Role of the transmembrane domains of prM and E proteins in the formation of yellow fever virus envelope

Baculovirus surface display of a chimeric E-NS1 protein of YFV protects against YFV infection

Yellow fever (YF) is a disease caused by the homonymous flavivirus that can be prevented by a vaccine containing attenuated viruses. Since some individuals cannot receive this vaccine, the development of alternatives is desirable. Here, we developed a recombinant baculovirus (rBV) surface display platform utilizing a chimeric E-NS1 protein as a vaccine candidate. A pBacPAK9 vector containing the baculoviral GP64 signal peptide, the YFV prM, E, NS1 and the ectodomain of VSV-G sequences was synthesized. This transfer plasmid and the bAcGOZA bacmid were cotransfected into Sf9 cells, and an rBV-E-NS1 was obtained, which was characterized by PCR, WB, IFI and FACS analysis. Mice immunized with rBV-E-NS1 elicited a specific humoral and cellular immune response and were protected after YFV infection. In summary, we have developed an rBV that expresses YFV major antigen proteins on its surface, which opens new alternatives that can be tested in a mouse model.

Multienzyme-like active MnO2 nanozyme with ROS scavenging for inflammatory injury therapy induced by avian flavivirus through antiviral function

Duck Tembusu virus (DTMUV) is an acute avian flavivirus that primarily infects poultry, mosquitoes, and some mammals including humans. The viral infection triggers reactive oxygen species (ROS) and inflammatory response that are crucial in mediating injury. Crafting multifunctional nanozymes that possess both ROS scavenging and anti-inflammatory activities presents formidable challenges. The study synthesized manganese dioxide cauliflowers (MnO2 Cfs) endowed with multiple enzyme-like activities (analogous to SOD, CAT, and GPX) that effectively alleviated the injury induced by DTMUV both in vitro and in vivo. Remarkably, MnO2 Cfs efficiently neutralized various ROS, encompassing hydrogen peroxide (H2O2), superoxide anion (O2·-) and hydroxyl radical (·OH). Our in vitro assessments showed that MnO2 Cfs could alleviate cytopathic effects and modulate the innate immune response during DTMUV infection through their ROS scavenging and anti-inflammatory properties. In vivo experiments supported these findings, demonstrating that ducklings therapied by MnO2 Cfs experienced alleviated injury during DTMUV infection. Importantly, MnO2 Cfs also effectively inhibited DTMUV replication in both laboratory and field conditions. This study presents a novel strategy for nanozyme design, promising significant therapeutic potential for treating viral inflammatory diseases.

A Bioreactor-Based Yellow Fever Virus-like Particle Production Process with Integrated Process Analytical Technology Based on Transient Transfection

Yellow Fever (YF) is a severe disease that, while preventable through vaccination, lacks rapid intervention options for those already infected. There is an urgent need for passive immunization techniques using YF-virus-like particles (YF-VLPs). To address this, we successfully established a bioreactor-based production process for YF-VLPs, leveraging transient transfection and integrating Process Analytical Technology. A cornerstone of this approach was the optimization of plasmid DNA (pDNA) production to a yield of 11 mg/L using design of experiments. Glucose, NaCl, yeast extract, and a phosphate buffer showed significant influence on specific pDNA yield. The preliminary work for VLP-production in bioreactor showed adjustments to the HEK cell density, the polyplex formation duration, and medium exchanges effectively elevated transfection efficiencies. The additive Pluronic F-68 was neutral in its effects, and anti-clumping agents (ACA) adversely affected the transfection process. Finally, we established the stirred-tank bioreactor process with integrated dielectric spectroscopy, which gave real-time insight in relevant process steps, e.g., cell growth, polyplex uptake, and harvest time. We confirmed the presence and integrity of YF-VLP via Western blot, imaging flow cytometry measurement, and transmission electron microscopy. The YF-VLP production process can serve as a platform to produce VLPs as passive immunizing agents against other neglected tropical diseases.

Genomic Determinants Potentially Associated with Clinical Manifestations of Human-Pathogenic Tick-Borne Flaviviruses

The tick-borne flavivirus group contains at least five species that are pathogenic to humans, three of which induce encephalitis (tick-borne encephalitis virus, louping-ill virus, Powassan virus) and another two species induce hemorrhagic fever (Omsk hemorrhagic fever virus, Kyasanur Forest disease virus). To date, the molecular mechanisms responsible for these strikingly different clinical forms are not completely understood. Using a bioinformatic approach, we performed the analysis of each amino acid (aa) position in the alignment of 323 polyprotein sequences to calculate the fixation index (F_st) per site and find the regions (determinants) where sequences belonging to two designated groups were most different. Our algorithm revealed 36 potential determinants (F_st ranges from 0.91 to 1.0) located in all viral proteins except a capsid protein. In an envelope (E) protein, most of the determinants were located on the virion surface regions (domains II and III) and one (absolutely specific site 457) was located in the transmembrane region. Another 100% specific determinant site (E63D) with F_st = 1.0 was located in the central hydrophilic domain of the NS2b, which mediates NS3 protease activity. The NS5 protein contains the largest number of determinants (14) and two of them are absolutely specific (T226S, E290D) and are located near the RNA binding site 219 (methyltransferase domain) and the extension structure. We assume that even if not absolutely, highly specific sites, together with absolutely specific ones (F_st = 1.0) can play a supporting role in cell and tissue tropism determination.

Development of a highly specific serodiagnostic ELISA for West Nile virus infection using subviral particles

West Nile virus (WNV), a member of the Japanese encephalitis virus (JEV) serocomplex group, causes lethal encephalitis in humans and horses. Because serodiagnosis of WNV and JEV is hampered by cross-reactivity, the development of a simple, secure, and WNV-specific serodiagnostic system is required. The coexpression of prM protein and E protein leads to the secretion of subviral particles (SPs). Deletion of the C-terminal region of E protein is reported to affect the production of SPs by some flaviviruses. However, the influence of such a deletion on the properties and antigenicity of WNV E protein is unclear. We analyzed the properties of full-length E protein and E proteins lacking the C-terminal region as novel serodiagnostics for WNV infection. Deletion of the C-terminal region of E protein suppressed the formation of SPs but did not affect the production of E protein. The sensitivity of an enzyme-linked immunosorbent assay (ELISA) using the full-length E protein was higher than that using the truncated E proteins. Furthermore, in the ELISA using full-length E protein, there was little cross-reactivity with anti-JEV antibodies, and the sensitivity was similar to that of the neutralization test.

