Although p27(Kip1) has been considered a general inhibitor of G1 and S phase 
cyclin-dependent kinases, we report that the interaction of p27 with two such 
kinases, cyclin A-Cdk2 and cyclin D-Cdk4, is different. In Mv1Lu cells 
containing a p27 inducible system, a 6-fold increase over the basal p27 level 
completely inhibited Cdk2 and cell cycle progression. In contrast, the same or a 
larger increase in p27 levels did not inhibit Cdk4 or its homologue Cdk6, 
despite extensive binding to these kinases. A p27-cyclin A-Cdk2 complex formed 
in vitro was essentially inactive, whereas a p27-cyclin D2-Cdk4 complex was 
active as a retinoblastoma kinase and served as a substrate for the 
Cdk-activating kinase Cak. High concentrations of p27 inhibited cyclin D2-Cdk4, 
apparently by conversion of active complexes into inactive ones by the binding 
of additional p27 molecules. In contrast to their differential interaction, 
cyclin A-Cdk2 and cyclin D2-Cdk4 were similarly inhibited by bound 
p21(Cip1/Waf1). Roles of cyclin A-Cdk2 as a p27 target and cyclin D2-Cdk4 as a 
p27 reservoir may result from the differential ability of bound p27 to inhibit 
the kinase subunit in these complexes.