The Cdk-activating kinase (CAK) from budding yeast

Activation of the cyclin-dependent kinases to promote cell cycle progression requires their association with cyclins as well as phosphorylation of a threonine (residue 161 in human p34(cdc2)). This phosphorylation 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. coli, and is not a component of the basal transcription factor, TFIIH. A temperature-sensitive mutation in CAK1 confers a G2 delay accompanied by low Cdc28p protein kinase activity and shows genetic interactions with altered expression of the gene for the major mitotic cyclin, CLB2. Our data raise the intriguing possibility that p40(MO15)-cyclin H-MAT1, identified as the predominant CAK in vertebrate cell extracts, may not function as a physiological CAK.