Serine15 phosphorylation stimulates p53 transactivation but does not directly influence interaction with HDM2

The p53 tumour suppressor protein is a labile transcription factor that is activated and stabilized in response to a wide range of cellular stresses, through a mechanism involving disruption of its interaction with MDM2, a ne gative regulatory partner. Induction of p53 by DNA damage additionally invo lves a series of phosphorylation and acetylation modifications, some of whi ch are thought to regulate MDM2 binding. Here we report the effects of intr oducing mutations at several known or putative N-terminal phosphorylation s ites on the transactivation function of p53. These studies highlight phosph orylation of Ser15, a key phosphorylation target during the p53 activation process, as being critical for p53-dependent transactivation. Biochemical d ata indicate that the mechanism by which phosphorylation of Ser15 stimulate s p53-dependent transactivation occurs through increased binding to the p30 0 coactivator protein. The data also indicate that Ser15-dependent regulati on of transactivation is independent of any involvement in modulating MDM2 binding, and that Ser15 phosphorylation alone is not sufficient to block th e p53-MDM2 interaction.