Post-translational modifications of the tumor suppressor p53: How p53 is activated in response to DNA damage

The tumor suppressor p53 is a transcriptional regulator whose ability to inhibit cell growth is dependent upon its transactivation function. p53 functions as a cell cycle checkpoint activated in response to DNA damage. p53 activation is regulated at the post translational level, where p53 is stabilized and its transcriptional and DNA binding activities are enhanced. We report that, in response to DNA damage, p53 forms a specific protein complex with the transcriptional coactivator CBP/p300 and its associated factor, P/CAF, leading to increased p53 transcriptional activity, and that the adenovirus E1a protein interferes directly with this complex formation. we also demonstrate that stabilization of p53 is mediated by phosphorylation of Ser20, which lies directly within the region required for interaction with the cellular oncoprotein MDM2, which targets p53 for ubiquitin-mediated degradation. We find that the kinase responsible for this phosphorylation is the ATM-dependent DNA damage checkpoint protein Chk2. In Vitro, Chk2 phosphorylates p53 on Ser20 and disrupts the interaction of p53 with Mdm2. In Vivo, Chk2 stabilizes p53 protein and induces G1 cell cycle arrest in a p53 dependent manner. Finally, we show evidence that upon γ-radiation-induced DNA damage; p53 is dephosphorylated within the C-terminal regulatory region at residue Ser376. This event creates a binding site for 14-3-3 proteins, highly conserved proteins that mediate signal transduction events by binding to phosphoserine-containing proteins. The interaction of p53 with 14-3-3 is functionally significant; mutating Ser residues within the 14-3-3 binding site abolishes p53-mediated G1 arrest. These studies contribute to our understanding of how p53 function is regulated in response to DNA damage