Deletion of the first cysteine-rich region of the varicella-zoster virus glycoprotein E ectodomain abolishes the gE and gI interaction and differentially affects cell-cell spread and viral entry

Varicella-zoster virus (VZV) glycoprotein E (gE) is the most abundant glycoprotein in infected cells and, in contrast to those of other alphaherpesviruses, is essential for viral replication. The gE ectodomain contains a unique N-terminal region required for viral replication, cell-cell spread, and secondary envelopment; this region also binds to the insulin-degrading enzyme (IDE), a proposed VZV receptor. To identify new functional domains of the gE ectodomain, the effect of mutagenesis of the first cysteine-rich region of the gE ectodomain (amino acids 208 to 236) was assessed using VZV cosmids. Deletion of this region was compatible with VZV replication in vitro, but cell-cell spread of the rOka-ΔCys mutant was reduced significantly. Deletion of the cysteine-rich region abolished the binding of the mutant gE to gI but not to IDE. Preventing gE binding to gI altered the pattern of gE expression at the plasma membrane of infected cells and the posttranslational maturation of gI and its incorporation into viral particles. In contrast, deletion of the first cysteine-rich region did not affect viral entry into human tonsil T cells in vitro or into melanoma cells infected with cell-free VZV. These experiments demonstrate that gE/gI heterodimer formation is essential for efficient cell-cell spread and incorporation of gI into viral particles but that it is dispensable for infectious varicella-zoster virion formation and entry into target cells. Blocking gE binding to gI resulted in severe impairment of VZV infection of human skin xenografts in SCIDhu mice in vivo, documenting the importance of cell fusion mediated by this complex for VZV virulence in skin.     

Varicella-zoster virus glycoprotein E is a critical determinant of virulence in the SCID mouse-human model of neuropathogenesis

Varicella-zoster virus (VZV) is a neurotropic alphaherpesvirus. VZV infection of human dorsal root ganglion (DRG) xenografts in immunodeficient mice models the infection of sensory ganglia. We examined DRG infection with recombinant VZV (recombinant Oka [rOka]) and the following gE mutants: gEΔ27-90, gEΔCys, gE-AYRV, and gE-SSTT. gEΔ27-90, which lacks the gE domain that interacts with a putative receptor insulin-degrading enzyme (IDE), replicated as extensively as rOka, producing infectious virions and significant cytopathic effects within 14 days of inoculation. Since neural cells express IDE, the gE/IDE interaction was dispensable for VZV neurotropism. In contrast, gEΔCys, which lacks gE/gI heterodimer formation, was significantly impaired at early times postinfection; viral genome copy numbers increased slowly, and infectious virus production was not detected until day 28. Delayed replication was associated with impaired cell-cell spread in ganglia, similar to the phenotype of a gI deletion mutant (rOkaΔgI). However, at later time points, infection of satellite cells and other supportive nonneuronal cells resulted in extensive DRG tissue damage and cell loss such that cytopathic changes observed at day 70 were more severe than those for rOka-infected DRG. The replication of gE-AYRV, which is impaired for trans-Golgi network (TGN) localization, and the replication of gE-SSTT, which contains mutations in an acidic cluster, were equivalent to that of rOka, causing significant cytopathic effects and infectious virus production by day 14; genome copy numbers were equivalent to those of rOka. These experiments suggest that the gE interaction with cellular IDE, gE targeting to TGN sites of virion envelopment, and phosphorylation at SSTT are dispensable for VZV DRG infection, whereas the gE/gI interaction is critical for VZV neurovirulence.     

