XRCC1 IS SPECIFICALLY ASSOCIATED WITH POLY(ADP-RIBOSE) POLYMERASE ANDNEGATIVELY REGULATES ITS ACTIVITY FOLLOWING DNA-DAMAGE

Poly(ADP-ribose) polymerase (PARP; EC 2.4.2.30) is a zinc-finger DNA-b inding protein that detects and signals DNA strand breaks generated di rectly or indirectly by genotoxic agents. In response to these breaks, the immediate poly(ADP-ribosyl)ation of nuclear proteins involved in chromatin architecture and DNA metabolism converts DNA damage into int racellular signals that can activate DNA repair programs or cell death options. To have greater insight into the physiological function of t his enzyme, we have used the two-hybrid system to find genes encoding proteins putatively interacting with PARP, We have identified a physic al association between PARP and the base excision repair (BER) protein XRCC1 (X-ray repair cross-complementing 1) in the Saccharomyces cerev isiae system, which was further confirmed to exist in mammalian cells, XRCC1 interacts with PARP by its central region (amino acids 301 to 4 02), which contains a BRCT (BRCA1 C terminus) module, a widespread mot if in DNA repair and DNA damage-responsive cell cycle checkpoint prote ins. Overexpression of XRCC1 in Cos-7 or HeLa cells dramatically decre ases PARP activity in vivo, reinforcing the potential protective funct ion of PARP at DNA breaks. Given that XRCC1 is also associated with DN A ligase III via a second BRCT module and with DNA polymerase beta, ou r results provide strong evidence that PARP is a member of a BER multi protein complex involved in the detection of DNA interruptions and pos sibly in the recruitment of XRCC1 and its partners for efficient proce ssing of these breaks in a coordinated manner. The modular organizatio ns of these interactors, associated,vith small conserved domains, may contribute to increasing the efficiency of the overall pathway.