The 110-kD spindle pole body component of Saccharomyces cerevisiae is a phosphoprotein that is modified in a cell cycle-dependent manner

Microtubule nucleation by γ-tubulin complexes and beyond

In this short review, we give an overview of microtubule nucleation within cells. It is nearly 30 years since the discovery of γ-tubulin, a member of the tubulin superfamily essential for proper microtubule nucleation in all eukaryotes. γ-tubulin associates with other proteins to form multiprotein γ-tubulin ring complexes (γ-TuRCs) that template and catalyse the otherwise kinetically unfavourable assembly of microtubule filaments. These filaments can be dynamic or stable and they perform diverse functions, such as chromosome separation during mitosis and intracellular transport in neurons. The field has come a long way in understanding γ-TuRC biology but several important and unanswered questions remain, and we are still far from understanding the regulation of microtubule nucleation in a multicellular context. Here, we review the current literature on γ-TuRC assembly, recruitment, and activation and discuss the potential importance of γ-TuRC heterogeneity, the role of non-γ-TuRC proteins in microtubule nucleation, and whether γ-TuRCs could serve as good drug targets for cancer therapy.

Novel phosphorylation states of the yeast spindle pole body

Phosphorylation regulates yeast spindle pole body (SPB) duplication and separation and likely regulates microtubule nucleation. We report a phosphoproteomic analysis using tandem mass spectrometry of enriched Saccharomyces cerevisiae SPBs for two cell cycle arrests, G1/S and the mitotic checkpoint, expanding on previously reported phosphoproteomic data sets. We present a novel phosphoproteomic state of SPBs arrested in G1/S by a cdc4-1 temperature-sensitive mutation, with particular focus on phosphorylation events on the γ-tubulin small complex (γ-TuSC). The cdc4-1 arrest is the earliest arrest at which microtubule nucleation has occurred at the newly duplicated SPB. Several novel phosphorylation sites were identified in G1/S and during mitosis on the microtubule nucleating γ-TuSC. These sites were analyzed in vivo by fluorescence microscopy and were shown to be required for proper regulation of spindle length. Additionally, in vivo analysis of two mitotic sites in Spc97 found that phosphorylation of at least one of these sites is required for progression through the cell cycle. This phosphoproteomic data set not only broadens the scope of the phosphoproteome of SPBs, it also identifies several γ-TuSC phosphorylation sites that influence microtubule formation.

Summary: A phosphoproteome of yeast spindle pole bodies in G1/S or M phase identifies phosphorylation sites involved in spindle length control and provides direction for future phosphorylation analyses of spindle pole components.

Higher-order oligomerization of Spc110p drives γ-tubulin ring complex assembly

Assembly of the microtubule-nucleating γ-tubulin ring complex (γTuRC) requires higher-order oligomerization of Spc110p, which connects γTuRC to the yeast spindle pole body (SPB). Because Spc110p is highly concentrated at the SPB, spatial regulation of microtubule nucleation is thus achieved by exclusive assembly of γTuRCs proximal to the SPB.

The microtubule (MT) cytoskeleton plays important roles in many cellular processes. In vivo, MT nucleation is controlled by the γ-tubulin ring complex (γTuRC), a 2.1-MDa complex composed of γ-tubulin small complex (γTuSC) subunits. The mechanisms underlying the assembly of γTuRC are largely unknown. In yeast, the conserved protein Spc110p both stimulates the assembly of the γTuRC and anchors the γTuRC to the spindle pole body. Using a quantitative in vitro FRET assay, we show that γTuRC assembly is critically dependent on the oligomerization state of Spc110p, with higher-order oligomers dramatically enhancing the stability of assembled γTuRCs. Our in vitro findings were confirmed with a novel in vivo γTuSC recruitment assay. We conclude that precise spatial control over MT nucleation is achieved by coupling localization and higher-order oligomerization of the receptor for γTuRC.

Interaction of CK1δ with γTuSC ensures proper microtubule assembly and spindle positioning

Casein kinase 1δ (CK1δ) family members associate with microtubule-organizing centers from yeast to humans. Budding yeast CK1δ, Hrr25, directly phosphorylated γTuSC proteins in vivo and in vitro, and this phosphorylation promoted δTuSC integrity and activity in biochemical assays.

