The role of cytoplasmic membranes in poliovirus biosynthesis

Comparison of centrifugation methods for molecular and morphological analysis of membranes associated with RNA replication of the flavivirus Kunjin

Kunjin virus-infected cells were lysed and the cytoplasmic extract was subjected to sedimentation analysis. After centrifugation at 16,000 x g for 10 min about 70% of the original RNA-dependent RNA polymerase (RDRP) was recovered in the pellet; most of this enzymic activity was recovered in the soluble fraction after treatment with NP40 detergent. Membrane fractions were prepared from cytoplasmic extracts by centrifugation in discontinuous density gradients comprising w/w or w/v sucrose solutions, either for 3 h (top-loaded on 4 ml 20-60% sucrose) or for 19 h (centre-loaded in 37 ml 0-60% sucrose). Similar separations of bands of light membranes were obtained in all gradients. Multi-layered heavy membrane bands obtained with w/w sucrose gradients were resolved into two well-separated bands (F4 and F5) using w/v sucrose gradients. Thin-section electron microscopy of embedded membrane fractions, gel analysis of intracellular RNA, and RDRP assays showed that the w/w centre loading method and the w/v top-loading (short spin) method produced similar recoveries and distributions of smooth and rough membranes, intact virus particles and RDRP activity. The distribution of intracellular viral RNA and proteins was coincident with the RDRP, all being located in the F4 and F5 bands which contained the characteristic membrane structures induced during flavivirus infection. Significant advantages of the preferred method (w/v sucrose, top loading and short spin) were its rapidity, good preservation of membranes and RDRP, and the concentrations of RDRP achieved in the small volume fractions collected from a total of 4.5 ml.

Inhibition of poliovirus RNA synthesis by brefeldin A

Brefeldin A (BFA), a fungal metabolite that blocks transport of newly synthesized proteins from the endoplasmic reticulum, was found to inhibit poliovirus replication 10(5)- to 10(6)-fold. BFA does not inhibit entry of poliovirus into the cell or translation of viral RNA. Poliovirus RNA synthesis, however, is completely inhibited by BFA. A specific class of membranous vesicles, with which the poliovirus replication complex is physically associated, is known to proliferate in poliovirus-infected cells. BFA may inhibit poliovirus replication by preventing the formation of these vesicles.

Molecular and ultrastructural analysis of heavy membrane fractions associated with the replication of Kunjin virus RNA

In subcellular extracts of Kunjin virus-infected cells prepared by lysis and differential centrifugation, the viral RNA polymerase, RNA and proteins were associated mainly with cytoplasm. When the cytoplasmic extract (500 g supernate) of infected cells labelled for 3 h from 24 h post-infection was further fractionated by rapid centrifugation through a sucrose density gradient, all viral products were located only in dense or "heavy membrane" fractions, which contained three types of virus-induced morphologically distinct membrane structures. These dense fractions were treated with 0.5% NP40 and the soluble material was again centrifuged through a sucrose gradient for analyses as before. Viral RNA polymerase activity was retained and was associated with replicative intermediate RNA and some replicative form RNA in the peak enzyme fractions sedimenting at 20S to 40S. Enrichment of NS3 and of the small nonstructural proteins NS2A and NS2B/NS4A was apparent in these fractions which were well separated from the slow sedimenting structural proteins. No detergent-resistant structures in the "heavy membrane" fractions other than ribosome-like particles were visible. The data show that the RNA polymerase complex cosedimented with virus-induced membrane structures and remained associated with specific nonstructural proteins and replicative intermediate RNA after detergent treatment.

Gold labelling of RNA in virus-induced mitochondrial vesicles in the sheep blowfly Lucilia cuprina

Membranous vesicles are thought to be the replication site for viral RNA of many plant and animal viruses. A relatively rare site of virus-associated vesiculation is that of the mitochondrial outer membrane. In this study, virus-induced mitochondrial vesicles of the blowfly, Lucilia cuprina, were labelled with ribonuclease/gold and an antibody against double stranded RNA (anti-polyinosinic:polycytidylic acid). Both methods showed the presence of RNA in the vesicles thus indicating they may be a site for viral RNA replication in Lucilia.

Effects of fatty acids on lipid synthesis and viral RNA replication in poliovirus-infected cells

Animal viruses profoundly modify the metabolism of lipids, the synthesis of new membranes, and membrane traffic. These alterations are related to the replication of viral genomes. Addition of oleic acid from the beginning of poliovirus infection inhibits the appearance of virus polypeptides at concentrations that do not affect translation in mock-infected HeLa cells. This inhibition is due to the blockade of viral RNA synthesis. Membranes made in poliovirus-infected cells in the presence of oleic acid differ in their buoyant density from control membranes. These results suggest that the incorporation of oleic acid into membranes leads to increased membrane fluidity and decreased buoyant density, making these membranes nonfunctional for poliovirus RNA replication.

In vitro membrane binding of the translation products of the carlavirus 7-kDa protein genes

Two double-stranded DNA copies of the genes potentially coding for the 7-kDa proteins of potato virus M (PVM) and potato virus S (PVS) were synthesized and cloned into T7 transcription vectors. Cell-free translation of the corresponding monocistronic transcripts yielded in both cases a single protein of approximately 7-8 kDa that contains a highly hydrophobic N-terminal segment. To analyze their membrane-binding potential, both proteins were synthesized in the membrane-enriched Krebs-2 extract. It was found that the smooth membrane fraction was enriched in the carlavirus 7-kDa proteins. The primary and predicted secondary structures of their N-terminal hydrophobic segments suggest that the latter can function as signals for translocation into the rough endoplasmic reticulum.

