The checkpoint protein MAD2 and the mitotic regulator CDC20 form a ternary complex with the anaphase-promoting complex to control anaphase initiation

How Do so Few Control so Many?

The separation of sister chromatids at the metaphase-to-anaphase transition is triggered by a protease called separase that is activated by the destruction of an inhibitory chaperone (securin). This process is mediated by a ubiquitin protein ligase called the anaphase-promoting complex or cyclosome (APC/C), along with a protein called Cdc20. It is vital that separase not be activated before every single chromosome has been aligned on the mitotic spindle. Kinetochores that have not yet attached to microtubules catalyze the sequestration of Cdc20 by an inhibitor called Mad2. Recent experiments shed important insight into how Mad2 molecules bound to centromeres through their association with a protein called Mad1 might be transferred to Cdc20 and thereby inhibit securin's destruction.

Checkpoint Signalling: Mad2 Conformers and Signal Propagation

The spindle checkpoint protein Mad2 has a tendency to form multimers and adopts at least two structural conformations. New work highlights the importance of the Mad2-Mad2 interaction, and suggests how spindle checkpoint signals are propagated away from kinetochores.

DNA damage-induced mitotic catastrophe is mediated by the Chk1-dependent mitotic exit DNA damage checkpoint

Mitotic catastrophe is the response of mammalian cells to mitotic DNA damage. It produces tetraploid cells with a range of different nuclear morphologies from binucleated to multimicronucleated. In response to DNA damage, checkpoints are activated to delay cell cycle progression and to coordinate repair. Cells in different cell cycle phases use different mechanisms to arrest their cell cycle progression. It has remained unclear whether the termination of mitosis in a mitotic catastrophe is regulated by DNA damage checkpoints. Here, we report the presence of a mitotic exit DNA damage checkpoint in mammalian cells. This checkpoint delays mitotic exit and prevents cytokinesis and, thereby, is responsible for mitotic catastrophe. The DNA damage-induced mitotic exit delay correlates with the inhibition of Cdh1 activation and the attenuated degradation of cyclin B1. We demonstrate that the checkpoint is Chk1-dependent.

Anaphase-promoting complex-dependent proteolysis of cell cycle regulators and genomic instability of cancer cells

Genomic instability can be found in most cancer cells. Cell proliferation is under tight control to ensure accurate DNA replication and chromosome segregation. Cyclin-dependent kinases (Cdks) and their activating subunits, the cyclins, are the driving forces of the cell division cycle. Regulation of cyclin oscillation by ubiquitin-dependent proteolysis thereby has a central role in cell cycle regulation. The anaphase-promoting complex (APC) is a specific ubiquitin ligase and is essential for chromosome segregation, exit from mitosis and a stable subsequent G1 phase allowing cell differentiation or accurate DNA replication in the following S phase. The APC is activated by the regulatory subunits Cdc20 (APC(Cdc20)) and Cdh1 (APC(Cdh1)) to target securin, mitotic cyclins and other cell cycle regulatory proteins for proteasomal degradation. This review is focused on the role of APC-dependent proteolysis in cell cycle regulation and how its deregulation may lead to genomic instability of cancer cells.

The yeast rRNA biosynthesis factor Ebp2p is also required for efficient nuclear division

Molecular genetic analysis of the yeast Ebp2 protein has revealed that it is an essential, nucleolar protein that functions in the rRNA biosynthesis pathway. Temperature-sensitive ebp2-1 mutants are defective in the processing of the 27 SA precursor rRNA, and the point substitutions that disrupt this activity cluster towards the central, more highly conserved region of the Ebp2 protein. We report here that other ebp2 mutants exhibit deficiencies associated with defects in chromosome segregation. Yeast cells bearing a 50 amino acid C-terminal truncation allele (ebp2 delta C50) display a slow-growth phenotype and exhibit an increased percentage of cells with the nucleus positioned at the bud neck. The ebp2-1 and ebp2 delta C50 alleles genetically complement each other, and ebp2 delta C50 mutants exhibit nuclear division defects that are distinct from the rRNA biosynthesis-related phenotypes of ebp2-1 mutants. Cytological and FACS analysis of the ebp2 delta C50 deletion mutants indicate that the chromosome segregation related activities of the Ebp2 protein are monitored by Mad2p, a mitotic checkpoint protein. The finding that yeast Ebp2p functions in nuclear division is consistent with the growing body of evidence that supports the role that human EBP2 plays in chromosome segregation.

The cytoskeleton-dependent localization of cdc2/cyclin B in blastomere cortex duringXenopus embryonic cell cycle

In the early development of the frog, Xenopus laevis, blastomeres undergo synchronous divisions at about the 12th cell cycle, followed by asynchronous divisions, which is referred to as mid-blastula transition (MBT). We investigated the distribution of several regulating factors for cell cycles around MBT using immunocytochemistry and confocal fluorescence microscopy. At the 8th cell cycle, most of the cdc2/cyclin B was localized in the cortical cytoplasm throughout the cell cycle, in the centrosomes and the nucleus at interphase and prometaphase, and in the spindles at metaphase and anaphase. Cdc2 was also localized in the chromatins at metaphase and anaphase. Cyclin B1 mRNA was localized in the periphery of the nucleus, but not in the cell cortex. At the 13th cell cycle, the amount of cdc2/cyclin B in the cortical cytoplasm decreased, and the inactive form of cdc2, phosphorylated at tyrosine 15, appeared in the nucleus and the centrosomes at interphase, indicating that the regulation of cdc2 by phosphorylation occurs around MBT. When the blastomeres were treated with nocodazole or latrunculin A at the 8th cell cycle, the amount of cortical cdc2 decreased, but that of cyclin B did not change. The cortical localization of cdc2 is dependent upon both microtubules and microfilaments. Most of the cdc27 was localized in the centrosomes, and in the spindle poles, but no significant difference was observed between the 8th and the 13th cell cycles. It is possible that the cortical MPF activity is regulated by the differential localization between cdc2 and cyclin B.

HTLV-I Tax directly binds the Cdc20-associated anaphase-promoting complex and activates it ahead of schedule

Expression of the human T lymphotropic virus type I (HTLV-I) transactivator/oncoprotein, Tax, leads to faulty mitosis as reflected by chromosome aneuploidy, cytokinesis failure, and formation of micro- and multinucleated cells. Here we show that HTLV-I-transformed T cells progress through S/G(2)/M phases of the cell cycle with a delay. This delay is correlated with a decrease in the levels of cyclin A, cyclin B1, and securin. In tax-expressing cells, the Cdc20-associated anaphase promoting complex (APC(Cdc20)), an E3 ubiquitin ligase that controls metaphase to anaphase transition, becomes active before cellular entry into mitosis as evidenced by premature cyclin B1 polyubiquitination and degradation during S/G(2). Consistent with the notion that Tax activates APC(Cdc20) directly, Tax is found to coimmunoprecipitate with Cdc20 and Cdc27/APC3. The APC(Cdc20) activity prematurely activated by Tax remains sensitive to spindle checkpoint inhibition. Unscheduled activation of APC(Cdc20) by Tax provides an explanation for the mitotic abnormalities in HTLV-I-infected cells and is likely to play an important role in the development of adult T cell leukemia.

