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

Structure of the native γ-tubulin ring complex capping spindle microtubules

Microtubule (MT) filaments, composed of α/β-tubulin dimers, are fundamental to cellular architecture, function and organismal development. They are nucleated from MT organizing centers by the evolutionarily conserved γ-tubulin ring complex (γTuRC). However, the molecular mechanism of nucleation remains elusive. Here we used cryo-electron tomography to determine the structure of the native γTuRC capping the minus end of a MT in the context of enriched budding yeast spindles. In our structure, γTuRC presents a ring of γ-tubulin subunits to seed nucleation of exclusively 13-protofilament MTs, adopting an active closed conformation to function as a perfect geometric template for MT nucleation. Our cryo-electron tomography reconstruction revealed that a coiled-coil protein staples the first row of α/β-tubulin of the MT to alternating positions along the γ-tubulin ring of γTuRC. This positioning of α/β-tubulin onto γTuRC suggests a role for the coiled-coil protein in augmenting γTuRC-mediated MT nucleation. Based on our results, we describe a molecular model for budding yeast γTuRC activation and MT nucleation.

A stable microtubule bundle formed through an orchestrated multistep process controls quiescence exit

Cells fine-tune microtubule assembly in both space and time to give rise to distinct edifices with specific cellular functions. In proliferating cells, microtubules are highly dynamics, and proliferation cessation often leads to their stabilization. One of the most stable microtubule structures identified to date is the nuclear bundle assembled in quiescent yeast. In this article, we characterize the original multistep process driving the assembly of this structure. This Aurora B-dependent mechanism follows a precise temporality that relies on the sequential actions of kinesin-14, kinesin-5, and involves both microtubule-kinetochore and kinetochore-kinetochore interactions. Upon quiescence exit, the microtubule bundle is disassembled via a cooperative process involving kinesin-8 and its full disassembly is required prior to cells re-entry into proliferation. Overall, our study provides the first description, at the molecular scale, of the entire life cycle of a stable microtubule structure in vivo and sheds light on its physiological function.

A role for β-1,6- and β-1,3-glucans in kinetochore function in Saccharomyces cerevisiae

Chromosome segregation is crucial for the faithful inheritance of DNA to the daughter cells after DNA replication. For this, the kinetochore, a megadalton protein complex, assembles on centromeric chromatin containing the histone H3 variant CENP-A, and provides a physical connection to the microtubules. Here, we report an unanticipated role for enzymes required for β-1,6- and β-1,3-glucan biosynthesis in regulating kinetochore function in Saccharomyces cerevisiae. These carbohydrates are the major constituents of the yeast cell wall. We found that the deletion of KRE6, which encodes a glycosylhydrolase/ transglycosidase required for β-1,6-glucan synthesis, suppressed the centromeric defect of mutations in components of the kinetochore, foremost the NDC80 components Spc24, Spc25, the MIND component Nsl1, and Okp1, a constitutive centromere-associated network protein. Similarly, the absence of Fks1, a β-1,3-glucan synthase, and Kre11/Trs65, a TRAPPII component, suppressed a mutation in SPC25. Genetic analysis indicates that the reduction of intracellular β-1,6- and β-1,3-glucans, rather than the cell wall glucan content, regulates kinetochore function. Furthermore, we found a physical interaction between Kre6 and CENP-A/Cse4 in yeast, suggesting a potential function for Kre6 in glycosylating CENP-A/Cse4 or another kinetochore protein. This work shows a moonlighting function for selected cell wall synthesis proteins in regulating kinetochore assembly, which may provide a mechanism to connect the nutritional status of the cell to cell-cycle progression and chromosome segregation.

Humanization reveals pervasive incompatibility of yeast and human kinetochore components

Kinetochores assemble on centromeres to drive chromosome segregation in eukaryotic cells. Humans and budding yeast share most of the structural subunits of the kinetochore, whereas protein sequences have diverged considerably. The conserved centromeric histone H3 variant, CenH3 (CENP-A in humans and Cse4 in budding yeast), marks the site for kinetochore assembly in most species. A previous effort to complement Cse4 in yeast with human CENP-A was unsuccessful; however, co-complementation with the human core nucleosome was not attempted. Previously, our lab successfully humanized the core nucleosome in yeast; however, this severely affected cellular growth. We hypothesized that yeast Cse4 is incompatible with humanized nucleosomes and that the kinetochore represented a limiting factor for efficient histone humanization. Thus, we argued that including the human CENP-A or a Cse4-CENP-A chimera might improve histone humanization and facilitate kinetochore function in humanized yeast. The opposite was true: CENP-A expression reduced histone humanization efficiency, was toxic to yeast, and disrupted cell cycle progression and kinetochore function in wild-type (WT) cells. Suppressors of CENP-A toxicity included gene deletions of subunits of 3 conserved chromatin remodeling complexes, highlighting their role in CenH3 chromatin positioning. Finally, we attempted to complement the subunits of the NDC80 kinetochore complex, individually and in combination, without success, in contrast to a previous study indicating complementation by the human NDC80/HEC1 gene. Our results suggest that limited protein sequence similarity between yeast and human components in this very complex structure leads to failure of complementation.

Proteomic profiling of centrosomes across multiple mammalian cell and tissue types by an affinity capture method

Centrosomes are the major microtubule-organizing centers in animals and play fundamental roles in many cellular processes. Understanding how their composition varies across diverse cell types and how it is altered in disease are major unresolved questions, yet currently available centrosome isolation protocols are cumbersome and time-consuming, and they lack scalability. Here, we report the development of centrosome affinity capture (CAPture)-mass spectrometry (MS), a powerful one-step purification method to obtain high-resolution centrosome proteomes from mammalian cells. Utilizing a synthetic peptide derived from CCDC61 protein, CAPture specifically isolates intact centrosomes. Importantly, as a bead-based affinity method, it enables rapid sample processing and multiplexing unlike conventional approaches. Our study demonstrates the power of CAPture-MS to elucidate cell-type-dependent heterogeneity in centrosome composition, dissect hierarchical interactions, and identify previously unknown centrosome components. Overall, CAPture-MS represents a transformative tool to unveil temporal, regulatory, cell-type- and tissue-specific changes in centrosome proteomes in health and disease.

Coordination between the Ndc80 complex and dynein is essential for microtubule plus-end capture by kinetochores during early mitosis

Mitotic kinetochores are initially captured by dynamic microtubules via a "search-and-capture" mechanism. The microtubule motor, dynein, is critical for kinetochore capture as it has been shown to transport microtubule-attached chromosomes toward the spindle pole during prometaphase. The microtubule-binding nuclear division cycle 80 (Ndc80) complex that is recruited to kinetochores in prophase is known to play a central role in forming kinetochore-microtubule (kMT) attachments in metaphase. It is not yet clear, however, how Ndc80 contributes to initial kMT capture during prometaphase. Here, by combining CRISPR/Cas9-mediated knockout and RNAi technology with assays specific to study kMT capture, we show that mitotic cells lacking Ndc80 exhibit substantial defects in this function during prometaphase. Rescue experiments show that Ndc80 mutants deficient in microtubule-binding are unable to execute proper kMT capture. While cells inhibited of dynein alone are predominantly able to make initial kMT attachments, cells co-depleted of Ndc80 and dynein show severe defects in kMT capture. Further, we use an in vitro total internal reflection fluorescence microscopy assay to reconstitute microtubule capture events, which suggest that Ndc80 and dynein coordinate with each other for microtubule plus-end capture and that the phosphorylation status of Ndc80 is critical for productive kMT capture. A novel interaction between Ndc80 and dynein that we identify in prometaphase extracts might be critical for efficient plus-end capture. Thus, our studies, for the first time, identify a distinct event in the formation of initial kMT attachments, which is directly mediated by Ndc80 and in coordination with dynein is required for efficient kMT capture and chromosome alignment.

