Envelope polypeptides of Friend leukemia virus: purification and structural analysis.

Roughly 10% of surface glycoproteins in the envelope of mature Friend murine leukemia virus are coupled to membrane polypeptides by disulfide bridges. The remaining 90% of these glycoproteins are associated noncovalently. However, they could also be linked to membrane polypeptides by the treatment of purified Friend murine leukemia virus with 2,2'dithiobis(m-nitropyridine). These amphiphilic heterodimer polypeptides, gp84/86, were recovered almost quantitatively in the form of aggregates, termed rosettes, when prepared by solubilization of the viral membrane with Triton X-100 and subsequent velocity sedimentation. gp69/71 and p12(E)/15(E) were purified from these protein micelles after reduction of the disulfide bonds by gel chromatography. Electron micrographs of rosettes, as well as of purified p12(E)/15(E), showed structures different from native viral knobs. Isolated gp84/86 could be reassociated and then displayed more similarity to these viral surface projections. As shown by peptide mapping, the primary structures of the glycoproteins gp69/71 are highly related as are those of the membrane polypeptides p12(E) and p15(E). Furthermore, it was shown by two-dimensional polyacrylamide gel electrophoresis and re-electrophoresis of purified gp84/86 that the larger component, gp86, was composed of gp71 associated with p15(E) and p12(E), whereas the smaller component, gp84, was formed by gp69 bound only to p12(E).     

Glycoproteins of friend murine leukemia virus: separation and NH2-terminal amino acid sequences of gp69 and gp71.

The NH2-terminal amino acid sequences (initial 23 residues) of Friend murine leukemia virus gp71 and gp69 were determined and found to be different but highly related. Friend murine leukemia virus gp71 differed from Rauscher murine leukemia virus gp70 in only one position. Friend murine leukemia virus gp69 showed approximately 41% homology to these glycoproteins but lacked the glycosylation site (sequon) occurring at position 12 in Rauscher murine leukemia virus gp70.     

Purification and serological characterization of the major envelope glycoprotein from AKR murine leukemia virus and its reactivity with autogenous immune sera from mice.

The major envelope glycoprotein (gp71) from AKR murine leukemia virus (MuLV) was purified and its serological reactivity with heterologous and autogenous immune mouse sera was examined. Homologous and interspecies competition radioimmunoassays using antisera to Rauscher-MulV gp69/71 or Friend-MuLV gp71 or antisera to feline leukemia virus to precipitate 125I-labeled gp71 from various MuLV showed that distinct differences exist between Rauscher- or Friend-MuLV and AKR-MuLV glycoproteins. Characteristically the AKR-MuLV gp71, in contrast to FLV or RLV gp71, does not compete fully in homologous or interspecies radioimmunoassays with iodinated Friend of Rauscher glycoproteins. Purified 125I-labeled AKR-MuLV gp71, in contrast to the Rauscher- or Friend-MuLV glycoproteins, reacts with normal (autogenous immune) mouse sera in direct radioimmune precipitation assays. Competition experiments further demonstrate that this is a predominant immunological reactivity of normal mouse sera which had previously been detected by radioimmune precipitation assay against intact virions.     

Role of carbohydrate in biological functions of Friend murine leukemia virus gp71.

Purified gp71 of Friend murine leukemia virus (FLV) can interfere with virus infection, absorb neutralizing antibody, and in the presence of group-specific anti-gp71 antibody, hemagglutinate sheep erythrocytes. Interference by FLV gp71 with several murine leukemia viruses (MuLV) was tested in the XC and S + L- assay systems. Treatment of gp71 with trypsin or Pronase eliminated its interfering capacity. However, treatment with neuraminidase or a mixture of glycosidase enzymes, which left the major serological properties of gp71 intact, did not reduce the interference potential of gp71 for FLV or AKR MuLV. The capacity of gp71 to absorb type- or group-specific virus-neutralizing antibodies was similarly affected by the various enzyme treatments. In contrast, indirect hemagglutination by gp71 was abolished not only by proteases but also by treatment with glycosidase enzymes, although neuraminidase had no effect. Preliminary data indicate that infectivity of FLV or xenotropic MuLV was not affected by short treatment with glycosidase enzymes.     

