Electrochemical modification of vesicular stomatitis virus glycoprotein by host cell transformation

Electrochemical properties of the glycoprotein of vesicular stomatitis virus (VSV) grown in Rous sarcoma virus (RSV)-transformed cells was compared with that of its counterpart grown in nontransformed cells. In DEAE-Sephadex column chromatography, the glycoproteins of VSV derived from transformed cells appeared more heterogeneous and had a tendency to elute with higher concentrations of NaCl than those from nontransformed cells. In isoelectric focussing, the glycoproteins of VSVs derived from transformed and nontransformed cells appeared as multiple components differing in the isoelectric point, and the glycoproteins from virus from transformed cells had isoelectric points that were more acidic than their counterparts from nontransformed cells. These results show that the glycoprotein of VSV consists of populations of molecules differing in charge and their isoelectric points were shifted to the acidic side by host cell transformation.     

Purification and composition of the proteins from Sindbis virus grown in chick and BHK cells.

Procedures are described for the purification of the Sindbis virus structural proteins. The amino acid and carbohydrate compositions of the purified proteins are presented for virus grown in BHK-21/13 and chicken embryo cells. Glycoprotein E1 from virus grown in BHK cells is deficient in a mannose-rich glycopeptide found on that glycoprotein when virus is grown in chicken embryo cells. The complex glactose-containing glycopeptides appear similar for virus grown in both hosts. However, when virus is grown in BHK cells, both glycoproteins are enriched in those glycopeptides containing more sialic acid. Since the two viral glycoproteins are difficult to separate cleanly during purification, it is suggested that there may be strong, but noncovalent, interactions between glycoproteins E1 and E2. It is also suggested that there may be an interaction between glycoprotein E2 and a component of the nucleocapsid.     

Carbohydrate Structure of Sindbis Virus Glycoprotein E2 from Virus Grown in Hamster and Chicken Cells

Sindbis virus was used as a probe to examine glycosylation processes in two different species of cultured cells. Parallel studies were carried out analyzing the carbohydrate added to Sindbis glycoprotein E2 when the virus was grown in chicken embryo cells and BHK cells. The Pronase glycopeptides of Sindbis glycoprotein E2 were purified by a combination of ion-exchange and gel filtration chromatography. Four glycopeptides were resolved, ranging in molecular weight from 1,800 to 2,700. Structures are proposed for each of the four glycopeptides, based on data obtained by quantitative composition analyses, methylation analyses, and degradation of the glycopeptides using purified exo- and endoglycosidases. The largest three glycopeptides (S1, S2, and S3) have similar structures but differ in the extent of sialylation. All three contain N-acetylglucosamine, mannose, galactose, and fucose, in a structure similar to oligosaccharides found on other glycoproteins. Glycopeptide S1 has two residues of sialic acid, whereas glycopeptides S2 and S3 contain 1 and 0 residues of sialic acid, respectively. The smallest glycopeptide, S4, contains only N-acetyglucosamine and mannose, and is also similar to mannose-rich oligosaccharides found on other glycoproteins. Each of the complex glycopeptides (S1, S2, or S3) from virus grown in BHK cells is indistinguishable from the corresponding glycopeptides derived from virus grown in chicken cells. Glycopeptide S4 is also very similar in size, composition, and sugar linkages from virus derived from the two hosts. These results suggest that chicken cells and BHK cells have similar glycosylation mechanisms and glycosylate Sindbis glycoprotein E2 in nearly identical ways.     

A comparison of membrane components of normal and transformed BALB/c cells.

