Poly(ADP-ribose)-mediated interplay of XPA and PARP1 leads to reciprocal regulation of protein function.

Poly(ADP-ribose) (PAR) is a complex and reversible post-translational modification that controls protein function and localization through covalent modification of, or noncovalent binding to target proteins. Previously, we and others characterized the noncovalent, high-affinity binding of the key nucleotide excision repair (NER) protein XPA to PAR. In the present study, we address the functional relevance of this interaction. First, we confirm that pharmacological inhibition of cellular poly(ADP-ribosyl)ation (PARylation) impairs NER efficacy. Second, we demonstrate that the XPA-PAR interaction is mediated by specific basic amino acids within a highly conserved PAR-binding motif, which overlaps the DNA damage-binding protein 2 (DDB2) and transcription factor II H (TFIIH) interaction domains of XPA. Third, biochemical studies reveal a mutual regulation of PARP1 and XPA functions showing that, on the one hand, the XPA-PAR interaction lowers the DNA binding affinity of XPA, whereas, on the other hand, XPA itself strongly stimulates PARP1 enzymatic activity. Fourth, microirradiation experiments in U2OS cells demonstrate that PARP inhibition alters the recruitment properties of XPA-green fluorescent protein to sites of laser-induced DNA damage. In conclusion, our results reveal that XPA and PARP1 regulate each other in a reciprocal and PAR-dependent manner, potentially acting as a fine-tuning mechanism for the spatio-temporal regulation of the two factors during NER.