Varicella-zoster virus gpI and herpes simplex virus gE: phosphorylation and Fc binding

Rhesus Cytomegalovirus-encoded Fcγ-binding glycoproteins facilitate viral evasion from IgG-mediated humoral immunity

Human cytomegalovirus (HCMV) encodes four viral Fc-gamma receptors (vFcγRs) that counteract antibody-mediated activation in vitro, but their role in infection and pathogenesis is unknown. To examine their in vivo function in an animal model evolutionarily closely related to humans, we identified and characterized Rh05, Rh152/151 and Rh173 as the complete set of vFcγRs encoded by rhesus CMV (RhCMV). Each one of these proteins displays functional similarities to their prospective HCMV orthologs with respect to antagonizing host FcγR activation in vitro. When RhCMV-naïve male rhesus macaques were infected with vFcγR-deleted RhCMV, peak plasma DNAemia levels and anti-RhCMV antibody responses were comparable to wildtype infections of both male and female animals. However, the duration of plasma DNAemia was significantly shortened in immunocompetent, but not in CD4 + T cell-depleted animals. Since vFcγRs were not required for superinfection of rhesus macaques, we conclude that these proteins can prolong lytic replication during primary infection by evading virus-specific adaptive immune responses, particularly antibodies.

Rhesus Cytomegalovirus-encoded Fcγ-binding glycoproteins facilitate viral evasion from IgG-mediated humoral immunity

Human cytomegalovirus (HCMV) encodes four viral Fc-gamma receptors (vFcγRs) that counteract antibody-mediated activation in vitro , but their role in infection and pathogenesis is unknown. To examine the in vivo function of vFcγRs in animal hosts closely related to humans, we identified and characterized vFcγRs encoded by rhesus CMV (RhCMV). We demonstrate that Rh05, Rh152/151 and Rh173 represent the complete set of RhCMV vFcγRs, each displaying functional similarities to their respective HCMV orthologs with respect to antagonizing host FcγR activation in vitro . When RhCMV-naïve rhesus macaques were infected with vFcγR-deleted RhCMV, peak plasma viremia levels and anti-RhCMV antibody responses were comparable to wildtype infections. However, the duration of plasma viremia was significantly shortened in immunocompetent, but not in CD4+ T cell-depleted animals. Since vFcγRs were not required for superinfection, we conclude that vFcγRs delay control by virus-specific adaptive immune responses, particularly antibodies, during primary infection.

Exocytosis of Progeny Infectious Varicella-Zoster Virus Particles via a Mannose-6-Phosphate Receptor Pathway without Xenophagy following Secondary Envelopment

The literature on the egress of different herpesviruses after secondary envelopment is contradictory. In this report, we investigated varicella-zoster virus (VZV) egress in a cell line from a child with Pompe disease, a glycogen storage disease caused by a defect in the enzyme required for glycogen digestion. In Pompe cells, both the late autophagy pathway and the mannose-6-phosphate receptor (M6PR) pathway are interrupted. We have postulated that intact autophagic flux is required for higher recoveries of VZV infectivity. To test that hypothesis, we infected Pompe cells and then assessed the VZV infectious cycle. We discovered that the infectious cycle in Pompe cells was remarkably different from that of either fibroblasts or melanoma cells. No large late endosomes filled with VZV particles were observed in Pompe cells; only individual viral particles in small vacuoles were seen. The distribution of the M6PR pathway (trans-Golgi network to late endosomes) was constrained in infected Pompe cells. When cells were analyzed with two different anti-M6PR antibodies, extensive colocalization of the major VZV glycoprotein gE (known to contain M6P residues) and the M6P receptor (M6PR) was documented in the viral highways at the surfaces of non-Pompe cells after maximum-intensity projection of confocal z-stacks, but neither gE nor the M6PR was seen in abundance at the surfaces of infected Pompe cells. Taken together, our results suggested that (i) Pompe cells lack a VZV trafficking pathway within M6PR-positive large endosomes and (ii) most infectious VZV particles in conventional cell substrates are transported via large M6PR-positive vacuoles without degradative xenophagy to the plasma membrane.IMPORTANCE The long-term goal of this research has been to determine why VZV, when grown in cultured cells, invariably is more cell associated and has a lower titer than other alphaherpesviruses, such as herpes simplex virus 1 (HSV1) or pseudorabies virus (PRV). Data from both HSV1 and PRV laboratories have identified a Rab6 secretory pathway for the transport of single enveloped viral particles from the trans-Golgi network within small vacuoles to the plasma membrane. In contrast, after secondary envelopment in fibroblasts or melanoma cells, multiple infectious VZV particles accumulated within large M6PR-positive late endosomes that were not degraded en route to the plasma membrane. We propose that this M6PR pathway is most utilized in VZV infection and least utilized in HSV1 infection, with PRV's usage being closer to HSV1's usage. Supportive data from other VZV, PRV, and HSV1 laboratories about evidence for two egress pathways are included.