Membrane glycolipids, glycoproteins, and surface proteins of normal and transformed BALB/c cell lines have been compared. Several virally and spontaneously transformed cell lines showed differences in membrane components compared to normal A31 cells. These differences consisted of increased amounts of simpler gangliosides, absence of the large external transformation sensitive (LETS) protein, and the appearance of a major new glycoprotein band of about 105 000 molecular weight. In contrast, the spontaneously transformed cell line that caused the fastest growing tumors in vivo and the most rapid animal death (3T12T) did not have these changes. A31 and 3T12T glycolipid profiles appear similar as did glycoproteins and cell surface proteins detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. When Pronase-generated glycopeptides were analyzed by Sephadex G-50 chromatography, and enrichment in faster-eluting species was seen in two killing tumor lines (c5T and 3T12T) compared to A31. Regressing tumor lines (MSC, c5) did not show this change. Isolated membrane glycoproteins yield glycopeptides of different sized after Pronase digestion. In addition, several 3T12T glycoproteins yield glycopeptides that are larger than those from the corresponding glycoproteins of A31 cells. It appears that glycopeptide alterations associated with transformation occur in several membrane glycoproteins.     

Differential effect of monensin on enveloped viruses that form at distinct plasma membrane domains

We have observed a striking differential effect of the ionophore, monensin, on replication of influenza virus and vesicular stomatitis virus (VSV) in Madin-Darby canine kidney (MDCK) and baby hamster kidney (BHK21) cells. In MDCK cells, influenza virus is assembled at the apical surfaces, whereas VSV particles bud from the basolateral membranes; no such polarity of maturation is exhibited in BHK21 cells. A 10(-6) M concentration of monensin reduces VSV yields in MDCK cells by greater than 90% as compared with controls, whereas influenza virus yields are unaffected. In BHK21 cells, monensin also inhibits VSV production, but influenza virus is also sensitive to the ionophore. Immunofluorescent staining of fixed and unfixed MDCK monolayers indicates that VSV glycoproteins are synthesized in the presence of monensin, but their appearance on the plasma membrane is blocked. Electron micrographs of VSV-infected MDCK cells treated with monensin show VSV particles aggregated within dilated cytoplasmic vesicles. Monensin-treated influenza virus-infected MDCK cells also contain dilated cytoplasmic vesicles, but virus particles were not found in these structures, and numerous influenza virions were observed budding at the cell surface. These results indicate that influenza virus glycoprotein transport is not blocked by monensin treatment, whereas there is a block in transport of VSV G protein. Thus it appears that at least two distinct pathways of transport of glycoproteins to the plasma membrane exist in MDCK cells, and only one of them is blocked by monensin.     

Membrane glycopeptides from virus-transformed hamster fibroblasts and the normal counterpart.

Comparisons of membrane glycopeptides from baby hamster kidney fibroblasts (BHK21/C13) and a clone transformed by Rous sarcoma virus (C13/B4) were made by using cells metabolically labeled with radioactive D-glucose and L-fucose. Most of the glycopeptides were metabolically labeled with both the general and the specific glycoprotein precursors. The glycopeptides obtained from the cell surface by controlled trypsinization were representative of the surface membrane as shown by comparing them with those of purified membrane preparations. The trypsin-removable glycopeptides from both cell types were further processed and examined by successive chromatography on Sephadex G-50 and DEAE-cellulose. The chromatographic distribution patterns showed that each cell type had glycopeptides of similar characteristics, although the proportions of the glycopeptides differed dramatically between the two cell types. After transformation there was an increase in the larger, more highly charged glycopeptides. This was verified by the increased sialic acid content in these glycopeptides. Some of the glycopeptides were homogeneous after the size and charge separations, since a variety of procedures did not separate them further. The apparent homogeneity and reasonably few species obtained may be due to the methods of isolation, with the procedures selecting particular glycopeptides from the external portion of the membrane. These results corroborate the concept and show for the first time that virus transformation is accompanied by an increase in certain species of glycopeptides rather than de novo synthesis.     

Glycosylation of VSV glycoprotein is similar in cystic fibrosis,heterozygous carrier,and normal human fibroblasts

The single envelope glycoprotein of vesicular stomatitis virus was used as a specific probe of glycosyltransferase activities in fibroblasts from two cystic fibrosis patients, an obligate heterozygous carrier and a normal individual. Gel filtration of pronasedigested glycopeptides from both purified virions and infected cell-associated VSV glycoprotein which had been labeled with [³H] glucosamine did not reveal any significant differences in the glycosylation patterns between the different cell cultures. All 4 cell lines were apparently able to synthesize the mannose- and glucosamine-containing core structure and branch chains terminating in sialic acid which are characteristic of asparagine-linked carbohydrate side chains in cellular glycoproteins. Analysis of tryptic glycopeptides by anion-exchange chromotography indicated that the same 2 major sites on the virus polypeptide were recognized and glycosylated in all 4 VSV-infected cell cultures. These studies suggest that the basic biochemical defect(s) in cystic fibrosis is not an absence or deficiency in enzymes responsible for the biosynthesis of complex carbohydrate side chains.     

