Differences in virion stability among Sindbis virus pathogenesis mutants.

The structure of closely related Sindbis virus strains, which differ in their virulence for neonatal mice, has been probed by measuring the sensitivity of virions to heat, varying concentrations of dithiothreitol (DTT), phenylglyoxal or low pH. Attenuated mutants (SB-RL and SB-FP) were much more sensitive to loss of infectivity after heat or DTT treatment than either the prototype virulent strain (SB) or same-site virulent revertants of SB-RL. Incubation of SB-RL virions in the presence of DTT increased their density and exposed sulfhydryl groups on both E1 and E2 glycoproteins as determined by [14C]iodoacetamide labeling. Incubation of SB-RL at pH 6.1 for 1 h at 37 degrees C resulted in a 50% decrease in titer, whereas an equivalent decrease in SB titer was obtained at pH 6.0. Attenuated (SB-RL) and virulent (SB) strains could not be distinguished on the basis of phenylglyoxal sensitivity. Comparison of the nucleotide sequences of these strains and analysis of strains derived from recombinant full-length cDNA clones demonstrated that reduced virion stability under these conditions was a consequence of an arginine for serine substitution at position 114 of the surface glycoprotein E2. This same amino acid substitution is responsible for attenuation in neonatal mice. However, experiments with a second-site virulent revertant of SB-RL, which retained E2 arginine 114 and the reduced stability phenotype, indicated that virion instability in itself was not directly responsible for reduced virulence in vivo.