Extensive mutagenesis of the HSV-1 gB ectodomain reveals remarkable stability of its postfusion form

Computational design of prefusion-stabilized Herpesvirus gB trimers

In the absence of effective vaccines, human-infecting members of the Herpesvirus family cause considerable morbidity and mortality worldwide. Herpesvirus infection relies on receptor engagement by a gH/gL glycoprotein complex which induces large-scale conformational changes of the gB glycoprotein to mediate fusion of the viral and host membranes and infection. The instability of all herpesvirus gBs have hindered biochemical and functional studies, thereby limiting our understanding of the infection mechanisms of these pathogens and preventing vaccine design. Here, we computationally stabilized and structurally characterized the Epstein-Barr virus prefusion gB ectodomain trimer, providing an atomic-level description of this key therapeutic target. We show that this stabilization strategy is broadly applicable to other herpesvirus gB trimers and identified conformational intermediates supporting a previously unanticipated mechanism of gB-mediated fusion. These findings provide a blueprint to develop vaccine candidates for these pathogens with major public health burden.

Targeted mutagenesis of the herpesvirus fusogen central helix captures transition states

Herpesviruses remain a burden for animal and human health, including the medically important varicella-zoster virus (VZV). Membrane fusion mediated by conserved core glycoproteins, the fusogen gB and the heterodimer gH-gL, enables herpesvirus cell entry. The ectodomain of gB orthologs has five domains and is proposed to transition from a prefusion to postfusion conformation but the functional relevance of the domains for this transition remains poorly defined. Here we describe structure-function studies of the VZV gB DIII central helix targeting residues 526EHV528. Critically, a H527P mutation captures gB in a prefusion conformation as determined by cryo-EM, a loss of membrane fusion in a virus free assay, and failure of recombinant VZV to spread in cell monolayers. Importantly, two predominant cryo-EM structures of gB[H527P] are identified by 3D classification and focused refinement, suggesting they represented gB conformations in transition. These studies reveal gB DIII as a critical element for herpesvirus gB fusion function.

Stabilisation of Viral Membrane Fusion Proteins in Prefusion Conformation by Structure-Based Design for Structure Determination and Vaccine Development

The membrane surface of enveloped viruses contains dedicated proteins enabling the fusion of the viral with the host cell membrane. Working with these proteins is almost always challenging because they are membrane-embedded and naturally metastable. Fortunately, based on a range of different examples, researchers now have several possibilities to tame membrane fusion proteins, making them amenable for structure determination and immunogen generation. This review describes the structural and functional similarities of the different membrane fusion proteins and ways to exploit these features to stabilise them by targeted mutational approaches. The recent determination of two herpesvirus membrane fusion proteins in prefusion conformation holds the potential to apply similar methods to this group of viral fusogens. In addition to a better understanding of the herpesviral fusion mechanism, the structural insights gained will help to find ways to further stabilise these proteins using the methods described to obtain stable immunogens that will form the basis for the development of the next generation of vaccines and antiviral drugs.

Nectin-1 and Non-muscle Myosin Heavy Chain-IIB: Major Mediators of Herpes Simplex Virus-1 Entry Into Corneal Nerves

Herpes Simplex Virus 1 (HSV-1) invades corneal nerves upon its infection of the cornea and then establishes latency in the trigeminal ganglion (TG). The latent virus in TG is often reactivated and travels back to the cornea, causing recurrent herpes simplex keratitis (HSK). The entry of HSV-1 into the corneal nerve is considered the initial step of infection resulting in HSV-1 latency and HSK recurrence. Several gD and gB receptors have been identified, including nectin-1, herpes virus entry medium (HVEM) and 3-O-sulfated heparan sulfate (3-OS-HS) as gD receptors, and non-muscle myosin heavy chain IIA (NMHC-IIA), NMHC-IIB and myelin-associated glycoprotein (MAG) as gB receptors. However, which receptors contribute to the entry of HSV-1 into corneal nerves are yet to be determined. This study observed that receptors nectin-1, HVEM, 3-OS-HS, NMHC-IIA, and NMHC-IIB, not MAG, were expressed in healthy corneal nerves. Further, we cultured TG neurons extracted from mice in vitro to screen for functional gD/gB receptors. Both in vitro siRNA knockdown and in vivo antibody blocking of either nectin-1 or NMHC-IIB reduced the entry and the replication of HSV-1 as shown by qPCR analysis and immunofluorescence measure, respectively. Also, we observed that the re-localization and the upregulation expression of NMHC-IIB after HSV-1 exposure were inhibited when gD receptor nectin-1 was knocked down. These data suggest that nectin-1 was the main gD receptor and NMHC-IIB was the main gB receptor in mediating HSV-1 entry and hold promise as therapeutic targets for resolving HSV-1 latency and HSK recurrence.

Herpes Simplex Virus Glycoprotein B Mutations Define Structural Sites in Domain I, the Membrane Proximal Region, and the Cytodomain That Regulate Entry

