Modification of membrane permeability in vaccinia virus-infected cells

Hirsutellin A: A Paradigmatic Example of the Insecticidal Function of Fungal Ribotoxins

The fungal pathogen Hirsutella thompsonii produces an insecticidal protein named hirsutellin A (HtA), which has been described to be toxic to several species of mites, insect larvae, and cells. On the other hand, on the basis of an extensive biochemical and structural characterization, HtA has been considered to be a member of the ribotoxins family. Ribotoxins are fungal extracellular ribonucleases, which inactivate ribosomes by specifically cleaving a single phosphodiester bond located at the large rRNA. Although ribotoxins were brought to light in the 1960s as antitumor agents, their biological function has remained elusive. Thus, the consideration of hirsutellin A, an insecticidal protein, as a singular ribotoxin recalled the idea of the biological activity of these toxins as insecticidal agents. Further studies have demonstrated that the most representative member of the ribotoxin family, α-sarcin, also shows strong toxic action against insect cells. The determination of high resolution structures, the characterization of a large number of mutants, and the toxicity assays against different cell lines have been the tools used for the study of the mechanism of action of ribotoxins at the molecular level. The aim of this review is to serve as a compilation of the facts that allow identification of HtA as a paradigmatic example of the insecticidal function of fungal ribotoxins.

Apoptosis and necrosis in vaccinia virus-infected HeLa G and BSC-40 cells

In most cells, vaccinia virus (VACV) infection is considered to cause a lytic cell death, an equivalent of necrosis. However, upon infection of the epithelial cell lines HeLa G and BSC-40 with VACV strain Western Reserve (WR), we have previously observed an increased activation of and activity attributable to caspases, a typical sign of apoptosis. In this paper, we have further analyzed the type of cell death in VACV-infected cells HeLa G and BSC-40. In a cell-based flow cytometric assay, we showed a specific activation of caspase-2 and 4 in HeLa G and BSC-40 cells infected with VACV, strain WR, while we did not find any effects of inhibitors of calpain and cathepsin D and E. The actual activity of the two caspases, but also of caspase-3, was then confirmed in lysates of infected HeLa G, but not in BSC-40 cells. Accordingly, poly(ADP)-ribose polymerase (PARP) cleavage was found increased only in infected HeLa G cells. Consequently, we have determined morphological features of apoptosis and/or activity of the executioner caspase-3 in infected HeLa G cells in situ, while only a background apoptosis was observed in infected BSC-40 cells. Finally, vaccination strains Dryvax and Praha were found to induce apoptosis in both HeLa G and BSC-40 cells, as characterized morphologically and by PARP cleavage. These findings may be important for understanding the differences in VACV-host interactions and post-vaccination complications in different individuals.

Lytic infection with vaccinia virus activates caspases in a Bcl-2-inhibitable manner

Vaccinia virus (VV) is considered to cause lytic infection of most cells, with lysis being regarded equivalent to necrosis. Activation of caspases has not been associated with necrosis. However, we observed the activation and activity of caspases in epithelial cells HeLa G and BSC-40 lytically infected with VV. Using three different flow-cytometric approaches, we characterized the distinct stages of caspase cascade in VV-infected cells: a cleaved, activated form of caspases detected using a fluorescent pan-caspase inhibitor; caspase activity assayed by cleavage of a non-fluorescent substrate into a fluorescent product; caspase-specific cleavage of death substrates characterized by a fluorescent antibody detecting a neo-epitope in cytokeratin-18. All of these approaches yielded an increased fluorescent signal in VV-infected cells compared to mock-infected controls. Additionally, the signal was decreased by the expression of Bcl-2. The cleavage of cytokeratin-18 was confirmed by western blotting, but another key protein involved in apoptosis, PARP, was not cleaved in VV-infected lytic cells. The necrotic phenotype of the cells was confirmed by increased cell membrane permeability and/or decreased mitochondrial membrane potential. In conclusion, our data suggest that VV infection of the epithelial cells HeLa G and BSC-40 initiates the apoptotic program, however, apoptosis is not completed and switches into necrosis.