Aurora kinase inhibitor ZM447439 blocks chromosome-induced spindle assembly, the completion of chromosome condensation, and the establishment of the spindle integrity checkpoint in Xenopus egg extracts

The Aurora family kinases contribute to accurate progression through several mitotic events. ZM447439 ("ZM"), the first Aurora family kinase inhibitor to be developed and characterized, was previously found to interfere with the mitotic spindle integrity checkpoint and chromosome segregation. Here, we have used extracts of Xenopus eggs, which normally proceed through the early embryonic cell cycles in the absence of functional checkpoints, to distinguish between ZM's effects on the basic cell cycle machinery and its effects on checkpoints. ZM clearly had no effect on either the kinetics or amplitude in the oscillations of activity of several key cell cycle regulators. It did, however, have striking effects on chromosome morphology. In the presence of ZM, chromosome condensation began on schedule but then failed to progress properly; instead, the chromosomes underwent premature decondensation during mid-mitosis. ZM strongly interfered with mitotic spindle assembly by inhibiting the formation of microtubules that are nucleated/stabilized by chromatin. By contrast, ZM had little effect on the assembly of microtubules by centrosomes at the spindle poles. Finally, under conditions where the spindle integrity checkpoint was experimentally induced, ZM blocked the establishment, but not the maintenance, of the checkpoint, at a point upstream of the checkpoint protein Mad2. These results show that Aurora kinase activity is required to ensure the maintenance of condensed chromosomes, the generation of chromosome-induced spindle microtubules, and activation of the spindle integrity checkpoint.

Phosphorylation of Cdc20 by Bub1 provides a catalytic mechanism for APC/C inhibition by the spindle checkpoint

To ensure the fidelity of chromosome segregation, the spindle checkpoint blocks the ubiquitin ligase activity of APC/C(Cdc20) in response to a single chromatid not properly attached to the mitotic spindle. Here we show that HeLa cells depleted for Bub1 by RNA interference are defective in checkpoint signaling. Bub1 directly phosphorylates Cdc20 in vitro and inhibits the ubiquitin ligase activity of APC/C(Cdc20) catalytically. A Cdc20 mutant with all six Bub1 phosphorylation sites removed is refractory to Bub1-mediated phosphorylation and inhibition in vitro. Upon checkpoint activation, Bub1 itself is hyperphosphorylated and its kinase activity toward Cdc20 is stimulated. Ectopic expression of the nonphosphorylatable Cdc20 mutant allows HeLa cells to escape from mitosis in the presence of spindle damage. Therefore, Bub1-mediated phosphorylation of Cdc20 is required for proper checkpoint signaling. We speculate that inhibition of APC/C(Cdc20) by Bub1 in a catalytic fashion may partly account for the exquisite sensitivity of the spindle checkpoint.

Kinetochore targeting of fission yeast Mad and Bub proteins is essential for spindle checkpoint function but not for all chromosome segregation roles of Bub1p

Several lines of evidence suggest that kinetochores are organizing centers for the spindle checkpoint response and the synthesis of a "wait anaphase" signal in cases of incomplete or improper kinetochore-microtubule attachment. Here we characterize Schizosaccharomyces pombe Bub3p and study the recruitment of spindle checkpoint components to kinetochores. We demonstrate by chromatin immunoprecipitation that they all interact with the central domain of centromeres, consistent with their role in monitoring kinetochore-microtubule interactions. Bub1p and Bub3p are dependent upon one another, but independent of the Mad proteins, for their kinetochore localization. We demonstrate a clear role for the highly conserved N-terminal domain of Bub1p in the robust targeting of Bub1p, Bub3p, and Mad3p to kinetochores and show that this is crucial for an efficient checkpoint response. Surprisingly, neither this domain nor kinetochore localization is required for other functions of Bub1p in chromosome segregation.

Transcriptional Abnormality of the hsMAD2 Mitotic Checkpoint Gene Is a Potential Link to Hepatocellular Carcinogenesis

MAD2 is localized to kinetochores of unaligned chromosomes, where it inactivates the anaphase-promoting complex/cyclosome, thus contributing to the production of a diffusible anaphase inhibitory signal. Disruption of MAD2 expression leads to defects in the mitotic checkpoint, chromosome missegregation, and tumorigenesis. However, the mechanism by which deregulation and/or abnormality of hsMAD2 expression remains to be elucidated. Here, we clone and analyze a approximately 0.5 kb fragment upstream of hsMAD2 and show that this fragment acts as a strong promoter. Transcriptional dysfunction of hsMAD2 is frequently observed in hepatocellular carcinoma cells, and down-regulation of hsMAD2 protein expression is correlated with transcriptional silencing of the hsMAD2 promoter by hypermethylation. These results imply a relationship between transcriptional abnormality of this mitotic checkpoint gene and mitotic abnormality in human cancers.

The centrosomal protein RAS association domain family protein 1A (RASSF1A)-binding protein 1 regulates mitotic progression by recruiting RASSF1A to spindle poles

The protein RAS association domain family protein 1A (RASSF1A), which is encoded by a gene that is frequently silenced in many types of sporadic tumor, functions in mitosis as a regulator of the anaphase-promoting complex (APC). With the use of a yeast two-hybrid screen, we identified a human protein, previously designated C19ORF5, that interacts with RASSF1A. This protein, here redesignated RASSF1A-binding protein 1 (RABP1), contains two microtubule-associated protein domains, and its association with RASSF1A was confirmed in mammalian cells by immunoprecipitation and immunofluorescence analyses. RABP1 was found to be localized to the centrosome throughout the cell cycle in a manner dependent on its microtubule-associated protein domains. Ectopic expression of RABP1 induced both stabilization of mitotic cyclins and mitotic arrest at prometaphase in a RASSF1A-dependent manner. It also increased the extent of association between RASSF1A and Cdc20. Conversely depletion of RABP1 by RNA interference prevented both the localization of RASSF1A to the spindle poles as well as its binding to Cdc20, resulting in premature destruction of mitotic cyclins and acceleration of mitotic progression. These findings indicate that RABP1 is required for the recruitment of RASSF1A to the spindle poles and for its inhibition of APC-Cdc20 activity during mitosis.

Elevated level of spindle checkprotein MAD2 correlates with cellular mitotic arrest, but not with aneuploidy and clinicopathological characteristics in gastric cancer

AIM: To study the relevance of spindle assembly checkprotein MAD2 to cellular mitotic status, aneuploidy and other clinicopathological characteristics in gastric cancer.

METHODS: Western blot analyses were performed to analyze the protein levels of MAD2 and cyclin B1 in the tumorous and adjacent nontumorous tissues of 34 gastric cancer patients. Cell cycle distribution and DNA ploidy of cancer tissues were also determined by flow cytometry. Conventional statistical methods were adopted to determine the relevance of abnormal MAD2 level to mitotic status, aneuploidy and clinicopathological parameters.

RESULTS: Out of 34 gastric cancer patients 25 (74%) exhibited elevated MAD2 levels in their tumorous tissues compared with the corresponding nontumorous tissues. Elevation of MAD2 levels significantly correlated with the increased levels of cyclin B1 expression and G₂/M-phase distribution (P = 0.038 and P = 0.033, respectively), but was not relevant to aneuploidy. The gastric cancer patients with elevated MAD2 levels showed a tendency toward better disease-free and overall survival (P > 0.05). However, no association was found between elevated MAD2 levels and patients’ clinicopathological characteristics.

