Regulation of the accumulation and function of p53 by phosphorylation of two residues within the domain that binds to Mdm2

The function and stability of the tumor suppressor p53 are tightly controlled by the negative regulator mouse double minute 2 (Mdm2), which binds to p53, blocking DNA binding and targeting p53 for proteosome-mediated degradation. Following DNA damage or cellular stress, p53 is phosphorylated within the Mdm2 binding domain on threonine 18 and serine 20. To analyze the roles of these phosphorylation events, residues IS and 20 were mutated to alanines. Transient transfection into p53-null cells demonstrated that the T18A protein can be expressed stably, but the S20A protein is very unstable, precluding further analysis. When expressed stably at low basal levels in p53-null human fibroblasts or fibrosarcoma cells, the T18A mutant accumulated 5-10-fold less well than wild-type p53 following exposure to UV. Analysis of p53-dependent transcription following UV revealed that the phosphorylation of threonine 18 is required for transactivation of the p21, Hdm2 (the human ortholog of Mdm2), and GADD45 genes. The phosphorylation of serine 33, another early event following DNA damage, is not required for p53 accumulation or p53-dependent transactivation following UV.