Fungal ribotoxins: molecular dissection of a family of natural killers

RNase T1 is the best known representative of a large family of ribonucleolytic proteins secreted by fungi, mostly Aspergillus and Penicillium species. Ribotoxins stand out among them by their cytotoxic character. They exert their toxic action by first entering the cells and then cleaving a single phosphodiester bond located within a universally conserved sequence of the large rRNA gene, known as the sarcin-ricin loop. This cleavage leads to inhibition of protein biosynthesis, followed by cellular death by apoptosis. Although no protein receptor has been found for ribotoxins, they preferentially kill cells showing altered membrane permeability, such as those that are infected with virus or transformed. Many steps of the cytotoxic process have been elucidated at the molecular level by means of a variety of methodological approaches and the construction and purification of different mutant versions of these ribotoxins. Ribotoxins have been used for the construction of immunotoxins, because of their cytotoxicity. Besides this activity, Aspf1, a ribotoxin produced by Aspergillus fumigatus, has been shown to be one of the major allergens involved in allergic aspergillosis-related pathologies. Protein engineering and peptide synthesis have been used in order to understand the basis of these pathogenic mechanisms as well as to produce hypoallergenic proteins with potential diagnostic and immunotherapeutic applications.

Evasion of innate immunity by vaccinia virus

Vaccinia virus, a member of the Poxviridae, expresses many proteins involved in immune evasion. In this review, we present a brief characterisation of the virus and its effects on host cells and discuss representative secreted and intracellular proteins expressed by vaccinia virus that are involved in modulation of innate immunity. These proteins target different aspects of the innate response by binding cytokines and interferons, inhibiting cytokine synthesis, opposing apoptosis or interfering with different signalling pathways, including those triggered by interferons and toll-like receptors.

Vaccinia virus induces Ca2+-independent cell-matrix adhesion during the motile phase of infection

Vaccinia virus (VV) induces two forms of cell motility: cell migration, which is dependent on the expression of early genes, and the formation of cellular projections, which requires the expression of late genes. The need for viral gene expression prior to cell motility suggests that VV proteins may affect how infected cells interact with the extracellular matrix. To address this, we have analyzed changes in cell-matrix adhesion after infection of BS-C-1 cells with VV. Whereas uninfected cells round up and detach from the culture flask in the presence of EGTA, infected cells remain attached to the culture flask with a stellate morphology. Ca2+-independent cell-matrix adhesion was evident by 10 h postinfection, after the onset of cell motility but before the formation of virus-induced cellular projections. Progression to Ca2+-independent adhesion required the expression of late viral genes but not the formation of intracellular enveloped virus particles or intracellular actin tails. Analyses of specific matrix proteins identified vitronectin and fibronectin as optimal ligands for Ca2+-independent adhesion and the formation of cellular projections. Adhesion to fibronectin was mediated via RGD motifs alone and was not inhibited by 500 micrograms of heparin/ml. Kistrin, a disintegrin which binds preferentially to the alphav beta3 (vitronectin/fibronectin) receptor inhibited the formation of cellular projections without disrupting preformed matrix interactions. Finally, we show that Ca2+-independent cell-matrix adhesion is a dynamic process which mediates changes in the morphology of VV-infected cells and uninfected cells which exhibit a transformed phenotype.

The human immunodeficiency virus type 1 Vpu protein enhances membrane permeability

Infection of T lymphocytes by the human immunodeficiency virus causes drastic alterations in the intracellular cation content of the infected cells. The human immunodeficiency virus type 1 genome encodes several accessory proteins, including Vpu, an integral membrane protein that forms ion channels in planar lipid bilayers. The effect of Vpu on the permeability of the plasma membrane to several molecules has been analyzed. Expression of vpu in Escherichia coli cells increases membrane permeability to a number of molecules such as 2-nitrophenyl beta-D-galactopyranoside, uridine, the impermeable translation inhibitor hygromycin B, and lysozyme. In addition, transient expression of Vpu in eukaryotic COS cells enhances entry of charged molecules such as hygromycin B and neurobiotin into these cells. The effect of Vpu on cell membrane permeability resembles that reported for other membrane-active proteins from different animal viruses, including influenza M2, Semliki Forest virus 6K, and poliovirus 2B and 3A proteins.

Rotavirus nonstructural glycoprotein NSP4 alters plasma membrane permeability in mammalian cells

The endoplasmic reticulum-localized transmembrane glycoprotein NSP4 of rotavirus is a key protein involved in rotavirus cytopathology. We have used a dual-recombinant vaccinia virus system to express NSP4 in monkey kidney epithelial cells at a level comparable to that observed during rotavirus infection. Expression of NSP4 results in loss of plasma membrane integrity, which can be demonstrated by release of both 51Cr and lactate dehydrogenase into the medium. The cytotoxic behavior of NSP4 is dose dependent, and morphological analysis reveals gross changes to cell ultrastructure, indicative of cell death. Thus, intracellular expression of a single rotavirus protein which localizes to the endoplasmic reticulum membrane has profound effects on the stability of the plasma membrane and cell viability. Analysis of NSP4 deletion mutants indicates that a membrane-proximal region located within the cytoplasmic domain may mediate cytotoxicity.

