The export pathway of the pseudorabies virus gB homolog gII involves oligomer formation in the endoplasmic reticulum and protease processing in the Golgi apparatus

Influence of N-glycosylation on Expression and Function of Pseudorabies Virus Glycoprotein gB

Envelope glycoprotein (g)B is conserved throughout the Herpesviridae and mediates fusion of the viral envelope with cellular membranes for infectious entry and spread. Like all viral envelope fusion proteins, gB is modified by asparagine (N)-linked glycosylation. Glycans can contribute to protein function, intracellular transport, trafficking, structure and immune evasion. gB of the alphaherpesvirus pseudorabies virus (PrV) contains six consensus sites for N-linked glycosylation, but their functional relevance is unknown. Here, we investigated the occupancy and functional relevance of N-glycosylation sites in PrV gB. To this end, all predicted N-glycosylation sites were inactivated either singly or in combination by the introduction of conservative mutations (N➔Q). The resulting proteins were tested for expression, fusion activity in cell-cell fusion assays and complementation of a gB-deficient PrV mutant. Our results indicate that all six sites are indeed modified. However, while glycosylation at most sites was dispensable for gB expression and fusogenicity, inactivation of N154 and N700 affected gB processing by furin cleavage and surface localization. Although all single mutants were functional in cell-cell fusion and viral entry, simultaneous inactivation of all six N-glycosylation sites severely impaired fusion activity and viral entry, suggesting a critical role of N-glycans for maintaining gB structure and function.

Common characteristics and unique features: A comparison of the fusion machinery of the alphaherpesviruses Pseudorabies virus and Herpes simplex virus

Membrane fusion is a fundamental biological process that allows different cellular compartments delimited by a lipid membrane to release or exchange their respective contents. Similarly, enveloped viruses such as alphaherpesviruses exploit membrane fusion to enter and infect their host cells. For infectious entry the prototypic human Herpes simplex viruses 1 and 2 (HSV-1 and -2, collectively termed HSVs) and the porcine Pseudorabies virus (PrV) utilize four different essential envelope glycoproteins (g): the bona fide fusion protein gB and the regulatory heterodimeric gH/gL complex that constitute the "core fusion machinery" conserved in all members of the Herpesviridae; and the subfamily specific receptor binding protein gD. These four components mediate attachment and fusion of the virion envelope with the host cell plasma membrane through a tightly regulated sequential activation process. Although PrV and the HSVs are closely related and employ the same set of glycoproteins for entry, they show remarkable differences in the requirements for fusion. Whereas the HSVs strictly require all four components for membrane fusion, PrV can mediate cell-cell fusion without gD. Moreover, in contrast to the HSVs, PrV provides a unique opportunity for reversion analyses of gL-negative mutants by serial cell culture passaging, due to a limited cell-cell spread capacity of gL-negative PrV not observed in the HSVs. This allows a more direct analysis of the function of gH/gL during membrane fusion. Unraveling the molecular mechanism of herpesvirus fusion has been a goal of fundamental research for years, and yet important mechanistic details remain to be uncovered. Nevertheless, the elucidation of the crystal structures of all key players involved in PrV and HSV membrane fusion, coupled with a wealth of functional data, has shed some light on this complex puzzle. In this review, we summarize and discuss the contemporary knowledge on the molecular mechanism of entry and membrane fusion utilized by the alphaherpesvirus PrV, and highlight similarities but also remarkable differences in the requirements for fusion between PrV and the HSVs.

Variations in glycoprotein B contribute to immunogenic difference between PRV variant JS-2012 and Bartha-K61

A newly emerged pseudorabies virus (PRV) variant has been identified in many Bartha-K61-vaccinated pig farms. This variant has caused great economic losses to the swine industry in China since 2011. Sequence analysis demonstrated that the gB gene of the emerging PRV variant JS-2012 had multiple variations compared with the vaccine strain Bartha-K61. In the study, a specific CRISPR/Cas9 system combined with homologous recombination was used to construct two recombinant viruses, BJB (Bartha-K61+JS-2012gB) and JBJ (JS-2012-ΔgE/gI+Bartha-K61gB), by interchanging the full-length gB genes between Bartha-K61 and JS-2012-ΔgE/gI. The two recombinant viruses showed similar characteristics in growth kinetics in vitro and similar pathogenicity in mice, as compared to their parental strains. Immunization of mice with inactivated BJB or JBJ followed by challenge of JS-2012 showed that BJB could increase protective efficacy to 80%, compared to only 40% protection by the parental Bartha-K61 strain. JBJ had a decreased protective efficacy of 65%, as compared to 90% protection by its parental JS-2012-ΔgE/gI strain. Exchange of the gB gene markedly altered the immunogenicity of the recombinant PRV. These data suggest that variations in gB might play an important role in the virulence of the reemergent PRV variant in China. Our results demonstrate the importance of gB in protective immunity and suggest that the recombinant virus BJB could be a promising vaccine candidate for eradication of the PRV variant.

Development of a competitive double antibody lateral flow assay for the detection of antibodies specific to glycoprotein B of Aujeszky's disease virus in swine sera

Three lateral flow assays (LFAs) for the detection of antibodies against glycoprotein B (gB) of Aujeszky's disease virus (ADV) in swine sera: a competitive double antibody sandwich LFA without a preincubation step (CDAS-gB-LFA), a CDAS-gB-LFA with a preincubation step (pCDAS-gB-LFA), and a competitive direct gB-LFA have been developed and were compared with each other and with a gB-ELISA. The assays are based on monoclonal antibodies to immunodominant epitopes of ADV gB. The pCDAS-gB-LFA proved to be the most specific and sensitive assay to detect antibodies directed to ADV gB. The specificity and sensitivity of the pCDAS-gB-LFA with the use of an LFA reader for test line intensity measurements were 97.6 and 94.9%, respectively. The lower diagnostic sensitivity of the pCDAS-gB-LFA compared to a gB-ELISA reflects its reduced analytical sensitivity, which was shown in titration experiments with positive sera. The pCDAS-gB-LFA, using the reader-based and visual detection modes, showed good agreement in respect to specificity; however, the LFA reader detection provided a higher diagnostic and analytical sensitivity compared to visual detection. The developed pCDAS-gB-LFA is a rapid, sensitive, and specific method for the detection of antibodies to ADV gB and can be used for screening ADV-infected swine in unvaccinated herds.

Production of pseudorabies virus recombinant glycoprotein B and its use in an agar gel immunodiffusion (AGID) test for detection of antibodies with sensitivity and specificity equal to the virus neutralization assay

Pseudorabies virus (PrV) causes Aujeszky's disease (AD), which affects mainly swine, but also cattle, sheep, and wild animals, resulting in substantial economic losses due to animal mortality and lost productivity worldwide. To combat PrV, eradication programs using PrV strains lacking the gene encoding glycoprotein E (gE) are ongoing in several countries. These eradication programs have generated a currently unmet demand for affordable, easy-to-use, and sensitive tests that can detect PrV infection in pigs infected with either wild-type virus or vaccine strain (gE-deleted) virus. To meet this demand, we used the baculovirus-insect cell system to produce recombinant glycoprotein B (gB) as antigen for an immune assay. The high GC-content (70% average) of the gB gene from the Argentinian PrV CL15 strain necessitated the use of betaine as a PCR enhancer to amplify the extracellular domain. Recombinant gB was expressed at high levels and reacted strongly with sera from PrV infected pigs. We used the recombinant gB to develop an agar gel immunodiffusion (AGID) test for detection of PrV antibodies. Compared to the gold standard virus neutralization (VN) assay, the AGID sensitivity and specificity were 95% and 96.6% respectively. Thus, recombinant gB produced in the baculovirus-insect cell system is a viable source of antigen for the detection of PrV antibodies in AGID tests. Considering its relatively lower cost, simplicity of use and result interpretation, our AGID is a valuable alternative tool to the VN assay.

Protection of pigs against pandemic swine origin H1N1 influenza A virus infection by hemagglutinin- or neuraminidase-expressing attenuated pseudorabies virus recombinants

Influenza is an important respiratory disease of pigs, and may lead to novel human pathogens like the 2009 pandemic H1N1 swine-origin influenza virus (SoIV). Therefore, improved influenza vaccines for pigs are required. Recently, we demonstrated that single intranasal immunization with a hemagglutinin (HA)-expressing pseudorabies virus recombinant of vaccine strain Bartha (PrV-Ba) protected pigs from H1N1 SoIV challenge (Klingbeil et al., 2014). Now we investigated enhancement of efficacy by prime-boost vaccination and/or intramuscular administration. Furthermore, a novel PrV-Ba recombinant expressing codon-optimized N1 neuraminidase (NA) was included. In vitro replication of this virus was only slightly affected compared to parental virus. Unlike HA, the abundantly expressed NA was efficiently incorporated into PrV particles. Immunization of pigs with the two PrV recombinants, either singly or in combination, induced B cell proliferation and the expected SoIV-specific antibodies, whose titers increased substantially after boost vaccination. After immunization of animals with either PrV recombinant H1N1 SoIV challenge virus replication was significantly reduced compared to PrV-Ba vaccinated or naïve controls. Protective efficacy of HA-expressing PrV was higher than of NA-expressing PrV, and not significantly enhanced by combination. Despite higher serum antibody titers obtained after intramuscular immunization, transmission of challenge virus to naïve contact animals was only prevented after intranasal prime-boost vaccination with HA-expressing PrV-Ba.

