Oligopeptides that specifically inhibit membrane fusion by paramyxoviruses: studies on the site of action

Monoclonal antipeptide antibodies recognize epitopes upon VP4 and VP7 of simian rotavirus SA11 in infected MA104 cells

To study morphogenetic events of rotavirus SA11-infected MA104 cells with strictly defined reagents we produced monoclonal antibodies against synthetic peptides from both outer capsid proteins VP4 (aa residues 228-241: QNTRNIVPVSIVSR) and VP7 (aa residues 319-326: SAAFYYRV) of simian rotavirus SA11. Two of the selected monoclonal antibodies proved to be reactive with determinants of SA11-infected MA104 rhesus monkey kidney cells, with purified SA11 as well as with the particular peptides used for immunization. The anti-VP4 antibody had a demonstrable neutralizing titer of 200 (50% focus reduction) whereas the anti-VP7 MuMAb revealed no detectable neutralizing activity. In peptide-inhibition experiments, the corresponding peptide inhibited its MuMAb whereas the noncorresponding peptide had no effect on antibody binding to intracellular viral antigen. Localization of VP7 was preceded by VP4 as shown by immunofluorescence microscopy.

The human fibroblast receptor for gp86 of human cytomegalovirus is a phosphorylated glycoprotein

A human embryonic lung (HEL) cell receptor for gp86 of human cytomegalovirus that functions in virus-cell fusion was further characterized. Anti-idiotype antibodies that mimic gp86 were used to immunoprecipitate the 92.5-kDa fibroblast membrane receptor for gp86, which was preincubated with various endoglycosidases. The receptor, which has a pI ranging from 5.3 to 5.6, appears to be a glycoprotein with primarily N-linked sugar residues, some of which have high concentrations of mannose and some of which are complex oligosaccharides. Western blots (immunoblots) of electrophoretically transferred receptor incubated with various biotinylated lectins confirmed the presence of sugar moieties, including N-acetylglucosamine, glucose or mannose, and galactose, but not fucose or N-acetylgalactosamine. This gp86 receptor from uninfected HEL cells also incorporated radiolabeled phosphate from orthophosphoric acid, indicating that it is a constitutively phosphorylated receptor.

Apolipoprotein A-1 interacts with the N-terminal fusogenic domains of SIV (simian immunodeficiency virus) GP32 and HIV (human immunodeficiency virus) GP41: implications in viral entry

Previous studies showed that apoA1, the major protein component of HDL (High Density Lipoprotein), inhibited HIV infectivity and virus-induced syncytia formation. The mechanism of inhibition is unknown. We bring here evidence that the amphipathic helices of apoA1 interact with the N-terminal peptides of SIV gp32 and HIV gp41. These peptides have been shown to be associated with the initial steps of the fusion between the host cell and the virus. Binding of apoA1 to these peptides prevents the insertion of the fusogenic domains into the cell membrane and inhibits the fusion and the entry of the virus into the host cell.

Influence of oligopeptide aldehydes on intracellular Ca2+ concentration in rat pituitary cells

We investigated the effects of some synthetic tripeptide aldehydes, earlier shown to influence pituitary hormone secretion and 45Ca2+ uptake, on the intracellular free Ca2+ concentration ([Ca2+]i) of rat anterior pituitary cells in suspension. Boc-D-Phe-Leu-Phenylalaninal or Boc-D-Phe-Leu-Prolinal in the tested range of 1-100 or 200 microM, respectively, were ineffective in influencing basal [Ca2+]i but caused a concentration-dependent inhibition in K+ (25 mM)-induced [Ca2+]i elevation. The IC50 of both effects was about 50 microM. In contrast, they did not interfere with the stimulation caused by the calcium channel agonist BAY K 8644 and were also ineffective in influencing the receptor-mediated stimulus of thyrotropin-releasing hormone on [Ca2+]i. On the basis of the present and foregoing results the possible involvement of calcium channels is discussed, but different mechanisms mediating the tripeptide aldehyde inhibition are also considered. A third tripeptide aldehyde, Boc-Gln-Leu-Lysinal (Boc-GLL), showed ionophore-like properties. This nontoxic substance caused a dose-dependent rise up to 400% (at 100 microM) in [Ca2+]i. Its effect is not mediated by voltage-dependent calcium channels, as it cannot be inhibited either by the classical calcium channel antagonists verapamil and nifedipine, or by the above-mentioned inhibitory tripeptide aldehydes. When we decreased the extracellular Ca2+ concentration by the addition of 4 mM EGTA, the effect was inverted and Boc-GLL caused a large fall in [Ca2+]i. We suggest that Boc-GLL may open cell membrane pores through which Ca2+ moves along the concentration gradient.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on the fusion peptide of a paramyxovirus fusion glycoprotein: roles of conserved residues in cell fusion

The role of residues in the conserved hydrophobic N-terminal fusion peptide of the paramyxovirus fusion (F) protein in causing cell-cell fusion was examined. Mutations were introduced into the cDNA encoding the simian virus 5 (SV5) F protein, the altered F proteins were expressed by using an eukaryotic vector, and their ability to mediate syncytium formation was determined. The mutant F proteins contained both single- and multiple-amino-acid substitutions, and they exhibited a variety of intracellular transport properties and fusion phenotypes. The data indicate that many substitutions in the conserved amino acids of the simian virus 5 F fusion peptide can be tolerated without loss of biological activity. Mutant F proteins which were not transported to the cell surface did not cause cell-cell fusion, but all of the mutants which were transported to the cell surface were fusion competent, exhibiting fusion properties similar to or better than those of the wild-type F protein. Mutant F proteins containing glycine-to-alanine substitutions had altered intracellular transport characteristics, yet they exhibited a great increase in fusion activity. The potential structural implications of this substitution and the possible importance of these glycine residues in maintaining appropriate levels of fusion activity are discussed.

