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

MiR-4763-3p accelerates lipopolysaccharide-induced cardiomyocyte apoptosis and inflammatory response by targeting IL10RA

In order to investigate miR-4763-3p and associated genes' roles in myocarditis, AC16 cell line was divided into LPS + miR-4763-3p inhibitor, LPS + NC inhibitor, LPS + miR-4763-3p inhibitor + si-IL10RA and NC groups, and Q-PCR was used to find out whether miR-4763-3p was expressed; Targetscan, Genecards, and MiRDB were used to estimate the miR-4763-3p target; Targetscan was used to display binding sites. Western blot assay was undertaken to detect Bax, Bcl-2, and IL10RA expression. Proliferation and apoptosis were processed using CCK8 and the flow cytometry assay, respectively. Migration and invasion were confirmed utilizing Transwell test. ELISA assay was processed to show the content of IL-6, IL-1ß, IL-10 and TGF-ß in the cell culture supernatant. After being exposed to LPS, cardiomyocyte cells expressed more miR-4763-3p. MiR-4763-3p inhibitor accelerated proliferation, migration and invasion behavior, while it also decreased apoptosis rate in LPS-treated cardiomyocyte cells. MiR-4763-3p inhibitor attenuated the inflammatory response by up-regulating Bax expression and down-regulating Bcl-2 level in LPS-treated cardiomyocyte cells. In cardiomyocyte cells treated with LPS, MiR-4763-3p expression was elevated. si-IL10RA The miR-4763-3p inhibitor restored its effects. MiR-4763-3p accelerates lipopolysaccharide-induced cardiomyocyte apoptosis and inflammatory response by targeting IL10RA, which might be a potential target for myocarditis.

The role of Aspergillus nidulans polo-like kinase PlkA in microtubule-organizing center control

Centrosomes are important microtubule-organizing centers (MTOC) in animal cells. In addition, non-centrosomal MTOCs (ncMTOCs) have been described in many cell types. The functional analogs of centrosomes in fungi are the spindle pole bodies (SPBs). In Aspergillus nidulans, additional MTOCs have been discovered at septa (sMTOC). Although the core components are conserved in both MTOCs, their composition and organization are different and dynamic. Here, we show that the polo-like kinase PlkA binds the γ-tubulin ring complex (γ-TuRC) receptor protein ApsB and contributes to targeting ApsB to both MTOCs. PlkA coordinates the activities of the SPB outer plaque and the sMTOC. PlkA kinase activity was required for astral MT formation involving ApsB recruitment. PlkA also interacted with the γ-TuRC inner plaque receptor protein PcpA. Mitosis was delayed without PlkA, and the PlkA protein was required for proper mitotic spindle morphology, although this function was independent of its catalytic activity. Our results suggest that the polo-like kinase is a regulator of MTOC activities and acts as a scaffolding unit through interaction with γ-TuRC receptors.

Keratin 17 regulates nuclear morphology and chromatin organization

Keratin 17 (KRT17; K17), a non-lamin intermediate filament protein, was recently found to occur in the nucleus. We report here on K17-dependent differences in nuclear morphology, chromatin organization, and cell proliferation. Human tumor keratinocyte cell lines lacking K17 exhibit flatter nuclei relative to normal. Re-expression of wild-type K17, but not a mutant form lacking an intact nuclear localization signal (NLS), rescues nuclear morphology in KRT17-null cells. Analyses of primary cultures of skin keratinocytes from a mouse strain expressing K17 with a mutated NLS corroborated these findings. Proteomics screens identified K17-interacting nuclear proteins with known roles in gene expression, chromatin organization and RNA processing. Key histone modifications and LAP2β (an isoform encoded by TMPO) localization within the nucleus are altered in the absence of K17, correlating with decreased cell proliferation and suppression of GLI1 target genes. Nuclear K17 thus impacts nuclear morphology with an associated impact on chromatin organization, gene expression, and proliferation in epithelial cells.This article has an associated First Person interview with the first author of the paper.

Orderly assembly underpinning built-in asymmetry in the yeast centrosome duplication cycle requires cyclin-dependent kinase

Asymmetric astral microtubule organization drives the polarized orientation of the S. cerevisiae mitotic spindle and primes the invariant inheritance of the old spindle pole body (SPB, the yeast centrosome) by the bud. This model has anticipated analogous centrosome asymmetries featured in self-renewing stem cell divisions. We previously implicated Spc72, the cytoplasmic receptor for the gamma-tubulin nucleation complex, as the most upstream determinant linking SPB age, functional asymmetry and fate. Here we used structured illumination microscopy and biochemical analysis to explore the asymmetric landscape of nucleation sites inherently built into the spindle pathway and under the control of cyclin-dependent kinase (CDK). We show that CDK enforces Spc72 asymmetric docking by phosphorylating Nud1/centriolin. Furthermore, CDK-imposed order in the construction of the new SPB promotes the correct balance of nucleation sites between the nuclear and cytoplasmic faces of the SPB. Together these contributions by CDK inherently link correct SPB morphogenesis, age and fate.

Tubgcp3 Is Required for Retinal Progenitor Cell Proliferation During Zebrafish Development

The centrosomal protein γ-tubulin complex protein 3 (Tubgcp3/GCP3) is required for the assembly of γ-tubulin small complexes (γ-TuSCs) and γ-tubulin ring complexes (γ-TuRCs), which play critical roles in mitotic spindle formation during mitosis. However, its function in vertebrate embryonic development is unknown. Here, we generated the zebrafish tubgcp3 mutants using the CRISPR/Cas9 system and found that the tubgcp3 mutants exhibited the small eye phenotype. Tubgcp3 is required for the cell cycle progression of retinal progenitor cells (RPCs), and its depletion caused cell cycle arrest in the mitotic (M) phase. The M-phase arrested RPCs exhibited aberrant monopolar spindles and abnormal distributed centrioles and γ-tubulin. Moreover, these RPCs underwent apoptosis finally. Our study provides the in vivo model for the functional study of Tubgcp3 and sheds light on the roles of centrosomal γ-tubulin complexes in vertebrate development.

