Role of N-linked glycans on bunyamwera virus glycoproteins in intracellular trafficking, protein folding, and virus infectivity

Culicoides-Specific Fitness Increase of Vesicular Stomatitis Virus in Insect-to-Insect Infections

Vesicular stomatitis virus (VSV) is an arthropod-borne virus affecting livestock. In the United States, sporadic outbreaks result in significant economic losses. During epizootics, Culicoides biting midges are biological vectors and key to the geographic expansion of outbreaks. Additionally, Culicoides may play a role in VSV overwintering because females and males are capable of highly efficient venereal transmission, despite their relatively low virus titers. We hypothesized that VSV propagated within a midge has increased fitness for subsequent midge infections. To evaluate the potential host-specific fitness increase, we propagated three viral isolates of VSV in porcine skin fibroblasts and Culicoides cell lines. We then evaluated the viral infection dynamics of the different cell-source groups in Culicoides sonorensis. Our results indicate that both mammalian- and insect-derived VSV replicate well in midges inoculated via intrathoracic injection, thereby bypassing the midgut barriers. However, when the virus was required to infect and escape the midgut barrier to disseminate after oral acquisition, the insect-derived viruses had significantly higher titers, infection, and dissemination rates than mammalian-derived viruses. Our research suggests that VSV replication in Culicoides cells increases viral fitness, facilitating midge-to-midge transmission and subsequent replication, and further highlights the significance of Culicoides midges in VSV maintenance and transmission dynamics.

Organisation of the orthobunyavirus tripodal spike and the structural changes induced by low pH and K+ during entry

Following endocytosis, enveloped viruses employ the changing environment of maturing endosomes as cues to promote endosomal escape, a process often mediated by viral glycoproteins. We previously showed that both high [K+] and low pH promote entry of Bunyamwera virus (BUNV), the prototypical bunyavirus. Here, we use sub-tomogram averaging and AlphaFold, to generate a pseudo-atomic model of the whole BUNV glycoprotein envelope. We unambiguously locate the Gc fusion domain and its chaperone Gn within the floor domain of the spike. Furthermore, viral incubation at low pH and high [K+], reminiscent of endocytic conditions, results in a dramatic rearrangement of the BUNV envelope. Structural and biochemical assays indicate that pH 6.3/K+ in the absence of a target membrane elicits a fusion-capable triggered intermediate state of BUNV GPs; but the same conditions induce fusion when target membranes are present. Taken together, we provide mechanistic understanding of the requirements for bunyavirus entry.

Mechanisms of bunyavirus morphogenesis and egress

Unlike many segmented negative-sense RNA viruses, most members of the Bunyavirales bud at Golgi membranes, as opposed to the plasma membrane. Central players in this assembly process are the envelope glycoproteins, Gn and Gc, which upon translation undergo proteolytic processing, glycosylation and trafficking to the Golgi, where they interact with ribonucleoprotein genome segments and bud into Golgi-derived compartments. The processes involved in genome packaging during virion assembly can lead to the generation of reassorted viruses, if a cell is co-infected with two different bunyaviruses, due to mismatching of viral genome segment packaging. This can lead to viruses with high pathogenic potential, as demonstrated by the emergence of Schmallenberg virus. This review focuses on the assembly pathways of tri-segmented bunyaviruses, highlighting some areas in need of further research to understand these important pathogens with zoonotic potential.

Structure, function, and evolution of the Orthobunyavirus membrane fusion glycoprotein

La Crosse virus, responsible for pediatric encephalitis in the United States, and Schmallenberg virus, a highly teratogenic veterinary virus in Europe, belong to the large Orthobunyavirus genus of zoonotic arthropod-borne pathogens distributed worldwide. Viruses in this under-studied genus cause CNS infections or fever with debilitating arthralgia/myalgia syndromes, with no effective treatment. The main surface antigen, glycoprotein Gc (∼1,000 residues), has a variable N-terminal half (Gc^S) targeted by the patients' antibody response and a conserved C-terminal moiety (Gc^F) responsible for membrane fusion during cell entry. Here, we report the X-ray structure of post-fusion La Crosse and Schmallenberg virus Gc^F, revealing the molecular determinants for hairpin formation and trimerization required to drive membrane fusion. We further experimentally confirm the role of residues in the fusion loops and in a vestigial endoplasmic reticulum (ER) translocation sequence at the Gc^S-Gc^F junction. The resulting knowledge provides essential molecular underpinnings for future development of potential therapeutic treatments and vaccines.

Oropouche Virus Glycoprotein Topology and Cellular Requirements for Glycoprotein Secretion

Oropouche virus (OROV; genus Orthobunyavirus) is the etiological agent of Oropouche fever, a debilitating febrile illness common in South America. We used recombinant expression of the OROV M polyprotein, which encodes the surface glycoproteins Gn and Gc plus the nonstructural protein NSm, to probe the cellular determinants for OROV assembly and budding. Gn and Gc self-assemble and are secreted independently of NSm. Mature OROV Gn has two predicted transmembrane domains that are crucial for glycoprotein translocation to the Golgi complex and glycoprotein secretion, and unlike related orthobunyaviruses, both transmembrane domains are retained during Gn maturation. Disruption of Golgi function using the drugs brefeldin A and monensin inhibits glycoprotein secretion. Infection studies have previously shown that the cellular endosomal sorting complexes required for transport (ESCRT) machinery is recruited to Golgi membranes during OROV assembly and that ESCRT activity is required for virus secretion. A dominant-negative form of the ESCRT-associated ATPase VPS4 significantly reduces recombinant OROV glycoprotein secretion and blocks virus release from infected cells, and VPS4 partly colocalizes with OROV glycoproteins and membranes costained with Golgi markers. Furthermore, immunoprecipitation and fluorescence microscopy experiments demonstrate that OROV glycoproteins interact with the ESCRT-III component CHMP6, with overexpression of a dominant-negative form of CHMP6 significantly reducing OROV glycoprotein secretion. Taken together, our data highlight differences in M polyprotein processing across orthobunyaviruses, indicate that Golgi and ESCRT function are required for glycoprotein secretion, and identify CHMP6 as an ESCRT-III component that interacts with OROV glycoproteins. IMPORTANCE Oropouche virus causes Oropouche fever, a debilitating illness common in South America that is characterized by high fever, headache, myalgia, and vomiting. The tripartite genome of this zoonotic virus is capable of reassortment, and there have been multiple epidemics of Oropouche fever in South America over the last 50 years, making Oropouche virus infection a significant threat to public health. However, the molecular characteristics of this arbovirus are poorly understood. We developed a recombinant protein expression system to investigate the cellular determinants of OROV glycoprotein maturation and secretion. We show that the proteolytic processing of the M polypeptide, which encodes the surface glycoproteins (Gn and Gc) plus a nonstructural protein (NSm), differs between OROV and its close relative Bunyamwera virus. Furthermore, we demonstrate that OROV M glycoprotein secretion requires the cellular endosomal sorting complexes required for transport (ESCRT) membrane-remodeling machinery and identify that the OROV glycoproteins interact with the ESCRT protein CHMP6.