Structure-based functional analyses of domains II and III of pseudorabies virus glycoprotein H

Enveloped viruses utilize membrane fusion for entry into, and release from, host cells. For entry, members of the Herpesviridae require at least three envelope glycoproteins: the homotrimeric gB and a heterodimer of gH and gL. The crystal structures of three gH homologues, including pseudorabies virus (PrV) gH, revealed four conserved domains. Domain II contains a planar β-sheet ("fence") and a syntaxin-like bundle of three α-helices (SLB), similar to those found in eukaryotic fusion proteins, potentially executing an important role in gH function. To test this hypothesis, we introduced targeted mutations into the PrV gH gene, which either disrupt the helices of the SLB by introduction of proline residues or covalently join them by artificial intramolecular disulfide bonds between themselves, to the adjacent fence region, or to domain III. Disruption of either of the three α-helices of the SLB (A250P, V275P, V298P) severely affected gH function in in vitro fusion assays and replication of corresponding PrV mutants. Considerable defects in fusion activity of gH, as well as in penetration kinetics and cell-to-cell spread of PrV mutants, were also observed after disulfide linkage of two α-helices within the SLB (A284C-S291C) or between SLB and domain III (H251C-L432C), as well as by insertions of additional cysteine pairs linking fence, SLB, and domain III. In vitro fusion activity of mutated gH could be partly restored by reduction of the artificial disulfide bonds. Our results indicate that the structure and flexibility of the SLB are relevant for the function of PrV gH in membrane fusion.

IMPORTANCE

Mutational analysis based on crystal structures of proteins is a powerful tool to understand protein function. Here, we continued our study of pseudorabies virus gH, a part of the core fusion machinery of herpesviruses. We previously showed that the "flap" region in domain IV of PrV gH is important for its function. We now demonstrate that mutations within domain II that interfere with integrity or flexibility of a syntaxin-like three-helix bundle also significantly impair gH function during fusion. These studies provide important insights into the structural requirements of gH for function in fusion.

Immunization of pigs with an attenuated pseudorabies virus recombinant expressing the haemagglutinin of pandemic swine origin H1N1 influenza A virus

Pigs can be severely harmed by influenza, and represent important reservoir hosts, in which new human pathogens such as the recent pandemic swine-origin H1N1 influenza A virus can arise by mutation and reassortment of genome segments. To obtain novel, safe influenza vaccines for pigs, and to investigate the antigen-specific immune response, we modified an established live-virus vaccine against Aujeszky's disease of swine, pseudorabies virus (PrV) strain Bartha (PrV-Ba), to serve as vector for the expression of haemagglutinin (HA) of swine-origin H1N1 virus. To facilitate transgene insertion, the genome of PrV-Ba was cloned as a bacterial artificial chromosome. HA expression occurred under control of the human or murine cytomegalovirus immediate early promoters (P-HCMV, P-MCMV), but could be substantially enhanced by synthetic introns and adaptation of the codon usage to that of PrV. However, despite abundant expression, the heterologous glycoprotein was not detectably incorporated into mature PrV particles. Replication of HA-expressing PrV in cell culture was only slightly affected compared to that of the parental virus strain. A single immunization of pigs with the PrV vector expressing the codon-optimized HA gene under control of P-MCMV induced high levels of HA-specific antibodies. The vaccinated animals were protected from clinical signs after challenge with a related swine-origin H1N1 influenza A virus, and challenge virus shedding was significantly reduced.

A wide extent of inter-strain diversity in virulent and vaccine strains of alphaherpesviruses

Alphaherpesviruses are widespread in the human population, and include herpes simplex virus 1 (HSV-1) and 2, and varicella zoster virus (VZV). These viral pathogens cause epithelial lesions, and then infect the nervous system to cause lifelong latency, reactivation, and spread. A related veterinary herpesvirus, pseudorabies (PRV), causes similar disease in livestock that result in significant economic losses. Vaccines developed for VZV and PRV serve as useful models for the development of an HSV-1 vaccine. We present full genome sequence comparisons of the PRV vaccine strain Bartha, and two virulent PRV isolates, Kaplan and Becker. These genome sequences were determined by high-throughput sequencing and assembly, and present new insights into the attenuation of a mammalian alphaherpesvirus vaccine strain. We find many previously unknown coding differences between PRV Bartha and the virulent strains, including changes to the fusion proteins gH and gB, and over forty other viral proteins. Inter-strain variation in PRV protein sequences is much closer to levels previously observed for HSV-1 than for the highly stable VZV proteome. Almost 20% of the PRV genome contains tandem short sequence repeats (SSRs), a class of nucleic acids motifs whose length-variation has been associated with changes in DNA binding site efficiency, transcriptional regulation, and protein interactions. We find SSRs throughout the herpesvirus family, and provide the first global characterization of SSRs in viruses, both within and between strains. We find SSR length variation between different isolates of PRV and HSV-1, which may provide a new mechanism for phenotypic variation between strains. Finally, we detected a small number of polymorphic bases within each plaque-purified PRV strain, and we characterize the effect of passage and plaque-purification on these polymorphisms. These data add to growing evidence that even plaque-purified stocks of stable DNA viruses exhibit limited sequence heterogeneity, which likely seeds future strain evolution.

Alphaherpesviruses such as herpes simplex virus (HSV) are ubiquitous in the human population. HSV causes oral and genital lesions, and has co-morbidities in acquisition and spread of human immunodeficiency virus (HIV). The lack of a vaccine for HSV hinders medical progress for both of these infections. A related veterinary alphaherpesvirus, pseudorabies virus (PRV), has long served as a model for HSV vaccine development, because of their similar pathogenesis, neuronal spread, and infectious cycle. We present here the first full genome characterization of a live PRV vaccine strain, Bartha, and reveal a spectrum of unique mutations that are absent from two divergent wild-type PRV strains. These mutations can now be examined individually for their contribution to vaccine strain attenuation and for potential use in HSV vaccine development. These inter-strain comparisons also revealed an abundance of short repetitive elements in the PRV genome, a pattern which is repeated in other herpesvirus genomes and even the unrelated Mimivirus. We provide the first global characterization of repeats in viruses, comparing both their presence and their variation among different viral strains and species. Repetitive elements such as these have been shown to serve as hotspots of variation between individuals or strains of other organisms, generating adaptations or even disease states through changes in length of DNA-binding sites, protein folding motifs, and other structural elements. These data suggest for the first time that similar mechanisms could be widely distributed in viral biology as well.

Cleavage of Epstein-Barr virus glycoprotein B is required for full function in cell-cell fusion with both epithelial and B cells

Glycoprotein B (gB) homologues within the herpesvirus family display high sequence conservation, and a number of gB homologues contain a cleavage motif R-X-K/R-R recognized by the cellular protease furin. Epstein-Barr virus (EBV) gB contains this motif and cleaved gB is found in EBV virions. To determine the functional significance of this cleavage motif in EBV gB, a deletion mutant (gB Deltafurin) was created lacking the motif. This cleavage mutant was expressed well in cell culture but was not cleaved. Experiments examining gB Deltafurin in a cell-fusion assay revealed that fusion was reduced by 52 % in epithelial and 28 % in B cells when compared with wild-type EBV gB. This decrease in cell-cell fusion is similar to that observed with multiple alphaherpesvirus gB cleavage mutants and supports a conserved function for cleaved gB.

Proteolytic cleavage of glycoprotein B is dispensable for in vitro replication, but required for syncytium formation of pseudorabies virus

Glycoprotein B (gB) is the most conserved glycoprotein among herpesviruses and it plays important roles in virus infectivity. In most herpesviruses, including pseudorabies virus (PRV), gB is cleaved by a cellular protease into two disulfide-linked subunits. In the present study, I found that the PRV gB generated in human colon carcinoma LoVo cells, which lack the ubiquitous protease furin, remained in the uncleaved form and the virus replicated in these cells without cell fusion. The uncleaved gB was converted into its subunits after furin digestion. The virus also replicated in Madin-Darby bovine kidney cells without cell fusion in the presence of a furin inhibitor, whereas distinct syncytia were formed in the absence of the inhibitor. LoVo cells constitutively expressing furin showed cell fusion when they were infected with the virus. Penetration kinetics assays revealed that the virus carrying uncleaved gB penetrated the cells at the same rate as the virus carrying cleaved gB. These results indicate that PRV gB is cleaved by furin and that the cleavage is dispensable for virus replication in vitro. Furthermore, gB cleavage is involved in syncytium formation but not in penetration kinetics, suggesting that different mechanisms operate between cell-cell fusion and virus-cell fusion by PRV.

