Evidence for a pre‐restriction point Cdk3 activity

AbstractWe have examined the activity of cyclin‐dependent kinase 3 (cdk3) during G1‐phase of the cell cycle in Chinese Hamster Ovary (CHO) fibroblasts. Histone H1 kinase activity associated with anti‐cdk3 immunoprecipitates peaked during a brief window of time, 2–3 h prior to the restriction point. In vitro cdk3 activity was sensitive to roscovitine, a drug previously shown to inhibit cdks 1, 2, and 5, but not cdk4 or 6. Early G1‐phase activation of cdk3 was downregulated by treatment of cells with MG132, an inhibitor of the proteasome, and by the protein synthesis inhibitor cycloheximide. These results provide evidence for a pre‐restriction point cdk3 activity that requires both the synthesis of a regulatory subunit and degradation of an inhibitor. J. Cell. Biochem. 85: 545–552, 2002. © 2002 Wiley‐Liss, Inc.