Enhancement of infectivity of insect cell-derived La Crosse Virus by human serum

Given the dual life cycle of arboviruses in insect and animal hosts and the importance of serum factors as a first line antiviral defense, we have examined the outcome of interactions between the arbovirus La Crosse Virus (LACV) and human serum. To mimic the life cycle between species, we used LACV derived from insect (I-LACV) and human keratinocyte (HaCaT) cells. Incubation of I-LACV with normal human serum did not result in neutralization, but instead stabilized I-LACV virions and enhanced the amount of infectious virus. Enhanced infectivity was also seen with heat-inactivated serum devoid of complement activity and with serum from a range of animals including mouse, ferret, and non-human primates. Depletion of antibodies from serum resulted in loss of enhancement of infectivity and sucrose gradient sedimentation assays showed IgG co-sedimenting with I-LACV particles. In agreement with our results with I-LACV, HaCaT-derived LACV was not neutralized by complement or antibodies in normal human serum. However, in contrast to I-LACV, HaCaT-derived LACV infectivity was stable when incubated alone and treatment with serum did not enhance infectivity. Our results indicate that LACV derived from insect cells differs substantially from virus derived from human cells, with I-LACV being dependent on serum factors to enhance infectivity. These findings suggest that understanding differential composition of insect versus animal cell-derived LACV may form the foundation for potential new antiviral approaches.

A shrimp glycosylase protein, PmENGase, interacts with WSSV envelope protein VP41B and is involved in WSSV pathogenesis

Viral glycoproteins are expressed by many viruses, and during infection they usually play very important roles, such as receptor attachment or membrane fusion. The mature virion of the white spot syndrome virus (WSSV) is unusual in that it contains no glycosylated proteins, and there are currently no reports of any glycosylation mechanisms in the pathogenesis of this virus. In this study, we cloned a glycosylase, mannosyl-glycoprotein endo-β-N-acetylglucosaminidase (ENGase, EC 3.2.1.96), from Penaeus monodon and found that it was significantly up-regulated in WSSV-infected shrimp. A yeast two-hybrid assay showed that PmENGase interacted with both structural and non-structural proteins, and GST-pull down and co-immunoprecipitation (Co-IP) assays confirmed its interaction with the envelope protein VP41B. In the WSSV challenge tests, the cumulative mortality and viral copy number were significantly decreased in the PmEngase-silenced shrimp, from which we conclude that shrimp glycosylase interacts with WSSV in a way that benefits the virus. Lastly, we speculate that the deglycosylation activity of PmENGase might account for the absence of glycosylated proteins in the WSSV virion.

Vaccinia Virus Glycoproteins A33, A34, and B5 Form a Complex for Efficient Endoplasmic Reticulum to trans-Golgi Network Transport

Orthopoxviruses produce two, antigenically distinct, infectious enveloped virions termed intracellular mature virions and extracellular virions. Extracellular virions are required for cell-to-cell spread and pathogenesis. Specific to the extracellular virion membrane, glycoproteins A33, A34, and B5 are highly conserved among orthopoxviruses and have roles during extracellular virion formation and subsequent infection. B5 is dependent on an interaction with either A33 or A34 for localization to the site of intracellular envelopment and incorporation into the envelope of released extracellular virions. In this report we show that an interaction between A33 and A34 can be detected in infected cells. Furthermore, we show that a three-protein complex between A33, A34, and B5 forms in the endoplasmic reticulum (ER) that disassociates post ER export. Finally, immunofluorescence reveals that coexpression of all three glycoproteins results in their localization to a juxtanuclear region that is presumably the site of intracellular envelopment. These results demonstrate the existence of two previously unidentified interactions: one between A33 and A34 and another simultaneous interaction between all three of the glycoproteins. Furthermore, these results indicate that interactions among A33, A34, and B5 are vital for proper intracellular trafficking and subcellular localization.IMPORTANCE The secondary intracellular envelopment of poxviruses at the trans-Golgi network to release infectious extracellular virus (EV) is essential for their spread and pathogenesis. Viral glycoproteins A33, A34, and B5 are critical for the efficient production of infectious EV and interactions among these proteins are important for their localization and incorporation into the outer extracellular virion membrane. We have uncovered a novel interaction between glycoproteins A33 and A34. Furthermore, we show that B5 can interact with the A33-A34 complex. Our analysis indicates that the three-protein complex has a role in ER exit and proper localization of the three glycoproteins to the intracellular site of wrapping. These results show that a complex set of interactions occur in the secretory pathway of infected cells to ensure proper glycoprotein trafficking and envelope content, which is important for the release of infectious poxvirus virions.

A Genome-Wide CRISPR-Cas9 Screen Identifies the Dolichol-Phosphate Mannose Synthase Complex as a Host Dependency Factor for Dengue Virus Infection