Synthesis of Semliki Forest virus RNA requires continuous lipid synthesis

The involvement of lipid biosynthesis in the replication of Semliki Forest virus (SFV) in HeLa cells has been analyzed by the use of cerulenin, an inhibitor of lipid synthesis. The presence of this agent from the beginning of infection blocks the appearance of viral proteins. However, when the antibiotic is added at later stages of infection it has no effect on protein synthesis, the cleavage of viral proteins and their acylation by palmitic acid. Cerulenin is a powerful inhibitor of viral RNA synthesis, as analyzed by [3H]uridine incorporation, incorporation of [32P]phosphate into viral replication complexes, or Northern blot analysis of viral RNAs hybridized with minus- or plus-stranded riboprobes. Finally, analysis of phospholipids made in SFV-infected cells indicates that viral infection clearly stimulates the synthesis of phosphatidyl choline and modifies the membrane formed as analyzed by sucrose gradient centrifugation. Cerulenin blocks the synthesis of phospholipids and inhibits the formation of new membranes. These results show that, when the synthesis of lipids is blocked by cerulenin, SFV RNA replication is hampered, suggesting that the synthesis of viral RNAs needs continuous lipid synthesis and membrane formation.

Role of a viral membrane polypeptide in strand-specific initiation of poliovirus RNA synthesis

A molecular genetic analysis has been combined with an in vitro biochemical approach to define the functional interactions required for nucleotidyl protein formation during poliovirus RNA synthesis. A site-directed lesion into the hydrophobic domain of a viral membrane protein produced a mutant virus that is defective in RNA synthesis at 39 degrees C. The phenotypic expression of this lesion affects initiation of RNA synthesis, in vitro uridylylation of the genome-linked protein (VPg), and the in vivo synthesis of plus-strand viral RNAs. Our results support a model that employs a viral membrane protein as carrier for VPg in the initiation of plus-strand RNA synthesis. Our data also suggest that a separate mechanism could be used in the initiation of minus-strand RNA synthesis, thereby providing a means for strand-specific regulation of picornavirus RNA replication.

Effect of glycosylation on the conformational epitopes of the glycoprotein of vesicular stomatitis virus (New Jersey serotype)

The conformational epitopes reactive with neutralizing monoclonal antibodies (MAbs) appear to be clustered at the middle third of the glycoprotein (G) of the New Jersey serotype of vesicular stomatitis virus (VSV-NJ) and are flanked by two N-linked carbohydrate chains (W. Keil and R.R. Wagner, Virology 170, 392-407, 1989). We report here studies on the effect of glycosylation on the reactivity of VSV-NJ G protein derived from released virions or immunoprecipitated from pulse-labeled cells was not significantly affected in its reactivity with MAbs directed to epitope IV mapped toward the amino-terminus, nor to the centrally located conformational epitopes VI, VIII, and IX. However, there was a 5- to 15-fold decrease in the reactivity with MAb of epitopes VI, VIII, and IX on unglycosylated G protein either isolated from a ribosome-enriched membrane fraction or immunoprecipitated from whole VSV-infected cells labeled for 15 hr in the presence of tunicamycin. In sharp contrast, epitope V and to a somewhat lesser extent epitope VII exhibited decreased reactivity with their respective MAbs when unglycosylated G protein was isolated from released viral particles or from pulse-labeled cells infected with VSV-NJ in the presence of tunicamycin. Enzymatic removal of preformed carbohydrate chains with N-glycanase had little or no effect on the MAb-reactivity of epitopes V and VII, indicating that the carbohydrate chains per se do not influence the antigenic specificity of VSV-NJ G protein. These data suggest that the formation of N-linked carbohydrate chains influences the structure of the VSV-NJ G protein in such a way that epitopes V and VII are shielded from reactivity with their specific MAbs from an early stage of G-protein processing and to a much lesser extent epitopes VI, VIII, and IX at late stages of intracellular processing. These results are compatible with, but do not prove, the hypothesis that N-linked glycosylation plays a key role in promoting the formation and the stability of the disulfide bonds that determine the epitope-specific conformational integrity of the VSV-NJ glycoprotein.

Induction of membrane proliferation in mouse CNS by gold sodium thiomalate with reference to increased virulence of the avirulent Semliki Forest virus

Separation of smooth membrane vesicles from whole mouse brain by isopycnic centrifugation in discontinuous sucrose density gradients show an increased membrane proliferation in gold sodium thiomalate (GSTM) treated mice. Induction of membrane proliferation by GSTM seems to be an important factor in converting the avirulent Semliki Forest virus infection into a lethal one.

Structural organization of poliovirus RNA replication is mediated by viral proteins of the P2 genomic region

Transcriptionally active replication complexes bound to smooth membrane vesicles were isolated from poliovirus-infected cells. In electron microscopic, negatively stained preparations, the replication complex appeared as an irregularly shaped, oblong structure attached to several virus-induced vesicles of a rosettelike arrangement. Electron microscopic immunocytochemistry of such preparations demonstrated that the poliovirus replication complex contains the proteins coded by the P2 genomic region (P2 proteins) in a membrane-associated form. In addition, the P2 proteins are also associated with viral RNA, and they can be cross-linked to viral RNA by UV irradiation. Guanidine hydrochloride prevented the P2 proteins from becoming membrane bound but did not change their association with viral RNA. The findings allow the conclusion that the protein 2C or 2C-containing precursor(s) is responsible for the attachment of the viral RNA to the vesicular membrane and for the spatial organization of the replication complex necessary for its proper functioning in viral transcription. A model for the structure of the viral replication complex and for the function of the 2C-containing P2 protein(s) and the vesicular membranes is proposed.

