The soluble glycoprotein of vesicular stomatitis virus is formed during or shortly after the translation process

Diving deep into fish bornaviruses: Uncovering hidden diversity and transcriptional strategies through comprehensive data mining

Recently, we discovered two novel orthobornaviruses in colubrid and viperid snakes using an in silico data-mining approach. Here, we present the results of a screening of more than 100,000 nucleic acid sequence datasets of fish samples from the Sequence Read Archive (SRA) for potential bornaviral sequences. We discovered the potentially complete genomes of seven bornavirids in datasets from osteichthyans and chondrichthyans. Four of these are likely to represent novel species within the genus Cultervirus, and we propose that one genome represents a novel genus within the family of Bornaviridae. Specifically, we identified sequences of Wǔhàn sharpbelly bornavirus in sequence data from the widely used grass carp liver and kidney cell lines L8824 and CIK, respectively. A complete genome of Murray-Darling carp bornavirus was identified in sequence data from a goldfish (Carassius auratus). The newly discovered little skate bornavirus, identified in the little skate (Leucoraja erinacea) dataset, contained a novel and unusual genomic architecture (N-Vp1-Vp2-X-P-G-M-L), as compared to other bornavirids. Its genome is thought to encode two additional open reading frames (tentatively named Vp1 and Vp2), which appear to represent ancient duplications of the gene encoding the viral glycoprotein (G). The datasets also provided insights into the possible transcriptional gradients of these bornavirids and revealed previously unknown splicing mechanisms.

A host-factor interaction and localization map for a plant-adapted rhabdovirus implicates cytoplasm-tethered transcription activators in cell-to-cell movement

To identify host factors that play critical roles in processes, including cell-to-cell movement of plant-adapted rhabdoviruses, we constructed and validated a high-resolution Nicotiana benthamiana yeast two-hybrid library. The library was screened with the putative movement protein (sc4), nucleocapsid (N), and matrix (M) proteins of Sonchus yellow net virus (SYNV). This resulted in identification of 31 potential host factors. Steady-state localization studies using autofluorescent protein fusions to full-length clones of interactors were conducted in transgenic N. benthamiana marker lines. Bimolecular fluorescence complementation assays were used to validate two-hybrid interactions. The sc4 interactor, sc4i21, localized to microtubules. The N interactor, Ni67, localized to punctuate loci on the endoplasmic reticulum. These two proteins are 84% identical homologues of the Arabidopsis phloem-associated transcription activator AtVOZ1, and contain functional nuclear localization signals. Sc4i17 is a microtubule-associated motor protein. The M interactor, Mi7, is a nuclear-localized transcription factor. Combined with a binary interaction map for SYNV proteins, our data support a model in which the SYNV nucleocapsids are exported from the nucleus and moved cell-to-cell by transcription activators tethered in the cytoplasm.

Spike protein assembly into the coronavirion: exploring the limits of its sequence requirements

The coronavirus spike (S) protein, required for receptor binding and membrane fusion, is incorporated into the assembling virion by interactions with the viral membrane (M) protein. Earlier we showed that the ectodomain of the S protein is not involved in this process. Here we further defined the requirements of the S protein for virion incorporation. We show that the cytoplasmic domain, not the transmembrane domain, determines the association with the M protein and suffices to effect the incorporation into viral particles of chimeric spikes as well as of foreign viral glycoproteins. The essential sequence was mapped to the membrane-proximal region of the cytoplasmic domain, which is also known to be of critical importance for the fusion function of the S protein. Consistently, only short C-terminal truncations of the S protein were tolerated when introduced into the virus by targeted recombination. The important role of the about 38-residues cytoplasmic domain in the assembly of and membrane fusion by this approximately 1300 amino acids long protein is discussed.

