Relationship between membrane integrity and the inhibition of host translation in virus-infected mammalian cells. Comparative studies between encephalomyocarditis virus and poliovirus

Viral and host proteins involved in picornavirus life cycle

Picornaviruses cause several diseases, not only in humans but also in various animal hosts. For instance, human enteroviruses can cause hand-foot-and-mouth disease, herpangina, myocarditis, acute flaccid paralysis, acute hemorrhagic conjunctivitis, severe neurological complications, including brainstem encephalitis, meningitis and poliomyelitis, and even death. The interaction between the virus and the host is important for viral replication, virulence and pathogenicity. This article reviews studies of the functions of viral and host factors that are involved in the life cycle of picornavirus. The interactions of viral capsid proteins with host cell receptors is discussed first, and the mechanisms by which the viral and host cell factors are involved in viral replication, viral translation and the switch from translation to RNA replication are then addressed. Understanding how cellular proteins interact with viral RNA or viral proteins, as well as the roles of each in viral infection, will provide insights for the design of novel antiviral agents based on these interactions.

Plasma membrane-porating domain in poliovirus 2B protein. A short peptide mimics viroporin activity

Picornavirus 2B, a non-structural protein required for effective viral replication, has been implicated in cell membrane permeabilization during the late phases of infection. Here, we have approached the molecular mechanism of this process by assessing the pore-forming activity of an overlapping peptide library that spanned the complete 2B sequence. At non-cytopathic concentrations, only the P3 peptide, spanning 2B residues 35-55, effectively assembled hydrophilic pores that allowed diffusion of low molecular mass solutes across the cell plasma membrane (IC(50) approximately 4x10(-7) M) and boundary liposome bilayers (starting at peptide to lipid molar ratios>1:10(4)). Circular dichroism data were consistent with its capacity to fold as a helix in a membrane-like environment. Furthermore, addition of this peptide to a sealed plasma-membrane model, consisting of retinal rod outer segments patch-clamped in a whole-cell configuration, induced ion channel activity within seconds at concentrations as low as 10(-8) M. Thus, we have established a "one-helix" 2B version that possesses the intrinsic pore-forming activity required to directly and effectively permeabilize the cell plasma membrane. We conclude that 2B viroporin can be classified as a genuine pore-forming toxin of viral origin, which is produced intracellularly at certain times post infection.

Mechanisms of membrane permeabilization by picornavirus 2B viroporin

Cell infection by picornaviruses leads to membrane permeabilization. Recent evidence suggests the involvement of the non-structural protein 2B in this process. We have recently reported the detection of 2B porin-like activity in isolated membrane-protein systems that lack other cell components. According to data derived from these model membranes, four self-aggregated 2B monomers (i.e. tetramers) would be sufficient to permeabilize a single lipid vesicle, allowing the free diffusion of solutes under ca. 1000 Da. Our findings also support a role for lipids in protein oligomerization and subsequent pore opening. The lipid dependence of these processes points to negatively charged cytofacial surfaces as 2B cell membrane targets.

Poliovirus induces an early impairment of mitochondrial function by inhibiting succinate dehydrogenase activity

Poliovirus infection of COS-1 and T47D cells caused a rapid decrease in total cell respiration, and this was attributed to an inhibition of mitochondrial respiration. The stimulation of mitochondrial respiration by pyruvate plus malate or succinate was impaired in saponin-permeabilised cells. However, this inhibition could be overcome by the addition of N,N,N',N'-tetramethyl-1, 4-phenylenediamine and ascorbate. The activity of succinate dehydrogenase was impaired in parallel with the inhibition of mitochondrial respiration during poliovirus infection. This shows that mitochondrial function is profoundly altered during poliovirus infection and that this occurs primarily through inhibition of electron flow at complex II of the mitochondrial respiratory chain.