Tyrosyl kinases acquired from anchorage-independent cells by a membrane-enveloped virus

Tyrosyl kinase activity in vesicular stomatitis virus (VSV) acquired from host cells that differ in morphology was investigated. VSV grown in baby hamster kidney (BHK) cells with rounded morphology and a high efficiency of colony formation in soft agar (Rous sarcoma virus [RSV]- transformed and suspension BHK cells) was compared with VSV grown in BHK cells with a flattened morphology and lower efficiency of colony formation in soft agar (RSV-infected revertant and control BHK cells). Tyrosyl kinase activity measured with the substrates angiotensin II peptide or casein was found at 7-10-fold higher levels in virus released from the anchorage-independent BHK cells. Most of the VSV- associated tyrosyl kinases acquired from the RSV-transformed BHK cells reacted with antiserum to pp60src, whereas the activity acquired from the suspension BHK cells was unaffected by anti-src serum. The overall levels of tyrosyl kinase in subcellular fractions of the host BHK cells were also measured. Like the VSV released from them, the RSV- transformed cell extracts contained high levels. The suspension cells, however, contained the same low levels of tyrosyl kinase as was found in the control BHK cell extracts. Therefore, tyrosyl kinase was concentrated and acquired by VSV from the anchorage-independent suspension BHK cells. VSV-associated protein kinases acquired from other cell types followed a similar pattern. Tyrosyl kinase levels were high in VSV released from suspension cultures (Chinese hamster ovary and HeLa) and from virally transformed cells (Kirsten murine sarcoma virus-transformed rat kidney cells) and low in VSV released from an anchorage-dependent primary cell culture (chick embryo fibroblasts).     

Comparison of the effects of Sindbis virus and Sindbis virus replicons on host cell protein synthesis and cytopathogenicity in BHK cells.

Infection of BHK cells by Sindbis virus leads to rapid inhibition of host cell protein synthesis and cytopathic effects (CPE). We have been studying these events to determine whether the expression of a specific viral gene is required and, in the present study, have focused our attention on the role of the structural proteins--the capsid protein and the two membrane glycoproteins. We tested a variety of Sindbis viruses and Sindbis virus replicons (virus particles containing an RNA that is self-replicating but with some or all of the viral structural protein genes deleted) for their abilities to inhibit host cell protein synthesis and cause CPE in infected BHK cells. Our results show that shutoff of host cell protein synthesis occurred in infected BHK cells when no viral structural proteins were synthesized and also under conditions in which the level of the viral subgenomic RNA was too low to be detected. These results support the conclusion that the early steps in viral gene expression are the ones required for the inhibition of host cell protein synthesis in BHK cells. In contrast, the Sindbis viruses and Sindbis virus replicons were clearly distinguished by the time at which CPE became evident. Viruses that synthesized high levels of the two membrane glycoproteins on the surface of the infected cells caused a rapid (12 to 16 h postinfection) appearance of CPE, and those that did not synthesize the glycoprotein spikes showed delayed (30 to 40 h) CPE.     

Specific changes in the oligosaccharide moieties of VSV grown in different lectin-resistnat CHO cells.