Insulin Degrading Enzyme Induces a Conformational Change in Varicella-Zoster Virus gE,and Enhances Virus Infectivity and Stability

Varicella-zoster virus (VZV) glycoprotein E (gE) is essential for virus infectivity and binds to a cellular receptor, insulin-degrading enzyme (IDE), through its unique amino terminal extracellular domain. Previous work has shown IDE plays an important role in VZV infection and virus cell-to-cell spread, which is the sole route for VZV spread in vitro. Here we report that a recombinant soluble IDE (rIDE) enhances VZV infectivity at an early step of infection associated with an increase in virus internalization, and increases cell-to-cell spread. VZV mutants lacking the IDE binding domain of gE were impaired for syncytia formation and membrane fusion. Pre-treatment of cell-free VZV with rIDE markedly enhanced the stability of the virus over a range of conditions. rIDE interacted with gE to elicit a conformational change in gE and rendered it more susceptible to proteolysis. Co-incubation of rIDE with gE modified the size of gE. We propose that the conformational change in gE elicited by IDE enhances infectivity and stability of the virus and leads to increased fusogenicity during VZV infection. The ability of rIDE to enhance infectivity of cell-free VZV over a wide range of incubation times and temperatures suggests that rIDE may be useful for increasing the stability of varicella or zoster vaccines.     

Unusual phosphorylation sequence in the gpIV (gI) component of the varicella-zoster virus gpI-gpIV glycoprotein complex (VZV gE-gI complex).

Varicella-zoster virus (VZV) glycoprotein gpIV, to be renamed VZV gI, forms a heterodimer with glycoprotein gpI (gE) which functions as an Fc receptor in virus-infected cells. Like VZV gpI (gE), this viral glycoprotein is phosphorylated in cell culture during biosynthesis. In this report, we investigated the nature and specificity of the phosphorylation event involving VZV gpIV (gI). Phosphoamino acid analysis indicated that gpIV (gI) was modified mainly on serine residues. To identify the precise location of the phosphorylation site on the 64-kDa protein, a step-by-step mutagenesis procedures was followed. Initially a tailless mutant was generated, and this truncated product was no longer phosphorylated. Thereafter, point mutations were made within the cytoplasmic tail of gpIV (gI) at potential phosphorylation sites. The phosphorylation site was localized to the following sequence: Ser-Pro-Pro (amino acids 343 to 345). Examination of the point mutants established that serine 343 in the cytoplasmic tail was the major phosphoacceptor. In addition, we found that the prolines located immediately to the C terminus of serine 343 were an integral part of the kinase recognition sequence. This site was located immediately N terminal to a predicted beta-turn secondary structure. By comparison with known substrate consensus sequences for various protein kinases, these data suggested that the phosphorylation of VZV gpIV (gI) was catalyzed by a proline-directed protein kinase. Computer homology analysis of other alphaherpesviruses demonstrated that a similar potential phosphorylation site was highly conserved in the cytoplasmic tails of herpes simplex virus type 1 gI, equine herpesvirus type 1 gI, and pseudorabies virus gp63.     

Mutagenesis of varicella-zoster virus glycoprotein I (gI) identifies a cysteine residue critical for gE/gI heterodimer formation, gI structure, and virulence in skin cells

Varicella-zoster virus (VZV) is the alphaherpesvirus that causes chicken pox (varicella) and shingles (zoster). The two VZV glycoproteins gE and gI form a heterodimer that mediates efficient cell-to-cell spread. Deletion of gI yields a small-plaque-phenotype virus, ΔgI virus, which is avirulent in human skin using the xenograft model of VZV pathogenesis. In the present study, 10 mutant viruses were generated to determine which residues were required for the typical function of gI. Three phosphorylation sites in the cytoplasmic domain of gI were not required for VZV virulence in vivo. Two deletion mutants mapped a gE binding region in gI to residues 105 to 125. A glycosylation site, N116, in this region did not affect virulence. Substitution of four cysteine residues highly conserved in the Alphaherpesvirinae established that C95 is required for gE/gI heterodimer formation. The C95A and Δ105-125 (with residues 105 to 125 deleted) viruses had small-plaque phenotypes with reduced replication kinetics in vitro similar to those of the ΔgI virus. The Δ105-125 virus was avirulent for human skin in vivo. In contrast, the C95A mutant replicated in vivo but with significantly reduced kinetics compared to those of the wild-type virus. In addition to abolished gE/gI heterodimer formation, gI from the C95A or the Δ105-125 mutant was not recognized by monoclonal antibodies that detect the canonical conformation of gI, demonstrating structural disruption of gI in these viruses. This alteration prevented gI incorporation into virus particles. Thus, residues C95 and 105 to 125 are critical for gI structure required for gE/gI heterodimer formation, virion incorporation, and ultimately, effective viral spread in human skin.     

