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

The Impact of Sodium Dodecyl Sulfate and 2-Mercaptoethanol on Antibody and Antigen Binding

OBJECTIVE

To evaluate the effect of sodium dodecyl sulfate (SDS) and 2-mercaptoethanol (2-ME) on antigen-antibody binding when incubated at 100°C, which is the pretreatment temperature required for western blots.

METHODS

Serum that tested positive for hepatitis B surface antigen (HBsAg) plus loading buffer were mixed at a ratio of 4:1 and incubated in a water bath. We then detected HBsAg using double immunodiffusion and ELISA.

RESULTS

The HBsAg titer was 1:512 in the control group when incubated at 37°C. Incubation with SDS at 100°C reduced the antigen titer to 1:32. The inhibitory effect on HBsAg titer reached 96.9% after incubation at 100°C with SDS and 2-ME.

CONCLUSION

We detected strong inhibition of antigens in western blots via SDS and 2-ME. It is likely that false-negative results will be obtained from western blots of antigens with weak resistance to these reagents.

Quaternary Structure Changes for PrPSc Predate PrPC Downregulation and Neuronal Death During Progression of Experimental Scrapie Disease

Prion diseases are fatal neurodegenerative diseases in mammals with the unique characteristics of misfolding and aggregation of the cellular prion protein (PrPC) to the scrapie prion (PrPSc). Although neuroinflammation and neuronal loss feature within the disease process, the details of PrPC/PrPSc molecular transition to generate different aggregated species, and the correlation between each species and sequence of cellular events in disease pathogenesis are not fully understood. In this study, using mice inoculated with the RML isolate of mouse-adapted scrapie as a model, we applied asymmetric flow field-flow fractionation to monitor PrPC and PrPSc particle sizes and we also measured seeding activity and resistance to proteases. For cellular analysis in brain tissue, we measured inflammatory markers and synaptic damage, and used the isotropic fractionator to measure neuronal loss; these techniques were applied at different timepoints in a cross-sectional study of disease progression. Our analyses align with previous reports defining significant decreases in PrPC levels at pre-clinical stages of the disease and demonstrate that these decreases become significant before neuronal loss. We also identified the earliest PrPSc assemblies at a timepoint equivalent to 40% elapsed time for the disease incubation period; we propose that these assemblies, mostly composed of proteinase K (PK)-sensitive species, play an important role in triggering disease pathogenesis. Lastly, we show that the PK-resistant assemblies of PrPSc that appear at timepoints close to the terminal stage have similar biophysical characteristics, and hence that preparative use of PK-digestion selects for this specific subpopulation. In sum, our data argue that qualitative, as well as quantitative, changes in PrP conformers occur at the midpoint of subclinical phase; these changes affect quaternary structure and may occur at the threshold where adaptive responses become inadequate to deal with pathogenic processes.

Purification of Tag-Free Chlamydia trachomatis Scc4 for Structural Studies Using Sarkosyl-Assisted on-Column Complex Dissociation

Chlamydia trachomatis is an obligate intracellular bacterial pathogen that causes the most common sexually transmitted bacterial disease in the world. The bacterium has a unique biphasic developmental cycle with a type III secretion system (T3SS) to invade host cells. Scc4 is a class I T3SS chaperone forming a heterodimer complex with Scc1 to chaperone the essential virulence effector, CopN. Scc4 also functions as an RNA polymerase binding protein to regulate σ⁶⁶-dependent transcription. Aggregation and low solubility of 6X-histidine-tagged Scc4 and the insolubility of 6X-histidine and FLAG-tagged Scc1 expressed in Escherichia coli have hindered the high-resolution nuclear magnetic resonance (NMR) structure determination of these proteins and motivated the development of an on-column complex dissociation method to produce tag-free Scc4 and soluble FLAG-tagged Scc1. By utilizing a 6X-histidine-tag on one protein, the coexpressed Scc4-Scc1 complex was captured on nickel-charged immobilized metal affinity chromatography resin, and the nondenaturing detergent, sodium N-lauroylsarcosine (sarkosyl), was used to dissociate and elute the non-6X-histidine-tagged protein. Tag-free Scc4 was produced in a higher yield and had better NMR spectral characteristics compared to 6X-histidine-tagged Scc4, and soluble FLAG-tagged Scc1 was purified for the first time in a high yield. The backbone structure of Scc4 after exposure to sarkosyl was validated using NMR spectroscopy, demonstrating the usefulness of the method to produce proteins for structural and functional studies. The sarkosyl-assisted on-column complex dissociation method is generally applicable to protein complexes with high affinity and is particularly useful when affinity tags alter the protein's biophysical properties or when coexpression is necessary for solubility.

