Selective solubilization with Tween 20 of membrane proteins from Acholeplasma laidlawaii

Tween-20 Induces the Structural Remodeling of Single Lipid Vesicles

The solubilization of lipid membranes by Tween-20 is crucial for a number of biotechnological applications, but the mechanistic details remain elusive. Evidence from ensemble assays supports a solubilization model that encompasses surfactant association with the membrane and the release of mixed micelles to solution, but whether this process also involves intermediate transitions between regimes is unanswered. In search of mechanistic origins, increasing focus is placed on identifying Tween-20 interactions with controllable membrane mimetics. Here, we employed ultrasensitive biosensing approaches, including single-vesicle spectroscopy based on fluorescence and energy transfer from membrane-encapsulated molecules, to interrogate interactions between Tween-20 and submicrometer-sized vesicles below the optical diffraction limit. We discovered that Tween-20, even at concentrations below the critical micellar concentration, triggers stepwise and phase-dependent structural remodeling events, including permeabilization and swelling, in both freely diffusing and surface-tethered vesicles, highlighting the substantial impact the surfactant has on vesicle conformation and stability prior to lysis.

Purification and characterization of spiralin, the main protein of the Spiroplasma citri membrane

The membrane proteins from Spiroplasma citri have been resolved into 16 components by SDS-polyacrylamide gel electrophoresis. By this technique it was also shown that the molecular weights of these proteins ranged from 13000 to 160 000. One of the proteins, which had an apparent molecular weight of 26 000 was the most abundant and represented more than 22% of total membrane protein. We have designated this protein spiralin. None of the proteins contained carbohydrate. Spiralin has been isolated by a procedure which involves removal of some membrane proteins with the neutral detergent Tween 20, selective solubilization of the Tween residue in DOC and fractionation of the DOC-soluble material by agarose-suspension electrophoresis. The homogeneity of spiralin was demonstrated by analytical polyacrylamide gel electrophoresis under different conditions and by crossed immunoelectrophoresis. Spiralin appeared to bind less DOC than the other membrane proteins of S. citri. This observation does not imply, however, that the binding of DOC to spiralin is weak. Spiralin was neither soluble in detergent-free buffers nor in Tween 20, which indicated that it is an intrinsic membrane protein. The amino-acid composition of spiralin was quite different from that of the membrane. Spiralin lacked methionine, histidine and tryptophan, and had a low content of glycine, leucine, tyrosine and phenylalanine, but a high content of threonine, alanine and valine.

Evaluation of hydrophobicity versus chaperonelike activity of bovine alphaA- and alphaB-crystallin

Calf lens alphaA-crystallin isolated by reversed-phase HPLC demonstrates a slightly more hydrophobic profile than alphaB-crystallin. Fluorescent probes in addition to bis-ANS, like cis-parinaric acid (PA) and pyrene, show higher quantum yields or Ham ratios when bound to alphaA-crystallin than to alphaB-crystallin at room temperature. Bis-ANS binding to both alphaA- and alphaB-crystallin decreases with increase in temperature. At room temperature, the chaperone-like activity of alphaA-crystallin is lower than that of alphaB-crystallin whereas at higher temperatures, alphaA-crystallin shows significantly higher protection against aggregation of substrate proteins compared to alphaB-crystallin. Therefore, calf lens alphaA-crystallin is more hydrophobic than alphaB-crystallin and chaperone-like activity of alpha-crystallin subunits is not quantitatively related to their hydrophobicity.

Purification of a phosphatase which hydrolyzes phosphatidic acid, a key intermediate in glucolipid synthesis in Acholeplasma laidlawii A membranes

A phosphatidic acid phosphatase (PAP; EC 3.1.3.4.), dephosphorylating phosphatidic acid (PA) to diacylglycerol (DAG), was identified and purified from the plasma membrane of Acholeplasma laidlawii A. After four purification steps, including membrane preparation, Tween 20 solubilization, preparative gel electrophoresis and electro-elution, PAP was purified about 400 times to near homogeneity. The molecular weight of PAP was according to SDS-polyacrylamide gel electrophoresis approximately 25 kDa and the enzyme was a stable and integral membrane protein. It is proposed to catalyze the first enzymatic step in the important glucolipid pathway of A. laidlawii. No essential cofactors or activator lipids were found. However, some divalent cations and phosphate analogues were potent inhibitors. Beside the in vivo substrate (PA), PAP was found to dephosphorylate p-nitrophenylphosphate. This less stringent specificity makes alternative in vivo functions for PAP plausible, the importance which is discussed.