The herpes virus Fc receptor gE-gI mediates antibody bipolar bridging to clear viral antigens from the cell surface

The Herpes Simplex Virus 1 (HSV-1) glycoprotein gE-gI is a transmembrane Fc receptor found on the surface of infected cells and virions that binds human immunoglobulin G (hIgG). gE-gI can also participate in antibody bipolar bridging (ABB), a process by which the antigen-binding fragments (Fabs) of the IgG bind a viral antigen while the Fc binds to gE-gI. IgG Fc binds gE-gI at basic, but not acidic, pH, suggesting that IgG bound at extracellular pH by cell surface gE-gI would dissociate and be degraded in acidic endosomes/lysosomes if endocytosed. The fate of viral antigens associated with gE-gI-bound IgG had been unknown: they could remain at the cell surface or be endocytosed with IgG. Here, we developed an in vitro model system for ABB and investigated the trafficking of ABB complexes using 4-D confocal fluorescence imaging of ABB complexes with transferrin or epidermal growth factor, well-characterized intracellular trafficking markers. Our data showed that cells expressing gE-gI and the viral antigen HSV-1 gD endocytosed anti-gD IgG and gD in a gE-gI-dependent process, resulting in lysosomal localization. These results suggest that gE-gI can mediate clearance of infected cell surfaces of anti-viral host IgG and viral antigens to evade IgG-mediated responses, representing a general mechanism for viral Fc receptors in immune evasion and viral pathogenesis.

Herpes simplex virus gE/gI sorts nascent virions to epithelial cell junctions, promoting virus spread

Alphaherpesviruses spread rapidly through dermal tissues and within synaptically connected neuronal circuitry. Spread of virus particles in epithelial tissues involves movement across cell junctions. Herpes simplex virus (HSV), varicella-zoster virus (VZV), and pseudorabies virus (PRV) all utilize a complex of two glycoproteins, gE and gI, to move from cell to cell. HSV gE/gI appears to function primarily, if not exclusively, in polarized cells such as epithelial cells and neurons and not in nonpolarized cells or cells that form less extensive cell junctions. Here, we show that HSV particles are specifically sorted to cell junctions and few virions reach the apical surfaces of polarized epithelial cells. gE/gI participates in this sorting. Mutant HSV virions lacking gE or just the cytoplasmic domain of gE were rarely found at cell junctions; instead, they were found on apical surfaces and in cell culture fluids and accumulated in the cytoplasm. A component of the AP-1 clathrin adapter complexes, mu1B, that is involved in sorting of proteins to basolateral surfaces was involved in targeting of PRV particles to lateral surfaces. These results are related to recent observations that (i) HSV gE/gI localizes specifically to the trans-Golgi network (TGN) during early phases of infection but moves out to cell junctions at intermediate to late times (T. McMillan and D. C. Johnson, J. Virol., in press) and (ii) PRV gE/gI participates in envelopment of nucleocapsids into cytoplasmic membrane vesicles (A. R. Brack, B. G. Klupp, H. Granzow, R. Tirabassi, L. W. Enquist, and T. C. Mettenleiter, J. Virol. 74:4004-4016, 2000). Therefore, interactions between the cytoplasmic domains of gE/gI and the AP-1 cellular sorting machinery cause glycoprotein accumulation and envelopment into specific TGN compartments that are sorted to lateral cell surfaces. Delivery of virus particles to cell junctions would be expected to enhance virus spread and enable viruses to avoid host immune defenses.

The C-terminal cytoplasmic tail of herpes simplex virus type 1 gE protein is phosphorylated in vivo and in vitro by cellular enzymes in the absence of other viral proteins

Herpes simplex virus 1 glycoprotein E (gE-1) is highly phosphorylated in culture cells during infection. In this report, it is shown that phosphorylation is mediated by host enzymes in human cells stably transfected with gE, in the absence of other herpesvirus products. In contrast, a tailless gE product (C terminus deletion mutant) is not phosphorylated. By using an in vitro kinase assay combined with linker-insertion mutagenesis, it is shown that casein kinase II catalyses the phosphorylation of the C-terminal domain of the protein. Also, it is demonstrated that the serine residues at positions 476 and/or 477 in the cytoplasmic portion of the protein are the major acceptors for the phosphate groups.

