Anatomy of the fungal microtubule organizing center, the spindle pole body

The fission yeast interphase spindle pole body (SPB) is a bipartite structure in which a bulky cytoplasmic domain is separated from a nuclear component by the nuclear envelope. During mitosis, the SPB is incorporated into a fenestra that forms within the envelope during mitotic commitment. Closure of this fenestra during anaphase B/mitotic exit returns the cytoplasmic component to the cytoplasmic face of an intact interphase nuclear envelope. Here we show that Brr6 is transiently recruited to SPBs at both SPB insertion and extrusion. Brr6 is required for both SPB insertion and nuclear envelope integrity during anaphase B/mitotic exit. Genetic interactions with apq12 and defective sterol assimilation suggest that Brr6 may alter envelope composition at SPBs to promote SPB insertion and extrusion. The restriction of the Brr6 domain to eukaryotes that use a polar fenestra in an otherwise closed mitosis suggests a conserved role in fenestration to enable a single microtubule organizing center to nucleate both cytoplasmic and nuclear microtubules on opposing sides of the nuclear envelope.

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Fission yeast MOZART1/Mzt1 is an essential γ-tubulin complex component required for complex recruitment to the microtubule organizing center, but not its assembly

Molecular Biology of the Cell ◽

10.1091/mbc.e13-05-0235 ◽

2013 ◽

Vol 24 (18) ◽

pp. 2894-2906 ◽

Cited By ~ 41

Author(s):

Hirohisa Masuda ◽

Risa Mori ◽

Masashi Yukawa ◽

Takashi Toda

Keyword(s):

Fission Yeast ◽

Spindle Pole Body ◽

Spindle Pole ◽

Temperature Sensitive ◽

Microtubule Organizing Center ◽

Microtubule Organization ◽

Unique Role ◽

Pole Body ◽

Organizing Center ◽

Core Components

γ-Tubulin plays a universal role in microtubule nucleation from microtubule organizing centers (MTOCs) such as the animal centrosome and fungal spindle pole body (SPB). γ-Tubulin functions as a multiprotein complex called the γ-tubulin complex (γ-TuC), consisting of GCP1–6 (GCP1 is γ-tubulin). In fungi and flies, it has been shown that GCP1–3 are core components, as they are indispensable for γ-TuC complex assembly and cell division, whereas the other three GCPs are not. Recently a novel conserved component, MOZART1, was identified in humans and plants, but its precise functions remain to be determined. In this paper, we characterize the fission yeast homologue Mzt1, showing that it is essential for cell viability. Mzt1 is present in approximately equal stoichiometry with Alp4/GCP2 and localizes to all the MTOCs, including the SPB and interphase and equatorial MTOCs. Temperature-sensitive mzt1 mutants display varying degrees of compromised microtubule organization, exhibiting multiple defects during both interphase and mitosis. Mzt1 is required for γ-TuC recruitment, but not sufficient to localize to the SPB, which depends on γ-TuC integrity. Intriguingly, the core γ-TuC assembles in the absence of Mzt1. Mzt1 therefore plays a unique role within the γ-TuC components in attachment of this complex to the major MTOC site.

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Analysis of Tub4p, a yeast gamma-tubulin-like protein: implications for microtubule-organizing center function.

The Journal of Cell Biology ◽

10.1083/jcb.134.2.443 ◽

1996 ◽

Vol 134 (2) ◽

pp. 443-454 ◽

Cited By ~ 114

Author(s):

L G Marschall ◽

R L Jeng ◽

J Mulholland ◽

T Stearns

Keyword(s):

Fluorescent Protein ◽

Spindle Pole Body ◽

Spindle Pole ◽

Dependent Manner ◽

Protein Fusion ◽

Microtubule Organizing Center ◽

Pole Body ◽

Monopolar Spindle ◽

Organizing Center ◽

Gamma Tubulin

gamma-Tubulin is a conserved component of microtubule-organizing centers and is thought to be involved in microtubule nucleation. A recently discovered Saccharomyces cerevisiae gene (TUB4) encodes a tubulin that is related to, but divergent from, gamma-tubulins. TUB4 is essential for cell viability, and epitope-tagged Tub4 protein (Tub4p) is localized to the spindle pole body (Sobel, S.G., and M. Snyder. 1995.J. Cell Biol. 131:1775-1788). We have characterized the expression of TUB4, the association of Tub4p with the spindle pole body, and its role in microtubule organization. Tub4p is a minor protein in the cell, and expression of TUB4 is regulated in a cell cycle-dependent manner. Wild-type Tub4p is localized to the spindle pole body, and a Tub4p-green fluorescent protein fusion is able to associate with a preexisting spindle pole body, suggesting that there is dynamic exchange between cytoplasmic and spindle pole body forms of Tub4p. Perturbation of Tub4p function, either by conditional mutation or by depletion of the protein, results in spindle as well as spindle pole body defects, but does not eliminate the ability of microtubules to regrow from, or remain attached to, the spindle pole body. The spindle pole bodies in tub4 mutant cells duplicate but do not separate, resulting in a monopolar spindle. EM revealed that one spindle pole body of the duplicated pair appears to be defective for the nucleation of microtubules. These results offer insight into the role of gamma-tubulin in microtubule-organizing center function.