Characterization of prokaryotic recombinant Aspergillus ribotoxin alpha-sarcin

The Aspergillus ribonuclease alpha-sarcin is toxic to intact mammalian cells but the mechanism by which it enters the cells to reach its ribosomal RNA substrate is unclear. Here we have compared the cytotoxicity of alpha-sarcin to that of ricin, another catalytic toxin that targets the same rRNA sequence but whose mechanism of cell entry is better understood. Intact ricin binds to cell surface components and enters the cells by receptor-mediated endocytosis, whereas the catalytic polypeptide of ricin (the A chain or RTA) which, like alpha-sarcin, is unable to bind to surface components directly and enters cells by fluid phase uptake. Recombinant alpha-sarcin was produced in Escherichia coli and purified to homogeneity. The protein was soluble, stable and its ability to inhibit in vitro protein synthesis was indistinguishable from that of native alpha-sarcin. Further, recombinant alpha-sarcin had the same in vitro protein synthesis inhibition activity as ricin A chain. The cytotoxicity of alpha-sarcin and ricin A chain to HeLa cells was also the same. The cytotoxicity of alpha-sarcin was due to its RNAase activity rather than to specific membrane effects at the cell surface, since a mutant containing a single substitution at a putative key catalytic residue had reduced ribonuclease activity and an equivalent reduction in cytotoxicity. One interpretation of the data is that a-sarcin enters mammalian cells in the same way as free ricin A chain.

Overexpression of the vaccinia virus A38L integral membrane protein promotes Ca2+ influx into infected cells

The vaccinia virus Western Reserve A38L protein is a hydrophobic integral membrane glycoprotein with amino acid similarity to mammalian integrin-associated protein. The protein has an N-terminal immunoglobulin superfamily domain, followed by five membrane-spanning domains and a short cytoplasmic tail. Deletion of the protein reduces virus plaque size but does not affect virus virulence (J. E. Parkinson, C. M. Sanderson, and G. L. Smith, Virology, in press). In this study, we have used a recombinant vaccinia virus in which the A38L gene may be inducibly overexpressed by addition of isopropyl-beta-D-thiogalactopyranoside (IPTG), to demonstrate that overexpression of the vaccinia virus A38L gene produces drastic changes in the morphology, permeability, and adhesion of infected cells. In particular, A38L overexpression caused swelling of cells, marginalization of nuclear chromatin, and vacuolization of the endoplasmic reticulum, features characteristic of cell necrosis. By 18 h postinfection, cells become permeable and lytic as defined by the free entry of propidium iodide and loss of the cytoplasmic enzyme lactate dehydrogenase. Chelation of extracellular Ca2+ 22 h postinfection inhibited further release of lactate dehydrogenase, showing that Ca2+ influx was required for A38L-induced lysis. Direct measurement of 45Ca2+ influx showed that the rate of Ca2+ uptake was directly related to the period of A38L induction. The A38L protein, therefore, promotes the formation of pores within the plasma membrane of cells, and these pores facilitate Ca2+ entry and induce necrosis. Addition of rifampin inhibited virus assembly but not the ability of A38L to induce necrosis, indicating that pore formation is independent of viral morphogenesis. Finally, overexpression of the A38L protein resulted in a reduced plaque size and a threefold decrease in production of infective particles in vitro. The A38L protein represents the first example of a virus protein which directly or indirectly promotes the influx of extracellular Ca2+.

Modification of membrane permeability by animal viruses

Animal viruses permeabilize cells at two well-defined moments during infection: (1) early, when the virus gains access to the cytoplasm, and (2) during the expression of the virus genome. The molecular mechanisms underlying both events are clearly different; early membrane permeability is induced by isolated virus particles, whereas late membrane leakiness is produced by newly synthesized virus protein(s) that possess activities resembling ionophores or membrane-active toxins. Detailed knowledge of the mechanisms, by which animal viruses permeabilize cells, adds to our understanding of the steps involved in virus replication. Studies on early membrane permeabilization give clues about the processes underlying entry of animal viruses into cells; understanding gained on the modification by viral proteins of membrane permeability during virus replication indicates that membrane leakiness is required for efficient virus release from infected cells or virus budding, in the case of enveloped viruses. In addition, the activity of these membrane-active virus proteins may be related to virus interference with host cell metabolism and with the cytopathic effect that develops after virus infection.

