The pestivirus glycoprotein Erns is anchored in plane in the membrane via an amphipathic helix

Host cell factors involved in classical swine fever virus entry

Classical swine fever virus (CSFV) is an ancient pathogen that continues to pose a threat to animal agriculture worldwide. The virus belongs to the genus Pestivirus and the family Flaviviridae. It causes a multisystemic disease that affects only pigs and is responsible for significant economic losses. CSFV infection is probably a multistep process that involves the proteins in the virus envelope and more than one receptor in the membrane of permissive cells. To date, the cellular receptors essential for CSFV entry and their detailed functions during this process remains unknown. All the viral envelope proteins Erns, E1 and E2 are involved in the entry process to some extent and the experimental approaches conducted until now have helped to unveil their contributions. This review aims to provide an overview of current knowledge on cellular molecules described to be involved in CSFV entry, including complement regulatory protein 46 (CD46), heparan sulphate (HS), Laminin receptor, Integrin ß3, Annexin II, MERKT and ADAM17. This knowledge would not only help to understand the molecular mechanisms involved in pestivirus infection, but also provide a rational basis for the development of nonvaccinal alternatives for CSFV control.

Attachment, Entry, and Intracellular Trafficking of Classical Swine Fever Virus

Classical swine fever virus (CSFV), which is a positive-sense, single-stranded RNA virus with an envelope, is a member of the Pestivirus genus in the Flaviviridae family. CSFV causes a severe and highly contagious disease in pigs and is prevalent worldwide, threatening the pig farming industry. The detailed mechanisms of the CSFV life cycle have been reported, but are still limited. Some receptors and attachment factors of CSFV, including heparan sulfate (HS), laminin receptor (LamR), complement regulatory protein (CD46), MER tyrosine kinase (MERTK), disintegrin, and metalloproteinase domain-containing protein 17 (ADAM17), were identified. After attachment, CSFV internalizes via clathrin-mediated endocytosis (CME) and/or caveolae/raft-dependent endocytosis (CavME). After internalization, CSFV moves to early and late endosomes before uncoating. During this period, intracellular trafficking of CSFV relies on components of the endosomal sorting complex required for transport (ESCRT) and Rab proteins in the endosome dynamics, with a dependence on the cytoskeleton network. This review summarizes the data on the mechanisms of CSFV attachment, internalization pathways, and intracellular trafficking, and provides a general view of the early events in the CSFV life cycle.

Host Cell Receptors Implicated in the Cellular Tropism of BVDV

Bovine viral diarrhea virus (BVDV) is one of the most hazardous viruses, which causes huge economic losses in the cattle industry around the world. In recent years, there has been a continuous increase in the diversity of pestivirus worldwide. As a member of the genus Pestivirus in the Flaviviridae family, BVDV has a wide range of host animals including cattle, goat, sheep, pig, camel and other cloven-hoofed animals, and it has multi-tissue tropism as well. The recognition of their permissive cells by viruses via interaction with the cellular receptors is a prerequisite for successful infection. So far, little is known about the cellular receptors essential for BVDV entry and their detailed functions during BVDV infection. Thus, discovery of the cellular receptors involved in the entry of BVDV and other pestiviruses is significant for development of the novel intervention. The viral envelope glycoprotein E^rns and E2 are crucial determinants of the cellular tropism of BVDV. The cellular proteins bound with E^rns and E2 potentially participate in BVDV entry, and their abundance might determine the cellular tropism of BVDV. Here, we summarize current knowledge regarding the cellular molecules have been described for BVDV entry, such as, complement regulatory protein 46 (CD46), heparan sulfate (HS), the low-density lipoprotein (LDL) receptor, and a disintegrin and metalloproteinase 17 (ADAM17). Furthermore, we focus on their implications of the recently identified cellular receptors for pestiviruses in BVDV life cycle. This knowledge provides a theoretical basis for BVDV prevention and treatment by targeting the cellular receptors essential for BVDV infection.

Proteome Analysis of Swine Macrophages after Infection with Two Genotype II African Swine Fever Isolates of Different Pathogenicity

Since the introduction of a highly pathogenic genotype II isolate of the African swine fever virus (ASFV) into Georgia in 2007, African swine fever (ASF) has gone panzootic. Outbreaks have been reported in Europe, Asia and, more recently, Latin America. Thus, ASFV has become a major threat to the pig industry worldwide, as broadly applicable vaccines are not available. While the majority of ASFV strains show high virulence in domestic pigs and wild boar, variations within the ASFV genome have resulted in the emergence of attenuated strains with low or moderate virulence. However, the molecular basis of the differences in virulence has not yet been discovered. To reveal virulence-associated protein expression patterns, we analysed the proteomes of the natural target cells of ASFV, primary porcine macrophages, after infection with two genotype II ASFV strains displaying high (Armenia 2008) and moderate (Estonia 2014) virulence using quantitative mass spectrometry. Very similar expression patterns were observed for the viral genes, and any differences were limited to the deletions within the Estonia 2014 genome. In addition to the canonical ASFV proteins, twelve novel protein products from recently described transcripts were confirmed in both isolates. Pathway analyses showed that both isolates evoked a similar host proteome response, despite their difference in virulence. However, subtle differences in the manipulation of the proteins involved in the proinflammatory response mediated by the MAPK14/p38 signalling cascade were observed.

Fifty Shades of Erns: Innate Immune Evasion by the Viral Endonucleases of All Pestivirus Species

The genus Pestivirus, family Flaviviridae, includes four historically accepted species, i.e., bovine viral diarrhea virus (BVDV)-1 and -2, classical swine fever virus (CSFV), and border disease virus (BDV). A large number of new pestivirus species were identified in recent years. A common feature of most members is the presence of two unique proteins, N^pro and E^rns, that pestiviruses evolved to regulate the host’s innate immune response. In addition to its function as a structural envelope glycoprotein, E^rns is also released in the extracellular space, where it is endocytosed by neighboring cells. As an endoribonuclease, E^rns is able to cleave viral ss- and dsRNAs, thus preventing the stimulation of the host’s interferon (IFN) response. Here, we characterize the basic features of soluble E^rns of a large variety of classified and unassigned pestiviruses that have not yet been described. Its ability to form homodimers, its RNase activity, and the ability to inhibit dsRNA-induced IFN synthesis were investigated. Overall, we found large differences between the various E^rns proteins that cannot be predicted solely based on their primary amino acid sequences, and that might be the consequence of different virus-host co-evolution histories. This provides valuable information to delineate the structure-function relationship of pestiviral endoribonucleases.

