Functional Region IV of Glycoprotein D from Herpes Simplex Virus Modulates Glycoprotein Binding to the Herpesvirus Entry Mediator

Glycoprotein D (gD) of herpes simplex virus (HSV) is essential for virus entry and has four functional regions (I to IV) important for this process. We previously showed that a truncated form of a functional region IV variant, gD1(Δ290-299t), had an enhanced ability to block virus entry and to bind to the herpesvirus entry mediator (HveAt; formerly HVEMt), a cellular receptor for HSV. To explore this phenotype further, we examined other forms of gD, especially ones with mutations in region IV. Variant proteins with deletions of amino acids between 277 and 300 (region IV), as well as truncated forms lacking C-terminal residues up to amino acid 275 of gD, were able to block HSV entry into Vero cells 1 to 2 logs better than wild-type gD1(306t). In contrast, gD truncated at residue 234 did not block virus entry into Vero cells. Using optical biosensor technology, we recently showed that gD1(Δ290-299t) had a 100-fold-higher affinity for HveAt than gD1(306t) (3.3 × 10⁻⁸ M versus 3.2 × 10⁻⁶ M). Here we found that the affinities of other region IV variants for HveAt were similar to that of gD1(Δ290-299t). Thus, the affinity data follow the same hierarchy as the blocking data. In each case, the higher affinity was due primarily to a faster k_on rather than to a slower k_off. Therefore, once the gDt-HveAt complex formed, its stability was unaffected by mutations in or near region IV. gD truncated at residue 234 bound to HveAt with a lower affinity (2.0 × 10⁻⁵ M) than did gD1(306t) due to a more rapid k_off. These data suggest that residues between 234 and 275 are important for maintaining stability of the gDt-HveAt complex and that functional region IV is important for modulating the binding of gD to HveA. The binding properties of any gD1(234t)-receptor complex could account for the inability of this form of gDt to block HSV infection.     

Examination of the Kinetics of Herpes Simplex Virus Glycoprotein D Binding to the Herpesvirus Entry Mediator, Using Surface Plasmon Resonance

Previously, we showed that truncated soluble forms of herpes simplex virus (HSV) glycoprotein D (gDt) bound directly to a truncated soluble form of the herpesvirus entry mediator (HveAt, formerly HVEMt), a cellular receptor for HSV. The purpose of the present study was to determine the affinity of gDt for HveAt by surface plasmon resonance and to compare and contrast the kinetics of an expanded panel of gDt variants in binding to HveAt in an effort to better understand the mechanism of receptor binding and virus entry. Both HveAt and gDt are dimers in solution and interact with a 2:1 stoichiometry. With HveAt, gD1(306t) (from the KOS strain of HSV-1) had a dissociation constant (K_D) of 3.2 × 10⁻⁶ M and gD2(306t) had a K_D of 1.5 × 10⁻⁶ M. The interaction between gDt and HveAt fits a 1:1 Langmuir binding model, i.e., two dimers of HveAt may act as one binding unit to interact with one dimer of gDt as the second binding unit. A gD variant lacking all signals for N-linked oligosaccharides had an affinity for HveAt similar to that of gD1(306t). A variant lacking the bond from cysteine 1 to cysteine 5 had an affinity for HveAt that did not differ from that of the wild type. However, variants with double cysteine mutations that eliminated either of the other two disulfide bonds showed decreased affinity for HveAt. This result suggests that two of the three disulfide bonds of gD are important for receptor binding. Four nonfunctional gDt variants, each representing one functional domain of gD, were also studied. Mutations in functional regions I and II drastically decreased the affinity of gDt for HveAt. Surprisingly, a variant with an insertion in functional region III had a wild-type level of affinity for HveAt, suggesting that this domain may function in virus entry at a step other than receptor binding. A variant with a deletion in functional region IV [gD1(Δ290-299t)] exhibited a 100-fold enhancement in affinity for HveAt (K_D = 3.3 × 10⁻⁸ M) due mainly to a 40-fold increase in its kinetic on rate. This agrees with the results of other studies showing the enhanced ability of gD1(Δ290-299t) to block infection. Interestingly, all the variants with decreased affinities for HveAt exhibited decreased kinetic on rates but only minor changes in their kinetic off rates. The results suggest that once the complex between gDt and HveAt forms, its stability is unaffected by a variety of changes in gD.     