Intracellular assembly and secretion of recombinant subviral particles from tick-borne encephalitis virus

It is believed that flavivirus assembly occurs by intracellular budding of the nucleocapsid into the lumen of the endoplasmic reticulum (ER). Recombinant expression of tick-borne encephalitis (TBE) virus envelope proteins prM and E in mammalian cells leads to their incorporation into enveloped recombinant subviral particles (RSPs), which have been used as a model system for studying assembly and entry processes and are also promising vaccine candidates. In this study, we analyzed the formation and secretion of TBE virus RSPs and of a membrane anchor-free E homodimer in mammalian cells. Immunofluorescence microscopy showed that E was accumulated in the lumen of the ER. RSPs were observed by electron microscopy in the rough and smooth ER and in downstream compartments of the secretory pathway. About 75% of the particles appeared to be of the size expected for RSPs (about 30 nm in diameter), but a number of larger particles and tubular structures were also observed in these compartments. Secretion of membrane anchor-free E dimers was detected 30 min after synthesis of prM and E, and secretion of RSPs was detected 1 h after synthesis of prM and E. We also found that the presence of the single N-linked oligosaccharide side chain on the E protein and its trimming by glucosidases was necessary for secretion of RSPs and truncated E dimers. Our results suggest that incorporation of prM and E into RSPs occurs at the ER membrane without other viral elements being required, followed by rapid transport along the compartments of the secretory pathway and secretion. Moreover, the carbohydrate side chain of E is involved in at least one assembly or transport step.

In vivo generation and characterization of a soluble form of the Semliki forest virus fusion protein

During infection of host cells, a number of enveloped animal viruses are known to produce soluble forms of viral membrane glycoproteins lacking the transmembrane domain. The roles of such soluble glycoproteins in viral life cycles are incompletely understood, but in several cases they are believed to modulate host immune response and viral pathogenesis. Semliki Forest virus (SFV) is an enveloped alphavirus that infects cells through low-pH-dependent fusion and buds from the plasma membrane. Fusion is mediated by the E1 subunit of the SFV spike protein. Previous studies described the in vivo generation of E1s, a truncated soluble form of E1, under conditions in which budding is inhibited in mammalian host cells. We have here examined the properties of E1s generation and the biological activity of E1s. E1s cleavage required spike protein transport out of the endoplasmic reticulum and was independent of virus infection. Cell surface E1 efficiently acted as a precursor for E1s. E1s generation was strongly pH dependent in BHK cells, with optimal cleavage at a pH of < or =7.0, conditions that inhibited the budding of SFV but not the budding of the rhabdovirus vesicular stomatitis virus. The pH dependence of E1s production and SFV budding was unaffected by the stability of the spike protein dimer but was a function of the host cell. Similar to the intact virus and in vitro-generated E1 ectodomain, treatment of E1s at low pH in the presence of target membranes triggered specific acid-dependent conformational changes. Thus, under a variety of conditions, SFV-infected cells can produce a soluble form of E1 that is biologically active.

Assembly of the coronavirus envelope: homotypic interactions between the M proteins

The viral membrane proteins M and E are the minimal requirements for the budding of coronavirus particles. Since the E protein occurs in particles only in trace amounts, the lateral interactions between the M proteins apparently generate the major driving force for envelope formation. By using coimmunoprecipitation and envelope incorporation assays, we provide extensive evidence for the existence of such M-M interactions. In addition, we determined which domains of the M protein are involved in this homotypic association, using a mutagenetic approach. Mutant M proteins which were not able to assemble into viruslike particles (VLPs) by themselves (C. A. M. de Haan, L. Kuo, P. S. Masters, H. Vennema, and P. J. M. Rottier, J. Virol. 72:6838-6850, 1998) were tested for the ability to associate with other M proteins and to be rescued into VLPs formed by assembly-competent M proteins. We found that M proteins lacking parts of the transmembrane cluster, of the amphipathic domain, or of the hydrophilic carboxy-terminal tail, or M proteins that had their luminal domain replaced by heterologous ectodomains, were still able to associate with assembly-competent M proteins, resulting in their coincorporation into VLPs. Only a mutant M protein in which all three transmembrane domains had been replaced lost this ability. The results indicate that M protein molecules interact with each other through multiple contact sites, particularly at the transmembrane level. Finally, we tested the stringency with which membrane proteins are selected for incorporation into the coronavirus envelope by probing the coassembly of some foreign proteins. The observed efficient exclusion from budding of the vesicular stomatitis virus G protein and the equine arteritis virus M protein indicates that envelope assembly is indeed a highly selective sorting process. The low but detectable incorporation of CD8 molecules, however, demonstrated that this process is not perfect.