The extracellular domain of herpes simplex virus gE is sufficient for accumulation at cell junctions but not for cell-to-cell spread

Herpes simplex virus (HSV) expresses a number of membrane glycoproteins, including gB, gD, and gH/gL, that function in both entry of virus particles and movement of virus from an infected cell to an uninfected cell (cell-to-cell spread). However, a complex of HSV glycoproteins gE and gI (gE/gI) is required for efficient cell-to-cell spread, especially between cells that form extensive cell junctions, yet it is not necessary for entry of extracellular virions. We previously showed that gE/gI has the capacity to localize specifically to cell junctions; the glycoprotein complex was found at lateral surfaces of cells in contact with other cells but not at those lateral surfaces not forming junctions or at apical surfaces. By virtue of these properties, gE/gI is an important molecular handle on the poorly understood process of cell-to-cell spread. Here, we show that the cytoplasmic domain of gE is important for the proper delivery of gE/gI to lateral surfaces of cells. Without this domain, gE/gI is found on the apical surface of epithelial cells, and more uniformly in the cytoplasm, although incorporation into the virion envelope is unaffected. However, even without proper trafficking signals, a substantial fraction of gE/gI retained the capacity to accumulate at cell junctions. Therefore, the extracellular domain of gE can mediate accumulation of gE/gI at cell junctions, if the glycoprotein can be delivered there, probably through interactions with ligands on the opposing cell. The role of phosphorylation of the cytoplasmic domain of gE was also studied. A second mutant HSV type 1 was constructed in which three serine residues that form a casein kinase II phosphorylation site were changed to alanine residues, reducing phosphorylation by 70 to 80%. This mutation did not affect accumulation at cell junctions or cell-to-cell spread.

Molecular mechanisms of neurotropic herpesvirus invasion and spread in the CNS

Pseudorabies virus (PRV) is a herpesvirus in the subfamily alphaherpesvirinae (the alpha herpesviruses). After primary infection at mucosal surfaces, PRV infects the peripheral nervous system in its natural host (swine) with occasional invasion of the central nervous system. When other hosts (including cows and rodents) are infected, the infection almost always gives rise to fatal disease in the CNS as a result of infection of peripheral neurons and subsequent spread to the brain. Part of the ability to cause fatal CNS disease can be attributed to a viral glycoprotein called gE. Viruses lacking gE are thought to be less virulent because they do not spread efficiently from cell to cell. Based on a set of gE mutations we have constructed, we suggest that these two phenotypes of cell-cell spread and virulence reflect separate functions of the gE protein. In this report, we show that viruses carrying these new gE mutations have marked reduction in virulence, yet spread efficiently in defined neural circuits in the rat brain. As such, they offer new insight and opportunities for understanding of viral disease and host response to injury, as well as in the construction of viral tracers of neuronal connections.

Regulation of endosome sorting by a specific PP2A isoform

The regulated sorting of proteins within the trans-Golgi network (TGN)/endosomal system is a key determinant of their biological activity in vivo. For example, the endoprotease furin activates of a wide range of proproteins in multiple compartments within the TGN/endosomal system. Phosphorylation of its cytosolic domain by casein kinase II (CKII) promotes the localization of furin to the TGN and early endosomes whereas dephosphorylation is required for efficient transport between these compartments (Jones, B.G., L. Thomas, S.S. Molloy, C.D. Thulin, M.D. Fry, K.A. Walsh, and G. Thomas. 1995. EMBO [Eur. Mol. Biol. Organ.] J. 14:5869-5883). Here we show that phosphorylated furin molecules internalized from the cell surface are retained in a local cycling loop between early endosomes and the plasma membrane. This cycling loop requires the phosphorylation state-dependent furin-sorting protein PACS-1, and mirrors the trafficking pathway described recently for the TGN localization of furin (Wan, L., S.S. Molloy, L. Thomas, G. Liu, Y. Xiang, S.L. Ryback, and G. Thomas. 1998. Cell. 94:205-216). We also demonstrate a novel role for protein phosphatase 2A (PP2A) in regulating protein localization in the TGN/endosomal system. Using baculovirus recombinants expressing individual PP2A subunits, we show that the dephosphorylation of furin in vitro requires heterotrimeric phosphatase containing B family regulatory subunits. The importance of this PP2A isoform in directing the routing of furin from early endosomes to the TGN was established using SV-40 small t antigen as a diagnostic tool in vivo. The role of both CKII and PP2A in controlling multiple sorting steps in the TGN/endosomal system indicates that the distribution of itinerant membrane proteins may be acutely regulated via signal transduction pathways.

Role of envelope protein gE endocytosis in the pseudorabies virus life cycle

Several groups have reported that certain herpesvirus envelope proteins do not remain on the surface of cells that express them but rather are internalized by endocytosis in a recycling process. The biological function of membrane protein endocytosis in the virus life cycle remains a matter of speculation and debate. In this report, we demonstrate that some, but not all, membrane proteins encoded by the alphaherpesvirus pseudorabies virus (PRV) are internalized after reaching the plasma membrane. Glycoproteins gE and gB are internalized from the plasma membrane of cells, while gI and gC are not internalized efficiently. We show for gE that the cytoplasmic domain of the protein is required for endocytosis. While the gI protein is incapable of endocytosis on its own, it can be internalized when complexed with gE. We demonstrate that endocytosis of the gE-gI complex and gB occurs early after infection of tissue culture cells but that this process stops completely after 6 h of infection, a time that correlates with significant shutoff of host protein synthesis. We also show that gE protein internalized at 4 h postinfection is not present in virions formed at a later time. We discuss the differences in PRV gE and gI endocytosis compared to that of the varicella-zoster virus homologs and the possible roles of glycoprotein endocytosis in the virus life cycle.