Very high frequency of reversion to guanidine resistance in clonal pools of guanidine-dependent type 1 poliovirus

We have carefully examined the frequency of guanidine-resistant revertants in six different clonal pools of guanidine-dependent mutants of type 1 poliovirus. The mutation frequency was (6.5 +/- 6.3) x 10(-4) (with all amino acid substitutions occurring at position 227). The minimal corrected base substitution frequency per single nucleotide site in the codon for amino acid 227 was (2.1 +/- 1.9) x 10(-4).

In vitro synthesis of Japanese encephalitis virus (JEV) RNA: membrane and nuclear fractions of JEV-infected cells possess high levels of virus-specific RNA polymerase activity

Japanese encephalitis virus (JEV)-specific RNAs (including 42S RNA) were synthesized in subcellular fractions prepared from infected C6/36 cells. This in vitro RNA synthesis essentially required Mg2+ and four ribonucleotides, and it was enhanced by K+. The amounts of RNA synthesized in vitro (in extracts from JEV-infected cells) increased as a function of time after infection. The RNA-synthetic activity in nuclear fractions was the highest among three kinds of subcellular fractions. Our data showed that nonstructural proteins NS3 and NS5 were membrane-associated proteins. In particular, NS3 was found almost exclusively in the nuclear and membrane fractions. Our results suggest that NS5 and NS3 may play specific role(s) in flavivirus RNA replication.

Double-stranded ribonuclease coinduced with interferon

Double-stranded (ds) RNA and many viruses are inducers of interferon (IFN), the latter presumably because they contain, or can form, dsRNA. Concomitant with the induction of IFN in chicken embryo cells was the induction of a novel double-stranded ribonuclease (dsRNase), which was released into the medium and continued to accumulate long after IFN production ceased. Only avian cells (chicken, quail, turkey, or duck) expressed high levels of this dsRNase; mammalian, turtle, or fish cells did not. Production of the nuclease was inducer dose-dependent. Optimum pH and cation requirements distinguished it from other dsRNase activities. Degradation of dsRNA was endonucleolytic. Activity resided in a molecule of an Mr of approximately 34,500. Low levels of a single-stranded (ss) RNase activity were inseparable from the dsRNase. The role for a dsRNA-inducible dsRNase released from cells is unknown.

Presence of a 43-kDa host-cell polypeptide in purified aphthovirions

A 43kDa cellular polypeptide (P43), which comigrates with host-cell actin in both SDS-PAGE and isoelectrofocusing slab gels, was found associated to 140 S aphthoviral particles purified from BHK 21 cells labeled with [35S]methionine prior to infection. Ultracentrifugation analysis of disrupted virions demonstrates that polypeptide P43 is not associated to VP1-3 containing 12 S subunits but remains, like viral polypeptide VP4, at the top of the sucrose gradients. In addition, in vitro iodination or trypsin treatment show that P43 is protected from the action of both procedures and therefore supports the hypothesis that host-cell polypeptide P43 is located within the viral particles.

The cellular secretory pathway is not utilized for biosynthesis, modification, or intracellular transport of the simian virus 40 large tumor antigen

Unlike most proteins, which are localized within a single subcellular compartment in the eucaryotic cell, the simian virus 40 (SV40) large tumor antigen (T-ag) is associated with both the nucleus and the plasma membrane. Current knowledge of protein processing would predict a role for the secretory pathway in the biosynthesis and transport of at least a subpopulation of T-ag to account for certain of its chemical modifications and for its ability to reach the cell surface. We have examined this prediction by using in vitro translation and translocation experiments. Preliminary experiments established that translation of T-ag was detectable with as little as 0.1 microgram of the total cytoplasmic RNA from SV40-infected cells. Therefore, by using a 100-fold excess of this RNA, the sensitivity of the assays was above the limits necessary to detect the theoretical fraction of RNA equivalent to the subpopulation of plasma-membrane-associated T-ag (2 to 5% of total T-ag). In contrast to a control rotavirus glycoprotein, the electrophoretic mobility of T-ag was not changed by the addition of microsomal vesicles to the in vitro translation mixture. Furthermore, T-ag did not undergo translocation in the presence of microsomal vesicles, as evidenced by its sensitivity to trypsin treatment and its absence in the purified vesicles. Identical results were obtained with either cytoplasmic RNA from SV40-infected cells or SV40 early RNA transcribed in vitro from a recombinant plasmid containing the SP6 promoter. SV40 early mRNA in infected cells was detected in association with free, but not with membrane-bound, polyribosomes. Finally, monensin, an inhibitor of Golgi function, failed to specifically prevent either glycosylation or cell surface expression of T-ag, although it did depress overall protein synthesis in TC-7 cells. We conclude from these observations that the constituent organelles of the secretory pathway are not involved in the biosynthesis, modification, or intracellular transport of T-ag. The initial step in the pathway of T-ag biosynthesis appears to be translation on free cytoplasmic polyribosomes. With the exclusion of the secretory pathway, we suggest that T-ag glycosylation, palmitylation, and transport to the plasma membrane are accomplished by previously unrecognized cellular mechanisms.

