Phosphorylation of p53 protein in response to ionizing radiation occurs atmultiple 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 mecha nism by which p53 protein senses the presence of damaged DNA is not underst ood. The possibility that p53 may be posttranslationally modified by enzyme s that are activated in response to DNA damage including DNA-dependent prot ein kinase (DNA-PK), poly(ADP-ribose) polymerase and stress activated prote in kinase has received considerable attention. Recent studies have indicate d that DNA-PK is not required for the transactivation or apoptosis-promotin g 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, fun ction and phosphorylation status of p53 protein in normal and DNA-PK-defici ent scid cells. While several novel p53 phosphoforms are generated in respo nse to DNA damage in normal cells, the same phosphoforms are observed in sc id cells.