An inhibitor of p34^CDC28 protein kinase activity from Saccharomyces cerevisiae

The p34^CDC28 protein from Saccharomyces cerevisiae is a homolog of the p34^cdc2 protein kinase, a fundamental regulator of cell division in all eukaryotic cells. Once activated it initiates the visible events of mitosis (chromosome condensation, nuclear envelope breakdown, and spindle formation). The p34^CDC28 protein also has a critical role in the initiation of DNA synthesis. The protein kinase activity is regulated by cycles of phosphorylation and dephosphorylation and by periodic association with cyclins. An endogenous 40-kilodalton protein (p40) originally identified as a substrate of the p34^CDC28 protein kinase was purified. The p40 protein bound tightly to p34^CDC28 and inhibited the activity of the kinase. The p40 protein may provide another mechanism to regulate p34 ^CDC28 protein kinase activity.