Sarkosyl: A milder detergent than SDS for identifying proteins with moderately high hyperstability using gel electrophoresis

Sodium dodecyl sulfate (SDS) is a detergent used as a strong denaturant of proteins in gel electrophoresis. It has previously been shown that certain hyperstable, also known as kinetically stable, proteins are resistant to SDS and thus require heating for their denaturation in the presence of SDS. Because of its high denaturing strength, relatively few proteins are resistant to SDS thereby limiting the current use of SDS-PAGE for identifying hyperstable degradation-resistant proteins. In this study, we show that sarkosyl, a milder detergent than SDS, is able to identify proteins with moderately high kinetic stability that lack SDS-resistance. Our assay involves running and subsequently comparing boiled and unheated protein samples containing sarkosyl, instead of SDS, on PAGE gels and identifying subsequent differences in protein migration. Our results also show that sarkosyl and SDS may be combined in PAGE experiments at varying relative percentages to obtain semi-quantitative information about a protein's kinetic stability in a range inaccessible by probing through native- or SDS-PAGE. Using protein extracts from various legumes as model systems, we detected proteins with a range of protein stability from nearly SDS-resistant to barely sarkosyl resistant.

Bilayer lipid composition modulates the activity of dermaseptins, polycationic antimicrobial peptides

The primary targets of defense peptides are plasma membranes, and the induced irreversible depolarization is sufficient to exert antimicrobial activity although secondary modes of action might be at work. Channels or pores underlying membrane permeabilization are usually quite large with single-channel conductances two orders of magnitude higher than those exhibited by physiological channels involved, e.g., in excitability. Accordingly, the ion specificity and selectivity are quite low. Whereas, e.g., peptaibols favor cation transport, polycationic or basic peptides tend to form anion-specific pores. With dermaseptin B2, a 33 residue long and mostly alpha-helical peptide isolated from the skin of the South American frog Phyllomedusa bicolor, we found that the ion specificity of its pores induced in bilayers is modulated by phospholipid-charged headgroups. This suggests mixed lipid-peptide pore lining instead of the more classical barrel-stave model. Macroscopic conductance is nearly voltage independent, and concentration dependence suggests that the pores are mainly formed by dermaseptin tetramers. The two most probable single-channel events are well resolved at 200 and 500 pS (in 150 mM NaCl) with occasional other equally spaced higher or lower levels. In contrast to previous molecular dynamics previsions, this study demonstrates that dermaseptins are able to form pores, although a related analog (B6) failed to induce any significant conductance. Finally, the model of the pore we present accounts for phospholipid headgroups intercalated between peptide helices lining the pore and for one of the most probable single-channel conductance.

Urothelial hinge as a highly specialized membrane: detergent-insolubility, urohingin association, and in vitro formation

Urothelial surface is covered by numerous plaques (consisting of asymmetric unit membranes or AUM) that are interconnected by ordinary looking hinge membranes. We describe an improved method for purifying bovine urothelial plaques using 2% sarkosyl and 25 mM NaOH to remove contaminating membrane and peripheral proteins selectively. Highly purified plaques interconnected by intact hinge areas were obtained, indicating that the hinges are as detergent-insoluble as the plaques. These plaque/hinge preparations contained uroplakins, an as yet uncharacterized 18-kDa plaque-associated protein, plus an 85-kDa glycoprotein that is known to be hinge-associated in situ. Examination of the isolated, in vitro-resealed bovine AUM vesicles by quick-freeze deep-etch showed that each AUM particle consists of a 16-nm, luminally exposed "head" anchored to the lipid bilayer via a 9-mm transmembranous "tail", and that an AUM plaque can break forming several smaller plaques separated by newly formed particle-free, hinge-like areas. These data lend support to our recently proposed three-dimensional model of mouse urothelial plaques. In addition, our findings suggest that urothelial plaques are dynamic structures that can rearrange giving rise to new plaques with intervening hinges; that the entire urothelial apical surface (both plaque and hinge areas) is highly specialized; and that these two membrane domains may be equally important in fulfilling some of the urothelial functions.

Topology and acylation of spiralin

Of the 51 polypeptides detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the plasma membrane of the helical mollicute Spiroplasma melliferum, 21 are acylated, predominantly with myristic (14:0) and palmitic (16:0) chains. This is notably the case for spiralin, the major membrane protein of this bacterium, which contains an average of 0.7 acyl chains per polypeptide, attached very probably by ester bonds to alcohol amino acids. The amphiphilicity of spiralin was demonstrated by the behavior of the protein in charge-shift electrophoresis, its incorporation into liposomes, and its ability to form in the absence of lipids and detergents, globular protein micelles (diameter, approximately 15 nm). The presence of epitopes on the two faces of the cell membrane, as probed by antibody adsorption and crossed immunoelectrophoresis, and the strong interaction between spiralin and the intracytoplasmic fibrils show that spiralin is a transmembrane protein. The mean hydropathy of the amino acid composition of spiralin (-0.30) is on the hydrophilic side of the scale. Surprisingly, the water-insoluble core of spiralin micelles, which is the putative membrane anchor, has a still more hydrophilic amino acid composition (mean hydropathy, -0.70) and is enriched in glycine and serine residues. Taking into account all these properties, we propose a topological model for spiralin featuring a transbilayer localization with hydrophilic domains protruding on the two faces of the membrane and connected by a small domain embedded within the apolar region of the lipid bilayer. In this model, the membrane anchoring of the protein is strengthened by a covalently bound acyl chain.

