SUMO-1 controls the protein stability and the biological function of phosducin

Phosducin regulates G beta gamma- stimulated signaling by binding to G beta gamma subunits of heterotrimeric G- proteins. Control of phosducin activity by phosphorylation is well established. However, little is known about other mechanisms that may control phosducin activity. Here we report that phosducin is regulated at the posttranslational level by modification with the small ubiquitin- related modifier, SUMO. We demonstrate modification with SUMO for phosducin in vitro expressed in cells and for native phosducin purified from retina and the heart. A consensus motif for SUMOylation was identified in phosducin at amino acid positions 32 - 35. Mutation of the conserved lysine 33 to arginine in this motif abolished SUMOylation of phosducin, indicating that SUMO is attached to lysine 33 of phosducin. In transfected cells the steady- state levels of the K33R mutant protein were much lower compared with wildtype phosducin. The investigation of the stability of wild- type phosducin and of phosducin(K33R) showed a decreased protein stability of the SUMOylation- deficient mutant. The decreased protein stability correlated with increased ubiquitinylation of the SUMOylation- deficient mutant. These findings indicate that SUMOylation protects phosducin from proteasomal degradation. SUMOylation of phosducin decreased its ability to bind G beta gamma. PhlP, a closely related member of the phosducin family, was not a target for SUMOylation, but its SUMOylation can be achieved by a single amino acid insertion in the conserved N terminus of PhlP. Together, these findings show that phosducin is a previously unrecognized target of SUMO modification and that SUMOylation controls phosducin stability in cells as well as its functional properties.