Herpes Simplex Virus Glycoprotein D Can Bind to Poliovirus Receptor-Related Protein 1 or Herpesvirus Entry Mediator, Two Structurally Unrelated Mediators of Virus Entry

Several cell membrane proteins have been identified as herpes simplex virus (HSV) entry mediators (Hve). HveA (formerly HVEM) is a member of the tumor necrosis factor receptor family, whereas the poliovirus receptor-related proteins 1 and 2 (PRR1 and PRR2, renamed HveC and HveB) belong to the immunoglobulin superfamily. Here we show that a truncated form of HveC directly binds to HSV glycoprotein D (gD) in solution and at the surface of virions. This interaction is dependent on the native conformation of gD but independent of its N-linked glycosylation. Complex formation between soluble gD and HveC appears to involve one or two gD molecules for one HveC protein. Since HveA also mediates HSV entry by interacting with gD, we compared both structurally unrelated receptors for their binding to gD. Analyses of several gD variants indicated that structure and accessibility of the N-terminal domain of gD, essential for HveA binding, was not necessary for HveC interaction. Mutations in functional regions II, III, and IV of gD had similar effects on binding to either HveC or HveA. Competition assays with neutralizing anti-gD monoclonal antibodies (MAbs) showed that MAbs from group Ib prevented HveC and HveA binding to virions. However, group Ia MAbs blocked HveC but not HveA binding, and conversely, group VII MAbs blocked HveA but not HveC binding. Thus, we propose that HSV entry can be mediated by two structurally unrelated gD receptors through related but not identical binding with gD.     

HveA (Herpesvirus Entry Mediator A), a Coreceptor for Herpes Simplex Virus Entry, also Participates in Virus-Induced Cell Fusion

The purpose of this study was to determine whether a cell surface protein that can serve as coreceptor for herpes simplex virus type 1 (HSV-1) entry, herpesvirus entry mediator (previously designated HVEM but renamed HveA), also mediates HSV-1-induced cell-cell fusion. We found that transfection of DNA from KOS-804, a previously described HSV-1 syncytial (Syn) strain whose Syn mutation was mapped to an amino acid substitution in gK, induced numerous large syncytia on HveA-expressing Chinese hamster ovary cells (CHO-HVEM12) but not on control cells (CHO-C8). Antibodies specific for gD as well as for HveA were effective inhibitors of KOS-804-induced fusion, consistent with previously described direct interactions between gD and HveA. Since mutations in gD determine the ability of HSV-1 to utilize HveA for entry, we examined whether the form of virally expressed gD also influenced the ability of HveA to mediate fusion. We produced a recombinant virus carrying the KOS-804 Syn mutation and the KOS-Rid1 gD mutation, which significantly reduces viral entry via HveA, and designated it KOS-SR1. KOS-SR1 DNA had a markedly reduced ability to induce syncytia on CHO-HVEM12 cells and a somewhat enhanced ability to induce syncytia on CHO-C8 cells. These results support previous findings concerning the relative abilities of KOS and KOS-Rid1 to infect CHO-HVEM12 and CHO-C8 cells. Thus, HveA mediates cell-cell fusion as well as viral entry and both activities of HveA are contingent upon the form of gD expressed by the virus.     

单纯疱疹病毒Ⅰ型糖蛋白D在酵母中的表达

从提取的HSV-1基因组中扩增得到编码gD蛋白胞外区1～314aa的基因gDt,将其插入毕赤酵母表达质粒pPIC9K的醇氧化酶(AOX1)启动子下游,构建携带gDt的重组载体,经电转化GS115菌株和G418筛选,得到了高效分泌表达gD蛋白的毕赤酵母菌株,表达量达到250mg/L,该目的蛋白可被gD单抗(1-I-9)特异性识别.表达产物经离子交换、金属螯合、分子筛柱层析纯化后得到纯度较高的重组蛋白.重组gD蛋白免疫BALB/c小鼠可诱生一定水平的特异性抗体,表明该蛋白具有较好的免疫原性,能够诱导小鼠产生体液免疫应答.     

