Stepwise dissociation of the Semliki Forest Virus membrane with trition X-100

Disruption Mechanisms of Enveloped Viruses by Ionic and Nonionic Surfactants

The world has witnessed multiple pandemics and endemics caused by enveloped viruses in the past century. To name a few, the ongoing COVID-19 pandemic and other pandemics/endemics caused by coronaviruses, influenza viruses, HIV-1, etc. The external and topical applications of surfactants have been effective in limiting the spread of viruses. While it is well-known that surfactants inactivate virus particles (virions), the mechanism of action of surfactants against enveloped virions has not yet been established. In this work, we have evaluated the surfactant-induced disruption mechanism of a cocktail of enveloped viruses containing particles of mumps, measles, and rubella viruses. We applied the total internal reflection fluorescence microscopy technique to trace the temporal changes in the fluorescence signal from single virions upon the addition of a surfactant solution. We report that surfactants solubilize either the viral lipid membrane, proteins, or both. Ionic surfactants, depending on their charge and interaction type with the viral lipids and proteins, can cause bursting or perforation of the viral envelope, whereas a nonionic surfactant can cause either symmetric expansion or perforation of the viral envelope depending on the surfactant concentration.

Detergent-Mediated Virus Inactivation in Biotechnological Matrices: More than Just CMC

For decades, the ability of detergents to solubilize biological membranes has been utilized in biotechnological manufacturing to disrupt the lipid envelope of potentially contaminating viruses and thus enhance the safety margins of plasma- and cell-derived drugs. This ability has been linked to detergent micelles, which are formed if the concentration of detergent molecules exceeds the critical micelle concentration (CMC). Traditionally, the CMC of detergents is determined in deionized water (ddH₂O), i.e., a situation considerably different from the actual situation of biotechnological manufacturing. This study compared, for five distinct detergents, the CMC in ddH₂O side-by-side with two biopharmaceutical process intermediates relevant to plasma-derived (Immunoglobulin) and cell-derived (monoclonal antibody) products, respectively. Depending on the matrix, the CMC of detergents changed by a factor of up to ~4-fold. Further, the CMC in biotechnological matrices did not correlate with antiviral potency, as Triton X-100 (TX-100) and similar detergents had comparatively higher CMCs than polysorbate-based detergents, which are known to be less potent in terms of virus inactivation. Finally, it was demonstrated that TX-100 and similar detergents also have virus-inactivating properties if applied below the CMC. Thus, the presence of detergent micelles might not be an absolute prerequisite for the disruption of virus envelopes.

BHK-21 Cell Clones Differ in Chikungunya Virus Infection and MXRA8 Receptor Expression

Baby hamster kidney-21 (BHK-21) cells are widely used to propagate and study many animal viruses using infection and transfection techniques. Among various BHK-21 cell clones, the fibroblast-like BHK-21/C-13 line and the epithelial-like BHK-21/WI-2 line are commonly used cell clones for alphavirus research. Here we report that BHK-21/WI-2 cells were significantly less susceptible to primary infection by the alphavirus chikungunya virus (CHIKV) than were BHK-21/C-13 cells. The electroporation efficiency of alphavirus RNA into BHK-21/WI-2 was also lower than that of BHK-21/C-13. The growth of CHIKV was decreased in BHK-21/WI-2 compared to BHK-21/C-13, while primary infection and growth of the alphavirus Sindbis virus (SINV) were equivalent in the two cell lines. Our results suggested that CHIKV entry could be compromised in BHK-21/WI-2. Indeed, we found that the mRNA level of the CHIKV receptor MXRA8 in BHK-21/WI-2 cells was much lower than that in BHK-21/C-13 cells, and exogenous expression of either human MXRA8 or hamster MXRA8 rescued CHIKV infection. Our results affirm the importance of the MXRA8 receptor for CHIKV infection, and document differences in its expression in two clonal cell lines derived from the original BHK-21 cell cultures. Our results also indicate that CHIKV propagation and entry studies in BHK-21 cells will be significantly more efficient in BHK-21/C-13 than in BHK-21/WI-2 cells.

Quantitation of residual sodium dodecyl sulfate in meningococcal polysaccharide by gas chromatography-mass spectrometry

Sodium dodecyl sulfate (SDS) is a commonly used surfactant in protein solubilization and also during the polysaccharide purification. A GC-MS method has been developed to quantitate residual SDS in meningococcal polysaccharide serogroups A,C,W,Y and X circumventing the need of spectroscopic assays and HPLC based methods which are either unstable or requires the confirmation by MS. The developed method is based on quantitative conversion of SDS to 1-dodecanol at elevated temperature. Meningococcal polysaccharides and SDS standards were treated with methanolic-HCl and extracted in n-Hexane. The conversion of SDS to 1-dodecanol was confirmed by mass spectra and separation was achieved using a DB-5ms column. The mass spectral analysis of 1-dodecanol showed characteristic ions at m/z 168, 140 and 125. The GC-MS method validation performed on intermediate and purified meningococcal polysaccharides showed linearity with r² > 0.99 over the concentration range of 2.5-200 μg/ml with LOD and LOQ of 1.27 and 3.85 respectively. The method was found to be precise, robust and accurate with spike recovery ranging 83-117%. The GC-MS method can be used in the quantitation of residual SDS during polysaccharide purification and provides valuable information about consistency of polysaccharide manufacturing process for development of pentavalent meningococcal conjugate vaccine.

