Two-step activation of ATM by DNA and the Mre11-Rad50-Nbs1 complex.

DNA double-strand breaks (DSBs) trigger activation of the ATM protein kinase, which coordinates cell-cycle arrest, DNA repair and apoptosis. We propose that ATM activation by DSBs occurs in two steps. First, dimeric ATM is recruited to damaged DNA and dissociates into monomers. The Mre11-Rad50-Nbs1 complex (MRN) facilitates this process by tethering DNA, thereby increasing the local concentration of damaged DNA. Notably, increasing the concentration of damaged DNA bypasses the requirement for MRN, and ATM monomers generated in the absence of MRN are not phosphorylated on Ser1981. Second, the ATM-binding domain of Nbs1 is required and sufficient to convert unphosphorylated ATM monomers into enzymatically active monomers in the absence of DNA. This model clarifies the mechanism of ATM activation in normal cells and explains the phenotype of cells from patients with ataxia telangiectasia-like disorder and Nijmegen breakage syndrome.