A fusion-defective mutant of the vesicular stomatitis virus glycoprotein

We have recently described an assay in which a temperature-sensitive mutant of vesicular stomatitis virus (VSV; mutant tsO45), encoding a glycoprotein that is not transported to the cell surface, can be rescued by expression of wild-type VSV glycoproteins from cDNA (M. Whitt, L. Chong, and J. Rose, J. Virol. 63:3569-3578, 1989). Here we examined the ability of mutant G proteins to rescue tsO45. We found that one mutant protein (QN-1) having an additional N-linked oligosaccharide at amino acid 117 in the extracellular domain was incorporated into VSV virions but that the virions containing this glycoprotein were not infectious. Further analysis showed that virus particles containing the mutant protein would bind to cells and were endocytosed with kinetics identical to those of virions rescued with wild-type G protein. We also found that QN-1 lacked the normal membrane fusion activity characteristic of wild-type G protein. The absence of fusion activity appears to explain lack of particle infectivity. The proximity of the new glycosylation site to a sequence of 19 uncharged amino acids (residues 118 to 136) that is conserved in the glycoproteins of the two VSV serotypes suggests that this region may be involved in membrane fusion. The mutant glycoprotein also interferes strongly with rescue of virus by wild-type G protein. The strong interference may result from formation of heterotrimers that lack fusion activity.