Mutations in the conserved carboxy-terminal hydrophobic region of glycoprotein gB affect infectivity of herpes simplex virus

Herpes Simplex Virus Cell Entry Mechanisms: An Update

Herpes simplex virus (HSV) can infect a broad host range and cause mild to life threating infections in humans. The surface glycoproteins of HSV are evolutionarily conserved and show an extraordinary ability to bind more than one receptor on the host cell surface. Following attachment, the virus fuses its lipid envelope with the host cell membrane and releases its nucleocapsid along with tegument proteins into the cytosol. With the help of tegument proteins and host cell factors, the nucleocapsid is then docked into the nuclear pore. The viral double stranded DNA is then released into the host cell’s nucleus. Released viral DNA either replicates rapidly (more commonly in non-neuronal cells) or stays latent inside the nucleus (in sensory neurons). The fusion of the viral envelope with host cell membrane is a key step. Blocking this step can prevent entry of HSV into the host cell and the subsequent interactions that ultimately lead to production of viral progeny and cell death or latency. In this review, we have discussed viral entry mechanisms including the pH-independent as well as pH-dependent endocytic entry, cell to cell spread of HSV and use of viral glycoproteins as an antiviral target.

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

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

Structural basis for membrane anchoring and fusion regulation of the herpes simplex virus fusogen gB

Viral fusogens merge viral and cell membranes during cell penetration. Their ectodomains drive fusion by undergoing large-scale refolding, but little is known about the functionally important regions located within or near the membrane. Here, we report the crystal structure of the full-length glycoprotein B, the fusogen from Herpes Simplex Virus, complemented by electron spin resonance measurements. The membrane-proximal (MPR), transmembrane (TMD), and cytoplasmic (CTD) domains form a uniquely folded trimeric pedestal beneath the ectodomain, which balances dynamic flexibility with extensive, stabilizing membrane interactions. Hyperfusogenic mutations within the CTD destabilize it, targeting trimeric interfaces, structural motifs, and membrane-interacting elements. Thus, we propose that the CTD trimer observed in the structure stabilizes gB in its prefusion state despite being appended to the postfusion ectodomain. Our data suggest a model for how this dynamic, membrane-dependent “clamp” controls the fusogenic refolding of gB.

Herpesvirus gB: A Finely Tuned Fusion Machine

Enveloped viruses employ a class of proteins known as fusogens to orchestrate the merger of their surrounding envelope and a target cell membrane. Most fusogens accomplish this task alone, by binding cellular receptors and subsequently catalyzing the membrane fusion process. Surprisingly, in herpesviruses, these functions are distributed among multiple proteins: the conserved fusogen gB, the conserved gH/gL heterodimer of poorly defined function, and various non-conserved receptor-binding proteins. We summarize what is currently known about gB from two closely related herpesviruses, HSV-1 and HSV-2, with emphasis on the structure of the largely uncharted membrane interacting regions of this fusogen. We propose that the unusual mechanism of herpesvirus fusion could be linked to the unique architecture of gB.

Interplay between the Herpes Simplex Virus 1 gB Cytodomain and the gH Cytotail during Cell-Cell Fusion

Herpesvirus entry into cells is mediated by the viral fusogen gB, which is thought to refold from the prefusion to the postfusion form in a series of large conformational changes that energetically couple refolding to membrane fusion. In contrast to most viral fusogens, gB requires a conserved heterodimer, gH/gL, as well as other nonconserved proteins. In a further mechanistic twist, gB-mediated cell-cell fusion appears restricted by its intraviral or cytoplasmic domain (cytodomain) because mutations within it result in a hyperfusogenic phenotype. Here, we characterized a panel of hyperfusogenic HSV-1 gB cytodomain mutants and show that they are fully functional in cell-cell fusion at shorter coincubation times and at lower temperatures than those for wild-type (WT) gB, which suggests that these mutations reduce the kinetic energy barrier to fusion. Despite this, the mutants require both gH/gL and gD. We confirm previous observations that the gH cytotail is an essential component of the cell-cell fusion mechanism and show that the N-terminal portion of the gH cytotail is critical for this process. Moreover, the fusion levels achieved by all gB constructs, WT and mutant, were proportionate to the length of the gH cytotail. Putting these results together, we propose that the gH cytotail, in addition to the gH/gL ectodomain, plays an essential role in gB activation, potentially acting as a "wedge" to release the gB cytodomain "clamp" and enable gB activation.

