The processing of pseudorabies virus glycoprotein gX in infected cells and in an uninfected cell line

Glycoprotein G from pseudorabies virus binds to chemokines with high affinity and inhibits their function

Pseudorabies virus (PRV), also known as suid herpesvirus, is the aetiological agent of Aujeszky's disease in swine. In other animals, except higher-order primates, PRV infection is often fatal. The mechanisms of PRV pathogenesis and immune modulation are largely unknown. PRV codes for 11 glycoproteins. Among them, glycoprotein G (gG) is the most abundant PRV protein found in the supernatant of PRV-infected cell cultures. PRV-gG has low amino acid sequence similarity with gG from other animal alphaherpesviruses and its function is unknown. gG from other animal alphaherpesviruses, with the exception of at least equine herpesvirus 4, binds to chemokines. We show here that PRV-gG binds to the human chemokine CL1 and several CC and CXC human chemokines with high affinity. Chemokine-binding activity can be detected in the supernatants of PRV-infected cell cultures, and insertional inactivation of the gene encoding gG from the PRV genome results in loss of chemokine-binding activity. Binding of PRV-gG to chemokines inhibits chemokine-mediated cell migration, suggesting a role for PRV-gG in immune evasion.

A TEF-1-element is required for activation of the promoter of pseudorabies virus glycoprotein X gene by IE180

The pseudorabies virus (PRV) immediate-early regulatory protein IE180 is able to transactivate the viral early and late genes. Using chloramphenicol acetyltransferase (CAT) assay, we investigated the transactivation function of IE180 to the promoter of PRV glycoprotein X (gX) gene, and our results showed that IE180 could significantly increase the expression of CAT gene which was under the control of gX promoter. To further identify the activation domains of IE180 protein that interact with the gX promoter sequences, various truncated mutants of IE180 gene and gX promoter gene were constructed and analyzed by CAT and gel retardation assay. Results revealed that the N-terminal amino acid residues from 133 to 736 of IE180 could interact with the binding site of transcriptional enhancer factor-1 (TEF-1) that resides in the gX promoter. Formation of protein-DNA complexes between the IE180 protein and the TEF-1 element of the gX promoter was observed using electrophoretic mobility shift assay (EMSA) as well as Southwestern blot analysis. These results indicated that a direct interaction occurred between IE180 and the TEF-1 element; and this interaction was abolished if the TEF-1 element was mutated. The association of IE180 with the TEF-1 element was further confirmed by the supershift of EMSA complexes using IE180 specific antibody. Taken together, our results suggested that formation of a complex between the IE180 protein and TEF-1 element in the gX promoter region was involved in the transcriptional regulation of the gX gene.

Insertions in the gG gene of pseudorabies virus reduce expression of the upstream Us3 protein and inhibit cell-to-cell spread of virus infection

The alphaherpesvirus Us4 gene encodes glycoprotein G (gG), which is conserved in most viruses of the alphaherpesvirus subfamily. In the swine pathogen pseudorabies virus (PRV), mutant viruses with internal deletions and insertions in the gG gene have shown no discernible phenotypes. We report that insertions in the gG locus of the attenuated PRV strain Bartha show reduced virulence in vivo and are defective in their ability to spread from cell to cell in a cell-type-specific manner. Similar insertions in the gG locus of the wild-type PRV strain Becker had no effect on the ability of virus infection to spread between cells. Insertions in the gG locus of the virulent NIA-3 strain gave results similar to those found with the Bartha strain. To examine the role of gG in cell-to-cell spread, a nonsense mutation in the gG signal sequence was constructed and crossed into the Bartha strain. This mutant, PRV157, failed to express gG yet had cell-to-cell spread properties indistinguishable from those of the parental Bartha strain. These data indicated that, while insertions in the gG locus result in decreased cell-to-cell spread, the phenotype was not due to loss of gG expression as first predicted. Analysis of gene expression upstream and downstream of gG revealed that expression of the upstream Us3 protein is reduced by insertion of lacZ or egfp at the gG locus. By contrast, expression of the gene immediately downstream of gG, Us6, which encodes glycoprotein gD, was not affected by insertions in gG. These data indicate that DNA insertions in gG have polar effects and suggest that the serine/threonine kinase encoded by the Us3 gene, and not gG, functions in the spread of viral infection between cells.

