The carboxy terminus of NBS1 is required for induction of apoptosis by the MRE11 complex

The MRE11 complex (MRE11, RAD50 and NBS1) and the ataxia-telangiectasia mutated (ATM) kinase function in the same DNA damage response pathway to effect cell cycle checkpoint activation and apoptosis(1-3). The functional interaction between the MRE11 complex and ATM has been proposed to require a conserved C-terminal domain of NBS1 for recruitment of ATM to sites of DNA damage(4,5). Human Nijmegen breakage syndrome (NBS) cells and those derived from multiple mouse models of NBS express a hypomorphic NBS1 allele that exhibits impaired ATM activity despite having an intact C-terminal domain(3,6-11). This indicates that the NBS1 C terminus is not sufficient for ATM function. We derived Nbs1(Delta C/Delta C) mice in which the C-terminal ATM interaction domain is deleted. Nbs1(Delta C/Delta C) cells exhibit intra-S-phase checkpoint defects, but are otherwise indistinguishable from wild-type cells with respect to other checkpoint functions, ionizing radiation sensitivity and chromosome stability. However, multiple tissues of Nbs1(Delta C/Delta C) mice showed a severe apoptotic defect, comparable to that of ATM- or CHK2-deficient animals. Analysis of p53 transcriptional targets and ATM substrates showed that, in contrast to the phenotype of Chk2(-/-) mice, NBS1(Delta C) does not impair the induction of proapoptotic genes. Instead, the defects observed in Nbs1(Delta C/Delta C) result from impaired phosphorylation of ATM targets including SMC1 and the proapoptotic factor, BID.