A Yellow Fever Virus 17D Infection and Disease Mouse Model Used to Evaluate a Chimeric Binjari-Yellow Fever Virus Vaccine

Despite the availability of an effective, live attenuated yellow fever virus (YFV) vaccine (YFV 17D), this flavivirus still causes up to ≈60,000 deaths annually. A number of new approaches are seeking to address vaccine supply issues and improve safety for the immunocompromised vaccine recipients. Herein we describe an adult female IFNAR-/- mouse model of YFV 17D infection and disease that recapitulates many features of infection and disease in humans. We used this model to evaluate a new YFV vaccine that is based on a recently described chimeric Binjari virus (BinJV) vaccine technology. BinJV is an insect-specific flavivirus and the chimeric YFV vaccine (BinJ/YFV-prME) was generated by replacing the prME genes of BinJV with the prME genes of YFV 17D. Such BinJV chimeras retain their ability to replicate to high titers in C6/36 mosquito cells (allowing vaccine production), but are unable to replicate in vertebrate cells. Vaccination with adjuvanted BinJ/YFV-prME induced neutralizing antibodies and protected mice against infection, weight loss and liver pathology after YFV 17D challenge.

A Molecular Determinant of West Nile Virus Secretion and Morphology as a Target for Viral Attenuation

West Nile virus (WNV) is a worldwide (re)emerging mosquito-transmitted Flavivirus causing fatal neurological diseases in humans. However, no human vaccine has been yet approved. One of the most effective live-attenuated vaccines was empirically obtained by serial passaging of wild-type yellow fever Flavivirus. However, such an approach is not acceptable nowadays, and the development of a rationally designed vaccine is necessary. Generating molecular infectious clones and mutating specific residues known to be involved in Flavivirus virulence constitute a powerful tool to promote viral attenuation. WNV membrane glycoprotein is thought to carry such essential determinants. Here, we identified two residues of this protein whose substitutions are key to the full and stable attenuation of WNV in vivo, most likely through inhibition of secretion and possible alteration of morphology. Applied to other flaviviruses, this approach should help in designing new vaccines against these viruses, which are an increasing threat to global human health.

West Nile virus (WNV), a member of the Flavivirus genus and currently one of the most common arboviruses worldwide, is associated with severe neurological disease in humans. Its high potential to reemerge and rapidly disseminate makes it a bona fide global public health problem. The surface membrane glycoprotein (M) has been associated with Flavivirus-induced pathogenesis. Here, we identified a key amino acid residue at position 36 of the M protein whose mutation impacts WNV secretion and promotes viral attenuation. We also identified a compensatory site at position M-43 whose mutation stabilizes M-36 substitution both in vitro and in vivo. Moreover, we found that introduction of the two mutations together confers a full attenuation phenotype and protection against wild-type WNV lethal challenge, eliciting potent neutralizing-antibody production in mice. Our study thus establishes the M protein as a new viral target for rational design of attenuated WNV strains.

IMPORTANCE West Nile virus (WNV) is a worldwide (re)emerging mosquito-transmitted Flavivirus causing fatal neurological diseases in humans. However, no human vaccine has been yet approved. One of the most effective live-attenuated vaccines was empirically obtained by serial passaging of wild-type yellow fever Flavivirus. However, such an approach is not acceptable nowadays, and the development of a rationally designed vaccine is necessary. Generating molecular infectious clones and mutating specific residues known to be involved in Flavivirus virulence constitute a powerful tool to promote viral attenuation. WNV membrane glycoprotein is thought to carry such essential determinants. Here, we identified two residues of this protein whose substitutions are key to the full and stable attenuation of WNV in vivo, most likely through inhibition of secretion and possible alteration of morphology. Applied to other flaviviruses, this approach should help in designing new vaccines against these viruses, which are an increasing threat to global human health.

Directional entry and release of Zika virus from polarized epithelial cells

Background

Both vector borne and sexual transmission of Zika virus (ZIKV) involve infection of epithelial cells in the initial stages of infection. Epithelial cells are unique in their ability to form polarized monolayers and their barrier function. Cell polarity induces an asymmetry in the epithelial monolayer, which is maintained by tight junctions and specialized sorting machinery. This differential localization can have a potential impact of virus infection. Asymmetrical distribution of a viral receptor can restrict virus entry to a particular membrane while polarized sorting can lead to a directional release of virions. The present study examined the impact of cell polarity on ZIKV infection and release.

Methods

A polarized Caco-2 cell model we described previously was used to assess ZIKV infection. Transepithelial resistance (TEER) was used to assess epithelial cell polarity, and virus infection was measured by immunofluorescence microscopy and qRT-PCR. Cell permeability was measured using a fluorescein leakage assay. Statistical significance was calculated using one-way ANOVA and significance was set at p < 0.05.

Results

Using the Caco-2 cell model for polarized epithelial cells, we report that Zika virus preferentially infects polarized cells from the apical route and is released vectorially through the basolateral route. Our data also indicates that release occurs without disruption of cell permeability.

Conclusions

Our results show that ZIKV has directional infection and egress in a polarized cell system. This mechanism of directional infection may be one of the mechanisms that enables the cross the epithelial barrier effectively without a disruption in cell monolayer integrity. Elucidation of entry and release characteristics of Zika virus in polarized epithelial cells can lead to better understanding of virus dissemination in the host, and can help in developing effective therapeutic interventions.

Electronic supplementary material

The online version of this article (10.1186/s12985-019-1200-2) contains supplementary material, which is available to authorized users.

A yellow fever-Zika chimeric virus vaccine candidate protects against Zika infection and congenital malformations in mice

The recent Zika virus (ZIKV) epidemic in the Americas led to an intense search for therapeutics and vaccines. Here we report the engineering of a chimeric virus vaccine candidate (YF-ZIKprM/E) by replacing the antigenic surface glycoproteins and the capsid anchor of YFV-17D with those of a prototypic Asian lineage ZIKV isolate. By intracellular passaging, a variant with adaptive mutations in the E protein was obtained. Unlike YFV-17D, YF-ZIKprM/E replicates poorly in mosquito cells. Also, YF-ZIKprM/E does not cause disease nor mortality in interferon α/β, and γ receptor KO AG129 mice nor following intracranial inoculation of BALB/c pups. A single dose as low as 1 × 10² PFU results, as early as 7 days post vaccination, in seroconversion to neutralizing antibodies and confers full protection in AG129 mice against stringent challenge with a lethal inoculum (10⁵ LD₅₀) of either homologous or heterologous ZIKV strains. Induction of multi-functional CD4⁺ and CD8⁺ T cell responses against ZIKV structural and YFV-17D non-structural proteins indicates that cellular immunity may also contribute to protection. Vaccine immunogenicity and protection was confirmed in other mouse strains, including after temporal blockade of interferon-receptors in wild-type mice to facilitate ZIKV replication. Vaccination of wild-type NMRI dams with YF-ZIKprM/E results in complete protection of foetuses against brain infections and malformations following a stringent intraplacental challenge with an epidemic ZIKV strain. The particular characteristic of YF-ZIKprM/E in terms of efficacy and its marked attenuation in mice warrants further exploration as a vaccine candidate.