Structural Proteins of Mammalian Oncogenic RNA Viruses: Murine Leukemia Virus Neutralization by Antisera Prepared Against Purified Envelope Glycoprotein

Goat and rabbit antisera prepared against a purified Rauscher murine leukemia virus glycoprotein (gp69/71) rapidly neutralized spleen focus-forming virus in Rauscher and Friend virus preparations. Absorption studies revealed that most of the neutralizing activity of goat anti-Rauscher virus gp69/71 serum was directed against type- and group-specific determinants.     

Possible cell surface receptor for Friend murine leukemia virus isolated with viral envelope glycoprotein complexes.

Water-soluble multimeric complexes of Friend leukemia virus envelope glycoprotein gp85 bind specifically to C57BL/6 mouse spleen leukocytes. Such complexes were used to isolate cell surface receptors for the virus, using an immunoprecipitation technique. The putative rceptor has a molecular weight of 14,000. Mouse H-2 histocompatibility antigens, which are receptors for Semliki Forest virus, are not receptors for Friend leukemia virus.     

Natural immunity in mice to the envelope glycoprotein of endogenous ecotropic type C viruses: neutralization of virus infectivity.

The ability of naturally immune mouse sera to neutralize ecotropic AKR murine leukemia virus (MuLV) was examined by using unfrozen virus preparations harvested for 1 h. In this assay several mouse sera significantly and consistently neutralized MuLV infectivity. The ability of these sera to neutralize was correlated with the presence of antibodies against MuLV detectable in a radioimmune precipitation assay using radioactively labeled intact virions. This neutralization was specific, in that either N- or B-tropic viruses, but not Friend MuLV, were neutralized. In addition, neutralization could be abrogated with purified AKR MuLV gp71 at concentrations that do not interfere with virus infectivity but could not be abrogated with Rauscher MuLV gp71. Neutralizing activity could be removed by absorption with intact AKR MuLV, but not by absorption with Friend MuLV, a BALB/c xenotropic virus, or with NZB xenotropic virus. All the neutralizing activity of (B6C3)F1 mouse sera was associated with the immunoglobulin G fraction.     

Isolation and Purification of the Envelope Proteins of Newcastle Disease Virus

A procedure has been developed for the isolation of Newcastle disease virus (NDV) envelope proteins. The two surface glycoproteins and the non-glycosylated membrane protein were solubilized with 2% Triton X-100 and 1 m KCl. Removal of the KCl by dialysis yielded by precipitation a pure preparation of the non-glycosylated membrane protein, which is insoluble in solutions of low ionic strength. The soluble fraction consisting of the two glycoproteins possessed full neuraminidase and hemagglutinating activities. The two glycoproteins could be separated by rate zonal sedimentation in a sucrose gradient containing 1% Triton X-100 and 1 m KCl. Under these conditions, the sedimentation coefficient of the larger glycoprotein, virus protein 1, was 9.3s, and that of the smaller, virus protein 2, was 6.1s. Both hemagglutinating and neuraminidase activities were associated with virus protein 1; virus protein 2 had neither activity. The results suggest that both activities reside on a single NDV glycoprotein. Similar results were obtained previously with another paramyxovirus, simian virus 5. These findings suggest that the association of hemagglutinating and neuraminidase activities with one glycoprotein is a general property of the paramyxovirus group.     

A cell surface integral membrane glycoprotein of 85,000 mol wt (gp85) associated with triton X-100-insoluble cell skeleton

The Triton X-100-insoluble skeleton of baby hamster kidney BHK cells consists of the nucleus, intermediate-size filaments, and actin fibers. By transmission electron microscopy, membrane fragments were found to be associated with these insoluble structures. When radioiodinated or [3H]glucosamine-labeled cells were extracted with 0.5% Triton, most plasma membrane glycoproteins were solubilized except for a glycoprotein with a molecular weight of 85,000 (gp85) that remained associated with the insoluble skeletons. Immunoprecipitation with a specific antiserum indicated that the gp85 is not a proteolytic degradation product of fibronectin, an extracellular matrix glycoprotein insoluble in detergent. A monoclonal antibody of BHK cells specific for gp85 was produced. Immunofluorescence analysis with this monoclonal antibody indicated that gp85 is not associated with the extracellular matrix, but is confined to the cell membrane. Both in fixed and unfixed intact cells, fluorescence was concentrated in dots preferentially aligned in streaks on the cell surface. Gp85 was found to behave as an integral membrane protein interacting with the hydrophobic core of the lipid bilayer since it was extracted from membrane preparations by ionic detergents such as SDS, but not by 0.1 N NaOH (pH 12) in the absence of detergents, a condition known to release peripheral molecules. Association of gp85 with the cell skeleton was unaffected by increasing the Triton concentration up to 5%, but it was affected when actin filaments were dissociated or when a protein-denaturing agent (6 M urea) was used in the presence of Triton, suggesting that protein-protein interactions are involved in the association of gp85 with the cell skeleton. We conclude that gp85 is an integral plasma membrane glycoprotein that might have a role in cell surface-cytoskeleton interaction.     

