Synthesis and processing of the equine herpesvirus 1 glycoprotein M

Human herpesvirus 6 glycoprotein M is essential for virus growth and requires glycoprotein N for its maturation

Human herpesvirus 6 (HHV-6) is a T-lymphotropic virus belonging to the betaherpesvirus family. Several HHV-6-encoded glycoproteins are required for cell entry and virion maturation. Glycoprotein M (gM) is conserved among all herpesviruses, and therefore thought to have important functions; however, the HHV-6 g has not been characterized. Here, we examined the expression of HHV-6 g, and examined its function in viral replication, using a mutant and revertant gM. HHV-6 g was expressed on virions as a glycoprotein modified with complex N-linked oligosaccharides. As in other herpesviruses, HHV-6 g formed a complex with glycoprotein N (gN), and was transported from the endoplasmic reticulum to the trans-Golgi network only when part of this complex. Finally, a gM mutant virus in which the gM start codon was destroyed was not reconstituted, although its revertant was, indicating that HHV-6 g is essential for virus production, unlike the gM of alphaherpesviruses.

In vitro and in vivo characterization of equine herpesvirus type 1 (EHV-1) mutants devoid of the viral chemokine-binding glycoprotein G (gG)

Glycoprotein G (gG) of equine herpesvirus type 1 (EHV-1), a structural component of virions and secreted from virus-infected cells, was shown to bind to a variety of different chemokines and as such might be involved in immune modulation. Little is known, however, about its role in the replication cycle and infection of EHV-1 in vivo. Here we report on the function of gG in context of virus infection in vitro and in vivo. A gG deletion mutant of pathogenic EHV-1 strain RacL11 (vL11DeltagG) was constructed and analyzed. Deletion of gG had virtually no effect on the growth properties of vL11DeltagG in cell culture when compared to parental virus or a rescuant virus vL11DeltagGR, respectively, and virus titers and plaque formation were unaffected in the absence of the glycoprotein. Similarly, in the murine model of EHV-1 infection, no significant differences in virulence between the gG deletion mutant and RacL11 or vL11DeltagGR were found at high doses of infection. However, infection of mice at lower doses revealed that the gG deletion mutant was able to replicate to higher titers in lungs of infected mice. Additionally, these mice lost significantly more weight than those infected with RacL11 and a more pronounced inflammatory response in lungs was observed. Therefore we concluded that deletion of gG in EHV-1 seems to lead to an exacerbation of respiratory disease in the mouse.

The carboxy-terminal domain of glycoprotein N of human cytomegalovirus is required for virion morphogenesis

Glycoproteins M and N (gM and gN, respectively) are among the few proteins that are conserved across the herpesvirus family. The function of the complex is largely unknown. Whereas deletion from most alphaherpesviruses has marginal effects on the replication of the respective viruses, both proteins are essential for replication of human cytomegalovirus (HCMV). We have constructed a series of mutants in gN to study the function of this protein. gN of HCMV is a type I glycoprotein containing a short carboxy-terminal domain of 14 amino acids, including two cysteine residues directly adjacent to the predicted transmembrane anchor at positions 125 and 126. Deletion of the entire carboxy-terminal domain as well as substitution with the corresponding region from alpha herpesviruses or mutations of both cysteine residues resulted in a replication-incompetent virus. Recombinant viruses containing point mutations of either cysteine residue could be generated. These viruses were profoundly defective for replication. Complex formation of the mutant gNs with gM and transport of the complex to the viral assembly compartment appeared unaltered compared to the wild type. However, in infected cells, large numbers of capsids accumulated in the cytoplasm that failed to acquire an envelope. Transiently expressed gN was shown to be modified by palmitic acid at both cysteine residues. In summary, our data suggest that the carboxy-terminal domain of gN plays a critical role in secondary envelopment of HCMV and that palmitoylation of gN appears to be essential for function in secondary envelopment of HCMV and virus replication.

