p21 regulates the affinity of PARP-1 for damaged DNA

The poly(ADP-ribose) polymerase 1 (PARP-1) is able to recognize damaged DNA and to recruit DNA repair, mainly base excision repair (BER) factors, to the lesion site. We have previously shown that p21 interacts with PARP-1 at the automodification/DNA-binding domain, and that the PAR activity of PARP-1 was higher in human p21-/- fibroblasts, than in parental cells damage, with concomitant BER defects. To investigate the role of p21 as regulator of PARP-1 activity, we have analyzed the influence of p21 on PARP-1 binding to BER enzymes, such as DNA pol ?, and DNA repair intermediates, with photoaffinity labeling technique, by using recombinant purified proteins or nuclear cell extracts. These results indicate that p21 stimulates the PARP-1 binding to DNA damage sites and that the interaction between PARP-1 and p21 occurs in the presence of PARylated PARP-1, although it has been observed also when PARP-1 activity is inhibited by Olaparib. These results suggest that p21 modulates PARP-1 affinity to the DNA damage site thereby promoting productive interaction of PARP-1 with BER factors, in order to allow efficient DNA repair.