Structure and replication of alpha-viruses

A model of the binding, entry, uncoating, and RNA synthesis of Semliki Forest virus in baby hamster kidney (BHK-21) cells

A quantitative understanding of viral trafficking would be useful in treating viral-mediated diseases, designing protocols for viral gene therapy, and optimizing heterologous protein production. In this article, a model for the trafficking of Semliki Forest virus and its RNA synthesis in baby hamster kidney (BHK-21) cells is presented. This model includes the various steps leading to infection such as attachment, endocytosis, and viral fusion in the endosome. The model estimates a mean fusion time of 4 to 6 min for the wild-type virus, and 38 min for Fus-1, an SFV mutant which requires a lower pH for fusion. These mean fusion times are consistent with the time-scale of endosomal acidification, suggesting viruses fuse almost instantaneously with the endosomal membrane as soon as the pH of the endosome drops below the pH threshold of the virus. Infection is most likely controlled at the level of viral uncoating, as shown by the close agreement between the efficiency of uncoating and the experimentally determined fraction of viruses that is infectious. The viral RNA synthesized per cell is best described by assuming that it depends on the number of uncoated viruses prior to the onset of replication according to a saturation-type expression. A Poisson distribution is used to determine the distribution of uncoated viruses among the cells. Because attachment is the rate-limiting step in the uncoating of the virus, increasing the attachment rate can lead to enhanced RNA synthesis and, hence, new virion production. Such an increase in the attachment rate may be obtained by lowering the medium pH or the addition of a polycation. (c) 1995 John Wiley & Sons, Inc.

Evidence that RNA 4 of alfalfa mosaic virus does not replicate autonomously

A mutant (Tbts7) of alfalfa mosaic virus, the coat protein of which is unable to activate the viral genome (the RNA species 1, 2, and 3, which need some coat protein for infectivity) at 30 degrees , can be rescued at this temperature by adding to the inoculum wild-type RNA 3 (the genome part that contains the coat protein cistron), but not adding wild-type RNA 4 (the subgenomic messenger for the coat protein). Unless RNA 3 of Tbts 7 has a second ts mutation at a site not occurring in RNA 4, it may be concluded from the above finding that RNA 4 does not replicate autonomously.

Viral RNA replication in association with cellular membranes

All plus-strand RNA viruses replicate in association with cytoplasmic membranes of infected cells. The RNA replication complex of many virus families is associated with the endoplasmic reticulum membranes, for example, picorna-, flavi-, arteri-, and bromoviruses. However, endosomes and lysosomes (togaviruses), peroxisomes and chloroplasts (tombusviruses), and mitochondria (nodaviruses) are also used as sites for RNA replication. Studies of individual nonstructural proteins, the virus-specific components of the RNA replicase, have revealed that the replication complexes are associated with the membranes and targeted to the respective organelle by the ns proteins rather than RNA. Many ns proteins have hydrophobic sequences and may transverse the membrane like polytopic integral membrane proteins, whereas others interact with membranes monotopically. Hepatitis C virus ns proteins offer examples of polytopic transmembrane proteins (NS2, NS4B), a “tip-anchored” protein attached to the membrane by an amphipathic α-helix (NS5A) and a “tail-anchored” posttranslationally inserted protein (NS5B). Semliki Forest virus nsP1 is attached to the plasma membrane by a specific binding peptide in the middle of the protein, which forms an amphipathic α-helix. Interaction of nsP1 with membrane lipids is essential for its capping enzyme activities. The other soluble replicase proteins are directed to the endo-lysosomal membranes only as part of the initial polyprotein. Poliovirus ns proteins utilize endoplasmic reticulum membranes from which vesicles are released in COPII coats. However, these vesicles are not directed to the normal secretory pathway, but accumulate in the cytoplasm. In many cases the replicase proteins induce membrane invaginations or vesicles, which function as protective environments for RNA replication.

Biogenesis of the Semliki Forest virus RNA replication complex

The nonstructural (ns) proteins nsP1 to -4, the components of Semliki Forest virus (SFV) RNA polymerase, were localized in infected cells by confocal microscopy using double labeling with specific antisera against the individual ns proteins. All ns proteins were associated with large cytoplasmic vacuoles (CPV), the inner surfaces of which were covered by small invaginations, or spherules, typical of alphavirus infection. All ns proteins were localized by immuno-electron microscopy (EM) to the limiting membranes of CPV and to the spherules, together with newly labeled viral RNA. Along with earlier observations by EM-autoradiography (P. M. Grimley, I. K. Berezesky, and R. M. Friedman, J. Virol. 2:326-338, 1968), these results suggest that individual spherules represent template-associated RNA polymerase complexes. Immunoprecipitation of radiolabeled ns proteins showed that each antiserum precipitated the other three ns proteins, implying that they functioned as a complex. Double labeling with organelle-specific and anti-ns-protein antisera showed that CPV were derivatives of late endosomes and lysosomes. Indeed, CPV frequently contained endocytosed bovine serum albumin-coated gold particles, introduced into the medium at different times after infection. With time, increasing numbers of spherules were also observed on the cell surfaces; they were occasionally released into the medium, probably by secretory lysosomes. We suggest that the spherules arise by primary assembly of the RNA replication complexes at the plasma membrane, guided there by nsP1, which has affinity to lipids specific for the cytoplasmic leaflet of the plasma membrane. Endosomal recycling and fusion of CPV with the plasma membrane can circulate spherules between the plasma membrane and the endosomal-lysosomal compartment.