Zika virus (ZIKV) infection generally results in mild symptoms but can cause serious developmental abnormalities in infants born to ZIKV infected mothers. Kai Dallmeier and colleagues at the KU Leuven in Belgium, engineered a chimeric live-attenuated vaccine (YF-ZIKprM/E) by swapping the glycoprotein from the Yellow Fever vaccine YFV-17D with that of a pre-epidemic ZIKV strain. YF-ZIKprM/E is very well tolerated with no adverse effects even following high dose intracranial infection. Mice highly susceptible to ZIKV infection—including AG129 and type I interferon receptor deficient strains—vaccinated with a single dose of YF-ZIKprM/E are fully protected from lethal ZIKV challenge. Protection can be achieved within 7 days and by low doses of YF-ZIKprM/E, is durable and generally results in sterilizing immunity. YF-ZIKprM/E elicits both neutralizing antibodies and robust cellular immunity. Finally, YF-ZIKprM/E can also prevent vertical transmission of ZIKV and achieve efficient protection of pups from neurological defects following intraplacental challenge.

Role of Capsid Anchor in the Morphogenesis of Zika Virus

The flavivirus capsid protein (C) is separated from the downstream premembrane (PrM) protein by a hydrophobic sequence named capsid anchor (Ca). During polyprotein processing, Ca is sequentially cleaved by the viral NS2B/NS3 protease on the cytosolic side and by signal peptidase on the luminal side of the endoplasmic reticulum (ER). To date, Ca is considered important mostly for directing translocation of PrM into the ER lumen. In this study, the role of Ca in the assembly and secretion of Zika virus was investigated using a pseudovirus-based approach. Our results show that, while Ca-mediated anchoring of C to the ER membrane is not needed for the production of infective particles, Ca expression in cis with respect to PrM is strictly required to allow proper assembly of infectious particles. Finally, we show that the presence of heterologous, but not homologous, Ca induces degradation of E through the autophagy/lysosomal pathway.IMPORTANCE The capsid anchor (Ca) is a single-pass transmembrane domain at the C terminus of the capsid protein (C) known to function as a signal for the translocation of PrM into the ER lumen. The objective of this study was to further examine the role of Ca in Zika virus life cycle, whether involved in the formation of nucleocapsid through association with C or in the formation of viral envelope. In this study, we show that Ca has a function beyond the one of translocation signal, controlling protein E stability and therefore its availability for assembly of infectious particles.

Development of a rapid and quantitative method for the analysis of viral entry and release using a NanoLuc luciferase complementation assay

Subviral particles (SVPs) self-assemble and are released from cells transfected with expression plasmids encoding flavivirus structural proteins. Flavivirus-like particles (VLPs), consisting of flavivirus structural proteins and a subgenomic replicon, can enter cells and cause single-round infections. Neither SVPs or VLPs possess complete viral RNA genomes, therefore are replication-incompetent systems; however, they retain the capacity to fuse and bud from target cells and follow the same maturation process as whole virions. SVPs and VLPs have been previously employed in studies analyzing entry and release steps of viral life cycles. In this study, we have developed quantitative methods for the detection of cellular entry and release of SVPs and VLPs by applying a luciferase complementation assay based on the high affinity interaction between the split NanoLuc luciferase protein, LgBiT and the small peptide, HiBiT. We introduced HiBiT into the structural protein of West Nile virus and generated SVPs and VLPs harboring HiBiT (SVP-HiBiT and VLP-HiBiT, respectively). As SVP-HiBiT emitted strong luminescence upon exposure to LgBiT and its substrate, the nascently budded SVP-HiBiT in the supernatant was readily quantified by luminometry. Similarly, the cellular entry of VLP-HiBiT generated luminescence when VLP-HiBiT was infected into LgBiT-expressing cells. These methods utilizing SVP-HiBiT and VLP-HiBiT will facilitate research into life cycles of flaviviruses, including WNV.

Membrane Anchors of the Structural Flavivirus Proteins and Their Role in Virus Assembly

The structural proteins of flaviviruses carry a unique set of transmembrane domains (TMDs) at their C termini that are derived from the mode of viral polyprotein processing. They function as internal signal and stop-transfer sequences during protein translation, but possible additional roles in protein interactions required during assembly and maturation of viral particles are ill defined. To shed light on the role of TMDs in these processes, we engineered a set of tick-borne encephalitis virus mutants in which these structural elements were replaced in different combinations by the homologous sequences of a distantly related flavivirus (Japanese encephalitis virus). The effects of these modifications were analyzed with respect to protein synthesis, viral particle secretion, specific infectivity, and acidic-pH-induced maturation processes. We provide evidence that interactions involving the double-membrane anchor of the envelope protein E (a unique feature compared to other viral fusion proteins) contribute substantially to particle assembly, stability, and maturation. Disturbances of the inter- and intra-TMD interactions of E resulted in the secretion of a larger proportion of capsidless subviral particles at the expense of whole virions, suggesting a possible role in the still incompletely understood mechanism of capsid integration during virus budding. In contrast, the TMD initially anchoring the C protein to the endoplasmic reticulum membrane does not appear to take part in envelope protein interactions. We also show that E TMDs are involved in the envelope protein rearrangements that are triggered by acidic pH in the trans-Golgi network and represent a hallmark of virus maturation.

IMPORTANCE

The assembly of flaviviruses occurs in the endoplasmic reticulum and leads to the formation of immature, noninfectious particles composed of an RNA-containing capsid surrounded by a lipid membrane, with the two integrated envelope proteins, prM and E, arranged in an icosahedral lattice. The mechanism by which the capsid is formed and integrated into the budding viral envelope is currently unknown. We provide evidence that the transmembrane domains (TMDs) of E are essential for the formation of capsid-containing particles and that disturbances of these interactions lead to the preferential formation of capsidless subviral particles at the expense of whole virions. E TMD interactions also appear to be essential for the envelope protein rearrangements required for virus maturation and for the generation of infectious virions. Our data thus provide new insights into the biological functions of E TMDs and extend their role during viral polyprotein processing to additional functions in particle assembly and maturation.

Picornavirus morphogenesis

The Picornaviridae represent a large family of small plus-strand RNA viruses that cause a bewildering array of important human and animal diseases. Morphogenesis is the least-understood step in the life cycle of these viruses, and this process is difficult to study because encapsidation is tightly coupled to genome translation and RNA replication. Although the basic steps of assembly have been known for some time, very few details are available about the mechanism and factors that regulate this process. Most of the information available has been derived from studies of enteroviruses, in particular poliovirus, where recent evidence has shown that, surprisingly, the specificity of encapsidation is governed by a viral protein-protein interaction that does not involve an RNA packaging signal. In this review, we make an attempt to summarize what is currently known about the following topics: (i) encapsidation intermediates, (ii) the specificity of encapsidation (iii), viral and cellular factors that are required for encapsidation, (iv) inhibitors of encapsidation, and (v) a model of enterovirus encapsidation. Finally, we compare some features of picornavirus morphogenesis with those of other plus-strand RNA viruses.

