Paradoxical mitotic exit induced by a small molecule inhibitor of APC/C Cdc20

The Anaphase Promoting Complex/Cyclosome (APC/C) is a mega-dalton ubiquitin ligase that initiates mitotic exit by targeting substrates for degradation. The APC/C is activated by Cdc20, which acts as a substrate receptor, and is inhibited by the mitotic checkpoint complex (MCC), which delays mitotic exit when the spindle assembly checkpoint (SAC) is activated. We previously identified apcin, a small molecule ligand of Cdc20, as an inhibitor of APC/CCdc20. Surprisingly, we found that apcin paradoxically accelerates substrate degradation and promotes mitotic exit in cells with high SAC activity. Biochemical studies indicate that apcin cooperates with p31comet to relieve MCC-dependent inhibition of APC/C. Apcin’s behavior as an antagonist of Cdc20 can thus result in either net inhibition of APC/C, delaying mitotic exit when SAC activity is slow, or net activation of APC/C, promoting mitotic exit when SAC activity is high. Genetic experiments suggest that the dual behaviors of apcin arise from targeting a common binding site in Cdc20 that is required for both substrate ubiquitination as well as efficient APC/C inhibition by MCC. We therefore establish a new mechanism through which a small molecule, by targeting a single site on a dynamic protein interface, can lead to opposing biological effects depending on the regulatory context.