Investigation of the role of glycans for the biological activity of Semliki Forest virus grown in Aedes albopictus cells using inhibitors of asparagine-linked oligosaccharides trimming

The Structural Biology of Eastern Equine Encephalitis Virus, an Emerging Viral Threat

Alphaviruses are arboviruses that cause arthritis and encephalitis in humans. Eastern Equine Encephalitis Virus (EEEV) is a mosquito-transmitted alphavirus that is implicated in severe encephalitis in humans with high mortality. However, limited insights are available into the fundamental biology of EEEV and residue-level details of its interactions with host proteins. In recent years, outbreaks of EEEV have been reported mainly in the United States, raising concerns about public safety. This review article summarizes recent advances in the structural biology of EEEV based mainly on single-particle cryogenic electron microscopy (cryoEM) structures. Together with functional analyses of EEEV and related alphaviruses, these structural investigations provide clues to how EEEV interacts with host proteins, which may open avenues for the development of therapeutics.

Mutation of N-glycosylation Sites in Salmonid Alphavirus (SAV) Envelope Proteins Attenuate the Virus in Cell Culture

Salmonid alphavirus (SAV) is the cause of pancreas disease and sleeping disease in farmed salmonid fish in Europe. The spread of these diseases has been difficult to control with biosecurity and current vaccination strategies, and increased understanding of the viral pathogenesis could be beneficial for the development of novel vaccine strategies. N-glycosylation of viral envelope proteins may be crucial for viral virulence and a possible target for its purposed attenuation. In this study, we mutated the N-glycosylation consensus motifs of the E1 and E2 glycoproteins of a SAV3 infectious clone using site-directed mutagenesis. Mutation of the glycosylation motif in E1 gave a complete inactivation of the virus as no viral replication could be detected in cell culture and infectious particles could not be rescued. In contrast, infectious virus particles could be recovered from the SAV3 E2 mutants (E2319Q, E2319A), but not if they were accompanied by lack of N-glycosylation in E1. Compared to the non-mutated infectious clone, the SAV3-E2319Q and SAV3-E2319A recombinant viruses produced less cytopathic effects in cell culture and lower amounts of infectious viral particles. In conclusion, the substitution in the N-linked glycosylation site in E2 attenuated SAV3 in cell culture. The findings could be useful for immunization strategies using live attenuated vaccines and testing in fish will be desirable to study the clone’s properties in vivo.

Role of N-linked glycosylation for sindbis virus infection and replication in vertebrate and invertebrate systems

Each Sindbis virus (SINV) surface glycoprotein has two sites for N-linked glycosylation (E1 positions 139 and 245 [E1-139 and E1-245] and E2 positions 196 and 318 [E2-196 and E2-318]). Studies of SINV strain TE12 mutants with each site eliminated identified the locations of carbohydrates by cryo-electron microscopy (S. V. Pletnev et al., Cell 105:127-136, 2001). In the current study, the effects of altered glycosylation on virion infectivity, growth in cells of vertebrates and invertebrates, heparin binding, virulence in mice, and replication in mosquitoes were assessed. Particle-to-PFU ratios for E1-139 and E2-196 mutant strains were similar to that for TE12, but this ratio for the E1-245 mutant was 100-fold lower than that for TE12. Elimination of either E2 glycosylation site increased virus binding to heparin and increased replication in BHK cells. Elimination of either E1 glycosylation site had no effect on heparin binding but resulted in an approximately 10-fold decrease in virus yield from BHK cells compared to the TE12 amount. No differences in pE2 processing were detected. E2-196 and E2-318 mutants were more virulent in mice after intracerebral inoculation, while E1-139 and E1-245 mutants were less virulent. The E1-245 mutant showed impaired replication in C7/10 mosquito cells and in Culex quinquefasciatus after intrathoracic inoculation. We conclude that the increased replication and virulence of E2-196 and E2-318 mutants are primarily due to increased efficiency of binding to heparan sulfate on mammalian cells. Lack of glycosylation at E1-139 or E1-245 impairs replication in vertebrate cells, while E1-245 also severely affects replication in invertebrate cells.

