Mdm2 binding to a conformationally sensitive domain on p53 can be modulated by RNA

Biochemical characterisation of the interaction of mdm2 protein with p53 protein has demonstrated that full‐length mdm2 does not bind stably to p53–DNA complexes, contrasting with C‐terminal truncations of mdm2 which do bind stably to p53–DNA complexes. In addition, tetrameric forms of the p53His175 mutant protein in the PAb1620+ conformation are reduced in binding to mdm2 protein. These data suggest that the mdm2 binding site in the BOX‐I domain of p53 becomes concealed when either p53 binds to DNA or when the core domain of p53 is unfolded by missense mutation. This further suggests that the C‐terminus of mdm2 protein contains a negative regulatory domain that affects mdm2 protein binding to a second, conformationally sensitive interaction site in the core domain of p53. We investigated whether there was a second docking site on p53 for mdm2 protein by examining the interaction of full‐length mdm2 with p53 lacking the BOX‐I domain. Although mdm2 protein did bind very weakly to p53 protein lacking the BOX‐I domain, addition of RNA activated mdm2 protein binding to this truncated form of p53. These data provide evidence for three previously undefined regulatory stages in the p53–mdm2 binding reaction: (1) conformational changes in p53 protein due to DNA binding or point mutation conceals a secondary docking site of mdm2 protein; (2) the C‐terminus of mdm2 is the primary determinant which confers this property upon mdm2 protein; and (3) mdm2 protein binding to this secondary interaction site within p53 can be stabilised by RNA.