The membrane-associated and secreted forms of the respiratory syncytial virus attachment glycoprotein G are synthesized from alternative initiation codons

Respiratory syncytial (RS) virus synthesizes two mature forms of its attachment glycoprotein G: an anchored type II integral membrane form and a smaller form that is secreted into the medium. Here we demonstrate that these two forms are synthesized as distinct primary translation products of a single species of G protein mRNA by initiation at either of two different AUGs. Mutant cDNAs which eliminated one of the other of the two AUG codons near the 5' end of the G gene open reading frame were constructed. Analysis of the proteins synthesized from these cDNAs, either by translation of transcripts in a cell-free system or in cells infected with recombinant vaccinia viruses containing either one of the mutant cDNAs, showed that elimination of either the first or the second of these AUG codons abrogated the synthesis of the membrane-anchored or the secreted form of the protein, respectively. Additionally, two unglycosylated forms of G protein which comigrated with the unglycosylated G proteins expressed by these recombinant viruses were detected in RS virus-infected cells. Since the second AUG encodes a methionine residue that lies near the middle of the signal/anchor domain, initiation at this codon resulted in a protein with a hydrophobic amino terminus. This form of the glycoprotein was efficiently secreted from cells infected with the vaccinia virus recombinant, and the amino-terminal sequence of this protein was identical to that of G protein secreted from RS virus-infected cells. Our results demonstrate that the secreted form of RS virus G protein is produced by initiation at the second AUG codon of the G open reading frame, followed by proteolytic removal of the signal/anchor domain.

The soluble interleukin-6 receptor is generated by shedding

The ligand-binding subunit (gp80) of the human interleukin-6 receptor (IL-6R) was transiently expressed in COS-7 cells. The metabolically labeled protein was shown to be quantitatively released from the membrane within 20 h. We identified the protein released from the transfected COS-7 cells after purification to homogeneity and N-terminal sequencing as a soluble form of the gp80/IL-6R. Shedding of the gp80 protein was strongly induced by 4 beta-phorbol-12-myristate-13-acetate, indicating that the process was regulated by protein kinase C (PKC). This was further corroborated by the finding that co-transfection of a PKC expression plasmid led to enhanced shedding of the gp80 protein. Since shedding of gp80 could not be prevented by treatment of the cells with inhibitors of all known classes of proteases, a novel protease seems to be involved. As a control, an unrelated membrane protein (vesicular stomatitis virus glycoprotein) was transfected into COS-7 cells and analyzed for shedding. Since the turnover of this protein was not mediated by shedding, we conclude that the release of gp80 from COS-7 cells is a specific process. The shed gp80 protein specifically binds IL-6, and this complex shows biological activity on human hepatoma cells. Human peripheral blood monocytes released a soluble form of the gp80 protein into the culture medium upon PMA treatment indicating that PKC-regulated shedding is the physiological mechanism of generation of the soluble IL-6R.

Formation of heterotrimers between the membrane-integrated and the soluble glycoproteins of vesicular stomatitis virus leads to their intracellular cotransport

BHK cells infected with vesicular stomatitis virus serotype Indiana generate intracellularly two different types of glycoproteins: the authentic membrane-integrated G protein of virions and a smaller soluble Gs protein lacking the transmembrane and cytoplasmic domains which is secreted into the growth medium. A Gs1 protein species which is formed during or shortly after translation in the endoplasmic reticulum lumen is modified in the same way as the G1 protein by endoglycosidase H-sensitive oligosaccharides of the high-mannose type. Both G1 and Gs1 are almost simultaneously transported, trimmed, and processed into G2 and Gs2 species which possess carbohydrate side chains of the complex type, making both glycoproteins resistant to endoglycosidase H cleavage. Secretion of Gs2 protein into the growth medium and arrival of G2 protein on the cell surface occur concomitantly. Membrane-integrated G protein and the soluble Gs protein molecules oligomerize intracellularly into heterotrimers which can be immunoprecipitated after chemical cross-linking. Gs protein seems to contain sufficient structural information for the formation of heterotrimers which are efficiently transported to the cell surface. Heterotrimer formation between G and Gs proteins explains the rapid secretion of Gs molecules.

