Phosphorylation and activation of Bub1 on unattached chromosomes facilitate the spindle checkpoint

Exploring novel biomarkers and immunotherapeutic targets for biofeedback therapies to reveal the tumor-associated immune microenvironment through a multimetric analysis of kidney renal clear cell carcinoma

BACKGROUND

Kidney renal clear cell carcinoma (KIRC) constitutes the primary subtype of renal cell carcinoma, representing 75% to 80% of cases and carrying a substantial cancer-specific mortality rate of up to 24%. Despite advancements in treatment options, KIRC displays notable resistance to conventional therapies, emphasizing the need for innovative targeted immunotherapeutic strategies. Chromatin regulators (CRs), pivotal proteins controlling gene expression and critical biological processes, play a crucial role in the initiation and progression of KIRC. This study employed a multi-omics approach to evaluate the impact of CR-associated genes on KIRC prognosis.

METHODS

The study utilized the TCGA-KIRC dataset and employed LASSO Cox regression to construct and validate a prognostic model that focuses on genes influencing KIRC prognosis. The research investigated interactions among gene characteristics, clinical parameters, the tumor microenvironment, targeted immunotherapy, and drug responsiveness. Experimental validation, encompassing various techniques such as cell culture, transient transfection, qPCR, Transwell assays, confirmed the robust predictive capability of the BRD9 gene.

RESULTS

The analysis identified the risk score of CRs as an independent factor determining KIRC prognosis. Furthermore, the study introduced a predictive Nomogram model that integrates clinical attributes and risk assessment. Significantly, BRD9 exhibited substantially elevated expression within KIRC cells, underscoring its role in driving cancer cell proliferation, invasion, and migration. These findings suggest the potential for tailored immunotherapy targeting BRD9 in the treatment of KIRC.

CONCLUSION

This study presents an innovative prognostic framework for KIRC based on multi-omics approaches, seamlessly incorporating CRs. This model holds promise for improving the accuracy of prognosis prediction for KIRC patients, laying a robust foundation for the development of targeted immunotherapies.

Negative regulation of APC/C activation by MAPK-mediated attenuation of Cdc20Slp1 under stress

Mitotic anaphase onset is a key cellular process tightly regulated by multiple kinases. The involvement of mitogen-activated protein kinases (MAPKs) in this process has been established in Xenopus egg extracts. However, the detailed regulatory cascade remains elusive, and it is also unknown whether the MAPK-dependent mitotic regulation is evolutionarily conserved in the single-cell eukaryotic organisms such as fission yeast (Schizosaccharomyces pombe). Here, we show that two MAPKs in S. pombe indeed act in concert to restrain anaphase-promoting complex/cyclosome (APC/C) activity upon activation of the spindle assembly checkpoint (SAC). One MAPK, Pmk1, binds to and phosphorylates Slp1Cdc20, the co-activator of APC/C. Phosphorylation of Slp1Cdc20 by Pmk1, but not by Cdk1, promotes its subsequent ubiquitylation and degradation. Intriguingly, Pmk1-mediated phosphorylation event is also required to sustain SAC under environmental stress. Thus, our study establishes a new underlying molecular mechanism of negative regulation of APC/C by MAPK upon stress stimuli, and provides a previously unappreciated framework for regulation of anaphase entry in eukaryotic cells.

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.

TGFBR2 mediated phosphorylation of BUB1 at Ser-318 is required for transforming growth factor-β signaling

BUB1 (budding uninhibited by benzimidazoles-1) is required for efficient TGF-β signaling, through its role in stabilizing the TGFBR1 and TGFBR2 complex. Here we demonstrate that TGFBR2 phosphorylates BUB1 at Serine-318, which is conserved in primates. S318 phosphorylation abrogates the interaction of BUB1 with TGFBR1 and SMAD2. Using BUB1 truncation domains (1–241, 241–482 and 482–723), we demonstrate that multiple contact points exist between BUB1 and TGF-β signaling components and that these interactions are independent of the BUB1 tetratricopeptide repeat (TPR) domain. Moreover, substitutions in the middle domain (241–482) encompassing S318 reveals that efficient interaction with TGFBR2 occurs only in its dephosphorylated state (241–482 S318A). In contrast, the phospho-mimicking mutant (241–482 S318D) exhibits efficient binding with SMAD2 and its over-expression results in a decrease in TGFBR1-TGFBR2 and TGFBR1-SMAD2 interactions. These findings suggest that TGFBR2 mediated BUB1 phosphorylation at S318 may serve as a switch for the dissociation of the SMAD2-TGFBR complex, and therefore represents a regulatory event for TGF-β signaling. Finally, we provide evidence that the BUB1-TGF-β signaling axis may mediate aggressive phenotypes in a variety of cancers.

Recent Progress on the Localization of the Spindle Assembly Checkpoint Machinery to Kinetochores

Faithful chromosome segregation during mitosis is crucial for maintaining genome stability. The spindle assembly checkpoint (SAC) is a surveillance mechanism that ensures accurate mitotic progression. Defective SAC signaling leads to premature sister chromatid separation and aneuploid daughter cells. Mechanistically, the SAC couples the kinetochore microtubule attachment status to the cell cycle progression machinery. In the presence of abnormal kinetochore microtubule attachments, the SAC prevents the metaphase-to-anaphase transition through a complex kinase-phosphatase signaling cascade which results in the correct balance of SAC components recruited to the kinetochore. The correct kinetochore localization of SAC proteins is a prerequisite for robust SAC signaling and, hence, accurate chromosome segregation. Here, we review recent progresses on the kinetochore recruitment of core SAC factors.