Replicase complex genes of Semliki Forest virus confer lethal neurovirulence

Semliki Forest virus (SFV) is a mosquito-transmitted pathogen of small rodents, and infection of adult mice with SFV4, a neurovirulent strain of SFV, leads to lethal encephalitis in a few days, whereas mice infected with the avirulent A7(74) strain remain asymptomatic. In adult neurons, A7(74) is unable to form virions and hence does not reach a critical threshold of neuronal damage. To elucidate the molecular mechanisms of neurovirulence, we have cloned and sequenced the entire 11,758-nucleotide genome of A7(74) and compared it to the highly neurovirulent SFV4 virus. We found several sequence differences and sought to localize determinants conferring the neuropathogenicity by using a panel of chimeras between SFV4 and a cloned recombinant, rA774. We first localized virulence determinants in the nonstructural region by showing that rA774 structural genes combined with the SFV4 nonstructural genome produced a highly virulent virus, while a reciprocal recombinant was asymptomatic. In addition to several amino acid mutations in the nonstructural region, the nsp3 gene of rA774 displayed an opal termination codon and an in-frame 21-nucleotide deletion close to the nsp4 junction. Replacement in rA774 of the entire nsp3 gene with that of SFV4 reconstituted the virulent phenotype, whereas an arginine at the opal position significantly increased virulence, leading to clinical symptoms in mice. Completion of the nsp3 deletion in rA774 did not increase virulence. We conclude that the opal codon and amino acid mutations other than the deleted residues are mainly responsible for the attenuation of A7(74) and that the attenuating determinants reside entirely in the nonstructural region.

Effects of mutations in the rubella virus E1 glycoprotein on E1-E2 interaction and membrane fusion activity

Rubella virus (RV) virions contain two glycosylated membrane proteins, E1 and E2, that exist as a heterodimer and form the viral spike complexes on the virion surface. Formation of an E1-E2 heterodimer is required for transport of E1 out of the endoplasmic reticulum lumen to the Golgi apparatus and plasma membrane. To investigate the nature of the E1-E2 interaction, we have introduced mutations in the internal hydrophobic region (residues 81 to 109) of E1. Substitution of serine at Cys82 (mutant C82S) or deletion of this hydrophobic domain (mutant dt) of E1 resulted in a disruption of the E1 conformation that ultimately affected E1-E2 heterodimer formation and cell surface expression of both E1 and E2. Substitution of either aspartic acid at Gly93 (G93D) or glycine at Pro104 (P104G) was found to impair neither E1-E2 heterodimer formation nor the transport of E1 and E2 to the cell surface. Fusion of RV-infected cells is induced by a brief treatment at a pH below 6. 0. To test whether this internal hydrophobic domain is involved in the membrane fusion activity of RV, transformed BHK cell lines expressing either wild-type or mutant spike proteins were exposed to an acidic pH and polykaryon formation was measured. No fusion activity was observed in the C82S, dt, and G93D mutants; however, the wild type and the P104G mutant exhibited fusogenic activities, with greater than 60% and 20 to 40% of the cells being fused, respectively, at pH 4.8. These results suggest that it is likely that the region of E1 between amino acids 81 and 109 is involved in the membrane fusion activity of RV and that it may be important for the interaction of that protein with E2 to form the E1-E2 heterodimer.

Alphaviruses induce apoptosis in Bcl-2-overexpressing cells: evidence for a caspase-mediated, proteolytic inactivation of Bcl-2

Bcl-2 oncogene expression plays a role in the establishment of persistent viral infection by blocking virus-induced apoptosis. This might be achieved by preventing virus-induced activation of caspase-3, an IL-1beta-converting enzyme (ICE)-like cysteine protease that has been implicated in the death effector phase of apoptosis. Contrary to this model, we show that three cell types highly overexpressing functional Bcl-2 displayed caspase-3 activation and underwent apoptosis in response to infection with alphaviruses Semliki Forest and Sindbis as efficiently as vector control counterparts. In all three cell types, overexpressed 26 kDa Bcl-2 was cleaved into a 23 kDa protein. Antibody epitope mapping revealed that cleavage occurred at one or two target sites for caspases within the amino acid region YEWD31 (downward arrow) AGD34 (downward arrow) A, removing the N-terminal BH4 region known to be essential for the death-protective activity of Bcl-2. Preincubation of cells with the caspase inhibitor Z-VAD prevented Bcl-2 cleavage and partially restored the protective activity of Bcl-2 against virus-induced apoptosis. Moreover, a murine Bcl-2 mutant having Asp31, Asp34 and Asp36 substituted by Glu was resistant to proteolytic cleavage and abrogated apoptosis following virus infection. These findings indicate that alphaviruses can trigger a caspase-mediated inactivation of Bcl-2 in order to evade the death protection imposed by this survival factor.

Distribution of Mayaro virus RNA in polysomes during heat shock

Mayaro virus (alphavirus) infection of Aedes albopictus cells results in inhibition of cell protein synthesis and viral proteins are preferably synthesized. When infected cells are heat shocked, however, there is also an inhibition of viral protein synthesis, and there is preferential synthesis of heat shock proteins. Based on these observations, the distribution of Mayaro viral RNA in polysomes and the association of p34 (capsid protein) with ribosomal fractions of the cells under such conditions have been analyzed. During infection, the viral RNA is mainly observed in light polysomes (60% of total viral RNA in the cell) and also in heavy polysomes (13%). However, when infected cells are heat-shocked, the viral RNA is strongly mobilized from heavy polysomes to the light polysomes fraction and an enrichment in the unbound fraction can be noticed. The amount of p34 associated with the ribosomal fraction was also shown to be decreased in the heat shocked cells. These data lead to the suggestion that two mechanisms could be involved in the inhibition of Mayaro virus protein synthesis in response to heat shock: (1) mobilization of Mayaro virus RNA from heavy to light polysomes; (2) a decrease in the amount of the p34 within the ribosomal fraction.

