Antigenic structure of the central conserved region of protein G of bovine respiratory syncytial virus.

Epitopes were resolved at the amino acid level for nine monoclonal antibodies (MAbs) directed against the central conserved region of protein G of bovine respiratory syncytial virus (BRSV-G). Peptide binding studies showed which amino acids in the epitope contributed to antibody binding. The details of the epitopes were compared with the high-resolution structure of a synthetic peptide corresponding to the central conserved region of BRSV-G, and this indicated which face of the central conserved region is the antigenic structure. The major linear epitope of the central conserved region of BRSV-G is located at the tip of the loop, overlapping a relatively flat surface formed by a double disulfide-bonded cystine noose. At least one, but possibly two sulfur atoms of a disulfide bridge that line the conserved pocket at the center of the flat surface, is a major contributor to antibody binding. Some of the residue positions in the epitope have mutated during the evolution of RSV-G, which suggests that the virus escaped antibody recognition with these mutations. Mutations that occur at positions 177 and 180 may have only a local effect on the antigenic surface, without influencing the structure of the backbone, whereas mutations at positions 183 and 184 will probably have major structural consequences. The study explains the antigenic, structural, and functional importance of each residue in the cystine noose which provides information for peptide vaccine design. Additionally, analysis of the epitopes demonstrated that two point mutations at positions 180 and 205 define the preliminary classification of BRSV subgroups.