Transmembrane envelope glycoproteins of human immunodeficiency virus type 2 and simian immunodeficiency virus SIV-mac exist as homodimers.

An 80-kilodalton glycoprotein (gp80) was produced in human immunodeficiency virus type 2 (HIV-2)-infected cells along with three envelope glycoproteins that we have recently reported: the extracellular glycoprotein (gp125), the envelope glycoprotein precursor (gp140), and the transient dimeric form of the precursor (gp300). gp125 and gp80 were detectable after the synthesis of gp140 and the formation of gp300. Using a specific monoclonal antibody, we showed here that gp80 is a dimeric form of the transmembrane glycoprotein gp36 of HIV-2. Dimerization of the envelope glycoprotein precursor and dimeric forms of the transmembrane glycoproteins were also observed in cells infected with simian immunodeficiency virus (SIV-mac), a virus closely related to HIV-2. Under routine conditions of our experiments (i.e., extraction by 1% Triton X-100 before polyacrylamide gel electrophoresis in sodium dodecyl sulfate [SDS]), monomeric forms of the transmembrane glycoprotein of HIV-2 and SIV-mac were only seldomly observed. Dimeric forms of the envelope precursors and the transmembrane glycoproteins are probably stabilized by extraction in the nonionic detergent Triton X-100 since such dimeric forms resist dissociation during subsequent electrophoresis in the presence of the ionic detergent SDS. However, the dissociation of these dimeric forms might occur when samples are prepared by extraction directly in 1% SDS or by incubation of the purified dimers at acidic pH. Dimerization of the envelope precursor might be required for its processing to give the mature envelope proteins, whereas the transmembrane dimer might be essential for optimal structure of the virion and thus its infectivity.     

Membrane association and defective transport of spleen focus-forming virus glycoproteins

The gp52 glycoprotein of the spleen focus-forming virus found in the Friend and Rauscher complexes of murine leukemia viruses (MuLV) has been previously identified as a recombinant molecule involving substitutions and deletions of the MuLV env gene. Unlike the MuLV structural glycoproteins, gp52 is defective in its transport to the cell surface. We have studied aspects of the intracellular transport and membrane association of gp52 to investigate the possible mechanisms underlying the defective transport process. It was found that a panel of monoclonal antibodies to different epitopes of p 15E, as well as an antiserum to a synthetic peptide corresponding to the carboxy terminus of MuLV envelope precursors, failed to react with gp52. Despite the possible absence of membrane-anchoring regions of MuLV envelope proteins known to reside on p 15E, gp52 was not found to be secreted into the culture fluids. Detergent extraction studies indicated that gp52 is associated with the membranes and not the contents of microsomal vesicles in speen focus-forming virus-infected cells. gp65, the processed form of gp52, could be labeled with [3H]palmitic acid, suggesting a membrane association. To determine whether a spontaneous denaturation occurs leading to aggregation and defective transport of gp52, we studied the surface expression of gp52 in cells grown at different temperatures, as well as the solubility of gp52 in low concentrations of Triton X-100. No evidence of aggregation or of a temperature-dependent difference in transport was obtained. gp52 appears to be a monotopic integral membrane protein, unlike MuLV envelope proteins which are bitopic integral membrane proteins; proteolytic digestion of intact microsomal vesicles did not reveal a detectable cytoplasmic tail under conditions where this could be demonstrated on MuLV envelope precursors. We suggest that a loss of putative signals involved in mediating intracellular transport is a likely cause for the defective transport of the spleen focus-forming virus glycoproteins.     

Role of carbohydrate in determining the immunochemical properties of the major glycoprotein (gp71) of Friend murine leukemia virus.

