Detergent Solubilization Research Articles

A novel label-free method to determine equilibrium dissociation constants of antibodies binding to cell surface proteins

Solution-based affinity assays are used for the selection and characterization of proteins that could be developed into therapeutic molecules. However, these assays have limitations for cell-surface proteins as in most cases their purification requires detergent solubilization and are unlikely to assume conformations in solution that resemble their native states in cell membranes. This report describes a novel electrochemiluminescence-based method, called MSD-CAT, for the affinity analysis of antibodies binding to cell-surface receptors. MSD-CAT was used to evaluate the binding of monoclonal antibodies, Fab fragments, and bispecific antibodies targeting the cell-surface receptor interleukin 3 receptor alpha (CD123) and the results were compared to data obtained using surface plasmon resonance (SPR). The data showed that MSD-CAT can be successfully applied to determine binding affinity on cells in a label free format and without the need for laborious solubilization procedures to generate recombinant antigen. In addition, this method has the potential for high-throughput application while enabling simultaneous determination of equilibrium dissociation constant (KD) and receptor density within the same experiment.

A novel method for expressing and purifying large quantities of functional and stable human voltage-gated proton channel (hHv1).

Purifying membrane proteins has been the limiting step for studying their structure and function. The challenges of the process include the low expression levels in heterologous systems and the requirement for their biochemical stabilization in solution. The human voltage-gated proton channel (hHv1) is a good example of that: the published protocols to express and purify hHv1 produce low protein quantities at high costs, which is an issue for systematically characterizing its structure and function. Based on a pipeline approach, we developed a novel method to produce large quantities of properly folded and fully functional hHv1. We found that using the correct Escherichia coli strain in an autoinduction medium at low temperatures maximized protein expression. Furthermore, solubilization screenings showed that the detergent Anzergent 3-12 was a better alternative than Fos-choline-12 to purify hHv1, considerably reducing the costs. Buffers with high ionic strength increased the protein extracted during detergent solubilization and the stability of hHv1 during downstream processing. Finally, a further improvement was achieved when an enterokinase cutting site was inserted at the N-terminus of the protein. Our novel method produces properly folded and fully functional hHv1, increasing the protein yield by 100 times and reducing the cost by 96% while improving the protein stability compared to the previously published protocols. Our work will accelerate studies on hHv1 and its possible future therapeutic use, while serving as an example for developing purification methodologies for other challenging membrane proteins.

Structures of methane and ammonia monooxygenases in native membranes

Methane- and ammonia-oxidizing bacteria play key roles in the global carbon and nitrogen cycles, respectively. These bacteria use homologous copper membrane monooxygenases to accomplish the defining chemical transformations of their metabolisms: the oxidations of methane to methanol by particulate methane monooxygenase (pMMO) and ammonia to hydroxylamine by ammonia monooxygenase (AMO), enzymes of prime interest for applications in mitigating climate change. However, investigations of these enzymes have been hindered by the need for disruptive detergent solubilization prior to structure determination, confounding studies of pMMO and precluding studies of AMO. Here, we overcome these challenges by using cryoEM to visualize pMMO and AMO directly in their native membrane arrays at 2.4 to 2.8 Å resolution. These structures reveal details of the copper centers, numerous bound lipids, and previously unobserved components, including identifiable and distinct supernumerary helices interacting with pMMO and AMO, suggesting a widespread role for these helices in copper membrane monooxygenases. Comparisons between these structures, their metallocofactors, and their unexpected protein-protein interactions highlight features that may govern activity or the formation of higher-order arrays in native membranes. The ability to obtain molecular insights within the native membrane will enable further understanding of these environmentally important enzymes.

Protein-Protein Interaction and Conformational Change in the Alpha-Helical Membrane Transporter BtuCD-F in the Native Cellular Envelope.