The alpha-TIF (VP16) homologue (ETIF) of equine herpesvirus 1 is essential for secondary envelopment and virus egress

The equine herpesvirus 1 (EHV-1) alpha-trans-inducing factor homologue (ETIF; VP16-E) is a 60-kDa virion component encoded by gene 12 (ORF12) that transactivates the immediate-early gene promoter. Here we report on the function of EHV-1 ETIF in the context of viral infection. An ETIF-null mutant from EHV-1 strain RacL11 (vL11deltaETIF) was constructed and analyzed. After transfection of vL11deltaETIF DNA into RK13 cells, no infectious virus could be reconstituted, and only single infected cells or small foci containing up to eight infected cells were detected. In contrast, after transfection of vL11deltaETIF DNA into a complementing cell line, infectious virus could be recovered, indicating the requirement of ETIF for productive virus infection. The growth defect of vL11deltaETIF could largely be restored by propagation on the complementing cell line, and growth on the complementing cell line resulted in incorporation of ETIF in mature and secreted virions. Low- and high-multiplicity infections of RK13 cells with phenotypically complemented vL11deltaETIF virus resulted in titers of virus progeny similar to those used for infection, suggesting that input ETIF from infection was recycled. Ultrastructural studies of vL11deltaETIF-infected cells demonstrated a marked defect in secondary envelopment at cytoplasmic membranes, resulting in very few enveloped virions in transport vesicles or extracellular space. Taken together, our results demonstrate that ETIF has an essential function in the replication cycle of EHV-1, and its main role appears to be in secondary envelopment.

The nonessential UL49.5 gene of infectious laryngotracheitis virus encodes an O-glycosylated protein which forms a complex with the non-glycosylated UL10 gene product

The UL10 and UL49.5 genes of avian infectious laryngotracheitis virus (ILTV) encode putative envelope proteins which are conserved in Alpha, Beta, and Gammaherpesvirinae. Many of the corresponding gene products have been shown to be glycosylated and to form heterodimeric protein complexes with each other. Unlike the homologous gM proteins of other herpesviruses, the UL10 protein of ILTV is not detectably glycosylated [Fuchs, W., Mettenleiter, T.C., 1999. 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. J. Gen. Virol. 80, 2173-2182]. Using a monospecific antiserum, we now identified the UL49.5 gene product of ILTV as an O-glycosylated membrane protein (gN). Correct processing of gN was shown to depend on the presence of the UL10 protein. Both gN and UL10 could be co-immunoprecipitated from ILTV-infected cell lysates with antisera against either of the proteins, indicating stable protein-protein interactions. For functional analysis parts of the UL10 and UL49.5 open reading frames were deleted from the ILTV genome, and replaced by a beta-galactosidase expression cassette. The resulting virus mutants were isolated and propagated in non-complementing chicken cells, which demonstrated that the UL10 and UL49.5 genes are not essential for in vitro replication of ILTV.

Complex formation by glycoproteins M and N of human cytomegalovirus: structural and functional aspects

The genomes of herpesviruses contain a number of genes which are conserved throughout the family of Herpesviridae, indicating that the proteins may serve important functions in the replication of these viruses. Among these are several envelope glycoproteins, including glycoprotein M (gM) and gN, which form a complex that is covalently linked via disulfide bonds in some herpesviruses. However, deletion of gM and/or gN from most alphaherpesviruses has limited effects on replication of the respective viruses in vitro. In contrast, insertional inactivation of the gM gene of the betaherpesvirus human cytomegalovirus (HCMV) results in a replication-incompetent virus. We have started to analyze the structural and functional aspects of the interaction between gM and gN of HCMV. We show that large parts of gM are dispensable for the formation of a gM/gN complex that is transported to distal parts of the cellular secretory pathway. In addition, we demonstrate that the disulfide bond is between the cysteine at position 44 in gM and cysteine 90 in gN. However, disulfide linkage is not a prerequisite for modification and transport of the gM/gN complex. Moreover, mutant viruses that lack a disulfide bridge between gM and gN replicate with efficiencies similar to that of wild-type viruses.