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.

The microtubule polymerase Stu2 promotes oligomerization of the γ-TuSC for cytoplasmic microtubule nucleation

Stu2/XMAP215/ZYG-9/Dis1/Alp14/Msps/ch-TOG family members in association with with γ-tubulin complexes nucleate microtubules, but we know little about the interplay of these nucleation factors. Here, we show that the budding yeast Stu2 in complex with the γ-tubulin receptor Spc72 nucleates microtubules in vitro without the small γ-tubulin complex (γ-TuSC). Upon γ-TuSC addition, Stu2 facilitates Spc72–γ-TuSC interaction by binding to Spc72 and γ-TuSC. Stu2 together with Spc72–γ-TuSC increases microtubule nucleation in a process that is dependent on the TOG domains of Stu2. Importantly, these activities are also important for microtubule nucleation in vivo. Stu2 stabilizes Spc72–γ-TuSC at the minus end of cytoplasmic microtubules (cMTs) and an in vivo assay indicates that cMT nucleation requires the TOG domains of Stu2. Upon γ-tubulin depletion, we observed efficient cMT nucleation away from the spindle pole body (SPB), which was dependent on Stu2. Thus, γ-TuSC restricts cMT assembly to the SPB whereas Stu2 nucleates cMTs together with γ-TuSC and stabilizes γ-TuSC at the cMT minus end.

Big Lessons from Little Yeast: Budding and Fission Yeast Centrosome Structure, Duplication, and Function

Centrosomes are a functionally conserved feature of eukaryotic cells that play an important role in cell division. The conserved γ-tubulin complex organizes spindle and astral microtubules, which, in turn, separate replicated chromosomes accurately into daughter cells. Like DNA, centrosomes are duplicated once each cell cycle. Although in some cell types it is possible for cell division to occur in the absence of centrosomes, these divisions typically result in defects in chromosome number and stability. In single-celled organisms such as fungi, centrosomes [known as spindle pole bodies (SPBs)] are essential for cell division. SPBs also must be inserted into the membrane because fungi undergo a closed mitosis in which the nuclear envelope (NE) remains intact. This poorly understood process involves events similar or identical to those needed for de novo nuclear pore complex assembly. Here, we review how analysis of fungal SPBs has advanced our understanding of centrosomes and NE events.

Intrinsic and Extrinsic Determinants Linking Spindle Pole Fate, Spindle Polarity, and Asymmetric Cell Division in the Budding Yeast S. cerevisiae

The budding yeast S. cerevisiae is a powerful model to understand the multiple layers of control driving an asymmetric cell division. In budding yeast, asymmetric targeting of the spindle poles to the mother and bud cell compartments respectively orients the mitotic spindle along the mother-bud axis. This program exploits an intrinsic functional asymmetry arising from the age distinction between the spindle poles-one inherited from the preceding division and the other newly assembled. Extrinsic mechanisms convert this age distinction into differential fate. Execution of this program couples spindle orientation with the segregation of the older spindle pole to the bud. Remarkably, similar stereotyped patterns of inheritance occur in self-renewing stem cell divisions underscoring the general importance of studying spindle polarity and differential fate in yeast. Here, we review the mechanisms accounting for this pivotal interplay between intrinsic and extrinsic asymmetries that translate spindle pole age into differential fate.

Synergistic role of fission yeast Alp16GCP6 and Mzt1MOZART1 in γ-tubulin complex recruitment to mitotic spindle pole bodies and spindle assembly

In fission yeast, γ-tubulin ring complex (γTuRC)-specific components Gfh1(GCP4), Mod21(GCP5), and Alp16(GCP6) are nonessential for cell growth. Of these deletion mutants, only alp16Δ shows synthetic lethality with temperature-sensitive mutants of Mzt1(MOZART1), a component of the γTuRC required for recruitment of the complex to microtubule-organizing centers. γ-Tubulin small complex levels at mitotic spindle pole bodies (SPBs, the centrosome equivalent in fungi) and microtubule levels for preanaphase spindles are significantly reduced in alp16Δ cells but not in gfh1Δ or mod21Δ cells. Furthermore, alp16Δ cells often form monopolar spindles and frequently lose a minichromosome when the spindle assembly checkpoint is inactivated. Alp16(GCP6) promotes Mzt1-dependent γTuRC recruitment to mitotic SPBs and enhances spindle microtubule assembly in a manner dependent on its expression levels. Gfh1(GCP4) and Mod21(GCP5) are not required for Alp16(GCP6)-dependent γTuRC recruitment. Mzt1 has an additional role in the activation of the γTuRC for spindle microtubule assembly. The ratio of Mzt1 to γTuRC levels for preanaphase spindles is higher than at other stages of the cell cycle. Mzt1 overproduction enhances spindle microtubule assembly without affecting γTuRC levels at mitotic SPBs. We propose that Alp16(GCP6) and Mzt1 act synergistically for efficient bipolar spindle assembly to ensure faithful chromosome segregation.

Requirement of the spindle pole body for targeting and/or tethering proteins to the inner nuclear membrane

Appropriate targeting of inner nuclear membrane (INM) proteins is important for nuclear function and architecture. To gain new insights into the mechanism(s) for targeting and/or tethering peripherally associated proteins to the INM, we screened a collection of temperature sensitive S. cerevisiae yeast mutants for defects in INM location of the peripheral protein, Trm1-II-GFP. We uncovered numerous genes encoding components of the Spindle Pole Body (SPB), the yeast centrosome. SPB alterations affect the localization of both an integral (Heh2) and a peripheral INM protein (Trm1-II-GFP), but not a nucleoplasmic protein (Pus1). In wild-type cells Trm1-II-GFP is evenly distributed around the INM, but in SPB mutants, Trm1-II-GFP mislocalizes as a spot(s) near ER-nucleus junctions, perhaps its initial contact site with the nuclear envelope. Employing live cell imaging over time in a microfluidic perfusion system to study protein dynamics, we show that both Trm1-II-GFP INM targeting and maintenance depend upon the SPB. We propose a novel targeting and/or tethering model for a peripherally associated INM protein that combines mechanisms of both integral and soluble nuclear proteins, and describe a role of the SPB in nuclear envelope dynamics that affects this process.