A double deletion prevents replication of the pestivirus bovine viral diarrhea virus in the placenta of pregnant heifers

In contrast to wild type bovine viral diarhea virus (BVDV) specific double deletion mutants are not able to establish persistent infection upon infection of a pregnant heifer. Our data shows that this finding results from a defect in transfer of the virus from the mother animal to the fetus. Pregnant heifers were inoculated with such a double deletion mutant or the parental wild type virus and slaughtered pairwise on days 6, 9, 10 and 13 post infection. Viral RNA was detected via qRT-PCR and RNAscope analyses in maternal tissues for both viruses from day 6 p.i. on. However, the double deletion mutant was not detected in placenta and was only found in samples from animals infected with the wild type virus. Similarly, high levels of wild type viral RNA were present in fetal tissues whereas the genome of the double deletion mutant was not detected supporting the hypothesis of a specific inhibition of mutant virus replication in the placenta. We compared the induction of gene expression upon infection of placenta derived cell lines with wild type and mutant virus via gene array analysis. Genes important for the innate immune response were strongly upregulated by the mutant virus compared to the wild type in caruncle epithelial cells that establish the cell layer on the maternal side at the maternal–fetal interface in the placenta. Also, trophoblasts which can be found on the fetal side of the interface showed significant induction of gene expression upon infection with the mutant virus although with lower complexity. Growth curves recorded in both cell lines revealed a general reduction of virus replication in caruncular epithelial cells compared to the trophoblasts. Compared to the wild type virus this effect was dramtic for the mutant virus that reached only a TCID₅₀ of 1.0 at 72 hours post infection.

Here we report on animal studies elucidating mechanisms preventing the transfer of a double deletion mutant of a pestivirus to the fetus in pregnant heifers. This mutant lacks both known factors engaged in blocking the innate immune response to pestiviral infection. As shown also in earlier studies, this mutant was not detected in the fetuses at any of the tested time points in contrast to the wild-type (wt) virus. However, similar to the wt the mutant was detected in a large variety of different maternal tissues. The only exception was the placenta where only wt but not mutant virus was detected. Using gene array analyses we showed that infection of two cell lines derived either from the maternal or the fetal site of the maternal-fetal interface with the mutant virus induces a significant antiviral gene expression response. The reaction of cells from the maternal side was more complex and virus replication in these cells was reduced, almost completly blocking the mutant virus. These results support the hypothesis that replication of the mutant virus is blocked in the placenta due to a highly active innate immune response and the prevention of replication also blocks transfer of the virus to the fetus.

The Molecular Basis for Erns Dimerization in Classical Swine Fever Virus

The pestivirus classical swine fever virus (CSFV) represents one of the most important pathogens of swine. Its virulence is dependent on the RNase activity of the essential structural glycoprotein E^rns that uses an amphipathic helix as a membrane anchor and forms homodimers via disulfide bonds employing cysteine 171. Dimerization is not necessary for CSFV viability but for its virulence. Mutant E^rns proteins lacking cysteine 171 are still able to interact transiently as shown in crosslink experiments. Deletion analysis did not reveal the presence of a primary sequence-defined contact surface essential for dimerization, but indicated a general importance of an intact ectodomain for efficient establishment of dimers. Pseudoreverted viruses reisolated in earlier experiments from pigs with mutations Cys171Ser/Ser209Cys exhibited partially restored virulence and restoration of the ability to form E^rns homodimers. Dimer formation was also observed for experimentally mutated proteins, in which other amino acids at different positions of the membrane anchor region of E^rns were replaced by cysteine. However, with one exception of two very closely located residues, the formation of disulfide-linked dimers was only observed for cysteine residues located at the same position of the helix.

Characterization of Membrane Topology and Retention Signal of Pestiviral Glycoprotein E1

Pestiviruses are members of the family Flaviviridae, a group of enveloped viruses that bud at intracellular membranes. Pestivirus particles contain three glycosylated envelope proteins, E^rns, E1, and E2. Among them, E1 is the least characterized concerning both biochemical features and function. E1 from bovine viral diarrhea virus (BVDV) strain CP7 was analyzed with regard to its intracellular localization and membrane topology. Here, it is shown that even in the absence of other viral proteins, E1 is not secreted or expressed at the cell surface but localizes predominantly in the endoplasmic reticulum (ER). Using engineered chimeric transmembrane domains with sequences from E1 and vesicular stomatitis virus G protein, the E1 ER-retention signal could be narrowed down to six fully conserved polar residues in the middle part of the transmembrane domain of E1. Retention was observed even when several of these polar residues were exchanged for alanine. Mutations with a strong impact on E1 retention prevented recovery of infectious viruses when tested in the viral context. Analysis of the membrane topology of E1 before and after the signal peptide cleavage via a selective permeabilization and an in vivo labeling approach revealed that mature E1 is a typical type I transmembrane protein with a single span transmembrane anchor at its C terminus, whereas it adopts a hairpin-like structure with the C terminus located in the ER lumen when the precleavage situation is mimicked by blocking the cleavage site between E1 and E2.

IMPORTANCE The shortage of specific antibodies against E1, making detection and further analysis of E1 difficult, resulted in a lack of knowledge on E1 compared to E^rns and E2 with regard to biosynthesis, structure, and function. It is known that pestiviruses bud intracellularly. Here, we show that E1 contains its own ER retention signal: six fully conserved polar residues in the middle part of the transmembrane domain are shown to be the determinants for ER retention of E1. Moreover, those six polar residues could serve as a functional group that intensely affect the generation of infectious viral particles. In addition, the membrane topology of E1 has been determined. In this context, we also identified dynamic changes in membrane topology of E1 with the carboxy terminus located on the luminal side of the ER in the precleavage state and relocation of this sequence upon signal peptidase cleavage. Our work provides the first systematic analysis of the pestiviral E1 protein with regard to its biochemical and functional characteristics.

