Distinct chromosome segregation roles for spindle checkpoint proteins

Aneuploidy of Specific Chromosomes is Beneficial to Cells Lacking Spindle Checkpoint Protein Bub3

Aneuploidy typically poses challenges for cell survival and growth. However, recent studies have identified exceptions where aneuploidy is beneficial for cells with mutations in certain regulatory genes. Our research reveals that cells lacking the spindle checkpoint gene BUB3 exhibit aneuploidy of select chromosomes. While the spindle checkpoint is not essential in budding yeast, the loss of BUB3 and BUB1 increases the probability of chromosome missegregation compared to wildtype cells. Contrary to the prevailing assumption that the aneuploid cells would be outcompeted due to growth defects, our findings demonstrate that bub3 Δ cells consistently maintained aneuploidy of specific chromosomes over many generations. We investigated whether the persistence of these additional chromosomes in bub3 Δ cells resulted from the beneficial elevated expression of certain genes, or mere tolerance. We identified several genes involved in chromosome segregation and cell cycle regulation that confer an advantage to Bub3-depleted cells. Overall, our results suggest that the upregulation of specific genes through aneuploidy may provide a survival and growth advantage to strains with poor chromosome segregation fidelity.

AUTHOR SUMMARY

Accurate chromosome segregation is crucial for the proper development of all living organisms. Errors in chromosome segregation can lead to aneuploidy, characterized by an abnormal number of chromosomes, which generally impairs cell survival and growth. However, under certain stress conditions, such as in various cancers, cells with specific mutations and extra copies of advantageous chromosomes exhibit improved survival and proliferation. In our study, we discovered that cells lacking the spindle checkpoint protein Bub3 became aneuploid, retaining specific chromosomes. This finding was unexpected because although bub3 Δ cells have a higher rate of chromosome mis-segregation, they were not thought to maintain an aneuploid karyotype. We investigated whether the increased copy number of specific genes on these acquired chromosomes offered a benefit to Bub3-deficient cells. Our results revealed that several genes involved in chromosome segregation and cell cycle regulation prevented the gain of chromosomes upon Bub3-depletion, suggesting that these genes confer a survival advantage. Overall, our study demonstrates that cells lacking Bub3 selectively retain specific chromosomes to increase the copy number of genes that promote proper chromosome segregation.

The TOG protein Stu2 is regulated by acetylation

Stu2 in S. cerevisiae is a member of the XMAP215/Dis1/CKAP5/ch-TOG family of MAPs and has multiple functions in controlling microtubules, including microtubule polymerization, microtubule depolymerization, linking chromosomes to the kinetochore, and assembly of γ-TuSCs at the SPB. Whereas phosphorylation has been shown to be critical for Stu2 localization at the kinetochore, other regulatory mechanisms that control Stu2 function are still poorly understood. Here, we show that a novel form of Stu2 regulation occurs through the acetylation of three lysine residues at K252, K469, and K870, which are located in three distinct domains of Stu2. Alteration of acetylation through acetyl-mimetic and acetyl-blocking mutations did not impact the essential function of Stu2. Instead, these mutations lead to a decrease in chromosome stability, as well as changes in resistance to the microtubule depolymerization drug, benomyl. In agreement with our in silico modeling, several acetylation-mimetic mutants displayed increased interactions with γ-tubulin. Taken together, these data suggest that Stu2 acetylation can govern multiple Stu2 functions, including chromosome stability and interactions at the SPB.

Microtubules are proteinaceous polymers that play several important roles in cell division and segregation of the genetic material to each daughter cell. The functions of microtubules are critically dependent upon their dynamic properties in which tubulin subunits are added or removed from the microtubule end, allowing microtubules to grow or shorten in length. These dynamic properties are controlled by several types of microtubule associated proteins. In this study using bakers yeast, we describe our discovery of a previously unappreciated way to regulate the microtubule associated protein Stu2 by a modification called acetylation. When we created mutations in the Stu2 protein that can’t be properly acetylated, the cell lost some of its chromosomes. Some of these mutations actually caused the microtubules to be resistant to drugs that normally disassemble the microtubule polymer. As similar versions of the Stu2 protein are found in diverse organisms that range from yeast and fungus, to plants, insects, mammals and humans, our work could provide unique insights into how microtubules malfunction in some human diseases. With further studies, this may provide a new understanding of chromosome loss in birth defects and/or cancer.

Minnelide synergizes with conventional chemotherapy by targeting both cancer and associated stroma components in pancreatic cancer

Addition of nab-paclitaxel to gemcitabine offers a survival benefit of only 6 weeks over gemcitabine alone at a cost of increased toxicity in PDAC. The goal of the present study is to evaluate the efficacy of Minnelide, a water-soluble prodrug of triptolide, in combination with the standard of care regimen for chemotherapy with the added advantage of reducing the doses of these drugs to minimize toxicity. Pancreatic cancer cell lines were implanted subcutaneously or orthotopically in athymic nude or C57BL/6J mice. Subsequently, animals were randomized and received saline or minnelide or full dose chemotherapy or low dose chemotherapy or minnelide in combination with low dose chemotherapy. Our results show that a combination of low doses of Minnelide with Gemcitabine + nab-paclitaxel significantly inhibited tumor progression and increased the survival of tumor-bearing mice in comparison with conventional chemotherapy alone. Moreover, combination therapy significantly reduced cancer-related morbidity by decreasing ascites and metastasis and effectively targeted both cancer and the associated stroma. In vitro studies with a combination of low doses of triptolide and paclitaxel significantly decreased the cell viability, increased apoptosis and led to significantly increased M-phase cell cycle arrest in various pancreatic cancer cell lines as compared to either drug alone. Our results show that Minnelide synergizes with conventional chemotherapy leading to a significant reduction in the doses of these toxic drugs, all the while achieving better efficacy in the treatment of PDAC. This combination effectively targeted both the cancer and the associated stromal components of pancreatic cancer.

Zombies Never Die: The Double Life Bub1 Lives in Mitosis

When eukaryotic cells enter mitosis, dispersed chromosomes move to the cell center along microtubules to form a metaphase plate which facilitates the accurate chromosome segregation. Meanwhile, kinetochores not stably attached by microtubules activate the spindle assembly checkpoint and generate a wait signal to delay the initiation of anaphase. These events are highly coordinated. Disruption of the coordination will cause severe problems like chromosome gain or loss. Bub1, a conserved serine/threonine kinase, plays important roles in mitosis. After extensive studies in the last three decades, the role of Bub1 on checkpoint has achieved a comprehensive understanding; its role on chromosome alignment also starts to emerge. In this review, we summarize the latest development of Bub1 on supporting the two mitotic events. The essentiality of Bub1 in higher eukaryotic cells is also discussed. At the end, some undissolved questions are raised for future study.

