Analysis of the influence of proteolytic cleavage on the structural organization of the surface of the West Nile flavivirus leads to the isolation of a protease-resistant E protein oligomer from the viral surface

Propagation, quantification, detection, and storage of West Nile virus

West Nile virus (WNV) is a member of the Flaviviridae family of enveloped, single-stranded, positive-sense RNA viruses. WNV, an emerging viral pathogen, is transmitted by mosquitoes to birds and mammals and is responsible for an increasing incidence of human disease in North America and Europe. Due to its ease of use in the laboratory and the availability of robust mouse models of disease, WNV provides an excellent experimental system for studying molecular virology and pathogenesis of infection by flaviviruses. Here, we describe common laboratory techniques used to propagate, quantify, detect, and store WNV. We also briefly describe appropriate safety precautions required for the laboratory use of WNV, which is classified as a Biosafety Level 3 pathogen by the United States Centers for Disease Control and Prevention.

Strategies for the plant-based expression of dengue subunit vaccines

Despite significant efforts in many countries, there is still no commercially viable dengue vaccine. Currently, attention is focused on the development of either live attenuated vaccines or live attenuated chimaeric vaccines using a variety of backbones. Alternate vaccine approaches, such as whole inactivated virus and subunit vaccines are in the early stages of development, and are each associated with different problems. Subunit vaccines offer the advantage of providing a uniform antigen of well-defined nature, without the added risk of introducing any genetic material into the person being inoculated. Preliminary trials of subunit vaccines (using dengue E protein) in rhesus monkeys have shown promising results. However, the primary disadvantages of dengue subunit vaccines are the low levels of expression of dengue proteins in mammalian or insect cells, as well as the added unknown risks of antigens produced from mammalian cells containing other potential sources of contamination. In the past two decades, plants have emerged as an alternative platform for expression of biopharmaceutical products, including antigens of bacterial, fungal or viral origin. In the present minireview, we highlight the current plant expression technologies used for expression of biopharmaceutical products, with an emphasis on plants as a production system for dengue subunit vaccines.

Exploring different strategies to express Dengue virus envelope protein in a plant system

Dengue virus envelope glycoprotein (E-protein) is the main protein associated with immunity induction. To produce a candidate for subunit vaccines and to provide an antigen for diagnostic kits, it was expressed in a novel plant system using deconstructed viral modules. A truncated version of the E-protein was designed to be expressed alone and co-expressed with Dengue virus structural proteins. As well, the critical domain III of E-protein was fused to hepatitis B core antigen (HBcore). The recombinant proteins were produced in Nicotiana benthamiana plants and were reactive with the anti-E antibody. The fusion was reactive with both anti-E and anti-HBcore antibodies.

Structures and mechanisms in flavivirus fusion

This chapter focuses on the work carried out with tick-borne encephalitis (TBE) virus, the structurally best characterized of the flaviviruses. The data is related to those obtained with other flaviviruses, which are assumed to have a conserved structural organization, and compare the characteristics of flavivirus fusion to those of other enveloped viruses. Fusion proteins from several different virus families, including Orthomyxoviridae , Paramyxoviridae , Retroviridae , and Filoviridae have been shown to exhibit striking structural similarities; they all use a common mechanism for inducing membrane fusion, and the same general model applies to all of these cases. The flavivirus genome is a positive-stranded RNA molecule consisting of a single, long open reading frame of more than 10,000 nucleotides flanked by noncoding regions at the 5′ and 3′ ends. The fusion properties of flaviviruses have been investigated using several different assay systems, including virus-induced cell–cell fusion and virus–liposome fusion. All of these studies indicate that flaviviruses require an acidic pH for fusion, consistent with their proposed mode of entry.

