Abstract
DNA double-strand breaks (DSBs) can lead to mutations, chromosomal rearrangements, genome instability, and ultimately cancer. Central to the sensing of DSBs are ATM (Ataxia-telangiectasia mutated) kinase, which belongs to the phosphatidylinositol 3-kinase-related protein kinase (PIKK) family, and the MRN (Mre11-Rad50-Nbs1) protein complex that activates ATM. How the MRN complex recruits and activates ATM kinase is poorly understood. Previous studies indicate that the FxF/Y motif of Nbs1 directly binds to ATM kinase and is required to retain active ATM at sites of DNA damage. Here, we report the 2.5 Å resolution cryo-EM structures of human ATM and its complex with the Nbs1 FxF/Y motif. In keeping with previous structures of ATM and its yeast homolog Tel1, the dimeric human ATM kinase adopts a symmetric, butterfly-shaped autoinhibited structure. The conformation of the ATM kinase domain is most similar to the inactive states of other PIKKs, suggesting that activation may involve an analogous realigning the N and C lobes along with relieving the blockage of the substrate-binding site. We show that the Nbs1 FxF/Y motif binds to a conserved hydrophobic cleft within the Spiral domain of ATM, suggesting an allosteric mechanism of activation. We evaluate the importance of these interactions with mutagenesis and biochemical assays.
Competing Interest Statement
The authors have declared no competing interest.