Semliki Forest virus capsid protein inhibits the initiation of translation by upregulating the double-stranded RNA-activated protein kinase (PKR)

We investigated the possible translational role which elevated concentrations of highly purified Semliki Forest virus (SFV) capsid (C)-protein molecules may play in a cell-free translation system. Here we demonstrate that in the absence of double-stranded RNA high concentrations of C protein triggered the phosphorylation of the interferon-induced, double-stranded RNA-activated protein kinase, PKR. Activated PKR in turn phosphorylated its natural substrate, the alpha subunit of eukaryotic initiation factor 2 (eIF-2), thereby inhibiting initiation of host cell translation. These findings were further strengthened by experiments showing that during natural infection with SFV the maximum phosphorylation of PKR coincided with the maximum synthesis of C protein 4-9 hours post infection. Thus, our results demonstrate that high concentrations of C-protein molecules may act in a hitherto novel mechanism on PKR to inhibit host cell protein synthesis during viral infection.

Analysis of the molecular basis of neuropathogenesis of RNA viruses in experimental animals: relevance for human disease?

RNA viruses with segmented genomes were the first model used for molecular analysis of viral neuropathogenesis, since they could be analysed genetically by reassortment. Four viruses with non-segmented genomes have been used as models of neurovirulence and demyelinating disease: JHM coronavirus, Theiler's virus, Sindbis virus and Semliki Forest virus (SFV). Virus gene expression in the central nervous system of infected animals has been measured by in situ hybridization and immunocytochemistry. Cell tropism has been analysed by neural cell culture. Infectious clones have been constructed for Theiler's virus, Sindbis virus and SFV, and these allow analysis of the sequences involved in the determination of neuropathogenesis, through the construction of chimeric viruses and site-specific mutagenesis. Measles and rubella viruses have been studied in animal systems because of their importance for human disease. The importance of two recently discovered mechanisms of neuropathogenesis, antibody-induced modulation of virus multiplication, and persistence of virus in the absence of multiplication, remains to be assessed.

Genetic evidence that epizootic Venezuelan equine encephalitis (VEE) viruses may have evolved from enzootic VEE subtype I-D virus

An important question pertaining to the natural history of Venezuelan equine encephalitis (VEE) virus concerns the source of epizootic, equine-virulent strains. An endemic source of epizootic virus has not been identified, despite intensive surveillance. One of the theories of epizootic strain origin is that epizootic VEE viruses evolve from enzootic strains. Likely enzootic sources of VEE virus occur in Colombia and Venezuela where many of the epizootic outbreaks of VEE have occurred. We have determined the nucleotide sequences of the entire genomes of epizootic VEE subtype I-C virus, strain P676, isolated in Venezuela, and of enzootic VEE subtype I-D virus, strain 3880, isolated in Panama. VEE subtype I-D viruses are maintained in enzootic foci in Panama, Colombia, and Venezuela. The genomes of P676 and 3880 viruses differ from that of VEE subtype I-AB virus, strain Trinidad donkey (TRD), by 417 (3.6%) and 619 (5.4%) nucleotides, respectively. The translated regions of P676 and 3880 genomes differ from those of TRD virus by 54 (1.4%) and 66 (1.8%) amino acids, respectively. This study and the oligonucleotide fingerprint analyses of South American I-C and I-D viruses (Rico-Hesse, Roehrig, Trent, and Dickerman, 1988, Am. J. Trop. Med. Hyg. 38, 187-194) provide the most conclusive evidence to date suggesting that equine-virulent strains of VEE virus arise naturally from minor variants present in populations of I-D VEE virus maintained in enzootic foci in northern South America.

Effect of heat shock on gene expression of Aedes albopictus cells infected with Mayaro virus

Three major Mayaro virus proteins of 62, 50 and 34 kDa were detected in Aedes albopictus cells after 48 h postinfection at 28 degrees C. When the infected cells were shifted from 28 to 37 degrees C for 90 min (heat shock conditions), the synthesis of two major heat shock proteins (HSP) 82 and 70 kDa was induced concomitantly with strong inhibition of virus and normal protein synthesis. Total cellular RNA was isolated from mock and infected cells incubated at 28 degrees C or under heat shock. Northern blot analysis with HSP genomic probes from Drosophila sp showed that (1) the probe for HSP 82 hybridized with an RNA of 2.6 kb present only in heat-shocked cells, (2) the HSP 70 probe hybridized with RNA species of 2.5 kb, present only in RNA from heat-shocked cells. These results showed that Mayaro virus was not able to alter the reprogrammation of gene expression induced by heat shock in A. albopictus cells.

The full-length nucleotide sequences of the virulent Trinidad donkey strain of Venezuelan equine encephalitis virus and its attenuated vaccine derivative, strain TC-83

Nucleotide sequence analysis of cDNA clones covering the entire genomes of Trinidad donkey (TRD) Venezuelan equine encephalitis (VEE) virus and its vaccine derivative, TC-83, has revealed 11 differences between the genomes of TC-83 virus and its parent. One nucleotide substitution and a single nucleotide deletion occurred in the 5'- and 3'-noncoding regions of the TC-83 genome, respectively. The deduced amino acid sequences of the nonstructural polypeptides of the two viruses differed only in a conservative Ser(TRD) to Thr(TC-83) substitution in nonstructural protein (nsP) three at amino acid position 260. The two silent mutations (one each in E1 and E2), one amino acid substitution in the E1 glycoprotein, and five substitutions in the E2 envelope glycoprotein of TC-83 virus were reported previously (B.J.B. Johnson, R.M. Kinney, C.L. Kost, and D.W. Trent, 1986, J. Gen. Virol. 67, 1951-1960). The genome of TRD virus was 11,444 nucleotides long with a 5'-noncoding region of 44 nucleotides. The carboxyl terminal portion of VEE nsP3 contained two peptide segments (7 and 34 amino acids long) that were repeated with high fidelity. The open reading frame of the nonstructural polyprotein was interrupted by an in-frame opal termination codon between nsP3 and nsP4, as has been reported for Sindbis, Ross River, and Middelburg viruses. The deduced amino acid sequences of the VEE TRD nsP1, nsP2, nsP3, and nsP4 polypeptides showed 60-66%, 57-58%, 35-44%, and 73-71% identity with the aligned sequences of the cognate polypeptides of Sindbis and Semliki Forest viruses, respectively. The lack of homology in the nsP3 of the viruses is due to sequence variation in the carboxyl terminal half of this polypeptide.

