CDK2 KINASE IS REQUIRED FOR ENTRY INTO MITOSIS AS A POSITIVE REGULATOR OF CDC2-CYCLIN-B KINASE-ACTIVITY

In higher eukaryotes, Cdk2 kinase plays an essential role in regulatin g the G1-S transition. Here, we use cycling Xenopus egg extracts to ex amine the requirement of Cdk2 kinase on progression into mitosis. Inte restingly, when Cdk2 kinase activity is inhibited by the Cdk-specific inhibitor, p21(Clp1), a block to mitosis occurs, and inactive Cdc2-cyc lin B accumulates. This block occurs in the absence of nuclei and is n ot due to direct inhibition of Cdc2 by Cip. Importantly, this block to mitosis is reversible by restoring Cdk2-cyclin E kinase activity to a Cip-treated cycling extract. Moreover, immunodepletion of Cdk2 from i nterphase extracts prevents activation of Cdc2 upon the addition of ex ogenous cyclin B. Thus, our data show that Cdk2 kinase is a positive r egulator of Cdc2-cyclin B complexes and establish a link between Cdk2 kinase and cell cycle progression into mitosis.