Dengue virus (DENV) is a mosquito-borne flavivirus responsible for dengue disease, a major human health concern for which no specific therapies are available. Like other viruses, DENV relies heavily on the host cellular machinery for productive infection. In this study, we performed a genome-wide CRISPR-Cas9 screen using haploid HAP1 cells to identify host genes important for DENV infection. We identified DPM1 and -3, two subunits of the endoplasmic reticulum (ER) resident dolichol-phosphate mannose synthase (DPMS) complex, as host dependency factors for DENV and other related flaviviruses, such as Zika virus (ZIKV). The DPMS complex catalyzes the synthesis of dolichol-phosphate mannose (DPM), which serves as mannosyl donor in pathways leading to N-glycosylation, glycosylphosphatidylinositol (GPI) anchor biosynthesis, and C- or O-mannosylation of proteins in the ER lumen. Mutation in the DXD motif of DPM1, which is essential for its catalytic activity, abolished DPMS-mediated DENV infection. Similarly, genetic ablation of ALG3, a mannosyltransferase that transfers mannose to lipid-linked oligosaccharide (LLO), rendered cells poorly susceptible to DENV. We also established that in cells deficient for DPMS activity, viral RNA amplification is hampered and truncated oligosaccharides are transferred to the viral prM and E glycoproteins, affecting their proper folding. Overall, our study provides new insights into the host-dependent mechanisms of DENV infection and supports current therapeutic approaches using glycosylation inhibitors to treat DENV infection.IMPORTANCE Dengue disease, which is caused by dengue virus (DENV), has emerged as the most important mosquito-borne viral disease in humans and is a major global health concern. DENV encodes only few proteins and relies on the host cell machinery to accomplish its life cycle. The identification of the host factors important for DENV infection is needed to propose new targets for antiviral intervention. Using a genome-wide CRISPR-Cas9 screen, we identified DPM1 and -3, two subunits of the DPMS complex, as important host factors for the replication of DENV as well as other related viruses such as Zika virus. We established that DPMS complex plays dual roles during viral infection, both regulating viral RNA replication and promoting viral structural glycoprotein folding/stability. These results provide insights into the host molecules exploited by DENV and other flaviviruses to facilitate their life cycle.

Evolutionary Dynamics of Oropouche Virus in South America

The emergence and reemergence of pathogens such as Zika virus, chikungunya virus, and yellow fever virus have drawn attention toward other cocirculating arboviruses in South America. Oropouche virus (OROV) is a poorly studied pathogen responsible for over a dozen outbreaks since the early 1960s and represents a public health burden to countries such as Brazil, Panama, and Peru. OROV is likely underreported since its symptomatology can be easily confounded with other febrile illnesses (e.g., dengue fever and leptospirosis) and point-of-care testing for the virus is still uncommon. With limited data, there is a need to optimize the information currently available. Analysis of OROV genomes can help us understand how the virus circulates in nature and can reveal the evolutionary forces that shape the genetic diversity of the virus, which has implications for molecular diagnostics and the design of potential vaccines.

The Amazon basin is home to numerous arthropod-borne viral pathogens that cause febrile disease in humans. Among these, Oropouche orthobunyavirus (OROV) is a relatively understudied member of the genus Orthobunyavirus, family Peribunyaviridae, that causes periodic outbreaks in human populations in Brazil and other South American countries. Although several studies have described the genetic diversity of the virus, the evolutionary processes that shape the OROV genome remain poorly understood. Here, we present a comprehensive study of the genomic dynamics of OROV that encompasses phylogenetic analysis, evolutionary rate estimates, inference of natural selective pressures, recombination and reassortment, and structural analysis of OROV variants. Our study includes all available published sequences, as well as a set of new OROV genome sequences obtained from patients in Ecuador, representing the first set of genomes from this country. Our results show differing evolutionary processes on the three segments that comprise the viral genome. We infer differing times of the most recent common ancestors of the genome segments and propose that this can be explained by cryptic reassortment. We also present the discovery of previously unobserved putative N-linked glycosylation sites, as well as codons that evolve under positive selection on the viral surface proteins, and discuss the potential role of these features in the evolution of OROV through a combined phylogenetic and structural approach.

IMPORTANCE The emergence and reemergence of pathogens such as Zika virus, chikungunya virus, and yellow fever virus have drawn attention toward other cocirculating arboviruses in South America. Oropouche virus (OROV) is a poorly studied pathogen responsible for over a dozen outbreaks since the early 1960s and represents a public health burden to countries such as Brazil, Panama, and Peru. OROV is likely underreported since its symptomatology can be easily confounded with other febrile illnesses (e.g., dengue fever and leptospirosis) and point-of-care testing for the virus is still uncommon. With limited data, there is a need to optimize the information currently available. Analysis of OROV genomes can help us understand how the virus circulates in nature and can reveal the evolutionary forces that shape the genetic diversity of the virus, which has implications for molecular diagnostics and the design of potential vaccines.

Exploitation of glycosylation in enveloped virus pathobiology

Glycosylation is a ubiquitous post-translational modification responsible for a multitude of crucial biological roles. As obligate parasites, viruses exploit host-cell machinery to glycosylate their own proteins during replication. Viral envelope proteins from a variety of human pathogens including HIV-1, influenza virus, Lassa virus, SARS, Zika virus, dengue virus, and Ebola virus have evolved to be extensively glycosylated. These host-cell derived glycans facilitate diverse structural and functional roles during the viral life-cycle, ranging from immune evasion by glycan shielding to enhancement of immune cell infection. In this review, we highlight the imperative and auxiliary roles glycans play, and how specific oligosaccharide structures facilitate these functions during viral pathogenesis. We discuss the growing efforts to exploit viral glycobiology in the development of anti-viral vaccines and therapies.

The CARD9-Associated C-Type Lectin, Mincle, Recognizes La Crosse Virus (LACV) but Plays a Limited Role in Early Antiviral Responses against LACV

La Crosse virus (LACV) is a mosquito-transmitted arbovirus and the main cause of virus-mediated neurological diseases in children. To date, little is known about the role of C-type lectin receptors (CLRs)—an important class of pattern recognition receptors—in LACV recognition. DC-SIGN remains the only well-described CLR that recognizes LACV. In this study, we investigated the role of additional CLR/LACV interactions. To this end, we applied a flow-through chromatography method for the purification of LACV to perform an unbiased high-throughput screening of LACV with a CLR-hFc fusion protein library. Interestingly, the CARD9-associated CLRs Mincle, Dectin-1, and Dectin-2 were identified to strongly interact with LACV. Since CARD9 is a common adaptor protein for signaling via Mincle, Dectin-1, and Dectin-2, we performed LACV infection of Mincle^−/− and CARD9^−/− DCs. Mincle^−/− and CARD9^−/− DCs produced less amounts of proinflammatory cytokines, namely IL-6 and TNF-α, albeit no reduction of the LACV titer was observed. Together, novel CLR/LACV interactions were identified; however, the Mincle/CARD9 axis plays a limited role in early antiviral responses against LACV.

