Glycoprotein B (gB) is the most conserved component of the complex cell-entry machinery of herpes viruses. A crystal structure of the gB ectodomain from herpes simplex virus type 1 reveals a multidomain trimer with unexpected homology to glycoprotein G from vesicular stomatitis virus (VSV G). An a-helical coiled-coil core relates gB to class I viral membrane fusion glycoproteins; two extended b hairpins with hydrophobic tips, homologous to fusion peptides in VSV G, relate gB to class II fusion proteins. Members of both classes accomplish fusion through a large-scale conformational change, triggered by a signal from a receptor-binding component. The domain connectivity within a gB monomer would permit such a rearrangement, including long-range translocations linked to viral and cellular membranes.H erpes simplex virus type 1 (HSV-1) is the prototype of the diverse herpesvirus family, which includes such notable human pathogens as cytomegalovirus (CMV), Epstein-Barr virus (EBV), and Kaposi_s sarcomaassociated herpesvirus (KSHV). Herpesviruses have an envelope, an outer lipid bilayer, bearing 12 surface glycoproteins. To deliver the capsid containing the double-stranded DNA genome into the host cell, HSV-1 must fuse its envelope with a cellular membrane. Among viral glycoproteins, only gC, gB, gD, gH, and gL participate in viral cell entry, and only the last four are required for fusion (1-4). All herpesviruses have gB, gH, and gL, which constitute the core fusion machinery (5). Of these, gB is the most highly conserved.The virus attaches to a cell through a nonessential interaction of gC with heparan sulfate proteoglycan and through an essential interaction of gD with one of three cellular receptors: nectin-1, herpesvirus entry mediator (HVEM), or a specifically modified heparan sulfate (6). Crystal structures of the soluble ectodomain of gD, unbound and in complex with the ectodomain of HVEM (7,8), show that binding of gD and receptor causes the former to undergo a conformational change in which a C-terminal segment of the ectodomain polypeptide chain is released from a strong intramolecular contact.The liberated C-terminal segment may interact with gB or the gH/gL complex to trigger molecular rearrangements and, ultimately, fusion. The precise functions of gB and gH/gL are unknown. Both are required for entry, and either or both presumably receive the signal from gD and respond by undergoing a conformational change; gD itself is thought not to participate in the fusion process (9, 10). Neither gB nor gH/gL has an obvious fusion peptide, but an indication that gB might be a fusion effector comes from the notable syncytial phenotype caused by certain mutations within the cytoplasmic domain of gB (1, 11-13).HSV-1 gB is a 904-residue protein. In the work reported here, we determined the crystal structure of a nearly full-length ectodomain of gB, residues Asp 103 to Ala 730 (14) (Fig. 1). Various features of the structure suggest that it is a fusion effector, an inference strengthened by its notable and unanticipated...

Binding of herpes simplex virus (HSV) glycoprotein D (gD) to a cell surface receptor is required to trigger membrane fusion during entry into host cells. Nectin-1 is a cell adhesion molecule and the main HSV receptor in neurons and epithelial cells. We report the structure of gD bound to nectin-1 determined by x-ray crystallography to 4.0 Å resolution. The structure reveals that the nectin-1 binding site on gD differs from the binding site of the HVEM receptor. A surface on the first Ig-domain of nectin-1, which mediates homophilic interactions of Ig-like cell adhesion molecules, buries an area composed by residues from both the gD N- and C-terminal extensions. Phenylalanine 129, at the tip of the loop connecting β-strands F and G of nectin-1, protrudes into a groove on gD, which is otherwise occupied by C-terminal residues in the unliganded gD and by N-terminal residues in the gD/HVEM complex. Notably, mutation of Phe129 to alanine prevents nectin-1 binding to gD and HSV entry. Together these data are consistent with previous studies showing that gD disrupts the normal nectin-1 homophilic interactions. Furthermore, the structure of the complex supports a model in which gD-receptor binding triggers HSV entry through receptor-mediated displacement of the gD C-terminal region.

Glycoprotein B (gB), along with gD, gH, and gL, is essential for herpes simplex virus (HSV) entry. The crystal structure of the gB ectodomain revealed it to be an elongated multidomain trimer. We generated and characterized a panel of 67 monoclonal antibodies (MAbs). Eleven of the MAbs had virus-neutralizing activity. To organize gB into functional regions within these domains, we localized the epitopes recognized by the entire panel of MAbs and mapped them onto the crystal structure of gB. Most of the MAbs were directed to continuous or discontinuous epitopes, but several recognized discontinuous epitopes that showed some resistance to denaturation, and we refer to them as pseudo-continuous. Each category contained some MAbs with neutralizing activity. To map continuous epitopes, we used overlapping peptides that spanned the gB ectodomain and measured binding by enzyme-linked immunosorbent assay. To identify discontinuous and pseudocontinuous epitopes, a purified form of the ectodomain of gB, gB(730t), was cleaved by ␣-chymotrypsin into two major fragments comprising amino acids 98 to 472 (domains I and II) and amino acids 473 to 730 (major parts of domains III, IV, and V). We also constructed a series of gB truncations to augment the other mapping strategies. Finally, we used biosensor analysis to assign the MAbs to competition groups. Together, our results identified four functional regions: (i) one formed by residues within domain I and amino acids 697 to 725 of domain V; (ii) a second formed by residues 391 to 410, residues 454 to 475, and a less-defined region within domain II; (iii) a region containing residues of domain IV that lie close to domain III; and (iv) the first 12 residues of the N terminus that were not resolved in the crystal structure. Our data suggest that multiple domains are critical for gB function.Herpes simplex virus (HSV) is a neurotropic agent responsible for episodic cold sores and genital legions. After primary infection of mucosal epithelial cells, the virus establishes lifelong latency in sensory neurons, from which it periodically reactivates. After reactivation, the virus migrates along the axons and infects cells at the site of primary infection, causing painful blisters on the surface of the lips in the case of HSV type 1 (HSV-1) or of the genital mucosa for the closely related HSV-2 (48).A critical event in the life cycle resides in the entry of the virus into target cells. Recent progress has been made in understanding the mechanism governing this process (reviewed in references 21 and 38). Of the 12 different glycoproteins of the viral envelope, 4 have essential functions for entry, namely, glycoprotein B (gB), gD, gH, and gL. First, the virion attaches by interaction of gC and gB with cell surface heparan sulfate proteoglycans (42). Although not essential for entry, this step provides stable interactions between the virion and the cell that favor the next steps. These include the association of gD with one of its three identified receptors-HVEM, nectin-1, and 3-O-sulfated...