Protein N-glycosylation in the baculovirus-insect cell system

One of the major advantages of the baculovirus-insect cell system is that it is a eukaryotic system that can provide posttranslational modifications, such as protein N-glycosylation. However, this is a vastly oversimplified view, which reflects a poor understanding of insect glycobiology. In general, insect protein glycosylation pathways are far simpler than the corresponding pathways of higher eukaryotes. Paradoxically, it is increasingly clear that various insects encode and can express more elaborate protein glycosylation functions in restricted fashion. Thus, the information gathered in a wide variety of studies on insect protein N-glycosylation during the past 25 years has provided what now appears to be a reasonably detailed, comprehensive, and accurate understanding of the protein N-glycosylation capabilities of the baculovirus-insect cell system. In this chapter, we discuss the models of insect protein N-glycosylation that have emerged from these studies and how this impacts the use of baculovirus-insect cell systems for recombinant glycoprotein production. We also discuss the use of these models as baselines for metabolic engineering efforts leading to the development of new baculovirus-insect cell systems with humanized protein N-glycosylation pathways, which can be used to produce more authentic recombinant N-glycoproteins for drug development and other biomedical applications.

N-Glycan processing by a lepidopteran insect alpha1,2-mannosidase

Protein glycosylation pathways are relatively poorly characterized in insect cells. As part of an overall effort to address this problem, we previously isolated a cDNA from Sf9 cells that encodes an insect alpha1,2-mannosidase (SfManI) which requires calcium and is inhibited by 1-deoxymannojirimycin. In the present study, we have characterized the substrate specificity of SfManI. A recombinant baculovirus was used to express a GST-tagged secreted form of SfManI which was purified from the medium using an immobilized glutathione column. The purified SfManI was then incubated with oligosaccharide substrates and the resulting products were analyzed by HPLC. These analyses showed that SfManI rapidly converts Man(9)GlcNAc(2)to Man(6)Glc-NAc(2)isomer C, then more slowly converts Man(6)GlcNAc(2)isomer C to Man(5)GlcNAc(2). The slow step in the processing of Man(9)GlcNAc(2)to Man(5)GlcNAc(2)by SfManI is removal of the alpha1,2-linked mannose on the middle arm of Man(9)GlcNAc(2). In this respect, SfManI is similar to mammalian alpha1,2-mannosidases IA and IB. However, additional HPLC and(1)H-NMR analyses demonstrated that SfManI converts Man(9)GlcNAc(2)to Man(5)GlcNAc(2)primarily through Man(7)GlcNAc(2)isomer C, the archetypal Man(9)GlcNAc(2)missing the lower arm alpha1,2-linked mannose residues. In this respect, SfManI differs from mammalian alpha1,2-mannosidases IA and IB, and is the first alpha1,2-mannosidase directly shown to produce Man(7)GlcNAc(2)isomer C as a major processing intermediate.

Linkage of an alphavirus host-range restriction to the carbohydrate-processing phenotypes of the host cell

The Sindbis virus mutant NE2G216 retains PE2 in place of E2 in its virion structure. NE2G216 is a host-range mutant that replicates with near-normal kinetics in vertebrate cells, but displays severely restricted growth in cultured mosquito cells (C6/36) due to defects in the virus maturation process. In this study we tested the hypothesis that the host-range phenotype of NE2G216 was linked to the differences in carbohydrate-processing phenotypes between vertebrate and arthropod cells. Arthropod cell-derived glycoproteins are distinguishable from those synthesized in vertebrate cells by the absence of complex- and hybrid-type N-linked oligosaccharides. To test our hypothesis we compared the growth of the wild-type virus, TRSB, NE2G216 and three PE2-containing, C6/36 cell-adapted variants, in vertebrate cells treated with 1-deoxymannojirimycin (1-dMM). 1-dMM inhibits the Golgi alpha-mannosidase I enzyme and limits oligosaccharide processing to high-mannose forms (Man(8-9)GlcNAc(2)). The growth of TRSB was not restricted by the action of 1-dMM; however, NE2G216 was restricted in a dose-dependent manner. In contrast, the growth of each PE2-containing, C6/36 cell-adapted mutant was enhanced by low concentrations of 1-dMM (up to 1500%) and was only slightly affected by the higher concentrations. These results demonstrate that virion maturation functions of NE2G216 are sensitive to the structure of cis-linked oligosaccharides, and indicate that the carbohydrate-processing phenotypes of the host cell can influence viral host-range and function as a selective pressure in alphavirus evolution.

