The Human PAXX Protein Mediates Nonhomologous End-Joining

Effective repair of DNA double-strand breaks (DSB) is essential for maintenance of genome integrity and cell viability. DSBs can be repaired by several mechanisms, including nonhomologous end-joining (NHEJ), which requires the functions of Ku70 [also known as x-ray repair cross-complementing protein 6 (XRCC6)], Ku80 (also known as XRCC5), XRCC4, XRCC-like factor 4 (XLF, also known as NHEJ factor 1), and DNA ligase IV (LIG4). Ochi, Blackford, and colleagues identified the human protein c9orf142 as an XRCC paralog containing a conserved N-terminal motif found in proteins of the XRCC4 superfamily. Analysis of the crystal structure of this protein, termed paralog of XRCC4 and XLF (PAXX), revealed its predominant conformation as a dimer and structural properties most similar to XRCC4. PAXX interacted directly with the Ku70–Ku80 (Ku) heterodimer via its evolutionarily conserved C-terminus and localized to sites of DNA damage in response to ionizing radiation (IR). Depletion of PAXX rendered cells radiosensitive, similar to XRCC4-deficient cells, and expression of wild-type PAXX, but not a PAXX variant harboring mutations in two C-terminal residues necessary for Ku binding, restored IR resistance in PAXX-depleted cells, suggesting a role for PAXX in NHEJ. Consistent with this idea, PAXX deletion conferred hypersensitivity to IR, which was not enhanced by concomitant deletion of XRCC4 or XLF, indicative of an epistatic relationship between these proteins in NHEJ, and resulted in defective resolution of γH2AX foci and impaired DSB repair in response to IR. Mechanistically, PAXX promoted NHEJ by stabilizing the assembly of Ku and other core NHEJ proteins at sites of DSBs and stimulated LIG4-mediated DNA ligation in a Ku-dependent manner. These results establish PAXX as an XRCC4 superfamily member and previously undescribed component of NHEJ that plays a critical role in DSB repair.

Ochi T, Blackford AN, Coates J, Jhujh S, Mehmood S, Tamura N, et al. PAXX, a paralog of XRCC4 and XLF, interacts with Ku to promote DNA double-strand break repair. Science 2015;347:185–8

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