Herpes simplex virus type 1 mediates fusion through a hemifusion intermediate by sequential activity of glycoproteins D, H, L, and B

Structure-function analysis of herpes simplex virus glycoprotein B with fusion-from-without activity

Fusion-from-without (FFWO) is the rapid induction of cell fusion by virions in the absence of viral protein synthesis. The combination of two amino acid mutations in envelope glycoprotein B (gB), one in the ectodomain and one in the cytoplasmic tail, can confer FFWO activity to wild type herpes simplex virus (HSV). In this report, we analyzed the entry and cell fusion phenotypes of HSV that contains FFWO gB, with emphasis on the cellular receptors for HSV, nectin-1, nectin-2 and HVEM. The ability of an HSV strain with FFWO gB to efficiently mediate FFWO via a specific gD-receptor correlated with its ability to mediate viral entry by that receptor. A FFWO form of gB was not sufficient to switch the entry of HSV from a pH-dependent, endocytic pathway to a direct fusion, pH-independent pathway. The conformation of gB with FFWO activity was not globally altered relative to wild type. However, distinct monoclonal antibodies had reduced reactivity with FFWO gB, suggesting an altered antigenic structure relative to wild type. FFWO was blocked by preincubation of virions with neutralizing antibodies to gB or gD. Together with previous studies, the results indicate that the roles of gB in FFWO and in virus-cell fusion during entry are related but not identical. This study also suggests that the FFWO function of gB is not a specific determinant for the selection of HSV entry pathway and that antigenic differences in FFWO gB may reflect its enhanced fusion activity.

Functional studies of per os infectivity factors of Helicoverpa armigera single nucleocapsid nucleopolyhedrovirus

A combined functional investigation on the four per os infectivity factors (PIFs) of Helicoverpa armigera single nucleocapsid nucleopolyhedrovirus (HearNPV) was conducted in this study. HearNPV bacmids with deletions of p74 (Ha20), pif1 (Ha111), pif2 (Ha132) and pif3 (Ha98) were constructed individually by homologous recombination in Escherichia coli cells. Repaired bacmids with respective pifs were also constructed. Western blot analyses revealed that all four PIFs were structural components of the envelope of HearNPV occlusion-derived virus (ODV). Electron microscopy showed that deletion of the pifs did not have any obvious effects on the morphology of the occlusion bodies (OBs). Bioassay analyses indicated that deletion of any of the above pifs resulted in loss of oral infectivity of OBs. The mixtures of the four pif-deletion mutants also resulted in deficiency of oral infectivity, implying that the four PIFs must be structural components of the same ODV to accomplish their function. Repairing of the respective genes into the pif-deletion bacmids could rescue the oral infectivity of the pif-deletion viruses. Calcofluor, which can damage the peritrophic membrane (PM), could not rescue the defects of the oral infectivity of the pif-deletion viruses, indicating that the PM is not likely to be the functional target of the PIFs.

Human cytomegalovirus glycoproteins gB and gH/gL mediate epithelial cell-cell fusion when expressed either in cis or in trans

Herpesviruses use a cascade of interactions with different cell surface molecules to gain entry into cells. In many cases, this involves binding to abundant glycosaminoglycans or integrins followed by interactions with more limited cell surface proteins, leading to fusion with cellular membranes. Human cytomegalovirus (HCMV) has the ability to infect a wide variety of human cell types in vivo. However, very little is known about which HCMV glycoproteins mediate entry into various cell types, including relevant epithelial and endothelial cells. For other herpesviruses, studies of cell-cell fusion induced by viral proteins have provided substantial information about late stages of entry. In this report, we describe the fusion of epithelial, endothelial, microglial, and fibroblast cells in which HCMV gB and gH/gL were expressed from nonreplicating adenovirus vectors. Fusion frequently involved the majority of cells, and gB and gH/gL were both necessary and sufficient for fusion, whereas no fusion occurred when either glycoprotein was omitted. Coexpression of UL128, UL130, and UL131 did not enhance fusion. We concluded that the HCMV core fusion machinery consists of gB and gH/gL. Coimmunoprecipitation indicated that HCMV gB and gH/gL can interact. Importantly, expression of gB and gH/gL in trans (gB-expressing cells mixed with other gH/gL-expressing cells) resulted in substantial fusion. We believe that this is the first description of a multicomponent viral fusion machine that can be split between cells. If gB and gH/gL must interact for fusion, then these molecules must reach across the space between apposing cells. Expression of gB and gH/gL in trans with different cell types revealed surface molecules that are required for fusion on HCMV-permissive cells but not on nonpermissive cells.

