Involvement of Golgi apparatus and a restructured nuclear envelope during biogenesis and transport of herpes simplex virus glycoproteins

The significance of the Golgi complex in envelopment of bovine herpesvirus 1 (BHV-1) as revealed by cryobased electron microscopy

Nucleocapsids of herpesviruses originate in the nucleus of host cells and bud through the inner nuclear membrane acquiring tegument and envelope. The release of the enveloped virus particle from the perinuclear space is unknown. Cryobased electron microscopic imaging revealed enveloped virus particles within cisterns associated with the perinuclear space, a pre-Golgi compartment connecting Golgi cisterns to the perinuclear space, and enveloped virus particles in Golgi cisterns where they are packaged into transport vacuoles by membrane fission. To our knowledge, our images show for the first time the connectivity from the perinuclear space to Golgi cisterns. The data strongly indicate an intracisternal transport of enveloped virus particles from the budding site to the packaging site. Budding starts by condensation at the inner membrane. Condensation involving the viral envelope and peripheral tegument was persistent in virus particles within perinuclear space and associated cisterns. Virus particles within Golgi cisterns and transport vacuoles originating by Golgi membrane fission, however, lacked condensation. Instead, spikes were clearly evident. The phenomenon of condensation is considered likely to be responsible for preventing fusion of the viral envelope with cisternal membranes and/or for driving virions from the perinuclear space to Golgi cisterns. Glycoprotein K is discussed to likely play a role in the intracisternal transportation of virions. In addition to the pathway including intracisternal transport and packaging, there were clear indications for the well-known pathway involving wrapping of cytoplasmic nucleocapsids by Golgi membranes. The origin of the cytoplasmic nucleocapsids, however, remains obscure. Lack of evidence for release of nucleocapsids at the outer nuclear membrane suggests that the process is very rapid, or that nucleocapsids pass the nucleocytoplasmic barrier via an alternative route.

Intra-nuclear localization of two envelope proteins, gB and gD, of herpes simplex virus

The envelopes of herpes simplex virus (HSV) particles are acquired from the inner nuclear membrane (INM) of the infected cell and virus-coded glycoproteins are present in the envelope of mature virions. Our ultrastructural study examined the process of virus envelopment and the targeting of two major viral glycoproteins, gB and gD, to the INM in HSV-infected human embryonic fibroblasts. It was shown that envelopment and transport of virus particles from the nucleus is facilitated by the formation of a dynamic tubulo-reticulum arising from the INM. Capsids were assembled in the nucleus and collected within INM tubules which protruded into the perinuclear space and thence into the cisternae of the endoplasmic reticulum (ER). Envelopment occurred by constriction and fusion of the tubular channel walls, releasing enveloped virions into the ER. Transport to the cell surface took place in membrane-bound compartments and probably followed the normal secretory pathway through the Golgi apparatus. Immunogold probes, tagged with specific monoclonal antibodies, were used to localize gB and gD during the process of virus maturation. Cytoplasmic membranes were not labelled, but probes bound inside the nucleus, mainly at sites of virus assembly. Labelling occurred on the nucleoplasmic side of the INM which surrounded capsids in the process of envelopment, but not on the outside of that membrane, although characteristic gB glycoprotein spikes were labelled on the envelopes of extracellular virus particles and on virions in trans-Golgi transport vesicles just prior to their release from the infected cell. gB was not detected on the surface of enveloped virions in the perinuclear space, or the cisternae of the ER or cis-Golgi, which suggests that the specific epitope was masked during that stage of intracellular processing. gD probes bound to virion envelopes and also to the tegument region of some particles found in both perinuclear and extracellular sites. We postulate the precursor core proteins for both gB and gD are transported first to the nucleus, and then, together with maturing capsids, are targeted to the INM, and later inserted into viral envelopes at the site of budding. Post-translational glycosylation of envelope proteins could occur as virus particles exit the nucleus and travel through the ER and Golgi compartments.