Characterization of the ectodomain of the envelope protein of dengue virus type 4: expression, membrane association, secretion and particle formation in the absence of precursor membrane protein

Background

The envelope (E) of dengue virus (DENV) is the major target of neutralizing antibodies and vaccine development. After biosynthesis E protein forms a heterodimer with precursor membrane (prM) protein. Recent reports of infection enhancement by anti-prM monoclonal antibodies (mAbs) suggest anti-prM responses could be potentially harmful. Previously, we studied a series of C-terminal truncation constructs expressing DENV type 4 prM/E or E proteins and found the ectodomain of E protein alone could be recognized by all 12 mAbs tested, suggesting E protein ectodomain as a potential subunit immunogen without inducing anti-prM response. The characteristics of DENV E protein ectodomain in the absence of prM protein remains largely unknown.

Methodology/Principal Findings

In this study, we investigated the expression, membrane association, glycosylation pattern, secretion and particle formation of E protein ectodomain of DENV4 in the presence or absence of prM protein. E protein ectodomain associated with membrane in or beyond trans-Golgi and contained primarily complex glycans, whereas full-length E protein associated with ER membrane and contained high mannose glycans. In the absence of prM protein, E protein ectodomain can secrete as well as form particles of approximately 49 nm in diameter, as revealed by sucrose gradient ultracentrifugation with or without detergent and electron microscopy. Mutational analysis revealed that the secretion of E protein ectodomain was affected by N-linked glycosylation and could be restored by treatment with ammonia chloride.

Conclusions/Significance

Considering the enhancement of DENV infectivity by anti-prM antibodies, our findings provide new insights into the expression and secretion of E protein ectodomain in the absence of prM protein and contribute to future subunit vaccine design.

A conserved region in the prM protein is a critical determinant in the assembly of flavivirus particles

Flaviviruses are assembled to bud into the lumen of the endoplasmic reticulum (ER) and are secreted through the vesicle transport pathway, but the details of the molecular mechanism of virion assembly remain largely unknown. In this study, a highly conserved region in the prM protein was identified among flaviviruses. In the subviral particle (SP) system of tick-borne encephalitis virus (TBEV) and Japanese encephalitis virus, secretion of SPs was impaired by a mutation in the conserved region in the prM protein. Viral proteins were sparse in the Golgi complex and accumulated in the ER. Ultrastructural analysis revealed that long filamentous structures, rather than spherical SPs, were observed in the lumen of the ER as a result of the mutation. The production of infectious virions derived from infectious cDNA of TBEV was also reduced by mutations in the conserved region. Molecular modelling analysis suggested that the conserved region is important for the association of prM-envelope protein heterodimers in the formation of a spike of immature virion. These results are the first demonstration that the conserved region in the prM protein is a molecular determinant for the flavivirus assembly process.

Pre-clinical efficacy and safety of experimental vaccines based on non-replicating vaccinia vectors against yellow fever

BACKGROUND

Currently existing yellow fever (YF) vaccines are based on the live attenuated yellow fever virus 17D strain (YFV-17D). Although, a good safety profile was historically attributed to the 17D vaccine, serious adverse events have been reported, making the development of a safer, more modern vaccine desirable.

METHODOLOGY/PRINCIPAL FINDINGS

A gene encoding the precursor of the membrane and envelope (prME) protein of the YFV-17D strain was inserted into the non-replicating modified vaccinia virus Ankara and into the D4R-defective vaccinia virus. Candidate vaccines based on the recombinant vaccinia viruses were assessed for immunogenicity and protection in a mouse model and compared to the commercial YFV-17D vaccine. The recombinant live vaccines induced γ-interferon-secreting CD4- and functionally active CD8-T cells, and conferred full protection against lethal challenge already after a single low immunization dose of 10(5) TCID(50). Surprisingly, pre-existing immunity against wild-type vaccinia virus did not negatively influence protection. Unlike the classical 17D vaccine, the vaccinia virus-based vaccines did not cause mortality following intracerebral administration in mice, demonstrating better safety profiles.

CONCLUSIONS/SIGNIFICANCE

The non-replicating recombinant YF candidate live vaccines induced a broad immune response after single dose administration, were effective even in the presence of a pre-existing immunity against vaccinia virus and demonstrated an excellent safety profile in mice.

The helical domains of the stem region of dengue virus envelope protein are involved in both virus assembly and entry

The envelope (E) of dengue virus (DENV) is a determinant of tropism and virulence. At the C terminus of E protein, there is a stem region containing two amphipathic α-helical domains (EH1 and EH2) and a stretch of conserved sequences in between. The crystal structure of E protein at the postfusion state suggested the involvement of the stem during the fusion; however, the critical domains or residues involved remain unknown. Site-directed mutagenesis was carried out to replace each of the stem residues at the hydrophobic face with an alanine or proline in a DENV serotype 4 (DENV4) precursor membrane (prM)/E expression construct. Most of the 15 proline mutations at either EH1 or EH2 severely affected the assembly of virus-like particles (VLPs). Radioimmunoprecipitation and membrane flotation assays revealed that EH1 mutations primarily affect prM-E heterodimerization and EH2 mutations affect the membrane binding of the stem. Introducing four proline mutations at either EH1 or EH2 into a DENV2 replicon packaging system greatly affects assembly and entry. Moreover, introducing these mutations into a DENV2 infectious clone confirmed the impairment in assembly and infectivity. Sequencing analysis of adaptive mutations in passage 5 viruses revealed a change to a leucine or wild-type residue at the original site, suggesting the importance of maintaining the helical structure. Collectively, these findings suggest that the EH1 and EH2 domains are involved in both assembly and entry steps of the DENV replication cycle; this feature, together with the high degree of sequence conservation, suggests that the stem region is a potential target of antiviral strategies.

The unique transmembrane hairpin of flavivirus fusion protein E is essential for membrane fusion

The fusion of enveloped viruses with cellular membranes is mediated by proteins that are anchored in the lipid bilayer of the virus and capable of triggered conformational changes necessary for driving fusion. The flavivirus envelope protein E is the only known viral fusion protein with a double membrane anchor, consisting of two antiparallel transmembrane helices (TM1 and TM2). TM1 functions as a stop-transfer sequence and TM2 as an internal signal sequence for the first nonstructural protein during polyprotein processing. The possible role of this peculiar C-terminal helical hairpin in membrane fusion has not been investigated so far. We addressed this question by studying TM mutants of tick-borne encephalitis virus (TBEV) recombinant subviral particles (RSPs), an established model system for flavivirus membrane fusion. The engineered mutations included the deletion of TM2, the replacement of both TM domains (TMDs) by those of the related Japanese encephalitis virus (JEV), and the use of chimeric TBEV-JEV membrane anchors. Using these mutant RSPs, we provide evidence that TM2 is not just a remnant of polyprotein processing but, together with TM1, plays an active role in fusion. None of the TM mutations, including the deletion of TM2, affected early steps of the fusion process, but TM interactions apparently contribute to the stability of the postfusion E trimer and the completion of the merger of the membranes. Our data provide evidence for both intratrimer and intertrimer interactions mediated by the TMDs of E and thus extend the existing models of flavivirus membrane fusion.