Ocular HSV-1 latency, reactivation and recurrent disease

Ocular infection with HSV-1 continues to be a serious clinical problem despite the availability of effective antivirals. Primary infection with HSV-1 can involve ocular and adenaxial sites and can manifest as blepharitis, conjunctivitis, or corneal epithelial keratitis. After initial ocular infection, HSV-1 can establish latent infection in the trigeminal ganglia for the lifetime of the host. During latency, the viral genome is retained in the neuron without producing viral proteins. However, abundant transcription occurs at the region encoding the latency-associated transcript, which may play significant roles in the maintenance of latency as well as neuronal reactivation. Many host and viral factors are involved in HSV-1 reactivation from latency. HSV-1 DNA is shed into tears and saliva of most adults, but in most cases this does not result in lesions. Recurrent disease occurs as HSV-1 is carried by anterograde transport to the original site of infection, or any other site innervated by the latently infected ganglia, and can reinfect the ocular tissues. Recurrent corneal disease can lead to corneal scarring, thinning, stromal opacity and neovascularization and, eventually, blindness. In spite of intensive antiviral and anti-inflammatory therapy, a significant percentage of patients do not respond to chemotherapy for herpetic necrotizing stromal keratitis. Therefore, the development of therapies that would reduce asymptomatic viral shedding and lower the risks of recurrent disease and transmission of the virus is key to decreasing the morbidity of ocular herpetic disease. This review will highlight basic HSV-1 virology, and will compare the animal models of latency, reactivation, and recurrent ocular disease to the current clinical data.

HCMV gH/gL/UL128-131 interferes with virus entry into epithelial cells: evidence for cell type-specific receptors

Human cytomegalovirus (HCMV) forms two different membrane protein complexes, gH/gL/gO and gH/gL/UL128/UL130/UL131, that function in different cell types. gH/gL/gO appears to be important for HCMV entry into or spread between fibroblasts, processes that occur at neutral pH. We demonstrated that HCMV entry into epithelial and endothelial cells requires gH/gL/UL128-131 and involves endocytosis and low pH. A complex of all five HCMV proteins, gH, gL, UL128, UL130, and UL131, is the functionally important mediator of this entry pathway into epithelial/endothelial cells. Here, we report that expression of gH/gL/UL128-131 in ARPE-19 epithelial cells causes the cells to be resistant to HCMV infection. Another HCMV glycoprotein, gB, did not interfere, and expression of all five gH/gL/UL128-131 proteins was required for this interference. gH/gL/UL128-131 interference was at the stage of virus entry into cells rather than the initial adsorption onto cell surfaces or after-entry defects. By contrast, expression of gH/gL/UL128-131 in primary human fibroblasts did not block HCMV infection. Previously, interference by retrovirus and herpes-simplex-virus entry mediators resulted from sequestration or obstruction of receptors. We concluded that epithelial cells express gH/gL/UL128-131 receptors that mediate HCMV entry. Fibroblasts either lack the gH/gL/UL128-131 receptors, the receptors are more numerous, or fibroblasts express other functional receptors.

Peptides containing membrane-interacting motifs inhibit herpes simplex virus type 1 infectivity

Herpes simplex virus (HSV) membrane fusion represents an attractive target for anti-HSV therapy. To investigate the structural basis of HSV membrane fusion and identify new targets for inhibition, we have investigated the different membranotropic domains of HSV-1 gH envelope glycoprotein. We observed that fusion peptides when added exogenously are able to inhibit viral fusion likely by intercalating with viral fusion peptides upon adopting functional structure in membranes. Interestingly, peptides analogous to the predicted HSV-1 gH loop region inhibited viral plaque formation more significantly. Their inhibitory effect appears to be a consequence of their ability to partition into membranes and aggregate within them. Circular dichroism spectra showed that peptides self-associate in aqueous and lipidic solutions, therefore the inhibition of viral entry may occur via peptides association with their counterpart on wild-type gH. The antiviral activity of HSV-1 peptides tested provides an attractive basis for the development of new fusion peptide inhibitors corresponding to regions outside the fusion protein heptad repeat regions.

Identification of restriction endonuclease with potential ability to cleave the HSV-2 genome: inherent potential for biosynthetic versus live recombinant microbicides

BACKGROUND

Herpes Simplex virus types 1 and 2 are enveloped viruses with a linear dsDNA genome of approximately 120-200 kb. Genital infection with HSV-2 has been denoted as a major risk factor for acquisition and transmission of HIV-1. Developing biomedical strategies for HSV-2 prevention is thus a central strategy in reducing global HIV-1 prevalence. This paper details the protocol for the isolation of restriction endunucleases (REases) with potent activity against the HSV-2 genome and models two biomedical interventions for preventing HSV-2.