Intracellular transport of the glycoproteins gE and gI of the varicella-zoster virus. gE accelerates the maturation of gI and determines its accumulation in the trans-Golgi network

The varicella-zoster virus (VZV) is the etiological agent of two different human pathologies, chickenpox (varicella) and shingles (zoster). This alphaherpesvirus is believed to acquire its lipidic envelope in the trans-Golgi network (TGN). This is consistent with previous data showing that the most abundant VZV envelope glycoprotein gE accumulates at steady-state in this organelle when expressed from cloned cDNA. In the present study, we have investigated the intracellular trafficking of gI, another VZV envelope glycoprotein. In transfected cells, this protein shows a very slow biosynthetic transport to the cell surface where it accumulates. However, upon co-expression of gE, gI experiences a dramatic increase in its exit rate from the endoplasmic reticulum, it accumulates in a sialyltransferase-positive compartment, presumably the TGN, and cycles between this compartment and the cell surface. This differential behavior results from the ability of gE and gI to form a complex in the early stages of the biosynthetic pathway whose intracellular traffic is exclusively determined by the sorting information in the tail of gE. Thus, gI provides the first example of a molecule localized to the TGN by means of its association with another TGN protein. We also show that, during the early stages of VZV infection, both proteins are also found in the TGN of the host cell. This suggests the existence of an intermediate stage during VZV biogenesis in which the envelope glycoproteins, transiently arrested in the TGN, could promote the envelopment of newly synthesized nucleocapsids into this compartment and, therefore, the assembly of infective viruses.

UL13 protein kinase of herpes simplex virus 1 complexes with glycoprotein E and mediates the phosphorylation of the viral Fc receptor: glycoproteins E and I

Herpes simplex virus 1 encodes a Fc receptor consisting of glycoproteins E (gE) and I (gI) and two protein kinases specified by UL13 and US3, respectively. We report the following: (i) Antibody to UL13 formed immune complexes containing gE and gI in addition to UL13 protein. Immune complexes formed by monoclonal antibody to gE, but not those formed by monoclonal antibody to gI, also contained the UL13 protein. This association may reflect direct interaction between gE and UL13 inasmuch as IgG in preimmune rabbit serum and an antiserum made against another viral protein which does not react with the UL13 protein directly also bound gE and UL13. (ii) In cells infected with the wild-type virus, gE formed two sharp bands and a diffuse, slower migrating band. The slower sharp band was undetectable, and the diffuse slower migrating forms of gE were diminished in lysates of cells infected with a mutant virus lacking the UL13 gene (DeltaUL13). (iii) Both gE and gI were labeled with 32Pi in cells infected with wild-type or the DeltaUL13 virus, but the labeling was significantly stronger in cells infected with the wild-type virus than in those infected with the DeltaUL13 virus. (iv) In an in vitro protein kinase assay, UL13 immunoprecipitated from cells infected with wild-type virus labeled gE in the presence of [gamma-32P]ATP. This activity was absent in precipitates from cells infected with DeltaUL13 virus. The labeled gE comigrated with the slower, sharp band of gE. (v) gI present in the UL13 immune complex was also phosphorylated in the in vitro kinase assay. (vi) The cytoplasmic domain of gE contains recognition sequences for phosphorylation by casein kinase II (CKII). Exogenous CKII phosphorylated gE in immune complexes from lysates of cells infected with the DeltaUL13 mutant or in immune complexes from lysates of cells infected with wild-type virus that had been heated to inactivate all endogenous kinase activity including that of UL13. In both instances, CKII phosphorylated gE in both the slow and fast migrating sharp bands. We conclude that UL13 physically associates with gE and mediates the phosphorylation of gE and gI. UL13 may also be a determinant in posttranslational processing of gE.

Characterization of pseudorabies virus mutants expressing carboxy-terminal truncations of gE: evidence for envelope incorporation, virulence, and neurotropism domains

Glycoprotein E (gE) gene of pseudorabies virus (PRV) is conserved among diverse alphaherpesviruses and therefore is predicted to be important for virus survival. gE contributes to viral spread from cell to cell in a variety of hosts and is responsible, in part, for increased virulence or pathogenesis of the virus. Virulence and spread mediated by gE are thought to be highly correlated. We initiated this study to explore the hypothesis that these two phenotypes might reflect separate functions of the gE protein. We did so by focusing on the role of the gE carboxy terminus in neuronal spread. Viruses harboring nonsense mutations affecting the expression of the gE cytoplasmic domain had several notable phenotypes. First, the truncated gE proteins expressed from these mutants are not found in virion envelopes. Second, the mutants retain the ability to spread to all retinorecipient regions of the rodent brain after retinal infection of rats. Third, the mutants have the reduced virulence phenotype of a gE deletion mutant in rats. Finally, the mutants have distinct plaque-size phenotypes on MDBK cells but not PK15 cells. Based on these observations, we suggest that gE-mediated virulence and spread may reflect separate functions that are not mediated by gE on virus particles.

