- Volume 77, Issue 9, 1996
Volume 77, Issue 9, 1996
- Review Article
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Reverse Transcriptase Jumps and Gaps
More LessIntroduction. Retroviruses are single-stranded RNA viruses of eukaryotes. Different subfamilies have been described. Avian or murine oncoviruses induce neoplasms, whereas lentiviruses, typified by human immunodeficiency virus and spumaviruses, produce persistent infections. Lentiviral infections may cause chronic disease whereas spumaviruses are apparently non-pathogenic. The retroviral life cycle is characterized by reverse transcription of their single-stranded plus (i.e. coding) RNA genome into a double-stranded DNA intermediate that integrates into the host genome. Over the last 25 years the mechanism of reverse transcription has been studied in great detail and these studies have led to the model shown in Fig. I. The result of reverse transcription is a linear double-stranded DNA molecule with a long terminal repeat (LTR) at each extremity. Synthesis of each DNA strand by the virus-encoded reverse transcriptase requires one template switch, also called a jump: the first jump is needed for synthesis of a minus DNA strand complementary to the viral RNA, the second jump for plus DNA strand synthesis.
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- Animal
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- RNA viruses
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Sodium Valproate, an Anticonvulsant Drug, Stimulates Human Immunodeficiency Virus Type 1 Replication Independently of Glutathione Levels
More LessSince modulation of the glutathione (GSH) level has been implicated in the regulation of human immuno-deficiency virus (HIV) transcription and expression, we have undertaken an analysis of the effect of sodium valproate (VPA) on HIV-1 replication. VPA, which is an anti-epileptic drug in widespread use in clinical medicine, has been shown to depress the activity of GSH reductase, an enzyme required for maintaining high cellular levels of reduced GSH. The effect of this drug on HIV-1 replication has been studied in primary infected cells, i.e. peripheral blood mononuclear cells (PBMC) and monocyte/macrophages, in the CEM-SS cell line, and in chronically infected stimulated and non-stimulated U1 cells. We have shown that VPA markedly enhanced viral replication in all infected cells tested. Virus production was induced in U1 cells by VPA treatment and the stimulatory effects of tumour necrosis factor-α, interleukin-6 and granulocyte/macrophage colony-stimulating factor were augmented. The LTR-driven gene expression in Jurkat T cells was increased. However, the elevated viral production did not correlate with the effect of VPA on the intracellular GSH level. Thus, VPA stimulated in vitro HIV-1 replication in acutely and chronically infected cells and enhanced LTR-driven gene expression. These effects were observed for concentrations that are reached in the plasma of VPA-treated patients. Therefore, although the clinical significance of these data remains to be demonstrated, these results should be considered in the choice of an anticonvulsant drug in HIV-infected individuals.
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Immunogenic Presentation of a Conserved gp41 Epitope of Human Immunodeficiency Virus Type 1 on Recombinant Surface Antigen of Hepatitis B Virus
In view of the high antigenic variability of human immunodeficiency virus type 1 (HIV-1), a vaccine against AIDS must induce an immune response to epitopes as invariable as possible among the various virus strains and clones. Previously the highly conserved six amino acid sequence Glu-Leu-Asp-Lys-Trp-Ala (ELDKWA) from gp41, defining the epitope of the human MAb 2F5, was shown to elicit HIV-1-neutralizing antibodies when presented on haemagglutinin of influenza virus. We investigated the immunogenic potential of the MAb 2F5 epitope and two of its major escape epitopes as internal fusions to the hepatitis B virus (HBV) surface antigen (HBsAg). Recombinant HBsAg-HIV proteins produced in the methylotrophic yeast Pichia pastoris self-assembled into 22 nm lipoprotein particles. Mice immunized with these particles developed an anti-HBsAg immune response in a range that is considered to be protective against HBV infection in humans. More importantly, antisera had extremely high titres of antibodies reactive with a structurally flexible form of the HIV-1 epitope, whereby strong cross-reactivity with the escape variants of the epitope was observed. Although HIV-1 gp160 and the ectodomain of gp41 containing the epitope were significantly recognized, the antisera failed to neutralize HIV-1 in vitro. These data, together with those on the haemagglutinin-ELDKWAS fusion suggest that the ability of the MAb 2F5 epitope to induce neutralizing antibodies depends on the molecular context in which it is presented. Therefore, further characterization of secondary and tertiary structure requirements of the epitope is indispensable for the full exploitation of its potential as a vaccine component.