Analysis of Mitotic Checkpoint Function in Xenopus Egg Extracts

Accurate sister chromatid segregation is pivotal in the faithful transmission of genetic information during each cell division. To ensure accurate segregation, eukaryotic organisms have evolved a "mitotic (or spindle assembly) checkpoint" to prevent premature advance to anaphase before successful attachment of every chromosome to the microtubules of the mitotic spindle. An unattached kinetochore generates a diffusible signal that inhibits ubiquitination of substrates such as cyclin B and securins. This protocol presents an in vitro assay for studying the mitotic checkpoint using Xenopus laevis egg extracts. Meiotic spindles assembled around nuclei added to egg extracts are synchronized at metaphase by an endogenous activity known as cytostatic factor (CSF). Normally, the mitotic checkpoint results in continued metaphase arrest following inactivation of CSF activity (by addition of calcium) and disassembly of spindle microtubules (with a microtubule inhibitor such as nocodazole). Simple DAPI staining for chromatin structure or biochemical analysis of Cdc2/cyclin B (cyclin-dependent kinase) histone H1 kinase activity can be used to evaluate the stages of the cell cycle and the status of the mitotic checkpoint. This cell-free system derived from Xenopus eggs has been successfully used to unravel the mechanisms of mitosis, and it provides a distinct advantage over cell-based studies in which perturbing kinetochore functions often results in lethality.

Kinase and Phosphatase Cross-Talk at the Kinetochore

Multiple kinases and phosphatases act on the kinetochore to control chromosome segregation: Aurora B, Mps1, Bub1, Plk1, Cdk1, PP1, and PP2A-B56, have all been shown to regulate both kinetochore-microtubule attachments and the spindle assembly checkpoint. Given that so many kinases and phosphatases converge onto two key mitotic processes, it is perhaps not surprising to learn that they are, quite literally, entangled in cross-talk. Inhibition of any one of these enzymes produces secondary effects on all the others, which results in a complicated picture that is very difficult to interpret. This review aims to clarify this picture by first collating the direct effects of each enzyme into one overarching schematic of regulation at the Knl1/Mis12/Ndc80 (KMN) network (a major signaling hub at the outer kinetochore). This schematic will then be used to discuss the implications of the cross-talk that connects these enzymes; both in terms of why it may be needed to produce the right type of kinetochore signals and why it nevertheless complicates our interpretations about which enzymes control what processes. Finally, some general experimental approaches will be discussed that could help to characterize kinetochore signaling by dissociating the direct from indirect effect of kinase or phosphatase inhibition in vivo. Together, this review should provide a framework to help understand how a network of kinases and phosphatases cooperate to regulate two key mitotic processes.

A sequential multi-target Mps1 phosphorylation cascade promotes spindle checkpoint signaling

The master spindle checkpoint kinase Mps1 senses kinetochore-microtubule attachment and promotes checkpoint signaling to ensure accurate chromosome segregation. The kinetochore scaffold Knl1, when phosphorylated by Mps1, recruits checkpoint complexes Bub1–Bub3 and BubR1–Bub3 to unattached kinetochores. Active checkpoint signaling ultimately enhances the assembly of the mitotic checkpoint complex (MCC) consisting of BubR1–Bub3, Mad2, and Cdc20, which inhibits the anaphase-promoting complex or cyclosome bound to Cdc20 (APC/C^Cdc20) to delay anaphase onset. Using in vitro reconstitution, we show that Mps1 promotes APC/C inhibition by MCC components through phosphorylating Bub1 and Mad1. Phosphorylated Bub1 binds to Mad1–Mad2. Phosphorylated Mad1 directly interacts with Cdc20. Mutations of Mps1 phosphorylation sites in Bub1 or Mad1 abrogate the spindle checkpoint in human cells. Therefore, Mps1 promotes checkpoint activation through sequentially phosphorylating Knl1, Bub1, and Mad1. This sequential multi-target phosphorylation cascade makes the checkpoint highly responsive to Mps1 and to kinetochore-microtubule attachment.

DOI:http://dx.doi.org/10.7554/eLife.22513.001

Probing the catalytic functions of Bub1 kinase using the small molecule inhibitors BAY-320 and BAY-524

The kinase Bub1 functions in the spindle assembly checkpoint (SAC) and in chromosome congression, but the role of its catalytic activity remains controversial. Here, we use two novel Bub1 inhibitors, BAY-320 and BAY-524, to demonstrate potent Bub1 kinase inhibition both in vitro and in intact cells. Then, we compared the cellular phenotypes of Bub1 kinase inhibition in HeLa and RPE1 cells with those of protein depletion, indicative of catalytic or scaffolding functions, respectively. Bub1 inhibition affected chromosome association of Shugoshin and the chromosomal passenger complex (CPC), without abolishing global Aurora B function. Consequently, inhibition of Bub1 kinase impaired chromosome arm resolution but exerted only minor effects on mitotic progression or SAC function. Importantly, BAY-320 and BAY-524 treatment sensitized cells to low doses of Paclitaxel, impairing both chromosome segregation and cell proliferation. These findings are relevant to our understanding of Bub1 kinase function and the prospects of targeting Bub1 for therapeutic applications.

DOI:http://dx.doi.org/10.7554/eLife.12187.001

The DNA in our cells is packaged into structures called chromosomes. When a cell divides, these chromosomes need to be copied and then correctly separated so that both daughter cells have a full set of genetic information. Errors in separating chromosomes can lead to the death of cells, birth defects or contribute to the development of cancer.

Chromosomes are separated by an array of protein fibers called the mitotic spindle. A surveillance mechanism known as the spindle assembly checkpoint prevents the cell from dividing until all the chromosomes have properly attached to the spindle. A protein called Bub1 is a central element of the SAC. However, it was not clear whether Bub1 works primarily as an enzyme or as a scaffolding protein.

Baron, von Schubert et al. characterized two new molecules that inhibit Bub1’s enzyme activity and used them to investigate what role the enzyme plays in the spindle assembly checkpoint in human cells. The experiments compared the effects of these inhibitors to the effects of other molecules that block the production of Bub1. Baron, von Schubert et al.’s findings suggest that Bub1 works primarily as a scaffolding protein, but that the enzyme activity is required for optimal performance.

Further experiments show that when the molecules that inhibit the Bub1 enzyme are combined with paclitaxel – a widely used therapeutic drug – cancer cells have more difficulties in separating their chromosomes and divide less often. The new inhibitors used by Baron, von Schubert et al. will be useful for future studies of this protein in different situations. Furthermore, these molecules may have the potential to be used as anti-cancer therapies in combination with other drugs.