Casein kinase 1δ (CK1δ) family members associate with microtubule-organizing centers (MTOCs) from yeast to humans, but their mitotic roles and targets have yet to be identified. We show here that budding yeast CK1δ, Hrr25, is a γ-tubulin small complex (γTuSC) binding factor. Moreover, Hrr25's association with γTuSC depends on its kinase activity and its noncatalytic central domain. Loss of Hrr25 kinase activity resulted in assembly of unusually long cytoplasmic microtubules and defects in spindle positioning, consistent with roles in regulation of γTuSC-mediated microtubule nucleation and the Kar9 spindle-positioning pathway, respectively. Hrr25 directly phosphorylated γTuSC proteins in vivo and in vitro, and this phosphorylation promoted γTuSC integrity and activity. Because CK1δ and γTuSC are highly conserved and present at MTOCs in diverse eukaryotes, similar regulatory mechanisms are expected to apply generally in eukaryotes.

Cell-cycle dependent phosphorylation of yeast pericentrin regulates γ-TuSC-mediated microtubule nucleation

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 (Kollman et al., 2010). 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 (Sawin et al., 2004), 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.

DOI:http://dx.doi.org/10.7554/eLife.02208.001

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.

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.

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.

DOI:http://dx.doi.org/10.7554/eLife.02208.002

Predicting kinase substrates using conservation of local motif density

MOTIVATION

Protein kinases represent critical links in cell signaling. A central problem in computational biology is to systematically identify their substrates.

RESULTS

This study introduces a new method to predict kinase substrates by extracting evolutionary information from multiple sequence alignments in a manner that is tolerant to degenerate motif positioning. Given a known consensus, the new method (ConDens) compares the observed density of matches to a null model of evolution and does not require labeled training data. We confirmed that ConDens has improved performance compared with several existing methods in the field. Further, we show that it is generalizable and can predict interesting substrates for several important eukaryotic kinases where training data is not available.

AVAILABILITY AND IMPLEMENTATION

ConDens can be found at http://www.moseslab.csb.utoronto.ca/andyl/.

CONTACT

alan.moses@utoronto.ca

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Budding yeast centrosome duplication requires stabilization of Spc29 via Mps1-mediated phosphorylation

Protein phosphorylation plays an important role in the regulation of centrosome duplication. In budding yeast, numerous lines of evidence suggest a requirement for multiple phosphorylation events on individual components of the centrosome to ensure their proper assembly and function. Here, we report the first example of a single phosphorylation event on a component of the yeast centrosome, or spindle pole body (SPB), that is required for SPB duplication and cell viability. This phosphorylation event is on the essential SPB component Spc29 at a conserved Thr residue, Thr(240). Mutation of Thr(240) to Ala is lethal at normal gene dosage, but an increased copy number of this mutant allele results in a conditional phenotype. Phosphorylation of Thr(240) was found to promote the stability of the protein in vivo and is catalyzed in vitro by the Mps1 kinase. Furthermore, the stability of newly synthesized Spc29 is reduced in a mutant strain with reduced Mps1 kinase activity. These results demonstrate the first evidence for a single phosphorylation event on an SPB component that is absolutely required for SPB duplication and suggest that the Mps1 kinase is responsible for this protein-stabilizing phosphorylation.

Phosphorylation of Spc110p by Cdc28p-Clb5p kinase contributes to correct spindle morphogenesis in S. cerevisiae

Spindle morphogenesis is regulated by cyclin-dependent kinases and monitored by checkpoint pathways to accurately coordinate chromosomal segregation with other events in the cell cycle. We have previously dissected the contribution of individual B-type cyclins to spindle morphogenesis in Saccharomyces cerevisiae. We showed that the S-phase cyclin Clb5p is required for coupling spindle assembly and orientation. Loss of Clb5p-dependent kinase abolishes intrinsic asymmetry between the spindle poles resulting in lethal translocation of the spindle into the bud with high penetrance in diploid cells. This phenotype was exploited in a screen for high dosage suppressors that yielded spc110(Delta)(13), encoding a truncation of the spindle pole body component Spc110p (the intranuclear receptor for the gamma-tubulin complex). We found that Clb5p-GFP was localised to the spindle poles and intranuclear microtubules and that Clb5p-dependent kinase promoted cell cycle dependent phosphorylation of Spc110p contributing to spindle integrity. Two cyclin-dependent kinase consensus sites were required for this phosphorylation and were critical for the activity of spc110(Delta)(13) as a suppressor. Together, our results point to the function of cyclin-dependent kinase phosphorylation of Spc110p and provide, in addition, support to a model for Clb5p control of spindle polarity at the level of astral microtubule organisation.