Immunoelectron microscopic localization of herpes simplex virus glycoprotein gB in the nuclear envelope of infected cells

Herpesvirus, such as herpes simplex type 1 (HSV-1) acquire their envelope by budding through a modified inner membrane of the nuclear envelope which forms thick and dense patches at the site of budding. This suggests that some of the viral envelope glycoproteins must be transported to the nuclear envelope in order to be incorporated into the virus. In an effort to establish the localization of the HSV-1 glycoprotein gB-1 in the nuclear envelope of HSV-1 infected cells directly, we have studied the distribution of the glycoprotein gB-1 by immunoelectron microscopy using a polyclonal anti gB-1 antibody. A specific accumulation of gB-1 in the nuclear envelope, which was five times more labeled than the plasma membrane was observed. The glycoprotein gB-1 was localized in both the outer and the inner membrane of the nuclear envelope. The labeling over the nuclear envelope was distributed evenly and no preferential concentration of gB-1 around or within the patches where the virus buds was detected. The nucleocapsids were found to be labeled only when they become associated with the nuclear envelope indicating that gB-1 is incorporated into the virus at this site.

A cellular function is required for pseudorabies virus envelope glycoprotein processing and virus egress

The mouse L-cell mutant gro29 is defective for egress of herpes simplex virus type 1 (HSV-1) virions and is significantly reduced in HSV-1 glycoprotein export (B. W. Banfield and F. Tufaro, J. Virol. 64:5716-5729, 1990). In this report, we demonstrate that pseudorabies virus (PRV), a distantly related alphaherpesvirus, shows a distinctive set of defects after infection of gro29 cells. Specifically, we identify defects in the rate and extent of viral glycoprotein export, infectious particle formation, plaque formation, and virus egress. The initial rate of viral glycoprotein synthesis was unaffected in gro29 cells, but the extent of export from the endoplasmic reticulum to the Golgi apparatus was impaired and export through the Golgi apparatus became essentially blocked late in infection. Moreover, by using a secreted variant of a viral membrane protein, we found that export from the Golgi apparatus out of the cell was also defective in gro29 cells. PRV does not form plaques on gro29 monolayers. A low level of infectious virus is formed and released early after infection, but further virus egress is blocked. Taken together, these observations suggest that the gro29 phenotype involves either multiple proteins or a single protein used at multiple steps in viral glycoprotein export and virus egress from cells. Moreover, this host cell protein is required by both HSV and PRV for efficient propagation in infected cells.

Overexpression in bacterial and identification in infected cells of the pseudorabies virus protein homologous to herpes simplex virus type 1 ICP18.5

The ICP18.5 gene (UL28) of herpes simplex virus type 1 is a member of a well-conserved gene family among herpesviruses and is thought to play a role in localization of viral glycoproteins. We have cloned, sequenced, and expressed the entire pseudorabies virus (PRV) ICP18.5 open reading frame in Escherichia coli as a Cro-ICP18.5 fusion protein. Rabbit antiserum against Cro-ICP18.5 immunoprecipitated a 79-kDa protein from PRV-infected cells as well as a 79-kDa protein from in vitro translation of a T7 RNA polymerase transcript of the ICP18.5 gene. ICP18.5 could be detected in infected cells by 2 h postinfection. Analysis by indirect immunofluorescence demonstrated that ICP18.5 became associated with the nucleus. Subcellular fractionation confirmed that ICP18.5 synthesized during a pulse-chase experiment appeared in the nuclear fraction with time and was stable for at least 2.5 h after synthesis. Pulse-chase analysis revealed that ICP18.5 was synthesized as a monomer during a 2-min pulse labeling but formed faster sedimenting complexes which were sensitive to sodium dodecyl sulfate (SDS) treatment. The majority of ICP18.5 appeared in complexes with an antigenically unrelated 70-kDa protein. Immunoblot analysis of total infected-cell extracts using polyvalent anti-ICP18.5 serum demonstrated that a 74-kDa cellular protein in addition to the 79-kDa ICP18.5 was detected. This cellular protein was present at similar levels in uninfected cells and in PRV-infected cells at least 12 h into the infectious cycle.

