Virus-specific glycoproteins associated with the nuclear fraction of herpes simplex virus type 1-infected cells

Extended Synaptotagmin 1 Interacts with Herpes Simplex Virus 1 Glycoprotein M and Negatively Modulates Virus-Induced Membrane Fusion

Enveloped viruses typically encode their own fusion machinery to enter cells. Herpesviruses are unusual, as they fuse with a number of cellular compartments throughout their life cycles. As uncontrolled fusion of the host membranes should be avoided in these events, tight regulation of the viral fusion machinery is critical. While studying herpes simplex virus 1 (HSV-1) glycoprotein gM, we identified the cellular protein E-Syt1 (extended synaptotagmin 1) as an interaction partner. The interaction took place in both infected and transfected cells, suggesting other viral proteins were not required for the interaction. Most interestingly, E-Syt1 is a member of the synaptotagmin family of membrane fusion regulators. However, the protein is known to promote the tethering of the endoplasmic reticulum (ER) to the plasma membrane. We now show that E-Syt1, along with the related E-Syt3, negatively modulates viral release into the extracellular milieu, cell-to-cell viral spread, and viral entry, all processes that implicate membrane fusion events. Similarly, these E-Syt proteins impacted the formation of virus-induced syncytia. Altogether, these findings hint at the modulation of the viral fusion machinery by the E-Syt family of proteins.IMPORTANCE Viruses typically encode their own fusion apparatus to enable them to enter cells. For many viruses, this means a single fusogenic protein. However, herpesviruses are large entities that express several accessory viral proteins to regulate their fusogenic activity. The present study hints at the additional participation of cellular proteins in this process, suggesting the host can also modulate viral fusion to some extent. Hence E-Syt proteins 1 and 3 seem to negatively modulate the different viral fusion events that take place during the HSV-1 life cycle. This could represent yet another innate immunity response to the virus.

Contributions of nucleotide excision repair, DNA polymerase eta, and homologous recombination to replication of UV-irradiated herpes simplex virus type 1

The effects of UV irradiation on herpes simplex virus type 1 (HSV-1) gene expression and DNA replication were examined in cell lines containing mutations inactivating the XPA gene product required for nucleotide-excision repair, the DNA polymerase eta responsible for translesion synthesis, or the Cockayne syndrome A and B (CSA and CSB) gene products required for transcription-coupled nucleotide excision repair. In the absence of XPA and CSA and CSB gene products, virus replication was reduced 10(6)-, 400-, and 100-fold, respectively. In DNA polymerase eta mutant cells HSV-1 plaque efficiency was reduced 10(4)-fold. Furthermore, DNA polymerase eta was strictly required for virus replication at low multiplicities of infection but dispensable at high multiplicities of infection. Knock down of Rad 51, Rad 52, and Rad 54 levels by RNA interference reduced replication of UV-irradiated HSV-1 150-, 100-, and 50-fold, respectively. We find that transcription-coupled repair efficiently supports expression of immediate early and early genes from UV-irradiated HSV-1 DNA. In contrast, the progression of the replication fork appears to be impaired, causing a severe reduction of late gene expression. Since the HSV-1 replisome does not make use of proliferating cell nuclear antigen, we attribute the replication defect to an inability to perform proliferating cell nuclear antigen-dependent translesion synthesis by polymerase switching at the fork. Instead, DNA polymerase eta may act during postreplication gap filling. Homologous recombination, finally, might restore the physical and genetic integrity of the virus chromosome.

Early, active, and specific localization of herpes simplex virus type 1 gM to nuclear membranes

Thirteen different glycoproteins are incorporated into mature herpes simplex virus type 1 (HSV-1) virions. Five of them play important roles during entry, while others intervene during egress of the virus. Although HSV-1 gM is not essential in cell culture, its deletion reduces viral yields and promotes syncytium formation. Furthermore, gM is conserved among herpesviruses, is essential for several of them, and can redirect the gD and gH/gL viral glycoproteins from the cell surface to the trans-Golgi network, where gM presumably modulates final capsid envelopment. Late in infection, gM reaches the nuclear envelope and decorates perinuclear virions. This process seemingly requires U(L)31 and U(L)34 and occurs when several markers of the trans-Golgi network have relocalized to the nucleus. However, the precise mechanism of gM nuclear targeting is unclear. We now report that gM is quickly and specifically targeted to nuclear membranes in a virus-dependent manner. This occurs prior to the HSV-1-induced reorganization of the trans-Golgi network and before gM enters the secretory pathway. The presence of a high-mannose glycosylation pattern on gM further corroborated these findings. While gM was targeted to the inner nuclear membrane early in infection, its partners gD, gH, gN, VP22, U(L)31, and U(L)34 did not colocalize with gM. These data suggest that nuclear gM fulfills an early nuclear function that is independent of its known interaction partners and its function in viral egress.

Herpesvirus gB-induced fusion between the virion envelope and outer nuclear membrane during virus egress is regulated by the viral US3 kinase

Herpesvirus capsids collect along the inner surface of the nuclear envelope and bud into the perinuclear space. Enveloped virions then fuse with the outer nuclear membrane (NM). We previously showed that herpes simplex virus (HSV) glycoproteins gB and gH act in a redundant fashion to promote fusion between the virion envelope and the outer NM. HSV mutants lacking both gB and gH accumulate enveloped virions in herniations, vesicles that bulge into the nucleoplasm. Earlier studies had shown that HSV mutants lacking the viral serine/threonine kinase US3 also accumulate herniations. Here, we demonstrate that HSV gB is phosphorylated in a US3-dependent manner in HSV-infected cells, especially in a crude nuclear fraction. Moreover, US3 directly phosphorylated the gB cytoplasmic (CT) domain in in vitro assays. Deletion of gB in the context of a US3-null virus did not add substantially to defects in nuclear egress. The majority of the US3-dependent phosphorylation of gB involved the CT domain and amino acid T887, a residue present in a motif similar to that recognized by US3 in other proteins. HSV recombinants lacking gH and expressing either gB substitution mutation T887A or a gB truncated at residue 886 displayed substantial defects in nuclear egress. We concluded that phosphorylation of the gB CT domain is important for gB-mediated fusion with the outer NM. This suggested a model in which the US3 kinase is incorporated into the tegument layer (between the capsid and envelope) in HSV virions present in the perinuclear space. By this packaging, US3 might be brought close to the gB CT tail, leading to phosphorylation and triggering fusion between the virion envelope and the outer NM.

Glycoproteins required for entry are not necessary for egress of pseudorabies virus

In the current perception of the herpesvirus replication cycle, two fusion processes are thought to occur during entry and nuclear egress. For penetration, glycoproteins gB and gH/gL have been shown to be essential, whereas a possible role of these glycoproteins in nuclear egress remains unclear. Viral envelope glycoproteins have been detected by immunolabeling in the nuclear membrane as well as in primary enveloped particles in several herpesviruses, indicating that they might be involved in the fusion process. Moreover, a herpes simplex virus type 1 mutant simultaneously lacking gB and gH was described to be deficient in nuclear egress (A. Farnsworth, T. W. Wisner, M. Webb, R. Roller, G. Cohen, R. Eisenberg, and D. C. Johnson, Proc. Natl. Acad. Sci. USA 104:10187-10192, 2007). To analyze the situation in the related alphaherpesvirus pseudorabies virus (PrV), mutants carrying single and double deletions of glycoproteins gB, gD, gH, and gL were constructed and characterized. We show here that the simultaneous deletion of gB and gD, gB and gH, gD and gH, or gH and gL has no detectable effect on PrV egress, implying that none of these glycoproteins either singly or in the tested combinations is required for nuclear egress. In addition, immunolabeling studies using different mono- or polyclonal sera raised against various PrV glycoproteins did not reveal the presence of viral glycoproteins in the inner nuclear membrane or in primary virions. Thus, our data strongly suggest that different fusion mechanisms are active during virus entry and egress.