DOI:http://dx.doi.org/10.7554/eLife.12187.002

Kinetochore function is controlled by a phospho-dependent coexpansion of inner and outer components

It is widely accepted that the kinetochore is built on CENP-A-marked centromeric chromatin in a hierarchical order from inner to outer kinetochore. Recruitment of many kinetochore proteins depends on microtubule attachment status, but it remains unclear how their assembly/disassembly is orchestrated. Applying 3D structured illumination microscopy to Xenopus laevis egg extracts, here we reveal that in the absence of microtubule attachment, proteins responsible for lateral attachment and spindle checkpoint signaling expand to form micrometer-scale fibrous structures over CENP-A-free chromatin, whereas a core module responsible for end-on attachment (CENP-A, CENP-T, and Ndc80) does not. Both outer kinetochore proteins (Bub1, BubR1, Mad1, and CENP-E) and the inner kinetochore component CENP-C are integral components of the expandable module, whose assembly depends on multiple mitotic kinases (Aurora B, Mps1, and Plx1) and is suppressed by protein phosphatase 1. We propose that phospho-dependent coexpansion of CENP-C and outer kinetochore proteins promotes checkpoint signal amplification and lateral attachment, whereas their selective disassembly enables the transition to end-on attachment.

Bub1 autophosphorylation feeds back to regulate kinetochore docking and promote localized substrate phosphorylation

During mitosis, Bub1 kinase phosphorylates histone H2A-T120 to promote centromere sister chromatid cohesion through recruitment of shugoshin (Sgo) proteins. The regulation and dynamics of H2A-T120 phosphorylation are poorly understood. Using quantitative phosphoproteomics we show that Bub1 is autophosphorylated at numerous sites. We confirm mitosis-specific autophosphorylation of a several residues and show that Bub1 activation is primed in interphase but fully achieved only in mitosis. Mutation of a single autophosphorylation site T589 alters kinetochore turnover of Bub1 and results in uniform H2A-T120 phosphorylation and Sgo recruitment along chromosome arms. Consequently, improper sister chromatid resolution and chromosome segregation errors are observed. Kinetochore tethering of Bub1-T589A refocuses H2A-T120 phosphorylation and Sgo1 to centromeres. Recruitment of the Bub1-Bub3-BubR1 axis to kinetochores has recently been extensively studied. Our data provide novel insight into the regulation and kinetochore residency of Bub1 and indicate that its localization is dynamic and tightly controlled through feedback autophosphorylation.

Deacetylation of the mitotic checkpoint protein BubR1 at lysine 250 by SIRT2 and subsequent effects on BubR1 degradation during the prometaphase/anaphase transition

Mitotic catastrophe, a form of cell death that occurs during mitosis and after mitotic slippage to a tetraploid state, plays an important role in the efficacy of cancer cell killing by microtubule inhibitors. Prolonged mitotic arrest at the spindle assembly checkpoint (SAC) is a well-known requirement for mitotic catastrophe and, thus, for conferring sensitivity to microtubule inhibitors. We previously reported that downregulation of SIRT2, a member of the sirtuin family of NAD+-dependent deacetylases, confers resistance to microtubule inhibitors by abnormally prolonging mitotic arrest and thus compromising the cell death pathway after mitotic slippage. Thus, turning off SAC activation after a defined period is an additional requirement for efficient post-slippage death. Here, we investigated whether SIRT2 deacetylates BubR1, which is a core component of the SAC; acetylation of BubR1 at lysine 250 (K250) during prometaphase inhibits its APC/C-dependent proteolysis and thus regulates timing in anaphase entry. We showed that SIRT2 deacetylates BubR1 K250 both in vitro and in vivo. We also found that SIRT2 knockdown leads to increased levels of BubR1 acetylation at prometaphase; however, this increase is not substantial to elevate the levels of total BubR1 or delay the transition from prometaphase to anaphase. The present study shows that SIRT2 is a deacetylase for BubR1 K250, although the abnormally prolonged SAC activation observed in SIRT2 knockdown cells is not accompanied by a change in BubR1 levels or by delayed progression from prometaphase to anaphase.

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.

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.

Bub1 kinase activity drives error correction and mitotic checkpoint control but not tumor suppression

Mice expressing a version of Bub1 that lacks kinase activity have increased chromosome segregation errors and aneuploidy but not increased susceptibility to tumors.

The mitotic checkpoint protein Bub1 is essential for embryogenesis and survival of proliferating cells, and bidirectional deviations from its normal level of expression cause chromosome missegregation, aneuploidy, and cancer predisposition in mice. To provide insight into the physiological significance of this critical mitotic regulator at a modular level, we generated Bub1 mutant mice that lack kinase activity using a knockin gene-targeting approach that preserves normal protein abundance. In this paper, we uncover that Bub1 kinase activity integrates attachment error correction and mitotic checkpoint signaling by controlling the localization and activity of Aurora B kinase through phosphorylation of histone H2A at threonine 121. Strikingly, despite substantial chromosome segregation errors and aneuploidization, mice deficient for Bub1 kinase activity do not exhibit increased susceptibility to spontaneous or carcinogen-induced tumorigenesis. These findings provide a unique example of a modular mitotic activity orchestrating two distinct networks that safeguard against whole chromosome instability and reveal the differential importance of distinct aneuploidy-causing Bub1 defects in tumor suppression.

BRK1, a Bub1-related kinase, is essential for generating proper tension between homologous kinetochores at metaphase I of rice meiosis

Bub1 (for budding uninhibited by benzimidazole 1), one of the main spindle checkpoint kinases, acts as a kinetochore scaffold for assembling other checkpoint proteins. Here, we identify a plant Bub1-related kinase 1 (BRK1) in rice (Oryza sativa). The brk1 mutants are sterile due to the precocious separation of sister chromatids at the onset of anaphase I. The centromeric recruitment of SHUGOSHIN1 and phosphorylation of histone H2A at Thr-134 (H2A-pT134) depend on BRK1. Although the homologs can faithfully separate from each other at the end of meiosis I, the uncorrected merotelic attachment of paired sister kinetochores at the early stage of metaphase I in brk1 reduces the tension across homologous kinetochores, causes the metaphase I spindle to be aberrantly shaped, and subsequently affects the synchronicity of homolog separation at the onset of anaphase I. In addition, the phosphorylation of inner centromeric histone H3 at Ser-10 (H3-pS10) during diakinesis depends on BRK1. Therefore, we speculate that BRK1 may be required for normal localization of Aurora kinase before the onset of metaphase I, which is responsible for correcting the merotelic attachment.