Quantitative determination of the surfactant-induced split ratio of influenza virus by fluorescence spectroscopy

The majority of marketed seasonal influenza vaccines are prepared using viruses that are chemically inactivated and treated with a surfactant. Treating with surfactants has important consequences: it produces 'split viruses' by solubilizing viral membranes, stabilizes free membrane proteins and ensures a low level of reactogenicity while retaining high vaccine potency. The formulation stability and potency of split influenza vaccines are largely determined by the specifics of this 'splitting' process; namely, the consequent conformational changes of proteins and interactions of solubilized particles, which may form aggregates. Robust methods to quantitatively determine the split ratio need to be developed before optimal splitting conditions can be investigated to streamline production of superior influenza vaccines. Here, we present a quantitative method, based on both steady-state and time-resolved fluorescence spectroscopy, to calculate the split ratio of the virus after surfactant treatment. We use the lipophilic dye Nile Red (NR) as a probe to elucidate molecular interactions and track changes in molecular environments. Inactivated whole influenza viruses obtained from a sucrose gradient were incubated with NR and subsequently treated with increasing concentrations of the surfactant Triton X-100 (TX-100) to induce virus splitting. NR's emission spectra showed that the addition of TX-100 caused ˜27 nm red-shifts in the emission peak, indicative of increasingly hydrophilic environments surrounding NR. The emission spectra of NR at different surfactant concentrations were analyzed with multi-peak fitting to ascertain the number of different micro-environments surrounding NR and track its population change in these different environments. Results from both the emission spectra and fluorescence lifetime spectroscopy revealed that NR showed presence in 3 distinct molecular environments. The split ratio of the virus was then calculated from the percentages of NR in these environments using both fluorescence emission and lifetime data. This study can pave the way for the development of robust methods to rapidly quantify splitting extent during vaccine manufacturing.

The effect of lipid environment in purple membrane on bacteriorhodopsin

The decay rate of the Bacteriorhodopsin (BR) photocycle intermediate M412 and proton, the proton pump efficiency (H+/M412), the ratios of M412 to other intermediates and the rotational correlation time (tauc) in purple membrane (PM) fragments treated by the zwitterionic detergent 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate (CHAPS) with different concentrations were studied. The results show that: (1) The largest effect of CHAPS on M412 decay rate and proton decay rate of BR, tauc of PM and the ratios of M412 to other intermediates in BR photocycle is in the range of its critical micelle concentration (CMC). This indicates that changes of the ratios of M412 to other intermediates, tauc, M412 decay and proton decay occur and are due to the variation of the lipid environment. (2) The dependency of proton yield on CHAPS concentrations is basically consistent with that of M412s%. This indicates the relation between proton pumping function and M412. These studies show the importance of maintaining a native environment.

Inhibition, reactivation, and determination of metal ions in membrane metalloproteases of bacterial origin using high-performance liquid chromatography coupled on-line with inductively coupled plasma mass spectrometry

High-performance liquid chromatography coupled on-line with inductively coupled plasma mass spectrometry (HPLC-ICP-MS) was used for the characterization of metal ions in several metalloproteases of bacterial origin. The different components of the bacterial extracts were separated on a size-exclusion column. The eluent of the HPLC system was continuously transported to the ICP-MS system for rapid, reproducible, and sensitive analyses of trace elements in the metalloproteases. Two different membrane proteases from Bacillus cereus and Pseudomonas aeruginosa were characterized to be zinc metalloproteases using enzymological methods and HPLC-ICP-MS. The zinc content was determined to be three molecules of zinc per protein molecule for the B. cereus protease and one molecule of zinc per protein molecule for the P. aeruginosa protease. For another purified protease, a periplasmic alanyl aminopeptidase of P. aeruginosa, the lack of protein-bound metal ions could be clearly determined-a confirmation that this main aminopeptidase of P. aeruginosa belongs to the cysteine protease family. The presence of nonionic detergents can influence the distribution of trace elements during the HPLC separation. Therefore, the use of these substances should be avoided during enzyme purification for metal analyses or they should be exchanged later for zwitterionic and ionic detergents with more strongly dissociating properties.

Mechanisms of animal cell damage associated with gas bubbles and cell protection by medium additives

Animal cell damage arising from gas sparging is considered to be a major barrier to large-scale production of recombinant biologicals in animal cell culture. Understanding sparging cell damage is therefore of significance to the application of animal cell culture. The paper reviews the hydrodynamics of bubble rupture, mechanisms of cell-bubble interaction, mathematical modelling and quantification of the sparging damage. Another interesting topic addressed in the paper is the protective effects of various medium additives against fluid mechanical cell damage, especially those surface-active polymers such as pluronic polyols, methylcellulose and polyethylene glycol. Experimental results obtained recently by the author and other researchers were examined to reveal the mechanisms of additive protection. The interactions of additives with air-liquid interfaces and the animal cells were analyzed with respect to their physical properties and chemical structure.