NS2 protein of hepatitis C virus interacts with structural and non-structural proteins towards virus assembly

Growing experimental evidence indicates that, in addition to the physical virion components, the non-structural proteins of hepatitis C virus (HCV) are intimately involved in orchestrating morphogenesis. Since it is dispensable for HCV RNA replication, the non-structural viral protein NS2 is suggested to play a central role in HCV particle assembly. However, despite genetic evidences, we have almost no understanding about NS2 protein-protein interactions and their role in the production of infectious particles. Here, we used co-immunoprecipitation and/or fluorescence resonance energy transfer with fluorescence lifetime imaging microscopy analyses to study the interactions between NS2 and the viroporin p7 and the HCV glycoprotein E2. In addition, we used alanine scanning insertion mutagenesis as well as other mutations in the context of an infectious virus to investigate the functional role of NS2 in HCV assembly. Finally, the subcellular localization of NS2 and several mutants was analyzed by confocal microscopy. Our data demonstrate molecular interactions between NS2 and p7 and E2. Furthermore, we show that, in the context of an infectious virus, NS2 accumulates over time in endoplasmic reticulum-derived dotted structures and colocalizes with both the envelope glycoproteins and components of the replication complex in close proximity to the HCV core protein and lipid droplets, a location that has been shown to be essential for virus assembly. We show that NS2 transmembrane region is crucial for both E2 interaction and subcellular localization. Moreover, specific mutations in core, envelope proteins, p7 and NS5A reported to abolish viral assembly changed the subcellular localization of NS2 protein. Together, these observations indicate that NS2 protein attracts the envelope proteins at the assembly site and it crosstalks with non-structural proteins for virus assembly.

Hepatitis C virus (HCV) causes major health problems worldwide. Understanding the major steps of the life cycle of this virus is essential to developing new and more efficient antiviral molecules. Virus assembly is the least understood step of the HCV life cycle. Growing experimental evidence indicates that, in addition to the physical virion components, the HCV non-structural proteins are intimately involved in orchestrating morphogenesis. Since it is dispensable for HCV RNA replication, the non-structural viral protein NS2 is suggested to play a central role in HCV particle assembly. Molecular interactions between NS2 and other HCV proteins were demonstrated. Furthermore, NS2 was shown to accumulate over time in endoplasmic reticulum-derived structures and to colocalize with the viral envelope glycoproteins and viral components of the replication complex in close proximity to the HCV core protein and lipid droplets. Importantly, specific mutations within NS2 that affected HCV infectivity could also alter the subcellular localization of NS2 protein and its interactions, suggesting that this subcellular localization and its interactions are essential for HCV particle assembly. Altogether, these observations indicate that NS2 protein plays an important role in connecting different viral components that are essential for virus assembly.

The C-terminal helical domain of dengue virus precursor membrane protein is involved in virus assembly and entry

The role of the α-helical domain (MH) of dengue virus (DENV) precursor membrane protein in replication was investigated by site-directed mutagenesis. Proline substitutions of three residues (120, 123 and 127) at the C-terminus, but not those at the N-terminus of MH domain, reduced the virus-like particles of DENV1, DENV2 and DENV4 detected in supernatants. In a DENV2 replicon trans-packaging system, these three mutations suppressed particles detected; two of them (I123P and V127P) also affected viral entry. In the context of DENV2 genome-length RNA, all three mutations reduced virion assembly and virus spreading in cell culture. Analysis of revertants showed that mutation A120P could partially support viral infection cycle; in contrast, mutations I123P and V127P were lethal, and adaptations of I123P→I123L and V127P→V127L were required to restore the viral infection cycle. These findings demonstrate that the C-terminus of the MH domain is involved in both assembly and entry of DENV.

Characterization of the GXXXG motif in the first transmembrane segment of Japanese encephalitis virus precursor membrane (prM) protein

The interaction between prM and E proteins in flavivirus-infected cells is a major driving force for the assembly of flavivirus particles. We used site-directed mutagenesis to study the potential role of the transmembrane domains of the prM proteins of Japanese encephalitis virus (JEV) in prM-E heterodimerization as well as subviral particle formation. Alanine insertion scanning mutagenesis within the GXXXG motif in the first transmembrane segment of JEV prM protein affected the prM-E heterodimerization; its specificity was confirmed by replacing the two glycines of the GXXXG motif with alanine, leucine and valine. The GXXXG motif was found to be conserved in the JEV serocomplex viruses but not other flavivirus groups. These mutants with alanine inserted in the two prM transmembrane segments all impaired subviral particle formation in cell cultures. The prM transmembrane domains of JEV may play importation roles in prM-E heterodimerization and viral particle assembly.

The length of and nonhydrophobic residues in the transmembrane domain of dengue virus envelope protein are critical for its retention and assembly in the endoplasmic reticulum

The morphogenesis of many enveloped viruses, in which viral nucleocapsid complex interacts with envelope (E) protein, is known to take place at various sites along the secretory pathway. How viral E protein retains in a particular intracellular organelle for assembly remains incompletely understood. In this study, we investigated determinants in the E protein of dengue virus (DENV) for its retention and assembly in the endoplasmic reticulum (ER). A chimeric experiment between CD4 and DENV precursor membrane/E constructs suggested that the transmembrane domain (TMD) of E protein contains an ER retention signal. Substitutions of three nonhydrophobic residues at the N terminus of the first helix (T1) and at either the N or C terminus of the second helix of the TMD with three hydrophobic residues, as well as an increase in the length of T1, led to the release of chimeric CD4 and E protein from the ER, suggesting that short length and certain nonhydrophobic residues of the TMD are critical for ER retention. The analysis of enveloped viruses assembled at the plasma membrane and of those assembled in the Golgi complex and ER revealed a trend of decreasing length and increasing nonhydrophobic residues of the TMD of E proteins. Taken together, these findings support a TMD-dependent sorting for viral E proteins along the secretory pathway. Moreover, similar mutations introduced into the TMD of DENV E protein resulted in the increased production of virus-like particles (VLPs), suggesting that modifications of TMD facilitate VLP production and have implications for utilizing flaviviral VLPs as serodiagnostic antigens and vaccine candidates.

Identification of a dominant endoplasmic reticulum-retention signal in yellow fever virus pre-membrane protein

Yellow fever virus (YFV) encodes two envelope proteins, pre-membrane (prM) and envelope (E), that accumulate in the endoplasmic reticulum (ER). The C termini of prM and E form two antiparallel transmembrane alpha-helices that contain ER-retention signals. To understand further the ER retention of the prME heterodimer, we characterized the subcellular localization of chimeric proteins made of a reporter protein fused to the transmembrane segments of YFV envelope proteins. We showed that at least three of the transmembrane segments of the prME heterodimer are ER-retention signals. Interestingly, increasing the length of these alpha-helices led to the export of the chimeric proteins out of the ER. Furthermore, adding a diacidic export signal at the C terminus of the first transmembrane segment of the E protein also induced export to the cell surface. However, adding this export signal at the C terminus of the first transmembrane segment of E in the context of prME did not change the subcellular localization of the prME heterodimer, suggesting the presence of a stronger ER-retention signal outside the first transmembrane segment of E. Importantly, the diacidic export motif added to the C terminus of the first transmembrane segment of the prM protein was not sufficient to export a chimeric protein out of the ER, indicating that this sequence is a dominant ER-retention signal. Together, these data indicate that a combination of several signals of different strengths contributes to the ER retention of the YFV envelope protein heterodimer.