The truncated form of glycoprotein gp2 of equine herpesvirus 1 (EHV-1) vaccine strain KyA is not functionally equivalent to full-length gp2 encoded by EHV-1 wild-type strain RacL11

Most equine herpesvirus 1 (EHV-1) strains, including the naturally occurring virulent RacL11 isolate, encode a large glycoprotein, gp2 (250 kDa), which is expressed from gene 71. Besides other alterations in the viral genome, the avirulent strain KyA harbors an in-frame deletion of 1,242 nucleotides in gene 71. To examine the contributions of gp2 variation to virus growth and virulence, mutant RacL11 and KyA viruses expressing full-length or truncated gp2 were generated. Western blot analyses demonstrated expression of a 250-kDa gp2 in cells infected with RacL11 virus or a mutant KyA virus harboring full-length gene 71, whereas a 75- to 80-kDa gp2 was detected in cells infected with KyA or mutant RacL11 virus expressing KyA gp2. The RacL11 gp2 precursor of 250 kDa in size and its truncated KyA counterpart of 80 kDa, as well as the 42-kDa carboxy-terminal gp2 subunit, were incorporated into virus particles. Absence of gp2 in RacL11 resulted in a 6-fold reduction of extracellular virus titers and a 13% reduction of plaque diameters, whereas gp2-negative KyA exhibited a 55% reduction in plaque diameter and a 51-fold decrease in extracellular virus titers. The massive growth defects of gp2-negative KyA could be restored by reinsertion of the truncated but not the full-length gp2 gene. The virulence of the generated gp2 mutant viruses was compared to the virulence of KyA and RacL11 in a murine infection model. RacL11 lacking gp2 was apathogenic for BALB/c mice, and insertion of the truncated KyA gp2 gene into RacL11 was unable to restore virulence. Similarly, replacement in the KyA genome of the truncated with the full-length RacL11 gene 71 did not result in the generation of virulent virus. From the results we conclude that full-length and truncated EHV-1 gp2 are not functionally equivalent and cannot compensate for the action of their homologues in allogeneic virus backgrounds.

The equine herpesvirus 1 UL34 gene product is involved in an early step in virus egress and can be efficiently replaced by a UL34-GFP fusion protein

The structure and function of the equine herpesvirus type 1 (EHV-1) UL34 homologous protein were characterized. A UL34 protein-specific antiserum reacted with an M(r)28,000 protein that could not be detected in purified extracellular virions. Confocal laser scanning microscopy demonstrated that UL34 reactivity mainly concentrated at the nuclear rim, which changed into a punctuate and filamentous pattern at late times after infection. These changes in UL34 distribution were especially prominent when analyzing the distribution of a GFP-UL34 fusion protein. A UL34-negative EHV-1 was generated by mutagenesis of a recently established BAC clone of EHV-1 strain RacH (pRacH). Release of extracellular infectious virus was severely impaired after infection of Rk13 cells with HDelta34. Electron microscopy revealed a virtual absence of virus particles in the cytoplasm of infected cells, whereas nucleocapsid formation and maturation within the nucleus appeared unaffected. A UL34-GFP fusion protein with GFP linked to the C-terminus of UL34 was able to complement for the UL34 deletion in trans, while a GFP-UL34-fusion protein with GFP linked to the N-terminus of UL34 was able to only partially restore virus growth. It was concluded that the EHV-1 UL34 product is essential for an early step in virus egress, i.e., release of capsids from infected-cell nuclei.

The gene 10 (UL49.5) product of equine herpesvirus 1 is necessary and sufficient for functional processing of glycoprotein M