The Erns Carboxyterminus: Much More Than a Membrane Anchor

Pestiviruses express the unique essential envelope protein E^rns, which exhibits RNase activity, is attached to membranes by a long amphipathic helix, and is partially secreted from infected cells. The RNase activity of E^rns is directly connected with pestivirus virulence. Formation of homodimers and secretion of the protein are hypothesized to be important for its role as a virulence factor, which impairs the host's innate immune response to pestivirus infection. The unusual membrane anchor of E^rns raises questions with regard to proteolytic processing of the viral polyprotein at the E^rns carboxy-terminus. Moreover, the membrane anchor is crucial for establishing the critical equilibrium between retention and secretion and ensures intracellular accumulation of the protein at the site of virus budding so that it is available to serve both as structural component of the virion and factor controlling host immune reactions. In the present manuscript, we summarize published as well as new data on the molecular features of E^rns including aspects of its interplay with the other two envelope proteins with a special focus on the biochemistry of the E^rns membrane anchor.

Charged Residues in the Membrane Anchor of the Pestiviral Erns Protein Are Important for Processing and Secretion of Erns and Recovery of Infectious Viruses

The pestivirus envelope protein E^rns is anchored in membranes via a long amphipathic helix. Despite the unusual membrane topology of the E^rns membrane anchor, it is cleaved from the following glycoprotein E1 by cellular signal peptidase. This was proposed to be enabled by a salt bridge-stabilized hairpin structure (so-called charge zipper) formed by conserved charged residues in the membrane anchor. We show here that the exchange of one or several of these charged residues reduces processing at the E^rns carboxy-terminus to a variable extend, but reciprocal mutations restoring the possibility to form salt bridges did not necessarily restore processing efficiency. When introduced into an E^rns-only expression construct, these mutations enhanced the naturally occurring E^rns secretion significantly, but again to varying extents that did not correlate with the number of possible salt bridges. Equivalent effects on both processing and secretion were also observed when the proteins were expressed in avian cells, which points at phylogenetic conservation of the underlying principles. In the viral genome, some of the mutations prevented recovery of infectious viruses or immediately (pseudo)reverted, while others were stable and neutral with regard to virus growth.

Generation and immunogenicity analysis of recombinant classical swine fever virus glycoprotein E2 and Erns expressed in baculovirus expression system

Classical swine fever (CSF) caused by the classical swine fever virus (CSFV) is a highly contagious swine disease resulting in large economical losses worldwide. The viral envelope glycoprotein E2 and E^rns are major targets for eliciting antibodies against CSFV in infected animals. In this report, the glycoprotein E2 and E^rns were expressed using the baculovirus system and their protective immunity in rabbits were tested. Twenty CSFV seronegative rabbits were randomly divided into five groups. Each rabbit was intramuscularly immunized with CSFV-E2, CSFV-E^rns, or their combination (CSFV-E2 + E^rns). Besides, a commercial CSFV vaccine (C-strain) and PBS were used as positive or negative controls, respectively. Four weeks after the second immunization, all the rabbits were challenged with 100 RID₅₀ of CSFV C-strain. High levels of CSFV E2-specific antibody, neutralizing antibody and cellular immune responses to CSFV were elicited in the rabbits inoculated with C-strain, CSFV-E2, and CSFV-E2 + E^rns. And the rabbits inoculated with the three vaccines received complete protection against CSFV C-strain. However, no neutralizing antibody was detected in the E^rns vaccinated rabbits and the rabbits exhibited fever typical of CSFV, suggesting the E^rns alone is not able to induce a protective immune response. Taken together, while the E^rns could not confer protection against CSFV, E2 and E2 + E^rns could not only elicit humoral and cell-mediated immune responses but also confer complete protection against CSFV C-strain in rabbits.

Downstream Sequences Control the Processing of the Pestivirus Erns-E1 Precursor

Like other enveloped viruses, pestiviruses employ cellular proteases for processing of their structural proteins. While typical signal peptidase cleavage motifs are present at the carboxy terminus of the signal sequence preceding E^rns and the E1/E2 and E2/P7 sites, the E^rns-E1 precursor is cleaved by signal peptidase at a highly unusual structure, in which the transmembrane sequence upstream of the cleavage site is replaced by an amphipathic helix. As shown before, the integrity of the amphipathic helix is crucial for efficient processing. The data presented here demonstrate that the E1 sequence downstream of this cleavage site is also important for the cleavage. Carboxy-terminal truncation of the E1 moiety as well as internal deletions in E1 reduced the cleavage efficiency to less than 30% of the wild-type (wt) level. Moreover, the C-terminal truncation by more than 30 amino acids resulted in strong secretion of the uncleaved fusion proteins. The reduced processing and increased secretion were even observed when 10 to 5 amino-terminal residues of E1 were left, whereas extensions by 1 or 3 E1 residues resulted in reduced processing but no significantly increased secretion. In contrast to the E1 sequences, a 10-amino-acid c-myc tag fused to the E^rns C terminus had only marginal effect on secretion but was also not processed efficiently. Mutation of the von Heijne sequence upstream of E2 not only blocked the cleavage between E1 and E2 but also prevented the processing between E^rns and E2. Thus, processing at the E^rns-E1 site is a highly regulated process.IMPORTANCE Cellular signal peptidase (SPase) cleavage represents an important step in maturation of viral envelope proteins. Fine tuning of this system allows for establishment of concerted folding and processing processes in different enveloped viruses. We report here on SPase processing of the E^rns-E1-E2 glycoprotein precursor of pestiviruses. E^rns-E1 cleavage is delayed and only executed efficiently when the complete E1 sequence is present. C-terminal truncation of the E^rns-E1 precursor impairs processing and leads to significant secretion of the protein. The latter is not detected when internal deletions preserving the E1 carboxy terminus are introduced, but also these constructs show impaired processing. Moreover, E^rns-E1 is only processed after cleavage at the E1/E2 site. Thus, processing of the pestiviral glycoprotein precursor by SPase is done in an ordered way and depends on the integrity of the proteins for efficient cleavage. The functional importance of this processing scheme is discussed in the paper.

