FUNCTIONAL ACTIVATION OF P53 VIA PHOSPHORYLATION FOLLOWING DNA-DAMAGEBY UV BUT NOT GAMMA-RADIATION

The tumor suppressor p53 is a nuclear phosphoprotein in which DNA-bind ing activity is increased on exposure to DNA-damaging agents such as U V or gamma radiation by unknown mechanisms. Because phosphorylation of p53 at the casein kinase (CK) II site activates p53 for DNA binding f unction in vitro, we sought to determine the in vivo relevance of phos phorylation at this site after UV and gamma radiation. A polyclonal an tibody was generated that binds to bacterially expressed p53 only when phosphorylated in vitro by CK II. Using this antibody, we showed that p53 is phosphorylated at the CK II site upon UV treatment of early pa ssage rat embryo fibroblasts and RKO cells. In addition, DNA-binding a ssays indicated that phosphorylated p53 bound to a p53-responsive elem ent, suggesting functional activation. However, gamma radiation, which also stabilizes p53, did not result in phosphorylation at the CK II s ite. These results indicate that phosphorylation at the CK II site is one of the post-translational mechanisms through which p53 is activate d in response to UV radiation and that different mechanisms activate p 53 after DNA damage by gamma radiation.