Expression of an olfactory receptor in Escherichia coli: purification, reconstitution, and ligand binding

Engineered Chimera Protein Constructs to Facilitate the Production of Heterologous Transmembrane Proteins in E. coli

To delve into the structure-function relationship of transmembrane proteins (TMPs), robust protocols are needed to produce them in a pure, stable, and functional state. Among all hosts that express heterologous TMPs, E. coli has the lowest cost and fastest turnover. However, many of the TMPs expressed in E. coli are misfolded. Several strategies have been developed to either direct the foreign TMPs to E. coli's membrane or retain them in a cytosolic soluble form to overcome this deficiency. Here, we summarize protein engineering methods to produce chimera constructs of the desired TMPs fused to either a signal peptide or precursor maltose binding protein (pMBP) to direct the entire construct to the periplasm, therefore depositing the fused TMP in the plasma membrane. We further describe strategies to produce TMPs in soluble form by utilizing N-terminally fused MBP without a signal peptide. Depending on its N- or C-terminus location, a fusion to apolipoprotein AI can either direct the TMP to the membrane or shield the hydrophobic regions of the TMP, maintaining the soluble form. Strategies to produce G-protein-coupled receptors, TMPs of Mycobacterium tuberculosis, HIV-1 Vpu, and other TMPs are discussed. This knowledge could increase the scope of TMPs' expression in E. coli.

Biosensors for Odor Detection: A Review

Animals can easily detect hundreds of thousands of odors in the environment with high sensitivity and selectivity. With the progress of biological olfactory research, scientists have extracted multiple biomaterials and integrated them with different transducers thus generating numerous biosensors. Those biosensors inherit the sensing ability of living organisms and present excellent detection performance. In this paper, we mainly introduce odor biosensors based on substances from animal olfactory systems. Several instances of organ/tissue-based, cell-based, and protein-based biosensors are described and compared. Furthermore, we list some other biological materials such as peptide, nanovesicle, enzyme, and aptamer that are also utilized in odor biosensors. In addition, we illustrate the further developments of odor biosensors.

Oriented Insertion of ESR-Containing Hybrid Proteins in Proteoliposomes

Microbial rhodopsins comprise a diverse family of retinal-containing membrane proteins that convert absorbed light energy to transmembrane ion transport or sensory signals. Incorporation of these proteins in proteoliposomes allows their properties to be studied in a native-like environment; however, unidirectional protein orientation in the artificial membranes is rarely observed. We aimed to obtain proteoliposomes with unidirectional orientation using a proton-pumping retinal protein from Exiguobacterium sibiricum, ESR, as a model. Three ESR hybrids with soluble protein domains (mCherry or thioredoxin at the C-terminus and Caf1M chaperone at the N-terminus) were obtained and characterized. The photocycle of the hybrid proteins incorporated in proteoliposomes demonstrated a higher pK_a of the M state accumulation compared to that of the wild-type ESR. Large negative electrogenic phases and an increase in the relative amplitude of kinetic components in the microsecond time range in the kinetics of membrane potential generation of ESR-Cherry and ESR-Trx indicate a decrease in the efficiency of transmembrane proton transport. On the contrary, Caf-ESR demonstrates a native-like kinetics of membrane potential generation and the corresponding electrogenic stages. Our experiments show that the hybrid with Caf1M promotes the unidirectional orientation of ESR in proteoliposomes.

Advances in the Production of Olfactory Receptors for Industrial Use

In biological olfactory systems, olfactory receptors (ORs) can recognize and discriminate between thousands of volatile organic compounds with very high sensitivity and specificity. The superior properties of ORs have led to the development of OR-based biosensors that have shown promising potential in many applications over the past two decades. In particular, newly designed technologies in gene synthesis, protein expression, solubilization, purification, and membrane mimetics for membrane proteins have greatly opened up the previously inaccessible industrial potential of ORs. In this review, gene design, expression and solubilization strategies, and purification and reconstitution methods available for modern industrial applications are examined, with a focus on ORs. The limitations of current OR production technology are also estimated, and future directions for further progress are suggested.

Wireless portable bioelectronic nose device for multiplex monitoring toward food freshness/spoilage

Monitoring food freshness/spoilage is important to ensure food quality and safety. Current methods of food quality monitoring are mostly time-consuming and labor intensive processes that require massive analytical equipment. In this study, we developed a portable bioelectronic nose (BE-nose) integrated with trace amine-associated receptor (TAAR) nanodiscs (NDs), allowing food quality monitoring via the detection of food spoilage indicators, including the biogenic amines cadaverine (CV) and putrescine (PT). The olfactory receptors TAAR13c and TAAR13d, which have specific affinities for CV and PT, were produced and successfully reconstituted in ND structures. TAAR13 NDs BE-nose-based side-gated field-effect transistor (SG-FET) system was constructed by utilizing a graphene micropattern (GM) into which two types of olfactory NDs (TAAR13c ND and TAAR13d ND) were introduced, and this system showed ultrahigh sensitivity for a limit of detection (LOD) of 1 fM for CV and PT. Moreover, the binding affinities between the TAAR13 NDs and the indicators were confirmed by a tryptophan fluorescence quenching assay and biosimulations, in which the specific binding site was confirmed. Gas-phase indicators were detected by the TAAR13 NDs BE-nose platform, and the LODs for CV and PT were confirmed to be 26.48 and 7.29 ppb, respectively. In addition, TAAR13 NDs BE-nose was fabricated with commercial gas sensors as a portable platform for the measurement of NH₃ and H₂S, multiplexed monitoring was achieved with similar performance, and the change ratio of the indicators was observed in a real sample. The integration of commercial gas sensors on a BE-nose enhanced the accuracy and reliability for the quality monitoring of real food samples. These results indicate that the portable TAAR13 NDs BE-nose can be used to monitor CV and PT over a wide range of concentrations, therefore, the electronic nose platform can be utilized for monitoring the freshness/spoilage step in various foods.