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

Functional replacement of fission yeast γ-tubulin small complex proteins Alp4 and Alp6 by human GCP2 and GCP3

Microtubule-organizing centers such as the γ-tubulin ring complex (γ-TuRC) act as a template for polarized growth and regulation of microtubules that are essential for diverse cellular structures and processes in eukaryotes. New structural models of the budding yeast γ-tubulin small complex (γ-TuSC) of the γ-TuRC combined with functional studies done in multiple eukaryotes are revealing the first mechanistic clues into control of microtubule nucleation and organization. Cross-species studies of human and budding yeast γ-TuSC proteins in fission yeast revealed conserved and divergent structural and functional features of the γ-TuSC. We show genetically that GCP3/Spc98 function is fully conserved with Alp6 across species but that functional differences exist between GCP2/Spc97 and Alp4. By further analysis of human γ-TuSC proteins, we found that GCP3 assembles normally into the >2000 kDa fission yeast γ-TuRC and that the GCP3 gene replaces fission yeast alp6. Interestingly, human GCP2 replaces the essential alp4 gene but is unable to rescue a normally recessive G1 defect of the alp4-1891 allele that results in loss of γ-TuRC from poles in subsequent cell cycles. Biochemically, GCP2 incorporation into fission yeast γ-TuRC is limited in the presence of Alp4; instead, the bulk of GCP2 fractionates as smaller complexes. By generating a functional Alp4-GCP2 chimeric protein we determined that the GCP2 N-terminal domain limits its ability to fully displace or compete with Alp4 during γ-TuRC assembly. Our findings have broad importance for understanding the essential domains of γ-TuSC proteins in the γ-TuRC mechanism.

Spindle pole body history intrinsically links pole identity with asymmetric fate in budding yeast

BACKGROUND

Budding yeast is a unique model for exploring differential fate in a cell dividing asymmetrically. In yeast, spindle orientation begins with the old spindle pole body (SPB) (from the preceding cell cycle) contacting the bud by its existing astral microtubules (aMTs) while the new pole delays astral microtubule organization. This appears to prime the inheritance of the old pole by the bud. The basis for this asymmetry and the discrimination of the poles by virtue of their history remain a mystery.

RESULTS

Here, we report that asymmetric aMT organization stems from an outstanding structural asymmetry linked to the SPB cycle. We show that the γ-tubulin nucleation complex (γTC) favors the old spindle pole, an asymmetry inherent to the outer plaque (the cytoplasmic face of the SPB). Indeed, Spc72 (the receptor for the γTC) is acquired by the new SPB outer plaque partway through spindle assembly. The significance of this asymmetry was explored in cells expressing an Spc72(1-276)-Cnm67 fusion that forced symmetric nucleation at the SPB outer plaques. This manipulation triggered simultaneous aMT organization by both spindle poles from the outset and led to symmetric contacts between poles and the bud, effectively disrupting the program for spindle polarity. Temporally symmetric aMT organization perturbed Kar9 polarization by randomizing the choice of the pole to be guided toward the bud. Accordingly, the pattern of SPB inheritance was also randomized.

CONCLUSIONS

Spc72 differential recruitment imparting asymmetric aMT organization represents the most upstream determinant linking SPB historical identity and fate.

Mitotic spindle form and function

The Saccharomyces cerevisiae mitotic spindle in budding yeast is exemplified by its simplicity and elegance. Microtubules are nucleated from a crystalline array of proteins organized in the nuclear envelope, known as the spindle pole body in yeast (analogous to the centrosome in larger eukaryotes). The spindle has two classes of nuclear microtubules: kinetochore microtubules and interpolar microtubules. One kinetochore microtubule attaches to a single centromere on each chromosome, while approximately four interpolar microtubules emanate from each pole and interdigitate with interpolar microtubules from the opposite spindle to provide stability to the bipolar spindle. On the cytoplasmic face, two to three microtubules extend from the spindle pole toward the cell cortex. Processes requiring microtubule function are limited to spindles in mitosis and to spindle orientation and nuclear positioning in the cytoplasm. Microtubule function is regulated in large part via products of the 6 kinesin gene family and the 1 cytoplasmic dynein gene. A single bipolar kinesin (Cin8, class Kin-5), together with a depolymerase (Kip3, class Kin-8) or minus-end-directed kinesin (Kar3, class Kin-14), can support spindle function and cell viability. The remarkable feature of yeast cells is that they can survive with microtubules and genes for just two motor proteins, thus providing an unparalleled system to dissect microtubule and motor function within the spindle machine.

The MPS1 family of protein kinases

MPS1 protein kinases are found widely, but not ubiquitously, in eukaryotes. This family of potentially dual-specific protein kinases is among several that regulate a number of steps of mitosis. The most widely conserved MPS1 kinase functions involve activities at the kinetochore in both the chromosome attachment and the spindle checkpoint. MPS1 kinases also function at centrosomes. Beyond mitosis, MPS1 kinases have been implicated in development, cytokinesis, and several different signaling pathways. Family members are identified by virtue of a conserved C-terminal kinase domain, though the N-terminal domain is quite divergent. The kinase domain of the human enzyme has been crystallized, revealing an unusual ATP-binding pocket. The activity, level, and subcellular localization of Mps1 family members are tightly regulated during cell-cycle progression. The mitotic functions of Mps1 kinases and their overexpression in some tumors have prompted the identification of Mps1 inhibitors and their active development as anticancer drugs.

