Novel sub-pathways of non-homologous end joining

Non-homologous end joining (NHEJ) is the major pathway for repair of DNA double-strand breaks in human cells. However, the full set of proteins required for the reaction has yet to be identified. Here we describe the purification of a novel factor that restores high levels of end joining activity in a biochemical complementation assay containing recombinant Ku and DNA ligase IV/XRCC4 complex. The active fractions do not contain detectable levels of the Mre11/Rad50/NBS1 (M/R/N) complex, which has previously been shown to stimulate end-joining in vitro. Thus, the new factor defines an independent, parallel sub-pathway of NHEJ repair that is distinct from the previously defined M/R/N dependent sub-pathway. Although the new sub-pathway operates constitutively in the absence of DNA-PKcs, purified DNA-PKcs imposes a restriction, or 'checkpoint' such that protein phosphorylation must occur before the reaction can proceed