Tyrosine 78 of premembrane protein is essential for assembly of West Nile virus

Flavivirus premembrane (prM) protein plays an important role in conformational folding of the envelope (E) protein and protects it against premature fusion in acidic vesicles of the Golgi network. Currently, molecular determinants on the prM protein ectodomain which mediate critical steps during the flavivirus assembly process are poorly characterized. In this study, bioinformatics analysis and alanine scanning mutagenesis showed that the amino acid triplet valine 76, tyrosine 78 and glycine 79 is absolutely conserved among flavivirus prM ectodomains. Triple mutations engineered at these residues in prM ectodomain of West Nile virus (WNV) completely abrogated virus infectivity. Site-directed mutagenesis of prM protein revealed that tyrosine 78 of the amino acid triplet was required for virus infectivity and secretion. The mutation did not affect folding, post-translational modifications and trafficking of the prM and E proteins. Ultrastructural studies using transmission electron microscopy confirmed that virus particle formation was blocked by tyrosine 78 mutation. Specificity of assembly defect conferred by tyrosine 78 mutation was demonstrated by positive and negative trans complementation studies. Collectively, these results defined tyrosine 78 as a novel critical determinant present on prM protein ectodomain that is required for flavivirus assembly. Molecular dissection of prM protein function provides the crucial knowledge much needed in the elucidation of flavivirus particle formation.

Yellow Fever: The Recurring Plague

Despite the availability of a safe and efficacious vaccine, yellow fever (YF) remains a disease of significant public health importance, with an estimated 200,000 cases and 30,000 deaths annually. The disease is endemic in tropical regions of Africa and South America; nearly 90% of YF cases and deaths occur in Africa. It is a significant hazard to unvaccinated travelers to these endemic areas. Virus transmission occurs between humans, mosquitoes, and monkeys. The mosquito, the true reservoir of YF, is infected throughout its life, and can transmit the virus transovarially through infected eggs. Man and monkeys, on the other hand, play the role of temporary amplifiers of the virus available for mosquito infection. Recent increases in the density and distribution of the urban mosquito vector, Aedes aegypti, as well as the rise in air travel increase the risk of introduction and spread of yellow fever to North and Central America, the Caribbean, the Middle East, Asia, Australia, and Oceania. It is an acute infectious disease characterized by sudden onset with a two-phase development, separated by a short period of remission. The clinical spectrum of yellow fever varies from very mild, nonspecific, febrile illness to a fulminating, sometimes fatal disease with pathognomic features. In severe cases, jaundice, bleeding diathesis, with hepatorenal involvement are common. The case fatality rate of severe yellow fever is 50% or higher. The pathogenesis and pathophysiology of the disease are poorly understood and have not been the subject of modern clinical research. There is no specific treatment for YF, making the management of YF patients extremely problematic. YF is a zoonotic disease that cannot be eradicated, therefore instituting preventive vaccination through routine childhood vaccination in endemic countries, can significantly reduce the burden of the disease. The distinctive properties of lifelong immunity after a single dose of yellow fever vaccination are the basis of the new applications of yellow fever 17D virus as a vector for foreign genes, "the chimeric vaccine,' and the promise of developing new vaccines against other viruses, and possibly against cancers.

A strong endoplasmic reticulum retention signal in the stem-anchor region of envelope glycoprotein of dengue virus type 2 affects the production of virus-like particles

Recombinant virus-like particles (VLPs) of flaviviruses have been shown to be produced efficiently by co-expressing the precursor membrane (PrM) and envelope (E) proteins with few exceptions, such as dengue virus type 2 (DENV2). It was reported previously that chimeric DENV2 PrM/E construct containing the stem-anchor region of E protein of Japanese encephalitis virus (JEV) produced VLPs efficiently (Chang, G. J., Hunt, A. R., Holmes, D. A., Springfield, T., Chiueh, T. S., Roehrig, J. T., and Gubler, D. J. 2003. Enhancing biosynthesis and secretion of premembrane and envelope proteins by the chimeric plasmid of dengue virus type 2 and Japanese encephalitis virus. Virology 306, 170-180.). We investigated the mechanisms involved and reported that compared with authentic DENV2 PrM/E-expressing cells, E protein in chimeric DENV2 PrM/E-expressing cells was also present in an endoglycosidase H (endo H)-resistant compartment and has shifted more to the pellets of the soluble fraction. Replacement of the transmembrane and cytoplasmic domains of CD4 with the stem-anchor of DENV2 (CD4D2) or JEV (CD4JEV) rendered the chimeric CD4 retained predominantly in the endoplasmic reticulum (ER). Flow cytometry revealed higher proportion of CD4JEV than CD4D2 expressed on the cell surface. Together, these findings suggested that the stem-anchor of DENV2 contained an ER retention signal stronger than that of JEV, which might contribute to the inefficient production of DENV2 VLPs. Moreover, co-expression of C protein can enhance the production of DENV2 VLPs, suggesting a mechanism of facilitating viral particle formation during DENV2 replication.

Construction and characterization of recombinant flaviviruses bearing insertions between E and NS1 genes

BACKGROUND

The yellow fever virus, a member of the genus Flavivirus, is an arthropod-borne pathogen causing severe disease in humans. The attenuated yellow fever 17D virus strain has been used for human vaccination for 70 years and has several characteristics that are desirable for the development of new, live attenuated vaccines. We described here a methodology to construct a viable, and immunogenic recombinant yellow fever 17D virus expressing a green fluorescent protein variant (EGFP). This approach took into account the presence of functional motifs and amino acid sequence conservation flanking the E and NS1 intergenic region to duplicate and fuse them to the exogenous gene and thereby allow the correct processing of the viral polyprotein precursor.

RESULTS

YF 17D EGFP recombinant virus was grew in Vero cells and reached a peak titer of approximately 6.45 +/- 0.4 log10 PFU/mL at 96 hours post-infection. Immunoprecipitation and confocal laser scanning microscopy demonstrated the expression of the EGFP, which was retained in the endoplasmic reticulum and not secreted from infected cells. The association with the ER compartment did not interfere with YF assembly, since the recombinant virus was fully competent to replicate and exit the cell. This virus was genetically stable up to the tenth serial passage in Vero cells. The recombinant virus was capable to elicit a neutralizing antibody response to YF and antibodies to EGFP as evidenced by an ELISA test. The applicability of this cloning strategy to clone gene foreign sequences in other flavivirus genomes was demonstrated by the construction of a chimeric recombinant YF 17D/DEN4 virus.

CONCLUSION

This system is likely to be useful for a broader live attenuated YF 17D virus-based vaccine development for human diseases. Moreover, insertion of foreign genes into the flavivirus genome may also allow in vivo studies on flavivirus cell and tissue tropism as well as cellular processes related to flavivirus infection.