Intracellular localization of varicella-zoster virus ORF39 protein and its functional relationship to glycoprotein K

Varicella-zoster virus (VZV) encodes two multiply inserted membrane proteins, open reading frame (ORF) 39 protein (ORF39p) and glycoprotein K (gK). The HSV-1 homologs of these proteins are believed to act in conjunction with each other during viral egress and cell-cell fusion, and they directly influence each other's intracellular trafficking. However, ORF39p and VZV gK have received very limited study largely due to difficulties in producing antibodies to these highly hydrophobic proteins. To overcome this obstacle, we introduced epitope tags into both ORF39p and gK and examined their intracellular distributions in transfected and infected cells. Our data demonstrate that both ORF39p and gK accumulate predominately in the ER of cultured cells when expressed in the absence of other VZV proteins or when coexpressed in isolation from other VZV proteins. Therefore, the transport of VZV ORF39p and gK does not exhibit the functional interdependence seen in their HSV-1 homologs. However, during infection, the primary distributions of ORF39p and gK shift from the ER to the Golgi, and they are also found in the plasma membrane indicating that their intracellular trafficking during infection depends on other VZV-encoded proteins. During infection, ORF39p and gK tightly colocalize with VZV envelope glycoproteins B, E and H; however, the coexpression of ORF39p or gK with other individual viral glycoproteins is insufficient to alter the transport of either ORF39p or gK.

Herpes simplex virus type 1 glycoprotein K and the UL20 protein are interdependent for intracellular trafficking and trans-Golgi network localization

Final envelopment of the cytoplasmic herpes simplex virus type 1 (HSV-1) nucleocapsid is thought to occur by budding into trans-Golgi network (TGN)-derived membranes. The highly membrane-associated proteins UL20p and glycoprotein K (gK) are required for cytoplasmic envelopment at the TGN and virion transport from the TGN to extracellular spaces. Furthermore, the UL20 protein is required for intracellular transport and cell surface expression of gK. Independently expressed gK or UL20p via transient expression in Vero cells failed to be transported from the endoplasmic reticulum (ER). Similarly, infection of Vero cells with either gK-null or UL20-null viruses resulted in ER entrapment of UL20p or gK, respectively. In HSV-1 wild-type virus infections and to a lesser extent in transient gK and UL20p coexpression experiments, both gK and UL20p localized to the Golgi apparatus. In wild-type, but not UL20-null, viral infections, gK was readily detected on cell surfaces. In contrast, transiently coexpressed gK and UL20p predominantly localized to the TGN and were not readily detected on cell surfaces. However, TGN-localized gK and UL20p originated from endocytosed gK and UL20p expressed at cell surfaces. Retention of UL20p to the ER through the addition of an ER retention motif forced total ER retention of gK, indicating that transport of gK is absolutely dependent on UL20p transport. In all experiments, gK and UL20p colocalized at intracellular sites, including the ER, Golgi, and TGN. These results are consistent with the hypothesis that gK and UL20p directly interact and that this interaction facilitates their TGN localization, an important prerequisite for cytoplasmic virion envelopment and egress.

The attenuated pseudorabies virus strain Bartha fails to package the tegument proteins Us3 and VP22

The Bartha strain of pseudorabies virus has several recognized mutations, including a deletion in the unique short region encompassing the glycoprotein I (gI), gE, Us9, and Us2 genes and point mutations in the gC, gM, and UL21 genes. We have determined that Bartha has mutations in the serine/threonine kinase encoded by the Us3 gene relative to the wild-type Becker strain. Our analysis revealed that Becker virions contain the Us3 protein, whereas Bartha virions do not. To test whether the mutations in the Bartha Us3 protein were responsible for this observation, we constructed a recombinant Bartha strain, PRV632, which expresses the Becker Us3 protein. PRV632 failed to package Us3 into the tegument, indicating that mutations other than those in the Us3 primary amino acid sequence were responsible for the failure of Bartha to package its Us3 protein. A recombinant Becker strain, PRV634, which expresses the Bartha Us3 protein, was constructed to test whether it was capable of being packaged into virions. The Bartha Us3 protein was not incorporated into PRV634 virions efficiently, suggesting that the primary sequence of the Bartha Us3 protein affects packaging into the tegument. To determine whether the packaging of other tegument proteins was affected in the Bartha strain, we examined VP22. Whereas Becker packaged VP22 into virions, Bartha had a severe deficiency in VP22 incorporation. Analysis of VP22 expression in Bartha-infected cells revealed that Bartha VP22 had a slower mobility on sodium dodecyl sulfate-polyacrylamide gels, indicating either primary sequence differences and/or different posttranslational modifications relative to Becker VP22. Taken together, these data indicate that, while the primary sequence of the Us3 protein does affect its incorporation into the tegument, other factors are involved. Furthermore, our data suggest that one or more of the gI, gE, Us9, or Us2 genes influences the localization of the Us3 protein in infected cells, and this effect may be important for the proper incorporation of Us3 into virions.

Human cytomegalovirus pp28 (UL99) localizes to a cytoplasmic compartment which overlaps the endoplasmic reticulum-golgi-intermediate compartment

Although the assembly of herpesviruses has remained an active area of investigation, considerable controversy continues to surround the cellular location of tegument and envelope acquisition. This controversy is particularly evident when the proposed pathways for alpha- and beta-herpesvirus assembly are compared. We have approached this aspect of human cytomegalovirus (HCMV) assembly, specifically, envelopment, by investigating the intracellular trafficking of viral tegument proteins which localize in the cytoplasms of infected cells. In this study we have demonstrated that the virion tegument protein pp28 (UL99), a true late protein, was membrane associated as a result of myristoylation. A mutation in this protein which prevented incorporation of [(3)H]myristic acid also altered the detergent solubility and intracellular distribution of the protein when it was expressed in transfected cells. Using a panel of markers for intracellular compartments, we could localize the expression of wild-type pp28 to an intracellular compartment which colocalized with the endoplasmic reticulum-Golgi-intermediate compartment (ERGIC), a dynamic compartment of the secretory pathway which interfaces with both the ER and Golgi apparatus. The localization of this viral tegument protein within an early secretory compartment of the cell provided further evidence that the assembly of the HCMV tegument likely includes a cytoplasmic phase. Because pp28 has been shown to be localized to a cytoplasmic assembly compartment in HCMV-infected cells, our findings also suggested that viral tegument protein interactions within the secretory pathway may have an important role in the assembly of the virion.

Accumulation of virion tegument and envelope proteins in a stable cytoplasmic compartment during human cytomegalovirus replication: characterization of a potential site of virus assembly

The assembly of human cytomegalovirus (HCMV) is thought to be similar to that which has been proposed for alphaherpesviruses and involve envelopment of tegumented subviral particles at the nuclear membrane followed by export from the cell by a poorly defined pathway. However, several studies have shown that at least two tegument virion proteins remain in the cytoplasm during the HCMV replicative cycle, thereby suggesting that HCMV cannot acquire its final envelope at the nuclear envelope. We investigated the assembly of HCMV by determining the intracellular trafficking of the abundant tegument protein pp150 (UL32) in productively infected human fibroblasts. Our results indicated that pp150 remained within the cytoplasm throughout the replicative cycle of HCMV and accumulated in a stable, juxtanuclear structure late in infection. Image analysis using a variety of cell protein-specific antibodies indicated that the pp150-containing structure was not a component of the endoplasmic reticulum, (ER), ER-Golgi intermediate compartment, cis or medial Golgi, or lysosomes. Partial colocalization of the structure was noted with the trans-Golgi network, and it appeared to lie in close proximity to the microtubule organizing center. Two additional tegument proteins (pp28 and pp65) and three envelope glycoproteins (gB, gH, and gp65) localized in this same structure late infection. This compartment appeared to be relatively stable since pp150, pp65, and the processed form of gB could be coisolated following cell fractionation. Our findings indicated that pp150 was expressed exclusively within the cytoplasm throughout the infectious cycle of HCMV and that the accumulation of the pp150 in this cytoplasmic structure was accompanied by at least five other virion proteins. These results suggested the possibility that this virus-induced structure represented a cytoplasmic site of virus assembly.

Characterization of pseudorabies virus glycoprotein B expressed by canine herpesvirus

A recombinant canine herpesvirus (CHV) which expressed glycoprotein B (gB) of pseudorabies virus (PrV) was constructed. The antigenicity of the PrV gB expressed by the recombinant CHV is similar to that of the native PrV. The expressed PrV gB was shown to be transported to the surface of infected cells as judged by an indirected immunofluorescence test. Antibodies raised in mice immunized with the recombinant CHV neutralized the infectivity of PrV in vitro. It is known that the authentic PrV gB exists as a glycoprotein complex, which consists of gBa, gBb and gBc. In MDCK cells, PrV gB expressed by the recombinant CHV was processed like authentic PrV gB, suggesting that the cleavage mechanism of PrV gB depends on a functional cleavage domain from PrV gB gene and protease from infected cells.