The budding yeast spindle pole body: structure, duplication, and function

Nucleation of microtubules by eukaryotic microtubule organizing centers (MTOCs) is required for a variety of functions, including chromosome segregation during mitosis and meiosis, cytokinesis, fertilization, cellular morphogenesis, cell motility, and intracellular trafficking. Analysis of MTOCs from different organisms shows that the structure of these organelles is widely varied even though they all share the function of microtubule nucleation. Despite their morphological diversity, many components and regulators of MTOCs, as well as principles in their assembly, seem to be conserved. This review focuses on one of the best-characterized MTOCs, the budding yeast spindle pole body (SPB). We review what is known about its structure, protein composition, duplication, regulation, and functions. In addition, we discuss how studies of the yeast SPB have aided investigation of other MTOCs, most notably the centrosome of animal cells.

The Saccharomyces cerevisiae spindle pole body is a dynamic structure

During spindle pole body (SPB) duplication, the new SPB is assembled at a distinct site adjacent to the old SPB. Using quantitative fluorescence methods, we studied the assembly and dynamics of the core structural SPB component Spc110p. The SPB core exhibits both exchange and growth in a cell cycle-dependent manner. During G1/S phase, the old SPB exchanges approximately 50% of old Spc110p for new Spc110p. In G2 little Spc110p is exchangeable. Thus, Spc110p is dynamic during G1/S and becomes stable during G2. The SPB incorporates additional Spc110p in late G2 and M phases; this growth is followed by reduction in the next G1. Spc110p addition to the SPBs (growth) also occurs in response to G2 and mitotic arrests but not during a G1 arrest. Our results reveal several dynamic features of the SPB core: cell cycle-dependent growth and reduction, growth in response to cell cycle arrests, and exchange of Spc110p during SPB duplication. Moreover, rather than being considered a conservative or dispersive process, the assembly of Spc110p into the SPB is more readily considered in terms of growth and exchange.

Cnm67p is a spacer protein of the Saccharomyces cerevisiae spindle pole body outer plaque

In Saccharomyces cerevisiae, the spindle pole body (SPB) is the functional homolog of the mammalian centrosome, responsible for the organization of the tubulin cytoskeleton. Cytoplasmic (astral) microtubules essential for the proper segregation of the nucleus into the daughter cell are attached at the outer plaque on the SPB cytoplasmic face. Previously, it has been shown that Cnm67p is an integral component of this structure; cells deleted for CNM67 are lacking the SPB outer plaque and thus experience severe nuclear migration defects. With the use of partial deletion mutants of CNM67, we show that the N- and C-terminal domains of the protein are important for nuclear migration. The C terminus, not the N terminus, is essential for Cnm67p localization to the SPB. On the other hand, only the N terminus is subject to protein phosphorylation of a yet unknown function. Electron microscopy of SPB serial thin sections reveals that deletion of the N- or C-terminal domains disturbs outer plaque formation, whereas mutations in the central coiled-coil domain of Cnm67p change the distance between the SPB core and the outer plaque. We conclude that Cnm67p is the protein that connects the outer plaque to the central plaque embedded in the nuclear envelope, adjusting the space between them by the length of its coiled-coil.

Yeast Mps1p phosphorylates the spindle pole component Spc110p in the N-terminal domain

The yeast spindle pole body (SPB) component Spc110p (Nuf1p) undergoes specific serine/threonine phosphorylation as the mitotic spindle apparatus forms, and this phosphorylation persists until cells enter anaphase. We demonstrate that the dual-specificity kinase Mps1p is essential for the mitosis-specific phosphorylation of Spc110p in vivo and that Mps1p phosphorylates Spc110p in vitro. Phosphopeptides generated by proteolytic cleavage were identified and sequenced by mass spectrometry. Ser(60), Thr(64), and Thr(68) are the major sites in Spc110p phosphorylated by Mps1p in vitro, and alanine substitution at these sites abolishes the mitosis-specific isoform in vivo. This is the first time that phosphorylation sites of an SPB component have been determined, and these are the first sites of Mps1p phosphorylation identified. Alanine substitution for any one of these phosphorylated residues, in conjunction with an alanine substitution at residue Ser(36), is lethal in combination with alleles of SPC97, which encodes a component of the Tub4p complex. Consistent with a specific dysfunction for the alanine substitution mutations, simultaneous mutation of all four serine/threonine residues to aspartate does not confer any defect. Sites of Mps1p phosphorylation and Ser(36) are located within the N-terminal globular domain of Spc110p, which resides at the inner plaque of the SPB and binds the Tub4p complex.