A single M protein mutation affects the acid inactivation threshold and growth kinetics of a chimeric flavivirus

Numerous viruses of the Flaviviridae family, including dengue, yellow fever, Japanese encephalitis, and West Nile, cause significant disease in humans and animals. The structure and function of the molecular components of the flavivirus envelope are therefore of significant interest. To our knowledge, a membrane (M) protein mutation which affects the pH at which flavivirus particles are inactivated in vitro has never been reported. Here we show that substitution of proline for glutamine at residue M5 (MQ5P) of a Japanese encephalitis-yellow fever chimera (ChimeriVax-JE) increases its acid sensitivity in vitro by 0.3 pH units (i.e., increases the pH at which virus titer is reduced by 50% from 6.08 to 6.38). In addition, growth kinetics of this mutant virus are accelerated in Vero cells, while neurovirulence and neuroinvasiveness measured in a mouse model are unaffected. A possible interpretation of these observations is that M can modulate the envelope (E) protein function during cell infection.

Construction and mutagenesis of an artificial bicistronic tick-borne encephalitis virus genome reveals an essential function of the second transmembrane region of protein e in flavivirus assembly

Flaviviruses have a monopartite positive-stranded RNA genome, which serves as the sole mRNA for protein translation. Cap-dependent translation produces a polyprotein precursor that is co- and posttranslationally processed by proteases to yield the final protein products. In this study, using tick-borne encephalitis virus (TBEV), we constructed an artificial bicistronic flavivirus genome (TBEV-bc) in which the capsid protein and the nonstructural proteins were still encoded in the cap cistron but the coding region for the surface proteins prM and E was moved to a separate translation unit under the control of an internal ribosome entry site element inserted into the 3' noncoding region. Mutant TBEV-bc was shown to produce particles that packaged the bicistronic RNA genome and were infectious for BHK-21 cells and mice. Compared to wild-type controls, however, TBEV-bc was less efficient in both RNA replication and infectious particle formation. We took advantage of the separate expression of the E protein in this system to investigate the role in viral assembly of the second transmembrane region of protein E (E-TM2), a second copy of which was retained in the cap cistron to fulfill its other role as an internal signal sequence in the polyprotein. Deletion analysis and replacement of the entire TBEV E-TM2 region with its counterpart from another flavivirus revealed that this element, apart from its role as a signal sequence, is important for virion formation.

Structural and functional characterization of transmembrane segment VII of the Na+/H+ exchanger isoform 1

The Na(+)/H(+) exchanger isoform 1 is an integral membrane protein that regulates intracellular pH by exchanging one intracellular H(+) for one extracellular Na(+). It is composed of an N-terminal membrane domain of 12 transmembrane segments and an intracellular C-terminal regulatory domain. We characterized the structural and functional aspects of the critical transmembrane segment VII (TM VII, residues 251-273) by using alanine scanning mutagenesis and high resolution NMR. Each residue of TM VII was mutated to alanine, the full-length protein expressed, and its activity characterized. TM VII was sensitive to mutation. Mutations at 13 of 22 residues resulted in severely reduced activity, whereas other mutants exhibited varying degrees of decreases in activity. The impaired activities sometimes resulted from low expression and/or low surface targeting. Three of the alanine scanning mutant proteins displayed increased, and two displayed decreased resistance to the Na(+)/H(+) exchanger isoform 1 inhibitor EMD87580. The structure of a peptide of TM VII was determined by using high resolution NMR in dodecylphosphocholine micelles. TM VII is predominantly alpha-helical, with a break in the helix at the functionally critical residues Gly(261)-Glu(262). The relative positions and orientations of the N- and C-terminal helical segments are seen to vary about this extended segment in the ensemble of NMR structures. Our results show that TM VII is a critical transmembrane segment structured as an interrupted helix, with several residues that are essential to both protein function and sensitivity to inhibition.

Interactions between virus proteins and host cell membranes during the viral life cycle

The structure and function of cells are critically dependent on membranes, which not only separate the interior of the cell from its environment but also define the internal compartments. It is therefore not surprising that the major steps of the life cycle of viruses of animals and plants also depend on cellular membranes. Indeed, interactions of viral proteins with host cell membranes are important for viruses to enter into host cells, replicate their genome, and produce progeny particles. To replicate its genome, a virus first needs to cross the plasma membrane. Some viruses can also modify intracellular membranes of host cells to create a compartment in which genome replication will take place. Finally, some viruses acquire an envelope, which is derived either from the plasma membrane or an internal membrane of the host cell. This paper reviews recent findings on the interactions of viral proteins with host cell membranes during the viral life cycle.

Histidine at residue 99 and the transmembrane region of the precursor membrane prM protein are important for the prM-E heterodimeric complex formation of Japanese encephalitis virus

The formation of the flavivirus prM-E complex is an important step for the biogenesis of immature virions, which is followed by a subsequent cleavage of prM to M protein through cellular protease to result in the production and release of mature virions. In this study, the intracellular formation of the prM-E complex of Japanese encephalitis virus was investigated by baculovirus coexpression of prM and E in trans in Sf9 insect cells as analyzed by anti-E antibody immunoprecipitation and sucrose gradient sedimentation analysis. A series of carboxyl-terminally truncated prM mutant baculoviruses was constructed to demonstrate that the truncations of the transmembrane (TM) region resulted in a reduction of the formation of the stable prM-E complex by approximately 40% for the TM1 (at residues 130 to 147 [prM130-147]) truncation and 20% for TM2 (at prM153-167) truncation. Alanine-scanning site-directed mutagenesis on the prM99-103 region indicated that the His99 residue was the critical prM binding element for stable prM-E heterodimeric complex formation. The single amino acid mutation at the His99 residue of prM abolishing the prM-E interaction was not due to reduced expression or different subcellular location of the mutant prM protein involved in prM-E interactions as characterized by pulse-chase labeling and confocal scanning microscopic analysis. Recombinant subviral particles were detected in the Sf9 cell culture supernatants by baculovirus coexpression of prM and E proteins but not with the prM H99A mutant. Sequence alignment analysis was further conducted with different groups of flaviviruses to show that the prM H99 residues are generally conserved. Our findings are the first report to characterize the minimum binding elements of the prM protein that are involved in prM-E interactions of flaviviruses. This information, concerning a molecular framework for the prM protein, is considered to elucidate the structure/function relationship of the prM-E complex synthesis and provide the proper trajectory for flavivirus assembly and maturation.