Autonomously Replicating RNAs of Bungowannah Pestivirus: ERNS Is Not Essential for the Generation of Infectious Particles

Autonomously replicating subgenomic Bungowannah virus (BuPV) RNAs (BuPV replicons) with deletions of the genome regions encoding the structural proteins C, E^RNS, E1, and E2 were constructed on the basis of an infectious cDNA clone of BuPV. Nanoluciferase (Nluc) insertion was used to compare the replication efficiencies of all constructs after electroporation of in vitro-transcribed RNA from the different clones. Deletion of C, E1, E2, or the complete structural protein genome region (C-E^RNS-E1-E2) prevented the production of infectious progeny virus, whereas deletion of E^RNS still allowed the generation of infectious particles. However, those ΔE^RNS viral particles were defective in virus assembly and/or egress and could not be further propagated for more than three additional passages in porcine SK-6 cells. These "defective-in-third-cycle" BuPV ΔE^RNS mutants were subsequently used to express the classical swine fever virus envelope protein E2, the N-terminal domain of the Schmallenberg virus Gc protein, and the receptor binding domain of the Middle East respiratory syndrome coronavirus spike protein. The constructs could be efficiently complemented and further passaged in SK-6 cells constitutively expressing the BuPV E^RNS protein. Importantly, BuPVs are able to infect a wide variety of target cell lines, allowing expression in a very wide host spectrum. Therefore, we suggest that packaged BuPV ΔE^RNS replicon particles have potential as broad-spectrum viral vectors.IMPORTANCE The proteins N^PRO and E^RNS are unique for the genus Pestivirus, but only N^PRO has been demonstrated to be nonessential for in vitro growth. While this was also speculated for E^RNS, it has always been previously shown that pestivirus replicons with deletions of the structural proteins E^RNS, E1, or E2 did not produce any infectious progeny virus in susceptible host cells. Here, we demonstrated for the first time that BuPV E^RNS is dispensable for the generation of infectious virus particles but still important for efficient passaging. The E^RNS-defective BuPV particles showed clearly limited growth in cell culture but were capable of several rounds of infection, expression of foreign genes, and highly efficient trans-complementation to rescue virus replicon particles (VRPs). The noncytopathic characteristics and the absence of preexisting immunity to BuPV in human populations and livestock also provide a significant benefit for a possible use, e.g., as a vector vaccine platform.

Changes Introduced in the Open Reading Frame of Bovine Viral Diarrhea Virus During Serial Infection of Pregnant Swine

Bovine viral diarrhea virus (BVDV) is one of the most economically important viruses of cattle, but this pathogen is also able to infect pigs, camelids, and a wide range of domestic and wild ruminants. BVDV isolates circulating in animal populations are genetically and antigenically highly diverse. Acute BVDV infections in cattle cause the introduction of many substitutions in the viral genome. Serial infection of pregnant sheep with a BVDV-1b isolate of bovine origin was also associated with great numbers of substitutions. To our knowledge, genomic changes arising during BVDV infections in swine have not been investigated. The purpose of this study was to investigate the changes occurring in the open reading frame (ORF) of BVDV during serial infection of pregnant swine with a BVDV isolate of bovine origin. The BVDV-1b isolate AU526 was serially passaged in six pregnant gilts, two of which gave birth to live piglets congenitally infected with BVDV. The complete ORF sequences of 14 BVDV isolates obtained from pregnant gilts and their piglets were determined. Their analysis revealed that serial transmission of AU526 in pregnant swine resulted in many genomic changes. All isolates of porcine origin shared 32 nucleotide and 12 amino acid differences with the virus inoculum AU526. These changes were detected after a single passage in pregnant swine and were conserved during the subsequent five passages. Amino acid changes occurred primarily in genomic regions encoding the BVDV structural proteins E2 and E ^rns . These results suggest that BVDV infections in pregnant swine may contribute significantly to the genetic variability of BVDV and lead to the appearance of adaptive changes.

HCV Pit Stop at the Lipid Droplet: Refuel Lipids and Put on a Lipoprotein Coat before Exit

The replication cycle of the liver-tropic hepatitis C virus (HCV) is tightly connected to the host lipid metabolism, during the virus entry, replication, assembly and egress stages, but also while the virus circulates in the bloodstream. This interplay coins viral particle properties, governs viral cell tropism, and facilitates immune evasion. This review summarizes our knowledge of these interactions focusing on the late steps of the virus replication cycle. It builds on our understanding of the cell biology of lipid droplets and the biosynthesis of liver lipoproteins and attempts to explain how HCV hijacks these organelles and pathways to assemble its lipo-viro-particles. In particular, this review describes (i) the mechanisms of viral protein translocation to and from the lipid droplet surface and the orchestration of an interface between replication and assembly complexes, (ii) the importance of the triglyceride mobilization from the lipid droplets for HCV assembly, (iii) the interplay between HCV and the lipoprotein synthesis pathway including the role played by apolipoproteins in virion assembly, and finally (iv) the consequences of these complex virus–host interactions on the virion composition and its biophysical properties. The wealth of data accumulated in the past years on the role of the lipid metabolism in HCV assembly and its imprint on the virion properties will guide vaccine design efforts and reinforce our understanding of the hepatic lipid metabolism in health and disease.

Glycoprotein 3 of Porcine Reproductive and Respiratory Syndrome Virus Exhibits an Unusual Hairpin-Like Membrane Topology

Glycoprotein 3 (GP3) of the arterivirus porcine reproductive and respiratory syndrome virus (PRRSV) consists of a cleaved signal peptide, a highly glycosylated domain, a short hydrophobic region, and an unglycosylated C-terminal domain. GP3 is supposed to form a complex with GP2 and GP4 in virus particles, but secretion of GP3 from cells has also been reported. We analyzed the membrane topology of GP3 from various PRRSV strains. A fraction of the protein is secreted from transfected cells, GP3 from PRRSV-1 strains to a greater extent than GP3 from PRRSV-2 strains. This secretion behavior is reversed after exchange of the variable C-terminal domain. A fluorescence protease protection assay shows that the C terminus of GP3, fused to green fluorescent protein (GFP), is resistant to proteolytic digestion in permeabilized cells. Furthermore, glycosylation sites inserted into the C-terminal part of GP3 are used. Both experiments indicate that the C terminus of GP3 is translocated into the lumen of the endoplasmic reticulum. Deletion of the conserved hydrophobic region greatly enhances secretion of GP3, and fusion of this domain to GFP promotes membrane anchorage. Bioinformatics suggests that the hydrophobic region forms an amphipathic helix. Accordingly, exchanging only a few amino acids in its hydrophilic face prevents secretion of GP3 and in its hydrophobic face enhances it. Exchanging the latter amino acids in the context of the viral genome did not affect release of virions, but released particles were not infectious. In sum, GP3 exhibits an unusual hairpin-like membrane topology that might explain why a fraction of the protein is secreted.IMPORTANCE PRRSV is the most important pathogen in the pork industry. It causes persistent infections that lead to reduced weight gain of piglets; highly pathogenic strains even kill 90% of an infected pig population. PRRSV cannot be eliminated from pig farms by vaccination due to the large amino acid variability between the existing strains, especially in the glycoproteins. Here, we analyzed basic structural features of GP3 from various PRRSV strains. We show that the protein exhibits an unusual hairpin-like membrane topology; membrane anchoring might occur via an amphipathic helix. This rather weak membrane anchor explains why a fraction of the protein is secreted from cells. Interestingly, PRRSV-1 strains secrete more GP3 than PRRSV-2. We speculate that secreted GP3 plays a role during PRRSV infection of pigs: it might serve as a decoy to distract antibodies away from virus particles.

