A COMPLEX OF CDC4P, SKP1P, AND CDC53P CULLIN CATALYZES UBIQUITINATIONOF THE PHOSPHORYLATED CDK INHIBITOR SIC1P/

In S. cerevisiae, the G1/S transition requires Cdc4p, Cdc34p, Cdc53p, Skp1p, and the Cln/Cdc28p cyclin-dependent kinase (Cdk). These protein s are thought to promote the proteolytic inactivation of the S-phase C dk inhibitor Sic1p. We show here that Cdc4p, Cdc53p, and Skp1p assembl e into a ubiquitin ligase complex named SCFCdc4p. When mixed together, SCFCdc4p sub- units, E1 enzyme, the E2 enzyme Cdc34p, and ubiquitin a re sufficient to reconstitute ubiquitination of Cdk-phosphorylated Sic 1p. Phosphorylated Sic1p substrate is specifically targeted for ubiqui tination by binding to a Cdc4p/Skp1p subcomplex. Taken together, these data illuminate the molecular basis for the G1/S transition in buddin g yeast and suggest a general mechanism for phosphorylation-targeted u biquitination in eukaryotes.