Alpha-helical membrane proteins perform numerous critical functions essential for the survival of living organisms. Traditionally, these proteins are extracted from membranes using detergent solubilization and reconstitution into liposomes or nanodiscs. However, these processes often obscure the effects of nanoconfinement and the native environment on the structure and conformational heterogeneity of the target protein. We demonstrate that pulsed dipolar electron spin resonance spectroscopy, combined with the Gd3+-nitroxide spin pair, enables the selective observation of the vitamin B12 importer BtuCD-F in its native cellular envelope. Despite the high levels of non-specific labeling in the envelope, this orthogonal approach combined with the long phase-memory time for the Gd3+ spin enables the observation of the target protein complex at a few micromolar concentrations with high resolution. In the native envelope, vitamin B12 induces a distinct conformational shift at the BtuCD-BtuF interface, which is not observed in the micelles. This approach offers a general strategy for investigating protein-protein and protein-ligand/drug interactions and conformational changes of the alpha-helical membrane proteins in their native envelope context.

DeFrND: detergent-free reconstitution into native nanodiscs with designer membrane scaffold peptides.

Membrane scaffold proteins-based nanodiscs (NDs) have facilitated unprecedented structural and biophysical analysis of membrane proteins in a near-native lipid environment. However, successful reconstitution of membrane proteins in NDs requires prior solubilization and purification in detergents, which may impact their physiological structure and function. Furthermore, the detergent-mediated reconstitution of NDs is unlikely to recapitulate the precise composition or asymmetry of native membranes. To circumvent this fundamental limitation of traditional ND technology, we herein describe the development of membrane-solubilizing peptides to directly extract membrane proteins from native cell membranes into nanoscale discoids. By systematically protein engineering and screening, we created a new class of chemically modified Apolipoprotein-A1 mimetic peptides to enable the formation of detergent-free NDs (DeFrNDs) with high efficiency. NDs generated with these engineered membrane scaffold peptides are suitable for obtaining high-resolution structures using single-particle cryo-EM with native lipids. To further highlight the versatility of DeFrNDs, we directly extract a sampling of membrane signaling proteins with their surrounding native membranes for biochemical and biophysical interrogations.

Understanding the biosynthesis of human IgM SAM-6 through a combinatorial expression of mutant subunits that affect product assembly and secretion.

Polymeric IgMs are secreted from plasma cells abundantly despite their structural complexity and intricate multimerization steps. To gain insights into IgM's assembly mechanics that underwrite such high-level secretion, we characterized the biosynthetic process of a natural human IgM, SAM-6, using a heterologous HEK293(6E) cell platform that allowed the production of IgMs both in hexameric and pentameric forms in a controlled fashion. By creating a series of mutant subunits that differentially disrupt secretion, folding, and specific inter-chain disulfide bond formation, we assessed their effects on various aspects of IgM biosynthesis in 57 different subunit chain combinations, both in hexameric and pentameric formats. The mutations caused a spectrum of changes in steady-state subcellular subunit distribution, ER-associated inclusion body formation, intracellular subunit detergent solubility, covalent assembly, secreted IgM product quality, and secretion output. Some mutations produced differential effects on product quality depending on whether the mutation was introduced to hexameric IgM or pentameric IgM. Through this systematic combinatorial approach, we consolidate diverse overlapping knowledge on IgM biosynthesis for both hexamers and pentamers, while unexpectedly revealing that the loss of certain inter-chain disulfide bonds, including the one between μHC and λLC, is tolerated in polymeric IgM assembly and secretion. The findings highlight the differential roles of underlying non-covalent protein-protein interactions in hexamers and pentamers when orchestrating the initial subunit interactions and maintaining the polymeric IgM product integrity during ER quality control steps, secretory pathway trafficking, and secretion.

Heterogenous Expression and Purification of Lipid II Flippase from Staphylococcus aureus.

