PHOSPHORYLATION OF SIC1, A CYCLIN-DEPENDENT KINASE (CDK) INHIBITOR, BY CDK INCLUDING PHO85 KINASE IS REQUIRED FOR ITS PROMPT DEGRADATION

In the yeast Saccharomyces cerevisiae, Sic1, an inhibitor of Clb-Cdc28 kinases, must be phosphorylated and degraded in G(1) for cells to ini tiate DNA replication, and Cln-Cdc28 kinase appears to be primarily re sponsible for phosphorylation of Sic1. The Pho85 kinase is a yeast cyc lin-dependent kinase (Cdk), which is not essential for cell growth unl ess both CLN1 and CLN2 are absent. We demonstrate that Pho85, when com plexed with Pcl1, a G(1) cyclin homologue, can phosphorylate Sic1 in v itro, and that Sic1 appears to be more stable in pho85 Delta cells. Th ree consensus Cdk phosphorylation sites present in Sic1 are phosphoryl ated in vivo, and two of them are required for prompt degradation of t he inhibitor. Pho85 and other G(1) Cdks appear to phosphorylate Sic1 a t different sites in vivo. Thus at least two distinct Cdks can partici pate in phosphorylation of Sic1 and may therefore regulate progression through G(1).