Homodimerisation-independent cleavage of dsRNA by a pestiviral nicking endoribonuclease

The glycoprotein E^rns plays a central role in the biology of the pestivirus bovine viral diarrhea virus (BVDV). This soluble endonuclease mediates the escape from an interferon (IFN) response in the infected fetus, thereby permitting the establishment of persistent infection. Viral single-stranded (ss) and double-stranded (ds) RNA act as potent IFN inducing signals and we previously showed that E^rns efficiently cleaves these substrates, thereby inhibiting an IFN response that is crucial for successful fetal infection. Considering that a large variety of RNases and DNases require dimerisation to cleave double-stranded substrates, the activity of E^rns against dsRNA was postulated to depend on homodimer formation mediated by disulfide bonds involving residue Cys171. Here, we show that monomeric E^rns is equally able to cleave dsRNA and to inhibit dsRNA-induced IFN synthesis as the wild-type form. Furthermore, both forms were able to degrade RNA within a DNA/RNA- as well as within a methylated RNA/RNA-hybrid, with the DNA and the methylated RNA strand being resistant to degradation. These results support our model that E^rns acts as 'nicking endoribonuclease' degrading ssRNA within double-stranded substrates. This efficiently prevents the activation of IFN and helps to maintain a state of innate immunotolerance in persistently infected animals.

Restoration of glycoprotein Erns dimerization via pseudoreversion partially restores virulence of classical swine fever virus

The classical swine fever virus (CSFV) represents one of the most important pathogens of swine. The CSFV glycoprotein E^rns is an essential structural protein and an important virulence factor. The latter is dependent on the RNase activity of this envelope protein and, most likely, its secretion from the infected cell. A further important feature with regard to its function as a virulence factor is the formation of disulfide-linked E^rns homodimers that are found in virus-infected cells and virions. Mutant CSFV lacking cysteine (Cys) 171, the residue responsible for intermolecular disulfide bond formation, were found to be attenuated in pigs (Tews BA, Schürmann EM, Meyers G. J Virol 2009;83:4823-4834). In the course of an animal experiment with such a dimerization-negative CSFV mutant, viruses were reisolated from pigs that contained a mutation of serine (Ser) 209 to Cys. This mutation restored the ability to form disulphide-linked E^rns homodimers. In transient expression studies E^rns mutants carrying the S209C change were found to form homodimers with about wt efficiency. Also the secretion level of the mutated proteins was equivalent to that of wt E^rns. Virus mutants containing the Cys171Ser/Ser209Cys configuration exhibited wt growth rates and increased virulence when compared with the Cys171Ser mutant. These results provide further support for the connection between CSFV virulence and E^rns dimerization.

Retention and topology of the bovine viral diarrhea virus glycoprotein E2

Pestiviruses are enveloped viruses that bud intracellularly. They have three envelope glycoproteins, E^rns, E1, and E2. E2 is the receptor binding protein and the main target for neutralizing antibodies. Both E^rns and E2 are retained intracellularly. Here, E2 of the bovine viral diarrhea virus (BVDV) strain CP7 was used to study the membrane topology and intracellular localization of the protein. E2 is localized in the ER and there was no difference between E2 expressed alone or in the context of the viral polyprotein. The mature E2 protein was found to possess a single span transmembrane anchor. For the mapping of a retention signal CD72-E2 fusion proteins, as well as E2 alone were analysed. This confirmed the importance of the transmembrane domain and arginine 355 for intracellular retention, but also revealed a modulating effect on retention through the cytoplasmic tail of the E2 protein, especially through glutamine 370. Mutants with a strong impact on retention were tested in the viral context and we were able to rescue BVDV with certain mutations that in E2 alone impaired intracellular retention and lead to export of E2 to the cells surface.

What can pestiviral endonucleases teach us about innate immunotolerance?

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In this review, we describe the identification of the PRRs involved in the recognition of pestiviruses, and the mechanisms of these viruses to prevent the activation of host’s innate immune response with special emphasis on viral RNases.

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Most importantly, we extend these data and present our model of innate immunotolerance requiring continuous prevention of detection of immunostimulatory self nucleic acids, in contrast to the well-known long-term tolerance of the adaptive immune system targeted predominantly against proteins.

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This hypothesis is very likely relevant beyond the bovine species and might answer more fundamental questions on the discrimination between “self” and “viral nonself RNA”, which are relevant also for the prevention and treatment of chronic IFN induction and autoimmunity induced by “self-RNAs”.

Pestiviruses including bovine viral diarrhea virus (BVDV), border disease virus (BDV) and classical swine fever virus (CSFV), occur worldwide and are important pathogens of livestock. A large part of their success can be attributed to the induction of central immunotolerance including B- and T-cells upon fetal infection leading to the generation of persistently infected (PI) animals. In the past few years, it became evident that evasion of innate immunity is a central element to induce and maintain persistent infection. Hence, the viral non-structural protease N^pro heads the transcription factor IRF-3 for proteasomal degradation, whereas an extracellularly secreted, soluble form of the envelope glycoprotein E^rns degrades immunostimulatory viral single- and double-stranded RNA, which makes this RNase unique among viral endoribonucleases. We propose that these pestiviral interferon (IFN) antagonists maintain a state of innate immunotolerance mainly pertaining its viral nucleic acids, in contrast to the well-established immunotolerance of the adaptive immune system, which is mainly targeted at proteins. In particular, the unique extension of ‘self’ to include the viral genome by degrading immunostimulatory viral RNA by E^rns is reminiscent of various host nucleases that are important to prevent inappropriate IFN activation by the host’s own nucleic acids in autoimmune diseases such as Aicardi-Goutières syndrome or systemic lupus erythematosus. This mechanism of “innate tolerance” might thus provide a new facet to the role of extracellular RNases in the sustained prevention of the body’s own immunostimulatory RNA to act as a danger-associated molecular pattern that is relevant across various species.

