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The crystal structure of the complex of replication protein A subunits RPA32 and RPA14 reveals a mechanism for single‐stranded DNA binding

Replication protein A (RPA), the eukaryote single‐stranded DNA‐binding protein (SSB), is a heterotrimer. The largest subunit, RPA70, which harbours the major DNA‐binding activity, has two DNA‐binding domains that each adopt an OB‐fold. The complex of the two smaller subunits, RPA32 and RPA14, has weak DNA‐binding activity but the mechanism of DNA binding is unknown. We have determined the crystal structure of the proteolytic core of RPA32 and RPA14, which consists of the central two‐thirds of RPA32 and the entire RPA14 subunit. The structure revealed that RPA14 and the central part of RPA32 are structural homologues. Each subunit contains a central OB‐fold domain, which also resembles the DNA‐binding domains in RPA70; an N‐terminal extension that interacts with the central OB‐fold domain; and a C‐terminal helix that mediate heterodimerization via a helix–helix interaction. The OB‐fold of RPA32, but not RPA14, possesses additional similarity to the RPA70 DNA‐binding domains, supporting a DNA‐binding role for RPA32. The discovery of a third and fourth OB‐fold in RPA suggests that the quaternary structure of SSBs, which in Bacteria and Archaea are also tetramers of OB‐folds, is conserved in evolution. The structure also suggests a mechanism for RPA trimer formation.