Phosphorylation of p53 protein in response to ionizing radiation occurs at multiple sites in both normal and DNA-PK deficient cells

The tumour suppressor gene product, p53, is involved in mediating cellular responses to DNA damage including growth arrest and/or apoptosis, The mechanism by which p53 protein senses the presence of damaged DNA is not understood. The possibility that p53 may be posttranslationally modified by enzymes that are activated in response to DNA damage including DNA-dependent protein kinase (DNA-PK), poly(ADP-ribose) polymerase and stress activated protein kinase has received considerable attention. Recent studies have indicated that DNA-PK is not required for the transactivation or apoptosis-promoting activities of p53 protein. However, the possibility that other functions of p53 may be dependent on phosphorylation by DNA-PK has not been explored. Here we describe a series of experiments that compares the expression, function and phosphorylation status of p53 protein in normal and DNA-PK-deficient scid cells. While several novel p53 phosphoforms are generated in response to DNA damage in normal cells, the same phosphoforms are observed in scid cells.