Measles virus inhibits mitogen-induced T cell proliferation but does not directly perturb the T cell activation process inside the cell

Measles virus (MV) inhibits lymphocyte function in patients, as well as in cells infected in vitro. The proliferation of phytohemagglutinin-stimulated T lymphocytes is suppressed by in vitro MV infection, as shown by the diminished incorporation of [3H]thymidine into DNA and the reduced frequency of cells in the S phase of the cell cycle, as compared with mock-infected cells. MV infection itself, however, does not completely block DNA synthesis in infected cells, because infected T cells expressing MV antigens on the cell surface, isolated by fluorescence-activated cell sorter, could still proliferate. Northern blot analysis indicated that the expression of genes induced during T cell activation, such as those encoding interleukin 2 (IL-2), c-myc, IL-2 receptor, IL-6, c-myb, and cdc-2, was not significantly suppressed in MV-infected cells, suggesting that MV does not interfere with the T cell activation process. When anti-MV serum or carbobenzoxy-D-Phe-L-Phe-Gly, a synthetic oligopeptide known to inhibit MV-induced fusion, was added 24 hr after infection, the inhibition of T cell proliferation was reversed in a dose-dependent manner. From these results we propose a model for the inhibition of T cell proliferation by MV; MV glycoproteins expressed on the cell surface of infected cells interact with the MV receptor or other molecules on the cell membrane of adjacent T cells, which in turn affects the proliferation of those T cells.

Anti-idiotype antibodies that mimic gp86 of human cytomegalovirus inhibit viral fusion but not attachment

Human cytomegalovirus (CMV) infects cells by sequential processes involving attachment, fusion with the cell membrane, and penetration of the capsid. We used two monoclonal anti-idiotype that mimic one of the CMV envelope glycoproteins, gp86, to study its role in the early phases of CMV infection. Neither of two such antibodies inhibited virus binding to human embryonic lung (HEL) fibroblasts; however, both antibodies inhibited the fusion of CMV with HEL cells, as measured by an assay in which viral envelope is labeled with a fluorescent amphiphile (octadecyl rhodamine B chloride, or R18), resulting in increased fluorescence during fusion of virus with the cell membrane. Because these anti-idiotype antibodies were shown previously to bind to specific receptors on HEL cell membranes, these findings suggest that both gp86 and its cell membrane receptor may function in the fusion of human CMV with HEL cells.

A structural correlation between lentivirus transmembrane proteins and natural cytolytic peptides

Although oncoviruses and lentiviruses replicate by similar mechanisms, they differ fundamentally in the usual fate of the infected host cell during productive natural infections. Oncoviruses typically establish persistent nonlytic infections in natural host cells, while lentivirus infections characteristically result in a variety of cytopathic effects ultimately leading to death of the target cell. Described here is a unique structural motif consisting of a strongly amphipathic and arginine-rich helical peptide segment in the carboxyl end of lentivirus TM proteins that is structurally similar to the family of cytolytic peptides produced as defensive agents by certain insects and amphibians. Also demonstrated is the lytic nature of synthetic peptides constructed from the transmembrane (TM) protein of human and simian immunodeficiency viruses (HIV and SIV). Thus, it appears that the cytopathic properties of lentiviruses may be in part attributed to the presence of lytic peptides within the TM protein, designated lentivirus lytic peptide (LLP) and that variations in this segment could account for some of the differences observed in the cytopathicity among variants of a particular lentivirus.

Inhibition of Sendai virus fusion with phospholipid vesicles and human erythrocyte membranes by hydrophobic peptides

Hydrophobic di- and tripeptides which are capable of inhibiting enveloped virus infection of cells are also capable of inhibiting at least three different types of membrane fusion events. Large unilamellar vesicles (LUV) of N-methyl dioleoylphosphatidylethanolamine (N-methyl DOPE), containing encapsulated 1-aminonaphthalene-3,6,8-trisulfonic acid (ANTS) and/or p-xylene bis(pyridinium bromide) (DPX), were formed by extrusion. Vesicle fusion (contents mixing) and leakage were then monitored with the ANTS/DPX fluorescence assay. Sendai virus fusion with lipid vesicles and Sendai virus fusion with human erythrocyte membranes were measured by following the relief of fluorescence quenching of virus labeled with octadecylrhodamine B chloride (R18), a lipid mixing assay for fusion. This study found that the effectiveness of the peptides carbobenzoxy-L-Phe-L-Phe (Z-L-Phe-L-Phe), Z-L-Phe, Z-D-Phe, and Z-Gly-L-Phe-L-Phe in inhibiting N-methyl DOPE LUV fusion or fusion of virus with N-methyl DOPE LUV also paralleled their reported ability to block viral infectivity. Furthermore, Z-D-Phe-L-PheGly and Z-Gly-L-Phe inhibited Sendai virus fusion with human erythrocyte membranes with the same relative potency with which they inhibited vesicle-vesicle and virus-vesicle fusion. The evidence suggests a mechanism by which these peptides exert their inhibition of plaque formation by enveloped viruses. This class of inhibitors apparently acts by inhibiting fusion of the viral envelope with the target cell membrane, thereby preventing viral infection. The physical pathway by which these peptides inhibit membrane fusion was investigated. 31P nuclear magnetic resonance (NMR) of proposed intermediates in the pathway for membrane fusion in LUV revealed that the potent fusion inhibitor Z-D-Phe-L-PheGly selectively altered the structure (or dynamics) of the hypothesized fusion intermediates and that the poor inhibitor Z-Gly-L-Phe did not. One possible interpretation of these 31P NMR results was that the inhibitory peptide stabilized a membrane structure with a large radius of curvature, when the fusion pathway demanded a membrane defect with a small radius of curvature. This hypothesis was tested by determining the influence of an inhibitory and a noninhibitory peptide on the formation of membraneous structures with small radii of curvature, through ultrasonic irradiation of phospholipid dispersions. The inhibitory peptide prevented the formation of membrane structures with small radii of curvature, while the noninhibitory peptide did not prevent the formation of such structures.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of virus-induced cell fusion by apolipoprotein A-I and its amphipathic peptide analogs