Selective irreversible inactivation of replicating mengovirus by nucleoside analogues: a new form of viral interference

We describe the selective irreversible inhibition of mengovirus growth in cultured cells by a combination of two pyrrolopyrimidine nucleoside analogues, 5-bromotubercidin (BrTu) and tubercidin (Tu). At a concentration of 5 microgram/ml, BrTu reversibly blocked the synthesis of cellular mRNA and rRNA but did not inhibit either mengovirus RNA synthesis or multiplication. BrTu is a potent inhibitor of adenosine kinase, and low concentrations of BrTu (e.g., 0.5 microgram/ml), which did not by themselves inhibit cell growth, blocked phosphorylation of Tu and thus protected uninfected cells against irreversible cytotoxicity resulting from Tu incorporation into nucleic acids. In contrast, in mengovirus-infected cells, BrTu did not completely inhibit Tu incorporation into mengovirus RNA, allowing the formation of Tu-containing functionally defective polynucleotides that aborted the virus development cycle. This increased incorporation of Tu coupled to mengovirus infection could be attributed either to a reduction in the inhibitory action of BrTu and/or its nucleotide derivatives at the level of nucleoside and nucleotide kinases and/or, perhaps, to an effect upon the nucleoside transport system. The virus life cycle in nucleoside-treated cells progressed to the point of synthesis of negative strands and probably to the production of a few defective new positive strands. Irreversible virus growth arrest was achieved if the nucleoside mixture of BrTu (0.5 to 10 microgram/ml) and Tu (1 to 20 microgram/ml) was added no later than 30 min after virus infection and maintained for periods of 2 to 8 h. The cultures thus "cured" of mengovirus infection could be maintained and transferred for several weeks, during which they neither produced detectable virus nor showed a visible cytopathic effect; however, the infected and cured cells themselves, while metabolically viable, were permanently impaired in RNA synthesis and unable to divide. Although completely resistant to superinfecting picornaviruses, they retained the ability to support the growth of several other viruses (vaccinia virus, reovirus, and vesicular stomatitis virus), showing that cured cells had, in general, retained the metabolic and structural machinery needed for virus production. The resistance of cured cells to superinfection with picornaviruses seemed attributable neither to interferon action nor to destruction or blockade of virus receptors but more likely to the consumption of some host factor(s) involved in the expression of early viral functions during the original infection.

Hygromycin B resistance mediates elimination of Leishmania virus from persistently infected parasites

A series of pX63-HYG derivatives encoding Leishmania RNA virus 1-4 (LRV1-4) sequences were electroporated into cells of Leishmania strain M4147, a virus-infected strain of L. guyanensis. After 6 weeks of drug selection (hygromycin B), transfected parasites lacked detectable quantities of viral genomic double-stranded RNA, viral capsid protein, and RNA-dependent RNA polymerase (RDRP) activity. Evidence of viral infection was not recovered upon removal of the drug. While viral RNA transcripts were produced from electroporated expression vectors, as determined by reverse transcription-PCR, viral antigens were not detected, suggesting that the antiviral effects of hygromycin B are mediated through translation inhibition. A short-term selection study suggests that the LRV1-4 elimination may not only be a function of hygromycin B as a protein synthesis inhibitor but also possibly related to the mechanism of hygromycin B resistance in Leishmania strains.

Cap-independent translation initiation in Xenopus oocytes

Eukaryotic cellular mRNAs contain a cap at their 5'-ends, but some viral and cellular mRNAs bypass the cap-dependent mechanism of translation initiation in favor of internal entry of ribosomes at specific RNA sequences. Cap-dependent initiation requires intact initiation factor eIF4G (formerly eIF-4gamma, eIF-4Fgamma or p220), whereas internal initiation can proceed with eIF4G cleaved by picornaviral 2A or L proteases. Injection of recombinant coxsackievirus B4 protease 2A into Xenopus oocytes led to complete cleavage of endogenous eIF4G, but protein synthesis decreased by only 35%. Co-injection of edeine reduced synthesis by >90%, indicating that eIF4G-independent synthesis involved ongoing initiation. The spectrum of endogenous proteins synthesized was very similar in the presence or absence of intact eIF4G. Translation of exogenous rabbit globin mRNA, by contrast, was drastically inhibited by eIF4G cleavage. The N-terminal cleavage product of eIF4G (cpN), which binds eIF4E, was completely degraded within 6-12 h, while the C-terminal cleavage product (cpC), which binds to eIF3 and eIF4A, was more stable over the same period. Thus, translation initiation of most endogenous mRNAs inXenopusoocytes requires no eIF4G, or perhaps only cpC, suggesting a cap-independent mechanism.