Sequence coevolution and structure stabilization modulate olfactory receptor expression

Olfactory receptors (ORs) belong to class A G-protein coupled receptors (GPCRs) and are activated by a variety of odorants. To date, there is no three-dimensional structure of an OR available. One of the major bottlenecks in obtaining purified protein for structural studies of ORs is their poor expression in heterologous cells. To design mutants that enhance expression and thereby enable protein purification, we first identified computable physical properties that recapitulate OR and class A GPCR expression and further conducted an iterative computational prediction-experimental test cycle and generated human OR mutants that express as high as biogenic amine receptors for which structures have been solved. In the process of developing the computational method to recapitulate the expression of ORs in membranes, we identified properties, such as amino acid sequence coevolution, and the strength of the interactions between intracellular loop 1 (ICL1) and the helix 8 region of ORs, to enhance their heterologous expression. We identified mutations that are directly located in these regions as well as other mutations not located in these regions but allosterically strengthen the ICL1-helix 8 enhance expression. These mutants also showed functional responses to known odorants. This method to enhance heterologous expression of mammalian ORs will facilitate high-throughput "deorphanization" of ORs, and enable OR purification for biochemical and structural studies to understand odorant-OR interactions.

Biotechnological applications of mammalian odorant-binding proteins

The olfactory system of mammals allows the detection and discrimination of thousands of odors from the environment. In mammals, odorant-binding proteins (OBPs) are considered responsible to carry odorant molecules across the aqueous nasal mucus to the olfactory receptors (ORs). The three-dimensional structure of these proteins presents eight antiparallel β-sheets and a short α-helical segment close to the C terminus, typical of the lipocalins family. The great ability of OBPs to bind differentiated ligand molecules has driven the research to understand the mechanisms underlying the OBP function in nature and the development of advanced biotechnological applications. This review describes the role of mammalian OBPs in the olfactory perception, highlighting the influence of several key parameters (amino acids, temperature, ionic strength, and pH) in the formation of the OBP/ligand complex. The information from the literature regarding OBP structure, affinity, the strength of binding, and stability inspiring the development of several applications herein detailed.

Efficient production of a functional G protein-coupled receptor in E. coli for structural studies

G protein-coupled receptors (GPCRs) are transmembrane signal transducers which regulate many key physiological process. Since their discovery, their analysis has been limited by difficulties in obtaining sufficient amounts of the receptors in high-quality, functional form from heterologous expression hosts. Albeit highly attractive because of its simplicity and the ease of isotope labeling for NMR studies, heterologous expression of functional GPCRs in E. coli has proven particularly challenging due to the absence of the more evolved protein expression and folding machinery of higher eukaryotic hosts. Here we first give an overview on the previous strategies for GPCR E. coli expression and then describe the development of an optimized robust protocol for the E. coli expression and purification of two mutants of the turkey β₁-adrenergic receptor (β₁AR) uniformly or selectively labeled in ¹⁵N or ²H,¹⁵N. These mutants had been previously optimized for thermal stability using insect cell expression and used successfully in crystallographic and NMR studies. The same sequences were then used for E. coli expression. Optimization of E. coli expression was achieved by a quantitative analysis of losses of receptor material at each step of the solubilization and purification procedure. Final yields are 0.2-0.3 mg receptor per liter culture. Whereas both expressed mutants are well folded and competent for orthosteric ligand binding, the less stable YY-β₁AR mutant also comprises the two native tyrosines Y^5.58 and Y^7.53, which enable G protein binding. High-quality ¹H-¹⁵N TROSY spectra were obtained for E. coli-expressed YY-β₁AR in three different functional states (antagonist, agonist, and agonist + G protein-mimicking nanobody-bound), which are identical to spectra obtained of the same forms of the receptor expressed in insect cells. NdeI and AgeI restriction sites introduced into the expression plasmid allow for the easy replacement of the receptor gene by other GPCR genes of interest, and the provided quantitative workflow analysis may guide the respective adaptation of the purification protocol.

Expression and purification of recombinant G protein-coupled receptors: A review

Given their extensive role in cell signalling, GPCRs are significant drug targets; despite this, many of these receptors have limited or no available prophylaxis. Novel drug design and discovery significantly rely on structure determination, of which GPCRs are typically elusive. Progress has been made thus far to produce sufficient quantity and quality of protein for downstream analysis. As such, this review highlights the systems available for recombinant GPCR expression, with consideration of their advantages and disadvantages, as well as examples of receptors successfully expressed in these systems. Additionally, an overview is given on the use of detergents and the styrene maleic acid (SMA) co-polymer for membrane solubilisation, as well as purification techniques.