TORC1 inactivation promotes APC/C-dependent mitotic slippage in yeast and human cells

Unsatisfied kinetochore-microtubule attachment activates the spindle assembly checkpoint to inhibit the metaphase-anaphase transition. However, some cells eventually override mitotic arrest by mitotic slippage. Here, we show that inactivation of TORC1 kinase elicits mitotic slippage in budding yeast and human cells. Yeast mitotic slippage was accompanied with aberrant aspects, such as degradation of the nucleolar protein Net1, release of phosphatase Cdc14, and anaphase-promoting complex/cyclosome (APC/C)-Cdh1-dependent degradation of securin and cyclin B in metaphase. This mitotic slippage caused chromosome instability. In human cells, mammalian TORC1 (mTORC1) inactivation also invoked mitotic slippage, indicating that TORC1 inactivation-induced mitotic slippage is conserved from yeast to mammalian cells. However, the invoked mitotic slippage in human cells was not dependent on APC/C-Cdh1. This study revealed an unexpected involvement of TORC1 in mitosis and provides information on undesirable side effects of the use of TORC1 inhibitors as immunosuppressants and anti-tumor drugs.

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Yeast TORC1 inhibition promotes Net1 degradation and Cdc14 release

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Yeast TORC1 inhibition invokes mitotic slippage in an APC/C-Cdh1-dependent manner

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Human mTORC1 inhibition also elicits mitotic slippage

On the Regulation of Mitosis by the Kinetochore, a Macromolecular Complex and Organising Hub of Eukaryotic Organisms

The kinetochore is the multiprotein complex of eukaryotic organisms that is assembled on mitotic or meiotic centromeres to connect centromeric DNA with microtubules. Its function involves the coordinated action of more than 100 different proteins. The kinetochore acts as an organiser hub that establishes physical connections with microtubules and centromere-associated proteins and recruits central protein components of the spindle assembly checkpoint (SAC), an evolutionarily conserved surveillance mechanism of eukaryotic organisms that detects unattached kinetochores and destabilises incorrect kinetochore-microtubule attachments. The molecular communication between the kinetochore and the SAC is highly dynamic and tightly regulated to ensure that cells can progress towards anaphase until each chromosome is properly bi-oriented on the mitotic spindle. This is achieved through an interplay of highly cooperative interactions and concerted phosphorylation/dephosphorylation events that are organised in time and space.This contribution discusses our current understanding of the function, structure and regulation of the kinetochore, in particular, how its communication with the SAC results in the amplification of specific signals to exquisitely control the eukaryotic cell cycle. This contribution also addresses recent advances in machine learning approaches, cell imaging and proteomics techniques that have enhanced our understanding of the molecular mechanisms that ensure the high fidelity and timely segregation of the genetic material every time a cell divides as well as the current challenges in the study of this fascinating molecular machine.

Research progress of Bub3 gene in malignant tumors

The spindle assembly checkpoint (SAC) is a highly conserved monitoring system that ensures a fidelity of chromosome segregation during mitosis. Bub3, a mitotic Checkpoint Protein, is a member of the Bub protein family, and an important factor in the SAC. Abnormal expression of Bub3 results in mitotic defects, defective spindle gate function, chromosomal instability and the development of aneuploidy cells. Aneuploidy is a state of abnormal karyotype that has long been considered as a marker of tumorigenesis. Karyotypic heterogeneity in tumor cells, known as "chromosomal instability" (CIN), can be used to distinguish cancerous cells from their normal tissue counterpart. In this review, we summarize the expression and clinical significance of Bub3 in a variety of tumors and suggest that it has potential in the treatment of cancer. This article is protected by copyright. All rights reserved.

Expression and prognosis analyses of BUB1, BUB1B and BUB3 in human sarcoma

Budding Uninhibited By Benzimidazoles are a group of genes encoding proteins that play central roles in spindle checkpoint during mitosis. Improper mitosis may lead to aneuploidy which is found in many types of tumors. As a key mediator in mitosis, the dysregulated expression of BUBs has been proven to be highly associated with various malignancies, such as leukemia, gastric cancer, breast cancer, and liver cancer. However, bioinformatic analysis has not been applied to explore the role of the BUBs in sarcomas. Herein, we investigate the transcriptional and survival data of BUBs in patients with sarcomas using Oncomine, Gene Expression Profiling Interactive Analysis, Cancer Cell Line Encyclopedia, Kaplan-Meier Plotter, LinkedOmics, and the Database for Annotation, Visualization and Integrated Discovery. We found that the expression levels of BUB1, BUB1B and BUB3 were higher in sarcoma samples and cell lines than in normal controls. Survival analysis revealed that the higher expression levels of BUB1, BUB1B and BUB3 were associated with lower overall and disease-free survival in patients with sarcomas. This study implies that BUB1, BUB1B and BUB3 are potential treatment targets for patients with sarcomas and are new biomarkers for the prognosis of sarcomas.

Differential requirement for Bub1 and Bub3 in regulation of meiotic versus mitotic chromosome segregation

Accurate chromosome segregation depends on the proper attachment of kinetochores to spindle microtubules before anaphase onset. The Ipl1/Aurora B kinase corrects improper attachments by phosphorylating kinetochore components and so releasing aberrant kinetochore–microtubule interactions. The localization of Ipl1 to kinetochores in budding yeast depends upon multiple pathways, including the Bub1–Bub3 pathway. We show here that in meiosis, Bub3 is crucial for correction of attachment errors. Depletion of Bub3 results in reduced levels of kinetochore-localized Ipl1 and concomitant massive chromosome missegregation caused by incorrect chromosome–spindle attachments. Depletion of Bub3 also results in shorter metaphase I and metaphase II due to premature localization of protein phosphatase 1 (PP1) to kinetochores, which antagonizes Ipl1-mediated phosphorylation. We propose a new role for the Bub1–Bub3 pathway in maintaining the balance between kinetochore localization of Ipl1 and PP1, a balance that is essential for accurate meiotic chromosome segregation and timely anaphase onset.

Establishing correct kinetochore-microtubule attachments in mitosis and meiosis

Faithful chromosome segregation in mitosis and meiosis requires that chromosomes properly attach to spindle microtubules. Initial kinetochore-microtubule attachments are often incorrect and rely on error correction mechanisms to release improper attachments, allowing the formation of new attachments. Aurora B kinase and, in mammalian germ cells, Aurora C kinase function as the enzymatic component of the Chromosomal Passenger Complex (CPC), which localizes to the inner centromere/kinetochore and phosphorylates kinetochore proteins for microtubule release during error correction. In this review, we discuss recent findings of the molecular pathways that regulate the chromosomal localization of Aurora B and C kinases in human cell lines, mice, fission yeast, and budding yeast. We also discuss differences in the importance of localization pathways between mitosis and meiosis.