A computationally directed screen identifying interacting coiled coils from Saccharomyces cerevisiae

Computational methods can frequently identify protein-interaction motifs in otherwise uncharacterized open reading frames. However, the identification of candidate ligands for these motifs (e.g., so that partnering can be determined experimentally in a directed manner) is often beyond the scope of current computational capabilities. One exception is provided by the coiled-coil interaction motif, which consists of two or more alpha helices that wrap around each other: the ligands for coiled-coil sequences are generally other coiled-coil sequences, thereby greatly simplifying the motif/ligand recognition problem. Here, we describe a two-step approach to identifying protein-protein interactions mediated by two-stranded coiled coils that occur in Saccharomyces cerevisiae. Coiled coils from the yeast genome are first predicted computationally, by using the multicoil program, and associations between coiled coils are then determined experimentally by using the yeast two-hybrid assay. We report 213 unique interactions between 162 putative coiled-coil sequences. We evaluate the resulting interactions, focusing on associations identified between components of the spindle pole body (the yeast centrosome).

Functional interaction between the PKC1 pathway and CDC31 network of SPB duplication genes

The earliest known step in yeast spindle pole body (SPB) duplication requires Cdc31p and Kar1p, two physically interacting SPB components, and Dsk2p and Rad23p, a pair of ubiquitin-like proteins. Components of the PKC1 pathway were found to interact with these SPB duplication genes in two independent genetic screens. Initially, SLG1 and PKC1 were obtained as high-copy suppressors of dsk2Delta rad23Delta and a mutation in MPK1 was synthetically lethal with kar1-Delta17. Subsequently, we demonstrated extensive genetic interactions between the PKC1 pathway and the SPB duplication mutants that affect Cdc31p function. The genetic interactions are unlikely to be related to the cell-wall integrity function of the PKC1 pathway because the SPB mutants did not exhibit cell-wall defects. Overexpression of multiple PKC1 pathway components suppressed the G2/M arrest of the SPB duplication mutants and mutations in MPK1 exacerbated the cell cycle arrest of kar1-Delta17, suggesting a role for the PKC1 pathway in SPB duplication. We also found that mutations in SPC110, which encodes a major SPB component, showed genetic interactions with both CDC31 and the PKC1 pathway. In support of the model that the PKC1 pathway regulates SPB duplication, one of the phosphorylated forms of Spc110p was absent in pkc1 and mpk1Delta mutants.

Dynamic localization of protein phosphatase type 1 in the mitotic cell cycle of Saccharomyces cerevisiae

Protein phosphatase type I (PP1), encoded by the single essential gene GLC7 in Saccharomyces cerevisiae, functions in diverse cellular processes. To identify in vivo subcellular location(s) where these processes take place, we used a functional green fluorescent protein (GFP)-Glc7p fusion protein. Time-lapse fluorescence microscopy revealed GFP-Glc7p localizes predominantly in the nucleus throughout the mitotic cell cycle, with the highest concentrations in the nucleolus. GFP-Glc7p was also observed in a ring at the bud neck, which was dependent upon functional septins. Supporting a role for Glc7p in bud site selection, a glc7-129 mutant displayed a random budding pattern. In alpha-factor treated cells, GFP-Glc7p was located at the base of mating projections, again in a septin-dependent manner. At the start of anaphase, GFP-Glc7p accumulated at the spindle pole bodies and remained there until cytokinesis. After anaphase, GFP-Glc7p became concentrated in a ring that colocalized with the actomyosin ring. A GFP-Glc7-129 fusion was defective in localizing to the bud neck and SPBs. Together, these results identify sites of Glc7p function and suggest Glc7p activity is regulated through dynamic changes in its location.