Structural instability and divergence from conserved residues underlie intracellular retention of mammalian odorant receptors

Mammalian odorant receptors are a diverse and rapidly evolving set of G protein-coupled receptors expressed in olfactory cilia membranes. Most odorant receptors show little to no cell surface expression in nonolfactory cells due to endoplasmic reticulum retention, which has slowed down biochemical studies. Here we provide evidence that structural instability and divergence from conserved residues of individual odorant receptors underlie intracellular retention using a combination of large-scale screening of odorant receptors cell surface expression in heterologous cells, point mutations, structural modeling, and machine learning techniques. We demonstrate the importance of conserved residues by synthesizing consensus odorant receptors that show high levels of cell surface expression similar to conventional G protein-coupled receptors. Furthermore, we associate in silico structural instability with poor cell surface expression using molecular dynamics simulations. We propose an enhanced evolutionary capacitance of olfactory sensory neurons that enable the functional expression of odorant receptors with cryptic mutations.

Bioelectronic sensor mimicking the human neuroendocrine system for the detection of hypothalamic-pituitary-adrenal axis hormones in human blood

In the neuroendocrine system, corticotropin-releasing hormone (CRH) and adrenocorticotropic hormone (ACTH) play important roles in the regulation of the hypothalamic-pituitary-adrenal (HPA) system. Disorders of the HPA system lead to physiological problems, such as Addison's disease and Cushing's syndrome. Therefore, detection of CRH and ACTH is essential for diagnosing disorders related to the HPA system. Herein, receptors of the HPA axis were used to construct a bioelectronic sensor system for the detection of CRH and ACTH. The CRH receptor, corticotropin-releasing hormone receptor 1 (CRHR1), and the ACTH receptor, melanocortin 2 receptor (MC2R), were produced using an Escherichia coli expression system, and were reconstituted using nanodisc (ND) technology. The receptor-embedded NDs were immobilized on a floating electrode of a carbon nanotube field-effect transistor (CNT-FET). The constructed sensors sensitively detected CRH and ACTH to a concentration of 1 fM with high selectivity in real time. Furthermore, the reliable detection of CRH and ACTH in human plasma by the developed sensors demonstrated their potential in clinical and practical applications. These results indicate that CRHR1 and MC2R-based bioelectronic sensors can be applied for rapid and efficient detection of CRH and ACTH.

Ultrasensitive, Selective, and Highly Stable Bioelectronic Nose That Detects the Liquid and Gaseous Cadaverine

Field-effect transistor (FET) devices based on conductive nanomaterials have been used to develop biosensors. However, development of FET-based biosensors that allow efficient stability, especially in the gas phase, for obtaining reliable and reproducible responses remains a challenge. In this study, we developed a nanodisc (ND)-functionalized bioelectronic nose (NBN) based on a nickel (Ni)-decorated carboxylated polypyrrole nanoparticle (cPPyNP)-FET that offers the detection of liquid and gaseous cadaverine (CV). The TAAR13c, specifically binding to CV, which is an indicator of food spoilage, was successfully constructed in NDs. The NBN was fabricated by the oriented assembly of TAAR13c-embedded NDs (T13NDs) onto the transistor with Ni/cPPyNPs. The NBN showed high performance in selectivity and sensitivity for the detection of CV, with excellent stability in both aqueous and gas phases. Moreover, the NBN allowed efficient measurement of corrupted real-food samples. It demonstrates the ND-based device can allow the practical biosensor that provides high stability in the gas phase.

Biochemical characterization of the mouse ABCF3 protein, a partner of the flavivirus-resistance protein OAS1B

Mammalian ATP-binding cassette (ABC) subfamily F member 3 (ABCF3) is a class 2 ABC protein that has previously been identified as a partner of the mouse flavivirus resistance protein 2',5'-oligoadenylate synthetase 1B (OAS1B). The functions and natural substrates of ABCF3 are not known. In this study, analysis of purified ABCF3 showed that it is an active ATPase, and binding analyses with a fluorescent ATP analog suggested unequal contributions by the two nucleotide-binding domains. We further showed that ABCF3 activity is increased by lipids, including sphingosine, sphingomyelin, platelet-activating factor, and lysophosphatidylcholine. However, cholesterol inhibited ABCF3 activity, whereas alkyl ether lipids either inhibited or resulted in a biphasic response, suggesting small changes in lipid structure differentially affect ABCF3 activity. Point mutations in the two nucleotide-binding domains of ABCF3 affected sphingosine-stimulated ATPase activity differently, further supporting different roles for the two catalytic pockets. We propose a model in which pocket 1 is the site of basal catalysis, whereas pocket 2 engages in ligand-stimulated ATP hydrolysis. Co-localization of the ABCF3-OAS1B complex to the virus-remodeled endoplasmic reticulum membrane has been shown before. We also noted that co-expression of ABCF3 and OAS1B in bacteria alleviated growth inhibition caused by expression of OAS1B alone, and ABCF3 significantly enhanced OAS1B levels, indirectly showing interaction between these two proteins in bacterial cells. As viral RNA synthesis requires large amounts of ATP, we conclude that lipid-stimulated ATP hydrolysis may contribute to the reduction in viral RNA production characteristic of the flavivirus resistance phenotype.