The herpes simplex virus UL37 protein is phosphorylated in infected cells

The herpes simplex virus type 1 (HSV-1) UL37 open reading frame encodes a 120-kDa late (gamma 1), nonstructural protein in infected cells. Recent studies in our laboratory have demonstrated that the UL37 protein interacts in the cytoplasm of infected cells with ICP8, the major HSV-1 DNA-binding protein. As a result of this interaction, the UL37 protein is transported to the nucleus and can be coeluted with ICP8 from single-stranded DNA columns. Pulse-labeling and pulse-chase studies of HSV-1-infected cells with [35S]methionine and 32Pi demonstrated that UL37 was a phosphoprotein which did not have a detectable rate of turnover. The protein was phosphorylated soon after translation and remained phosphorylated throughout the viral replicative cycle. UL37 protein expressed from a vaccinia virus recombinant was also phosphorylated during infection, suggesting that the UL37 protein was phosphorylated by a cellular kinase and that interaction with the ICP8 protein was not a prerequisite for UL37 phosphorylation.

Identification of the phosphorylation sequence in the cytoplasmic tail of the varicella-zoster virus Fc receptor glycoprotein gpI

Varicella-zoster virus (VZV) glycoprotein gpI, the homolog of herpes simplex virus gE, functions as a receptor for the Fc portion of immunoglobulin G. Like other cell surface receptors, this viral receptor is highly phosphorylated in cell culture. To identify the precise location of the cellular kinase-mediated phosphorylation, we generated a tailless deletion mutant and several point mutants which had altered serine and threonine residues within the cytoplasmic domain of gpI. The mutated and wild-type genes of gpI were transfected and expressed within a vaccinia virus-T7 polymerase transfection system in order to determine what effect these mutations had on the phosphorylation state of the protein in vivo and in vitro. Truncation of the cytoplasmic domain of gpI diminished the phosphorylation of gpI in vivo. Examination of the point mutants established that the major phosphorylation sequence of gpI was located between amino acids 593 and 598, a site which included four phosphorylatable serine and threonine residues. Phosphorylation analyses of the mutant and wild-type glycoproteins confirmed that gpI was a substrate for casein kinase II, with threonines 596 and 598 being critical residues. Although the mutant glycoproteins were phosphorylated by casein kinase I, protease V8 partial digestion profiles suggested that casein kinase II exerted the major effect. Thus, these mutagenesis studies demonstrated that the gpI cytoplasmic sequence Ser-Glu-Ser-Thr-Asp-Thr was phosphorylated in mammalian cells in the absence of any other herpesvirus products. Since the region defined by transfection was consistent with results obtained with in vitro phosphorylation by casein kinase II, we propose that VZV gpI is a physiologic substrate for casein kinase II. Immunofluorescence and pulse-chase experiments demonstrated that the mutant glycoproteins were processed and transported to the outer cell membrane.

Specific pseudorabies virus infection of the rat visual system requires both gI and gp63 glycoproteins

Transneuronal transport of pseudorabies virus (PRV) from the retina to visual centers that mediate visual discrimination and reflexes requires specific genes in the unique short region of the PRV genome. In contrast, these same viral genes are not required to infect retinorecipient areas of the brain involved in circadian rhythm regulation. In this report, we demonstrate that viral mutants carrying defined deletions of the genes encoding glycoprotein gI or gp63, or both, result in the same dramatic transport defect. Efficient export of either gI or gp63 from the endoplasmic reticulum to the Golgi apparatus in a fibroblast cell line requires the presence of both proteins. We also show that gI and gp63 physically interact, as demonstrated by pulse-chase and sucrose gradient sedimentation experiments. Complex formation is rapid compared with homodimerization of PRV glycoprotein gII. We suggest that gI and gp63 function in concert to affect neurotropism in the rat visual circuitry and that a heterodimer is likely to be the unit of function.

Syncytium-inducing mutations localize to two discrete regions within the cytoplasmic domain of herpes simplex virus type 1 glycoprotein B

Herpes simplex virus type 1 glycoprotein B (gB) is essential for virus entry, an event involving fusion of the virus envelope with the cell surface membrane, and virus-induced cell-cell fusion, resulting in polykaryocyte, or syncytium, formation. The experiments described in this report employed a random mutagenesis strategy to develop a more complete genetic map of mutations resulting in the syn mutant phenotype. The results indicate that syn mutations occur within two essential and highly conserved hydrophilic, alpha-helical regions of the gB cytoplasmic domain. Region I is immediately proximal to the transmembrane domain and includes residues R796 to E816/817. Region II is localized centrally in the cytoplasmic domain and includes residues A855 and R858. Positively charged residues were particularly affected in both regions, suggesting that charge interactions may be required to suppress the syn mutant phenotype. No syn mutations were identified within the transmembrane domain. A virus containing a rate of entry (roe) mutation at residue A851, either within or immediately proximal to syn region II, was isolated. Since roe mutations have also been discovered in the external domain of gB, it appears likely that the external and cytoplasmic domains cooperate in virus penetration. Moreover, the observation that both roe and syn mutations occur in the cytoplasmic domain further suggests that gB functions in an analogous manner in both membrane fusion events. It might be predicted from these observations that membrane fusion involves transduction of a fusion signal along the gB molecule through the transmembrane domain. Communication between the external and cytoplasmic domain may thus be required for gB-mediated membrane fusion events.