Unusual chromatographic behaviour and one-step purification of a novel membrane proteinase from Bacillus cereus

Cell envelopes of Bacillus cereus contain a casein-cleaving membrane proteinase (CCMP) and an insulin-cleaving membrane proteinase (ICMP), which differ in their substrate and inhibitor specificity from all Bacillus proteinases described previously. They remained localized in the cytoplasmic membrane after treatment with lysozyme and mutanolysin and they are strongly attached to the membrane compared with other known membrane proteinases. Only high a concentration of the Zwitterionic detergent sulfobetain SB-12 enabled an effective solubilization of both membrane proteinases. The usual conventional purification methods, such as chromatofocusing, ion-exchange chromatography and hydrophobic interaction chromatography in the presence of detergent concentrations beyond their critical micelle concentration, could not be applied to the purification, because the solubilized membrane proteinases bound strongly and irreversibly to the chromatographic matrix. In the search for other purification methods, we used a tentacle ion-exchanger (EMD trimethylaminoethyl-Fractogel) to reduce the hydrophobic interactions between the proteinases and the matrix. All contaminating proteins could be removed by a first gradient of sodium chloride without elution of CCMP; a second gradient with isopropanol and a decreasing salt concentration resulted in an efficiently purified CCMP. The ICMP was irreversibly denaturated. Purified CCMP is a member of the metalloproteinase family with a pH optimum in the neutral range and a temperature optimum of 40 degrees C, whose properties differ from the serine-type membrane proteinase of Bacillus subtilis described by Shimizu et al. [Agric. Biol. Chem., 47 (1983) 1775]. It consists of two subunits in sodiumdodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under reducing conditions (Mr 53,000 and 65,000); however, the molecular mass of the purified enzyme could not be determined by size exclusion or SDS-PAGE, because the purified enzyme aggregated at the top of the gel matrix. CCMP solubilized before the purification process, could be eluted in the presence of 0.1% octylphenol-poly(ethyleneglycol ether)9-10 (Triton X-100) in two peaks of Mr 56,000 and 128,000, respectively. We discuss this special chromatographic behaviour of the CCMP from Bacillus cereus, with regard to the strong hydrophobic interactions of the enzyme with the chromatographic matrix and additional self-aggregation, which could only be dissolved by solvents such as isopropanol.

Isolation of a matrix that binds medial Golgi enzymes

Rat liver Golgi stacks were extracted with Triton X-100 at neutral pH. After centrifugation the low speed pellet contained two medial-Golgi enzymes, N-acetylglucosaminyltransferase I and mannosidase II, but no enzymes or markers from other parts of the Golgi apparatus. Both were present in the same structures which appeared, by electron microscopy, to be small remnants of cisternal membranes. The enzymes could be removed by treatment with low salt, leaving behind a salt pellet, which we term the matrix. Removal of salt caused specific re-binding of both enzymes to the matrix, with an apparent dissociation constant of 3 nM for mannosidase II. Re-binding was abolished by pretreatment of intact Golgi stacks with proteinase K, suggesting that the matrix was present between the cisternae.

Disulfide bonds are essential for the stability of the Sindbis virus envelope

Sindbis virus is a membrane-containing virus which has two glycoproteins organized in an icosahedral lattice. Protein-protein associations have been identified which participate in the formation of the icosahedron and these associations are stabilized by intramolecular disulfide bridges (Anthony, R. P., and Brown, D. T., 1990, J. Virol. 65, 1187-1194). The present study further examines the role of disulfides in the structure and function of Sindbis virus by following the effect of dithiothreitol on the protease sensitivity of envelope proteins as well as the electron microscopic appearance and infectivity of Sindbis virus. Treatment of isolated virus with 5 mM dithiothreitol for 6 hr causes a marked increase in trypsin sensitivity of both E1 and E2, profound morphological alterations in the viral envelope, increased susceptibility of the nucleocapsid to RNase, and 95% loss of infectivity. These effects are greatly enhanced and accelerated when treatment with DTT is preceded by a brief exposure of the virus to pH 5.3, suggesting that acid-induced conformational changes render structurally critical disulfides more accessible to reductive cleavage by DTT. When compared to other manipulations known to change the conformation of the viral envelope, such as heating to 51 or 60 degrees or exposure to acid pH, only the exposure to DTT with or without prior acid treatment caused marked structural changes correlated with a loss of infectivity. These data provide electron microscopic and functional evidence that intact disulfide bonds are critical for the stability of the virus envelope and suggest that the cleavage of critical disulfide(s) may play a role in the process of virus infection.

Spike protein-nucleocapsid interactions drive the budding of alphaviruses

Semliki Forest virus (SFV) particles are released from infected cells by budding of nucleocapsids through plasma membrane regions that are modified by virus spike proteins. The budding process was studied with recombinant SFV genomes which lacked the nucleocapsid protein gene or, alternatively, the spike genes. No subviral particles were released from cells which expressed only the nucleocapsid protein or the spike proteins. Virus release was found to be strictly dependent on the coexpression of the nucleocapsid and the spike proteins. These results provide direct proof for the hypothesis that the alphavirus budding is driven by nucleocapsid-spike interactions. The importance of the viral 42S RNA for virus assembly and budding was investigated by using the heterologous vaccinia virus-T7 expression system for the synthesis of the SFV structural proteins. The results demonstrate that the viral genome is not absolutely required for formation of budding competent nucleocapsids, since small amounts of viruslike particles were assembled in the absence of 42S RNA.

The mass of the Sindbis virus nucleocapsid suggests it has T = 4 icosahedral symmetry

The mass of the nucleocapsid of the alphavirus Sindbis was determined by scanning transmission electron microscopy. The nucleocapsid was found to have an average mass of 10.54 x 10(6) Da with the largest percentage of particles showing a mass of 10.7 to 10.9 x 10(6) Da. These data predict that the nucleocapsid contains 240 copies of protein and is a T = 4 icosahedron.

Protein-protein interactions in an alphavirus membrane

Using homobifunctional chemical cross-linkers with various span distances, we have determined the near-neighbor associations and planar organization of the E1 and E2 envelope glycoproteins which compose the icosahedral surface of Sindbis virus. We have found that E1-E2 heterodimers, which form the virus protomeric units, exist in two conformationally distinct forms, reflecting their nonequivalent positions in the icosahedron. Three of these heterodimers form the trimeric morphologic units (capsomeres) which are held together by central E1-E1 interactions. In addition, we present data which suggest that E2-E2 interactions organize the capsomeres into pentameric and hexameric geometric units and that E1-E1 interactions between capsomeres maintain the icosahedral lattice in mature virions.