The viral fusion protein glycoprotein B (gB) is conserved in all herpesviruses and is essential for virus entry. During entry, gB fuses viral and host cell membranes by refolding from a prefusion to a postfusion form. We previously introduced three structure-based mutations (gB-I671A/H681A/F683A) into the domain V arm of the gB ectodomain that resulted in reduced cell-cell fusion. A virus carrying these three mutations (called gB3A) displayed a small-plaque phenotype and remarkably delayed entry into cells. To identify mutations that could counteract this phenotype, we serially passaged the gB3A virus and selected for revertant viruses with increased plaque sizes. Genomic sequencing revealed that the revertant viruses had second-site mutations in gB, including E187A, M742T, and S383F/G645R/V705I/V880G. Using expression constructs encoding these mutations, only gB-V880G was shown to enhance cell-cell fusion. In contrast, all of the revertant viruses showed enhanced entry kinetics, underscoring the fact that cell-cell fusion and virus-cell fusion are different. The results indicate that mutations in three different regions of gB (domain I, the membrane proximal region, and the cytoplasmic tail domain) can counteract the slow-entry phenotype of gB3A virus. Mapping these compensatory mutations to prefusion and postfusion structural models suggests sites of intramolecular functional interactions with the gB domain V arm that may contribute to the gB fusion function. IMPORTANCE The nine human herpesviruses are ubiquitous and cause a range of diseases in humans. Glycoprotein B (gB) is an essential viral fusion protein that is conserved in all herpesviruses. During host cell entry, gB mediates virus-cell membrane fusion by undergoing a conformational change. Structural models for the prefusion and postfusion forms of gB exist, but the details of how the protein converts from one to the other are unclear. We previously introduced structure-based mutations into gB that inhibited virus entry and fusion. By passaging this entry-deficient virus over time, we selected second-site mutations that partially restore virus entry. The locations of these mutations suggest regulatory sites that contribute to fusion and gB refolding during entry. gB is a target of neutralizing antibodies, and defining how gB refolds during entry could provide a basis for the development of fusion inhibitors for future research or clinical use.

In Vitro Viral Evolution Identifies a Critical Residue in the Alphaherpesvirus Fusion Glycoprotein B Ectodomain That Controls gH/gL-Independent Entry

Herpesvirus entry and spread requires fusion of viral and host cell membranes, which is mediated by the conserved surface glycoprotein B (gB). Upon activation, gB undergoes a major conformational change and transits from a metastable prefusion to a stable postfusion conformation. Although gB is a structural homolog of low-pH-triggered class III fusogens, its fusion activity depends strictly on the presence of the conserved regulatory gH/gL complex and nonconserved receptor binding proteins, which ensure that fusion occurs at the right time and space. How gB maintains its prefusion conformation and how gB fusogenicity is controlled remain poorly understood. Here, we report the isolation and characterization of a naturally selected pseudorabies virus (PrV) gB able to mediate efficient gH/gL-independent virus-cell and cell-cell fusion. We found that the control exerted on gB by the accompanying viral proteins is mediated via its cytosolic domain (CTD). Whereas gB variants lacking the CTD are inactive, a single mutation of a conserved asparagine residue in an alpha-helical motif of the ectodomain recently shown to be at the core of the gB prefusion trimer compensated for CTD absence and uncoupled gB from regulatory viral proteins, resulting in a hyperfusion phenotype. This phenotype was transferred to gB homologs from different alphaherpesvirus genera. Overall, our data propose a model in which the central helix acts as a molecular switch for the gB pre-to-postfusion transition by conveying the structural status of the endo- to the ectodomain, thereby governing their cross talk for fusion activation, providing a new paradigm for herpesvirus fusion regulation.

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.

The structural basis of herpesvirus entry

Herpesviruses are ubiquitous, double-stranded DNA, enveloped viruses that establish lifelong infections and cause a range of diseases. Entry into host cells requires binding of the virus to specific receptors, followed by the coordinated action of multiple viral entry glycoproteins to trigger membrane fusion. Although the core fusion machinery is conserved for all herpesviruses, each species uses distinct receptors and receptor-binding glycoproteins. Structural studies of the prototypical herpesviruses herpes simplex virus 1 (HSV-1), HSV-2, human cytomegalovirus (HCMV) and Epstein-Barr virus (EBV) entry glycoproteins have defined the interaction sites for glycoprotein complexes and receptors, and have revealed conformational changes that occur on receptor binding. Recent crystallography and electron microscopy studies have refined our model of herpesvirus entry into cells, clarifying both the conserved features and the unique features. In this Review, we discuss recent insights into herpesvirus entry by analysing the structures of entry glycoproteins, including the diverse receptor-binding glycoproteins (HSV-1 glycoprotein D (gD), EBV glycoprotein 42 (gp42) and HCMV gH-gL-gO trimer and gH-gL-UL128-UL130-UL131A pentamer), as well gH-gL and the fusion protein gB, which are conserved in all herpesviruses.

The prefusion structure of herpes simplex virus glycoprotein B

Cell entry of enveloped viruses requires specialized viral proteins that mediate fusion with the host membrane by substantial structural rearrangements from a metastable pre- to a stable postfusion conformation. This metastability renders the herpes simplex virus 1 (HSV-1) fusion glycoprotein B (gB) highly unstable such that it readily converts into the postfusion form, thereby precluding structural elucidation of the pharmacologically relevant prefusion conformation. By identification of conserved sequence signatures and molecular dynamics simulations, we devised a mutation that stabilized this form. Functionally locking gB allowed the structural determination of its membrane-embedded prefusion conformation at sub-nanometer resolution and enabled the unambiguous fit of all ectodomains. The resulting pseudo-atomic model reveals a notable conservation of conformational domain rearrangements during fusion between HSV-1 gB and the vesicular stomatitis virus glycoprotein G, despite their very distant phylogeny. In combination with our comparative sequence-structure analysis, these findings suggest common fusogenic domain rearrangements in all class III viral fusion proteins.

Expression, Purification, and Crystallization of HSV-1 Glycoproteins for Structure Determination

Herpes simplex viruses utilize glycoproteins displayed on the viral envelope to perform a variety of functions in the viral infectious cycle. Structural and functional studies of these viral glycoproteins can benefit from biochemical, biophysical, and structural analysis of purified proteins. Here, we describe a general protocol for expression and purification of viral glycoproteins from insect cells based on those developed for the HSV-1 gB and HSV-2 gH/gL ectodomains as well as the protocol for crystallization of these glycoproteins. This protocol can be used for generating milligram amounts of wild-type (WT) or mutant gB and gH/gL ectodomains or can be adapted to produce purified ectodomains of glycoproteins from HSV or other herpesviruses for biochemical and structural studies.