Intracellular formation of two soluble glycoproteins in BHK cells infected with vesicular stomatitis virus serotype New Jersey

Infection of BHK 21 cells with VSV serotype New Jersey gave rise to three intracellular viral glycoproteins: the membrane-integrated G protein and the two soluble glycoproteins Gs and Gss which lacked the cytoplasmic and transmembrane domains as was deduced from limited chemical cleavage of the glycoproteins by hydroxylamine. Both soluble glycoproteins were completely protected by the microsomal membrane against proteolytic digestion. The soluble glycoproteins were formed in the endoplasmic reticulum because both were fully endo H sensitive after a 5-min pulse with [35S]methionine. Protease inhibitors and lysosomorphic agents had no effect on the yield of Gs and Gss. Tunicamycin treatment of VSV-infected cells reduced extensively viral particle maturation without affecting significantly the release of Gs and Gss. Two other glycosylation inhibitors, swainsonine and deoxynojirimycin did not decrease virus particle formation and secretion of both soluble glycoproteins. Since the glycosylation inhibitors showed a differential effect on the processing and transport of the glycoproteins a precursor-product relationship between G protein and soluble glycoproteins is highly unlikely. Both soluble glycoproteins were also synthesized in vitro in a reticulocyte lysate without microsomal membranes when primed with RNA extracted from VSV-infected cells or with newly transcribed mRNA from nucleocapsids in a coupled transcription system. Thus, proteases localized in the lumen of the ER seemed to be not essential for the generation of both soluble glycoproteins.

Distinct transport vesicles mediate the delivery of plasma membrane proteins to the apical and basolateral domains of MDCK cells

Immunoisolation techniques have led to the purification of apical and basolateral transport vesicles that mediate the delivery of proteins from the trans-Golgi network to the two plasma membrane domains of MDCK cells. We showed previously that these transport vesicles can be formed and released in the presence of ATP from mechanically perforated cells (Bennett, M. K., A. Wandinger-Ness, and K. Simons, 1988. EMBO (Euro. Mol. Biol. Organ.) J. 7:4075-4085). Using virally infected cells, we have monitored the purification of the trans-Golgi derived vesicles by following influenza hemagglutinin or vesicular stomatitis virus (VSV) G protein as apical and basolateral markers, respectively. Equilibrium density gradient centrifugation revealed that hemagglutinin containing vesicles had a slightly lower density than those containing VSV-G protein, indicating that the two fractions were distinct. Antibodies directed against the cytoplasmically exposed domains of the viral spike glycoproteins permitted the resolution of apical and basolateral vesicle fractions. The immunoisolated vesicles contained a subset of the proteins present in the starting fraction. Many of the proteins were sialylated as expected for proteins existing the trans-Golgi network. The two populations of vesicles contained a number of proteins in common, as well as components which were enriched up to 38-fold in one fraction relative to the other. Among the unique components, a number of transmembrane proteins could be identified using Triton X-114 phase partitioning. This work provides evidence that two distinct classes of vesicles are responsible for apical and basolateral protein delivery. Common protein components are suggested to be involved in vesicle budding and fusion steps, while unique components may be required for specific recognition events such as those involved in protein sorting and vesicle targeting.