An extended γ-tubulin ring functions as a stable platform in microtubule nucleation

γ-Tubulin complexes are essential for microtubule (MT) nucleation. The γ-tubulin small complex (γ-TuSC) consists of two molecules of γ-tubulin and one molecule each of Spc97 and Spc98. In vitro, γ-TuSCs oligomerize into spirals of 13 γ-tubulin molecules per turn. However, the properties and numbers of γ-TuSCs at MT nucleation sites in vivo are unclear. In this paper, we show by fluorescence recovery after photobleaching analysis that γ-tubulin was stably integrated into MT nucleation sites and was further stabilized by tubulin binding. Importantly, tubulin showed a stronger interaction with the nucleation site than with the MT plus end, which probably provides the basis for MT nucleation. Quantitative analysis of γ-TuSCs on single MT minus ends argued for nucleation sites consisting of approximately seven γ-TuSCs with approximately three additional γ-tubulin molecules. Nucleation and anchoring of MTs required the same number of γ-tubulin molecules. We suggest that a spiral of seven γ-TuSCs with a slight surplus of γ-tubulin nucleates MTs in vivo.

Nucleocytoplasmic shuttling of cytoskeletal proteins: molecular mechanism and biological significance

Various nuclear functional complexes contain cytoskeletal proteins as regulatory subunits; for example, nuclear actin participates in transcriptional complexes, and actin-related proteins are integral to chromatin remodeling complexes. Nuclear complexes such as these are involved in both basal and adaptive nuclear functions. In addition to nuclear import via classical nuclear transport pathways or passive diffusion, some large cytoskeletal proteins spontaneously migrate into the nucleus in a karyopherin-independent manner. The balance of nucleocytoplasmic distribution of such proteins can be altered by several factors, such as import versus export, or capture and release by complexes. The resulting accumulation or depletion of the nuclear populations thereby enhances or attenuates their nuclear functions. We propose that such molecular dynamics constitute a form of cytoskeleton-modulated regulation of nuclear functions which is mediated by the translocation of cytoskeletal components in and out of the nucleus.

Phosphorylation of the yeast γ-tubulin Tub4 regulates microtubule function

The yeast γ-tubulin Tub4 is assembled with Spc97 and Spc98 into the small Tub4 complex. The Tub4 complex binds via the receptor proteins Spc72 and Spc110 to the spindle pole body (SPB), the functional equivalent of the mammalian centrosome, where the Tub4 complex organizes cytoplasmic and nuclear microtubules. Little is known about the regulation of the Tub4 complex. Here, we isolated the Tub4 complex with the bound receptors from yeast cells. Analysis of the purified Tub4 complex by mass spectrometry identified more than 50 phosphorylation sites in Spc72, Spc97, Spc98, Spc110 and Tub4. To examine the functional relevance of the phosphorylation sites, phospho-mimicking and non-phosphorylatable mutations in Tub4, Spc97 and Spc98 were analyzed. Three phosphorylation sites in Tub4 were found to be critical for Tub4 stability and microtubule organization. One of the sites is highly conserved in γ-tubulins from yeast to human.

Structural mutants of the spindle pole body cause distinct alteration of cytoplasmic microtubules and nuclear dynamics in multinucleated hyphae

To determine how microtubule (MT) nucleation and nuclear migration are controlled in multinucleated hyphae we deleted genes encoding MTOC subunits and AgStu2. The novel phenotypes we observed in these mutants compared with analogous deletions in budding yeast allowed us to assign functions to the two types of cMTs that we observe in A. gossypii.

In the multinucleate fungus Ashbya gossypii, cytoplasmic microtubules (cMTs) emerge from the spindle pole body outer plaque (OP) in perpendicular and tangential directions. To elucidate the role of cMTs in forward/backward movements (oscillations) and bypassing of nuclei, we constructed mutants potentially affecting cMT nucleation or stability. Hyphae lacking the OP components AgSpc72, AgNud1, AgCnm67, or the microtubule-stabilizing factor AgStu2 grew like wild- type but showed substantial alterations in the number, length, and/or nucleation sites of cMTs. These mutants differently influenced nuclear oscillation and bypassing. In Agspc72Δ, only long cMTs were observed, which emanate tangentially from reduced OPs; nuclei mainly moved with the cytoplasmic stream but some performed rapid bypassing. Agnud1Δ and Agcnm67Δ lack OPs; short and long cMTs emerged from the spindle pole body bridge/half-bridge structures, explaining nuclear oscillation and bypassing in these mutants. In Agstu2Δ only very short cMTs emanated from structurally intact OPs; all nuclei moved with the cytoplasmic stream. Therefore, long tangential cMTs promote nuclear bypassing and short cMTs are important for nuclear oscillation. Our electron microscopy ultrastructural analysis also indicated that assembly of the OP occurs in a stepwise manner, starting with AgCnm67, followed by AgNud1 and lastly AgSpc72.

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.

Nuclear export receptor Xpo1/Crm1 is physically and functionally linked to the spindle pole body in budding yeast

The spindle pole body (SPB) represents the microtubule organizing center in the budding yeast Saccharomyces cerevisiae. It is a highly structured organelle embedded in the nuclear membrane, which is required to anchor microtubules on both sides of the nuclear envelope. The protein Spc72, a component of the SPB, is located at the cytoplasmic face of this organelle and serves as a receptor for the gamma-tubulin complex. In this paper we show that it is also a binding partner of the nuclear export receptor Xpo1/Crm1. Xpo1 binds its cargoes in a Ran-dependent fashion via a short leucine-rich nuclear export signal (NES). We show that binding of Spc72 to Xpo1 depends on Ran-GTP and a functional NES in Spc72. Mutations in this NES have severe consequences for mitotic spindle morphology in vivo. This is also the case for xpo1 mutants, which show a reduction in cytoplasmic microtubules. In addition, we find a subpopulation of Xpo1 localized at the SPB. Based on these data, we propose a functional link between Xpo1 and the SPB and discuss a role for this exportin in spindle biogenesis in budding yeast.

