Structural Insights into γH2Ax containing Nucleosomes

Abstract

MDC1 is a key mediator of DNA-damage signaling. When DNA double-strand breaks (DSB) occur, the histone variant H2AX on the nucleosome is phosphorylated on its C-terminus at residue Ser139 to form the γH2AX nucleosome. This phosphorylated form is specifically recognized by the tandem BRCT repeats of MDC1. The MDC1-bound nucleosome serves as a docking platform to promote the localization of other DNA repair factors. To further characterize the nucleosome- BRCT interaction, we developed a time efficient two-step modified native chemical ligation protocol to prepare phosphorylated nucleosomes. Our binding studies show that BRCT interacts with the nucleosome with a higher affinity than the phosphorylated peptide. Using cryogenic electron microscopy (cryo-EM), we obtained structures of the γH2AX nucleosome revealing the structural basis for nucleosome-nucleosome stacking promoted by interactions of the H4 Nterminal of one nucleosome with its stacked partner. In contrast, we show that binding of the MDC1 BRCT domain disrupts this stacking, suggesting that histone/DNA dynamics are integral to DNA damage signaling.