Staphylococcus aureus is a common pathogen with strains that are resistant to existing antibiotics. MurJ from S. aureus (SaMurJ), an integral membrane protein functioning as Lipid II flippase, is a potential target for developing new antibacterial agents against this pathogen. Successful expression and purification of this protein shall be useful in the development of drugs against this target. In this study, we demonstrated the optimized expression and purification procedures of SaMurJ, identified suitable detergent for extracting and solubilizing the protein, and examined the peptidisc system to generate a detergent-free environment. SaMurJ fused with N-terminal ten-His tag was expressed without induction. Six detergents were selected for screening the most efficient candidate for extraction and solubilization of the protein. The thermostability of the detergent-solubilized protein was assessed by evaluated temperature incubation. Different ratios of peptidisc bi-helical peptide (NSPr) to SaMurJ were mixed and the on-bead peptidisc assembly method was applied. SaMurJ expressed in BL21(DE3) was confirmed by peptide fingerprinting, with a yield of 1 mg SaMurJ per liter culture. DDM was identified as the optimum detergent for solubilization and the nickel affinity column enabled SaMurJ purification with a purity of ~88%. However, NSPr could not stabilize SaMurJ. The expression and purification of SaMurJ were successful, with high purity and good yield. SaMurJ can be solubilized and stabilized by a DDM-containing buffer.

Bacterial Envelope Fractionation.

Numerous bioinformatics tools allow predicting the localization of membrane proteins in the outer or inner membrane of Escherichia coli with high precision. Nevertheless, it might be desirable to experimentally verify such predictions or to assay the correct localization of recombinant or mutated variants of membrane proteins. Here we describe two methods (preferential detergent solubilization and sucrose-gradient fractionation) that allow to fractionate Gram-negative bacterial membranes and subsequently to enrich inner or outer membrane proteins.

Structural determinants of mitochondrial STAT3 targeting and function

Signal transducer and activator of transcription (STAT) 3 has been found within mitochondria in addition to its canonical role of shuttling between cytoplasm and nucleus during cytokine signaling. Mitochondrial STAT3 has been implicated in modulation of cellular metabolism, largely through effects on the respiratory electron transport chain. However, the structural requirements underlying mitochondrial targeting and function have remained unclear. Here, we show that mitochondrial STAT3 partitions between mitochondrial compartments defined by differential detergent solubility, suggesting that mitochondrial STAT3 is membrane associated. The majority of STAT3 was found in an SDS soluble fraction copurifying with respiratory chain proteins, including numerous components of the complex I NADH dehydrogenase, while a minor component was found with proteins of the mitochondrial translation machinery. Mitochondrial targeting of STAT3 required the amino-terminal domain, and an internal linker domain motif also directed mitochondrial translocation. However, neither the phosphorylation of serine 727 nor the presence of mitochondrial DNA was required for the mitochondrial localization of STAT3. Two cysteine residues in the STAT3 SH2 domain, which have been previously suggested to be targets for protein palmitoylation, were also not required for mitochondrial translocation, but were required for its function as an enhancer of complex I activity. These structural determinants of STAT3 mitochondrial targeting and function provide potential therapeutic targets for disrupting the activity of mitochondrial STAT3 in diseases such as cancer.

Detection of Lipid-Bound Bacteriorhodopsin Trimer Complex Directly from Purple Membrane by Native Mass Spectrometry.

Native mass spectrometry (MS) was used to detect the membrane protein, bacteriorhodopsin (bR), in its 27 kDa monomeric form and trimeric assemblies directly from lipid-containing purple membranes (PMs) from the halophilic archaeon, Halobacterium salinarum. Trimer bR ion populations bound to lipid molecules were detected with n-octyl β-d-glucopyranoside as the solubilizing detergent; the use of octyl tetraethylene glycol monooctyl ether or n-dodecyl-β-d-maltopyranoside resulted in only detection of monomeric bR. The archaeal lipids phosphotidylglycerolphosphate methyl ester and 3-HSO3-Galp-β1,6-Manp-α1,2-Glcp-α1,1-sn-2,3-diphytanylglycerol were the only lipids in the PMs found to bind to bR, consistent with previous high-resolution structural studies. Removal of the lipids from the sample resulted in the detection of only the bR monomer, highlighting the importance of specific lipids for stabilizing the bR trimer. To the best of our knowledge, this is the first report of the detection of the bR trimer with resolved lipid-bound species by MS.