The carbohydrate moieties of the G glycoprotein of vesicular stomatitis virus (VSV) grown in three distinct lectin-resistant (LecR) Chinese hamster ovary (CHO) cell lines have been compared by fine structural analysis of radiolabeled glycopeptides. The mutant WgaRIII, selected for resistance to wheat germ agglutinin (WGA), produces VSV containing G glycoprotein specifically lacking in sialic acid. The mutant PhaRI, selected for resistance to phytohemagglutinin (PHA) and previously shown to lack a particular glycoprotein N-acetyl-glucosaminyl-transferase activity, produces VSV containing G glycoprotein specifically lacking terminal N-acetylglucosamine-galactose-sialic acid sequences and possessing an increased number of mannose residues in the "core" region of its carbohydrate moieties. The mutant PhaRIConARII, a "double" mutant selected from PhaRI cells for resistance to concanavalin A (ConA), produces VSV containing G glycoprotein with a further alteration in the mannose residues of the "core" oligosaccharide region. We discuss the relevance of these findings to the mechanisms of glycoprotein biosynthesis in mammalian cells and to the biochemical bases of lectin resistance in CHO cells.     

Glucose-regulated membrane properties of untransformed and virus-transformed BHK21 cells.

BHK21 fibroblasts transformed by hamster sarcoma virus have a higher rate of uptake of hexoses than their untransformed counterparts, and therefore rapidly exhaust glucose from the culture medium. The effects of culturing normal and transformed BHK cells, both in limiting and in excess glucose, on several membrane properties related to malignant transformation have been studied. The increase in the rate of hexose uptake in transformed cells is partially but not entirely dependent on extracellular glucose concentration. Two transformation-increased membrane proteins of molecular weights 95 000 and 78 000 are shown to be regulated by extracellular glucose concentration in both normal and transformed cells. The loss of LETS-protein, the high density of intramembranous particles, the increase in the amount of a 177K integral plasma membrane protein and the increase in the amount of high molecular weight surface glycopeptides in transformed cells, are not related to glucose depletion of the medium. Beside LETS, another iodinated protein, of molecular weight 160 000, is decreased in transformed cells. The exposure of this protein increased in both normal and transformed cells when arrested in G1 by asparagine deprivation.     

Chemical and immunological studies of cell surfaces from normal and transformed cells

Immunological and chemical studies of cell surfaces from normal and transformed BALB/c fibroblasts have shown alterations associated with transformation. The cells studied include normal lines which do not cause tumors when injected into BALB/c mice, viral transformants, and spontaneous transformants which cause tumors that either regress or grow progressively, killing the host. The spontaneously transformed progressors include cell lines which are immunogenic and nonimmunogenic as determined by the ability of tumor excision to protect an animal from subsequent rechallenge by tumor cells. Tumor-bearing mice produce lymphocytes which are nonspecifically cytotoxic for all the normal and transformed lines. Some of the cell lines induce specific antibody formation in BALB/c hosts. Antisera have been prepared in rabbits which are specific for the transformed cell lines. These antisera can be used to determine specific surface changes on the transformed cells. Chemical studies have shown glycolipid alterations between the normal cells and some, but not all, of the transformants. Glycoproteins labeled by lactoperoxidase-¹²⁵ I or [³H] glucosamine were compared by SDS gel electrophoresis. Results from these studies do not show changes associated with malignancy. Individual glycoprotein regions from gels were treated with pronase, and the glycopeptides compared by Sephadex G-50 chromatography. Alterations in glycopeptides from several cellular glycoproteins are the only changes which appear to be associated with malignancy.     

Polarized sorting of viral glycoproteins to the axon and dendrites of hippocampal neurons in culture

Cultured hippocampal neurons were infected with a temperature-sensitive mutant of vesicular stomatitis virus (VSV) and a wild-type strain of the avian influenza fowl plague virus (FPV). The intracellular distribution of viral glycoproteins was monitored by immunofluorescence microscopy. In mature, fully polarized neurons the VSV glycoprotein (a basolateral protein in epithelial MDCK cells) moved from the Golgi complex to the dendritic domain, whereas the hemagglutinin protein of FPV (an apically sorted protein in MDCK cells) was targeted preferentially, but not exclusively, to the axon. The VSV glycoprotein appeared in clusters on the dendritic surface, while the hemagglutinin was distributed uniformly along the axonal membrane. Based on the finding that the same viral glycoproteins are sorted in a polarized fashion in both neuronal and epithelial cells, we propose that the molecular mechanisms of surface protein sorting share common features in the two cell types.     