METHODS AND RESULTS

Using the whole genome of HSV-2, 289 REases and the bioinformatics software Webcutter2; we searched for potential recognition sites by way of genome wide palindromics. REase application in HSV-2 biomedical therapy was modeled concomitantly. Of the 289 enzymes analyzed; 77(26.6%) had potential to cleave the HSV-2 genome in > 100 but < 400 sites; 69(23.9%) in > 400 but < 700 sites; and the 9(3.1%) enzymes: BmyI, Bsp1286I, Bst2UI, BstNI, BstOI, EcoRII, HgaI, MvaI, and SduI cleaved in more than 700 sites. But for the 4: PacI, PmeI, SmiI, SwaI that had no sign of activity on HSV-2 genomic DNA, all 130(45%) other enzymes cleaved < 100 times. In silico palindromics has a PPV of 99.5% for in situ REase activity (2) Two models detailing how the REase EcoRII may be applied in developing interventions against HSV-2 are presented: a nanoparticle for microbicide development and a "recombinant lactobacillus" expressing cell wall anchored receptor (truncated nectin-1) for HSV-2 plus EcoRII.

CONCLUSION

Viral genome slicing by way of these bacterially- derived R-M enzymatic peptides may have therapeutic potential in HSV-2 infection; a cofactor for HIV-1 acquisition and transmission.

Analysis of a membrane interacting region of herpes simplex virus type 1 glycoprotein H

Glycoprotein H (gH) of herpes simplex virus type I (HSV-1) is involved in the complex mechanism of membrane fusion of the viral envelope with the host cell. Membrane interacting regions and potential fusion peptides have been identified in HSV-1 gH as well as glycoprotein B (gB). Because of the complex fusion mechanism of HSV-1, which requires four viral glycoproteins, and because there are only structural data for gB and glycoprotein D, many questions regarding the mechanism by which HSV-1 fuses its envelope with the host cell membrane remain unresolved. Previous studies have shown that peptides derived from certain regions of gH have the potential to interact with membranes, and based on these findings we have generated a set of peptides containing mutations in one of these domains, gH-(626-644), to investigate further the functional role of this region. Using a combination of biochemical, spectroscopic, and nuclear magnetic resonance techniques, we showed that the alpha-helical nature of this stretch of amino acids in gH is important for membrane interaction and that the aromatic residues, tryptophan and tyrosine, are critical for induction of fusion.

Structures and mechanisms of viral membrane fusion proteins: multiple variations on a common theme

Recent work has identified three distinct classes of viral membrane fusion proteins based on structural criteria. In addition, there are at least four distinct mechanisms by which viral fusion proteins can be triggered to undergo fusion-inducing conformational changes. Viral fusion proteins also contain different types of fusion peptides and vary in their reliance on accessory proteins. These differing features combine to yield a rich diversity of fusion proteins. Yet despite this staggering diversity, all characterized viral fusion proteins convert from a fusion-competent state (dimers or trimers, depending on the class) to a membrane-embedded homotrimeric prehairpin, and then to a trimer-of-hairpins that brings the fusion peptide, attached to the target membrane, and the transmembrane domain, attached to the viral membrane, into close proximity thereby facilitating the union of viral and target membranes. During these conformational conversions, the fusion proteins induce membranes to progress through stages of close apposition, hemifusion, and then the formation of small, and finally large, fusion pores. Clearly, highly divergent proteins have converged on the same overall strategy to mediate fusion, an essential step in the life cycle of every enveloped virus.

The Murid Herpesvirus-4 gL regulates an entry-associated conformation change in gH

The glycoprotein H (gH)/gL heterodimer is crucial for herpesvirus membrane fusion. Yet how it functions is not well understood. The Murid Herpesvirus-4 gH, like that of other herpesviruses, adopts its normal virion conformation by associating with gL. However, gH switched back to a gL-independent conformation after virion endocytosis. This switch coincided with a conformation switch in gB and with capsid release. Virions lacking gL constitutively expressed the down-stream form of gH, prematurely switched gB to its down-stream form, and showed premature capsid release with poor infectivity. These data argue that gL plays a key role in regulating a gH and gB functional switch from cell binding to membrane fusion.

Native 3D intermediates of membrane fusion in herpes simplex virus 1 entry

The concerted action of four viral glycoproteins and at least one cellular receptor is required to catalyze herpes simplex virus 1 entry into host cells either by fusion at the plasma membrane or intracellularly after internalization by endocytosis. Here, we applied cryo electron tomography to capture 3D intermediates from Herpes simplex virus 1 fusion at the plasma membrane in their native environment by using two model systems: adherent cells and synaptosomes. The fusion process was delineated as a series of structurally different steps. The incoming capsid separated from the tegument and was closely surrounded by the cortical cytoskeleton. After entry, the viral membrane curvature changed concomitantly with a reorganization of the envelope glycoprotein spikes. Individual glycoprotein complexes in transitional conformations during pore formation and dilation revealed the complex viral fusion mechanism in action. Snapshots of the fusion intermediates provide unprecedented details concerning the overall structural changes occurring during herpesvirus entry. Moreover, our data suggest that there are two functional "poles" of the asymmetric herpesvirion: one related to cell entry, and the other formed during virus assembly.