The copy-number and varied strengths of MELT motifs in Spc105 balance the strength and responsiveness of the spindle assembly checkpoint

During mitosis, the Spindle Assembly Checkpoint (SAC) maintains genome stability while also ensuring timely anaphase onset. To maintain genome stability, the SAC must be strong to delay anaphase even if just one chromosome is unattached, but for timely anaphase onset, it must promptly respond to silencing mechanisms. How the SAC meets these potentially antagonistic requirements is unclear. Here we show that the balance between SAC strength and responsiveness is determined by the number of ‘MELT’ motifs in the kinetochore protein Spc105/KNL1 and their Bub3-Bub1 binding affinities. Many strong MELT motifs per Spc105/KNL1 minimize chromosome missegregation, but too many delay anaphase onset. We demonstrate this by constructing a Spc105 variant that trades SAC responsiveness for much more accurate chromosome segregation. We propose that the necessity of balancing SAC strength and responsiveness drives the dual evolutionary trend of the amplification of MELT motif number, but degeneration of their functionally optimal amino acid sequence.

Mitotic kinase anchoring proteins: the navigators of cell division

The coordinated activities of many protein kinases, acting on multiple protein substrates, ensures the error-free progression through mitosis of eukaryotic cells. Enormous research effort has thus been devoted to studying the roles and regulation of these mitotic kinases, and to the identification of their physiological substrates. Central for the timely deployment of specific protein kinases to their appropriate substrates during the cell division cycle are the many anchoring proteins, which serve critical regulatory roles. Through direct association, anchoring proteins are capable of modulating the catalytic activity and/or sub-cellular distribution of the mitotic kinases they associate with. The key roles of some anchoring proteins in cell division are well-established, whilst others are still being unearthed. Here, we review the current knowledge on anchoring proteins for some mitotic kinases, and highlight how targeting anchoring proteins for inhibition, instead of the mitotic kinases themselves, could be advantageous for disrupting the cell division cycle.

BUB-1 promotes amphitelic chromosome biorientation via multiple activities at the kinetochore

Accurate chromosome segregation relies on bioriented amphitelic attachments of chromosomes to microtubules of the mitotic spindle, in which sister chromatids are connected to opposite spindle poles. BUB-1 is a protein of the Spindle Assembly Checkpoint (SAC) that coordinates chromosome attachment with anaphase onset. BUB-1 is also required for accurate sister chromatid segregation independently of its SAC function, but the underlying mechanism remains unclear. Here we show that, in Caenorhabditis elegans embryos, BUB-1 accelerates the establishment of non-merotelic end-on kinetochore-microtubule attachments by recruiting the RZZ complex and its downstream partner dynein-dynactin at the kinetochore. In parallel, BUB-1 limits attachment maturation by the SKA complex. This activity opposes kinetochore-microtubule attachment stabilisation promoted by CLS-2^CLASP-dependent kinetochore-microtubule assembly. BUB-1 is therefore a SAC component that coordinates the function of multiple downstream kinetochore-associated proteins to ensure accurate chromosome segregation.

Polymorphisms in mitotic checkpoint-related genes can influence survival outcomes of early-stage non-small cell lung cancer

This study was conducted to investigate the association between variants in mitotic checkpoint-related genes and clinical outcomes of non-small cell lung cancer (NSCLC). A total of 766 patients with NSCLC who underwent curative surgery were enrolled. Among the 73 variants evaluated, 4 variants were related with survival outcomes. BUB3 rs7897156C>T was associated with worse overall survival under a recessive model (adjusted hazard ratio = 1.58, 95% confidence interval = 1.07–2.33, P = 0.02). AURKB rs1059476G>A was associated with better overall survival under a recessive model (adjusted hazard ratio = 0.64, 95% confidence interval = 0.41–0.99, P = 0.05). PTTG1 rs1895320T>C and RAD21 rs1374297C>G were associated with worse disease-free survival. In the functional study, relative luciferase activity was higher at the BUB3 rs7897156T allele compared to that at the C allele. Western blot showed that the phosphorylation of AKT and mTOR in the AURKB variant-type (M²⁹⁸) was significantly lower than in the AURKB wild-type (T²⁹⁸). We found that 4 variants of mitotic checkpoint-related genes were associated with survival outcomes in patients with surgically resected NSCLC. Particularly, our results suggest that BUB3 rs7897156C>T and AURKB rs1059476G>A are functional variants.

Parallel reorganization of protein function in the spindle checkpoint pathway through evolutionary paths in the fitness landscape that appear neutral in laboratory experiments

Regulatory networks often increase in complexity during evolution through gene duplication and divergence of component proteins. Two models that explain this increase in complexity are: 1) adaptive changes after gene duplication, such as resolution of adaptive conflicts, and 2) non-adaptive processes such as duplication, degeneration and complementation. Both of these models predict complementary changes in the retained duplicates, but they can be distinguished by direct fitness measurements in organisms with short generation times. Previously, it has been observed that repeated duplication of an essential protein in the spindle checkpoint pathway has occurred multiple times over the eukaryotic tree of life, leading to convergent protein domain organization in its duplicates. Here, we replace the paralog pair in S. cerevisiae with a single-copy protein from a species that did not undergo gene duplication. Surprisingly, using quantitative fitness measurements in laboratory conditions stressful for the spindle-checkpoint pathway, we find no evidence that reorganization of protein function after gene duplication is beneficial. We then reconstruct several evolutionary intermediates from the inferred ancestral network to the extant one, and find that, at the resolution of our assay, there exist stepwise mutational paths from the single protein to the divergent pair of extant proteins with no apparent fitness defects. Parallel evolution has been taken as strong evidence for natural selection, but our results suggest that even in these cases, reorganization of protein function after gene duplication may be explained by neutral processes.

Role of Intrinsic and Extrinsic Factors in the Regulation of the Mitotic Checkpoint Kinase Bub1

The spindle assembly checkpoint (SAC) monitors microtubule attachment to kinetochores to ensure accurate sister chromatid segregation during mitosis. The SAC members Bub1 and BubR1 are paralogs that underwent significant functional specializations during evolution. We report an in-depth characterization of the kinase domains of Bub1 and BubR1. BubR1 kinase domain binds nucleotides but is unable to deliver catalytic activity in vitro. Conversely, Bub1 is an active kinase regulated by intra-molecular phosphorylation at the P+1 loop. The crystal structure of the phosphorylated Bub1 kinase domain illustrates a hitherto unknown conformation of the P+1 loop docked into the active site of the Bub1 kinase. Both Bub1 and BubR1 bind Bub3 constitutively. A hydrodynamic characterization of Bub1:Bub3 and BubR1:Bub3 demonstrates both complexes to have 1:1 stoichiometry, with no additional oligomerization. Conversely, Bub1:Bub3 and BubR1:Bub3 combine to form a heterotetramer. Neither BubR1:Bub3 nor Knl1, the kinetochore receptor of Bub1:Bub3, modulate the kinase activity of Bub1 in vitro, suggesting autonomous regulation of the Bub1 kinase domain. We complement our study with an analysis of the Bub1 substrates. Our results contribute to the mechanistic characterization of a crucial cell cycle checkpoint.