Apolipoprotein A-I (apoA-I), the major protein component of serum high-density lipoproteins (HDL), was found to inhibit herpes simplex virus (HSV)-induced cell fusion at physiological (approximately 1 microM) concentrations, whereas HDL did not exert any inhibitory effect. Lipid-associating, synthetic amphipathic peptides corresponding to residues 1-33 (apoA-I[1-33]) or residues 66-120 (apoA-I[66-120]) of apoA-I, also inhibited HSV-induced cell fusion, whereas a peptide corresponding to residues 8-33 of apoA-I (apoA-I[8-33]), which fails to associate with lipids, did not exert any inhibitory effect. These results suggest that lipid binding may be a prerequisite for peptide-mediated fusion inhibition. Consistent with this idea, a series of lipid-binding 22-amino-acid-residue-long synthetic amphipathic peptides that correspond to the amphipathic helical domains of apoA-I (A-I consensus series), or 18-residue-long model amphipathic peptides (18A series), were found to exert variable levels of fusion-inhibitory activity. The extent of fusion-inhibitory activity did not correlate with hydrophobic moment, hydrophobicity of the nonpolar face, helix-forming ability, or lipid affinity of the different peptides. Peptides in which the nonpolar face was not interrupted by a charged residue displayed greater fusion-inhibitory activity. Also, the presence of positively charged residues at the polar-nonpolar interface was found to correlate with higher fusion-inhibitory activity.

Oligopeptide inhibitors of HIV-induced syncytium formation

The human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein is essential for virus entry and the formation of multinucleated giant cells by cell fusion, one of the major virus-induced cytopathic effects. To study the effects of potential fusion inhibitors, a vaccinia virus recombinant expressing the envelope glycoprotein was generated and used to infect HeLa CD4+ cells. Syncytium induction was observed as early as 4 h postinfection and continued until the entire monolayer was fused. The N-terminus of the gp41 subunit of the HIV envelope protein is very hydrophobic, and appears to be involved in virus-induced membrane fusion. We synthesized several oligopeptide analogs of the N-terminal region of gp41 and determined their ability to inhibit HIV-induced cell fusion in CD4+ HeLa cells. A hexapeptide which was identical in amino acid sequence to the N-terminus of gp41 was found to completely inhibit cell fusion, whereas peptides with altered sequences showed reduced inhibitory activity. These peptides had no effect on protein synthesis, processing, or transport to the cell surface, and showed no signs of toxicity to cells even at very high concentrations. These results indicate that oligopeptides which are homologous to the fusion peptide of HIV inhibit virus-induced cytopathology, and should be evaluated further as potential antiviral agents.

Apolipoprotein A-I and its amphipathic helix peptide analogues inhibit human immunodeficiency virus-induced syncytium formation

The envelope (membrane) glycoprotein of HIV is essential for virus attachment and entry into host cells. Additionally, when expressed on the plasma membrane of infected cells, the envelope protein is responsible for mediating cell-cell fusion which leads to the formation of multinucleated giant cells, one of the major cytopathic effects of HIV infections. The envelope glycoproteins of HIV contain regions that can fold into amphipathic alpha-helixes, and these regions have been suggested to play a role in subunit associations and in virus-induced cell fusion and cytopathic effects of HIV. We therefore tested the possibility that amphipathic helix-containing peptides and proteins may interfere with the HIV amphipathic peptides and inhibit those steps of HIV infection involving membrane fusion. Apolipoprotein A-I, the major protein component of high density lipoprotein, and its amphipathic peptide analogue were found to inhibit cell fusion, both in HIV-1-infected T cells and in recombinant vaccinia-virus-infected CD4+ HeLa cells expressing HIV envelope protein on their surfaces. The amphipathic peptides inhibited the infectivity of HIV-1. The inhibitory effects were manifest when the virus, but not cells, was pretreated with the peptides. Also, a reduction in HIV-induced cell killing was observed when virus-infected cell cultures were maintained in presence of amphipathic peptides. These results have potential implications for HIV biology and therapy.

Oligopeptides interfering with calcium channels inhibit prolactin and growth hormone release by cultured anterior pituitary cells of the rat

A number of oligopeptides, protected at their N termini and possessing an aldehyde residue at their C terminal amino acids, are able to inhibit 45Ca2+ influx into anterior pituitary cells grown in monolayer culture and depolarized with high extracellular potassium concentration. In addition, the same oligopeptides interfere with hormone release, especially with that produced by lactotrophs. Our findings imply that oligopeptides may represent a new class of calcium channel ligands, and the pituitary cells are sensitive targets for them.

Prevention of HIV-1 infection and preservation of CD4 function by the binding of CPFs to gp120

Infection by human immunodeficiency virus type-1 (HIV-1) is initiated when its envelope protein, gp120, binds to its receptor, the cell surface glycoprotein CD4. Small molecules, termed N-carbomethoxycarbonyl-prolyl-phenylalanyl benzyl esters (CPFs), blocked this binding. CPFs interacted with gp120 and did not interfere with the binding of CD4 to class II major histocompatibility complex molecules. One CPF isomer, CPF(DD), preserved CD4-dependent T cell function while inhibiting HIV-1 infection of H9 tumor cells and human T cells. Although the production of viral proteins in infected T cells is unaltered by CPF(DD), this compound prevents the spread of infection in an in vitro model system.