Illuminating the Energy Landscape of GPCRs: The Key Contribution of Solution-State NMR Associated with Escherichia coli as an Expression Host

Conformational dynamics of GPCRs are central to their function but are difficult to explore at the atomic scale. Solution-state NMR has provided the major contribution in that area of study during the past decade, despite nonoptimized labeling schemes due to the use of insect cells and, to a lesser extent, yeast as the main expression hosts. Indeed, the most efficient isotope-labeling scheme ever to address energy landscape issues for large proteins or protein complexes relies on the use of ¹³CH₃ probes immersed in a perdeuterated dipolar environment, which is essentially out of reach of eukaryotic expression systems. In contrast, although its contribution has been underestimated because of technical issues, Escherichia coli is by far the best-adapted host for such labeling. As it is now tightly controlled, we show in this review that bacterial expression can provide an NMR spectral resolution never achieved in the GPCR field.

Expression, Purification and Characterization of the Human Cannabinoid 1 Receptor

The human cannabinoid 1 receptor (hCB1) is involved in numerous physiological processes and therefore provides a wide scope of potential therapeutic opportunities to treat maladies such as obesity, cardio-metabolic disorders, substance abuse, neuropathic pain, and multiple sclerosis. Structure-based drug design using the current knowledge of the hCB1 receptor binding site is limited and requires purified active protein. Heterologous expression and purification of functional hCB1 has been the bottleneck for ligand binding structural studies using biophysical methods such as mass spectrometry, x-ray crystallography and NMR. We constructed several plasmids for in-cell or in vitro Escherichia coli (E. coli) based expression of truncated and stabilized hCB1 receptor (hΔCB1 and hΔCB1_T4L) variants and evaluated their competency to bind the CP-55,940 ligand. MALDI-TOF MS analysis of in vitro expressed and purified hΔCB1_T4Lhis6 variants, following trypsin digestion, generated ~80% of the receptor sequence coverage. Our data demonstrate the feasibility of a cell-free expression system as a promising part of the strategy for the elucidation of ligand binding sites of the hCB1 receptor using a "Ligand Assisted Protein Structure" (LAPS) approach.

The bioelectronic nose and tongue using olfactory and taste receptors: Analytical tools for food quality and safety assessment

Food intake is the primary method for obtaining energy and component materials in the human being. Humans evaluate the quality of food by combining various facets of information, such as an item of food's appearance, smell, taste, and texture in the mouth. Recently, bioelectronic noses and tongues have been reported that use human olfactory and taste receptors as primary recognition elements, and nanoelectronics as secondary signal transducers. Bioelectronic sensors that mimic human olfaction and gustation have sensitively and selectively detected odor and taste molecules from various food samples, and have been applied to food quality assessment. The portable and multiplexed bioelectronic nose and tongue are expected to be used as next-generation analytical tools for rapid on-site monitoring of food quality. In this review, we summarize recent progress in the bioelectronic nose and tongue using olfactory and taste receptors, and discuss the potential applications and future perspectives in the food industry.

Dopamine Receptor D1 Agonism and Antagonism Using a Field-Effect Transistor Assay

The field-effect transistor (FET) has been used in the development of diagnostic tools for several decades, leading to high-performance biosensors. Therefore, the FET platform can provide the foundation for the next generation of analytical methods. A major role of G-protein-coupled receptors (GPCRs) is in the transfer of external signals into the cell and promoting human body functions; thus, their principle application is in the screening of new drugs. The research community uses efficient systems to screen potential GPCR drugs; nevertheless, the need to develop GPCR-conjugated analytical devices remains for next-generation new drug screening. In this study, we proposed an approach for studying receptor agonism and antagonism by combining the roles of FETs and GPCRs in a dopamine receptor D1 (DRD1)-conjugated FET system, which is a suitable substitute for conventional cell-based receptor assays. DRD1 was reconstituted and purified to mimic native binding pockets that have highly discriminative interactions with DRD1 agonists/antagonists. The real-time responses from the DRD1-nanohybrid FET were highly sensitive and selective for dopamine agonists/antagonists, and their maximal response levels were clearly different depending on their DRD1 affinities. Moreover, the equilibrium constants (K) were estimated by fitting the response levels. Each K value indicates the variation in the affinity between DRD1 and the agonists/antagonists; a greater K value corresponds to a stronger DRD1 affinity in agonism, whereas a lower K value in antagonism indicates a stronger dopamine-blocking effect.

The diversified function and potential therapy of ectopic olfactory receptors in non-olfactory tissues

Olfactory receptors (ORs) are mainly distributed in olfactory neurons and play a key role in detecting volatile odorants, eventually resulting in the production of smell perception. Recently, it is also reported that ORs are expressed in non-olfactory tissues including heart, lung, sperm, skin, and cancerous tissues. Interestingly, ectopic ORs are associated with the development of diseases in non-olfactory tissues. For instance, ectopic ORs initiate the hypoxic ventilatory responses and maintain the oxygen homeostasis of breathing in the carotid body when oxygen levels decline. Ectopic ORs induce glucose homeostasis in diabetes. Ectopic ORs regulate systemic blood pressure by increasing renin secretion and vasodilation. Ectopic ORs participate in the process of tumor cell proliferation, apoptosis, metastasis, and invasiveness. Ectopic ORs accelerate the occurrence of obesity, angiogenesis and wound-healing processes. Ectopic ORs affect fetal hemoglobin levels in sickle cell anemia and thalassemia. Finally, we also elaborate some ligands targeting for ORs. Obviously, the diversified function and related signal pathway of ectopic ORs may play a potential therapeutic target in non-olfactory tissues. Thus, this review focuses on the latest research results about the diversified function and therapeutic potential of ectopic ORs in non-olfactory tissues.

