Comparison of the oligosaccharide structure of the glycoprotein of vesicular stomatitis virus and a thermolabile mutant (tl-17)

Establishing Rules for Self-Adhesion and Aggregation of N-Glycan Sugars Using Virus Glycan Shields

The surfaces of cells and pathogens are covered with short polymers of sugars known as glycans. Complex N-glycans have a core of three mannose sugars with distal repeats of N-acetylglucosamine and galactose sugars terminating with sialic acid (SA). Long-range tough and short-range brittle self-adhesions were observed between SA and mannose residues, respectively, in ill-defined artificial monolayers. We investigated if and how these adhesions translate when the residues are presented in N-glycan architecture with SA at the surface and mannose at the core and with other glycan sugars. Two pseudotyped viruses with complex N-glycan shields were brought together in force spectroscopy (FS). At higher ramp rates, slime-like adhesions were observed between the shields, whereas Velcro-like adhesions were observed at lower rates. The higher approach rates compress the virus as a whole, and the self-adhesion between the surface SA is sampled. At the lower ramp rates, however, the complex glycan shield is penetrated and adhesion from the mannose core is accessed. The slime-like and Velcro-like adhesions were lost when SA and mannose were cleaved, respectively. While virus self-adhesion in forced contact was modulated by glycan penetrability, the self-aggregation of the freely diffusing virus was only determined by the surface sugar. Mannose-terminal viruses self-aggregated in solution, and SA-terminal ones required Ca2+ ions to self-aggregate. Viruses with galactose or N-acetylglucosamine surfaces did not self-aggregate, irrespective of whether or not a mannose core was present below the N-acetylglucosamine surface. Well-defined rules appear to govern the self-adhesion and -aggregation of N-glycosylated surfaces, regardless of whether the sugars are presented in an ill-defined monolayer, or N-glycan, or even polymer architecture.

Solution structure of the monovalent lectin microvirin in complex with Man(alpha)(1-2)Man provides a basis for anti-HIV activity with low toxicity

Lectins that bind surface envelope glycoprotein gp120 of HIV with high avidity can potently inhibit viral entry. Yet properties such as multivalency that facilitate strong interactions can also cause nonspecific binding and toxicity. The cyanobacterial lectin microvirin (MVN) is unusual as it potently inhibits HIV-1 with negligible toxicity compared with cyanovirin-N (CVN), its well studied antiviral homolog. To understand the structural and mechanistic basis for these differences, we solved the solution structure of MVN free and in complex with its ligand Manα(1-2)Man, and we compared specificity and time windows of inhibition with CVN and Manα(1-2)Man-specific mAb 2G12. We show by NMR and analytical ultracentrifugation that MVN is monomeric in solution, and we demonstrate by NMR that Manα(1-2)Man-terminating carbohydrates interact with a single carbohydrate-binding site. Synchronized infectivity assays show that 2G12, MVN, and CVN inhibit entry with distinct kinetics. Despite shared specificity for Manα(1-2)Man termini, combinations of the inhibitors are synergistic suggesting they recognize discrete glycans and/or dynamic glycan conformations on gp120. Entry assays employing amphotropic viruses show that MVN is inactive, whereas CVN potently inhibits both. In addition to demonstrating that HIV-1 can be inhibited through monovalent interactions, given the similarity of the carbohydrate-binding site common to MVN and CVN, these data suggest that gp120 behaves as a clustered glycan epitope and that multivalent-protein interactions achievable with CVN but not MVN are required for inhibition of some viruses.

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.