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 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.

Monitoring the fidelity of mitotic chromosome segregation by the spindle assembly checkpoint

Accurate chromosome segregation relies on activity of the spindle assembly checkpoint, a surveillance mechanism that prevents premature anaphase onset until all chromosomes are properly attached to the mitotic spindle apparatus and aligned at the metaphase plate. Defects in this mechanism contribute to chromosome instability and aneuploidy, a hallmark of malignant cells. Here, we review the molecular mechanisms of activation and silencing of the spindle assembly checkpoint and its relationship to tumourigenesis.

Mutual regulation between the spindle checkpoint and APC/C

Accurate chromosome segregation during mitosis is critical for maintaining genomic stability. The spindle checkpoint is a cellular surveillance system that ensures the fidelity of chromosome segregation. In response to sister chromatids not properly captured by spindle microtubules, the spindle checkpoint interferes with the functions of Cdc20, the mitotic activator of the anaphase-promoting complex or cyclosome (APC/C), thereby blocking APC/C-mediated degradation of securin and cyclin B to delay anaphase onset. This review summarizes the recent progress on the mechanisms by which checkpoint proteins inhibit APC/C, the conformational and enzymatic activation of checkpoint proteins, and the emerging roles of APC/C-dependent ubiquitination in checkpoint inactivation.

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.

BUB1 and BUBR1: multifaceted kinases of the cell cycle

The multidomain protein kinases BUB1 and BUBR1 (Mad3 in yeast, worms and plants) are central components of the mitotic checkpoint for spindle assembly (SAC). This evolutionarily conserved and essential self-monitoring system of the eukaryotic cell cycle ensures the high fidelity of chromosome segregation by delaying the onset of anaphase until all chromosomes are properly bi-oriented on the mitotic spindle. Despite their amino acid sequence conservation and similar domain organization, BUB1 and BUBR1 perform different functions in the SAC. Recent structural information provides crucial molecular insights into the regulation and recognition of BUB1 and BUBR1, and a solid foundation to dissect the roles of these proteins in the control of chromosome segregation in normal and oncogenic cells.

Sgo1 establishes the centromeric cohesion protection mechanism in G2 before subsequent Bub1-dependent recruitment in mitosis

Bub1 was one of the first protein kinases identified as a component of the spindle-assembly checkpoint, a surveillance mechanism that delays anaphase onset until all chromosomes are stably attached to spindle microtubules. Whereas the kinase activity of Bub1 is not required for checkpoint function in yeast, its requirement in mammalian cells is still unclear. Using a complementation assay with bona fide BUB1-null mouse embryonic fibroblasts, we show that the kinase activity of Bub1 is not required for checkpoint function or chromosome alignment. Its activity is, however, required for centromeric localisation of Sgo1, a known protector of centromeric cohesion. Despite the absence of Sgo1 from mitotic centromeres in cells devoid of Bub1 activity, centromeric cohesion is still maintained until anaphase. An explanation for this comes from observations showing that Sgo1 is first recruited to centromeric heterochromatin in G2, but then becomes diffusely localised throughout the nucleus in early prophase, before returning to centromeres later in prophase. Importantly, whereas centromeric localisation of Sgo1 in prophase is dependent on the kinase activity of Bub1, its recruitment to centromeric heterochromatin in G2 is not. Rather, the localisation of Sgo1 in G2 is abolished when heterochromatin protein 1 is not bound to centromeric heterochromatin. Thus, it seems that Sgo1 sets up the centromeric protection mechanism in G2, but that its Bub1-dependent localisation to centromeres during mitosis is not required to maintain cohesion.

Overcoming inhibition in the spindle checkpoint

Spindle checkpoint silencing is a critical step during mitosis that initiates chromosome segregation, yet surprisingly little is known about its mechanism. Protein phosphatase I (PP1) was shown recently to be a key player in this process, and in this issue of Genes & Deverlopment, Akiyoshi and colleagues (pp. 2887-2899) identify budding yeast Fin1p as a kinetochore-localized regulator of PP1 activity toward checkpoint targets. Here we review recent mechanistic insights and propose a working model for spindle checkpoint silencing.

Dividing the goods: co-ordination of chromosome biorientation and mitotic checkpoint signalling by mitotic kinases

Error-free chromosome segregation during cell division relies on chromosome biorientation and mitotic checkpoint activity. A group of unrelated kinases controls various aspects of both processes. The present short review outlines our current understanding of the roles of these kinases in maintaining chromosomal stability.

Bub1 regulates chromosome segregation in a kinetochore-independent manner

The kinetochore-bound protein kinase Bub1 performs two crucial functions during mitosis: it is essential for spindle checkpoint signaling and for correct chromosome alignment. Interestingly, Bub1 mutations are found in cancer tissues and cancer cell lines. Using an isogenic RNA interference complementation system in transformed HeLa cells and untransformed RPE1 cells, we investigate the effect of structural Bub1 mutants on chromosome segregation. We demonstrate that Bub1 regulates mitosis through the same mechanisms in both cell lines, suggesting a common regulatory network. Surprisingly, Bub1 can regulate chromosome segregation in a kinetochore-independent manner, albeit at lower efficiency. Its kinase activity is crucial for chromosome alignment but plays only a minor role in spindle checkpoint signaling. We also identify a novel conserved motif within Bub1 (amino acids 458–476) that is essential for spindle checkpoint signaling but does not regulate chromosome alignment, and we show that several cancer-related Bub1 mutants impair chromosome segregation, suggesting a possible link to tumorigenesis.