Dissociation of NS5 from cell fractions containing West Nile virus-specific polymerase activity

West Nile virus replication complexes were partially purified from cytoplasmic extracts of virus-infected cells by centrifugation through a 20% glycerol cushion. Numerous cell proteins, as well as the largest nonstructural protein, NS5, were separated from the replication complexes without significant loss of in vitro West Nile virus polymerase activity.

Association of polioviral proteins of the P2 genomic region with the viral replication complex and virus-induced membrane synthesis as visualized by electron microscopic immunocytochemistry and autoradiography

Using high resolution electron microscopic autoradiography and immunocytochemistry with monoclonal antibodies against poliovirus proteins of the P2 genomic region, the location of these proteins in respect to the virus-induced vesicle formation and the viral RNA synthesis was followed during the viral replication cycle. It was found that P2 proteins become rER associated soon after their synthesis. At the site of protein and rER interaction, electron-dense patches appear. Simultaneously, membrane protrusions grow and form vesicles which finally budd off, carrying the patches on their outer surface. As shown by autoradiography, these patches are the site of viral RNA replication and, therefore, they represent the poliovirus replication complex. The vesicles with the replication complex, including replicating and replicated viral RNA, move away from the rER to form a continuously growing vesiculated area in the center of the infected cell, where virus maturation takes place. A likely function of the 2C protein is to attach the replication complex, or some of its components, to the vesicular membranes.

Temperature-sensitive defect of influenza A/Ann Arbor/6/60 cold-adapted variant leads to a blockage of matrix polypeptide incorporation into the plasma membrane of the infected cells

A temperature-sensitive (ts) defect in growth of the A/Ann Arbor/6/60 (A/AA/60) cold-adapted (ca) and ts variant strain has been studied. At the restrictive temperature of 38.5 degrees C, the variant synthesized all the viral polypeptides in normal amounts within the infected cells, but the virions released into the culture fluid contained greatly reduced amounts of the matrix (M1) polypeptide and showed significantly low infectivity per unit hemagglutinin activity. Cell fractionation experiments revealed that incorporation of the M1 polypeptide into plasma membranes of the variant-infected cells was selectively reduced at 38.5 degrees C, whilst it occurred normally at 34 degrees C. The ts reassortants between the A/AA/60 variant and the A/AA/1/80 wild type (wt) strain (non-ts), which had the M gene derived from the wt parent, also showed similar patterns. These results suggest that the ts defect of the variant and its ts reassortants involves the process of incorporation of the M1 polypeptide into the plasma membranes of the infected cells and that this defect is not attributable to the M gene of the variant.

Guanidine-dependent mutants of poliovirus: identification of three classes with different growth requirements

Four mutants resistant to high (2.0 mM) guanidine were derived from a mutant resistant to intermediate (0.53 mM) levels of this drug. One of these mutants was found to be resistant to high guanidine and was shown to contain a mutation within 2C seen previously in this class of mutants, while lacking the mutation seen in the intermediate parent. The other three mutants were dependent on guanidine for growth and contained the mutation in 2C seen in the parental virus as well as a mutation seen previously in another dependent mutant. Comparison of the newly isolated dependent mutants to two previously described dependent mutants revealed that three classes of dependent mutants which vary in their requirements for optimal growth can be observed. We present a model for the interaction of guanidine with 2C that explains the occurrence of the three classes of dependent mutants.

The effect of mengovirus infection on lipid synthesis in cultured Ehrlich ascites tumor cells

The concept of generally increased lipid synthesis during the initial 2/3 of picornaviral infectious cycles, held by several authors, needs differentiation. In mengovirus-infected Ehrlich ascites tumor cells, an increase in the rate of synthesis of phosphatidylcholine could be confirmed, but for phosphatidylethanolamine constant to decreasing rates of synthesis were found. Moreover, phosphatidylinositol was increasingly synthesized in the midst of the infectious cycle. The changes observed might have their functional expression in the proliferation of smooth cytoplasmic membrane systems that provide the structural framework for the replication of picornaviral RNA and virus assembly. The alterations in the labeling patterns of phosphatidylinositol, phosphatidylglycerol and diphosphatidylglycerol late in virus infection point to increased turnover of these compounds, possibly mediated by phospholipase D. The formation of lysophosphatidylcholine (cytolytic effect) and bis(monoacylglyceryl)phosphate in the final phase of the infectious cycle might be correlated with the liberation of lysosomal enzymes and the development of the cytopathic effect.

Production of guanidine-resistant and -dependent poliovirus mutants from cloned cDNA: mutations in polypeptide 2C are directly responsible for altered guanidine sensitivity

cDNA fragments representing the region in polypeptide 2C containing mutations in a guanidine-resistant or -dependent mutant were cloned into the wild-type background of an infectious clone. Transfection of COS-1 cells with these plasmids yielded viruses that were either completely resistant to 2.0 mM guanidine hydrochloride or dependent on this concentration of drug for growth.