The production of recombinant dengue virus E protein using Escherichia coli and Pichia pastoris

The dengue virus envelope protein was expressed as a GST fusion protein using E. coli and P. pastoris as expression hosts. In E. coli the recombinant E protein is expressed initially as a soluble 81 kDa GST fusion protein. Treatment of the fusion protein with thrombin released a 55 kDa protein, which is the expected size for correctly processed, non-glycosylated recombinant E protein. The antiserum from animals immunised with this recombinant E protein was found to specifically recognise the dengue virus E protein in virus-infected cells, thus demonstrating the immunogenic nature of the recombinant E protein. This expression system allowed production of up to 2 mg of purified recombinant E protein from a 1 1 bacterial culture. In contrast, expression of this GST fusion protein in P. pastoris is associated with extensive proteolytic degradation of the recombinant E protein. However, this proteolytic degradation was not observed in the truncated E protein sequences which were expressed. One of these recombinant fusion proteins, GST E401 was secreted into the culture medium at levels of up to 100 microg/l of growth medium.

Characterization of Langat virus antigenic determinants defined by monoclonal antibodies to E, NS1 and preM and identification of a protective, non-neutralizing preM-specific monoclonal antibody

Hybridomas secreting monoclonal antibodies (MAb) to the tick-borne encephalitis (TBE) group virus, Langat virus (LGTV), were prepared. Of more than 200 MAb screened, 19 antibodies, which cross-reacted with the etiologic agent of Central European encephalitis, were selected for further characterization. Of these MAb, 15 were specific for LGTV E glycoprotein, two for the NS1 protein, and three for preM protein. The two NS1-specific MAb and two of the E-specific MAb reacted with all six of the other TBE group viruses tested while the remainder of the E-specific MAb failed to recognize at least one of the viruses. None of the MAb neutralized LGTV in cell culture assays, but one of the preM-specific MAb protected weanling mice against a virulent LGTV challenge. Although protective antibodies to E and NS1 proteins of TBE viruses were reported, our data provided the first evidence for protection by a non-neutralizing antibody to the preM or M protein of any of the tick-borne flaviviruses.

Analysis of flavivirus envelope proteins reveals variable domains that reflect their antigenicity and may determine their pathogenesis

Studies on the molecular basis of flavivirus neutralisation, attenuation and tropism indicate that amino acid substitutions, in different parts of the envelope gene, may be responsible for the altered phenotypes. However, the association of particular substitutions with individual characteristics has proven difficult. Comparative analysis of all known tick-borne flavivirus envelope proteins through sequence alignment and a sliding window, reveals clusters of amino acid variation distributed throughout the envelope protein coding region. Further comparison with mosquito-borne flaviviruses reveals essentially the same profile of variability throughout the envelope protein sequence although there is a major difference within the postulated B domain of these viruses which may reflect their different evolutionary development. Most phenotypically variant properties, such as serotypic differences, variants characteristic of vaccine strains, altered tropisms and neutralisation escape mutants, map within the variable clusters. Thus, we propose that natural mutagenesis and selection may occur at specific sites that do not destroy the secondary and tertiary E protein structure and that the variable clusters represent the exposed surface amino acids of the envelope protein defining antigenicity, tropicity and pathogenesis.

Oligomeric rearrangement of tick-borne encephalitis virus envelope proteins induced by an acidic pH

The flavivirus envelope protein E undergoes irreversible conformational changes at a mildly acidic pH which are believed to be necessary for membrane fusion in endosomes. In this study we used a combination of chemical cross-linking and sedimentation analysis to show that the envelope proteins of the flavivirus tick-borne encephalitis virus also change their oligomeric structure when exposed to a mildly acidic environment. Under neutral or slightly alkaline conditions, protein E on the surface of native virions exists as a homodimer which can be isolated by solubilization with the nonionic detergent Triton X-100. Solubilization with the same detergent after pretreatment at an acidic pH, however, yielded homotrimers rather than homodimers, suggesting that exposure to an acidic pH had induced a simultaneous weakening of dimeric contacts and a strengthening of trimeric ones. The pH threshold for the dimer-to-trimer transition was found to be 6.5. Because the pH dependence of this transition parallels that of previously observed changes in the conformation and hydrophobicity of protein E and that of virus-induced membrane fusion, it appears likely that the mechanism of fusion with endosomal membranes involves a specific rearrangement of the proteins in the viral envelope. Immature virions in which protein E is associated with the uncleaved precursor (prM) of the membrane protein M did not undergo a low-pH-induced rearrangement. This is consistent with a protective role of protein prM for protein E during intracellular transport of immature virions through acidic compartments of the trans-Golgi network.

