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

Abstract

The p53 tumor suppressor protein is stabilized in response to ionizing radiation and accumulates in the nucleus. Stabilization is thought to involve disruption of the interaction between the p53 protein and Mdm2, which targets p53 for degradation. Here we show that the direct association between a p53 N-terminal peptide and Mdm2 is disrupted by phosphorylation of the peptide on Thr18 but not by phosphorylation at other N-terminal sites, including Ser¹⁵ and Ser³⁷. Thr¹⁸ was phosphorylated in vitro by casein kinase (CK1); this process required the prior phosphorylation of Ser¹⁵. Thr¹⁸ was phosphorylated in vivo in response to DNA damage, and such phosphorylation required Ser¹⁵. Our results suggest that stabilization of p53 after ionizing radiation may result, in part, from an inhibition of Mdm2 binding through a phosphorylation-phosphorylation cascade involving DNA damage-activated phosphorylation of p53 Ser¹⁵ followed by phosphorylation of Thr¹⁸.