Molecular analysis of structural protein genes of the Yamagata-1 strain of defective subacute sclerosing panencephalitis virus. IV. Nucleotide sequence of the fusion gene

The full-length cDNA corresponding to the mRNA of the fusion (F) protein of the Yamagata-1 strain of subacute sclerosing panencephalitis (SSPE) virus was cloned, and its complete nucleotide sequence was determined. The F gene was composed of 2369 nucleotides and contained a single large coding region, which is located between two noncoding regions. The 5'-terminal noncoding region consisted of 584 nucleotides comprising 44.9% cytosine, and had several inverted repetitious sequences. The 3'-terminal noncoding region had a relatively low homology of 91.7% with the MV. The coding region was expanded for nucleotides 585-2189, which encoded 534 amino acids with a molecular weight of 57,963. The homology of the amino acid sequence of the F protein between the MV and SSPE virus was 96.27%, and the positions of cysteine and proline were almost identical in the two viruses. The functional domains of SSPE-virus F protein closely resembled those of MV F protein, including the cleavage site, a signal sequence, the fusion-related stretch, the transmembrane region, and four potential glycosylation sites. Four antigenic epitopes on the MV F protein were also conserved on the SSPE-virus F protein. However, deletion of one nucleotide (position 2155) of the SSPE virus was found when compared with the MV, and shifted the coding frame, causing the substitutions of 27 C-terminal amino acids of the MV F protein with 11 different residues. The variations of the C-terminal region of the F protein were observed with two other SSPE viruses, suggesting that this may be a common property of SSPE virus that differs from MV.

Characterization of the fusion domain of the human immunodeficiency virus type 1 envelope glycoprotein gp41

The human immunodeficiency virus transmembrane glycoprotein gp41 has at its amino terminus a strongly hydrophobic stretch of 28 amino acids flanked by a highly conserved series of polar amino acids. To investigate the role in syncytium formation of the hydrophobic amino terminus of gp41 and the polar border of this hydrophobic region, we introduced eight single-amino acid substitutions and one double-amino acid substitution in the amino-terminal 31 amino acids of gp41. The mutant envelope glycoproteins were expressed from two distinct human immunodeficiency virus type 1 envelope glycoprotein expression vectors; the effects of the mutations on syncytium formation, envelope glycoprotein transport, secretion, and CD4 receptor-binding were analyzed. Results showed that polar substitutions throughout the hydrophobic amino terminus of gp41 greatly reduced or blocked syncytium formation mediated by the human immunodeficiency virus type 1 envelope glycoproteins, as did nonconservative mutations in the polar border of the hydrophobic amino terminus. Mutations at gp41 amino acids 15, 26, and 29 also significantly increased the extent of gp120 secretion into the extracellular medium. None of the mutations detectably affected envelope glycoprotein processing or envelope glycoprotein binding to CD4.

Antiviral effects of apolipoprotein A-I and its synthetic amphipathic peptide analogs

Apolipoprotein A-I (apo A-I), the major protein component of serum high density lipoproteins, was found to inhibit herpes simplex virus (HSV)-induced cell fusion at physiological (approximately 1 microM) concentrations. An 18 amino acid-long synthetic amphipathic alpha-helical peptide analog of apo A-I (18A) was also found to inhibit HSV-induced cell fusion at similar concentration (approximately 2 microM). Dimers of 18A connected via a proline (37pA) or an alanine (37aA) residue also inhibited virus-induced cell fusion at similar concentration, suggesting that the presence of a proline turn does not influence the antiviral activity of the amphipathic peptides. However, a peptide analog 18R, in which the distribution of charged residues was reversed, inhibited virus-induced cell fusion only at a higher (approximately 125 microM) concentration, suggesting that the anti-viral activity of the amphipathic peptide is strongly influenced by the nature of the charge distribution at the polar-nonpolar interface. Consistent with their ability to inhibit virus-induced cell fusion, the peptides inhibited the spread of HSV infection as demonstrated by a 10-fold reduction in the virus yield, when virus-infected cells were maintained in the presence of amphipathic peptides. The amphipathic peptides also inhibited penetration of virus into cells, but did not exert any effect on virus adsorption. A nearly complete inhibition of virus penetration was observed when the virus, or both virus and cells, was pretreated with the peptide, suggesting that the peptides may have a direct effect on the virus. The results indicate that amphipathic helices may be useful in designing novel antiviral agents that inhibit penetration and spread of enveloped viruses.

Evaluation of the antiviral effect of synthetic oligopeptides whose sequences are derived from paramyxovirus F1 N termini

We examined the antiviral effects of three oligopeptides, carbobenzoxy(Z)-D-Phe-Ile-Gly, Z-D-Leu-Ile-Gly and Z-D-Phe-Phe-Gly, which mimic the N-terminal regions of F1 glycoproteins of two Newcastle disease virus strains (Miyadera and D26) and Sendai virus, respectively. Only one of these peptides, Z-D-Phe-Phe-Gly, significantly and with a similar potency inhibited viruses of homologous and heterologous F1 N-terminal sequences, suggesting no strict sequence requirement for inhibition. Furthermore, the enveloped RNA viruses of several different families showed essentially the same sensitivity to the three peptides as the paramyxoviruses, while a non-enveloped RNA virus was not susceptible to any of them. In addition, the Z-D-Phe-Phe-Gly peptides was effective only when the virus particles had been pretreated before infection.

Measles virus-substance P receptor interactions. Possible novel mechanism of viral fusion

Measles virus (MV) encodes the fusion protein (F) that mediates cell fusion and intercellular spread of the virus, and is homologous to the carboxy terminus of the neuropeptide substance P (SP). In addition, the oligopeptide Z-D-Phe-L-Phe-Gly, also homologous to F and SP, inhibits MV fusion with target cells. These observations raise the question of whether MV uses the SP receptor (SPR) during a specific phase of its infectious cycle. In this report, we examine the structural and functional consequences of this interaction and show, using cross-linking studies, that MV and SP specifically bind to a 52-58-kD protein, previously reported to comprise the SPR on human IM-9 lymphoblasts. Moreover, bound MV and SP are shown to reciprocally displace each other from these cells. In addition, we demonstrate that anti-SP antisera inhibits the cell-to-cell spread of MV, and that SP blocks MV fusion with target cells. These results indicate the presence of MV-SPR interactions during viral fusion, and suggest possible novel mechanisms for viral entry into cells.

