PAXX Is an Accessory c-NHEJ Factor that Associates with Ku70 and Has Overlapping Functions with XLF

In mammalian cells, classical non-homologous end joining (c-NHEJ) is critical for DNA double-strand break repair induced by ionizing radiation and during V(D)J recombination in developing B and T lymphocytes. Recently, PAXX was identified as a c-NHEJ core component. We report here that PAXX-deficient cells exhibit a cellular phenotype uncharacteristic of a deficiency in c-NHEJ core components. PAXX-deficient cells display normal sensitivity to radiomimetic drugs, are proficient in transient V(D)J recombination assays, and do not shift toward higher micro-homology usage in plasmid repair assays. Although PAXX-deficient cells lack c-NHEJ phenotypes, PAXX forms a stable ternary complex with Ku bound to DNA. Formation of this complex involves an interaction with Ku70 and requires a bare DNA extension for stability. Moreover, the relatively weak Ku-dependent stimulation of LIG4/XRCC4 activity by PAXX is unmasked by XLF ablation. Thus, PAXX plays an accessory role during c-NHEJ that is largely overlapped by XLF’s function. [Display omitted] •PAXX interaction with Ku threaded onto DNA requires a bare DNA extension•PAXX interacts with the Ku70 subunit•PAXX is dispensable for V(D)J recombination and has no core c-NHEJ phenotype•PAXX’s function in c-NHEJ is masked by XLF Tadi et al. find that PAXX interacts with Ku threaded on DNA via the Ku70 subunit and requires a bare DNA extension for stability. They show that PAXX’s function overlaps with c-NHEJ factor XLF, explaining why its ablation has only a mild effect on NHEJ.