Mammalian p55CDC mediates association of the spindle checkpoint protein Mad2 with the cyclosome/anaphase-promoting complex, and is involved in regulating anaphase onset and late mitotic events

The anaphase promoting complex/cyclosome: a machine designed to destroy

The anaphase promoting complex/cyclosome (APC/C) is a ubiquitin ligase that has essential functions in and outside the eukaryotic cell cycle. It is the most complex molecular machine that is known to catalyse ubiquitylation reactions, and it contains more than a dozen subunits that assemble into a large 1.5-MDa complex. Recent discoveries have revealed an unexpected multitude of mechanisms that control APC/C activity, and have provided a first insight into how this unusual ubiquitin ligase recognizes its substrates.

Tumor suppressor candidate TSSC5 is regulated by UbcH6 and a novel ubiquitin ligase RING105

The region of human chromosome 11p15.5 is linked with Beckwith-Wiedemann syndrome that is associated with susceptibility to Wilms' tumor, rhabdomyosarcoma and hepatoblastoma. TSSC5 (tumor-suppressing subchromosomal transferable fragment cDNA; also known as ORCTL2/IMPT1/BWR1A/SLC22A1L) is located in the region. The expression of TSSC5 and other genes in the region is regulated through paternal imprinting. Mutations and/or reduced expression of TSSC5 have been found in certain tumors. TSSC5 encodes an efflux transporter-like protein with 10 transmembrane domains, whose regulation may affect drug sensitivity, cellular metabolism and growth. Here, we present evidences indicating that RING105, a novel conserved RING-finger protein with a PA (protease-associated) domain and a PEST sequence, is a ubiquitin ligase for TSSC5 that can function in concert with the ubiquitin-conjugating enzyme UbcH6. The polyubiquitin target site on TSSC5 was mapped to a region in the 6th hydrophilic loop. Ectopic expression of RING105 in HeLa cells caused an accumulation of cells during G1 that was not observed with the expression of a form of RING105 in which a residue within the RING finger was mutated to inactivate its ligase activity. UbcH6-RING105 may define a novel ubiquitin-proteasome pathway that targets TSSC5 in mammalian cells.

Inhibition of TRIP1/S8/hSug1, a component of the human 19S proteasome, enhances mitotic apoptosis induced by spindle poisons

Mitotic spindle poisons (e.g., Taxol and vinblastine), used as chemotherapy drugs, inhibit mitotic spindle function, activate the mitotic spindle checkpoint, arrest cells in mitosis, and then cause cell death by mechanisms that are poorly understood. By expression cloning, we identified a truncated version of human TRIP1 (also known as S8, hSug1), an AAA (ATPases associated with diverse cellular activities) family ATPase subunit of the 19S proteasome regulatory complex, as an enhancer of spindle poison-mediated apoptosis. Stable expression of the truncated TRIP1/S8/hSug1 in HeLa cells [OP-TRIP1(88-406)] resulted in a decrease of measurable cellular proteasome activity, indicating that OP-TRIP1(88-406) had a dominant-negative effect on proteasome function. OP-TRIP1(88-406) revealed an increased apoptotic response after treatment with spindle poisons or with proteasome inhibitors. The increased apoptosis coincided with a significant decrease in expression of BubR1, a kinase required for activation and maintenance of the mitotic spindle checkpoint in response to treatment with spindle poisons. Small interfering RNA (siRNA)-mediated knockdown of TRIP1/S8/hSug1 resulted in a reduction of general proteasome activity and an increase in mitotic index. The siRNA treatment also caused increased cell death after spindle poison treatment. These results indicate that inhibition of TRIP1/S8/hSug1 function by expression of a truncated version of the protein or by siRNA-mediated suppression enhances cell death in response to spindle poison treatment. Current proteasome inhibitor drugs in trial as anticancer agents target elements of the 20S catalytic subcomplex. Our results suggest that targeting the ATPase subunits in 19S regulatory complex in the proteasome may enhance the antitumor effects of spindle poisons.

Determinants of conformational dimerization of Mad2 and its inhibition by p31comet

The spindle assembly checkpoint (SAC) monitors chromosome attachment to spindle microtubules. SAC proteins operate at kinetochores, scaffolds mediating chromosome-microtubule attachment. The ubiquitous SAC constituents Mad1 and Mad2 are recruited to kinetochores in prometaphase. Mad2 sequesters Cdc20 to prevent its ability to mediate anaphase onset. Its function is counteracted by p31comet (formerly CMT2). Upon binding Cdc20, Mad2 changes its conformation from O-Mad2 (Open) to C-Mad2 (Closed). A Mad1-bound C-Mad2 template, to which O-Mad2 binds prior to being converted into Cdc20-bound C-Mad2, assists this process. A molecular understanding of this prion-like property of Mad2 is missing. We characterized the molecular determinants of the O-Mad2:C-Mad2 conformational dimer and derived a rationalization of the binding interface in terms of symmetric and asymmetric components. Mutation of individual interface residues abrogates the SAC in Saccharomyces cerevisiae. NMR chemical shift perturbations indicate that O-Mad2 undergoes a major conformational rearrangement upon binding C-Mad2, suggesting that dimerization facilitates the structural conversion of O-Mad2 required to bind Cdc20. We also show that the negative effects of p31comet on the SAC are based on its competition with O-Mad2 for C-Mad2 binding.