Surface Plasmon Resonance Reveals Direct Binding of Herpes Simplex Virus Glycoproteins gH/gL to gD and Locates a gH/gL Binding Site on gD

Herpes simplex virus (HSV) requires fusion between the viral envelope and host membrane. Four glycoproteins, gD, gH/gL, and gB, are essential for this process. To initiate fusion, gD binds its receptor and undergoes a conformational change that hypothetically leads to activation of gH/gL, which in turn triggers the fusion protein gB to undergo rearrangements leading to membrane fusion. Our model predicts that gD must interact with both its receptor and gH/gL to promote fusion. In support of this, we have shown that gD is structurally divided into two "faces": one for the binding receptor and the other for its presumed interaction with gH/gL. However, until now, we have been unable to demonstrate a direct interaction between gD and gH/gL. Here, we used surface plasmon resonance to show that the ectodomain of gH/gL binds directly to the ectodomain of gD when (i) gD is captured by certain anti-gD monoclonal antibodies (MAbs) that are bound to a biosensor chip, (ii) gD is bound to either one of its receptors on a chip, and (iii) gD is covalently bound to the chip surface. To localize the gH/gL binding site on gD, we used multiple anti-gD MAbs from six antigenic communities and determined which ones interfered with this interaction. MAbs from three separate communities block gD-gH/gL binding, and their epitopes encircle a geographical area on gD that we propose comprises the gH/gL binding domain. Together, our results show that gH/gL interacts directly with gD, supporting a role for this step in HSV entry.IMPORTANCE HSV entry is a multistep process that requires the actions of four glycoproteins, gD, gH/gL, and gB. Our current model predicts that gD must interact with both its receptor and gH/gL to promote viral entry. Although we know a great deal about how gD binds its receptors, until now we have been unable to demonstrate a direct interaction between gD and gH/gL. Here, we used a highly sensitive surface plasmon resonance technique to clearly demonstrate that gD and gH/gL interact. Furthermore, using multiple MAbs with defined epitopes, we have delineated a domain on gD that is independent of that used for receptor binding and which likely represents the gH/gL interaction domain. Targeting this interaction to prevent fusion may enhance both therapeutic and vaccine strategies.

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.

Oncolytic herpes simplex virus tumor targeting and neutralization escape by engineering viral envelope glycoproteins

Oncolytic herpes simplex viruses (oHSVs) have been approved for clinical usage and become more and more popular for tumor virotherapy. However, there are still many issues for the oHSVs used in clinics and clinical trials. The main issues are the limited anti-tumor effects, intratumor injection, and some side effects. To overcome such challenges, here we review the genetic engineering of the envelope glycoproteins for oHSVs to target tumors specifically, and at the same time we summarize the many neutralization antibodies against the envelope glycoproteins and align the neutralization epitopes with functional domains of the respective glycoproteins for future identification of new functions of the glycoproteins and future engineering of the epitopes to escape from host neutralization.

Different functional states of fusion protein gB revealed on human cytomegalovirus by cryo electron tomography with Volta phase plate

Human cytomegalovirus (HCMV) enters host by glycoprotein B (gB)-mediated membrane fusion upon receptor-binding to gH/gL-related complexes, causing devastating diseases such as birth defects. Although an X-ray crystal structure of the recombinant gB ectodomain at postfusion conformation is available, the structures of prefusion gB and its complex with gH/gL on the viral envelope remain elusive. Here, we demonstrate the utility of cryo electron tomography (cryoET) with energy filtering and the cutting-edge technologies of Volta phase plate (VPP) and direct electron-counting detection to capture metastable prefusion viral fusion proteins and report the structures of glycoproteins in the native environment of HCMV virions. We established the validity of our approach by obtaining cryoET in situ structures of the vesicular stomatitis virus (VSV) glycoprotein G trimer (171 kD) in prefusion and postfusion conformations, which agree with the known crystal structures of purified G trimers in both conformations. The excellent contrast afforded by these technologies has enabled us to identify gB trimers (303kD) in two distinct conformations in HCMV tomograms and obtain their in situ structures at up to 21 Å resolution through subtomographic averaging. The predominant conformation (79%), which we designate as gB prefusion conformation, fashions a globular endodomain and a Christmas tree-shaped ectodomain, while the minority conformation (21%) has a columnar tree-shaped ectodomain that matches the crystal structure of the "postfusion" gB ectodomain. We also observed prefusion gB in complex with an "L"-shaped density attributed to the gH/gL complex. Integration of these structures of HCMV glycoproteins in multiple functional states and oligomeric forms with existing biochemical data and domain organization of other class III viral fusion proteins suggests that gH/gL receptor-binding triggers conformational changes of gB endodomain, which in turn triggers two essential steps to actuate virus-cell membrane fusion: exposure of gB fusion loops and unfurling of gB ectodomain.

Natural Selection of Glycoprotein B Mutations That Rescue the Small-Plaque Phenotype of a Fusion-Impaired Herpes Simplex Virus Mutant