Effect of human apolipoprotein E genotype on the pathogenesis of experimental ocular HSV-1

The isoform-specific role of human apolipoprotein E (apoE) has been assessed in a mouse model of ocular herpes. Female, age-matched transgenic mice knocked-in for the human allele apoE3 or apoE4 and their parent C57Bl/6 mice were inoculated corneally with HSV-1 strain KOS. Ocular HSV-1 pathogenesis was monitored through viral replication and clinical progression of stromal opacity and neovascularization by slit-lamp examination. Establishment of latency was determined by analysis of HSV-1 DNA (copy number) by specific real-time PCR in the cornea, trigeminal ganglia (TG), and brain. Representative groups of transgenic mice were sacrificed for the analysis of gene expression of vascular endothelial growth factor (VEGF) by reverse-transcription PCR, and apoE expression by Western blot analysis. At 6days post-infection (P.I.), the ocular infectious HSV-1 titer was significantly higher (p<0.05) in apoE4 mice compared with apoE3 and C57Bl/6 mice. Corneal neovascularization in apoE4 mice was significantly higher (p<0.05) than apoE3 and C57Bl/6 mice. The onset of corneal opacity in apoE4 mice was accelerated during days 9-11 P.I.; however, no significant difference in severity was seen on P.I. days 15 and beyond. At 28 days P.I., infected mice of all genotypes had no significant differences in copy numbers (range 0-15) of HSV-1 DNA in their corneas, indicating that HSV-1 DNA copy numbers in cornea are independent of apoE isoform regulation. At 28 days P.I., both apoE4 and C57Bl/6 mice had a significantly higher (p=0.001) number of copies of HSV-1 DNA in TG compared with apoE3. ApoE4 mice also had significantly higher (p=0.001) copies of HSV-1 DNA in their TGs compared with C57Bl/6 mice. In brain, both apoE4 and C57Bl/6 mice had significantly higher numbers (p<or=0.03) of copies of HSV-1 DNA compared with apoE3 mice. However, the number of HSV-1 DNA copies in the brain of C57Bl/6 mice was not significantly different than that of apoE4 (p=0.1). Comparative molecular analysis between apoE3 and apoE4 mice on selected days between 7 and 28 P.I., inclusive, revealed that the corneas of apoE4 mice expressed VEGF. None of the corneas in the apoE3 mice expressed VEGF during this time. Western blot analysis showed proteolytic cleavage of the apoE protein in the corneas of the apoE4 mice. Through days 14-28 P.I., a approximately 29 kDa C-terminal truncated apoE fragment was present in the corneas of apoE4 mice, but not in apoE3 mice. ApoE4 is a risk factor for ocular herpes, in part, through increased replication of virus in the eye, an earlier onset in clinical opacity, significantly higher neovascularization, and increased HSV-1 DNA load in TG and brain than that of apoE3. Increased pathogenesis of ocular herpes in apoE4 mice was also mediated, in part through up-regulated expression of VEGF and apoE proteolysis in the cornea. This is the first report linking a human gene, apoE4, as a risk factor for ocular herpes pathogenesis in a transgenic mouse model.

The identification and characterization of fusogenic domains in herpes virus glycoprotein B molecules

The molecular mechanism of entry of herpes viruses requires a multicomponent fusion system. Virus entry and cell-cell fusion of Herpes simplex virus (HSV) requires four glycoproteins: gD, gB and gH/gL. The role of gB remained elusive until recently, when the crystal structure of HSV-1 gB became available. Glycoprotein B homologues represent the most highly conserved group of herpes virus glycoproteins; however, despite the high degree of sequence and structural conservation, differences in post-translational processing are observed for different members of this virus family. Whereas gB of HSV is not proteolytically processed after oligomerization, most other gB homologues are cleaved by a cellular protease into subunits that remain linked through disulfide bonds. Proteolytic cleavage is common for activation of many other viral fusion proteins, so it remains difficult to envisage a common role for different herpes virus gB structures in the fusion mechanism. We selected bovine herpes virus type 1 (BoHV-1) and herpes simplex virus type 1 (HSV-1) as representative viruses expressing cleaved and uncleaved gBs, and have screened their amino acid sequences for regions of highly interfacial hydrophobicity. Synthetic peptides corresponding to such regions were tested for their ability to induce the fusion of large unilamellar vesicles and to inhibit herpes virus infection. These results underline that several regions of the gB protein are involved in the mechanism of membrane interaction.

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.

PILRalpha is a herpes simplex virus-1 entry coreceptor that associates with glycoprotein B

Glycoprotein B (gB) is one of the essential components for infection by herpes simplex virus-1 (HSV-1). Although several cellular receptors that associate with glycoprotein D (gD), such as herpes virus entry mediator (HVEM) and Nectin-1, have been identified, specific molecules that mediate HSV-1 infection by associating with gB have not been elucidated. Here, we found that paired immunoglobulin-like type 2 receptor (PILR) alpha associates with gB, and cells transduced with PILRalpha become susceptible to HSV-1 infection. Furthermore, HSV-1 infection of human primary cells expressing both HVEM and PILRalpha was blocked by either anti-PILRalpha or anti-HVEM antibody. Our results demonstrate that cellular receptors for both gB and gD are required for HSV-1 infection and that PILRalpha plays an important role in HSV-1 infection as a coreceptor that associates with gB. These findings uncover a crucial aspect of the mechanism underlying HSV-1 infection.