IMPORTANCE

Herpesviruses infect their hosts for life and cause a substantial disease burden. Herpes simplex viruses cause oral and genital sores as well as rare yet severe encephalitis and a panoply of ocular ailments. Infection initiates when the viral envelope fuses with the host cell membrane in a process orchestrated by the viral fusogen gB, assisted by the viral glycoproteins gH, gL, and gD and a cellular gD receptor. This process is more complicated than that of most other viruses and is subject to multiple regulatory inputs. Antiviral and vaccine development would benefit from a detailed mechanistic knowledge of this process and how it is regulated.

The structure of herpesvirus fusion glycoprotein B-bilayer complex reveals the protein-membrane and lateral protein-protein interaction

Glycoprotein B (gB) is a key component of the complex herpesvirus fusion machinery. We studied membrane interaction of two gB ectodomain forms and present an electron cryotomography structure of the gB-bilayer complex. The two forms differed in presence or absence of the membrane proximal region (MPR) but showed an overall similar trimeric shape. The presence of the MPR impeded interaction with liposomes. In contrast, the MPR-lacking form interacted efficiently with liposomes. Lateral interaction resulted in coat formation on the membranes. The structure revealed that interaction of gB with membranes was mediated by the fusion loops and limited to the outer membrane leaflet. The observed intrinsic propensity of gB to cluster on membranes indicates an additional role of gB in driving the fusion process forward beyond the transient fusion pore opening and subsequently leading to fusion pore expansion.

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Full-length gB ectodomain has a structure similar to the ectodomain lacking the MPR

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The gB-bilayer structure reveals that the interaction is limited to the outer leaflet

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gB trimers have an intrinsic propensity to interact laterally and form protein arrays

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Arrays of gB trimers on membranes render the fusion pore open state irreversible

The membrane-proximal region (MPR) of herpes simplex virus gB regulates association of the fusion loops with lipid membranes

Glycoprotein B (gB), gD, and gH/gL constitute the fusion machinery of herpes simplex virus (HSV). Prior studies indicated that fusion occurs in a stepwise fashion whereby the gD/receptor complex activates the entire process, while gH/gL regulates the fusion reaction carried out by gB. Trimeric gB is a class III fusion protein. Its ectodomain of 773 amino acids contains a membrane-proximal region (MPR) (residues 731 to 773) and two fusion loops (FLs) per protomer. We hypothesized that the highly hydrophobic MPR interacts with the FLs, thereby masking them on virions until fusion begins. To test this hypothesis, we made a series of deletion, truncation, and point mutants of the gB MPR. Although the full-length deletion mutants were expressed in transfected cells, they were not transported to the cell surface, suggesting that removal of even small stretches of the MPR was highly detrimental to gB folding. To circumvent this limitation, we used a baculovirus expression system to generate four soluble proteins, each lacking the transmembrane region and cytoplasmic tail. All retained the FLs and decreasing portions of the MPR [gB(773t) (gB truncated at amino acid 773), gB(759t), gB(749t), and gB(739t)]. Despite the presence of the FLs, all were compromised in their ability to bind liposomes compared to the control, gB(730t), which lacks the MPR. We conclude that residues 731 to 739 are sufficient to mask the FLs, thereby preventing liposome association. Importantly, mutation of two aromatic residues (F732 and F738) to alanine restored the ability of gB(739t) to bind liposomes. Our data suggest that the MPR is important for modulating the association of gB FLs with target membranes.

To successfully cause disease, a virus must infect host cells. Viral infection is a highly regulated, multistep process. For herpesviruses, genetic material transfers from the virus to the target cell through fusion of the viral and host cell lipid membranes. Here, we provide evidence that the ability of the herpes simplex virus (HSV) glycoprotein B (gB) fusion protein to interact with the host membrane is regulated by its membrane-proximal region (MPR), which serves to cover or shield its lipid-associating moieties (fusion loops). This in turn prevents the premature binding of gB with host cells and provides a level of regulation to the fusion process. These findings provide important insight into the complex regulatory steps required for successful herpesvirus infection.

Structural rearrangement within an enveloped virus upon binding to the host cell

We have made the surprising discovery that the interactions of herpes simplex virus with its initial cell attachment receptor induce a rapid and highly efficient structural change in the tegument, the region of the virion situated between the membrane and the capsid. It has been known for nearly a decade that viruses can trigger host signaling pathways when they bind to receptors on the cell surface; however, until now there has been no evidence that a signal can be sent in reverse--from the "outside in"--across a viral membrane. Evidence for this signaling event was found during studies of UL16, a tegument protein that is conserved among all the herpesviruses. Previous work has demonstrated that UL16 is bound to capsids isolated from the cytoplasm of infected cells, but this interaction is destabilized during subsequent egress steps, leading to release of the extracellular virion. Pretreatment with N-ethylmaleimide, a small, membrane-permeating compound that covalently modifies free cysteines, restabilizes the interaction, thereby permitting the capsid-UL16 complex to be isolated following disruption of virions with NP-40. In the experiments described here, we found that the natural signal for release of UL16 from capsids is sent when virions merely bind to cells at 4 degrees C. The internal change was also observed upon binding to immobilized heparin in a manner that requires viral glycoprotein C. This represents the first example of signaling across a viral envelope following receptor binding.