单纯疱疹病毒Ⅰ型糖蛋白D在酵母中的表达

从提取的HSV-1基因组中扩增得到编码gD蛋白胞外区1～314aa的基因gDt,将其插入毕赤酵母表达质粒pPIC9K的醇氧化酶(AOX1)启动子下游,构建携带gDt的重组载体,经电转化GS115菌株和G418筛选,得到了高效分泌表达gD蛋白的毕赤酵母菌株,表达量达到250mg/L,该目的蛋白可被gD单抗(1-I-9)特异性识别。表达产物经离子交换、金属螯合、分子筛柱层析纯化后得到纯度较高的重组蛋白。重组gD蛋白免疫BALB/c小鼠可诱生一定水平的特异性抗体,表明该蛋白具有较好的免疫原性,能够诱导小鼠产生体液免疫应答。     

Monoclonal Antibodies to Distinct Sites on Herpes Simplex Virus (HSV) Glycoprotein D Block HSV Binding to HVEM

HVEM (for herpesvirus entry mediator) is a member of the tumor necrosis factor receptor superfamily and mediates entry of many strains of herpes simplex virus (HSV) into normally nonpermissive Chinese hamster ovary (CHO) cells. We used sucrose density centrifugation to demonstrate that purified HSV-1 KOS virions bind directly to a soluble, truncated form of HVEM (HVEMt) in the absence of any other cell-associated components. Therefore, HVEM mediates HSV entry by serving as a receptor for the virus. We previously showed that soluble, truncated forms of HSV glycoprotein D (gDt) bind to HVEMt in vitro. Here we show that antibodies specific for gD, but not the other entry glycoproteins gB, gC, or the gH/gL complex, completely block HSV binding to HVEM. Thus, virion gD is the principal mediator of HSV binding to HVEM. To map sites on virion gD which are necessary for its interaction with HVEM, we preincubated virions with gD-specific monoclonal antibodies (MAbs). MAbs that recognize antigenic sites Ib and VII of gD were the only MAbs which blocked the HSV-HVEM interaction. MAbs from these two groups failed to coprecipitate HVEMt in the presence of soluble gDt, whereas the other anti-gD MAbs coprecipitated HVEMt and gDt. Previous mapping data indicated that site VII includes amino acids 11 to 19 and site Ib includes 222 to 252. The current experiments indicate that these sites contain residues important for HSV binding to HVEM. Group Ib and VII MAbs also blocked HSV entry into HVEM-expressing CHO cells. These results suggest that the mechanism of neutralization by these MAbs is via interference with the interaction between gD in the virus and HVEM on the cell. Group Ia and II MAbs failed to block HSV binding to HVEM yet still neutralized HVEM-mediated entry, suggesting that these MAbs block entry at a step other than HVEM binding.     

Herpes Simplex Virus Glycoprotein B Associates with Target Membranes via Its Fusion Loops