Insulin degrading enzyme is a cellular receptor mediating varicella-zoster virus infection and cell-to-cell spread

Varicella-zoster virus (VZV) causes chickenpox and shingles. While varicella is likely spread as cell-free virus to susceptible hosts, the virus is transmitted by cell-to-cell spread in the body and in vitro. Since VZV glycoprotein E (gE) is essential for virus infection, we postulated that gE binds to a cellular receptor. We found that insulin-degrading enzyme (IDE) interacts with gE through its extracellular domain. Downregulation of IDE by siRNA, or blocking of IDE with antibody, with soluble IDE protein extracted from liver, or with bacitracin inhibited VZV infection. Cell-to-cell spread of virus was also impaired by blocking IDE. Transfection of cell lines impaired for VZV infection with a plasmid expressing human IDE resulted in increased entry and enhanced infection with cell-free and cell-associated virus. These studies indicate that IDE is a cellular receptor for both cell-free and cell-associated VZV.     

Mutational analysis of the role of glycoprotein I in varicella-zoster virus replication and its effects on glycoprotein E conformation and trafficking.

The contributions of the glycoproteins gI (ORF67) and gE (ORF68) to varicella-zoster virus (VZV) replication were investigated in deletion mutants made by using cosmids with VZV DNA derived from the Oka strain. Deletion of both gI and gE prevented virus replication. Complete deletion of gI or deletions of 60% of the N terminus or 40% of the C terminus of gI resulted in a small plaque phenotype as well as reduced yields of infectious virus. Melanoma cells infected with gI deletion mutants formed abnormal polykaryocytes with a disrupted organization of nuclei. In the absence of intact gI, gE became localized in patches on the cell membrane, as demonstrated by confocal microscopy. A truncated N-terminal form of gI was transported to the cell surface, but its expression did not restore plaque morphology or infectivity. The fusogenic function of gH did not compensate for gI deletion or the associated disruption of the gE-gI complex. These experiments demonstrated that gI was dispensable for VZV replication in vitro, whereas gE appeared to be required. Although VZV gI was dispensable, its deletion or mutation resulted in a significant decrease in infectious virus yields, disrupted syncytium formation, and altered the conformation and distribution of gE in infected cells. Normal cell-to-cell spread and replication kinetics were restored when gI was expressed from a nonnative locus in the VZV genome. The expression of intact gI, the ORF67 gene product, is required for efficient membrane fusion during VZV replication.     

Glycoprotein E of varicella-zoster virus enhances cell-cell contact in polarized epithelial cells

Varicella-zoster virus (VZV) infection involves the cell-cell spread of virions, but how viral proteins interact with the host cell membranes that comprise intercellular junctions is not known. Madin-Darby canine kidney (MDCK) cells were constructed to express the glycoproteins gE, gI, or gE/gI constitutively and were used to examine the effects of these VZV glycoproteins in polarized epithelial cells. At low cell density, VZV gE induced partial tight junction (TJ) formation under low-calcium conditions, whether expressed alone or with gI. Although most VZV gE was intracellular, gE was also shown to colocalize with the TJ protein ZO-1 with or without concomitant expression of gI. Freeze fracture electron microscopy revealed normal TJ strand morphology in gE-expressing MDCK cells. Functionally, the expression of gE was associated with a marked acceleration in the establishment of maximum transepithelial electrical resistance (TER) in MDCK-gE cells; MDCK-gI and MDCK-gE/gI cells exhibited a similar pattern of early TER compared to MDCK cells, although peak resistances were lower than those of gE alone. VZV gE expression altered F-actin organization and lipid distribution, but coexpression of gI modulated these effects. Two regions of the gE ectodomain, amino acids (aa) 278 to 355 and aa 467 to 498, although lacking Ca(2+) binding motifs, exhibit similarities with corresponding regions of the cell adhesion molecules, E-cadherin and desmocollin. These observations suggest that VZV gE and gE/gI may contribute to viral pathogenesis by facilitating epithelial cell-cell contacts.     