The functional cooperation of equine herpesvirus 1 (EHV-1) glycoprotein M (gM) and the gene 10 (UL49.5) product was analyzed. Transient-transfection experiments using gM and UL49.5 expression plasmids as well as RK13 cell lines constitutively expressing UL49.5 (RK49.5) or gM (RKgM) demonstrated that the endo-beta-N-acetylglucosaminidase H (endo H)-resistant mature form of gM was detectable only after coexpression of the two proteins. Deletion of the EHV-1 UL49.5-homologous gene 10 in strain KyA resulted in a small-plaque phenotype and up to 190-fold-reduced virus titers. The growth defects of the mutant KyA Delta 49.5 virus, which were very similar to those of a gM-negative KyA virus, could be completely compensated for by growth of the mutant virus on RK49.5 cells or by repairing the deletion of gene 10 in the revertant virus KyA Delta 49.5R. Analysis of cells infected with the UL49.5-negative EHV-1 demonstrated that gM was not transported to the trans-Golgi network in the absence of the UL49.5 product. In contrast, gM was efficiently transported and processed to the endo H-resistant mature form in KyA Delta 49.5-infected RK49.5 cells. Furthermore, radioimmunoprecipitation experiments demonstrated that gM maturation was observed only if a 10,000-M(r) protein was coprecipitated with gM in KyA- or KyA Delta 49.5R-infected cells or virions. This protein was absent in cells infected with Ky Delta 49.5 or KyA Delta gM, suggesting that it was the EHV-1 UL49.5 product. Taken together, our results demonstrate that the expression of the EHV-1 UL49.5 product is necessary and sufficient for gM processing and that it is required for efficient virus replication.

Equine herpesvirus type 1 devoid of gM and gp2 is severely impaired in virus egress but not direct cell-to-cell spread

Experiments were conducted to analyze the effects of a simultaneous deletion of glycoprotein M (gM) and glycoprotein 2 (gp2) of equine herpesvirus type 1 (EHV-1). EHV-1 strain RacH was cloned as a bacterial artificial chromosome (pRacH) by homologous recombination of a mini F plasmid into the unique short region of the genome, thereby deleting gene 71 encoding gp2. Upon transfection of the pRacH DNA into rabbit kidney RK13 cells, virus plaques were visible from day 1 after transfection. The mutant RacH virus (H Delta gp2) reconstituted from pRacH lacked gene 71 and did not express gp2 as assayed by indirect immunofluorescence analysis using gp2-specific monoclonal antibodies. The H Delta gp2 virus exhibited 10-fold reduced extracellular titers and an approximately 10% reduction in mean plaque diameters when compared to parental or gp2-revertant virus. The gM open reading frame was deleted from pRacH by recE/T mediated mutagenesis in Escherichia coli. The gM-gp2 double negative virus mutant (H Delta gp2gM) did not express either of the deleted glycoproteins as demonstrated by indirect immunofluorescence analysis. The H Delta gp2gM virus exhibited a 200-fold reduction of end-point extracellular titers when compared to parental RacH virus, which could not be compensated for by growth of the mutant virus on gM-expressing cells. After restoration of the gM open reading frame, however, growth of the mutant virus was comparable to the H Delta gp2 virus. Plaque diameters of the gM-gp2 double-negative mutant were reduced by only 16% when compared to that of parental RacH virus. From the results it was concluded that the simultaneous absence of gM and gp2 had an additive effect on egress but not secondary envelopment or cell-to-cell spread of EHV-1.

Deletion of gene 52 encoding glycoprotein M of equine herpesvirus type 1 strain RacH results in increased immunogenicity

The immunogenicity of equine herpesvirus type 1 (EHV-1) strain RacH was compared to a RacH virus in which gene 52 encoding glycoprotein M (gM) was interrupted by insertion of LacZ (HDeltagM-Ins) and a RacH with 75% of gene 52 was deleted and replaced by LacZ (HDeltagM-HS). HDeltagM-Ins failed to produce full-length gM, but the carboxy-terminal portion was still expressed. No gM expression was detected in HDeltagM-HS-infected cells. Mice were immunised once with 1x10(3) to 1x10(5) plaque-forming units (PFU) of RacH or mutant viruses and challenged with virulent RacL11 virus 29 days later. A dose-dependence of protection was observed in RacH-immunised mice, and following immunisation with 1x10(4) or 1x10(3) PFU body weight losses and increased virus titres in lungs were observed after challenge infection. HDeltagM-HS-immunised mice were completely protected even after immunisation with 1x10(3) PFU. Mice immunised with 1x10(3) PFU of HDeltagM-Ins but not the higher doses showed signs of disease after challenge infection.