ICP22 is required for wild-type composition and infectivity of herpes simplex virus type 1 virions

The immediate-early regulatory protein ICP22 is required for efficient replication of herpes simplex virus type 1 in some cell types (permissive) but not in others (restrictive). In mice infected via the ocular route, the pathogenesis of an ICP22- virus, 22/n199, was altered relative to that of wild-type virus. Specifically, tear film titers of 22/n199-infected mice were significantly reduced at 3 h postinfection relative to those of mice infected with wild-type virus. Further, 22/n199 virus titers were below the level of detection in trigeminal ganglia (TG) during the first 9 days postinfection. On day 30 postinfection, TG from 22/n199-infected mice contained reduced viral genome loads and exhibited reduced expression of latency-associated transcripts and reduced reactivation efficiency relative to TG from wild-type virus-infected mice. Notably, the first detectable alteration in the pathogenesis of 22/n199 in these tests occurred in the eye prior to the onset of nascent virus production. Thus, ICP22- virions appeared to be degraded, cleared, or adsorbed more rapidly than wild-type virions, implying potential differences in the composition of the two virion types. Analysis of the protein composition of purified extracellular virions indicated that ICP22 is not a virion component and that 22/n199 virions sediment at a reduced density relative to wild-type virions. Although similar to wild-type virions morphologically, 22/n199 virions contain reduced amounts of two gamma2 late proteins, US11 and gC, and increased amounts of two immediate-early proteins, ICP0 and ICP4, as well as protein species not detected in wild-type virions. Although ICP22- viruses replicate to near-wild-type levels in permissive cells, the virions produced in these cells are biochemically and physically different from wild-type virions. These virion-specific differences in ICP22- viruses add a new level of complexity to the functional analysis of this immediate-early viral regulatory protein.

A three-residue signal confers localization of a reporter protein in the inner nuclear membrane

By alignment to the carboxy-terminal-deduced aa sequence of human cytomegalovirus glycoprotein B (gB), conserved hexameric aa motifs with putative function for localization in the inner nuclear membrane (INM) were identified in the nucleoplasmic tails of herpes simplex virus type 1 gB and of the cellular lamin B receptor. Fusion of the respective hexamers to CD8 as a reporter redirected transport of the chimeras into the INM, suggesting their functioning as modular signal motifs. Consecutive experiments showed that the three-residue motif RxR represents a consensus sequence which is sufficient for localization of the CD8 reporter in the INM.

Identification of a novel signal sequence that targets transmembrane proteins to the nuclear envelope inner membrane

Herpesvirus maturation requires translocation of glycoprotein B homologue from the endoplasmic reticulum to the inner nuclear membrane. Glycoprotein B of human cytomegalovirus was used in this context as a model protein. To identify a specific signal sequence within human cytomegalovirus glycoprotein B acting in a modular fashion, coding sequences were recombined with reporter proteins. Immunofluorescence and cell fractionation demonstrated that a short sequence element within the cytoplasmic tail of human cytomegalovirus glycoprotein B was sufficient to translocate the membrane protein CD8 to the inner nuclear membrane. This carboxyl-terminal sequence had no detectable nuclear localization signal activity for soluble beta-Galactosidase and could not be substituted by the nuclear localization signal of SV40 T antigen. For glycoprotein B of herpes simplex virus, a carboxyl-terminal element with comparable properties was found. Further experiments showed that the amino acid sequence DRLRHR of human cytomegalovirus glycoprotein B (amino acids 885-890) was sufficient for nuclear envelope translocation. Single residue mutations revealed that the arginine residues in positions 4 and 6 of the DRLRHR sequence were essential for its function. These results support the view that transmembrane protein transport to the inner nuclear membrane is controlled by a mechanism different from that of soluble proteins.

Role of the membrane anchoring and cytoplasmic domains in intracellular transport and localization of viral glycoproteins

Nuclear factor-κB (NF-κB) binds to nucleotide sequences between -80 and -70 bp upstream of the transcriptional start site in the interleukin-8 (IL-8) promoter and is crucial for transcription of the IL-8 gene. We showed that exogenous nitric oxide in the form of a nitric oxide donor significantly reduced IL-8 mRNA in cytokine-activated ECV304. Similarly, nitric oxide significantly reduced migration of polymorphonuclear neutrophils through cytokine-activated ECV304 monolayers, an IL-8-dependent process. Using a luciferase reporter construct containing the NF-κB site of the IL-8 gene, we showed that exposing cytokine-activated ECV304 to exogenous nitric oxide resulted in significant reduction of NF-κB binding. Follow-up studies using a luciferase reporter construct possessing a mutated NF-κB site confirmed that the luciferase activity observed in the NF-κB reporter resulted from NF-κB binding. These studies demonstrate that nitric oxide, supplied exogenously into reactions containing activated endothelium, down-regulates pro-inflammatory activity, such as the secretion of chemokines, and functional activity, such as transendothelial migration of neutrophils. Key words: interleukin-8, nuclear factor κ B, transendothelial migration, nitric oxide.

Herpes simplex virus type 1 glycoprotein K is not essential for infectious virus production in actively replicating cells but is required for efficient envelopment and translocation of infectious virions from the cytoplasm to the extracellular space

We characterized the glycoprotein K (gK)-null herpes simplex virus type 1 [HSV-1] (KOS) delta gK and compared it to the gK-null virus HSV-1 F-gKbeta (L. Hutchinson et al., J. Virol. 69:5401-5413, 1995). delta gK and F-gKbeta mutant viruses produced small plaques on Vero cell monolayers at 48 h postinfection. F-gKbeta caused extensive fusion of 143TK cells that was sensitive to melittin, a specific inhibitor of gK-induced cell fusion, while delta gK virus did not fuse 143TK cells. A recombinant plasmid containing the truncated gK gene specified by F-gKbeta failed to rescue the ICP27-null virus KOS (d27-1), while a plasmid with the delta gK deletion rescued the d27-1 virus efficiently. delta gK virus yield was approximately 100,000-fold lower in stationary cells than in actively replicating Vero cells. The plaquing efficiencies of delta gK and F-gKbeta virus stocks on VK302 cells were similar, while the plaquing efficiency of F-gKbeta virus stocks on Vero cells was reduced nearly 10,000-fold in comparison to that of delta gK virus. Mutant delta gK and F-gKbeta infectious virions accumulated within Vero and HEp-2 cells but failed to translocate to extracellular spaces. delta gK capsids accumulated in the nuclei of Vero but not HEp-2 cells. Enveloped delta gK virions were visualized in the cytoplasms of both Vero and HEp-2 cells, and viral capsids were found in the cytoplasm of HEp-2 cells within vesicles. Glycoproteins B, C, D, and H were expressed on the surface of delta gK-infected Vero cells in amounts similar to those for KOS-infected Vero cells. These results indicate that gK is involved in nucleocapsid envelopment, and more importantly in the translocation of infectious virions from the cytoplasm to the extracellular spaces, and that actively replicating cells can partially compensate for the envelopment but not for the cellular egress deficiency of the delta gK virus. Comparison of delta gK and F-gKbeta viruses suggests that the inefficient viral replication and plaquing efficiency of F-gKbeta virus in Vero cells and its syncytial phenotype in 143TK- cells are most likely due to expression of a truncated gK.

Purification of herpes simplex virus type 1 by density gradient centrifugation and estimation of the sedimentation coefficient of the virion

Purification of herpes simplex virus type 1 (HSV-1) is often performed by centrifugation in gradients of various materials. A major problem with such procedures is a gradual decrease in the infectivity of the virus, probably due to the influence of the gradient material. In the present work we have compared Nycodenz gradients with Ficoll gradients for HSV-1 purification. Both gradient materials have low osmolarities. The purity of the preparations recovered from the two gradients was similar, as measured by electron microscopy and two-dimensional gel electrophoresis, and the yield of infectious virus was approximately the same. Szilágyi & Cunningham (22) reported previously that Ficoll gradients separate one band of complete virions and another band containing light particles devoid of nucleocapsids. In Nycodenz gradients we observed the same bands, and an additional third band which contained aggregates and disrupted particles. In this respect, the separation in Nycodenz is slightly superior to that in Ficoll. The purified HSV-1 virions were subjected to analytical differential centrifugation and an S-value of 670S was calculated for the virus.