Comparison of Schmallenberg virus sequences isolated from mammal host and arthropod vector

Schmallenberg virus (SBV) is the member of Peribunyaviridae family, which comprises pathogens of importance for human and veterinary medicine. The virus is transmitted only between animals and mainly by biting midges of the genus Culicoides. This study was performed in order to determine SBV genetic diversity and elucidate the host–vector adaptation. All three viral segments were analysed for sequence variability and phylogenetic relations. The Polish SBV strains obtained from acute infections of cattle, congenital cases in sheep, and from Culicoides midges were sequenced using Sanger and next-generation sequencing (NGS) methods. The obtained sequences were genetically similar (99.2–100% identity) to the first-detected strain BH80/11—4 from German cattle. The sampling year and origin of Polish sequences had no effect on molecular diversity of SBV. Considering all analysed Polish as well as European sequences, ovine-derived sequences were the most variable, while the midge ones were more conserved and encompassed unique substitutions located mainly in nonstructural protein S. SBV sequences isolated from Culicoides are the first submitted to GenBank and reported.

Prediction of N-linked glycosylation sites using position relative features and statistical moments

Glycosylation is one of the most complex post translation modification in eukaryotic cells. Almost 50% of the human proteome is glycosylated as glycosylation plays a vital role in various biological functions such as antigen’s recognition, cell-cell communication, expression of genes and protein folding. It is a significant challenge to identify glycosylation sites in protein sequences as experimental methods are time taking and expensive. A reliable computational method is desirable for the identification of glycosylation sites. In this study, a comprehensive technique for the identification of N-linked glycosylation sites has been proposed using machine learning. The proposed predictor was trained using an up-to-date dataset through back propagation algorithm for multilayer neural network. The results of ten-fold cross-validation and other performance measures such as accuracy, sensitivity, specificity and Mathew’s correlation coefficient inferred that the accuracy of proposed tool is far better than the existing systems such as Glyomine, GlycoEP, Ensemble SVM and GPP.

Entry of severe fever with thrombocytopenia syndrome virus

Severe fever with thrombocytopenia syndrome virus (SFTSV) is a globe-shaped virus covered by a dense icosahedral array of glycoproteins Gn/Gc that mediate the attachment of the virus to host cells and the fusion of viral and cellular membranes. Several membrane factors are involved in virus entry, including C-type lectins and nonmuscle myosin heavy chain IIA. The post-fusion crystal structure of the Gc protein suggests that it is a class II membrane fusion protein, similar to the E/E1 protein of flaviviruses and alphaviruses. The virus particles are internalized into host cell endosomes through the clathrin-dependent pathway, where the low pH activates the fusion of the virus with the cell membrane. With information from studies on other bunyaviruses, herein we will review our knowledge of the entry process of SFTSV.

The Role of Phlebovirus Glycoproteins in Viral Entry, Assembly and Release

Bunyaviruses are enveloped viruses with a tripartite RNA genome that can pose a serious threat to animal and human health. Members of the Phlebovirus genus of the family Bunyaviridae are transmitted by mosquitos and ticks to humans and include highly pathogenic agents like Rift Valley fever virus (RVFV) and severe fever with thrombocytopenia syndrome virus (SFTSV) as well as viruses that do not cause disease in humans, like Uukuniemi virus (UUKV). Phleboviruses and other bunyaviruses use their envelope proteins, Gn and Gc, for entry into target cells and for assembly of progeny particles in infected cells. Thus, binding of Gn and Gc to cell surface factors promotes viral attachment and uptake into cells and exposure to endosomal low pH induces Gc-driven fusion of the viral and the vesicle membranes. Moreover, Gn and Gc facilitate virion incorporation of the viral genome via their intracellular domains and Gn and Gc interactions allow the formation of a highly ordered glycoprotein lattice on the virion surface. Studies conducted in the last decade provided important insights into the configuration of phlebovirus Gn and Gc proteins in the viral membrane, the cellular factors used by phleboviruses for entry and the mechanisms employed by phlebovirus Gc proteins for membrane fusion. Here, we will review our knowledge on the glycoprotein biogenesis and the role of Gn and Gc proteins in the phlebovirus replication cycle.

Bunyamwera orthobunyavirus glycoprotein precursor is processed by cellular signal peptidase and signal peptide peptidase

The M genome segment of Bunyamwera virus (BUNV)-the prototype of both the Bunyaviridae family and the Orthobunyavirus genus-encodes the glycoprotein precursor (GPC) that is proteolytically cleaved to yield two viral structural glycoproteins, Gn and Gc, and a nonstructural protein, NSm. The cleavage mechanism of orthobunyavirus GPCs and the host proteases involved have not been clarified. In this study, we investigated the processing of BUNV GPC and found that both NSm and Gc proteins were cleaved at their own internal signal peptides (SPs), in which NSm domain I functions as SP(NSm) and NSm domain V as SP(Gc) Moreover, the domain I was further processed by a host intramembrane-cleaving protease, signal peptide peptidase, and is required for cell fusion activities. Meanwhile, the NSm domain V (SP(Gc)) remains integral to NSm, rendering the NSm topology as a two-membrane-spanning integral membrane protein. We defined the cleavage sites and boundaries between the processed proteins as follows: Gn, from residue 17-312 or nearby residues; NSm, 332-477; and Gc, 478-1433. Our data clarified the mechanism of the precursor cleavage process, which is important for our understanding of viral glycoprotein biogenesis in the genus Orthobunyavirus and thus presents a useful target for intervention strategies.

N-Glycans on the Rift Valley Fever Virus Envelope Glycoproteins Gn and Gc Redundantly Support Viral Infection via DC-SIGN

Rift Valley fever is a mosquito-transmitted, zoonotic disease that infects humans and ruminants. Dendritic cell specific intercellular adhesion molecule 3 (ICAM-3) grabbing non-integrin (DC-SIGN) acts as a receptor for members of the phlebovirus genus. The Rift Valley fever virus (RVFV) glycoproteins (Gn/Gc) encode five putative N-glycan sequons (asparagine (N)–any amino acid (X)–serine (S)/threonine (T)) at positions: N438 (Gn), and N794, N829, N1035, and N1077 (Gc). The N-glycosylation profile and significance in viral infection via DC-SIGN have not been elucidated. Gc N-glycosylation was first evaluated by using Gc asparagine (N) to glutamine (Q) mutants. Subsequently, we generated a series of recombinant RVFV MP-12 strain mutants, which encode N-to-Q mutations, and the infectivity of each mutant in Jurkat cells stably expressing DC-SIGN was evaluated. Results showed that Gc N794, N1035, and N1077 were N-glycosylated but N829 was not. Gc N1077 was heterogeneously N-glycosylated. RVFV Gc made two distinct N-glycoforms: “Gc-large” and “Gc-small”, and N1077 was responsible for “Gc-large” band. RVFV showed increased infection of cells expressing DC-SIGN compared to cells lacking DC-SIGN. Infection via DC-SIGN was increased in the presence of either Gn N438 or Gc N1077. Our study showed that N-glycans on the Gc and Gn surface glycoproteins redundantly support RVFV infection via DC-SIGN.

