THE CDK-ACTIVATING KINASE (CAK) FROM BUDDING YEAST

Activation of the cyclin-dependent kinases to promote cell cycle progr ession requires their association with cyclins as well as phosphorylat ion of a threonine (residue 161 in human p34(cdc2)). This phosphorylat ion is carried out by CAK, the Cdk-activating kinase. We have purified and cloned CAK from S. cerevisiae. Unlike CAKs from other organisms, Cak1p is active as a monomer, has full activity when expressed in E. c oli, and is not a component of the basal transcription factor, TFIIH. A temperature-sensitive mutation in CAK1 confers a G2 delay accompanie d by low Cdc28p protein kinase activity and shows genetic interactions with altered expression of the gene for the major mitotic cyclin, CLB 2. Our data raise the intriguing possibility that p40(MO15)-cyclin H-M AT1, identified as the predominant CAK in vertebrate cell extracts, ma y not function as a physiological CAK.