Mutational inactivation of the p53 gene product is one of the most common genetic aberations so far identified in human cancers. The p53 protein suppresses the transformed phenotype by transactivation or repression of genes involved in cell growth control. Missense mutations in the p53 protein coding sequence observed in human cancers are clustered within a central conserved (conformational) domain spanning amino acid residues 100-300 of a total of 393. Using the conformational domain of p53 fused with protein A, we have shown that the p53 conformational domain possesses Zn+2-dependent, sequence-specific DNA-binding activity. In addition to binding DNA, this domain interacts with at least five cellular proteins ranging in sizes from 30K to 90K M(r) and with the SV40 large T antigen viral oncoprotein. We investigated these cellular proteins for their modulatory effects on the sequence-specific DNA binding activity of full-length wild-type p53. A mixture of p53 conformational domain-binding proteins in bulk enhanced the DNA-binding activity of p53 greater than two-fold. Selective elution of the p53-binding proteins from the p53 hybrid protein by using a sequential step-wise NaCl gradient implicated one protein of 35K M(r) as contributing to a greater than four-fold activation of p53 DNA-binding activity. A p53 conformational domain protein containing a tumor-derived mutation at amino acid 175 failed to associate with the 35K M(r) protein. We propose that proteins interacting with the conformational domain of wild type p53 regulate the DNA-binding activity of p53, thus providing a biochemical basis for the alterations in its function induced by point mutations.