Antiviral effects of 6-diazo-5-oxo-L-norleucin on replication of herpes simplex virus type 1

An L-glutamine antagonist, 6-diazo-5-oxo-L-norleucin (L-DON), inhibits replication of vesicular stomatitis virus, poliovirus and paramyxoviruses in cultured cells. We tested the antiviral activity of L-DON against different strains of herpes simplex virus type 1 (HSV-1) in Vero cells. In the presence of a physiological plasma concentration of L-glutamine (0.5mM) L-Don inhibited 50% production of virus plaques at concentrations ranging from 7.9 to 16 microM. At concentrations of 40 microM L-Don inhibited infectious virus yield by 99%. The antiviral activity of L-DON decreased with increasing L-glutamine concentrations. A concentration of 5000 microM of L-Don had no significant effects on the viability of Vero cells. Transmission electron microscopical investigations showed that L-DON prevented mainly envelopment of viral nucleocapsids in the cytoplasm. The immunoprecipitation experiments demonstrated selective inhibition of synthesis of HSV-1 glycoproteins in L-DON treated cells. The results showed that L-DON inhibits HSV-1 replication at a late stage in the virus replication cycle, probably the cytoplasmic maturation of virions and subsequent virion egress from the cells.

Intracellular compartmentalization of the glycoprotein B of herpesvirus Simian agent 8 expressed with a baculovirus vector in insect cells

The intracellular localization of the glycoprotein B of herpesvirus simian agent 8 expressed with a baculovirus system in insect cells was studied. Cell fractionation and immunoprecipitation revealed that gB is present in microsomal as well as in nuclear membranes. Both fractions contain oligomers, probably dimers, of gB with endoglycosidase-H sensitive, mannose-rich carbohydrates. Nuclear transport of gB was further analysed by immuno electron microscopy of recombinant baculovirus-infected cells. The glycoprotein is present both in the outer and the inner nuclear membrane as well as in cytoplasmic structures and at the cell surface. This study precludes the possibility that glycosylation and/or oligomerisation of SA8 gB are responsible for nuclear targeting.

Intra-nuclear localization of two envelope proteins, gB and gD, of herpes simplex virus

The envelopes of herpes simplex virus (HSV) particles are acquired from the inner nuclear membrane (INM) of the infected cell and virus-coded glycoproteins are present in the envelope of mature virions. Our ultrastructural study examined the process of virus envelopment and the targeting of two major viral glycoproteins, gB and gD, to the INM in HSV-infected human embryonic fibroblasts. It was shown that envelopment and transport of virus particles from the nucleus is facilitated by the formation of a dynamic tubulo-reticulum arising from the INM. Capsids were assembled in the nucleus and collected within INM tubules which protruded into the perinuclear space and thence into the cisternae of the endoplasmic reticulum (ER). Envelopment occurred by constriction and fusion of the tubular channel walls, releasing enveloped virions into the ER. Transport to the cell surface took place in membrane-bound compartments and probably followed the normal secretory pathway through the Golgi apparatus. Immunogold probes, tagged with specific monoclonal antibodies, were used to localize gB and gD during the process of virus maturation. Cytoplasmic membranes were not labelled, but probes bound inside the nucleus, mainly at sites of virus assembly. Labelling occurred on the nucleoplasmic side of the INM which surrounded capsids in the process of envelopment, but not on the outside of that membrane, although characteristic gB glycoprotein spikes were labelled on the envelopes of extracellular virus particles and on virions in trans-Golgi transport vesicles just prior to their release from the infected cell. gB was not detected on the surface of enveloped virions in the perinuclear space, or the cisternae of the ER or cis-Golgi, which suggests that the specific epitope was masked during that stage of intracellular processing. gD probes bound to virion envelopes and also to the tegument region of some particles found in both perinuclear and extracellular sites. We postulate the precursor core proteins for both gB and gD are transported first to the nucleus, and then, together with maturing capsids, are targeted to the INM, and later inserted into viral envelopes at the site of budding. Post-translational glycosylation of envelope proteins could occur as virus particles exit the nucleus and travel through the ER and Golgi compartments.

Redistribution of microtubules and Golgi apparatus in herpes simplex virus-infected cells and their role in viral exocytosis

Earlier studies have shown that the Golgi apparatus was fragmented and dispersed in herpes simplex virus 1-infected Vero and HEp-2 cells but not in human 143TK- cells, that the fragmentation and dispersal required viral functions expressed concurrently with or after the onset of DNA synthesis (G. Campadelli-Fiume, R. Brandimarti, C. Di Lazzaro, P. L. Ward, B. Roizman, and M. R. Torrisi, Proc. Natl. Acad. Sci. USA 90:2798-2802, 1993), and that in 143TK- cells, but not Vero or HEp-2 cells, infected with viral mutants lacking the UL20 gene virions were glycosylated and transported to extracellular space (J. D. Baines, P. L. Ward, G. Campadelli-Fiume, and B. Roizman, J. Virol. 65:6414-6424, 1991; E. Avitabile, P. L. Ward, C. Di Lazzaro, M. R. Torrisi, B. Roizman, and G. Campadelli-Fiume, J. Virol. 68:7397-7405, 1994). Experiments designed to elucidate the role of the microtubules and of intact or fragmented Golgi apparatus in the exocytosis of virions showed the following. (i) In all cell lines tested (Vero, 143TK-, BHK, and Hep-2) microtubules underwent fragmentation particularly evident at the cell periphery and then reorganized into bundles which circumvent the nucleus. This event was not affected by inhibitors of viral DNA synthesis. We conclude that redistribution of microtubules may be required but is not sufficient for the fragmentation and dispersal of the Golgi apparatus. (ii) In all infected cell lines tested, nocodazole caused fragmentation and dispersal of the Golgi and a far more extensive depolymerization of the microtubules than was seen in untreated, infected Vero or HEp-2 cells. Taxol precluded the depolymerization of the microtubules and fragmentation of the Golgi in both infected cell lines. Neither nocodazole nor taxol affected the exocytosis of infectious virus from Vero, HEp-2, or 143TK- cells infected with wild-type virus. We conclude that the effects of nocodazole or of taxol are dominant over the effects of viral infection in the cell lines tested and that viral exocytosis is independent of the organization of microtubules or of the integrity of the Golgi apparatus. Lastly, the data suggest that herpes simplex viruses have evolved an exocytic pathway for which the UL20 protein is a component required in some cells but not others and in which this protein does not merely compensate for the fragmentation and dispersal of the Golgi apparatus.

Envelopment of varicella-zoster virus: targeting of viral glycoproteins to the trans-Golgi network

Previous studies suggested that varicella-zoster virus derives its final envelope from the trans-Golgi network (TGN) and that envelope glycoproteins (gps) are transported to the TGN independently of nucleocapsids. We tested the hypothesis that gpI is targeted to the TGN as a result of a signal sequence or patch encoded in its cytosolic domain. cDNAs encoding gpI wild type (wt) and a truncated mutant gpI(trc) lacking transmembrane and cytosolic domains were cloned by using the PCR. Cells transfected with cDNA encoding gpI(wt) or gpI(trc) synthesized and N glycosylated the proteins. gpI(wt) accumulated in the TGN, some reached the plasmalemma, but none was secreted. In contrast, gpI(trc) was retained and probably degraded in the endoplasmic reticulum; none was found on cell surfaces, but some was secreted. The distribution of gpI(trc) was not affected by deletion of potential glycosylation sites. To locate a potential gpI-targeting sequence, cells were transfected with cDNA encoding chimeric proteins in which the ectodomain of a plasmalemmal marker, the interleukin-2 receptor (tac), was fused to different domains of gpI. A chimeric protein in which tac was fused with the transmembrane and cytoplasmic domains of gpI was targeted to the TGN. In contrast, a chimeric protein in which tac was fused only with the gpI transmembrane domain passed through the TGN and concentrated in endosomes. We conclude that gpI is targeted to the TGN as a result of a targeting sequence or patch in its cytosolic domain.

