Following exposure to ionizing radiation, a number of recombination/repair proteins and complexes relocalize to nuclear foci that are believed to correspond to the sites of double-strand breaks. These proteins include gamma-H2AX, ATM, RAD51, BRCA1, BRCA2, NBS1, RPA, and MDC1/NFBD1 and the MRE11-RAD50-NBS1. Gamma-H2AX, the phosphorylated form of H2AX, is one of the first proteins to appear within nuclear foci and plays an essential role in recruiting other repair proteins (Paull et al., 2000).
One model illustrating the organization of the recruited repair proteins (Van den Bosch et al., 2003) is shown here. (A) Undamaged section of a chromosome, showing two chromatin loops and an inactive ATM dimer. (B,C) Induction of a DNA double-stranded break (DSB), modification of chromatin, activation of ATM and recruitment of both ATM and MRE11/RAD50/NBS1(MRN). The possibility that MRN binds before ATM is shown, but the activation of ATM appears to require the MDC1 and the MRN complex (Uziel, et al 2003; Mochan et al. 2003). The thin black line indicates modified chromatin. (D,E) Following DNA damage, a wave of H2AX phosphorylation occurs and the mediator proteins (mediator of DNA damage checkpoint protein 1 (MDC1), p53-binding protein 1(53BP1), and breast-cancer-associated protein 1(BRCA1)) are recruited to the growing focus where they are phosphorylated in an ATM-dependent manner. The molecular architecture of the focus is unknown. (F) Disassembly of the focus, ATM inactivation and chromatin remodeling. The model suggests at least two distinct forms of soluble ATM: an inactive oligomer and an active monomer, and at least two distinct active, insoluble forms: one directly at the lesion and another integral to the growing focus. Note that the MRN complex is also a component of the growing focus, but for clarity, has been omitted here. Complex persistent lesions are thought to be more difficult to repair, and this is reflected in the size attained by the growing focus.