Investigation of the role of glycans for the biological activity of Semliki Forest virus grown in Aedes albopictus cells using inhibitors of asparagine-linked oligosaccharides trimming

The effects of N-linked-oligosaccharide-processing inhibitors on the formation of Semliki Forest virus (SFV) in C6/36 Aedes albopictus cells were investigated. The glycosidase inhibitors deoxynojirimycin, deoxymannojirimycin and swainsonine prevented the formation of Endo-H resistant structures, but had little effect on virus formation and on the biological activities of the virus. Tunicamycin greatly inhibited virus formation, but had little effect on cell-cell fusion from within and the cleavage of p 62. These results indicate that correct glycosylation is not a prerequisite for biological activities of SFV, whereas glycosylation per se is needed for virus production.

Time course of virus-specific macromolecular synthesis during rubella virus infection in Vero cells

Virus specific macromolecular synthesis was studied in Vero cells infected with plaque-purified rubella virus under one-step multiplication conditions. Under these conditions, the rate of virus production was found to increase rapidly until 24 hr postinfection after which time the rate of virus production rose more slowly, reaching a peak level at 48 hr postinfection. This peak rate of virus production was maintained through 72 hr postinfection. A majority of the cells remained alive through 96 hr postinfection, although a 20 to 30% decrease in the number of living cells occurred between 24 and 48 hr postinfection, the time period at which cytopathic effect was first observed. The virus structural proteins were first detected intracellularly at 16 hr postinfection. The rate of synthesis of these proteins was already maximal at 16 hr postinfection and remained constant through 48 hr postinfection. By immunofluorescence, cells expressing virus proteins were first observed at 12 hr postinfection. At 24 hr postinfection, 35 to 50% of the cells in the infected culture were exhibiting immunofluorescence, at 36 hr postinfection, 65 to 90% of the cells were exhibiting immunofluorescence, and at 48 hr postinfection, all of the cells were exhibiting immunofluorescence. The virus genomic and subgenomic RNA species were first detectable by 12 hr postinfection. The rate of synthesis of both of these species peaked at 26 hr postinfection. Rubella virus infection was found to have no effect on total cell RNA synthesis. However, a modest inhibition of total cell protein synthesis which reached 40% by 48 hr postinfection was observed. When Northern analysis of RNA extracted from infected cells was performed, a negative-polarity, virus-specific RNA probe hybridized only to the virus genomic and subgenomic RNA species. A positive-polarity, virus-specific RNA probe hybridized predominantly to a negative-polarity RNA of genome length indicating that both the genomic and subgenomic RNAs are synthesized from a genome-length negative-polarity template. Defective interfering (DI) RNAs were not detected in infected cells through 96 hr postinfection or in cells onto which virus released through 96 hr postinfection was passaged. Thus, the generation of DI particles by rubella virus appears to play no role in the slow, noncytopathic replication of this virus or in the ability of rubella virus-infected cells to survive for extended periods of time.

Nucleotide sequence of the 26 S mRNA of the virulent Trinidad donkey strain of Venezuelan equine encephalitis virus and deduced sequence of the encoded structural proteins

A cDNA clone containing all of the 26 S mRNA coding region of the RNA genome of Venezuelan equine encephalitis (VEE) virus, virulent strain Trinidad donkey (TRD), has been constructed and sequenced. The nucleotide and deduced amino acid sequences of the 26 S RNA of VEE virus conform to the general organization of the alphavirus subgenomic mRNA. Excluding the poly(A) tail, the VEE 26 S RNA is 3913 nucleotides long with a protein coding region of 3762 nucleotides. Codon usage in the translated region is nonrandom and correlates well with that reported for Sindbis (SIN), Semliki Forest (SF), and Ross River (RR) alphaviruses. Highly conserved sequences of 19 to 22 nucleotides representing putative replicase recognition sites occur at the 26 S RNA junction region of the 42 S genomic RNA and at the 3' terminus immediately preceding the poly(A) tail. The conserved sequence at the 26 S/42 S junction region of VEE virus differs from that of other alphaviruses in that an ochre termination codon (UAA) is substituted for a GGU (Gly) codon present in the other viruses. The 5' and 3' noncoding regions (30 and 121 nucleotides, respectively) of the VEE 26 S RNA are shorter than has been reported for several other alphaviruses. The approximate transmembrane domains of the VEE E1 and E2 envelope glycoproteins have been identified. VEE E1 contains a single asparagine-linked glycosylation site, whereas E2 has three such sites, all of which are apparently glycosylated. The deduced amino acid sequence of the VEE polyprotein shows an overall homology of 44 to 46% with the precursor polyproteins of SIN, SF, and RR viruses. VEE virus capsid, E1, and E2 structural proteins show 43 to 46%, 50 to 53%, and 36 to 41% homology, respectively, with the cognate proteins of SIN, SF, and RR viruses.