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The Non-Producer Phenotype of the Human Immunodeficiency Virus Type 1 Provirus F12/HIV-1 is the Result of Multiple Genetic Variations
More LessA cell clone (Hut-78/F12) chronically infected with a non-producer human immunodeficiency virus type 1 (HIV-1) variant showed an abnormal pattern of virus structural proteins and released no detectable virus particles. Exchanges of homologous parts of the F12/HIV provirus and a replication-competent HIV (strain NL4-3) were undertaken to define the genetic determinants of the F12/HIV phenotype. The non-infectious phenotype was reproduced by replacing an NL4-3 genomic fragment encoding the C terminus of gp120 and the N terminus of gp41 with the corresponding parts of the F12/HIV provirus. Conversely, a much more extended genomic fragment (encompassing the vif, pol and env genes) was necessary to convert the F12/HIV phenotype. These results demonstrate that the F12/HIV non-producer phenotype is the result of mutations scattered along most of the genome, rendering the conversion to an infectious phenotype a very unlikely event. The F12/HIV genome is thus a reliable model for preclinical studies of anti-HIV gene therapy.
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Human Immunodeficiency Virus Grown in CD4-Expressing Cells is Associated with CD4
Using a CD4-capture immunoassay for gp120, several strains of human immunodeficiency virus type 1 (HIV-1) grown in CD4-expressing T lymphoblastoid cells were found to contain little CD4-reactive gp120 (0.3–1.0 ng/ml) relative to virus titre (103.2–105.0 TCID50/ml) and p24 antigen (80–1000 ng/ml). The measured CD4-reactive gp120 concentrations of HIV-1 suspensions grown in CD4-negative human neuroblastoma cells were 100- to 10000-fold greater than those of HIV-1 grown in CD4-positive lymphoblastoid cells, even though both virus suspensions contained abundant viral gp120 as shown by immunoblot assay. It was postulated that CD4 derived from host cells might be associated with virions, concealing the binding domains of gp120. CD4 association with HIV-1 virions grown in CD4-positive cells was demonstrated directly by immunoblot assay of sucrose gradient-purified virus suspensions and by specific co-sedimentation of 125I-labelled OKT4 with virions propagated in CD4-expressing cells. CD4 coating of primary HIV-1 isolates grown in peripheral blood mononuclear cells was also observed. The biological significance of CD4 coating of HIV particles remains to be determined.
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Probing the Conformation of the Human T-Lymphotropic Virus I Envelope Protein Complex with Monoclonal Antibodies
More LessWe are investigating the binding of a series of monoclonal antibodies to native and detergent-treated human T-lymphotropic virus I (HTLV-I) envelope proteins to explore their conformation. A comparison of our data with previously published findings suggests that a central neutralization domain (aa 175–200) is folded such that only short stretches are exposed at the surface of the native envelope protein complex. However, the complete domain becomes accessible after treatment with mild non-ionic detergents, suggesting that envelope subunit interaction may partially obscure this domain. We further provide immunochemical evidence that a region containing a heptad repeat in the extracellular part of the transmembrane protein is folded towards the interior of the HTLV-I envelope complex.
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Genetic Diversity of Argentine Isolates of Feline Immunodeficiency Virus
We report the nucleotide sequence and genetic diversity of part of the envelope (env) gene of four strains of feline immunodeficiency virus (FIV) isolated from Argentine domestic cats. The DNA encoding the V3 to V5 regions of the env gene of the FIV isolates were amplified by PCR, cloned and sequenced. Phylogenetic analysis revealed that the Argentine isolates did not cluster into a single group; one isolate clustered with subtype B FIV isolated in the USA and Japan, whereas the others formed a new cluster of FIV which might represent a prototype sequence for subtype E.
