Baculovirus expression of pestivirus non-structural proteins

Multiple recombinant ELISA for the detection of bovine viral diarrhoea virus antibodies in cattle sera

The most immunogenic proteins (E0, E2 and NS3) of bovine viral diarrhoea virus (BVDV) (NADL strain) were expressed in the baculovirus/insect cells system. Recombinant antigens were applied to the design of enzyme immunoabsorbent assays (ELISAs) for the detection of specific antibodies in cattle sera. The assays developed were shown to be highly sensitive and specific in comparison with the viral neutralization test, which is the reference test for the serological diagnosis of BVDV. The present results demonstrate the contribution of each recombinant antigen to determine clearly the pattern of anti-BVDV antibodies in bovine serum samples.

A single amino acid is critical for the expression of B-cell epitopes on the helicase domain of the pestivirus NS3 protein

Truncated NS3 proteins, expressed by recombinant baculoviruses, were used to investigate the location of conserved B-cell epitopes on this non-structural bovine viral diarrhoea virus (BVDV) protein. A goat anti-pestivirus antiserum, and a panel of anti-NS3 monoclonal antibodies, including the BVDV-1 specific antibody P1D8, were used to verify the presence or absence of the epitopes. Interestingly, the monoclonal antibodies reacted only with the truncated protein encompassing the helicase domain of NS3. Expression of the B-cell epitopes was dependent on, but not within, a 57 amino acid sequence at the carboxy-terminal end of this protein, supporting observations that these conserved epitopes are conformational in nature. A comparison of deduced amino acid sequences of the helicase domain from BVDV-1, BVDV-2, BDV and CSFV isolates highlighted a single amino acid that appeared to be unique to P1D8-reactive BVDV-1 isolates. Site-directed mutagenesis studies confirmed that this amino acid is critical for the expression of the BVDV-1 specific NS3 epitope recognised by the P1D8 monoclonal antibody. Surprisingly, the amino acid was also important for an epitope recognised by two group-specific monoclonal antibodies, P1H11 and P4A11. Protein modelling studies, based on the structure of the hepatitis C NS3 helicase domain, indicated that this amino acid occupies a prominent position on the surface of the protein.

NS3 serine protease of bovine viral diarrhea virus: characterization of active site residues, NS4A cofactor domain, and protease-cofactor interactions

The gene expression of bovine viral diarrhea virus (BVDV), a pestivirus, occurs via translation of a hypothetical polyprotein that is processed cotranslationally and posttranslationally by viral and cellular enzymes. A protease located in the N-terminal region of nonstructural (NS) protein NS3 catalyzes the cleavages, leading to the release of NS4A, NS4B, NS5A, and NS5B. Our study provides experimental evidence that histidine at position 1658 and aspartic acid at position 1686 constitute together with the previously identified serine at position 1752 (S1752) the catalytic triad of the pestiviral NS3 serine protease. Interestingly, a mutant protease encompassing an exchange of the active site S1752 to threonine still showed residual activity. This finding links the NS3 protease of pestiviruses to the capsid protease of Sindbis virus. Furthermore, we observed that the minimal protease domain of NS3 encompasses about 209 amino acids. The NS3 protease was found to be sensitive to N-terminal truncation because a deletion of 6 amino acids significantly reduced the cleavage efficiency at the NS4A/4B site. Larger N-terminal deletions also impaired the activity of the enzyme with respect to the other cleavage sites but to a different degree at each site. The NS3 protease of BVDV has previously been shown to depend on NS4A as cofactor. We demonstrate here that the central region of NS4A represents the cofactor domain. Furthermore, coprecipitation studies strongly suggest an interaction between NS4A and the N-terminal region of NS3. Besides the remarkable similarities observed between the pestiviral NS3 protease and the corresponding enzyme of hepatitis C virus (HCV), our results suggest a common ancestry between these enzymes and the capsid protease of Sindbis virus.