Effect of monensin on the assembly of Uukuniemi virus in the Golgi complex

The effect of the carboxylic ionophore monensin on the maturation of Uukuniemi virus, a bunyavirus, and the transport of its two membrane glycoproteins, G1 and G2, were studied in chicken embryo fibroblasts and baby hamster kidney cells. Virus maturation, which occurs in the Golgi complex (E. Kuismanen, K. Hedman, J. Saraste, and R. F. Pettersson, Mol. Cell. Biol. 2:1444-1458, 1982; E. Kuismanen, B. Bång, M. Hurme, and R. F. Pettersson, J. Virol. 51:137-146, 1984), was effectively inhibited by the drug (1 or 10 microM) as studied by electron microscopy and by assaying the release of infectious or radiolabeled virus. Immunoelectron microscopy showed that association of viral nucleocapsids with the cytoplasmic surface of glycoprotein-containing Golgi membranes, a prerequisite for virus budding, was unaffected by monensin. In the presence of the drug, the virus glycoproteins assembled into long, tubular structures extending into the lumen of Golgi-derived vacuoles, suggesting that monensin inhibited a terminal step in the assembly of the virus. Intracellular transport and expression of the virus membrane glycoproteins G1 and G2 at the cell surface were not inhibited by monensin as studied by immunocytochemical and radiolabeling techniques. Pulse-chase experiments in the presence of monensin showed that intracellular G1 acquired only partially endo-H-resistant glycans. The sialylation of G1 appearing on the cell surface in the presence of the drug was decreased, whereas sialylation of G2 apparently was inhibited to a lesser extent, as shown by external labeling of the cells with the periodate-boro[3H]hydride method. Thus, monensin exerted a differential effect on the terminal glycosylation of G1 and G2. Unlike several membrane and secretory glycoproteins, both G1 and G2 could enter a functional transport pathway in the presence of monensin and become expressed at the cell surface.

Rubella virus RNA: effect of high multiplicity passage

Evidence for the amplification of defective interfering particles of rubella virus after passage at high multiplicity has been obtained. The process is associated with the production of subgenomic rubella RNA species.

Posttranslational processing of Uukuniemi virus glycoproteins G1 and G2

Uukuniemi virus, which matures specifically in the Golgi complex, contains two species of envelope glycoproteins, G1 (Mr, 70,000) and G2 (Mr, 65,000). These are translated as a polyprotein, p110, from an mRNA which is complementary to the medium-sized segment of the virion RNAs. By synchronized initiation of protein synthesis and pulse-labeling, it was shown that glycoprotein G1 is amino terminal in precursor protein p110. Apparently, the nonglycosylated forms of these proteins (Mr, 54,000 to 57,000), synthesized in the presence of tunicamycin, comigrate in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, because a similar-sized protein could be isolated by immunoprecipitation with monoclonal antibodies directed against either G1 or G2. The G1 protein, which in the virion contains exclusively endoglycosidase H (endo H)-resistant glycans, was converted to the endo H-resistant form in a half time of about 45 min. The G2 protein, which in the virion has a heterogeneous glycosylation pattern as revealed by endo H digestion, attained this partial endo H resistance only after 90 to 150 min of chase. The transport time of Uukuniemi virus glycoproteins from the endoplasmic reticulum to the Golgi complex was considerably longer than that for alpha and rhabdovirus glycoproteins. Determination of the transport time of G1 and G2 to extracellular virions revealed that G1 is incorporated into mature virions about 10 min faster than G2, suggesting that G1 and G2 are transported with different kinetics to the site of virus maturation.

Infection of neuroblastoma cells by Semliki Forest virus. The interference of viral capsid protein with the binding of host messenger RNAs into initiation complexes is the cause of the shut-off of host protein synthesis

From ribosomal washes of neuroblastoma cells infected with Semliki Forest virus (SFV) a protein of Mr 33000 was purified, which comigrated with the viral capsid protein on sodium dodecyl sulfate/polyacrylamide gels and was recognized by antibodies against the capsid protein of SFV. This protein selectively inhibits the translation of host and early viral 42S mRNA in vitro, but has no effect on late viral 26S and encephalomyocarditis virus mRNA translation. Eukaryotic initiation factor 4B and cap-binding protein restore the translation of host and 42S mRNA to control levels. The capsid protein specifically prevents the binding of host mRNA into 80S initiation complexes, but has no effect on that of late viral mRNA. We propose that the capsid protein is the component responsible for the shut-off of host protein synthesis in SFV-infected cells and for the decreased translational activity of the crude ribosomal washes from these cells.

Subcellular fractions in rubella immunoassays

Rubella virus-infected cells were fractionated by differential and sucrose gradient centrifugations. Rubella virus antigens distributed into all fractions but particulate material in the 100,000 x g pellet was shown to be enriched about two-fold for rubella virus antigen. Similarly, sucrose gradient fractions for rough endoplasmic reticulum and smooth cellular membranes were enriched for rubella virus antigens. The 100,000 x g pellet and the isolated cellular membranes proved to be useful when different fractions were used in solid-phase immunoassays for rubella virus-specific IgG or IgM. These fractions were equal in quality of the semipurified rubella virus preparations in the IgG assays but inferior to those in the IgM assays. However, simultaneous use of 35/25% sucrose fractions from infected and non-infected cells reveals non-specific binding of IgM to the antigens and renders the IgM tests more specific for rubella virus.

Rubella virus 40S genome RNA specifies a 24S subgenomic mRNA that codes for a precursor to structural proteins

We have analyzed the structure of the rubella virus genome RNA and the virus-specific RNA species synthesized in B-Vero cells infected with rubella virus. A single-stranded, capped, and polyadenylated RNA species sedimenting at 40S in a sucrose gradient was released from purified virions treated with sodium dodecyl sulfate. This RNA species migrated with an Mr of about 3.8 X 10(6) in an agarose gel after denaturation with glyoxal and dimethyl sulfoxide. Infected cells labeled with [3H]uridine in the presence of actinomycin D contained, in addition to the 40S RNA, a single-stranded polyadenylated 24S RNA species as shown by sucrose gradient analysis. In a Northern blot analysis, this RNA hybridized to a cDNA probe derived from the 3' portion of the genomic 40S RNA. In vitro translation of the 24S RNA species yielded a 110,000-dalton polypeptide, in addition to some smaller products which were immunoprecipitated with an antiserum prepared against the structural proteins E1, E2a, E2b, and C. Since the sum of the molecular weights of the nonglycosylated envelope proteins and the capsid protein has been estimated to be about 116,000 (C. Oker-Blom et al., J. Virol. 46:964-973, 1983), these results suggest that the 24S RNA species represents a subgenomic mRNA coding for a precursor (p110) to the structural proteins of rubella virus. Thus, the strategy of gene expression of rubella virus appears to be similar to that of the alphaviruses.