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Variations in Lentiviral Gene Expression in Monocyte-derived Macrophages from Naturally Infected Sheep
More LessSeventy-nine 1-year-old lambs from three individual farms and a feedlot were examined for natural lentivirus infection. We used three different methods to detect infection and to identify the stage of the ovine lentivirus life cycle in blood-derived macrophages. Cytopathic infectious virus was obtained from 14/14 Border Leicester animals obtained from a naturally infected flock. Neither virus particles, virus proteins, virus specific antibodies nor viral DNA were detected in samples from 34 lambs from two South Kansas City farms. However, among 31 feedlot lambs, we identified 11 infected animals. Specific viral proteins were immunoprecipitated from macrophages of one animal, but no infectious cytopathic virus was isolated from these cells. Cells from ten of the other feedlot animals harboured viral DNA but neither viral particles nor proteins could be detected by our techniques. Thus, in these naturally infected animals, the virus life cycle either proceeded to completion, subject to differentiation of infected precursor cells in blood, or remained arrested at the DNA stage despite maturation of monocytes to macrophages. Sequence analysis of the env gene of viral genomes from two of the ten feedlot sheep showed sequences distinct from those of known ovine and caprine lentiviruses. Surprisingly, these sequences have a higher identity (of nucleotide and derived amino acid sequences) to caprine arthritis-encephalitis virus than to the ovine prototype, maedi-visna virus. These data suggest that the ovine and caprine lentiviruses found in North American sheep may have a common ancestral genotype that is closely related to the caprine virus.
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Recombinant Baculovirus-Synthesized African Horsesickness Virus (AHSV) Outer-capsid Protein VP2 Provides Protection Against Virulent AHSV Challenge
More LessAfrican horsesickness virus serotype 4 (AHSV-4) outer-capsid proteins VP2 or VP2 and VP5, prepared from single or dual recombinant baculovirus expression vectors grown in Sf9 insect cells, were administered in different amounts to horses and the neutralizing antibody responses were measured. Control and vaccinated horses were challenged with virulent AHSV-4 6 months later and monitored post challenge. The results indicated that two inoculations of extracts containing VP2 and VP5, or VP2 alone, in doses of 5 µg VP2 or more per horse, were sufficient to elicit protection against African horsesickness (AHS) disease. The recombinant VP2 protein is a potential candidate vaccine for AHS in horses.
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Phosphorylation Generates Different Forms of Rotavirus NSP5
More LessNSP5 (non-structural protein 5) is one of two proteins encoded by genome segment 11 of group A rotaviruses. In virus-infected cells NSP5 accumulates in the virosomes and is found as two polypeptides with molecular masses of 26 and 28 kDa (26K and 28K proteins). NSP5 has been previously shown to be post-translationally modified by the addition of O-linked monosaccharide residues of N-acetylglucosamine and also by phosphorylation. We have now found that, as a consequence of phosphorylation, a complex modification process gives rise to previously unidentified forms of NSP5, with molecular masses of up to 34 kDa. Treatment with phosphatases of NSP5 obtained from virus-infected cells produced a single band of 26 kDa. NSP5 could be phosphorylated in vitro by incubation of immuno-precipitates with [γ-32P]ATP, producing mainly phosphorylated products of 28 and 32–34 kDa (32–34K). In both in vivo and in vitro phosphorylated NSP5, phosphates were only found attached via serine and threonine residues. The in vitro translated NSP5 precursor polypeptide, molecular mass 25 kDa (25K), could also be phosphorylated and transformed into a 28K protein by incubation with extracts obtained from virus-infected cells, but not from non-infected cells. In addition, NSP5 labelled in vivo with [1,6-3H]glucosamine showed mainly the presence of the 26K and 28K proteins (converted to 26K by protein phosphatase treatment) suggesting that the type of protein produced is regulated according to the level of phosphorylation and/or O-glycosylation. The results also suggest that NSP5 is autophosphorylated.