The ubiquitin-proteasome pathway in cell cycle control

Ubiquitin-mediated proteolysis is one of the key mechanisms underlying cell cycle control. The removal of barriers posed by accumulation of negative regulators, as well as the clearance of proteins when they are no longer needed or deleterious, are carried out via the ubiquitin-proteasome system. Ubiquitin conjugating enzymes and protein-ubiquitin ligases collaborate to mark proteins destined for degradation by the proteasome by covalent attachment of multi-ubiquitin chains. Most regulated proteolysis during the cell cycle can be attributed to two families of protein-ubiquitin ligases. The anaphase promoting complex/cyclosome (APC/C) is activated during mitosis and G1 where it is responsible for eliminating proteins that impede mitotic progression and that would have deleterious consequences if allowed to accumulate during G1. SCF (Skp1/Culin/F-box protein) protein-ubiquitin ligases ubiquitylate proteins that are marked by phosphorylation at specific sequences known as phosphodegrons. Targeting of proteins for destruction by phosphorylation provides a mechanism for linking cell cycle regulation to internal and external signaling pathways via regulated protein kinase activities.

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.

Essential tension and constructive destruction: the spindle checkpoint and its regulatory links with mitotic exit

Replicated genetic material must be partitioned equally between daughter cells during cell division. The precision with which this is accomplished depends critically on the proper functioning of the mitotic spindle. The assembly, orientation and attachment of the spindle to the kinetochores are therefore constantly monitored by a surveillance mechanism termed the SCP (spindle checkpoint). In the event of malfunction, the SCP not only prevents chromosome segregation, but also inhibits subsequent mitotic events, such as cyclin destruction (mitotic exit) and cytokinesis. This concerted action helps to maintain temporal co-ordination among mitotic events. It appears that the SCP is primarily activated by either a lack of occupancy or the absence of tension at kinetochores. Once triggered, the inhibitory circuit bifurcates, where one branch restrains the sister chromatid separation by inhibiting the E3 ligase APC(Cdc20) (anaphase-promoting complex activated by Cdc20) and the other impinges on the MEN (mitotic exit network). A large body of investigations has now led to the identification of the control elements, their targets and the functional coupling among them. Here we review the emerging regulatory network and discuss the remaining gaps in our understanding of this effective mechanochemical control system.

Explaining the oligomerization properties of the spindle assembly checkpoint protein Mad2

Mad2 is an essential component of the spindle assembly checkpoint (SAC), a molecular device designed to coordinate anaphase onset with the completion of chromosome attachment to the spindle. Capture of chromosome by microtubules occur on protein scaffolds known as kinetochores. The SAC proteins are recruited to kinetochores in prometaphase where they generate a signal that halts anaphase until all sister chromatid pairs are bipolarly oriented. Mad2 is a subunit of the mitotic checkpoint complex, which is regarded as the effector of the spindle checkpoint. Its function is the sequestration of Cdc20, a protein required for progression into anaphase. The function of Mad2 in the checkpoint correlates with a dramatic conformational rearrangement of the Mad2 protein. Mad2 adopts a closed conformation (C-Mad2) when bound to Cdc20, and an open conformation (O-Mad2) when unbound to this ligand. Checkpoint activation promotes the conversion of O-Mad2 to Cdc20-bound C-Mad2. We show that this conversion requires a C-Mad2 template and we identify this in Mad1-bound Mad2. In our proposition, Mad1-bound C-Mad2 recruits O-Mad2 to kinetochores, stimulating Cdc20 capture, implying that O-Mad2 and C-Mad2 form dimers. We discuss Mad2 oligomerization and link our discoveries to previous observations related to Mad2 oligomerization.

Polo-like kinase 1 creates the tension-sensing 3F3/2 phosphoepitope and modulates the association of spindle-checkpoint proteins at kinetochores

BACKGROUND

In mitosis, a mechanochemical system recognizes tension that is generated by bipolar microtubule attachment to sister kinetochores. This is translated into multiple outputs including the stabilization of microtubule attachments, changes in kinetochore protein dynamics, and the silencing of the spindle checkpoint. How kinetochores sense tension and translate this into various signals represent critical unanswered questions. The kinetochores of chromosomes not under tension are specifically phosphorylated at an epitope recognized by the 3F3/2 monoclonal antibody. Determining the kinase that generates the 3F3/2 phosphoepitope at kinetochores should reveal an important component of this system that regulates mitotic progression.