Varicella-zoster virus glycoprotein gpI/gpIV receptor: expression, complex formation, and antigenicity within the vaccinia virus-T7 RNA polymerase transfection system

The unique short region of the varicella-zoster virus (VZV) genome contains two open reading frames which encode glycoproteins designated gpI and gpIV (herpes simplex virus homologs gE and gI, respectively). Like its herpesviral counterpart gE, the VZV gpI gene product functions as a cell surface receptor (V. Litwin, W. Jackson, and C. Grose, J. Virol. 66:3643-3651, 1992). To evaluate the biosynthesis of the two VZV glycoproteins and further explore their relationship to one another, the two glycoprotein genes were individually cloned into a pTM1 vector under control of the T7 promoter. Transfection of the cloned gpI or gpIV construct into HeLa cells previously infected with vaccinia recombinant virus expressing bacteriophage T7 polymerase resulted in a much higher level expression of each VZV glycoprotein than previously achieved. Synthesis of both gpI and gpIV included intermediary partially glycosylated forms and mature N- and O-linked final product. Transfections in the presence of 32Pi demonstrated that the mature forms of both gpI and gpIV were phosphorylated, while similar experiments with [35S]sulfate showed that only the mature gpI was sulfated. When gpI and gpIV were coexpressed in the same cell, the two glycoproteins were complexed to each other, as both proteins could be immunoprecipitated by antibodies against either gpI or gpIV. Coprecipitation did not occur as a result of a shared epitope, because gpI expressed alone was not precipitated by antibody to gpIV, and gpIV expressed alone was not precipitated by antibody to gpI. Pulse-chase analysis demonstrated that the gpI-gpIV association occurred early in processing; furthermore, this complex formation interfered with posttranslational modifications and thereby reduced the M(r)s of the mature forms of both gpI and gpIV. Similarly, the molecular masses of the cotransfected gene products corresponded with those of the infected cell glycoproteins, a result which suggested that authentic gpI and gpIV were ordinarily found within a complex. Thus, the adjacent open reading frames 67 and 68 code for two glycoproteins which in turn form a distinctive sulfated and phosphorylated cell surface complex with receptor properties.

Herpesviral Fc receptors and their relationship to the human Fc receptors

Nearly two decades ago, it was observed that cells infected with herpes simplex virus (HSV) acquired an IgG Fc binding activity. The properties of the viral Fc receptor (FcR) have now been characterized by several laboratories. The Fc binding activity appears on the surface of the infected cell prior to formation of progeny virions. The FcR induced by HSV has been identified as the HSV glycoprotein, gE. When HSV gE forms a complex with a second HSV glycoprotein, gI, the receptor binds IgG with higher affinity. Varicella-zoster virus (VZV), which is closely related to HSV, has also been shown to induce an FcR. Like the HSV FcR, the FcR specified by VZV possesses characteristics common to viral glycoproteins. VZV encodes two glycoproteins, gpI and gpIV, which are the homologs of HSV gE and gI. The VZV glycoproteins have many properties common to cell surface receptors, including O-linked glycans and phosphorylation sites. However, extensive computer-assisted analyses of the amino acid sequences of VZV gpI and gpIV did not uncover regions of homology to the human cellular Fc receptors for IgG.

Receptor properties of two varicella-zoster virus glycoproteins, gpI and gpIV, homologous to herpes simplex virus gE and gI

The varicella-zoster virus (VZV) genome contains 70 reading frames (ORF), 5 of which encode the glycoproteins gpI, gpII, gpIII, gpIV, and gpV. ORF 67 and 68 lie adjacent to each other in the unique short region of the VZV genome and code for gpIV and gpI, respectively. These two genes, which are contained within the HindIII C fragment of the VZV genome, were subcloned in the correct orientation downstream from the promoter regions of the eukaryotic expression vectors pCMV5 and pBJ. After transfection, 5 to 20% of the Cos cells bound antibody specific for the given glycoprotein. In this study, it was shown that only the cells transfected with the gpI construct bound to the Fc fragment of human immunoglobulin G. Neither the transfected gpIV gene product nor the vector only bound to the Fc fragment. Thus, VZV gpI is confirmed to be the VZV-encoded Fc-binding glycoprotein. Like the wild-type form of gpI expressed in VZV-infected cells, gpI precipitated from transfected cells contained both N-linked and O-linked glycans and was heavily sialated. In addition, the transfected gpI gene product was phosphorylated both in cell culture and in protein kinase assays by mammalian casein kinases I and II. Extensive computer-assisted analyses of the VZV gpI sequence, as well as those of alphaherpesviral homolog glycoproteins, disclosed properties similar to those of other cell surface receptors; these included (i) exocytoplasmic regions rich in cysteine residues, (ii) membrane-proximal regions with potential O-linked glycosylation sites, and (iii) cytoplasmic domains with consensus phosphorylation sites.