Saccharomyces cerevisiae MPS2 encodes a membrane protein localized at the spindle pole body and the nuclear envelope

The MPS2 (monopolar spindle two) gene is one of several genes required for the proper execution of spindle pole body (SPB) duplication in the budding yeast Saccharomyces cerevisiae (). We report here that the MPS2 gene encodes an essential 44-kDa protein with two putative coiled-coil regions and a hydrophobic sequence. Although MPS2 is required for normal mitotic growth, some null strains can survive; these survivors exhibit slow growth and abnormal ploidy. The MPS2 protein was tagged with nine copies of the myc epitope, and biochemical fractionation experiments show that it is an integral membrane protein. Visualization of a green fluorescent protein (GFP) Mps2p fusion protein in living cells and indirect immunofluorescence microscopy of 9xmyc-Mps2p revealed a perinuclear localization with one or two brighter foci of staining corresponding to the SPB. Additionally, immunoelectron microscopy shows that GFP-Mps2p localizes to the SPB. Our analysis suggests that Mps2p is required as a component of the SPB for insertion of the nascent SPB into the nuclear envelope.

Molecular characteristics of the centrosome

As an organizer of the microtubule cytoskeleton in animals, the centrosome has an important function. From the early light microscopic observation of the centrosome to examination by electron microscopy, the centrosome field is now in an era of molecular identification and precise functional analyses. Tables compiling centrosomal proteins and reviews on the centrosome are presented here and demonstrate how active the field is. However, despite this intense research activity, many classical questions are still unanswered. These include those regarding the precise function of centrioles, the mechanism of centrosome duplication and assembly, the origin of the centrosome, and the regulation and mechanism of the centrosomal microtubule nucleation activity. Fortunately, these questions are becoming elucidated based on experimental data discussed here. Given the fact that the centrosome is primarily a site of microtubule nucleation, special focus is placed on the process of microtubule nucleation and on the regulation of centrosomal microtubule nucleation capacity during the cell cycle and in some tissues.

Characterization of protein phosphatases in mouse oocytes

Okadaic acid (OA) enhances the resumption of meiosis in mouse oocytes, indicating that serine/threonine protein phosphatase-1 (PP1) and/or PP2A is involved. However, specific identification of PP1 and/or PP2A in mouse oocytes has not been reported. Here we demonstrate that fully grown germinal vesicle-intact (GVI) mouse oocytes contain mRNA corresponding to two isotypes of PP1, PP1alpha and PP1gamma. In addition, the transcript for PP2A was also present. At the protein level only PP1alpha and PP2A were recognized in fully grown GVI oocytes by Western blot analysis. Neither of the PP1gamma spliced variant proteins, PP1gamma1 and PP1gamma2, was detectable. Immunohistochemical analysis of ovarian tissue from gonadotropin-stimulated adult mice resulted in subcellular localization of both PP1alpha and PP2A, but not PP1gamma, in oocytes from all stages of folliculogenesis. In primordial oocytes, PP1alpha and PP2A were present in the cytoplasm. In more advanced stages of oogenesis, PP1alpha, although still present in the cytoplasm, was highly concentrated in the nucleus, whereas PP2A was predominantly cytoplasmic with a distinct reduction in the nuclear area. Both PP1alpha and PP2A were immunodetectable in oocytes during the prepubertal period. Eleven-day-old mouse oocytes, considered OA-insensitive and germinal vesicle breakdown (GVB)-incompetent, displayed both PP1alpha and PP2A predominantly in the cytoplasm. By 15 days of age mouse oocytes, which are beginning to acquire OA sensitivity and GVB competence, showed a relocation of PP1alpha into the nucleoplasm while PP2A remained predominantly cytoplasmic. This is the first specific identification of PP1alpha and PP2A in mouse oocytes. The differential localization of PP1alpha and PP2A, in addition to the relocation of PP1alpha during the acquisition of meiotic competence, suggests that these PPs have distinct regulatory roles during the resumption of meiosis.