Induction of antibodies in mice by a recombinant baculovirus expressing pseudorabies virus glycoprotein B in mammalian cells

The glycoprotein gB of pseudorabies virus (PrV) was expressed in various mammalian cells by a recombinant baculovirus carrying the PrV gB gene under the control of the CAG promoter. When the recombinant baculovirus was inoculated into the stable porcine kidney cell line CPK, expression of PrV gB was detected by immunofluorescent antibody analysis and a 155 kDa of protein, which has the same molecular mass as the native PrV gB, was detected by Western blotting. High levels of expression of PrV gB were observed in BHK-21, HmLu-1 and SK-H cell lines. Furthermore, anti-PrV gB-specific antibodies against PrV gB protein were detected by the enzyme-linked immunosorbent assay in mice inoculated the recombinant baculovirus. The recombinant baculovirus containing the PrV glycoprotein gB gene under the CAG promoter could be a candidate for a pseudorabies vaccine.

DNA sequence of the UL6 to UL20 genes of infectious laryngotracheitis virus and characterization of the UL10 gene product as a nonglycosylated and nonessential virion protein

The 24 kbp KpnI restriction fragment A from the unique long genome region of infectious laryngotracheitis virus (ILTV, gallid herpesvirus-1) has been sequenced. The analysed region contains 14 open reading frames sharing homology with conserved alphaherpesvirus genes. Arrangement of the UL6 to UL20 homologues of ILTV is almost identical to that found in the herpes simplex virus type 1 genome. As in other herpesviruses the UL15 gene consists of two exons and is expressed from a spliced mRNA. However, the UL16 gene, which is usually localized within the intron sequence of UL15, is not conserved at this position of the ILTV genome. Another unique feature is the absence of any putative N-glycosylation motifs within the deduced ILTV UL10 gene product, which is the homologue of the conserved herpesvirus glycoprotein M. After preparation of a monospecific antiserum, two distinct UL10 proteins with apparent molecular masses of 36 and 31 kDa were identified in ILTV-infected cells as well as in purified virions. None of these UL10 gene products is modified by N- or O-linked glycosylation. Isolation of a green fluorescent protein-expressing UL10 deletion mutant of ILTV revealed that this gene is not required for virus replication in cell culture.

Human herpesvirus-8 glycoprotein B interacts with Epstein-Barr virus (EBV) glycoprotein 110 but fails to complement the infectivity of EBV mutants

To characterize human herpesvirus 8 (HHV-8) gB, the open reading frame was PCR amplified from the HHV-8-infected cell line BCBL-1 and cloned into an expression vector. To facilitate detection of expressed HHV-8 gB, the cytoplasmic tail of the glycoprotein was tagged with the influenza hemagglutinin (HA) epitope. Expression of tagged HHV-8 gB (gB-HA), as well as the untagged form, was readily detected in CHO-K1 cells and several lymphoblastoid cell lines (LCLs). HHV-8 gB-HA was sensitive to endoglycosidase H treatment, and immunofluorescence revealed that HHV-8 gB-HA was detectable in the perinuclear region of CHO-K1 cells. These observations suggest that HHV-8 gB is not processed in the Golgi and localizes to the endoplasmic reticulum or nuclear membrane. Because both HHV-8 and EBV are gamma-herpesviruses, the ability of HHV-8 gB to interact with and functionally complement EBV gp110 was examined. HHV-8 gB-HA and EBV gp110 co-immunoprecipitated, indicating formation of hetero-oligomers. However, HHV-8 gB-HA and HHV-8 gB failed to restore the infectivity of gp110-negative EBV mutants. These findings indicate that although HHV-8 gB and EBV gp110 have similar patterns of intracellular localization and can interact, there is not sufficient functional homology to allow efficient complementation.

Glycoproteins M and N of pseudorabies virus form a disulfide-linked complex

Genes homologous to the herpes simplex virus UL49.5 open reading frame are conserved throughout the Herpesviridae. In the alphaherpesvirus pseudorabies virus (PrV), the UL49.5 product is an O-glycosylated structural protein of the viral envelope, glycoprotein N (gN) (A. Jöns, H. Granzow, R. Kuchling, and T. C. Mettenleiter, J. Virol. 70:1237-1241, 1996). For functional characterization of gN, a gN-negative PrV mutant, PrV-gNbeta, and the corresponding rescuant, PrV-gNbetaR, were constructed, gN-negative PrV was able to productively replicate on noncomplementing cells, and one-step growth in cell culture was only slightly reduced compared to that of wild-type PrV. However, penetration was significantly delayed. In indirect immunofluorescence assays with rabbit serum directed against baculovirus-expressed gN, specific staining of wild-type PrV-infected cells occurred only after permeabilization of cells, whereas live cells failed to react with the antiserum. This indicates the lack of surface accessibility of gN in the plasma membrane of a PrV-infected cell. Western blot analyses and radioimmunoprecipitation experiments under reducing and nonreducing conditions led to the discovery of a heteromeric complex composed of gM and gN. The complex was stable in the absence of 2-mercaptoethanol but dissociated after the addition of the reducing agent, indicating that the partners are linked by disulfide bonds. Finally, gN was absent from gM-negative PrV virions, whereas gM was readily detected in virions in the absence of gN. Thus, gM appears to be required for virion localization of gN.

Glycoprotein B of Aujeszky's disease virus: topographical epitope mapping and epitope-specific antibody response

A panel of 26 monoclonal antibodies (mAbs) against glycoprotein B (gB) of Aujeszky's disease (pseudorabies) virus (ADV), a glycoprotein complex consisting of three glycoproteins, gBa, gBb, and gBc, was produced by two research groups and was used for the topographical epitope mapping of gB. An epitope map was constructed in which the identified epitopes of gB were situated in 14 topologically distinct antigenic domains; ten antigenic domains represented by 22 mAbs were localized on gBc, while four antigenic domains represented by four mAbs resided on gBb of the gB complex. All the epitopes located on gBc appeared to be conformation-dependent, whereas all the epitopes on gBb were conformation-independent. The identified epitopes of gB were conserved among laboratory, vaccine and field ADV strains. Conformation-dependent epitopes were shown to contribute largely to the overall antibody response to gB in naturally infected swine and immunized mice. Moreover, it was found that most of the infected animals responded relatively weakly to the identified conformation-independent epitopes of gB, while a group of immunodominant epitopes that induced a strong antibody response was represented exclusively by conformation-dependent epitopes from different antigenic domains. The results clearly demonstrated that conformation-dependent epitopes of gBc play a crucial role in inducing the humoral immune response to gB of ADV during the natural infection of swine and immunization of mice. The application of mAbs of our panel as research and diagnostic tools is discussed.

Functional analysis of the transmembrane anchor region of bovine herpesvirus 1 glycoprotein gB

In herpesviruses, homologues of glycoprotein B (gB) are essential membrane proteins which are involved in fusion. However, there is no clear evidence regarding the location of the fusogenic domain on gB. By using bovine herpesvirus 1 (BHV-1) as a model, we studied the relationship between the structure and the fusogenic activity of gB. This was achieved by expressing genes of different gB derivatives containing specific truncations at the end of segments 2 or 3 of the transmembrane region in Madin-Darby bovine kidney cells under the control of the bovine heat-shock protein hsp70A gene promoter. All expressed gB products were structurally similar to authentic gB. One truncated form of gB, gBt, which contains residues 1-763, was efficiently secreted. However, gBtM (residues 1-807), which includes the first two segments at the carboxyl terminus, showed unstable retention on the cell surface, whereas gBtMA (residues 1 829), which contains all three membrane-spanning segments, was mostly intracellularly retained with some unstable surface anchorage. Another truncated gB, gBtDAF, which has gB residues 1-763 (gBt) and a human decay-accelerating factor (DAF) carboxyl tail, was also expressed. The DAF fragment provided a signal for the addition of a glycosyl phosphatidylinositol-based membrane anchor, which could target the gBt chimeric protein on the cell membrane. Immunofluorescence staining and pulse-chase kinetic studies support the theory that gBtM, gBtMA, and gBtDAF are retained on nuclear and cellular membranes via different segments of the transmembrane region or the DAF fragment, respectively. For the cells expressing gBt or gBtM, no cell fusion was observed, whereas cells expressing gBtMA clearly showed fusion. However, in gBtDAF cells, the overexpression and cellular accumulation of recombinant gB products did not cause fusion either, which supports our contention that the fusion phenomenon in gBtMA cells is caused by the fusogenic activity of the expressed gBtMA. With the help of sequence analysis, our results indicate that segment 2 of the transmembrane anchor region might be a fusogenic domain, whereas the real anchor is segment 3.

