Poly(ADP-ribosyl)ation is an immediate cellular reaction to DNA strand-breakage induced by alkylating agents, ionizing radiation or endogenous oxidants. This DNA damage-dependent posttranslational modification of proteins contributes to the survival of proliferating cells via its involvement in the DNA base excision repair (BER). In this DNA repair pathway PARP1 and PARP2 interact with the scaffold protein XRCC1, a central factor that orchestrates the overall pathway. This interaction with PARPs and/or ADP-ribose polymers is supposed to initiate the recruitment of key players of the BER (DNA polymerase b and DNA ligase III) and to induce the sequential assembly of these factors coordinated by XRCC1. However, this long-time assumption drawn from in vitro studies remained to be demonstrated in vivo. To gain further insight into the functional role of poly(ADP-ribose) synthesis in response to DNA damage, we locally introduced DNA strand-breaks in restricted areas of nuclei in intact living cells. We subsequently followed, in space and time, the fate of XRCC1 (either endogenous or GFP-tagged) versus poly (ADP-ribose) synthesis. We provide the first visual evidence that the recruitment of XRCC1, mediated by its BRCT-1 domain, at DNA damaged sites occurs within 30 seconds and is strongly dependent upon local poly (ADP-ribose) synthesis.Therefore, at the nucleus level, poly(ADP-ribose) acts as a signaling molecule emanating from DNA breaks that rapidly triggers the local recruitment of XRCC1 at damage sites.References: 1.Caldecott KW: Bioessays, 2001; 23: 447-455 2.Dantzer F et al: Biochimie, 1999; 81: 69-75 3.Masson M et al: Mol Cell Biol, 1998; 18: 3563-3571 4.Schreiber V et al: J Biol Chem, 2002; 277: 23028-23036 5.Thompson LH, West MG: Mutat Res, 2000; 459: 1-18

Keywords: XRCC1, Poly(ADP-ribose), local irradiation, recruitment