Effect of detergents on the structure of integral membrane proteins of Sendai virus studied with size-exclusion high-performance liquid chromatography and monoclonal antibodies

The integral membrane proteins of Sendai virus, the fusion protein F (Mr = 65,000) and the haemagglutinin-neuraminidase protein HN (Mr = 68,000), were used as a model protein mixture. They were subjected to size-exclusion high-performance liquid chromatography on Superose 6HR columns with eluents containing various additives in order to solubilize the proteins. The effect of the additives on the structure of the membrane proteins was investigated with conformation-dependent monoclonal antibodies, either directed against F or HN protein, and by determination of the haemagglutinating capacity of the HN protein. The results show that the structure of the HN protein is more easily disturbed by eluents than that of the F protein. When the elution conditions are mild, e.g., 0.1% octylglucoside, the structure of both proteins is conserved but no separation is obtained. Elution with a buffer containing 0.05% sarkosyl (dodecyl methylglycine sodium salt) did not affect the structure and resulted in pure F protein. Pretreatment of the Amberlite XAD-2-treated Sendai virus envelope extract with 4% sodium dodecyl sulphate (SDS) and elution with 0.1% SDS in 50 mM sodium phosphate (pH 6.5) altered the structure of the HN protein but resulted in purification of the tetramer and the dimer of the HN protein, and the monomer of the F protein.

Highly selective extraction of spiralin from the Spiroplasma citri cell membrane with alkyl-N-sulfobetaines

The extraction of proteins from the membrane of the mollicute (mycoplasma) Spiroplasma citri by sodium N-dodecyl-N,N-dimethyl-3-amino-1-propane sulfonate (SB12) and sodium N-tetradecyl-N,N-dimethyl-3-amino-1-propane sulfonate (SB14) was studied with electrophoretic methods. The membranes were prepared by osmotic lysis of the cells and depleted of the bulk of extrinsic proteins. It was possible to extract up to 35 and 45% of membrane proteins with SB12 and SB14, respectively. Maximal yield was obtained in both cases with detergent concentrations greater than or equal to 5 mumoles/mg of membrane protein. Spiralin, the major protein in the S. citri membrane, was highly selectively solubilized without the loss of antigenicity, with a yield of about 90% with SB12 and close to 100% with SB14, for a detergent concentration greater than or equal to 0.2 M. The degree of selectivity in favour of spiralin was higher with SB12 (purity approximately equal to 70%) than with SB14 (purity approximately equal to 50%). Treatment of the S. citri membrane with high concentrations of SB12 is a simple and fast procedure for partial purification of spiralin. This example shows that, in some cases, it should be possible to modulate the selectivity of the extraction of membrane proteins simply by varying the relative concentration of detergent.

Adsorption of proteins from the Spiroplasma citri cell membrane by magnesium lauroyl-sarcosinate crystals

Interactions between the ionic detergent Sarkosyl (sodium lauroyl sarcosinate), Mg2+ ions, and the Spiroplasma citri cell membrane were analyzed microscopically and electrophoretically. Studies were performed under conditions where membrane proteins were apparently not released from the membrane by the detergent (molar ratio of MgCl2/Sarkosyl = 0.5). Although the S. citri membrane interfered with the crystallization phenomenon to some extent, the formation of Sarkosyl-Mg2+ crystals occurred regardless to the sequence of addition of the three components. Concomitantly the structure of the membrane disintegrated and membrane components were adsorbed to the crystal surfaces. The membrane protein fraction bound to the crystals was composed of the majority of the putatively intrinsic polypeptides, including the amphiphilic protein spiralin, and several extrinsic polypeptides. The polypeptide compositions of M-bands (crystal fractions loaded with membrane material) prepared from S. citri cells and from isolated S. citri membranes were similar, as shown by sodium dodecyl-sulfate electrophoresis and crossed immunoelectrophoresis. These results show that, the S. citri cell membrane, in contrast to bacterial membranes, is not protected from the effect of Sarkosyl by Mg2+ ions.

Amphilphilic nature of spiralin, the major protein of the Spiroplasma citri cell membrane

Spiralin could not be solubilized in the absence of detergents, and it was shown by charge-shift crossed immunoelectrophoresis that this protein was capable of binding detergents under nondenaturing conditions. These properties indicate the amphiphilic nature of spiralin, which therefore should be regarded as an intrinsic membrane protein. The efficiency of mild (ionic and neutral) detergents to solubilize spiralin was as follows: deoxycholate greater than lauroyl sarcosinate, cholate, taurocholate, taurodeoxycholate greater than Triton X-100 greater than Brij 58 greater than Tween 20, indicating that mild ionic detergents were more effective than neutral ones. Solubilization of spiralin was quantitative with sodium deoxycholate. It was also shown that although a membrane protein is not extractable by a given detergent from the membrane, this does not necessarily mean that the protein is not soluble in this detergent.