Viral membrane glycoproteins: comparison of the amino terminal amino acid sequences of the precursor and mature glycoproteins of three serotypes of vesicular stomatitis virus

Membrane fusion activity of vesicular stomatitis virus glycoprotein G is induced by low pH but not by heat or denaturant

The fusogenic envelope glycoprotein G of the rhabdovirus vesicular stomatitis virus (VSV) induces membrane fusion at acidic pH. At acidic pH the G protein undergoes a major structural reorganization leading to the fusogenic conformation. However, unlike other viral fusion proteins, the low-pH-induced conformational change of VSV G is completely reversible. As well, the presence of an alpha-helical coiled-coil motif required for fusion by a number of viral and cellular fusion proteins was not predicted in VSV G protein by using a number of algorithms. Results of pH dependence of the thermal stability of G protein as determined by intrinsic Trp fluorescence and circular dichroism (CD) spectroscopy show that the G protein is equally stable at neutral or acidic pH. Destabilization of G structure at neutral pH with either heat or urea did not induce membrane fusion or conformational change(s) leading to membrane fusion. Taken together, these data suggest that the mechanism of VSV G-induced fusion is distinct from the fusion mechanism of fusion proteins that involve a coiled-coil motif.

Structure, expression and phylogenetic analysis of the glycoprotein gene of Cocal virus

A cDNA copy of the mRNA of the glycoprotein G of Cocal virus, a rhabdovirus, has been cloned, sequenced and expressed in mammalian cells. The deduced amino acid sequence shows a typical transmembrane glycoprotein, 512 amino acids in length, containing two potential N-linked glycosylation sites. The amino acid sequence showed a high degree of identity with that of the prototype vesicular stomatitis virus serotype Indiana [VSV (IND)] G protein. In addition, phylogenetic analysis of amino acid sequence differences among the G proteins of vesiculoviruses indicated that Cocal virus represents a distinct lineage within the VSV (IND) serotype. Expression of the cloned Cocal G gene in mammalian cells produced a glycoprotein of mol.wt 71000 which was not palmitylated but induced cell fusion at acid pH.

The glycoprotein G of rhabdoviruses

Rhabdoviruses show an RNA-containing helically-wound nucleocapsid either enclosed by or enclosing a membrane M protein, surrounded by a lipid bilayer through which dynamic protein trimers made up of non-covalently associated monomers of glycoprotein G (G) project outside. Mature monomeric rhabdoviral G has more than 500 amino acids, 2-6 potential glycosylation sites, 12-16 highly conserved cysteine residues, 2-3 stretches of a-d hydrophobic heptad-repeats, a removed amino terminal hydrophobic signal peptide, a close to the carboxy terminal hydrophobic transmembrane sequence and a carboxy terminal short hydrophylic cytoplasmic domain. Association-dissociation between monomers-trimers and displacement of the trimers along the plane of the lipid membrane, are induced by changes in the external conditions (pH, temperature, detergents, etc.). Throughout conformational changes the G trimers are responsible for the virus attachment to cell receptors, for low-pH membrane fusion and for reacting with host neutralizing monoclonal antibodies (MAbs). Antigenic differences could exist between monomers and trimers, which may have implications for future vaccine developments. The family Rhabdoviridae is made up of the Lyssavirus (rabies), the Vesiculovirus (vesicular stomatitis virus, VSV) and many rhabdoviruses infecting fish, plants, and arthropod insects. All these reasons make the G of rhabdoviruses an ideal subject to study comparative virology and to investigate new vaccine technologies.

Membrane anchoring domain of herpes simplex virus glycoprotein gB is sufficient for nuclear envelope localization

We have used the glycoprotein gB of herpes simplex virus type 1 (gB-1), which buds from the inner nuclear membrane, as a model protein to study localization of membrane proteins in the nuclear envelope. To determine whether specific domains of gB-1 glycoprotein are involved in localization in the nuclear envelope, we have used deletion mutants of gB-1 protein as well as chimeric proteins constructed by replacing the domains of the cell surface glycoprotein G of vesicular stomatitis virus with the corresponding domains of gB. Mutant and chimeric proteins expressed in COS cells were localized by immunoelectron microscopy. A chimeric protein (gB-G) containing the ectodomain of gB and the transmembrane and cytoplasmic domains of G did not localize in the nuclear envelope. When the ectodomain of G was fused to the transmembrane and cytoplasmic domains of gB, however, the resulting chimeric protein (G-gB) was localized in the nuclear envelope. Substitution of the transmembrane domain of G with the 69 hydrophobic amino acids containing the membrane anchoring domain of gB allowed the hybrid protein (G-tmgB) to be localized in the nuclear envelope, suggesting that residues 721 to 795 of gB can promote retention of proteins in the nuclear envelope. Deletion mutations in the hydrophobic region further showed that a transmembrane segment of 21 hydrophobic amino acids, residues 774 to 795 of gB, was sufficient for localization in the nuclear envelope. Since wild-type gB and the mutant and chimeric proteins that were localized in the nuclear envelope were also retained in the endoplasmic reticulum, the membrane spanning segment of gB could also influence retention in the endoplasmic reticulum.