Complete Reversible Refolding of a G-Protein Coupled Receptor on a Solid Support

The factors defining the correct folding and stability of integral membrane proteins are poorly understood. Folding of only a few select membrane proteins has been scrutinised, leaving considerable deficiencies in knowledge for large protein families, such as G protein coupled receptors (GPCRs). Complete reversible folding, which is problematic for any membrane protein, has eluded this dominant receptor family. Moreover, attempts to recover receptors from denatured states are inefficient, yielding at best 40–70% functional protein. We present a method for the reversible unfolding of an archetypal family member, the β₁-adrenergic receptor, and attain 100% recovery of the folded, functional state, in terms of ligand binding, compared to receptor which has not been subject to any unfolding and retains its original, folded structure. We exploit refolding on a solid support, which could avoid unwanted interactions and aggregation that occur in bulk solution. We determine the changes in structure and function upon unfolding and refolding. Additionally, we employ a method that is relatively new to membrane protein folding; pulse proteolysis. Complete refolding of β₁-adrenergic receptor occurs in n-decyl-β-D-maltoside (DM) micelles from a urea-denatured state, as shown by regain of its original helical structure, ligand binding and protein fluorescence. The successful refolding strategy on a solid support offers a defined method for the controlled refolding and recovery of functional GPCRs and other membrane proteins that suffer from instability and irreversible denaturation once isolated from their native membranes.

Large-scale production and protein engineering of G protein-coupled receptors for structural studies

Structural studies of G protein-coupled receptors (GPCRs) gave insights into molecular mechanisms of their action and contributed significantly to molecular pharmacology. This is primarily due to technical advances in protein engineering, production and crystallization of these important receptor targets. On the other hand, NMR spectroscopy of GPCRs, which can provide information about their dynamics, still remains challenging due to difficulties in preparation of isotopically labeled receptors and their low long-term stabilities. In this review, we discuss methods used for expression and purification of GPCRs for crystallographic and NMR studies. We also summarize protein engineering methods that played a crucial role in obtaining GPCR crystal structures.

Membrane protein structure from rotational diffusion

The motional averaging of powder pattern line shapes is one of the most fundamental aspects of sold-state NMR. Since membrane proteins in liquid crystalline phospholipid bilayers undergo fast rotational diffusion, all of the signals reflect the angles of the principal axes of their dipole-dipole and chemical shift tensors with respect to the axis defined by the bilayer normal. The frequency span and sign of the axially symmetric powder patterns that result from motional averaging about a common axis provide sufficient structural restraints for the calculation of the three-dimensional structure of a membrane protein in a phospholipid bilayer environment. The method is referred to as rotationally aligned (RA) solid-state NMR and demonstrated with results on full-length, unmodified membrane proteins with one, two, and seven trans-membrane helices. RA solid-state NMR is complementary to other solid-state NMR methods, in particular oriented sample (OS) solid-state NMR of stationary, aligned samples. Structural distortions of membrane proteins from the truncations of terminal residues and other sequence modifications, and the use of detergent micelles instead of phospholipid bilayers have also been demonstrated. Thus, it is highly advantageous to determine the structures of unmodified membrane proteins in liquid crystalline phospholipid bilayers under physiological conditions. RA solid-state NMR provides a general method for obtaining accurate and precise structures of membrane proteins under near-native conditions.

Folding and stability of integral membrane proteins in amphipols

Amphipols (APols) are a family of amphipathic polymers designed to keep transmembrane proteins (TMPs) soluble in aqueous solutions in the absence of detergent. APols have proven remarkably efficient at (i) stabilizing TMPs, as compared to detergent solutions, and (ii) folding them from a denatured state to a native, functional one. The underlying physical-chemical mechanisms are discussed.

Mistic's membrane association and its assistance in overexpression of a human GPCR are independent processes

The interaction of the Bacillus subtilis protein Mistic with the bacterial membrane and its role in promoting the overexpression of other membrane proteins are still matters of debate. In this study, we aimed to determine whether individual helical fragments of Mistic are sufficient for its interaction with membranes in vivo and in vitro. To this end, fragments encompassing each of Mistic's helical segments and combinations of them were produced as GFP-fusions, and their cellular localization was studied in Escherichia coli. Furthermore, peptides corresponding to the four helical fragments were synthesized by solid-phase peptide synthesis, and their ability to acquire secondary structure in a variety of lipids and detergents was studied by circular dichroism spectroscopy. Both types of experiments demonstrate that the third helical fragment of Mistic interacts only with LDAO micelles but does not partition into lipid bilayers. Interestingly, the other three helices interact with membranes in vivo and in vitro. Nevertheless, all of these short sequences can replace full-length Mistic as N-terminal fusions to achieve overexpression of a human G-protein-coupled receptor in E. coli, although with different effects on quantity and quality of the protein produced. A bioinformatic analysis of the Mistic family expanded the number of homologs from 4 to 20, including proteins outside the genus Bacillus. This information allowed us to discover a highly conserved Shine-Dalgarno sequence in the operon mstX-yugO that is important for downstream translation of the potassium ion channel yugO.