RESULTS

We demonstrate that Polo-like kinase 1 (Plk1) creates the 3F3/2 phosphoepitope on mitotic kinetochores. In a permeabilized in vitro cell system, the depletion of Xenopus Plk1 from M phase extract leads to the loss of 3F3/2 kinase activity. Purified recombinant Plk1 is sufficient to generate the 3F3/2 phosphoepitope in this system. Using siRNA, we show that the reduction of Plk1 protein levels significantly diminishes 3F3/2 phosphoepitope expression at kinetochores. The consensus phosphorylation sites of Plk1 show strong similarity to the 3F3/2 phosphoepitope sequence determined by phosphopeptide mapping. The inhibition of Plk1 by siRNA alters the normal kinetochore association of Mad2, Cenp-E, Hec1/Ndc80, Spc24, and Cdc20 and induces a spindle-checkpoint-mediated mitotic arrest.

CONCLUSIONS

Plk1 generates the 3F3/2 phosphoepitope at kinetochores that are not under tension and contributes to the normal kinetochore association of several key proteins important in checkpoint signaling. Mechanical tension regulates Plk1 accumulation at kinetochores and possibly its kinase activity.

The anaphase-promoting complex: a key factor in the regulation of cell cycle

Events controlling cell division are governed by the degradation of different regulatory proteins by the ubiquitin-dependent pathway. In this pathway, the attachment of a polyubiquitin chain to a substrate by an ubiquitin-ligase targets this substrate for degradation by the 26S proteasome. Two different ubiquitin ligases play an important role in the cell cycle: the SCF (Skp1/Cullin/F-box) and the anaphase-promoting complex (APC). In this review, we describe the present knowledge about the APC. We pay particular attention to the latest results concerning APC structure, APC regulation and substrate recognition, and we discuss the implication of these findings in the understanding the APC function.

Changes in the localization of the Saccharomyces cerevisiae anaphase-promoting complex upon microtubule depolymerization and spindle checkpoint activation

The anaphase-promoting complex/cyclosome (APC/C) is an E3 ubiquitin ligase in the ubiquitin-mediated proteolysis pathway (UMP). To understand how the APC/C was targeted to its substrates, we performed a detailed analysis of one of the APC/C components, Cdc23p. In live cells, Cdc23-GFP localized to punctate nuclear spots surrounded by homogenous nuclear signal throughout the cell cycle. These punctate spots colocalized with two outer kinetochore proteins, Slk19p and Okp1p, but not with the spindle pole body protein, Spc42p. In late anaphase, the Cdc23-GFP was also visualized along the length of the mitotic spindle. We hypothesized that spindle checkpoint activation may affect the APC/C nuclear spot localization. Localization of Cdc23-GFP was disrupted upon nocodazole treatment in the kinetochore mutant okp1-5 and in the cdc20-1 mutant. Cdc23-GFP nuclear spot localization was not affected in the ndc10-1 mutant, which is defective in spindle checkpoint function. Additional studies using a mad2Delta strain revealed a microtubule dependency of Cdc23-GFP spot localization, whether or not the checkpoint response was activated. On the basis of these data, we conclude that Cdc23p localization was dependent on microtubules and was affected by specific types of kinetochore disruption.

Centromere DNA, proteins and kinetochore assembly in vertebrate cells

The centromere is a specialized region of the chromosome that is essential for faithful chromosome segregation during mitosis and meiosis in eukaryotic cells. It is the site at which the kinetochore, the functional nucleoprotein complex responsible for microtubule binding and chromosome movement, is assembled through complex molecular mechanisms. Herein, I review recent advances in our understanding of centromeric DNAs as sites for kinetochore assembly and the mechanisms underlying kinetochore assembly in vertebrate cells.

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.

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.

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.

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.

Survivin dynamics increases at centromeres during G2/M phase transition and is regulated by microtubule-attachment and Aurora B kinase activity

The inhibitor of apoptosis protein survivin is implicated in two key biological events: in the control of cell proliferation and in the regulation of cell lifespan. Although the details of mitotic roles of survivin are unclear, the protein appears to modulate microtubule function and might participate in regulating the spindle checkpoint. Survivin physically associates with Aurora B, a serine-threonine kinase involved in microtubule attachment to centromeres and regulation of chromosome segregation. Here we have examined the dynamics and localization of a survivin-GFP chimera using high-resolution fluorescence microscopy and photobleaching. Survivin forms a bi-partite structure at the inner centromere that undergoes significant stretching during mitosis. Photobleaching experiments revealed marked changes in rates of survivin turnover at centromeres. These were regulated by stage of the cell cycle, microtubule attachment, and Aurora B kinase activity. We hypothesize that changes in the turnover of survivin at centromeres influence the stability of kinetochore-microtubule attachment and signaling of the spindle checkpoint.

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.

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.

Assembly of kinetochores in vertebrate cells

The centromere is a specialized region of each chromosome that is essential for faithful chromosome segregation during mitosis and meiosis in eukaryotic cells. Centromeres are the site at which kinetochores are formed. The kinetochore is responsible for microtubule binding and chromosome movement. In this review, I will focus on recent advances in our understanding of centromere DNAs as sites for kinetochore assembly and the mechanism underlying kinetochore assembly in vertebrate cells.