The herpes simplex virus receptor nectin-1 is down-regulated after trans-interaction with glycoprotein D

During herpes simplex virus (HSV) entry, membrane fusion occurs either on the cell surface or after virus endocytosis. In both cases, binding of glycoprotein D (gD) to a receptor such as nectin-1 or HVEM is required. In this study, we co-cultured cells expressing gD with nectin-1 expressing cells to investigate the effects of gD on nectin-1 at cell contacts. After overnight co-cultures with gD expressing cells, there was a down-regulation of nectin-1 in B78H1-C10, SY5Y, A431 and HeLa cells, which HSV enters by endocytosis. In contrast, on Vero cells, which HSV enters at the plasma membrane, nectin-1 was not down-regulated. Further analysis of B78H1-derived cells showed that nectin-1 down-regulation corresponds to the ability of gD to bind nectin-1 and is achieved by internalization and low-pH-dependent degradation of nectin-1. Moreover, gD is necessary for virion internalization in B78H1 cells expressing nectin-1. These data suggest that the determinants of gD-mediated internalization of nectin-1 may direct HSV to an endocytic pathway during entry.

Mutagenic analysis of herpes simplex virus type 1 glycoprotein L reveals the importance of an arginine-rich region for function

Herpes simplex virus type 1 (HSV-1) glycoproteins H and L (gH and gL) are required for virus-induced membrane fusion. Expression of gH at the virion or infected cell surface is mediated by the chaperone-like activity of gL. We have previously shown that a region between amino acids 155 and 161 is critical for gL chaperone-like activity. Here, we conducted Ala substitution mutagenesis of residues in this region and found that substitution of Cys160, Arg156, Arg158, or Arg156/158/159 with Ala resulted in a gL mutant that bound gH but displayed a reduced ability in gH trafficking and membrane fusion. Substitution of Arg156 with another positively charged amino acid, Lys, restored function. Substitution of Arg158 with Lys restored function in gH trafficking and cell fusion but not virus entry. These results indicate that an arginine-rich region of gL is critical for function.

Epigallocatechin gallate inactivates clinical isolates of herpes simplex virus

In the absence of a fully effective herpes simplex virus (HSV) vaccine, topical microbicides represent an important strategy for preventing HSV transmission. (-)-Epigallocatechin gallate (EGCG) (molecular weight, 458.4) is the primary catechin in green tea. The present study shows that EGCG has greater anti-HSV activity than other green tea catechins and inactivates multiple clinical isolates of HSV type 1 (HSV-1) and HSV-2. EGCG reduced HSV-2 titers by >or=1,000-fold in 10 to 20 min and reduced HSV-1 titers by the same amount in 30 to 40 min. The anti-HSV activity of EGCG is due to a direct effect on the virion, and incubating Vero and CV1 cells with EGCG for 48 h prior to infection with HSV-1 and HSV-2, respectively, does not reduce HSV production. Electron microscopic (EM) studies showed that purified virions exposed to EGCG were damaged, and EM immunogold labeling of the envelope glycoproteins gB and gD was significantly reduced following EGCG treatment while capsid protein labeling was unchanged. When purified HSV-1 envelope glycoproteins gB and gD were incubated with EGCG and then examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, lower-molecular-weight gB and gD bands decreased and new higher-molecular-weight bands appeared, indicating the EGCG-dependent production of macromolecular complexes. gB and gD are essential for HSV infectivity, and these results suggest that EGCG could inactivate HSV virions by binding to gB, gD, or another envelope glycoprotein. EGCG is stable in the pH range found in the vagina and appears to be a promising candidate for use in a microbicide to reduce HSV transmission.

Viral and developmental cell fusion mechanisms: conservation and divergence

Membrane fusion is a fundamental requirement in numerous developmental, physiological, and pathological processes in eukaryotes. So far, only a limited number of viral and cellular fusogens, proteins that fuse membranes, have been isolated and characterized. Despite the diversity in structures and functions of known fusogens, some common principles of action apply to all fusion reactions. These can serve as guidelines in the search for new fusogens, and may allow the formulation of a cross-species, unified theory to explain divergent and convergent evolutionary principles of membrane fusion.