Yeast alpha-tubulin suppressor Ats1/Kti13 relates to the Elongator complex and interacts with Elongator partner protein Kti11

The alpha-tubulin suppressor 1 (ATS1) gene and the killer toxin-insensitive 13 (KTI13) locus from Saccharomyces cerevisiae are allelic. The Ats1/Kti13 gene product interacts with the cell polarity factor Nap1 and promotes growth inhibition of S. cerevisiae by zymocin, a tRNAse toxin complex from Kluyveromyces lactis. Kti13 removal causes zymocin resistance, a trait that is typical of defects in the Elongator complex. Here, we show that Kti13 co-purifies with the Elongator partner protein Kti11 and that the Kti11 interaction, not the Nap1 partnership, requires the C-terminus of Kti13. Moreover, Kti13 functionally relates to roles of the Elongator complex in tRNA wobble uridine modification, tRNA suppression of nonsense (SUP4) and missense (SOE1) mutations and tRNA restriction by zymocin. Also, inactivation of Kti13 or Elongator rescues the thermosensitive growth defect of secretory mutants (sec2-59(ts), sec12-4(ts)), suggesting that Kti13 and Elongator affect secretion processes that depend on the GTP exchange factors Sec2 and Sec12 respectively. Distinct from tandem deletions in KTI13 and Elongator genes, a kti13Delta kti11Delta double deletion induces synthetic sickness or lethality. In sum, our data suggest that Kti13 and Kti11 support Elongator functions and that they both share Elongator-independent role(s) that are important for cell viability.

Nucleocytoplasmic shuttling of the Rpb4p and Rpb7p subunits of Saccharomyces cerevisiae RNA polymerase II by two pathways

Rpb4p and Rpb7p are subunits of the RNA polymerase II of Saccharomyces cerevisiae that form a dissociable heterodimeric complex. Whereas the only reported function of Rpb7p is related to transcription, Rpb4p has been found to also act in mRNA export and in the major mRNA decay pathway that operates in the cytoplasm, thus raising the possibility that Rpb4p links between the nuclear and cytoplasmic processes. Here we show that both Rpb4p and Rpb7p shuttle between the nucleus and the cytoplasm. Shuttling kinetics of the two proteins are similar as long as their interaction is possible, suggesting that they shuttle as a heterodimer. Under normal conditions, shuttling of Rpb4p and Rpb7p depends on ongoing transcription. However, during severe stresses of heat shock, ethanol, and starvation, the two proteins shuttle via a transcription-independent pathway. Thus, Rpb4p and Rpb7p shuttle via two pathways, depending on environmental conditions.

Modulation of Alp4 function in Schizosaccharomyces pombe induces novel phenotypes that imply distinct functions for nuclear and cytoplasmic gamma-tubulin complexes

The gamma-tubulin complex acts as a nucleation unit for microtubule assembly. It remains unknown, however, how spatial and temporal regulation of the complex activity affects microtubule-mediated cellular processes. Alp4 is one of the essential components of the S. pombe gamma-tubulin complex. We show here that overproduction of a carboxy-terminal form of Alp4 (Alp4C) and its derivatives tagged to a nuclear localization signal or to a nuclear export signal affect localization of gamma-tubulin complexes and induces novel phenotypes that reflect distinct functions of nuclear and cytoplasmic gamma-tubulin complexes. Nuclear Alp4C induces a Wee1-dependent G2 delay, reduces the levels of the gamma-tubulin complex at the spindle pole body, and results in defects in mitotic progression including spindle assembly, cytoplasmic microtubule disassembly, and chromosome segregation. In contrast, cytoplasmic Alp4C induces oscillatory nuclear movement and affects levels of cell polarity markers, Bud6 and Tip1, at the cell ends. These results demonstrate that regulation of nuclear gamma-tubulin complex activity is essential for cell cycle progression through the G2/M boundary and M phase, whereas regulation of cytoplasmic gamma-tubulin complex activity is important for nuclear positioning and cell polarity control during interphase.

Intranuclear accumulation of plant tubulin in response to low temperature

Concurrently with cold-induced disintegration of microtubular structures in the cytoplasm, gradual tubulin accumulation was observed in a progressively growing proportion of interphase nuclei in tobacco BY-2 cells. This intranuclear tubulin disappeared upon rewarming. Simultaneously, new microtubules rapidly emerged from the nuclear periphery and reconstituted new cortical arrays, as was shown by immunofluorescence. A rapid exclusion of tubulin from the nucleus during rewarming was also observed in vivo in cells expressing GFP-tubulin. Nuclei were purified from cells that expressed GFP fused to an endoplasmic-reticulum retention signal (BY-2-mGFP5-ER), and green-fluorescent protein was used as a diagnostic marker to confirm that the nuclear fraction was not contaminated by nuclear-envelope proteins. These purified, GFP-free nuclei contained tubulin when isolated from cold-treated cells, whereas control nuclei were void of tubulin. Furthermore, highly conserved putative nuclear-export sequences were identified in tubulin sequences. These results led us to interpret the accumulation of tubulin in interphasic nuclei, as well as its rapid nuclear export, in the context of ancient intranuclear tubulin function during the cell cycle progression.

Cotransport of the heterodimeric small subunit of the Saccharomyces cerevisiae ribonucleotide reductase between the nucleus and the cytoplasm

Ribonucleotide reductase (RNR) catalyzes the rate-limiting step in de novo deoxyribonucleotide biosynthesis and is essential in DNA replication and repair. Cells have evolved complex mechanisms to modulate RNR activity during normal cell cycle progression and in response to genotoxic stress. A recently characterized mode of RNR regulation is DNA damage-induced RNR subunit redistribution. The RNR holoenzyme consists of a large subunit, R1, and a small subunit, R2. The Saccharomyces cerevisiae R2 is an Rnr2:Rnr4 heterodimer. Rnr2 generates a diferric-tyrosyl radical cofactor required for catalysis; Rnr4 facilitates cofactor assembly and stabilizes the resulting holo-heterodimer. Upon DNA damage, Rnr2 and Rnr4 undergo checkpoint-dependent, nucleus-to-cytoplasm redistribution, resulting in colocalization of R1 and R2. Here we present evidence that Rnr2 and Rnr4 are transported between the nucleus and the cytoplasm as one protein complex. Tagging either Rnr2 or Rnr4 with a nuclear export sequence causes cytoplasmic localization of both proteins. Moreover, mutations at the Rnr2:Rnr4 heterodimer interface can affect the localization of both proteins without disrupting the heterodimeric complex. Finally, the relocalization of Rnr4 appears to involve both active export and blockage of nuclear import. Our findings provide new insights into the mechanism of DNA damage-induced RNR subunit redistribution.