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Cell Culture Isolation of Piscine Neuropathy Nodavirus from Juvenile Sea Bass, Dicentrarchus labrax
More LessA virus causing a vacuolating encephalopathy and retinopathy in juvenile sea bass, Dicentrarchus labrax, was isolated from brain tissue in a fish cell line (SSN-1) derived from striped snakehead, Channa striatus. The isometric, non-enveloped, 30 nm diameter virus particles were resistant to pH 2–9 and heating at 56 °C for 30 min. Infectious particles had a buoyant density of approximately 1.31 g/cm3 in CsCl. Two structural polypeptides of molecular mass 40 and 42 kDa were identified and the ssRNA consisted of two fragments of molecular mass 1.10 and 0.51 × 106 Da. From these characteristics the virus was identified as a nodavirus. Due to the broad range of susceptible fish hosts and the consistent neuropathology of the disease condition, the generic term piscine neuropathy nodavirus (PNN) is proposed for this infectious agent.
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Heterogeneity of Hepatitis C Virus Genotype 2 Variants in West Central Africa (Guinea Conakry)
An overall anti-hepatitis C virus (HCV) prevalence of 6.7% was found in a sero-epidemiological study carried out in the town of Conakry (Guinea Conakry, West Central Africa) on 1421 subjects who were either blood donors, pregnant women or in- and outpatients receiving treatment for conditions other than liver disease. Seven HCV isolates from a subsample of 73 sterile sera from this population were studied for genetic characterization and classification. The 5′NCR was analysed by the Line Probe Assay. This method assigned the isolates to genotype 2. Analysis of the 5′NCR sequences alone was unable to give a more accurate classification. Comparison of NS5b region sequences (nucleotides 7575–8196), from Guinea isolates and genotype 2 database sequences, showed evolutionary distances in the range 0.15–0.26. There was a high level of subtype heterogeneity among the genotype 2 Guinea HCV isolates. Four of the subtypes were possibly new.
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Characterizationof the NTPase Activity of Japanese Encephalitis Virus NS3 Protein
More LessJapanese encephalitis (JE) virus NS3 protein and two N-terminally truncated (Δ1–148 and Δ1-323) forms of NS3 were engineered and expressed in E. coli as fusion proteins with a histidine tag at the N terminus. The purified recombinant proteins his-NS3 and his-NS3(Δ1–148) were found to possess NTPase activity which was stimulated by single-stranded RNA, whereas NS3(Δ1–323) did not. The requirements for MgCl2 and MnCl2 and the salt and pH ranges necessary for optimal activity of the enzyme were determined and shown to be slightly different from those of the NTPases of other flaviviruses. Poly(U) and poly(C) were better than poly(A) at stimulating the NTPase activities, in contrast to other flaviviral NTPases. The substrate preference was in the order GTP > ATP ≫ UTP > CTP. Interestingly, we found that Ca2+ could not substitute for Mg2+; on the contrary, it inhibited NTPase activity. The removal of the N-terminal 148 amino acids enhanced NTPase activity, but further deletion of the region (amino acids 148–323) completely abolished the activity. Therefore, amino acids 148–323 contain a critical region required for NTPase activity.
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A Mouse-attenuated Envelope Protein Variant of Murray Valley Encephalitis Virus with Altered Fusion Activity
More LessA neutralization escape variant of Murray Valley encephalitis virus (MVE), of low neuroinvasiveness in mice and with low haemagglutination activity, had a reduced rate of replication in cultured cells during the early phase of infection compared to wild-type MVE. The variant was internalized by Vero cells at a similar rate to wild-type MVE at pH 7.4, but had reduced pH-dependent membrane fusion activity. In fusion-from-within experiments in infected mosquito (C6/36) cells, the variant had a lowered pH threshold for induction of fusion, which occurred at a reduced rate and to a lesser extent than for wild-type virus. Fusion was inhibited by monoclonal antibodies specific for envelope protein epitopes E-5 and E-8, which were implicated as determinants of fusion. These observations are discussed in relation to the regulation of MVE replication by fusion of the viral envelope with endosome membranes and, in turn, how rates of replication may affect neuroinvasion.
