INHIBITION OF CYCLIN-DEPENDENT KINASES BY P21

p21(Cip1) is a cyclin-dependent kinase (Cdk) inhibitor that is transcr iptionally activated by p53 in response to DNA damage. We have explore d the interaction of p21 with the currently known Cdks. p21 effectivel y inhibits Cdk2, Cdk3, Cdk4, and Cdk6 kinases (K-i 0.5-15 nM) but is m uch less effective toward Cdc2/cyclin B (K-i similar to 400 nM) and Cd k5/p35 (K-i > 2 mu M), and does not associate with Cdk7/cyclin H. Over expression of p21 arrests cells in G1. Thus, p21 is not a universal in hibitor of Cdks but displays selectivity for G1/S Cdk/cyclin complexes . Association of p21 with Cdks is greatly enhanced by cyclin binding. This property is shared by the structurally related inhibitor p27, sug gesting a common biochemical mechanism for inhibition. With respect to Cdk2 and Cdk4 complexes, p27 shares the inhibitory potency of p21 but has slightly different kinase specificities. In normal diploid fibrob lasts, the vast majority of active Cdk2 is associated with p21, but th is active kinase can be fully inhibited by addition of exogenous p21. Reconstruction experiments using purified components indicate that mul tiple molecules of p21 can associate with Cdk/cyclin complexes and ina ctive complexes contain more than one molecule of p21. Together, these data suggest a model whereby p21 functions as an inhibitory buffer wh ose levels determine the threshold kinase activity required for cell c ycle progression.