THE MO15 GENE ENCODES THE CATALYTIC SUBUNIT OF A PROTEIN-KINASE THAT ACTIVATES CDC2 AND OTHER CYCLIN-DEPENDENT KINASES (CDKS) THROUGH PHOSPHORYLATION OF THR161 AND ITS HOMOLOGS

Phosphorylation of Thr161, a residue conserved in all members of the c dc2 family, has been reported to be absolutely required for the cataly tic activity of cdc2, the major regulator of eukaryotic cell cycle. In the present work, we have purified from starfish oocytes a kinase tha t specifically activates cdc2 in a cyclin-dependent manner through pho sphorylation of its Thr161 residue. Our most highly purified preparati on contained only two major proteins of apparent M(r) 37 and 40 kDa (p 37 and p40), which could not be separated from each other without loss of activity. The purified kinase was found to phosphorylate not only cdc2, but also cdk2 and a divergent cdc2-like protein from Caenorhabdi tis, in chimeric complexes including both mitotic and G1/S cyclins. Ex tensive microsequencing of p40 did not reveal any convincing homology with any known protein. In contrast, p37 is the starfish homologue of the M015 gene product, a kinase previously cloned by homology probing from a Xenopus cDNA library. As expected, immunodepletion of the M015 protein depleted Xenopus egg extracts of CAK (cdk-activating kinase) a ctivity, which was recovered in immunoprecipitates. Taken together, th e above results demonstrate that MO15 is a gene conserved throughout e volution (at least from echinoderms to vertebrates) that encodes the c atalytic subunit of a protein kinase that activates cdc2-cdks complexe s through phosphorylation of Thr161 (or its homologues).