Regulation of REGγ cellular distribution and function by SUMO modification

Discovery of emerging REG gamma-regulated proteins has accentuated the REG gamma-proteasome as an important pathway in multiple biological processes, including cell growth, cell cycle regulation, and apoptosis. However, little is known about the regulation of the REG gamma-proteasome pathway. Here we demonstrate that REG gamma can be SUMOylated in vitro and in vivo by SUMO-1, SUMO-2, and SUMO-3. The SUMO-E3 protein inhibitor of activated STAT (PIAS)1 physically associates with REG gamma and promotes SUMOylation of REG gamma. SUMOylation of REG gamma was found to occur at multiple sites, including K6, K14, and K12. Mutation analysis indicated that these SUMO sites simultaneously contributed to the SUMOylation status of REG gamma in cells. Posttranslational modification of REG gamma by SUMO conjugation was revealed to mediate cytosolic translocation of REG gamma and to cause increased stability of this proteasome activator. SUMOylation-deficient REG gamma displayed attenuated ability to degrade p21(Waf//Cip1) due to reduced affinity of the REG gamma SUMOylation-defective mutant for p21. Taken together, we report a previously unrecognized mechanism regulating the activity of the proteasome activator REG gamma. This regulatory mechanism may enable REG gamma to function as a more potent factor in protein degradation with a broader substrate spectrum.