Palmitoylation of Semliki Forest virus glycoproteins in insect cells (C6/36) occurs in an early compartment and is coupled to the cleavage of the precursor p62

The acylation of the envelope proteins of Semliki Forest virus by palmitic acid in infected mosquito (C6/36) cells was investigated. It is shown that in these cells palmitic acid was incorporated post-translationally via hydroxylamine-labile linkages onto cysteines in the inner domains of the viral envelope proteins. The kinetics of incorporation, however, differed considerably as compared to higher eukaryotic cells. (i) The precursor of the envelope proteins E2 and E3, p62, was weakly and incompletely palmitoylated irrespective of the duration of labeling. (ii) Under all conditions tested complete acylation of E2 was delayed as compared to E1. (iii) Heavy protein complexes were formed consisting of unacylated p62 and partially unacylated E1. From this data, we conclude that during the maturation of SFV glycoproteins in mosquito cells differently acylated intermediates of p62/E2 exist. Furthermore, acylation of p62/E2 and cleavage of p62 are coupled events, occurring in an early compartment and allowing the release of the envelope oligomers for transport.

Chemically induced plasma membrane vesicles as a useful tool for the investigation of virus binding to susceptible cells

Binding to the surface of susceptible cells, the initial step of viral infections, represents an yet poorly understood event in the case of alphaviruses. In intact cells, the binding per se can only be studied when endocytosis is inhibited, e.g., at low incubation temperatures. However, nonphysiologically low temperatures may give only an imperfect picture of the events taking place at the cell surface during binding. In this report we present the application of chemically induced plasma membrane vesicles (PMV) for binding studies. PMV produced overnight from Vero cells give high yields of homogeneous vesicles. PMV represent cellular plasma membranes as far as protein composition and enzyme activities are concerned, but are not competent for endocytosis. Binding experiments to PMV were performed using Semliki Forest virus (SFV), a prototype of alphaviruses. The results show that the binding sites at the PMV surface are saturable and not competed by bovine serum albumin. Binding appeared to be specific and biologically relevant since fusion between viral and PMV membranes could be induced by lowering the incubation pH. Our model is of general interest since many cell types that are susceptible to viral infection may be induced to release PMV after adapting the method.

Asparagine-linked oligosaccharides of Semliki Forest virus grown in mosquito cells

The structure of the N-linked oligosaccharides of Semliki Forest viral glycoproteins produced in infected mosquito cells (C6/36) was investigated by biosynthetic labeling, enzymic deglycosylation using endo-beta-N-acetylglucosaminidases H, D, F/glycopeptidase F, exoglycosidase and analysis of the sugars on Concanavalin A-Sepharose columns and by gel filtration chromatography. The results demonstrated that the glycoproteins decorating the virus shed from infected cells have N-linked glycans with a trimannosyl core similar to the core glycans produced by vertebrate and yeast cells. However, the E1 glycoprotein produced by infected C6/36 cells exhibited both a trimannosyl core and a modified trimannosyl core most probably with terminal N-acetylglucosamine. The carbohydrate side chains of Semliki Forest envelope proteins displayed two types of structural heterogeneities existing either at different N-glycosylation sites as in the case of E2, or at the same N-glycosylation site as in the case of E1. In the presence of 1-deoxymannojirimycin, no structural heterogeneities in the glycan chains were found. This strongly suggests that the glycosylation events that lead to the observed sugar heterogeneities occur in the Golgi membranes.

The cleavage of p62, the precursor of E2 and E3, is an early and continuous event in Semliki Forest virus-infected Aedes albopictus cells

The cleavage of p62 of Semliki Forest virus (SFV) in C6/36 (Aedes albopictus) cells was investigated by pulse-chase labeling experiments and analysis of the sugar side chain of E1 using endoglycosidases. Similar to vertebrates, E1, E2, and p62 are transported as complexes in C6/36 cells. This observation allows the use of E1 as a positional marker for the transport and processing of E2 and p62. The oligosaccharide on the viral spike E1 protein was modified first to an Endo-D-sensitive (35 min) and then to an Endo-H-resistant structure (55 min), whereas the oligosaccharides of p62 remained sensitive towards Endo-H the whole time. E2 could be detected already at 10-20 min post synthesis, suggesting that p62 cleavage starts early, probably before the protein has been transported to the Golgi apparatus. This is in contrast to the cleavage taking place later mainly near the plasma membrane of higher eukaryotes. The spike proteins finally appeared in extracellular virions after about 70-90 min post synthesis.

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.