CONCLUSION: Elevation of MAD2 level is present in 74% of gastric cancer patients, and correlates with increased mitotic checkpoint activity. However, elevation of MAD2 level is not associated with patients aneuploidy and any of the clinicopathological characteristics.

Association of MAD2 expression with mitotic checkpoint competence in hepatoma cells

Chromosomal instability (CIN) refers to high rates of chromosomal gains and losses and is a major cause of genomic instability of cells. It is thought that CIN caused by loss of mitotic checkpoint contributes to carcinogenesis. In this study, we evaluated the competence of mitotic checkpoint in hepatoma cells and investigated the cause of mitotic checkpoint defects. We found that 6 (54.5%) of the 11 hepatoma cell lines were defective in mitotic checkpoint control as monitored by mitotic indices and flow-cytometric analysis after treatment with microtubule toxins. Interestingly, all 6 hepatoma cell lines with defective mitotic checkpoint showed significant underexpression of mitotic arrest deficient 2 (MAD2), a key mitotic checkpoint protein. The level of MAD2 underexpression was significantly associated with defective mitotic checkpoint response (p < 0.001). In addition, no mutations were found in the coding sequences of MAD2 in all 11 hepatoma cell lines. Our findings suggest that MAD2 deficiency may cause a mitotic checkpoint defect in hepatoma cells.

Speriolin Is a Novel Spermatogenic Cell-specific Centrosomal Protein Associated with the Seventh WD Motif of Cdc20

The fundamental mechanisms of mitosis are conserved throughout evolution in eukaryotes, including ubiquitin-mediated proteolysis of cell cycle regulators by the anaphase-promoting complex/cyclosome. The spindle checkpoint protein Cdc20 activates the anaphase-promoting complex/cyclosome in a substrate-specific manner. It is present in the cytoplasm and concentrated in the centrosomes throughout the cell cycle, accumulates at the kinetochores in metaphase, and is no longer detected following anaphase. However, it is unknown whether Cdc20 has the same activities and distribution during meiosis in male germ cells. We found that in mice, Cdc20 accumulates in the cytoplasm of pachytene spermatocytes during meiosis I, is distributed throughout spermatocytes undergoing meiotic division, and is present in the cytoplasm of postmeiotic spermatids. Several proteins bind to and regulate the function of Cdc20 during mitosis. We identified speriolin and determined that it is a novel spermatogenic cell-specific Cdc20-binding protein, is present in the cytoplasm, and is concentrated at the centrosomes of spermatocytes and spermatids and that a leucine zipper domain is required to target speriolin to the centrosome. The seven tandem WD motifs of Cdc20 probably fold into a seven-blade beta-propeller structure, and we determined that they are required for speriolin binding and for localization of Cdc20 to the centrosomes and nucleus, suggesting that speriolin might regulate or stabilize the folding of Cdc20 during meiosis in spermatogenic cells.

The Response of Malignant B Lymphocytes to Ionizing Radiation: Cell Cycle Arrest, Apoptosis and Protection against the Cytotoxic Effects of the Mitotic Inhibitor Nocodazole

Ionizing radiation and mitotic inhibitors are used for the treatment of lymphoma. We have studied cell cycle arrest and apoptosis of three human B-lymphocyte cell lines after X irradiation and/or nocodazole treatment. Radiation (4 and 6 Gy) caused arrest in the G(2) phase of the cell cycle as well as in G(1) in Reh cells with an intact TP53 response. Reh cells, but not U698 and Daudi cells with defects in the TP53 pathway, died by apoptosis after exposure to 4 or 6 Gy radiation (>15% apoptotic Reh cells and <5% apoptotic U698/Daudi cells 24 h postirradiation). Lower doses of radiation (0.5 and 1 Gy) caused a transient delay in the G(2) phase of the cell cycle for the three cell lines but did not induce apoptosis (<5% apoptotic cells at 24 h postirradiation). Cells of all three cell lines died by apoptosis after exposure to 1 microg/ml nocodazole, a mitotic blocker that acts by inhibiting the polymerization of tubulin (>25% apoptotic cells after 24 h). When X irradiation with 4 or 6 Gy was performed at the time of addition of nocodazole to U698 and Daudi cells, X rays protected against the apoptosis-inducing effects of the microtubule inhibitor (<5% and 15% apoptotic cells, respectively, 24 h incubation). U698 and Daudi cells apparently have some error(s) in the signaling pathway inducing apoptosis after irradiation, and our results suggest that the arrest in G(2) prevents the cells from entering mitosis and from apoptosis in the presence of microtubule inhibitors. This arrest was overcome by caffeine, which caused U698 cells to enter mitosis (after irradiation) and become apoptotic in the presence of nocodazole (26% apoptotic cells, 24 h incubation). These results may have implications for the design of clinical multimodality protocols involving ionizing radiation for the treatment of cancer.

Crosstalk of the mitotic spindle assembly checkpoint with p53 to prevent polyploidy

Treatment of cells with microtubule inhibitors results in activation of the mitotic spindle assembly checkpoint, leading to mitotic arrest before anaphase. Upon prolonged treatment, however, cells can adapt and exit mitosis aberrantly, resulting in the occurrence of tetraploid cells in G1. Those cells subsequently arrest in postmitotic G1 due to the activation of a p53-dependent G1 checkpoint. Failure of the G1 checkpoint leads to endoreduplication and further polyploidization. Using HCT116 and isogenic p53-deficient or spindle checkpoint compromised derivatives, we show here that not only p53 but also a functional spindle assembly checkpoint is required for postmitotic G1 checkpoint function. During transient mitotic arrest, p53 stabilization and activation is triggered by a pathway independent of ATM/ATR, Chk1 and Chk2. We further show that a prolonged spindle checkpoint-mediated mitotic arrest is required for proper postmitotic G1 checkpoint function. In addition, we demonstrate that polyploid cells are inhibited to re-enter mitosis by an additional checkpoint acting in G2. Thus, during a normal cell cycle, polyploidization and subsequent aneuploidization is prevented by the function of the mitotic spindle checkpoint, a p53-dependent G1 checkpoint and an additional G2 checkpoint.

Dynamics of centromere and kinetochore proteins; implications for checkpoint signaling and silencing

BACKGROUND

The mitotic checkpoint prevents the onset of anaphase before all chromosomes are attached to spindle microtubules. The checkpoint is thought to act by the catalytic generation at unattached kinetochores of a diffusible "wait signal" that prevents anaphase. Mad2 and Cdc20, two candidate proteins for components of a diffusible wait signal, have previously been shown to be recruited to and rapidly released from unattached kinetochores.

RESULTS

Fluorescence recovery after photobleaching demonstrated that Mad1, Bub1, and a portion of Mad2, all essential mitotic-checkpoint components, are stably bound elements of unattached kinetochores (as are structural centromere components such as Centromere protein C [CENP-C]). After microtubule attachment, Mad1 and Mad2 are released from kinetochores and relocalize to spindle poles, whereas Bub1 remains at kinetochores.

