THE CAK1P PROTEIN-KINASE IS REQUIRED AT G(1) S AND G(2)/M IN THE BUDDING YEAST-CELL CYCLE/

The CAK1 gene encodes the major CDK-activating kinase (CAK) in budding yeast and is required for activation of Cdc28p for cell cycle progres sion from G(2) to M phase. Here we describe the isolation of a mutant allele of CAK1 in a synthetic lethal screen with the Sit4 protein phos phatase. Analysis of several different cak1 mutants shows that althoug h the G(2) to M transition appears most sensitive to loss of Cak1p fun ction, Cak1p is also required for activation of Cdc28p for progression from G(1) into S phase. Further characterization of these mutants sug gests that, unlike the CAK identified from higher eukaryotes, Cak1p of budding yeast may not play a role in general transcription. Finally, although Cak1 protein levels and in vitro protein kinase activity do n ot fluctuate during the cell cycle, at least a fraction of Cak1p assoc iates with higher molecular weight proteins, which may be important fo r its in vivo function.