Influence of tetramerisation on site-specific post-translational modifications of p53: comparison of human and murine p53 tumor suppressor protein.

The tumor suppressor protein p53 is considered the "Guardian of the Genome", crucial for cell cycle control and mutated in over 50% of human cancers. Following cellular stress, post-translational modifications such as phosphorylation and acetylation stabilise and activate p53 for cell cycle arrest, DNA repair, apoptosis or senescence. p53 protein functions as a tetramer and we have shown that loss of tetramerisation and changes at the N-terminus influence the recovery of wild type p53 'status'. To investigate the relationship between tetramerisation and post-translational modifications we examined a range of site-specific modifications in wild type and dimeric mutant (M340Q/L344R) murine p53 expressed in MEFs p53(-/-) and in wild type, monomeric (L344P) and dimeric (M340Q/L344R) human p53 expressed in HCT116 p53(-/-) cells. Using site-specific antibodies we demonstrate that in murine p53, S15 is phosphorylated in a tetramerisation-dependent manner. In contrast, human p53 S15 phosphorylation is not tetramerisation-dependent. Inability to form tetramers in human p53 proteins reduced site-specific N-terminal phosphorylation at S6, S9 and S46 and reduced C-terminal phosphorylation and acetylation at S315 and K382 respectively. In addition, p53 tetramerisation is required for efficient p21 and hdm2 transcription and protein expression and recruitment of p53 to specific promoter regions of p21 and hdm2.