Effects of solvents and detergents on the contractions of isolated smooth muscle preparations

In testing poorly soluble substances in-vitro on isolated organs, organic solvents and solubilizers are used to increase water-solubility. To facilitate selection of appropriate substances, the effects of eleven of these chemicals have been studied in the following isolated smooth muscle preparations: guinea-pig ileum stimulated by carbachol, histamine, 5-HT or single field stimuli; rat fundus stimulated by 5-HT; and mouse vas deferens stimulated by noradrenaline or trains of field stimuli. Nine solvents (acetone, diethyleneglycol monoethylether, dimethyl sulphoxide, ethanol, glycerol, methanol, polyethylene glycol 400, 1,2-propanediol, Tetraglycol (tetrahydrofurfurylalcohol polyethyleneglycolether)) and two detergents (Triton-X 100 and Tween 80) were examined. The vas deferens proved to be most resistant, whereas rat fundus and guinea-pig ileum were more sensitive to the effects of solvent when present from 1 to 10 g L-1. Most solvents caused non-specific, concentration-dependent reversible inhibition of contractions. Dimethyl sulphoxide in high concentrations increased the contractile responses of guinea-pig ileum stimulated by 5-HT and in both experiments with electrical stimulation. Polyethylene glycol 400 augmented the response of mouse vas deferens to electrical stimulation. Overall, 1,2-propylene glycol and polyethylene glycol 400 had the least effect and can be used in a concentration of 3 g L-1, and in qualitative studies even up to 10 g L-1. Glycerol, both monohydric alcohols and dimethyl sulphoxide produced more intense effects and should not exceed concentrations of 1-3 g L-1. Stronger inhibition was caused by diethyleneglycol monoethylether, acetone and Tetraglycol, and the bath concentrations of these substances should not exceed 0.5-1g L-1. Of the detergents only Tween 80 is suitable as a solubilizer in smooth muscle preparations in-vitro, forming micelles at 10 mg L-1 a concentration tolerated by isolated organs in this study.

Asymmetric extraction of membrane lipids by CHAPS

We have characterized and quantitated the lipids which are cosolubilized with serotonin 5-HT1A sites from sheep brain using 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS). Dialysis of the CHAPS extract produced a [3H]8-hydroxy(2-di-n-propylamino)tetralin [( 3H]8-OH-DPAT) binding vesicular preparation of the protein. Quantitative analysis of the lipids present in the CHAPS extract by HPTLC and transmittance-densitometry revealed extraction of phosphatidylethanolamine (PE), phosphatidylcholine (PC), phosphatidylinositol (PI), phosphatidyl serine (PS) and phosphatidic acid (PA) in striking preference over cholesterol, galactosylceramides, sulfatides and sphingomyelin. All lipids present in the clear CHAPS-extract were coeluted with the [3H]8-OH-DPAT binding preparation were separated by centrifugation, 95-100% of the [3H]8-OH-DPAT binding protein was retained in the vesicle-containing pellet. The supernatant contained small amounts of cholesterol, PE and PC, but virtually no PS, PI, or PA, whereas the vesicular pellet contained all the lipids mentioned, indicating that PS, PI and PA are more tightly bound to the vesicles than PE, PC and cholesterol. SDS-PAGE analysis of the pellet revealed two major protein bands, at 58 kDa and 33.5 kDa, respectively. Our report outlines a simple and improved densitometric assay used for the first detailed analysis of lipids cosolubilized with an active, membrane protein, and also, a simple assay for CHAPS.

Form-determining functions in Sindbis virus nucleocapsids: nucleosomelike organization of the nucleocapsid

Purified intact Sindbis virus nucleocapsids were treated at different pH values or with various concentrations of divalent cations, cation chelators, salt, or formamide. The resulting structures were examined by velocity sedimentation, electron microscopy, and protein-protein cross-linking. Changes in each of the test conditions led to alterations in the sedimentation profile of treated nucleocapsids. Appropriate concentrations of formamide or divalent cations generated beaded strandlike structures similar in morphology to those generated from adenovirus cores and nucleosomes. The capsid protein and RNA remained associated with each other at NaCl concentrations less than or equal to 1 M or after treatment of the structures with alkaline pH up to and including pH 10.7. Protein and RNA were dissociated by salt concentrations of greater than 1 M, suggesting that the arginine-rich, amino-terminal portion of the capsid protein is responsible for binding the RNA. Protein-protein cross-linking also indicated that the capsid proteins remained associated in small aggregates under some of the conditions that caused dissociation of the nucleocapsid and suggested the presence of more than one type of protein-protein interaction in the nucleocapsids. Collectively, these data suggest that, like histones and adenovirus core proteins, the Sindbis virus capsid protein serves to package segments of the genome into nucleoprotein beads which are capable of interacting with each other to form the nucleocapsid structure.