The Murray Valley encephalitis virus prM protein confers acid resistance to virus particles and alters the expression of epitopes within the R2 domain of E glycoprotein

To study the role of the precursor to the membrane protein (prM) in flavivirus maturation, we inhibited the proteolytic processing of the Murray Valley encephalitis (MVE) virus prM to membrane protein in infected cells by adding the acidotropic agent ammonium chloride late in the virus replication cycle. Viruses purified from supernatants of ammonium chloride-treated cells contained prM protein and were unable to fuse C6/36 mosquito cells from without. When ammonium chloride was removed from the cells, both the processing of prM and the fusion activity of the purified viruses were partially restored. By using monoclonal antibodies (MAbs) specific for the envelope (E) glycoprotein of MVE virus, we found that at least three epitopes were less accessible to their corresponding antibodies in the prM-containing MVE virus particles. Amino-terminal sequencing of proteolytic fragments of the E protein which were reactive with sequence-specific peptide antisera or MAb enabled us to estimate the site of the E protein interacting with the prM to be within amino acids 200 to 327. Since prM-containing viruses were up to 400-fold more resistant to a low pH environment, we conclude that the E-prM interaction might be necessary to protect the E protein from irreversible conformational changes caused by maturation into the acidic vesicles of the exocytic pathway.

The vaccinia virus K2L gene encodes a serine protease inhibitor which inhibits cell-cell fusion

In certain circumstances, cells infected with vaccinia virus (VV) undergo fusion, but this does not occur in tissue cultures infected with wild-type VV. The VV genome includes three genes (B24R, B13R, and K2L) encoding polypeptides that are structurally related to members of the plasma serine proteases inhibitor (SPI) superfamily. In this study, we demonstrate by deleting these genes singly or in combination that the K2L gene encoding SPI-3, but not the B24R or B13R genes encoding SPI-1 and SPI-2, inhibits cell-cell fusion in VV-infected cells. A VV-encoded hemagglutinin (HA) has previously been demonstrated to inhibit cell-cell fusion, but fusion-promoting VVs with K2L gene deletions had normal expression and cellular location of the VV HA. As both HA and SPI-3 independently inhibit cell-cell fusion in VV-infected cells, there must be at least two fusion-promoting mechanisms encoded by VV. These may play different roles in virus-cell fusion and in cell-cell fusion after VV infection.

The flavivirus envelope protein E: isolation of a soluble form from tick-borne encephalitis virus and its crystallization

By the use of limited trypsin digestion of purified virions, we generated a membrane anchor-free and crystallizable form of the tick-borne encephalitis virus envelope glycoprotein E. It retained its reactivity with a panel of monoclonal antibodies, and only subtle structural differences from the native protein E were recognized. Treatment with the bifunctional cross-linker dimethylsuberimidate resulted in the formation of a dimer. Crystallization experiments yielded hexagonal rod-shaped crystals suitable for X-ray diffraction analysis.

The dengue viruses

Dengue, a major public health problem throughout subtropical and tropical regions, is an acute infectious disease characterized by biphasic fever, headache, pain in various parts of the body, prostration, rash, lymphadenopathy, and leukopenia. In more severe or complicated dengue, patients present with a severe febrile illness characterized by abnormalities of hemostasis and increased vascular permeability, which in some instances results in a hypovolemic shock. Four distinct serotypes of the dengue virus (dengue-1, dengue-2, dengue-3, and dengue-4) exist, with numerous virus strains found worldwide. Molecular cloning methods have led to a greater understanding of the structure of the RNA genome and definition of virus-specific structural and nonstructural proteins. Progress towards producing safe, effective dengue virus vaccines, a goal for over 45 years, has been made.