Genetic and Phylogenetic Characterization of Tataguine and Witwatersrand Viruses and Other Orthobunyaviruses of the Anopheles A, Capim, Guamá, Koongol, Mapputta, Tete, and Turlock Serogroups

The family Bunyaviridae has more than 530 members that are distributed among five genera or remain to be classified. The genus Orthobunyavirus is the most diverse bunyaviral genus with more than 220 viruses that have been assigned to more than 18 serogroups based on serological cross-reactions and limited molecular-biological characterization. Sequence information for all three orthobunyaviral genome segments is only available for viruses belonging to the Bunyamwera, Bwamba/Pongola, California encephalitis, Gamboa, Group C, Mapputta, Nyando, and Simbu serogroups. Here we present coding-complete sequences for all three genome segments of 15 orthobunyaviruses belonging to the Anopheles A, Capim, Guamá, Kongool, Tete, and Turlock serogroups, and of two unclassified bunyaviruses previously not known to be orthobunyaviruses (Tataguine and Witwatersrand viruses). Using those sequence data, we established the most comprehensive phylogeny of the Orthobunyavirus genus to date, now covering 15 serogroups. Our results emphasize the high genetic diversity of orthobunyaviruses and reveal that the presence of the small nonstructural protein (NSs)-encoding open reading frame is not as common in orthobunyavirus genomes as previously thought.

Genomic characterisation of three Mapputta group viruses, a serogroup of Australian and Papua New Guinean bunyaviruses associated with human disease

The Mapputta serogroup tentatively contains the mosquito-associated viruses Mapputta, Maprik, Trubanaman and Gan Gan. Interestingly, this serogroup has previously been associated with an acute epidemic polyarthritis-like illness in humans; however, there has been no ensuing genetic characterisation. Here we report the complete genome sequences of Mapputta and Maprik viruses, and a new Mapputta group candidate, Buffalo Creek virus, previously isolated from mosquitoes and detected by serology in a hospitalised patient. Phylogenetic analyses indicate that the group is one of the earliest diverged groups within the genus Orthobunyavirus of the family Bunyaviridae. Analyses show that these three viruses are related to the recently sequenced Australian bunyaviruses from mosquitoes, Salt Ash and Murrumbidgee. A notable feature of the Mapputta group viruses is the absence of the NSs (non-structural) ORF commonly found on the S segment of other orthobunyaviruses. Viruses of the Mapputta group have been isolated from geographically diverse regions ranging from tropical Papua New Guinea to the semi-arid climate of south-eastern Australia. The relevance of this group to human health in the region merits further investigation.

Imaging RNA virus replication assemblies: bunyaviruses and reoviruses

ABSTRACT 

RNA viruses replicate in the cytoplasm in close association with host cell membranes. Both viral and cellular factors generate organelle-like structures termed viral factories, viral inclusions or viroplasms. Biochemical, light and electron microscopy analyses, including 3D models, have improved our understanding of the architecture and function of RNA virus replication factories. In these structures, the virus compartmentalizes genome replication and transcription, thus enhancing replication efficiency and protection from host defenses. Recent studies with diverse RNA viruses have elucidated the ultrastructure of replication organelles and shown how these structures act in close coordination with virion assembly. This review focuses on a general description of RNA virus factories and summarizes recent progress in the characterization of those assembled by bunyaviruses and reoviruses. We describe how these viruses modify intracellular membranes; we highlight similarities with the structures induced by viruses of other families, and discuss how these structures might be formed.

Schmallenberg virus-two years of experiences

In autumn 2011, a novel species of the genus Orthobunyavirus of the Simbu serogroup was discovered close to the German/Dutch border and named Schmallenberg virus (SBV). Since then, SBV has caused a large epidemic in European livestock. Like other viruses of the Simbu serogroup, SBV is transmitted by insect vectors. Adult ruminants may show a mild transient disease, while an infection during a critical period of pregnancy can lead to severe congenital malformation, premature birth or stillbirth. The current knowledge about the virus, its diagnosis, the spread of the epidemic, the impact and the possibilities for preventing infections with SBV is described and discussed.

Role of N-linked glycosylation of the human parainfluenza virus type 3 hemagglutinin-neuraminidase protein

Human parainfluenza virus type 3 (hPIV-3) is a major respiratory tract pathogen that affects infants and young children. The hPIV-3 hemagglutinin-neuraminidase (HN) protein is a multifunctional protein mediating hemadsorption (HAD), neuraminidase (NA), and fusion promotion activities, each of which affects the ability of HN to promote viral fusion and entry. The hPIV-3 HN protein contains four potential sites (N308, N351, N485 and N523) for N-linked glycosylation. Electrophoretic mobility analysis of mutated HN proteins indicated that N308, N351 and N523 sites, but not the N485 site in HN protein, were targeted for the addition of glycans in BHK-21 cells. These functional glycosylation sites were systematically eliminated in various combinations from HN to form a panel of mutants in which the roles of individual carbohydrate chains and groups of carbohydrate chains could be analyzed. Removal of individual or multiple N-glycans on the hPIV-3 HN protein had no effects on transport to the cell surface, expression and NA activity. Single glycosylation site mutants (G1, G2 and G4) not only impaired fusion promotion activity but also reduced HAD activity of HN protein, which was even more obvious for all three double mutants (G12, G14 and G24) and the triple mutant (G124). In addition, every mutant protein retained F-interactive capability that was equal to the wild-type protein capability. Interestingly, the F protein that could be co-immunoprecipitated with the G12 mutated protein or immunoprecipitated with anti-F antibody was not efficiently cleaved. For G14, G24 and G124, little cleaved F protein was detected in co-immuoprecipitation F protein assay and its total amounts where in the cell lysates. The mechanism underlying hPIV-3 HN and F protein remained associated before and after receptor engagement and the strength of the HN-receptor interaction modulated the activation of F the protein which could determine the extent of fusion. Finally, we demonstrated that single or multiple N-glycosylation site mutations inhibited fusion at the earliest stages. Taken together, these results indicated that N-glycosylation of hPIV-3 HN is critical to its receptor recognition activity, cleavage of the F protein, and fusion promotion activity, but had no influence on its interaction with the homologous F protein and NA activity.