Phospholipid cofactor solubilization inhibits formation of native prions.

Cofactor molecules are required to generate infectious mammalian prions in vitro. Mouse and hamster prions appear to have different cofactor preferences: Whereas both mouse and hamster prions can use phosphatidylethanolamine (PE) as a prion cofactor, only hamster prions can also use single-stranded RNA as an alternative cofactor. Here, we investigated the effect of detergent solubilization on rodent prion formation in vitro. We discovered that detergents that can solubilize PE (n-octylglucoside, n-octylgalactoside, and CHAPS) inhibit mouse prion formation in serial protein misfolding cyclic amplification (sPMCA) reactions using bank vole brain homogenate substrate, whereas detergents that are unable to solubilize PE (Triton X-100 and IPEGAL) have no effect. For all three PE-solubilizing detergents, inhibition of RML mouse prion formation was only observed above the critical micellar concentration (CMC). Two other mouse prion strains, Me7 and 301C, were also inhibited by the three PE-solubilizing detergents but not by Triton X-100 or IPEGAL. In contrast, none of the detergents inhibited hamster prion formation in parallel sPMCA reactions using the same bank vole brain homogenate substrate. In reconstituted sPMCA reactions using purified substrates, n-octylglucoside inhibited hamster prion formation when immunopurified bank vole PrPC substrate was supplemented with brain phospholipid but not with RNA. Interestingly, phospholipid cofactor solubilization had no effect in sPMCA reactions using bacterially expressed recombinant PrP substrate, indicating that the inhibitory effect of solubilization requires PrPC post-translational modifications. Overall, these in vitro results show that the ability of PE to facilitate the formation of native but not recombinant prions requires phospholipid bilayer integrity, suggesting that membrane structure may play an important role in prion formation in vivo.

Productive, Qualitative, and In Vitro Fermentation Traits of Amaranthus Grains as Potential Ingredients for Pig Diet

The present work compared the agronomic traits, chemical composition, fatty acid profile, and in vitro fermentation characteristics of twelve accessions of Amaranthus spp., belonging to A. cruentus, A. hybridus, A. hypochondriacus, and A. tricolor, grown in a semiarid Mediterranean area. Among accessions, Benin and Arizona (A. cruentus) and Pennsylvania (A. hypochondriacus) showed the highest seed yield (on average, 322.1 g m−2), while Taiwan (A. tricolor) and India and Iowa (A. hypochondriacus) the highest thousand seed weight (on average, 0.81 g). Among the species, A. hypochondriacus showed the highest crude protein (16 g 100g−1), starch (51.5 g 100g−1), and soluble detergent fiber (2.03 g 100g−1) contents and the most favorable in vitro fermentation characteristics with the highest short-chain fatty acid (SCFA 52.6 mmol g−1) and butyric acid (20.7% SCFA) production together with the lowest crude fiber (4.93 g 100g−1) and insoluble dietary fiber (12.5 g 100g−1) content. Arizona (A. cruentus) showed the highest level of monounsaturated fatty acids (32.67 g 100g−1), Ohio (A. hybridus) had the highest levels of polyunsaturated fatty acids (44.62 g 100g−1) and n6-PUFA (44.21 g 100g−1), and India (A. hypochondriacus) had the highest level of n3-PUFA (0.63 g 100g−1). A. hypochondriacus exhibited not only desirable nutritive characteristics, agronomic traits, and suitability to Mediterranean growing conditions, but also a potential beneficial effect. Nonetheless, it is recommended to run longer-term field trials to confirm these findings and to assess the genotype by environment interaction either with current accessions or others from the wide Amaranth germplasm available.