Membrane-bound complement regulatory activity is decreased on vaccinia virus-infected cells

Decay accelerating factor (DAF), membrane cofactor protein (MCP), complement receptor 1 and mouse Crry are cell surface-bound complement regulatory proteins capable of inhibiting C3 convertase activity on cell membranes, and therefore provide a substantial protection from attack by homologous complement activated either by the classical or by the alternative pathway. Decrease in complement regulatory activity might lead to spontaneous complement deposition and subsequent cell injury. MoAb 5I2 can inhibit the complement regulatory activity of molecules on rat cells, resulting in deposition of homologous complement. The antigen recognized by 5I2 MoAb in rats is homologous to mouse Crry. Fifteen to 20 h after infection with vaccinia virus, in vitro cultured KDH-8 rat hepatoma cells show a strong decrease in expression of Crry-like antigen, and proved to be sensitive to complement deposition when 1:5 diluted normal rat serum was added to the culture medium as a source of complement. Addition of complement to the cultured KDH-8 cells infected with a very low dose of vaccinia virus (1 plaque-forming unit (PFU)/1000 cells) substantially reduced spreading of virus infection in the cell culture, while inactivation of complement by heat or zymosan treatment abrogated the protective effect.

Entry of animal viruses and macromolecules into cells

The entry of animal viruses into cells is mediated by conformational changes in certain virion-particle components. These changes are triggered by the binding of virions to receptors and are influenced by low pH during receptor-mediated endocytosis. These conformational alterations promote the interaction of some viral proteins with cellular membranes thereby leading to transient pore formation and the disruption of ionic and pH gradients. The entry of toxins that do not possess receptors on the cell surface is promoted during the translocation of the virus genome or the nucleocapsid to the cytoplasm. A model is now presented which indicates that efficient virus translocation through cellular membranes requires energy, that may be generated by a protonmotive force. The entry of some animal viruses, as promoted by low pH, should thus only take place when a pH gradient and/or a membrane potential exist, but will not take place if these are dissipated, even if virion particles are present in an acidic environment.

Picornavirus inhibitors

Picornaviruses are among the best understood animal viruses in molecular terms. A number of important human and animal pathogens are members of the Picornaviridae family. The genome organization, the different steps of picornavirus growth and numerous compounds that have been reported as inhibitors of picornavirus functions are reviewed. The picornavirus particles and several agents that interact with them have been solved at atomic resolution, leading to computer-assisted drug design. Picornavirus inhibitors are useful in aiding a better understanding of picornavirus biology. In addition, some of them are promising therapeutic agents. Clinical efficacy of agents that bind to picornavirus particles has already been demonstrated.

Translocation of alpha-sarcin across the lipid bilayer of asolectin vesicles

alpha-Sarcin is a cytotoxic protein produced by the mould Aspergillus giganteus. Insertion of alpha-sarcin into asolectin membranes has been demonstrated by protein labelling with photoreactive phospholipids. alpha-Sarcin added externally to tRNA-containing asolectin liposomes degrades the entrapped tRNA. Trypsin-containing asolectin liposomes were also prepared. Encapsulated trypsin degrades alpha-sarcin, even in the presence of a large excess of external hen egg-white trypsin inhibitor to prevent any alpha-sarin degradation outside the vesicles. These processes occur only with acidic phospholipids and were not observed when phosphatidylcholine vesicles were used. These results indicate that alpha-sarcin penetrates the lipid bilayer and becomes exposed to the lumen of negatively charged liposomes.