CONCLUSIONS

A long residence time at kinetochores identifies Bub1, Mad1, and a portion of Mad2 as part of a catalytic platform that recruits, activates, and releases a diffusible wait signal that is partly composed of the rapidly exchanging portion of Mad2. The release of Mad1 and Mad2, but not Bub1, from kinetochores upon attachment separates the elements of this "catalytic platform" and thereby silences generation of the anaphase inhibitor despite continued rapid cycling of Mad2 at spindle poles.

Heat shock protein 90 regulates the metaphase-anaphase transition in a polo-like kinase-dependent manner

We have shown previously that the molecular chaperone heat shock protein 90 (Hsp90) is required for a proper centrosome function. Indeed, this Hsp90 function seems to be reflected in Polo-like kinase stability. Inhibition of Hsp90 in HeLa cells results in cell cycle arrest either in G2 stage or at the metaphase-anaphase transition. Here, we show that this inhibition leads to inactivation of the anaphase-promoting complex or cyclosome by both dephosphorylation and induction of the spindle assembly checkpoint. Hsp90 inhibition compromises two of the main mitotic kinases, Polo-like kinase 1 (Plk1) and cdc2. Interestingly, this mitotic arrest does not occur in certain tumor cell lines where Hsp90 and Plk1 are not associated. Those cells are able to process mitosis successfully and have an active Plk1 despite Hsp90 inactivation. Therefore, it seems that Hsp90 regulates completion of mitosis depending on its association with Plk1.

Kinetochore localization of spindle checkpoint proteins: who controls whom?

The spindle checkpoint prevents anaphase onset until all the chromosomes have successfully attached to the spindle microtubules. The mechanisms by which unattached kinetochores trigger and transmit a primary signal are poorly understood, although it seems to be dependent at least in part, on the kinetochore localization of the different checkpoint components. By using protein immunodepletion and mRNA translation in Xenopus egg extracts, we have studied the hierarchic sequence and the interdependent network that governs protein recruitment at the kinetochore in the spindle checkpoint pathway. Our results show that the first regulatory step of this cascade is defined by Aurora B/INCENP complex. Aurora B/INCENP controls the activation of a second regulatory level by inducing at the kinetochore the localization of Mps1, Bub1, Bub3, and CENP-E. This localization, in turn, promotes the recruitment to the kinetochore of Mad1/Mad2, Cdc20, and the anaphase promoting complex (APC). Unlike Aurora B/INCENP, Mps1, Bub1, and CENP-E, the downstream checkpoint protein Mad1 does not regulate the kinetochore localization of either Cdc20 or APC. Similarly, Cdc20 and APC do not require each other to be localized at these chromosome structures. Thus, at the last step of the spindle checkpoint cascade, Mad1/Mad2, Cdc20, and APC are recruited at the kinetochores independently from each other.

Haploinsufficiency for tumour suppressor genes: when you don't need to go all the way

Classical tumour suppressor genes are thought to require mutation or loss of both alleles to facilitate tumour progression. However, it has become clear over the last few years that for some genes, haploinsufficiency, which is loss of only one allele, may contribute to carcinogenesis. These effects can either be directly attributable to the reduction in gene dosage or may act in concert with other oncogenic or haploinsufficient events. Here we describe the genes that undergo this phenomenon and discuss possible mechanisms that allow haploinsufficiency to display a phenotype and facilitate the pathogenesis of cancer.

Spindle checkpoint regulates Cdc20p stability in Saccharomyces cerevisiae

The spindle checkpoint arrests cells at the metaphase-to-anaphase transition until all chromosomes have properly attached to the mitotic spindle. Checkpoint proteins Mad2p and Mad3p/BubR1p bind and inhibit Cdc20p, an activator for the anaphase-promoting complex (APC). We find that upon spindle checkpoint activation by microtubule inhibitors benomyl or nocodazole, wild-type Saccharomyces cerevisiae contains less Cdc20p than spindle checkpoint mutants do, whereas their CDC20 mRNA levels are similar. The difference in Cdc20p levels correlates with their difference in the half-lives of Cdc20p, indicating that the spindle checkpoint destabilizes Cdc20p. This process requires the association between Cdc20p and Mad2p, and functional APC, but is independent of the known destruction boxes in Cdc20p and the other APC activator Cdh1p. Importantly, destabilization of Cdc20p is important for the spindle checkpoint, because a modest overexpression of Cdc20p causes benomyl sensitivity and premature Pds1p degradation in cells treated with nocodazole. Our study suggests that the spindle checkpoint reduces Cdc20p to below a certain threshold level to ensure a complete inhibition of Cdc20p before anaphase.

Conformation-specific binding of p31(comet) antagonizes the function of Mad2 in the spindle checkpoint

The spindle checkpoint ensures accurate chromosome segregation by delaying anaphase in response to misaligned sister chromatids during mitosis. Upon checkpoint activation, Mad2 binds directly to Cdc20 and inhibits the anaphase-promoting complex or cyclosome (APC/C). Cdc20 binding triggers a dramatic conformational change of Mad2. Consistent with an earlier report, we show herein that depletion of p31(comet) (formerly known as Cmt2) by RNA interference in HeLa cells causes a delay in mitotic exit following the removal of nocodazole. Purified recombinant p31(comet) protein antagonizes the ability of Mad2 to inhibit APC/C(Cdc20) in vitro and in Xenopus egg extracts. Interestingly, p31(comet) binds selectively to the Cdc20-bound conformation of Mad2. Binding of p31(comet) to Mad2 does not prevent the interaction between Mad2 and Cdc20 in vitro. During checkpoint inactivation in HeLa cells, p31(comet) forms a transient complex with APC/C(Cdc20)-bound Mad2. Purified p31(comet) enhances the activity of APC/C isolated from nocodazole-arrested HeLa cells without disrupting the Mad2-Cdc20 interaction. Therefore, our results suggest that p31(comet) counteracts the function of Mad2 and is required for the silencing of the spindle checkpoint.

Spindle Checkpoint Protein Dynamics at Kinetochores in Living Cells

BACKGROUND

To test current models for how unattached and untense kinetochores prevent Cdc20 activation of the anaphase-promoting complex/cyclosome (APC/C) throughout the spindle and the cytoplasm, we used GFP fusions and live-cell imaging to quantify the abundance and dynamics of spindle checkpoint proteins Mad1, Mad2, Bub1, BubR1, Mps1, and Cdc20 at kinetochores during mitosis in living PtK2 cells.

RESULTS

Unattached kinetochores in prometaphase bound on average only a small fraction (estimated at 500-5000 molecules) of the total cellular pool of each spindle checkpoint protein. Measurements of fluorescence recovery after photobleaching (FRAP) showed that GFP-Cdc20 and GFP-BubR1 exhibit biphasic exponential kinetics at unattached kinetochores, with approximately 50% displaying very fast kinetics (t1/2 of approximately 1-3 s) and approximately 50% displaying slower kinetics similar to the single exponential kinetics of GFP-Mad2 and GFP-Bub3 (t1/2 of 21-23 s). The slower phase of GFP-Cdc20 likely represents complex formation with Mad2 since it was tension insensitive and, unlike the fast phase, it was absent at metaphase kinetochores that lack Mad2 but retain Cdc20 and was absent at unattached prometaphase kinetochores for the Cdc20 derivative GFP-Cdc20delta1-167, which lacks the major Mad2 binding domain but retains kinetochore localization. GFP-Mps1 exhibited single exponential kinetics at unattached kinetochores with a t1/2 of approximately 10 s, whereas most GFP-Mad1 and GFP-Bub1 were much more stable components.