Real-time tracking of drug binding to influenza A M2 reveals a high energy barrier

The drug Rimantadine binds to two different sites in the M2 protein from influenza A, a peripheral site and a pore site that is the primary site of efficacy. It remained enigmatic that pore binding did not occur in certain detergent micelles, and in particular incomplete binding was observed in a mixture of lipids selected to match the viral membrane. Here we show that two effects are responsible, namely changes in the protein upon pore binding that prevented detergent solubilization, and slow binding kinetics in the lipid samples. Using 55–100 kHz magic-angle spinning NMR, we characterize kinetics of drug binding in three different lipid environments: DPhPC, DPhPC with cholesterol and viral mimetic membrane lipid bilayers. Slow pharmacological binding kinetics allowed the characterization of spectral changes associated with non-specific binding to the protein periphery in the kinetically trapped pore-apo state. Resonance assignments were determined from a set of proton-detected 3D spectra. Chemical shift changes associated with functional binding in the pore of M2 were tracked in real time in order to estimate the activation energy. The binding kinetics are affected by pH and the lipid environment and in particular cholesterol. We found that the imidazole-imidazole hydrogen bond at residue histidine 37 is a stable feature of the protein across several lipid compositions. Pore binding breaks the imidazole-imidazole hydrogen bond and limits solubilization in DHPC detergent.

Affinity Selection Mass Spectrometry (AS-MS) as a Tool for Prospecting Target Ligands

Affinity selection mass spectrometry (AS-MS) has been shown to be a powerful tool for identifying bioactive molecules in synthetic and/or natural libraries. The selection provided by the formation of the target-ligand complex allows the identification of hits irrespective of their functional effect. Moreover, it precludes the use of label, since the binders are identified by their exact mass.1 The binders are determined by an affinity or index ratio calculated through control assays. The target protein can be used in solution or immobilized in a solid support (Figure 1). Both approaches have pros and cons. Unlike most conventional high-throughput screening assays, AS-MS has fewer or no limitations when it comes to target selection. It is important, however, to understand the implications of choosing membrane proteins as targets. Membrane proteins correspond to 42% of all drug targets listed in DrugBank. Moreover, they are likely to be selected as protein targets due to their participation in many disease pathways, acting as ion channels, molecular transporters, solute carriers, receptors, and anchors. One of the bottlenecks in working with membrane proteins comes from the need to use a detergent for solubilization, folding, and structure maintenance. Detergents are usually used above the critical micelle concentration, which can lead to empty micelles and thus to false positive results, caused by nonspecific interactions with the detergent micelles.8 Interference in the ionization of the binders also needs to be examined.

Single-molecule investigation of ligand-activation mechanisms for membrane proteins in cell-derived nanovesicles.

Molecular recognition of ligands for specific binding sites in membrane proteins is a cornerstone of biological signaling processes. Despite structural and functional evidence often painting a relatively clear picture of the binding sites, the mechanism by which ligand binding to multiple active sites confers a change in protein activity is in many cases still poorly understood. This is due in part to the fact that individual binding events are rarely directly observed, and their asynchronous dynamics are occluded in ensemble-averaged measures. For membrane proteins, single-molecule approaches that resolve these dynamics are challenged by dysfunction in non-native lipid environments, lack of access to intracellular sites, and costly sample preparation. Here, we describe an approach for resolving individual binding events of fluorescently labeled ligands at single membrane proteins in cell-derived nanovesicles. The approach involves simple transient transfection, does not use detergent solubilization so that proteins are maintained throughout in their native lipid bilayer, and provides solution access to either intracellular or extracellular binding domains. Furthermore, we describe a fully automated approach to idealization of fluorescence time series describing the binding dynamics at many molecules imaged simultaneously. As an exemplar, we apply these approaches to cyclic nucleotide binding to TAX-4 ion channels critical for sensory transduction and discuss mechanistic insights from kinetic modeling of their binding dynamics. This approach is broadly applicable to studies of binding dynamics for membrane proteins with extracellular or intracellular domains in native cell membrane.

Copolymeric nanodiscs do not retain the native membrane environment, but do maintain protein-lipid interactions.