Semiliki forest virus vector carrying the bovine viral diarrhea virus NS3 (p80) cDNA induced immune responses in mice and expressed BVDV protein in mammalian cells

Bovine viral diarrhea virus (BVDV) is a primary pathogen responsible for bovine enteric, respiratory and reproductive failure. A genetic region is encoding the p80 (NS3) of BVDV as the most conserved protein among Pestiviruses. BVDV infection in cattle induces NS3 specific lymphocyte proliferation and humoral responses. To generate a DNA vaccine against BVDV, the gene for BVDV-NADL NS3 was cloned into an eukaryotic expression vector of Semiliki Forest virus (pSFV-1). Quadriceps muscles of BALB/c mice were injected with recombinant DNA generated statistically significant cytotoxic T-lymphocyte activity (CTL) and cell mediated immune (CMI) responses against cytopathic and noncytopathic BVDV. Whereas, the BVDV-NS3 did not generate neutralizing antibodies against BVDVin mice. pSFV-1-NS3 DNA was subjected to in vitro transcription into mRNA. The mRNA was transfected into baby hamster kidney cells (BHK-21) and Madin-Darby bovine kidney cells (MDBK). The recombinant cells were used in the detection of DNA antigen responses by immunological assays. This report establishes the ability of BVDV-NS3 DNA inoculation to induce a strong cellular immune responses in mice.

Identification and characterization of an RNA-dependent RNA polymerase activity within the nonstructural protein 5B region of bovine viral diarrhea virus

Nonstructural protein 5B (NS5B) of bovine viral diarrhea virus (BVDV) contains sequence motifs that are predictive of an RNA-dependent RNA polymerase activity. We describe the expression and purification of the BVDV NS5B protein derived from an infectious cDNA clone of BVDV (NADL strain). BVDV NS5B protein was active in an in vitro RNA polymerase assay using homopolymeric RNA or BVDV minigenomic RNA templates. The major product was a covalently linked double-stranded molecule generated by a "copy-back" mechanism from the input template RNA. In addition, a nucleotide-nonspecific and template-independent terminal nucleotidyl transferase activity was observed with the BVDV NS5B preparation.

Detection of antibodies to bovine viral diarrhoea virus (BVDV) and characterization of genomes of BVDV from Brazil

An ELISA for the detection of antibodies to bovine viral diarrhea virus (BVDV) was developed based on antigens derived from a genotype I BVDV strain isolated in Switzerland. Using monoclonal antibodies we showed that this antigen contained the conserved non-structural protein NS3 whereas it essentially lacked the more strain-specific E2 surface glycoprotein. This ELISA has a sensitivity of 97.5% and a specificity of 99.2% as compared to the serum neutralization test (SNT). Preliminary experiments showed that this ELISA reliably detects antibodies to BVDV strains circulating in Brazil. Serum samples obtained from 430 adult cattle on 19 farms of the State of Rio Grande do Sul (Brazil) and one farm from Corrientes (Argentina) were tested for antibodies by means of this ELISA. We found antibodies in 56% +/- 15.1% of the cattle sera tested, which indicates that, in Brazil, the prevalence of infection with BVDV is similar to that found in Europe and the USA. Our sequence analysis of two BVDV isolates showed that BVDV of both genotypes I and II circulate in Brazil.

Application of recombinant bovine viral diarrhea virus proteins in the diagnosis of bovine viral diarrhea infection in cattle