CONCLUSIONS

Our data support catalytic models of checkpoint activation where Mad1 and Bub1 are mainly resident, Mad2 free of Mad1, BubR1 and Bub3 free of Bub1, Cdc20, and Mps1 dynamically exchange as part of the diffuse wait-anaphase signal; and Mad2 interacts with Cdc20 at unattached kinetochores.

The spindle checkpoint, aneuploidy, and cancer

Cancer cells contain abnormal number of chromosomes (aneuploidy), which is a prevalent form of genetic instability in human cancers. Defects in a cell cycle surveillance mechanism called the spindle checkpoint contribute to chromosome instability and aneuploidy. In response to straying chromosomes in mitosis, the spindle checkpoint inhibits the ubiquitin ligase activity of the anaphase-promoting complex or cyclosome (APC/C), thus preventing precocious chromosome segregation and ensuring the accurate partition of the genetic material. We review recent progress toward the understanding of the molecular mechanism of the spindle checkpoint and its role in guarding genome integrity at the chromosome level.

Dependence of paclitaxel sensitivity on a functional spindle assembly checkpoint

Paclitaxel stabilizes microtubules, causing mitotic arrest and activating the spindle assembly checkpoint. We determined whether suppression of the checkpoint genes Mad2 and BubR1 affects paclitaxel resistance and whether overexpression of Mad2 protein in checkpoint-defective cells enhances paclitaxel sensitivity. Suppression of Mad2 and BubR1 in paclitaxel-treated cancer cells abolished checkpoint function, resulting in paclitaxel resistance that correlated with suppression of cyclin-dependent kinase-1 activity. In contrast, overexpression of Mad2 in cells with a checkpoint defect attributable to low Mad2 expression restored checkpoint function, resulting in enhanced paclitaxel sensitivity that correlated with enhanced cyclin-dependent kinase-1 activity. However, overexpression of Mad2 failed to enhance paclitaxel sensitivity via checkpoint activation in Mad2-independent checkpoint-defective and -intact cells. Thus, checkpoint function is required for paclitaxel sensitivity. These findings show that any molecules that could interfere with the spindle assembly checkpoint could generate paclitaxel resistance in any patient.

Mammalian Cdh1/Fzr mediates its own degradation

The Anaphase-Promoting Complex/Cyclosome (APC/C) ubiquitin ligase mediates degradation of cell cycle proteins during mitosis and G1. Cdc20/Fzy and Cdh1/Fzr are substrate-specific APC/C activators. The level of mammalian Cdh1 is high in mitosis, but it is inactive and does not bind the APC/C. We show that when Cdh1 is active in G1 and G0, its levels are considerably lower and almost all of it is APC/C associated. We demonstrate that Cdh1 is subject to APC/C-specific degradation in G1 and G0, and that this degradation depends upon two RXXL-type destruction boxes. We further demonstrate that addition of Cdh1 to Xenopus interphase extracts, which have an inactive APC/C, activates it to degrade Cdh1. These observations indicate that Cdh1 mediates its own degradation by activating the APC/C to degrade itself. Elevated levels of Cdh1 are deleterious for cell cycle progression in various organisms. This auto-regulation of Cdh1 could thus play a role in ensuring that the level of Cdh1 is reduced during G1 and G0, allowing it to be switched off at the correct time.

Different spindle checkpoint proteins monitor microtubule attachment and tension at kinetochores in Drosophila cells

The spindle assembly checkpoint detects errors in kinetochore attachment to the spindle including insufficient microtubule occupancy and absence of tension across bi-oriented kinetochore pairs. Here, we analyse how the kinetochore localization of the Drosophila spindle checkpoint proteins Bub1, Mad2, Bub3 and BubR1, behave in response to alterations in microtubule binding or tension. To analyse the behaviour in the absence of tension, we treated S2 cells with low doses of taxol to disrupt microtubule dynamics and tension, but not kinetochore-microtubule occupancy. Under these conditions, we found that Mad2 and Bub1 do not accumulate at metaphase kinetochores whereas BubR1 does. Consistently, in mono-oriented chromosomes, both kinetochores accumulate BubR1 whereas Bub1 and Mad2 only localize at the unattached kinetochore. To study the effect of tension we analysed the kinetochore localization of spindle checkpoint proteins in relation to tension-sensitive kinetochore phosphorylation recognised by the 3F3/2 antibody. Using detergent-extracted S2 cells as a system in which kinetochore phosphorylation can be easily manipulated, we observed that BubR1 and Bub3 accumulation at kinetochores is dependent on the presence of phosphorylated 3F3/2 epitopes. However, Bub1 and Mad2 localize at kinetochores regardless of the 3F3/2 phosphorylation state. Altogether, our results suggest that spindle checkpoint proteins sense distinct aspects of kinetochore interaction with the spindle, with Mad2 and Bub1 monitoring microtubule occupancy while BubR1 and Bub3 monitor tension across attached kinetochores.

Complete loss of the tumor suppressor MAD2 causes premature cyclin B degradation and mitotic failure in human somatic cells

MAD2 inhibits the anaphase-promoting complex when chromosomes are unattached to the mitotic spindle. It acts as a tumor suppressor gene because MAD2+/-cells enter anaphase early and display chromosome instability, leading to the formation of lung tumors in mice. Complete MAD2 inactivation has not been identified in human tumors, although partial defects are prevalent. By employing RNA interference in human somatic cells, we found that severe reduction of MAD2 protein levels results in mitotic failure and extensive cell death arising from defective spindle formation, incomplete chromosome condensation, and premature mitotic exit leading to multinucleation. Cyclin B is degraded prematurely in the MAD2 short interfering RNA-treated cells but not in MAD2+/- cells, suggesting an explanation for the spindle failure and mitotic catastrophe in the MAD2 knockdown cells. Thus, anaphase-promoting complex substrates exhibit distinct sensitivities in the presence of different MAD2 doses, which in turn determine MAD2's role as either a tumor suppressor or an essential gene.

The Mad2 spindle checkpoint protein has two distinct natively folded states

The spindle checkpoint delays chromosome segregation in response to misaligned sister chromatids during mitosis, thus ensuring the fidelity of chromosome inheritance. Through binding to Cdc20, the Mad2 spindle checkpoint protein inhibits the target of this checkpoint, the ubiquitin protein ligase APC/C(Cdc20). We now show that without cofactor binding or covalent modification Mad2 adopts two distinct folded conformations at equilibrium (termed N1-Mad2 and N2-Mad2). The structure of N2-Mad2 has been determined by NMR spectroscopy. N2-Mad2 is much more potent in APC/C inhibition. Overexpression of a Mad2 mutant that specifically sequesters N2-Mad2 partially blocks checkpoint signaling in living cells. The two Mad2 conformers interconvert slowly in vitro, but interconversion is accelerated by a fragment of Mad1, an upstream regulator of Mad2. Our results suggest that the unusual two-state behavior of Mad2 is critical for spindle checkpoint signaling.