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.

Mitotic regulation of the human anaphase-promoting complex by phosphorylation

The anaphase-promoting complex (APC) or cyclosome is a ubiquitin ligase that initiates anaphase and mitotic exit. APC activation is thought to depend on APC phosphorylation and Cdc20 binding. We have identified 43 phospho-sites on APC of which at least 34 are mitosis specific. Of these, 32 sites are clustered in parts of Apc1 and the tetratricopeptide repeat (TPR) subunits Cdc27, Cdc16, Cdc23 and Apc7. In vitro, at least 15 of the mitotic phospho-sites can be generated by cyclin-dependent kinase 1 (Cdk1), and 3 by Polo-like kinase 1 (Plk1). APC phosphorylation by Cdk1, but not by Plk1, is sufficient for increased Cdc20 binding and APC activation. Immunofluorescence microscopy using phospho-antibodies indicates that APC phosphorylation is initiated in prophase during nuclear uptake of cyclin B1. In prometaphase phospho-APC accumulates on centrosomes where cyclin B ubiquitination is initiated, appears throughout the cytosol and disappears during mitotic exit. Plk1 depletion neither prevents APC phosphorylation nor cyclin A destruction in vivo. These observations imply that APC activation is initiated by Cdk1 already in the nuclei of late prophase cells.

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.

Overexpressed pituitary tumor-transforming gene causes aneuploidy in live human cells

The mammalian securin, pituitary tumor-transforming gene (PTTG), is overexpressed in several tumors and transforms cells in vitro and in vivo. To test the hypothesis that PTTG overexpression causes aneuploidy, enhanced green fluorescent protein (EGFP)-tagged PTTG (PTTG-EGFP) was expressed in human H1299 cancer cells (with undetectable endogenous PTTG expression) and mitosis of individual live cells observed. Untransfected cells and cells expressing EGFP alone exhibited appropriate mitosis. PTTG-EGFP markedly prolonged prophase and metaphase, indicating that PTTG blocks progression of mitosis to anaphase. In cells that underwent apparently normal mitosis (35 of 65 cells), PTTG-EGFP was degraded about 1 min before anaphase onset. Cells that failed to degrade PTTG-EGFP exhibited asymmetrical cytokinesis without chromosome segregation (18 of 65 cells) or chromosome decondensation without cytokinesis (9 of 65 cells), resulting in appearance of a macronucleus. Fifty-one of 55 cells expressing a nondegradable mutant PTTG exhibited asymmetrical cytokinesis without chromosome segregation, and some (4 of 55) decondensed chromosomes, both resulting in macronuclear formation. During this abnormal cytokinesis, all chromosomes and spindles and both centrosomes moved to one daughter cell, suggesting potential chaos in the subsequent mitosis. In conclusion, failure of PTTG degradation or enhanced PTTG accumulation, as a consequence of overexpression, inhibits mitosis progression and chromosome segregation but does not directly affect cytokinesis, resulting in aneuploidy. These results demonstrate that PTTG induces aneuploidy in single, live, human cancer cells.

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.

Dynamic behavior of Nuf2-Hec1 complex that localizes to the centrosome and centromere and is essential for mitotic progression in vertebrate cells

Nuf2 and Hec1 are evolutionarily conserved centromere proteins. To clarify the functions of these proteins in vertebrate cells, we characterized them in chicken DT40 cells. We generated GFP fusion constructs of Nuf2 and Hec1 to examine in detail the localization of these proteins during the cell cycle. We found that Nuf2 is associated with Hec1 throughout the cell cycle and that this complex is localized to the centrosomes during G1 and S phases and then moves through the nuclear membrane to the centromere in G2 phase. During mitosis, this complex is localized to the centromere. We also created conditional loss-of-function mutants of Nuf2 and Hec1. In both mutants, the cell cycle arrested at prometaphase, suggesting that the Nuf2-Hec1 complex is essential for mitotic progression. The inner centromere proteins CENP-A, -C, and -H and checkpoint protein BubR1 were localized to chromosomes in the mutant cells arrested at prometaphase, but Mad2 localization was abolished. Furthermore, photobleaching experiments revealed that the Nuf2-Hec1 complex is stably associated with the centromere and that interaction of this complex with the centrosome is dynamic.

Activating and silencing the mitotic checkpoint through CENP-E-dependent activation/inactivation of BubR1

The mitotic checkpoint prevents advance to anaphase prior to successful attachment of every centromere/kinetochore to mitotic spindle microtubules. Using purified components and Xenopus egg extracts, the kinetochore-associated microtubule motor CENP-E is now shown to be the activator of the essential checkpoint kinase BubR1. Since kinase activity and the checkpoint are silenced following CENP-E-dependent microtubule attachment in extracts or binding of CENP-E antibodies that do not disrupt CENP-E association with BubR1, CENP-E mediates silencing of BubR1 signaling. Checkpoint signaling requires the normal level of BubR1 containing a functional Mad3 domain implicated in Cdc20 binding, but only a small fraction need be kinase competent. This supports bifunctional roles for BubR1 in the checkpoint: an enzymatic one requiring CENP-E-dependent activation of its kinase activity at kinetochores and a stoichiometric one as a direct inhibitor of Cdc20.

