In 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, p21(Cip1), 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.
Guadagno, T. M., & Newport, J. W. (1996). Cdk2 kinase is required for entry into mitosis as a positive regulator of Cdc2-cyclin B kinase activity. Cell, 84(1), 73–82. https://doi.org/10.1016/S0092-8674(00)80994-0