Kinetic study of the cytotoxic effect of alpha-sarcin, a ribosome inactivating protein from Aspergillus giganteus, on tumour cell lines: protein biosynthesis inhibition and cell binding

alpha-Sarcin is a ribosome inactivating protein produced by the mould Aspergillus giganteus. The effect of this protein on eight different tumour cell lines has been studied in the absence of any agent affecting membrane permeability. The protein is cytotoxic for all the tumour cell lines considered. alpha-Sarcin modifies the cell proliferation pattern by inhibiting the protein biosynthesis of the cultured cells. No membrane damage produced by alpha-sarcin has been observed by measuring lactic dehydrogenase leakage. Alteration on the cell mitochondrial activity has not been detected upon treatment with alpha-sarcin. Differences on the extent of the protein binding to the cells have been observed by flow cytometric measurements. The kinetic analysis of the protein biosynthesis inhibition produced by alpha-sarcin reveals an alpha-sarcin concentration-dependent lag phase followed by a first order decrease of the protein synthesis rate. This parameter is dependent on the external alpha-sarcin concentration. A saturable component for the action of alpha-sarcin is also deduced from these experiments. Results are discussed in terms of the protein passage across the cell membrane as the potential rate-limiting step for the action of alpha-sarcin.

Inhibitors of poliovirus uncoating efficiently block the early membrane permeabilization induced by virus particles

The entry of animal viruses into cells is associated with permeabilization of the infected cells to protein toxins such as alpha-sarcin (C. Fernández-Puentes and L. Carrasco, Cell 20:769-775, 1980). This phenomenon has been referred to as "the early permeabilization by animal viruses" (L. Carrasco, Virology 113:623-629, 1981). A number of inhibitors of poliovirus growth such as WIN 51711 6-(3,4-dichlorophenoxy)-3-(ethylthio)-2-pyridincarbonitrile (DEPC) and Ro 09-0410 specifically block the uncoating step of poliovirus but have no effect on attachment or entry of poliovirus particles into cells. These agents are potent inhibitors of the early permeabilization induced by poliovirus to the toxin alpha-sarcin. Thus, the uncoating of poliovirus is required for the permeabilization of cell membranes to proteins. The increased entry of labeled heparin promoted by virus entry is not blocked by these agents, indicating that poliovirus binds to its receptor and is internalized along with heparin in endosomes in the presence of WIN 51711, DEPC, or Ro 09-0410. We conclude that the delivery to the cytoplasm of some molecules that coenter with virion particles does not take place if the uncoating process is hindered.

Poxvirus pathogenesis

Poxviruses are a highly successful family of pathogens, with variola virus, the causative agent of smallpox, being the most notable member. Poxviruses are unique among animal viruses in several respects. First, owing to the cytoplasmic site of virus replication, the virus encodes many enzymes required either for macromolecular precursor pool regulation or for biosynthetic processes. Second, these viruses have a very complex morphogenesis, which involves the de novo synthesis of virus-specific membranes and inclusion bodies. Third, and perhaps most surprising of all, the genomes of these viruses encode many proteins which interact with host processes at both the cellular and systemic levels. For example, a viral homolog of epidermal growth factor is active in vaccinia virus infections of cultured cells, rabbits, and mice. At least five virus proteins with homology to the serine protease inhibitor family have been identified and one, a 38-kDa protein encoded by cowpox virus, is thought to block a host pathway for generating a chemotactic substance. Finally, a protein which has homology with complement components interferes with the activation of the classical complement pathway. Poxviruses infect their hosts by all possible routes: through the skin by mechanical means (e.g., molluscum contagiosum infections of humans), via the respiratory tract (e.g., variola virus infections of humans), or by the oral route (e.g., ectromelia virus infection of the mouse). Poxvirus infections, in general, are acute, with no strong evidence for latent, persistent, or chronic infections. They can be localized or systemic. Ectromelia virus infection of the laboratory mouse can be systemic but inapparent with no mortality and little morbidity, or highly lethal with death in 10 days. On the other hand, molluscum contagiosum virus replicates only in the stratum spinosum of the human epidermis, with little or no involvement of the dermis, and does not spread systemically from the site of infection. The host response to infection is progressive and multifactorial. Early in the infection process, interferons, the alternative pathway of complement activation, inflammatory cells, and natural killer cells may contribute to slowing the spread of the infection. The cell-mediated response involving learned cytotoxic T lymphocytes and delayed-type hypersensitivity components appears to be the most important in recovery from infection. A significant role for specific antiviral antibody and antibody-dependent cell-mediated cytotoxicity has yet to be demonstrated in recovery from a primary infection, but these responses are thought to be important in preventing reinfection.