Treatment of Friend leukemia virus gp71 with protease-free glycosidase enzymes results in removal of the major portion of the carbohydrate without affecting the amount of protein present. The digested material migrates as protein of about 60,000 to 65,000 molecular weight on sodium dodecyl sulfatepolyacrylamide gel electrophoresis. Analyses of the serological properties of gp71 after enzyme treatment indicated that the type, group, and interspecies determinants were not destroyed. In contrast, treatment with proteolytic enzymes led to the complete destruction of the gp71 molecule, including the total elimination of its serological reactivity as measured by direct and competition radioimmunoassay and by a serum cytotoxicity assay. We conclude that the carbohydrate portion of gp71 is not of major significance in defining the antigenic determinants of this viral glycoprotein.     

Isolation and immunochemical characterization of fractions from membranes of Aspergillus fumigatus with protease activity

Two fractions exhibiting acid protease activity (AFPI and AFPII) were isolated by extraction of membrane vesicles of Aspergillus fumigatus with Triton X-100. These two fractions produced single bands in both polyacrylamide and sodium dodecyl sulfate polyacrylamide gel electrophoresis and showed apparent molecular weights of 73,000 and 43,000, respectively. Molecular weights determined by gel filtration in the absence and presence of Triton X-100 and sedimentation velocities in analytical ultracentrifugation indicated hydrophobic characteristics, since both fractions readily aggregated and complexed with Triton X-100; both exhibited elevated enzyme activities in the presence of Triton X-100. Carbohydrate content was 93% for AFPI and 85% for AFPII. The enzymatic fractions demonstrated different pH optima in the acid range as well as different temperature stabilities. Both protease fractions cross reacted in double immunodiffusion, while in crossed immunoelectrophoresis both demonstrated five precipitin peaks, each with similar patterns. AFPI demonstrated two additional precipitin peaks in crossed immunoelectrophoresis. As determined by crossed immunoaffinoelectrophoresis, the protease fractions demonstrated galactose and mannose residues. In biotin-avidin enzyme-linked immunosorbent assay both fractions reacted with allergic bronchopulmonary aspergillosis and aspergilloma sera. It can be concluded that two fractions with protease activity of A. fumigatus reported here may be of significance in Aspergillus-induced diseases.     

Interspecies determinants of Friend leukemia virus antigens involved in cytolysis of virus-pfoducing cells.

The cytolytic reactivity of a complex goat anti-feline leukemia virus (FeLV) antiserum for mouse cells (Eveline) releasing large quantities of Friend leukemia virus (FLV) was analyzed by the sensitive [14C]nicotinamide release microcytotoxicity assay. Whereas this interspecies killing reactivity could be blocked by absorption of the goat anti-FeLV serum with a preparation of disrupted FLV, absorption with purified FLV gp71, the major envelope glycoprotein, had no effect. Subsequent serum absorptions with the individual FLV structural polypeptides revealed that the lysis of the Eveline cells by the goat anti-FeLV serum is mediated by antibodies recognizing the interspecies determinant of p30, the major internal capsid protein. The expression of this internal viral component at the surface of virus-producing cells is discussed further. The results also demonstrated that removal of approximately 70% of the carbohydrate portion of gp71 with a preparation of glycosidases did not affect the integrity of its interspecies determinant; these results are in agreement with an earlier study (Bolgnesi et al., 1975) that examined primarily the group- and type-specific sites.     

Triton X-100 induced dissociation of beef heart cytochrome c oxidase into monomers

Purified beef heart cytochrome c oxidase, when solubilized with at least 5 mg of Triton X-100/mg of protein, was found to be a monodisperse complex containing 180 molecules of bound Triton X-100 with a protein molecular weight of 200 000, a Stokes radius of 66-72 A, and an s(0)20,w = 8.70 S. These values were determined by measurement of the protein molecular weight by sedimentation equilibrium in the presence of D2O, evaluation of the sedimentation coefficient, S(0)20,w, by sedimentation velocity with correction for its dependence upon the concentration of protein and detergent, and measurement of the effective radius by calibrated Sephacryl S-300 gel chromatography. The monomeric complex was judged to be homogeneous and monodisperse since the effective mass of the complex was independent of the protein concentration throughout the sedimentation equilibrium cell and a single protein schlieren peak was observed during sedimentation velocity. These results are interpreted in terms of a fully active monomeric complex that exhibits typical biphasic cytochrome c kinetics and contains 2 heme a groups and stoichiometric amounts of the 12 subunits normally associated with cytochrome c oxidase. With lower concentrations of Triton X-100, cytochrome c oxidase dimers and higher aggregates can be present together with the monomeric complex. Monomers and dimers can be separated by sedimentation velocity but cannot be separated by Sephacryl S-300 gel filtration, probably because the size of the Triton X-100 solubilized dimer is not more than 20% larger than the Triton X-100 solubilized monomer.(ABSTRACT TRUNCATED AT 250 WORDS)     

