Cdk2 Kinase Is Required for Entry into Mitosis as a Positive Regulator of Cdc2–Cyclin B Kinase Activity

AbstractIn higher eukaryotes, Cdk2 kinase plays an essential role in regulating the G1–S transition. Here, we use cycling Xenopus egg extracts to examine the requirement of Cdk2 kinase on progression into mitosis. Interestingly, when Cdk2 kinase activity is inhibited by the Cdk-specific inhibitor, p21Cip1, a block to mitosis occurs, and inactive Cdc2–cyclin B accumulates. This block occurs in the absence of nuclei and is not 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 interphase extracts prevents activation of Cdc2 upon the addition of exogenous cyclin B. Thus, our data show that Cdk2 kinase is a positive regulator of Cdc2–cyclin B complexes and establish a link between Cdk2 kinase and cell cycle progression into mitosis