Polymer stabilized lipid-protein nanoparticles have become invaluable tools in membrane biology since they can isolate individual proteins with their surrounding lipids. These nanoparticles, termed nanodiscs, have historically relied on membrane scaffolding proteins, but can now be formed using amphipathic copolymers that directly intercalate into membranes. Since copolymeric nanodiscs are formed directly from membranes without requiring detergent solubilization, they have been presumed to retain the native membrane environment, but this assumption has not been thoroughly tested. To test if native membrane lipid packing is retained in styrene-maleic acid (SMA) and diisobutylene-maleic acid (DIBMA) stabilized nanodiscs, lipid packing was analyzed using solvatochromic dyes Laurdan, C-Laurdan, and Pro12A. SMA and DIBMA copolymeric nanodiscs were formed from membranes increasing in complexity from single and binary lipid mixture, to biomimetic ternary mixtures composed of palmitoyl-sphingomyelin (PSM), 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), and cholesterol, and ultimately to mammalian cellular membranes. For all synthetic model membranes, lipid packing of fluid-disordered membranes was significantly increased in both SMA and DIBMA nanodiscs compared to intact membranes, with DIBMA being the least perturbing. In contrast, lipid packing in ordered membranes was unaffected. For cellular membranes, lipid packing was also higher inside nanodiscs than in native membranes. Our data challenges the assumption that native membrane properties are retained inside nanodiscs. While not maintaining native membrane properties, nanodiscs can be useful models for studying protein-lipid interactions, as they are composed of membrane lipids. Interestingly, nanodiscs can exchange lipids through collisions, allowing tuning of their lipid compositions. We used this capability to analyze the preferred surrounding lipidome of multi-pass transmembrane proteins. Altogether, our studies demonstrate that copolymeric nanodiscs have different properties to those of intact membranes, but they are useful models in studying the paralipidomes of membrane proteins.

Miniprep assisted proteomics (MAP) for rapid proteomics sample preparation.

Complete enzymatic digestion of proteins for bottom-up proteomics is substantially improved by use of detergents for denaturation and solubilization. Detergents however, are incompatible with many proteases and highly detrimental to LC-MS/MS. Recently; filter-based methods have seen wide use due to their capacity to remove detergents and harmful reagents prior to digestion and mass spectrometric analysis. We hypothesized that non-specific protein binding to negatively charged silica-based filters would be enhanced by addition of lyotropic salts, similar to DNA purification. We sought to exploit these interactions and investigate if low-cost DNA purification spin-filters, 'Minipreps,' efficiently and reproducibly bind proteins for digestion and LC-MS/MS analysis. We propose a new method, Miniprep Assisted Proteomics (MAP), for sample preparation. We demonstrate binding capacity, performance, recovery and identification rates for proteins and whole-cell lysates using MAP. MAP recovered equivalent or greater protein yields from 0.5-50 μg analyses benchmarked against commercial trapping preparations. Nano UHPLC-MS/MS proteome profiling of lysates of Escherichia coli had 99.3% overlap vs. existing approaches and reproducibility of replicate minipreps was 98.8% at the 1% FDR protein level. Label Free Quantitative proteomics was performed and 91.2% of quantified proteins had a %CV <20% (2044/2241). Miniprep Assisted Proteomics can be performed in minutes, shows low variability, high recovery and proteome depth. This suggests a significant role for adventitious binding in developing new proteomics sample preparation techniques. MAP represents an efficient, ultra-low-cost alternative for sample preparation in a commercially obtainable device that costs ∼$0.50 (USD) per miniprep.

Recent Advances in the Digestive, Metabolic and Therapeutic Effects of Farnesoid X Receptor and Fibroblast Growth Factor 19: From Cholesterol to Bile Acid Signaling.