Membrane permeabilization by poliovirus proteins 2B and 2BC

Poliovirus infection leads to drastic alterations in membrane permeability late during infection. Transient expression of each nonstructural protein of poliovirus by means of recombinant vaccinia virus encoding the T7 RNA polymerase indicates that proteins 2B and 2BC strongly enhance membrane permeability to hygromycin B in HeLa cells. Almost no effect on expression of proteins 2C, 3A, 3AB, and 3C was found. Deletions and point mutations in 2B and 2BC have identified sequences in 2B involved in membrane permeabilization. Regions located at both ends of 2B are necessary to bring about these permeability alterations. A deletion of 11 amino acids of 2BC at the junction between 2B and 2C, as well as long deletions in 2C encompassing the GTPase motifs of this protein, do not impair the capacity of 2BC to modify the permeability of the membrane. The release of compounds such as choline or uridine from preloaded cells is also augmented by 2B and 2BC expression.

Mutations in the hydrophobic domain of poliovirus protein 3AB abrogate its permeabilizing activity

Poliovirus protein 3AB contains a predicted amphipathic helix that could lead to pore formation in membranes. We have introduced various mutations in the hydrophobic domain of the protein and the membrane-modifying properties of the resulting mutants have been analyzed. Expression of wild type 3AB protein in E. coli increases the influx and efflux of different molecules such as nucleosides, lactose analogues and antibiotics. Thus, 3AB expression makes E. coli cells two orders of magnitude more sensitive to hygromycin B, a non-permeant inhibitor of translation, and causes a 15-20-fold enhancement in the efflux of uridine. Changes in membrane permeability take place under conditions where no cellular lysis is detected and when other molecules such as beta-galactosidase or polyribonucleotides are kept inside the cell. These membrane modifications can be blocked to different extents by amino acid substitutions in the membrane-spanning region of the protein. These results suggest that poliovirus protein 3AB could possess an intrinsic ability to form pores in natural membranes, thus allowing the flux of small hydrophylic molecules through them.

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.

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.

Lack of direct correlation between p220 cleavage and the shut-off of host translation after poliovirus infection

Poliovirus induces a drastic inhibition of host protein synthesis soon after infection of susceptible cells. The correlation between this inhibition and the cleavage of p220, a polypeptide that forms part of protein synthesis initiation factor elF-4F, has been examined in detail. Measurements of protein synthesis at half-hourly intervals after infection with poliovirus show the lack of direct correlation between p220 cleavage and the blockade of cellular translation. Moreover, the use of inhibitors of poliovirus RNA synthesis helped to dissociate those two events more clearly. Thus, in the presence of guanidine or Ro 09-0179 when little shut-off was induced by poliovirus extensive proteolytic degradation of p220 took place. When HeLa cells infected with poliovirus are placed at 28 degrees the inhibition of host protein synthesis is prevented and cellular translation continues for at least 8 hr, albeit at a reduced level compared to cells incubated at 37 degrees. At 28 degrees, cleavage of p220 is observed and about 80% of p220 is degraded after 6 hr of incubation at that temperature. Strikingly, when cells in which more than 50% of p220 is cleaved are shifted to 37 degrees, cellular translation recuperates to 100%, in spite of the fact that no detectable p220 is present. Furthermore, if poliovirus-infected cells are incubated for 2 hr at 37 degrees to permit the cleavage of p220 and then are shifted to 28 degrees in the presence of guanidine, cellular proteins are synthesized at the same level as uninfected HeLa cells incubated at 28 degrees. These results show that translation of cellular mRNAs takes place in cells containing a cleaved p220 and indicate that this cleavage is not directly responsible for the shut-off of host translation induced by poliovirus.

Killing of Burkitt-lymphoma-derived Daudi cells by ultraviolet-inactivated vaccinia virus

Interaction of active and UV-inactivated vaccinia virus at high multiplicity caused cytological changes and inhibition in cellular protein and DNA synthesis, thus arresting the multiplication of Burkitt-lymphoma-derived Daudi cells and eventually killing the cells. Adsorption to the cells but the lack of penetration was evident by immunofluorescence, electron microscopy and [3H]thymidine-labeled virus incorporation. Viral DNA synthesis or virus replication was not demonstrated. Thus, it appears that the massive adsorption of viral particles, active or UV-inactivated, or possibly a "toxic" component that resides in the virion, damages the plasma membrane and may be responsible for killing the cells by a mechanism of lysis from without.