Mutational analysis of the vesicular stomatitis virus glycoprotein G for membrane fusion domains

The spike glycoprotein G of vesicular stomatitis virus (VSV) induces membrane fusion at low pH. We used linker insertion mutagenesis to characterize the domain(s) of G glycoprotein involved in low-pH-induced membrane fusion. Two or three amino acids were inserted in frame into various positions in the extracellular domain of G, and 14 mutants were isolated. All of the mutants expressed fully glycosylated proteins in COS cells. However, only seven mutant G glycoproteins were transported to the cell surface. Two of these mutants, D1 and A6, showed wild-type fusogenic properties. The mutant A2 had a temperature-sensitive defect in the transport of the mutant G glycoprotein to the cell surface. The other four mutants, H2, H5, H10, and A4, although present in cell surface, failed to induce cell fusion when cells expressing these mutant glycoproteins were exposed to acidic pH. These four mutant G proteins could form trimers, indicating that the defect in fusion was not due to defective oligomerization. One of these mutations, H2, is within a region of conserved, uncharged amino acids that has been proposed as a possible fusogenic sequence. The mutation in H5 was about 70 amino acids downstream of the mutation in H2, while mutations in H10 and A4 were about 300 amino acids downstream of the mutation in H2. Conserved sequences were also noted in the H10 and A4 segment. The results suggest that in the case of VSV G glycoprotein, the fusogenic activity may involve several spatially separated regions in the extracellular domain of the protein.

Fusion of enveloped viruses with cells and liposomes. Activity and inactivation

The fusion of viruses with cells and liposomes is reviewed with focus on the analysis of the final extents and kinetics of fusion. Influenza virus and Sendai virus exhibit 100% of fusion capacity with cells at pH 5 and pH 7.5, respectively. On the other hand, there may be in certain cases, a limit on the number of virions that can fuse with a single cell, that is significantly below the limit on binding. It still remains to be resolved whether this limit reflects a limited number of possible fusion sites, or a saturation limit on the amount of viral glycoproteins that can be incorporated in the cellular membrane, like the case of virus fusion with pure phospholipid vesicles, in which the fusion products were shown to consist of a single virus and several liposomes. Both viruses demonstrate incomplete fusion activity towards liposomes of a variety of compositions. In the case of Sendai virus, fusion inactive virions bind essentially irreversibly to liposomes. Yet, preliminary results revealed that such bound, unfused virions can be released by sucrose gradient centrifugation. The separated unfused virions subsequently fuse when incubated with a "fresh" batch of liposomes. We conclude, therefore, that the fraction of initially bound unfused virions does not consist of dective particles, but rather of particles bound to liposomes via "inactive" sites. Details of the low pH inactivation of fusion capacity of influenza virus towards cells and liposomes are presented. This inactivation is caused by protonation and exposure of the hydrophobic segment of HA2, and affects primarily the fusion rate constants. Some degree of inactivation also occurs when virions are bound to cellular membranes.

Membrane fusion of enveloped viruses: especially a matter of proteins

To infect mammalian cells, enveloped viruses have to deposit their nucleocapsids into the cytoplasm of a host cell. Membrane fusion represents a key element in this entry mechanism. The fusion activity resides in specific, virally encoded membrane glycoproteins. Some molecular properties of these fusion proteins will be briefly described. These properties will then be correlated to the ability of a virus to fuse with target membranes, and to induce cell-cell fusion. Some molecular and physical parameters affecting virus fusion--at the level of either viral or target membrane or both--and the significance of modelling virus fusion by using synthetic peptides resembling viral fusion peptides, will also be discussed.

Entry mechanisms of enveloped viruses. Implications for fusion of intracellular membranes

Enveloped viruses infect cells by a mechanism involving membrane fusion. This process is mediated and triggered by specific viral membrane glycoproteins. Evidence is accumulating that fusion of intracellular membranes, as occurs during endocytosis and transport between intracellular organelles, also requires the presence of specific proteins. The relevance of elucidating the mechanisms of virus fusion for a better understanding of fusion of intracellular membranes is discussed.

Amino-terminal mutation of the vesicular stomatitis virus glycoprotein does not affect its fusion activity

Earlier studies demonstrated that synthetic peptides corresponding to the amino terminus of the vesicular stomatitis virus glycoprotein (G protein) have a pH-dependent hemolytic activity that is thought to be related to the fusion activity of G protein (R. Schlegel and M. Wade, J. Biol. Chem. 259: 4691-4694, 1984; R. Schlegel and M. Wade, J. Virol. 53: 319-323, 1985). A single amino acid change (lysine to glutamic acid at the amino terminus) abolishes the hemolytic activity of the peptide. Here we used oligonucleotide-directed mutagenesis to create a DNA encoding G protein with this same amino acid change at its amino terminus. The mutant protein encoded by this gene was expressed transiently in a monkey fibroblast cell line (COS) and was found to have a pH-dependent fusion activity indistinguishable from wild-type G protein. This result indicates that the hemolytic activity of the synthetic peptides was not related to the fusion activity of the G protein.

Biologically active peptides of the vesicular stomatitis virus glycoprotein

A peptide corresponding to the amino-terminal 25 amino acids of the mature vesicular stomatitis virus glycoprotein has recently been shown to be a pH-dependent hemolysin. In the present study, we analyzed smaller constituent peptides and found that the hemolytic domain resides within the six amino-terminal amino acids. Synthesis of variant peptides indicates that the amino-terminal lysine can be replaced by another positively charged amino acid (arginine) but that substitution with glutamic acid results in the total loss of the hemolytic function. Peptide-induced hemolysis was dependent upon buffer conditions and was inhibited when isotonicity was maintained with mannitol, sucrose, or raffinose. In sucrose, all hemolytic peptides were also observed to mediate hemagglutination. The large 25-amino acid peptide is also a pH-dependent cytotoxin for mammalian cells and appears to effect gross changes in cell permeability. Conservation of the amino terminus of vesicular stomatitis virus and rabies virus suggests that the membrane-destabilizing properties of this domain may be important for glycoprotein function.