Accessibility to proteases of the cytoplasmic G protein domain of vesicular stomatitis virus is increased during intracellular transport

G1 and G2 are two forms of the membrane-integrated G protein of vesicular stomatitis virus that migrate differently in gel electrophoresis because G1 is modified by high-mannose and G2 by complex-type oligosaccharide side chains. The cytoplasmic domain in G1 is less exposed to cleavage by several proteases than in G2 molecules. Acylation by palmitic acid as well as inhibition of carbohydrate processing by swainsonine and deoxynojirimycin resulted in the same pattern of proteolytic sensitivity of both glycoproteins as in untreated cells. In contrast, accessibility of the cytoplasmic domain to proteases did not change when the intracellular transport of the G protein was blocked in carbonyl cyanide m-chlorophenylhydrazone- or monensin-treated BHK-21 cells, respectively. The results suggest that the increase in accessibility of the cytoplasmic tail of the G protein occurs after the monensin block in the trans-Golgi and might reflect a conformational change of functional significance--i.e., making the cytoplasmic domain of the viral spike protein competent for its interaction with the viral core, inducing thereby the formation of the budding virus particle.

Fatty acid acylation at the single cysteine residue in the cytoplasmic domain of the glycoprotein of vesicular-stomatitis virus

The cysteine residue in the cytoplasmic domain at position 489 of the sequence of the glycoprotein (G protein) isolated from vesicular-stomatitis virions is completely blocked for carboxymethylation. After release of covalently bound fatty acids by hydroxylamine at pH 6.8, this cysteine residue could be specifically labelled by iodo[14C]acetic acid. Reaction products were analysed after specific cleavage of labelled G protein at asparagine-glycine bonds by hydroxylamine at pH 9.3, which generated a C-terminal peptide of Mr 15,300 containing only the single cysteine residue. Bromelain digestion of [3H]palmitic acid-labelled membrane fractions of vesicular-stomatitis-virus-infected baby-hamster kidney cells removed almost completely the 3H radioactivity from the cytoplasmic domain of the G protein, whereas the ectodomain was completely protected by the microsomal membrane. This result indicates that the acylation site of the G protein is exposed on the cytoplasmic side of intracellular membranes. Taken together, both biochemical techniques strongly suggest that the single cysteine-489 residue, which is located six amino acid residues distal to the putative transmembrane domain, is the acylation site. The thioester bond between palmitic acid and the G protein is quite resistant to hydroxylamine treatment (0.32 M at pH 6.8 for 1 h at 37 degrees C) compared with the reactivity of the thioester linkage in palmitoyl-CoA, which is cleaved at relatively low concentrations of hydroxylamine (0.05 M).

Similarities in glycosylation and transport between the secreted and plasma membrane forms of the epidermal growth factor receptor in A-431 cells

We have studied the synthesis and oligosaccharide processing of the 110,000 dalton form of the epidermal growth factor (EGF) receptor that is secreted into the medium of A-431 cells. Its 90,000 dalton precursor is soluble within the lumen of intracellular membrane vesicles shortly after synthesis, indicating that it lacks a membrane anchor. Analysis of labeled glycopeptides reveals that the glycosylation of the 110,000 dalton, secreted receptor is very similar to that of the 170,000 dalton, plasma membrane receptor. Based on Concanavalin A-Sepharose elution profiles of its glycopeptides, the secreted receptor has both complex and high-mannose N-linked oligosaccharides. Also, like the plasma membrane receptor, the secreted receptor contains N-acetylgalactosamine residues in its complex chains. Not only are major features of oligosaccharide processing of the soluble and membrane-bound forms of the receptor similar, but the kinetics of transport to the cell exterior is the same for each. These data indicate that the glycosylation pattern and kinetics of cellular transport of the EGF receptor are determined by factors other than the sequence of its cytoplasmic and transmembrane domains.