The function of a spindle checkpoint gene bub-1 in C. elegans development

BACKGROUND

The serine/threonine kinase BUB1 (Budding Uninhibited by Benzimidazole 1) was originally identified in yeast as a checkpoint protein, based on its mutant's incapacity of delaying the cell cycle in response to loss of microtubules. Our understanding of its function is primarily from studies carried out in yeast S. cerevisiae. It has been shown that it is a component of the mitotic spindle checkpoint and regulates the separation of sister chromatids through its downstream molecules. However, its roles in multi-cellular organisms remain unclear.

METHODS AND FINDINGS

In nematode C. elegans, rapid cell divisions primarily occur in embryos and in germline of postembryonic larvae and adults. In addition, a select set of cells undergo a few rounds of cell division postembryonically. One common phenotype associated with impaired cell division is described as Stu (Sterile and Uncoordinated) [1], [2]. We conducted a genetic screen for zygotic mutants that displayed Stu phenotype in C. elegans. We isolated seven Stu mutants that fell into five complementation groups. We report here that two mutations, FanWang5 (fw5) and FanWang8 (fw8) affect the bub-1 gene, a homolog of yeast BUB1. Both mutant alleles of fw5 and fw8 exhibited variable behavioral defects, including developmental arrest, uncoordination and sterility. The number of postembryonically born neurons in the ventral cord decreased and their axon morphology was abnormal. Also, the decrease of neurons in the ventral cord phenotype could not be suppressed by a caspase-3 loss-of-function mutant. In addition, bub-1(fw5 and fw8) mutants showed widespread effects on postembryonic development in many cell lineages. We found that bub-1 functioned maternally in several developmental lineages at the embryonic stage in C. elegans. Studies in yeast have shown that BUB1 functions as a spindle checkpoint protein by regulating the anaphase promoting complex/cyclosome (APC/C). We performed double mutant analysis and observed that bub-1 genetically interacted with several downstream genes, including fzy-1/CDC20, mat-2/APC1 and emb-27/APC6.

CONCLUSIONS

Our results demonstrate a conserved role of bub-1 in cell-cycle regulation and reveal that C. elegans bub-1 is required both maternally and zygotically. Further, our genetic analysis is consistent with that the function of bub-1 in C. elegans is likely similar to its yeast and mammalian homologs.

Kinase signaling in the spindle checkpoint

The spindle checkpoint is a cell cycle surveillance system that ensures the fidelity of chromosome segregation. In mitosis, it elicits the "wait anaphase" signal to inhibit the anaphase-promoting complex or cyclosome until all chromosomes achieve bipolar microtubule attachment and align at the metaphase plate. Because a single kinetochore unattached to microtubules activates the checkpoint, the wait anaphase signal is thought to be generated by this kinetochore and is then amplified and distributed throughout the cell to inhibit the anaphase-promoting complex/cyclosome. Several spindle checkpoint kinases participate in the generation and amplification of this signal. Recent studies have begun to reveal the activation mechanisms of these checkpoint kinases. Increasing evidence also indicates that the checkpoint kinases not only help to generate the wait anaphase signal but also actively correct kinetochore-microtubule attachment defects.

Impaired Bub1 function in vivo compromises tension-dependent checkpoint function leading to aneuploidy and tumorigenesis

Bub1 is a serine/threonine kinase originally described as a core component of the spindle assembly checkpoint (SAC) mechanism in yeast. Bub1 binding at kinetochores has been reported to be required for SAC function and localization of other SAC components. A proper SAC is believed to be essential for murine embryonic development, as all previously described null mutations in SAC components in mice cause embryonic lethality. We produced mice harboring a Bub1 mutant allele lacking exons 2 and 3, resulting in a hypomorphic mutant expressed at <5% of wild-type levels. Despite this significant reduction, homozygous mutant animals are viable on a mixed 129P2/B6 or FVB background but display increased tumorigenesis with aging, whereas mice with a C57Bl/6J background die perinatally. Bub1 mutant murine embryonic fibroblasts (MEFs) display defects in chromosome congression to the metaphase plate, severe chromosome missegregation, and aneuploidy accompanied by high levels of premature senescence. Mutant MEFs have a robust SAC in response to nocodazole treatment but an impaired response to Taxol. Mutant MEFs also show reduced kinetochore localization of BubR1, but not of Mad2. The significant reduction in SAC response to Taxol, but not nocodazole, coupled with the reduced binding of BubR1, but not Mad2, indicates that Bub1 is particularly critical for the SAC response to a lack of tension on kinetochores. Thus, Bub1 is essential for proper chromosome segregation, a defect that can lead to severe phenotypes, including perinatal lethality and a predisposition to cancer.

Structure and substrate recruitment of the human spindle checkpoint kinase Bub1

In mitosis, the spindle checkpoint detects a single unattached kinetochore, inhibits the anaphase-promoting complex or cyclosome (APC/C), and prevents premature sister chromatid separation. The checkpoint kinase Bub1 contributes to checkpoint sensitivity through phosphorylating the APC/C activator, Cdc20, and inhibiting APC/C catalytically. We report here the crystal structure of the kinase domain of Bub1, revealing the requirement of an N-terminal extension for its kinase activity. Though the activation segment of Bub1 is ordered and has structural features indicative of active kinases, the C-terminal portion of this segment sterically restricts substrate access to the active site. Bub1 uses docking motifs, so-called KEN boxes, outside its kinase domain to recruit Cdc20, one of two known KEN box receptors. The KEN boxes of Bub1 are required for the spindle checkpoint in human cells. Therefore, its unusual active-site conformation and mode of substrate recruitment suggest that Bub1 has an exquisitely tuned specificity for Cdc20.

Linking kinetochore-microtubule binding to the spindle checkpoint

The spindle checkpoint blocks cell-cycle progression until chromosomes are properly attached to the mitotic spindle. Popular models propose that checkpoint proteins associate with kinetochores to produce a "wait anaphase" signal that inhibits anaphase. Recent data suggest that a two-state switch results from using the same kinetochore proteins to bind microtubules and checkpoint proteins. At least eight protein kinases are implicated in spindle checkpoint signaling, arguing that a traditional signal transduction cascade is integral to spindle checkpoint signaling.