A genetic analysis of interactions with Spc110p reveals distinct functions of Spc97p and Spc98p, components of the yeast gamma-tubulin complex

The spindle pole body (SPB) in Saccharomyces cerevisiae functions as the microtubule-organizing center. Spc110p is an essential structural component of the SPB and spans between the central and inner plaques of this multilamellar organelle. The amino terminus of Spc110p faces the inner plaque, the substructure from which spindle microtubules radiate. We have undertaken a synthetic lethal screen to identify mutations that enhance the phenotype of the temperature-sensitive spc110-221 allele, which encodes mutations in the amino terminus. The screen identified mutations in SPC97 and SPC98, two genes encoding components of the Tub4p complex in yeast. The spc98-63 allele is synthetic lethal only with spc110 alleles that encode mutations in the N terminus of Spc110p. In contrast, the spc97 alleles are synthetic lethal with spc110 alleles that encode mutations in either the N terminus or the C terminus. Using the two-hybrid assay, we show that the interactions of Spc110p with Spc97p and Spc98p are not equivalent. The N terminus of Spc110p displays a robust interaction with Spc98p in two different two-hybrid assays, while the interaction between Spc97p and Spc110p is not detectable in one strain and gives a weak signal in the other. Extra copies of SPC98 enhance the interaction between Spc97p and Spc110p, while extra copies of SPC97 interfere with the interaction between Spc98p and Spc110p. By testing the interactions between mutant proteins, we show that the lethal phenotype in spc98-63 spc110-221 cells is caused by the failure of Spc98-63p to interact with Spc110-221p. In contrast, the lethal phenotype in spc97-62 spc110-221 cells can be attributed to a decreased interaction between Spc97-62p and Spc98p. Together, these studies provide evidence that Spc110p directly links the Tub4p complex to the SPB. Moreover, an interaction between Spc98p and the amino-terminal region of Spc110p is a critical component of the linkage, whereas the interaction between Spc97p and Spc110p is dependent on Spc98p.

Receptors determine the cellular localization of a gamma-tubulin complex and thereby the site of microtubule formation

The yeast microtubule organizing centre (MTOC), known as the spindle pole body (SPB), organizes the nuclear and cytoplasmic microtubules which are functionally and spatially distinct. Microtubule organization requires the yeast gamma-tubulin complex (Tub4p complex) which binds to the nuclear side of the SPB at the N-terminal domain of Spc110p. Here, we describe the identification of the essential SPB component Spc72p whose N-terminal domain interacts with the Tub4p complex on the cytoplasmic side of the SPB. We further report that this Tub4p complex-binding domain of Spc72p is essential and that temperature-sensitive alleles of SPC72 or overexpression of a binding domain-deleted variant of SPC72 (DeltaN-SPC72) impair cytoplasmic microtubule formation. Consequently, polynucleated and anucleated cells accumulated in these cultures. In contrast, overexpression of the entire SPC72 results in more cytoplasmic microtubules compared with wild-type. Finally, exchange of the Tub4p complex-binding domains of Spc110p and Spc72p established that the Spc110p domain, when attached to DeltaN-Spc72p, was functional at the cytoplasmic site of the SPB, while the corresponding domain of Spc72p fused to DeltaN-Spc110p led to a dominant-negative effect. These results suggest that different components of MTOCs act as receptors for gamma-tubulin complexes and that they are essential for the function of MTOCs.

Analysis of the Saccharomyces spindle pole by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry

A highly enriched spindle pole preparation was prepared from budding yeast and fractionated by SDS gel electrophoresis. Forty-five of the gel bands that appeared enriched in this fraction were analyzed by high-mass accuracy matrix-assisted laser desorption/ ionization (MALDI) peptide mass mapping combined with sequence database searching. This identified twelve of the known spindle pole components and an additional eleven gene products that had not previously been localized to the spindle pole. Immunoelectron microscopy localized eight of these components to different parts of the spindle. One of the gene products, Ndc80p, shows homology to human HEC protein (Chen, Y., D.J. Riley, P-L. Chen, and W-H. Lee. 1997. Mol. Cell Biol. 17:6049-6056) and temperature-sensitive mutants show defects in chromosome segregation. This is the first report of the identification of the components of a large cellular organelle by MALDI peptide mapping alone.