Cap-independent translation of mRNA conferred by encephalomyocarditis virus 5' sequence improves the performance of the vaccinia virus/bacteriophage T7 hybrid expression system

A recombinant vaccinia virus that directs the synthesis of bacteriophage T7 RNA polymerase provides the basis for the expression of genes that are regulated by T7 promoters in mammalian cells. The T7 transcripts, which account for as much as 30% of the total cytoplasmic RNA at 24 hr after infection, are largely uncapped. To improve the translatability of the uncapped RNA, the encephalomyocarditis virus (EMCV) untranslated region (UTR) was inserted between the T7 promoter and the chloramphenicol acetyltransferase (CAT) gene. Experiments with a reticulocyte extract demonstrated that the EMCV UTR conferred efficient and cap-independent translatability to CAT RNA synthesized in vitro by T7 RNA polymerase. In cells infected with recombinant vaccinia viruses containing the T7 promoter-regulated CAT gene, the EMCV UTR increased the amount of CAT RNA on polyribosomes. The polyribosome-derived CAT RNA, which contained the EMCV UTR, was translated in vitro in a cap-independent fashion as well. Use of the EMCV UTR significantly enhanced the vaccinia/T7 hybrid expression system as it resulted in a 4- to 7-fold increase in total CAT activity. A further approximately 2-fold improvement was achieved by incubating the cells in hypertonic medium, which favors the translation of uncapped picornavirus RNA over cellular mRNAs. With this newly modified expression system, CAT was the predominant protein synthesized by infected cells and within 24 hr accounted for greater than 10% of the total cell protein.

Alteration of intracellular monovalent cation concentrations by a poliovirus mutant which encodes a defective 2A protease

Poliovirus mutant 2A-1, which encodes a defective protease 2A, fails to inhibit translation of capped mRNAs selectively. Despite the failure of 2A-1 to inactivate cap-dependent translation, a reduction in the overall rate of protein synthesis, both virus and cell-specified, does occur after 2A-1 infection. This global reduction in protein synthesis is temporally correlated with an increase in [Na+]i and a decrease in [K+]i. The extensive global shutoff of protein synthesis is not observed in 2A-1 infected cells incubated in low NaCl medium or medium containing an elevated concentration of KCl which compensate for the virally-induced alterations in intracellular monovalent cation concentrations. Furthermore, 2A-1-specified protein synthesis is only partly resistant to hypertonic NaCl media which increase [Na+]i, in contrast to protein synthesis specified by wild-type poliovirus. These results suggest that shutoff of host and viral protein synthesis during infection by poliovirus mutant 2A-1 is a consequence of the virus-induced changes in intracellular monovalent cation concentrations.

Sensitivity of Burkitt lymphoma Daudi cells to inactive influenza virus

Interaction of UV-inactivated influenza A/X47 virus at high multiplicity caused a rapid inhibition in cellular protein and DNA synthesis, thus arresting Burkitt-lymphoma-derived Daudi cell multiplication, and eventually killing the cells. The mechanism of the cytolytic effect is presumably, linked to the increase in cell membrane permeability indicated by elevation in 51Cr release. This might be the consequence of the mass adsorption and/or penetration of viral particles.

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.

Regulation of protein synthesis in virus-infected animal cells

This chapter summarizes the structural features that govern the translation of viral mRNAs: where the synthesis of a protein starts and ends, how many proteins can be produced from one mRNA, and how efficiently. It focuses on the interplay between viral and cellular mRNAs and the translational machinery. That interplay, together with the intrinsic structure of viral mRNAs, determines the patterns of translation in infected cells. It also points out some possibilities for translational regulation that can only be glimpsed at present, but are likely to come into focus in the future. The mechanism of selecting the initiation site for protein synthesis appears to follow a single formula. The translational machinery displays a certain flexibility that is exploited more frequently by viral than by cellular mRNAs. Although some of the parameters that determine efficiency have been identified, how efficiently a given mRNA will be translated cannot be predicted by summing the known parameters.

