Anaphase-promoting complex/cyclosome-dependent proteolysis of human cyclina starts at the beginning of mitosis and is not subject to the spindle assembly checkpoint

Cyclin A is a stable protein in S and G2 phases, but is destabilized when c ells enter mitosis and is almost completely degraded before the metaphase t o anaphase transition. Microinjection of antibodies against subunits of the anaphase-promoting complex/cyclosome (APC/C) or against human Cdc20 (fizzy ) arrested cells at metaphase and stabilized both cyclins A and B1, Cyclin A was efficiently polyubiquitylated by Cdc20 or Cdh1-activated APC/C in vit ro. but in contrast to cyclin Blithe proteolysis of cyclin A was not delaye d by the spindle assembly checkpoint. The degradation of cyclin B1 was acce lerated by inhibition of the spindle assembly checkpoint. These data sugges t that the APC/C is activated as cells enter mitosis and immediately target s cyclin A for degradation, whereas the spindle assembly checkpoint delays the degradation of cyclin B1 until the metaphase to anaphase transition. Th e "destruction box" (D-box) of cyclin A is 10-20 residues longer than that of cyclin B. Overexpression of wild-type cyclin A delayed the metaphase to anaphase transition, whereas expression of cyclin A mutants lacking a D-box arrested cells in anaphase.