Initial characterization of the membrane-associated form of ICP4 of herpes simplex virus type 1

The immediate-early gene product, ICP4, of herpes simplex virus type 1 (HSV-1) is one of the major transcriptional regulatory proteins in the virus replicative process and is localized primarily within the nucleus soon after its synthesis. Earlier studies have shown that detectable amounts of ICP4 are also associated with the plasma membranes of infected cells (F. Yao and R. J. Courtney, J. Virol. 65:1516-1524, 1991). To extend our understanding of the properties of the membrane-associated ICP4, we have used various electrophoretic techniques, including sodium dodecyl sulfate-polyacrylamide gel electrophoresis, two-dimensional gel electrophoresis, and isoelectric focusing, to compare the membrane- and nuclear-associated forms of ICP4. The data from all of these methods revealed that a single unique form of ICP4 associates with plasma membranes of HSV-1-infected cells. While multiple forms of ICP4 were detected in infected cell nuclei, the membrane-associated form of ICP4 appeared to have a lower apparent molecular weight and a more acidic pI than the various forms of ICP4 found in infected cell nuclei. These results suggest that a novel form of ICP4 may associate with plasma membranes of HSV-1-infected cells. A recombinant adenovirus, AdICP4 (encoding an ICP4 protein), was used to determine the role that other herpesvirus proteins may play in the membrane association of ICP4. The results suggest that the expression of other HSV-1 proteins is not required for the membrane association of ICP4.

Herpes simplex virus glycoprotein K promotes egress of virus particles

Herpes simplex virus (HSV) glycoprotein K (gK) is thought to be intimately involved in the process by which infected cells fuse because HSV syncytial mutations frequently alter the gK (UL53) gene. Previously, we characterized gK produced in cells infected with wild-type HSV or syncytial HSV mutants and found that the glycoprotein was localized to nuclear and endoplasmic reticulum membranes and did not reach the cell surface (L. Hutchinson, C. Roop, and D. C. Johnson, J. Virol. 69:4556-4563, 1995). In this study, we have characterized a mutant HSV type 1, denoted F-gK beta, in which a lacZ gene cassette was inserted into the gK coding sequences. Since gK was found to be essential for virus replication, F-gK beta was propagated on complementing cells which can express gK. F-gK beta produced normal plaques bounded by nonfused cells when plated on complementing cells, although syncytia were observed when the cells produced smaller amounts of gK. In contrast, F-gK beta produced only microscopic plaques on Vero cells and normal human fibroblasts (which do not express gK) and these plaques were reduced by 10(2) to 10(6) in number. Further, large numbers of nonenveloped capsids accumulated in the cytoplasm of F-gK beta-infected Vero cells, virus particles did not reach the cell surface, and the few enveloped particles that were produced exhibited a reduced capacity to enter cells and initiate an infection of complementing cells. Overexpression of gK in HSV-infected cells also caused defects in virus egress, although particles accumulated in the perinuclear space and large multilamellar membranous structures juxtaposed with the nuclear envelope were observed. Together, these results demonstrate that gK regulates or facilitates egress of HSV from cells. How this property is connected to cell fusion is not clear. In this regard, gK may alter cell surface transport of viral particles or other viral components directly involved in the fusion process.

Interaction of herpes simplex virus glycoprotein gC with mammalian cell surface molecules

The entry of herpes simplex virus (HSV) into mammalian cells is a multistep process beginning with an attachment step involving glycoproteins gC and gB. A second step requires the interaction of glycoprotein gD with a cell surface molecule. We explored the interaction between gC and the cell surface by using purified proteins in the absence of detergent. Truncated forms of gC and gD, gC1(457t), gC2(426t), and gD1(306t), lacking the transmembrane and carboxyl regions were expressed in the baculovirus system. We studied the ability of these proteins to bind to mammalian cells, to bind to immobilized heparin, to block HSV type 1 (HSV-1) attachment to cells, and to inhibit plaque formation by HSV-1. Each of these gC proteins bound to conformation-dependent monoclonal antibodies and to human complement component C3b, indicating that they maintained the same conformation of gC proteins expressed in mammalian cells. Biotinylated gC1(457t) and gC2(426t) each bind to several cell lines. Binding was inhibited by an excess of unlabeled gC but not by gD, indicating specificity. The attachment of gC to cells involves primarily heparan sulfate proteoglycans, since heparitinase treatment of cells reduced gC binding by 50% but had no effect on gD binding. Moreover, binding of gC to two heparan sulfate-deficient L-cell lines, gro2C and sog9, both of which are mostly resistant to HSV infection, was markedly reduced. Purified gD1 (306t), however, bound equally well to the two mutant cell lines. In contrast, saturating amounts of gC1(457t) interfered with HSV-1 attachment to cells but failed to block plaque formation, suggesting a role for gC in attachment but not penetration. A mutant form of gC lacking residues 33 to 123, gC1(delta 33-123t), expressed in the baculovirus system, bound significantly less well to cells than did gC1(457t) and competed poorly with biotinylated gC1(457t) for binding. These results suggest that residues 33 to 123 are important for gC attachment to cells. In contrast, both the mutant and wild-type forms of gC bound to immobilized heparin, indicating that binding of these proteins to the cell surface involves more than a simple interaction with heparin. To determine that the contribution of the N-terminal region of gC is important for HSV attachment, we compared several properties of a mutant HSV-1 which contains gC lacking amino acids 33 to 123 to those of its parental virus, which contains full-length gC. The mutant bound less well to cells than the parental virus but exhibited normal growth properties.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane anchoring domain of herpes simplex virus glycoprotein gB is sufficient for nuclear envelope localization

We have used the glycoprotein gB of herpes simplex virus type 1 (gB-1), which buds from the inner nuclear membrane, as a model protein to study localization of membrane proteins in the nuclear envelope. To determine whether specific domains of gB-1 glycoprotein are involved in localization in the nuclear envelope, we have used deletion mutants of gB-1 protein as well as chimeric proteins constructed by replacing the domains of the cell surface glycoprotein G of vesicular stomatitis virus with the corresponding domains of gB. Mutant and chimeric proteins expressed in COS cells were localized by immunoelectron microscopy. A chimeric protein (gB-G) containing the ectodomain of gB and the transmembrane and cytoplasmic domains of G did not localize in the nuclear envelope. When the ectodomain of G was fused to the transmembrane and cytoplasmic domains of gB, however, the resulting chimeric protein (G-gB) was localized in the nuclear envelope. Substitution of the transmembrane domain of G with the 69 hydrophobic amino acids containing the membrane anchoring domain of gB allowed the hybrid protein (G-tmgB) to be localized in the nuclear envelope, suggesting that residues 721 to 795 of gB can promote retention of proteins in the nuclear envelope. Deletion mutations in the hydrophobic region further showed that a transmembrane segment of 21 hydrophobic amino acids, residues 774 to 795 of gB, was sufficient for localization in the nuclear envelope. Since wild-type gB and the mutant and chimeric proteins that were localized in the nuclear envelope were also retained in the endoplasmic reticulum, the membrane spanning segment of gB could also influence retention in the endoplasmic reticulum.

Characterization of baculovirus-expressed herpes simplex virus type 1 glycoprotein K

The DNA region encoding the complete herpes simplex virus type 1 (HSV-1) glycoprotein K (gK) was inserted into a baculovirus transfer vector, and recombinant viruses expressing gK were isolated. Four gK-related recombinant baculovirus-expressed peptides of 29, 35, 38, and 40 kDa were detected with polyclonal antibody to gK. The 35-, 38-, and 40-kDa species were susceptible to tunicamycin treatment, suggesting that they were glycosylated. The 38- and 40-kDa species corresponded to partially glycosylated precursor gK (pgK) and mature gK, respectively. The 29-kDa peptide probably represented a cleaved, unglycosylated peptide. The 35-kDa peptide probably represented a cleaved, glycosylated peptide that may be a precursor to pgK. Indirect immunofluorescence with polyclonal antibody to gK peptides indicated that the recombinant baculovirus-expressed gK was abundant on the surface of the insect cells in which it was expressed. Mice vaccinated with the baculovirus-expressed gK produced very low levels (< 1:10) of HSV-1 neutralizing antibody. Nonetheless, these mice were partially protected from lethal challenge with HSV-1 (75% survival). This protection was significant (P = 0.02). Despite some protection against death, gK-vaccinated mice showed no protection against the establishment of latency. Surprisingly, gK-vaccinated mice that were challenged ocularly with a stromal disease-producing strain of HSV-1 had significantly higher levels of ocular disease (herpes stromal keratitis) than did mock-vaccinated mice. In summary, this is the first report to show that vaccination with HSV-1 gK can provide protection against lethal HSV-1 challenge and that vaccination with an HSV-1 glycoprotein can significantly increase the severity of HSV-1-induced ocular disease.

