THE CDK2 KINASE IS REQUIRED FOR THE G1-TO-S TRANSITION IN MAMMALIAN-CELLS

In the cell cycle of fission and budding yeast, the p34cdc2/cDC28 kina se is required for both the G1-to-S and G2-to-M phase transitions. In vertebrates, the homologous p34cdc2 kinase is required for G2-to-M pha se transitions but appears to be dispensible for DNA synthesis. We hav e investigated the function of a related kinase, p33cdk2, using serum- stimulated quiescent human fibroblasts. While the p33cdk2 protein was expressed at constant levels throughout the cell cycle, p33cdk2 kinase activity was first detected a few hours prior to the onset of DNA syn thesis. Microinjection of anti-p33cdk2 antibodies blocked cells from e ntering S phase. Pre-adsorbtion of these antibodies with cdk2 protein abrogated their blocking effect suggesting that the G1 arrest caused b y these antibodies is cdk2-specific. These results indicate that p33cd k2 is required for the G1-to-S phase transition in mammalian cells. We also show evidence to suggest that the cyclin E/p33cdk2 complex is li kely to be required for entry into S phase since the timing of the cyc lin E-associated kinase activity was coincident with that of p33cdk2 a nd preclearing of either component abolished the majority of the histo ne H1 kinase activity present in the lysates harvested from the late G 1.