Werner's syndrome (WS) is an inherited disease characterized by genomic instability and premature aging. The WS gene encodes a protein (WRN) with helicase and exonuclease activities. We have previously reported that WRN interacts with Ku70/80 and this interaction strongly stimulates WRN exonuclease activity. To gain further insight on the function of WRN and its relationship with the Ku heterodimer, we established a cell line expressing tagged WRN(H), a WRN point mutant lacking helicase activity, and used affinity purification, immunoblot analysis and mass spectroscopy to identify WRN-associated proteins. To this end, we identified three proteins that are stably associated with WRN in nuclear extracts. Two of these proteins, Ku70 and Ku80, were identified by immunoblot analysis. The third polypeptide, which was identified by mass spectrometry analysis, is identical to poly(ADP-ribose) polymerase-1(PARP-1), a 113-kDa enzyme that functions as a sensor of DNA damage. Biochemical fractionation studies and immunoprecipitation assays and studies confirmed that endogenous WRN is associated with subpopulations of PARP-1 and Ku70/80 in the cell. Protein interaction assays with purified proteins further indicated that PARP-1 binds directly to WRN and assembles in a complex with WRN and Ku70/80. In the presence of DNA and NAD(+), PARP-1 poly(ADP-ribosyl)ates itself and Ku70/80 but not WRN, and gel-shift assays showed that poly-(ADP-ribosyl)ation of Ku70/80 decreases the DNA-binding affinity of this factor. Significantly, (ADP-ribosyl)ation of Ku70/80 reduces the ability of this factor to stimulate WRN exonuclease, suggesting that covalent modification of Ku70/80 by PARP-1 may play a role in the regulation of the exonucleolytic activity of WRN.