Cell Type- and Promoter-specific Roles of Ser super(18) Phosphorylation in Regulating p53 Responses

Phosphorylation of mouse p53 at Ser super(18) occurs after DNA damage. To determine the physiological roles of this phosphorylation event in p53-dependent DNA damage responses, a Ser super(18) to Ala missense mutation was introduced into the germline of mice. Thymocytes and fibroblasts from the knock-in mice show reduced transactivation of many p53 target genes following DNA damage. p53 protein stabilization and DNA binding are similar in knock-in and wild type mice, but C-terminal acetylation was defective, consistent with a role for Ser super(18) in the recruitment of transcriptional co-activators. The apoptotic response of knock-in thymocytes to ionizing radiation is intermediate between that of wild type and p53 null thymocytes. Despite impaired transcriptional and apoptotic responses, the knock-in mice are not prone to spontaneous tumorigenesis. This indicates that neither phosphorylation of p53 on Ser super(18) by ATM nor a full transcriptional response is essential to prevent spontaneous tumor formation in mice.