Sindbis virus pathogenesis: phenotypic reversion of an attenuated strain to virulence by second-site intragenic suppressor mutations.

Monoclonal antibodies (MAbs) specific for the E2c neutralizing antigenic site on the Sindbis virus E2 glycoprotein define a pathogenesis domain that affects neonatal mouse virulence. Sequence analysis of E2c MAb escape mutants showed that the domain included E2 amino acids 62, 96 and 159. The pathogenesis domain is also influenced by changes at E2 position 114. Mutation of E2 residues Asn 62 to Asp or Lys 159 to Glu results in suppression of the attenuated phenotype conferred by a mutation from Ser to Arg at E2 position 114. Possible mechanisms of phenotypic suppression within the E2c pathogenesis domain were investigated by using site-directed mutagenesis to determine the effects of specific combinations of positively charged, negatively charged and uncharged amino acid substitutions at E2 positions 62, 114 and 159. Phenotypic reversion to virulence by second-site suppressor mutations at E2 amino acids 62 or 159 was not dependent on ionic interaction with the residue at E2 114. Rather, suppression appeared to be the result of independent virulence effects mediated by specific residues.