Establishment and analysis of a system which allows assembly and disassembly of alphavirus core-like particles under physiological conditions in vitro

Core-like (CL) particles which closely resemble alphavirus cores in size, shape, and relative amount of nucleic acid and protein have been assembled in vitro from Sindbis (SIN) virus core (C) protein and single-stranded nucleic acids in buffer containing 1 M urea [G. Wengler, U. Boege, G. Wengler, H. Bischoff, and K. Wahn (1982) Virology 118, 401-410]. We have now analyzed the interaction of SIN virus C protein and nucleic acids in vitro under conditions designed to resemble those present in the cell during core assembly. In buffer containing 100 mM K-acetate, 1.7 mM Mg-acetate, pH 7.4, CL particles are efficiently assembled from all single-stranded nucleic acids analyzed, and even heparin and polyvinylsulfate are incorporated into such particles. A reticulocyte lysate translates SIN virus-specific mRNA into C protein under these ionic conditions. Interactions of C protein with nucleic acids and ribosomes in a reticulocyte lysate have also been analyzed. The following conclusions can be drawn from these analyses: (1) In accordance with earlier findings [N. Glanville and I. Ulmanen (1976) Biochem. Biophys. Res. Commun. 71, 393-399] the C protein translated in vitro efficiently binds to ribosomes. (2) Exogenously added C protein binds to the large subunit of the ribosomes in the lysate. (3) CL particles can be assembled in the lysate from exogenous added 42 S genome RNA and exogenous added C protein if both components are present at sufficiently high concentrations. (4) The C protein translated from viral mRNA in the lysate is transferred from the ribosomes into preassembled CL particles containing 42 S RNA in the lysate. (5) If only small amounts of CL particles are added into a lysate these particles disaggregate and core protein molecules are transferred from the particles to the large subunit of the ribosomes. The results on the assembly of CL particles in vitro allow the formulation of some hypotheses concerning the assembly and disassembly of core particles in vivo.

Semliki Forest virus: a probe for membrane traffic in the animal cell

The traffic among the cellular compartments is thought to be mediated by membrane vesicles, which bud from one compartment and fuse with the next. Despite the continuous exchange of membrane components among them, the organelles maintain their characteristic protein and lipid compositions such that the traffic remains selective, thus, avoiding intermixing of components. This membrane traffic recycles components from the cell surface to the interior of the cell and back to the cell surface again. The membrane traffic between the ER and the cell surface involves a major sorting problem. Little is known of how the animal cell has solved this problem in molecular terms. One experimental tool in this direction is provided by some enveloped animal viruses, which mature at the cell surface of infected cells. Such viruses include influenza virus, Semliki Forest virus (SFV), Sindbis virus, and vesicular stomatitis virus (VSV). They are extremely simple in makeup and hence are very well characterized. The purpose of this article is to illustrate the use of the enveloped viruses as tools in the study of membrane traffic in the animal cell. This is done in the context of the life cycle of the virus in the host cell. The article will be concerned mainly with Semliki Forest virus (SFV), which is the virus that has been worked upon in the chapter. SFV belongs to the alphaviruses, a genus of the togavirus family.

Ribosome topography in baby hamster kidney cells infected with Sindbis and vesicular stomatitis viruses

The topography of polysomal ribosomes in mock-infected and in Sindbis virus- and vesicular stomatitis virus-infected BHK cells was investigated using a double, radioactive labelling technique. Ribosomal proteins in intact polysomes were surface labelled by reductive methylation using [14C]formaldehyde. Following removal of ribosomal RNA, proteins were denatured in 6 M guanidine and labelled with [3H]borohydride. Labelled ribosomal proteins were separated by electrophoresis in two-dimensional gels and the 3H/14C ratio for each ribosomal protein was taken as an index of its relative surface exposure in intact ribosomes. Comparison of the ratios for individual ribosomal proteins in Sindbis virus-infected vs. control polysomes indicated that proteins L7, L8, L17, L26 and S19 became more 'buried' and others such as L4, L29, L36, S2 and S26 became more 'exposed' in infected cells. Most of the topographical alterations occurred in the large ribosomal subunit. In contrast, infection of BHK cells with vesicular stomatitis virus induced little or no topographical alteration.

Identification of a unique guanine-7-methyltransferase in Semliki Forest virus (SFV) infected cell extracts

The methylation of the 5' terminal guanosine residue of the cap structure of Semliki Forest virus (SFV) mRNAs has been shown to occur in vitro concomitantly with their synthesis (R. K. Cross and P. J. Gomatos, Virology, 114, 542-554, 1981). The enzyme responsible for this methylation, a guanine-7-methyltransferase, is associated with the SFV replication complex which contains both the virus-specified polymerase and RNA template in a mitochondrial pellet fraction, P-15, from infected cell lysates. In the present report, evidence has been obtained demonstrating that a virus-specified function is required for this methylating activity. First, the methyltransferase enzyme in these infected P-15 extracts has been found to differ in substrate specificity from that of the BHK host cell enzyme. This enzyme was able to catalyze the methylation of GTP to m7GTP in vitro whereas the cellular enzyme could not methylate GTP. The incorporation of a methyl group onto GTP occurred linearly for at least 2 hr at 30 degrees under conditions of neutral pH and added GTP substrate. Second, a study of the kinetics of appearance of this activity, has demonstrated that the capacity to methylate GTP did not appear until 1 hr after infection and reached maximal levels by about 3 hr. Third, de novo protein synthesis was required. Addition of the protein synthesis inhibitor, cycloheximide, prevented the appearance and subsequent increase in the methylating activity. However, once formed the methyltransferase was found to be stable for at least 3 hr. These results suggest that an early viral function, perhaps a nonstructural polypeptide is required for this novel guanine-7-methyltransferase activity in SFV infected cell extracts.