PIM3 proto-oncogene kinase is a common transcriptional target of divergent EWS/ETS oncoproteins

Despite significant structural diversity, present evidence suggests that EWS/ETS fusion proteins promote oncogenesis by transcriptionally modulating a common set of target genes. In order to identify these genes, microarray expression analyses were performed on NIH 3T3 polyclonal populations expressing one of three EWS/ETS fusion genes. The majority of these genes can be grouped into seven functional categories, including cellular metabolism and signal transduction. The biologic significance of these target genes was pursued. The effects of modulating genes involved in metabolism were assessed by flux studies and demonstrated shifts in glucose utilization and lactate production as a result of EWS/FLI1 expression. The proto-oncogene coding for serine/threonine kinase PIM3 was found to one of several genes encoding signal transduction proteins that were up-regulated by EWS/ETS fusions. PIM3 was found to be expressed in a panel of human Ewing's family tumor cell lines. Forced expression of PIM3 promoted anchorage-independent growth. Coexpression of a kinase-deficient PIM3 mutant attenuated EWS/FLI1-mediated NIH 3T3 tumorigenesis in immunodeficent mice.

Differential gene expression underlying the functional distinctions of primary human CD34+ hematopoietic stem and progenitor cells from peripheral blood and bone marrow

The restorative capacity of human CD34(+) hematopoietic cells is clinically used in the autologous and allogeneic transplant setting to support cytotoxic therapy. We examined gene expression patterns of highly enriched bone marrow CD34(+) (BM-CD34(+)) or G-CSF-mobilized peripheral blood CD34(+) (PB-CD34(+)) cells by cDNA array technology, quantitative real-time RT-PCR, and flow cytometry, to identify molecular causes underlying the functional differences between circulating and sedentary hematopoietic stem and progenitor cells. The greater cell cycle and DNA synthesis activity of BM-CD34(+) compared to PB-CD34(+) cells was reflected by the 2- to 5-fold higher expression of 9 genes involved in cell cycle, 11 genes regulating DNA synthesis, and the cell cycle-initiating transcription factor E2F-1. The 2- to 3-fold greater expression of 5 pro-apoptotic genes in PB-CD34(+) cells indicated a higher apoptotic activity, which could functionally be corroborated by apoptosis assays. Thrombin receptor (PAR1), known to play a role in trafficking of malignant cells, was 3.6-fold higher expressed in circulating CD34(+) cells than in BM-CD34(+) cells. Guidance via thrombin receptor might molecularly mediate stem cell migration. In summary, our study provides gene expression profiles of primary human CD34(+) hematopoietic cells of blood and marrow. Our data molecularly confirm and explain the finding that CD34(+) cells residing in the bone marrow are cycling more rapidly, whereas circulating CD34(+) cells consist of a higher number of quiescent stem and progenitor cells. Moreover, our data give novel molecular insights into stem cell migration and differentiation.

Nuclear localization of the cell cycle regulator CDH1 and its regulation by phosphorylation

The anaphase-promoting complex activated by CDC20 and CDH1 is a major ubiquitination system that controls the destruction of cell cycle regulators. Exactly how ubiquitination is regulated in time and space is incompletely understood. Here we report on the cell cycle-dependent localization of CDH1 and its regulation by phosphorylation. CDH1 localizes dynamically to the nucleus during interphase and to the centrosome during metaphase and anaphase. The nuclear accumulation of CDH1 correlates with a reduction in the steady-state amount of cyclin A, but not of cyclin E. A nuclear localization signal conserved in various species was identified in CDH1, and it sufficiently targets green fluorescent protein to the nucleus. Interestingly, a CDH1-4D mutant mimicking the hyperphosphorylated form was constitutively found in the cytoplasm. In further support of the notion that phosphorylation inhibits nuclear import, the nuclear localization signal of CDH1 with two phospho-accepting serine/threonine residues changed into aspartates was unable to drive heterologous protein into the nucleus. On the other hand, abolition of the cyclin-binding ability of CDH1 has no influence on its nuclear localization. Taken together, our findings document the phosphorylation-dependent localization of CDH1 in vertebrate cells.

[Mitosis under control]

The mitotic checkpoint is essential to ensure accurate chromosome segregation by allowing a mitotic delay in response to a spindle defect. This checkpoint postpones the onset of anaphase until all the chromosomes are attached and correctly aligned onto the mitotic spindle. The checkpoint functions by preventing an ubiquitin ligase called the anaphase-promoting complex (APC) from ubiquitinylating proteins whose degradation is required for anaphase onset. Loss of this checkpoint results in chromosome missegregation in higher eukaryotes and may contribute to the genomic instability observed in most of the tumour cells.