Bub1 mediates cell death in response to chromosome missegregation and acts to suppress spontaneous tumorigenesis

The physiological role of the mitotic checkpoint protein Bub1 is unknown. To study this role, we generated a series of mutant mice with a gradient of reduced Bub1 expression using wild-type, hypomorphic, and knockout alleles. Bub1 hypomorphic mice are viable, fertile, and overtly normal despite weakened mitotic checkpoint activity and high percentages of aneuploid cells. Bub1 haploinsufficient mice, which have a milder reduction in Bub1 protein than Bub1 hypomorphic mice, also exhibit reduced checkpoint activity and increased aneuploidy, but to a lesser extent. Although cells from Bub1 hypomorphic and haploinsufficient mice have similar rates of chromosome missegregation, cell death after an aberrant separation decreases dramatically with declining Bub1 levels. Importantly, Bub1 hypomorphic mice are highly susceptible to spontaneous tumors, whereas Bub1 haploinsufficient mice are not. These findings demonstrate that loss of Bub1 below a critical threshold drives spontaneous tumorigenesis and suggest that in addition to ensuring proper chromosome segregation, Bub1 is important for mediating cell death when chromosomes missegregate.

Rapid effects of acute anoxia on spindle kinetochore interactions activate the mitotic spindle checkpoint

The dramatic chromosome instability in certain tumors might reflect a synergy of spindle checkpoint defects with hypoxic conditions. In Caenorhabditis elegans and Drosophila melanogaster, spindle checkpoint activation has been implicated in the response to acute anoxia. The activation mechanism is unknown. Our analyses in D. melanogaster demonstrate that oxygen deprivation affects microtubule organization within minutes. The rapid effects of anoxia are identical in wild-type and spindle checkpoint-deficient Mps1 mutant embryos. Therefore, the anoxia effects on the mitotic spindle are not a secondary consequence of spindle checkpoint activation. Some motor, centrosome and kinetochore proteins (dynein, Kin-8, Cnn, TACC, Cenp-C, Nuf2) are rapidly relocalized after oxygen deprivation. Kinetochores congress inefficiently into the metaphase plate and do not experience normal pulling forces. Spindle checkpoint proteins accumulate mainly within the spindle midzone and inhibit anaphase onset. In checkpoint-deficient embryos, mitosis is still completed after oxygen deprivation, although accompanied by massive chromosome missegregation. Inhibitors of oxidative phosphorylation mimic anoxia effects. We conclude that oxygen deprivation impairs the chromosome segregation machinery more rapidly than spindle checkpoint function. Although involving adenosine triphosphate (ATP)-consuming kinases, the spindle checkpoint can therefore be activated by spindle damage in response to acute anoxia and protect against aneuploidies.

Cdc20: a WD40 activator for a cell cycle degradation machine

Cdc20 is an essential cell-cycle regulator required for the completion of mitosis in organisms from yeast to man and contains at its C terminus a WD40 repeat domain that mediates protein-protein interactions. In mitosis, Cdc20 binds to and activates the ubiquitin ligase activity of a large molecular machine called the anaphase-promoting complex/cyclosome (APC/C) and enables the ubiquitination and degradation of securin and cyclin B, thus promoting the onset of anaphase and mitotic exit. APC/C(Cdc20) is temporally and spatially regulated during the somatic and embryonic cell cycle by numerous mechanisms, including the spindle checkpoint and the cytostatic factor (CSF). Therefore, Cdc20 serves as an integrator of multiple intracellular signaling cascades that regulate progression through mitosis. This review summarizes recent progress toward the understanding of the functions of Cdc20, the mechanisms by which it activates APC/C, and its regulation by phosphorylation and by association with its binding proteins.

Preparation of monoclonal antibodies against the spindle checkpoint kinase Bub1

The Bub1 kinase is a critical component of the spindle checkpoint involved in monitoring the separation of sister chromatids at mitosis. The viral oncoprotein Simian virus 40 large T antigen (LT) can bind and perturb the spindle checkpoint function of Bub1. We have developed three highly specific monoclonal antibodies against the Bub1 protein and have demonstrated that they can all detect Bub1 via Western blotting and immunofluorescence, in addition to their ability to immunoprecipitate Bub1.

BubR1 deficiency results in enhanced activation of MEK and ERKs upon microtubule stresses

Disruption of microtubules activates the spindle checkpoint, of which BubR1 is a major component. Our early studies show that BubR1 haplo-insufficiency results in enhanced mitotic slippage in vitro and tumorigenesis in vivo.

OBJECTIVE

Given that both MAPKs/ERKs and MEK play an important role during mitosis, we investigated whether there existed regulatory relationship between the MAPK signalling pathway and BubR1.

METHOD AND RESULTS

Here, we have demonstrated that BubR1 deficiency is correlated with enhanced activation of MEK and ERKs after disruption of microtubule dynamics. Specifically, treatment with nocodazole and paclitaxel resulted in hyper-activation of ERKs and MEK in BubR1(+/-) murine embryonic fibroblasts (MEF) compared to that of wild-type MEFs. This enhanced activation of ERKs and MEK was at least partly responsible for more successful proliferation completion when cells were treated with nocodazole. BubR1 knockdown via RNAi resulted in enhanced activation of ERKs and MEK in HeLa cells, correlating with inhibition of PP1, a negative regulator of MEK. Moreover, when BubR1 was partially inactivated due to premature missegregation of chromosomes after Sgo1 depletion, phosphorylation of ERKs and MEK was enhanced in mitotic cells; in contrast, little, if any activated ERKs and MEK were detected in mitotic cells induced by nocodazole. Furthermore, BubR1, activated ERKs and activated MEK all localized to spindle poles during mitosis, and also, the proteins physically interacted with each other.

CONCLUSION

Our studies suggest that there exists a cross-talk between spindle checkpoint components and ERKs and MEK and that BubR1 may play an important role in mediating the cross-talk.

