VESICULAR STOMATITIS-VIRUS GLYCOPROTEIN MUTATIONS THAT AFFECT MEMBRANE-FUSION ACTIVITY AND ABOLISH VIRUS INFECTIVITY

We have introduced amino acid substitutions into two regions of the ex tracellular domain of the vesicular stomatitis virus (VSV) glycoprotei n (G protein) and examined the effect of these mutations on protein tr ansport, low-pH-induced stability of G protein oligomers, and membrane fusion activity. We suggested previously that the region between amin o acids 118 and 139 may be important for the membrane fusion activity of G protein, on the basis of the characterization of a fusion-defecti ve G protein mutant (M. A. Whitt, P. Zagouras, B. Crise, and J. K. Ros e, J. Virol. 64:4907-4913, 1990). It has also been postulated by other s that this region as well as the region between amino acids 181 and 2 12 may constitute putative internal fusion domains of VSV G protein. I n this report, we show that three different amino acids substitutions between residues 118 and 139 (G-124-->E, P-127-->D, and A-133-->K) eit her altered or abolished Low-pH-dependent membrane fusion activity. In contrast, substitutions between residues 192 and 212 resulted either in G proteins that had wild-type fusion activity or in mutant proteins in which the mutation prevented transport of G protein to the cell su rface, Two of the substitutions between residues 118 and 139 (G-124--> E and P-127-->D) resulted in G proteins that were fusion defective at pH 5.7, although syncytia were observed after cells were treated with fusion buffer at pH 5.5, albeit at levels significantly less than that induced by wild-type G protein. Interestingly, when either G-124-->E or P-127-->D was incorporated into ts045 virions, the resulting partic les were not infectious, presumably because the viral envelope was not able to fuse with the proper intracellular membrane. These results su pport the hypothesis that the region between amino acids 118 and 139 i s important for the membrane fusion activity of VSV G protein and may constitute an internal fusion domain.