Figure 4.

A model of the mechanism by which mad2 overexpression blocks the metaphase-to-anaphase transition. APC targets specific proteins for degradation, which is required for three different postmetaphase events. Cyclin B degradation is required for the exit from mitosis. Cut2p degradation is required for sister chromosome separation. The protein whose degradation is required for spindle elongation has not yet been identified. mad2 overexpression mimics activation of the spindle checkpoint and blocks the metaphase-to-anaphase transition by directly or indirectly inhibiting APC.