Cdk1 is essential for mammalian cyclosome/APC regulation

The cyclosome/APC (anaphase-promoting complex), the major component of cell -cycle-specific ubiquitin-mediated proteolysis of mitotic cyclins and of ot her cell cycle proteins, is essential for sister chromatid separation and f or exit from mitosis. Cyclosome activity and substrate specificity are modu lated by phosphorylation and by transient interactions with Fizzy/cdc20 (Fz y) and Fizzy-related/Hct1/Cdh1 (Fzr). This regulation has been studied so f ar in Drosophila embryos, in yeast, and in cell-free extracts in. vitro. St udying cyclosome regulation in mammalian cells in vivo we found that both F zr overexpression and Cdk1 inhibition can override the prometaphase checkpo int. We further show that Fzr activation of the cyclosome is negatively reg ulated by Cdk1. Finally, we show that the mammalian cdc14 phosphatase, like its budding yeast homologue, plays a role in cyclosome pathway regulation. These results suggest that Cdk1 is essential for coupling various activiti es of the cyclosome and in particular for preventing Fzr from short-circuit ing the spindle pole checkpoint. Cdk1-cyclin B is thus an inhibitor, activa tor, and substrate of the cyclosome. (C) 2000 Academic Press.