Fluorescent foci quantitation for high-throughput analysis

A number of cellular proteins localize to discrete foci within cells, for example DNA repair proteins, microtubule organizing centers, P bodies or kinetochores. It is often possible to measure the fluorescence emission from tagged proteins within these foci as a surrogate for the concentration of that specific protein. We wished to develop tools that would allow quantitation of fluorescence foci intensities in high-throughput studies. As proof of principle we have examined the kinetochore, a large multi-subunit complex that is critical for the accurate segregation of chromosomes during cell division. Kinetochore perturbations lead to aneuploidy, which is a hallmark of cancer cells. Hence, understanding kinetochore homeostasis and regulation are important for a global understanding of cell division and genome integrity. The 16 budding yeast kinetochores colocalize within the nucleus to form a single focus. Here we have created a set of freely-available tools to allow high-throughput quantitation of kinetochore foci fluorescence. We use this 'FociQuant' tool to compare methods of kinetochore quantitation and we show proof of principle that FociQuant can be used to identify changes in kinetochore protein levels in a mutant that affects kinetochore function. This analysis can be applied to any protein that forms discrete foci in cells.

Genetic instability in budding and fission yeast-sources and mechanisms

Cells are constantly confronted with endogenous and exogenous factors that affect their genomes. Eons of evolution have allowed the cellular mechanisms responsible for preserving the genome to adjust for achieving contradictory objectives: to maintain the genome unchanged and to acquire mutations that allow adaptation to environmental changes. One evolutionary mechanism that has been refined for survival is genetic variation. In this review, we describe the mechanisms responsible for two biological processes: genome maintenance and mutation tolerance involved in generations of genetic variations in mitotic cells of both Saccharomyces cerevisiae and Schizosaccharomyces pombe. These processes encompass mechanisms that ensure the fidelity of replication, DNA lesion sensing and DNA damage response pathways, as well as mechanisms that ensure precision in chromosome segregation during cell division. We discuss various factors that may influence genome stability, such as cellular ploidy, the phase of the cell cycle, transcriptional activity of a particular region of DNA, the proficiency of DNA quality control systems, the metabolic stage of the cell and its respiratory potential, and finally potential exposure to endogenous or environmental stress.

The stability of budding and fission yeast genomes is influenced by two contradictory factors: (1) the need to be fully functional, which is ensured through the replication fidelity pathways of nuclear and mitochondrial genomes through sensing and repairing DNA damage, through precise chromosome segregation during cell division; and (2) the need to acquire changes for adaptation to environmental challenges.

Kinetochore-localized BUB-1/BUB-3 complex promotes anaphase onset in C. elegans

The conserved Bub1/Bub3 complex is recruited to the kinetochore region of mitotic chromosomes, where it initiates spindle checkpoint signaling and promotes chromosome alignment. Here we show that, in contrast to the expectation for a checkpoint pathway component, the BUB-1/BUB-3 complex promotes timely anaphase onset in Caenorhabditis elegans embryos. This activity of BUB-1/BUB-3 was independent of spindle checkpoint signaling but required kinetochore localization. BUB-1/BUB-3 inhibition equivalently delayed separase activation and other events occurring during mitotic exit. The anaphase promotion function required BUB-1's kinase domain, but not its kinase activity, and this function was independent of the role of BUB-1/BUB-3 in chromosome alignment. These results reveal an unexpected role for the BUB-1/BUB-3 complex in promoting anaphase onset that is distinct from its well-studied functions in checkpoint signaling and chromosome alignment, and suggest a new mechanism contributing to the coordination of the metaphase-to-anaphase transition.

Bub3 promotes Cdc20-dependent activation of the APC/C in S. cerevisiae

Bub3 and its kinetochore localization are required for the normal timing of anaphase onset and for normal binding of APC/C and Cdc20 in S. cerevisiae.

The spindle checkpoint ensures accurate chromosome segregation by sending a signal from an unattached kinetochore to inhibit anaphase onset. Numerous studies have described the role of Bub3 in checkpoint activation, but less is known about its functions apart from the spindle checkpoint. In this paper, we demonstrate that Bub3 has an unexpected role promoting metaphase progression in budding yeast. Loss of Bub3 resulted in a metaphase delay that was not a consequence of aneuploidy or the activation of a checkpoint. Instead, bub3Δ cells had impaired binding of the anaphase-promoting complex/cyclosome (APC/C) with its activator Cdc20, and the delay could be rescued by Cdc20 overexpression. Kinetochore localization of Bub3 was required for normal mitotic progression, and Bub3 and Cdc20 colocalized at the kinetochore. Although Bub1 binds Bub3 at the kinetochore, bub1Δ cells did not have compromised APC/C and Cdc20 binding. The results demonstrate that Bub3 has a previously unknown function at the kinetochore in activating APC/C-Cdc20 for normal mitotic progression.

Single-Cell Based Quantitative Assay of Chromosome Transmission Fidelity

Errors in mitosis are a primary cause of chromosome instability (CIN), generating aneuploid progeny cells. Whereas a variety of factors can influence CIN, under most conditions mitotic errors are rare events that have been difficult to measure accurately. Here we report a green fluorescent protein−based quantitative chromosome transmission fidelity (qCTF) assay in budding yeast that allows sensitive and quantitative detection of CIN and can be easily adapted to high-throughput analysis. Using the qCTF assay, we performed genome-wide quantitative profiling of genes that affect CIN in a dosage-dependent manner and identified genes that elevate CIN when either increased (icCIN) or decreased in copy number (dcCIN). Unexpectedly, qCTF screening also revealed genes whose change in copy number quantitatively suppress CIN, suggesting that the basal error rate of the wild-type genome is not minimized, but rather, may have evolved toward an optimal level that balances both stability and low-level karyotype variation for evolutionary adaptation.

Dissecting the roles of human BUB1 in the spindle assembly checkpoint

Mitotic chromosome segregation is initiated by the anaphase promoting complex/cyclosome (APC/C) and its co-activator CDC20. APC/CCDC20 is inhibited by the spindle assembly checkpoint (SAC) when chromosomes have not attached to spindle microtubules. Unattached kinetochores catalyze the formation of a diffusible APC/CCDC20 inhibitor that is composed of BUBR1, BUB3, MAD2 and a second molecule of CDC20. Kinetochore recruitment of these proteins as well as SAC activation rely on the mitotic kinase BUB1, but the molecular mechanism by which BUB1 accomplishes this in human cells is unknown. We show that BUBR1 and BUB3 kinetochore recruitment by BUB1 is dispensable for SAC activation. Unlike its yeast and nematode orthologs, human BUB1 does not associate stably with the MAD2 activator MAD1 and, although required for accelerating loading of MAD1 onto kinetochores, is dispensable for its steady-state levels there. Instead, we identify a 50 amino acid segment harboring the recently reported ABBA motif close to a KEN box as critical for BUB1's role in SAC signaling. The presence of this segment correlates with SAC activity and efficient binding of CDC20 but not MAD1 to kinetochores.