Glycoprotein B (gB) is a conserved viral fusion protein that is required for herpesvirus entry. To mediate fusion with the cellular membrane, gB refolds from a prefusion to a postfusion conformation. We hypothesize that an interaction between the C-terminal arm and the central coiled coil of the herpes simplex virus 1 (HSV-1) gB ectodomain is critical for fusion. We previously reported that three mutations in the C-terminal arm (I671A/H681A/F683A, called gB3A) greatly reduced cell-cell fusion and that virus carrying these mutations had a small-plaque phenotype and delayed entry into cells. By serially passaging gB3A virus, we selected three revertant viruses with larger plaques. These revertant viruses acquired mutations in gB that restore the fusion function of gB3A, including gB-A683V, gB-S383F/G645R/V705I/A855V, and gB-T509M/N709H. V705I and N709H are novel mutations that map to the portion of domain V that enters domain I in the postfusion structure. S383F, G645R, and T509M are novel mutations that map to an intersection of three domains in a prefusion model of gB. We introduced these second-site mutations individually and in combination into wild-type gB and gB3A to examine the impact of the mutations on fusion and expression. V705I and A855V (a known hyperfusogenic mutation) restored the fusion function of gB3A, whereas S383F and G645R dampened fusion and T509M and N709H worked in concert to restore gB3A fusion. The results identify two regions in the gB ectodomain that modulate the fusion activity of gB, potentially by impacting intramolecular interactions and stability of the prefusion and/or postfusion gB trimer.IMPORTANCE Glycoprotein B (gB) is an essential viral protein that is conserved in all herpesviruses and is required for virus entry. gB is thought to undergo a conformational change that provides the energy to fuse the viral and cellular membranes; however, the details of this conformational change and the structure of the prefusion and intermediate conformations of gB are not known. Previously, we demonstrated that mutations in the gB "arm" region inhibit fusion and impart a small-plaque phenotype. Using serial passage of a virus carrying these mutations, we identified revertants with restored plaque size. The revertant viruses acquired novel mutations in gB that restored fusion function and mapped to two sites in the gB ectodomain. This work supports our hypothesis that an interaction between the gB arm and the core of gB is critical for gB refolding and provides details about the function of gB in herpesvirus-mediated fusion and subsequent virus entry.

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.

A comparative review of viral entry and attachment during large and giant dsDNA virus infections

Viruses enter host cells via several mechanisms, including endocytosis, macropinocytosis, and phagocytosis. They can also fuse at the plasma membrane and can spread within the host via cell-to-cell fusion or syncytia. The mechanism used by a given viral strain depends on its external topology and proteome and the type of cell being entered. This comparative review discusses the cellular attachment receptors and entry pathways of dsDNA viruses belonging to the families Adenoviridae, Baculoviridae, Herpesviridae and nucleocytoplasmic large DNA viruses (NCLDVs) belonging to the families Ascoviridae, Asfarviridae, Iridoviridae, Phycodnaviridae, and Poxviridae, and giant viruses belonging to the families Mimiviridae and Marseilleviridae as well as the proposed families Pandoraviridae and Pithoviridae. Although these viruses have several common features (e.g., topology, replication and protein sequence similarities) they utilize different entry pathways to infect wide-range of hosts, including humans, other mammals, invertebrates, fish, protozoa and algae. Similarities and differences between the entry methods used by these virus families are highlighted, with particular emphasis on viral topology and proteins that mediate viral attachment and entry. Cell types that are frequently used to study viral entry are also reviewed, along with other factors that affect virus-host cell interactions.

The Fusion Loops of the Initial Prefusion Conformation of Herpes Simplex Virus 1 Fusion Protein Point Toward the Membrane

All enveloped viruses, including herpesviruses, must fuse their envelope with the host membrane to deliver their genomes into target cells, making this essential step subject to interference by antibodies and drugs. Viral fusion is mediated by a viral surface protein that transits from an initial prefusion conformation to a final postfusion conformation. Strikingly, the prefusion conformation of the herpesvirus fusion protein, gB, is poorly understood. Herpes simplex virus (HSV), a model system for herpesviruses, causes diseases ranging from mild skin lesions to serious encephalitis and neonatal infections. Using cryo-electron tomography and subtomogram averaging, we have characterized the structure of the prefusion conformation and fusion intermediates of HSV-1 gB. To this end, we have set up a system that generates microvesicles displaying full-length gB on their envelope. We confirmed proper folding of gB by nondenaturing electrophoresis-Western blotting with a panel of monoclonal antibodies (MAbs) covering all gB domains. To elucidate the arrangement of gB domains, we labeled them by using (i) mutagenesis to insert fluorescent proteins at specific positions, (ii) coexpression of gB with Fabs for a neutralizing MAb with known binding sites, and (iii) incubation of gB with an antibody directed against the fusion loops. Our results show that gB starts in a compact prefusion conformation with the fusion loops pointing toward the viral membrane and suggest, for the first time, a model for gB’s conformational rearrangements during fusion. These experiments further illustrate how neutralizing antibodies can interfere with the essential gB structural transitions that mediate viral entry and therefore infectivity.

The herpesvirus family includes herpes simplex virus (HSV) and other human viruses that cause lifelong infections and a variety of diseases, like skin lesions, encephalitis, and cancers. As enveloped viruses, herpesviruses must fuse their envelope with the host membrane to start an infection. This process is mediated by a viral surface protein that transitions from an initial conformation (prefusion) to a final, more stable, conformation (postfusion). However, the prefusion conformation of the herpesvirus fusion protein (gB) is poorly understood. To elucidate the structure of the prefusion conformation of HSV type 1 gB, we have employed cryo-electron microscopy to study gB molecules expressed on the surface of vesicles. Using different approaches to label gB’s domains allowed us to model the structures of the prefusion and intermediate conformations of gB. Overall, our findings enhance our understanding of HSV fusion and lay the groundwork for the development of new ways to prevent and block HSV infection.