Modification of membrane permeability during Semliki Forest virus infection

Modification of membrane permeability has been analyzed in Semliki Forest virus (SFV)-infected cells by means of translation inhibitors not permeable to normal cells. A higher inhibition of protein synthesis in the infected cells is only observed with those antibiotics that do not easily pass the cell membrane, but not with others, permeable to cells, such as anisomycin, cycloheximide, trichodermin, etc. It does not, therefore, seem that the suggestion of M. A. Gray, K. J. Micklem, and C. A. Pasternak [Eur. J. Biochem. 135, 299-302, (1983)] that protein synthesis in virus-infected cells is more susceptible to translation inhibitors in general is correct. Both low- and high-molecular weight compounds enter the cell very early during SFV infection. This permeabilization is blocked by compounds known to increase the pH of coated vesicles, such as NH4Cl and chloroquine. Inhibition of energy production by means of N3Na and 2'-deoxyglucose also blocks this process. The optimal external pH for this early permeabilization is around 7-8. Acidic pH inhibits the entry of these impermeant antibiotics promoted by SFV. Analysis of 86Rb+ content in SFV-infected HeLa cells also indicates that a drastic decline in this cation takes place, in agreement with previous findings, but disagreeing with the previous results. A parallel between the decrease in this cation and the blockade of protein synthesis is apparent, throughout the course of infection. In addition to the early permeabilization that takes place during virus entry, increased entry of hygromycin B and alpha-sarcin also occurs in SFV-infected cells from 2 to 3 hr postinfection, but not when late viral replication is blocked by means of interferon treatment.

New antiviral compounds

Several potent and selective antiviral agents against herpes virus infections have been developed. However, the majority of compounds against other viral diseases has not yet reached such high standard. Based on progress in molecular virology it can, however, be anticipated that similar concepts of selective inhibition will also be developed for other virus groups. In addition to virus-induced enzymes, viral proteins other than enzymes with specific activities will be identified. The identification of active sites will lead to the design of new and specific inhibitors. Moreover, studies on the mode of action of the huge number of known antiviral compounds may provide the basis for new and potent approaches to specific virus chemotherapy. New inhibitors of viral replication may also be derived from 2'-5'A and other mediators of the interferon induced antiviral state. However, since 2'-5'A does not enter cells, is rapidly degraded by phosphodiesterases, and affects viral and cellular protein synthesis, only analogs which do not have these disadvantages may qualify as antiviral drugs. In addition to refinements at the molecular level quantitative assays for a better evaluation of antiviral agents for clinical use are required. For clinical trials, rapid diagnosis, early initiation of treatment, and quantitative evaluation of the antiviral effects of a drug need to be developed. Moreover, new methods of drug delivery and/or drug targeting will improve potency and selectivity of antiviral compounds. Drug carriers have already successfully been used in cancer therapy (Poste and Fidler, 1981) they should be also applicable to virus chemotherapy. Finally, a better understanding of the pathogenesis and the natural course of viral diseases will contribute to the development of more effective and safe antiviral agents.

Encephalomyocarditis virus replication complexes preferentially utilizing nucleoside diphosphates as substrates for viral RNA synthesis. Nucleotide kinases specifically associated with the complex channel RNA precursor

Replication complexes (RC) of the encephalomyocarditis (EMC) virus were shown previously to contain components that exhibit marked preference for nucleoside diphosphates over nucleoside triphosphates (NTP) as substrates for viral RNA synthesis [Koonin and Agol (1983), Virology 129, 309-318]. These NDP-preferring components have now been found to posses the following properties. When RC preparations were fractionated by sucrose density gradient centrifugation, the fractions containing NDP-preferring components exhibited a considerably higher nucleotide kinase activity as compared to either the fractions containing NTP-preferring components or corresponding fractions from mock-infected cells. When NDP-preferring RC were incubated with ADP and three other NTP, very low concentrations of endogenously generated ATP ensured a greater rate of RNA synthesis than did much higher concentrations of exogenous ATP. When an equimolar mixture of differently labelled UDP and UTP was used as a substrate for NDP-preferring RC, the label from UDP predominated in the newly synthesized RNA, even though the UDP-derived UTP constituted a minor portion of the total UTP pool. When labelled UDP was diluted with unlabelled uridine nucleotides, unlabelled UTP proved to be far less efficient than unlabelled UDP in diminishing the specific radioactivity of UMP incorporated into RNA by NDP-preferring RC. These data are interpreted in the sense that the NTP generated by the built-in nucleotide kinase system are not freed into the external milieu but rather form a separate pool preferentially used for synthesis of viral RNA by NDP-preferring RC. It is suggested that this functional compartmentation of NTP may be significant for the replication of viral RNA in vivo.