Engineered disulfide bonds in herpes simplex virus type 1 gD separate receptor binding from fusion initiation and viral entry

Glycoprotein D (gD) is the receptor binding protein of herpes simplex virus (HSV) and binds to at least two distinct protein receptors, herpesvirus entry mediator (HVEM) and nectin-1. While both receptor binding regions are found within the first 234 amino acids, a crystal structure shows that the C terminus of the gD ectodomain normally occludes the receptor binding sites. Receptor binding must therefore displace the C terminus, and this conformational change is postulated to be required for inducing fusion via gB and gH/gL. When cysteine residues are introduced at positions 37 and 302 of gD, a disulfide bond is formed that stabilizes the C terminus and prevents binding to either receptor. We speculated that if disulfide bonds were engineered further upstream, receptor binding might be separated from the induction of fusion. To test this, we made five additional double cysteine mutants, each potentially introducing a disulfide bond between the ectodomain C terminus and the core of the gD ectodomain. The two mutants predicted to impose the greatest constraint were unable to bind receptors or mediate cell-cell fusion. However, the three mutants with the most flexible C terminus bound well to both HVEM and nectin-1. Two of these mutants were impaired in cell-cell fusion and null-virus complementation. Importantly, a third mutant in this group was nonfunctional in both assays. This mutant clearly separates the role of gD in triggering fusion from its role in receptor binding. Based upon the properties of the panel of mutants we conclude that fusion requires greater flexibility of the gD ectodomain C terminus than does receptor binding.

Bimolecular complementation reveals that glycoproteins gB and gH/gL of herpes simplex virus interact with each other during cell fusion

Herpes simplex virus entry into cells requires four glycoproteins, gB, gD, gH, and gL. Binding of gD to one of its receptors triggers steps requiring the core fusion proteins, gB and the gH/gL heterodimer. There is evidence that gH/gL initiates hemifusion of cells, but whether this complex interacts physically with gB to cause complete fusion is unknown. We used bimolecular complementation (BiMC) of enhanced yellow fluorescent protein (EYFP) to detect glycoprotein interactions during cell-cell fusion. The N- or C-terminal half of EYFP was fused to the C terminus of gD, gB, and gH to form six chimeric proteins (Dn, Dc, Bn, Bc, Hn, and Hc). BiMC was detected by confocal microscopy. Receptor-bearing (C10) cells cotransfected with Dn and Bc or Dn, Hc, and untagged gL exhibited EYFP fluorescence, indicative of interactions between gD and gB and between gD and gH/gL. EYFP complementation did not occur in cells transfected with gL, Bc, and Hn. However, when gD was coexpressed with these other three proteins, cell-cell fusion occurred and the syncytia exhibited bright EYFP fluorescence. To separate glycoprotein expression from fusion, we transfected C10 cells with gL, Bc, and Hn for 20 h and then added soluble gD to trigger fusion. We detected fluorescent syncytia within 10 min, and both their number and size increased with exposure time to gD. Thus, when gD binds its receptor, the core fusion machinery is triggered to form a multiprotein complex as a step in fusion and possibly virus entry.

Characterization of the human cytomegalovirus gH/gL/UL128-131 complex that mediates entry into epithelial and endothelial cells

The entry of human cytomegalovirus (HCMV) into biologically relevant epithelial and endothelial cells involves endocytosis followed by low-pH-dependent fusion. This entry pathway is facilitated by the HCMV UL128, UL130, and UL131 proteins, which form one or more complexes with the virion envelope glycoprotein gH/gL. gH/gL/UL128-131 complexes appear to be distinct from the gH/gL/gO complex, which likely facilitates entry into fibroblasts. In order to better understand the assembly and protein-protein interactions of gH/gL/UL128-131 complexes, we generated HCMV mutants lacking UL128-131 proteins and nonreplicating adenovirus vectors expressing gH, gL, UL128, UL130, and UL131. Our results demonstrate that UL128, UL130, and UL131 can each independently assemble onto gH/gL scaffolds. However, the binding of individual UL128-131 proteins onto gH/gL can significantly affect the binding of other proteins; for example, UL128 increased the binding of both UL130 and UL131 to gH/gL. Direct interactions between gH/UL130, UL130/UL131, gL/UL128, and UL128/UL130 were also observed. The export of gH/gL complexes from the endoplasmic reticulum (ER) to the Golgi apparatus and cell surface was dramatically increased when all of UL128, UL130, and UL131 were coexpressed with gH/gL (with or without gO expression). Incorporation of gH/gL complexes into the virion envelope requires transport beyond the ER. Thus, we concluded that UL128, UL130, and UL131 must all bind simultaneously onto gH/gL for the production of complexes that can function in entry into epithelial and endothelial cells.