The spindle checkpoint: from normal cell division to tumorigenesis

Faithful chromosome segregation during each cell division is regulated by the spindle checkpoint. This surveillance mechanism monitors kinetochore-microtubule attachment and the integrity of the mitotic apparatus, delaying mitotic exit until all chromosomes are properly aligned at the metaphase plate. Failure of this mechanism can generate gross aneuploidy. Since its discovery, mutations in genes involved in the spindle checkpoint response were predicted to be serious candidates for the chromosomal instability phenotype observed in many tumors. During the last few years, significant advances have been made in understanding the molecular basis of the spindle checkpoint. However, many studies of tumor cell lines and primary cancer isolates have failed to show a direct correlation with mutations in spindle checkpoint components. Nevertheless, it was shown that many tumor cells have an abnormal spindle checkpoint. Therefore, better understanding of the molecular mechanisms involved in regulation of spindle checkpoint response are expected to provide important clues regarding the mechanisms underlying the emergence of neoplasia.

Recycling the cell cycle: cyclins revisited

I discuss advances in the cell cycle in the 21 years since cyclin was discovered. The surprising redundancy amongst the classical cyclins (A, B, and E) and cyclin-dependent kinases (Cdk1 and Cdk2) show that the important differences between these proteins are when and where they are expressed rather than the proteins they phosphorylate. Although the broad principles of the cell cycle oscillator are widely accepted, we are surprisingly ignorant of its detailed mechanism. This is especially true of the anaphase promoting complex (APC), the machine that triggers chromosome segregation and the exit of mitosis by targeting securin and mitotic cyclins for destruction. I discuss how a cyclin/Cdk-based engine could have evolved to assume control of the cell cycle from other, older protein kinases.

The spindle assembly and spindle position checkpoints

The mitotic spindle segregates chromosomes to opposite ends of the cell in preparation for cell division. Chromosome attachment to the spindle is monitored by the spindle assembly checkpoint, and at least in yeast cells, penetration of one spindle pole into the bud is monitored by the spindle position checkpoint. We review the historical origins of these checkpoints and recent progress in understanding their surveillance pathways. We also highlight fascinating but as yet unresolved questions, and examine crosstalk between the checkpoints.

Mitotic degradation of human thymidine kinase 1 is dependent on the anaphase-promoting complex/cyclosome-CDH1-mediated pathway

The expression of human thymidine kinase 1 (hTK1) is highly dependent on the growth states and cell cycle stages in mammalian cells. The amount of hTK1 is significantly increased in the cells during progression to the S and M phases, and becomes barely detectable in the early G(1) phase by a proteolytic control during mitotic exit. This tight regulation is important for providing the correct pool of dTTP for DNA synthesis at the right time in the cell cycle. Here, we investigated the mechanism responsible for mitotic degradation of hTK1. We show that hTK1 is degraded via a ubiquitin-proteasome pathway in mammalian cells and that anaphase-promoting complex/cyclosome (APC/C) activator Cdh1 is not only a necessary but also a rate-limiting factor for mitotic degradation of hTK1. Furthermore, a KEN box sequence located in the C-terminal region of hTK1 is required for its mitotic degradation and interaction capability with Cdh1. By in vitro ubiquitinylation assays, we demonstrated that hTK1 is targeted for degradation by the APC/C-Cdh1 ubiquitin ligase dependent on this KEN box motif. Taken together, we concluded that activation of the APC/C-Cdh1 complex during mitotic exit controls timing of hTK1 destruction, thus effectively minimizing dTTP formation from the salvage pathway in the early G(1) phase of the cell cycle in mammalian cells.

Spindle checkpoint proteins Mad1 and Mad2 are required for cytostatic factor-mediated metaphase arrest

In cells containing disrupted spindles, the spindle assembly checkpoint arrests the cell cycle in metaphase. The budding uninhibited by benzimidazole (Bub) 1, mitotic arrest-deficient (Mad) 1, and Mad2 proteins promote this checkpoint through sustained inhibition of the anaphase-promoting complex/cyclosome. Vertebrate oocytes undergoing meiotic maturation arrest in metaphase of meiosis II due to a cytoplasmic activity termed cytostatic factor (CSF), which appears not to be regulated by spindle dynamics. Here, we show that microinjection of Mad1 or Mad2 protein into early Xenopus laevis embryos causes metaphase arrest like that caused by Mos. Microinjection of antibodies to either Mad1 or Mad2 into maturing oocytes blocks the establishment of CSF arrest in meiosis II, and immunodepletion of either protein blocked the establishment of CSF arrest by Mos in egg extracts. A Mad2 mutant unable to oligomerize (Mad2 R133A) did not cause cell cycle arrest in blastomeres or in egg extracts. Once CSF arrest has been established, maintenance of metaphase arrest requires Mad1, but not Mad2 or Bub1. These results suggest a model in which CSF arrest by Mos is mediated by the Mad1 and Mad2 proteins in a manner distinct from the spindle checkpoint.

Probing the precision of the mitotic clock with a live-cell fluorescent biosensor

Precise timing of mitosis is essential for high-fidelity cell duplication. However, temporal measurements of the mitotic clock have been challenging. Here we present a fluorescent mitosis biosensor that monitors the time between nuclear envelope breakdown (NEB) and re-formation using parallel total internal reflection fluorescence (TIRF) microscopy. By tracking tens to hundreds of mitotic events per experiment, we found that the mitotic clock of unsynchronized rat basophilic leukemia cells has a marked precision with 80% of cells completing mitosis in 32 +/- 6 min. This assay further allowed us to observe delays in mitotic timing at Taxol concentrations 100 times lower than previous minimal effective doses, explaining why Taxol is clinically active at low concentrations. Inactivation of the spindle checkpoint by targeting the regulator Mad2 with RNAi consistently shortened mitosis, providing direct evidence that the internal mitotic timing mechanism is much faster in cells that lack the checkpoint.

The tumour suppressor RASSF1A regulates mitosis by inhibiting the APC-Cdc20 complex

The tumour suppressor gene RASSF1A is frequently silenced in lung cancer and other sporadic tumours as a result of hypermethylation of a CpG island in its promoter. However, the precise mechanism by which RASSF1A functions in cell cycle regulation and tumour suppression has remained unknown. Here we show that RASSF1A regulates the stability of mitotic cyclins and the timing of mitotic progression. RASSF1A localizes to microtubules during interphase and to centrosomes and the spindle during mitosis. The overexpression of RASSF1A induced stabilization of mitotic cyclins and mitotic arrest at prometaphase. RASSF1A interacts with Cdc20, an activator of the anaphase-promoting complex (APC), resulting in the inhibition of APC activity. Although RASSF1A does not contribute to either the Mad2-dependent spindle assembly checkpoint or the function of Emi1 (ref. 1), depletion of RASSF1A by RNA interference accelerated the mitotic cyclin degradation and mitotic progression as a result of premature APC activation. It also caused a cell division defect characterized by centrosome abnormalities and multipolar spindles. These findings implicate RASSF1A in the regulation of both APC-Cdc20 activity and mitotic progression.