A gentle fixation and permeabilization method for combined cell surface and intracellular staining with improved precision in DNA quantification

A method was developed for gentle fixation of mammalian cells and permeabilization of their membranes. The method is useful for staining of intracellular antigens or quantification of DNA content simultaneously with cell surface staining. Cells are treated for 1 h at 4 degrees C with 0.25% buffered paraformaldehyde then for 15 min at 37 degrees C with 0.2% Tween 20 detergent in PBS. The procedure permits excellent staining of intracellular proteins, very low coefficients of variation (CV) on the G0G1-peak of DNA distributions, and preservation of the integrity of cell surface antigens. The low vs. 90 degrees angle light scatter profile of cell clusters is maintained thereby allowing discrimination of different cell populations including human peripheral blood lymphocytes and monocytes for gating and analytic purposes. The method was successfully used on a variety of other cell types, including human thymocytes, murine thymocytes and spleen cells, and several leukemic cell lines. Dual-color surface antigen staining combined with DNA staining with 7-amino-actinomycin D (7-AAD) on peripheral blood mononuclear cells (PBMC) cultured with tetanus toxoid allowed the determination of the cell subset that was preferentially stimulated. Staining for internal antigens was done on CCRF-CEM for expression of CD3 epsilon and on NALM-6 for expression of mu. The technique we developed gave bright and specific staining of internal antigens in the examples presented here. It is particularly suited for correlations of internal antigen staining with DNA staining and/or surface immunofluorescence.

Reversed-phase and hydrophobic-interaction high-performance liquid chromatography of elapid cardiotoxins

The separation of proteins by hydrophobic-interaction HPLC and reversed-phase HPLC depends upon differences in the hydrophobicity of accessible surface groups. The elution order of a group of snake venom cardiotoxins was found to vary between these two HPLC methods. Circular dichroism spectroscopy showed that the eluant acetonitrile-trifluoroacetic acid used for reversed-phase HPLC altered the conformation of the toxins, whereas the salt-buffer eluting medium used for hydrophobic-interaction HPLC did not affect toxin conformation. The retention times of cardiotoxins on reversed-phase HPLC are therefore influenced by their conformational instability in the eluting medium which causes partial or complete unfolding. Hydrophobic interaction is clearly the preferred method with which to correlate the "surface hydrophobicity" of cardiotoxins and their biological effects.

Identification of selectively solubilised syncytiotrophoblast plasma membrane proteins as potential antigenic targets during normal human pregnancy

Syncytiotrophoblast plasma membranes prepared from term placentae were selectively solubilised in non-ionic detergents. The solubilised proteins and the insoluble residue were tested in an ELISA assay for their ability to function as antigenic targets for anti-trophoblast antibodies present in normal first trimester pregnancy sera. The soluble proteins were fractionated by gel filtration and four major antigen forms were identified. The antigens were reactive with affinity purified anti-trophoblast antibody isolated from maternal sera and hence were termed maternally-recognised trophoblast antigens (MRTA); these were designated MRTA-I (Mr = 400,000 D), MRTA-II (Mr = 142,000), MRTA-III (Mr = 50,000) and MRTA-IV (Mr = 13,000). The relationship between MRTA-I, II, III and IV and antigens identified in maternal sera in the form of immune complexes is discussed.

Solubilization of phospholipids by detergents. Structural and kinetic aspects

Most amphiphiles in biological membranes including phospholipids, steroids, and membrane proteins are insoluble amphiphiles and would form liquid crystals or insoluble precipitates alone in aqueous media. Detergents are soluble amphiphiles and above a critical concentration and temperature form micelles of various sizes and shapes. Much of the recent progress in studying the insoluble amphiphiles is due to the formation of thermodynamically stable isotropic solutions of these compounds in the presence of detergents. This process, which is commonly denoted as "solubilization,' involves transformation of lamellar structures into mixed micelles. The information available to date on the solubilization of phospholipids, which constitute the lipid skeleton of biomembranes, by the common detergents is discussed in this review, both with respect to the kinetics of this process and the structure of the various phospholipid-detergent mixed micelles formed. It is hoped that this discussion will lead to somewhat more useful, although still necessarily fairly empirical, approaches to the solubilization of phospholipids by detergents.