Membrane fusion of enveloped viruses: especially a matter of proteins

To infect mammalian cells, enveloped viruses have to deposit their nucleocapsids into the cytoplasm of a host cell. Membrane fusion represents a key element in this entry mechanism. The fusion activity resides in specific, virally encoded membrane glycoproteins. Some molecular properties of these fusion proteins will be briefly described. These properties will then be correlated to the ability of a virus to fuse with target membranes, and to induce cell-cell fusion. Some molecular and physical parameters affecting virus fusion--at the level of either viral or target membrane or both--and the significance of modelling virus fusion by using synthetic peptides resembling viral fusion peptides, will also be discussed.

Effect of drugs which inhibit cholesterol synthesis on syncytia formation in vero cells infected with measles virus

We found that nontoxic doses of two inhibitors of cholesterol synthesis, namely W-7 and cerulenin, delayed syncytia formation in vero cells infected with measles virus. To correlate syncytia formation and lipidic membrane changes induced by these drugs, we labelled cell lipids with [14C]acetate. Measles virus infection increased the incorporation of radiolabel into fatty acids, triacylglycerol, cholesterol ester, and decreased its incorporation into cholesterol and 1,2-diacylglycerol. The ratios phosphatidylcholine/sphingomyelin and free cholesterol/lanosterol-dihydrolanosterol also decreased during the infection. W-7 and cerulenin greatly altered lipid metabolism. Both decreased the phosphatidylcholine to sphingomyelin and the cholesterol to lanosterol-dihydrolanosterol ratios. Z-D-Phe-L-Phe-L-Gly, a tripeptide which corresponds to the N-terminal sequence of the viral fusion protein (responsible for syncytia formation) and which inhibits virus-induced cell fusion without affecting virus synthesis also perturbed cholesterol metabolism. The tripeptide reversed the phosphatidylcholine to sphingomyelin ratio in infected cells. At non-toxic doses, W-7 inhibited the synthesis of infectious virus. Cerulenin which inhibited strongly the lipid synthesis did not. Finally, the well characterized inhibitors of cholesterol synthesis, mevinolin, ketoconazole and miconazole were shown to inhibit the syncytia formation. We conclude that the inhibition of syncytia by W-7 and cerulenin is associated with their capacity to alter the cholesterol metabolism, whereas the antiviral effect of W-7 does not seem related to this capacity.

Rotavirus gene structure and function

Knowledge of the structure and function of the genes and proteins of the rotaviruses has expanded rapidly. Information obtained in the last 5 years has revealed unexpected and unique molecular properties of rotavirus proteins of general interest to virologists, biochemists, and cell biologists. Rotaviruses share some features of replication with reoviruses, yet antigenic and molecular properties of the outer capsid proteins, VP4 (a protein whose cleavage is required for infectivity, possibly by mediating fusion with the cell membrane) and VP7 (a glycoprotein), show more similarities with those of other viruses such as the orthomyxoviruses, paramyxoviruses, and alphaviruses. Rotavirus morphogenesis is a unique process, during which immature subviral particles bud through the membrane of the endoplasmic reticulum (ER). During this process, transiently enveloped particles form, the outer capsid proteins are assembled onto particles, and mature particles accumulate in the lumen of the ER. Two ER-specific viral glycoproteins are involved in virus maturation, and these glycoproteins have been shown to be useful models for studying protein targeting and retention in the ER and for studying mechanisms of virus budding. New ideas and approaches to understanding how each gene functions to replicate and assemble the segmented viral genome have emerged from knowledge of the primary structure of rotavirus genes and their proteins and from knowledge of the properties of domains on individual proteins. Localization of type-specific and cross-reactive neutralizing epitopes on the outer capsid proteins is becoming increasingly useful in dissecting the protective immune response, including evaluation of vaccine trials, with the practical possibility of enhancing the production of new, more effective vaccines. Finally, future analyses with recently characterized immunologic and gene probes and new animal models can be expected to provide a basic understanding of what regulates the primary interactions of these viruses with the gastrointestinal tract and the subsequent responses of infected hosts.

Identification of amino acids recognized by syncytium-inhibiting and neutralizing monoclonal antibodies to the human parainfluenza type 3 virus fusion protein

Neutralizing monoclonal antibodies specific for the fusion (F) glycoprotein of human parainfluenza type 3 virus (PIV3) were used to select neutralization-resistant antigenic variants. Sequence analysis of the F genes of the variants indicated that their resistance to antibody binding, antibody-mediated neutralization or to both was a result of specific amino acid substitutions within the neutralization epitopes of the F1 and F2 subunits. Comparison of the locations of PIV3 neutralization epitopes with those of Newcastle disease and Sendai viruses indicated that the antigenic organization of the fusion proteins of paramyxoviruses is similar. Furthermore, some of the PIV3 epitopes recognized by syncytium-inhibiting monoclonal antibodies are located in an F1 cysteine cluster region which corresponds to an area of the measles virus F protein involved in fusion activity.

CD4-independent infection of human neural cells by human immunodeficiency virus type 1

A number of studies have indicated that central nervous system-derived cells can be infected with human immunodeficiency virus type 1 (HIV-1). To determine whether CD4, the receptor for HIV-1 in lymphoid cells, was responsible for infection of neural cells, we characterized infectable human central nervous system tumor lines and primary fetal neural cells and did not detect either CD4 protein or mRNA. We then attempted to block infection with anti-CD4 antibodies known to block infection of lymphoid cells; we noted no effect on any of these cultured cells. The results indicate that CD4 is not the receptor for HIV-1 infection of the glioblastoma line U373-MG, medulloblastoma line MED 217, or primary human fetal neural cells.

Entry mechanisms of enveloped viruses. Implications for fusion of intracellular membranes

Enveloped viruses infect cells by a mechanism involving membrane fusion. This process is mediated and triggered by specific viral membrane glycoproteins. Evidence is accumulating that fusion of intracellular membranes, as occurs during endocytosis and transport between intracellular organelles, also requires the presence of specific proteins. The relevance of elucidating the mechanisms of virus fusion for a better understanding of fusion of intracellular membranes is discussed.