p55CDC/hCDC20 mutant induces mitotic catastrophe by inhibiting the MAD2-dependent spindle checkpoint activity in tumor cells

Nondisjunction of chromosomes results in aneuploidy in mammalian cells causing genomic instability. The spindle checkpoint, one of the surveillance systems to maintain genomic stability, prevents missegregation of chromosomes until all the kinetochores are properly attached with bipolar spindles. When this condition is not met, MAD2, a component of the spindle checkpoint complex, associates with p55CDC/hCDC20 to inhibit ubiquitination of substrates by the anaphase-promoting complex (APC). In this study, we have focused on the biological role of the MAD2-binding domain in p55CDC/hCDC20 in the maintenance of genomic stability. Based on previous studies, we constructed a truncated p55CDC/hCDC20 mutant (F2) that harbors only the MAD2-binding domain. Interestingly, we found that in the yeast two-hybrid system, the interaction of F2 and MAD2 was stronger than that of intact p55CDC/hCDC20. We also found that in the presence of the microtubule-disrupting drug, nocodazole, U2OS cells expressing p55CDC/hCDC20 mutants bypassed the mitotic arrest and showed apoptotic morphologies, whereas cells harboring vector alone arrested at metaphase. In particular, the apoptotic phenomena were dramatically enhanced in the F2-expressing cells. These mitotic catastrophes also occurred in cells treated with other microtubule disrupting agents, such as taxol and vinblastine. In addition, the mutant cells exhibited chromosomal missegregation during mitosis, even in the absence of nocodazole. Taken together, these results suggest that agents blocking the spindle checkpoint response may induce tumor cells to become more sensitive to spindle poison drugs, providing a powerful tool to improve chemotherapy.

Activation of m-calpain is required for chromosome alignment on the metaphase plate during mitosis

Calpains form a superfamily of Ca(2+)-dependent intracellular cysteine proteases with various isoforms. Two isoforms, micro- and m-calpains, are ubiquitously expressed and known as conventional calpains. It has been previously shown that the mammalian calpains are activated during mitosis by transient increases in cytosolic Ca(2+) concentration. However, it is still unknown whether the activation of calpains contributes to particular events in mitosis. With the use of RNA interference (RNAi), we investigated the roles of calpains in mitosis. Cells reduced the levels of m-calpain, but not mu-calpain, arrested at prometaphase and failed to align their chromosomes at the spindle equator. Specific peptidyl calpain inhibitors also induced aberrant mitosis with chromosome misalignment. Although both m-calpain RNAi and calpain inhibitors affected neither the separation of centrosomes nor the assembly of bipolar spindles, Mad2 was detected on the kinetochores of the misaligned chromosomes, indicating that the prometaphase arrest induced by calpain inhibition is due to activation of the spindle assembly checkpoint. Furthermore, when calpain activity was inhibited in cells having monopolar spindles, chromosomes were clustered adjacent to the centrosome, suggesting that calpain activity is involved in a polar ejection force for metaphase alignment of chromosomes. Based on these findings, we propose that activation of m-calpain during mitosis is required for cells to establish the chromosome alignment by regulating some molecules that generate polar ejection force.

The spindle checkpoint requires cyclin-dependent kinase activity

The spindle checkpoint prevents anaphase onset until completion of mitotic spindle assembly by restraining activation of the ubiquitin ligase anaphase-promoting complex/cyclosome-Cdc20 (APC/CCdc20). We show that the spindle checkpoint requires mitotic cyclin-dependent kinase (cdk) activity. Inhibiting cdk activity overrides checkpoint-dependent arrest in Xenopus egg extracts and human cells. Following inhibition, the interaction between APC/C and Cdc20 transiently increases while the inhibitory checkpoint protein Mad2 dissociates from Cdc20. Cdk inhibition also overcomes Mad2-induced mitotic arrest. In addition, in vitro cdk1-phosphorylated Cdc20 interacts with Mad2 rather than APC/ C. Thus, cdk activity is required to restrain APC/CCdc20 activation until completion of spindle assembly.

A novel yeast mutant that is defective in regulation of the Anaphase-Promoting Complex by the spindle damage checkpoint

The accurate segregation of sister chromatids at the metaphase to anaphase transition in Saccharomyces cerevisiae is regulated by the activity of the anaphase-promoting complex or cyclosome (APC/C). In the event of spindle damage or monopolar spindle attachment, the spindle checkpoint is activated and inhibits APC/C activity towards the anaphase inhibitor Pds1p, resulting in a cell cycle arrest at metaphase. We have identified a novel allele of a gene for an APC/C subunit, cdc16-183, in S. cerevisiae. cdc16-183 mutants arrest at metaphase at 37 degrees C, and are supersensitive to the spindle-damaging agent nocodazole, which activates the spindle checkpoint, at lower temperatures. This supersensitivity to nocodazole cannot be explained by impairment of the spindle checkpoint pathway, as cells respond normally to spindle damage with a stable metaphase arrest and high levels of Pds1p. Despite showing metaphase arrest at G2/M at 37 degrees C, cdc16-183 mutants are able to perform tested G1 functions normally at this temperature. This is the first demonstration that a mutation in a core APC/C subunit can result in a MAD2-dependent arrest at the restrictive temperature. Our results suggest that the cdc16-183 mutant may have a novel APC/C defect(s) that mimics or activates the spindle checkpoint pathway.

Cks1 is degraded via the ubiquitin-proteasome pathway in a cell cycle-dependent manner

BACKGROUND

Recent work has demonstrated the role of cdc kinase subunit 1 (Cks1) in the ubiquitin-proteasome dependent degradation of CDK inhibitor p27Kip1 protein as an essential cofactor for SCFSkp2 ubiquitin ligase. Although over-expression of Cks1 protein as well as it of Skp2 might be associated with tumour progression via p27Kip1 protein degradation, it is unknown how the cellular level of Cks1 is regulated.

RESULTS

Here we show that Cks1 protein is degraded via the ubiquitin-proteasome pathway. Degradation of Cks1 protein was markedly inhibited by proteasome inhibitors. In addition, Cks1 protein was modified with polyubiquitin chains both in vivo and in vitro. Furthermore, we found that degradation of Cks1 protein via the ubiquitin-proteasome pathway was facilitated in M phase during the cell cycle.

CONCLUSION

These observations suggest that the level of expression of Cks1 protein is regulated at not only the transcriptional level but also the post-translational level via the ubiquitin-proteasome pathway in a cell-cycle-dependent manner.

