SEPARATE DOMAINS OF P21 INVOLVED IN THE INHIBITION OF CDK KINASE AND PCNA

THE protein p21 (WAF1, CIP1 or sdi1), induced by the tumour-suppressor protein p53, interacts with and inhibits two different targets essent ial for cell-cycle progression(1-8). One of these is the cyclin-Cdk fa mily of kinases and the other is the essential DNA replication factor, proliferating-cell nuclear antigen (PCNA). We report here that separa te domains of p21 are responsible for interacting with and inhibiting the two targets. An amino-terminal domain inhibits cyclin-Cdk kinases and a carboxy-terminal domain inhibits PCNA. Using these separated dom ains, we have determined that p21 inhibits different biological system s through different targets. The PCNA-binding domain is sufficient for inhibition of DNA replication based on sinian virus 40, whereas the C dk2-binding domain is sufficient for inhibition of DNA replication bas ed on Xenopus egg extract and for growth suppression in transformed hu man cells.