The dynamics of signal amplification by macromolecular assemblies for the control of chromosome segregation

The control of chromosome segregation relies on the spindle assembly checkpoint (SAC), a complex regulatory system that ensures the high fidelity of chromosome segregation in higher organisms by delaying the onset of anaphase until each chromosome is properly bi-oriented on the mitotic spindle. Central to this process is the establishment of multiple yet specific protein-protein interactions in a narrow time-space window. Here we discuss the highly dynamic nature of multi-protein complexes that control chromosome segregation in which an intricate network of weak but cooperative interactions modulate signal amplification to ensure a proper SAC response. We also discuss the current structural understanding of the communication between the SAC and the kinetochore; how transient interactions can regulate the assembly and disassembly of the SAC as well as the challenges and opportunities for the definition and the manipulation of the flow of information in SAC signaling.

Mad2, Bub3, and Mps1 regulate chromosome segregation and mitotic synchrony in Giardia intestinalis, a binucleate protist lacking an anaphase-promoting complex

The binucleate pathogen Giardia intestinalis is a highly divergent eukaryote with a semiopen mitosis, lacking an anaphase-promoting complex/cyclosome (APC/C) and many of the mitotic checkpoint complex (MCC) proteins. However, Giardia has some MCC components (Bub3, Mad2, and Mps1) and proteins from the cohesin system (Smc1 and Smc3). Mad2 localizes to the cytoplasm, but Bub3 and Mps1 are either located on chromosomes or in the cytoplasm, depending on the cell cycle stage. Depletion of Bub3, Mad2, or Mps1 resulted in a lowered mitotic index, errors in chromosome segregation (including lagging chromosomes), and abnormalities in spindle morphology. During interphase, MCC knockdown cells have an abnormal number of nuclei, either one nucleus usually on the left-hand side of the cell or two nuclei with one mislocalized. These results suggest that the minimal set of MCC proteins in Giardia play a major role in regulating many aspects of mitosis, including chromosome segregation, coordination of mitosis between the two nuclei, and subsequent nuclear positioning. The critical importance of MCC proteins in an organism that lacks their canonical target, the APC/C, suggests a broader role for these proteins and hints at new pathways to be discovered.

Cdh1 is an antagonist of the spindle assembly checkpoint

The spindle assembly checkpoint (SAC) monitors unsatisfied connections of microtubules to kinetochores and prevents anaphase onset by inhibition of the ubiquitin ligase E3 anaphase-promoting complex or cyclosome (APC/C) in association with the activator Cdc20. Another APC/C activator, Cdh1, exists permanently throughout the cell cycle but it becomes active from telophase to G1. Here, we show that Cdh1 is partially active and mediates securin degradation even in SAC-active metaphase cells. Additionally, Cdh1 mediates Cdc20 degradation in metaphase, promoting formation of the APC/C-Cdh1. These results indicate that Cdh1 opposes the SAC and promotes anaphase transition.

Kaposi's sarcoma-associated herpesvirus-encoded LANA can induce chromosomal instability through targeted degradation of the mitotic checkpoint kinase Bub1

Kaposi's sarcoma-associated herpesvirus (KSHV) has a significant contributory role in the development of three major human neoplastic or lymphoproliferative diseases: Kaposi's sarcoma (KS), primary effusion lymphoma (PEL), and multicentric Castleman's disease (MCD). These diseases are associated with chromosomal instability, a hallmark of human cancer. The latency-associated nuclear antigen (LANA) encoded by KSHV plays a key role in regulating a number of cellular pathways critical for oncogenesis. KSHV LANA alone can induce the development of B-cell hyperplasia and lymphoma in mice expressing LANA. LANA also induces chromosomal instability, thus promoting oncogenesis. However, the precise mechanism underlying LANA-mediated chromosomal instability remains uncharted. Here we report that LANA promoted the induction of chromosomal instability and the formation of micronuclei and multinucleation through its interaction with one of the critical spindle checkpoint proteins, Bub1, and the resulting degradation of Bub1. This interaction occurs through the Knl and kinase domains of Bub1, identified as important for stability and degradation. These results suggest that LANA can dysregulate Bub1 activity, which leads to aberrant chromosome replication and aneuploidy, thus contributing to KSHV-mediated oncogenesis.

IMPORTANCE

This work represents the first set of results identifying a novel mechanism by which LANA, a latency-associated antigen encoded by KSHV, can induce the degradation of Bub1, a spindle checkpoint protein that is important for spindle checkpoint signaling and chromosome segregation. The downregulation of Bub1 mediated by LANA resulted in chromosomal instability, a hallmark of cancer. We further investigated the specific domains of Bub1 that are required for the interaction between LANA and Bub1. The results demonstrated that the Knl and kinase domains of Bub1 are required for the interaction between LANA and Bub1. In addition, we also investigated the mechanism by which LANA promoted Bub1 degradation. Our results showed that LANA interacted physically with the anaphase-promoting complex (APC/C), thus promoting the degradation of Bub1 in a ubiquitin-dependent process.

A Bub1-Mad1 interaction targets the Mad1-Mad2 complex to unattached kinetochores to initiate the spindle checkpoint

A Bub1–Mad1 interaction targets the Mad1–Mad2 complex to unattached kinetochores to initiate the spindle checkpoint.

Recruitment of Mad1–Mad2 complexes to unattached kinetochores is a central event in spindle checkpoint signaling. Despite its importance, the mechanism that recruits Mad1–Mad2 to kinetochores is unclear. In this paper, we show that MAD-1 interacts with BUB-1 in Caenorhabditis elegans. Mutagenesis identified specific residues in a segment of the MAD-1 coiled coil that mediate the BUB-1 interaction. In addition to unattached kinetochores, MAD-1 localized between separating meiotic chromosomes and to the nuclear periphery. Mutations in the MAD-1 coiled coil that selectively disrupt interaction with BUB-1 eliminated MAD-1 localization to unattached kinetochores and between meiotic chromosomes, both of which require BUB-1, and abrogated checkpoint signaling. The identified MAD-1 coiled-coil segment interacted with a C-terminal region of BUB-1 that contains its kinase domain, and mutations in this region prevented MAD-1 kinetochore targeting independently of kinase activity. These results delineate an interaction between BUB-1 and MAD-1 that targets MAD-1–MAD-2 complexes to kinetochores and is essential for spindle checkpoint signaling.

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.

