P21 BINDING TO PCNA CAUSES G1 AND G2 CELL-CYCLE ARREST IN P53-DEFICIENT CELLS

A unique feature of p21 that distinguishes it from the other cyclin-de pendent kinase (CDK) inhibitors is its ability to associate with the p roliferating cell nuclear antigen (PCNA), an auxiliary factor for DNA polymerases delta and epsilon. While it is now well established that i nhibition of cyclin/CDK complexes by p21 can result in G1 cell cycle a rrest, the consequences of p21/PCNA interaction on cell cycle progress ion have not yet been determined. Here, we show, using a tetracycline- regulated system, that expression of wild-type p21 in p53-deficient DL D1 human colon cancer cells inhibits DNA synthesis and causes G1 and G 2 cell cycle arrest. Similar effects are observed in cells expressing p21(CDK-) mutant impaired in the interaction with CDKs, but not in cel ls expressing p21(PCNA-), a mutant deficient for the interaction with PCNA. Analysis of cells treated with a p21-derived PCNA-binding peptid e provides additional evidence that the growth inhibitory effects of p 21 and p21(CDK-) result from their ability to bind to PCNA. Our result s suggest that p21 might inhibit cell cycle progression by two indepen dent mechanisms, inhibition of cyclin/CDK complexes, and inhibition of PCNA function resulting in both G1 and G2 arrest.