The spindle-assembly checkpoint in space and time

In eukaryotes, the spindle-assembly checkpoint (SAC) is a ubiquitous safety device that ensures the fidelity of chromosome segregation in mitosis. The SAC prevents chromosome mis-segregation and aneuploidy, and its dysfunction is implicated in tumorigenesis. Recent molecular analyses have begun to shed light on the complex interaction of the checkpoint proteins with kinetochores--structures that mediate the binding of spindle microtubules to chromosomes in mitosis. These studies are finally starting to reveal the mechanisms of checkpoint activation and silencing during mitotic progression.

Expression changes of the MAD mitotic checkpoint gene family in renal cell carcinomas characterized by numerical chromosome changes

Papillary and chromophobe renal cell carcinomas are characterized by multiple trisomies and monosomies, respectively, but the molecular mechanisms behind the acquisition of these numerical chromosome changes are unknown. To evaluate the role of mitotic checkpoint defects for the karyotypic patterns characteristic of these two renal cell cancer subtypes, we analyzed the messenger RNA expression levels of the major mitotic checkpoint genes of the budding uninhibited by benzimidazole family (BUB1, BUBR1, BUB3) and of the mitotic arrest deficiency family (MAD1, MAD2L1, MAD2L2) by real-time quantitative polymerase chain reaction in 30 renal cell cancer samples (11 chromophobe and 19 papillary) and 36 normal kidney tissue samples. MAD1, MAD2L1, and MAD2L2 showed significant expression differences in tumor tissue compared to controls. Chromophobe tumors presented underexpression of MAD1, and MAD2L2, whereas papillary tumors showed overexpression of MAD2L1. The expression level of the BUB gene family did not differ significantly from that of normal kidney. We conclude that expression changes in mitotic arrest deficiency genes (MAD1, MAD2L1, and MAD2L2) play a role in renal carcinogenesis characterized by multiple numerical chromosome abnormalities.

KEN-Box-dependent Degradation of the Bub1 Spindle Checkpoint Kinase by the Anaphase-promoting Complex/Cyclosome

The spindle checkpoint is a cell cycle surveillance mechanism that ensures the fidelity of chromosome segregation during mitosis and meiosis. Bub1 is a protein serine-threonine kinase that plays multiple roles in chromosome segregation and the spindle checkpoint. In response to misaligned chromosomes, Bub1 directly inhibits the ubiquitin ligase activity of the anaphase-promoting complex or cyclosome (APC/C) by phosphorylating its activator Cdc20. The protein level and the kinase activity of Bub1 are regulated during the cell cycle; they peak in mitosis and are low in G1/S phase. Here we show that Bub1 is degraded during mitotic exit and that degradation of Bub1 is mediated by APC/C in complex with its activator Cdh1 (APC/C(Cdh1)). Overexpression of Cdh1 reduces the protein levels of ectopically expressed Bub1, whereas depletion of Cdh1 by RNA interference increases the level of the endogenous Bub1 protein. Bub1 is ubiquitinated by immunopurified APC/C(Cdh1) in vitro. We further identify two KEN-box motifs on Bub1 that are required for its degradation in vivo and ubiquitination in vitro. A Bub1 mutant protein with both KEN-boxes mutated is stable in cells but fails to elicit a cell cycle phenotype, indicating that degradation of Bub1 by APC/C(Cdh1) is not required for mitotic exit. Nevertheless, our study clearly demonstrates that Bub1, an APC/C inhibitor, is also an APC/C substrate. The antagonistic relationship between Bub1 and APC/C may help to prevent the premature accumulation of Bub1 during G1.

Bub1 up-regulation and hyperphosphorylation promote malignant transformation in SV40 tag-induced transgenic mouse models

Rodents do not naturally develop prostate cancer. Currently, most widely used genetically engineered mouse prostate cancer models use SV40 T/tag oncogene. To understand the mechanism underlying prostate cancer development in transgenic and knock-in SV40 Tag mouse models, we did cDNA microarray analyses, comparing gene expression profiles of prostate cancer tissues from early-, late-, and advance-stage androgen-independent prostate cancers. Of the 67 genes that were up-regulated by > or = 10-fold, 40 are known to be required for chromosome stability. In particular, the spindle checkpoint component Bub1 was persistently up-regulated from early to advanced androgen-independent prostate cancer lesions. Significantly, Bub1, which is required for accurate chromosome segregation during mitosis, has recently been reported to bind SV40 Tag. Consistent with a spindle checkpoint defect, flow cytometry experiments indicate that advanced androgen-independent prostate cancer tumors exhibit aneuploidy, along with up-regulation of levels of both Bub1 mRNA and Bub1 protein or hyperphosphorylation. Importantly, up-regulation and hyperphosphorylation of Bub1 were also observed in established human prostate cancer cell lines and in clinical studies. Furthermore, analysis of human prostate cancer lines showed impaired spindle checkpoint function and endoreduplication following exposure to spindle toxins. Small interfering RNA-mediated repression of Bub1 in the human prostate cancer line PC-3 restrained cell proliferation, an effect mimicked by inhibition of mitogen-activated protein kinase, an upstream activator of Bub1. Thus, by perturbing Bub1 function, our observations suggest a new mechanism whereby the SV40 Tag oncoprotein promotes chromosomal instability and aneuploidy in transgenic mouse prostate cancer models. Whereas the exact details of this mechanism remain unclear, our novel findings raise the possibility of exploiting Bub1 as a new therapeutic target in the treatment of prostate cancer, the most common cancer in adult men in North America.

Role of Hcn1 and its phosphorylation in fission yeast anaphase-promoting complex/cyclosome function

The anaphase-promoting complex/cyclosome (APC/C) is a conserved multisubunit ubiquitin ligase required for the degradation of key cell cycle regulators. The APC/C becomes active at the metaphase/anaphase transition and remains active during G(1) phase. One mechanism linked to activation of the APC/C is phosphorylation. Although many sites of mitotic phosphorylation have been identified in core components of the APC/C, the consequence of any individual phosphorylation event has not been elucidated in vivo. In this study, we show that Hcn1 is an essential core component of the fission yeast APC/C and is critical for maintaining complex integrity. Moreover, Hcn1 is a phosphoprotein in vivo. Phosphorylation of Hcn1 occurs at a single Cdk1 site in vitro and in vivo. Mutation of this site to alanine, but not aspartic acid, compromises APC/C function and leads to a specific defect in the completion of cell division.