Channel catfish virus gene 50 encodes a secreted, mucin-like glycoprotein

Cells infected with the wild-type (WT) strain of channel catfish virus (CCV) secreted a glycoprotein with an apparent molecular mass (MM) superior to 200 kDa into the culture medium. This protein, designated gp250, was the sole viral glycoprotein detected in the culture medium after [3H]mannose labeling of the infected cells. When cells were infected with the attenuated V60 strain, a glycoprotein of 135 kDa (designated gp135) was detected instead of gp250. Because WT gene 50 is predicted to encode a secreted, mucin-type glycoprotein, we expressed this gene transiently and detected a glycoprotein of the same apparent MM as gp250 in the culture medium of transfected catfish cells. The increased mobility in SDS-PAGE of the secreted V60 glycoprotein correlated with the presence of a major deletion in V60 gene 50. Therefore, we concluded that gp250 in the WT and gp135 in the V60 strains are both likely encoded by gene 50. An important shift in the relative mobility of gp250 in SDS-PAGE was observed after tunicamycin treatment of infected cells labeled with [3H]glucosamine, confirming the presence of N-linked sugars on gp250. We observed variations in the size of PCR products derived from gene 50 amplification in three different field isolates. Such genetic variations are a characteristic feature of mucin genes and are linked to crossing-over events between internal repeated sequences, such as those present in gene 50.

Effect of maternally acquired Aujeszky's disease (pseudorabies) virus-specific antibody in pigs on establishment of latency and seroconversion to differential glycoproteins after low dose challenge

This study investigated whether (1) passively immune pigs could become latently infected after challenge with low doses of wild type pseudorabies virus (PRV) and (2) if seroconversion to PRV could be consistently detected using two commercially available differential diagnostic ELISAs. Three litters of piglets with passively acquired PRV serum neutralizing (SN) antibody (geometric mean titers 47.03 to 95.10) were challenged at 6 to 12 days of age with 236 to 500 TCID50 of Shope strain virus; pigs were vaccinated at 11 weeks of age with a commercially available genetically engineered vaccine (TK- gE- gG- Iowa S62 strain PRV). Vaccination was intended to reduce the risk of reactivation of latent infection resulting in spread of virulent PRV infection to previously uninfected pigs during the experiment. Vaccination at this age also approximated common field practices in infected herds. After 15 weeks, all challenged pigs were seropositive on the PRV glycoprotein (g or gp) E differential ELISA but were seronegative on the gG differential ELISA. All three challenge groups had pigs that were latently infected as evidenced by the detection of PRV DNA by polymerase chain reaction (PCR) assay of their trigeminal ganglia (TG). There was a significant inverse relationship observed for age at challenge and the proportion of PCR positive pigs in the group 15 weeks postchallenge (p = 0.0004). This trend was independent of the passively acquired PRV SN antibody titers at challenge. In this study, passively acquired antibody did not provide protection against establishment of latent infection in piglets after exposure to low doses of virulent PRV. These latent infections were detected serologically by only one of two available differential diagnostic ELISA.

Bovine herpesvirus 1 U(s) open reading frame 4 encodes a glycoproteoglycan

Sequence analysis of the short unique (Us) segment of the bovine herpesvirus 1 (BHV-1) genome predicted that the Us open reading frame (ORF) 4 encodes a protein with homology to glycoprotein G (gG) of other alpha-herpesviruses (P. Leung-Tack, J.-C. Audonnet, and M. Riviere, Virology 199:409-421, 1994). RNA analysis showed that the Us ORF4 is contained within two transcripts of 3.5 and 1.8 kb. The 3.5 kb RNA represents a structurally bicistronic RNA which encompasses the Us ORF3 and Us ORF4, whereas the 1.8-kb RNA constitutes the monocistronic Us ORF4 mRNA. To identify the predicted BHV-I gG, recombinant vaccinia virus expressing the Us ORF4 was used to raise specific antibodies in rabbits. The antiserum recognized a 65-kDa polypeptide and a very diffusely migrating species of proteins with an apparent molecular mass of between 90 and greater than 240 kDa in supernatants of BHV-1-infected cells which was also precipitated together with 61- and 70-kDa polypeptides from cell-associated proteins. The specificity of the reaction was demonstrated by the absence of these proteins from the supernatant of cells infected with the Us ORF4 deletion mutant BHV-l/gp1-8. Treatment of the immunoprecipitated proteins with glycosidases and chondroitinase AC showed that the 65-kDa protein constitutes gG, which contains both N- and O-linked carbohydrates, and that the high-molecular-mass proteins contain glycosaminoglycans linked to a 65-kDa glycoprotein that is antigenically related to gG. These molecules were therefore named glycoproteoglycan C (gpgG). Pulse chase experiments indicated that gG and gpgG were processed from a common precursor molecule with an apparent molecular mass of 61 kDa via a 70-kDa intermediate. Both gG and gpgG could not be found associated with purified virions. In summary, our results identify the BHV-I gG protein and demonstrate the presence of a form of posttranslational modification, glycosamino-glycosylation, that has not yet been described for a herpesvirus-encoded protein.