The National Animal Disease Laboratory (NADL) vaccine strain of bovine viral diarrhea virus (BVDV) genes for gp48 and p80 were expressed in Escherichia coli. The BVDV-NADL gene for gp62 was integrated into a baculovirus genome for expression in Spodoptera frugiperda (Sf-9) insect ovarian cells. The antigenicity of baculovirus expressed BVDV protein was detected by anti-BVDV specific antibodies in an enzyme-linked immunosorbent assay (ELISA), indirect immunofluorescent assay (IFA) and radio-immunoprecipitation (RIP). The recombinant proteins isolated from bacteria showed antigenic properties when analyzed by ELISA and immunoblotting using BVDV antibodies. The recombinant proteins were then used in ELISA or IFA to detect BVDV infection by testing 54 independent bovine serum samples. The baculovirus-expressed BVDV protein was used as an ELISA and IFA antigen, and the bacteria-expressed proteins were used as ELISA antigens. BVDV-NADL-infected Madin-Darby bovine kidney (MDBK) cell monolayers served as a control antigen. Statistical analysis showed a high degree of correlation between the reactivity of recombinants and natural antigens in ELISA using bovine sera. The results of ELISA or IFA proved there is a high degree of correlation with the virus neutralization. In the comparative ELISA assays, the insect-cell-mediated expression revealed greater specificity and sensitivity than the bacterial expression or the natural BVDV antigens produced by cell cultures.

Bovine viral diarrhea virus NS3 serine proteinase: polyprotein cleavage sites, cofactor requirements, and molecular model of an enzyme essential for pestivirus replication

Members of the Flaviviridae encode a serine proteinase termed NS3 that is responsible for processing at several sites in the viral polyproteins. In this report, we show that the NS3 proteinase of the pestivirus bovine viral diarrhea virus (BVDV) (NADL strain) is required for processing at nonstructural (NS) protein sites 3/4A, 4A/4B, 4B/5A, and 5A/5B but not for cleavage at the junction between NS2 and NS3. Cleavage sites of the proteinase were determined by amino-terminal sequence analysis of the NS4A, NS4B, NS5A, and NS5B proteins. A conserved leucine residue is found at the P1 position of all four cleavage sites, followed by either serine (3/4A, 4B/5A, and 5A/5B sites) or alanine (4A/4B site) at the P1' position. Consistent with this cleavage site preference, a structural model of the pestivirus NS3 proteinase predicts a highly hydrophobic P1 specificity pocket. trans-Processing experiments implicate the 64-residue NS4A protein as an NS3 proteinase cofactor required for cleavage at the 4B/5A and 5A/5B sites. Finally, using a full-length functional BVDV cDNA clone, we demonstrate that a catalytically active NS3 serine proteinase is essential for pestivirus replication.

Cytopathogenicity of a pestivirus correlates with a 27-nucleotide insertion

Cytopathogenic (cp) bovine viral diarrhea virus (BVDV) strains are generated in cattle persistently infected with noncytopathogenic (noncp) BVDV.cp BVDV strains are considered crucial for the development of fatal mucosal disease. Comparative analysis of cp and noncp BVDV strains isolated from one animal suffering from mucosal disease revealed that the genomes of the cp BVDV strain (CP7) and the corresponding noncp BVDV strain (NCP7) are highly homologous. However, only the genome of CP7 contains an insertion of 27 nucleotides in the NS2 coding region. The inserted sequence represents a duplication of bases 4064 to 4090 of the viral genome, located between the formerly neighboring nucleotides 4353 and 4354. Parts of the viral polyproteins of CP7 and NCP7 were expressed in the T7 vaccinia virus system. These studies revealed that the insertion identified in the CP7 genome is necessary and sufficient for the induction of NS2-3 cleavage. Since the expression of NS3 is strictly correlated to cp BVDV, the insertion identified in the genome of BVDV CP7 represents most likely the relevant mutation leading to the evolvement of CP7 from NCP7.