Three interacting regions of the Ndc80 and Dam1 complexes support microtubule tip-coupling under load

Accurate mitosis requires kinetochores to make persistent, load-bearing attachments to dynamic microtubule tips, thereby coupling chromosome movements to tip growth and shortening. This tip-coupling behavior depends on the conserved Ndc80 complex and, in budding yeast, on the Dam1 complex, which bind each other directly via three distinct interacting regions. The functional relevance of these multiple interactions was mysterious. Here we show that interactions between two of these regions support the high rupture strengths that occur when applied force is rapidly increased and also support the stability of tip-coupling when force is held constant over longer durations. The contribution of either of these two regions to tip-coupling is reduced by phosphorylation by Aurora B kinase. The third interaction region makes no apparent contribution to rupture strength, but its phosphorylation by Aurora B kinase specifically decreases the long-term stability of tip-coupling. The specific reduction of long-term stability relative to short-term strength might have important implications for mitotic error correction.

AtNUF2 modulates spindle microtubule organization and chromosome segregation during mitosis

The NDC80 complex is a conserved eukaryotic complex composed of four subunits (NUF2, SPC25, NDC80, and SPC24). In yeast and animal cells, the complex is located at the outer layer of the kinetochore, connecting the inner layer of the kinetochore and spindle microtubules (MTs) during cell division. In higher plants, the relationship of the NDC80 complex with MTs is still unclear. In this study, we characterized the biological function of AtNUF2, a subunit of the Arabidopsis NDC80 complex. We found that AtNUF2 is widely expressed in various organs, especially in different stages of embryonic development. It was verified that AtNUF2 co-localized with α-tubulin on MTs during mitosis by immunohistochemical assays. Mutation of AtNUF2 led to severe mitotic defects, not only in the embryo and endosperm, but also in seedlings, resulting in seed abortion and stagnating seedling growth. Furthermore, the biological function of AtNUF2 was studied using partially complemented nuf2-3/-^{DD45;ABI3pro::AtNUF2} (nuf2-3/-^DA ) seedlings. The chromosome bridge and lagging chromatids occurred in nuf2-3/-^DA root apical meristem cells, along with aberration of spindle MTs, resulting in blocked root growth. Meanwhile, the direct binding of AtNUF2 and AtSPC25 to MTs was determined by an MT co-sedimentation assay in vitro. This study revealed the function of AtNUF2 in mitosis and the underlying mechanisms, modulating spindle MT organization and ensuring chromosome segregation during embryo, endosperm, and root development, laying the foundation for subsequent research of the NDC80 complex.

Microtubule-associated proteins and motors required for ectopic microtubule array formation in Saccharomyces cerevisiae

The mitotic spindle is resilient to perturbation due to the concerted, and sometimes redundant, action of motors and microtubule-associated proteins. Here, we utilize an inducible ectopic microtubule nucleation site in the nucleus of Saccharomyces cerevisiae to study three necessary steps in the formation of a bipolar array: the recruitment of the γ-tubulin complex, nucleation and elongation of microtubules (MTs), and the organization of MTs relative to each other. This novel tool, an Spc110 chimera, reveals previously unreported roles of the microtubule-associated proteins Stu2, Bim1, and Bik1, and the motors Vik1 and Kip3. We report that Stu2 and Bim1 are required for nucleation and that Bik1 and Kip3 promote nucleation at the ectopic site. Stu2, Bim1, and Kip3 join their homologs XMAP215, EB1 and kinesin-8 as promoters of microtubule nucleation, while Bik1 promotes MT nucleation indirectly via its role in SPB positioning. Furthermore, we find that the nucleation activity of Stu2 in vivo correlates with its polymerase activity in vitro. Finally, we provide the first evidence that Vik1, a subunit of Kar3/Vik1 kinesin-14, promotes microtubule minus end focusing at the ectopic site.

Aurora B-dependent Ndc80 degradation regulates kinetochore composition in meiosis

The kinetochore complex is a conserved machinery that connects chromosomes to spindle microtubules. During meiosis, the kinetochore is restructured to accommodate a specialized chromosome segregation pattern. In budding yeast, meiotic kinetochore remodeling is mediated by the temporal changes in the abundance of a single subunit called Ndc80. We previously described the regulatory events that control the timely synthesis of Ndc80. Here, we report that Ndc80 turnover is also tightly regulated in meiosis: Ndc80 degradation is active in meiotic prophase, but not in metaphase I. Ndc80 degradation depends on the ubiquitin ligase APC^Ama1 and is mediated by the proteasome. Importantly, Aurora B-dependent Ndc80 phosphorylation, a mark that has been previously implicated in correcting erroneous microtubule-kinetochore attachments, is essential for Ndc80 degradation in a microtubule-independent manner. The N terminus of Ndc80, including a 27-residue sequence and Aurora B phosphorylation sites, is both necessary and sufficient for kinetochore protein degradation. Finally, defects in Ndc80 turnover predispose meiotic cells to chromosome mis-segregation. Our study elucidates the mechanism by which meiotic cells modulate their kinetochore composition through regulated Ndc80 degradation, and demonstrates that Aurora B-dependent regulation of kinetochores extends beyond altering microtubule attachments.

Use of Mass Spectrometry to Study the Centromere and Kinetochore

A number of paths have led to the present list of centromere proteins, which is essentially complete for constitutive structural proteins, but still may be only partial if we consider the many other proteins that briefly visit the centromere and kinetochore to fine-tune the chromatin and adjust other functions. Elegant genetics led to the description of the budding yeast point centromere in 1980. In the same year was published the serendipitous discovery of antibodies that stained centromeres of human mitotic chromosomes in antisera from CREST patients. Painstaking biochemical analyses led to the identification of the human centromere antigens several years later, with the first yeast proteins being described 6 years after that. Since those early days, the discovery and cloning of centromere and kinetochore proteins has largely been driven by improvements in technology. These began with expression cloning methods, which allowed antibodies to lead to cDNA clones. Next, functional screens for kinetochore proteins were made possible by the isolation of yeast centromeric DNAs. Ultimately, the completion of genome sequences for humans and model organisms permitted the coupling of biochemical fractionation with protein identification by mass spectrometry. Subsequent improvements in mass spectrometry have led to the current state where virtually all structural components of the kinetochore are known and where a high-resolution map of the entire structure will likely emerge within the next several years.