Open reading frames encoding a protein kinase, homolog of glycoprotein gX of pseudorabies virus, and a novel glycoprotein map within the unique short segment of equine herpesvirus type 1

DNA sequence analysis of the unique short (Us) segment of the genome of equine herpesvirus type 1 Kentucky A strain (EHV-1) by our laboratory and strains Kentucky D and AB1 by other workers identifies a total of nine open reading frames (ORF). In this report, we present the DNA sequence of three of these newly identified ORFs, designated EUS 2, EUS 3, and EUS 4. The EUS 2 ORF is 1146 nucleotides (nt) in length and encodes a potential protein of 382 amino acids. Cis-regulatory sequences upstream of the putative ATG start codon include a G/C box 112 nt upstream and two potential TATA-like elements located between 15 and 90 nt before the ATG. The EUS 2 translation product exhibits significant homology to Ser/Thr protein kinases encoded within the Us segments of other herpesviruses, such as herpes simplex virus (26% homology) and pseudorabies virus (PRV), (45% homology), and possesses sequence domains conserved in protein kinases of cellular and viral origin. The EUS 3 ORF begins 127 nt downstream from the EUS 2 stop codon and ends at a stop codon 1119 nt further downstream. A single TATA-like element maps 61 nt upstream of the ORF. This ORF encodes a potential protein of 373 amino acids and is a homolog of glycoprotein gX of PRV, as judged by overall homology of amino acid residues, cysteine displacement, and presence of potential glycosylation sites and signal sequence. Interestingly, the EUS 4 ORF encodes a potential membrane glycoprotein that does not exhibit homology to any reported protein sequence. The EUS 4 ORF encodes a 383 amino acid polypeptide with a sequence indicative of a signal sequence at its amino terminal end, glycosylation sites for N-linked oligosaccharides, and a transmembrane domain near its carboxyl terminus. Several cis-acting regulatory sequences lie upstream of this ORF. These findings support the observation that the short region of alphaherpesviruses show considerable variation in their genetic content and gene organization.

Marker vaccines, virus protein-specific antibody assays and the control of Aujeszky's disease

Vaccination of pigs is widely practised to control Aujeszky's disease (AD). Molecular biological research revealed that several conventionally attenuated virus vaccines harbour deletions in their genomes. The deleted genes are nonessential for virus replication and can be involved in the expression of virulence. These findings have prompted several groups to construct well-characterized deletion mutants of AD virus that do not express either glycoprotein gI, gX or gIII. These mutants have also been rendered thymidine kinase negative. Although data on vaccine efficacy and safety have been published, widely varying test conditions have made it impossible to identify the most efficacious deletion mutant vaccine(s). Vaccination enhances the amount of virus required for infection and reduces, but does not prevent, the shedding of virulent virus and the establishment of latency in pigs infected with virulent AD virus. Therefore, while a vaccination programme will reduce the circulation of virus in the field, it will not eliminate AD virus from pig populations. To eradicate AD, the ability to differentiate infected from vaccinated pigs is crucial. The use of marker vaccines enables us to identify infected pigs in vaccinated populations by detecting antibodies against the protein whose gene is deleted from vaccine strains. The antibody response to gI appears to persist for more than 2 years, and all of about 300 field strains tested so far express gI. The use of vaccines lacking gI in combination with an enzyme linked immunosorbent assay to detect antibodies to gI and culling of gI-seropositive pigs, may help to eradicate AD in countries where vaccination is widely practised.

