CDK-INTERACTING PROTEIN-1 DIRECTLY BINDS WITH PROLIFERATING CELL NUCLEAR ANTIGEN AND INHIBITS DNA-REPLICATION CATALYZED BY THE DNA-POLYMERASE-DELTA HOLOENZYME

Cdk-interacting protein 1 (Cip1) is a p53-regulated 21-kDa protein tha t inhibits several members of the cyclin-dependent kinase (CDK) family . It was initially observed in complexes containing CDK4, cyclin D, an d proliferating cell nuclear antigen (PCNA). PCNA, in conjunction with activator 1, acts as a processivity factor for eukaryotic DNA polymer ase (pol) delta, and these three proteins constitute the pol delta hol oenzyme. In this report, we demonstrate that Cip1 can also directly in hibit DNA synthesis in vitro by binding to PCNA. Cip1 efficiently inhi bits sinian virus 40 replication dependent upon pol alpha, activator 1 , PCNA, and pol delta, and this inhibition can be overcome by addition al PCNA. Simian virus 40 DNA replication, catalyzed solely by high lev els of pol alpha-primase complex, is unaffected by Cip1. Using the sur face plasmon resonance technique, a direct physical interaction of PCN A and Cip1 was detected. We have observed that Cip1 efficiently inhibi ts synthesis of long (7.2 kb) but not short (10 nt) templates, suggest ing that its association with PCNA is likely to impair the processive movement of pol delta during DNA chain elongation, as opposed to block ing assembly of the pol delta holoenzyme. The implications of the Cip1 -PCNA interaction with respect to regulation of DNA synthesis, cell cy cle checkpoint control, and DNA repair are discussed.