The equine herpesvirus 1 UL45 homolog encodes a glycosylated type II transmembrane protein and is involved in virus egress

Experiments to analyze the product of the equine herpesvirus type 1 (EHV-1) UL45 homolog were conducted. Using an antiserum generated against the carboxylterminal 114 amino acids of the EHV-1 UL45 protein, proteins of M(r) 32,000, 40,000, and 43,000 were detected specifically in EHV-1-infected cells. Neither form of the protein was located in purified virions of EHV-1 wild-type strain RacL22 or the modified live vaccine strain RacH, but UL45 was demonstrated to be expressed as a late (gamma-2) protein. Fractionation of infected cells and deglycosylation experiments demonstrated that the EHV-1 UL45 protein represents a type II membrane glycoprotein. Deletion of the UL45 gene in RacL22 and RacH (LDelta45 and HDelta45) showed that UL45 is nonessential for EHV-1 growth in vitro, but that deletion reduced the viruses' replication efficiency. A marked reduction of virus release was observed although no significant influence was noticed either on plaque size or on the syncytial phenotype of the EHV-1 strain RacH.

Equine herpesvirus 1 (EHV-1) glycoprotein M: effect of deletions of transmembrane domains

Equine herpesvirus 1 (EHV-1) recombinants that carry either a deletion of glycoprotein M (gM) or express mutant forms of gM were constructed. The recombinants were derived from strain Kentucky A (KyA), which also lacks genes encoding gE and gI. Plaques on RK13 cells induced by the gM-negative KyA were reduced in size by 80%, but plaque sizes were restored to wild-type levels on gM-expressing cells. Electron microscopic studies revealed a massive defect in virus release after the deletion of gM in the gE- and gI-negative KyA, which was caused by a block in secondary envelopment of virions at Golgi vesicles. Recombinant KyA expressing mutant gM with deletions of predicted transmembrane domains was generated and characterized. It was shown that mutant gM was expressed and formed dimeric and oligomeric structures. However, subcellular localization of mutant gM proteins differed from that of wild-type gM. Mutant glycoproteins were not transported to the Golgi network and consequently were not incorporated into the envelope of extracellular virions. Also, a small plaque phenotype of mutant viruses that was indistinguishable from that of the gM-negative KyA was observed. Plaque sizes of mutant viruses were restored to wild-type levels by plating onto RK13 cells constitutively expressing full-length EHV-1 gM, indicating that mutant proteins did not exert a transdominant negative effect on wild-type gM.

Complex formation by human cytomegalovirus glycoproteins M (gpUL100) and N (gpUL73)

The envelope glycoproteins of human cytomegalovirus (HCMV) virions are incompletely characterized. We have analyzed complex formation between glycoprotein M (gM or gpUL100) and a second glycoprotein. gM-homologous proteins are conserved throughout the herpesvirus family and represent type III membrane proteins containing multiple hydrophobic sequences. In extracellular HCMV particles, gM was found to be complexed through disulfide bonds to a second protein with an apparent molecular mass of 50 to 60 kDa. The 50- to 60-kDa protein was found to be derived from reading frame UL73 of HCMV strain AD169. UL73-homologous genes are also conserved within herpesviruses. When transiently expressed by itself, the UL73 gene product consisted of a protein of 18 kDa. However, in the presence of gM, the UL73 gene product was posttranslationally modified to the 50- to 60-kDa species. Thus, gM and the UL73 gene product, which represents the gN homolog of herpesviruses, form a disulfide-linked complex in HCMV virions. Transient expression of gM and gN followed by fluorescence imaging with monoclonal antibodies against either protein demonstrated that complex formation was required for transport of the proteins from the endoplasmic reticulum to the Golgi and trans-Golgi compartments. Finally, we tested the gM-gN complex for reactivity with sera from HCMV-seropositive donors. Whereas most sera failed to react with either gM or gN when expressed alone, 62% of sera were positive for the gM-gN complex. Because a murine monoclonal antibody reactive with gN in the gM-gN complex efficiently neutralizes infectious virus, the gM-gN complex may represent a major antigenic target of antiviral antibody responses.