Structure-Based Mutations in the Herpes Simplex Virus 1 Glycoprotein B Ectodomain Arm Impart a Slow-Entry Phenotype

Glycoprotein B (gB) is the conserved herpesvirus fusion protein, and it is required for the entry of herpesviruses. The structure of the postfusion conformation of gB has been solved for several herpesviruses; however, the gB prefusion crystal structure and the details of how the protein refolds from a prefusion to a postfusion form to mediate fusion have not been determined. Using structure-based mutagenesis, we previously reported that three mutations (I671A, H681A, and F683A) in the C-terminal arm of the gB ectodomain greatly reduced cell-cell fusion. This fusion deficit could be rescued by the addition of a hyperfusogenic mutation, suggesting that the gB triple mutant was not misfolded. Using a bacterial artificial chromosome (BAC), we constructed two independent herpes simplex virus 1 mutant strains (gB 3A) carrying the three arm mutations. The gB 3A viruses have 200-fold smaller plaques than the wild-type virus and demonstrate remarkably delayed entry into cells. Single-step and multistep growth curves show that gB 3A viruses have delayed replication kinetics. Interestingly, incubation at 40°C promoted the entry of the gB 3A viruses. We propose that the gB 3A viruses' entry deficit is due to a loss of interactions between residues in the gB C-terminal arm and the coiled-coil core of gB. The results suggest that the triple alanine mutation may destabilize the postfusion gB conformation and/or stabilize the prefusion gB conformation and that exposure to elevated temperatures can overcome the defect in gB 3A viruses.IMPORTANCE Because of its complexity, the mechanism of herpesvirus entry into cells is not well understood. Our study investigated one of the most important unanswered questions about herpesvirus entry; i.e., how does the herpesvirus fusion protein gB mediate membrane fusion? gB is an essential protein that is conserved in all herpesviruses and is thought to undergo a conformational change to provide the energy to fuse the viral and cellular membranes. Using our understanding of the structure of gB, we designed mutations in the gB "arm" region that we predicted would impede gB function. We introduced these mutations into herpes simplex virus 1 by using a bacterial artificial chromosome, and the mutant virus exhibited a drastically delayed rate of entry. This entry defect was rescued by incubation at elevated temperatures, supporting a hypothesis that the engineered mutations altered the energetics of gB refolding. This study supports our hypothesis that an interaction between the gB arm and the core of gB is critical for gB refolding and the execution of membrane fusion, thus providing key details about the function of gB in herpesvirus-mediated fusion and subsequent virus entry.

Hendra virus fusion protein transmembrane domain contributes to pre-fusion protein stability

Enveloped viruses utilize fusion (F) proteins studding the surface of the virus to facilitate membrane fusion with a target cell membrane. Fusion of the viral envelope with a cellular membrane is required for release of viral genomic material, so the virus can ultimately reproduce and spread. To drive fusion, the F protein undergoes an irreversible conformational change, transitioning from a metastable pre-fusion conformation to a more thermodynamically stable post-fusion structure. Understanding the elements that control stability of the pre-fusion state and triggering to the post-fusion conformation is important for understanding F protein function. Mutations in F protein transmembrane (TM) domains implicated the TM domain in the fusion process, but the structural and molecular details in fusion remain unclear. Previously, analytical ultracentrifugation was utilized to demonstrate that isolated TM domains of Hendra virus F protein associate in a monomer-trimer equilibrium (Smith, E. C., Smith, S. E., Carter, J. R., Webb, S. R., Gibson, K. M., Hellman, L. M., Fried, M. G., and Dutch, R. E. (2013) J. Biol. Chem. 288, 35726-35735). To determine factors driving this association, 140 paramyxovirus F protein TM domain sequences were analyzed. A heptad repeat of β-branched residues was found, and analysis of the Hendra virus F TM domain revealed a heptad repeat leucine-isoleucine zipper motif (LIZ). Replacement of the LIZ with alanine resulted in dramatically reduced TM-TM association. Mutation of the LIZ in the whole protein resulted in decreased protein stability, including pre-fusion conformation stability. Together, our data suggest that the heptad repeat LIZ contributed to TM-TM association and is important for F protein function and pre-fusion stability.

The Glycoprotein B Cytoplasmic Domain Lysine Cluster Is Critical for Varicella-Zoster Virus Cell-Cell Fusion Regulation and Infection

The conserved glycoproteins gB and gH-gL are essential for herpesvirus entry and cell-cell fusion induced syncytium formation, a characteristic of varicella-zoster virus (VZV) pathology in skin and sensory ganglia. VZV syncytium formation, which has been implicated in the painful condition of postherpetic neuralgia, is regulated by the cytoplasmic domains of gB (gBcyt) via an immunoreceptor tyrosine-based inhibition motif (ITIM) and gH (gHcyt). A lysine cluster (K894, K897, K898, and K900) in the VZV gBcyt was identified by sequence alignment to be conserved among alphaherpesviruses, suggesting a functional role. Alanine and arginine substitutions were used to determine if the positive charge and susceptibility to posttranslational modifications of these lysines contributed to gB/gH-gL cell-cell fusion. Critically, the positive charge of the lysine residues was necessary for fusion regulation, as alanine substitutions induced a 440% increase in fusion compared to that of the wild-type gBcyt while arginine substitutions had wild-type-like fusion levels in an in vitro gB/gH-gL cell fusion assay. Consistent with these results, the alanine substitutions in the viral genome caused exaggerated syncytium formation, reduced VZV titers (-1.5 log₁₀), and smaller plaques than with the parental Oka (pOka) strain. In contrast, arginine substitutions resulted in syncytia with only 2-fold more nuclei, a -0.5-log₁₀ reduction in titers, and pOka-like plaques. VZV mutants with both an ITIM mutation and either alanine or arginine substitutions had reduced titers and small plaques but differed in syncytium morphology. Thus, effective VZV propagation is dependent on cell-cell fusion regulation by the conserved gBcyt lysine cluster, in addition to the gBcyt ITIM and the gHcyt.

IMPORTANCE

Varicella-zoster virus (VZV) is a ubiquitous pathogen that causes chickenpox and shingles. Individuals afflicted with shingles risk developing the painful condition of postherpetic neuralgia (PHN), which has been difficult to treat because the underlying cause is not well understood. Additional therapies are needed, as the current vaccine is not recommended for immunocompromised individuals and its efficacy decreases with the age of the recipient. VZV is known to induce the formation of multinuclear cells in neuronal tissue, which has been proposed to be a factor contributing to PHN. This study examines the role of a lysine cluster in the cytoplasmic domain of the VZV fusion protein, gB, in the formation of VZV induced multinuclear cells and in virus replication kinetics and spread. The findings further elucidate how VZV self-regulates multinuclear cell formation and may provide insight into the development of new PHN therapies.