Cytomegalovirus UL131-128 products promote gB conformational transition and gB-gH interaction during entry into endothelial cells

Herpesviruses use gB and gH-gL glycoproteins to execute fusion. Other virus-specific glycoproteins are required for receptor binding and fusion activation. The human cytomegalovirus (HCMV) UL131-128 proteins are essential for the infection of leukocytes, endothelial cells (ECs), and many epithelial cell lines. Here we show that UL131-128 play a role in a chain of events involving gB and gH during HCMV entry into ECs. An HCMV strain bearing the wild-type (wt) UL131-128 locus exhibited a gB transition from a protease-resistant to protease-sensitive form, a conformational change that was suppressed by a thiourea inhibitor of fusion (WY1768); in contrast, gH was susceptible to proteolysis throughout entry. Moreover, gB and gH transiently interacted, and a lipid mixing assay showed that the wt strain had carried out fusion by 60 min postinfection. However, these events were greatly altered when UL131-128-defective strains were used for infection or when there was an excess of soluble pUL128 during wt infection: the gB conformational change became WY1768 resistant, the gB-gH complex was no longer observed, and fusion was prevented. Both gB and gH in this case showed late protease resistance, related to their endocytic uptake. Our data point to the involvement of UL131-128 proteins in driving gB through a WY1768-sensitive fold transition, thus promoting a short-lived gB-gH complex and fusion; they also suggest that HCMV fuses with the EC plasma membrane and that endocytosis ensues only when the virus cannot trigger UL131-128-dependent steps.

Complexes between herpes simplex virus glycoproteins gD, gB, and gH detected in cells by complementation of split enhanced green fluorescent protein

The interactions between herpes simplex virus gD and its nectin1 receptor or between gD, gB, and gH were analyzed by complementation of the N and C portions of split enhanced green fluorescent protein (EGFP) fused to the glycoproteins. The gD(N)-Nect(C) complex was readily detected; the gD(N)-gC(C) complex was undetectable, highlighting the specificity of the assay. Split EGFP complementation was detected between proteins designated gD(N)+gH(C), gD(N)+gB(C), and gH(N)+gB(C)+wtgD (gB was deleted of endocytosis motifs), both in cells transfected with two-tree glycoproteins and in syncytia. The in situ assay provides evidence that gD interacts with gH and gB independently of each other and supports a model whereby gH and gB in complex exert their activities to gD.

Random linker-insertion mutagenesis to identify functional domains of herpes simplex virus type 1 glycoprotein B

Herpes simplex virus glycoprotein B (gB) is one of four glycoproteins essential for viral entry and cell fusion. Recently, an x-ray structure of the nearly full-length trimeric gB ectodomain was determined. Five structural domains and two linker regions were identified in what is probably a postfusion conformation. To identify functional domains of gB, we performed random linker-insertion mutagenesis. Analyses of 81 mutants revealed that only 27 could fold to permit processing and transport of gB to the cell surface. These 27 mutants fell into three categories. Insertions into two regions excluded from the solved structure (the N terminus and the C-terminal cytoplasmic tail) had no negative effect on cell fusion and viral entry activity, identifying regions that can tolerate altered structure without loss of function. Insertions into a disordered region in domain II and the adjacent linker region also permitted partial cell fusion and viral entry activity. Insertions at 16 other positions resulted in loss of cell fusion and viral entry activity, despite detectable levels of cell surface expression. Four of these insertion sites were not included in the solved structure. Two were between residues exposed to a cavity that is too small to accommodate the 5-amino acid insertions, consistent with the solved structure being different from the native prefusion structure. Ten were between residues exposed to the surface of the trimer, identifying regions that may be critical for interactions with other viral proteins or cellular components or for transitions from the prefusion to postfusion state.

Binding-site interactions between Epstein-Barr virus fusion proteins gp42 and gH/gL reveal a peptide that inhibits both epithelial and B-cell membrane fusion

Herpesviruses require membrane-associated glycoproteins gB, gH, and gL for entry into host cells. Epstein-Barr virus (EBV) gp42 is a unique protein also required for viral entry into B cells. Key interactions between EBV gp42 and the EBV gH/gL complex were investigated to further elucidate their roles in membrane fusion. Deletion and point mutants within the N-terminal region of gp42 revealed residues important for gH/gL binding and membrane fusion. Many five-residue deletion mutants in the N-terminal region of gp42 that exhibit reduced membrane fusion activity retain binding with gH/gL but map out two functional stretches between residues 36 and 96. Synthetic peptides derived from the gp42 N-terminal region were studied in in vitro binding experiments with purified gH/gL and in cell-cell fusion assays. A peptide spanning gp42 residues 36 to 81 (peptide 36-81) binds gH/gL with nanomolar affinity, comparable to full-length gp42. Peptide 36-81 efficiently inhibits epithelial cell membrane fusion and competes with soluble gp42 to inhibit B-cell fusion. Additionally, this peptide at low nanomolar concentrations inhibits epithelial cell infection by intact virus. Shorter gp42 peptides spanning the two functional regions identified by deletion mutagenesis had little or no binding to soluble gH/gL and were also unable to inhibit epithelial cell fusion, nor could they complement gp42 deletion mutants in B-cell fusion. These studies identify key residues of gp42 that are essential for gH/gL binding and membrane fusion activation, providing a nanomolar inhibitor of EBV-mediated membrane fusion.