Epstein-Barr virus that lacks glycoprotein gN is impaired in assembly and infection

The Epstein-Barr virus (EBV) glycoproteins N and M (gN and gM) are encoded by the BLRF1 and BBRF3 genes. To examine the function of the EBV gN-gM complex, recombinant virus was constructed in which the BLRF1 gene was interrupted with a neomycin resistance cassette. Recombinant virus lacked not only gN but also detectable gM. A significant proportion of the recombinant virus capsids remained associated with condensed chromatin in the nucleus of virus-producing cells, and cytoplasmic vesicles containing enveloped virus were scarce. Virus egress was impaired, and sedimentation analysis revealed that the majority of the virus that was released lacked a complete envelope. The small amount of virus that could bind to cells was also impaired in infectivity at a step following fusion. These data are consistent with the hypothesis that the predicted 78-amino-acid cytoplasmic tail of gM, which is highly charged and rich in prolines, interacts with the virion tegument. It is proposed that this interaction is important both for association of capsids with cell membrane to assemble and release enveloped particles and for dissociation of the capsid from the membrane of the newly infected cell on its way to the cell nucleus. The phenotype of EBV lacking the gN-gM complex is more striking than that of most alphaherpesviruses lacking the same complex but resembles in many respects the phenotype of pseudorabies virus lacking glycoproteins gM, gE, and gI. Since EBV does not encode homologs for gE and gI, this suggests that functions that may have some redundancy in alphaherpesviruses have been concentrated in fewer proteins in EBV.

Pseudorabies virus glycoprotein M inhibits membrane fusion

A transient transfection-fusion assay was established to investigate membrane fusion mediated by pseudorabies virus (PrV) glycoproteins. Plasmids expressing PrV glycoproteins under control of the immediate-early 1 promoter-enhancer of human cytomegalovirus were transfected into rabbit kidney cells, and the extent of cell fusion was quantitated 27 to 42 h after transfection. Cotransfection of plasmids encoding PrV glycoproteins B (gB), gD, gH, and gL resulted in formation of polykaryocytes, as has been shown for homologous proteins of herpes simplex virus type 1 (HSV-1) (A. Turner, B. Bruun, T. Minson, and H. Browne, J. Virol. 72:873-875, 1998). However, in contrast to HSV-1, fusion was also observed when the gD-encoding plasmid was omitted, which indicates that PrV gB, gH, and gL are sufficient to mediate fusion. Fusogenic activity was enhanced when a carboxy-terminally truncated version of gB (gB-008) lacking the C-terminal 29 amino acids was used instead of wild-type gB. With gB-008, only gH was required in addition for fusion. A very rapid and extended fusion was observed after cotransfection of plasmids encoding gB-008 and gDH, a hybrid protein consisting of the N-terminal 271 amino acids of gD fused to the 590 C-terminal amino acids of gH. This protein has been shown to substitute for gH, gD, and gL function in the respective viral mutants (B. G. Klupp and T. C. Mettenleiter, J. Virol. 73:3014-3022, 1999). Cotransfection of plasmids encoding PrV gC, gE, gI, gK, and UL20 with gB-008 and gDH had no effect on fusion. However, inclusion of a gM-expressing plasmid strongly reduced the extent of fusion. An inhibitory effect was also observed after inclusion of plasmids encoding gM homologs of equine herpesvirus 1 or infectious laryngotracheitis virus but only in conjunction with expression of the gM complex partner, the gN homolog. Inhibition by PrV gM was not limited to PrV glycoprotein-mediated fusion but also affected fusion induced by the F protein of bovine respiratory syncytial virus, indicating a general mechanism of fusion inhibition by gM.

