Two critical hydrophobic amino acids in the N-terminal domain of the p53 protein are required for the gain of function phenotypes of human p53 mutants.

Some mutant forms of the p53 protein have been shown to gain new functions that are not shared by the wild-type p53 protein; (1) mutant p53 proteins can transcriptionally transactivate the multi-drug resistance gene-1 (MDR-1) and (2) when expressed in non-tumorigenic cells with no endogenous p53 protein, mutant p53 proteins can enhance the tumorigenic potential of these cells (Dittmer et al., 1993). It has recently been shown (Lin et al., 1994b) that the transcriptional activator domain of the p53 protein contains two amino acids, leu-22 and trp-23, which are required by the wild-type p53 protein for transcriptional activity. To determine whether these same amino acid residues are utilized by mutant p53 proteins for their gain of function phenotype, the triple mutant p53 protein (at residues 22 and 23 in the transactivation domain and residue 281 in the DNA binding domain--a gain of function mutant) was made. While the p53-281 mutant transcriptionally activates the MDR-1 gene and enhances the tumorigenic potential of cells it is expressed in, the 22, 23, 281 triple mutant failed to carry out either of these functions.