Approaches to define the viral genetic basis of classical swine fever virus virulence

Classical swine fever (CSF), a highly contagious disease of pigs caused by the classical swine fever virus (CSFV), can lead to important economic losses in the pig industry. Numerous CSFV isolates with various degrees of virulence have been isolated worldwide, ranging from low virulent strains that do not result in any apparent clinical signs to highly virulent strains that cause a severe peracute hemorrhagic fever with nearly 100% mortality. Knowledge of the molecular determinants of CSFV virulence is an important issue for effective disease control and development of safe and effective marker vaccines. In this review, the latest studies in the field of CSFV virulence are discussed. The topic of virulence is addressed from different angles; nonconventional approaches like codon pair usage and quasispecies are considered. Future research approaches in the field of CSFV virulence are proposed.

A mutation 'hot spot' in the Schmallenberg virus M segment

In the autumn of 2011, Schmallenberg virus (SBV), a novel orthobunyavirus of the Simbu serogroup, was identified by metagenomic analysis in Germany. SBV has since been detected in ruminants all over Europe, and investigations on phylogenetic relationships, clinical signs and epidemiology have been conducted. However, until now, only comparative sequence analysis of SBV genome segments with other species of the Simbu serogroup have been performed, and detailed data on the S and M segments, relevant for virus-host-cell interaction, have been missing. In this study, we investigated the S- and M-segment sequences obtained from 24 SBV-positive field samples from sheep, cattle and a goat collected from all over Germany. The results obtained indicated that the overall genome variability of SBV is neither regionally nor host species dependent. Nevertheless, we characterized for the first time a region of high sequence variability (a mutation 'hot spot') within the glycoprotein Gc encoded by the M segment.

Inhibition of α-glucosidase and α-amylase by diaryl derivatives of imidazole-thione and 1,2,4-triazole-thiol

The in vivo and in vitro effects of 4,5-diphenylimidazole-2-thione (1), 4,5-diphenyl-1,2,4-triazole-3-thiol (2) and 5-(2-hydroxyphenyl)-4-phenyl-1,2,4-triazole-3-thiol (3) on α-glucosidase and α-amylase were investigated. The in vivo inhibition has been found to be dose-dependent and to occur at a value less than LD50. The in vitro treatment of the enzymes by 4,5-diphenylimidazole-2-thione exhibited a reversible inhibition of the non-competitive type with Ki value of 3.5 and 6.5×10(-5) M for α-glucosidase and α-amylase, respectively. 4,5-diphenyl-1,2,4-triazole-3-thione showed a reversible inhibition of the competitive and non-competitive types, with Ki value of 10(-5) M magnitude, for α-glucosidase and α-amylase. On the other hand, 5-(o-hydroxyphenyl)-4-phenyl-1,2,4-triazole-3-thione did not display an inhibitory effect towards α-amylase but showed a potent inhibition of the competitive type for hepatic α-glucosidase with 10(-5) M magnitude of Ki value.

Role of the NSs protein in the zoonotic capacity of Orthobunyaviruses

The family Bunyaviridae contains over 350 named isolates, classified into five genera: Orthobunyavirus, Hantavirus, Nairovirus, Phlebovirus and Tospovirus. The Orthobunyavirus genus contains some 170 isolates that are mainly transmitted by mosquitoes and are responsible for a range of disease syndromes in humans including self-limiting febrile illness, encephalitis and haemorrhagic fever. The viruses have a tripartite, negative-sense RNA genome. Analyses of viruses in four serogroups (Bunyamwera, California, Group C and Simbu) showed that the smallest (S) RNA segment encodes the nucleocapsid protein (N) and a non-structural protein called (NSs). The NSs protein of Bunyamwera virus (BUNV) has been shown to play a role in shut-off of host cell protein synthesis in mammalian cells, but no protein shut-off is observed in BUNVinfected mosquito cells (Aedes albopictus C6/36 cells). Protein shut-off in infected mammalian cells is achieved by global inhibition of RNA polymerase II-mediated transcription and enables the virus to overcome the host innate immune response. As innate defence mechanisms constitute a significant barrier to virus infection of different hosts, NSs would appear to play a key role in determining the zoonotic capacity of orthobunyaviruses.

Efficient cellular release of Rift Valley fever virus requires genomic RNA

The Rift Valley fever virus is responsible for periodic, explosive epizootics throughout sub-Saharan Africa. The development of therapeutics targeting this virus is difficult due to a limited understanding of the viral replicative cycle. Utilizing a virus-like particle system, we have established roles for each of the viral structural components in assembly, release, and virus infectivity. The envelope glycoprotein, Gn, was discovered to be necessary and sufficient for packaging of the genome, nucleocapsid protein and the RNA-dependent RNA polymerase into virus particles. Additionally, packaging of the genome was found to be necessary for the efficient release of particles, revealing a novel mechanism for the efficient generation of infectious virus. Our results identify possible conserved targets for development of anti-phlebovirus therapies.

Glycosylation of minor envelope glycoproteins of porcine reproductive and respiratory syndrome virus in infectious virus recovery, receptor interaction, and immune response

The role of N-glycosylation of the three minor envelope glycoproteins (GP2, GP3, and GP4) of porcine reproductive and respiratory syndrome virus (PRRSV) on infectious virus production, interactions with the receptor CD163, and neutralizing antibody production in infected pigs was examined. By mutation of the glycosylation sites in these proteins, the studies show that glycan addition at N184 of GP2, N42, N50 and N131 of GP3 is necessary for infectious virus production. Although single-site mutants of GP4 led to infectious virus production, mutation of any two sites in GP4 was lethal. Furthermore, the glycosylation of GP2 and GP4 was important for efficient interaction with CD163. Unlike PRRSVs encoding hypoglycosylated form of GP5 that induced significantly higher levels of neutralizing antibodies in infected piglets, PRRSVs encoding hypoglycosylated forms of GP2, GP3 or GP4 did not. These studies reveal the importance of glycosylation of these minor GPs in the biology of PRRSV.