Membrane asymmetry. A survey and critical appraisal of the methodology. I. Methods for assessing the asymmetric orientation and distribution of proteins

This and the companion article are aimed at surveying the methods used for the study of membrane asymmetry. The techniques employed for the assessment of the asymmetric distribution and orientation of membrane proteins are reviewed in this article, whereas those pertaining to the unequal distribution of lipids are detailed in the companion paper. The use of immunological techniques and lectins, functions of proteins and their perturbations, chemical reagents, enzymatic isotopic labeling and enzymatic cleavage of membrane proteins and physical techniques are discussed and illustrated using recent examples of their application. Whenever appropriate, problems involving crypticity and non-availability or non-reactivity of functional sites, relevant chemical functions or protein fragments to appropriate ligands, reagents or modifying enzymes are envisaged and possible modification of the exposure of proteins during preparation of ghosts and other drawbacks are discussed, the use of different techniques and control experiments in conjunction is recommended for a more realistic assessment of the distribution and orientation of proteins.

Membrane composition and virus susceptibility of Acholeplasma laidlawii

The membrane composition of 11 strains of Acholeplasma laidlawii, including three strains persistently infected with mycoplasmaviruses MVL51, MVL2, and MVL3, was studied and correlated with mycoplasmavirus sensitivity. Membranes of the strains had similiar sodium dodecyl sulfate-polyacrylamide gel electrophoresis patterns, and all strains were inhibited by an antiserum produced against membranes from one of the strains. The amounts of integral membrane proteins solubilized by the nonionic detergent Tween 20 differed considerably. Therefore, characteristic crossed immunoelectrophoresis patterns were obtained for each strain. Strains persistently infected with MVL2 and MVL3 were notably different from the noninfected host. The ability to propagate any of the viruses was not correlated with sodium dodecyl sulfate-polyacrylamide gel electrophoresis or crossed immunoelectrophoresis patterns. The persistently infected strains had a characteristic lipid composition. MVL51-resistant strains, including a resistant clone selected from a sensitive strain, were characterized by a large monoglucosyldiglyceride/diglucosyldiglyceride ratio and trace amounts of diphosphatidylglyceol (as opposed to the sensitive strains). Differences in lipid composition in A. laidlawii seem to affect the relationship between cells and viruses.

Characterization of the strain-specific and common surface antigens of Mycoplasma arginini

A combination of quantitative immunoelectrophoresis and sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis was used to determine location and molecular weights of surface membrane antigens of four strains of Mycoplasma arginini. Two major surface antigens were identified for M. arginini by absorption of antiserum with whole cells: one surface antigen was strain specific, electrophoretically fast, and prominently located on the surface, whereas the other surface antigen was common to the four strains and of intermediate electrophoretic mobility. Three of the four strains of M. arginini (G-230, 23243, and 27389) possessed immunologically strain-specific antigens which did not cross-react, whereas the leonis strain lacked an immunologically detectable unique surface antigen. A monospecific antiserum prepared against immune precipitates of the strain-specific antigen of strain G-230 detected three polypeptides of 74,000, 44,000, and 17,000 daltons in SDS-polyacrylamide gels of membrane preparations. All four strains shared the common surface antigen which appeared considerably more hydrophobic than the strain-specific surface antigen because it could only be demonstrated by charge-shift immunoelectrophoretic conditions (addition of deoxycholate to the nonionic detergent). Monospecific antiserum to the common antigen of strain G-230 reacted with all four M. arginini strains, but did not react with two other arginine-utilizing species, and recognized three polypeptides of 40,000, 29,000, and 20,000 daltons in membranes of strain G-230. Whereas the common surface antigen is a likely target for conventional serological reactions used for identification of the species M. arginini, strain-specific antigen cannot fulfill this role but must participate in other surface reactions.