Membrane anchors of vesicular stomatitis virus: characterization and incorporation into virions

Wild-type vesicular stomatitis virus-infected cells contained multiple carboxy-terminal fragments of the envelope glycoprotein G. They migrated in 16% polyacrylamide gels with two dominant apparent molecular weights, 14,000 and 9,000. Both fragments were immunoprecipitated by two antibodies, anti-G(COOH) and anti-G(stem), made against the last 15 amino acids at the carboxy terminus and against the first 22 amino acids of the ectodomain adjacent to the transmembrane region of G, respectively. Pulse-chase experiments in the presence and absence of tunicamycin indicated that the higher-molecular-weight fragment, Gal, was generated first, presumably in the rough endoplasmic reticulum, and then apparently chased into the faster-migrating, stable fragment, Ga2. Exposure of infected cells to radioactive palmitic acid labeled Ga2. Ga2 was detected in purified virions. These results show that a polypeptide approximately 71 amino acids long is transported and incorporated into budding virions. What signals are operative and whether this C-terminal fragment of G protein is transported as a complex with other viral or host cell proteins are presently unknown.

Delayed formation of defective interfering particles in vesicular stomatitis virus-infected cells: kinetic studies of viral protein and RNA synthesis during autointerference

The time course of defective interfering (DI) particle and B particle release from vesicular stomatitis virus-infected BHK-21 cells was studied at different multiplicities of defective and infective particles. Particle release was progressively delayed in cells infected with an increasing DI-to-B particle ratio. The delayed particle release during interference was found to be connected with a reduced but prolonged synthesis of viral proteins, a slower accumulation of viral proteins, and a delayed shutoff of cellular protein synthesis. The relative synthesis of M and G proteins was reduced during interference, whereas the relative synthesis of N and NS proteins was increased. On the level of genomic RNA replication, we found that DI RNA was replicated more slowly during interference than the standard genomic RNA was during acute infection. The ratio of DI particles to B particles which were released increased throughout the infectious cycle. At a given time in the infectious cycle, this ratio was independent of the multiplicity of infecting DI and B particles. On the basis of the kinetic studies, we argue that cells infected with higher amounts of DI particles compared with B particles synthesize a higher DI-to-B particle ratio and release these progeny particles later than cells infected with a low DI-to-B particle ratio.

Effects of transport inhibitors on the generation and transport of a soluble viral glycoprotein

The generation and transport of the soluble glycoprotein (Gs) of wild-type vesicular stomatitis virus (VSV) were studied using cell fractionation and transport inhibitors. Gs was found in the rough endoplasmic reticulum (RER) and the Golgi-enriched membrane fractions of infected Chinese hamster ovary cells. The identity of intracellular Gs was confirmed by its precipitation with a monoclonal antibody to the ectodomain but not with a anti-peptide antibody directed against the first 15 amino acids at the carboxy terminus of the VSV transmembrane glycoprotein G. Their extracellular appearance was affected in a concentration-dependent manner by monensin and carbonyl cyanide m-chlorophenylhydrazone (CCCP) and was completely inhibited by incubation at 20 degrees. Inhibitors failed to dissociate the transport of Gs from G. These experiments indicate that in fibroblast cells Gs can be generated intracellularly, probably in the RER, and that Gs, like G, is transported from there to the Golgi complex and then presumably to the extracellular environment.

Cell surface expression of membrane-anchored v-sis gene products: glycosylation is not required for cell surface transport

The v-sis gene is able to transform cells by production of a growth factor that is structurally related to platelet-derived growth factor. This growth factor has been detected in the conditioned media of v-sis transformed cells, and is able to stimulate the autophosphorylation of the platelet-derived growth factor receptor. We have used the v-sis gene product to analyze the role of protein-encoded signals in cell surface transport. We constructed several gene fusions that encode transmembrane forms of the v-sis gene product. These membrane-anchored forms of the v-sis gene product are properly folded into a native structure, as indicated by their dimerization, glycosylation, and NH2-terminal proteolytic processing. Indirect immunofluorescence demonstrated that several of these membrane-anchored gene products are transported to the cell surface. Removal of the N-linked glycosylation site from the v-sis gene product did not prevent cell surface transport. Several of these mutant genes are able to induce focus formation in NIH3T3 cells, providing further evidence that the membrane-anchored proteins are properly folded. These results demonstrate that N-linked glycosylation is not required for the cell surface transport of a protein that is in a native, biologically active conformation. These results provide a correlation between cell surface expression of the membrane-anchored v-sis gene products and transformation.