Rubella virus contains one capsid protein and three envelope glycoproteins, E1, E2a, and E2b

We have analyzed the structure of rubella virus proteins labeled metabolically with [35S]methionine, [3H]mannose, and [3H]glucosamine or externally with [3H]borohydride after galactose oxidase treatment. Four structural proteins, with MrS of about 58,000 (E1), 47,000 (E2a), 42,000 (E2b), and 33,000 (C), were resolved on sodium dodecyl sulfate-polyacrylamide gels. Tryptic peptide maps obtained from [35S]methionine-labeled proteins indicated that E1 and C were unrelated to each other and to E2a and E2b, whereas the latter two gave similar, if not identical, maps. E1, E2a, and E2b were associated with the envelope and were located externally on the virus particle, whereas the C protein was associated with the RNA in the nucleocapsid. Solubilization of the virus with Triton X-100, followed by removal of the nucleocapsid and the detergent, resulted in the formation of soluble envelope protein complexes (rosettes) containing E1, E2a, and E2b. Although external labeling with [3H]borohydride and metabolic labeling with [3H]glucosamine suggested that all three proteins were glycosylated, only E1 and E2b were efficiently labeled with [3H]mannose. It is thus possible that the difference in migration between E2a and E2b is due to differences in glycosylation. Analysis by immunoprecipitation and sodium dodecyl sulfate-gel electrophoresis of intracellular [35S]methionine-labeled structural proteins synthesized in the presence and absence of tunicamycin supported the conclusion that E1 and E2 are glycoproteins. Unglycosylated E1 and E2 had an Mr of about 53,000 and 30,000, respectively.

Uukuniemi virus maturation: accumulation of virus particles and viral antigens in the Golgi complex

We studied the maturation of Uukuniemi virus and the localization of the viral surface glycoproteins and nucleocapsid protein in infected cells by electron microscopy, indirect immunofluorescence, and immunoelectron microscopy with specific antisera prepared in rabbits against the two glycoproteins G1 and G2 and the nucleocapsid protein N. Electron microscopy of thin sections from infected cells showed virus particles maturing at smooth-surfaced membranes close to the nucleus. Localization of the G1/G2 and N proteins by indirect immunofluorescence at different stages after infection showed the antigens to be present throughout the cell interior but concentrated in the juxtanuclear region. The G1/G2 antiserum also appeared to stain the nuclear and plasma membranes. Double staining with tetramethylrhodamine isothiocyanate-conjugated wheat germ agglutinin, which preferentially stains the Golgi complex, and fluorescein isothiocyanate-conjugated anti-rabbit immunoglobulin G, which stained the G1/G2 or N proteins, showed that the staining of the juxtanuclear region coincided. Similarly, double staining for thiamine pyrophosphatase, an enzyme activity specific for the Golgi complex, showed the fluorescence and the cytochemical stain to coincide in the juxtanuclear region. Immunoperoxidase electron microscopy of cells permeabilized with saponin revealed that the viral glycoproteins were present in the rough endoplasmic reticulum and the nuclear and Golgi membranes; the latter was heavily stained. With this method, the N protein was localized to the cytoplasm, especially around smooth-surfaced vesicles in the Golgi region. Taken together, the results indicate that Uukuniemi virus and its structural proteins accumulate in the Golgi complex, supporting the idea that this compartment rather than the plasma membrane is the site of virus maturation. This raises the interesting possibility that deficient transport of the glycoproteins to the plasma membrane and hence their accumulation in the Golgi complex determines the site of virus maturation.

Uukuniemi virus maturation: accumulation of virus particles and viral antigens in the Golgi complex

We studied the maturation of Uukuniemi virus and the localization of the viral surface glycoproteins and nucleocapsid protein in infected cells by electron microscopy, indirect immunofluorescence, and immunoelectron microscopy with specific antisera prepared in rabbits against the two glycoproteins G1 and G2 and the nucleocapsid protein N. Electron microscopy of thin sections from infected cells showed virus particles maturing at smooth-surfaced membranes close to the nucleus. Localization of the G1/G2 and N proteins by indirect immunofluorescence at different stages after infection showed the antigens to be present throughout the cell interior but concentrated in the juxtanuclear region. The G1/G2 antiserum also appeared to stain the nuclear and plasma membranes. Double staining with tetramethylrhodamine isothiocyanate-conjugated wheat germ agglutinin, which preferentially stains the Golgi complex, and fluorescein isothiocyanate-conjugated anti-rabbit immunoglobulin G, which stained the G1/G2 or N proteins, showed that the staining of the juxtanuclear region coincided. Similarly, double staining for thiamine pyrophosphatase, an enzyme activity specific for the Golgi complex, showed the fluorescence and the cytochemical stain to coincide in the juxtanuclear region. Immunoperoxidase electron microscopy of cells permeabilized with saponin revealed that the viral glycoproteins were present in the rough endoplasmic reticulum and the nuclear and Golgi membranes; the latter was heavily stained. With this method, the N protein was localized to the cytoplasm, especially around smooth-surfaced vesicles in the Golgi region. Taken together, the results indicate that Uukuniemi virus and its structural proteins accumulate in the Golgi complex, supporting the idea that this compartment rather than the plasma membrane is the site of virus maturation. This raises the interesting possibility that deficient transport of the glycoproteins to the plasma membrane and hence their accumulation in the Golgi complex determines the site of virus maturation.

