Low levels of cyclin D and nonfunctional Rb protein affect cdk6 association with cyclin-dependent kinase inhibitor p27(Kip1)

p27(Kip1) associates with cyclin/cdk complexes and inhibiting cdk activity, and overexpression of p27(Kip1) induces G1 arrest. We found that p27(Kip1) overexpression inhibits cdk2 kinase activity, but not cdk6 kinase activity in HeLa cells. The amount of p27(Kip1) associated with cdk2 was significan tly higher than that associated with cdk6. cdk6 complexes contained detecta ble amounts of p27(Kip1) in all human cell lines examined, except in HeLa c ells where p27(Kip1) preferentially associated with cdk2. It appears that i n HeLa cells overexpressed p27(Kip1) fails to inhibit cdk6 kinase activity because of low binding affinity of cdk6 to p27(Kip1). The low binding affin ity is due to a low level of the cdk6/cyclin D complexes. Functional inacti vation of pRb has an effect on p27(Kip1) association with cdk6/cyclin D com plexes. (C) 2001 Academic Press.