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 gB-liposome 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 peptides induced the fusion of large unilamellar vesicles and inhibited herpesvirus infection (23, 24). Thus, it appears that both gB and gH/gL may be fusion proteins, a theory strengthened by data showing that either gB or gH/gL is sufficient for membrane fusion during nuclear egress (22). Additionally, gB730t blocks virus entry into cells deficient in heparan sulfate proteoglycans (HSPGs), suggesting that it competes with virion gB for an obligate cell surface receptor (9). A recent study suggested that paired immunoglobulin-like type 2 receptor alpha (PILRα) may serve this role for at least some cell types (47).The crystal structure of gB is now known for both HSV type 1 (HSV-1) (32) and Epstein-Barr virus (EBV) (6). Interestingly, gB is structurally related to two other viral fusion proteins, the vesicular stomatitis virus (VSV) G protein (45) and the baculovirus gp64 protein (34). VSV G, gB, and most recently, baculovirus gp64 were placed into a newly formed group of fusion proteins, the class III proteins. Class III fusion proteins have similar individual domain structures and contain a central three-stranded coiled coil reminiscent of the class I proteins. Whereas class I proteins have an N-terminal fusion peptide, class III proteins have internal bipartite fusion loops within domain I (shown in Fig. ​Fig.1A1A for gB) which are similar to the single fusion loop of class II fusion proteins. However, the class II fusion loop is composed entirely of hydrophobic amino acids, whereas the fusion loops of gB have both hydrophobic and charged residues (32, 34, 45). Unlike G or gp64, which are the sole fusion proteins for their respective viruses, gB requires gH/gL to function in fusion and entry.Open in a separate windowFIG. 1.HSV gB hydrophobic ridge is surrounded by charged residues on the surface of the molecule. A ribbon diagram of the HSV protomer (A) and molecular surface representation of the trimer (B) are shown. In each, one protomer is colored by secondary structure succession, using blue (domain I), green (domain II), yellow (domain III), orange (domain IV), and red (domain V). The box in panel A shows the primary amino acid sequences of the fusion loops. The box in panel B shows the base of the gB trimer, rotated 90°. For the boxes in both panels A and B, highlighted hydrophobic residues are colored in blue and charged residues are shown in red. All structural figures were generated, in part, using PyMOL Molecular Graphics System software.In our previous study, we used site-directed mutagenesis to show that three hydrophobic amino acids within the gB loops (W174, Y179, and A261) are essential for gB function (29). Similar studies of VSV G, gp64, and EBV gB support the notion that hydrophobic amino acids of both fusion loops are critical for fusion (34, 44, 51) and together constitute a fusion domain. Recently, bimolecular complementation was used to show that gB and gH/gL interact with each other concomitantly with fusion and that this interaction is triggered by binding of gD to its cellular receptor (3, 4). Thus, gB may function cooperatively with gH/gL, yet each may have some fusogenic potential on its own.The goal of the experiments reported here was twofold. First, we wanted to complete our mutagenic analysis of all of the residues in the two putative fusion loops of HSV gB. Our data show that the two fusion loops constitute a structural “subdomain” wherein key hydrophobic amino acids form a ridge that is supported on both sides by charged residues. We hypothesize that two charged residues on one side of the ridge enhance the ability of the hydrophobic residues to interact with target membranes and to function in fusion.Our second goal was to assess the effects of mutations in the fusion loops on the function of gB in cell binding, blocking of entry, and insertion into lipid membranes. Therefore, we constructed recombinant baculoviruses, with each carrying the gene for a truncated version (residues 31 to 730) of one of four mutant forms of gB (W174R, Y179S, H263A, and R264A). We found that the mutant proteins were able to efficiently block virus entry, suggesting that the fusion loops do not participate in protein-receptor binding. However, all four mutant proteins were impaired in cell binding compared to WT gB730t. Whereas WT gB730t associated with liposomes in a flotation assay, soluble truncated forms of HSV gD and gH/gL did not, consistent with our previous finding that gB730t associates with lipid rafts on cell surfaces (8). In contrast to WT gB730t, the gB mutant proteins were either impaired or unable to bind liposomes. Our data suggest that gB has an intrinsic ability to associate with a target membrane via its fusion domain.     

Three-Dimensional Structure of Herpes Simplex Virus Type 1 Glycoprotein D at 2.4-Nanometer Resolution

Herpes simplex virus type 1 glycoprotein D (gD) is essential for virus infectivity and is responsible for binding to cellular membrane proteins and subsequently promoting fusion between the virus envelope and the cell. No structural data are available for gD or for any other herpesvirus envelope protein. Here we present a three-dimensional model for the baculovirus-expressed truncated protein gD1(306t) based on electron microscopic data. We demonstrate that gD1(306t) appears as a homotetramer containing a pronounced pocket in the center of the molecule. Monoclonal antibody binding demonstrates that the molecule is oriented such that the pocket protrudes away from the virus envelope.     

单纯疱疹病毒糖蛋白D的表达及免疫学鉴定

单纯疱疹病毒(herpes simplex virus,HSV)是TORCH综合征的病原体之一.新生儿可通过宫内、产道和出生后等多种途径感染,大部分患儿呈现症状,如皮炎、角膜炎、口唇疱疹,也可发生涉及多个器官的播散性感染,严重者出现疱疹性脑膜炎,并常导致婴幼儿死亡.目前尚无全身用的特效药物和有效的防范措施.HSV包膜糖蛋白D(glycoprotein D,gD)是极为保守的免疫原性蛋白,在体内可诱导高滴度的中和抗体,因此gD基因成为近些年来诊断研究的靶基因.本文尝试将gD蛋白在酵母菌中表达,并分析其抗原性,为建立快速易行的重组抗原诊断试剂盒奠定基础.此外,利用该表达系统表达的HSV gD蛋白,可为HSV基因工程重组疫苗的研制提供依据,对优生优育、提高人口出生质量具有重要的理论及实际意义.     

