HUMAN CYCLIN-DEPENDENT KINASE-2 IS ACTIVATED DURING THE S-PHASE AND G(2)-PHASE OF THE CELL-CYCLE AND ASSOCIATES WITH CYCLIN-A

We have analyzed the cell cycle regulation of human cyclin-dependent k inase 2 (CDK2), a protein closely related to the cell cycle-regulatory protein kinase CDC2. We find that CDK2 activity, like that of CDC2, o scillates during the cell cycle in cultured mammalian fibroblasts. Unl ike CDC2 activity (which peaks during mitosis), CDK2 activity rises in late G1 or early S phase and declines during mitosis. Active S-phase CDK2 migrates in multiple large complexes on gel filtration, and CDK2 in one of these complexes is associated with cyclin A. These findings suggest that CDK2 and CDC2, in association with distinct cyclins, regu late separate functions in the mammalian cell cycle.