Mps1 Phosphorylation by MAP Kinase Is Required for Kinetochore Localization of Spindle-Checkpoint Proteins

The spindle checkpoint delays anaphase onset until all chromosomes have achieved bipolar attachment to the spindle microtubules. Unattached kinetochores activate the spindle checkpoint by recruiting several spindle-checkpoint proteins, including Mps1, Mad1, Mad2, Bub1, Bub3, and BubR1 (Mad3 in yeast). In vertebrate cells, active MAP kinase (MAPK) is also enriched at unattached kinetochores and is required for the spindle checkpoint. It has been shown that the kinase activity of Mps1 is required for the spindle checkpoint and for kinetochore localization of Bub1, Bub3, Mad1, and Mad2 . We herein demonstrate that MAPK phosphorylates Mps1 at S844 in Xenopus egg extracts. Interestingly, changing S844 to unphosphorylatable alanine (S844A) has no effect on the kinase activity of Mps1, although it abolishes the checkpoint function of Mps1. Biochemical and immunofluorescence studies show that S844A mutation perturbs kinetochore localization of Mps1 and other spindle-checkpoint proteins, whereas the phosphorylation-mimicking S844D mutant restores their functions. Our studies suggest that Mps1 phosphorylation by MAPK at S844 might create a phosphoepitope that allows Mps1 to interact with kinetochores. In addition, our results indicate that active Mps1 must localize to kinetochores in order to execute its checkpoint function.

Phosphorylation- and polo-box-dependent binding of Plk1 to Bub1 is required for the kinetochore localization of Plk1

Polo-like kinase 1 (Plk1) is required for the generation of the tension-sensing 3F3/2 kinetochore epitope and facilitates kinetochore localization of Mad2 and other spindle checkpoint proteins. Here, we investigate the mechanism by which Plk1 itself is recruited to kinetochores. We show that Plk1 binds to budding uninhibited by benzimidazole 1 (Bub1) in mitotic human cells. The Plk1-Bub1 interaction requires the polo-box domain (PBD) of Plk1 and is enhanced by cyclin-dependent kinase 1 (Cdk1)-mediated phosphorylation of Bub1 at T609. The PBD-dependent binding of Plk1 to Bub1 facilitates phosphorylation of Bub1 by Plk1 in vitro. Depletion of Bub1 in HeLa cells by RNA interference (RNAi) diminishes the kinetochore localization of Plk1. Ectopic expression of the wild-type Bub1, but not the Bub1-T609A mutant, in Bub1-RNAi cells restores the kinetochore localization of Plk1. Our results suggest that phosphorylation of Bub1 at T609 by Cdk1 creates a docking site for the PBD of Plk1 and facilitates the kinetochore recruitment of Plk1.

MPS1-dependent mitotic BLM phosphorylation is important for chromosome stability

Spindle assembly checkpoint (SAC) ensures bipolar attachment of chromosomes to the mitotic spindle and is essential for faithful chromosome segregation, thereby preventing chromosome instability (CIN). Genetic evidence suggests a causal link between compromised SAC, CIN, and cancer. Bloom syndrome (BS) is a genetic disorder that predisposes affected individuals to cancer. BS cells exhibit elevated rates of sister chromatid exchange, chromosome breaks, and CIN. The BS gene product, BLM, is a member of the RecQ helicases that are required for maintenance of genome stability. The BLM helicase interacts with proteins involved in DNA replication, recombination, and repair and is required for the repair of stalled-replication forks and in the DNA damage response. Here we present biochemical evidence to suggest a role of BLM phosphorylation during mitosis in maintaining chromosome stability. BLM is associated with the SAC kinase MPS1 and is phosphorylated at S144 in a MPS1-dependent manner. Phosphorylated BLM interacts with polo-like kinase 1, a mitotic kinase that binds to phosphoserine/threonine through its polo-box domain (PBD). Furthermore, BS cells expressing BLM-S144A show normal levels of sister chromatid exchange but fail to maintain the mitotic arrest when SAC is activated and exhibit a broad distribution of chromosome numbers. We propose that MPS1-dependent BLM phosphorylation is important for ensuring accurate chromosome segregation, and its deregulation may contribute to cancer.

Protein kinases involved in mitotic spindle checkpoint regulation

A number of checkpoint controls function to preserve the genome by restraining cell cycle progression until prerequisite events have been properly completed. Chromosome attachment to the mitotic spindle is monitored by the spindle assembly checkpoint. Sister chromatid separation in anaphase is initiated only once all chromosomes have been attached to both poles of the spindle. Premature separation of sister chromatids leads to the loss or gain of chromosomes in daughter cells (aneuploidy), a prevalent form of genetic instability of human cancer. The spindle assembly checkpoint ensures that cells with misaligned chromosomes do not exit mitosis and divide to form aneuploid cells. A number of protein kinases and checkpoint phosphoproteins are required for the function of the spindle assembly checkpoint. This review discusses the recent progress in understanding the role of protein kinases of the mitotic checkpoint complex in the surveillance pathway of the checkpoint.

On the road to cancer: aneuploidy and the mitotic checkpoint

Abnormal chromosome content - also known as aneuploidy - is the most common characteristic of human solid tumours. It has therefore been proposed that aneuploidy contributes to, or even drives, tumour development. The mitotic checkpoint guards against chromosome mis-segregation by delaying cell-cycle progression through mitosis until all chromosomes have successfully made spindle-microtubule attachments. Defects in the mitotic checkpoint generate aneuploidy and might facilitate tumorigenesis, but more severe disabling of checkpoint signalling is a possible anticancer strategy.

Bub1 and the multilayered inhibition of Cdc20-APC/C in mitosis

To ensure the accuracy of chromosome segregation in mitosis, the spindle checkpoint blocks the activity of the anaphase-promoting complex APC/C until all chromosomes are properly bi-orientated on the metaphase spindle. How the checkpoint machinery actually inhibits the APC/C is still unclear. A new paper by Tang and coworkers helps further our understanding of this complex and fundamental process.