IDENTIFICATION OF BINDING DOMAINS ON THE P21(CIP1) CYCLIN-DEPENDENT KINASE INHIBITOR

Members of the recently discovered family of cyclin-dependent kinases inhibitors (CKIs) appear to play an essential regulatory role in the c ontrol of cell proliferation. To investigate the molecular basis of th e interaction between these proteins and the cyclin-dependent kinases (CDKs), we performed a systematic mutagenesis of the CKI family member p21Cip1 using the alanine-scanning strategy. We have examined the int eraction between in vitro translated human cdk2, cyclins A and D1, pur ified proliferating cell nuclear antigen (PCNA) and a set of human p21 Cip1 mutants fused to glutathione S-transferase. Independent domains t hat are required for the interaction with cdk2 and with PCNA have been identified. The cdk2 binding domain is located in the N-terminal part of the protein, between residues 45 and 60, a region that is fully co nserved in the p27Kip1 inhibitor. A PCNA binding region was localised to the C-terminus of the protein, between residues 142 and 163. These findings define protein motifs that are highly conserved between membe rs of the CKI family and that are likely to play an essential function in the regulation of the G1/S transition.