MAD2 TRANSIENTLY ASSOCIATES WITH AN APC P55CDC COMPLEX DURING MITOSIS/

Activation of the mitotic checkpoint pathway in response to mitotic sp indle damage in eukaryotic cells delays the exit from mitosis in an at tempt to prevent chromosome missegregation, One component of this path way, hsMad2, has been shown in mammalian cells to physically associate with components of a ubiquitin ligase activity; (termed the anaphase promoting complex or APC) when the checkpoint is activated, thereby pr eventing the degradation of inhibitors of the mitotic exit machinery. In the present report, we demonstrate that the inhibitory association between Mad2 and the APC component Cdc27 also takes place transiently: during the early stages of a normal mitosis and is lost before mitoti c exit, We also show that Mad2 associates with the APC regulatory prot ein p55Cdc in mammalian cells as has been reported in yeast. In contra st,however,this complex is present only in nocodazole-arrested or earl y mitotic cells and is associated with the APC as a Mad2/p55Cdc/Cdc27 ternary complex, Evidence for a Mad2/Cdc27 complex that forms independ ent of p55Cdc also is presented, These results suggest a model for the regulation of the APC by Mad2 and may explain how the spindle assembl y checkpoint apparatus controls the timing of mitosis under normal gro wth conditions.