Host cell selection of Murray Valley encephalitis virus variants altered at an RGD sequence in the envelope protein and in mouse virulence

We have passaged the prototype strain of Murray Valley encephalitis virus in SW13 (human) cells, sequenced the E and M genes, and examined the virulence of the passaged virus for 21-day-old mice following intracranial and intraperitoneal inoculation. Six independent passage series were carried out: four in the presence of mouse hyperimmune ascitic fluid and two without antibody. Changes were observed in the E protein deduced amino acid sequence for each of the six 10th passage stocks sequenced. Eleven changes were observed in total for the six stocks sequenced; these were at residues 117, 118, 390, 423, and 460. Nine of the changes were nonconservative. Five of the six passaged variants were altered at Asp 390 which is part of an Arg-Gly-Asp (RGD) sequence. This change resulted from adaptation to SW13 cells rather than from antibody selection. The RGD sequence (and residue 423) falls within a region which is highly conserved between flaviviruses and is strongly hydrophilic. All five variants which were altered at Asp 390 were attenuated in 21-day-old mice following i.p. inoculation. We propose that the domain of E encompassing the RGD sequence is an important determinant of flavivirus pathogenicity.

Serologically defined linear epitopes in the envelope protein of dengue 2 (Jamaica strain 1409)

Antisera from dengue patients and dengue virus infected rabbits recognized octapeptides corresponding to linear amino acid sequences in the envelope protein of dengue 2 (Jamaica 1409). Although no peptide was recognized by sera from all dengue infected hosts, two peptides (216LPLPWLPG223 and 448FSGVSWTM455) were recognized by sera from all dengue 2 infected rabbits. One of these 448FSGVSWTM455 was also recognized by sera from both the dengue 2 patients tested. No peptides were identified which reacted exclusively with all dengue 2 infected animals. Use of a mouse monoclonal antibody (1B7) enabled identification of two regions (50AKQPATLR57 and 127GKVVLPEN134) and possibly a third (349GRLITVNP356) in the envelope protein of dengue 2 likely to be involved in haemagglutination inhibition and virus neutralization in vitro.

Cell-associated West Nile flavivirus is covered with E+pre-M protein heterodimers which are destroyed and reorganized by proteolytic cleavage during virus release

Flaviviruses are enveloped viruses which accumulate in cellular vacuoles prior to release. The membrane of cell-associated virus contains the proteins pre-M and E. During release of virus the pre-M protein is cleaved, and only its carboxy-terminal segment remains associated with the virus as M protein. Studies of the association of membrane proteins of intracellular and extracellular particles of West Nile virus show that in cell-associated virus the pre-M and E proteins are present as E+pre-M heterodimers. Cleavage of pre-M during release leads to dissociation of the heterodimers: the amino-terminal region of the pre-M protein is lost from the virus, whereas the proteins M and E remain associated with the viral membrane as separate molecules. The E protein of extracellular virus has a tendency to oligomerize into trimers, and both E-protein monomers and trimers are present on extracellular virions. We have prepared partially purified extracellular virus without loss of viral infectivity. These preparations contain approximately 600 physical particles for each PFU. Since purification of cell-associated virus results in significant loss of PFU, an inactivation of virus may occur during this procedure. Preparations of cell-associated virus contained approximately 40,000 physical particles for each PFU.

Analyses of the terminal sequences of West Nile virus structural proteins and of the in vitro translation of these proteins allow the proposal of a complete scheme of the proteolytic cleavages involved in their synthesis

The proteolytic processes involved in the synthesis of the structural proteins of the West Nile (WN) flavivirus were analyzed: The carboxy-terminal sequences of the structural proteins were determined and the proteins translated in vitro in the presence of membranes from a mRNA coding for the structural polyprotein were analyzed. The results obtained indicate that the following proteolytic activities are involved in the synthesis and assembly of WN virus structural proteins: The growing peptide chain which contains the sequences of the structural proteins in the order C-pre-M-E is cleaved at three places by cellular signalase(s). This cleavage generates the primary amino acid sequence of the mature structural proteins pre-M and E (and the amino-terminus of the ensuing nonstructural protein NS 1). The amino-terminal part of the polyprotein containing the amino acid residues 1 to 123 is released as a molecule which migrates slightly slower than the mature viral core protein and which presumably is associated to the RER membranes via its carboxy-terminal sequence. This protein is called the anchored C virus particles the anchored C protein is converted into mature C protein by removal of the carboxy-terminal hydrophobic segment containing the amino acid residues 106 to 123. Presumably a virus-coded protease which can cleave the polyprotein after two basic amino acid residues is responsible for this cleavage. The cell-associated WN virus particles are constructed from the proteins C, pre-M, and E which contain the amino residues 1-105, 124-290, and 291-787 of the polyprotein, respectively. Cleavage of the pre-M protein between amino acid residues 215 and 216, presumably by a cellular enzyme located in the Golgi vesicles, and loss of the amino-terminal fragment of this protein are associated with the release of virus from the cells.