Immunobiology of pseudorabies (Aujeszky's disease)

Aujeszky's Disease (AD), a serious illness of pigs causing significant economic losses in the pig industry, is caused by Pseudorabies Virus (PrV). PrV belongs to the alphaherpesvirus subfamily of the herpesviruses with a double-stranded DNA genome in an enveloped capsid capable of encoding approximately 70 proteins. For disease control, vaccination with live and killed vaccines is performed. Recently, 'marked' vaccines have become available for use in eradication programs based on the differentiation between infected and vaccinated animals. PrV is also used as a viral vector for the development of multivalent vaccines. Despite the effectiveness of PrV vaccines, relatively little is known about the immune response against PrV infection. Several viral envelope glycoproteins have been shown to represent targets for antibody responses, and a number of isolated glycoproteins as well as genetically engineered proteins were able to elicit protective immunity. The nature of the cellular immune response is even less defined. Using viral mutants genetically engineered to lack specific antigens, it has been shown that glycoprotein C (gC) acts as a target for cytotoxic T-lymphocytes, and gB, gC, gD, and gH appear to be involved in stimulation of in vitro proliferation of PBMC from immune animals. In addition, gB and gC have been implicated in recognition of infected cells by lymphokine-activated killer (LAK) cells. In summary, the data indicate a prominent role for viral envelope glycoproteins in eliciting humoral and cellular immune responses in the animal host. A complicating factor is the ability of PrV to productively infect cells of the hematopoietic system, which may impair immune responses and might also play a role in persistent or latent infection.

The receptor-binding domain of pseudorabies virus glycoprotein gC is composed of multiple discrete units that are functionally redundant

Many herpesviruses attach to cells in a two-step process, using the glycoprotein gC family of homologs to bind the primary receptor, heparan sulfate (HS) proteoglycan, and glycoprotein gD homologs to bind an unknown secondary receptor. We have previously shown by deletion analysis that the amino-terminal one-third of gC from pseudorabies virus (PRV), a swine herpesvirus, includes at least the principal HS receptor-binding domain. This portion of PRV gC contains three discrete clusters of basic residues that exactly or nearly match proposed consensus sequences for heparin-binding domains (HBDs); four additional potential HBDs lie in the distal two-thirds of the glycoprotein. We now specifically implicate each of the three amino-terminal HBDs in virus attachment. Mutational analysis demonstrated that any one of the three HBDs could mediate efficient virus infectivity; HS-dependent PRV attachment to cells was eliminated only after all three amino-terminal HBDs were altered. Furthermore, the binding dysfunction was due to a disruption of the specific HBDs and not to total charge loss. Thus, unlike previously described viral receptor-binding domains, the PRV gC receptor-binding domain is composed of multiple, discrete units that can function independently of one another. These units may function redundantly either to increase binding affinity or perhaps to effectively increase the virus's host range.

Postoligomerization folding of human cytomegalovirus glycoprotein B: identification of folding intermediates and importance of disulfide bonding

Human cytomegalovirus glycoprotein B (gB or UL55) has been demonstrated to be a disulfide-linked homodimer within the envelope of mature virions. Previously, it has been shown that gB undergoes a rapid dimerization nearly coincident with its synthesis. Following dimerization, the molecule slowly folds into a form which can be transported from the endoplasmic reticulum. In this study we have examined the prolonged folding of gB by using a set of defined gB-reactive murine monoclonal antibodies and gB expressed as a recombinant protein in the absence of other human cytomegalovirus proteins. Our results have documented a folding pathway consistent with the relatively rapid dimerization of the translation product followed by delayed conversion into a fully folded molecule. Assembly of the dominant antigenic domain of gB, AD-1, preceded dimerization and folding of the molecule. The fully folded dimer was heat stable, but its conformation was altered by treatment with 2% sodium dodecyl sulfate (SDS), whereas an oligomeric folding intermediate was both heat and SDS stable. Postoligomerization disulfide bond formation could be demonstrated during folding of gB, suggesting that the formation of these covalent bonds could contribute to the prolonged folding of this glycoprotein.

Characterization of cell-binding properties of bovine herpesvirus 1 glycoproteins B, C, and D: identification of a dual cell-binding function of gB

Previous studies have suggested that the attachment of bovine herpesvirus 1 (BHV-1) to permissive cells is mediated by its major glycoproteins B (gB), C (gC), and D (gD). In order to gain further insight into the mechanism of the BHV-1 attachment process, we purified authentic gB, gC, and gD from BHV-1-infected cells and membrane anchor-truncated, soluble gB, gC, and gD from stably transfected cell lines by affinity chromatography and examined their cell-binding properties on Madin-Darby bovine kidney cells. All of the glycoproteins tested exhibited saturable binding to Madin-Darby bovine kidney cells. All of the glycoproteins tested exhibited saturable binding to Madin-Darby bovine kidney cells. Addition of exogenous heparin or treatment of cells with heparinase to remove cellular heparan sulfate (HS) prevented both gC and gB from binding to cells but had no effect on gD binding. An assessment of competition between gB, gC, and gD for cell binding revealed that gC was able to inhibit gB binding, whereas other combinations showed no effect. Cell-bound gC could be dissociated by heparin or heparinase treatment. The response of bound gB to heparin and heparinase treatments differed for the authentic and soluble forms; while soluble gB was susceptible to the treatment, a significant portion of cell-bound authentic gB was resistant to the treatment. Binding affinity analysis showed that soluble gB and both forms of gC and gD each had single binding kinetics with comparable dissociation constants (Kds), ranging from 1.5 x 10(-7) to 5.1 x 10(-7) M, whereas authentic gB exhibited dual binding kinetics with Kd1 = 5.2 x 10(-7) M and Kd2 = 4.1 x 10(-9) M. These results demonstrate that BHV-1 gC binds only to cellular HS, gD binds to a non-HS component, and gB initially binds to HS and then binds with high affinity to a non-HS receptor. Furthermore, we found that while authentic gB was able to inhibit viral plaque formation, soluble gB, which retains the HS-binding property but lacks the high-affinity binding property, was defective in this respect. These results suggest that the interaction between gB and its high-affinity receptor may play a critical role in the virus entry process.

Evidence for antiviral activity of glutathione: in vitro inhibition of herpes simplex virus type 1 replication

The role of glutathione (GSH) in the in vitro infection and replication of human herpes simplex virus type 1 (HSV-1) was investigated. Intracellular endogenous GSH levels dramatically decreased in the first 24 h after virus adsorption, starting immediately after virus challenge. The addition of exogenous GSH was not only able to restore its intracellular levels almost up to those found in uninfected cells, but also to inhibit > 99% the replication of HSV-1. This inhibition was concentration-dependent, not related to toxic effects on host cells and also maintained if the exogenous GSH was added as late as 24 h after virus challenge, i.e. when virus infection was fully established. Electron microscopic examination of HSV-1-infected cells showed that GSH dramatically reduced the number of extracellular and intracytoplasmic virus particles, whereas some complete nucleocapsids were still detected within the nuclei of GSH-treated cells. Consistent with this observation, immunoblot analysis showed that the expression of HSV-1-glycoprotein B, crucial for the release and the infectivity of virus particles, was significantly decreased. Data suggest that exogenous GSH inhibits the replication of HSV-1 by interfering with very late stages of the virus life cycle, without affecting cellular metabolism.

Characterization of pseudorabies virus glycoprotein gII expressed by recombinant baculovirus

The gene encoding the complete glycoprotein gII (homologue of gB of herpes simplex virus) of pseudorabies virus (PrV) was inserted into a baculovirus transfer vector, and a recombinant virus expressing gII was isolated. Three gII-related recombinant baculovirus-expressed peptides of 100, 60, and 45 to 50 kDa were detected with a polyclonal antibody against gII; these correspond to the authentic subunits gIIa and its cleavage products gIIb and gIIc, respectively. These proteins were subjected to N-terminal sequencing, and the results showed that the protease cleavage sites were identical to those of authentic gII. The expressed gII was shown to be transported to the surface of infected cells as judged by an indirect immunofluorescence test. Antibodies raised in mice immunized with the recombinant gII neutralized the infection of PrV in vitro. Mice inoculated with the recombinant gII were completely protected from lethal challenge with PrV.

A heterologous heparin-binding domain can promote functional attachment of a pseudorabies virus gC mutant to cell surfaces

The efficient attachment of pseudorabies virus to cultured cells is dependent on an electrostatic interaction between negatively charged cell surface heparan sulfate and the viral envelope glycoprotein gC. Deletion of the first one-third of gC severely impairs virus attachment, but the mutant virions are still capable of entering cells and establishing an infection via a gC-independent pathway. This region of gC contains three clusters of positively charged amino acids that exactly or nearly conform to proposed consensus motifs for heparin-binding domains (HBDs), and the loss of one or more of these potential HBDs may be responsible for the observed attachment defect. To more directly show the involvement of HBDs in pseudorabies virus attachment to cells, we replaced the first one-third of gC with a single, biochemically defined HBD from apolipoprotein B-100. On the basis of the results of attachment, penetration, and heparin competition assays, the heterologous HBD mediated heparan sulfate-dependent virus attachment, but not to fully wild-type levels. Although the intermediate phenotype is not understood, the apolipoprotein B-100 HBD may represent the smallest defined amino acid sequence that promotes functional herpesvirus attachment to cultured cells.

Mutational analysis of the proteolytic cleavage site of glycoprotein B (gB) of Marek's disease virus

The Marek's disease virus (MDV) glycoprotein B (gB) precursor, gp100, is proteolytically cleaved into two disulfide-linked subunits, gp60 and gp49. In the gB homologs of most other herpesviruses, a tetrapeptide, Arg-Xaa-Arg-Arg, is immediately upstream from the predicted cleavage site. We have investigated the specificity of the proteolytic cleavage in gp100 by introducing mutations within its predicted cleavage site (Arg-Leu-Arg-Arg) and expressed these mutants in recombinant fowlpox virus (FPV). The results show that all three Arg residues at the predicted cleavage site play an important role in the specific proteolytic cleavage of gp100. Furthermore, we demonstrated that the cleavage of gp100 is not necessary for transport of gB to the cell surface.