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.

Crystal Structure of the Human Cytomegalovirus Glycoprotein B

Human cytomegalovirus (HCMV), a dsDNA, enveloped virus, is a ubiquitous pathogen that establishes lifelong latent infections and caused disease in persons with compromised immune systems, e.g., organ transplant recipients or AIDS patients. HCMV is also a leading cause of congenital viral infections in newborns. Entry of HCMV into cells requires the conserved glycoprotein B (gB), thought to function as a fusogen and reported to bind signaling receptors. gB also elicits a strong immune response in humans and induces the production of neutralizing antibodies although most anti-gB Abs are non-neutralizing. Here, we report the crystal structure of the HCMV gB ectodomain determined to 3.6-Å resolution, which is the first atomic-level structure of any betaherpesvirus glycoprotein. The structure of HCMV gB resembles the postfusion structures of HSV-1 and EBV homologs, establishing it as a new member of the class III viral fusogens. Despite structural similarities, each gB has a unique domain arrangement, demonstrating structural plasticity of gB that may accommodate virus-specific functional requirements. The structure illustrates how extensive glycosylation of the gB ectodomain influences antibody recognition. Antigenic sites that elicit neutralizing antibodies are more heavily glycosylated than those that elicit non-neutralizing antibodies, which suggest that HCMV gB uses glycans to shield neutralizing epitopes while exposing non-neutralizing epitopes. This glycosylation pattern may have evolved to direct the immune response towards generation of non-neutralizing antibodies thus helping HCMV to avoid clearance. HCMV gB structure provides a starting point for elucidation of its antigenic and immunogenic properties and aid in the design of recombinant vaccines and monoclonal antibody therapies.

Human cytomegalovirus (HCMV) establishes lifelong infection in a majority of the world’s population and causes disease in neonates and the immunocompromised patients such as organ transplant recipients or persons with AIDS. There is no vaccine against HCMV, and current HCMV antivirals are toxic and an increasing prevalence of resistance. Glycoprotein B (gB), displayed on the viral surface is a major viral immunogen and is necessary for viral penetration into cells. The crystal structure of gB reported here provides a detailed 3D map of gB. A thick glycan layer covers a large surface area, which may explain why anti-gB neutralizing antibodies are relatively rare. The structure is expected to aid in the development of a HCMV vaccine and monoclonal antibody therapies.

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.

Forces and Structures of the Herpes Simplex Virus (HSV) Entry Mechanism

This paper discusses physical and structural aspects of the mechanisms herpes simplex virus (HSV) uses for membrane fusion. Calculations show that herpes simplex virus glycoprotein D has such avidity for its receptors that it can hold the virion against the plasma membrane of a neuron strongly enough for glycoprotein B (gB) to disrupt both leaflets of the bilayer. The strong electric field generated by the cell potential across perforations at this disruption would break the hydrogen bonds securing the gB fusion loops, leading to fusion of the plasma and viral membranes. This mechanism agrees with the high stability of the tall trimeric spike structure of gB and is consistent with the probable existence of a more compact initial conformation that would allow it to closely approach the plasma membrane. The release of the fusion domains by disruption of hydrogen bonds is shared with the endocytotic entry pathway where, for some cell types not punctured by gB, the virus is able to induce inward forces that cause endocytosis and the fusion loops are released by acidification. The puncture-fusion mechanism requires low critical strain or high tissue strain, matching primary tropism of neural processes at the vermillion border. In support of this mechanism, this paper proposes a functional superstructure of the antigens essential to entry and reviews its consistency with experimental evidence.

Identification of a neutralizing epitope within antigenic domain 5 of glycoprotein B of human cytomegalovirus

Human cytomegalovirus (HCMV) is an important, ubiquitous pathogen that causes severe clinical disease in immunocompromised individuals, such as organ transplant recipients and infants infected in utero. The envelope glycoprotein B (gB) of HCMV is a major antigen for the induction of virus-neutralizing antibodies. We have begun to define target structures within gB that are recognized by virus-neutralizing antibodies. Antigenic domain 5 (AD-5) of gB has been identified as an important target for neutralizing antibodies in studies using human monoclonal antibodies (MAbs). Anti-AD-5 MAbs share a target site on gB, despite originating from different, healthy, HCMV-infected donors. Mutational analysis of AD-5 identified tyrosine 280 in combination with other surface-exposed residues (the YNND epitope) as critical for antibody binding. The YNND epitope is strictly conserved among different HCMV strains. Recombinant viruses carrying YNND mutations in AD-5 were resistant to virus-neutralizing MAbs. Competition enzyme-linked immunosorbent assays (ELISAs) with human HCMV-convalescent-phase sera from unselected donors confirmed the conserved antibody response for the YNND epitope in HCMV-infected individuals and, because a significant fraction of the gB AD-5 response was directed against the YNND epitope, further argued that this epitope is a major target of anti-AD-5 antibody responses. In addition, affinity-purified polyclonal anti-AD-5 antibodies prepared from individual sera showed reactivity to AD-5 and neutralization activity toward gB mutant viruses that were similar to those of AD-5-specific MAbs. Taken together, our data indicate that the YNND epitope represents an important target for anti-gB antibody responses as well as for anti-AD-5 virus-neutralizing antibodies.

IMPORTANCE

HCMV is a major global health concern, and a vaccine to prevent HCMV disease is a widely recognized medical need. Glycoprotein B of HCMV is an important target for neutralizing antibodies and hence an interesting molecule for intervention strategies, e.g., vaccination. Mapping the target structures of neutralizing antibodies induced by naturally occurring HCMV infection can aid in defining the properties required for a protective capacity of vaccine antigens. The data presented here extend our knowledge of neutralizing epitopes within gB to include AD-5. Collectively, our data will contribute to optimal vaccine design and development of antibody-based therapies.