Spc98p directs the yeast gamma-tubulin complex into the nucleus and is subject to cell cycle-dependent phosphorylation on the nuclear side of the spindle pole body

In the yeast Saccharomyces cerevisiae, microtubules are organized by the spindle pole body (SPB), which is embedded in the nuclear envelope. Microtubule organization requires the gamma-tubulin complex containing the gamma-tubulin Tub4p, Spc98p, and Spc97p. The Tub4p complex is associated with cytoplasmic and nuclear substructures of the SPB, which organize the cytoplasmic and nuclear microtubules. Here we present evidence that the Tub4p complex assembles in the cytoplasm and then either binds to the cytoplasmic side of the SPB or is imported into the nucleus followed by binding to the nuclear side of the SPB. Nuclear import of the Tub4p complex is mediated by the essential nuclear localization sequence of Spc98p. Our studies also indicate that Spc98p in the Tub4p complex is phosphorylated at the nuclear, but not at the cytoplasmic, side of the SPB. This phosphorylation is cell cycle dependent and occurs after SPB duplication and nucleation of microtubules by the new SPB and therefore may have a role in mitotic spindle function. In addition, activation of the mitotic checkpoint stimulates Spc98p phosphorylation. The kinase Mps1p, which functions in SPB duplication and mitotic checkpoint control, seems to be involved in Spc98p phosphorylation. Our results also suggest that the nuclear and cytoplasmic Tub4p complexes are regulated differently.

Tyrosine phosphorylation regulates cell cycle-dependent nuclear localization of Cdc48p

Cdc48p from Saccharomyces cerevisiae and its highly conserved mammalian homologue VCP (valosin-containing protein) are ATPases with essential functions in cell division and homotypic fusion of endoplasmic reticulum vesicles. Both are mainly attached to the endoplasmic reticulum, but relocalize in a cell cycle-dependent manner: Cdc48p enters the nucleus during late G1; VCP aggregates at the centrosome during mitosis. The nuclear import signal sequence of Cdc48p was localized near the amino terminus and its function demonstrated by mutagenesis. The nuclear import is regulated by a cell cycle-dependent phosphorylation of a tyrosine residue near the carboxy terminus. Two-hybrid studies indicate that the phosphorylation results in a conformational change of the protein, exposing the nuclear import signal sequence previously masked by a stretch of acidic residues.

A mutational analysis identifies three functional regions of the spindle pole component Spc110p in Saccharomyces cerevisiae

The central coiled coil of the essential spindle pole component Spc110p spans the distance between the central and inner plaques of the Saccharomyces cerevisiae spindle pole body (SPB). The carboxy terminus of Spc110p, which binds calmodulin, resides at the central plaque, and the amino terminus resides at the inner plaque from which nuclear microtubules originate. To dissect the functions of Spc110p, we created temperature-sensitive mutations in the amino and carboxy termini. Analysis of the temperature-sensitive spc110 mutations and intragenic complementation analysis of the spc110 alleles defined three functional regions of Spc110p. Region I is located at the amino terminus. Region II is located at the carboxy-terminal end of the coiled coil, and region III is the previously defined calmodulin-binding site. Overexpression of SPC98 suppresses the temperature sensitivity conferred by mutations in region I but not the phenotypes conferred by mutations in the other two regions, suggesting that the amino terminus of Spc110p is involved in an interaction with the gamma-tubulin complex composed of Spc97p, Spc98p, and Tub4p. Mutations in region II lead to loss of SPB integrity during mitosis, suggesting that this region is required for the stable attachment of Spc110p to the central plaque. Our results strongly argue that Spc110p links the gamma-tubulin complex to the central plaque of the SPB.

Spc98p and Spc97p of the yeast gamma-tubulin complex mediate binding to the spindle pole body via their interaction with Spc110p

Previously, we have shown that the yeast gamma-tubulin, Tub4p, forms a 6S complex with the spindle pole body components Spc98p and Spc97p. In this paper we report the purification of the Tub4p complex. It contained one molecule of Spc98p and Spc97p, and two or more molecules of Tub4p, but no other protein. We addressed how the Tub4p complex binds to the yeast microtubule organizing center, the spindle pole body (SPB). Genetic and biochemical data indicate that Spc98p and Spc97p of the Tub4p complex bind to the N-terminal domain of the SPB component Spc110p. Finally, we isolated a complex containing Spc110p, Spc42p, calmodulin and a 35 kDa protein, suggesting that these four proteins interact in the SPB. We discuss in a model, how the N-terminus of Spc110p anchors the Tub4p complex to the SPB and how Spc110p itself is embedded in the SPB.

