THE CDC42P EFFECTOR GIC2P IS TARGETED FOR UBIQUITIN-DEPENDENT DEGRADATION BY THE SCFGRR1 COMPLEX

Cdc42p, a Rho-related GTP-binding protein, regulates cytoskeletal pola rization and rearrangements in eukaryotic cells. In yeast, Gic1p and G ic2p are effecters of Cdc42p involved in actin polarization at bud eme rgence. Gic2p is expressed in a cell cycle-dependent manner and rapidl y disappears shortly after bud emergence concomitant with the activati on of the G(1) cyclin-dependent kinase Cdc28p-CInp, Here we have shown that the rapid disappearance of Gic2p results from ubiquitin-dependen t proteolysis. Biochemical and genetic evidence demonstrates that degr adation of Gic2p required the (S) under bar kp1-(c) under bar ullin-(F ) under bar-box protein complex: (SCF) components Cdc34p, Cd53p, Skp1p and Grr1p, but not Cdc4p, Phosphorylation of several C-terminal sites of Gic2p served as part of the recognition signal for ubiquitination, In addition, binding of Gic2p to Cdc42p was a prerequisite for degrad ation, suggesting that specifically the active form of Gic2p is target ed for destruction. Finally, our data indicate that degradation of Gic 2p may be part of a mechanism which restricts cytoskeletal polarizatio n in the G(1) phase of the cell cycle.