Herpes simplex virus (HSV) entry requires the interaction of glycoprotein D (gD) with a cellular receptor such as herpesvirus entry mediator (HVEM or HveA) or nectin-1 (HveC). However, the fusion mechanism is still not understood. Since cholesterol-enriched cell membrane lipid rafts are involved in the entry of other enveloped viruses such as human immunodeficiency virus and Ebola virus, we tested whether HSV entry proceeds similarly. Vero cells and cells expressing either HVEM or nectin-1 were treated with cholesterolsequestering drugs such as methyl-␤-cyclodextrin or nystatin and then exposed to virus. In all cases, virus entry was inhibited in a dose-dependent manner, and the inhibitory effect was fully reversible by replenishment of cholesterol. To examine the association of HVEM and nectin-1 with lipid rafts, we analyzed whether they partitioned into nonionic detergent-insoluble glycolipid-enriched membranes (DIG). There was no constitutive association of either receptor with DIG. Binding of soluble gD or virus to cells did not result in association of nectin-1 with the raft-containing fractions. However, during infection, a fraction of gB but not gC, gD, or gH associated with DIG. Similarly, when cells were incubated with truncated soluble glycoproteins, soluble gB but not gC was found associated with DIG. Together, these data favor a model in which HSV uses gB to rapidly mobilize lipid rafts that may serve as a platform for entry and cell signaling. It also suggests that gB may interact with a cellular molecule associated with lipid rafts.

Herpes simplex virus (HSV) glycoproteins gB, gD, and gH/gL are necessary and sufficient for virus entry into cells. Structural features of gB are similar to those of vesicular stomatitis virus G and baculovirus gp64, and together they define the new class III group of fusion proteins. Previously, we used mutagenesis to show that three hydrophobic residues (W174, Y179, and A261) within the putative gB fusion loops are integral to gB function. Here we expanded our analysis, using site-directed mutagenesis of each residue in both gB fusion loops. Mutation of most of the nonpolar or hydrophobic amino acids (W174, F175, G176, Y179, and A261) had severe effects on gB function in cell-cell fusion and null virus complementation assays. Of the six charged amino acids, mutation of H263 or R264 also negatively affected gB function. To further analyze the mutants, we cloned the ectodomains of the W174R, Y179S, H263A, and R264A mutants into a baculovirus expression system and compared them with the wild-type (WT) form, gB730t. As shown previously, gB730t blocks virus entry into cells, suggesting that gB730t competes with virion gB for a cell receptor. All four mutant proteins retained this function, implying that fusion loop activity is separate from gB-receptor binding. However, unlike WT gB730t, the mutant proteins displayed reduced binding to cells and were either impaired or unable to bind naked, cholesterol-enriched liposomes, suggesting that it may be gB-lipid binding that is disrupted by the mutations. Furthermore, monoclonal antibodies with epitopes proximal to the fusion loops abrogated gBliposome binding. Taken together, our data suggest that gB associates with lipid membranes via a fusion domain of key hydrophobic and hydrophilic residues and that this domain associates with lipid membranes during fusion.Herpes simplex virus (HSV) entry into cells requires four viral envelope glycoproteins (gB, gD, and the heterodimer gH/gL) as well as a cell surface gD receptor (reviewed in references 31, 42, 43, and 49). When gD binds its receptor, it undergoes conformational changes that are essential to activate the fusion machinery, gB and gH/gL. In addition to being essential for virus entry, both gH/gL and gB play important roles in primary fusion events that occur during egress of the capsid from the nuclei of infected cells (22). gB and gH/gL constitute the core fusion machinery of all members of the Herpesviridae.The mechanisms by which gB and gH/gL function individually and in concert during fusion are topics of intense investigations. Peptides based on predicted heptad repeats in gH block virus entry and have the ability to bind and disrupt model membranes (24,26,27). In addition, gH/gL can achieve hemifusion of adjacent cells in the absence of other herpesvirus proteins (50). These studies imply that gH/gL has fusogenic properties. Previously, we showed that both virion gB and soluble wild-type (WT) gB (gB730t), but not gD or gH/gL, bind to cells and associate with lipid rafts (10). Like gH/gL, several synthetic gB pe...