Functional interaction between Ku and the Werner syndrome protein in DNA end processing

Werner syndrome (WS) is an autosomal recessive disease characterized by premature aging. The gene responsible for the syndrome was recently cloned and shown to encode a protein, with strong homology to DNA/RNA helicases. In addition, the Werner syndrome protein (WRN) possesses an exonuclease activity. Eased on the homology to helicases it has been proposed that WRN functions in some aspects of DNA replication, recombination, or repair. However, there is currently no evidence of a role of WRN in any of these processes; therefore, its biological function remains unknown. Using a biochemical approach, we have identified two polypeptides that bind to the WRN protein. Peptide sequence analysis indicates that the two proteins are identical to Ku70 and Ku80, a heterodimer involved in double strand DNA break repair by non-homologous DNA end joining. Protein-protein interaction studies reveal that WRN binds directly to Ku80 and that this interaction is mediated by the amino terminus of WRN. In addition, we show that the binding of Ku alters the specificity of the WRN exonuclease. These results suggest a potential involvement of WRN in the repair of double strand DNA breaks.