Oxidative in vitro folding of a cysteine deficient variant of the G protein-coupled neuropeptide Y receptor type 2 improves stability at high concentration

In vitro folding of G protein-coupled receptors into a detergent environment represents a promising strategy for obtaining sufficient amounts of functional receptor molecules for structural studies. Typically, these preparations exhibit a poor long-term stability especially at the required high protein concentration. Here, we report a protocol for the stabilization of the Escherichia coli-expressed and subsequently folded neuropeptide Y receptor type 2. We identified the free cysteines in the receptor as one major reason for intermolecular protein aggregation. Therefore, six out of the eight cysteine residues were mutated to alanine or serine without any significant loss of functionality of the receptor as demonstrated in cell culture models. Furthermore, the disulfide bond between the remaining two cysteines was irreversibly formed by applying oxidative in vitro folding. Applying this strategy, the stability of the functionally folded Y2 receptor could be increased to 20 days at a concentration of 15 μm in a micelle environment consisting of 1,2-diheptanoyl-sn-glycero-3-phosphocholine and n-dodecyl-ß-D-maltoside.

Lipid bilayer preparations of membrane proteins for oriented and magic-angle spinning solid-state NMR samples

Solid-state NMR spectroscopy has been used successfully for characterizing the structure and dynamics of membrane proteins as well as their interactions with other proteins in lipid bilayers. Such an environment is often necessary for achieving native-like structures. Sample preparation is the key to this success. Here we present a detailed description of a robust protocol that results in high-quality membrane protein samples for both magic-angle spinning and oriented-sample solid-state NMR. The procedure is demonstrated using two proteins: CrgA (two transmembrane helices) and Rv1861 (three transmembrane helices), both from Mycobacterium tuberculosis. The success of this procedure relies on two points. First, for samples for both types of NMR experiment, the reconstitution of the protein from a detergent environment to an environment in which it is incorporated into liposomes results in 'complete' removal of detergent. Second, for the oriented samples, proper dehydration followed by rehydration of the proteoliposomes is essential. By using this protocol, proteoliposome samples for magic-angle spinning NMR and uniformly aligned samples (orientational mosaicity of <1°) for oriented-sample NMR can be obtained within 10 d.

Cell-free expression of a mammalian olfactory receptor and unidirectional insertion into small unilamellar vesicles (SUVs)

Although the identification of the multigene family encoding mammalian olfactory receptors were identified more than 20 years ago, we are far from understanding olfactory perception because of the difficulties in functional expression of these receptors in heterologous cell systems. Cell-free (CF) or in vitro expression systems offer an elegant alternative route to cell based protein expression, as the functional expression of membrane proteins can be directly achieved from the genetic template without the need of cell cultivation and protein isolation. Here we investigated in detail the cell-free expression and membrane insertion of the olfactory receptor OR5 in dependence of different experimental conditions like probing different origins of the cell-free expression system (from bacteria, via plants and insects toward mammalian system) and lipid composition of the respective extracts. We provided substantial biochemical indications by radioactive labeling based on [(35)S]-methionine, followed by proteolytic digestion, and we found that the insertion of the olfactory receptor OR5 into liposomes resulted in an unidirectional orientation with the binding side exposed into the aqueous space, resembling the native orientation in the cilia of the olfactory neurons. We report the different results in synthesis capacity for the different in vitro systems employed as we like to demonstrate the first in vitro kit toward and ex situ and ex vivo odorant receptor array.

BB0172, a Borrelia burgdorferi outer membrane protein that binds integrin α3β1

Lyme disease is a multisystemic disorder caused by Borrelia burgdorferi infection. Upon infection, some B. burgdorferi genes are upregulated, including members of the microbial surface components recognizing adhesive matrix molecule (MSCRAMM) protein family, which facilitate B. burgdorferi adherence to extracellular matrix components of the host. Comparative genome analysis has revealed a new family of B. burgdorferi proteins containing the von Willebrand factor A (vWFA) domain. In the present study, we characterized the expression and membrane association of the vWFA domain-containing protein BB0172 by using in vitro transcription/translation systems in the presence of microsomal membranes and with detergent phase separation assays. Our results showed evidence of BB0172 localization in the outer membrane, the orientation of the vWFA domain to the extracellular environment, and its function as a metal ion-dependent integrin-binding protein. This is the first report of a borrelial adhesin with a metal ion-dependent adhesion site (MIDAS) motif that is similar to those observed in eukaryotic integrins and has a similar function.

Magic angle spinning nuclear magnetic resonance spectroscopy of G protein-coupled receptors

G protein-coupled receptors (GPCRs) represent the largest family of membrane receptors and mediate a diversity of cellular processes. These receptors have a common seven-transmembrane helix structure, yet have evolved to respond to literally thousands of different ligands. In this chapter, we describe the use of magic angle spinning solid-state NMR spectroscopy for characterizing the structure and dynamics of GPCRs. Solid-state NMR spectroscopy is well suited for structural measurements in both detergent micelles and membrane bilayer environments. We first outline the methods for large-scale production of stable, functional receptors containing (13)C- and (15)N-labeled amino acids. The expression methods make use of eukaryotic HEK293S cell lines that produce correctly folded, fully functional receptors. We subsequently describe the basic methods used for magic angle spinning solid-state NMR measurements of chemical shifts and dipolar couplings, which reveal detailed information on GPCR structure and dynamics.