A dominant mutation to ricin resistance in Chinese hamster ovary cells induces UDP-GlcNAc:glycopeptide beta-4-N-acetylglucosaminyltransferase III activity

A biochemical basis for the LEC10 mutant phenotype of Chinese hamster ovary cells has been identified. Independent LEC10 mutants, originally selected for resistance to the toxicity of ricin, have been shown to exhibit reduced binding of 125I-ricin at the cell surface. Although this is indicative of structural changes in cell-surface carbohydrates, labeling of plasma membranes with galactose oxidase/[3H]borohydride revealed no significant differences between mutant and parental cells. Alterations in the carbohydrates synthesized by LEC10 cells were, however, resolved by lectin-affinity chromatography of glycopeptides from the G glycoprotein of vesicular stomatitis virus (VSV) grown in LEC10. LEC10/VSV glycopeptides contain a fraction which is not bound to concanavalin A-Sepharose but is strongly retarded on E-PHA (erythroagglutinin from Proteus vulgaris)-agarose. In contrast, CHO/VSV glycopeptides or those from a LEC 10 revertant (R.LEC 10/VSV) do not contain carbohydrates with these properties. High-field 1H NMR spectroscopy of the novel LEC10/VSV carbohydrates showed that they are complex, biantennary structures containing N-acetylglucosamine in beta(1,4)-linkage to the beta-linked core mannose residue. The presence of these structures correlates with the expression of the enzyme responsible for the addition of this "bisecting" GlcNAc residue, UDP-GlcNAc:glycopeptide beta-4-N-acetylglucosaminyltransferase III (GlcNAc-TIII). Parental Chinese hamster ovary cells and the LEC10 revertant possess no detectable GlcNAc-TIII activity. The combined evidence suggests that the LEC10 mutation induces the expression of the GlcNAc-TIII enzyme in Chinese hamster ovary cells.     

Modulation of glycosylation and transport of viral membrane glycoproteins by a sodium ionophore

Analysis of viral glycoprotein expression on surfaces of monensin- treated cells using a fluorescence-activated cell sorter (FACS) demonstrated that the sodium ionophore completely inhibited the appearance of the vesicular stomatitis virus (VSV) G protein on (Madin- Darby canine kidney) MDCK cell surfaces. In contrast, the expression of the influenza virus hemagglutinin (HA) glycoprotein on the surfaces of MDCK cells was observed to occur at high levels, and the time course of its appearance was not altered by the ionophore. Viral protein synthesis was not inhibited by monensin in either VSV- or influenza virus-infected cells. However, the electrophoretic mobilities of viral glycoproteins were altered, and analysis of pronase-derived glycopeptides by gel filtration indicated that the addition of sialic acid residues to the VSV G protein was impaired in monensin-treated cells. Reduced incorporation of fucose and galactose into influenza virus HA was observed in the presence of the ionophore, but the incompletely processed HA protein was cleaved, transported to the cell surface, and incorporated into budding virus particles. In contrast to the differential effects of monensin on VSV and influenza virus replication previously observed in monolayer cultures of MDCK cells, yields of both viruses were found to be significantly reduced by high concentrations of monensin in suspension cultures, indicating that cellular architecture may play a role in determining the sensitivity of virus replication to the drug. Nigericin, an ionophore that facilitates transport of potassium ions across membranes, blocked the replication of both influenza virus and VSV in MDCK cell monolayers, indicating that the ion specificity of ionophores influences their effect on the replication of enveloped viruses.     

Oligosaccharide moieties of the glycoprotein of vesicular stomatitis virus.

Vesicular stomatitis virus contains a single structural glycoprotein whose carbohydrate sequences are probably specified by the host cell. The glycopeptides derived by Pronase digestion of the glycoprotein of vesicular stomatitis virus grown in HeLa cells have an average molecular weight of 1,800. There are multiple oligosaccharide chains on the vesicular stomatitis virus glycoprotein with protein-carbohydrate linkages that are cleaved only by strong alkali under reducing conditions, suggesting that they contain asparagine and N-acetylglucosamine. The oligosaccharide moieties, in addition, appear to be heterogeneous in sequence on the basis of their mobilities during electrophoresis and their sensitivities to cleavage by an endoglycosidase. The carbohydrate-peptide linkage region of the major class of oligosaccharides of the vesicular stomatitis virus glycoprotein has the proposed sequence: (see article).     