Varicella-zoster virus glycoprotein I is essential for growth of virus in Vero cells.

Varicella-zoster virus (VZV) encodes at least six glycoproteins. Glycoprotein I (gI), the product of open reading frame 67, is a 58- to 62-kDa glycoprotein found in VZV-infected cells. We constructed two VZV gI deletion mutants. Immunoprecipitation of VZV gE from infected cells indicated that cells infected with VZV deleted for gI expressed a gE that was larger (100 kDa) than that expressed in cells infected with the parental virus (98 kDa). Cell-associated or cell-free VZV deleted for gI grew to lower titers in melanoma cells than did parental VZV. While VZV deleted for gI replicated in other human cells, the mutant virus replicated to very low titers in primary guinea pig and monkey cells and did not replicate in Vero cells. When compared with the parental virus, rescued viruses, in which the gI deletion was restored with a wild-type allele, showed a similarly sized gE and comparable growth patterns in melanoma and Vero cells. VZV deleted for gI entered Vero cells; however, viral DNA synthesis was impaired in these cells. The VZV gI mutant was slightly impaired for adsorption to human cells. Thus, VZV gI is required for replication of the virus in Vero cells, for efficient replication of the virus in nonhuman cells, and for normal processing of gE.     

Varicella-Zoster Virus Glycoprotein I Is Essential for Spread in Dorsal Root Ganglia and Facilitates Axonal Localization of Structural Virion Components in Neuronal Cultures

Neurons of the sensory ganglia are the major site of varicella-zoster virus (VZV) latency and may undergo productive infection during reactivation. Although the VZV glycoprotein E/glycoprotein I (gE/gI) complex is known to be critical for neurovirulence, few studies have assessed the roles of these proteins during infection of dorsal root ganglia (DRG) due to the high human specificity of the virus. Here, we show that the VZV glycoprotein I gene is an important neurotropic gene responsible for mediating the spread of virus in neuronal cultures and explanted DRG. Inoculation of differentiated SH-SY5Y neuronal cell cultures with a VZV gI gene deletion strain (VZV rOkaΔgI) showed a large reduction in the percentage of cells infected and significantly smaller plaque sizes in a comparison with cultures infected with the parental strain (VZV rOka). In contrast, VZV rOkaΔgI was not significantly attenuated in fibroblast cultures, demonstrating a cell type-specific role for VZV gI. Analysis of rOkaΔgI protein localization by immunofluorescent staining revealed aberrant localization of viral glycoprotein and capsid proteins, with little or no staining present in the axons of differentiated SH-SY5Y cells infected with rOkaΔgI, yet axonal vesicle trafficking was not impaired. Further studies utilizing explanted human DRG indicated that VZV gI is required for the spread of virus within DRG. These data demonstrate a role for VZV gI in the cell-to-cell spread of virus during productive replication in neuronal cells and a role in facilitating the access of virion components to axons.     