A putative murine ecotropic retrovirus receptor gene encodes a multiple membrane-spanning protein and confers susceptibility to virus infection

Murine type C ecotropic retrovirus infection is initiated by virus envelope binding to a membrane receptor expressed on mouse cells. We have identified a cDNA clone that may encode for this receptor through a strategy combining gene transfer of mouse NIH 3T3 DNA into nonpermissive human EJ cells, selection of EJ clones that have acquired susceptibility to infection by retrovirus vectors containing drug resistance genes, and identification of the putative receptor cDNA clone through linkage to a mouse repetitive DNA sequence. Human EJ cells that express the cDNA acquire a million-fold increase in MuLV infectivity. The predicted 622 amino acid sequence of the putative receptor protein is extremely hydrophobic; 14 potential membrane-spanning domains have been identified. A computer-based search of sequence data banks did not identify a protein with significant similarity to the putative receptor. We conclude that a novel membrane protein determines susceptibility to ecotropic MuLV infection by binding and/or fusion with the virus envelope.

Mutations located on both F1 and F2 subunits of the Newcastle disease virus fusion protein confer resistance to neutralization with monoclonal antibodies

The fusion gene sequence of six Newcastle disease virus escape mutants revealed that residues important for the integrity of antigenic site 1 and antigenic site 2 were located, respectively, on the F2 subunit and within the cysteine-rich domain of the F1 subunit. We further report the antibody-binding capacity of these mutants.

Molecular biology of Friend viral erythroleukemia

Friend virus clearly provides an important model for understanding the molecular biology of cancer. Moreover, the most important aspects of the erythroleukemia can be caused by a single SFFV infection in the absence of any helper virus. The SFFV env gene encodes a membrane glycoprotein, gp55. This glycoprotein, when expressed on erythroblast surfaces, causes a constitutive mitogenesis. However, SFFV infections only rarely increase the cell's self-renewal capability or abrogate its commitment to differentiate. Therefore, the consequence of infection is initially a polyclonal erythroblastosis. This polyclonal proliferation usually leads to cell differentiation and to recovery unless helper virus is present to cause continuing infection of new erythroblasts. Extremely rare SFFV proviral integrations, however, result in abrogation of the cell's commitment to differentiate and in the concomitant acquisition of cell immortality. These immortalizing proviral integrations occur at only a small number of sites in the mouse genome. Therefore, the mitogenic and immortalizing stages of erythroleukemia are now known to be caused by discrete genetic events--the first involving the SFFV env gene and the second involving the rare proviral integration sites. In early investigations of Friend virus, the first stage always preceded the second stage by at least several weeks. Now it is known that this delay in onset of the second stage is caused solely by statistics. Every SFFV-infected erythroblast is mitogenically activated, yet only rarely does the SFFV proviral integration produce immortality. Both steps in leukemogenesis can be caused simultaneously in an erythroblast by a rare single SFFV proviral integration. There has been an explosion of interest in retroviral env gene-mediated pathogenesis. Such pathogenesis has been recently associated with most of the naturally transmitted retroviral diseases including AIDS. Such pathogenesis involves in different viruses immunosuppression, anemia, neuropathy, and leukemia (Mathes et al. 1978; Simon et al. 1984, 1987; Weiss et al. 1985; Lifson et al. 1986; Riedel et al. 1986; Sitbon et al. 1986; Sodroski et al. 1986; Mitani et al. 1987; Schmidt et al. 1987; Klase et al. 1988; Overbaugh et al. 1988a, b). The shuffling and dynamic env gene rearrangements that have been associated with murine retroviral leukemogenesis have also now been seen in FeLV-FAIDS and HIV (Fisher et al. 1988; Overbaugh et al. 1 t88b; Saag et al. 1988; Tersmette et al. 1988). Friend virus provides an important established example of such env gene pathogenesis. Although we still do not understand precisely how gp55 causes erythroblast mitosis, workers in this field have discovered important clues that may lead to answers.(ABSTRACT TRUNCATED AT 400 WORDS)

RNA virus evolution and the control of viral disease

RNA viruses and other RNA genetic elements must be viewed as organized distributions of sequences termed quasi-species. This means that the viral genome is statistically defined but individually indeterminate. Stable distributions may be maintained for extremely long time periods under conditions of population equilibrium. Perturbation of equilibrium results in rapid distribution shifts. This genomic organization has many implications for viral pathogenesis and disease control. This review has emphasized the problem of selection of viral mutants resistant to antiviral drugs and the current difficulties encountered in the design of novel synthetic vaccines. Possible strategies for antiviral therapy and vaccine development have been discussed.

A synthetic peptide corresponding to the cleavage region of VP3 from rotavirus SA11 induces neutralizing antibodies

Antibodies were elicited in rabbits by immunization with the synthetic tetradecapeptide Gln-Asn-Thr-Arg-Asn-Ile-Val-Pro-Val-Ser-Ile-Val-Ser-Arg, corresponding to amino acids 228 to 241 of SA11-VP3. Protein specificity of the antipeptide serum is demonstrated. The antipeptide serum revealed neutralizing activity directed against SA11 in a neutralization assay. Human rotavirus strains Wa, S2, and Hochi and bovine strains NCDV and UK were not neutralized, demonstrating the strain-specific neutralizing activity of the raised antipeptide serum. Upon immune electron microscopy, aggregation of SA11 particles was observed.

Structure, function, and intracellular processing of paramyxovirus membrane proteins

Paramyxoviruses are a fascinating group of viruses with diverse hosts and disease manifestations. They are valuable systems for studying viral pathogenesis, molecular mechanisms of negative strand viral replication, and glycoprotein structure and function. In the past few years this group of viruses has received increased attention and as a result there is a wealth of new information. For example, most of the genes of many paramyxoviruses have been cloned and sequenced. The recent availability of sequence information from a number of paramyxoviruses now allows the direct comparison of the amino acid sequence and determinants of secondary structure of analogous genes across the family of viruses. Such comparisons are revealing for two reasons. First, results provide clues to the evolution of these viruses. Second, and more importantly, comparisons of analogous genes may point to sequences and structural determinants that are central to the function of the individual proteins. Below is a comparison of five of the paramyxovirus genes with a discussion of the implications of common structural determinants for function, intracellular processing, and evolutionary origin. The focus is on the paramyxovirus membrane proteins, although other proteins are discussed briefly.

