Accumulation of Checkpoint Protein 53BP1 at DNA Breaks Involves Its Binding to Phosphorylated Histone H2AX

53BP1 participates in the cellular response to DNA damage. Like many proteins involved in the DNA damage response, 53BP1 becomes hyperphosphorylated after radiation and colocalizes with phosphorylated H2AX in megabase regions surrounding the sites of DNA strand breaks. However, it is not yet clear whether the phosphorylation status of 53BP1 determines its localization or vice versa. In this study we mapped a region upstream of the 53BP1 C terminus that is required and sufficient for the recruitment of 53BP1 to these DNA break areas. In vitro assays revealed that this region binds to phosphorylated but not unphosphorylated H2AX. Moreover, using H2AX-deficient cells reconstituted with wild-type or a phosphorylation-deficient mutant of H2AX, we have shown that phosphorylation of H2AX at serine 140 is critical for efficient 53BP1 foci formation, implying that a direct interaction between 53BP1 and phosphorylated H2AX is required for the accumulation of 53BP1 at DNA break sites. On the other hand, radiation-induced phosphorylation of the 53BP1 N terminus by the ATM (ataxia-telangiectasia mutated) kinase is not essential for 53BP1 foci formation and takes place independently of 53BP1 redistribution. Thus, these two damage-induced events, hyperphosphorylation and relocalization of 53BP1, occur independently in the cell.