Inactivation of human immunodeficiency virus in serum specimens as a safety measure for diagnostic immunoassays

Since the human immunodeficiency virus (HIV) may be transmitted accidentally to laboratory personnel analyzing patient sera, the efficiency of a non-ionic detergent, Triton X-100, in inactivation of HIV in human serum as a safety measure was studied. Semliki Forest virus, an enveloped toga virus, was used as a model virus to create optimal treatment conditions. In the presence of 50% serum, complete inactivation (i.e. no residual virus detected, greater than 7 log reduction of virus titre) was achieved by incubation with 0.2% Triton X-100 for 1 h at 37 degrees C. Under these conditions HIV was also completely inactivated (i.e. no residual infectious virus detected, greater than or equal to 5 log reduction of virus titre). Both treated and untreated serum specimens were also tested with several enzyme immunoassays used in virological laboratories to determine whether the inactivation treatment interfered with the assays. The treated specimens, further diluted as recommended for each assay, were subjected to 15 enzyme immunoassays for microbial antibodies and antigens (HIV IgG, hepatitis A IgG and IgM, hepatitis B s, c, and e antigens and antibodies, cytomegalovirus IgG, mumps virus IgG, poliovirus IgG, rubellavirus IgM, toxoplasma IgG, and chlamydia IgG). Clearly decreased sensitivity was found only with two hepatitis B tests (e antigen and antibody to the surface antigen). It is concluded that safe inactivation of HIV in serum is achieved by 0.2% Triton X-100, but the treatment may decrease the sensitivity of some tests in which low specimen dilution is used.

Organization of the Sindbis virus nucleocapsid as revealed by bifunctional cross-linking agents

Purified Sindbis virus nucleocapsids were reacted with a variety of bifunctional protein-specific cross-linking agents. The products were analyzed in concentration-gradient polyacrylamide gels and amounts of various products determined. These studies indicated that available lysine residues within adjacent capsid proteins in purified intact nucleocapsids are separated by 6 A. The capsid proteins in intact nucleocapsids are cross-linked in a pattern predicted for discrete monomeric entities, rather than in dimeric or trimeric aggregates. Purified, soluble capsid protein exists in a conformation that differs from the arrangement of protein within nucleocapsids. These conformational differences suggest that topological changes may occur in the capsid protein during virus maturation. Cross-linked nucleocapsids that were treated with RNases resulted in the generation of RNA-free protein shells that retained hexagonal morphology, indicating that, together, the RNA and protein form the outer surface of the nucleocapsid. These data are used to produce a model of the Sindbis virus nucleocapsid in which the proteins are arranged quasi-equivalently in a T = 4 icosahedral shell.

Detection of chikungunya virus antigen in Aedes albopictus mosquitoes by enzyme-linked immunosorbent assay

Double-antibody sandwich and modified sandwich systems of enzyme-linked immunosorbent assay for detecting chikungunya virus antigen present in female mosquitoes, Aedes albopictus (Oahu strain), were evaluated as simple and rapid methods of selection of a highly susceptible mosquito line. Both assays were capable of detecting 3.9 X 10(1) ng (4.0 X 10(6) PFU) or more of the purified antigen. An inhibition system was less sensitive, and a direct system with adsorption of test specimens on the solid phase was not useful. Positive reactions were observed in 16 (48.5%) of 33 infected mosquitoes with with 10(6) to 10(7) PFU, which correspond to the highly susceptible group of this strain. Mosquitoes with less than 10(6) PFU were all negative, indicating the usefulness of the sandwich techniques for identifying high-titered mosquitoes.

Subunit vaccines against enveloped viruses: virosomes, micelles and other protein complexes

The envelope proteins (the peplomers) of enveloped viruses are the components that are important for induction of protective immunity. This article reviews methods and problems of making subunit vaccines of peplomers. In the first section, the solubilization of enveloped viruses with detergent is discussed. The preparation of envelope proteins into defined different physical forms is described, i.e. monomeric and micelle forms and the reconstitution of the protein into lipid vesicles (virosomes). Finally, the preparation of a new type of complex is described (named iscom), which is highly immunogenic. In the following sections the efficacy of the different physical forms are reviewed and it is concluded that monomeric forms must be avoided since they are poorly immunogenic and they may even have a suppressive effect on the immune response. The multimeric micelles, virosomes and iscoms are all immunogenic. The iscom is an interesting new concept that can be used to produce efficient subunit vaccines.

A mild, rapid, and efficient method of lipid extraction for use in determining vitamin E/lipid ratios

A new, general method for lipid extraction and measurement of vitamin E/total lipid ratios in tissue and cell samples has been developed. The new extraction procedure uses a combination of sodium dodecylsulfate, ethanol and n-heptane, and is mild, clean, convenient, efficient and rapid (less than or equal to 5 min). The efficiency of the new method has been confirmed for human plasma, red blood cells and rat liver homogenate by the comparison of the yields of vitamin E, O-acyl lipid and cholesterol with the yields obtained following conventional extraction procedures. Extraction efficiency also has been confirmed for multilamellar vesicles composed of known quantities of vitamin E, egg lecithin and cholesterol.

Semliki Forest virus: a probe for membrane traffic in the animal cell

The traffic among the cellular compartments is thought to be mediated by membrane vesicles, which bud from one compartment and fuse with the next. Despite the continuous exchange of membrane components among them, the organelles maintain their characteristic protein and lipid compositions such that the traffic remains selective, thus, avoiding intermixing of components. This membrane traffic recycles components from the cell surface to the interior of the cell and back to the cell surface again. The membrane traffic between the ER and the cell surface involves a major sorting problem. Little is known of how the animal cell has solved this problem in molecular terms. One experimental tool in this direction is provided by some enveloped animal viruses, which mature at the cell surface of infected cells. Such viruses include influenza virus, Semliki Forest virus (SFV), Sindbis virus, and vesicular stomatitis virus (VSV). They are extremely simple in makeup and hence are very well characterized. The purpose of this article is to illustrate the use of the enveloped viruses as tools in the study of membrane traffic in the animal cell. This is done in the context of the life cycle of the virus in the host cell. The article will be concerned mainly with Semliki Forest virus (SFV), which is the virus that has been worked upon in the chapter. SFV belongs to the alphaviruses, a genus of the togavirus family.

