Natural and induced variation in the fusion glycoprotein gene of human respiratory syncytial virus subgroup A (1994)

The nucleotide sequences of the fusion (F) protein encoding gene of five isolates of subgroup A respiratory syncytial (RS) virus, representing the five lineages associated with current epidemics of RS virus-associated disease, were determined and compared with F gene sequences from laboratory strains. Overall natural variation in the fusion protein gene among subgroup A isolates is low. Amino acid identities of between 97% to 99.5% were deduced. The relationship of the five lineage isolates, based on the sequence of their fusion protein genes, agrees broadly with evolutionary relationships inferred from comparison of their G, SH and N protein genes.&#13;\nThere is a region of high amino acid variation at the C-terminal end of the F2- subunit, specifically between residues #101 and #105. None of the apparent amino acid coding changes are located in known epitopes. Using predictive structural models it is suggested that the few amino acid changes observed may alter the fusion protein structure especially in the C-terminal domain of the F2-subunit. The secondary structure of the Fi-subunit is predicted to remain unaltered. It is hypothesised that amino acid variation in the F2-subunit may result in antigenic variation, by altering a potential conformational epitope formed by interaction between the N-terminal region of Fi and the C-terminal region of F2.&#13;\nInduced variation in the fusion protein gene of two candidate vaccine strains was investigated. Temperature-sensitive mutant MAI, derived by intensive mutagenesis from the A2 strain, has two distinctive phenotypes; temperature-sensitivity and a retarded fusion protein mobility in non-reducing gels. Previous analysis of the phenotypic characteristics of tsA1 indicated that the F gene might be the site of the ts mutation. The sequence data derived in this study suggest that the site of ts lesion is located at residue #66 (Glu —&gt; Lys) and that the mobility phenotype is located at residue #102 (Pro —► Ser). Mutant tsA1 exhibits a complex pattern of reversion with two classes of revertant viruses observed; a fully revertant virus which has wild-type growth characteristics but still retains retarded fusion protein mobility; and a partially revertant virus that possesses near wild-type growth characteristics and wild-type mobility. In fully revertant viruses the correction of the ts phenotype has been identified as the reversion of amino acid #66 to the wild-type residue (Lys —&gt; Glu) whilst the coding change at amino acid #102 is retained, resulting in a mobility phenotype similar to that found in mutant tsA1. In partially revertant viruses the coding changes at amino acid #66 and amino acid #102 are retained. In partially revertant viruses the coding change correcting for the mobility phenotype, and partially correcting for the ts phenotype, has been tentatively identified as additional coding changes at residues #103 (Thr —► Ala) and/or #105 (Asn —&gt; Ser). In vitro expression of the fusion gene products of mutant, revertant and wild-type viruses in mammalian cells has confirmed that the mobility phenotype is solely a consequence of changes in the fusion protein gene.&#13;\nTemperature-sensitive mutant MIC is a triple ts mutant derived from the RSS-2 strain. The mutations detected in the fusion gene of mutant MIC are conservative in nature and are not located in known epitopes. Therefore it is unlikely that the coding changes observed in the fusion protein gene account for the ts-phenotype or viral attenuation. It is also thought that the induced mutations have not altered the antigenic properties of the virus.&#13;\n