Different evolutionary patterns of classical swine fever virus envelope proteins

Classical swine fever virus (CSFV) is the causative agent of classical swine fever, which is a highly contagious disease of the domestic pig as well as wild boar. The proteins E(rns), E1, and E2 are components of the viral envelope membrane. They are also implicated in virus attachment and entry, replication, and (or) anti-immune response. Here, we studied the genetic variations of these envelope proteins in the evolution of CSFV. The results reveal that the envelope proteins underwent different evolutionary fates. In E(rns) and E1, but not E2, a number of amino acid sites experienced functional divergence. Furthermore, the diversification in E(rns) and E1 was generally episodic because the divergence-related changes of E1 only occurred with the separation of 2 major groups of CSFV and that of E(rns) took place with the division of 1 major group. The major divergence-related sites of E(rns) are located on one of the substrate-binding regions of the RNase domain and C-terminal extension. These functional domains have been reported to block activation of the innate immune system and attachment and entry into host cells, respectively. Our results may shed some light on the divergent roles of the envelope proteins.

Lipid Binding of the Amphipathic Helix Serving as Membrane Anchor of Pestivirus Glycoprotein Erns

Pestiviruses express a peculiar protein named E^rns representing envelope glycoprotein and RNase, which is important for control of the innate immune response and persistent infection. The latter functions are connected with secretion of a certain amount of E^rns from the infected cell. Retention/secretion of E^rns is most likely controlled by its unusual membrane anchor, a long amphipathic helix attached in plane to the membrane. Here we present results of experiments conducted with a lipid vesicle sedimentation assay able to separate lipid-bound from unbound protein dissolved in the water phase. Using this technique we show that a protein composed of tag sequences and the carboxyterminal 65 residues of E^rns binds specifically to membrane vesicles with a clear preference for compositions containing negatively charged lipids. Mutations disturbing the helical folding and/or amphipathic character of the anchor as well as diverse truncations and exchange of amino acids important for intracellular retention of E^rns had no or only small effects on the proteins membrane binding. This result contrasts the dramatically increased secretion rates observed for E^rns proteins with equivalent mutations within cells. Accordingly, the ratio of secreted versus cell retained E^rns is not determined by the lipid affinity of the membrane anchor.

Structures and Functions of Pestivirus Glycoproteins: Not Simply Surface Matters

Pestiviruses, which include economically important animal pathogens such as bovine viral diarrhea virus and classical swine fever virus, possess three envelope glycoproteins, namely E^rns, E1, and E2. This article discusses the structures and functions of these glycoproteins and their effects on viral pathogenicity in cells in culture and in animal hosts. E2 is the most important structural protein as it interacts with cell surface receptors that determine cell tropism and induces neutralizing antibody and cytotoxic T-lymphocyte responses. All three glycoproteins are involved in virus attachment and entry into target cells. E1-E2 heterodimers are essential for viral entry and infectivity. E^rns is unique because it possesses intrinsic ribonuclease (RNase) activity that can inhibit the production of type I interferons and assist in the development of persistent infections. These glycoproteins are localized to the virion surface; however, variations in amino acids and antigenic structures, disulfide bond formation, glycosylation, and RNase activity can ultimately affect the virulence of pestiviruses in animals. Along with mutations that are driven by selection pressure, antigenic differences in glycoproteins influence the efficacy of vaccines and determine the appropriateness of the vaccines that are currently being used in the field.

Identification of amino acid changes in the envelope glycoproteins of bovine viral diarrhea viruses isolated from alpaca that may be involved in host adaptation

Bovine viral diarrhea viruses (BVDV) are most commonly associated with infections of cattle. However, BVDV are often isolated from closely related ruminants with a number of BVDV-1b viruses being isolated from alpacas that were both acutely and persistently infected. The complete nucleotide sequence of the open reading frame of eleven alpaca-adapted BVDV isolates and the region encoding the envelope glycoproteins of an additional three isolates were determined. With the exception of one, all alpaca isolates were >99.2% similar at the nucleotide level. The Hercules isolate was more divergent, with 95.7% sequence identity to the other viruses. Sequence similarity of the 14 viruses indicated they were isolates of a single BVDV strain that had adapted to and were circulating through alpaca herds. Hercules was a more distantly related strain that has been isolated only once in Canada and represented a separate adaptation event that possessed the same adaptive changes. Comparison of amino acid sequences of alpaca and bovine-derived BVDV strains revealed three regions with amino acid sequences unique to all alpaca isolates. The first contained two small in-frame deletions near the N-terminus of the E2 glycoprotein. The second was found near the C-terminus of the E2 protein where four altered amino acids were located within a 30 amino acid domain that participates in E2 homodimerization. The third region contained three variable amino acids in the C-terminus of the E(rns) within the amphipathic helix membrane anchor. These changes were found in the polar side of the amphipathic helix and resulted in an increased charge within the polar face. Titration of bovine and alpaca viruses in both bovine and alpaca cells indicated that with increased charge in the amphipathic helix, the ability to infect alpaca cells also increased.

The Molecular Biology of Pestiviruses

Pestiviruses are among the economically most important pathogens of livestock. The biology of these viruses is characterized by unique and interesting features that are both crucial for their success as pathogens and challenging from a scientific point of view. Elucidation of these features at the molecular level has made striking progress during recent years. The analyses revealed that major aspects of pestivirus biology show significant similarity to the biology of human hepatitis C virus (HCV). The detailed molecular analyses conducted for pestiviruses and HCV supported and complemented each other during the last three decades resulting in elucidation of the functions of viral proteins and RNA elements in replication and virus-host interaction. For pestiviruses, the analyses also helped to shed light on the molecular basis of persistent infection, a special strategy these viruses have evolved to be maintained within their host population. The results of these investigations are summarized in this chapter.

Modeling of the Ebola virus delta peptide reveals a potential lytic sequence motif

Filoviruses, such as Ebola and Marburg viruses, cause severe outbreaks of human infection, including the extensive epidemic of Ebola virus disease (EVD) in West Africa in 2014. In the course of examining mutations in the glycoprotein gene associated with 2014 Ebola virus (EBOV) sequences, a differential level of conservation was noted between the soluble form of glycoprotein (sGP) and the full length glycoprotein (GP), which are both encoded by the GP gene via RNA editing. In the region of the proteins encoded after the RNA editing site sGP was more conserved than the overlapping region of GP when compared to a distant outlier species, Tai Forest ebolavirus. Half of the amino acids comprising the “delta peptide”, a 40 amino acid carboxy-terminal fragment of sGP, were identical between otherwise widely divergent species. A lysine-rich amphipathic peptide motif was noted at the carboxyl terminus of delta peptide with high structural relatedness to the cytolytic peptide of the non-structural protein 4 (NSP4) of rotavirus. EBOV delta peptide is a candidate viroporin, a cationic pore-forming peptide, and may contribute to EBOV pathogenesis.