Centromeres and kinetochores: from epigenetics to mitotic checkpoint signaling

The centromere is a chromosomal locus that ensures delivery of one copy of each chromosome to each daughter at cell division. Efforts to understand the nature and specification of the centromere have demonstrated that this central element for ensuring inheritance is itself epigenetically determined. The kinetochore, the protein complex assembled at each centromere, serves as the attachment site for spindle microtubules and the site at which motors generate forces to power chromosome movement. Unattached kinetochores are also the signal generators for the mitotic checkpoint, which arrests mitosis until all kinetochores have correctly attached to spindle microtubules, thereby representing the major cell cycle control mechanism protecting against loss of a chromosome (aneuploidy).

Mad2 phosphorylation regulates its association with Mad1 and the APC/C

Improper attachment of the mitotic spindle to the kinetochores of paired sister chromatids in mitosis is monitored by a checkpoint that leads to an arrest in early metaphase. This arrest requires the inhibitory association of Mad2 with the anaphase promoting complex/cyclosome (APC/C). It is not known how the association of Mad2 with the kinetochore and the APC/C is regulated in mitosis. Here, we demonstrate that human Mad2 is modified through phosphorylation on multiple serine residues in vivo in a cell cycle dependent manner and that only unphosphorylated Mad2 interacts with Mad1 or the APC/C in vivo. A Mad2 mutant containing serine to aspartic acid mutations mimicking the C-terminal phosphorylation events fails to interact with Mad1 or the APC/C and acts as a dominant-negative antagonist of wild-type Mad2. These data suggest that the phosphorylation state of Mad2 regulates its checkpoint activity by modulating its association with Mad1 and the APC/C.

Analysis of Bub3 spindle checkpoint function in Xenopus egg extracts

The spindle checkpoint delays the onset of anaphase if there are any defects in the interactions between spindle microtubules and kinetochores. This checkpoint has been reconstituted in vitro in Xenopus egg extracts, and here we use antibodies to Xenopus Bub3 (XBub3) to show that this protein is required for both the activation and the maintenance of a spindle checkpoint arrest in egg extracts. We detect two forms of XBub3 in egg extracts and find both to be complexed with the XBub1 and XBubR1 kinases. Only one form of XBub3 is apparent in Xenopus tissue culture (XTC) cells, and localisation studies reveal that, unlike the Mad proteins, which are concentrated at the nuclear periphery, XBub3 is diffusely localised throughout the nucleus during interphase. During early prophase it is recruited to kinetochores, where it remains until chromosomes align at the metaphase plate. We discuss the mechanism by which our alpha-XBub3 antibodies interfere with the checkpoint and possible roles for XBub3 in the spindle checkpoint pathway.

MAD2-induced sensitization to vincristine is associated with mitotic arrest and Raf/Bcl-2 phosphorylation in nasopharyngeal carcinoma cells

Mitotic arrest deficient 2 (MAD2) is thought to be a key component of the mitotic checkpoint, which ensures accurate chromosome segregation. Reduced expression of MAD2 protein is associated with mitotic checkpoint abrogation and chromosomal instability in certain types of human cancers. To explore the possibility of developing a novel strategy for the treatment of cancer based on selective killing of mitotic checkpoint-defective or -competent cells, here we have investigated the effect of MAD2 expression on cellular sensitivity to checkpoint-targeting anticancer drugs. We reintroduced MAD2 protein in a mitotic checkpoint-defective nasopharyngeal carcinoma cell line, CNE2, using an inducible expression vector. We found that overexpression of MAD2 led to an increased sensitivity to vincristine, which was accompanied by increased mitotic index and G2/M cell cycle arrest. In addition, increased phosphorylation of Raf, MEK1/2 and Bcl-2 was observed in MAD2-overexpressing cells in response to vincristine. Furthermore, inhibition of phosphorylation of MEK1/2 by its inhibitor PD098059 led to reduced sensitivity to vincristine, which was associated with decreased Bcl-2 phosphorylation. Our data suggest a role for MAD2 in the sensitization of cancer cells to certain mitotic checkpoint-targeting anticancer drugs.

Identification of a MAD2-binding protein, CMT2, and its role in mitosis

MAD2 is a key component of the spindle checkpoint that delays the onset of anaphase until all the kinetochores are attached to the spindle. It binds to human p55CDC and prevents it from promoting destruction of an anaphase inhibitor, securin. Here we report the characterization of a novel MAD2-binding protein, CMT2. Upon the completion of spindle attachment, formation of the CMT2-MAD2 complex coincides with dissociation of the p55CDC-MAD2 complex. Overexpression of CMT2 in cells arrested by the spindle checkpoint causes premature destruction of securin and allows exit from mitosis without chromosome segregation. Depletion of CMT2 induces cell death following a transient delay in the onset of anaphase. These results indicate that CMT2 interacts with the spindle checkpoint and coordinates cell cycle events in late mitosis.

