Molecular Analysis of Pc2 Function

Sumoylation is a post-translational modification that occurs on many proteins. Pc2 (Cbx4) is a member of the chromobox family of polycomb proteins, and is a SUMO E3 ligase for the transcriptional corepressor, CtBP. We have data that suggests the serine/threonine kinase Akt, which is activated by growth factor signaling, phosphorylates both Pc2 and CtBP. Phosphorylation of CtBP is dramatically stimulated by Pc2, and we show that this requires interaction of both CtBP and Akt1 with Pc2. Mutation of the phosphorylated residue in CtBP (T176) to a phosphomimic results in decreased dimerization and decreased transcriptional repression by CtBP in addition to increased levels of ubiquitination on CtBP. In part, Pc2 promotes CtBP phosphorylation by preventing inactivation of Akt1 by phosphatases, thus maintaining a pool of active Akt1 within the nucleus. These results provide a novel mechanism by which nuclear Akt1 activity is regulated, and suggest that Pc2 may act as a nuclear platform to coordinate multiple enzymatic activities. Recent research has revealed that many proteins contain SUMO interaction motifs (SIMs), and these small hydrophobic protein domains are important for protein function and cellular localization. Components of the sumoylation machinery such as the SUMO E3s PIAS and RanBP2 contain SIMs, alluding to their importance in the SUMO pathway. The SUMO E3 Pc2 contains 2 SIMs, and we show that both SIM1 and SIM2 are essential for SUMO E3 activity. Deletion of SIM2 abolished Pc2 autosumoylation, whereas deletion of either SIM1 and/or SIM2 abolishes CtBP modification. Pc2 and SUMO have been shown to colocalize at nuclear polycomb bodies, and Pc2 SIM mutants have a reduced capacity to recruit SUMO1 or SUMO2 to polycomb bodies. Pc2 SIM mutants iii can have an altered cellular localization in large nuclear foci, and inhibiting the SUMO pathway induces another polycomb protein, Bmi1, to form similar foci. Taken together, this suggests SUMO binding is important for E3 function and proper protein localization.

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