The herpes simplex virus type 1 (HSV-1) glycoprotein K(gK) is essential for viral corneal spread and neuroinvasiveness

PURPOSE

To determine the role of herpes simplex virus-1 (HSV-1) glycoprotein K(gK) in corneal infection, neuroinvasion, and virus latency in trigeminal ganglia of mice.

METHODS

The recombinant virus HSV-1 (McKrae) Delta gK (MKDelta gK) carrying a deletion of the gK gene was constructed by insertional/deletion mutagenesis and replaced by a gene cassette constitutively expressing the enhanced green fluorescence protein. The gK deletion of the MKDelta gK virus was rescued to produce the wild-type-like virus MKgK. Balb/c mice were infected ocularly with either virus, and the infection pattern in the eye, clinical disease progression, and establishment of viral latency was monitored.

RESULTS

Mice infected with the MKDelta gK strain produced in a gK complementing cell line did not exhibit clinical signs when compared with mice infected with the MKgK virus. Direct visualization of infected eyes revealed that the MKDelta gK virus was unable to spread in mouse corneas, while the MKgK rescued virus spread efficiently. Nineteen of 20 scarified and 5/12 unscarified mice infected with the MKgK virus produced infectious virus after coculture with permissive cells, while 0/20 scarified and 0/12 unscarified mice infected with the MKDelta gK virus produced infectious virus. HSV DNA was detected in trigeminal ganglia by PCR in 19/20 scarified and 9/12 unscarified mice inoculated with MKgK, while HSV DNA was detected in the trigeminal ganglia of 3/20 scarified and 0/12 unscarified mice inoculated with MKDelta gK.

CONCLUSIONS

The results show that HSV-1 gK is essential for efficient replication and spread in the corneal epithelium and trigeminal ganglia neuroinvasion in MKDelta gK inoculated mice.

Mutational evidence of internal fusion loops in herpes simplex virus glycoprotein B

Herpes simplex virus type 1 (HSV-1) glycoprotein B (gB) is one of four glycoproteins necessary and sufficient for HSV cellular entry. Recently, the crystal structures of HSV-1 gB and vesicular stomatitis virus glycoprotein G were determined. Surprisingly, the two proteins share remarkable structural homology. Both proteins are homotrimeric and center about a long alpha-helix, features reminiscent of class I fusion proteins, such as influenza virus hemagglutinin or paramyxovirus F. However, these structures revealed that G has internal fusion loops, similar to the fusion loops of the class II fusion proteins, and that these loops are structurally conserved in gB. To examine whether these putative fusion loops are important for gB function, we mutated potential membrane-interacting (hydrophobic) residues to charged amino acids. Of most interest were mutant gB proteins that were expressed on the cell surface and were recognized by monoclonal antibodies against conformational epitopes but lacked the ability to function in cell-cell fusion assays. We find that three of the five hydrophobic amino acids targeted in these loops, tryptophan 174, tyrosine 179, and alanine 261, are integral in the function of gB. Our data suggest that they are part of an important functional domain. We hypothesize that two loops in domain 1 of HSV gB function as fusion loops. Our data are further evidence that gB is a viral fusogen and suggest clues as to how gB may function.

Identification of functional domains in herpes simplex virus 2 glycoprotein B

Glycoprotein B (gB) is one of four membrane proteins that are essential for the entry of herpes simplex viruses (HSV) into cells, and coexpression of the same combination of proteins in transfected cells results in cell fusion. The latter effect is reminiscent of the ability of virus infection to cause cell fusion, particularly since the degree of fusion is greatly increased by syncytial mutations in gB. Despite intensive efforts with the gB homologs of HSV and some other herpesviruses, information about functionally important regions in the 700-amino-acid ectodomain of this protein is very limited at present. This is largely due to the misfolding of the majority of the mutants examined. It was shown previously that the percentage of correctly folded mutants could be increased by targeting only predicted loop regions (i.e., not alpha-helix or beta-strand), and by using this approach new functional domains in HSV-2 gB have now been identified.