Secretion of noninfectious dengue virus-like particles and identification of amino acids in the stem region involved in intracellular retention of envelope protein

DNA plasmids that express flavivirus premembrane/membrane (prM/M) and envelope (E) proteins in the form of virus-like particles (VLPs) have an excellent potential as DNA vaccine candidates against virus infection. The plasmid-expressed VLPs are also useful as safe, noninfectious antigens in serodiagnostic assays. We have constructed plasmids containing the prM/M and E gene regions for DENV-1, -3, and -4 that express and secrete VLPs when electroporated into Chinese hamster ovary cells. Constructs containing the full-length DENV-1 E protein gene did not secrete VLPs into tissue culture fluid effectively. However, a 16-fold increase in ELISA titers of DENV-1 VLPs was achieved after replacing the carboxy-terminal 20% region of DENV-1 E protein gene with the corresponding sequence of Japanese encephalitis virus (JEV). DENV-3 plasmids containing either the full-length DENV-3 E protein gene or the 20% JEV sequence replacement secreted VLPs to similarly high levels. Whereas DENV-4 VLPs were secreted to high levels by plasmids containing the full-length DENV-4 E protein gene but not by the chimeric plasmid containing 20% JEV E replacement. Domain substitutions by replacing prM/M protein stem-anchor region with the corresponding prM/M stem-anchor region of JEV or DENV-2 in the chimeric DENV-4 construct failed to promote the secretion of DENV-4 VLPs. Using the DENV-2 chimeric plasmid with carboxy-terminal 10% of JEV E gene, the sequence responsible for intracellular localization of E protein was mapped onto the E-H1 alpha-helix domain of DENV-2 E protein. Substitution of three amino acids from the DENV-2 sequence to the corresponding amino acids in the JEV sequence (I398L, M401A, and M412L) in the E-H1 was sufficient to promote extracellular secretion and resulted in detectable titers of DENV-2 VLP secretion.

A structural perspective of the flavivirus life cycle

Dengue, Japanese encephalitis, West Nile and yellow fever belong to the Flavivirus genus, which is a member of the Flaviviridae family. They are human pathogens that cause large epidemics and tens of thousands of deaths annually in many parts of the world. The structural organization of these viruses and their associated structural proteins has provided insight into the molecular transitions that occur during the viral life cycle, such as assembly, budding, maturation and fusion. This review focuses mainly on structural studies of dengue virus.

Histidine 39 in the dengue virus type 2 M protein has an important role in virus assembly

The mature flavivirus particle comprises a nucleocapsid core surrounded by a lipid bilayer containing the membrane (M) (derived from the precursor prM) and envelope (E) proteins. The formation of intracellular prM/E heterodimers occurs rapidly after translation and is believed to be important for the assembly and secretion of immature virus particles. In this study, the role of the His residue at position 39 in the M protein (M39) of dengue virus type 2 (DENV-2) in the virus life cycle was investigated. Mutations encoding basic (Arg), non-polar (Leu and Pro) and uncharged polar (Asn, Gln and Tyr) amino acids at M39 were introduced into a DENV-2 genomic-length cDNA clone and their effects on virus replication were examined. Substitution of the His residue with non-polar amino acids abolished virus replication, whereas substitution with basic or uncharged polar amino acids decreased virus replication moderately ( approximately 2 log(10) p.f.u. ml(-1) decrease in viral titre for Arg and Asn) or severely (>3.5 log(10) p.f.u. ml(-1) decrease in viral titre for Gln and Tyr). Selected mutations were introduced into a prM-E gene cassette and expressed transiently in COS cells to investigate whether the mutations impaired prM/E association or secretion. None of the mutations was found to disrupt the formation of intracellular prM/E heterodimers. However, the mutations that abolished virus replication prevented secretion of prM/E complexes. The results of this study pinpoint a critical residue in the M protein that potentially plays a role in viral morphogenesis, secretion and entry.

The transmembrane domains of the prM and E proteins of yellow fever virus are endoplasmic reticulum localization signals

The immature flavivirus particle contains two envelope proteins, prM and E, that are associated as a heterodimer. Virion morphogenesis of the flaviviruses occurs in association with endoplasmic reticulum (ER) membranes, suggesting that there should be accumulation of the virion components in this compartment. This also implies that ER localization signals must be present in the flavivirus envelope proteins. In this work, we looked for potential subcellular localization signals in the yellow fever virus envelope proteins. Confocal immunofluorescence analysis of the subcellular localization of the E protein in yellow fever virus-infected cells indicated that this protein accumulates in the ER. Similar results were obtained with cells expressing only prM and E. Chimeric proteins containing the ectodomain of CD4 or CD8 fused to the transmembrane domains of prM or E were constructed, and their subcellular localization was studied by confocal immunofluorescence and by analyzing the maturation of their associated glycans. Although a small fraction was detected in the ER-to-Golgi intermediate and Golgi compartments, these chimeric proteins were located mainly in the ER. The C termini of prM and E form two antiparallel transmembrane alpha-helices. Interestingly, the first transmembrane passage contains enough information for ER localization. Taken altogether, these data indicate that, besides their role as membrane anchors, the transmembrane domains of yellow fever virus envelope proteins are ER retention signals. In addition, our data show that the mechanisms of ER retention of the flavivirus and hepacivirus envelope proteins are different.

Single point mutation in tick-borne encephalitis virus prM protein induces a reduction of virus particle secretion

Flaviviruses are assembled to bud into the lumen of the endoplasmic reticulum (ER) and are secreted through the vesicle transport pathway. Virus envelope proteins play important roles in this process. In this study, the effect of mutations in the envelope proteins of tick-borne encephalitis (TBE) virus on secretion of virus-like particles (VLPs), using a recombinant plasmid expression system was analysed. It was found that a single point mutation at position 63 in prM induces a reduction in secretion of VLPs. The mutation in prM did not affect the folding of the envelope proteins, and chaperone-like activity of prM was maintained. As observed by immunofluorescence microscopy, viral envelope proteins with the mutation in prM were scarce in the Golgi complex, and accumulated in the ER. Electron microscopic analysis of cells expressing the mutated prM revealed that many tubular structures were present in the lumen. The insertion of the prM mutation at aa 63 into the viral genome reduced the production of infectious virus particles. This data suggest that prM plays a crucial role in the virus budding process.

Topology of hepatitis C virus envelope glycoproteins

Hepatitis C virus encodes two envelope glycoproteins, E1 and E2, that are released from a polyprotein precursor after cleavage by host signal peptidase(s). These proteins contain a large N-terminal ectodomain and a C-terminal transmembrane domain, and they assemble as a noncovalent heterodimer. The transmembrane domains of hepatitis C virus envelope glycoproteins have been shown to be multifunctional: (1) they are membrane anchors, (2) they bear ER retention signals, (3) they contain a signal sequence function, and (4) they are involved in E1-E2 heterodimerisation. Due to these multiple functions, the topology adopted by these transmembrane domains has given rise to much controversy. They are less than 30 amino acid residues long and are composed of two stretches of hydrophobic residues separated by a short segment containing one or two fully conserved positively charged residues. The presence of a signal sequence function in the C-terminal half of the transmembrane domains of E1 and E2 had suggested that these domains are composed of two membrane spanning segments. However, the two hydrophobic stretches are too short to make two membrane spanning alpha-helices. These discrepancies can now be explained by a dynamic model, based on experimental data, describing the early steps of the biogenesis of hepatitis C virus envelope glycoproteins. In this model, the transmembrane domains of E1 and E2 form a hairpin structure before cleavage by a signal peptidase, and a reorientation of the second hydrophobic stretch occurs after cleavage to produce a single membrane spanning domain.