Radiation injury to cells enhances C-terminal phosphorylation of p53 at both Ser315 and Ser392 in vivo, suggesting the existence of two cooperating DNA damage-responsive pathways that play a role in stimulating p53-dependent gene expression. Our previous data has shown that cyclin A-cdk2 is the major enzyme responsible for modifying p53 at Ser315 in vivo after irradiation damage and in this report we dissect the mechanism of cyclinA-cdk2 binding to and phosphorylation of p53. Although cyclin B(1)-dependent protein kinases can phosphorylate small peptides containing the Ser315 site, cyclin A-cdk2 does not phosphorylate such small peptides suggesting that additional determinants are required for cyclin A-cdk2 interaction with p53. Peptide competition studies have localized a cyclin A interaction site to a Lys381Lys382Leu383Met384Phe385 sequence within C-terminal negative regulatory domain of human p53. An alanine mutation at any one of four key positions abrogates the efficacy of a synthetic peptide containing this motif as an inhibitor of cyclin A-cdk2 phosphorylation of p53 protein. Single amino acid mutations of full-length p53 protein at Lys382, Leu383, or Phe385 decreases cyclin A-cdk2 dependent phosphorylation at Ser315. Cyclin B(1)-cdk2 complexes are not inhibited by KKLMF motif-containing peptides nor is p53 phosphorylation by cyclin B-cdk2 reduced by mutation of the cyclin A interaction site. These data identifying a KKLMF cyclin A docking site on p53 protein highlight a common cyclin A interaction motif that is shared between the tumour suppressor proteins pRb and p53.
Copyright 2000 Academic Press.