Differential roles of two conserved glycine residues in the fusion peptide of Semliki Forest Virus

Semliki Forest Virus (SFV) is an enveloped alphavirus that infects cells by a low-pH-dependent membrane fusion reaction. SFV fusion is catalyzed by the spike protein E1 subunit, which contains a putative fusion peptide between residues 79 and 97. Prior mutagenesis studies demonstrated that an E1 G91 D mutation blocks both virus-membrane fusion and the formation of a highly stable E1 trimer believed to be a critical fusion intermediate. We have here demonstrated that the G91D mutant was also inactive in hemifusion, suggesting that the E1 homotrimer is important in the initial stages of lipid mixing. Revertant analysis of a G91 deletion mutant indicated that G91 was crucial for the viability of SFV. In contrast, a G83D mutation produced infectious virus with both efficient fusion and homotrimer formation. Thus, the G83 position, although highly conserved among alphaviruses, was functional if replaced with a charged amino acid.