Journal
CURRENT BIOLOGY
Volume 16, Issue 12, Pages 1194-1200Publisher
CELL PRESS
DOI: 10.1016/j.cub.2006.04.043
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Funding
- NIGMS NIH HHS [R37 GM040198-22, R37 GM040198] Funding Source: Medline
- PHS HHS [NIH/GMS 40198] Funding Source: Medline
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In the presence of unattached/weakly attached kineto-chores, 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 [1-5], and likely Drosophila [6-8], 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.
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