Insulin-Degrading Enzyme Binds to the Nonglycosylated Precursor of Varicella-Zoster Virus gE Protein Found in the Endoplasmic Reticulum

Insulin degradation enzyme (IDE) is a 110-kDa zinc metalloprotease found in the cytosol of all cells. IDE degrades insulin and a variety of small proteins including amyloid-β. Recently, IDE has been proposed as the receptor for varicella-zoster virus (VZV) attachment. During our reassessment, some of the original studies were repeated and expanded in scope. We first confirmed that IDE antibody reduced VZV spread. For additional controls, we repeated the same experiments with herpes simplex virus (HSV)-infected cells as well as uninfected cells. There was a visible reduction in HSV spread but less than seen in the VZV system. Of greater importance, IDE antibody also inhibited the growth of uninfected cells. Second, we repeated the coprecipitation assays. We confirmed that antibodies to VZV gE (open reading frame 68) coprecipitated IDE and that anti-IDE antibody coprecipitated gE. However, the detected gE protein was not the mature 98-kDa form; rather, it was a precursor 73-kDa gE form found in the endoplasmic reticulum. Additional control experiments included VZV-infected cell cultures treated with tunicamycin to block gE glycosylation in the endoplasmic reticulum; again, the anti-IDE antibody coprecipitated a 73-kDa gE product. Finally, Orbitrap mass spectrometry analysis of a chromatographically purified gE sample revealed four cellular proteins associated with the unfolded protein response: BiP (HSPA5), HSPA8, HSPD1, and PPIA (peptidyl-propyl cis-trans isomerase). We conclude that IDE protease binds to the 73-kDa gE precursor and that this event occurs in the cytosol but not as a receptor/ligand interaction.Insulin degradation enzyme (IDE) has been proposed as a virus receptor. IDE is a zinc metalloprotease that is known to degrade a number of small proteins (<6 kDa) including insulin and amyloid-β (6, 18). Because of these observations, IDE has been intensively studied as a participant in the pathogenesis of diabetes and Alzheimer''s syndrome (9, 30). In 2006, IDE was reported to be a cellular receptor of varicella-zoster virus (VZV), which mediated infection and cell-to-cell spread of the virus (19). This report was based on several experiments. Initially, a truncated VZV gE protein was immobilized onto protein A-Sepharose beads with anti-gE monoclonal antibody (MAb). When the beads were incubated with cell lysates, a protein identified as IDE was attached to the complex. The authors also performed coimmunoprecipitation assays. When gE was precipitated from VZV-infected melanoma cells, the investigators observed that IDE was also detectable in the precipitate. When a rabbit polyclonal anti-IDE antibody was added to a monolayer prior to VZV infection, VZV spread was subsequently inhibited by 30 to 45%. Knockdown of IDE by small interfering RNA (siRNA) inhibited VZV infection and cell-to-cell spread. Similarly, bacitracin, a compound that inhibits IDE, also inhibited VZV infection and cell-to-cell spread. Finally, these deficiencies were corrected by expression of exogenous IDE. In a subsequent publication, the IDE binding domain in gE was localized to amino acids 32 to 71 (20). Removal of that gE domain from a recombinant virus limited cell spread (1).In this report, we present additional data about the VZV gE-IDE interaction. We first confirmed the interaction. But we found that the interaction occurred predominantly in the cytosol with a precursor nonglycosylated gE form (73 kDa) rather than the mature gE glycoprotein (98 kDa). In fact, the gE-IDE interaction occurred when VZV-infected cells were treated with tunicamycin to prevent the biosynthesis of mature glycosylated gE. The above results suggested that the VZV gE-IDE interaction may occur either in the endoplasmic reticulum (ER) or adjacent to the ER, a site where IDE is known to interact with other proteins such as amyloid-β.     