Effects of deletions in the carboxy-terminal hydrophobic region of herpes simplex virus glycoprotein gB on intracellular transport and membrane anchoring

The gB glycoprotein of herpes simplex virus type 1 is involved in viral entry and fusion and contains a predicted membrane-anchoring sequence of 69 hydrophobic amino acids, which can span the membrane three times, near the carboxy terminus. To define the membrane-anchoring sequence and the role of this hydrophobic stretch, we have constructed deletion mutants of gB-1, lacking one, two, or three predicted membrane-spanning segments within the 69 amino acids. Expression of the wild-type and mutant glycoproteins in COS-1 cells show that mutant glycoproteins lacking segment 3 (amino acids 774 to 795 of the gB-1 protein) were secreted from the cells. Protease digestion and alkaline extraction of microsomes containing labeled mutant proteins further showed that segment 3 was sufficient for stable membrane anchoring of the glycoproteins, indicating that this segment may specify the transmembrane domain of the gB glycoprotein. Also, the mutant glycoproteins containing segment 3 were localized in the nuclear envelop, which is the site of virus budding. Deletion of any of the hydrophobic segments, however, affected the intracellular transport and processing of the mutant glycoproteins. The mutant glycoproteins, although localized in the nuclear envelope, failed to complement the gB-null virus (K082). These results suggest that the carboxy-terminal hydrophobic region contains essential structural determinants of the functional gB glycoprotein.

Immunoelectron microscopic localization of herpes simplex virus glycoprotein gB in the nuclear envelope of infected cells

Herpesvirus, such as herpes simplex type 1 (HSV-1) acquire their envelope by budding through a modified inner membrane of the nuclear envelope which forms thick and dense patches at the site of budding. This suggests that some of the viral envelope glycoproteins must be transported to the nuclear envelope in order to be incorporated into the virus. In an effort to establish the localization of the HSV-1 glycoprotein gB-1 in the nuclear envelope of HSV-1 infected cells directly, we have studied the distribution of the glycoprotein gB-1 by immunoelectron microscopy using a polyclonal anti gB-1 antibody. A specific accumulation of gB-1 in the nuclear envelope, which was five times more labeled than the plasma membrane was observed. The glycoprotein gB-1 was localized in both the outer and the inner membrane of the nuclear envelope. The labeling over the nuclear envelope was distributed evenly and no preferential concentration of gB-1 around or within the patches where the virus buds was detected. The nucleocapsids were found to be labeled only when they become associated with the nuclear envelope indicating that gB-1 is incorporated into the virus at this site.

An immortalized human fibroblast cell line is permissive for human cytomegalovirus infection

Human foreskin fibroblasts (HFF) were immortalized via retrovirus-mediated gene transfer of the E6 and E7 genes of human papillomavirus type 16. An immortalized fibroblast (IF) cell line which was morphologically akin to the parental cell line was isolated. The IF cell line was evaluated for permissiveness to human cytomegalovirus (HCMV) infection after the IF cell line surpassed the normal passage limitation of diploid fibroblasts. Western immunoblot analysis of representative HCMV-encoded immediate-early (72-kDa), early (gB), and late (gH) gene products demonstrated that the IF cell line produced these proteins analogous to those produced by the parental HFF cells. Similar quantities of infectious virus were produced in the IF and HFF cell lines as determined in one-step growth curve experiments. Compared with the HFF cells, morphologically identical plaques were produced in the IF cell line in approximately 10 to 12 days postinfection. These findings indicate that fibroblast cell lines immortalized with transforming genes of human papillomavirus retain complete permissiveness to HCMV infection and support plaque formation. The IF cell line will be useful for future genetic analysis of HCMV.

Orientation of the cleavage site of the herpes simplex virus glycoprotein G-2

During the synthesis of glycoprotein G-2 (gG-2) of herpes simplex virus type 2, the 104,000-Da gG-2 precursor (104K precursor) is cleaved to generate the 72K and the 31K intermediates. The 72K product is processed to generate the mature gG-2 (molecular mass, 108,000 Da), while the 31K product is additionally processed and secreted into the extracellular medium as the 34K component (H. K. Su, R. Eberle, and R. J. Courtney, J. Virol. 61:1735-1737, 1987). In this study, the orientations of the 31K and 72K products on the 104K precursor were determined by using two antipeptide sera produced in rabbits and a monoclonal antibody, 13 alpha C6, directed against gG-2. The sera prepared against synthetic peptides corresponding to the terminal amino acid residues 67 to 78 and an internal peptide at amino acids 247 to 260 of gG-2 recognized the 104K precursor and the 31K cleavage product but not the 72K intermediate. In contrast, 13 alpha C6 detected the 72K cleavage product and the uncleaved precursor but not the 31K cleavage component. The epitope recognized by 13 alpha C6 was mapped within amino acids 486 to 566. These results suggest that the 31K cleavage product is derived from the amino-terminal portion of the 104K precursor molecule and that the 72K intermediate is derived from the carboxyl terminus. In support of our model described above for the synthesis of gG-2, antibodies recognizing either of the cleavage products reacted with the uncleaved precursor but not with the other cleavage product. By using partial endo-beta-N-acetylglucosaminidase H analysis, two N-linked glycosylation sites were found on each of the cleavage products. The distribution of the N-linked glycosylation sites and the reactivities of the antipeptide sera allowed the cleavage region on the precursor to be mapped to within amino acids 260 to 437.

Analysis of the UL36 open reading frame encoding the large tegument protein (ICP1/2) of herpes simplex virus type 1

Using peptide antisera specific for regions within the N terminus and C terminus of the predicted UL36 gene product, immunoblotting experiments were performed to demonstrate definitively that ICP1/2 is encoded by the UL36 gene. These data also suggest that both the cell- and the virion-associated forms of ICP1/2 are colinear with the complete predicted amino acid sequence of the UL36 gene. Computer-assisted analyses of the predicted amino acid sequence of the UL36 gene revealed the presence of two putative leucine zipper-type motifs and a potential ATP-binding domain. The possible functions of these consensus domains will also be discussed.

Studies on endoplasmic reticulum--Golgi complex cycling pathway in herpes simplex virus-infected and brefeldin A-treated human fibroblast cells

Brefeldin A (BFA), a fungal metabolite, significantly inhibited the release of herpes simplex virus type 1 (HSV-1) from infected human fibroblast cells. Electron micrographs of HSV-1-infected and BFA-treated human cells demonstrated the presence of enveloped particles trapped between outer and inner nuclear membranes. Analyses of viral glycoproteins B, C, and D (gB, gC, and gD) showed faster migrating, immature forms in BFA-treated cells when compared to the mature glycoproteins, as observed in the untreated control cells. The shift in mobilities of the glycoproteins in BFA-treated cells apparently was due to the disassembly of the Golgi complex when evaluated by an indirect immunofluorescence assay. The immature forms of gB, gC, and gD could not be detected on the surface of BFA-treated human fibroblast cells. Removal of BFA resulted in a reorganization of the Golgi complex and formation of fully glycosylated gB, gC, and gD. Moreover, the HSV-1 particles released from the treated cells after the removal of BFA completely restored the infectivity of the viral particles. Our results indicate that human fibroblast cells have an endoplasmic reticulum-Golgi cycling pathway.