Intracellular maturation and sorting of two herpes simplex virus type 1 glycoproteins. Immunogold staining of ultrathin cryosections

Simultaneous immunocytochemical triple staining of ultrathin cryosections of herpes simplex virus type 1-infected cells was carried out using monoclonal antibodies specific for glycoprotein C, glycoprotein D and alpha + beta tubulin. The viral glycoproteins were identified in the cytoplasm, in the Golgi sacs, on the plasma membrane and on the surface of intra- and extracellular virus particles, but not on the nuclear membrane. The glycoproteins identified in the cytoplasm outside the Golgi region were not always confined to the membranes of vesicles, but were often located in close proximity to the tubulin-labelled structures. The glycoproteins C and D were usually codistributed in the cytoplasm, and both accumulated in the Golgi sacs in the same membrane domains. As the glycoproteins occur in close proximity to the microtubular structures, we speculate that these might be directly involved in the intracellular transport of viral glycoproteins.

Effect of brefeldin A on alphaherpesvirus membrane protein glycosylation and virus egress

In this work we used brefeldin A (BFA), a specific inhibitor of export to the Golgi apparatus, to study pseudorabies virus viral glycoprotein processing and virus egress. BFA had little effect on initial synthesis and cotranslational modification of viral glycoproteins in the endoplasmic reticulum (ER), but it disrupted subsequent glycoprotein maturation and export. Additionally, single-step growth experiments demonstrated that after the addition of BFA, accumulation of infectious virus stopped abruptly. BFA interruption of virus egress was reversible. Electron microscopic analysis of infected cells demonstrated BFA-induced disappearance of the Golgi apparatus accompanied by a dramatic accumulation of enveloped virions between the inner and outer nuclear membranes and also in the ER. Large numbers of envelope-free capsids were also present in the cytoplasm of all samples. In control samples, these capsids were preferentially associated with the forming face of Golgi bodies and acquired a membrane envelope derived from the trans-cisternae. Our results are consistent with a multistep pathway for envelopment of pseudorabies virus that involves initial acquisition of a membrane by budding of capsids through the inner leaf of the nuclear envelope followed by deenvelopment and release of these capsids from the ER into the cytoplasm in proximity to the trans-Golgi. The released capsids then acquire a bilaminar double envelope containing mature viral glycoproteins at the trans-Golgi. The resulting double-membraned virus is transported to the plasma membrane, where membrane fusion releases a mature, enveloped virus particle from the cell.

Varicella-zoster virus glycoprotein oligosaccharides are phosphorylated during posttranslational maturation

Varicella-zoster virus (VZV)-infected human embryonic lung fibroblasts (HELF) do not release infectious virions into their growth medium. Extracellular virions are pleomorphic, suggesting that they are partially degraded before their release from cells. To examine the intracellular pathway of viral maturation, [2-3H]mannose-labeled virus-encoded glycoproteins were isolated from VZV-infected HELF. Oligosaccharides attached to the glycoproteins were processed to complex-type units, some of which were phosphorylated. The major intracellular site of accumulation of VZV gpI was found to be perinuclear and to correspond to that of the cation-independent mannose 6-phosphate (Man 6-P) receptor. Subsets of VZV-containing cytoplasmic vacuoles were coated, Golgi-associated, or accessible to endocytic tracers. Phosphorylated monosaccharides protected HELF from the cytopathic effect of VZV in proportion to their ability to block Man 6-P receptor-mediated endocytosis. These data suggest that the unusual phosphorylated oligosaccharides mediate an interaction between VZV and Man 6-P receptors of the host cell; this interaction may be responsible for withdrawal of newly synthesized virions from the secretory pathway and for their diversion to prelysosomal structures.