Proteins of Vesicular Stomatitis Virus and of Phenotypically Mixed Vesicular Stomatitis Virus-Simian Virus 5 Virions

The identity of the glycoprotein of vesicular stomatitis virus (VSV) as the spike protein has been confirmed by the removal of the spikes with a protease from Streptomyces griseus, leaving bullet-shaped particles bounded by a smooth membrane. This treatment removes the glycoprotein but does not affect the other virion proteins, apparently because they are protected from the enzyme by the lipids in the viral membrane. The proteins of phenotypically mixed, bullet-shaped virions produced by cells mixedly infected with VSV and the parainfluenza virus simian virus 5 (SV5) have been analyzed by polyacrylamide gel electrophoresis. These virions contain all the VSV proteins plus the two SV5 spike proteins, both of which are glycoproteins. The finding of the SV5 spike glycoproteins on virions with the typical morphology of VSV indicates that there is not a stringent requirement that only the VSV glycoprotein can be used to form the bullet-shaped virion. On the other hand, the SV5 nucleocapsid protein and the major non-spike protein of the SV5 envelope were not detected in the phenotypically mixed virions, and this suggests that a specific interaction between the VSV nucleocapsid and regions of the cell membrane which contain the nonglycosylated VSV envelope protein is necessary for assembly of the bullet-shaped virion.     

A fusion-defective mutant of the vesicular stomatitis virus glycoprotein.

We have recently described an assay in which a temperature-sensitive mutant of vesicular stomatitis virus (VSV; mutant tsO45), encoding a glycoprotein that is not transported to the cell surface, can be rescued by expression of wild-type VSV glycoproteins from cDNA (M. Whitt, L. Chong, and J. Rose, J. Virol. 63:3569-3578, 1989). Here we examined the ability of mutant G proteins to rescue tsO45. We found that one mutant protein (QN-1) having an additional N-linked oligosaccharide at amino acid 117 in the extracellular domain was incorporated into VSV virions but that the virions containing this glycoprotein were not infectious. Further analysis showed that virus particles containing the mutant protein would bind to cells and were endocytosed with kinetics identical to those of virions rescued with wild-type G protein. We also found that QN-1 lacked the normal membrane fusion activity characteristic of wild-type G protein. The absence of fusion activity appears to explain lack of particle infectivity. The proximity of the new glycosylation site to a sequence of 19 uncharged amino acids (residues 118 to 136) that is conserved in the glycoproteins of the two VSV serotypes suggests that this region may be involved in membrane fusion. The mutant glycoprotein also interferes strongly with rescue of virus by wild-type G protein. The strong interference may result from formation of heterotrimers that lack fusion activity.     

Surface Membrane Glycopeptides Which Coincide with Virus Transformation and Tumorigenesis

Glycopeptides from the surface of clones of hamster embryo cells were examined at various intervals after infection with polyoma virus. Two types of transformed cells were examined: (i) clones that showed delayed transformation or an initially low tumorigenicity, and (ii) clones that were rapidly transformed showing an initially high tumorigenicity. The glycopeptides were removed from the cell surface by trypsin and, after Pronase digestion, were examined by filtration through Sephadex G-50. With delayed transformation, a specific group of glycopeptides was increasingly evident over an 85-day period as the cells showed phenotypic properties of transformation and the ability to form tumors. In the other series, all but one clone of hamster embryo cells showed rapid transformation after infection with polyoma virus. This clone was less tumorigenic and showed little of the specific glycopeptides. In all cases of delayed or rapid transformation examined, the specific group of glycopeptides increased proportionately to the ability of the cells to form tumors. All of the cells derived from progressively growing tumors formed by injection of these transformed hamster cells into adult animals showed an abundance of this group of glycopeptides. These results suggest that specific surface membrane glycopeptides accompany viral transformation and tumorigenesis.