ATM phosphorylation of Nijmegen breakage syndrome protein is required in aDNA damage response

Nijmegen breakage syndrome (NBS) is characterized by extreme radiation sens itivity, chromosomal instability and cancer(1). The phenotypes are similar to those of ataxia telangiectasia mutated (ATM) disease, where there is a d eficiency in a protein kinase that is activated by DNA damage, indicating t hat the Nbs and Atm proteins may participate in common pathways. Here we re port that Nbs is specifically phosphorylated in response to g-radiation, ul traviolet light and exposure to hydroxyurea. Phosphorylation of Nbs mediate d by g-radiation, but not that induced by hydroxyurea or ultraviolet light, was markedly reduced in ATM cells. In vivo, Nbs was phosphorylated on many serine residues, of which S343, S397 and S615 were phosphorylated by Atm i n vitro. At least two of these sites were underphosphorylated in ATM cells. Inactivation of these serines by mutation partially abrogated Atm-dependen t phosphorylation. Reconstituting NBS cells with a mutant form of Nbs that cannot be phosphorylated at selected, ATM-dependent serine residues led to a specific reduction in clonogenic survival after g-radiation. Thus, phosph orylation of Nbs by Atm is critical for certain responses of human cells to DNA damage.