Membrane-surfactant interactions. The effect of Triton X-100 on sarcoplasmic reticulum vesicles

The effect of Triton X-100 on purified sarcoplasmic reticulum vesicles has been studied by means of chemical, ultrastructural and enzymic techniques. At low detergent/membrane ratios (about 1 Triton X-100 per 60 phospholipid molecules) the only effect observed is an increase in vesicle permeability. Higher surfactant concentrations, up to a 1:1 detergent/phospholipid ratio, produce a large enhancement of ATPase activity. Membrane solubilization occurs as a critical phenomenon when the surfactant/phospholipid molar ratio reaches a value around 1.5:1, corresponding to 2 mumol Triton X-100/mg protein. At this point, the suspension turbidity drops, virtually all the protein and phospholipid is solubilized and every organized structure disappears. Simultaneously, a dramatic increase in the specific activity of the solubilized ATPase is observed. The sudden solubilization of almost all the bilayer components at a given detergent concentration is attributed to the relative simplicity of this membrane system. Solubilization takes place at the same surfactant/membrane ratio, at least between 0.5 and 4 mg membrane protein/ml. The non-solubilized residue seems to consist mainly of delipidized aggregated forms of ATPase.

Monensin inhibits Semliki Forest virus penetration into culture cells

The carboxylic ionophores monensin and nigericin, at concentrations higher than 10 and 6 muM, respectively, prevent the penetration of the Semliki Forest virus (SFV) genome into the cytosol of baby hamster kidney (BHK-21) cells and thereby inhibit viral replication. In the absence of inhibitors, the entry of SFV is known to proceed by adsorptive endocytosis in coated vesicles, followed by acid-triggered membrane fusion in intracellular vacuoles or lysosomes. The results show that binding of the virus to the cell surface, adsorptive endocytosis, and intracellular transport of viruses to the lysosomes are only marginally affected by the ionophores. No direct virucidal effect is observed, nor is the membrane fusion activity of the virus at low pH directly affected. Sequential addition of monensin and ammonium chloride (a non-related lysosomotropic inhibitor of SFV entry) indicates that both inhibitors affect the same step in the entry pathway. On the basis of these data and the known effects of carboxylic ionophores and lysosomotropic weak bases on cellular pH gradients, we conclude that monensin inhibits penetration by increasing the pH in endocytic vacuoles and lysosomes above pH 6, which is the pH threshold for the viral membrane fusion activity.

Expression of the structural proteins of Semliki Forest virus from cloned cDNA microinjected into the nucleus of baby hamster kidney cells

The three structural proteins of Semliki Forest virus--i.e., the capsid, p62, and E1 proteins--were expressed in baby hamster kidney cells from cloned DNA transcribed from the virus-specific 4.1-kilobase mRNA. The cDNA was engineered into an expression vector developed by others [Mulligan, R. C. & Berg, P. (1980) Science 209, 1422--1427] downstream from the simian virus 40 early promoter and was introduced into cell nuclei by microneedle injection. Immunofluorescence analysis of injected cells showed that the capsid protein was located in the cell cytoplasm, whereas the membrane proteins were associated with cellular membranes. The p62 protein was shown to be transported from the rough endoplasmic reticulum to the plasma membrane, whereas the E1 protein remained in the rough endoplasmic reticulum.

Asymmetric and symmetric membrane reconstitution by detergent elimination. Studies with Semliki-Forest-virus spike glycoprotein and penicillinase from the membrane of Bacillus licheniformis

The dissociation and reconstitution of the Semliki Forest virus membrane using the nonionic detergent octyl beta-D-glucoside was studied by sucrose density gradient centrifugation. The dissociation occurred in three stages: lysis at a free equilibrium octyl glucoside concentration of 14--18 mM, solubilization at 18--20 mM, and delipidation of the spike glycoproteins at the critical micellar concentration (22 mM) or higher. After solubilization the spike glycoproteins were present as soluble complexes with sedimentation coefficients of 19 S and 6 S. The 6-S form probably corresponded to a glycoprotein monomer complexed to detergent and the 19-S form consisted of oligomeric detergent-protein complexes. The two forms were in slow equilibrium with each other. When the soluble spike protein complexes and egg lecithin solubilized with octyl glucoside were mixed and the octyl glucoside concentration lowered either by dialysis or by dilution, reconstitution occurred. Three types of products were obtained: vesicles with 30% of the spike protein facing inwards and 70% facing outwards, vesicles with virtually all (95%) of the spike proteins pointing outwards, and small protein-rich soluble aggregates [Helenius et al. (1977) J. Cell Biol. 75, 866]. It was demonstrated that during reconstitution the symmetric vesicles were formed at 19 mM free equilibrium octyl glucoside by the association of the 6-S protein complexes with the phospholipids, and the asymmetric vesicles were formed at 10--16 mM octyl glucoside when the 19-S complexes associated with the lipids. Asymmetric membrane vesicles were also obtained when membrane penicillinase from Bacillus licheniformis was reconstituted with egg lecithin using octyl glucoside. It could be shown that the penicillinase was oligomeric at the octyl glycoside concentration where the reconstitution occurred. The results demonstrate that different mechanisms of reconstitution give rise to the symmetric and the asymmetric vesicles. The critical factor in determining the mechanism is the state of aggregation of the proteins at the octyl glucoside concentration where membranes begin to form from the solubilized lipids.

