APC/CCdc20 controls the ubiquitin-mediated degradation of p21 in prometaphase

During the G1/S transition, p2l proteolysis is mediated by Skp2; however, p2l reaccumulates in G2 and is degraded again in prometaphase. How p2l degradation is controlled in mitosis remains unexplored. We found that Cdc20 (an activator of the ubiquitin ligase APC/C) binds p2l in cultured cells and identified a D box motif in p2l necessary for APC/ C-cdc20- mediated ubiquitylation of p2l. Overexpression of Cdc20 or Skp2 destabilized wildtype p2l; however, only Skp2, but not Cdc20, was able to destabilize a p2l(D box) mutant. Silencing of Cdc20 induced an accumulation of p2l, increased the fraction of p2l bound to Cdkl, and inhibited Cdkl activity in p21(+/+) prometaphase cells, but not in p21(-/-) cells. Thus, in prometaphase Cdc20 positively regulates Cdkl by mediating the degradation of p2l. We propose that the ApC/C-cdc20- mediated degradation of p2l contributes to the full activation of Cdk1 necessary for mitotic events and prevents mitotic slippage during spindle checkpoint activation.