Mad1 contribution to spindle assembly checkpoint signalling goes beyond presenting Mad2 at kinetochores

The spindle assembly checkpoint inhibits anaphase until all chromosomes have become attached to the mitotic spindle. A complex between the checkpoint proteins Mad1 and Mad2 provides a platform for Mad2:Mad2 dimerization at unattached kinetochores, which enables Mad2 to delay anaphase. Here, we show that mutations in Bub1 and within the Mad1 C-terminal domain impair the kinetochore localization of Mad1:Mad2 and abrogate checkpoint activity. Artificial kinetochore recruitment of Mad1 in these mutants co-recruits Mad2; however, the checkpoint remains non-functional. We identify specific mutations within the C-terminal head of Mad1 that impair checkpoint activity without affecting the kinetochore localization of Bub1, Mad1 or Mad2. Hence, Mad1 potentially in conjunction with Bub1 has a crucial role in checkpoint signalling in addition to presenting Mad2.

HP1 knockdown is associated with abnormal condensation of almost all chromatin types in a grasshopper (Eyprepocnemis plorans)

Heterochromatin protein 1 (HP1) is a highly conserved family of eukaryotic proteins required for heterochromatic gene silencing and euchromatic gene transcription regulation. In addition, HP1 is involved in chromatin organization and protection of chromosome integrity during cell division. Here, we present a cytological and molecular analysis of the effects of HP1 knockdown in Eyprepocnemis plorans, a grasshopper species polymorphic for supernumerary heterochromatic chromosomes. Our results revealed contrasting effects of HP1 knockdown on gene activity. While the Bub1 gene decreased in expression level in HP1 knockdown animals, NOR activity, rRNA and, contrarily to previous reports in Drosophila, Hsp70 gene expression remained unchanged. Furthermore, HP1 knockdown resulted in abnormal chromatin condensation, chromosomal bridges, higher frequency of macrospermatids, loss of muscle mass and hemolymph amount as well as a low number of dividing cells and survival reduction. All these phenotypes are very likely due to the chromatin condensation disruption observed for almost all kinds of chromatin.

Mad1 kinetochore recruitment by Mps1-mediated phosphorylation of Bub1 signals the spindle checkpoint

The mitotic spindle checkpoint ensures accurate chromosome segregation and genomic integrity. Understanding the regulation of checkpoint protein Mad1 recruitment to the kinetochore has been an outstanding question in the field. Here, London and Biggins show that Bub1 is a receptor for Mad1, and the Bub1–Mad1 interaction at kinetochores is driven by Mps1-mediated phosphorylation of Bub1. This work reveals a long-sought mechanism that determines kinetochore activation of the spindle checkpoint.

The spindle checkpoint is a conserved signaling pathway that ensures genomic integrity by preventing cell division when chromosomes are not correctly attached to the spindle. Checkpoint activation depends on the hierarchical recruitment of checkpoint proteins to generate a catalytic platform at the kinetochore. Although Mad1 kinetochore localization is the key regulatory downstream event in this cascade, its receptor and mechanism of recruitment have not been conclusively identified. Here, we demonstrate that Mad1 kinetochore association in budding yeast is mediated by phosphorylation of a region within the Bub1 checkpoint protein by the conserved protein kinase Mps1. Tethering this region of Bub1 to kinetochores bypasses the checkpoint requirement for Mps1-mediated kinetochore recruitment of upstream checkpoint proteins. The Mad1 interaction with Bub1 and kinetochores can be reconstituted in the presence of Mps1 and Mad2. Together, this work reveals a critical mechanism that determines kinetochore activation of the spindle checkpoint.

Chromosome segregation in budding yeast: sister chromatid cohesion and related mechanisms

Studies on budding yeast have exposed the highly conserved mechanisms by which duplicated chromosomes are evenly distributed to daughter cells at the metaphase-anaphase transition. The establishment of proteinaceous bridges between sister chromatids, a function provided by a ring-shaped complex known as cohesin, is central to accurate segregation. It is the destruction of this cohesin that triggers the segregation of chromosomes following their proper attachment to microtubules. Since it is irreversible, this process must be tightly controlled and driven to completion. Furthermore, during meiosis, modifications must be put in place to allow the segregation of maternal and paternal chromosomes in the first division for gamete formation. Here, I review the pioneering work from budding yeast that has led to a molecular understanding of the establishment and destruction of cohesion.

Formation of multiprotein assemblies in the nucleus: the spindle assembly checkpoint

Specific interactions within the cell must occur in a crowded environment and often in a narrow time-space framework to ensure cell survival. In the light that up to 10% of individual protein molecules present at one time in mammalian cells mediate signal transduction, the establishment of productive, specific interactions is a remarkable achievement. The spindle assembly checkpoint (SAC) is an evolutionarily conserved and essential self-monitoring system of the eukaryotic cell cycle that ensures the high fidelity of chromosome segregation by delaying the onset of anaphase until all chromosomes are properly bi-oriented on the mitotic spindle. The function of the SAC involves communication with the kinetochore, an essential multiprotein complex crucial for chromosome segregation that assembles on mitotic or meiotic centromeres to link centromeric DNA with microtubules. Interactions in the SAC and kinetochore-microtubule network often involve the reversible assembly of large multiprotein complexes in which regions of the polypeptide chain that exhibit low structure complexity undergo a disorder-to-order transition. The confinement and high density of protein molecules in the cell has a profound effect on the stability, folding rate, and biological functions of individual proteins and protein assemblies. Here, I discuss the role of large and highly flexible surfaces that mediate productive intermolecular interactions in SAC signaling and postulate that macromolecular crowding contributes to the exquisite regulation that is required for the timely and accurate segregation of chromosomes in higher organisms.

Aneuploidy in health, disease, and aging

Aneuploidy, an aberrant number of chromosomes, has been recognized as a feature of human malignancies for over a century, but compelling evidence for causality was largely lacking until mouse models for chromosome number instability were used. These in vivo studies have not only uncovered important new insights into the extremely complex aneuploidy-cancer relationship but also into the molecular mechanisms underlying proper and aberrant chromosome segregation. A series of diverse mouse models for the mitotic checkpoint protein BubR1 has provided evidence for a provocative novel link between aneuploidization and the development of age-related pathologies.

