Gene V protein (GVP) encoded by the filamentous phage Ff (M13, f1, fd) is a homodimeric protein of 87 amino acids that binds to single-stranded DNA (ssDNA) nonspecifically and cooperatively. The structure (monoclinic C2 form) of the wild-type protein has been determined and refined at 1.8-Å resolution [Skinner et al. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 2071–2075], The monomer structure consists of a somewhat distorted five-stranded β-barrel core with three prominent loops: a DNA-binding loop, a dyad loop, and a dimer contact loop. The amino acid residue at position 41 plays an important role in the dimer-dimer interactions of the protein-ssDNA complex. Two Y41 mutant structures have been studied by X-ray crystallography. The Y41F GVP structure has been refined to an R-factor of 0.180 at 2.2-Å resolution and is very similar to the wild-type (wt) structure (rmsd of all C., atoms = 0.30 Å). In contrast, Y41H GVP forms a new crystal lattice in the space group P212121 with a = 77.18 Å, b = 84.17 Å, and c = 28.62 Å. Its structure has been solved by the molecular replacement method and refined to an R-factor of 0.170 at 2.5-Å resolution. The two monomers of Y41H are crystallographically independent, and their structures remain similar to wt-GVP but with significant differences, particularly in the DNA-binding hairpin region. In both crystals, the loop (residues 36–43) that contains the Y41 residue is involved in the crystal dimer packings but in a different manner. The dimer-dimer contacts found in the wt-GVP crystal may be important for GVP aggregation in the absence of DNA. In the presence of DNA, the dimer-dimer contacts may switch to the type found in the Y41H crystal, allowing the G VP-ssDN A complex to form cooperatively. A model of the complex, consistent with existing biochemical and biophysical data, has been constructed from those crystal packing data.
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