xref ref-type="fig" rid="fig5s1">Figure 5–figure supplement 1A</xref>). We therefore turned to mass spectrometry to determine the cell cycle dependent phosphorylation of Spc110. We have fractionated yeast cells into a supernatant fraction and a SPB containing pellet. We have extracted SPB proteins from the pellet. Spc110-GFP was enriched form the supernatant and the SPB faction. As shown in <xref ref-type="fig" rid="fig5s1">Figure 5–figure supplement 1B,C</xref> T18 of Spc110 is predominately phosphorylated in mitosis. This fits with the data from Keck et al., which showed that Spc110-T18 was phosphorylated in mitosis.</p><p><italic>4) The biological significance of T18 phosphorylation to inhibit microtubule nucleation is unclear. The dynamics of spindle microtubules are high during this mid-mitosis, thus to support this property it is likely that nucleation activities from the SPB should be high as well</italic>.</p><p>As outlined under 1) we have changed this part of the paper. In budding yeast, nuclear MTs are only dynamic from the plus end because of the blocking cap of γ-TuSC at the MT minus end. Data from T. Stearns (Murphy et al., 1998) suggest that in budding yeast MT nucleation mainly happens in S phase when SPBs duplicate but probably not in other cell cycle phases. The T18 phosphorylation could inhibit excess MT nucleation by SPB-associated γ-TuSC in mitosis. SPBs organize relative constant numbers of 20 MTs in mitosis (<xref ref-type="bibr" rid="bib87">Winey et al., 1995</xref>).</p><p>Because T18A is not behaving as a non-phosphorylatable mutation, we unfortunately cannot test this model. However, we have discussed in line 510 of the Discussion the possibility that pT18 restricts the number of nuclear MTs in mitosis.</p></body></sub-article></article> |