Multiple activities contribute to Pc2 E3 function.

Pc2 is a polycomb protein, which has SUMO E3 activity for the corepressors CtBP and CtBP2. Here we demonstrate that, in vivo, Pc2 adapter function contributes to enhancement of CtBP sumoylation. Mutation of the CtBP binding site on Pc2 abolishes E3 activity toward CtBP. However, a carboxyl-terminal fragment of Pc2 that recruits both Ubc9 and CtBP lacks E3 activity. We identify a second domain, which, when coexpressed with the carboxyl-terminal adapter region, restores E3 function. In vitro, this domain has E3 activity in isolation, suggesting that it is a functional domain, and that adapter function is required to selectively corecruit E2 and substrate in vivo. These results demonstrate the presence of two domains in Pc2 that contribute to full in vivo E3 activity, and suggest that SUMO E3s are more than simple platforms to which E2 and substrate bind.