PARP-1 enhances the mismatch-dependence of 5′-directed excision in human mismatch repair in vitro

End-directed mismatch-provoked excision has been reconstituted in several purified systems. While 3′-directed excision displays a mismatch dependence similar to that observed in nuclear extracts (≈20-fold), the mismatch dependence of 5′-directed excision is only 3–4-fold, significantly less than that in extracts (8–10-fold). Utilizing a fractionation-based approach, we have isolated a single polypeptide that enhances mismatch dependence of reconstituted 5′-directed excision and have shown it to be identical to poly[ADP-ribose] polymerase 1 (PARP-1). Titration of reconstituted excision reactions or PARP-1-depleted HeLa nuclear extract with purified PARP-1 showed that the protein specifically enhances mismatch dependence of 5′-directed excision. Analysis of a set of PARP-1 mutants revealed that the DNA binding domain and BRCT fold contribute to the regulation of excision specificity. Involvement of the catalytic domain is restricted to its ability to poly(ADP-ribosyl)ate PARP-1 in the presence of NAD+, likely through interference with DNA binding. Analysis of protein-protein interactions demonstrated that PARP-1 interacts with mismatch repair proteins MutSα, exonuclease 1, replication protein A (RPA), and as previously shown by others, replication factor C (RFC) and proliferating cell nuclear antigen (PCNA) as well. The BRCT fold plays an important role in the interaction of PARP-1 with the former three proteins.

► PARP-1 enhances mismatch dependence of 5′-directed excision by MutSα-activated Exo1. ► Mismatch specificity enhancement depends on PARP-1 DNA-binding and BRCT domains. ► PARP-1 BRCT domain interacts with MutSα, Exo1, and RPA.