The spindle pole body of Schizosaccharomyces pombe enters and leaves the nuclear envelope as the cell cycle proceeds

The cycle of spindle pole body (SPB) duplication, differentiation, and segregation in Schizosaccharomyces pombe is different from that in some other yeasts. Like the centrosome of vertebrate cells, the SPB of S. pombe spends most of interphase in the cytoplasm, immediately next to the nuclear envelope. Some gamma-tubulin is localized on the SPB, suggesting that it plays a role in the organization of interphase microtubules (MTs), and serial sections demonstrate that some interphase MTs end on or very near to the SPB. gamma-Tubulin is also found on osmiophilic material that lies near the inner surface of the nuclear envelope, immediately adjacent to the SPB, even though there are no MTs in the interphase nucleus. Apparently, the MT initiation activities of gamma-tubulin in S. pombe are regulated. The SPB duplicates in the cytoplasm during late G2 phase, and the two resulting structures are connected by a darkly staining bridge until the mitotic spindle forms. As the cell enters mitosis, the nuclear envelope invaginates beside the SPB, forming a pocket of cytoplasm that accumulates dark amorphous material. The nuclear envelope then opens to form a fenestra, and the duplicated SPB settles into it. Each part of the SPB initiates intranuclear MTs, and then the two structures separate to lie in distinct fenestrae as a bipolar spindle forms. Through metaphase, the SPBs remain in their fenestrae, bound to the polar ends of spindle MTs; at about this time, a small bundle of cytoplasmic MTs forms in association with each SPB. These MTs are situated with one end near to, but not on, the SPBs, and they project into the cytoplasm at an orientation that is oblique to the simple axis. As anaphase proceeds, the nuclear fenestrae close, and the SPBs are extruded back into the cytoplasm. These observations define new fields of enquiry about the control of SPB duplication and the dynamics of the nuclear envelope.

Mini review: mitosis and the spindle pole body in Saccharomyces cerevisiae

Cell duplication is characteristic of life. The coordination of cell growth with cell duplication and, specifically, the ordered steps necessary for this process are termed the cell cycle. Central to this process is the faithful replication and segregation of the chromosomes. The cycle consists of four phases: G1, where the decision to enter the cell cycle, which is known as Start, is made; S phase, during which the DNA is replicated; G2, during which controls assuring the completion of S phase operate; and M, or the mitotic phase, which is characterized by chromosome segregation, nuclear division, and cytokinesis. The budding yeast Saccharomyces cerevisiae has been developed into a model genetic system for the study of the cell division cycle (Hartwell et al. ["73] Genetics, 74:267-286). Here I review the basic processes by which chromosomes are segregated, with an emphasis on the physical structures fundamental to this process.

Genetic analysis of the mitotic spindle

Much of our understanding of the molecular basis of mitotic spindle function has been achieved within the past decade. Studies utilizing genetically tractable organisms have made important contributions to this field and these studies form the basis of this review. We focus upon three areas of spindle research: spindle poles, centromeres, and spindle motors. The structure and duplication mechanisms of spindle poles are considered as well as their roles in organizing spindle microtubules. Centromeres vary considerably in their size and complexity. We describe recent progress in our understanding of the relatively simple centromeres of the yeast Saccharomyces cerevisiae and the complex centromeres that are more typical of eukaryotic cells. Microtubule-based motor proteins that generate the characteristic spindle movements have been identified in recent years and can be grouped into families defined by conserved primary sequence and mitotic function.

Spc110p: assembly properties and role in the connection of nuclear microtubules to the yeast spindle pole body

Spc110p is an essential component of the budding yeast spindle pole body (SPB). It binds calmodulin and contains a long central coiled-coil rod which acts as a spacer element between the central plaque of the SPB and the ends of the nuclear or spindle microtubules. This suggests that the essential function of Spc110p is to connect the nuclear microtubules to the SPB. To confirm this, we examined the phenotype of ts alleles of SPC110, one of which contains a mutation in the calmodulin binding site and was suppressed by overexpression of calmodulin. The alleles fail to form a functional mitotic spindle because spindle microtubules are not properly connected to the SPB. We also examined the phenotype of the toxic overexpression of either the wild-type or a truncated version of Spc110p containing a deletion of most of the coiled-coil domain. Both of these proteins form large ordered spheroidal polymers in the nucleus. The polymerization of the truncated Spc110p appears to be initiated inside the SPB from the position where Spc110p is normally located, and as the polymer grows in size it severs the connection between the nuclear microtubules and the SPB. The polymers were purified and are composed of Spc110p and calmodulin. A model for the structure of the polymer is proposed.