Comparative Biology of Centrosomal Structures in Eukaryotes

The centrosome is not only the largest and most sophisticated protein complex within a eukaryotic cell, in the light of evolution, it is also one of its most ancient organelles. This special issue of "Cells" features representatives of three main, structurally divergent centrosome types, i.e., centriole-containing centrosomes, yeast spindle pole bodies (SPBs), and amoebozoan nucleus-associated bodies (NABs). Here, I discuss their evolution and their key-functions in microtubule organization, mitosis, and cytokinesis. Furthermore, I provide a brief history of centrosome research and highlight recently emerged topics, such as the role of centrioles in ciliogenesis, the relationship of centrosomes and centriolar satellites, the integration of centrosomal structures into the nuclear envelope and the involvement of centrosomal components in non-centrosomal microtubule organization.

Identification and localization of SAS-6 in the microsporidium Nosema bombycis

The centriole in eukaryotes functions as the cell's microtubule-organizing center (MTOC) to nucleate spindle assembly. The evolutionarily conserved protein SAS-6 constitutes the center of the cartwheel assembly that scaffolds centrioles early in their biogenesis. Microsporidia possess the spindle plaque instead of centriole as their MTOC to nucleate spindle assembly. However, little is known about the components of spindle plaques in microsporidia. In our present study, we identified a SAS-6 protein in the microsporidium Nosema bombycis and named it as NSAS-6. The NSAS-6 gene contains a complete ORF of 1104 bp in length that encodes a 367-amino acid polypeptide. NSAS-6 consists of a conserved N-terminal domain and a coiled-coil domain. The high identity of SAS-6 homologous sequences from microsporidia indicates that SAS-6 is a conserved protein in microsporidia. Immunolocalization in sporoplasms, intracellular stages and mature spores showed that NSAS-6 probably localizes to the nucleus of N. bombycis and exists throughout the life cycle of N. bombycis. These results suggest that NSAS-6 is required in cell morphogenesis and division in N. bombycis. The function and structure of NSAS-6 should be the focus for further studies, which is essential to elucidate the role of SAS-6 in spindle plaque assembly.

MUN (MERISTEM UNSTRUCTURED), encoding a SPC24 homolog of NDC80 kinetochore complex, affects development through cell division in Arabidopsis thaliana

Kinetochore, a protein super-complex on the centromere of chromosomes, mediates chromosome segregation during cell division by providing attachment sites for spindle microtubules. The NDC80 complex, composed of four proteins, NDC80, NUF2, SPC24 and SPC25, is localized at the outer kinetochore and connects spindle fibers to the kinetochore. Although it is conserved across species, functional studies of this complex are rare in Arabidopsis. Here, we characterize a recessive mutant, meristem unstructured-1 (mun-1), exhibiting an abnormal phenotype with unstructured shoot apical meristem caused by ectopic expression of the WUSCHEL gene in unexpected tissues. mun-1 is a weak allele because of the insertion of T-DNA in the promoter region of the SPC24 homolog. The mutant exhibits stunted growth, embryo arrest, DNA aneuploidy, and defects in chromosome segregation with a low cell division rate. Null mutants of MUN from TALEN and CRISPR/Cas9-mediated mutagenesis showed zygotic embryonic lethality similar to nuf2-1; however, the null mutations were fully transmissible via pollen and ovules. Interactions among the components of the NDC80 complex were confirmed in a yeast two-hybrid assay and in planta co-immunoprecipitation. MUN is co-localized at the centromere with HTR12/CENH3, which is a centromere-specific histone variant, but MUN is not required to recruit HTR12/CENH3 to the kinetochore. Our results support that MUN is a functional homolog of SPC24 in Arabidopsis, which is required for proper cell division. In addition, we report the ectopic generations of stem cell niches by the malfunction of kinetochore components.

Systems Level Modeling of the Cell Cycle Using Budding Yeast

Proteins involved in the regulation of the cell cycle are highly conserved across all eukaryotes, and so a relatively simple eukaryote such as yeast can provide insight into a variety of cell cycle perturbations including those that occur in human cancer. To date, the budding yeast Saccharomyces cerevisiae has provided the largest amount of experimental and modeling data on the progression of the cell cycle, making it a logical choice for in-depth studies of this process. Moreover, the advent of methods for collection of high-throughput genome, transcriptome, and proteome data has provided a means to collect and precisely quantify simultaneous cell cycle gene transcript and protein levels, permitting modeling of the cell cycle on the systems level. With the appropriate mathematical framework and sufficient and accurate data on cell cycle components, it should be possible to create a model of the cell cycle that not only effectively describes its operation, but can also predict responses to perturbations such as variation in protein levels and responses to external stimuli including targeted inhibition by drugs. In this review, we summarize existing data on the yeast cell cycle, proteomics technologies for quantifying cell cycle proteins, and the mathematical frameworks that can integrate this data into representative and effective models. Systems level modeling of the cell cycle will require the integration of high-quality data with the appropriate mathematical framework, which can currently be attained through the combination of dynamic modeling based on proteomics data and using yeast as a model organism.

Kinetochore inactivation by expression of a repressive mRNA

Differentiation programs such as meiosis depend on extensive gene regulation to mediate cellular morphogenesis. Meiosis requires transient removal of the outer kinetochore, the complex that connects microtubules to chromosomes. How the meiotic gene expression program temporally restricts kinetochore function is unknown. We discovered that in budding yeast, kinetochore inactivation occurs by reducing the abundance of a limiting subunit, Ndc80. Furthermore, we uncovered an integrated mechanism that acts at the transcriptional and translational level to repress NDC80 expression. Central to this mechanism is the developmentally controlled transcription of an alternate NDC80 mRNA isoform, which itself cannot produce protein due to regulatory upstream ORFs in its extended 5' leader. Instead, transcription of this isoform represses the canonical NDC80 mRNA expression in cis, thereby inhibiting Ndc80 protein synthesis. This model of gene regulation raises the intriguing notion that transcription of an mRNA, despite carrying a canonical coding sequence, can directly cause gene repression.

Polo-like kinase Cdc5 regulates Spc72 recruitment to spindle pole body in the methylotrophic yeast Ogataea polymorpha

Cytoplasmic microtubules (cMT) control mitotic spindle positioning in many organisms, and are therefore pivotal for successful cell division. Despite its importance, the temporal control of cMT formation remains poorly understood. Here we show that unlike the best-studied yeast Saccharomyces cerevisiae, position of pre-anaphase nucleus is not strongly biased toward bud neck in Ogataea polymorpha and the regulation of spindle positioning becomes active only shortly before anaphase. This is likely due to the unstable property of cMTs compared to those in S. cerevisiae. Furthermore, we show that cMT nucleation/anchoring is restricted at the level of recruitment of the γ-tubulin complex receptor, Spc72, to spindle pole body (SPB), which is regulated by the polo-like kinase Cdc5. Additionally, electron microscopy revealed that the cytoplasmic side of SPB is structurally different between G1 and anaphase. Thus, polo-like kinase dependent recruitment of γ-tubulin receptor to SPBs determines the timing of spindle orientation in O. polymorpha.

