XPF-ERCC1 and XPG proteins are nucleases that are involved in human nucleotide excision repair. In this study, we characterized the structure-specific junction-cutting activities of both nucleases using DNA substrates containing a bubble or loop structure. We found that the junction-cutting activities of XPF-ERCC1 and XPG were greatly stimulated by human replication protein A (RPA), while heterologous single-stranded DNA-binding proteins could not substitute for human RPA. To test for specific interaction between RPA and XPF-ERCC1 as is known to occur between RPA and XPG, we employed a pull-down assay with immobilized "bubble" substrate. We found that the binding of XPF-ERCC1 complex to the bubble substrate was enhanced by RPA, suggesting a possible mechanism for RPA in the excision nuclease system, that is the targeting of the nuclease subunits to their specific sites of action. Furthermore, the RPA-promoted junction cutting by XPF-ERCC1 and XPG nucleases was observed with "loop" substrates as well, raising the possibility that XPF-ERCC1, XPG, and RPA may function in removing loop structures from DNA, independent of the other subunits of the human excinuclease