Visualizing the replication cycle of bunyamwera orthobunyavirus expressing fluorescent protein-tagged Gc glycoprotein

The virion glycoproteins Gn and Gc of Bunyamwera virus (BUNV), the prototype of the Bunyaviridae family and also of the Orthobunyavirus genus, are encoded by the medium (M) RNA genome segment and are involved in both viral attachment and entry. After their synthesis Gn and Gc form a heterodimer in the endoplasmic reticulum (ER) and transit to the Golgi compartment for virus assembly. The N-terminal half of the Gc ectodomain was previously shown to be dispensable for virus replication in cell culture (X. Shi, J. Goli, G. Clark, K. Brauburger, and R. M. Elliott, J. Gen. Virol. 90:2483-2492, 2009.). In this study, the coding sequence for a fluorescent protein, either enhanced green fluorescent protein (eGFP) or mCherry fluorescent protein, was fused to the N terminus of truncated Gc, and two recombinant BUNVs (rBUNGc-eGFP and rBUNGc-mCherry) were rescued by reverse genetics. The recombinant viruses showed bright autofluorescence under UV light and were competent for replication in various mammalian cell lines. rBUNGc-mCherry was completely stable over 10 passages, whereas internal, in-frame deletions occurred in the chimeric Gc-eGFP protein of rBUNGc-eGFP, resulting in loss of fluorescence between passages 5 and 7. Autofluorescence of the recombinant viruses allowed visualization of different stages of the infection cycle, including virus attachment to the cell surface, budding of virus particles in Golgi membranes, and virus-induced morphological changes to the Golgi compartment at later stages of infection. The fluorescent protein-tagged viruses will be valuable reagents for live-cell imaging studies to investigate virus entry, budding, and morphogenesis in real time.

Nucleotide variability of Tahyna virus (Bunyaviridae, Orthobunyavirus) small (S) and medium (M) genomic segments in field strains differing in biological properties

Tahyna virus (TAHV), a mosquito-borne bunyavirus (California group), is frequently associated with inapparent or influenza-like (Valtice fever) infections in humans, rarely leading to atypical pneumonia or meningitis. Field TAHV strains exhibit a high variability in their biological properties with respect to virulence for laboratory mouse, temperature-sensitivity or character of plaques in cell culture. In consideration of the variations in the antigenic properties TAHV and its potential genetic variability, we analyzed complete nucleotide sequences of the small (S) and medium (M) genomic segments of field TAHV strains with different combinations of phenotypic markers. S segment was highly conservative in all analyzed TAHV strains. Within the M segment, the highest variability was observed in the G(C) gene encoding viral envelope protein and to a less extent also in the NSm gene. However, 5' and 3' non-coding regions of M segment, as well as in G(N) gene exhibited highly conservative pattern, indicating its functional importance, but minor or no role in the determination of biological properties of TAHV field strains.

Mutational analysis of the Bunyamwera orthobunyavirus nucleocapsid protein gene

The bunyavirus nucleocapsid protein, N, is a multifunctional protein that encapsidates each of the three negative-sense genome segments to form ribonucleoprotein complexes that are the functional templates for viral transcription and replication. In addition, N protein molecules interact with themselves to form oligomers, with the viral L (RNA polymerase) protein, with the carboxy-terminal regions of either or both of the virion glycoproteins, and probably also with host cell proteins. Bunyamwera virus (BUNV), the prototype bunyavirus, encodes an N protein of 233 amino acids in length. To learn more about the roles of individual amino acids in the different interactions of N, we performed a wide-scale mutagenic analysis of the protein, and 110 single-point mutants were obtained. When the mutants were employed in a minireplicon assay to examine their effects on viral RNA synthesis, a wide range of activities compared to those of wild-type N protein were observed; changes at nine amino acid positions resulted in severely impaired RNA synthesis. Seventy-seven mutant clones were selected for use in the bunyavirus reverse genetics system, and 57 viable recombinant viruses were recovered. The recombinant viruses displayed a range of plaque sizes and titers in cell culture (from approximately 10(3) to 10(8) PFU/ml), and a number of viruses were shown to be temperature sensitive. Different assays were applied to determine why 20 mutant N proteins could not be recovered into infectious virus. Based on these results, a preliminary domain map of the BUNV N protein is proposed.

Functional analysis of the Bunyamwera orthobunyavirus Gc glycoprotein

The virion glycoproteins Gn and Gc of Bunyamwera orthobunyavirus (family Bunyaviridae) are encoded by the M RNA genome segment and have roles in both viral attachment and membrane fusion. To investigate further the structure and function of the Gc protein in viral replication, we generated 12 mutants that contain truncations from the N terminus. The effects of these deletions were analysed with regard to Golgi targeting, low pH-dependent membrane fusion, infectious virus-like particle (VLP) formation and virus infectivity. Our results show that the N-terminal half (453 residues) of the Gc ectodomain (909 residues in total) is dispensable for Golgi trafficking and cell fusion. However, deletions in this region resulted in a significant reduction in VLP formation. Four mutant viruses that contained N-terminal deletions in their Gc proteins were rescued, and found to be attenuated to different degrees in BHK-21 cells. Taken together, our data indicate that the N-terminal half of the Gc ectodomain is dispensable for replication in cell culture, whereas the C-terminal half is required to mediate cell fusion. A model for the domain structure of the Gc ectodomain is proposed.

Generation and analysis of recombinant Bunyamwera orthobunyaviruses expressing V5 epitope-tagged L proteins

The L protein of Bunyamwera virus (BUNV; family Bunyaviridae) is an RNA-dependent RNA polymerase, 2238 aa in length, that catalyses transcription and replication of the negative-sense, tripartite RNA genome. To learn more about the molecular interactions of the L protein and to monitor its intracellular distribution we inserted a 14 aa V5 epitope derived from parainfluenza virus type 5, against which high-affinity antibodies are available, into different regions of the protein. Insertion of the epitope at positions 1935 or 2046 resulted in recombinant L proteins that retained functionality in a minireplicon assay. Two viable recombinant viruses, rBUNL4V5 and rBUNL5V5, expressing the tagged L protein were rescued by reverse genetics, and characterized with respect to their plaque size, growth kinetics and protein synthesis profile. The recombinant viruses behaved similarly to wild-type (wt) BUNV in BHK-21 cells, but formed smaller plaques and grew to lower titres in Vero E6 cells compared with wt BUNV. Immunofluorescent staining of infected cells showed the L protein to have a punctate to reticular distribution in the cytoplasm, and cell fractionation studies indicated that the L protein was present in both soluble and microsomal fractions. Co-immunoprecipitation and confocal microscopic assays confirmed an interaction between BUNV L and N proteins. The recombinant viruses expressing tagged L protein will be highly valuable reagents for the detailed dissection of the role of the BUNV L protein in virus replication.

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.