Before a cell divides, it needs to duplicate its genetic material to provide the new daughter cell with a full set of genetic information. To do so, the cell forms a complex of proteins called the spindle apparatus, which is made up of string-like microtubules that divide the chromosomes evenly. In many organisms, the position of the spindle determines where in the cell this separation happens.

However, in baker’s yeast, the location where the cell will divide is determined well before the spindle is formed. Unlike many other eukaryotic cells, these yeast cells divide asymmetrically and create buds that will form the new daughter cells. The position of this bud determines where the spindle should be located and where the chromosomes separate.

The spindle itself is then organised by a structure called the spindle pole body, which connects to microtubules inside the cell nucleus and microtubules in the cell plasma. Several proteins control where and how the spindle forms, including a protein called the spindle pole component 72, or Spc72 for short, and an enzyme called Cdc5. However, until now it was unclear how spindle formation is timed and controlled in other yeast species.

Now, Maekawa et al. have used fluorescent markers and time lapse microscopy to examine how the spindle forms in the yeast species Ogataea polymorpha, an important industrial yeast used to produce medicines and alcohol. The results show that in O. polymorpha, the positioning and orientation of the spindle only occurred very late in the cell cycle and the microtubules in the cell plasma remained unstable until the chromosomes were about to separate. This was linked to changes in the level of Spc72, which increased at the spindle pole body before the chromosomes separated and then dropped again. This was controlled by Cdc5.

Understanding when and where microtubules are formed is an important step in understanding how cells divide. This is the first example of a budding yeast that creates new microtubules in the cell plasma every time the cell divides. Unravelling the molecular differences between yeast species could lead to new ways to optimise the use of industrial yeasts like O. polymorpha, or to combat disease-causing ones.

The unconventional kinetoplastid kinetochore: from discovery toward functional understanding

The kinetochore is the macromolecular protein complex that drives chromosome segregation in eukaryotes. Its most fundamental function is to connect centromeric DNA to dynamic spindle microtubules. Studies in popular model eukaryotes have shown that centromere protein (CENP)-A is critical for DNA-binding, whereas the Ndc80 complex is essential for microtubule-binding. Given their conservation in diverse eukaryotes, it was widely believed that all eukaryotes would utilize these components to make up a core of the kinetochore. However, a recent study identified an unconventional type of kinetochore in evolutionarily distant kinetoplastid species, showing that chromosome segregation can be achieved using a distinct set of proteins. Here, I review the discovery of the two kinetochore systems and discuss how their studies contribute to a better understanding of the eukaryotic chromosome segregation machinery.

A ternary membrane protein complex anchors the spindle pole body in the nuclear envelope in budding yeast

In budding yeast (Saccharomyces cerevisiae) the multilayered spindle pole body (SPB) is embedded in the nuclear envelope (NE) at fusion sites of the inner and outer nuclear membrane. The SPB is built from 18 different proteins, including the three integral membrane proteins Mps3, Ndc1, and Mps2. These membrane proteins play an essential role in the insertion of the new SPB into the NE. How the huge core structure of the SPB is anchored in the NE has not been investigated thoroughly until now. The present model suggests that the NE protein Mps2 interacts via Bbp1 with Spc29, one of the coiled-coil proteins forming the central plaque of the SPB. To test this model, we purified and reconstituted the Mps2-Bbp1 complex from yeast and incorporated the complex into liposomes. We also demonstrated that Mps2-Bbp1 directly interacts with Mps3 and Ndc1. We then purified Spc29 and reconstituted the ternary Mps2-Bbp1-Spc29 complex, proving that Bbp1 can simultaneously interact with Mps2 and Spc29 and in this way link the central plaque of the SPB to the nuclear envelope. Interestingly, Bbp1 induced oligomerization of Spc29, which may represent an early step in SPB duplication. Together, this analysis provides important insights into the interaction network that inserts the new SPB into the NE and indicates that the Mps2-Bbp1 complex is the central unit of the SPB membrane anchor.

Aberrant Meiotic Modulation Partially Contributes to the Lower Germination Rate of Pollen Grains in Maize (Zea mays L.) Under Low Nitrogen Supply

Pollen germination is an essential step towards successful pollination during maize reproduction. How low niutrogen (N) affects pollen germination remains an interesting biological question to be addressed. We found that only low N resulted in a significantly lower germination rate of pollen grains after 4 weeks of low N, phosphorus or potassium treatment in maize production. Importantly, cytological analysis showed 7-fold more micronuclei in male meiocytes under the low N treatment than in the control, indicating that the lower germination rate of pollen grains was partially due to numerous chromosome loss events resulting from preceding meiosis. The appearance of 10 bivalents in the control and low N cells at diakinesis suggested that chromosome pairing and recombination in meiosis I was not affected by low N. Further gene expression analysis revealed dramatic down-regulation of Nuclear Division Cycle 80 (Ndc80) and Regulator of Chromosome Condensation 1 (Rcc1-1) expression and up-regulation of Cell Division Cycle 20 (Cdc20-1) expression, although no significant difference in the expression level of kinetochore foundation proteins Centromeric Histone H3 (Cenh3) and Centromere Protein C (Cenpc) and cohesion regulators Recombination 8 (Rec8) and Shugoshin (Sgo1) was observed. Aberrant modulation of three key meiotic regulators presumably resulted in a high likelihood of erroneous chromosome segregation, as testified by pronounced lagging chromosomes at anaphase I or cell cycle disruption at meiosis II. Thus, we proposed a cytogenetic mechanism whereby low N affects male meiosis and causes a higher chromosome loss frequency and eventually a lower germination rate of pollen grains in a staple crop plant.

A Cell Biological Perspective on Past, Present and Future Investigations of the Spindle Assembly Checkpoint

The spindle assembly checkpoint (SAC) is a quality control mechanism that ensures accurate chromosome segregation during cell division. It consists of a mechanochemical signal transduction mechanism that senses the attachment of chromosomes to the spindle, and a signaling cascade that inhibits cell division if one or more chromosomes are not attached. Extensive investigations of both these component systems of the SAC have synthesized a comprehensive understanding of the underlying molecular mechanisms. This review recounts the milestone results that elucidated the SAC, compiles a simple model of the complex molecular machinery underlying the SAC, and highlights poorly understood facets of the biochemical design and cell biological operation of the SAC that will drive research forward in the near future.