Prolonged activity of the pestiviral RNase Erns as an interferon antagonist after uptake by clathrin-mediated endocytosis

The RNase activity of the envelope glycoprotein E(rns) of the pestivirus bovine viral diarrhea virus (BVDV) is required to block type I interferon (IFN) synthesis induced by single-stranded RNA (ssRNA) and double-stranded RNA (dsRNA) in bovine cells. Due to the presence of an unusual membrane anchor at its C terminus, a significant portion of E(rns) is also secreted. In addition, a binding site for cell surface glycosaminoglycans is located within the C-terminal region of E(rns). Here, we show that the activity of soluble E(rns) as an IFN antagonist is not restricted to bovine cells. Extracellularly applied E(rns) protein bound to cell surface glycosaminoglycans and was internalized into the cells within 1 h of incubation by an energy-dependent mechanism that could be blocked by inhibitors of clathrin-dependent endocytosis. E(rns) mutants that lacked the C-terminal membrane anchor retained RNase activity but lost most of their intracellular activity as an IFN antagonist. Surprisingly, once taken up into the cells, E(rns) remained active and blocked dsRNA-induced IFN synthesis for several days. Thus, we propose that E(rns) acts as an enzymatically active decoy receptor that degrades extracellularly added viral RNA mainly in endolysosomal compartments that might otherwise activate intracellular pattern recognition receptors (PRRs) in order to maintain a state of innate immunotolerance.

IMPORTANCE

The pestiviral RNase E(rns) was previously shown to inhibit viral ssRNA- and dsRNA-induced interferon (IFN) synthesis. However, the localization of E(rns) at or inside the cells, its species specificity, and its mechanism of interaction with cell membranes in order to block the host's innate immune response are still largely unknown. Here, we provide strong evidence that the pestiviral RNase E(rns) is taken up within minutes by clathrin-mediated endocytosis and that this uptake is mostly dependent on the glycosaminoglycan binding site located within the C-terminal end of the protein. Remarkably, the inhibitory activity of E(rns) remains for several days, indicating the very potent and prolonged effect of a viral IFN antagonist. This novel mechanism of an enzymatically active decoy receptor that degrades a major viral pathogen-associated molecular pattern (PAMP) might be required to efficiently maintain innate and, thus, also adaptive immunotolerance, and it might well be relevant beyond the bovine species.

Structure of the membrane anchor of pestivirus glycoprotein E(rns), a long tilted amphipathic helix

E(rns) is an essential virion glycoprotein with RNase activity that suppresses host cellular innate immune responses upon being partially secreted from the infected cells. Its unusual C-terminus plays multiple roles, as the amphiphilic helix acts as a membrane anchor, as a signal peptidase cleavage site, and as a retention/secretion signal. We analyzed the structure and membrane binding properties of this sequence to gain a better understanding of the underlying mechanisms. CD spectroscopy in different setups, as well as Monte Carlo and molecular dynamics simulations confirmed the helical folding and showed that the helix is accommodated in the amphiphilic region of the lipid bilayer with a slight tilt rather than lying parallel to the surface. This model was confirmed by NMR analyses that also identified a central stretch of 15 residues within the helix that is fully shielded from the aqueous layer, which is C-terminally followed by a putative hairpin structure. These findings explain the strong membrane binding of the protein and provide clues to establishing the E(rns) membrane contact, processing and secretion.

Folding and self-assembly of the TatA translocation pore based on a charge zipper mechanism

We propose a concept for the folding and self-assembly of the pore-forming TatA complex from the Twin-arginine translocase and of other membrane proteins based on electrostatic "charge zippers." Each subunit of TatA consists of a transmembrane segment, an amphiphilic helix (APH), and a C-terminal densely charged region (DCR). The sequence of charges in the DCR is complementary to the charge pattern on the APH, suggesting that the protein can be "zipped up" by a ladder of seven salt bridges. The length of the resulting hairpin matches the lipid bilayer thickness, hence a transmembrane pore could self-assemble via intra- and intermolecular salt bridges. The steric feasibility was rationalized by molecular dynamics simulations, and experimental evidence was obtained by monitoring the monomer-oligomer equilibrium of specific charge mutants. Similar "charge zippers" are proposed for other membrane-associated proteins, e.g., the biofilm-inducing peptide TisB, the human antimicrobial peptide dermcidin, and the pestiviral E(RNS) protein.

Crystal structure of the pestivirus envelope glycoprotein E(rns) and mechanistic analysis of its ribonuclease activity

Pestiviruses, which belong to the Flaviviridae family of RNA viruses, are important agents of veterinary diseases causing substantial economical losses in animal farming worldwide. Pestivirus particles display three envelope glycoproteins at their surface: E(rns), E1, and E2. We report here the crystal structure of the catalytic domain of E(rns), the ribonucleolytic activity of which is believed to counteract the innate immunity of the host. The structure reveals a three-dimensional fold corresponding to T2 ribonucleases from plants and fungi. Cocrystallization experiments with mono- and oligonucleotides revealed the structural basis for substrate recognition at two binding sites previously identified for T2 RNases. A detailed analysis of poly-U cleavage products using (31)P-NMR and size exclusion chromatography, together with molecular docking studies, provides a comprehensive mechanistic picture of E(rns) activity on its substrates and reveals the presence of at least one additional nucleotide binding site.

New insights into the antigenic structure of the glycoprotein E(rns) of classical swine fever virus by epitope mapping

The E(rns) glycoprotein of classical swine fever virus (CSFV) has been studied in detail concerning biochemical and functional properties, whereas less is known about its antigenic structure. In order to define epitopes recognized by CSFV-specific antibodies, the binding sites of seven E(rns)-specific monoclonal antibodies were investigated. Mapping experiments using chimeric E(rns) proteins, site-directed mutagenesis and an overlapping peptide library identified one antigenic region located between amino acids (aa) 55 to 110 on the E(rns) protein of CSFV Alfort/187. The domain comprises three linear motifs *(64)TNYTCCKLQ(72), (73)RHEWNKHGW(81), and (88)DPWIQLMNR(96), respectively, and two aa at position 102 and 107 that are crucial for the interaction with antibodies. Additionally, the presentation of the epitope in a correct conformation is mandatory for an efficient antibody binding. These findings allow a better understanding of the organization and the structure of the E(rns) and provide valuable information with regard to the development of E(rns)-based diagnostic tests.