Nucleotide sequence of the Marek's disease virus (MDV) RB-1B A antigen gene and the identification of the MDV A antigen as the herpes simplex virus-1 glycoprotein C homologue

The A antigen gene from a very virulent strain of Marek's disease virus, RB-1B, has been cloned and the nucleotide sequence determined. The predicted amino acid sequence showed 99% identity to that determined for the MDV GA A antigen (Coussens and Velicer, J. Virol. 62, 2373-2379, 1988) over all but the carboxy-terminal region where the sequence diverged extensively. The divergence results from three nucleotide frameshifts in the reported sequence of the MDV GA gene which are not present in a cloned copy of the MDV GA A antigen gene sequenced by us. The MDV A antigen shows significant homology to a number of herpes virus gC homologues, the homology being most extensive in the carboxy-halves of the proteins.

Quantitation of putative glycoprotein X in bioengineered pseudorabies vaccine virus culture medium by ELISA

An enzyme-linked immunosorbent assay has been developed for the detection and quantitation of putative pseudorabies glycoprotein X (gX) in bulk bioengineered PRV delta gX delta tk-1 pseudorabies vaccine virus culture medium supernatants. The assay has a dynamic range of 0.2-25 ng, with a best linear region of 0.4-12.5 ng (correlation coefficient = 0.99) which permits 1 ppm discrimination for gX.

A herpesvirus vector for expression of glycosylated membrane antigens: fusion proteins of pseudorabies virus gIII and human immunodeficiency virus type 1 envelope glycoproteins

We describe experiments using the swine herpesvirus, pseudorabies virus (PRV), as a vector for expression of hybrid membrane protein genes. In particular, we present the construction and analysis of three infectious PRV mutants expressing chimeric viral membrane proteins composed of portions of the PRV envelope glycoprotein gIII and of the human retrovirus, human immunodeficiency virus type 1 (HIV-1), envelope glycoproteins gp120 and gp41. All of the chimeric genes contain the transcription control sequences and the first 157 codons of PRV gIII (known to contain signals sufficient for efficient export of the encoded peptide out of the cell) fused to different regions of the HIV-1 envelope. The mutant viruses express novel glycosylated fusion proteins that are immunoprecipitated by polyvalent sera specific for gIII, as well as acquired immunodeficiency syndrome patient sera. The levels of expression are lower than expected due primarily to instability or altered processing of the hybrid mRNA. We could not detect cleavage of chimeric proteins carrying the gp120-gp41 protease processing site. The use of localization signals contained within herpesvirus membrane proteins to direct chimeric proteins to desired cellular locations is discussed.

Inducible expression of herpes simplex virus type 2 glycoprotein gene gG-2 in a mammalian cell line

The gG-2 glycoprotein gene of herpes simplex virus type 2 (HSV-2) was cloned into the mammalian expression vector pMSG under the control of the inducible mouse mammary tumor virus promoter. Transfection of this cloned gG-2 construct into NIH 3T3 cells resulted in the stable expression of gG-2 upon induction with dexamethasone. In addition, the 104,000-molecular-weight (104K) and 72K gG-2 precursors as well as the 34K secreted component were generated in the transformed cells. The synthesis of gG-2 in these transformed cells appeared to follow the same cleavage-processing pathway as gG-2 synthesis during an HSV-2 infection. These results indicate that the processing of gG-2 can occur in the absence of an HSV-2 infection.

Analysis of pseudorabies virus glycoprotein gIII localization and modification by using novel infectious viral mutants carrying unique EcoRI sites

We have constructed two pseudorabies virus (PRV) mutants, each with a unique EcoRI restriction site in the nonessential gIII envelope glycoprotein gene. Since no natural PRV isolate has been reported to contain EcoRI sites, the isolation and single-step growth curve analysis of these mutants established that PRV can carry such a site with little ill effect in tissue culture. Virus carrying these defined mutations produced novel gIII proteins that enabled us to begin functional assignment of protein localization information within the gIII gene. Specifically, one viral mutant contained an in-frame synthetic EcoRI linker sequence that was flanked on one side by the first one-third of the gIII gene and on the other side by the last one-third of the gene. The resulting protein lacked the middle one-third of the parental species, including five of eight putative N-linked glycosylation signals, but was still glycosylated and found in enveloped virions; it was not secreted into the medium. A second viral mutant contained an in-frame synthetic EcoRI linker sequence that additionally specified a nonsense codon at position 158, producing a gIII protein that was glycosylated and secreted into the medium; the fragment was not found in enveloped virions. By endoglycosidase and pulse-chase analyses, we established a precursor-product relationship between the various forms of gIII expressed in the parental and mutant strains, and perhaps determined certain features of the gIII protein that are required for its efficient export within the cell.