Endoproteolytic cleavage of gp160 is required for the activation of human immunodeficiency virus

The envelope protein of human immunodeficiency virus (HIV) is synthesized as a polyprotein (gp160) and cleaved intracellularly to a gp120-gp41 heterodimer. In this study, the tryptic-like endoproteolytic cleavage site was removed by site-directed mutagenesis and replaced with a chymotryptic-like site. The resultant mutant, RIP7/mut10, was found to be indistinguishable from wild-type HIV when analyzed at the level of proviral replication, RNA processing, protein expression, and viral assembly. However, the gp160 polyprotein was not cleaved and the mutated virions were biologically inactive, until and unless they were exposed to limiting concentrations of chymotrypsin. As is the case for other enveloped mammalian viruses, endoproteolytic cleavage of the HIV envelope protein and release of a unique hydrophobic domain appear to be necessary for the full expression of viral infectivity.

Mode of insertion into a lipid membrane of the N-terminal HIV gp41 peptide segment

The complete amino-acid sequence of the gp160 polyprotein of HIV (strain WMJ1) has been analyzed by the Eisenberg procedure. The region surrounding the cleavage site between the gp120 and the gp41 subunit contains a receptor-like region immediately followed by a transmembrane-like region containing approximately 13 residues. These two regions are separated by the cleavage site between gp120 and gp41. Since the same arrangement exists in some paramyxoviruses (unpublished observation) and since the effective cleavage between a receptor-like region and the transmembrane-like region is required in paramyxoviruses to generate fusogenic segment (located at the N-terminal sequence of the transmembrane-like region), we have focused our analysis on the conformational properties of the N-terminal peptide segment of HIV gp41. This peptide segment, which consists of a helical structure according to Garnier prediction, was oriented at the lipid-water interface using a theoretical analysis method that we recently developed. Analysis of the transmembrane peptide determined by Eisenberg method shows that the helical segment orients itself in the lipid monolayer obliquely with respect to the lipid-water interface. Since this rather unusual orientation for a membrane segment of a protein is also found in the fusogenic peptide of the Newcastle Disease Virus (Virus Genes, in press) and seems to possess membrane destabilizing properties, it is in agreement with previous reports suggesting a fusogenic role for the N-terminal part of gp41.

Resistance of a measles virus mutant to fusion inhibitory oligopeptides is not associated with mutations in the fusion peptide

The nucleotide sequence and predicted amino acid sequence has been obtained for the fusion (F) protein gene of the R93 strain of measles virus and compared to that of the parental strain, Edmonston B. The R93 strain is a mutant measles virus which is able to grow and induce cell fusion in the presence of the fusion inhibiting oligopeptide, Z-D-Phe-L-Phe-L-(NO2)Arg (SV4814). Primer extension sequencing on isolated R93 mRNA demonstrated the presence of three nucleotide changes leading to three amino acid changes, none of which are in the hydrophobic NH2-terminal region of the F1 polypeptide.

Structural comparison of the cleavage-activation site of the fusion glycoprotein between virulent and avirulent strains of Newcastle disease virus

The nucleotide sequence of the mRNA encoding the fusion (F0) protein of a virulent strain of Newcastle disease virus was determined. A single open reading frame in the sequence encodes a protein of 553 amino acids with a calculated molecular weight of 59058. The amino acid sequence predicted several structural features involving the fusion-inducing hydrophobic stretch (residues 117-142) and the cleavage-activation site (residues 112-116) to generate the disulfide-linked F1 and F2 subunits. The cleavage-activation site as well as a part of the fusion-inducing sequence were compared among a series of virulent and avirulent strains by the chain-termination method using a synthetic oligonucleotide primer. It was found that without exception, the cleavage-activation site of virulent strains consisted of two dibasic residues with an intervening glutamine, Arg-Arg-Gln-Arg-Arg, whereas the corresponding region of avirulent strains was made of a sequence with single basic residues scattered among uncharged residues, Gly-LysArg-Gln-GlySer-Arg. On the basis of these observations and the previous results showing a strict correlation between the pathogenicity and the cleavability of the fusion protein of NDV (Y. Nagai, H-D. Klenk, and R. Rott, Virology, 72, 494-508, 1976), we propose the importance of the dibasic residues for efficient proteolytic activation of the fusion protein and for the pantropic property of NDV. Some strains were found to have Leu-Ile-Gly as the N-terminus of F1, whereas others contained Phe-Ile-Gly, indicating that Phe-X-Gly is not always conserved at F1 N-terminus of paramyxovirus.

The fusion glycoprotein of Sendai virus: sequence analysis of an epitope involved in fusion and virus neutralization

To localize the amino acid residues on the F glycoprotein that are involved in Sendai virus fusion and virus neutralization, an anti-F monoclonal antibody which inhibits these functions was used to select three antigenic variants. Sequence analysis of the entire F gene of the three variants identified a single mutation that was responsible for the loss of antibody binding. The mutation, a proline to glutamine substitution at residue 399, was at a position in the primary sequence far removed from the hydrophobic F1-NH2 terminus thought to be directly involved in fusion. A synthetic peptide, comprising amino acid sequences in the region of the mutation, bound to the antibody used to select the variants, suggesting that the site of mutation is also the site of antibody binding. This information suggests that in the three-dimensional structure of the F molecule the amino acid residues around proline 399 are located close to the F1-NH2 terminus, and that fusion is directly inhibited by antibody binding. Other less likely alternatives are discussed.