[Handling G-protein-coupled receptors: expression, purification and in vitro stabilization]

Among the different classes of integral membrane proteins, G protein-coupled receptors (GPCR) constitute the largest family. They are involved in most essential physiological functions and particularly play a key role in cell-to-cell communication and sensory signal transduction. They represent targets for approximately 30% of currently marketed drugs. In order to better understand their functioning, define their tridimensional structure and develop novel selective and efficient therapeutic compounds, it is crucial to purify these proteins for a full characterization. However, this biochemical step is not trivial since GPCR are present in membranes at very low levels and they require detergents to be extracted from their natural lipid environment and be handled as functional proteins. No universal strategy for GPCR production, purification and stabilization is currently available; each single GPCR possesses a unique set of physicochemical characteristics, preference for some detergents upon solubilization and specific conditions for purification. During the last decade, major breakthroughs regarding overexpression, purification and above all GPCR stabilization, thanks to amphipols and nanodiscs, opened very exciting perspectives for structural and dynamic investigations of these membrane proteins. The aim of this chapter is to provide an overview of the different aspects of GPCR handling.

Structural transitions of transmembrane helix 6 in the formation of metarhodopsin I

Absorption of light by the visual pigment rhodopsin triggers a rapid cis-trans photoisomerization of its retinal chromophore and a series of conformational changes in both the retinal and protein. The largest structural change is an outward tilt of transmembrane helix H6 that increases the separation of the intracellular ends of H6 and H3 and opens up the G-protein binding site. In the dark state of rhodopsin, Glu247 at the intracellular end of H6 forms a salt bridge with Arg135 on H3 to tether H6 in an inactive conformation. The Arg135-Glu247 interaction is broken in the active state of the receptor, and Arg135 is then stabilized by interactions with Tyr223, Met257, and Tyr306 on helices H5, H6, and H7, respectively. To address the mechanism of H6 motion, solid-state NMR measurements are undertaken of Metarhodopsin I (Meta I), the intermediate preceding the active Metarhodopsin II (Meta II) state of the receptor. (13)C NMR dipolar recoupling measurements reveal an interhelical contact of (13)Cζ-Arg135 with (13)Cε-Met257 in Meta I but not with (13)Cζ-Tyr223 or (13)Cζ-Tyr306. These observations suggest that helix H6 has rotated in the formation of Meta I but that structural changes involving helices H5 and H7 have not yet occurred. Together, our results provide insights into the sequence of events leading up to the outward motion of H6, a hallmark of G protein-coupled receptor activation.

Assessment of a fully active class A G protein-coupled receptor isolated from in vitro folding

We provide a protocol for the preparation of fully active Y2 G protein-coupled receptors (GPCRs). Although a valuable target for pharmaceutical research, information about the structure and dynamics of these molecules remains limited due to the difficulty in obtaining sufficient amounts of homogeneous and fully active receptors for in vitro studies. Recombinant expression of GPCRs as inclusion bodies provides the highest protein yields at lowest costs. But this strategy can only successfully be applied if the subsequent in vitro folding results in a high yield of active receptors and if this fraction can be isolated from the nonactive receptors in a homogeneous form. Here, we followed that strategy to provide large quantities of the human neuropeptide Y receptor type 2 and determined the folding yield before and after ligand affinity chromatography using a radioligand binding assay. Directly after folding, we achieved a proportion of ~25% active receptor. This value could be increased to ~96% using ligand affinity chromatography. Thus, a very homogeneous sample of the Y2 receptor could be prepared that exhibited a K(D) value of 0.1 ± 0.05 nM for the binding of polypeptide Y, which represents one of the natural ligands of the Y2 receptor.

Amphipols from A to Z

Amphipols (APols) are short amphipathic polymers that can substitute for detergents to keep integral membrane proteins (MPs) water soluble. In this review, we discuss their structure and solution behavior; the way they associate with MPs; and the structure, dynamics, and solution properties of the resulting complexes. All MPs tested to date form water-soluble complexes with APols, and their biochemical stability is in general greatly improved compared with MPs in detergent solutions. The functionality and ligand-binding properties of APol-trapped MPs are reviewed, and the mechanisms by which APols stabilize MPs are discussed. Applications of APols include MP folding and cell-free synthesis, structural studies by NMR, electron microscopy and X-ray diffraction, APol-mediated immobilization of MPs onto solid supports, proteomics, delivery of MPs to preexisting membranes, and vaccine formulation.

Biophysical characterization of G-protein coupled receptor-peptide ligand binding

G-protein coupled receptors (GPCRs) are ubiquitous membrane proteins allowing intracellular responses to extracellular factors that range from photons of light to small molecules to proteins. Despite extensive exploitation of GPCRs as therapeutic targets, biophysical characterization of GPCR-ligand interactions remains challenging. In this minireview, we focus on techniques that have been successfully used for structural and biophysical characterization of peptide ligands binding to their cognate GPCRs. The techniques reviewed include solution-state nuclear magnetic resonance (NMR) spectroscopy, solid-state NMR, X-ray diffraction, fluorescence spectroscopy and single-molecule fluorescence methods, flow cytometry, surface plasmon resonance, isothermal titration calorimetry, and atomic force microscopy. The goal herein is to provide a cohesive starting point to allow selection of techniques appropriate to the elucidation of a given GPCR-peptide interaction.