Mutations in mákos, a Drosophila gene encoding the Cdc27 subunit of the anaphase promoting complex, enhance centrosomal defects in polo and are suppressed by mutations in twins/aar, which encodes a regulatory subunit of PP2A

The gene mákos (mks) encodes the Drosophila counterpart of the Cdc27 subunit of the anaphase promoting complex (APC/C). Neuroblasts from third-larval-instar mks mutants arrest mitosis in a metaphase-like state but show some separation of sister chromatids. In contrast to metaphase-checkpoint-arrested cells, such mutant neuroblasts contain elevated levels not only of cyclin B but also of cyclin A. Mutations in mks enhance the reduced ability of hypomorphic polo mutant alleles to recruit and/or maintain the centrosomal antigens gamma-tubulin and CP190 at the spindle poles. Absence of the MPM2 epitope from the spindle poles in such double mutants suggests Polo kinase is not fully activated at this location. Thus, it appears that spindle pole functions of Polo kinase require the degradation of early mitotic targets of the APC/C, such as cyclin A, or other specific proteins. The metaphase-like arrest of mks mutants cannot be overcome by mutations in the spindle integrity checkpoint gene bub1, confirming this surveillance pathway has to operate through the APC/C. However, mutations in the twins/aar gene, which encodes the 55kDa regulatory subunit of PP2A, do suppress the mks metaphase arrest and so permit an alternative means of initiating anaphase. Thus the APC/C might normally be required to inactivate wild-type twins/aar gene product.

Degradation of cyclin B is required for the onset of anaphase in Mammalian cells

Recently, it has been shown that cyclin B1 was degraded mainly before the onset of anaphase in mammalian cells. When a nondegradable form of cyclin B1 was introduced into cells, the metaphase-anaphase transition was blocked. This blockage was not due to a failure in activating anaphase-promoting complex, nor was it due to a failure of degradation of securin. To resolve the question of whether this blockage by overexpressing the nondegradable form of cyclin B1 is physiologically relevant or not, we developed a novel method to estimate the relative protein level of the overexpressed cyclin B1 mutant within an individual cell. We found that a low level of nondegradable cyclin B1 (less than 30% of the endogenous cyclin B1) was sufficient to block the metaphase-anaphase transition, implying that the blockage of anaphase onset by the nondegradable cyclin B1 was not due to an artifact of excessive M-phase-promoting factor activity. This result suggests that, in mammalian cells, the majority of cyclin B1 must be destroyed before the cell can enter anaphase.

Metaphase I Arrest upon Activation of the Mad2-Dependent Spindle Checkpoint in Mouse Oocytes

BACKGROUND

The importance of mitotic spindle checkpoint control has been well established during somatic cell divisions. The metaphase-to-anaphase transition takes place only when all sister chromatids have been properly attached to the bipolar spindle and are aligned at the metaphase plate. Failure of this checkpoint may lead to unequal separation of sister chromatids. On the contrary, the existence of such a checkpoint during the first meiotic division in mammalian oocytes when homologous chromosomes are segregated has remained controversial.

RESULTS

Here, we show that mouse oocytes respond to spindle damage by a transient and reversible cell cycle arrest in metaphase I with high Maturation Promoting Factor (MPF) activity. Furthermore, the mitotic checkpoint protein Mad2 is present throughout meiotic maturation and is recruited to unattached kinetochores. Overexpression of Mad2 in meiosis I leads to a cell cycle arrest in metaphase I. Expression of a dominant-negative Mad2 protein interferes with proper spindle checkpoint arrest.

CONCLUSIONS

Errors in meiosis I cause missegregation of chromosomes and can result in the generation of aneuploid embryos with severe birth defects. In human oocytes, failures in spindle checkpoint control may be responsible for the generation of trisomies (e.g., Down Syndrome) due to chromosome missegregation in meiosis I. Up to now, the mechanisms ensuring correct separation of chromosomes in meiosis I remained unknown. Our study shows for the first time that a functional Mad2-dependent spindle checkpoint exists during the first meiotic division in mammalian oocytes.

Centromere-associated protein-E is essential for the mammalian mitotic checkpoint to prevent aneuploidy due to single chromosome loss

Centromere-associated protein-E (CENP-E) is an essential mitotic kinesin that is required for efficient, stable microtubule capture at kinetochores. It also directly binds to BubR1, a kinetochore-associated kinase implicated in the mitotic checkpoint, the major cell cycle control pathway in which unattached kinetochores prevent anaphase onset. Here, we show that single unattached kinetochores depleted of CENP-E cannot block entry into anaphase, resulting in aneuploidy in 25% of divisions in primary mouse fibroblasts in vitro and in 95% of regenerating hepatocytes in vivo. Without CENP-E, diminished levels of BubR1 are recruited to kinetochores and BubR1 kinase activity remains at basal levels. CENP-E binds to and directly stimulates the kinase activity of purified BubR1 in vitro. Thus, CENP-E is required for enhancing recruitment of its binding partner BubR1 to each unattached kinetochore and for stimulating BubR1 kinase activity, implicating it as an essential amplifier of a basal mitotic checkpoint signal.

Adenovirus ADP protein (E3-11.6K), which is required for efficient cell lysis and virus release, interacts with human MAD2B

The human subgroup C adenovirus (Ad) protein named adenovirus death protein (ADP) (previously named E3-11.6K) is synthesized at very late stages of infection when it mediates efficient lysis of cells and release of adenovirus to infect other cells. ADP is an integral membrane N-linked, O-linked palmitoylated glycoprotein of 101 amino acids (aa) that localizes to the nuclear membrane, endoplasmic reticulum (ER), and Golgi. It has a single membrane spanning region (roughly aa 40-60) and is oriented with aa 1-40 in the lumen and aa 61-101 in the nucleoplasm and cytoplasm. Using aa 61-101 of Ad2 ADP as bait in a yeast two-hybrid screen, we isolated a cDNA for a 211-aa protein that initially was not in the database but has now been published by others with the names human MAD2B, MAD2L2, and REV7. ADP binds strongly to human MAD2B not only in yeast but also in GST pull-down experiments and in coimmunoprecipitations of ADP and MAD2B synthesized in vitro or in vivo. ADP mutants with deletions throughout the bait region do not interact with human MAD2B, whereas a Pro69Pro70 to Ala69Ala70 mutant in the "basic-proline" domain of ADP does interact. Northern blot analyses indicate that human MAD2B is expressed ubiquitously. Human MAD2B is about 25% identical to human MAD2, a spindle assembly checkpoint protein. Two human A549 cell lines were made that constitutively overexpress MAD2B. Wild-type adenovirus lyses these cells significantly more slowly than it lyses parental A549 cells, raising the possibility that ADP and MAD2B act in opposition and suggesting that the ADP-MAD2B interaction is biologically relevant.

Phosphorylation of Cdc20 is required for its inhibition by the spindle checkpoint

The spindle checkpoint delays anaphase until all chromosomes are properly attached to spindle microtubules. When the spindle checkpoint is activated at unattached kinetochores, the checkpoint proteins BubR1, Bub3 and Mad2 bind and inhibit Cdc20, an activator of the anaphase-promoting complex (APC). Here, we show that Xenopus laevis Cdc20 is phosphorylated at Ser 50, Thr 64, Thr 68 and Thr 79 during mitosis and that mitogen-activated protein kinase (MAPK) contributes to the phosphorylation at Thr 64 or Thr 68. Cdc20 mutants that are phosphorylation-deficient are able to activate the APC in X. laevis egg extracts. However, Cdc20 mutants in which any of the four phosphorylation sites were altered to Ala or Val failed to respond to the spindle checkpoint signal, owing to their reduced affinity for the spindle checkpoint proteins. This study demonstrates that the spindle checkpoint stops anaphase by inhibiting fully-phosphorylated Cdc20. Our results also have implications for the spindle checkpoint silencing mechanism.