Assembly of glycoprotein-80 adhesion complexes in Dictyostelium. Receptor compartmentalization and oligomerization in membrane rafts

The phospholipid-anchored membrane glycoprotein (gp)-80 mediates cell-cell adhesion through a homophilic trans-interaction mechanism during Dictyostelium development and is enriched in a Triton X-100-insoluble floating fraction. To elucidate how gp80 adhesion complexes assemble in the plasma membrane, gp80-gp80 and gp80-raft interactions were investigated. A low density raft-like membrane fraction was isolated using a detergent-free method. It was enriched in sterols, the phospholipid-anchored proteins gp80, gp138, and ponticulin, as well as DdCD36 and actin, corresponding to components found in the Triton X-100-insoluble floating fraction. Chemical cross-linking revealed that gp80 oligomers were enriched in the raft-like membrane fraction, implicating stable oligomer-raft interactions. However, gp80 oligomers resisted sterol sequestration and were partially dissociated with Triton X-100, suggesting that compartmentalization in rafts was not solely responsible for their formation. The trans-dimer known to mediate adhesion was identified, but cis-oligomerization predominated and displayed greater accumulation during development. In fact, oligomerization was dependent on the level of gp80 expression and occurred among isolated gp80 extracellular domains, indicating that it was mediated by direct gp80-gp80 interactions. Rafts existed in gp80-null cells and such pre-existent membrane domains may provide optimal microenvironments for gp80 cis-oligomerization and the assembly of adhesion complexes.     

Insights into virus inactivation by polysorbate 80 in the absence of solvent

Triton X-100 has long been used either alone or in combination with solvent to inactivate enveloped viruses in biopharmaceutical manufacturing. However, European Chemicals Agency (ECHA) officially placed Triton X-100 on the Annex XIV authorization list in 2017 because 4-(1,1,3,3-tetramethylbutyl) phenol, a degradation product of Triton X-100, is of harmful endocrine disrupting activities. As a result, any use of Triton X-100 in the European Economic Area would require an ECHA issued authorization after the sunset date of January 4, 2021. In search of possible replacements for Triton X-100, we discovered that polysorbate 80 (PS80) in absence of any solvents was able to effectively inactive enveloped viruses such as xenotropic murine leukemia virus and pseudorabies virus with comparable efficacy as measured by log reduction factors. Interestingly, PS80 did not show any virucidal activities in phosphate buffered saline (PBS) while achieving robust virus inactivation in cell-free Chinese hamster ovary (CHO) bioreactor harvests. This intriguing observation led us to speculate that virus inactivation by PS80 involved components in the cell-free CHO bioreactor harvests that were absent in PBS. Specifically, we hypothesized that esterase and/or lipases in the cell-free bioreactor harvests hydrolyzed PS80 to yield oleic acid, a known potent virucidal agent, which in turn inactivated viruses. This theory was confirmed using purified recombinant lysosomal phospholipase A2 isomer (rLPLA2) in PBS. Subsequent characterization work has indicated that virus inactivation by PS80 is effective and robust within temperature and concentration ranges comparable to those of Triton X-100. Similar to Triton X-100, virus inactivation by PS80 is dually dependent on treatment time and temperature. Unlike Triton X-100, PS80 inactivation does not correlate with concentrations in a simple manner. Additionally, we have demonstrated that PS20 exhibits similar virus inactivation activities as PS80. Based on the findings described in the current work, we believe that PS80 is potentially a viable replacement for Triton X-100 and can be used in manufacturing processes for wide spectrum of biopharmaceuticals to achieve desirable virus clearance. Finally, the advantages and disadvantages of using PS80 for virus inactivation are discussed in the contexts of GMP manufacturing.     

Subcellular localization of the env-related glycoproteins in Friend erythroleukemia cells.

