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 modulation of gene expression. Casein kinase II phosphorylates the C-terminal serine of p53 (residue 389 for murine p53) and mutation of this site abolishes p53 growth suppressor function. DNA binding by purified p53 is 'activated' by casein kinase II, suggesting that the carboxyl terminus of p53 represents a critical regulatory domain for sequence-specific DNA binding and hence for growth suppressor function. In the present study we have substituted serine 389 with either aspartic acid (mimics 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 p53ala389 and p53asp389 were both indistinguishable from wild type p53 on the basis of size fractionation and immunoreactivity with PAb421, PAb246 and PAb1620. Both mutants also exhibited specific binding for the DNA consensus p53-CON. Since p53ala389 retains the ability to bind DNA and yet is known to lack growth suppressor function we conclude that phosphorylation by casein kinase II is important for p53 growth suppressor function via a mechanism which is ancillary to p53 sequence-specific DNA binding.