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Constitutive dynein activity inshe1mutants reveals differences in microtubule attachment at the yeast spindle pole body

Molecular Biology of the Cell ◽

10.1091/mbc.e12-03-0223 ◽

2012 ◽

Vol 23 (12) ◽

pp. 2319-2326 ◽

Cited By ~ 10

Author(s):

Zane J. Bergman ◽

Xue Xia ◽

I. Alexandra Amaro ◽

Tim C. Huffaker

Keyword(s):

Spindle Pole Body ◽

Spindle Pole ◽

Microtubule Assembly ◽

Microtubule Organizing Center ◽

Microtubule Attachment ◽

Pole Body ◽

Organizing Center ◽

Cytoplasmic Microtubules ◽

G1 Cells

The organization of microtubules is determined in most cells by a microtubule-organizing center, which nucleates microtubule assembly and anchors their minus ends. In Saccharomyces cerevisiae cells lacking She1, cytoplasmic microtubules detach from the spindle pole body at high rates. Increased rates of detachment depend on dynein activity, supporting previous evidence that She1 inhibits dynein. Detachment rates are higher in G1 than in metaphase cells, and we show that this is primarily due to differences in the strengths of microtubule attachment to the spindle pole body during these stages of the cell cycle. The minus ends of detached microtubules are stabilized by the presence of γ-tubulin and Spc72, a protein that tethers the γ-tubulin complex to the spindle pole body. A Spc72–Kar1 fusion protein suppresses detachment in G1 cells, indicating that the interaction between these two proteins is critical to microtubule anchoring. Overexpression of She1 inhibits the loading of dynactin components, but not dynein, onto microtubule plus ends. In addition, She1 binds directly to microtubules in vitro, so it may compete with dynactin for access to microtubules. Overall, these results indicate that inhibition of dynein activity by She1 is important to prevent excessive detachment of cytoplasmic microtubules, particularly in G1 cells.

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Assembly of Yeast Spindle Pole Body and its Components Revealed by Electron Cryo-Tomography

10.1101/442574 ◽

2018 ◽

Author(s):

Sam Li ◽

Jose-Jesus Fernandez

Keyword(s):

Cell Cycle ◽

Spindle Pole Body ◽

Spindle Pole ◽

Microtubule Organizing Center ◽

3D Images ◽

Averaging Methods ◽

Cellular Processes ◽

Pole Body ◽

Organizing Center ◽

Subtomogram Averaging

AbstractSpindle pole body (SPB) is the microtubule organizing center (MTOC) in yeast. It plays essential roles during many cellular processes, ranging from mitosis to karyogamy. Here, we used electron cryo-tomography (cryo-ET) and image processing to study SPB and its component purified from the budding yeast. The 3D images and models of SPB at various cell cycle stages were reconstructed by cryo-ET and were analyzed. The results reveal SPB as a cylindrical shaped structure composed of multiple layers. The central layers are arranged with a degree of crystalline order. By using subtomogram averaging methods, we studied the purified “sheet” from over-expressed Spc42p. Our analysis of the SPBs and its components provides new insights into the assembly of this organelle and its cellular function as an MTOC.

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γ-Tubulin and the fungal microtubule cytoskeleton

Canadian Journal of Botany ◽

10.1139/b95-267 ◽

1995 ◽

Vol 73 (S1) ◽

pp. 352-358 ◽

Cited By ~ 6

Author(s):

Berl R. Oakley

Keyword(s):