A scheme was developed for the subcellular fractionation of murine erythroleukemia cells transformed by Friend leukemia virus. The subcellular localization of the env-related glycoproteins was determined by immune precipitation with antiserum against gp70, the envelope glycoprotein of the helper virus, followed by gel electrophoresis. In cells labeled for 2 h with [35S]methionine, the glycoprotein encoded by the defective spleen focus-forming virus, gp55SFFV, was found primarily in the nuclear fraction and in fractions containing dense cytoplasmic membranes such as endoplasmic reticulum. A similar distribution was noted for gp85env, the precursor to gp70. The concentration of viral glycoproteins in the nuclear fraction could not be accounted for by contamination with endoplasmic reticulum. In pulse-chase experiments, neither glycoprotein underwent major redistribution. However, labeled gp85env disappeared from intracellular membranes with a half-time of 30 min to 1 h, whereas labeled gp55SFFV was stable during a 2-h chase. In plasma membrane preparations with very low levels of contamination with endoplasmic reticulum, gp70 was the major viral env-related glycoprotein detected; a minor amount of gp55SFFV and no gp85env could be detected. The unexpected result of these experiments is the amount of viral glycoproteins found in the nuclear fraction. Presence of viral proteins in the nucleus could be relevant to the mechanism of viral leukemogenesis.     

Identification and characterization of a Triton X-100 replacement for virus inactivation

Triton X-100 detergent treatment is a robust enveloped virus inactivation unit operation included in biopharmaceutical manufacturing processes. However, the European Commission officially placed Triton X-100 on the Annex XIV authorization list in 2017 because a degradation product of Triton X-100, 4-(1,1,3,3-tetramethylbutyl) phenol (also known as 4-tert-octylphenol), is considered to have harmful endocrine disrupting activities. As a result, the use of Triton X-100 in the European Economic Area (EEA) would not be allowed unless an ECHA issued authorization was granted after the sunset date of January 4, 2021. This has prompted biopharmaceutical manufacturers to search for novel, environment-friendly alternative detergents for enveloped virus inactivation. In this study, we report the identification of such a novel detergent, Simulsol SL 11W. Simulsol SL 11W is an undecyl glycoside surfactant produced from glucose and C11 fatty alcohol. We report here that Simulsol SL 11W was able to effectively inactive enveloped viruses, such as xenotropic murine leukemia virus (XMuLV) and pseudorabies virus (PRV). By using XMuLV as a representative enveloped virus, the influence of various parameters on the effectiveness of virus inactivation was evaluated. Virus inactivation by Simulsol SL 11W was effective across different clarified bioreactor harvests at broad concentrations, pH, and temperature ranges. Simulsol SL 11W concentration, temperature of inactivation, and treatment time were identified as critical process parameters for virus inactivation. Removal of Simulsol SL 11W was readily achieved by Protein A chromatography and product quality was not affected by detergent treatment. Taken together, these results have shown the potential of Simulsol SL 11W as a desirable alternative to Triton X-100 for enveloped virus inactivation that could be readily implemented into biopharmaceutical manufacturing processes.     

Effects of monensin on morphogenesis and infectivity of Friend murine leukemia virus.

The transport of the gp70 glycoprotein to the cell surface and concomitant release of infectious virus was inhibited by treatment of Friend murine leukemia virus-infected Eveline cells with the sodium ionophore monensin. Virus yields were reduced more than 50-fold by 10(-5) M monensin, whereas particle production was reduced by 50% in monensin-treated cells. The resulting particles failed to incorporate newly synthesized gp70 and p15(E), whereas the other structural proteins, p30, p15, p12, and p10, were incorporated into virions. However, monensin did not inhibit the incorporation into virions of preformed gp70. A reduction in the efficiency of cleavage of the PrENV glycoprotein precursor and a defect in the processing of simple endo-H-sensitive to complex endo-H-resistant oligosaccharides suggest that intracellular transport of gp70 may be blocked before its entry into the Golgi apparatus. Fewer particles were found to bud from the cell surface, but intracellular vacuoles with budding virions were detected. Ferritin labeling and pulse-chase studies suggested a cell surface origin for these vacuoles. These experiments indicate that monensin inhibits the transport of Friend murine leukemia virus glycoproteins at an early stage, with a resultant block in the assembly and release of infectious virus.