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A porcine CD8+ T Cell Clone with Heterotypic Specificity for Foot-and-mouth Disease Virus
Foot-and-mouth disease virus (FMDV)-specific T cell lines and clones have been obtained from a swine lymphocyte antigen (SLA) inbred miniature pig vaccinated with chemically inactivated virus. One of the clones obtained, CE3, showed a specific and heterotypic proliferation against infectious but not inactivated FMDV in the presence of syngeneic peripheral blood mononuclear cells (PBMC). Adherent cells from PBMC were sufficient to support specific activation of the clone and the proliferation was abolished when the contact between CE3 and adherent cells was prevented. Phenotypic characterization of CE3 cells revealed expression of CD2, CD25 (interleukin-2 receptor), SLA class I and SLA class II. Furthermore, the cells were highly CD8 positive but showed low expression of CD4. The expression of T cell receptor (TCR) α and β genes confirmed their T cell nature. Consistent with the CD8 phenotype, the proliferative response of CE3 was inhibited with MAbs to SLA class I and CD8. Altogether, these results indicate that CE3 is a porcine SLA class I-restricted CD8+ T cell clone, that recognizes a heterotypic FMDV antigen.
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A Study of the Cellular Immune Response to Enteroviruses in Humans: Identification of Cross-reactive T Cell Epitopes on the Structural Proteins of Enteroviruses
More LessWe have attempted to extend our understanding of the enteroviral cross-reactive T cell response in humans. Peripheral blood mononuclear cells (PBMC) from healthy donors were stimulated in vitro with six different serotypes of enterovirus and 15 synthetic peptides representing conserved regions in the four structural proteins of these viruses. Upon challenge with different antigens, PBMC from donors responded specifically with proliferation and production of interferon-γ (IFN-γ). In contrast, synthesis of interleukin-4 (IL-4) or IL-10 was not detected. A T cell response to each enterovirus serotype was recorded in all individuals even though not all individuals had serum neutralizing antibody against each virus. These data confirmed previous findings that human T cells recognize enteroviral cross-reactive epitopes. Analysis of the peptide-induced IFN-γ production and proliferative response showed that the cross-reactive T cell epitopes are localized mainly in capsid protein VP2 and VP3 and to a lesser extent in VP1. Surprisingly, T cell epitopes were not identified in the most conserved structural protein of enterovirus, VP4. Immune responses were mediated by CD4+ T cells in association with MHC class II molecules. The sources of IFN-γ in response to the most immunodominant cross-reactive T cell epitopes were CD4+, CD8+ and NK cells. The two latter subsets produced IFN-γ provided CD4+ T cells were present. Since T helper 1 (Th1) cells can mediate an in vivo protective immune response against poliovirus infection in mice, our novel findings in humans merit further detailed characterization of T cells that recognize the enteroviral cross-reactive T cell epitopes.
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Screening for Membrane-Permeabilizing Mutants of the Poliovirus Protein 3AB
More LessSynthesis of the poliovirus polypeptide 3AB in bacterial cells results in an increase in membrane permeability. The alterations observed resemble those elicited by bacteriophage lytic proteins, which are presumed to cause pore formation in biological membranes. This property has been exploited in the development of an in vivo screening system that allows morphological differentiation of Escherichia coli clones expressing either wild-type 3AB or variant 3AB proteins lacking the ability to permeabilize bacteria. Expression of the wild-type 3AB gene in the presence of a chromogenic β-galactosidase substrate causes E. coli clones to stain dark blue. In contrast, bacterial mutants that synthesize 3AB proteins with alterations in the hydrophobic domain lack pore-forming activity and stain to a light blue colour, allowing differentiation from wild-type clones. This phenotypic property correlates with the rate of entry of the β-galactosidase substrate into the bacteria. The method developed here was used to screen more than 8000 E. coli clones after random PCR mutagenesis of the poliovirus 3AB gene. Our results show the existence of three different domains involved in the permeabilizing activity of 3AB protein. Twenty individual amino acid substitutions were identified in clones that showed the mutant phenotype and such bacteria displayed different reduced levels of permeability towards ONPG, hygromycin B, lysozyme and uridine. The procedure reported here may be of general interest to understand structure-function relationships in other eukaryotic proteins known to form pores.