Structural basis for the recognition of human cytomegalovirus glycoprotein B by a neutralizing human antibody

Human cytomegalovirus (HCMV) infections are life-threating to people with a compromised or immature immune system. Upon adhesion, fusion of the virus envelope with the host cell is initiated. In this step, the viral glycoprotein gB is considered to represent the major fusogen. Here, we present for the first time structural data on the binding of an anti-herpes virus antibody and describe the atomic interactions between the antigenic domain Dom-II of HCMV gB and the Fab fragment of the human antibody SM5-1. The crystal structure shows that SM5-1 binds Dom-II almost exclusively via only two CDRs, namely light chain CDR L1 and a 22-residue-long heavy chain CDR H3. Two contiguous segments of Dom-II are targeted by SM5-1, and the combining site includes a hydrophobic pocket on the Dom-II surface that is only partially filled by CDR H3 residues. SM5-1 belongs to a series of sequence-homologous anti-HCMV gB monoclonal antibodies that were isolated from the same donor at a single time point and that represent different maturation states. Analysis of amino acid substitutions in these antibodies in combination with molecular dynamics simulations show that key contributors to the picomolar affinity of SM5-1 do not directly interact with the antigen but significantly reduce the flexibility of CDR H3 in the bound and unbound state of SM5-1 through intramolecular side chain interactions. Thus, these residues most likely alleviate unfavorable binding entropies associated with extra-long CDR H3s, and this might represent a common strategy during antibody maturation. Models of entire HCMV gB in different conformational states hint that SM5-1 neutralizes HCMV either by blocking the pre- to postfusion transition of gB or by precluding the interaction with additional effectors such as the gH/gL complex.

Human cytomegalovirus (HCMV) belongs to the family of β-herpes viruses. HCMV infections are not only life threatening to people with a compromised immune system but also the most common viral cause of congenital defects in newborns. Hence, the development of HCMV vaccines was ranked top priority by the US Institute of Medicine in 1999. Virtually all infected individuals develop antibodies against the envelope protein gB, which plays a crucial role in the infection process. Here, we describe the crystal structure of a fragment of the virus neutralizing antibody SM5-1 in complex with an antigenic determinant of gB, namely Dom-II. The structure shows that antigen antibody interactions are concentrated within two CDRs of SM5-1. Computational methods and an analysis of additional antibody sequences from the same lineage reveal that additional key contributions to high affinity binding are provided by residues that stiffen the extra-long CDR H3 loop without directly contacting the antigen. We suggest that the optimization of such indirect contributions represents a common and yet undervalued principle of the antibody maturation process. Furthermore our data suggest that the neutralizing effect of SM5-1 either originates from blocking membrane fusion or from preventing interaction of gB with other envelope proteins.

Functional fluorescent protein insertions in herpes simplex virus gB report on gB conformation before and after execution of membrane fusion

Entry of herpes simplex virus (HSV) into a target cell requires complex interactions and conformational changes by viral glycoproteins gD, gH/gL, and gB. During viral entry, gB transitions from a prefusion to a postfusion conformation, driving fusion of the viral envelope with the host cell membrane. While the structure of postfusion gB is known, the prefusion conformation of gB remains elusive. As the prefusion conformation of gB is a critical target for neutralizing antibodies, we set out to describe its structure by making genetic insertions of fluorescent proteins (FP) throughout the gB ectodomain. We created gB constructs with FP insertions in each of the three globular domains of gB. Among 21 FP insertion constructs, we found 8 that allowed gB to remain membrane fusion competent. Due to the size of an FP, regions in gB that tolerate FP insertion must be solvent exposed. Two FP insertion mutants were cell-surface expressed but non-functional, while FP insertions located in the crown were not surface expressed. This is the first report of placing a fluorescent protein insertion within a structural domain of a functional viral fusion protein, and our results are consistent with a model of prefusion HSV gB constructed from the prefusion VSV G crystal structure. Additionally, we found that functional FP insertions from two different structural domains could be combined to create a functional form of gB labeled with both CFP and YFP. FRET was measured with this construct, and we found that when co-expressed with gH/gL, the FRET signal from gB was significantly different from the construct containing CFP alone, as well as gB found in syncytia, indicating that this construct and others of similar design are likely to be powerful tools to monitor the conformation of gB in any model system accessible to light microscopy.

Viral fusion proteins undergo complicated conformational changes in order to fuse viral and host membranes during viral entry. Conformational changes between prefusion and postfusion states also allow the virus to hide critical regions of the fusion machinery from the immune system. The structure of herpes simplex virus fusion protein gB is known only in its postfusion state, while the prefusion structure is unknown. To study the prefusion state, we created fluorescent protein (FP) insertions within gB and tested them for fusion activity. Due to the size of the fluorescent protein insertion, regions in gB that tolerate this insertion must be solvent exposed, thereby describing structural features of the prefusion structure. We created functional gB constructs with FP insertions in two of the three globular domains of gB, while non-functional insertions in the third domain suggested that it may be buried in the prefusion structure. Additionally, we created a dual-labeled FP gB construct which we found to report on the conformation of gB before and after fusion. Using this dual-labeled gB construct, we have demonstrated how fluorescence-based methods can be used to directly study dynamics of viral fusion proteins in living cells.

Macromolecular crystallography beamline X25 at the NSLS

Beamline X25 at the NSLS is one of the five beamlines dedicated to macromolecular crystallography operated by the Brookhaven National Laboratory Macromolecular Crystallography Research Resource group. This mini-gap insertion-device beamline has seen constant upgrades for the last seven years in order to achieve mini-beam capability down to 20 µm × 20 µm. All major components beginning with the radiation source, and continuing along the beamline and its experimental hutch, have changed to produce a state-of-the-art facility for the scientific community.