The Drosophila gamma-tubulin small complex subunit Dgrip84 is required for structural and functional integrity of the spindle apparatus

Gamma-tubulin, a protein critical for microtubule assembly, functions within multiprotein complexes. However, little is known about the respective role of gamma-tubulin partners in metazoans. For the first time in a multicellular organism, we have investigated the function of Dgrip84, the Drosophila orthologue of the Saccharomyces cerevisiae gamma-tubulin-associated protein Spc97p. Mutant analysis shows that Dgrip84 is essential for viability. Its depletion promotes a moderate increase in the mitotic index, correlated with the appearance of monopolar or unpolarized spindles, impairment of centrosome maturation, and increase of polyploid nuclei. This in vivo study is strengthened by an RNA interference approach in cultured S2 cells. Electron microscopy analysis suggests that monopolar spindles might result from a failure of centrosome separation and an unusual microtubule assembly pathway via centriolar triplets. Moreover, we point to an involvement of Dgrip84 in the spindle checkpoint regulation and in the maintenance of interphase microtubule dynamics. Dgrip84 also seems essential for male meiosis, ensuring spindle bipolarity and correct completion of cytokinesis. These data sustain that Dgrip84 is required in some aspects of microtubule dynamics and organization both in interphase and mitosis. The nature of a minimal gamma-tubulin complex necessary for proper microtubule organization in the metazoans is discussed.

CDK phosphorylation of a novel NLS-NES module distributed between two subunits of the Mcm2-7 complex prevents chromosomal rereplication

Cyclin-dependent kinases (CDKs) use multiple mechanisms to block reassembly of prereplicative complexes (pre-RCs) at replication origins to prevent inappropriate rereplication. In Saccharomyces cerevisiae, one of these mechanisms promotes the net nuclear export of a pre-RC component, the Mcm2-7 complex, during S, G2, and M phases. Here we identify two partial nuclear localization signals (NLSs) on Mcm2 and Mcm3 that are each necessary, but not sufficient, for nuclear localization of the Mcm2-7 complex. When brought together in cis, however, the two partial signals constitute a potent NLS, sufficient for robust nuclear localization when fused to an otherwise cytoplasmic protein. We also identify a Crm1-dependent nuclear export signal (NES) adjacent to the Mcm3 NLS. Remarkably, the Mcm2-Mcm3 NLS and the Mcm3 NES are sufficient to form a transport module that recapitulates the cell cycle-regulated localization of the entire Mcm2-7 complex. Moreover, we show that CDK regulation promotes net export by phosphorylation of the Mcm3 portion of this module and that nuclear export of the Mcm2-7 complex is sufficient to disrupt replication initiation. We speculate that the distribution of partial transport signals among distinct subunits of a complex may enhance the specificity of protein localization and raises the possibility that previously undetected distributed transport signals are used by other multiprotein complexes.

Mannosyl-diinositolphospho-ceramide, the major yeast plasma membrane sphingolipid, governs toxicity of Kluyveromyces lactis zymocin

Kluyveromyces lactis zymocin, a trimeric (alphabetagamma) protein toxin complex, inhibits proliferation of Saccharomyces cerevisiae cells. Here we present an analysis of kti6 mutants, which resist exogenous zymocin but are sensitive to intracellular expression of its inhibitory gamma-toxin subunit, suggesting that KTI6 encodes a factor needed for toxin entry into the cell. Consistent with altered cell surface properties, kti6 cells resist hygromycin B, syringomycin E, and nystatin, antibiotics that require intact membrane potentials or provoke membrane disruption. KTI6 is allelic to IPT1, coding for mannosyl-diinositolphospho-ceramide [M(IP)(2)C] synthase, which produces M(IP)(2)C, the major plasma membrane sphingolipid. kti6 membranes lack M(IP)(2)C and sphingolipid mutants that have reduced levels of M(IP)(2)C precursors, including the sphingolipid building block ceramide survive zymocin. In addition, kti6/ipt1 cells allow zymocin docking but prevent import of its toxic gamma-subunit. Genetic analysis indicates that Kti6 is likely to act upstream of lipid raft proton pump Kti10/Pma1, a previously identified zymocin sensitivity factor. In sum, M(IP)(2)C operates in a plasma membrane step that follows recognition of cell wall chitin by zymocin but precedes the involvement of elongator, the potential toxin target.

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.

After chitin docking, toxicity of Kluyveromyces lactis zymocin requires Saccharomyces cerevisiae plasma membrane H+-ATPase

Zymocin, a three-subunit (alpha beta gamma) toxin complex from Kluyveromyces lactis, imposes a cell cycle block on Saccharomyces cerevisiae. Phenotypic analysis of the resistant kti10 mutant implies a membrane defect, suggesting that KTI10 represents a gene involved early in the zymocin response. Consistently, KTI10 is shown here to be allelic to PMA1 encoding H(+)-ATPase, a plasma membrane H(+) pump vital for membrane energization (Delta Psi). Like pma1 mutants, kti10 cells lose viability at low pH, indicating a pH homeostasis defect, and resist the antibiotic hygromycin B, uptake of which is known to be Pma1 and Delta Psi sensitive. Similar to kti10 cells, pma1 mutants with reported H(+) pump defects survive in the presence of exozymocin but do not resist endogenous expression of its lethal gamma-toxin subunit. Based on DNA sequence data, kti10 cells are predicted to produce a malfunctional Pma1 variant with expression levels that are normal. Intriguingly, zymocin protection of kti10 cells is suppressed by excess H(+), a scenario ineffective in bypassing resistance of chitin or toxin target mutants. Together with unaltered zymocin docking and gamma-toxin import events in kti10 cells, our data suggest that Pma1's role in zymocin action is likely to involve activation of gamma-toxin in a step following its cellular uptake.