Regulation of APC-Cdc20 by the spindle checkpoint

The spindle checkpoint ensures the fidelity of chromosome segregation in mitosis and meiosis. In response to defects in the mitotic apparatus, it blocks the activity of the anaphase-promoting complex, a large ubiquitin ligase required for chromosome segregation. Recent studies indicate that the spindle checkpoint monitors both the attachment of chromosomes to the mitotic spindle and the tension across the sister chromatid generated by microtubules. Upon checkpoint activation, checkpoint protein complexes containing BubR1(Mad3), Bub3, Mad2 and Cdc20 directly bind to the anaphase-promoting complex and inhibit its ligase activity. Therefore, the checkpoint proteins form a complex intracellular signalling network to inhibit the anaphase-promoting complex.

The spindle checkpoint: structural insights into dynamic signalling

Chromosome segregation is a complex and astonishingly accurate process whose inner working is beginning to be understood at the molecular level. The spindle checkpoint plays a key role in ensuring the fidelity of this process. It monitors the interactions between chromosomes and microtubules, and delays mitotic progression to allow extra time to correct defects. Here, we review and integrate findings on the dynamics of checkpoint proteins at kinetochores with structural information about signalling complexes.

Role of Hec1 in spindle checkpoint signaling and kinetochore recruitment of Mad1/Mad2

The spindle checkpoint delays sister chromatid separation until all chromosomes have undergone bipolar spindle attachment. Checkpoint failure may result in chromosome mis-segregation and may contribute to tumorigenesis. We showed that the human protein Hec1 was required for the recruitment of Mps1 kinase and Mad1/Mad2 complexes to kinetochores. Depletion of Hec1 impaired chromosome congression and caused persistent activation of the spindle checkpoint, indicating that high steady-state levels of Mad1/Mad2 complexes at kinetochores were not essential for checkpoint signaling. Simultaneous depletion of Hec1 and Mad2 caused catastrophic mitotic exit, making Hec1 an attractive target for the selective elimination of spindle checkpoint-deficient cells.

Rapid microtubule-independent dynamics of Cdc20 at kinetochores and centrosomes in mammalian cells

Cdc20 is a substrate adaptor and activator of the anaphase-promoting complex/cyclosome (APC/C), the E3 ubiquitin ligase whose activity is required for anaphase onset and exit from mitosis. A green fluorescent protein derivative, Cdc20-GFP, bound to centrosomes throughout the cell cycle and to kinetochores from late prophase to late telophase. We mapped distinct domains of Cdc20 that are required for association with kinetochores and centrosomes. FRAP measurements revealed extremely rapid dynamics at the kinetochores (t1/2 = 5.1 s) and spindle poles (t1/2 = 4.7 s). This rapid turnover is independent of microtubules. Rapid transit of Cdc20 through kinetochores may ensure that spindle checkpoint signaling at unattached/relaxed kinetochores can continuously inhibit APC/CCdc20 targeting of anaphase inhibitors (securins) throughout the cell until all the chromosomes are properly attached to the mitotic spindle.

Rapid exchange of mammalian topoisomerase II alpha at kinetochores and chromosome arms in mitosis

A stable cell line (GT2-LPk) derived from LLC-Pk was created in which endogenous DNA topoisomerase II alpha (topoII alpha) protein was downregulated and replaced by the expression of topoII alpha fused with enhanced green fluorescent protein (EGFP-topoII alpha). The EGFP-topoII alpha faithfully mimicked the distribution of the endogenous protein in both interphase and mitosis. In early stages of mitosis, EGFP-topoII alpha accumulated at kinetochores and in axial lines extending along the chromosome arms. During anaphase, EGFP-topoII alpha diminished at kinetochores and increased in the cytoplasm with a portion accumulating into large circular foci that were mobile and appeared to fuse with the reforming nuclei. These cytoplasmic foci appearing at anaphase were coincident with precursor organelles of the reforming nucleolus called nucleolus-derived foci (NDF). Photobleaching of EGFP-topoII alpha associated with kinetochores and chromosome arms showed that the majority of the protein rapidly exchanges (t1/2 of 16 s). Catalytic activity of topoII alpha was essential for rapid dynamics, as ICRF-187, an inhibitor of topoII alpha, blocked recovery after photobleaching. Although some topoII alpha may be stably associated with chromosomes, these studies indicate that the majority undergoes rapid dynamic exchange. Rapid mobility of topoII alpha in chromosomes may be essential to resolve strain imparted during chromosome condensation and segregation.