Expression, purification, and crystallization of HSV-1 glycoproteins for structure determination

HSV glycoproteins play important roles in the viral infectious cycle, particularly viral entry into the cell. Here we describe the protocol for expression, purification, and crystallization of viral glycoproteins based on those developed for the HSV-1 gB and HSV-2 gH/gL ectodomains. These protocols can be used for generating milligram amounts of wild-type (WT) or mutant gB and gH/gL ectodomains or can be adapted to produce purified ectodomains of other HSV glycoproteins for biochemical and structural studies.

Mechanism of neutralization of herpes simplex virus by antibodies directed at the fusion domain of glycoprotein B

Glycoprotein B (gB), the fusogen of herpes simplex virus (HSV), is a class III fusion protein with a trimeric ectodomain of known structure for the postfusion state. Seen by negative-staining electron microscopy, it presents as a rod with three lobes (base, middle, and crown). gB has four functional regions (FR), defined by the physical location of epitopes recognized by anti-gB neutralizing monoclonal antibodies (MAbs). Located in the base, FR1 contains two internal fusion loops (FLs) and is the site of gB-lipid interaction (the fusion domain). Many of the MAbs to FR1 are neutralizing, block cell-cell fusion, and prevent the association of gB with lipid, suggesting that these MAbs affect FL function. Here we characterize FR1 epitopes by using electron microscopy to visualize purified Fab-gB ectodomain complexes, thus confirming the locations of several epitopes and localizing those of MAbs DL16 and SS63. We also generated MAb-resistant viruses in order to localize the SS55 epitope precisely. Because none of the epitopes of our anti-FR1 MAbs mapped to the FLs, we hyperimmunized rabbits with FL1 or FL2 peptides to generate polyclonal antibodies (PAbs). While the anti-FL1 PAb failed to bind gB, the anti-FL2 PAb had neutralizing activity, implying that the FLs become exposed during virus entry. Unexpectedly, the anti-FL2 PAb (and the anti-FR1 MAbs) bound to liposome-associated gB, suggesting that their epitopes are accessible even when the FLs engage lipid. These studies provide possible mechanisms of action for HSV neutralization and insight into how gB FR1 contributes to viral fusion.

IMPORTANCE

For herpesviruses, such as HSV, entry into a target cell involves transfer of the capsid-encased genome of the virus to the target cell after fusion of the lipid envelope of the virus with a lipid membrane of the host. Virus-encoded glycoproteins in the envelope are responsible for fusion. Antibodies to these glycoproteins are important biological tools, providing a way of examining how fusion works. Here we used electron microscopy and other techniques to study a panel of anti-gB antibodies. Some, with virus-neutralizing activity, impair gB-lipid association. We also generated a peptide antibody against one of the gB fusion loops; its properties provide insight into the way the fusion loops function as gB transits from its prefusion form to an active fusogen.

Trimeric transmembrane domain interactions in paramyxovirus fusion proteins: roles in protein folding, stability, and function

Paramyxovirus fusion (F) proteins promote membrane fusion between the viral envelope and host cell membranes, a critical early step in viral infection. Although mutational analyses have indicated that transmembrane (TM) domain residues can affect folding or function of viral fusion proteins, direct analysis of TM-TM interactions has proved challenging. To directly assess TM interactions, the oligomeric state of purified chimeric proteins containing the Staphylococcal nuclease (SN) protein linked to the TM segments from three paramyxovirus F proteins was analyzed by sedimentation equilibrium analysis in detergent and buffer conditions that allowed density matching. A monomer-trimer equilibrium best fit was found for all three SN-TM constructs tested, and similar fits were obtained with peptides corresponding to just the TM region of two different paramyxovirus F proteins. These findings demonstrate for the first time that class I viral fusion protein TM domains can self-associate as trimeric complexes in the absence of the rest of the protein. Glycine residues have been implicated in TM helix interactions, so the effect of mutations at Hendra F Gly-508 was assessed in the context of the whole F protein. Mutations G508I or G508L resulted in decreased cell surface expression of the fusogenic form, consistent with decreased stability of the prefusion form of the protein. Sedimentation equilibrium analysis of TM domains containing these mutations gave higher relative association constants, suggesting altered TM-TM interactions. Overall, these results suggest that trimeric TM interactions are important driving forces for protein folding, stability and membrane fusion promotion.

Dual split protein-based fusion assay reveals that mutations to herpes simplex virus (HSV) glycoprotein gB alter the kinetics of cell-cell fusion induced by HSV entry glycoproteins

Herpes simplex virus (HSV) entry and cell-cell fusion require glycoproteins gD, gH/gL, and gB. We propose that receptor-activated changes to gD cause it to activate gH/gL, which then triggers gB into an active form. We employed a dual split-protein (DSP) assay to monitor the kinetics of HSV glycoprotein-induced cell-cell fusion. This assay measures content mixing between two cells, i.e., fusion, within the same cell population in real time (minutes to hours). Titration experiments suggest that both gD and gH/gL act in a catalytic fashion to trigger gB. In fact, fusion rates are governed by the amount of gB on the cell surface. We then used the DSP assay to focus on mutants in two functional regions (FRs) of gB, FR1 and FR3. FR1 contains the fusion loops (FL1 and FL2), and FR3 encompasses the crown at the trimer top. All FL mutants initiated fusion very slowly, if at all. However, the fusion rates caused by some FL2 mutants increased over time, so that total fusion by 8 h looked much like that of the WT. Two distinct kinetic patterns, "slow and fast," emerged for mutants in the crown of gB (FR3), again showing differences in initiation and ongoing fusion. Of note are the fusion kinetics of the gB syn mutant (LL871/872AA). Although this mutant was originally included as an ongoing high-rate-of-fusion control, its initiation of fusion is so rapid that it appears to be on a "hair trigger." Thus, the DSP assay affords a unique way to examine the dynamics of HSV glycoprotein-induced cell fusion.