Visualization of glycoproteins after tunicamycin and monensin treatment of herpes simplex virus infected cells

The effects of tunicamycin and monensin on the morphogenesis of herpes simplex virus type 1 and on the ultrastructure and function of host cell membranes was investigated by conventional technics of electron microscopy and cytochemical localization of glycoproteins with thiocarbohydrazide-SO2. Infected RS 537 rabbit fibroblasts were treated with tunicamycin, which inhibits the glycosylation of many glycoproteins, or monensin, which inhibits the transport of proteins to the cell surface, and were compared with untreated infected cells. Tunicamycin treatment almost entirely suppresses the perinuclear envelopment of viral capsids, induces the nuclear export of unusually numerous naked viral capsids, and prevents the proliferation of the Golgi apparatus. On the other hand, perinuclear envelopment of viral capsids still occurs following a monensin treatment; however, enveloped viral capsids are not released into the extracellular space; in addition this treatment induces the proliferation of the rough endoplasmic reticulum (RER). The number of structures stained for glycoproteins in tunicamycin-treated cells is markedly lower than that in nontreated infected cells, whereas an unusual additional staining of the entire outer nuclear membrane and of the RER occurs following monensin treatment.

Role of cytoplasmic vacuoles in varicella-zoster virus glycoprotein trafficking and virion envelopment

Varicella-zoster virus (VZV) encodes several glycoproteins which are present on both mature viral envelopes and the surfaces of infected cell membranes. Mechanisms of VZV glycoprotein transport and virion envelopment were investigated by both continuous radiolabeling and pulse-chase analyses with tritiated fucose in VZV-infected cells. We studied in detail the large cytoplasmic vacuoles which were present in infected cells but absent from uninfected cells. The specific activity in each subcellular compartment was defined by quantitative electron microscope autoradiography, using a cross-fire probability matrix analysis to more accurately assess the individual compartment demarcated by the silver grains. By these techniques, we documented a progression of activity originating in the Golgi apparatus and traveling through the post-Golgi region into virus-induced cytoplasmic vacuoles and finally to areas of the cellular membrane associated with the egress of viral particles. Significant amounts of radiolabel were not observed in the nucleus, and only low levels of radiolabel were associated with the cellular membrane not involved with the egress of viral particles. In addition, immunolabeling of Lowicryl-embedded VZV-infected cells demonstrated the presence of VZV glycoproteins within cytoplasmic vacuole membranes as well as on virion envelopes. These observations suggested that cytoplasmic vacuoles harbored VZV-specified glycoproteins and were also the predominant site of VZV virion envelopment within the infected cell. Neither enveloped nor unenveloped viral particles were observed within the Golgi apparatus itself.

Characterization of herpesvirus sylvilagus glycoproteins released into the culture medium of infected cells: antisera to gp13 and gp32 neutralize viral infectivity in vitro and identify antigens on plasma membranes of infected cells

Polypeptides released into the culture medium of herpesvirus sylvilagus-infected cells were identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of extracellular fluid from [35S]methionine- and [3H]glucosamine-labeled cell cultures. Virus-induced glycoproteins 31, 32, and 33 (molecular weights of 62,000, 59,000, and 54,000, respectively) were the most abundant species and appeared predominantly in the culture medium. This observation, together with the known cell-associated nature of herpesvirus sylvilagus, suggested that virus-induced glycoproteins 31, 32, and 33 were specifically released. Immunization of rabbits with virus-induced glycoproteins 13 (molecular weight of 130,000) and 32 resulted in the production of antibodies that neutralized viral infectivity in vitro. Both antiserum to gp13 and antiserum to gp32 immunoprecipitated gp13, gp26, gp33a, gp45, and virus-induced polypeptide 39 (molecular weights of 130,000, 77,000, 49,000, 27,000, and 36,000, respectively) from [35S]methionine-labeled cell extracts as well as virus-induced glycoproteins 31, 32, and 33 from the culture medium. In addition, membrane immunofluorescence assays indicate that an antigen(s) reactive with anti-gp13/32 serum was located on the plasma membrane of infected cells.