SPECIFIC DNA-BINDING BY P53 IS INDEPENDENT OF MUTATION AT SERINE-389,THE CASEIN KINASE-II SITE

The best understood function of p53 is that of cell growth suppression and this is likely to involve sequence-specific DNA binding and modul ation of gene expression. Casein kinase II phosphorylates the C-termin al serine of p53 (residue 389 for murine p53) and mutation of this sit e abolishes p53 growth suppressor function. DNA binding by purified p5 3 is 'activated' by casein kinase II, suggesting that the carboxyl ter minus of p53 represents a critical regulatory domain for sequence-spec ific DNA binding and hence for growth suppressor function. In the pres ent study we have substituted serine 389 with either aspartic acid (mi mics phosphoserine and partially conserves p53 suppressor function) or with alanine, a non-phosphorylable residue which abolishes suppressor function (Milne et al., 1992; Nucleic Acids Research 20, 5565-5570). When expressed in vitro p53(ala389) and p53(asp389) were both indistin guishable from wild type p53 on the basis of size fractionation and im munoreactivity with PAb421, PAb246 and PAb1620. Both mutants also exhi bited specific binding for the DNA consensus p53-CON. Since p53(ala389 ) retains the ability to bind DNA and yet is known to lack growth supp ressor function we conclude that phosphorylation by casein kinase II i s important for p53 growth suppressor function via a mechanism which i s ancilliary to p53 sequence-specific DNA binding.