Cell Cycle ◽

Mitotic Spindle ◽

Spindle Pole Body ◽

Spindle Pole ◽

Microtubule Organizing Center ◽

Pole Body ◽

Spindle Pole Bodies ◽

Key Words Tubulin ◽

Organizing Center ◽

Chromosomal Condensation

γ-Tubulin is present in phylogenetically diverse eukaryotes. It is a component of microtubule organizing centers such as the spindle pole bodies of fungi. In Aspergillus nidulans and Schizosaccharomyces pombe, it is essential for nuclear division, and, thus, for viability. In A. nidulans, nuclei carrying a γ-tubulin disruption can be maintained in heterokaryons, and the phenotypes caused by the disruption can be determined in uninucleate spores produced by the heterokaryons. Experiments with heterokaryons created in strains with mutations that allow synchronization of the cell cycle reveal that γ-tubulin is not required for the transition from the G1 phase of the cell cycle through S phase to G2, nor for the entry into mitosis as judged by chromosomal condensation. It is, however, required for the formation of the mitotic spindle and for the successful completion of mitosis. Staining with the MPM-2 monoclonal antibody reveals that spindle pole body replication occurs in the absence of functional γ-tubulin. Finally, human γ-tubulin functions in fission yeast, and this indicates that γ-tubulin has similar functions in widely divergent organisms. Key words: tubulin, microtubule, spindle pole body, microtubule organizing center.

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Genetic interactions between CDC31 and KAR1, two genes required for duplication of the microtubule organizing center in Saccharomyces cerevisiae.

Genetics ◽

10.1093/genetics/137.2.407 ◽

1994 ◽

Vol 137 (2) ◽

pp. 407-422 ◽

Cited By ~ 8

Author(s):

E A Vallen ◽

W Ho ◽

M Winey ◽

M D Rose

Keyword(s):

Copy Number ◽

Gene Dosage ◽

Spindle Pole Body ◽

Direct Interaction ◽

Spindle Pole ◽

High Copy Number ◽

Temperature Sensitive ◽

Microtubule Organizing Center ◽

Recessive Mutations ◽

Organizing Center

Abstract KAR1 encodes an essential component of the yeast spindle pole body (SPB) that is required for karyogamy and SPB duplication. A temperature-sensitive mutation, kar1-delta 17, mapped to a region required for SPB duplication and for localization to the SPB. To identify interacting SPB proteins, we isolated 13 dominant mutations and 3 high copy number plasmids that suppressed the temperature sensitivity of kar1-delta 17. Eleven extragenic suppressor mutations mapped to two linkage groups, DSK1 and DSK2. The extragenic suppressors were specific for SPB duplication and did not suppress karyogamy-defective alleles. The major class, DSK1, consisted of mutations in CDC31. CDC31 is required for SPB duplication and encodes a calmodulin-like protein that is most closely related to caltractin/centrin, a protein associated with the Chlamydomonas basal body. The high copy number suppressor plasmids contained the wild-type CDC31 gene. One CDC31 suppressor allele conferred a temperature-sensitive defect in SPB duplication, which was counter-suppressed by recessive mutations in KAR1. In spite of the evidence for a direct interaction, the strongest CDC31 alleles, as well as both DSK2 alleles, suppressed a complete deletion of KAR1. However, the CDC31 alleles also made the cell supersensitive to KAR1 gene dosage, arguing against a simple bypass mechanism of suppression. We propose a model in which Kar1p helps localize Cdc31p to the SPB and that Cdc31p then initiates SPB duplication via interaction with a downstream effector.

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Mobility, Microtubule Nucleation and Structure of Microtubule-organizing Centers in Multinucleated Hyphae ofAshbya gossypii

Molecular Biology of the Cell ◽

10.1091/mbc.e09-01-0063 ◽

2010 ◽

Vol 21 (1) ◽

pp. 18-28 ◽

Cited By ~ 29

Author(s):

Claudia Lang ◽

Sandrine Grava ◽

Tineke van den Hoorn ◽

Rhonda Trimble ◽

Peter Philippsen ◽

...

Keyword(s):

Spindle Pole Body ◽

Hyphal Growth ◽

Spindle Pole ◽

Microtubule Organizing Center ◽

Microtubule Nucleation ◽

Cytoplasmic Microtubule ◽

Organizing Center ◽

Cytoplasmic Side ◽

Independent Mode

We investigated the migration of multiple nuclei in hyphae of the filamentous fungus Ashbya gossypii. Three types of cytoplasmic microtubule (cMT)-dependent nuclear movements were characterized using live cell imaging: short-range oscillations (up to 4.5 μm/min), rotations (up to 180° in 30 s), and long-range nuclear bypassing (up to 9 μm/min). These movements were superimposed on a cMT-independent mode of nuclear migration, cotransport with the cytoplasmic stream. This latter mode is sufficient to support wild-type-like hyphal growth speeds. cMT-dependent nuclear movements were led by a nuclear-associated microtubule-organizing center, the spindle pole body (SPB), which is the sole site of microtubule nucleation in A. gossypii. Analysis of A. gossypii SPBs by electron microscopy revealed an overall laminar structure similar to the budding yeast SPB but with distinct differences at the cytoplasmic side. Up to six perpendicular and tangential cMTs emanated from a more spherical outer plaque. The perpendicular and tangential cMTs most likely correspond to short, often cortex-associated cMTs and to long, hyphal growth-axis–oriented cMTs, respectively, seen by in vivo imaging. Each SPB nucleates its own array of cMTs, and the lack of overlapping cMT arrays between neighboring nuclei explains the autonomous nuclear oscillations and bypassing observed in A. gossypii hyphae.