Characterization of Herpes Virus Entry Mediator as a Factor Linked to Obesity

Herpes virus entry mediator (HVEM) is a member of the tumor necrosis factor (TNF) receptor superfamily (TNFRSF14), which serves as a receptor for herpes viruses and cytokines such as lymphotoxin‐α (LT‐α) and LIGHT (lymphotoxin‐like inducible protein that competes with glycoprotein D for herpes virus entry on T cells). We aimed to explore the associations of HVEM with human obesity. HVEM gene expression and protein levels were studied in total adipose tissue and in their fractions (isolated adipocytes and stromovascular cells (SVCs)) obtained from 81 subjects during elective surgical procedures. HVEM ?241GA and ?14AG gene polymorphisms were also studied and associated with obesity measures in 840 subjects. Visceral adipose tissue had significantly higher expression of HVEM than subcutaneous adipose tissue (P < 0.0001). Obese patients had significantly higher subcutaneous HVEM gene expression (P = 0.03) and protein levels (P = 0.01) than lean subjects. HVEM gene expression and protein levels were found in both isolated adipocytes and SVCs. These findings were confirmed in primary cultures from human preadipocytes, in which a significant increase in HVEM was observed during the differentiation process. HVEM ?241GA and ?14AG gene polymorphisms were associated with obesity, diastolic pressure, several inflammatory parameters (C‐reactive protein and interleukin 18 (IL‐18)), and circulating LIGHT concentrations. A sample of men with the G241A gene polymorphism also showed an increased serum titer of IgG antiherpes virus 1. These results provide evidences of an existing relationship between HVEM and obesity, which suggest that this TNF superfamily receptor could be involved in the pathogenesis of obesity and inflammation‐related activity.     

Contributions of Herpes Simplex Virus 1 Envelope Proteins to Entry by Endocytosis

Herpes simplex virus (HSV) proteins specifically required for endocytic entry but not direct penetration have not been identified. HSVs deleted of gE, gG, gI, gJ, gM, UL45, or Us9 entered cells via either pH-dependent or pH-independent endocytosis and were inactivated by mildly acidic pH. Thus, the required HSV glycoproteins, gB, gD, and gH-gL, may be sufficient for entry regardless of entry route taken. This may be distinct from entry mechanisms employed by other human herpesviruses.     

Herpes Simplex Virus Glycoproteins: Participation of Individual Herpes Simplex Virus Type 1 Glycoprotein Antigens in Immunocytolysis and Their Correlation with Previously Identified Glycopolypeptides

Tissue culture cells infected with herpes simplex type 1 virus express virus-specified glycoprotein antigens on the plasma membrane. Three of these have been previously identified and have been designated as Ag-11, Ag-8, and Ag-6. In the present study, immunoglobulins to each of the antigens were shown to be capable of mediating immunocytolysis in the presence of either complement (antibody-dependent complement-mediated cytotoxicity) or peripheral blood mononuclear cells (antibody-dependent cell-mediated cytotoxicity [ADCC]). Two herpes simplex virus type 1 strains, VR-3 and F, reacted similarly in the ADCC test in the presence of immunoglobulins to Ag-11, Ag-8, and Ag-6 in both infected Chang liver cells and HEp-2 cells. Anti-Ag-6, however, produced a lower ADCC reaction in HEp-2 cells than in Chang liver cells, suggesting differences in the Ag-6 surface expression in, or release from, these cells. Chang liver and HEp-2 cells infected with the MP mutant strain of herpes simplex virus type 1 showed reduced ADCC in the presence of anti-Ag-11 and anti-Ag-8, but no reactivity at all with anti-Ag-6. Crossed immunoelectrophoretic analysis showed that MP-infected cell extracts contain Ag-11 and Ag-8, but lack Ag-6. Polypeptide analysis of herpes simplex virus type 1 strains F, VR-3, and MP showed that Ag-11 consists of the glycoproteins gA and gB, that Ag-8 consists of gD, and that Ag-6 consists of gC. In conclusion, the present study demonstrates that either one of the glycoproteins (gC, gD, and a mixture of gA and gB) can function as a target for immunocytolysis and that the antibody preparation to gC (Ag-6) does not cross-react with any of the other glycoproteins.