Low Levels of Cyclin D and Nonfunctional Rb Protein Affect cdk6 Association with Cyclin-Dependent Kinase Inhibitor p27Kip1

Abstract

p27Kip1 associates with cyclin/cdk complexes and inhibiting cdk activity, and overexpression of p27Kip1 induces G1 arrest. We found that p27Kip1 overexpression inhibits cdk2 kinase activity, but not cdk6 kinase activity in HeLa cells. The amount of p27Kip1 associated with cdk2 was significantly higher than that associated with cdk6. cdk6 complexes contained detectable amounts of p27Kip1 in all human cell lines examined, except in HeLa cells where p27Kip1 preferentially associated with cdk2. It appears that in HeLa cells overexpressed p27Kip1 fails to inhibit cdk6 kinase activity because of low binding affinity of cdk6 to p27Kip1. The low binding affinity is due to a low level of the cdk6/cyclin D complexes. Functional inactivation of pRb has an effect on p27Kip1 association with cdk6/cyclin D complexes.