Expression at the cell surface of native fusion protein of the Newcastle disease virus (NDV) strain Italien from cloned cDNA

A cDNA library was constructed with poly(A)+-mRNAs from NDV-Italien infected BHK-21 cells. A clone, that hybridized to the F gene mRNA, was sequenced. A long open reading frame encodes for a protein of 553 amino acids, with a calculated molecular weight of 59,153, consisting of twelve cysteine residues and six potential glycosylation sites. The protein sequence contains a hydrophobic region at the N-terminus of F1 and a presumptive long transmembrane fragment near the C-terminus. Comparison of the F proteins from NDV strains Italien and Australia-Victoria shows that the sequences are very similar, with conservation of most cysteine residues and of the potential glycosylation sites. The F coding sequence was inserted into the genome of vaccinia virus under the control of vaccinia P7.5 transcriptional regulatory sequences. Expression of F protein was demonstrated by indirect immunofluorescence with five anti-F monoclonal antibodies known to react with conformational epitopes.

High protection of animals lethally infected with influenza virus by aprotinin-rimantadine combination

The successful therapeutic synergism of aprotinin and rimantadine, which are known to attack different viral targets, was demonstrated in influenza-virus-infected animals. Combined treatment with these drugs of mice infected with a highly lethal dose of mouse-adapted influenza virus prevented the development of fatal haemorrhagic pneumonia and protected about 75% of animals; whereas the separate administration of aprotinin and rimantadine induced 35% and 15% protection, respectively.

Protease activation mutants of Sendai virus: sequence analysis of the mRNA of the fusion protein (F) gene and direct identification of the cleavage-activation site

Trypsin cleaves the fusion protein (F) of wild-type Sendai virus into two disulfide-linked polypeptides, F1 and F2, and thereby activates the membrane fusion activity of the virus. A. Scheid and P.W. Choppin [1976). Virology, 265-277) selected mutant viruses of which the F protein could be activated by different proteases, either elastase, chymotrypsin, or plasmin. Herein, we have further characterized five of these mutants. Sequencing of each mutant mRNA encoding the 60-70 amino acids surrounding the cleavage site revealed one or two amino acid changes near or at the cleavage sites. Virions cleaved in vitro by the appropriate proteases were assayed of their fusion activity by hemolysis, and the cleavage sites were determined by amino acid sequencing. In three cases, the change of protease specificity can be accounted for by changed amino acids right at the cleavage site, whereas several other mutations that potentiate cleavage at new sites by new proteases are somewhat removed from the actual cleavage site. We surmise that such mutations might alter local polypeptide conformation, thereby allowing the proteases access to existing sites. Cleavage at new sites produced fusion proteins with novel F1 NH-termini. We found that a mutant with a charged residue at the third position of this normally hydrophobic NH-terminal sequence retains activity in the hemolysis assay, whereas a mutant with a charged residue at the first position does not.

The nucleotide sequence of the mRNA encoding the fusion protein of measles virus (Edmonston strain): a comparison of fusion proteins from several different paramyxoviruses

Membrane fusion is the primary cytopathic effect observed in cells infected with measles virus. The viral protein responsible for this process has previously been defined as the fusion (F) protein. Fusion is activated by the proteolytic cleavage of a precursor molecule (F0) to yield two disulfide-linked polypeptides (F1 and F2). In this paper the mRNA for the membrane fusion protein has been cloned and the resulting cDNAs were sequenced. A mRNA composed of 2377 nucleotides was found to contain one open reading frame which could potentially code for a protein of 550 amino acids. This corresponding gene product was identified as the fusion protein through use of antibodies directed against a synthetic peptide which was constructed from the deduced amino acid sequence. A long and rather G-C rich 5' terminus was found on the mRNA and this noncoding region may play some role in regulation of protein synthesis at the translational level. Protein sequence data derived from the cDNA clones revealed a highly conserved F1 amino terminus which is characteristic of most paramyxoviruses. Very little amino acid homology (except for the conservation of the F1 terminus and 9 cysteines) was evident when the sequence was compared to other paramyxovirus fusion proteins. However an overall hydrophobic nature was characteristic of all the F proteins and hydrophobicity plots for the fusion proteins of 4 different paramyxoviruses were very similar. Computer analysis was also employed to analyze the secondary structure of the measles virus F protein. Large stretches of alpha helix were characteristic of the regions which purportedly interact with membranes. The functional domains of the F protein and their similarity to those of the influenza hemagglutinin protein are discussed in this communication. We concluded that the distribution of hydrophobic regions capable of spanning biological membranes determines the fusogenic nature of the F protein.

Conservation in rotaviruses of the protein region containing the two sites associated with trypsin enhancement of infectivity

The primary structure of the region in the outer layer protein VP3, containing the two sites associated with trypsin enhancement of infectivity of rotavirus was found to be greatly conserved in cultivable human rotavirus serotypes 1 (Wa), 2 (DS1), and 3 (P) and in four human rotaviruses directly purified from feces. Significant differences with this conserved sequence were found in human rotavirus serotype 4 (ST3), isolated from an asymptomatic neonate, and in seven animal rotaviruses. However, the two trypsin cleavage sites were conserved in every rotavirus VP3 sequence analyzed.

Potential targets for antiviral chemotherapy

Serendipity and random screening have been successful in producing effective antiviral agents. The increase in our knowledge of the basic biochemistry of viral replication and of virus-host interrelationships has revealed not only an understanding of the targets upon which existing antiviral agents exert their inhibitory effect, but also has uncovered new potential targets. The hope is that such molecular understanding will afford the synthesis of compounds with selective antiviral activity. A review of various viral targets which are potentially susceptible to attack, and a few approaches for development of antiviral agents are presented.

Alphavirus replication in cultured cells and infected animals is inhibited by antiproteinase agents

The influence of different antiproteinase agents on alphavirus replication was examined. Sindbis virus multicycle replication in cultured cells was suppressed by N-tosyl-phenylalanine chloromethyl ketone (TPCK), an inhibitor of chymotrypsin-like proteinases, and by aprotinin, an inhibitor of a wide spectrum of proteinases. Antiviral activity of TPCK was also demonstrated in Sindbis virus-infected animals. Parenteral injections of TPCK in infected mice reduced virus titers in brain and blood. The possible mechanism(s) of antiviral action of the antiproteinase agents are discussed.