Characterization of the large tegument protein (ICP1/2) of herpes simplex virus type 1

ICP1/2 (also designated VP1/2) is a 270-kDa structural protein of herpes simplex virus type 1 (HSV-1) which is located in the tegument region of the virion. In this report we describe the production of a polyclonal antiserum specific for ICP1/2 and the use of this antiserum to examine the synthesis, processing, and intracellular localization of the viral polypeptide. Pulse-labeling studies indicated that ICP1/2 is synthesized late during infection, being initially detectable between 8 and 9 hr postinfection with the rate of synthesis continuing to increase until 11 to 12 hr postinfection. Further studies on the expression of ICP1/2 in the presence or absence of viral DNA replication indicated that the synthesis of the polypeptide is absolutely dependent on viral DNA replication. These results suggest that ICP1/2 represents a gamma 2 (true late) gene product. Additionally, we have performed experiments to determine if ICP1/2 is post-translationally modified in HSV-infected cells. These studies indicated that ICP1/2 is phosphorylated on serine residues; however, we found no evidence to suggest that the protein is glycosylated. Using subcellular fractionation and indirect immunofluorescence techniques, we have determined that ICP1/2 is diffusely distributed throughout the nucleus and cytoplasm of HSV-infected cells with no specific compartmentalization of the polypeptide.

Posttranslational modification and subcellular localization of the p12 capsid protein of herpes simplex virus type 1

We have previously shown that the 12-kDa capsid protein (p12) of herpes simplex virus type 1 (HSV-1) is a gamma 2 (true late) gene product encoded by the UL35 open reading frame (D. S. McNabb and R. J. Courtney, J. Virol. 66:2653-2663, 1992). To extend the characterization of p12, we have investigated the posttranslational modifications and intracellular localization of the 12-kDa polypeptide. These studies have demonstrated that p12 is modified by phosphorylation at serine and threonine residues. In addition, analysis of p12 by acid-urea gel electrophoresis has indicated that the protein can be resolved into three components, designated p12a, p12b, and p12c. Using isotopic-labeling and alkaline phosphatase digestion experiments, we have determined that p12a and p12b are phosphorylated forms of the protein, and p12c is likely to represent the unphosphorylated polypeptide. The kinetics of phosphorylation was examined by pulse-chase radiolabeling, and these studies indicated that p12c can be completely converted into p12a and p12b following a 4-h chase. All three species of p12 were found to be associated with purified HSV-1 virions; however, p12b and p12c represented the most abundant forms of the protein within viral particles. We have also examined the intracellular localization of p12 by cell fractionation and indirect immunofluorescence techniques. These results indicated that p12 is predominantly localized in the nucleus of HSV-1-infected cells and appears to be restricted to specific regions within the nucleus.

Baculovirus expressed herpes simplex virus type 1 glycoprotein C protects mice from lethal HSV-1 infection

A recombinant baculovirus (vAc-gC1) was constructed that expresses the glycoprotein C (gC) gene of herpes simplex virus type 1 (HSV-1). When Sf9 cells were infected with this recombinant, a protein that was smaller in size than authentic HSV-1 gC was detected by Western blotting using anti-gC polyclonal antibody. The recombinant gC was susceptible to tunicamycin, partially resistant to Endo-H, and was found on the membrane of Sf9 cells. Antibodies raised in mice to recombinant gC reacted with gC from HSV-1 infected cells and neutralized the infectivity of HSV-1 in vitro. Immunized mice were protected from lethal challenge with HSV-1.

Association of ICP0 but not ICP27 with purified virions of herpes simplex virus type 1

Recent studies have shown that ICP4, one of the major immediate-early proteins of herpes simplex virus type 1 is present within the tegument region of the virion (F. Yao and R. J. Courtney, J. Virol. 63:3338-3344, 1989). With monoclonal antibodies to two additional immediate-early proteins, ICP0 and ICP27, and Western blot (immunoblot) analysis, ICP0, but not ICP27, was also found to be associated with purified virus particles. In an effort to localize the ICP0 within the virion, purified virions were treated with trypsin in the presence and absence of detergent. The data suggest that ICP0 is located within the tegument region of the virion and is not localized in the envelope or within the nucleocapsid. The number of molecules of ICP0 per virion was estimated to be approximately 150.

Identification and characterization of the herpes simplex virus type 1 virion protein encoded by the UL35 open reading frame

The UL35 open reading frame (ORF) of herpes simplex virus type 1 (HSV-1) has been predicted from DNA sequence analysis to encode a small polypeptide with a molecular weight of 12,095. We have investigated the protein product of the UL35 ORF by using a trpE-UL35 gene fusion to produce a corresponding fusion protein in Escherichia coli. The TrpE-UL35 chimeric protein was subsequently isolated and used as a source of immunogen for the production of rabbit polyclonal antiserum directed against the UL35 gene product. The TrpE-UL35 antiserum was found to recognize a 12-kDa protein which was specifically present in HSV-1-infected cells. By utilizing the TrpE-UL35 antiserum, the kinetics of synthesis of the UL35 gene product was examined, and these studies indicate that UL35 is expressed as a gamma 2 (true late) gene. The 12-kDa protein recognized by the TrpE-UL35 antiserum was associated with purified HSV-1 virions and type A and B capsids, suggesting that the UL35 ORF may encode the 12-kDa capsid protein variably designated p12, NC7, or VP26. To confirm this assignment, immunoprecipitation and immunoblotting studies were performed to demonstrate that the TrpE-UL35 antiserum reacts with the same polypeptide as an antiserum directed against the purified p12 capsid protein (anti-NC7) (G.H. Cohen, M. Ponce de Leon, H. Diggelmann, W.C. Lawrence, S.K. Vernon, and R.J. Eisenberg, J. Virol. 34:521-531, 1980). Furthermore, the anti-NC7 serum was also found to react with the TrpE-UL35 chimeric protein isolated from E. coli, providing additional evidence that the UL35 gene encodes p12. On the basis of these studies, we conclude that UL35 represents a true late gene which encodes the 12-kDa capsid protein of HSV-1.

Herpes simplex virus envelopment and maturation studied by fracture label

Herpes simplex virus envelopment and maturation were investigated by thin-section fracture label. The distribution of glycoproteins B and D was analyzed by labeling with antibodies; the precursor and mature forms of the glycoproteins were differentiated by labeling with the lectins concanavalin A (ConA) and wheat germ agglutinin (WGA), respectively. We report that the two glycoproteins were readily detected in the intracellular virion, whether located between the inner and outer nuclear membranes or within cytoplasmic membrane-bound vesicles and in the inner and outer nuclear membranes themselves. The enveloped virion between the inner and outer nuclear membranes labeled with ConA but not with WGA. During the transit to the extracellular space the reactivity of the virion membranes with ConA decreased and that with WGA ensued. The results document that herpes simplex viruses acquire at the inner nuclear membrane an envelope carrying the immature forms of the glycoproteins and that during the transit to the extracellular space the envelope glycoproteins become of the fully processed type.

Expression of a human cytomegalovirus receptor correlates with infectibility of cells

Previous studies have demonstrated that human cytomegalovirus (HCMV) specifically binds to a fibroblast membrane glycoprotein(s) with a molecular mass from 30 to 34 kDa. In this study, the distribution of the putative receptor proteins was analyzed in a variety of cell types, including cell types representative of those that are infected in vivo. Using a sensitive microbinding assay (to score virus attachment) and an indirect detection method (to score HCMV-binding proteins), we found that the 34- and 32-kDa HCMV binding proteins are ubiquitous molecules, broadly distributed among diverse cell types. In addition, the level of virus attachment was found to correlate with the abundance of the 34- and 32-kDa cellular proteins, while the ability of the virus to penetrate cells and initiate infection did not. The results support the hypothesis that the 34- and 32-kDa cellular proteins represent the HCMV (attachment) receptor. The data also support the notion that additional cellular components are required for virus entry and fusion.

Brefeldin A arrests the maturation and egress of herpes simplex virus particles during infection

Herpes simplex virus (HSV) requires the host cell secretory apparatus for transport and processing of membrane glycoproteins during the course of virus assembly. Brefeldin A (BFA) has been reported to induce retrograde movement of molecules from the Golgi to the endoplasmic reticulum and to cause disassembly of the Golgi complex. We examined the effects of BFA on propagation of HSV type 1. Release of virions into the extracellular medium was blocked by as little as 0.3 microgram of BFA per ml when present from 2 h postinfection. Characterization of infected cells revealed that BFA inhibited infectious viral particle formation without affecting nucleocapsid formation. Electron microscopic analyses of BFA-treated and untreated cells (as in control cells) demonstrated that viral particles were enveloped at the inner nuclear membrane in BFA-treated cells and accumulated aberrantly in this region. Most of the progeny virus particles observed in the cytoplasm of control cells, but not that of BFA-treated cells, were enveloped and contained within membrane vesicles, whereas many unenveloped nucleocapsids were detected in the cytoplasm of BFA-treated cells. This suggests that BFA prevents the transport of enveloped particles from the perinuclear space to the cytoplasmic vesicles. These findings indicate that BFA-induced retrograde movement of molecules from the Golgi complex to the endoplasmic reticulum early in infection arrests the ability of host cells to support maturation and egress of enveloped viral particles. Furthermore, we demonstrate that the effects of BFA on HSV propagation are not fully reversible, indicating that maturation and egress of HSV type 1 particles relies on a series of events which cannot be easily reconstituted after the block to secretion is relieved.