Initiation of poliovirus plus-strand RNA synthesis in a membrane complex of infected HeLa cells

An in vitro poliovirus RNA-synthesizing system derived from a crude membrane fraction of infected HeLa cells was used to analyze the mechanism of initiation of poliovirus plus-strand RNA synthesis. This system contains an activity that synthesizes the nucleotidyl proteins VPg-pU and VPg-pUpU. These molecules represent the 5'-terminal structure of nascent RNA molecules and of virion RNA. The membranous replication complex is also capable of synthesizing nucleotidyl proteins containing nine or more of the poliovirus 5'-proximal nucleotides as assayed by the formation of the RNase T1-resistant oligonucleotide VPg-pUUAAAACAGp or by fingerprint analysis of the in vitro-synthesized RNA. Incubation of preformed VPg-pUpU with unlabeled nucleoside triphosphates resulted in the formation of VPg-pUUAAAACAGp. This reaction, which appeared to be an elongation of VPg-pUpU, was stimulated by the addition of a soluble fraction (S-10) obtained from uninfected HeLa cells. Preformed VPg-pU could be chased into VPg-pUpU in the presence of UTP. Our data are consistent with a model that VPg-pU can function as a primer for poliovirus plus-strand RNA synthesis in the membranous replication complex and that the elongation reaction may be stimulated by a host cellular factor.

A reassortant between influenza A viruses (H7N2) synthesizing an enzymatically inactive neuraminidase at 40 degrees which is not incorporated into infectious particles

Cells infected with a reassortant (113/Ho, H7N2) between A/fowl plague/Rostock/34 (FPV, H7N1) and A/Hong Kong/1/68 (H3N2) carrying RNA segments 1 and 6 of the Hong Kong virus and the residual genes of FPV, synthesized at 40 degrees a neuraminidase (NA) which is enzymatically not active and which is not incorporated into infectious particles. At 40 degrees NA accumulates in the rough endoplasmic reticulum. It contains mainly carbohydrate side chains of the mannose type, and fucose is only scarcely incorporated. At 33 degrees NA of the reassortant is overproduced, and at least some of it is active and is incorporated into viral particles. Under nonreducing conditions during PAGE its NA migrates to the same position as after heating with mercaptoethanol, in contrast to the Hong Kong parent virus. It is speculated that at 40 degrees the tetramerization of the NA in the rough endoplasmic reticulum does not function, and in this way its migration to the cytoplasmic membrane and its incorporation into infectious particles does not occur. Since 113/Ho is as pathogenic for the chicken (body temperature of 41 degrees) as is FPV, the question arises which role the NA plays in virus replication and spread in the infected organism.

Guanidine-selected mutants of poliovirus: mapping of point mutations to polypeptide 2C

Sequence analysis of the genomic RNA of interstrain guanidine-resistant and antibody-resistant variant recombinants of poliovirus type 1 mapped the resistance of mutants capable of growth in 2.0 mM guanidine hydrochloride to a region located 3' of nucleotide 4444. This region of the viral genome specifies the nonstructural protein 2C. The sequence of genomic RNA encoding 2C from six independently isolated mutants resistant to 2.0 mM guanidine was determined. All six isolates contained a mutation in 2C at the same position in all cases, resulting in two types of amino acid changes. Dependent mutants were examined and found to contain two amino acid changes each within 2C. Mutants resistant to 0.53 mM guanidine were isolated and found to lack the mutations seen in variants resistant to 2.0 mM guanidine. A comparison of the amino acid sequences of the 2C proteins of poliovirus, foot-and-mouth disease virus, rhinovirus types 2 and 14, and encephalomyocarditis virus revealed a strong homology over regions totaling 115 residues. All of the mutations observed in guanidine-selected mutants were contained within this region. The amino acid region containing the mutations observed in poliovirus mutants resistant to 2.0 mM guanidine was compared with the homologous region in the other picornaviruses; a strong correlation was found between the amino acid present at this position and the sensitivity of the virus to 2.0 mM guanidine.

Inhibition of the assembly of Newcastle disease virus by monensin

Monensin inhibits the intracellular transport of the glycoproteins of Newcastle disease virus between cis and trans Golgi stacks of infected BHK cells, as evidenced by its effect upon their post-translational modifications such as fatty acid acylation, glycosylation and proteolytic cleavage. Thus the drug has markedly altered the subcellular distribution of the glycoproteins so that they accumulate in the internal smooth membranes but are virtually absent in the plasma membrane. These glycoproteins that accumulated in intracellular membranes have a cytoplasmic domain susceptible to protease digestion and thus are transmembranous. Under such conditions, the behavior of M protein, which plays a crucial role in virus assembly (Y. Nagai et al., 1976, Virology 69, 523-538), has been analyzed. It has been found that the M protein can neither associate with the internal membranes nor bind to the plasma membrane. Thus no virus budding has been observed, either at the plasma membranes or at internal membranes. These results substantiate the view that the interaction between M and glycoproteins is of great importance for virus assembly and suggest further that this interaction is possibly only when the glycoproteins have been incorporated into the plasma membrane.

