Successful mitosis requires the right protein be degraded at the right time. Central to this is the spindle checkpoint that prevents the destruction of securin and cyclin B1 when there are improperly attached chromosomes. The principal target of the checkpoint is Cdc20, which activates the anaphase-promoting complex/cyclosome (APC/C). A Drosophila Cdc20/fizzy mutant arrests in mitosis with high levels of cyclins A and B, but paradoxically the spindle checkpoint does not stabilize cyclin A. Here, we investigated this paradox and found that Cdc20 is rate limiting for cyclin A destruction. Indeed, Cdc20 binds efficiently to cyclin A before and in mitosis, and this complex has little associated Mad2. Furthermore, the cyclin A complex must bind to a Cks protein to be degraded independently of the checkpoint. Thus, we identify a crucial role for the Cks proteins in mitosis and one mechanism by which the APC/C can target substrates independently of the spindle checkpoint.
Cell Cycle Proteins/metabolism , Cell Cycle/physiology , Cyclin A/metabolism , Cyclin-Dependent Kinases/metabolism , Spindle Apparatus/metabolism , Adenomatous Polyposis Coli Protein/genetics , Adenomatous Polyposis Coli Protein/metabolism , Animals , CDC2-CDC28 Kinases , Carrier Proteins/genetics , Carrier Proteins/metabolism , Cdc20 Proteins , Cell Cycle Proteins/genetics , Cell Line , Cyclin A/genetics , Cyclin A2 , Cyclin-Dependent Kinase 2/genetics , Cyclin-Dependent Kinase 2/metabolism , Cyclin-Dependent Kinases/genetics , Drosophila melanogaster/genetics , Drosophila melanogaster/metabolism , Humans , Mitosis/physiology , RNA Interference , Recombinant Fusion Proteins/genetics , Recombinant Fusion Proteins/metabolism