Pestivirus NS3 (p80) protein possesses RNA helicase activity

The pestivirus bovine viral diarrhea virus (BVDV) p80 protein (referred to here as the NS3 protein) contains amino acid sequence motifs predictive of three enzymatic activities: serine proteinase, nucleoside triphosphatase, and RNA helicase. We have previously demonstrated that the former two enzymatic activities are associated with this protein. Here, we show that a purified recombinant BVDV NS3 protein derived from baculovirus-infected insect cells possesses RNA helicase activity. BVDV NS3 RNA helicase activity was specifically inhibited by monoclonal antibodies to the p80 protein. The activity was dependent on the presence of nucleoside triphosphate and divalent cation, with a preference for ATP and Mn2+. Hydrolysis of the nucleoside triphosphate was necessary for strand displacement. The helicase activity required substrates with an un-base-paired region on the template strand 3' of the duplex region. As few as three un-base-paired nucleotides were sufficient for efficient oligonucleotide displacement. However, the enzyme did not act on substrates having a single-stranded region only to the 5' end of the duplex or on substrates lacking single-stranded regions altogether (blunt-ended duplex substrates), suggesting that the directionality of the BVDV RNA helicase was 3' to 5' with respect to the template strand. The BVDV helicase activity was able to displace both RNA and DNA oligonucleotides from RNA template strands but was unable to release oligonucleotides from DNA templates. The possible role of this activity in pestivirus replication is discussed.

Nonstructural protein 3 of the hepatitis C virus encodes a serine-type proteinase required for cleavage at the NS3/4 and NS4/5 junctions

We have studied processing of the nonstructural (NS) polyprotein of the hepatitis C virus. A series of cDNAs corresponding to predicted NS2/3/4 or NS3/4 regions were constructed, and processing of the polyproteins was studied in an in vitro transcription-translation system. We report that a catalytically active serine-type proteinase is encoded by the NS3 region. Substitution of the serine residue of the putative catalytic triad (H, D, and S) by alanine blocked cleavage at the NS3/4 junction, while processing between NS2 and NS3 was not affected. Thus, cleavage at the NS2/3 junction is mediated either by cellular enzymes or by an NS-2 inherent proteinase activity. Deletion analysis of an NS3/4 cDNA construct mapped the amino terminus of the enzymatically active proteinase between amino acids 1049 and 1065 of the polyprotein. As internal deletions of variable segments of the presumed helicase domain prevented processing at the NS314 junction, a continuous NS3 region appears to be required for processing at this site. To analyze hepatitis C virus polyprotein cleavage in vivo, recombinant vaccinia viruses expressing NS2/3/4 or NS3/4/5 proteins were generated. In agreement with the in vitro data, cleavage between NS2 and NS3 was independent of a catalytically active NS3 proteinase, whereas substitution of the active-site serine residue by the amino acid alanine completely blocked processing at the NS3/4 and NS4/5 junctions. These results demonstrate that NS3 encodes the viral proteinase essential for generating the amino termini of NS4 and NS5.

Characterization of the hepatitis C virus-encoded serine proteinase: determination of proteinase-dependent polyprotein cleavage sites

Processing of the hepatitis C virus (HCV) H strain polyprotein yields at least nine distinct cleavage products: NH2-C-E1-E2-NS2-NS3-NS4A-NS4B-NS5A-NS5B-CO OH. As described in this report, site-directed mutagenesis and transient expression analyses were used to study the role of a putative serine proteinase domain, located in the N-terminal one-third of the NS3 protein, in proteolytic processing of HCV polyproteins. All four cleavages which occur C terminal to the proteinase domain (3/4A, 4A/4B, 4B/5A, and 5A/5B) were abolished by substitution of alanine for either of two predicted residues (His-1083 and Ser-1165) in the proteinase catalytic triad. However, such substitutions have no observable effect on cleavages in the structural region or at the 2/3 site. Deletion analyses suggest that the structural and NS2 regions of the polyprotein are not required for the HCV NS3 proteinase activity. NS3 proteinase-dependent cleavage sites were localized by N-terminal sequence analysis of NS4A, NS4B, NS5A, and NS5B. Sequence comparison of the residues flanking these cleavage sites for all sequenced HCV strains reveals conserved residues which may play a role in determining HCV NS3 proteinase substrate specificity. These features include an acidic residue (Asp or Glu) at the P6 position, a Cys or Thr residue at the P1 position, and a Ser or Ala residue at the P1' position.