Association of a major transcriptional regulatory protein, ICP4, of herpes simplex virus type 1 with the plasma membrane of virus-infected cells

A major transcriptional regulatory protein, ICP4, of herpes simplex virus type 1 (HSV-1) is localized primarily within the nucleus soon after its synthesis. Recent studies have shown that approximately 100 to 200 molecules of ICP4 are located in the tegument region of purified virions (F. Yao and R. J. Courtney, J. Virol. 63:3338-3344, 1989). As an extension to these studies, we present data suggesting that ICP4 may also associate with the plasma membrane of HSV-1-infected cells. The experimental approaches used included the isolation and purification of plasma membranes from HSV-1-infected cells, the isolation of purified vesicular stomatitis virus containing ICP4, and immunofluorescence of HSV-1-infected cells following selective permeabilization with detergent. The results from the above studies support the suggestion that detectable amounts of ICP4 are associated with the inner surface of the plasma membrane of HSV-1-infected cells.

Origin of unenveloped capsids in the cytoplasm of cells infected with herpes simplex virus 1

In cells infected with herpes simplex viruses the capsids acquire an envelope at the nuclear membrane and are usually found in the cytoplasm in structures bound by membranes. Infected cells also accumulate unenveloped capsids alone or juxtaposed to cytoplasmic membranes. The juxtaposed capsids have been variously interpreted as either undergoing terminal deenvelopment resulting from fusion of the envelope with the membrane of the cytoplasmic vesicles or undergoing sequential envelopment and deenvelopment as capsids transit the cytoplasm into the extracellular space. Recent reports have shown that (i) wild-type virus attaches to but does not penetrate cells expressing glycoprotein D (G. Campadelli-Fiume, M. Arsenakis, F. Farabegoli, and B. Roizman, J. Virol. 62:159-167, 1988) and that (ii) a mutation in glycoprotein D enables the mutant virus to productively infect cells expressing the wild-type glycoprotein (G. Campadelli-Fiume, S. Qi, E. Avitabile, L. Foa-Tomasi, R. Brandimarti, and B. Roizman, J. Virol. 64:6070-6079, 1990). If the unenveloped capsids in the cytoplasm result from fusion of the cytoplasmic membranes with the envelopes of viruses transiting the cytoplasm, cells infected with virus carrying the mutation in glycoprotein D should contain many more unenveloped capsids in the cytoplasm inasmuch as there would be little or no restriction in the fusion of the envelope with cytoplasmic membranes. Comparison of thin sections of baby hamster kidney cells infected with wild-type and mutant viruses indicated that this was the case. Moreover, in contrast to the wild-type parent, the mutant virus was not released efficiently from infected cells. The conclusion that the unenveloped capsids are arrested forms of deenveloped capsids is supported by the observation that the unenveloped capsids were unstable in that they exhibited partially extruded DNA.

Herpes simplex virus particles are unable to traverse the secretory pathway in the mouse L-cell mutant gro29

The mouse L-cell mutant gro29 was selected for its ability to survive infection by herpes simplex virus type 1 (HSV-1) and is defective in the propagation of HSV-1 and vesicular stomatitis virus (F. Tufaro, M. D. Snider, and S. L. McKnight, J. Cell Biol. 105:647-657, 1987). In this report, we show that gro29 cells harbor a lesion that inhibits the egress of HSV-1 virions during infection. We also found that HSV-1 glycoprotein D was slow to traverse the secretory pathway en route to the plasma membrane of infected gro29 cells. The movement of glycoproteins was not blocked entirely, however, and immunofluorescence experiments revealed that infected gro29 cells contained roughly 10% of the expected amount of glycoprotein D on their cell surface at 12 h postinfection. Furthermore, nucleocapsids and virions assembled inside the cells during infection, suggesting that the lesion in gro29 cells impinged on a late step in virion maturation. Electron micrographs of infected cells revealed that many of the intracellular virions were contained in irregular cytoplasmic vacuoles, similar to those that accumulate in HSV-1-infected cells treated with the ionophore monensin. We conclude from these results that gro29 harbors a defect that blocks the egress of HSV-1 virions from the infected cell without seriously impeding the flux of individual glycoproteins to the cell surface. We infer that HSV-1 maturation and egress require a host cell component that is either reduced or absent in gro29 cells and that this lesion, although not lethal to the host cell, cannot be tolerated by HSV-1 during its life cycle.

Oligomerization of herpes simplex virus glycoprotein B occurs in the endoplasmic reticulum and a 102 amino acid cytosolic domain is dispensable for dimer assembly

Glycoprotein B (gB) is an essential protein specified by herpes simplex virus and a major envelope component of the virions. It is known to assemble into noncovalently associated dimers. The aim of this study was to investigate the role of topogenic domains of gB in dimer assembly and the intracellular location at which gB dimers are assembled. Therefore, dimer analyses were performed on intact gB and its three COOH-terminus-truncated gB derivatives encoding NH2-terminal 772, 586, and 477 amino acids (aa) of the mature gB, using SDS-polyacrylamide gel electrophoresis and sucrose gradient assays. Dimers were detected in gB and in tgB(772 aa), but were absent from tgB(586 aa) and from tgB(477 aa). These results showed that a 102 aa cytosolic domain (aa 773-874) is not required for the assembly of gB dimers. In addition, using endoglycosidase H treatment and dimer analysis of gB synthesized during 7 min pulse-labeling period, we have demonstrated that ER is the subcellular organelle at which gB monomers are assembled into dimeric forms.

Synthesis and processing of equine herpesvirus type 1 glycoprotein 14

Glycoprotein 14 (gp14) of equine herpesvirus type 1 (EHV-1), the homolog of herpes simplex virus (HSV) glycoprotein B (gB), was investigated employing a panel of monoclonal antibodies to ascertain the regulatory class, rate of synthesis, and type of glycosylation of this polypeptide. Application of immunoprecipitation, Western blot, and SDS-PAGE analysis in conjunction with the use of metabolic inhibitors (cycloheximide, antinomycin D, phosphonoacetic acid, tunicamycin, and monensin), and time-course and pulse-chase experiments revealed the following information: (1) Three gp14-related polypeptides with molecular weights of 138 kilodaltons (K), 77-75K, and 55-53K are present in EHV-1-infected cell extracts. (2) All three species are synthesized in the presence of the DNA synthesis inhibitor phosphonoacetic acid although their synthesis is enhanced by DNA replication, indicative of a beta-gamma class molecule. (3) The 138K species is synthesized first as a precursor of the smaller species of gp14, the 77-75K and 55-53K forms. (4) Use of glycosylation inhibitors and digestion of immunoprecipitated gp14 with endoglycosidases indicate that the primary translation product is a 118K molecule which is cotranslationally glycosylated to the 138K form by the addition of high mannose oligosaccharides. (5) The 77-75K species contains both high mannose and hybrid oligosaccharides while the 55-53K form of gp14 contains some complex oligosaccharides. (6) In the absence of a reducing agent, the 138K polypeptide and a large 145K species are observed in both infected cell extracts and purified virions. Thus, EHV-1 gp14 appears to be synthesized as a large precursor molecule of 138K and is proteolytically cleaved to two smaller forms, 77-75K and 55-53K, which are linked by a disulfide bond(s) to form a 145K complex. This model of gp14 synthesis and maturation is similar to those proposed for a number of HSV gB equivalents found in the Alphaherpesvirnae.