Herpes simplex virus gE/gI must accumulate in the trans-Golgi network at early times and then redistribute to cell junctions to promote cell-cell spread

Herpes simplex virus (HSV) glycoprotein heterodimer gE/gI is necessary for virus spread in epithelial and neuronal tissues. Deletion of the relatively large gE cytoplasmic (CT) domain abrogates the ability of gE/gI to mediate HSV spread. The gE CT domain is required for the sorting of gE/gI to the trans-Golgi network (TGN) in early stages of virus infection, and there are several recognizable TGN sorting motifs grouped near the center of this domain. Late in HSV infection, gE/gI, other viral glycoproteins, and enveloped virions redistribute from the TGN to epithelial cell junctions, and the gE CT domain is also required for this process. Without the gE CT domain, newly enveloped virions are directed to apical surfaces instead of to cell junctions. We hypothesized that the gE CT domain promotes virus envelopment into TGN subdomains from which nascent enveloped virions are sorted to cell junctions, a process that enhances cell-to-cell spread. To characterize elements of the gE CT domain involved in intracellular trafficking and cell-to-cell spread, we constructed a panel of truncation mutants. Specifically, these mutants were used to address whether sorting to the TGN and redistribution to cell junctions are necessary, and sufficient, for gE/gI to promote cell-to-cell spread. gE-519, lacking 32 C-terminal residues, localized normally to the TGN early in infection and then trafficked to cell junctions at late times and mediated virus spread. By contrast, mutants gE-495 (lacking 56 C-terminal residues) and gE-470 (lacking 81 residues) accumulated in the TGN but did not traffic to cell junctions and did not mediate cell-to-cell spread. A fourth mutant, gE-448 (lacking most of the CT domain), did not localize to cell junctions and did not mediate virus spread. Therefore, the capacity of gE/gI to promote cell-cell spread requires early localization to the TGN, but this is not sufficient for virus spread. Additionally, gE CT sequences between residues 495 and 519, which contain no obvious cell sorting motifs, are required to promote gE/gI traffic to cell junctions and cell-to-cell spread.     

Essential role played by the C-terminal domain of glycoprotein I in envelopment of varicella-zoster virus in the trans-Golgi network: interactions of glycoproteins with tegument

Varicella-zoster virus (VZV) is enveloped in the trans-Golgi network (TGN). Here we report that glycoprotein I (gI) is required within the TGN for VZV envelopment. Enveloping membranous TGN cisternae were microscopically identified in cells infected with intact VZV. These sacs curved around, and ultimately enclosed, nucleocapsids. Tegument coated the concave face of these sacs, which formed the viral envelope, but the convex surface was tegument-free. TGN cisternae of cells infected with VZV mutants lacking gI (gI(Delta)) or its C (gI(DeltaC))- or N-terminal (gI(DeltaN))-terminal domains were uniformly tegument coated and adhered to one another, forming bizarre membranous stacks. Viral envelopment was compromised, and no virions were delivered to post-Golgi structures. The TGN was not gI-immunoreactive in cells infected with the gI(Delta) or gI(DeltaN) mutants, but it was in cells infected with gI(DeltaC) (because the ectodomains of gI and gE interact). The presence in the TGN of gI lacking a C-terminal domain, therefore, was not sufficient to maintain enveloping cisternae. In cells infected with intact VZV or with gI(Delta), gI(DeltaN), or gI(DeltaC) mutants, ORF10p immunoreactivity was concentrated on the cytosolic face of TGN membranes, suggesting that it interacts with the cytosolic domains of glycoproteins. Because of the gE-gI interaction, cotransfected cells that expressed gE or gI were able to target truncated forms of the other to the TGN. Our data suggest that the C-terminal domain of gI is required to segregate viral and cellular proteins in enveloping TGN cisternae.     

Herpes simplex virus type 1 encodes two Fc receptors which have different binding characteristics for monomeric immunoglobulin G (IgG) and IgG complexes.