Synthesis and processing of the major envelope glycoprotein of murine cytomegalovirus

We have characterized the synthesis and processing pathway of the major envelope glycoprotein complex of murine cytomegalovirus (gp52/105/150). We have demonstrated that it belongs to the "late" kinetic class of MCMV proteins, and is initially synthesized as a 128K glycoprotein (gp128) which contains N-linked, high-mannose type oligosaccharide chains and is phosphorylated predominantly at serine residues. A fraction of the gp128 molecules also contains O-linked GalNAc residues. The majority of the gp128 molecules appears to be retained in the endoplasmic reticulum as evidenced by their sensitivity to endoglycosidase H digestion. The formation of disulfide linkages and dimerization allow the transport of gp128 to the Golgi compartments where modification of N-linked carbohydrate structures and extension of O-linked oligosaccharide chains take place, cumulating in the appearance of the mature gp150. The final processing step involves the cleavage of gp150 into gp52 and gp105. By blocking the transport of the glycoprotein precursor to the trans-Golgi compartments with the ionophore monensin, the cleavage process is inhibited, suggesting that the trans-Golgi compartment is the site where gp150 is cleaved. However, the cleavage process is incomplete, resulting in the formation of multiple disulfide-linked complexes made up of different combinations of gp52, gp105, and gp150. Therefore, the processing of the major envelope glycoprotein complexes of murine cytomegalovirus resembles that of the gcl/gp55-116 complex of human cytomegalovirus.

Cell surface expression of the varicella-zoster virus glycoproteins and Fc receptor

Varicella-zoster virus (VZV) specifies the synthesis of viral glycoproteins which are important antigens for induction of the host immune response. In this report the technology of laser-activated flow cytometry has been employed to measure the membrane expression of VZV glycoproteins gpI, gpII, gpIII, and gpIV. By use of biotinylated monoclonal antibodies as probes, all four glycoproteins were demonstrated on the infected cell surface. The temporal appearance of the viral glycoproteins was defined in a time course experiment and shown to be maximal about 24 hr postinfection. The issue whether VZV induces the cell surface expression of an Fc receptor (FcR) was investigated with biotinylated nonimmune human IgG, followed by streptavidin-phycoerythrin. By this technique a 10-fold increase in fluorescence intensity was seen in the VZV-infected cells as compared to the mock-infected controls. When the experiment was repeated with purified human Fc fragment rather than whole IgG, a similar degree of binding was seen. Both the VZV glycoproteins and the VZV FcR were exquisitely sensitive to trypsin treatment (1 mg/ml); likewise, the cell surface expression of these VZV products was diminished by treatment of the infected cultures with monensin, an inhibitor of glycoprotein transport. In order to prove that VZV infection was not causing the induction of a cellular Fc gamma R, the VZV-infected and mock-infected cells were stained with monoclonal antibodies directed against each of the three human cellular IgG FcR, but no differences were observed. Therefore, the FcR activity seen in the infected culture was not due to one of the known cellular Fc gamma R.

Domain structure of herpes simplex virus 1 glycoprotein B: neutralizing epitopes map in regions of continuous and discontinuous residues

Herpes simplex virus 1 (HSV-1) glycoprotein B (gB) is a multifunctional glycoprotein required for infectivity; it is thought to promote fusion of the viral envelope with the cell membrane and entry of virions into cells. To map the antigenic and functional domains on gB, we constructed amino terminal derivatives lacking the entire carboxyl terminus and internal deletion mutants lacking defined regions of the extracellular and transmembrane domains. Transient expression of the mutants in COS-1 cells revealed that the amino terminal derivatives were released into the medium whereas those with deletions in the extracellular domain were mostly retained within the transfected cells. Analysis of intact gB and the amino terminal derivatives showed that the intact molecule formed dimers whereas the mutant derivatives did not. Reactions of the derivatives with a panel of well-characterized monoclonal antibodies to gB showed that the neutralizing epitopes cluster in two domains. The first maps in the amino terminal 190 residues and contains seven continuous epitopes, five of which are HSV-1-specific. Reactions of antibodies with a set of oligopeptides fine-mapped the epitopes between residues 1 and 47. The second domain is composed of discontinuous epitopes and was expressed by amino terminal derivatives that were at least 457 residues in length or longer. Eleven epitopes map in this region, including those of four potent neutralizing antibodies whose cognitive sites mapped between residues 273 and 298 in mapping studies using antibody-resistant mutants. Results of the present study indicate that the cognitive sites of these antibodies are assembled into the discontinuous domain by juxtaposing residues from the amino-terminal half of gB monomers.