The functions of oligosaccharide chains associated with influenza C viral glycoproteins. II. The role of carbohydrates in the antigenic properties of influenza C viral glycoproteins

The antigenic properties of influenza C viral glycoprotein gp88 were compared with those of its nonglycosylated counterpart T76 synthesized in infected cells treated with tunicamycin. Radioimmunoprecipitation experiments with three different monoclonal antibodies against gp88 revealed that an antibody designated Q-5 precipitated gp88 but not T76, indicating the requirement for glycosylation for the binding of this antibody to gp88. It is unlikely, however, that the antigenic determinant recognized by Q-5 is carbohydrate moiety since the ability of the antibody to bind to gp88 varied depending on the virus strain, and trypsin-treatment of gp88 eliminated its reactivity with Q-5. Gel electrophoretic analysis under nonreducing conditions showed that T76 underwent the formation of disulfide-linked multimers in the absence of reducing agent while gp88 behaved as monomers, suggesting that glycosylation is required for gp88 molecules to attain an appropriate conformation. These observations, altogether, suggests that glycosylation is important in determining the immunological specificity of gp88 presumably by influencing the folding of this glycoprotein.

Effects of glycosylation on the conformation and antigenicity of influenza C viral glycoproteins

The antigenicity of influenza C viral glycoprotein gp88 was compared with that of its non-glycosylated counterpart T76 by immunoprecipitation utilizing monoclonal antibodies against gp88. Of the three monoclonal antibodies tested, an antibody designated Q-5 was found to precipitate gp88 but not T76, indicating the requirement for glycosylation for the binding of Q-5 to gp88. However, the antigenic determination recognized by Q-5 did not appear to be carbohydrates since trypsin-treatment of gp88 eliminated its reactivity with this antibody. These results suggest that glycosylation is important in determining the antigenicity of gp88 presumably by influencing the folding of the glycoproteins.

Membrane-bound virions of coxsackievirus B4: cellular localization, analysis of the genomic RNA, genome-linked protein, and effect on host macromolecular synthesis

Hela cells infected with several group B coxsackieviruses contain, in addition to standard virions, a population of virus-specific ribonucleoprotein particles which we (5) designated membrane-bound virions (MBV). MBVs differ from standard virions in buoyant density, yield, appearance, protein composition and infectivity. Here we present several new features of MBVs of coxsackievirus B4. The MBVs are lighter (rho about 1.30) and are localized in rough membranes, intermixed with virions. They contain 35S virion RNA covalently linked with a small protein, VPg. The VPg contain two proteins of different charge. MBV VPg is considerably smaller than the 5300-dalton virion VPg. MBV RNA is homologous to the base sequence present in B4 virus double-stranded RNA. The T1 oligonucleotide fingerprint of MBV RNA is distinguishable from that of virion RNA by one oligonucleotide. Several oligonucleotides of virion RNA appear to occur in submolar quantities in MBV RNA. MBVs are 75 to greater than 200 times less infective; they inhibit host cell macromolecular synthesis less efficiently than virions. In coinfected cells, the extent of inhibition of host synthesis is less severe than in cells infected with virions alone, which suggest interference by MBV particles.

Association of poliovirus proteins with the endoplasmic reticulum

Poliovirus proteins, except P3-7c, are associated with the endoplasmic reticulum after extraction of the cytoplasm and centrifugation of membranes to equilibrium in sucrose gradients. Proteins P3-2, P2-X, and P3-9 are found preferentially among the rough endoplasmic reticulum, whereas P3-7c is located in smooth endoplasmic reticulum fractions. P3-7c is probably not membrane associated, since it can be separated from membranes after centrifugation in buffer. However, P3-4a, P2-5b, P2-X, and P3-9 are avidly bound to membranes and cannot be dislodged with high-ionic-strength buffer containing EDTA or 4 M urea. These proteins are digested by trypsin, indicating peripheral rather than internal localization.

Post-translational glycosylation of coronavirus glycoprotein E1: inhibition by monensin

The intracellular sites of biosynthesis of the structural proteins of murine hepatitis virus A59 have been analyzed using cell fractionation techniques. The nucleocapsid protein N is synthesized on free polysomes, whereas the envelope glycoproteins E1 and E2 are translated on the rough endoplasmic reticulum (RER). Glycoprotein E2 present in the RER contains N-glycosidically linked oligosaccharides of the mannose-rich type, supporting the concept that glycosylation of this protein is initiated at the co-translational level. In contrast, O-glycosylation of E1 occurs after transfer of the protein to smooth intracellular membranes. Monensin does not interfere with virus budding from the membranes of the endoplasmic reticulum, but it inhibits virus release and fusion of infected cells. The oligosaccharide side chains of E2 obtained under these conditions are resistant to endoglycosidase H and lack fucose suggesting that transport of this glycoprotein is inhibited between the trans Golgi cisternae and the cell surface. Glycoprotein E1 synthesized in the presence of monensin is completely carbohydrate-free. This observation suggests that the intracellular transport of this glycoprotein is also blocked by monensin.

A mutant of fowl plague virus (influenza A) with an altered glycosylation pattern in its hemagglutinin

A temperature-sensitive mutant (ts 1/1) with a defect in the hemagglutinin (HA) gene, which was obtained by undiluted passage of fowl plague virus (FPV) at 33 degrees, is described. At 33 degrees proteolytic cleavage of the abnormal HA yielded an altered HA2 (XHA2) which migrated ahead of the NS1 protein and lacked the complex oligosaccharide side chain. At the nonpermissive temperature of 40 degrees, the migration of the HA of ts 1/1 from the rough endoplasmic reticulum (RER) via the Golgi apparatus to the cell surface was rate limiting for virus maturation. The HA was only slowly cleaved and migrated during polyacrylamide gel electrophoresis ahead of the HA of wild type FPV. Some revertants of ts 1/1 exhibited the same protein pattern as the mutant, others resembled wild type FPV, while one revertant gave rise to a mixture of HA2 and XHA2 at 40 degrees. These results suggest that (1) the loss of the complex oligosaccharide side chain is not responsible for the ts phenotype, (2) the mutation is presumably not at the site where the oligosaccharide side chain is linked to the protein backbone, and (3) ts 1/1 presumably carries a mutation located in RNA segment 4, which by pseudoreversion (suppressor mutation) in the same gene leads to different ts+ phenotypes.

