Identification of a CK2 phosphorylation site in mdm2

Mdm2 is a cellular oncoprotein the most obvious function of which is the do wn-regulation of the growth suppressor protein p53. It represents a highly phosphorylated protein but only little is yet known about the sites phospho rylated in vivo, the kinases that are responsible for the phosphorylation o r the functional relevance of the phosphorylation status, Recently, we have shown that mdm2 is a good-substrate for protein kinase CK2 at least in vit ro. Computer analysis of the primary amino acid sequence of mdm2 revealed 1 9 putative CK2 phosphorylation sites. By using deletion mutants of mdm2 and a peptide library we identified the serine residue at position 269 which l ies within a canonical CK2 consensus sequence (EGQELSDEDDE) as the most imp ortant CK2 phosphorylation site. Moreover, by using the mdm2 S269A mutant f or in vitro phosphorylation assays this site was shown to be phosphorylated by CK2. Binding studies revealed-that phosphorylation of mdm2 at S269 does not have any influence on the binding of p53 to mdm2.