Bile acids (BA) are amphiphilic molecules synthesized in the liver (primary BA) starting from cholesterol. In the small intestine, BA act as strong detergents for emulsification, solubilization and absorption of dietary fat, cholesterol, and lipid-soluble vitamins. Primary BA escaping the active ileal re-absorption undergo the microbiota-dependent biotransformation to secondary BA in the colon, and passive diffusion into the portal vein towards the liver. BA also act as signaling molecules able to play a systemic role in a variety of metabolic functions, mainly through the activation of nuclear and membrane-associated receptors in the intestine, gallbladder, and liver. BA homeostasis is tightly controlled by a complex interplay with the nuclear receptor farnesoid X receptor (FXR), the enterokine hormone fibroblast growth factor 15 (FGF15) or the human ortholog FGF19 (FGF19). Circulating FGF19 to the FGFR4/β-Klotho receptor causes smooth muscle relaxation and refilling of the gallbladder. In the liver the binding activates the FXR-small heterodimer partner (SHP) pathway. This step suppresses the unnecessary BA synthesis and promotes the continuous enterohepatic circulation of BAs. Besides BA homeostasis, the BA-FXR-FGF19 axis governs several metabolic processes, hepatic protein, and glycogen synthesis, without inducing lipogenesis. These pathways can be disrupted in cholestasis, nonalcoholic fatty liver disease, and hepatocellular carcinoma. Thus, targeting FXR activity can represent a novel therapeutic approach for the prevention and the treatment of liver and metabolic diseases.

A method for selective 19 F-labeling absent of probe sequestration (SLAPS).

Fluorine (19F) offers several distinct advantages for biomolecular nuclear magnetic resonance spectroscopy such as no background signal, 100% natural abundance, high sensitivity, and a large chemical shift range. Exogenous cysteine‐reactive 19F‐probes have proven especially indispensable for characterizing large, challenging systems that are less amenable to other isotopic labeling strategies such as G protein‐coupled receptors. As fluorine linewidths are inherently broad, limiting reactions with offsite cysteines is critical for spectral simplification and accurate deconvolution of component peaks—especially when analyzing systems with intermediate to slow timescale conformational exchange. Here, we uncovered noncovalent probe sequestration by detergent proteomicelles as a second source of offsite labeling when using the popular 19F‐probe BTFMA (2‐bromo‐N‐(4‐[trifluoromethyl]phenyl)acetamide). The chemical shift and relaxation rates of these unreacted 19F‐BTFMA molecules are insufficient to distinguish them from protein‐conjugates, but they can be easily identified using mass spectrometry. We present a simple four‐step protocol for Selective Labeling Absent of Probe Sequestration (SLAPS): physically disrupt cell membranes in the absence of detergent, incubate membranes with cysteine‐reactive 19F‐BTFMA, remove excess unreacted 19F‐BTFMA molecules via ultracentrifugation, and finally solubilize in the detergent of choice. Our approach builds upon the in‐membrane chemical modification method with the addition of one crucial step: removal of unreacted 19F‐probes by ultracentrifugation prior to detergent solubilization. SLAPS is broadly applicable to other lipophilic cysteine‐reactive probes and membrane protein classes solubilized in detergent micelles or lipid mimetics.

New insights into the Tat protein transport cycle from characterizing the assembled Tat translocon.

The twin-arginine protein translocation (Tat) system transports folded proteins across the bacterial cytoplasmic membrane and the thylakoid membrane of chloroplasts. The Tat translocation site is transiently assembled by the recruitment of multiple TatA proteins to a substrate-activated TatBC receptor complex in a process requiring the protonmotive force. The ephemeral nature of the Tat translocation site has so far precluded its isolation. We now report that detergent solubilization of membranes during active transport allows the recovery of receptor complexes that are associated with elevated levels of TatA. We apply this biochemical analysis in combination with live cell fluorescence imaging to Tat systems trapped in the assembled state. We resolve sub-steps in the Tat translocation cycle and infer that TatA assembly precedes the functional interaction of TatA with a polar cluster site on TatC. We observe that dissipation of the protonmotive force releases TatA oligomers from the assembled translocation site demonstrating that the stability of the TatA oligomer does not depend on binding to the receptor complex and implying that the TatA oligomer is assembled at the periphery of the receptor complex. This work provides new insight into the Tat transport cycle and advances efforts to isolate the active Tat translocon.