Guanidine-resistant poliovirus mutants produce modified 37-kilodalton proteins

Eighteen spontaneous, guanidine-resistant mutants of poliovirus were obtained by plaque selection. Isoelectric focusing demonstrated charge changes in a 37-kilodalton protein, pX, among three of the mutants. The precursor of pX, NCVP5b , also exhibited charge changes among the three mutants. pX of 12 mutants was also examined by peptide mapping with Staphylococcus aureus V8 protease. Nine of the mutants presented modified maps, and seven of these maps were identical. The demonstration of mutational changes in pX in 12 of 18 mutants suggests a role for this protein in determining the guanidine trait of poliovirus and corroborates studies with foot-and-mouth disease virus.

On the role of oligosaccharide trimming in the maturation of Sindbis and influenza virus

The alpha-glucosidase inhibitor bromoconduritol inhibits the formation of the N-linked, complex-type oligosaccharides of the glycoproteins from influenza viruses (fowl plague virus, influenza virus PR-8) and from sindbis virus. Viral glycoproteins produced in bromoconduritol-treated chicken-embryo and baby-hamster kidney cells are fully glycosylated, but accumulate N-linked, high-mannose oligosaccharides of the composition Glc1Manx (GlcNAc)2 (x = 7, 8, and 9). Other alpha-glucosidase inhibitors (nojirimycin, deoxynojirimycin, acarbose) were not specific inhibitors of oligosaccharide processing under the conditions used in the present investigation. In bromoconduritol-treated, sindbis virus-infected chicken-embryo and baby-hamster kidney cells, the sindbis glycoproteins are metabolically stable. Specific proteolytic cleavage of the polyprotein precursors to form E2 and E1 occurs in bromoconduritol-treated chicken-embryo cells, but cleavage of PE2 to E2 is prevented in the infected baby-hamster kidney cells. Yet, release of infectious sindbis virus particles is inhibited in both cell types indicating that the formation of complex oligosaccharides is required for a late step in virus formation. The release of virus particles from influenza virus PR-8-infected bromoconduritol-treated chicken-embryo cells is not inhibited, and virus with only high-mannose oligosaccharides is formed. In contrast, when chicken-embryo cells were infected with the influenza virus fowl plague virus, release of infectious particles was inhibited. The fowl plague virus hemagglutinin is cleaved in chicken-embryo cells, in contrast to the hemagglutinin of the PR-8 virus. However, the cleavage products HA1 and HA2 do not reach the cell surface. In addition, or as a consequence, HA1 and HA2 are proteolytically broken down, whereas uncleaved hemagglutinin of PR-8 appeared metabolically stable. These results may explain the decrease in formation of fowl plague virus particles and the lack of effect on PR-8 virus in bromoconduritol-treated cells. This work thus shows different biological roles for oligosaccharide processing.

Membrane-dependent uridylylation of the genome-linked protein VPg of poliovirus

A small nucleotidyl-protein has been synthesized in vitro in a membrane fraction of poliovirus-infected HeLa cells. Analyses of the nucleotides and polypeptide have shown that the nucleotidyl-protein is VPg-pUpU: the genome-linked protein of poliovirion RNA covalently bound to the first two 5'-terminal nucleotides of poliovirus RNA. Synthesis of VPg-pUpU in vitro was sensitive to nonionic detergent. We suggest that VPg-pUpU is part of the initiation complex in poliovirus RNA replication in a membranous environment.

Intracellular distribution of poliovirus proteins and the induction of virus-specific cytoplasmic structures

In a susceptible cell, enteroviruses induce a vesiculated region (the "virus-induced vesicles") which is both the site of viral RNA synthesis as well as the site referred to morphologically, as the "cytopathic effect." Proteins of poliovirus (type I, Mahoney) were shown to migrate into the region of the virus-induced vesicles of infected HEp-2 cells. Five proteins (P2-5b, P3-4b, P3-6a, P3-7c, P3-9) were found to be associated with the vesicles themselves, either as intrinsic membrane protein (P3-9) or in a soluble form within the vesicles (P3-4b, P3-7c, and, partially, P3-6a) or bound to a DOC-resistant structure (P2-5b and a small amount of P3-6a). Partial inhibition of the cleavage of the viral polyprotein with ZnCl2 was used to alter the viral protein pattern within the cells. The data obtained indicate that P2-5b is the protein responsible for the formation of the virus-induced vesicles.

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.

