Practice question

Mitotic checkpoint complex inhibition of anaphase-promoting complex relies on multiple contacts that block substrate recognition and catalytic activity. MCC consists of BubR1, Bub3, Mad2, and Cdc20, assembled at unattached kinetochores. Closed conformer of Mad2, generated through Mad1-Mad2 template mechanism, wraps around Cdc20 as safety belt, sequestering it and presenting inhibitory surface. Although BubR1 contains KEN boxes that mimic APC/C substrates and directly blocks substrate binding via pseudo-substrate inhibition, Mad2 provides crucial initial locking of Cdc20 in MCC, preventing it from associating with and activating APC/C. Studies using purified components show Mad2 alone can partially inhibit APC/C, but full potency requires BubR1 which inserts into APC/C central cavity and obstructs E2 ubiquitin-conjugating enzyme UbcH10 recruitment. Bub3 serves as adaptor recognizing phosphorylated MELT motifs on Knl1. Mutations that prevent closed Mad2 formation bypass checkpoint, allowing premature anaphase. Therefore, within MCC, Mad2 is considered direct inhibitor that captures Cdc20 and nucleates complex assembly that culminates in APC/C suppression until bi-orientation achieved.