Phosphorylation of varicella-zoster virus glycoprotein gpI by mammalian casein kinase II and casein kinase I

Varicella-zoster virus (VZV) glycoprotein gpI is the predominant viral glycoprotein within the plasma membranes of infected cells. This viral glycoprotein is phosphorylated on its polypeptide backbone during biosynthesis. In this report, we investigated the protein kinases which participate in the phosphorylation events. Under in vivo conditions, VZV gpI was phosphorylated on its serine and threonine residues by protein kinases present within lysates of either VZV-infected or uninfected cells. Because this activity was diminished by heparin, a known inhibitor of casein kinase II, isolated gpI was incubated with purified casein kinase II and shown to be phosphorylated in an in vitro assay containing [gamma-32P]ATP. The same glycoprotein was phosphorylated when [32P]GTP was substituted for [32P]ATP in the protein kinase assay. We also tested whether VZV gpI was phosphorylated by two other ubiquitous mammalian protein kinases--casein kinase I and cyclic AMP-dependent kinase--and found that only casein kinase I modified gpI. When the predicted 623-amino-acid sequence of gpI was examined, two phosphorylation sites known to be optimal for casein kinase II were observed. Immediately upstream from each of the casein kinase II sites was a potential casein kinase I phosphorylation site. In summary, this study showed that VZV gpI was phosphorylated by each of two mammalian protein kinases (casein kinase I and casein kinase II) and that potential serine-threonine phosphorylation sites for each of these two kinases were present in the viral glycoprotein.

Existence of similar antigenic-sites on varicella-zoster virus gpI and gpIV

We previously identified the antibody-binding site of a monoclonal antibody (mAb 79.0) on varicella-zoster virus (VZV) glycoprotein I (gpI) and showed that this monoclonal antibody binds to both VZV gpI and gpIV (Vafai et al., J. Virol. 62, 2544, 1988). In this study, a synthetic peptide comprising the mAb 79.0 binding site (designated el) was prepared and anti-peptide antibodies (RAnti-el) were raised in rabbit. RAnti-el recognized the primary translation products encoded by VZV genes 67 (gpIV) and 68 (gpI). To further localize the binding site of RAnti-el on VZV gpIV, the gpIV gene cloned in pGEM transcription vector was cleaved at different locations to generate four truncated DNA fragments. RNA was transcribed from each truncated gpIV fragment, translated in vitro and immunoprecipitated with RAnti-el. The results indicated that RAnti-el binds an antigenic determinant within the first 153 amino acid residues on the primary translation product of VZV gpIV. In addition, RAnti-el recognized the high-mannose intermediate but not the mature from of gpI in the infected cells or the translation products of gpIV glycosylated in vitro in the presence of canine microsomal membrane. These results: (a) confirmed the existence of a shared antigenic determinant on both VZV gpI and gpIV; and (b) indicated that the addition of terminal sugar modification may influence the conformation of gpI and gpIV with respect to the antigenic determinant recognized by RAnti-el.

Recognition of similar epitopes on varicella-zoster virus gpI and gpIV by monoclonal antibodies

Two monoclonal antibodies, MAb43.2 and MAb79.0, prepared against varicella-zoster virus (VZV) proteins were selected to analyze VZV gpIV and gpI, respectively. MAb43.2 reacted only with cytoplasmic antigens, whereas MAb79.0 recognized both cytoplasmic and membrane antigens in VZV-infected cells. Immunoprecipitation of in vitro translation products with MAb43.2 revealed only proteins encoded by the gpIV gene, whereas MAb79.0 precipitated proteins encoded by the gpIV and gpI genes. Pulse-chase analysis followed by immunoprecipitation of VZV-infected cells indicated reactivity of MAb43.2 with three phosphorylated precursor species of gpIV and reactivity of MAb79.0 with the precursor and mature forms of gpI and gpIV. These results indicated that (i) MAb43.2 and MAb79.0 recognize different epitopes on VZV gpIV, (ii) glycosylation of gpIV ablates recognition by MAb43.2, and (iii) gpIV is phosphorylated. To map the binding site of MAb79.0 on gpI, the pGEM transcription vector, containing the coding region of the gpI gene, was linearized, and three truncated gpI DNA fragments were generated. RNA was transcribed from each truncated fragment by using SP6 RNA polymerase, translated in vitro in a rabbit reticulocyte lysate, and immunoprecipitated with MAb79.0 and human sera. The results revealed the existence of an antibody-binding site within 14 amino acid residues located between residues 109 to 123 on the predicted amino acid sequences of gpI. From the predicted amino acid sequences, 14 residues on gpI (residues 107 to 121) displayed a degree of similarity (36%) to two regions (residues 55 to 69 and 245 to 259) of gp IV. Such similarities may account for the binding of MAb79.0 to both VZV gpI and gpIV.