Hepatitis B polypeptide vaccine preparation in micelle form

The immunoprophylaxis of hepatitis B is hampered by the lack of a technique for growing hepatitis B virus (HBV) in tissue culture. Plasma from persistently infected individuals, one source of viral antigen, contains characteristic 22-nm spherical particles which share a common antigen (the hepatitis B surface antigen or HBsAg) with the outer envelope of the 42-nm double-shelled DNA virus. Highly purified inactivated 22-nm particles have been shown to be safe and to confer protective immunity against HBV in a recent large-scale clinical trial. We have already described the extraction from the particles of a complex of two proteins which are antigenic determinants of HBV--the polypeptide with molecular weight (MW) between 22,000 and 24,000 (called p23) and the glycosylated polypeptide (called gp28) with MW in the range 26,000--29,000 which is thought to be the glycosylated form of p23. We now report the preparation from this complex of water-soluble protein micelles which may be a suitable basis for a second-generation hepatitis B vaccine.

Structure of the lipid-containing bacteriophage phi 6. Disruption by Triton X-100 treatment

The structure of the lipid-containing bacteriophage phi 6 was studied by means of controlled Triton X-100 disruption and subsequent isolation of subviral particles. Rate-zonal centrifugation yielded two fractions, a nucleocapsid fraction with RNA, proteins P1, P2, P4, P7, P8, and about half of the protein P5 and a membrane fraction with associated proteins P3, P6, P9, P10, and the rest of the protein P5. Following isopycnic sucrose gradient centrifugation, an empty capsid fraction was obtained which lacked RNA but contained a protein composition similar to the nucleocapsid except for the absence of P5. The membrane fraction isolated after isopycnic centrifugation was morphologically indistinguishable from that isolated after rate-zonal centrifugation but contained only proteins P3, P6, P9 and P10. By treating phi 6 with Triton X-100 prior to isopycnic sucrose gradient centrifugation the viral membrane was further separated into submembrane structures and the attachment protein, P3, could be isolated in rather pure form.

On the mechanism of inactivation of Chikungunya virus by tannic acid

The mechanism of the action of tannic acid (TA) Chikungunya virus (CHIKV) was investigated. Both infectivity and hemagglutination (HA) activity of CHIKV were reduced by treatment with TA in vitro. Aggregation of the TA-treated virus particles was observed by electron microscopy. The reaction was reversible, depending on the pH of the mixture. However, mere dilution of the TA-virus mixture or addition of other protein, such as bovine serum albumin, did not restore the lost infectivity. TA also suppressed the infectivity of RNA extracted from the virus and the HA activity of the viral membrane. The affinity between TA and CHIKV structural proteins (E1, E2, and C) was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis in a discontinuous buffer system and the order of affinity was found to be C greater than E1 greater than E2. Specific conditions for binding of TA to each of the virus proteins were investigated.

Purification of the envelope glycoproteins of western equine encephalitis virus by glass wool column chromatography

Glass wool column chromatography was used for separation of the two glycoproteins of western equine encephalitis virus. Cross-contamination of each protein separated was confirmed to be negligible by sodium dodecyl sulfate-polyacrylamide gel electrophoresis.

Studies on structural proteins of Chikungunya Virus. I. Separation of three species of proteins and their preliminary characterization

Chikungunya virus (CHIKV) was purified and subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis in discontinuous buffer systems. Three bands were revealed by staining with Coomassie blue; two of them (E1 and E2) were associated with the membrane, and one (C) with the core. Their molecular weights were estimated to be 56,000 (E1), 52,000 (E2), and 36,000 (C), irrespective of the concentration of acrylamide in the gels. The molar ratios of E1, E2, and C were almost equal when the sample buffer was Tris-HCl, whereas they were different when phosphate buffer was used.

Triton X-100 solubilization of mitochondrial inner and outer membranes

Rat liver mitochondrial inner and outer membranes were subjected to the solubilizing effect of the nonionic detergent Triton X-100 under various conditions. After centrifugation, the supernatants (containing the solubilized fraction) and pellets were characterized chemically and/or ultrastructurally. The detergent seems to act by inducing a phase transition from membrane lamellae to mixed protein-lipid-detergent micelles. Different electron-microscopy patterns are shown by the inner membranes after treatment with different amounts of surfactant, whereas the corresponding images from outer membranes vary but slightly. Selective solubilization of various components is observed, especially in the case of the inner membrane. Some membrane lipids (e.g., cardiolipin) are totally solubilized at detergent concentrations when others, such as sphyngomyelin, remain in the membrane. Other inner-membrane components (flavins, cytochromes, coenzyme Q) show different solubilization patterns. This allows the selection of conditions for optimal solubilization of a given membrane component with some degree of selectivity. The influence of Triton X-100 on various mitochondrial inner-membrane activities was studied. The detergent seems to act especially through disruption of the topology of the functional complexes, although the activity of the individual enzymes appears to be preserved. Relatively simple enzyme activities, such as ATPase, are more or less solubilized according to the detergent concentration, whereas the more complex succinate-cytochrome c reductase activity practically disappears even at low Triton X-100 concentrations.

