Stable MCC binding to the APC/C is required for a functional spindle assembly checkpoint

Abstract The spindle assembly checkpoint (SAC) delays progression into anaphase until all chromosomes have aligned on the metaphase plate by inhibiting Cdc20, the mitotic co-activator of the APC/C. Mad2 and BubR1 bind and inhibit Cdc20, thereby forming the mitotic checkpoint complex (MCC), which can bind stably to the APC/C. Whether MCC formation per se is sufficient for a functional SAC or MCC association with the APC/C is required remains unclear. Here, we analyze the role of two conserved motifs in Cdc20, IR and C-Box, in binding of the MCC to the APC/C. Mutants in both motifs assemble the MCC normally, but IR motif integrity is particularly important for stable binding to the APC/C. Cells expressing Cdc20 with a mutated IR motif have a compromised SAC, as uninhibited Cdc20 can compete with the MCC for APC/C binding and activate it. We thus show that stable MCC association with the APC/C is critical for a functional SAC. Synopsis image Whether formation of the mitotic checkpoint complex (MCC) is sufficient for a functional spindle assembly checkpoint (SAC) remains unclear. This study shows that the MCC needs to stably interact with the APC/C for proper checkpoint function, ensuring accurate chromosome segregation during mitosis. The IR motif of Cdc20 is important for stable MCC binding to the APC/C Stable MCC-APC/C interaction is required for proper checkpoint function