A new type of intracellular retention signal identified in a pestivirus structural glycoprotein

Sorting of membrane proteins into intracellular organelles is crucial for cell function. Viruses exploit intracellular transport and retention systems to concentrate envelope proteins at the site of virus budding. In pestiviruses, a group of important pathogens of pigs and ruminants closely related to human hepatitis C virus, the E(rns) protein translated from the viral RNA is secreted from the infected cells and found in the serum of infected animals. Secretion of the protein is regarded as crucial for its function as a viral virulence factor associated with its RNase activity. However, ∼95% of the E(rns) molecules are retained within the infected cell. Fusion of different E(rns) fragments to the C terminus of CD72 allowed identification of a retention signal within the C-terminal 65 aa of the viral protein. This C-terminal sequence represents its membrane anchor and folds into an amphipathic helix binding in-plane to the membrane surface. Residues L183, I190, and L208 are important for intracellular location of E(rns). Presentation of the retention signal on the cytoplasmic instead of the luminal face of the ER membrane in CD8α fusion proteins still led to retention. Thus, E(rns) contains in its C-terminal amphipathic helix an intracellular retention signal that is active on both faces of the membrane.

Complementation studies with the novel "Bungowannah" virus provide new insights in the compatibility of pestivirus proteins

In recent years several atypical pestiviruses have been described. Bungowannah virus is the most divergent virus in this group. Therefore, heterologous complementation was used to clarify the phylogenetic relationship and to analyze the exchangeability of genome regions encoding structural proteins. Using a BVDV type 1 backbone, chimeric constructs with substituted envelope proteins E(rns), E1 and E2, were investigated. While all constructs replicated autonomously, infectious high titer chimeric virus could only be observed after exchanging the complete E1-E2 encoding region. The complementation of E1 and E2 alone resulted only in replicons. Complementation of BVDV-E(rns) was only efficient if Bungowannah virus-E(rns) was expressed from a bicistronic construct. Our data provide new insights in the compatibility of pestivirus proteins and demonstrate that heterologous complementation could be useful to characterize new pestiviruses.

Viral RNase Involvement in Strategies of Infection

The overwhelming majority of RNase activity is engaged in catabolic processes. Viruses have no metabolism of their own, but rely completely on host cellular energy and substrate provision to support the biochemical processes necessary for virus replication. It is therefore obvious that RNA hydrolysis does not represent an obligate step in the viral life cycle that would have to be governed by viral proteins. Accordingly, RNases are found only rarely in the viral proteomes and serve special functions. In this chapter, several virus-specific RNases will be described and their role in the viral life cycle discussed. The text will concentrate on RNases of members of the nidoviruses, herpesviruses, pestiviruses, and several viruses with segmented negative-strand RNA genome including influenza virus. These enzymes are involved in specific steps of viral gene expression, viral genome replication, shutoff of host cellular gene expression, and interference with the host’s immune response to virus infection.

A new type of signal peptidase cleavage site identified in an RNA virus polyprotein

Pestiviruses, a group of enveloped positive strand RNA viruses belonging to the family Flaviviridae, express their genes via a polyprotein that is subsequently processed by proteases. The structural protein region contains typical signal peptidase cleavage sites. Only the site at the C terminus of the glycoprotein E(rns) is different because it does not contain a hydrophobic transmembrane region but an amphipathic helix functioning as the E(rns) membrane anchor. Despite the absence of a hydrophobic region, the site between the C terminus of E(rns) and E1, the protein located downstream in the polyprotein, is cleaved by signal peptidase, as demonstrated by mutagenesis and inhibitor studies. Thus, E(rns)E1 is processed at a novel type of signal peptidase cleavage site showing a different membrane topology. Prevention of glycosylation or introduction of mutations into the C-terminal region of E(rns) severely impairs processing, presumably by preventing proper membrane interaction or disturbing a conformation critical for the protein to be accepted as a substrate by signal peptidase.

Mutation of cysteine 171 of pestivirus E rns RNase prevents homodimer formation and leads to attenuation of classical swine fever virus

Pestiviruses represent important pathogens of farm animals that have evolved unique strategies and functions to stay within their host populations. E(rns), a structural glycoprotein of pestiviruses, exhibits RNase activity and represents a virulence factor of the viruses. E(rns) forms disulfide linked homodimers that are found in virions and virus-infected cells. Mutation or deletion of cysteine 171, the residue engaged in intermolecular disulfide bond formation, results in loss of dimerization as tested in coprecipitation and native protein gel electrophoresis analyses. Nevertheless, stable virus mutants with changes affecting cysteine codon 171 could be recovered in tissue culture. These mutants grew almost as well as the parental viruses and exhibited an RNase-positive phenotype. E(rns) dimerization-negative mutants of classical swine fever virus were found to be attenuated in pigs even though the virus clearly replicated and induced a significant neutralizing antibody response in the animals.

Functionality of Chimeric E2 Glycoproteins of BVDV and CSFV in Virus Replication

An intriguing difference between the E2 glycoprotein of CSFV and the other groups of pestiviruses (nonCSFV) is a lack of two cysteine residues on positions cysteine 751 and 798. Other groups of pestivirus are not restricted to one species as swine, whereas CSFV is restricted to swine and wild boar. We constructed chimeric CSFV/BVDV E2 genes based on a 2D model of E2 proposed by van Rijn et al. (van Rijn et al. 1994, J Virol 68, 3934–42) and confirmed their expression by immunostaining of plasmid-transfected SK6 cells. No equivalents for the antigenic units B/C and A were found on E2 of BVDVII. This indicates major structural differences in E2. However, the immunodominant BVDVII domain A, containing epitopes with essential amino acids between position 760–764, showed to be dependent on the presence of the region defined by amino acids 684 to 796. As for the A domain of CSFV, the BVDVII A-like domain seemed to function as a separate unit. These combined domains in E2 proved to be the only combination which was functional in viral background of CSFV C-strain. The fitness of this virus (vflc36BVDVII 684–796) seemed to be reduced compared to vflc9 (with the complete antigenic region of BVDVII).