Glycoprotein D receptor-dependent, low-pH-independent endocytic entry of herpes simplex virus type 1

Two herpes simplex virus type 1 (HSV-1) entry pathways have been described: direct fusion between the virion envelope and the plasma membrane, as seen on Vero cells, and low-pH-dependent endocytosis, as seen on CHO nectin-1 and HeLa cells. In this paper, we studied HSV entry into C10 murine melanoma cells and identified a third entry pathway for this virus. During entry into C10 cells, virion envelope glycoproteins rapidly became protected from the membrane-impermeable chemical cross-linker BS3 and from proteinase K. Protection was gD receptor dependent, and the time taken to detect protected protein was proportional to the rate of virus entry. Ultrastructural examination revealed that virions attached to the surface of C10 cells were localized to membrane invaginations, whereas those on the surface of receptor-negative B78 cells were peripherally attached. Virus entry into C10 cells was energy dependent, and intracellular enveloped virions were seen within membrane-bound vesicles consistent with endocytic entry. Entry was not inhibited by bafilomycin A1 or ammonium chloride, showing that passage of the virion through a low-pH environment was not required for infection. Resistance to similar reagents should therefore not be taken as proof of HSV entry by a nonendosomal pathway. These data define a novel gD receptor-dependent acid-independent endocytic entry pathway for HSV.

A synthetic peptide from a heptad repeat region of herpesvirus glycoprotein B inhibits virus replication

Glycoprotein B (gB) is the most conserved glycoprotein of herpesviruses and plays important roles in virus infectivity. Two intervening heptad repeat (HR) sequences were found in the C-terminal half of all herpesvirus gBs analysed. A synthetic peptide derived from the HR region (aa 477-510) of bovine herpesvirus type 1 (BoHV-1) gB was studied for its ability to inhibit virus replication. The peptide interfered with cell-to-cell spread and consistently inhibited replication of BoHV-1, with a 50 % effective concentration value (EC(50)) of 5 microM. Inhibition of replication was obtained not only with herpesviruses including pseudorabies virus and herpes simplex virus type 1 but also partly with Newcastle disease virus. Possible mechanisms of membrane fusion inhibition by the peptide are discussed.

Effects of mutations in the cytoplasmic domain of herpes simplex virus type 1 glycoprotein B on intracellular transport and infectivity

Herpes simplex virus type 1 (HSV-1) is a human pathogen of the alphaherpesvirus family which infects and spreads in the nervous system. Glycoproteins play a key role in the process of assembly and maturation of herpesviruses, which is essential for neuroinvasion and transneuronal spread. Glycoprotein B (gB) is a main component of the HSV-1 envelope and is necessary for the production of infectious particles. The cytoplasmic domain of gB, the longest one among HSV-1 glycoproteins, contains several highly conserved peptide sequences homologous to motifs involved in intracellular sorting. To determine the specific roles of these motifs in processing, subcellular localization, and the capacity of HSV-1 gB to complement a gB-null virus, we generated truncated or point mutated forms of a green fluorescent protein (GFP)-tagged gB. GFP-gB with a deletion in the acidic cluster DGDADEDDL (amino acids [aa] 896 to 904) behaved the same as the parental form. Deletion or disruption of the YTQV motif (aa 889 to 892) abolished internalization and reduced complementation by 60%. Disruption of the LL motif (aa 871 to 872) impaired the return of the protein to the trans-Golgi network (TGN) while enhancing its recycling to the plasma membrane. Truncations from residue E 857 abolished transport and processing of the truncated proteins, which had null complementation activity, through the Golgi complex. Altogether, our results favor a model in which HSV-1 gets its final envelope in the TGN, and they suggest that endocytosis, albeit not necessary, might play a role in infectivity.

The transmembrane domain and cytoplasmic tail of herpes simplex virus type 1 glycoprotein H play a role in membrane fusion

Herpes simplex virus glycoprotein H (gH) is one of the four virion envelope proteins which are required for virus entry and for cell-cell fusion in a transient system. In this report, the role of the transmembrane and cytoplasmic tail domains of gH in membrane fusion was investigated by generating chimeric constructs in which these regions were replaced with analogous domains from other molecules and by introducing amino acid substitutions within the membrane-spanning sequence. gH molecules which lack the authentic transmembrane domain or cytoplasmic tail were unable to mediate cell-cell fusion when coexpressed with gB, gD, and gL and were unable to rescue the infectivity of a gH-null virus as efficiently as a wild-type gH molecule. Many amino acid substitutions of specific amino acid residues within the transmembrane domain also affected cell-cell fusion, in particular, those introduced at a conserved glycine residue. Some gH mutants that were impaired in cell-cell fusion were nevertheless able to rescue the infectivity of a gH-negative virus, but these pseudotyped virions entered cells more slowly than wild-type virions. These results indicate that the fusion event mediated by the coexpression of gHL, gB, and gD in cells shares common features with the fusion of the virus envelope with the plasma membrane, they point to a likely role for the membrane-spanning and cytoplasmic tail domains of gH in both processes, and they suggest that a conserved glycine residue in the membrane-spanning sequence is crucial for efficient fusion.