The Cdc42p effector Gic2p is targeted for ubiquitin-dependent degradation by the SCFGrr1 complex

Cdc42p, a Rho-related GTP-binding protein, regulates cytoskeletal polarization and rearrangements in eukaryotic cells. In yeast, Gic1p and Gic2p are effecters of Cdc42p involved in actin polarization at bud emergence. Gic2p is expressed in a cell cycle-dependent manner and rapidly disappears shortly after bud emergence concomitant with the activation of the G(1) cyclin-dependent kinase Cdc28p-CInp, Here we have shown that the rapid disappearance of Gic2p results from ubiquitin-dependent proteolysis. Biochemical and genetic evidence demonstrates that degradation 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 degradation, suggesting that specifically the active form of Gic2p is targeted for destruction. Finally, our data indicate that degradation of Gic2p may be part of a mechanism which restricts cytoskeletal polarization in the G(1) phase of the cell cycle.