Two herpes simplex virus type 1 glycoproteins, gE and gI, have been shown to form a complex that binds the Fc domain of immunoglobulin G (IgG). We demonstrate that this complex is required for the binding of monomeric nonimmune IgG but that gE alone is sufficient for binding polymeric IgG in the form of IgG complexes. Evidence that gE but not gI is required for binding IgG complexes is as follows. IgG complexes bound equally well to cells infected with gI-negative mutants or with wild-type virus, whereas cells infected with gE-negative mutants did not bind IgG complexes. Furthermore, L cells transiently transfected to express gE bound IgG complexes. Additional evidence that gI fails to augment binding of IgG complexes comes from experiments in which the gI gene was inducibly expressed in cells after infection. Inducible gI expression failed to increase binding of IgG complexes to infected cells in comparison with cells not capable of inducible gI expression. In contrast, expression of both gE and gI was necessary for binding of monomeric IgG, as demonstrated by flow cytometry using cells infected with gE-negative and gI-negative mutants. These observations demonstrate that herpes simplex virus type 1 Fc receptors (FcRs) have different binding characteristics for monomeric IgG and IgG complexes. Furthermore, it appears that gE is the FcR for IgG complexes and that gE and gI form the FcR for monomeric IgG.     

A functional YNKI motif in the short cytoplasmic tail of varicella-zoster virus glycoprotein gH mediates clathrin-dependent and antibody-independent endocytosis

The trafficking of varicella-zoster virus (VZV) gH was investigated under both infection and transfection conditions. In initial endocytosis assays performed in infected cells, the three glycoproteins gE, gI, and gB served as positive controls for internalization from the plasma membrane. Subsequently, we discovered that gH in VZV-infected cells was also internalized and followed a similar trafficking pattern. This observation was unexpected because all herpesvirus gH homologues have short endodomains not known to contain trafficking motifs. Further investigation demonstrated that VZV gH, when expressed alone with its chaperone gL, was capable of endocytosis in a clathrin-dependent manner, independent of gE, gI, or gB. Upon inspection of the short gH cytoplasmic tail, we discovered a putative tyrosine-based endocytosis motif (YNKI). When the tyrosine was replaced with an alanine, endocytosis of gH was blocked. Utilizing an endocytosis assay dependent on biotin labeling, we further documented that endocytosis of VZV gH was antibody independent. In control experiments, we showed that gE, gI, and gB also internalized in an antibody-independent manner. Alignment analysis of the VZV gH cytoplasmic tail to other herpesvirus gH homologues revealed two important findings: (i) herpes simplex virus type 1 and 2 homologues lacked an endocytosis motif, while all other alphaherpesvirus gH homologues contained a potential motif, and (ii) the VZV gH and simian varicella virus gH cytoplasmic tails were likely longer in length (18 amino acids) than predicted in the original sequence analyses (12 and 16 amino acids, respectively). The longer tails provided the proper context for a functional endocytosis motif.     

Complex Formation Facilitates Endocytosis of the Varicella-Zoster Virus gE:gI Fc Receptor

Open reading frames within the unique short segment of alphaherpesvirus genomes participate in egress and cell-to-cell spread. The case of varicella-zoster virus (VZV) is of particular interest not only because the virus is highly cell associated but also because its most prominent cell surface protein, gE, bears semblance to the mammalian Fc receptor FcγRII. A previous study demonstrated that when expressed alone in cells, VZV gE was endocytosed from the cell surface through a tyrosine localization motif in its cytoplasmic tail (J. K. Olson and C. Grose, J. Virol. 71:4042–4054, 1997). Since VZV gE is normally found in association with gI in the infected cell, the present study was directed at defining the trafficking of the VZV gE:gI protein complex. First, VZV gI underwent endocytosis and recycling when it was expressed alone in cells, and interestingly, VZV gI contained a methionine-leucine internalization motif in its cytoplasmic tail. Second, VZV gI was found by confocal microscopy to colocalize with VZV gE during endocytosis and recycling in cells. Third, by a quantitative internalization assay, VZV gE:gI was shown to undergo endocytosis more efficiently (steady state, 55 to 60%) than either gE alone (steady state, ~32%) or gI alone (steady state, ~45%). Further, examination of endocytosis-deficient mutant proteins demonstrated that VZV gI exerted a more pronounced effect than gE on internalization of the complex. Most importantly, therefore, these studies suggest that VZV gI behaves as an accessory component by facilitating the endocytosis of the major constituent gE and thereby modulating the trafficking of the entire cell surface gE:gI Fc receptor complex.     