Varicella-zoster virus glycoprotein oligosaccharides are phosphorylated during posttranslational maturation

Varicella-zoster virus (VZV)-infected human embryonic lung fibroblasts (HELF) do not release infectious virions into their growth medium. Extracellular virions are pleomorphic, suggesting that they are partially degraded before their release from cells. To examine the intracellular pathway of viral maturation, [2-3H]mannose-labeled virus-encoded glycoproteins were isolated from VZV-infected HELF. Oligosaccharides attached to the glycoproteins were processed to complex-type units, some of which were phosphorylated. The major intracellular site of accumulation of VZV gpI was found to be perinuclear and to correspond to that of the cation-independent mannose 6-phosphate (Man 6-P) receptor. Subsets of VZV-containing cytoplasmic vacuoles were coated, Golgi-associated, or accessible to endocytic tracers. Phosphorylated monosaccharides protected HELF from the cytopathic effect of VZV in proportion to their ability to block Man 6-P receptor-mediated endocytosis. These data suggest that the unusual phosphorylated oligosaccharides mediate an interaction between VZV and Man 6-P receptors of the host cell; this interaction may be responsible for withdrawal of newly synthesized virions from the secretory pathway and for their diversion to prelysosomal structures.

Relatedness of glycoproteins expressed on the surface of simian herpes-virus virions and infected cells to specific HSV glycoproteins

The antigenic relatedness of the surface glycoprotein antigens of six herpesviruses indigenous to human and nonhuman primates was examined. Binding of anti-viral sera to viral antigens expressed on the surface of infected cells demonstrated that the surface antigens of herpes simplex virus type 1 (HSV 1), HSV 2, simian agent 8 (SA8), and Herpesvirus simiae (B virus) exhibit extensive cross-reactivity. Surface antigens of two viruses isolated from South American primates, H. saimiri 1 (HVS 1) and H. ateles 1 (HVA 1), were comparatively more virus-specific in their antigenic reactivity. Endpoint neutralization tests performed in the presence and absence of complement confirmed these results. Immunoprecipitation of viral proteins was used to identify those representing cross-reactive surface antigens. A glycoprotein of approximately 110,000-125,000 Daltons (110-125 k) was immunoprecipitated from cells infected with each of the six primate herpesvirus by antisera to each of the viruses. Using monospecific antisera, these glycoproteins were shown to be antigenically related to the gB glycoproteins of HSV. Although these glycoproteins were antigenically conserved among all six viruses, antibodies to the gB glycoproteins did not cross-neutralize heterologous viruses. A glycoprotein of approximately 60-70 k was precipitated from HSV 1, HSV 2, SA8, and B virus infected cells by antisera to each of these four viruses. These SA8 and B virus glycoproteins were shown to be antigenically related to the gD glycoproteins of HSV 1 and HSV 2 and to be involved in cross-neutralization among these viruses. Antisera to HVS 1 and HVA 1 did not recognize these gD glycoproteins nor was a glycoprotein of similar molecular weight precipitable from HVS 1 or HVA 1 infected cells by antisera to the other four viruses. Southern blot hybridizations using probes for HSV glycoprotein genes confirmed the conservation of the gB glycoproteins among all the simian viruses and of the gD gene in SA8 and B virus. A glycoprotein of approximately 75-80 k was, however, precipitated from HVS 1 and HVA 1 infected cells by antisera to either of these two viruses. In addition, at least one glycoprotein which appeared to be predominantly virus-specific in its reactivity was identified for five of the viruses.

A major transcriptional regulatory protein (ICP4) of herpes simplex virus type 1 is associated with purified virions

Herpes simplex virus type 1 was purified by density gradient centrifugation, and the virion-associated proteins were resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. By Western blot (immunoblot) analysis with an anti-ICP4 monospecific serum, the results indicated that ICP4, one of the five immediate-early proteins of herpes simplex virus type 1, was associated with the purified virions. To define the location of ICP4 within the virion, trypsin digestion experiments were performed. Purified virions were treated with trypsin in the presence or absence of detergent. The virus envelope appeared to protect ICP4 from the trypsin, since virus-associated ICP4 was sensitive to digestion only after detergent treatment. In addition, ICP4 remained associated with the virus particle when the virion-specific glycoproteins were removed after detergent treatment. Finally, ICP4 was not detected in purified preparations of type A and B capsids isolated from the nuclear fraction of virus-infected cells. The above-mentioned data suggest that detectable amounts of ICP4 are present within the tegument region of the virion.

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

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

Role of cytoplasmic vacuoles in varicella-zoster virus glycoprotein trafficking and virion envelopment

Varicella-zoster virus (VZV) encodes several glycoproteins which are present on both mature viral envelopes and the surfaces of infected cell membranes. Mechanisms of VZV glycoprotein transport and virion envelopment were investigated by both continuous radiolabeling and pulse-chase analyses with tritiated fucose in VZV-infected cells. We studied in detail the large cytoplasmic vacuoles which were present in infected cells but absent from uninfected cells. The specific activity in each subcellular compartment was defined by quantitative electron microscope autoradiography, using a cross-fire probability matrix analysis to more accurately assess the individual compartment demarcated by the silver grains. By these techniques, we documented a progression of activity originating in the Golgi apparatus and traveling through the post-Golgi region into virus-induced cytoplasmic vacuoles and finally to areas of the cellular membrane associated with the egress of viral particles. Significant amounts of radiolabel were not observed in the nucleus, and only low levels of radiolabel were associated with the cellular membrane not involved with the egress of viral particles. In addition, immunolabeling of Lowicryl-embedded VZV-infected cells demonstrated the presence of VZV glycoproteins within cytoplasmic vacuole membranes as well as on virion envelopes. These observations suggested that cytoplasmic vacuoles harbored VZV-specified glycoproteins and were also the predominant site of VZV virion envelopment within the infected cell. Neither enveloped nor unenveloped viral particles were observed within the Golgi apparatus itself.

Herpes simplex virus type 1-specific cytotoxic T lymphocytes recognize virus nonstructural proteins

The specificity of herpes simplex virus type 1-specific cytotoxic T cells was examined with target cells expressing either input viral structural antigens or antigens resulting from permissive infection or cells from an interrupted infection in which they expressed predominantly nonstructural immediate-early proteins. These studies indicated that only an insignificant minority of cytotoxic T cells recognized the input viral antigens, whereas a significant proportion (20 to 35%) recognized target cells that expressed the immediate-early proteins despite the absence of serologically detectable viral antigens upon the infected cell surface. The finding that a significant proportion of cytotoxic T-cell populations obtained from the draining lymph nodes of mice acutely infected with herpes simplex virus type 1 also recognized immediately-early gene-expressing target cells indicates the importance of nonstructural herpes simplex virus proteins to antiviral immunity in vivo.

Abundant expression of herpes simplex virus glycoprotein gB using an adenovirus vector

Herpes simplex virus type 1 (HSV-1) glycoprotein B (gB) is a major component of infected cell membranes and virion envelopes. Glycoprotein B is known to be essential for entry of viruses into cells and may play important roles in virus-induced cell fusion and other alterations in cell morphology. In order to study the biochemical and immunological properties of gB in isolation from other HSV-1 polypeptides we have constructed human adenovirus vectors capable of expressing high levels of gB. The gB gene was coupled to the SV40 early promoter and inserted into the E3 region of two adenovirus vectors, one in which the E1 region was deleted (AdgB-1) and another which contained E1 sequences (AdgB-2). In AdgB-1 the orientation of the chimeric gB-SV40 gene was right to left, i.e., opposite to the direction of late and E3 mRNA transcription, whereas in AdgB-2 the orientation was left to right. Human 293 cells which express E1 functions supported replication of AdgB-1 and gB was expressed in these cells but not in mouse cells and only at very low levels in human cells other than 293. Replication of AdgB-2 was not limited to 293 cells and the virus was able to induce synthesis of gB at levels equal to or higher than those expressed in HSV-1-infected human or mouse cells. Microscopic examination of AdgB-2-infected cells revealed extensive vacuolization in a manner completely uncharacteristic of adenovirus-infected cells, and fluorescent antibody staining indicated that gB was not only present at the cell surface but also concentrated in the cytoplasmic vacuoles.