Dephosphorylation of cyclin-dependent kinases by type 2C protein phosphatases

Activating phosphorylation of cyclin-dependent protein kinases (CDKs) is ne cessary for their kinase activity and cell cycle progression. This phosphor ylation is carried out by the Cdk-activating kinase (CAK); in contrast, lit tle is known about the corresponding protein phosphatase. We show that type 2C protein phosphatases (PP2Cs) are responsible for this dephosphorylation of Cdc28p, the major budding yeast CDK. Two yeast PP2Cs, Ptc2p and Ptc3p, display Cdc28p phosphatase activity in vitro and in vivo, and account for s imilar to 90% of Cdc28p phosphatase activity in yeast extracts. Overexpress ion of PTC2 or PTC3 results in synthetic lethality in strains temperature-s ensitive for yeast CAK1, and disruptions of PTC2 and PTC3 suppress the grow th defect of a cak1 mutant. Furthermore, PP2C-like enzymes are the predomin ant phosphatases toward human Cdk2 in HeLa cell extracts, indicating that t he substrate specificity of PP2Cs toward CDKs is evolutionarily conserved.