Mitotic checkpoint slippage in humans occurs via cyclin B destruction in the presence of an active checkpoint.
In the presence of unattached/weakly attached kinetochores, the spindle assembly checkpoint (SAC) delays exit from mitosis by preventing the anaphase-promoting complex (APC)-mediated proteolysis of cyclin B, a regulatory subunit of cyclin-dependent kinase 1 (Cdk1). Like all checkpoints, the SAC does not arrest cells permanently, and escape from mitosis in the presence of an unsatisfied SAC requires that cyclin B/Cdk1 activity be inhibited. In yeast , and likely Drosophila, this occurs through an "adaptation" process involving an inhibitory phosphorylation on Cdk1 and/or activation of a cyclin-dependent kinase inhibitor (Cdki). The mechanism that allows vertebrate cells to escape mitosis when the SAC cannot be satisfied is unknown. To explore this issue, we conducted fluorescence microscopy studies on rat kangaroo (PtK) and human (RPE1) cells dividing in the presence of nocodazole. We find that in the absence of microtubules (MTs), escape from mitosis occurs in the presence of an active SAC and requires cyclin B destruction. We also find that cyclin B is progressively destroyed during the block by a proteasome-dependent mechanism. Thus, vertebrate cells do not adapt to the SAC. Rather, our data suggest that in normal cells, the SAC cannot prevent a slow but continuous degradation of cyclin B that ultimately drives the cell out of mitosis.
Brito
Daniela A
DA
Department of Biomedical Sciences, School of Public Health, State University of New York, Albany, New York 12222, USA.
Rieder
Conly L
CL
D000818:Animals; D002135:Calcium-Binding Proteins / Q000378:metabolism; D018797:Cell Cycle Proteins / Q000378:metabolism; D002460:Cell Line; D019926:Cyclin B / Q000378:metabolism*; D006801:Humans; D018386:Kinetochores / Q000378:metabolism / Q000648:ultrastructure; D064246:Mad2 Proteins; D008870:Microtubules / Q000648:ultrastructure; D008938:Mitosis / Q000502:physiology*; D015739:Nocodazole / Q000494:pharmacology; D048328:Potoroidae; D011494:Protein Kinases / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases; D012097:Repressor Proteins / Q000378:metabolism; D008941:Spindle Apparatus / Q000502:physiology / Q000648:ultrastructure
D002135:Calcium-Binding Proteins; D018797:Cell Cycle Proteins; D019926:Cyclin B; C577697:MAD2L1 protein, human; D064246:Mad2 Proteins; D012097:Repressor Proteins; D011494:Protein Kinases; C090464:Bub1 spindle checkpoint protein; D017346:Protein Serine-Threonine Kinases; D015739:Nocodazole