Alterations of 86Rb+ fluxes in poliovirus-infected HeLa cells and their dependence on virus replication

Components of the 86Rb+ influx were investigated subsequent to poliovirus infection in the presence and absence of guanidine-HCl, both under normal steady-state conditions and after Na+ preloading of the cells. Measurements of the ouabain-sensitive 86Rb+ uptake indicated a biphasic change in the activity of the Na+, K+ pump in the course of virus infection: a transient increase in the second hour postinfection, that was detectable only after Na+ preloading and inhibition after 3 hr. The enhanced activity of the Na+, K+ pump was not affected, while the decrease later was fully prevented by the antiviral agent guanidine-HCl. The piretanide-sensitive 86Rb+ uptake due to the Na+, K+, 2 Cl- cotransport system also became strongly inhibited beginning in the second hour postinfection. The inhibition of this transport system was partially antagonized by guanidine-HCl. The remaining 86Rb+ influx in the presence of ouabain and piretanide increased in the third hour postinfection. The latter change in 86Rb+ influx, indicating an increased permeability to monovalent cations was completely abolished by guanidine-HCl.

Action of oligomycin on cultured mammalian cells. Permeabilization to translation inhibitors

Oligomycin, an inhibitor of ATP synthesis, has been used as a model to study the effects of ATP depletion on macromolecular synthesis and modification of membrane permeability. Protein synthesis is totally blocked by the antibiotic, whereas RNA and DNA synthesis are less inhibited. Different concentrations of monovalent and divalent cations do not revert the inhibition of protein synthesis. Measurement of cellular ATP and 86Rb+ content indicate that the blockade of translation depends on the ATP content. A significant decrease in cellular ATP does not lead to the reduction of monovalent ions in the cell, although hyperpolarization of the cell membrane does take place. An increased membrane permeability to some inhibitors develops when the cells are hyperpolarized by oligomycin.

Effect of interferon treatment on blockade of protein synthesis induced by poliovirus infection

Treatment of HeLa cells with lymphoblastoid interferon leads to a drastic inhibition of infective poliovirus. Even relatively high concentrations of human lymphoblastoid interferon HuIFN-alpha (Ly) (400 IU/ml) do not prevent destruction of the cell monolayer after most of the cells have been infected with poliovirus. Analysis of macromolecular synthesis in a single step growth cycle of poliovirus in interferon-treated cells detected no viral protein synthesis. In spite of this inhibition of viral translation, the shut-off of host protein synthesis in interferon-treated cells is apparent when they are infected both at low and high multiplicities. Although viral RNA synthesis is inhibited considerably in cells treated with interferon, a certain amount is detected, suggesting that some viral replication takes place. Analysis of membrane permeability after poliovirus infection shows a leakage to 86Rb+ ions and modification of membrane permeability to the translation inhibitor hygromycin B at the moment when the bulk of virus protein synthesis occurs. These changes are delayed and even prevented if cells are pretreated with interferon. A situation is described in which host protein synthesis is shut-down with no major changes in membrane permeability, as studied by the two tests mentioned above. Prevention of viral gene expression by inactivation with ultraviolet light of the input virus or by treatment with cycloheximide blocks the shut-off of protein synthesis. This does not occur in the presence of 3 mM guanidine. These observations are in agreement with the idea that some poliovirus protein synthesis takes place in interferon-treated cells and this early gene expression is necessary to block cellular protein synthesis.

Encephalomyocarditis virus replication complexes that prefer nucleoside diphosphates as substrates for viral RNA synthesis

Replication complexes (RC) containing endogenous viral RNA templates and the viral RNA polymerase were isolated from encephalomyocarditis (EMC) virus-infected Krebs II cells by two different procedures one of which included removal of the bulk of the associated proteins by treatment with 2 M LiCl. Replacement of one or all the four nucleoside triphosphate (NTP) substrates by corresponding nucleoside diphosphates (NDPs) did not eliminate the ability of the RC to synthesize viral RNA products. Moreover, the complexes were shown to accept as substrates even nucleoside monophosphates (NMPs), provided at least one NTP was present in the system. These results suggested that nucleotide kinases, NMP kinase and NDP kinase, were associated with the RC, and this suggestion was confirmed directly. The RC could be resolved, by sucrose gradient centrifugation, into distinct components that exhibited marked preference for either NDPs or NTPs as substrates for RNA synthesis. It is suggested that, in the NDP-preferring components, the NTP molecules generated by the built-in nucleotide kinase system are channeled directly to the vicinity of the replication fork.

Antiviral effects of hygromycin B, a translation inhibitor nonpermeant to uninfected cells

Hygromycin B, a preferential translation inhibitor in virus-infected cells, has been tested for its antiviral effects against herpes simplex virus type 2 and poliovirus. The activity has been compared with other antiviral agents such as vidarabine and iododeoxyuridine.