Quantitation of virus-specific classes of antibodies following immunization of mice with attenuated equine herpesvirus 1 and viral glycoprotein D

The antibody responses of CBA/J mice infected intranasally (i.n.) with either the attenuated KyA strain or the pathogenic RacL11 strain of equine herpesvirus 1 (EHV-1) or immunized with recombinant glycoprotein D (rgD) were investigated using the ELISPOT assay to measure EHV-1-specific antibody-secreting cells (ASC) in the regional lymphoid tissue of the respiratory tract. IgG, IgA, and IgM ASC specific for EHV-1 were detected in the mediastinal lymph nodes (MLN) and lungs 2 weeks after i.n. infection with EHV-1 strain KyA or RacL11, or immunization with heat-killed KyA or rgD. EHV-1-specific ASC were present in the MLN and lungs at 4 and 8 weeks, but declined in frequency by fivefold in the lung at 8 weeks. However, i.n. immunized (2 x 10(6) pfu KyA or 50 microgram rgD/mouse) mice infected at 8 weeks with pathogenic EHV-1 RacL11 resisted challenge and showed eight- and tenfold increases in MLN ASC and lung ASC, respectively, by 3 days after challenge. In contrast to the intranasal route of immunization, intraperitoneal immunization yielded ASC frequencies in the MLN and lungs that were only slightly above those of nonimmunized control mice. These data indicate that immunization with infectious or heat-killed EHV-1 KyA, or rgD, induces significant levels of virus-specific ASC both in the MLN and lungs, a specific memory B-cell response, and long-term protective immunity. The finding that the numbers of ASC induced by the pathogenic strain versus the attenuated strain of EHV-1, which were virtually identical, indicated that the ability to generate a B-cell response is independent of and does not contribute to EHV-1 virulence.

Construction and characterization of an equine herpesvirus 1 glycoprotein C negative mutant

An equine herpesvirus 1 (EHV-1) strain RacL 11 mutant was constructed that carries the Escherichia coli LacZ gene instead of the open reading frame encoding glycoprotein C (gC). The engineered virus mutant (L11(delta)gC) lacked codons 46-440 of the 1404 bp gene. On rabbit kidney cell line Rk13 and equine dermal cell line Edmin337, the L11(delta)gC virus grew to titers which were reduced by approximately 5- to 10-fold compared with wild-type RacL11 virus or a repaired virus (R-L11(delta)gC). However, when L11(delta)gC growth properties were analyzed on primary equine cells a decrease of viral titers was observed such that extracellular L11(delta)gC titers were reduced by 48- to 210-fold compared with those of wild-type or repaired virus. Heparin sensitive and heparin resistant attachment was assessed by binding studies using radiolabeled virion preparations. These studies revealed that EHV-1 gC is important for heparin sensitive attachment to the target cell. Similar results were obtained when cellular glycosaminoglycan (GAG) synthesis was inhibited by chlorate treatment or when cells defective in GAG synthesis were used. L11(delta)gC also exhibited significantly delayed penetration kinetics on Rk13 and primary equine cells. Infection of mice with L11(delta)gC did not cause EHV-1-related disease, whereas mice infected with either RacL11 or R-L11(delta)gC exhibited massive bodyweight losses, high virus titers in the lungs, and viremia. Taken together, EHV-1 gC was shown to play important roles in the early steps of infection and in release of virions, especially in primary equine cells, and contributes to EHV-1 virulence.

Bovine herpesvirus 1 glycoprotein M forms a disulfide-linked heterodimer with the U(L)49.5 protein

Nine glycoproteins (gB, gC, gD, gE, gG, gH, gI, gK, and gL) have been identified in bovine herpesvirus 1 (BHV-1). gM has been identified in many other alpha-, beta-, and gammaherpesviruses, in which it appears to play a role in membrane penetration and cell-to-cell fusion. We sought to express BHV-1 open reading frame U(L)10, which encodes gM, and specifically identify the glycoprotein. We corrected a frameshift error in the published sequence and used the corrected sequence to design coterminal peptides from the C terminus. These were expressed as glutathione S-transferase fusion proteins in Escherichia coli. The fusion protein containing the 63 C-terminal amino acids from the corrected gM sequence engendered antibodies that immunoprecipitated a 30-kDa protein from in vitro translation reactions programmed with the U(L)10 gene. Proteins immunoprecipitated by this antibody from virus-infected cells ran at 36 and 43 kDa in reducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and 43 and 48 kDa in nonreducing SDS-PAGE. Only the larger of the pair was present in virions. A 7-kDa protein was released from gM by reducing agents. The 7-kDa protein was not recognized in Western blots probed with the anti-gM antibody but reacted specifically with antibodies prepared against BHV-1 U(L)49.5, previously reported to be a 9-kDa protein associated with an unidentified 39-kDa protein (X. Liang, B. Chow, C. Raggo, and L. A. Babiuk, J. Virol. 70:1448-1454, 1996). This is the first report of a small protein covalently bound to any herpesvirus gM. Similar patterns of hydrophobic domains and cysteines in all known gM and U(L)49.5 homologs suggest that these two proteins may be linked by disulfide bonds in all herpesviruses.