Inappropriate expression of the translation elongation factor 1A disrupts genome stability and metabolism

The translation elongation factor eEF1A is one of the most abundant proteins found within cells, and its role within protein synthesis is well documented. Levels of eEF1A are tightly controlled, with inappropriate expression linked to oncogenesis. However, the mechanisms by which increased eEF1A expression alters cell behaviour are unknown. Our analyses in yeast suggest that elevation of eEF1A levels leads to stabilisation of the spindle pole body and changes in nuclear organisation. Elevation of the eEF1A2 isoform also leads to altered nuclear morphology in cultured human cells, suggesting a conserved role in maintaining genome stability. Gene expression and metabolomic analyses reveal that the level of eEF1A is crucial for the maintenance of metabolism and amino acid levels in yeast, most likely because of its role in the control of vacuole function. Increased eEF1A2 levels trigger lysosome biogenesis in cultured human cells, also suggesting a conserved role within metabolic control mechanisms. Taken together, our data suggest that the control of eEF1A levels is important for the maintenance of a number of cell functions beyond translation and that its de-regulation might contribute to its oncogenic properties.

Summary: The translation elongation factor eEF1A is elevated in some cancers. We use yeast and human cell models to show that eEF1A elevation leads to genome instability and metabolic alterations that might affect its oncogenic properties.

Polyglutamine Tract Expansion Increases S-Nitrosylation of Huntingtin and Ataxin-1

Expansion of the polyglutamine (polyQ) tract in the huntingtin (Htt) protein causes Huntington’s disease (HD), a fatal inherited movement disorder linked to neurodegeneration in the striatum and cortex. S-nitrosylation and S-acylation of cysteine residues regulate many functions of cytosolic proteins. We therefore used a resin-assisted capture approach to identify these modifications in Htt. In contrast to many proteins that have only a single S-nitrosylation or S-acylation site, we identified sites along much of the length of Htt. Moreover, analysis of cells expressing full-length Htt or a large N-terminal fragment of Htt shows that polyQ expansion strongly increases Htt S-nitrosylation. This effect appears to be general since it is also observed in Ataxin-1, which causes spinocerebellar ataxia type 1 (SCA1) when its polyQ tract is expanded. Overexpression of nitric oxide synthase increases the S-nitrosylation of normal Htt and the frequency of conspicuous juxtanuclear inclusions of Htt N-terminal fragments in transfected cells. Taken together with the evidence that S-nitrosylation of Htt is widespread and parallels polyQ expansion, these subcellular changes show that S-nitrosylation affects the biology of this protein in vivo.

Sec66-Dependent Regulation of Yeast Spindle-Pole Body Duplication Through Pom152

In closed mitotic systems such as Saccharomyces cerevisiae, the nuclear envelope (NE) does not break down during mitosis, so microtubule-organizing centers such as the spindle-pole body (SPB) must be inserted into the NE to facilitate bipolar spindle formation and chromosome segregation. The mechanism of SPB insertion has been linked to NE insertion of nuclear pore complexes (NPCs) through a series of genetic and physical interactions between NPCs and SPB components. To identify new genes involved in SPB duplication and NE insertion, we carried out genome-wide screens for suppressors of deletion alleles of SPB components, including Mps3 and Mps2. In addition to the nucleoporins POM152 and POM34, we found that elimination of SEC66/SEC71/KAR7 suppressed lethality of cells lacking MPS2 or MPS3. Sec66 is a nonessential subunit of the Sec63 complex that functions together with the Sec61 complex in import of proteins into the endoplasmic reticulum (ER). Cells lacking Sec66 have reduced levels of Pom152 protein but not Pom34 or Ndc1, a shared component of the NPC and SPB. The fact that Sec66 but not other subunits of the ER translocon bypass deletion mutants in SPB genes suggests a specific role for Sec66 in the control of Pom152 levels. Based on the observation that sec66∆ does not affect the distribution of Ndc1 on the NE or Ndc1 binding to the SPB, we propose that Sec66-mediated regulation of Pom152 plays an NPC-independent role in the control of SPB duplication.

Lessons from yeast: the spindle pole body and the centrosome

The yeast spindle pole body (SPB) is the functional equivalent of the centrosome. Most SPB components have been identified and their functions partly established. This involved a large variety of techniques which are described here, and the potential use of some of these in the centrosome field is highlighted. In particular, very useful structural information on the SPB was obtained from a reconstituted complex, the γ-tubulin complex, and also from a sub-particle, SPB cores, prepared by extraction of an enriched SPB preparation. The labelling of SPB proteins with GFP at the N or C termini, using GFP tags inserted into the genome, gave informative electron microscopy localization and fluorescence resonance energy transfer data. Examples are given of more precise functional data obtained by removing domains from one SPB protein, Spc110p, without affecting its essential function. Finally, a structural model for SPB duplication is described and the differences between SPB and centrosome duplication discussed.

Centrin: another target of monastrol, an inhibitor of mitotic spindle

Monastrol, a cell-permeable inhibitor, considered to specifically inhibit kinesin Eg5, can cause mitotic arrest and monopolar spindle formation, thus exhibiting antitumor properties. Centrin, a ubiquitous protein associated with centrosome, plays a critical role in centrosome duplication. Moreover, a correlation between centrosome amplification and cancer has been reported. In this study, it is proposed for the first time that centrin may be another target of the anticancer drug monastrol since monastrol can effectively inhibit not only the growth of the transformed Escherichia coli cells in vivo, but also the Lu(3+)-dependent self-assembly of EoCen in vitro. The two closely related compounds (Compounds 1 and 2) could not take the same effect. Fluorescence titration experiments suggest that four monastrols per protein is the optimum binding pattern, and the binding constants at different temperatures were obtained. Detailed thermodynamic analysis indicates that hydrophobic force is the main acting force between monastrol and centrin, and the extent of monastrol inhibition of centrin self-assembly is highly dependent upon the hydrophobic region of the protein, which is largely exposed by the binding of metal ions.

Remodeling Cildb, a popular database for cilia and links for ciliopathies

Background

New generation technologies in cell and molecular biology generate large amounts of data hard to exploit for individual proteins. This is particularly true for ciliary and centrosomal research. Cildb is a multi–species knowledgebase gathering high throughput studies, which allows advanced searches to identify proteins involved in centrosome, basal body or cilia biogenesis, composition and function. Combined to localization of genetic diseases on human chromosomes given by OMIM links, candidate ciliopathy proteins can be compiled through Cildb searches.

Methods

Othology between recent versions of the whole proteomes was computed using Inparanoid and ciliary high throughput studies were remapped on these recent versions.

Results

Due to constant evolution of the ciliary and centrosomal field, Cildb has been recently upgraded twice, with new species whole proteomes and new ciliary studies, and the latter version displays a novel BioMart interface, much more intuitive than the previous ones.

Conclusions

This already popular database is designed now for easier use and is up to date in regard to high throughput ciliary studies.