Synthesis of Semliki-forest virus in polyamine-depleted baby-hamster kidney cells

The role of polyamines in macromolecular synthesis has been studied using the synthesis of Semliki-Forest virus (SF virus) in normal and alpha-difluoromethylornithine-treated baby-hamster kidney (BHK21) cells as a model system. The activities of ornithine decarboxylase and S-adenosylmethionine decarboxylase, the rate-limiting enzymes in polyamine biosynthesis, decreased rapidly in mock- and SF-virus-infected cells, indicating that virus production in BHK21 cells was not dependent on polyamines formed after infection. A prolonged treatment of BHK21 cells with alpha-difluoro-methylornithine, a specific inhibitor of polyamine synthesis, resulted in a marked inhibition of the initial rate of virus production, which appeared 72 h after the beginning of the treatment. This inhibition was reversed by putrescine, spermidine and spermine, and at last partially by several other diamines and polyamine homologues. Polyamine-depletion also markedly reduced viral RNA polymerase activity in SF-virus infected cells. Addition of spermidine to the culture medium rapidly increased viral RNA polymerase activity in the inhibitor-treated cells but had no effect on the enzyme activity when added directly to the assay mixture. The results indicated that polyamines are needed for maximum initial rate of SF-virus replication and suggest that the inhibition of virus production in polyamine-depleted cells is at least partly due to malfunction of the protein-synthetic machinery of the host cell.

Equine arteritis virus-infected cells contain six polyadenylated virus-specific RNAs

The kinetics of equine arteritis virus growth and virus-specific RNA synthesis at 40° were determined in BHK-21 cells. Maximum titers of infectious virus (∼10⁷ PFU/ml) were observed at 12 hr p.i., while incorporation of [³H]uridine into virus-specific RNA became detectable at 4 hr p.i. and increased to reach a maximum rate at 8 hr p.i. This RNA was labeled between 2.5 and 7 hr p.i. and isolated from infected cells. About 44% bound to oligo(dT)-cellulose; this material was denatured using glyoxal and dimethyl sulfoxide and analyzed by electrophoresis in a 1% agarose-urea gel. Six virus-specific RNA species were found having the following molecular weights: 4.3 × 10⁶ (RNA1), 1.3 × 10⁶ (RNA2), 0.9 × 10⁶ (RNA3), 0.7 × 10⁶ (RNA4), 0.3 × 10⁶ (RNA5), and 0.2 × 10⁶ (RNA6). RNA1 comigrated with the viral genome. Artifacts caused by defective interfering particles or breakdown of RNA were excluded. Subsequently, the target sizes of the templates for the synthesis of the genome-sized RNA and the five subgenomic RNAs were determined by uv transcription mapping. Infected cells were irradiated at 6.5 hr p.i. The effect o of reasing uv doses on the RNA synthesis was determined by quantitation of the individual RNAs after separation by agarose gel electrophoresis. The uv target sizes calculated for the templates for RNAs 2–5 were very close to the physical size of RNA1. The target size of the template of RNA6 was smaller (2.8 × 106 daltons), although much greater than its physical size. The data are consistent with a model in which the individual RNAs are derived from a larger precursor RNA molecule. The consequences of these findings for the taxonomy of Togaviridae are discussed.

Semliki forest virus neurovirulence mutants have altered cytopathogenicity for central nervous system cells

We have analyzed the pathogenicity and host range properties of four neurovirulence mutants of Semliki Forest virus which, unlike the wild type (WT), allow the survival of weanling mice injected intraperitoneally with 10(2) PFU. The mutant M9 showed a sustained multiplication in the brains of infected mice. It produced paralysis in 35%, and 8% died. Demyelination occurred in 94% of the surviving mice and was associated with the destruction of oligodendrocytes. All of the mutants showed a restricted ability to multiply in BHK, C1300 (neuroblastoma), and G26-24 (oligodendroglioma) cells as compared with the WT, and this was not associated with differential interferon production or action. C1300 cells infected with the mutants survived, whereas WT-infected cells were killed. In G26-24 cells all of the mutants and the WT produced a rapid cytopathic effect which was inhibited by pretreatment with 10 U of mouse interferon. Extensive RNA synthesis was detected for all of the mutants and the WT in BHK and C1300 cells, but it was only detectable in G26-24 cells in small amounts early in the infection. The mutant M4 had a defect in the nucleocapsid assembly, whereas M9 had a defect in total RNA synthesis. M136 was defective in the synthesis of 26S RNA, and M103 showed defective synthesis of viral core protein in C1300 cells. It is concluded that C1300 cells can tolerate viral RNA synthesis by a defective virus without showing a cytopathic effect, but the fully virulent WT virus is cytopathic. G26-24 cells are sensitive to small amounts of viral RNA synthesis. These properties of the WT and mutant viruses correlate with changes produced in the neurons and oligodendrocytes of the central nervous system: the virulence of the WT is due to its ability to destroy both neurons and oligodendrocytes, whereas the demyelination produced by the mutants M9 and M136 is due to the destruction of oligodendrocytes alone.

Structural complexity of defective-interfering RNAs of Semliki Forest virus as revealed by analysis of complementary DNA

The 18S defective interfering RNA of Semliki Forest virus has been reverse transcribed to cDNA, which was shown to be heterogeneous by restriction enzyme analysis. After transformation to E.coli, using pBR322 as a vector, two clones, pKTH301 and pKTH309 with inserts of 1.7 kb and 2 kb, were characterized, respectively. The restriction maps of the two clones were different but suggested that both contained repeating units. At the 3' terminus, pKTH301 had preserved 106 nucleotides and pKTH309 102 nucleotides from the 3' end of the viral 42S genome. The conserved 3' terminal sequence was joined to a different sequence in the two clones, and these sequences were not derived from the region coding for the viral structural proteins. The DI RNAs represented by the two clones are generated from the viral 42S RNA by several noncontinuous internal deletions, since the largest colinear regions with 42S RNA are 320 nucleotides in pKTH301, and 430 and 340 nucleotides in pKTH309. All these fragments had unique RNase T1 oligonucleotide fingerprints, suggesting that they were derived from different regions of 42S RNA.

Evidence for the presence of the minor capsid protein of Western equine encephalitis virus

A minor capsid protein was found in Western equine encephalitis virus. The minor capsid protein appeared to be produced by proteolytic cleavage of part of the newly synthesized capsid protein in infected cells and to be incorporated into nucleocapsids.