FIGURE 7.

The Rad50 zinc hook domain is important for DSB repair because of its essential role in recruiting MRN to chromosomal DSBs. A, Rad50 hook mutant cells are sensitive to IR as revealed by clonogenic survival assay. U2OS cells expressing Rad50WT, Rad50HK-NA, or vector were mock-treated (Ctrl) or silenced for endogenous Rad50 by shRNAs. The cells were irradiated at the indicated doses, and the surviving colonies were counted after 2 weeks. The error bars represent the standard deviation from three independent experiments. B, model of MRN complex recruitment to the DSB ends. MRN complexes bind to the DSB ends and activate ATM, leading to γH2AX phosphorylation. Phosphorylated γH2AX activates MDC1, which recruits more MRN to the DSB-flanking chromatin. The Rad50 hook mutants are defective in ATM activation and downstream signaling cascade and thus fail to activate MRN complexes at DSB ends and to recruit more MRN complexes to DSB-flanking chromatin, resulting in DSB repair defects.