Kinetochore biorientation in Saccharomyces cerevisiae requires a tightly folded conformation of the Ndc80 complex

Accurate transmission of genetic material relies on the coupling of chromosomes to spindle microtubules by kinetochores. These linkages are regulated by the conserved Aurora B/Ipl1 kinase to ensure that sister chromatids are properly attached to spindle microtubules. Kinetochore-microtubule attachments require the essential Ndc80 complex, which contains two globular ends linked by large coiled-coil domains. In this study, we isolated a novel ndc80 mutant in Saccharomyces cerevisiae that contains mutations in the coiled-coil domain. This ndc80 mutant accumulates erroneous kinetochore-microtubule attachments, resulting in misalignment of kinetochores on the mitotic spindle. Genetic analyses with suppressors of the ndc80 mutant and in vitro cross-linking experiments suggest that the kinetochore misalignment in vivo stems from a defect in the ability of the Ndc80 complex to stably fold at a hinge in the coiled coil. Previous studies proposed that the Ndc80 complex can exist in multiple conformations: elongated during metaphase and bent during anaphase. However, the distinct functions of individual conformations in vivo are unknown. Here, our analysis revealed a tightly folded conformation of the Ndc80 complex that is likely required early in mitosis. This conformation is mediated by a direct, intracomplex interaction and involves a greater degree of folding than the bent form of the complex at anaphase. Furthermore, our results suggest that this conformation is functionally important in vivo for efficient error correction by Aurora B/Ipl1 and, consequently, to ensure proper kinetochore alignment early in mitosis.

The kinetochore

A critical requirement for mitosis is the distribution of genetic material to the two daughter cells. The central player in this process is the macromolecular kinetochore structure, which binds to both chromosomal DNA and spindle microtubule polymers to direct chromosome alignment and segregation. This review will discuss the key kinetochore activities required for mitotic chromosome segregation, including the recognition of a specific site on each chromosome, kinetochore assembly and the formation of kinetochore-microtubule connections, the generation of force to drive chromosome segregation, and the regulation of kinetochore function to ensure that chromosome segregation occurs with high fidelity.

"Uno, nessuno e centomila": the different faces of the budding yeast kinetochore

"One, no one and one hundred thousand" is a masterpiece of Italian literature, written by Luigi Pirandello. The central theme is that in each individual there are multiple personalities, since one's perception of one's self differs from the view of others. As a consequence, a unique identity does not exist, but rather one hundred thousand. This concept can be very well applied to the kinetochore, one of the largest macromolecular complexes conserved in eukaryotes. The kinetochore is essential during cell division and fulfills different sophisticated functions, including linking chromosomes to spindle microtubules and delaying anaphase onset in case of incorrect bi-orientation. In order to perform these tasks, the kinetochore shapes its structure by recruiting different subunits, such as the components of the spindle assembly checkpoint (SAC) or the monopolin complex during meiosis. It also modifies its internal organization by rearranging intramolecular connections and acquiring a distinct identity at different time points of cell division. In this review, we describe recent insights into the changes in composition and configuration of the kinetochore in mitosis and meiosis, focusing on the kinetochore of Saccharomyces cerevisiae.

The composition, functions, and regulation of the budding yeast kinetochore

The propagation of all organisms depends on the accurate and orderly segregation of chromosomes in mitosis and meiosis. Budding yeast has long served as an outstanding model organism to identify the components and underlying mechanisms that regulate chromosome segregation. This review focuses on the kinetochore, the macromolecular protein complex that assembles on centromeric chromatin and maintains persistent load-bearing attachments to the dynamic tips of spindle microtubules. The kinetochore also serves as a regulatory hub for the spindle checkpoint, ensuring that cell cycle progression is coupled to the achievement of proper microtubule-kinetochore attachments. Progress in understanding the composition and overall architecture of the kinetochore, as well as its properties in making and regulating microtubule attachments and the spindle checkpoint, is discussed.

Cell cycle: proteomics gives it a spin

The eukaryotic cell division cycle has been studied at the molecular level for over 30 years, most fruitfully in model organisms. In the past 5 years, developments in mass spectrometry-based proteomics have been applied to the study of protein interactions and post-translational modifications involving key cell cycle regulators such as cyclin-dependent kinases and the anaphase-promoting complex, as well as effectors such as centrosomes, the kinetochore and DNA replication forks. In addition, innovations in chemical biology, functional proteomics and bioinformatics have been employed to study the cell cycle at the proteome level. This review surveys the contributions of proteomics to cell cycle research. The near future should see the application of more quantitative proteomic approaches to probe the dynamic aspects of the molecular system that underlie the cell cycle in model organisms and in human cells.

Mzt1/Tam4, a fission yeast MOZART1 homologue, is an essential component of the γ-tubulin complex and directly interacts with GCP3(Alp6)

In humans, MOZART1 plays an essential role in mitotic spindle formation as a component of the γ-tubulin ring complex. We report that the fission yeast homologue of MOZART1, Mzt1/Tam4, is located at microtubule-organizing centers (MTOCs) and coimmunoprecipitates with γ-tubulin Gtb1 from cell extracts. We show that mzt1/tam4 is an essential gene in fission yeast, encoding a 64-amino acid peptide, depletion of which leads to aberrant microtubule structure, including malformed mitotic spindles and impaired interphase microtubule array. Mzt1/Tam4 depletion also causes cytokinesis defects, suggesting a role of the γ-tubulin complex in the regulation of cytokinesis. Yeast two-hybrid analysis shows that Mzt1/Tam4 forms a complex with Alp6, a fission yeast homologue of γ-tubulin complex protein 3 (GCP3). Biophysical methods demonstrate that there is a direct interaction between recombinant Mzt1/Tam4 and the N-terminal region of GCP3(Alp6). Together our results suggest that Mzt1/Tam4 contributes to the MTOC function through regulation of GCP3(Alp6).

Profiling of the mammalian mitotic spindle proteome reveals an ER protein, OSTD-1, as being necessary for cell division and ER morphology

Cell division is important for many cellular processes including cell growth, reproduction, wound healing and stem cell renewal. Failures in cell division can often lead to tumors and birth defects. To identify factors necessary for this process, we implemented a comparative profiling strategy of the published mitotic spindle proteome from our laboratory. Of the candidate mammalian proteins, we determined that 77% had orthologs in Caenorhabditis elegans and 18% were associated with human disease. Of the C. elegans candidates (n=146), we determined that 34 genes functioned in embryonic development and 56% of these were predicted to be membrane trafficking proteins. A secondary, visual screen to detect distinct defects in cell division revealed 21 genes that were necessary for cytokinesis. One of these candidates, OSTD-1, an ER resident protein, was further characterized due to the aberrant cleavage furrow placement and failures in division. We determined that OSTD-1 plays a role in maintaining the dynamic morphology of the ER during the cell cycle. In addition, 65% of all ostd-1 RNAi-treated embryos failed to correctly position cleavage furrows, suggesting that proper ER morphology plays a necessary function during animal cell division.

CEP proteins: the knights of centrosome dynasty

Centrosome forms the backbone of cell cycle progression mechanism. Recent debates have occurred regarding the essentiality of centrosome in cell cycle regulation. CEP family protein is the active component of centrosome and plays a vital role in centriole biogenesis and cell cycle progression control. A total of 31 proteins have been categorized into CEP family protein category and many more are under candidate evaluation. Furthermore, by the recent advancements in genomics and proteomics researches, several new CEP proteins have also been characterized. Here we have summarized the importance of CEP family proteins and their regulation mechanism involved in proper cell cycle progression. Further, we have reviewed the detailed molecular mechanism behind the associated pathological phenotypes and the possible therapeutic approaches. Proteins such as CEP57, CEP63, CEP152, CEP164, and CEP215 have been extensively studied with a detailed description of their molecular mechanisms, which are among the primary targets for drug discovery. Moreover, CEP27, CEP55, CEP70, CEP110, CEP120, CEP135, CEP192, CEP250, CEP290, and CEP350 also seem promising for future drug discovery approaches. Since the overview implicates that the overall researches on CEP proteins are not yet able to present significant details required for effective therapeutics development, thus, it is timely to discuss the importance of future investigations in this field.