Alteration of the N-linked glycosylation condition in E1 glycoprotein of Classical Swine Fever Virus strain Brescia alters virulence in swine

E1, along with E(rns) and E2 is one of the three envelope glycoproteins of Classical Swine Fever Virus (CSFV). Previously we showed that glycosylation status of virulent CSFV strain Brescia E2 or E(rns) affects virus virulence. Here, the three putative glycosylation sites of E1 were serially removed by means of site directed mutagenesis of a CSFV Brescia infectious clone (BICv) and their effect on virulence assessed in swine. Removal of all three putative glycosylation sites in E1, at CSFV positions N500, N513 and N594, yielded nonviable progeny, while single or dual site mutants excluding N594 were viable. Individual N594A (E1.N3 virus) or combined N500A/N513A (E1.N1N2 virus) substitutions resulted in BICv attenuation. Furthermore infection with E1.N3 or E1.N1N2 viruses efficiently protected swine from challenge with virulent BICv at 3 and 28 days post-infection. As previously observed with E(rns) and E2 and here with E1 data suggest that modification of glycosylation patterns could be used for developing CSFV live-attenuated vaccines.

Amino acid substitutions in the s2 region enhance severe acute respiratory syndrome coronavirus infectivity in rat angiotensin-converting enzyme 2-expressing cells

To clarify the molecular basis of severe acute respiratory syndrome coronavirus (SARS-CoV) adaptation to different host species, we serially passaged SARS-CoV in rat angiotensin-converting enzyme 2 (ACE2)-expressing cells. After 15 passages, the virus (Rat-P15) came to replicate effectively in rat ACE2-expressing cells. Two amino acid substitutions in the S2 region were found on the Rat-P15 S gene. Analyses of the infectivity of the pseudotype-bearing S protein indicated that the two substitutions in the S2 region, especially the S950F substitution, were responsible for efficient infection. Therefore, virus adaptation to different host species can be induced by amino acid substitutions in the S2 region.

Role of the cytoplasmic tail domains of Bunyamwera orthobunyavirus glycoproteins Gn and Gc in virus assembly and morphogenesis

The M RNA genome segment of Bunyamwera virus (BUNV), the prototype of the Bunyaviridae family, encodes a precursor polyprotein that is proteolytically cleaved to yield two structural proteins, Gn and Gc, and a nonstructural protein called NSm. Gn and Gc are type I integral transmembrane glycoproteins. The Gn protein contains a predicted cytoplasmic tail (CT) of 78 residues, and Gc has a shorter CT of 25 residues. Little is known about the role of the Gn and Gc CT domains in the virus replication cycle. We generated a series of mutant glycoprotein precursor constructs containing either deletions or alanine substitutions in the CT domains of Gn and Gc. We examined the effects of these mutations on glycoprotein maturation, cell surface expression, and low pH-induced syncytium formation. In addition, the effects of these mutations were also assessed using a reverse genetics-based virus assembly assay and a virus rescue system. Our results show that the CT domains of both Gn and Gc play crucial roles in BUNV-mediated membrane fusion, virus assembly, and morphogenesis.

Analysis of glycoproteins of viruses in the family Bunyaviridae

The membrane glycoproteins (Gn and Gc) of viruses in the family Bunyaviridae form projections on the virion envelope and are involved in virus entry and eliciting protective immunity. The glycoproteins are modified by N-linked glycosylation and accumulate in the Golgi complex where virions mature and bud. In this chapter, we describe the methods that have been used in our laboratory for the study of the glycoproteins of Bunyamwera virus, the prototype of the family. The protocols cover the expression of viral glycoproteins, examination of intracellular localization by immnunofluorescent confocal microscopy, radiolabeling, immunoprecipitation, and SDS-PAGE analysis of the proteins, and the improved reverse genetic system to rescue recombinant viruses that contain mutations at N-linked glycosylation sites.

N-linked glycosylation of Gn (but not Gc) is important for Crimean Congo hemorrhagic fever virus glycoprotein localization and transport

The mature Gn glycoprotein of Crimean Congo hemorrhagic fever (CCHF) virus contains two predicted glycosylation sites (557N and 755N). Of these, N-glycans are added only at 557N, as evidenced by abrogation of Gn-glycosylation by mutation of 557N but not 755N site. Mutational block of Gn-glycosylation at 557N did not significantly affect Gn proteolytic processing but did result in mislocalization and retention of Gn and other proteins synthesized from the virus M segment ORF (GP160, GP85, GP38 and Gc) in the endoplasmic reticulum. In contrast to Gn, similar mutational analysis demonstrated that, while N-glycosylation occurs at the two predicted sites in Gc, abrogation of their glycosylation did not alter localization of any of the CCHF virus glycoproteins. Studies of Gn expressed in the absence of Gc demonstrate that, while Gn processing and localization are independent of Gc, all the CCHF virus glycoproteins appear dependent on N-glycosylation of Gn for correct folding, localization and transport.

Virus glycosylation: role in virulence and immune interactions

The study of N-linked glycosylation as it relates to virus biology has become an area of intense interest in recent years due to its ability to impart various advantages to virus survival and virulence. HIV and influenza, two clear threats to human health, have been shown to rely on expression of specific oligosaccharides to evade detection by the host immune system. Additionally, other viruses such as Hendra, SARS-CoV, influenza, hepatitis and West Nile rely on N-linked glycosylation for crucial functions such as entry into host cells, proteolytic processing and protein trafficking. This review focuses on recent findings on the importance of glycosylation to viral virulence and immune evasion for several prominent human pathogens.

N-linked glycosylation status of classical swine fever virus strain Brescia E2 glycoprotein influences virulence in swine

E2 is one of the three envelope glycoproteins of classical swine fever virus (CSFV). Previous studies indicate that E2 is involved in several functions, including virus attachment and entry to target cells, production of antibodies, induction of protective immune response in swine, and virulence. Here, we have investigated the role of E2 glycosylation of the highly virulent CSFV strain Brescia in infection of the natural host. Seven putative glycosylation sites in E2 were modified by site-directed mutagenesis of a CSFV Brescia infectious clone (BICv). A panel of virus mutants was obtained and used to investigate whether the removal of putative glycosylation sites in the E2 glycoprotein would affect viral virulence/pathogenesis in swine. We observed that rescue of viable virus was completely impaired by removal of all putative glycosylation sites in E2 but restored when mutation N185A reverted to wild-type asparagine produced viable virus that was attenuated in swine. Single mutations of each of the E2 glycosylation sites showed that amino acid N116 (N1v virus) was responsible for BICv attenuation. N1v efficiently protected swine from challenge with virulent BICv at 3 and 28 days postinfection, suggesting that glycosylation of E2 could be modified for development of classical swine fever live attenuated vaccines.