Two Interaction Surfaces between XPA and RPA Organize the Preincision Complex in Nucleotide Excision Repair

The XPA and RPA proteins fulfill essential roles in the assembly of the preincision complex in the nucleotide excision repair pathway. We have previously characterized the two interaction surfaces between XPA and RPA, with the RPA32 and RPA70AB subunits. Here we show that the mutations in the two individual interaction surfaces reduce NER activity in biochemical and cellular systems, and that combining mutations in two domains leads to an additive inhibition of NER, suggesting that they fulfill distinct roles. Our data suggest that the interaction between XPA and RPA32 is important for the initial association of XPA with NER complexes, while the interaction between XPA and RPA70 is needed for structural organization of the complex to license the dual incision reaction. SAXS analysis of complexes of XPA and RPA bound to ss/dsDNA junction substrates reveals the architecture of XPA and RPA in the preincision complex and shows that the two interaction domains between RPA and XPA are located at opposite sides of the two molecules. We propose a structure for the overall NER preincision complex that shows that the two strands of the NER bubble assume a U-shape with the two ss/dsDNA junctions localized in close proximity, with the interaction between XPA and RPA70 as one of the key organizing elements.