Mcm10 self-association is mediated by an N-terminal coiled-coil domain

Minichromosome maintenance protein 10 (Mcm10) is an essential eukaryotic DNA-binding replication factor thought to serve as a scaffold to coordinate enzymatic activities within the replisome. Mcm10 appears to function as an oligomer rather than in its monomeric form (or rather than as a monomer). However, various orthologs have been found to contain 1, 2, 3, 4, or 6 subunits and thus, this issue has remained controversial. Here, we show that self-association of Xenopus laevis Mcm10 is mediated by a conserved coiled-coil (CC) motif within the N-terminal domain (NTD). Crystallographic analysis of the CC at 2.4 Å resolution revealed a three-helix bundle, consistent with the formation of both dimeric and trimeric Mcm10 CCs in solution. Mutation of the side chains at the subunit interface disrupted in vitro dimerization of both the CC and the NTD as monitored by analytical ultracentrifugation. In addition, the same mutations also impeded self-interaction of the full-length protein in vivo, as measured by yeast-two hybrid assays. We conclude that Mcm10 likely forms dimers or trimers to promote its diverse functions during DNA replication.

Coupling unbiased mutagenesis to high-throughput DNA sequencing uncovers functional domains in the Ndc80 kinetochore protein of Saccharomyces cerevisiae

During mitosis, kinetochores physically link chromosomes to the dynamic ends of spindle microtubules. This linkage depends on the Ndc80 complex, a conserved and essential microtubule-binding component of the kinetochore. As a member of the complex, the Ndc80 protein forms microtubule attachments through a calponin homology domain. Ndc80 is also required for recruiting other components to the kinetochore and responding to mitotic regulatory signals. While the calponin homology domain has been the focus of biochemical and structural characterization, the function of the remainder of Ndc80 is poorly understood. Here, we utilized a new approach that couples high-throughput sequencing to a saturating linker-scanning mutagenesis screen in Saccharomyces cerevisiae. We identified domains in previously uncharacterized regions of Ndc80 that are essential for its function in vivo. We show that a helical hairpin adjacent to the calponin homology domain influences microtubule binding by the complex. Furthermore, a mutation in this hairpin abolishes the ability of the Dam1 complex to strengthen microtubule attachments made by the Ndc80 complex. Finally, we defined a C-terminal segment of Ndc80 required for tetramerization of the Ndc80 complex in vivo. This unbiased mutagenesis approach can be generally applied to genes in S. cerevisiae to identify functional properties and domains.

The SYSCILIA gold standard (SCGSv1) of known ciliary components and its applications within a systems biology consortium

The multinational SYSCILIA consortium aims to gain a mechanistic understanding of the cilium. We utilize multiple parallel high-throughput (HTP) initiatives to develop predictive models of relationships between complex genotypes and variable phenotypes of ciliopathies. The models generated are only as good as the wet laboratory data fed into them. It is therefore essential to orchestrate a well-annotated and high-confidence dataset to be able to assess the quality of any HTP dataset. Here, we present the inaugural SYSCILIA gold standard of known ciliary components as a public resource.

The Mub1/Ubr2 ubiquitin ligase complex regulates the conserved Dsn1 kinetochore protein

The kinetochore is the macromolecular complex that assembles onto centromeric DNA and orchestrates the segregation of duplicated chromosomes. More than 60 components make up the budding yeast kinetochore, including inner kinetochore proteins that bind to centromeric chromatin and outer proteins that directly interact with microtubules. However, little is known about how these components assemble into a functional kinetochore and whether there are quality control mechanisms that monitor kinetochore integrity. We previously developed a method to isolate kinetochore particles via purification of the conserved Dsn1 kinetochore protein. We find that the Mub1/Ubr2 ubiquitin ligase complex associates with kinetochore particles through the CENP-C^Mif2 protein. Although Mub1/Ubr2 are not stable kinetochore components in vivo, they regulate the levels of the conserved outer kinetochore protein Dsn1 via ubiquitylation. Strikingly, a deletion of Mub1/Ubr2 restores the levels and viability of a mutant Dsn1 protein, reminiscent of quality control systems that target aberrant proteins for degradation. Consistent with this, Mub1/Ubr2 help to maintain viability when kinetochores are defective. Together, our data identify a previously unknown regulatory mechanism for the conserved Dsn1 kinetochore protein. We propose that Mub1/Ubr2 are part of a quality control system that monitors kinetochore integrity, thus ensuring genomic stability.

The flawless execution of cell division is essential to the survival of all organisms. The loss or gain of a single chromosome, the state called aneuploidy, is a hallmark of cancer cells and is the leading cause of spontaneous miscarriages and hereditary birth defects. Segregation is mediated by the kinetochore, the macromolecular complex that assembles on each chromosome and attaches to spindle microtubules to pull chromosomes to opposite poles when cells divide. It is therefore critical to understand how kinetochores are assembled and maintained. Here, we find that the levels of a conserved kinetochore protein are regulated by proteolysis. We propose that cells have quality control systems that ensure kinetochore integrity and thus genome stability.

Meiosis I chromosome segregation is established through regulation of microtubule-kinetochore interactions

During meiosis, a single round of DNA replication is followed by two consecutive rounds of nuclear divisions called meiosis I and meiosis II. In meiosis I, homologous chromosomes segregate, while sister chromatids remain together. Determining how this unusual chromosome segregation behavior is established is central to understanding germ cell development. Here we show that preventing microtubule-kinetochore interactions during premeiotic S phase and prophase I is essential for establishing the meiosis I chromosome segregation pattern. Premature interactions of kinetochores with microtubules transform meiosis I into a mitosis-like division by disrupting two key meiosis I events: coorientation of sister kinetochores and protection of centromeric cohesin removal from chromosomes. Furthermore we find that restricting outer kinetochore assembly contributes to preventing premature engagement of microtubules with kinetochores. We propose that inhibition of microtubule-kinetochore interactions during premeiotic S phase and prophase I is central to establishing the unique meiosis I chromosome segregation pattern.DOI:http://dx.doi.org/10.7554/eLife.00117.001.

Establishment of the vertebrate kinetochores

The centromere is essential for accurate chromosome segregation during mitosis and meiosis to achieve transmission of genetic information to daughter cells. To facilitate accurate chromosome segregation, the centromere serves several specific functions, including microtubule binding, spindle-checkpoint control, and sister chromatid cohesion. The kinetochore is formed on the centromere to achieve these functions. To understand kinetochore structure and function, it is critical to identify the protein components of the kinetochore and characterize the functional properties of each component. Here, we review recent progress with regard to the molecular architecture of the kinetochore and discuss the future directions for centromere biology.