A comparative study of entry modes into C6/36 cells by Semliki Forest and Japanese encephalitis viruses

The entry modes of Semliki Forest virus and Japanese encephalitis virus into C6/36 cells were compared by electron microscopic observation. At physiological pH, the two viruses showed characteristically different entry modes. Following attachment to the plasma membrane, many SF virions appeared within plasma membrane invaginations and cytoplasmic vesicles; on the other hand, JE virions remained to be found exclusively at the cell surface, with no virions appearing within cytoplasmic vesicles. Electron microscopic observation, therefore, indicated that SF virus entered C6/36 cells by receptor-mediated endocytosis, while JE virus penetrated the cells at the surface and disintegrated at or near the adsorption sites. At pH 5.8, SF virus also entered C6/36 cells by direct penetration at the cell surface. On the basis of the present and other findings, the following working hypotheses are presented for future investigations: (a) at physiological pH, the fusion protein of SF virus is in an inactive state and needs to be activated by acidic pH within the endosome in order to act on the host-cell membrane, but that of JE virus is in an active state and is capable of dissolving the host plasma membrane at the cell surface immediately after the attachment; (b) the states of viral fusion proteins (inactive or active) at the time of viral attachment to the cell surface determine which of the two entry modes these viruses follow.

Antigenic structure of the flavivirus envelope protein E at the molecular level, using tick-borne encephalitis virus as a model

A model of the tick-borne encephalitis virus envelope protein E is presented that contains information on the structural organization of this flavivirus protein and correlates epitopes and antigenic domains to defined sequence elements. It thus reveals details of the structural and functional characteristics of the corresponding protein domains. The localization of three antigenic domains (composed of 16 distinct epitopes) within the primary structure was performed by (i) amino-terminal sequencing of three immunoreactive fragments of protein E and (ii) sequencing the protein E-coding regions of seven antigenic variants of tick-borne encephalitis virus that had been selected in the presence of neutralizing monoclonal antibodies directed against the E protein. Further information about variable and conserved regions was obtained by a comparative computer analysis of flavivirus E protein amino acid sequences. The search for potential T-cell determinants revealed at least one sequence compatible with an amphipathic alpha-helix which is conserved in all flaviviruses sequenced so far. By combining these data with those on the location of disulfide bridges (T. Nowak and G. Wengler, Virology 156:127-137, 1987) and the structural characteristics of epitopes, such as dependency on conformation or on intact disulfide bridges or both, a model was established that goes beyond the location of epitopes in the primary sequence and reveals features of the folding of the polypeptide chain, including the generation of discontinuous protein domains.

Virus entry into animal cells

In addition to its many other functions, the plasma membrane of eukaryotic cells serves as a barrier against invading parasites and viruses. It is not permeable to ions and to low molecular weight solutes, let alone to proteins and polynucleotides. Yet it is clear that viruses are capable of transferring their genome and accessory proteins into the cytosol or into the nucleus, and thus infect the cell. While the detailed mechanisms remain unclear for most animal viruses, a general theme is apparent like other stages in the replication cycle; their entry depends on the activities of the host cell. In order to take up nutrients, to communicate with other cells, to control the intracellular ion balance, and to secrete substances, cells have a variety of mechanisms for bypassing and modifying the barrier properties imposed by their plasma membrane. It is these mechanisms, and the molecules involved in them, that viruses exploit.