| p53 functions as the central tumor suppressor transcription factor coordinating cellular responses to DNA damage and other genomic aberrations, and its activity is governed by a dense network of post-translational modifications rather than by transcriptional induction alone, since basal p53 levels remain low in unstressed cells through continuous MDM2-mediated ubiquitination and proteasomal degradation. DNA damage triggers phosphorylation of human p53 at serine 15, and this modification directly reduces p53's interaction with MDM2 both in cells and in purified biochemical reconstitution; using purified DNA-dependent protein kinase, phosphorylation of p53 at serine 15 together with serine 37 measurably impairs MDM2's ability to inhibit p53-dependent transcriptional activation, with the effect attributed to a conformational change in p53 induced by phosphorylation rather than to altered surface charge alone, since a phosphomimetic serine-to-aspartate substitution at this position fails to prevent MDM2-mediated degradation the way genuine phosphorylation does. This same N-terminal region is phosphorylated by the related kinases ATM, ATR, and DNA-PK, connecting multiple upstream DNA damage-sensing kinases to a shared mechanism of MDM2 disengagement, while the checkpoint kinases Chk1 and Chk2 phosphorylate p53 at a separate site, serine 20, enhancing p53 tetramerization, stability, and transcriptional activity through a mechanism distinct from direct MDM2 binding disruption. Independently of phosphorylation, p53 is acetylated at its C-terminal lysine 382 following DNA damage, and this acetylation is catalyzed by the acetyltransferases p300 and CBP; the deacetylase hSIR2/SIRT1 binds p53 directly and reverses this modification with specificity for the same Lys382 residue, and expression of catalytically active SIRT1 in human cells reduces p53 transcriptional activity, while expression of a catalytically dead SIRT1 mutant instead potentiates p53-dependent apoptosis and radiosensitivity, directly demonstrating that SIRT1's deacetylase activity actively restrains p53 function and that this Lys382 acetylation-deacetylation cycle operates as a genuine on-off regulatory switch rather than a passive modification. |