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Temperature-Sensitive Parainfluenza Type 1 Vaccine Virus Directly Accesses the Central Nervous System by Infecting Olfactory Neurons
More LessImmunohistochemical investigation showed that intranasal inoculation of mice with a temperature-sensitive (ts) mutant of parainfluenza type 1 vaccine virus resulted in infection of some olfactory neurons as well as respiratory epithelial cells. It also disclosed the presence of viral antigens in glomeruli of the olfactory bulb but not in the secondary neurons (mitral and tufted cells). Polymerase chain reaction demonstrated the persistence of virus-specific nucleic acids in the olfactory bulb. These observations lead to the conclusion that parainfluenza virus, even with a ts phenotype, gains access to the central nervous system by infecting olfactory neurons.
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The Highly Inducible Member of the 70 kDa Family of Heat Shock Proteins Increases Canine Distemper Virus Polymerase Activity
More LessThe cellular stress response is characterized by the production of heat shock proteins (HSP) which serve important cytoprotective functions. Paradoxically, in vitro induction of the stress response promotes cytopathic effect mediated by infection with canine distemper virus (CDV). The stress-mediated increase in cytopathic effect is correlated to the formation of complexes between the viral nucleocapsid (NC) and the major inducible member of the ≈ 70 kDa family of HSP (hsp72). The objective of the present study was to document the functional significance of CDV NC-HSP interaction. Cytoplasmic NC was purified from Vero cells lytically infected with the Onderstepoort strain of CDV. Both ultrastructural variants of CDV NC interacted with both hsp72 and the constitutively expressed member of the ≈ 70 kDa family of HSP (hsp73) in a reversible and ATP-dependent manner. An effect of hsp72/73 on NC polymerase activity was demonstrated using cell-free assays derived from either Vero or HeLa cell lines. Antibody specific to hsp72 suppressed both basal and stress-enhanced polymerase activity whereas hsp73-specific antibody had no affect. Supplementation of purified hsp72/73, but not hsp73 alone, enhanced basal polymerase activity in a dosage-dependent manner. Using purified NC variants, polymerase activity was demonstrated in pre-formed hsp72/73-NC complexes but not in NC devoid of HSP. These results suggest that the stimulatory effect of the stress response upon CDV gene expression may, in part, be mediated by a reversible and direct interaction between hsp72 and the viral core particle.
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Studies on Unusual Cytoplasmic Structures which Contain Rabies Virus Envelope Proteins
More LessWe investigated unusual structures produced in BHK-21 cells infected with rabies virus (HEP-Flury strain). Sellers’ staining of the cells revealed, in addition to Negri body-like structures (inclusion bodies), production of a fuchsin-stained cytoplasmic structure (FCPS) which encircled the nucleus. The frequency of the FCPS-forming cells increased as replication progressed. The FCPS was different from the inclusion body because the former contained the viral glycoprotein (G) and matrix protein (M2) antigens, while the latter contained nucleocapsid antigens. In the early phase of infection, we observed accumulation of viral envelope antigens in a cytoplasmic structure that was considered to be expanded rough endoplasmic reticulum (rER) because of its concomitant increase in BiP content. Time-course studies suggested that the envelope antigen-containing structure, which was not stained with basic fuchsin, translocated to the perinuclear region to form the FCPS. FCPS formation was dependent on incubation temperature and was decreased at 30°C, while the development of virus-induced cytopathic effect (CPE) was delayed. When the incubation temperature was shifted up to 37°C, FCPS formation was induced again and progression of CPE was accelerated in approximate proportion to the increasing number of FCPS-positive cells. From these studies, we conclude that viral G proteins gradually accumulate in the rER with M2 protein and the expanded rER converts eventually into the FCPS, which may be closely related to accelerated host cell death.
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