The yeast elongator histone acetylase requires Sit4-dependent dephosphorylation for toxin-target capacity

Kluyveromyces lactis zymocin, a heterotrimeric toxin complex, imposes a G1 cell cycle block on Saccharomyces cerevisiae that requires the toxin-target (TOT) function of holo-Elongator, a six-subunit histone acetylase. Here, we demonstrate that Elongator is a phospho-complex. Phosphorylation of its largest subunit Tot1 (Elp1) is supported by Kti11, an Elongator-interactor essential for zymocin action. Tot1 dephosphorylation depends on the Sit4 phosphatase and its associators Sap185 and Sap190. Zymocin-resistant cells lacking or overproducing Elongator-associator Tot4 (Kti12), respectively, abolish or intensify Tot1 phosphorylation. Excess Sit4.Sap190 antagonizes the latter scenario to reinstate zymocin sensitivity in multicopy TOT4 cells, suggesting physical competition between Sit4 and Tot4. Consistently, Sit4 and Tot4 mutually oppose Tot1 de-/phosphorylation, which is dispensable for integrity of holo-Elongator but crucial for the TOT-dependent G1 block by zymocin. Moreover, Sit4, Tot4, and Tot1 cofractionate, Sit4 is nucleocytoplasmically localized, and sit4Delta-nuclei retain Tot4. Together with the findings that sit4Delta and totDelta cells phenocopy protection against zymocin and the ceramide-induced G1 block, Sit4 is functionally linked to Elongator in cell cycle events targetable by antizymotics.

MAPping the Eukaryotic Tree of Life: Structure, Function, and Evolution of the MAP215⧸Dis1 Family of Microtubule-Associated Proteins

The MAP215/Dis1 family of proteins is an evolutionarily ancient family of microtubule-associated proteins, with characterized members in all major kingdoms of eukaryotes, including fungi (Stu2 in S. cerevisiae, Dis1 and Alp14 in S. pombe), Dictyostelium (DdCP224), plants (Mor1 in A. thaliana and TMBP200 in N. tabaccum), and animals (Zyg9 in C. elegans, Msps in Drosophila, XMAP215 in Xenopus, and ch-TOG in humans). All MAP215/Dis1 proteins (with the exception of those in plants) localize to microtubule-organizing centers (MTOCs), including spindle pole bodies in yeast and centrosomes in animals, and all bind to microtubules in vitro and?or in vivo. Diverse roles in regulating microtubule assembly and organization have been proposed for individual family members, and a substantial body of evidence suggests that MAP215/Dis1-related proteins play critical roles in the assembly and function of the meiotic/mitotic spindles and/or cell division. An extensive search of public databases (including both EST and genome databases) identified partial sequences predicted to encode more than three dozen new members of the MAP215/Dis1 family, including putative MAP215/Dis1-related proteins in Giardia lamblia and four other protists, sixteen additional species of fungi, six plants, and twelve animals. The structure and function of MAP215/Dis1 proteins are discussed in relation to the evolution of this ancient family of microtubule-associated proteins.

Elongator's toxin-target (TOT) function is nuclear localization sequence dependent and suppressed by post-translational modification

The toxin target (TOT) function of the Saccharomyces cerevisiae Elongator complex enables Kluyveromyces lactis zymocin to induce a G1 cell cycle arrest. Loss of a ubiquitin-related system (URM1-UBA4 ) and KTI11 enhances post-translational modification/proteolysis of Elongator subunit Tot1p (Elp1p) and abrogates its TOT function. Using TAP tagging, Kti11p contacts Elongator and translational proteins (Rps7Ap, Rps19Ap Eft2p, Yil103wp, Dph2p). Loss of YIL103w and DPH2 (involved in diphtheria toxicity) suppresses zymocicity implying that both toxins overlap in a manner mediated by Kti11p. Among the pool that co-fractionates with RNA polymerase II (pol II) and nucleolin, Nop1p, unmodified Tot1p dominates. Thus, modification/proteolysis may affect association of Elongator with pol II or its localization. Consistently, an Elongator-nuclear localization sequence (NLS) targets green fluorescent protein (GFP) to the nucleus, and its truncation yields TOT deficiency. Similarly, KAP120 deletion rescues cells from zymocin, suggesting that Elongator's TOT function requires NLS- and karyopherin-dependent nuclear import.

Spindle formation in Aspergillus is coupled to tubulin movement into the nucleus

In many important organisms, including many algae and most fungi, the nuclear envelope does not disassemble during mitosis. This fact raises the possibility that mitotic onset and/or exit might be regulated, in part, by movement of important mitotic proteins into and out of the nucleoplasm. We have used two methods to determine whether tubulin levels in the nucleoplasm are regulated in the fungus Aspergillus nidulans. First, we have used benomyl to disassemble microtubules and create a pool of free tubulin that can be readily observed by immunofluorescence. We find that tubulin is substantially excluded from interphase nuclei, but is present in mitotic nuclei. Second, we have observed a green fluorescent protein/alpha-tubulin fusion in living cells by time-lapse spinning-disk confocal microscopy. We find that tubulin is excluded from interphase nuclei, enters the nucleus seconds before the mitotic spindle begins to form, and is removed from the nucleoplasm during the M-to-G1 transition. Our data indicate that regulation of intranuclear tubulin levels plays an important, perhaps essential, role in the control of mitotic spindle formation in A. nidulans. They suggest that regulation of protein movement into the nucleoplasm may be important for regulating mitotic onset in organisms with intranuclear mitosis.

