Recruitment of multiple enzymatic activities to polycomb group complexes by Pc2

Sumoylation is a post translational modification that has been implicating in regulating a rapidly growing list of proteins. The polycomb protein Pc2 can recruit the corepressor CtBP and the SUMO E2 conjugating enzyme, Ubc9 to polycomb group (PcG) bodies. By colocalizing both substrate and enzyme Pc2 is able to increase the amount of sumoylated CtBP, in vivo. Recombinant Pc2 also enhances the amount of SUMO modified CtBP, in vitro, indicating that it is a SUMO E3. Pc2 requires adapter function to increase CtBP sumoylation since deletion of the CtBP interaction motif (PIDLR) or a proposed Ubc9 docking domain, abolishes E3 activity. Still, this is not sufficient since a carboxyl terminal fragment of Pc2 that colocalizes CtBP and Ubc9 to PcG bodies cannot increase sumoylation of CtBP, in vivo. The amino terminus of Pc2 has E3 activity in vitro . It interacts directly with Ubc9 and stimulates the transfer of SUMO from SUMO loaded Ubc9 to CtBP. Mutation of a noncovalent SUMO binding motif in the amino terminus of Pc2 significantly reduces E3 activity. We propose a mechanistic model where the carboxyl terminus of Pc2 recruits all necessary components (CtBP and Ubc9) allowing the amino terminus of Pc2 to facilitate the transfer of SUMO from Ubc9 to Ctl3P.; Multiple enzymatic activities are contained within PcG bodies that may be important for epigenetic regulation. The serine/threonine kinase Akt is recruited to PcG bodies by Pc2. Akt phosphorylates both Pc2 and CtBP. Akt kinase activity for CtBP is greatly stimulated by Pc2. This may in part be due to the colocalization of CtBP and Akt at PcG bodies. However, Pc2 increases or protects phosphorylation of Akt on T308. Phosphorylation of T308 is required for activation of Akt, suggesting that Pc2 is stimulating Akt kinase activity. Akt is sumoylated, and in vivo the sumoylation of Akt is significantly enhanced by Pc2. Mutation of the primary sumoylated residue (K64) increases kinase activity of Akt, suggesting that sumoylation is negatively regulating Akt. PcG bodies may serve as a platform to modify Akt signaling by either activating the kinase through T308 phosphorylation or inhibiting activity by SUMO modification.