Solubility characteristics of Micrococcus lysodeikticus membrane components in detergents and chaotropic salts analyzed by immunoelectrophoresis

In order to evalute the effectiveness and selectivity of various reagents in the solubilization of bacterial membranes, membranes of Micrococcus lysodeikticus were treated with detergents and chaotropic agents. The composition of the extracts so obtained was analyzed by rocket and two-dimensional immunoelectrophoretic techniques. Recoveery of succinate-, malate-, and reduced nicotinamide adenine dinucleotide- (NADH) dehydrogenases, ATPase, succinylated lipomannan and cytochromes in the extracts was measured. Treatment with a variety of non-denaturing detergents produced extracts that were generally qualitatively uniform although quantitative differences were observed. The degree of extraction of various components was correlated with the hydrophile-lipophile balance. Several chaotropic agents were also evaluated as reagents for membrane solubilization. These agents were less effective in extraction of bulk protein, but produced extracts enriched in some membrane components.

Crossed immunoelectrophoresis, in the presence of tween 20 or sodium deoxycholate, of purified membrane proteins from Acholeplasma laidlawii

Five membrane proteins from Acholeplasma laidlawii have been previously purified on a large scale. These proteins have been used to establish the relationship between the precipitation lines obtained by crossed immunoelectrophoresis of solubilized cell membrane proteins from A. laidlawii in the presence of the neutral detergent Tween 20 or those obtained in the presence of the anionic detergent sodium deoxycholate. This relationship, which was unambiguously established for four of the five proteins, was determined by tandem or "parallel" crossed immunoelectrophoresis of the sodium deoxycholate-solubilized membrane together with the purified proteins. Membranes from strain A of A. laidlawii were composed of proteins, which were immunologically related to and probably identical to membrane proteins from strain B of this organism.

Solubilization of Spiroplasma citri cell membrane proteins with the anionic detergent sodium lauroyl-sarcosinate (Sarkosyl)

1. Up to 90 per cent of the membrane proteins from Spiroplasma citri could be solubilized with the anionic detergent Sarkosyl (sodium lauroyl-sarcosinate). Maximal solubilization was obtained with 6 to 20 mumoles of of detergent per mg of membrane protein. The insoluble residue, comprising about 10 per cent of the membrane protein, contained mainly the protein spiralin, which is quantitatively the major one of this membrane. 2. Mg2+ ions completely prevented solubilization of the membrane proteins at a molar ratio of MgCl2/Sarkosyl greater than 0.5. 3. The selectivity of Sarkosyl was also tested at low detergent concentrations and in the presence of Mg2+ ions. Spiralin was the least soluble protein also under these conditions. Other proteins were not selectively solubilized. 4. An electrophoretical and immunoelectrophoretical approach was used to study the interaction between Sarkosyl and membrane proteins. The results indicated that Sarkosyl should be considered as a mild detergent which usually solubilizes membrane proteins without gross donformational changes. This hypothesis was supported by experiments with a membrane-bound enzyme in the presence of Sarkosyl.

Determination of Triton X-100 binding to membrane proteins by polyacrylamide gel electrophoresis

The molecular weight of proteins in protein-detergent complexes can be determined from ultracentrifugation experiments if the amount of bound detergent is known. A new sensitive method to measure the binding of the nonionic detergent Triton X-100 to proteins has been developed. For the membrane proteins studied, less than 50 mug of protein was required to achieve an accuracy of 10% in the determination of the detergent-protein weight ratio. The proteins were equilibrated with the detergent by electrophoresis into polyacrylamide gels containing radioactively labelled Triton X-100. The gels were then sliced and the amount of bound detergent calculated from the increase in radio-activity in the slices containing the protein zone. The amounts of protein were determined by amino acid analysis of identical protein zones cut from gels running parallel.

Carbohydrate composition of central nervous system synapses. Analysis of isolated synaptic junctional complexes and postsynaptic densities

The composition of specialized structures present at synapses within the central nervous system was elucidated by biochemical analysis of fractions enriched in synaptic junctional complexes and postsynaptic densities. The results indicate that the synaptic junctional complex is primarily protein together with some glycoproteins. The synaptic junctional complex proteins are similar in amino acid composition to synaptic membrane proteins; they are not expecially rich in basic residues, as previously suggested. The major carbohydrates present in the synaptic junctional complex and postsynaptic density glycoproteins are mannose, galactose, and glucosamine, with lesser amounts of fucose, N-acetylneuraminic acid, and galactosamine. Comparison with the synaptic membrane fraction indicates that galactose is more concentrated in the synaptic junctional complex and mannose in the postsynaptic density. Glucose is dramatically enriched in both these fractions. Sucrose binding during isolation may partially account for the glucose enrichment.