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Protein G of respiratory syncytial virus (RSV) is an envelope glycoprotein that is structurally very different from its counterparts (haemagglutinin-neuraminidase and haemagglutinin) in other paramyxoviruses. In this study, we put forward a model for this unique viral envelope protein. We propose that protein G of RSV contains several independently folding regions, with the ectodomain consisting of a conserved central hydrophobic region located between two polymeric mucinlike regions. The central conserved region is probably the only relatively fixed and folded part of the ectodomain of RSV-G. This central conserved region contains four conserved cysteine residues which can form two disulphide bridges. Analysis of the proteolytic digestion products of a peptide corresponding to the central conserved region showed that one of the three theoretically possible combinations of disulphide connections could be eliminated. The final disulphide bridge assignment was established by affinity measurements with peptide variants in which different disulphide connections were formed. Additionally, peptide binding studies were used to map the binding site, at the amino acid level, of a monoclonal antibody directed against the central conserved region. These studies indicated the level of surface exposure of the amino acid side-chains. The surface exposure agreed with the structural model. The proteolytic digestion, the peptide binding studies and the affinity measurements with structural peptide variants support a structural model with disulphide connections that correspond to a structural motif called a cystine noose. This model provides a structural explanation for the location and molecular details of important antigenic sites.

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