Making an effective switch at the kinetochore by phosphorylation and dephosphorylation

The kinetochore, the proteinaceous structure on the mitotic centromere, functions as a mechanical latch that hooks onto microtubules to support directional movement of chromosomes. The structure also brings in a number of signaling molecules, such as kinases and phosphatases, which regulate microtubule dynamics and cell cycle progression. Erroneous microtubule attachment is destabilized by Aurora B-mediated phosphorylation of multiple microtubule-binding protein complexes at the kinetochore, such as the KMN network proteins and the Ska/Dam1 complex, while Plk-dependent phosphorylation of BubR1 stabilizes kinetochore-microtubule attachment by recruiting PP2A-B56. Spindle assembly checkpoint (SAC) signaling, which is activated by unattached kinetochores and inhibits the metaphase-to-anaphase transition, depends on kinetochore recruitment of the kinase Bub1 through Mps1-mediated phosphorylation of the kinetochore protein KNL1 (also known as Blinkin in mammals, Spc105 in budding yeast, and Spc7 in fission yeast). Recruitment of protein phosphatase 1 to KNL1 is necessary to silence the SAC upon bioriented microtubule attachment. One of the key unsolved questions in the mitosis field is how a mechanical change at the kinetochore upon microtubule attachment is converted to these and other chemical signals that control microtubule attachment and the SAC. Rapid progress in the field is revealing the existence of an intricate signaling network created right on the kinetochore. Here we review the current understanding of phosphorylation-mediated regulation of kinetochore functions and discuss how this signaling network generates an accurate switch that turns on and off the signaling output in response to kinetochore-microtubule attachment.

Microtubule attachment and spindle assembly checkpoint signalling at the kinetochore

In eukaryotes, chromosome segregation during cell division is facilitated by the kinetochore, a multiprotein structure that is assembled on centromeric DNA. The kinetochore attaches chromosomes to spindle microtubules, modulates the stability of these attachments and relays the microtubule-binding status to the spindle assembly checkpoint (SAC), a cell cycle surveillance pathway that delays chromosome segregation in response to unattached kinetochores. Recent studies are shaping current thinking on how each of these kinetochore-centred processes is achieved, and how their integration ensures faithful chromosome segregation, focusing on the essential roles of kinase-phosphatase signalling and the microtubule-binding KMN protein network.

The HLJ1-targeting drug screening identified Chinese herb andrographolide that can suppress tumour growth and invasion in non-small-cell lung cancer

HLJ1 is a novel tumour suppressor and is a potential druggable target for non-small-cell lung cancer (NSCLC). In this report, using a promoter-containing enhancer region as the HLJ1-targeting drug-screening platform, we identified several herbal compounds from a Chinese herbal bank with the capacity to enhance HLJ1 promoter activity and suppress tumour growth and invasion of NSCLC. Among the herbal drugs identified, the andrographolide (from Andrographis paniculata [Burm. f.] Nees.) most significantly induced HLJ1 expression and suppressed tumorigenesis both in vitro and in vivo. The andrographolide upregulates HLJ1 via JunB activation, which modulates AP-2α binding at the MMP-2 promoter and represses the expression of MMP-2. In addition, silencing of HLJ1 partially reverses the inhibition of cancer-cell invasion by andrographolide. Microarray transcriptomic analysis was performed to comprehensively depict the andrographolide-regulated signalling pathways. We showed that andrographolide can affect 939 genes (analysis of variance, false discovery rate < 0.05) that are dominantly involved in the cell cycle, apoptosis and adhesion-related biological signalling, including mitogen-activated protein kinase, focal adhesion and tight junction pathways, indicating the diverse effects of andrographolide on anticancer invasion and proliferation. In conclusion, the HLJ1-targeting drug-screening platform is useful for screening of novel anticancer compounds. Using this platform, we identified andrographolide is a promising new anticancer agent that could suppress tumour growth and invasion in NSCLC.

Evolution and function of the mitotic checkpoint

The mitotic checkpoint evolved to prevent cell division when chromosomes have not established connections with the chromosome segregation machinery. Many of the fundamental molecular principles that underlie the checkpoint, its spatiotemporal activation, and its timely inactivation have been uncovered. Most of these are conserved in eukaryotes, but important differences between species exist. Here we review current concepts of mitotic checkpoint activation and silencing. Guided by studies in model organisms and our phylogenomics analysis of checkpoint constituents and their functional domains and motifs, we highlight ancient and taxa-specific aspects of the core checkpoint modules in the context of mitotic checkpoint function.

The Mad1-Mad2 balancing act--a damaged spindle checkpoint in chromosome instability and cancer

Cancer cells are commonly aneuploid. The spindle checkpoint ensures accurate chromosome segregation by controlling cell cycle progression in response to aberrant microtubule-kinetochore attachment. Damage to the checkpoint, which is a partial loss or gain of checkpoint function, leads to aneuploidy during tumorigenesis. One form of damage is a change in levels of the checkpoint proteins mitotic arrest deficient 1 and 2 (Mad1 and Mad2), or in the Mad1:Mad2 ratio. Changes in Mad1 and Mad2 levels occur in human cancers, where their expression is regulated by the tumor suppressors p53 and retinoblastoma 1 (RB1). By employing a standard assay, namely the addition of a mitotic poison at mitotic entry, it has been shown that checkpoint function is normal in many cancer cells. However, in several experimental systems, it has been observed that this standard assay does not always reveal checkpoint aberrations induced by changes in Mad1 or Mad2, where excess Mad1 relative to Mad2 can lead to premature anaphase entry, and excess Mad2 can lead to a delay in entering anaphase. This Commentary highlights how changes in the levels of Mad1 and Mad2 result in a damaged spindle checkpoint, and explores how these changes cause chromosome instability that can lead to aneuploidy during tumorigenesis.

Up-regulation of the mitotic checkpoint component Mad1 causes chromosomal instability and resistance to microtubule poisons

The mitotic checkpoint is the major cell cycle checkpoint acting during mitosis to prevent aneuploidy and chromosomal instability, which are hallmarks of tumor cells. Reduced expression of the mitotic checkpoint component Mad1 causes aneuploidy and promotes tumors in mice [Iwanaga Y, et al. (2007) Cancer Res 67:160-166]. However, the prevalence and consequences of Mad1 overexpression are currently unclear. Here we show that Mad1 is frequently overexpressed in human cancers and that Mad1 up-regulation is a marker of poor prognosis. Overexpression of Mad1 causes aneuploidy and chromosomal instability through weakening mitotic checkpoint signaling caused by mislocalization of the Mad1 binding partner Mad2. Cells overexpressing Mad1 are resistant to microtubule poisons, including currently used chemotherapeutic agents. These results suggest that levels of Mad1 must be tightly regulated to prevent aneuploidy and transformation and that Mad1 up-regulation may promote tumors and cause resistance to current therapies.

Spindle assembly checkpoint: the third decade

The spindle assembly checkpoint controls cell cycle progression during mitosis, synchronizing it with the attachment of chromosomes to spindle microtubules. After the discovery of the mitotic arrest deficient (MAD) and budding uninhibited by benzymidazole (BUB) genes as crucial checkpoint components in 1991, the second decade of checkpoint studies (2001–2010) witnessed crucial advances in the elucidation of the mechanism through which the checkpoint effector, the mitotic checkpoint complex, targets the anaphase-promoting complex (APC/C) to prevent progression into anaphase. Concomitantly, the discovery that the Ndc80 complex and other components of the microtubule-binding interface of kinetochores are essential for the checkpoint response finally asserted that kinetochores are crucial for the checkpoint response. Nevertheless, the relationship between kinetochores and checkpoint control remains poorly understood. Crucial advances in this area in the third decade of checkpoint studies (2011–2020) are likely to be brought about by the characterization of the mechanism of kinetochore recruitment, activation and inactivation of checkpoint proteins, which remains elusive for the majority of checkpoint components. Here, we take a molecular view on the main challenges hampering this task.

