Abstract
DNA double-strand break repair involves phosphorylation of histone variant H2AX ('γH2AX'), which accumulates in foci at sites of DNA damage. In current models, the recruitment of multiple DNA repair proteins to γH2AX foci depends mainly on recognition of this 'mark' by a single protein, MDC1. However, DNA repair proteins accumulate at γH2AX sites without MDC1, suggesting that other 'readers' of this mark exist. Here, we use a quantitative chemical proteomics approach to profile direct, phospho-selective γH2AX binders in native proteomes. We identify γH2AX binders, including the DNA repair mediator 53BP1, which we show recognizes γH2AX through its BRCT domains. Furthermore, we investigate the targeting of wild-type 53BP1, or a mutant form deficient in γH2AX binding, to chromosomal breaks resulting from endogenous and exogenous DNA damage. Our results show how direct recognition of γH2AX modulates protein localization at DNA damage sites, and suggest how specific chromatin mark-reader interactions contribute to essential mechanisms ensuring genome stability.