Thyroid hormone receptor interacting protein 13 (TRIP13) AAA-ATPase is a novel mitotic checkpoint-silencing protein.

The mitotic checkpoint (or spindle assembly checkpoint) is a fail-safe mechanism to prevent chromosome missegregation by delaying anaphase onset in the presence of defective kinetochore-microtubule attachment. The target of the checkpoint is the E3 ubiquitin ligase anaphase-promoting complex/cyclosome. Once all chromosomes are properly attached and bioriented at the metaphase plate, the checkpoint needs to be silenced. Previously, we and others have reported that TRIP13 AAA-ATPase binds to the mitotic checkpoint-silencing protein p31(comet). Here we show that endogenous TRIP13 localizes to kinetochores. TRIP13 knockdown delays metaphase-to-anaphase transition. The delay is caused by prolonged presence of the effector for the checkpoint, the mitotic checkpoint complex, and its association and inhibition of the anaphase-promoting complex/cyclosome. These results suggest that TRIP13 is a novel mitotic checkpoint-silencing protein. The ATPase activity of TRIP13 is essential for its checkpoint function, and interference with TRIP13 abolished p31(comet)-mediated mitotic checkpoint silencing. TRIP13 overexpression is a hallmark of cancer cells showing chromosomal instability, particularly in certain breast cancers with poor prognosis. We suggest that premature mitotic checkpoint silencing triggered by TRIP13 overexpression may promote cancer development.

Two different mitotic checkpoint inhibitors of the anaphase-promoting complex/cyclosome antagonize the action of the activator Cdc20.

The mitotic checkpoint system ensures the fidelity of chromosome segregation by preventing the completion of mitosis in the presence of any misaligned chromosome. When activated, it blocks the initiation of anaphase by inhibiting the ubiquitin ligase anaphase-promoting complex/cyclosome (APC/C). Little is known about the biochemical mechanisms by which this system inhibits APC/C, except for the existence of a mitotic checkpoint complex (MCC) inhibitor of APC/C composed of the APC/C activator Cdc20 associated with the checkpoint proteins Mad2, BubR1, and Bub3. We have been studying the mechanisms of the mitotic checkpoint system in extracts that reproduce its downstream events. We found that inhibitory factors are associated with APC/C in the checkpoint-arrested state, which can be recovered from immunoprecipitates. Only a part of the inhibitory activity was caused by MCC [Braunstein I, Miniowitz S, Moshe Y, Hershko A (2007) Proc Natl Acad Sci USA 104:4870-4875]. Here, we show that during exit from checkpoint, rapid disassembly of MCC takes place while APC/C is still inactive. This observation suggested the possible involvement of multiple factors in the regulation of APC/C by the mitotic checkpoint. We have separated a previously unknown inhibitor of APC/C from MCC. This inhibitor, called mitotic checkpoint factor 2 (MCF2), is associated with APC/C only in the checkpoint-arrested state. The inhibition of APC/C by both MCF2 and MCC was decreased at high concentrations of Cdc20. We propose that both MCF2 and MCC inhibit APC/C by antagonizing Cdc20, possibly by interaction with the Cdc20-binding site of APC/C.

Human MPS1 kinase is required for mitotic arrest induced by the loss of CENP-E from kinetochores.

We have determined that the previously identified dual-specificity protein kinase TTK is the human orthologue of the yeast MPS1 kinase. Yeast MPS1 (monopolar spindle) is required for spindle pole duplication and the spindle checkpoint. Consistent with the recently identified vertebrate MPS1 homologues, we found that hMPS1 is localized to centrosomes and kinetochores. In addition, hMPS1 is part of a growing list of kinetochore proteins that are localized to nuclear pores. hMPS1 is required by cells to arrest in mitosis in response to spindle defects and kinetochore defects resulting from the loss of the kinesin-like protein, CENP-E. The pattern of kinetochore localization of hMPS1 in CENP-E defective cells suggests that their interaction with the kinetochore is sensitive to microtubule occupancy rather than kinetochore tension. hMPS1 is required for MAD1, MAD2 but not hBUB1, hBUBR1 and hROD to bind to kinetochores. We localized the kinetochore targeting domain in hMPS1 and found that it can abrogate the mitotic checkpoint in a dominant negative manner. Last, hMPS1 was found to associate with the anaphase promoting complex, thus raising the possibility that its checkpoint functions extend beyond the kinetochore.

Human CENP-I specifies localization of CENP-F, MAD1 and MAD2 to kinetochores and is essential for mitosis.

The kinetochore, a macromolecular complex located at the centromere of chromosomes, provides essential functions for accurate chromosome segregation. Kinetochores contain checkpoint proteins that monitor attachments between the kinetochore and microtubules to ensure that cells do not exit mitosis in the presence of unaligned chromosomes. Here we report that human CENP-I, a constitutive protein of the kinetochore that shares limited similarity with Mis6 of Schizosaccharomyces pombe, is required for the localization of CENP-F and the checkpoint proteins MAD1 and MAD2 to kinetochores. Depletion of CENP-I from kinetochores causes the cell cycle to delay in G2. Although monopolar chromosomes in CENP-I-depleted cells fail to establish bipolar connections, the cells are unable to arrest in mitosis. These cells are transiently delayed in mitosis in a MAD2-dependent manner, even though their kinetochores are depleted of MAD2. The delay is extended considerably when the number of unattached kinetochores is increased. This suggests that no single unattached kinetochore in CENP-I-depleted cells can arrest mitosis. The collective output from many unattached kinetochores is required to reach a threshold signal of 'wait for anaphase' to sustain a prolonged mitotic arrest.