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.