Hydrophobic residues that form putative fusion loops of Epstein-Barr virus glycoprotein B are critical for fusion activity

To test the importance of the hydrophobic residues within the putative Epstein-Barr virus (EBV) glycoprotein B (gB) fusion loops in membrane fusion, WY(112-113) and WLIW(193-196) were mutated into alanine, glutamic acid, or the analogous residues from herpes simplex virus type 1 (HSV-1) gB (HR and RVEA). All gB variants exhibited cell surface expression, demonstrating that the substitutions did not perturb gB trafficking. None of six gB variants was, however, capable of mediating fusion with either epithelial or B cells. These data demonstrate that the bulky and hydrophobic EBV loop residues, which differ from the more hydrophilic HSV-1 residues and appear more compatible with membrane insertion, are essential for EBV gB-dependent fusion.

Herpes simplex virus glycoproteins gB and gH function in fusion between the virion envelope and the outer nuclear membrane

Herpesviruses must traverse the nuclear envelope to gain access to the cytoplasm and, ultimately, to exit cells. It is believed that herpesvirus nucleocapsids enter the perinuclear space by budding through the inner nuclear membrane (NM). To reach the cytoplasm these enveloped particles must fuse with the outer NM and the unenveloped capsids then acquire a second envelope in the trans-Golgi network. Little is known about the process by which herpesviruses virions fuse with the outer NM. Here we show that a herpes simplex virus (HSV) mutant lacking both the two putative fusion glycoproteins gB and gH failed to cross the nuclear envelope. Enveloped virions accumulated in the perinuclear space or in membrane vesicles that bulged into the nucleoplasm (herniations). By contrast, mutants lacking just gB or gH showed only minor or no defects in nuclear egress. We concluded that either HSV gB or gH can promote fusion between the virion envelope and the outer NM. It is noteworthy that fusion associated with HSV entry requires the cooperative action of both gB and gH, suggesting that the two types of fusion (egress versus entry) are dissimilar processes.

Cholesterol dependence of varicella-zoster virion entry into target cells

The entry of inhaled virions into airway cells is presumably the initiating step of varicella-zoster infection. In order to characterize viral entry, we studied the relative roles played by lipid rafts and clathrin-mediated transport. Virus and target cells were pretreated with agents designed to perturb selected aspects of endocytosis and membrane composition, and the effects of these perturbations on infectious focus formation were monitored. Infectivity was exquisitely sensitive to methyl-beta-cyclodextrin (M beta CD) and nystatin, which disrupt lipid rafts by removing cholesterol. These agents inhibited infection by enveloped, but not cell-associated, varicella-zoster virus (VZV) in a dose-dependent manner and exerted these effects on both target cell and viral membranes. Inhibition by M beta CD, which could be reversed by cholesterol replenishment, rapidly declined as a function of time after exposure of target cells to VZV, suggesting that an early step in viral infection requires cholesterol. No effect of cholesterol depletion, however, was seen on viral binding; moreover, there was no reduction in the surface expression or internalization of mannose 6-phosphate receptors, which are required for VZV entry. Viral entry was energy dependent and showed concentration-dependent inhibition by chlorpromazine, which, among other actions, blocks clathrin-mediated endocytosis. These data suggest that both membrane lipid composition and clathrin-mediated transport are critical for VZV entry. Lipid rafts are likely to contribute directly to viral envelope integrity and, in the host membrane, may influence endocytosis, evoke downstream signaling, and/or facilitate membrane fusion.

The multipartite system that mediates entry of herpes simplex virus into the cell

The multipartite entry-fusion system of herpes simplex virus is made of a quartet of glycoproteins-gD, gB, gH.gL-and three alternative gD receptors, herpesvirus entry mediator (HVEM), nectin1 and modified sites on heparan sulphate. This multipartite system recapitulates the basic steps of virus-cell fusion, i.e. receptor recognition, triggering of fusion and fusion execution. Specifically, in addition to serving as the receptor-binding glycoprotein, gD triggers fusion through a specialised domain, named pro-fusion domain (PFD), located C-terminally in the ectodomain. In the unliganded gD the C-terminal region folds around the N-terminal region, such that gD adopts a closed autoinhibited conformation. In HVEM- and nectin1-bound gD the C-terminal region is displaced (opened conformation). gD is the tool for modification of HSV tropism, through insertion of ligands to heterologous tumour-specific receptors. It is discussed whether gD responds to the interaction with the natural and the heterologous receptors by adopting similar conformations, and whether the closed-to-open switch in conformation is a generalised mechanism of activation. A peculiar recombinant highlighted that the central Ig-folded core of gD may not encode executable functions for entry and that the 219-314 aa segment may be sufficient to trigger fusion. With respect to fusion execution, gB appears to be a prospective fusogen based on its coiled-coil trimeric structure, similar to that of another fusion glycoprotein. On the other hand, gH exhibits molecular elements typical of class 1 fusion glycoproteins, in particular heptad repeats and strong tendency to interact with lipids. Whether fusion execution is carried out by gB or gH.gL, or both glycoproteins in complex or sequentially remains to be determined.