Human adenovirus-host cell interactions: comparative study with members of subgroups B and C

Host cell interactions of human adenovirus serotypes belonging to subgroups B (adenovirus type 3 [Ad3] and Ad7) and C (Ad2 and Ad5) were comparatively analyzed at three levels: (i) binding of virus particles with host cell receptors; (ii) cointernalization of macromolecules with adenovirions; and (iii) adenovirus-induced cytoskeletal alterations. The association constants with human cell receptors were found to be similar for Ad2 and Ad3 (8 x 10(9) to 9 x 10(9) M-1), and the number of receptor sites per cell ranged from 5,000 (Ad2) to 7,000 (Ad3). Affinity blottings, competition experiments, and immunofluorescence stainings suggested that the receptor sites for adenovirus were distinct for members of subgroups B and C. Adenovirions increased the permeability of cells to macromolecules. We showed that this global effect could be divided into two distinct events: (i) cointernalization of macromolecules and virions into endocytotic vesicles, a phenomenon that occurred in a serotype-independent way, and (ii) release of macromolecules into the cytoplasm upon adenovirus-induced lysis of endosomal membranes. The latter process was found to be type specific and to require unaltered and infectious virus particles of serotype 2 or 5. Perinuclear condensation of the vimentin filament network was observed at early stages of infection with Ad2 or Ad5 but not with Ad3, Ad7, and noninfectious particles of Ad2 or Ad5, obtained by heat inactivation of wild-type virions or with the H2 ts1 mutant. This phenomenon appeared to be a cytological marker for cytoplasmic transit of infectious virions within adenovirus-infected cells. It could be experimentally dissociated from vimentin proteolysis, which was found to be serotype dependent, occurring only with members of subgroup C, regardless of the infectivity of the input virus.

Modification of membrane permeability by animal viruses

Animal viruses modify membrane permeability during lytic infection. There is a co-entry of macromolecules and virion particules during virus penetration and a drastic change in transport and membrane permeability at the late stages of the lytic cycle. Both events are of importance to understand different molecular aspects of viral infection, as virus entry into the cell and the interference of virus infection with cellular metabolism. Other methods of cell permeabilization of potential relevance to understand the mechanism of viral damage of the membrane are also discussed.

Exogenous phospholipase C permeabilizes mammalian cells to proteins

Mammalian cells treated with low concentrations of phospholipase C become permeable to the protein toxin alpha-sarcin. A similar permeabilization is not induced upon treatment with other lipases such as phospholipase A2, sphingomyelinase, or cholesterol esterase. Concentrations of 10 micrograms/ml alpha-sarcin almost completely blocked translation in HeLa cells treated with 0.3 U/ml phospholipase C (PL-C) for 1 h. In contrast, 200 micrograms/ml of alpha-sarcin had no effect at all on protein synthesis in untreated cells. Other macromolecules such as horseradish peroxidase and luciferase also enter into cells if they are treated with phospholipase C. This permeabilization method is fully reversible. As soon as 5 min after PL-C removal, the cells become impermeable to alpha-sarcin. Other metabolites such as uridine nucleotides are partially released after PL-C incubation, whereas the content of 86Rb+ remains at control levels, probably because the Na+/K+ ATPase activity increases.

Proteins are cointernalized with virion particles during early infection

Mammalian cells infected with enveloped or naked animal viruses become permeabilized to several proteins. The entry of alpha-sarcin, horseradish peroxidase, and luciferase is greatly increased during the early stages of viral infection. This process is promoted by uv-inactivated SFV, but not by heat-inactivated virions, suggesting that the process does not require viral gene expression. The entry of alpha-sarcin has been monitored both by its effects on protein synthesis and by indirect immunofluorescence. Increased entry of alpha-sarcin and luciferase is clearly observed in animal virus-infected cells by fluorescence microscopy. Chloroquine blocks the coentry of alpha-sarcin with enveloped, but not with naked, viruses. These results have implications to elucidate the mechanisms involved in virus entry.

L929 cells infected with temperature sensitive mutants of vesicular stomatitis virus: virus replication is necessary for induction of changes in membrane permeability