Mad2 and Mad3 cooperate to arrest budding yeast in mitosis

BACKGROUND

The spindle checkpoint ensures accurate chromosome transmission by delaying chromosome segregation until all chromosomes are correctly aligned on the mitotic spindle. The checkpoint is activated by kinetochores that are not attached to microtubules or are attached but not under tension and arrests cells at metaphase by inhibiting the anaphase-promoting complex (APC) and its coactivator Cdc20. Despite numerous studies, we still do not understand how the checkpoint proteins coordinate with each other to inhibit APC(Cdc20) activity.

RESULTS

To ask how the checkpoint components induce metaphase arrest, we constructed fusions of checkpoint proteins and expressed them in the budding yeast Saccharomyces cerevisiae to mimic possible protein interactions during checkpoint activation. We found that expression of a Mad2-Mad3 protein fusion or noncovalently linked Mad2 and Mad3, but not the overexpression of the two separate proteins, induces metaphase arrest that is independent of functional kinetochores or other checkpoint proteins. We further showed that artificially tethering Mad2 to Cdc20 also arrests cells in metaphase independently of other checkpoint components.

CONCLUSION

Our results suggest that Mad3 is required for the stable binding of Mad2 to Cdc20 in vivo, which is sufficient to inhibit APC activity and is the most downstream event in spindle checkpoint activation.

The kinetochore protein Bub1 participates in the DNA damage response

The DNA damage response (DDR) and the spindle assembly checkpoint (SAC) are two critical mechanisms by which mammalian cells maintain genome stability. There is a growing body of evidence that DDR elements and SAC components crosstalk. Here we report that Bub1 (budding uninhibited by benzimidazoles 1), one of the critical kinetochore proteins essential for SAC, is required for optimal DDRs. We found that knocking-down Bub1 resulted in prolonged H2AX foci and comet tail formation as well as hypersensitivity in response to ionizing radiation (IR). Further, we found that Bub1-mediated Histone H2A Threonine 121 phosphorylation was induced after IR in an ATM-dependent manner. We demonstrated that ATM phosphorylated Bub1 on serine 314 in response to DNA damage in vivo. Finally, we showed that ATM-mediated Bub1 serine 314 phosphorylation was required for IR-induced Bub1 activation and for the optimal DDR. Together, we elucidate the molecular mechanism of DNA damage-induced Bub1 activation and highlight a critical role of Bub1 in DDR.

Bub1 and BubR1: at the interface between chromosome attachment and the spindle checkpoint

The spindle checkpoint ensures genome fidelity by temporarily halting chromosome segregation and the ensuing mitotic exit until the last kinetochore is productively attached to the mitotic spindle. At the interface between proper chromosome attachment and the metaphase-to-anaphase transition are the mammalian spindle checkpoint kinases. Compelling evidence indicates that the checkpoint kinases Bub1 and BubR1 have the added task of regulating kinetochore-microtubule attachments. However, the debate on the requirement of kinase activity is in full swing. This minireview summarizes recent advances in our understanding of the core spindle checkpoint kinases Bub1 and BubR1 and considers evidence that supports and opposes the role of kinase activity in regulating their functions during mitosis.

Reduced Mad2 expression keeps relaxed kinetochores from arresting budding yeast in mitosis

A study of the consequences of removing one copy of Mad2 in diploid budding yeast shows that MAD2 haploinsufficiency is due to subunit imbalance in the Mad1–Mad2 complex and that a normal Mad2:Mad1 ratio is essential for cells to respond to a loss of tension on mitotic chromosomes but is dispensable for a response to unattached chromosomes.

Chromosome segregation depends on the spindle checkpoint, which delays anaphase until all chromosomes have bound microtubules and have been placed under tension. The Mad1–Mad2 complex is an essential component of the checkpoint. We studied the consequences of removing one copy of MAD2 in diploid cells of the budding yeast, Saccharomyces cerevisiae. Compared to MAD2/MAD2 cells, MAD2/mad2Δ heterozygotes show increased chromosome loss and have different responses to two insults that activate the spindle checkpoint: MAD2/mad2Δ cells respond normally to antimicrotubule drugs but cannot respond to chromosomes that lack tension between sister chromatids. In MAD2/mad2Δ cells with normal sister chromatid cohesion, removing one copy of MAD1 restores the checkpoint and returns chromosome loss to wild-type levels. We conclude that cells need the normal Mad2:Mad1 ratio to respond to chromosomes that are not under tension.

Bub1, Sgo1, and Mps1 mediate a distinct pathway for chromosome biorientation in budding yeast

The conserved mitotic kinase Bub1 performs multiple functions that are only partially characterized. Besides its role in the spindle assembly checkpoint and chromosome alignment, Bub1 is crucial for the kinetochore recruitment of multiple proteins, among them Sgo1. Both Bub1 and Sgo1 are dispensable for growth of haploid and diploid budding yeast, but they become essential in cells with higher ploidy. We find that overexpression of SGO1 partially corrects the chromosome segregation defect of bub1Δ haploid cells and restores viability to bub1Δ tetraploid cells. Using an unbiased high-copy suppressor screen, we identified two members of the chromosomal passenger complex (CPC), BIR1 (survivin) and SLI15 (INCENP, inner centromere protein), as suppressors of the growth defect of both bub1Δ and sgo1Δ tetraploids, suggesting that these mutants die due to defects in chromosome biorientation. Overexpression of BIR1 or SLI15 also complements the benomyl sensitivity of haploid bub1Δ and sgo1Δ cells. Mutants lacking SGO1 fail to biorient sister chromatids attached to the same spindle pole (syntelic attachment) after nocodazole treatment. Moreover, the sgo1Δ cells accumulate syntelic attachments in unperturbed mitoses, a defect that is partially corrected by BIR1 or SLI15 overexpression. We show that in budding yeast neither Bub1 nor Sgo1 is required for CPC localization or affects Aurora B activity. Instead we identify Sgo1 as a possible partner of Mps1, a mitotic kinase suggested to have an Aurora B-independent function in establishment of biorientation. We found that Sgo1 overexpression rescues defects caused by metaphase inactivation of Mps1 and that Mps1 is required for Sgo1 localization to the kinetochore. We propose that Bub1, Sgo1, and Mps1 facilitate chromosome biorientation independently of the Aurora B-mediated pathway at the budding yeast kinetochore and that both pathways are required for the efficient turnover of syntelic attachments.