Catalytic Assembly of the Mitotic Checkpoint Inhibitor BubR1-Cdc20 by a Mad2-Induced Functional Switch in Cdc20

The mitotic checkpoint acts to maintain chromosome content by generation of a diffusible anaphase inhibitor. Unattached kinetochores catalyze a conformational shift in Mad2, converting an inactive open form into a closed form that can capture Cdc20, the mitotic activator of the APC/C ubiquitin ligase. Mad2 binding is now shown to promote a functional switch in Cdc20, exposing a previously inaccessible site for binding to BubR1’s conserved Mad3 homology domain. BubR1, but not Mad2, binding to APC/CCdc20 is demonstrated to inhibit ubiquitination of cyclin B. Closed Mad2 is further shown to catalytically amplify production of BubR1-Cdc20 without necessarily being part of the complex. Thus, the mitotic checkpoint is produced by a cascade of two catalytic steps: an initial step acting at unattached kinetochores to produce a diffusible Mad2-Cdc20 intermediate and a diffusible step in which that intermediate amplifies production of BubR1-Cdc20, the inhibitor of cyclin B ubiquitination, by APC/CCdc20. •Mad2 binding induces a functional switch in Cdc20 that uncovers a BubR1 binding site•BubR1, not Mad2, is the mitotic checkpoint-produced inhibitor of APC/CCdc20•Cytoplasmic amplification of the BubR1-Cdc20 mitotic checkpoint inhibitor by Mad2•Dual catalytic steps by Mad2 produce the mitotic checkpoint inhibitor BubR1-Cdc20