We previously reported that protein kinase CK2 phosphorylates the human mdm2 (hdm2) protein at serine residue 269. This phosphorylation site is located in the central acidic, highly-conserved region of mdm2, which is responsible for the interaction with a number of proteins. Studying the influence of phosphorylation of mdm2 by CK2 upon interaction with some of these binding partners, we found that the retinoblastoma (Rb) protein bound more strongly to the unphosphorylated mdm2 than to its CK2-phosphorylated counterpart. An S269 phosphospecific antibody was generated, and reacted with a 60 kDa subpopulation of mdm2 in human cells. We created a mutant mdm2 with a serine to aspartic acid exchange at position 269, which was used to transfect mdm2−/− cells. Cells transfected with the S269D mutant exhibited a different growth behavior than wild-type mdm2-expressing cells, which might be attributed to the altered Rb-mdm2 interaction.

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