Centrosomal proteins CG-NAP and kendrin provide microtubule nucleation sites by anchoring gamma-tubulin ring complex

Microtubule assembly is initiated by the gamma-tubulin ring complex (gamma-TuRC). In yeast, the microtubule is nucleated from gamma-TuRC anchored to the amino-terminus of the spindle pole body component Spc110p, which interacts with calmodulin (Cmd1p) at the carboxy-terminus. However, mammalian protein that anchors gamma-TuRC remains to be elucidated. A giant coiled-coil protein, CG-NAP (centrosome and Golgi localized PKN-associated protein), was localized to the centrosome via the carboxyl-terminal region. This region was found to interact with calmodulin by yeast two-hybrid screening, and it shares high homology with the carboxyl-terminal region of another centrosomal coiled-coil protein, kendrin. The amino-terminal region of either CG-NAP or kendrin indirectly associated with gamma-tubulin through binding with gamma-tubulin complex protein 2 (GCP2) and/or GCP3. Furthermore, endogenous CG-NAP and kendrin were coimmunoprecipitated with each other and with endogenous GCP2 and gamma-tubulin, suggesting that CG-NAP and kendrin form complexes and interact with gamma-TuRC in vivo. These proteins were localized to the center of microtubule asters nucleated from isolated centrosomes. Pretreatment of the centrosomes by antibody to CG-NAP or kendrin moderately inhibited the microtubule nucleation; moreover, the combination of these antibodies resulted in stronger inhibition. These results imply that CG-NAP and kendrin provide sites for microtubule nucleation in the mammalian centrosome by anchoring gamma-TuRC.

The yeast protein kinase Mps1p is required for assembly of the integral spindle pole body component Spc42p

Saccharomyces cerevisiae MPS1 encodes an essential protein kinase that has roles in spindle pole body (SPB) duplication and the spindle checkpoint. Previously characterized MPS1 mutants fail in both functions, leading to aberrant DNA segregation with lethal consequences. Here, we report the identification of a unique conditional allele, mps1-8, that is defective in SPB duplication but not the spindle checkpoint. The mutations in mps1-8 are in the noncatalytic region of MPS1, and analysis of the mutant protein indicates that Mps1-8p has wild-type kinase activity in vitro. A screen for dosage suppressors of the mps1-8 conditional growth phenotype identified the gene encoding the integral SPB component SPC42. Additional analysis revealed that mps1-8 exhibits synthetic growth defects when combined with certain mutant alleles of SPC42. An epitope-tagged version of Mps1p (Mps1p-myc) localizes to SPBs and kinetochores by immunofluorescence microscopy and immuno-EM analysis. This is consistent with the physical interaction we detect between Mps1p and Spc42p by coimmunoprecipitation. Spc42p is a substrate for Mps1p phosphorylation in vitro, and Spc42p phosphorylation is dependent on Mps1p in vivo. Finally, Spc42p assembly is abnormal in a mps1-1 mutant strain. We conclude that Mps1p regulates assembly of the integral SPB component Spc42p during SPB duplication.

Roles of cytoskeletal and junctional plaque proteins in nuclear signaling

Cytoplasmic junctional plaque proteins play an important role at intercellular junctions. They link transmembrane cell adhesion molecules to components of the cytoskeleton, thereby playing an important role in the control of many cellular processes. Recent studies on the subcellular distribution of some plaque proteins have revealed that a number of these proteins are able to localize in the nucleus. This dual location indicates that in addition to promoting adhesive interactions, plaque proteins may also play a direct role in nuclear processes, and in particular in the transfer of signals from the membrane to the nucleus. Therefore, translocation of plaque proteins into the nucleus in response to extracellular signals could represent a novel and direct mechanism by which signals can be transmitted from the plasma membrane to the nucleus. This could allow cells to respond to changing environmental conditions in a rapid and efficient way. In addition, conditional sequestration of karyophilic proteins at the sites of cell-cell and cell-substratum adhesion may represent a general mechanism for the regulation of nucleocytoplasmic transport.

Prospore membrane formation linked to the leading edge protein (LEP) coat assembly

In yeast, the differentiation process at the end of meiosis generates four daughter cells inside the boundaries of the mother cell. A meiosis-specific plaque (MP) at the spindle pole bodies (SPBs) serves as the starting site for the formation of the prospore membranes (PSMs) that are destined to encapsulate the post-meiotic nuclei. Here we report the identification of Ady3p and Ssp1p, which are functional components of the leading edge protein (LEP) coat, that covers the ring-shaped opening of the PSMs. Ssp1p is required for the assembly of the LEP coat, which consists of at least three proteins (Ssp1p, Ady3p and Don1p). The assembly of the LEP coat starts with the formation of cytosolic precursors, which then bind in an Ady3p-dependent manner to the SPBs. Subsequent processes at the SPBs leading to functional LEP coats require Ssp1p and the MP components. During growth of the PSMs, the LEP coat functions in formation of the cup-shaped membrane structure that is indispensable for the regulated cellularization of the cytoplasm around the post-meiotic nuclei.

Regulating microtubule properties by modifying their organizing minus ends

In the November 2001 issue of Developmental Cell, Vogel et al. describe that the budding yeast gamma-tubulin, Tub4p, is phosphorylated at a conserved tyrosine in G1 phase of the cell cycle. The results suggest that gamma-tubulin phosphorylation regulates the number and dynamics of microtubules.

Phosphorylation of gamma-tubulin regulates microtubule organization in budding yeast

gamma-Tubulin is essential for microtubule nucleation in yeast and other organisms; whether this protein is regulated in vivo has not been explored. We show that the budding yeast gamma-tubulin (Tub4p) is phosphorylated in vivo. Hyperphosphorylated Tub4p isoforms are restricted to G1. A conserved tyrosine near the carboxy terminus (Tyr445) is required for phosphorylation in vivo. A point mutation, Tyr445 to Asp, causes cells to arrest prior to anaphase. The frequency of new microtubules appearing in the SPB region and the number of microtubules are increased in tub4-Y445D cells, suggesting this mutation promotes microtubule assembly. These data suggest that modification of gamma-tubulin is important for controlling microtubule number, thereby influencing microtubule organization and function during the yeast cell cycle.

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.