Equine herpesvirus 1 mutants devoid of glycoprotein B or M are apathogenic for mice but induce protection against challenge infection

Equine herpesvirus 1 (EHV-1) mutants devoid of the open reading frames (ORFs) of either glycoprotein (g) B or M were constructed and tested for their immunogenic potential in a murine model of EHV-1 infection. The mutant viruses were engineered using the virulent EHV-1 strain RacL11 or the modified live vaccine strain RacH by inserting the Escherichia coli LacZ gene into the viral ORFs. RacL11-infected mice showed signs typical of an EHV-1 infection, whereas mice infected with the EHV-1 gB- or gM-negative mutants or with RacH did not develop disease. No difference in the pathogenic potential of RacL11 gB- and gM-negative viruses was observed after application of either phenotypically completed or negative viruses. However, revertant RacL11 viruses in which the gB or gM gene had been restored caused EHV-1-related symptoms that were indistinguishable from those induced by RacL11. Mice that had been immunized with phenotypically negative gB- and gM-deficient EHV-1 were challenged with the RacL11 virus 25 days after immunization. Mock-immunized mice developed EHV-1 disease and high virus loads in their lungs were observed. In contrast, mice developed not exhibit EHV-1-caused disease. It was concluded (i) that deletion of either gB or gM abolished the virulence of strain RacL11 and (ii) that immunization with gB- or gM-negative EHV-1 elicited a protective immunity that was reflected by both virus-neutralizing antibodies and EHV-1-specific T-cells in spleens of immunized mice.

Intracellular processing of pseudorabies virus glycoprotein M (gM): gM of strain Bartha lacks N-glycosylation

Genes encoding homologs of the herpes simplex virus type 1 UL10 product, glycoprotein M, are conserved in all herpesviruses investigated so far. Recently, we identified pseudorabies virus (PrV) gM as a 45-kDa structural component of purified virions. A gM-PrV mutant could be propagated in cell culture, albeit at lower titers and with delayed penetration kinetics. Thus, gM has a nonessential but modulatory function in PrV infection. PrV gM is modified by addition of an N-linked glycan at a consensus sequence located between the predicted first and second hydrophobic region of the protein. This N-glycosylation site is conserved in all gM homologs sequenced so far, indicating an important functional role. To analyze intracellular processing of PrV gM, Western blot analyses were performed. In PrV-infected cells, mature 45-kDa gM as well as 33- and 35-kDa precursor forms were detectable. Presumably dimeric 90- and 70-kDa proteins were also observed. The 33- and 35-kDa proteins represent nonglycosylated and glycosylated precursors as shown by endoglycosidase digestions. Investigation of several PrV strains revealed that the UL10 product of PrV strain Bartha, an attenuated virus used as vaccine, was not modified by N-glycosylation. Sequence analysis showed that the N-glycosylation consensus sequence was altered from NDT to NDA, which resulted in loss of the N-glycosylation signal. To our knowledge, this is the only gM homolog identified so far which is not N-glycosylated. To investigate whether this form of the protein is functionally competent, the UL10 gene of strain Bartha was inserted into PrV strain Kaplan by substitution of the wild-type UL10 gene. The resulting recombinant expressed a UL10 protein lacking N-glycans. In vitro replication analyses did not reveal any difference in virus production, but plaque size and penetration kinetics were slightly reduced. In summary, we show that wild-type gM is modified by N-glycosylation at one conserved site. However, although this site is highly conserved throughout the herpesviruses, loss of N-glycans due to mutation of the consensus sequence had only a minor effect on propagation of PrV in cell culture.