Proteolytic cleavage of bovine herpesvirus 1 (BHV-1) glycoprotein gB is not necessary for its function in BHV-1 or pseudorabies virus

Glycoprotein B homologs represent the most highly conserved group of herpesvirus glycoproteins. They exist in oligomeric forms based on a dimeric structure. Despite the high degree of sequence and structural conservation, differences in posttranslational processing are observed. Whereas gB of herpes simplex virus is not proteolytically processed after oligomerization, most other gB homologs are cleaved by a cellular protease into subunits that remain linked via disulfide bonds. Proteolytic cleavage is common for activation of viral fusion proteins, and it has been shown that herpesvirus gB homologs are essential for membrane fusion events during infection, e.g., virus penetration and direct viral cell-to-cell spread. To analyze the importance of proteolytic cleavage for the function of gB homologs, we isolated a mutant bovine herpesvirus 1 (BHV-1) expressing a BHV-1 gB that is no longer proteolytically processed because of a deletion of the proteolytic cleavage site and analyzed its phenotype in cell culture. We showed previously that BHV-1 gB can functionally substitute for the homologous glycoprotein in pseudorabies virus (PrV), based on the isolation of a PrV gB-negative PrV recombinant that expresses BHV-1 gB (A. Kopp and T. C. Mettenleiter, J. Virol, 66:2754-2762, 1992). Therefore, we also isolated a mutant PrV lacking PrV gB but expressing a noncleavable BHV-1 gB. Our results show that cleavage of BHV-1 gB is not essential for its function in either a BHV-1 or a PrV background. Compared with the PrV recombinant expressing cleavable BHV-1 gB, deletion of the cleavage site in the recombinant PrV did not detectably alter the viral phenotype, as analyzed by plaque assays, one-step growth kinetics, and penetration kinetics. In the BHV-1 mutant, the uncleaved BHV-1 gB was functionally equivalent to the wild-type protein with regard to penetration and showed only slightly delayed one-step growth kinetics compared with parental wild-type BHV-1. However, the resulting plaques were significantly smaller, indicating a role for proteolytic cleavage of BHV-1 gB in cell-to-cell spread of BHV-1.

Glycoprotein gB (gII) of pseudorabies virus can functionally substitute for glycoprotein gB in herpes simplex virus type 1

Glycoproteins homologous to gB of herpes simplex virus (HSV) constitute the most highly conserved family of herpesvirus glycoproteins. All gB homologs analyzed so far have been shown to play essential roles in penetration and direct viral cell-to-cell spread. In studies aimed at assessing whether the high sequence homology is also indicative of functional homology, we analyzed the ability of the gB-homologous glycoprotein (former designation gII) of pseudorabies virus (PrV) to complement a gB- HSV type 1 (HSV-1) mutant and vice versa. The results show that a PrV gB-expressing cell line phenotypically complemented the lethal defect in gB- HSV-1 whereas reciprocal complementation of a gB- PrV mutant by HSV-1 gB was not observed.

The amino-terminal one-third of pseudorabies virus glycoprotein gIII contains a functional attachment domain, but this domain is not required for the efficient penetration of Vero cells

We have examined the attachment and penetration phenotypes of several glycoprotein gIII mutants of pseudorabies virus (PRV) and have identified the first one-third of gIII as a region that mediates efficient virus attachment to PK15 and Vero cells. This portion of gIII, amino acids 25 through 157 of the wild-type sequence, appeared to support attachment by binding to heparinlike molecules on cell surfaces. Virions containing the first one-third of gIII were sensitive to heparin competition and showed greatly reduced infectivity on cells treated with heparinase. PRV virions lacking the first one-third of the mature glycoprotein exhibited only residual binding to cells if challenged by vigorous washing with phosphate-buffered saline at 2 h postinfection at 4 degrees C. This residual binding was resistant to heparin competition, and strains lacking the first one-third of gIII were able to infect cells treated with heparinase as effectively as untreated cells. When we determined the penetration phenotypes for each strain, we found that gIII-mediated virus attachment was necessary for timely penetration of PK15 cells but remarkably was not required for efficient virus penetration of Vero cells. Moreover, wild-type PRV was actually prohibited from rapid penetration of Vero cells by a gIII-heparan sulfate interaction. Our results indicate that initial virus binding to heparan sulfate via glycoprotein gIII is not required for efficient PRV infection of all cell types and may in fact be detrimental in some instances.

Oligomerization of the human cytomegalovirus major envelope glycoprotein complex gB (gp55-116)

The disulfide-linked glycoprotein B (gB; gp55-116) complex of human cytomegalovirus represents the most abundant and immunogenic component of the virion envelope. We have studied the oligomerization and transport of this molecule, using a series of murine monoclonal antibodies. Our results indicated that oligomerization of this molecule occurred shortly after its synthesis, with a half-time of maximal formation of approximately 25 min. The oligomeric form had an estimated mass of 340,000 Da and likely consisted of a homodimer of the gp55-116 complex. By using a conformation-specific monoclonal antibody, postoligomerization folding of this molecule was demonstrated. This event exhibited an unusually prolonged half-maximal time of approximately 160 min. Both oligomerization and folding occurred in the endoplasmic reticulum. Oligomerization and folding occurred in the absence of carbohydrate modifications, although likely at lower efficiency. Finally, the oligomeric and folded forms were shown to be transported to the surface of infected cells and infectious virions.

Structural, functional, and immunological characterization of bovine herpesvirus-1 glycoprotein gl expressed by recombinant baculovirus

The major glycoprotein complex gl of bovine herpesvirus-1 was expressed at high levels (36 micrograms per 1 x 10(6) cells) in insect cells using a recombinant baculovirus. The recombinant gl had an apparent molecular weight of 116 kDa and was partially cleaved to yield 63-kDa (glb) and 52-kDa (glc) subunits. This processing step was significantly less efficient in insect cells than the analogous step in mammalian cells, even though the cleavage sites of authentic and recombinant gl were shown to be identical. The oligosaccharide linkages were mostly endoglycosidase-H-sensitive, in contrast to those of authentic gl, which has mostly endoglycosidase-H-resistant linkages and an apparent molecular weight of 130/74/55 kDa. Despite the reduced cleavage and altered glycosylation, the recombinant glycoprotein was transported and expressed on the surface of infected insect cells. These surface molecules were biologically active as demonstrated by their ability to induce cell-cell fusion. Fusion was inhibited by three monoclonal antibodies specific for antigenic domains I and IV on gl. Domain I maps to the extracellular region of the carboxy terminal fragment glc and domain IV to the very amino terminus of the glb fragment, indicating that domains mapping in two distinct regions of gl function in cell fusion. Monoclonal antibodies specific for eight different epitopes recognized recombinant gl, indicating that the antigenic characteristics of the recombinant and authentic glycoproteins are similar. In addition, the recombinant gl was as immunogenic as the authentic gl, resulting in the induction of gl-specific antibodies in cattle.

Expression of the Marek's disease virus homolog of herpes simplex virus glycoprotein B in Escherichia coli and its identification as B antigen

Marek's disease (MD) is an oncogenic disease of chickens caused by MD virus (MDV). Among the major glycoproteins found in MDV-infected cells are gp100, gp60, and gp49, detected by immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis with antisera previously shown to be reactive with B antigen in immunodiffusion analysis. Following treatment with tunicamycin (TM), an inhibitor of N-linked glycosylation, the same sera were reported to detect two molecules called pr88 and pr44. However, the gene encoding B antigen was not unequivocally identified. Recently, an MDV homolog of the gene encoding herpes simplex virus glycoprotein B (gB) was identified and sequenced (L. J. N. Ross, M. Sanderson, S. D. Scott, M. M. Binns, T. Doel, and B. Milne, J. Gen. Virol. 70:1789-1804, 1989). To determine whether the MDV gB homolog gene might encode the B antigen, antisera against trpE fusion proteins of the MDV gB homolog (trpE-MDV-gB) were prepared. These antisera immunoprecipitated gp100, gp60, gp49, and a 92-kDa precursor polypeptide (pr88, now designated 92-kDa pr88, in the presence of TM) from MDV-infected cell lysates. On the basis of size comparison, trpE-MDV-gB competition and blocking assays, and the fact that gp100, gp60, gp49, and 92-kDa pr88 could be detected in MDV-infected cells with antisera specific to both MDV B antigen and the gB homolog, it was concluded that (i) the MDV gB homolog gene encodes MDV B antigen and (ii) 92-kDa pr88 is the primary precursor polypeptide. The antisera against trpE-MDV-gB also contained antibody reactive with the herpesvirus of turkey gB homolog, consistent with the known antigenic relatedness between the MDV and herpesvirus of turkey B antigens. TM inhibition data and results from pulse-chase analysis with MDV-infected cells show that MDV gB homolog processing involves cotranslational glycosylation of 92-kDa pr88 to form gp100, which is then cleaved to form gp60 and gp49, the N- and C-terminal halves, respectively, of gp100. This processing pathway is consistent with those of other gB homologs, further supporting the gene identification described above. The conclusions of this study will facilitate future research on the immunobiology of MD, especially studies on the mechanism of immunoprotection.

