Fission yeast Csk1 is a CAK-activating kinase (CAKAK)

Cell cycle progression is dependent on the sequential activity of cyclin-de pendent kinases (CDKs), For full activity, CDKs require an activating phosp horylation of a conserved residue (corresponding to Thr160 in human CDK2) c arried out by the CDK-activating kinase (CAK), Two distinct CAK kinases hav e been described: in budding yeast Saccharomyces cerevisiae, the Cak1/Civ1 kinase is responsible for CAK activity. In several other species including human, Xenopus, Drosophila and fission yeast Schizosaccharomyces pombe, CAK has been identified as a complex homologous to CDK7-cyclin H (Mcs6-Mcs2 in fission yeast). Here we identify the fission yeast Csk1 kinase as an in vi vo activating kinase of the Mcs6-Mcs2 CAK defining Csk1 as a CAK-activating kinase (CAKAK).