Infection of L929 murine cells with vesicular stomatitis virus (VSV) results in inhibition of host protein synthesis and appearance of membrane alterations at a time when cells are still actively engaged in viral protein synthesis. VSV temperature-sensitive (ts) mutants have been used to explore the role(s) played by the virus-coded proteins in the genesis of these effects. Cells were infected with each of five ts mutants representing the known complementation groups of VSV Indiana serotype, and incubated at permissive (32 degrees C) and non-permissive temperatures (39 degrees C). Protein synthesis in the presence and absence of Hygromycin B (Hyg. B) was analyzed during virus infection via incorporation of 35S-methionine in acid-precipitable material and SDS-polyacrylamide gel electrophoresis. Data indicate that mutants belonging to groups I (L protein), II (NS protein) and IV (N protein) do not inhibit host protein synthesis and do not induce any membrane changes when grown at the non-permissive temperature. Mutants of group III (M protein) and V (G protein), instead, do inhibit cell protein synthesis and induce membrane changes also when grown at the non-permissive temperature; this suggests that these effects do not correlate with the biological activity of these proteins and their interaction with the cellular membrane. On the other hand, mutants exhibiting defective steps of nucleocapsid replication are apparently unable to induce these effects once more suggesting that virus replication per se is essential, as also indirectly shown by experiments employing cycloheximide to mimic shut-off.

A 14,000-Mr envelope protein of vaccinia virus is involved in cell fusion and forms covalently linked trimers

A monoclonal antibody, MAbC3, that reacts with a 14,000-molecular-weight envelope protein (14K protein) of vaccinia virus completely inhibited virus-induced cell fusion during infection. Immunoblot and immunofluorescence studies revealed that the 14K protein was synthesized at about 6 to 7 h postinfection and transported from the cytoplasm to the cell surface. Synthesis and transport of the 14K protein during infection occurred in the presence of rifampin, an inhibitor of virus maturation. One- and two-dimensional gel electrophoretic analyses demonstrated that the 14K protein forms largely trimers (42K) that are covalently linked by disulfide bonds. The facts that MAbC3 prevents virus uncoating and blocks virus-induced cell fusion but does not prevent virus attachment to cells and the 14K envelope protein forms trimers all suggest that this protein plays major role in virus penetration.

Early interactions between animal viruses and the host cell: relevance to viral vaccines

Viral recognition of specific receptors in the host cell plasma membrane is the first step in virus infection. Attachment is followed by a redistribution or capping of virus particles on the cell surface which may play a role in the uptake process. Certain viruses penetrate the plasma membrane directly but many, both enveloped and non-enveloped viruses, are endocytosed at coated pits and subsequently pass into endosomes. The low pH environment of the endosome facilitates passage of the viral genome into the cytoplasm. For some viruses the mechanism of membrane penetration is now known to be linked to a pH-mediated conformational change in external virion proteins. As a consequence of infection there are alterations in the permeability of the plasma membrane which may contribute to cellular damage. Recent advances in the understanding of these processes are reviewed and their relevance to the development of new strategies for vaccines emphasised.

Changes in membrane permeability in cells infected by human cytomegalovirus

To analyze whether human cytomegalovirus could induce changes in membrane permeability we studied both Hygromycin B (HyB) inhibition of protein synthesis and glycine and choline uptake during viral replication. A higher sensitivity to HyB was observed both immediately after virus adsorption and in concomitance with the release of viral progeny. HyB sensitivity immediately after virus adsorption was not associated with changes in membrane permeability to glycine and choline while 60 hours p.i. a remarkable increase in choline uptake was detected.

Vaccinia virus antigens on the plasma membrane of infected cells. I. Viral antigens transferred from infecting virus particles and synthesized after infection

SDS-polyacrylamide gel electrophoretic analysis of plasma membranes prepared from L cells infected with radioiodinated vaccinia virus particles showed that at 2.0 hr postinfection, 125I-labeled virion polypeptides with molecular weights of 58K-60K, 32K-34K, 17K, and 12K-14K were associated with infected cell plasma membranes. By 4 hr postinfection, only the 32- to 34-kDa polypeptide, derived from infecting virus particles, could be detected on infected cell surfaces. A variety of techniques were applied to analyzing purified plasma cell membranes to define the viral antigens expressed on cell surfaces after infection, including (a) surface radioiodination of infected cells; (b) immune or Western blotting; (c) specific immunoprecipitation of viral proteins present in nonionic detergent extracts of membranes purified from [35S]methionine-labeled, virus-infected cells. It was determined that vaccinia virus-specified polypeptides with molecular weights of 78K-82K, 65K, 50K, 42K-45K, 35K-37K, 32K-34K, 30K, 20K, and 17K-18K were expressed by 3 hr postadsorption, on the plasma membranes of infected cells and were accessible to binding by exogenous antiviral antibodies. Viral antigens with molecular weights similar to those expressed on cell surfaces were secreted or shed from infected cells and could be detected in the medium harvested from virus-infected mouse L-cell cultures.