A cellular function is required for pseudorabies virus envelope glycoprotein processing and virus egress

The mouse L-cell mutant gro29 is defective for egress of herpes simplex virus type 1 (HSV-1) virions and is significantly reduced in HSV-1 glycoprotein export (B. W. Banfield and F. Tufaro, J. Virol. 64:5716-5729, 1990). In this report, we demonstrate that pseudorabies virus (PRV), a distantly related alphaherpesvirus, shows a distinctive set of defects after infection of gro29 cells. Specifically, we identify defects in the rate and extent of viral glycoprotein export, infectious particle formation, plaque formation, and virus egress. The initial rate of viral glycoprotein synthesis was unaffected in gro29 cells, but the extent of export from the endoplasmic reticulum to the Golgi apparatus was impaired and export through the Golgi apparatus became essentially blocked late in infection. Moreover, by using a secreted variant of a viral membrane protein, we found that export from the Golgi apparatus out of the cell was also defective in gro29 cells. PRV does not form plaques on gro29 monolayers. A low level of infectious virus is formed and released early after infection, but further virus egress is blocked. Taken together, these observations suggest that the gro29 phenotype involves either multiple proteins or a single protein used at multiple steps in viral glycoprotein export and virus egress from cells. Moreover, this host cell protein is required by both HSV and PRV for efficient propagation in infected cells.

Protection of mice and swine from pseudorabies virus conferred by vaccinia virus-based recombinants

Glycoproteins gp50, gII, and gIII of pseudorabies virus (PRV) were expressed either individually or in combination by vaccinia virus recombinants. In vitro analysis by immunoprecipitation and immunofluorescence demonstrated the expression of a gII protein of approximately 120 kDa that was proteolytically processed to the gIIb (67- to 74-kDa) and gIIc (58-kDa) mature protein species similar to those observed in PRV-infected cells. Additionally, the proper expression of the 90-kDa gIII and 50-kDa gp50 was observed. All three of these PRV-derived glycoproteins were detectable on the surface of vaccinia virus-PRV recombinant-infected cells. In vivo, mice were protected against a virulent PRV challenge after immunization with the PRV glycoprotein-expressing vaccinia virus recombinants. The coexpression of gII and gIII by a single vaccinia virus recombinant resulted in a significantly reduced vaccination dose required to protect mice against PRV challenge. Inoculation of piglets with the various vaccinia virus-PRV glycoprotein recombinants also resulted in protection against virulent PRV challenge as measured by weight gain. The simultaneous expression of gII and gp50 in swine resulted in a significantly enhanced level of protection as evaluated by weight evolution following challenge with live PRV.

Pseudorabies virus envelope glycoproteins gp50 and gII are essential for virus penetration, but only gII is involved in membrane fusion

To investigate the function of the envelope glycoproteins gp50 and gII of pseudorabies virus in the entry of the virus into cells, we used linker insertion mutagenesis to construct mutant viruses that are unable to express these proteins. In contrast to gD mutants of herpes simplex virus, gp50 mutants, isolated from complementing cells, were able to form plaques on noncomplementing cells. However, progeny virus released from these cells was noninfectious, although the virus was able to adsorb to cells. Thus, the virus requires gp50 to penetrate cells but does not require it in order to spread by cell fusion. This finding indicates that fusion of the virus envelope with the cell membrane is not identical to fusion of the cell membranes of infected and uninfected cells. In contrast to the gp50 mutants, the gII mutant was unable to produce plaques on noncomplementing cells. Examination by electron microscopy of cells infected by the gII mutant revealed that enveloped virus particles accumulated between the inner and outer nuclear membranes. Few noninfectious virus particles were released from the cell, and infected cells did not fuse with uninfected cells. These observations indicate that gII is involved in several membrane fusion events, such as (i) fusion of the viral envelope with the cell membrane during penetration, (ii) fusion of enveloped virus particles with the outer nuclear membrane during the release of nucleocapsids into the cytoplasm, and (iii) fusion of the cell membranes of infected and uninfected cells.

Mutations in the C-terminal hydrophobic domain of pseudorabies virus gIII affect both membrane anchoring and protein export

The transmembrane and anchor region of pseudorabies virus gIII is postulated to be in the 35 hydrophobic amino acids (residues 436 to 470) found near the carboxy terminus of the 479-amino-acid envelope protein. In this study, we used a genetic approach to localize the functional gIII membrane anchor between amino acids 443 and 466. Mutant gIII proteins lacking the membrane anchor were not associated with virus particles, indicating that membrane retention is a prerequisite for virion localization. Unexpectedly, the specific hydrophobic gIII sequence defined by these deletions was not required for membrane anchor function since the entire region could be replaced with leucine residues without affecting gIII membrane retention, export, or virion localization. The hydrophobic region appears to encode more than the membrane anchor domain since both efficiency of posttranslational processing and localization to virions are affected by mutations in this region. We speculate that the composition of the hydrophobic domain influences the overall conformation of gIII, which in turn effects the efficiency of gIII export and processing. The virion localization phenotype is probably indirect and reflects the efficiency of protein processing. This conclusion provides insight into the mechanism of glycoprotein incorporation into virions.

Identification of the infectious laryngotracheitis virus glycoprotein gB gene by the polymerase chain reaction

The infectious laryngotracheitis virus (ILTV) homologue of the herpes simplex virus type 1 (HSV-1) glycoprotein B (gB) gene was identified by PCR amplification of genomic ILTV DNA. A 488-bp amplified DNA fragment was used to identify and clone two adjacent PstI fragments from genomic ILTV DNA. Sequence analysis of the region surrounding the amplified fragment identified a 2619-bp open reading frame that has 39% homology with both the nucleotide and amino-acid sequences of the HSV-1 gB gene. Northern blot analysis using a portion of the open reading frame as a probe identified a 2.7-kb RNA transcript in ILTV-infected chicken embryo liver cells. Analysis of the predicted amino acid sequence of the ILTV protein indicated that it shares structural features with the gB glycoproteins of other herpesviruses.

Analysis of the intracellular maturation of the herpes simplex virus type 1 glycoprotein gH in infected and transfected cells

We have expressed the HSV-1 glycoprotein, gH, in transiently transfected COS-1 cells. The expressed protein was retained intracellularly, contained unprocessed carbohydrate, and was unrecognized by the monoclonal antibody, LP11. In addition, the protein was aggregated. These properties suggest that unlike other HSV glycoproteins, gH is misfolded in transfected cells. Pulse-chase studies of HSV-1-infected cells indicate that the kinetics of processing of gH are comparable to those of gB, gC, and gD. Rescue studies suggest that gH may interact with another protein during maturation in infected cells. However, we were unable to detect any stable interaction, although analysis of gH on neutral sucrose gradients shortly after synthesis indicated a possible transient association with a high molecular weight molecule or complex. The processing and cell surface expression of gH were also analyzed in HSV-1 virus mutants lacking gB, gC, or gD. Our results indicate that the maturation and cell surface transport of gH did not require the presence of these HSV-1 glycoproteins. In addition, three truncation mutants were constructed by linker insertion mutagenesis. Each of the three truncated proteins was synthesized, but the proteins were aggregated, contained only endo H-sensitive carbohydrate, and none were secreted.

Antigenic variation of pseudorabies virus glycoproteins g II and g III demonstrated by neutralizing monoclonal antibodies

Antigenic variation of pseudorabies virus glycoprotein g II was demonstrated in field isolates by neutralizing monoclonal antibodies. Mutants capable of avoiding neutralization by the monoclonal antibodies specific for glycoproteins g II and g III could be obtained from the parent virus by mutagenesis.

Oligomer formation of the gB glycoprotein of herpes simplex virus type 1

Oligomer formation of the gB glycoprotein of herpes simplex virus type 1 was studied by sedimentation analysis of radioactively labeled infected cell and virion lysates. Fractions from sucrose gradients were precipitated with a pool of gB-specific monoclonal antibodies and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Pulse-labeled gB from infected cell was synthesized as monomers and converted to oligomers posttranslationally. The oligomers from infected cells and from virions sedimented as dimers, and there was no evidence of higher-molecular-weight forms. To identify amino acid sequences of gB that contribute to oligomer formation, pairs of mutant plasmids were transfected into Vero cells and superinfected with a gB-null mutant virus to stimulate plasmid-specified gene expression. Radioactively labeled lysates were precipitated with antibodies and examined by SDS-PAGE. Polypeptides from cotransfections were precipitated with an antibody that recognized amino acid sequences present in only one of the two polypeptides. A coprecipitated polypeptide lacking the antibody target epitope was presumed to contain the sequences necessary for oligomer formation. Using this technique, two noncontiguous sites for oligomer formation were detected. An upstream site was localized between residues 93 and 282, and a downstream site was localized between residues 596 and 711. Oligomer formation resulted from molecular interactions between two upstream sites, between two downstream sites, and between an upstream and a downstream site. A schematic diagram of a gB oligomer is presented that is consistent with these data.

