XRCC1 stimulates human polynucleotide kinase activity at damaged DNA termini and accelerates DNA single-strand break repair

XRCC1 protein is required for DNA single-strand break repair and genetic st ability but its biochemical role is unknown. Here, we report that XRCC1 int eracts with human polynucleotide kinase in addition to its established inte ractions with DNA polymerase-beta and DNA ligase III. Moreover, these four proteins are coassociated in multiprotein complexes in human cell extract a nd together they repair single-strand breaks typical of those induced by re active oxygen species and ionizing radiation. Strikingly, XRCC1 stimulates the DNA kinase and DNA phosphatase activities of polynucleotide kinase at d amaged DNA termini and thereby accelerates the overall repair reaction. The se data identify a novel pathway for mammalian single-strand break repair a nd demonstrate a concerted role for XRCC1 and PNK in the initial step of pr ocessing damaged DNA ends.