Dephosphorylation of p53 at Ser20 after cellular exposure to low levels ofnon-ionizing radiation

Induction of the transactivation function of p53 after cellular irradiation was studied under conditions in which upstream signaling events modulating p53 activation were uncoupled from those regulating stabilization. This in vestigation prompted the discovery of a novel radiation-responsive kinase p athway targeting Ser20 that results in the masking of the DO-1 epitope in u ndamaged cells. Unmasking of the DO-1 epitope via dephosphorylation occurs in response to low doses of nonionizing radiation. Our data show that phosp horylation at Ser20 reduces binding of the mdm2 protein, suggesting that a function of the Ser20-kinase pathway may be to produce a stable pool of ina ctive p53 in undamaged cells which can be readily activated after cellular injury. Phospho-specific monoclonal antibodies were used to determine wheth er the Ser20 signaling pathway is coupled to the Ser15 and Ser392 radiation -responsive kinase pathways. These results demonstrated that: (1) dephospho rylation at Ser20 is co-ordinated with an increased steady-state phosphoryl ation at Ser392 after irradiation, without p53 protein stabilization, and ( 2) stabilization of p53 protein can occur without Ser15 phosphorylation at higher doses of radiation. These data show that the Ser20 and Ser392 phosph orylation sites are both targeted by an integrated network of signaling pat hways which is acutely sensitive to radiation injury.