Varicella-Zoster Virus Fc Receptor Component gI Is Phosphorylated on Its Endodomain by a Cyclin-Dependent Kinase

Varicella-zoster virus (VZV) glycoprotein gI is a type 1 transmembrane glycoprotein which is one component of the heterodimeric gE:gI Fc receptor complex. Like VZV gE, VZV gI was phosphorylated in both VZV-infected cells and gI-transfected cells. Preliminary studies demonstrated that a serine 343-proline 344 sequence located within the gI cytoplasmic tail was the most likely phosphorylation site. To determine which protein kinase catalyzed the gI phosphorylation event, we constructed a fusion protein, consisting of glutathione-S-transferase (GST) and the gI cytoplasmic tail, called GST-gI-wt. When this fusion protein was used as a substrate for gI phosphorylation in vitro, the results demonstrated that GST-gI-wt fusion protein was phosphorylated by a representative cyclin-dependent kinase (CDK) called P-TEFb, a homologue of CDK1 (cdc2). When serine 343 within the serine-proline phosphorylation site was replaced with an alanine residue, the level of phosphorylation of the gI fusion protein was greatly reduced. Subsequent experiments with individually immunoprecipitated mammalian CDKs revealed that the VZV gI fusion protein was phosphorylated best by CDK1, to a lesser degree by CDK2, and not at all by CDK6. Transient-transfection assays carried out in the presence of the specific CDK inhibitor roscovitine strongly supported the prior results by demonstrating a marked decrease in gI phosphorylation while gI protein expression was unaffected. Finally, the possibility that VZV gI contained a CDK phosphorylation site in its endodomain was of further interest because its partner, gE, contains a casein kinase II phosphorylation site in its endodomain; prior studies have established that CDK1 can phosphorylate casein kinase II.     

The extracellular domain of herpes simplex virus gE is indispensable for efficient cell-to-cell spread: evidence for gE/gI receptors

Herpes simplex virus (HSV) spreads rapidly and efficiently within epithelial and neuronal tissues. The HSV glycoprotein heterodimer gE/gI plays a critical role in promoting cell-to-cell spread but does not obviously function during entry of extracellular virus into cells. Thus, gE/gI is an important molecular handle on the poorly understood process of cell-to-cell spread. There was previous evidence that the large extracellular (ET) domains of gE/gI might be important in cell-to-cell spread. First, gE/gI extensively accumulates at cell junctions, consistent with being tethered there. Second, expression of gE/gI in trans interfered with HSV spread between epithelial cells. To directly test whether the gE ET domain was necessary for gE/gI to promote virus spread, a panel of gE mutants with small insertions in the ET domain was constructed. Cell-to-cell spread was reduced when insertions were made within either of two regions, residues 256 to 291 or 348 to 380. There was a strong correlation between loss of cell-to-cell spread function and binding of immunoglobulin. gE ET domain mutants 277, 291, and 348 bound gI, produced mature forms of gE that reached the cell surface, and were incorporated into virions yet produced plaques similar to gE null mutants. Moreover, all three mutants were highly restricted in spread within the corneal epithelium, in the case of mutant 277 to only 4 to 6% of the number of cells compared with wild-type HSV. Therefore, the ET domain of gE is indispensable for efficient cell-to-cell spread. These observations are consistent with our working hypothesis that gE/gI can bind extracellular ligands, so-called gE/gI receptors that are concentrated at epithelial cell junctions. This fits with similarities in structure and function of gE/gI and gD, which is a receptor binding protein.