A systematic assessment of mature MBP in membrane protein production: overexpression, membrane targeting and purification

Obtaining enough membrane protein in native or native-like status is still a challenge in membrane protein structure biology. Maltose binding protein (MBP) has been widely used as a fusion partner in improving membrane protein production. In the present work, a systematic assessment on the application of mature MBP (mMBP) for membrane protein overexpression and purification was performed on 42 membrane proteins, most of which showed no or poor expression level in membrane fraction fused with an N-terminal Histag. It was found that most of the small membrane proteins were overexpressed in the native membrane of Escherichia coli when using mMBP. In addition, the proteolysis of the fusions were performed on the membrane without solubilization with detergents, leading to the development of an efficient protocol to directly purify the target membrane proteins from the membrane fraction through a one-step affinity chromatography. Our results indicated that mMBP is an excellent fusion partner for overexpression, membrane targeting and purification of small membrane proteins. The present expression and purification method may be a good solution for the large scale preparation of small membrane proteins in structural and functional studies.

New advances in production and functional folding of G-protein-coupled receptors

G-protein-coupled receptors (GPCRs), the largest family of integral membrane proteins, participate in the regulation of many physiological functions and are the targets of approximately 30% of currently marketed drugs. However, knowledge of the structural and molecular bases of GPCR functions remains limited owing to difficulties related to their overexpression, purification and stabilization. The development of new strategies aimed at obtaining large amounts of functional GPCRs is therefore crucial. Here, we review the most recent advances in the production and functional folding of GPCRs from Escherichia coli inclusion bodies. Major breakthroughs open exciting perspectives for structural and dynamic investigations of GPCRs. In particular, combining targeting to bacterial inclusion bodies with amphipol-assisted folding is emerging as a highly powerful strategy.

Contemporary methods in structure determination of membrane proteins by solution NMR

Integral membrane proteins are vital to life, being responsible for information and material exchange between a cell and its environment. Although high-resolution structural information is needed to understand how these functions are achieved, membrane proteins remain an under-represented subset of the protein structure databank. Solution NMR is increasingly demonstrating its ability to help address this knowledge shortfall, with the development of a diverse array of techniques to counter the challenges presented by membrane proteins. Here we document the advances that are helping to define solution NMR as an effective tool for membrane protein structure determination. Developments introduced over the last decade in the production of isotope-labeled samples, reconstitution of these samples into the growing selection of NMR-compatible membrane-mimetic systems, and the approaches used for the acquisition and application of structural restraints from these complexes are reviewed.

Expression of G-protein coupled receptors in Escherichia coli for structural studies

To elaborate a high-performance system for expression of genes of G-protein coupled receptors (GPCR), methods of direct and hybrid expression of 17 GPCR genes in Escherichia coli and selection of strains and bacteria cultivation conditions were investigated. It was established that expression of most of the target GPCR fused with the N-terminal fragment of OmpF or Mistic using media for autoinduction provides high output (up to 50 mg/liter).

Fusion expression and immunogenicity of Bordetella pertussis PTS1-FHA protein: implications for the vaccine development

Mutants of pertussis toxin (PT) S1 subunit and filamentous hemagglutinin (FHA) type I immunodominant domain from Bordetella pertussis (B. pertussis) are considered to be effective candidate antigens for acellular pertussis vaccines; however, the substantial progress is hampered in part for the lack of a suitable in vitro expression system. In this paper, the gene sequences of a S1 mutant C180-R9K/E129G (mS1) and a truncated peptide named Fs from FHA type I immunodominant domain were linked together and constructed to pET22b expression vector as a fusion gene; after inducing with IPTG, it was highly expressed in E. coli BL21 (DE3) as inclusion body. The fusion protein FsmS1 was purified from cell lysates and refolded successfully. The result of Western blotting indicate that it was able to react with both anti-S1 and anti-FHA McAbs; antiserum produced from New Zealand white rabbits immunized with this protein was able to recognize both native PT and FHA antigens as determined by western blotting. These data have provided a novel feasible method to produce PT S1 subunit and FHA type I immunodominant domain in large scale in vitro, which is implicated for the development of multivalent subunit vaccines candidate against B. pertussis infection.

A reconstitution protocol for the in vitro folded human G protein-coupled Y2 receptor into lipid environment

Although highly resolved crystal structures of G protein-coupled receptors have become available within the last decade, the need for studying these molecules in their natural membrane environment, where the molecules are rather dynamic, has been widely appreciated. Solid-state NMR spectroscopy is an excellent method to study structure and dynamics of membrane proteins in their native lipid environment. We developed a reconstitution protocol for the uniformly (15)N labeled Y(2) receptor into a bicelle-like lipid structure with high yields suitable for NMR studies. Milligram quantities of target protein were expressed in Escherichia coli using an optimized fermentation process in defined medium yielding in over 10mg/L medium of purified Y(2) receptor solubilized in SDS micelles. The structural integrity of the receptor molecules was strongly increased through refolding and subsequent reconstitution into phospholipid membranes. Specific ligand binding to the integrated receptor was determined using radioligand affinity assay. Further, by NMR measurement a dispersion of the (15)N signals comparable to native rhodopsin was shown. The efficiency of the reconstitution could also be inferred from the fact that reasonable (13)C NMR spectra at natural abundance could be acquired.