Membrane fractions active in poliovirus RNA replication contain VPg precursor polypeptides

The poliovirus specific polypeptide P3-9 is of special interest for studies of viral RNA replication because it contains a hydrophobic region and, separated by only seven amino acids from that region, the amino acid sequence of the genome-linked protein VPg. Membraneous complexes of poliovirus-infected HeLa cells that contain poliovirus RNA replicating proteins have been analyzed for the presence of P3-9 by immunoprecipitation. Incubation of a membrane fraction rich in P3-9 with proteinase leaves the C-terminal 69 amino acids of P3-9 intact, an observation suggesting that this portion is protected by its association with the cellular membrane. These studies have also revealed two hitherto undescribed viral polypeptides consisting of amino acid sequences of the P2 and P3 regions of the polyprotein. Sequence analysis of stepwise Edman degradation show that these proteins are 3b/9 (Mr 77,000) and X/9 (Mr 50,000). 3b/9 and X/9 are membrane bound and are turned over rapidly and may be direct precursors to proteins P2-X and P3-9 of the RNA replication complex. P2-X, a polypeptide void of hydrophobic amino acid sequences but also found associated with membranes, is rapidly degraded when the membraneous complex is treated with trypsin. It is speculated that P2-X is associated with membranes by its affinity to the N-terminus of P3-9.

Effect of myocrisin (sodium auro-thio-malate) on the morphogenesis of avirulent Semliki Forest virus in mouse brain: an electron microscopical study

Adult mice, infected intracerebrally or intraperitoneally with avirulent Semliki Forest virus, do not show mature virus or advanced stages of viral replication in the brain. If myocrisin is given intraperitoneally 3 h before the virus there is enhancement of all stages of viral replication and budding of virus and mature virions are seen. Compared with controls many intracytoplasmic smooth membrane vesicles were seen in the parenchymal cells of the brain treated with myocrisin or with myocrisin and virus. Myocrisin was visible in the brain and has a membrane proliferating effect which may enhance viral synthesis in the early stages of replication, as well as help in the assembly and budding of mature virus. Increased numbers of infiltrating cells were observed in myocrisin treated mice infected with SFV. The formation of mature virus, and its virulence, appears to be related to the degree of membrane proliferation of the brain cells. The inflammation associated with the increased number of infiltrating cells is secondary to this, the whole process promoting the death of the animals rather than survival.

Poliovirus RNA synthesis in vitro: structural elements and antibody inhibition

The poliovirus RNA polymerase complex has been analyzed by immunoautoradiography using antibody probes derived from purified replicase (P3) region viral polypeptides. Antibody preparations made against the polio RNA polymerase, P3-4b, detected a previously unreported cellular protein that copurifies with the RNA polymerase. An IgG fraction purified from rabbit antiserum to polypeptide P3-2, a precursor of the RNA polymerase, specifically inhibits poliovirus RNA synthesis in vitro. We have also immunoprecipitated a 60,000-dalton protein (P3-4a) with antiserum to protein P3-4b and have determined the precise genomic map position of this protein by automated Edman degradation. Protein P3-4a originates by cleavage of the RNA polymerase precursor at a glutamine-glycine amino acid pair not previously reported to be a viral cleavage site.

Effects of hexose starvation and the role of sialic acid in influenza virus release

We previously reported that growth of influenza virus in the presence of cytochalasin B (CB), a drug that disrupts microfilaments and blocks hexose transport, yields particles with glycoproteins that are heterogeneous and unlabeled by [3H]glucosamine. When the virus was grown in glucose-free medium, we observed reduced virus titers similar to those produced by CB. In contrast, treatment of cells with cytochalasin D (CD) and dihydrocytochalasin B (H2CB), drugs which are known to inhibit microfilament function without affecting hexose transport, did not cause a reduction in virus titers or a change in the electrophoretic mobility of viral glycoproteins. Partial inhibition of glycosylation of viral glycoproteins resulting from either CB-induced inhibition of hexose transport or from glucose starvation resulted in the formation of aggregates of virions on cell surfaces. These aggregates can be dissociated by exogenous neuraminidase. Under these conditions the virions contained a functional hemagglutinin glycoprotein (HA) but an inactive neuraminidase glycoprotein (NA) which was not able to cleave sialic acid, the HA receptor, from viral glycoproteins, or from cellular glycoproteins and glycolipids. Neuraminidase treatment of membrane fractions of CB-treated cells did not cause a shift in the electrophoretic mobility of HA or in the gel elution profile of HA glycopeptides obtained after extensive pronase digestion from HA synthesized in glucose-free medium. These findings suggest that sialic acid is not present on labeled glycoproteins in either of these preparations. We obtained evidence that the sialic acid to which HA binds when NA is inactive is on glycoproteins and glycolipids of cellular origin. Our results support the idea that even when NA is functional, sialylated cellular components impede influenza virus release.

Association of the transforming proteins of the ST and GA strains of feline sarcoma virus and their in vitro associated protein kinase activities with cellular membranes

The translation products of the Snyder-Theilen (ST) and Gardner-Arnstein (GA) strains of feline sarcoma virus (FeSV), termed gag-fes proteins, are high molecular weight polyproteins containing different amounts of the amino terminus of the feline leukemia virus (FeLV) gag gene-coded precursor protein linked to a similar sarcoma virus-specific polypeptide. Both polyproteins are phosphoproteins with indistinguishable in vitro associated tyrosine-specific protein kinase activities. The polyproteins are extremely hydrophobic proteins which are intimately associated with the plasma membrane fraction of transformed cells. Approximately 10% of the proteins are modified by glycosylation and expressed on the cell surface where they are accessible to lactoperoxidase-mediated radio-iodination and trypsinization. Cell surface localization of the polyproteins does not appear to be necessary for transformation. However, preliminary evidence suggests that the amount of FeLV p30 sequences at the amino end of the proteins may have some effect on the intracellular distribution of the gag-fes polyproteins and on the phenotype of the transformed cell.