INVESTIGATION OF THE CELL-CYCLE REGULATION OF CDK3-ASSOCIATED KINASE-ACTIVITY AND THE ROLE OF CDK3 IN PROLIFERATION AND TRANSFORMATION

The G1-S transition in mammalian cells has been demonstrated to requir e the cyclin-dependent kinases cdk2, cdk3 and cdk4/6. Here we show tha t a novel kinase activity associated with cdk3 fluctuates throughout t he cell cycle differently from the expression of cyclin D1-, E- and A- associated kinase activities. Cdk3 kinase activity is neither affected by p16 (in contrast to cdk4/6) nor by E2F-1 (in contrast to cdk2), bu t is downregulated upon transient p27 expression. We found cdk3 to bin d to p21 and p27, We provide evidence that p27 could be involved in th e regulation of the cell cycle fluctuation of cdk3 activity: cdk3 prot ein does not fluctuate and interaction of cdk3 with p27, but not with p21, is lost when cdk3 kinase becomes active during the cell cycle. In Myc-overexpressing cells, but not in normal Rat1 cells, constitutive ectopic expression of cdk3 induces specific upregulation of cdk3-assoc iated kinase activity that is still cell cycle phase dependent. Ectopi c cdk3, but not cdk2, enhances Myc-induced proliferation and anchorage -independent growth associated with Myc activation, without effects on cyclin D1, E- and A protein expression or kinase activities. High lev els of cdk3 in Myc-overexpressing cells trigger up- and deregulation o f E2F-dependent transcription without inducing the E2F-DNA binding cap acity. In contrast to all other studied positive G1 regulators, cdk3 i s unable to cooperate with ras in fibroblast transformation suggesting a function of cdk3 in G1 progression that is different from cyclin Do r E-associated kinase activities. Our data provide first insights into the regulation of cdk3-associated kinase activity and suggest a model how cdk3 participates in the regulation of the G1-S transition.