Overexpression in bacterial and identification in infected cells of the pseudorabies virus protein homologous to herpes simplex virus type 1 ICP18.5

The ICP18.5 gene (UL28) of herpes simplex virus type 1 is a member of a well-conserved gene family among herpesviruses and is thought to play a role in localization of viral glycoproteins. We have cloned, sequenced, and expressed the entire pseudorabies virus (PRV) ICP18.5 open reading frame in Escherichia coli as a Cro-ICP18.5 fusion protein. Rabbit antiserum against Cro-ICP18.5 immunoprecipitated a 79-kDa protein from PRV-infected cells as well as a 79-kDa protein from in vitro translation of a T7 RNA polymerase transcript of the ICP18.5 gene. ICP18.5 could be detected in infected cells by 2 h postinfection. Analysis by indirect immunofluorescence demonstrated that ICP18.5 became associated with the nucleus. Subcellular fractionation confirmed that ICP18.5 synthesized during a pulse-chase experiment appeared in the nuclear fraction with time and was stable for at least 2.5 h after synthesis. Pulse-chase analysis revealed that ICP18.5 was synthesized as a monomer during a 2-min pulse labeling but formed faster sedimenting complexes which were sensitive to sodium dodecyl sulfate (SDS) treatment. The majority of ICP18.5 appeared in complexes with an antigenically unrelated 70-kDa protein. Immunoblot analysis of total infected-cell extracts using polyvalent anti-ICP18.5 serum demonstrated that a 74-kDa cellular protein in addition to the 79-kDa ICP18.5 was detected. This cellular protein was present at similar levels in uninfected cells and in PRV-infected cells at least 12 h into the infectious cycle.

Characterization and sequence analyses of antibody-selected antigenic variants of herpes simplex virus show a conformationally complex epitope on glycoprotein H

Thirteen antigenic variants of herpes simplex virus which were resistant to neutralization by monoclonal antibody 52S or LP11 were isolated and characterized. The antibodies in the absence of complement potently neutralize infectivity of wild-type virus as well as inhibit the transfer of virus from infected to uninfected cells ("plaque inhibition") and decrease virus-induced cell fusion by syncytial strains. The first variant isolated arose in vivo. Of 66 type 1 isolates analyzed from typing studies of 100 clinical isolates, one was identified as resistant to neutralization by LP11 antibody. The glycoprotein H (gH) sequence was derived and compared with those of wild-type and syncytial laboratory strains SC16, strain 17, and HFEM. The sequences were highly conserved in contrast to the diversity observed between gH sequences from herpesviruses of different subgroups. Only four coding changes were present in any of the comparisons, and only one unique coding change was observed between the laboratory strains and the clinical isolate (Asp-168 to Gly). These sequences were compared with those of antigenic variants selected by antibody in tissue culture. Twelve variants were independently selected with antibody LP11 or 52S from parent strain SC16 or HFEM. For each variant, the gH nucleotide sequence was derived and a point mutation was identified giving rise to a single amino acid substitution. The LP11-resistant viruses encoded gH sequences with amino acid substitutions at sites distributed over one-half of the gH external domain, Glu-86, Asp-168, or Arg-329, while the 52S-resistant mutant viruses had substitutions at adjacent positions Ser-536 and Ala-537. One LP11 mutant virus had a point mutation in the gH gene that was identical to that of the clinical isolate, giving rise to a substitution of Asp-168 with Gly. Both LP11 and 52S appeared to recognize distinct gH epitopes as mutant virus resistant to neutralization and immunoprecipitation with LP11 remained sensitive to 52S and the converse was shown for the 52S-resistant mutant virus. This is consistent with previous studies which showed that while the 52S epitope could be formed in the absence of other virus products, virus gene expression was required for stable presentation of the LP11 epitope, and for transport of gH to the cell surface (Gompels and Minson, J. Virol. 63:4744-4755, 1989). All mutant viruses produced numbers of infectious particles that were similar to those produced by the wild-type virus, with the exception of one variant which produced lower yields.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of brefeldin A on alphaherpesvirus membrane protein glycosylation and virus egress

In this work we used brefeldin A (BFA), a specific inhibitor of export to the Golgi apparatus, to study pseudorabies virus viral glycoprotein processing and virus egress. BFA had little effect on initial synthesis and cotranslational modification of viral glycoproteins in the endoplasmic reticulum (ER), but it disrupted subsequent glycoprotein maturation and export. Additionally, single-step growth experiments demonstrated that after the addition of BFA, accumulation of infectious virus stopped abruptly. BFA interruption of virus egress was reversible. Electron microscopic analysis of infected cells demonstrated BFA-induced disappearance of the Golgi apparatus accompanied by a dramatic accumulation of enveloped virions between the inner and outer nuclear membranes and also in the ER. Large numbers of envelope-free capsids were also present in the cytoplasm of all samples. In control samples, these capsids were preferentially associated with the forming face of Golgi bodies and acquired a membrane envelope derived from the trans-cisternae. Our results are consistent with a multistep pathway for envelopment of pseudorabies virus that involves initial acquisition of a membrane by budding of capsids through the inner leaf of the nuclear envelope followed by deenvelopment and release of these capsids from the ER into the cytoplasm in proximity to the trans-Golgi. The released capsids then acquire a bilaminar double envelope containing mature viral glycoproteins at the trans-Golgi. The resulting double-membraned virus is transported to the plasma membrane, where membrane fusion releases a mature, enveloped virus particle from the cell.

Pseudorabies virus mutants lacking the essential glycoprotein gII can be complemented by glycoprotein gI of bovine herpesvirus 1

The genome of pseudorabies virus (PrV) encodes at least seven glycoproteins. The glycoprotein complex gII consists of three related polypeptides, two of them derived by proteolytic cleavage from a common precursor and linked via disulfide bonds. It is homologous to herpes simplex virus (HSV) gB and is therefore thought to be essential for PrV replication, as is gB for HSV replication. To isolate PrV mutants deficient in gII expression, we established cell lines that stably carry the PrV gII gene. Line N7, of Vero cell origin, contains the gII gene under its own promoter and expresses gII after transactivation by herpesviral functions after infection. MDBK-derived line MT3 contains the gII gene under control of the mouse metallothionein promoter. However, it has essentially lost inducibility and constitutively produces high amounts of correctly processed glycoprotein gII. We used a beta-galactosidase expression cassette inserted into a partially deleted cloned copy of the gII gene for cotransfection with PrV DNA. gII- PrV mutants were isolated from viral progeny by taking advantage of their blue-plaque phenotype when incubated under an agarose overlay containing a chromogenic substrate. Analysis of these mutants proved that gII is indeed essential for PrV replication, since the gII- mutants grew normally on gII-complementing cells but were unable to produce plaques on noncomplementing cells. Surprisingly the PrV gII- mutants were also able to grow on a cell line constitutively expressing the gB-homologous glycoprotein gI from bovine herpesvirus 1 (BHV-1) to the same extent as on cells expressing PrV gII. gII- PrV propagated on cells expressing BHV-1 gI became susceptible to neutralization by anti-BHV-1 gI monoclonal antibodies. We also found that BHV-1 gI is present in the envelope of purified gII- pseudorabies virions grown on cells expressing BHV-1 gI, as judged by radioimmunoprecipitation and immunoelectron microscopy. These results prove that BHV-1 gI is integrated into the PrV envelope and can functionally replace glycoprotein gII of PrV.

Synthesis and processing of the major envelope glycoprotein of murine cytomegalovirus

We have characterized the synthesis and processing pathway of the major envelope glycoprotein complex of murine cytomegalovirus (gp52/105/150). We have demonstrated that it belongs to the "late" kinetic class of MCMV proteins, and is initially synthesized as a 128K glycoprotein (gp128) which contains N-linked, high-mannose type oligosaccharide chains and is phosphorylated predominantly at serine residues. A fraction of the gp128 molecules also contains O-linked GalNAc residues. The majority of the gp128 molecules appears to be retained in the endoplasmic reticulum as evidenced by their sensitivity to endoglycosidase H digestion. The formation of disulfide linkages and dimerization allow the transport of gp128 to the Golgi compartments where modification of N-linked carbohydrate structures and extension of O-linked oligosaccharide chains take place, cumulating in the appearance of the mature gp150. The final processing step involves the cleavage of gp150 into gp52 and gp105. By blocking the transport of the glycoprotein precursor to the trans-Golgi compartments with the ionophore monensin, the cleavage process is inhibited, suggesting that the trans-Golgi compartment is the site where gp150 is cleaved. However, the cleavage process is incomplete, resulting in the formation of multiple disulfide-linked complexes made up of different combinations of gp52, gp105, and gp150. Therefore, the processing of the major envelope glycoprotein complexes of murine cytomegalovirus resembles that of the gcl/gp55-116 complex of human cytomegalovirus.