Damage-mediated phosphorylation of human p53 threonine 18 through a cascade mediated by a casein 1-like kinase - Effect on Mdm2 binding

The p53 tumor suppressor protein is stabilized in response to ionizing radi ation and accumulates in the nucleus. Stabilization is thought to involve d isruption of the interaction between the p53 protein and Mdm2, which target s p53 for degradation. Here we show that the direct association between a p 53 N-terminal peptide and Mdm2 is disrupted by phosphorylation of the pepti de on Thr's but not by phosphorylation at other N-terminal sites, including Ser(15) and Ser(37). Thr(18) was phosphorylated in vitro by casein kinase (CK1); this process required the prior phosphorylation of Ser's Thr's was p hosphorylated in vivo in response to DNA damage, and such phosphorylation r equired Ser's. Our results suggest that stabilization of p53 after ionizing radiation may result, in part, from an inhibition of Mdm(2) binding throug h a phosphorylation-phosphorylation cascade involving DNA damage-activated phosphorylation of p53 Ser(15) followed by phosphorylation of Thr(18).