Binding of the Triton X series of nonionic surfactants to bovine serum albumin

The binding of a series of Triton X nonionic surfactants (NIS) tobivine serum albumin (BSA) has been studied by equilibrium dialysis and titration calorimetry. At pH 7.0, Triton X molecules bind to two classes of sites, the first 2 molecules binding with positive cooperativity to high-affinity sites following by the binding of approximately 15 additional molecules to lower affinity, thermodynamically identical, and independent sites. The strength of the binding decreases as the number of oxyethylene units is increased in the surfactants Triton X-114, X-100, X-102, and X-165. Calorimetric measurements show the enthalpy change for the NIS-BSA interaction to be small and endothermic. Increasing the hydrophilic oxyethylene chain length results in a more endothermic enthalpy change and a smaller association constant. Electron spin resonance studies of Triton X binding to BSA, covalently spin-labeled with N-(2,2,6,6-tetramethyl-piperidinyl-1-oxy)maleimide, indicated that the protein conformation in the vicinity of the labeled sulfhydryl was insensitive to NIS binding from dilute monomeric solutions. Calorimetric experiments near the critical micelle concentration indicate, however, that the protein probably undergoes a conformational change associated with the population of the lower affinity NIS binding sites.

Isolation of coronavirus envelope glycoproteins and interaction with the viral nucleocapsid

The two envelope glycoproteins and the viral nucleocapsid of the coronavirus A59 were isolated by solubilization of the viral membrane with Nonidet P-40 at 4 degrees C followed by sucrose density gradient sedimentation. Isolated E2 consisted of rosettes of peplomers, whereas E1, the membrane glycoprotein, was irregular and amorphous. Under certain conditions significant interactions occurred between components of Nonidet P-40-disrupted virions. Incubation of the Nonidet P-40-disrupted virus at 37 degrees C resulted in formation of a complex between one of the viral glycoproteins, E1, and the viral nucleocapsid. This was caused by a temperature-dependent conformational change in E1, resulting in aggregation of E1 and interaction with the viral RNA in the nucleocapsid. E1 also bound rRNA. The E1-nucleocapsid complexes can be distinguished on sucrose and Renografin density gradients from native viral nucleocapsids. The separation of the membrane glycoprotein E1 from the peplomeric glycoprotein E2 permitted preparation of antisera against these isolated proteins. A model is proposed for the arrangement of the three major structural proteins in the coronavirus A59 virion in relation to the viral envelope and RNA.

Distribution of terminal glycosyltransferases in hepatic Golgi fractions

The distribution of the three glycosyltransferases synthesizing the terminal trisaccharide sialic acid yields D-galactose yields N-acetylglucosamine present in many glycoproteins was determined in Golgi fractions prepared from rat liver homogenates by a modification of the procedure of Ehrenreich et al. (1973, J. Cell Biol. 70:671--684). The enzymes were assayed with asialofetuin, ovomucoid, and Smith-degraded ovomucoid as sugar acceptors. Careful adjustment of the pH of all sucrose solutions to 7.0 +/- 0.1 prevented enzyme inactivation, and allowed quantitative recoveries at every isolation step. The three morphologically and functionally different Golgi fractions GJ1, GF2, and GF3 showed (in that order) decreasing specific activities of all three enzymes, but the relative amounts and relative specific activities of the three transferases in any given fraction were nearly identical. Two marginal fractions, one extra heavy (collected on the gradient below GF3) and the other extra light (isolated by flotation from the postmicrosomal supernate) were found to contain recognizable Golgi elements. An enrichment of any transferase over the two others was not detected in either preparation. A partial release of content from a combined GF1+2 was achieved by treatment with the nonionic detergent Triton X-100. Low Triton/phospholipid ratios (less than 2 mg/mg) led to lysis of the vesicles and cisternae and loss of very low density lipoprotein particles (ascertained by electron microscopy), but failed to separate the transferases from each other; the three enzymes sedimented together with a population of empty vesicles to a density of approximately 1.08 g/ml.

Interaction of diphtheria toxin fragments A, B and protein crm 45 with liposomes

Diphtheria toxin, its fragments A, B and the protein serologically related to toxin, crm 45, have been studied for their hydrophobicity using the method of charge shift electrophoresis. These molecules were then assayed for liposome interaction. The results have shown that the diphtheria toxin B fragment behaves as an amphiphatic protein because it contains a hydrophobic domain located in that portion of the B chain which remains in protein crm 45. Toxin fragment A is hydrophilic. Incubation of protein crm 45 or toxin fragment B with preformed liposomes leads to association of these proteins with lipid vesicles. Fragment A does not interact with liposomes. Binding of protein crm 45 with lipid vesicles is dependent on time and temperature. Protein crm 45 is unidirectionally associated with liposomes, its enzymic fragment A directed outside the liposome. Fragment B or protein crm 45, upon binding with liposomes, does not affect the permeability of the vesicles.

Isolation of the structural proteins of western equine encephalitis virus by isoelectric focusing

Western equine encephalitis virus was disrupted with Triton X-100 and subjected to isoelectric focusing in a sucrose or urea gradient. The two envelope proteins, E1 and E2 were not well separated in a sucrose gradient, while the E1 and E2 proteins were distinguished as two major peaks which focused in a urea gradient at about pH 7.5 and 10, respectively. Isolated E1 protein refocused at pH 6.5 in a sucrose gradient isoelectric focusing column. When Western equine encephalitis virus was treated with Triton X-100 in 0.01 M phosphate buffer (pH6), hemagglutinating E1 protein was solubilized, which isoelectrofocused at pH 6.5. Purified nucleocapsids focused at pH 4 in a sucrose gradient on an isoelectric focusing column. After ribonuclease treatment of the purified nucleocapsid more than 95 per cent of the viral RNA was acid-soluble, and hte nucleocapsid protein isoelectrofocused at about pH 4.