The roles of Fzy/Cdc20 and Fzr/Cdh1 in regulating the destruction of cyclin B in space and time

In Drosophila cells cyclin B is normally degraded in two phases: (a) destruction of the spindle-associated cyclin B initiates at centrosomes and spreads to the spindle equator; and (b) any remaining cytoplasmic cyclin B is degraded slightly later in mitosis. We show that the APC/C regulators Fizzy (Fzy)/Cdc20 and Fzy-related (Fzr)/Cdh1 bind to microtubules in vitro and associate with spindles in vivo. Fzy/Cdc20 is concentrated at kinetochores and centrosomes early in mitosis, whereas Fzr/Cdh1 is concentrated at centrosomes throughout the cell cycle. In syncytial embryos, only Fzy/Cdc20 is present, and only the spindle-associated cyclin B is degraded at the end of mitosis. A destruction box-mutated form of cyclin B (cyclin B triple-point mutant [CBTPM]-GFP) that cannot be targeted for destruction by Fzy/Cdc20, is no longer degraded on spindles in syncytial embryos. However, CBTPM-GFP can be targeted for destruction by Fzr/Cdh1. In cellularized embryos, which normally express Fzr/Cdh1, CBTPM-GFP is degraded throughout the cell but with slowed kinetics. These findings suggest that Fzy/Cdc20 is responsible for catalyzing the first phase of cyclin B destruction that occurs on the mitotic spindle, whereas Fzr/Cdh1 is responsible for catalyzing the second phase of cyclin B destruction that occurs throughout the cell. These observations have important implications for the mechanisms of the spindle checkpoint.

Crystal structure of the tetrameric Mad1-Mad2 core complex: implications of a 'safety belt' binding mechanism for the spindle checkpoint

The spindle checkpoint protein Mad1 recruits Mad2 to unattached kinetochores and is essential for Mad2-Cdc20 complex formation in vivo but not in vitro. The crystal structure of the Mad1-Mad2 complex reveals an asymmetric tetramer, with elongated Mad1 monomers parting from a coiled-coil to form two connected sub-complexes with Mad2. The Mad2 C-terminal tails are hinged mobile elements wrapping around the elongated ligands like molecular 'safety belts'. We show that Mad1 is a competitive inhibitor of the Mad2-Cdc20 complex, and propose that the Mad1-Mad2 complex acts as a regulated gate to control Mad2 release for Cdc20 binding. Mad1-Mad2 is strongly stabilized in the tetramer, but a 1:1 Mad1-Mad2 complex slowly releases Mad2 for Cdc20 binding, driven by favourable binding energies. Thus, the rate of Mad2 binding to Cdc20 during checkpoint activation may be regulated by conformational changes that destabilize the tetrameric Mad1-Mad2 assembly to promote Mad2 release. We also show that unlocking the Mad2 C-terminal tail is required for ligand release from Mad2, and that the 'safety belt' mechanism may prolong the lifetime of Mad2-ligand complexes.

Induction of apoptosis in a human erythroleukemic cell line K562 by tylophora alkaloids involves release of cytochrome c and activation of caspase 3

Tylophora alkaloids are plant products known for their antiasthamatic and antiproliferative activities. The underlying cellular changes resulting from inhibition of proliferation were investigated. Tylophora alkaloids induced apoptosis in K562 cells with characteristic apoptotic features like nuclear condensation, apoptotic body formation, flipping of membrane phosphatidylserine, activation of caspase 3 and release of mitochondrial cytochrome c. These studies suggest that the Tylophora alkaloids, in addition to their antiproliferative effects also induce apoptosis in erythroleukemic cells. These observations imply that Tylophora alkaloids could be useful molecules for their antiproliferative activity and for induction of apoptosis in tumor cells.

The anaphase-promoting complex: proteolysis in mitosis and beyond

Key events in mitosis such as sister chromatid separation and subsequent inactivation of cyclin-dependent kinase 1 are regulated by ubiquitin-dependent proteolysis. These events are mediated by the anaphase-promoting complex (APC), a cell cycle-regulated ubiquitin ligase that assembles multiubiquitin chains on regulatory proteins such as securin and cyclins and thereby targets them for destruction by the 26S proteasome.

Control of localization of a spindle checkpoint protein, Mad2, in fission yeast

To ensure accurate chromosome segregation, the spindle checkpoint delays the onset of sister chromatid separation when the spindle is not attached to a kinetochore. Mad2, a component of the checkpoint, targets fission yeast Slp1/budding yeast Cdc20/human p55CDC and prevents it from promoting proteolysis, which is a prerequisite to sister chromatid separation. The protein is localized to unattached kinetochores in higher eukaryotes, and it is thought to be required for activation of the checkpoint as well. In this study, Mad2 and its target Slp1 were visualized in a tractable organism,fission yeast Schizosaccharomyces pombe. When cells were arrested at a prometaphase-like stage, the Mad2-Slp1 complex was stable and the two proteins were colocalized to unattached kinetochores. When the spindle attachment was completed, the complex was no longer detectable and only Mad2 was found associated to the spindle. These results would suggest that unattached kinetochores provide sites for assembly of the Mad2-Slp1 complex. During interphase, Mad2 was localized to the nuclear periphery as well as to the chromatin domain. This localization was abolished in a yeast strain lacking Mad1, a protein that physically interacts with Mad2. Mad1 may anchor Mad2 to the nuclear membrane and regulate its entry into the nucleus.