Hsp110 is required for spindle length control

Systematic affinity purification combined with mass spectrometry analysis of N- and C-tagged cytoplasmic Hsp70/Hsp110 chaperones was used to identify new roles of Hsp70/Hsp110 in the cell. This allowed the mapping of a chaperone-protein network consisting of 1,227 unique interactions between the 9 chaperones and 473 proteins and highlighted roles for Hsp70/Hsp110 in 14 broad biological processes. Using this information, we uncovered an essential role for Hsp110 in spindle assembly and, more specifically, in modulating the activity of the widely conserved kinesin-5 motor Cin8. The role of Hsp110 Sse1 as a nucleotide exchange factor for the Hsp70 chaperones Ssa1/Ssa2 was found to be required for maintaining the proper distribution of kinesin-5 motors within the spindle, which was subsequently required for bipolar spindle assembly in S phase. These data suggest a model whereby the Hsp70-Hsp110 chaperone complex antagonizes Cin8 plus-end motility and prevents premature spindle elongation in S phase.

A pericentrin-related protein homolog in Aspergillus nidulans plays important roles in nucleus positioning and cell polarity by affecting microtubule organization

Pericentrin is a large coiled-coil protein in mammalian centrosomes that serves as a multifunctional scaffold for anchoring numerous proteins. Recent studies have linked numerous human disorders with mutated or elevated levels of pericentrin, suggesting unrecognized contributions of pericentrin-related proteins to the development of these disorders. In this study, we characterized AnPcpA, a putative homolog of pericentrin-related protein in the model filamentous fungus Aspergillus nidulans, and found that it is essential for conidial germination and hyphal development. Compared to the hyphal apex localization pattern of calmodulin (CaM), which has been identified as an interactive partner of the pericentrin homolog, GFP-AnPcpA fluorescence dots are associated mainly with nuclei, while the accumulation of CaM at the hyphal apex depends on the function of AnPcpA. In addition, the depletion of AnPcpA by an inducible alcA promoter repression results in severe growth defects and abnormal nuclear segregation. Most interestingly, in mature hyphal cells, knockdown of pericentrin was able to significantly induce changes in cell shape and cytoskeletal remodeling; it resulted in some enlarged compartments with condensed nuclei and anucleate small compartments as well. Moreover, defects in AnPcpA significantly disrupted the microtubule organization and nucleation, suggesting that AnPcpA may affect nucleus positioning by influencing microtubule organization.

Failure of amino acid homeostasis causes cell death following proteasome inhibition

The ubiquitin-proteasome system targets many cellular proteins for degradation and thereby controls most cellular processes. Although it is well established that proteasome inhibition is lethal, the underlying mechanism is unknown. Here, we show that proteasome inhibition results in a lethal amino acid shortage. In yeast, mammalian cells, and flies, the deleterious consequences of proteasome inhibition are rescued by amino acid supplementation. In all three systems, this rescuing effect occurs without noticeable changes in the levels of proteasome substrates. In mammalian cells, the amino acid scarcity resulting from proteasome inhibition is the signal that causes induction of both the integrated stress response and autophagy, in an unsuccessful attempt to replenish the pool of intracellular amino acids. These results reveal that cells can tolerate protein waste, but not the amino acid scarcity resulting from proteasome inhibition.

Functional analysis of centrosomal kinase substrates in Drosophila melanogaster reveals a new function of the nuclear envelope component otefin in cell cycle progression

Phosphorylation is one of the key mechanisms that regulate centrosome biogenesis, spindle assembly, and cell cycle progression. However, little is known about centrosome-specific phosphorylation sites and their functional relevance. Here, we identified phosphoproteins of intact Drosophila melanogaster centrosomes and found previously unknown phosphorylation sites in known and unexpected centrosomal components. We functionally characterized phosphoproteins and integrated them into regulatory signaling networks with the 3 important mitotic kinases, cdc2, polo, and aur, as well as the kinase CkIIβ. Using a combinatorial RNA interference (RNAi) strategy, we demonstrated novel functions for P granule, nuclear envelope (NE), and nuclear proteins in centrosome duplication, maturation, and separation. Peptide microarrays confirmed phosphorylation of identified residues by centrosome-associated kinases. For a subset of phosphoproteins, we identified previously unknown centrosome and/or spindle localization via expression of tagged fusion proteins in Drosophila SL2 cells. Among those was otefin (Ote), an NE protein that we found to localize to centrosomes. Furthermore, we provide evidence that it is phosphorylated in vitro at threonine 63 (T63) through Aurora-A kinase. We propose that phosphorylation of this site plays a dual role in controlling mitotic exit when phosphorylated while dephosphorylation promotes G(2)/M transition in Drosophila SL2 cells.

The structure of purified kinetochores reveals multiple microtubule-attachment sites

Chromosomes must be accurately partitioned to daughter cells to prevent aneuploidy, a hallmark of many tumors and birth defects. Kinetochores are the macromolecular machines that segregate chromosomes by maintaining load-bearing attachments to the dynamic tips of microtubules. Here, we present the structure of isolated budding yeast kinetochore particles as visualized by electron microscopy (EM) and electron tomography of negatively stained preparations. The kinetochore appears as a ~126 nm particle containing a large central hub surrounded by multiple outer globular domains. In the presence of microtubules, some particles also have a ring that encircles the microtubule. Our data show that kinetochores bind to microtubules via multivalent attachments and lay the foundation to uncover the key mechanical and regulatory mechanisms by which kinetochores control chromosome segregation and cell division.

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.

New insights into subcomplex assembly and modifications of centrosomal proteins

This review provides a brief overview of the recent work on centrosome proteomics, protein complex identification and functional characterization with an emphasis on the literature of the last three years. Proteomics, genetic screens and comparative genomics studies in different model organisms have almost exhaustively identified the molecular components of the centrosome. However, much knowledge is still missing on the protein-protein interactions, protein modifications and molecular changes the centrosome undergoes throughout the cell cycle and development. The dynamic nature of this large multi-protein complex is reflected in the variety of annotated subcellular locations and biological processes of its proposed components. Some centrosomal proteins and complexes have been studied intensively in different organisms and provided detailed insight into centrosome functions. For example, the molecular, structural and functional characterization of the γ-Tubulin ring complex (γ-TuRC) and the the discovery of the Augmin/HAUS complex has advanced our understanding of microtubule (MT) capture, nucleation and organization. Surprising findings revealed new functions and localizations of proteins that were previously regarded as bona fide centriolar or centrosome components, e.g. at the kinetochore or in the nuclear pore complex regulating MT plus end capture or mRNA processing. Many centrosome components undergo posttranslational modifications such as phosphorylation, SUMOylation and ubiquitylation that are critical in modulating centrosome function and biology. A wealth of information has recently become available driven by new developments in technologies such as mass spectrometry, light and electron microscopy providing more detailed molecular and structural definition of the centrosome and particular roles of proteins throughout the cell cycle and development.