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Analysis of a Spindle Pole Body Mutant Reveals a Defect in Biorientation and Illuminates Spindle Forces

Molecular Biology of the Cell ◽

10.1091/mbc.e04-08-0703 ◽

2005 ◽

Vol 16 (1) ◽

pp. 141-152 ◽

Cited By ~ 22

Author(s):

Tennessee J. Yoder ◽

Mark A. McElwain ◽

Susan E. Francis ◽

Joy Bagley ◽

Eric G.D. Muller ◽

...

Keyword(s):

Fluorescent Protein ◽

Spindle Pole Body ◽

Resonance Energy Transfer ◽

Resonance Energy ◽

Spindle Pole ◽

Microtubule Organizing Center ◽

Bipolar Spindle ◽

Pole Body ◽

Cytoplasmic Microtubules ◽

Mutant Cells

The spindle pole body (SPB) is the microtubule organizing center in Saccharomyces cerevisiae. An essential task of the SPB is to ensure assembly of the bipolar spindle, which requires a proper balancing of forces on the microtubules and chromosomes. The SPB component Spc110p connects the ends of the spindle microtubules to the core of the SPB. We previously reported the isolation of a mutant allele spc110-226 that causes broken spindles and SPB disintegration 30 min after spindle formation. By live cell imaging of mutant cells with green fluorescent protein (GFP)-Tub1p or Spc97p-GFP, we show that spc110-226 mutant cells have early defects in spindle assembly. Short spindles form but do not advance to the 1.5-μm stage and frequently collapse. Kinetochores are not arranged properly in the mutant cells. In 70% of the cells, no stable biorientation occurs and all kinetochores are associated with only one SPB. Examination of the SPB remnants by electron microscopy tomography and fluorescence microscopy revealed that the Spc110-226p/calmodulin complex is stripped off of the central plaque of the SPB and coalesces to from a nucleating structure in the nucleoplasm. The central plaque components Spc42p and Spc29p remain behind in the nuclear envelope. The delamination is likely due to a perturbed interaction between Spc42p and Spc110-226p as detected by fluorescence resonance energy transfer analysis. We suggest that the force exerted on the SPB by biorientation of the chromosomes pulls the Spc110-226p out of the SPB; removal of force exerted by coherence of the sister chromatids reduced fragmentation fourfold. Removal of the forces exerted by the cytoplasmic microtubules had no effect on fragmentation. Our results provide insights into the relative contributions of the kinetochore and cytoplasmic microtubules to the forces involved in formation of a bipolar spindle.

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Translational control of MPS1 links protein synthesis with the initiation of cell division and spindle pole body duplication in yeast

10.1101/2020.12.29.424704 ◽

2020 ◽

Author(s):

Heidi M. Blank ◽

Annabel Alonso ◽

Mark Winey ◽

Michael Polymenis

Keyword(s):

Cell Cycle ◽

Protein Synthesis ◽

Cell Division ◽

Translational Control ◽

Spindle Pole Body ◽

Spindle Pole ◽

Microtubule Organizing Center ◽

Structured Illumination Microscopy ◽

Couple Growth ◽

Pole Body

ABSTRACTProtein synthesis underpins cell growth and controls when cells commit to a new round of cell division at a point in late G1 of the cell cycle called Start. Passage through Start also coincides with the duplication of the microtubule-organizing centers, the yeast spindle pole bodies, which will form the two poles of the mitotic spindle that segregates the chromosomes in mitosis. The conserved Mps1p kinase governs the duplication of the spindle pole body in Saccharomyces cerevisiae. Here, we show that the MPS1 transcript has a short upstream open reading frame that represses the synthesis of Mps1p. Mutating the MPS1 uORF makes the cells smaller, accelerates the appearance of Mps1p in late G1, and promotes completion of Start. The accelerated Start of MPS1 uORF mutants depends on the G1 cyclin Cln3p. Monitoring the spindle pole body in the cell cycle using structured illumination microscopy revealed that mutating the MPS1 uORF enabled cells to duplicate their spindle pole body earlier, at a smaller cell size. For the first time, these results identify growth inputs in mechanisms that control duplication of the microtubule-organizing center and implicate these processes in general mechanisms that couple growth with division.

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