Single-stranded DNA-protein interactions in canine parvovirus

Background: Parvoviruses are small icosahedral single-stranded (ss) DNA viruses which replicate in rapidly proliferating cells, causing a variety of serious and often lethal diseases in mammals, including humans. The structure of canine parvovirus (CPV) showed an 11-nucleotide oligomeric fragment of its genome bound to 60 equivalent binding sites on the inside surface of the capsid. This provides an opportunity to study the conformation of ssDNA, its interactions with protein, and its role in viral assembly. Results The icosahedrally ordered part of CPV ssDNA has an unusual loop conformation with the bases pointing outwards and the phosphates surrounding metal ions on the inside. The protein interacts with the bases, making 15 putative hydrogen bonds. The DNA electron density indicates preferences for particular base types in parts of the binding site. Statistical analysis of the genome yields ∼30 regions with sequences similar to that observed in the structure, demonstrating a low level of sequence specificity for binding to capsid protein. Conclusion ssDNA can adopt unusual conformations upon association with protein by using phosphoribose backbone rotamers that are found in tRNA, but not in DNA duplexes. The CPV DNA-protein interactions differ from the non-specific backbone interactions seen in some plant and insect viruses. The sequence specificity, albeit low level, of the protein for CPV DNA may contribute both to distinguishing the viral DNA from other nucleic acids and to the DNA packaging process during viral assembly.