Mammalian G protein-coupled receptor expression in Escherichia coli: II. Refolding and biophysical characterization of mouse cannabinoid receptor 1 and human parathyroid hormone receptor 1

Design of an effective small expression tag to enhance GPCR production in E. coli-based cell-free and whole cell expression systems

G protein-coupled receptors (GPCRs) play crucial roles in sensory, immune, and tumor metastasis processes, making them valuable targets for pharmacological and sensing applications in various industries. However, most GPCRs have low production yields in Escherichia coli (E. coli) expression systems. To overcome this limitation, we introduced AT10 tag, an effective fusion tag that could significantly enhance expression levels of various GPCRs in E. coli and its derived cell-free protein synthesis (CFPS) system. This AT10 tag consisted of an A/T-rich gene sequence designed via optimization of translation initiation rate. It is translated into a short peptide sequence of 10 amino acids at the N-terminus of GPCRs. Additionally, effector proteins could be utilized to suppress cytotoxicity caused by membrane protein expression, further boosting GPCR production in E. coli. Enhanced expression of various GPCRs using this AT10 tag is a promising approach for large-scale production of functional GPCRs in E. coli-based CFPS and whole cell systems, enabling their potential utilization across a wide range of industrial applications.

Ni-rGO Sensor Combined with Human Olfactory Receptor-Embedded Nanodiscs for Detecting Gas-Phase DMMP as a Simulant of Nerve Agents

Nerve agents are organophosphorus toxic chemicals that can inhibit acetylcholinesterase, leading to paralysis of the nervous system and death. Early detection of nerve agents is important for safety issues. Dimethyl methylphosphonate (DMMP) is widely used as a simulant of nerve agents, and many studies have been conducted using DMMP as a substitute for detecting nerve agents. Despite many studies on sensors for detecting DMMP, they have limitations in sensitivity and selectivity. To overcome these limitations, a nickel-decorated reduced graphene oxide (Ni-rGO) sensor with human olfactory receptor hOR2T7 nanodiscs was utilized to create a bioelectronic nose platform for DMMP gas detection. hOR2T7 was produced and reconstituted into nanodiscs for enhancing the sensor's stability, especially for detection in a gas phase. It could detect DMMP gas selectively and repeatedly at a concentration of 1 ppb. This sensitive and selective bioelectronic nose can be applied as a practical tool for the detection of gaseous chemical warfare agents in military and safety fields.

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.

Single-chain Fv antibody covalently linked to antigen peptides and its structural evaluation

The monoclonal antibody G2 specifically recognizes different peptides. The single-chain Fv (scFv) antibodies of G2 covalently linked to antigen peptides, Pep18mer and Pep395, via a flexible linker were expressed in Escherichia coli in the insoluble fraction, and were solubilized using guanidine HCl, followed by refolding. We analyzed the folding thermodynamics of the refolded proteins, purified as monomers using size-exclusion chromatography (SEC). The results of the differential scanning calorimetry (DSC) showed that the thermal stabilities of antigen peptide-linked G2 scFvs were higher than those of antigen-free G2 scFv in the absence or presence of antigen peptides. The folding thermodynamics further indicated how the antigen-antibody affinity affect the intramolecular interactions. The combination of SEC and DSC experiments could confirm the folding correctness of antigen peptide-linked G2 scFvs and could be applied for "structural screening" of refolded proteins in the case that the "functional screening" like antigen binding is difficult to apply. The present method to covalently link the peptide would contribute to the stable complex structure, and would be widely applied to other antibodies recognizing peptide antigens.

Bio-Inspired Strategies for Improving the Selectivity and Sensitivity of Artificial Noses: A Review

Artificial noses are broad-spectrum multisensors dedicated to the detection of volatile organic compounds (VOCs). Despite great recent progress, they still suffer from a lack of sensitivity and selectivity. We will review, in a systemic way, the biomimetic strategies for improving these performance criteria, including the design of sensing materials, their immobilization on the sensing surface, the sampling of VOCs, the choice of a transduction method, and the data processing. This reflection could help address new applications in domains where high-performance artificial noses are required such as public security and safety, environment, industry, or healthcare.

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.

Parathyroid Hormone Senses Extracellular Calcium To Modulate Endocrine Signaling upon Binding to the Family B GPCR Parathyroid Hormone 1 Receptor

Parathyroid hormone (PTH) binds to a family B G protein coupled receptor, parathyroid hormone 1 receptor (PTH1R). One of its functions is to regulate Ca2+ homeostasis in bone remodeling, during which Ca2+ can reach up to 40 mM. A truncated version of PTH, PTH(1-34), can fully activate PTH1R and has been used for osteoporosis treatments. Here, we used fluorescence anisotropy to examine the binding of PTH(1-34) to PTH1R purified in nanodiscs (PTH1R-ND) and found that the affinity increases 5-fold in the presence of 15 mM Ca2+. However, PTHrP(1-36), another truncated endogenous agonist for PTH1R, does not show this Ca2+ effect. Mutations of Glu19 and Glu22 in PTH(1-34) that are not conserved in PTHrP(1-36) largely abolished the Ca2+ effect. The results support that PTH(1-34) not only activates PTH1R but also uniquely senses Ca2+. This dual function of a peptide hormone is a novel observation that couples changes in extracellular environment with endocrine signaling. Understanding this can potentially reveal the complex role of PTH signaling in bone remodeling and improve the PTH(1-34) treatment for osteoporosis.

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.

Segmental Isotope Labeling of Insoluble Proteins for Solid-State NMR by Protein Trans-Splicing

Solid-state NMR spectroscopy (ssNMR) is uniquely suited for atomic-resolution structural investigations of large protein assemblies, which are notoriously difficult to study due to their insoluble and non-crystalline nature. However, assignment ambiguities because of limited resolution and spectral crowding are currently major hurdles that quickly increase with the length of the polypeptide chain. The line widths of ssNMR signals are independent of proteins size, making segmental isotope labeling a powerful approach to overcome this limitation. It allows a scalable reduction of signal overlap, aids the assignment of repetitive amino acid sequences, and can be easily combined with other selective isotope labeling strategies. Here we present a detailed protocol for segmental isotope labeling of insoluble proteins using protein trans-splicing. Our protocol exploits the ability of many insoluble proteins, such as amyloid fibrils, to fold correctly under in vitro conditions. In combination with the robust trans-splicing efficiency of the intein DnaE from Nostoc punctiforme, this allows for high yields of segmentally labeled protein required for ssNMR analysis.

Bioelectronic Nose Using Olfactory Receptor-Embedded Nanodiscs

Olfactory receptors (ORs) are the largest family of the G protein-coupled receptors (GPCRs), which are significantly involved in many human diseases and 40% of all drug targets. A platform containing stable and high-quality OR would be a powerful tool for the development of a practical biosensor that can be applied to various applications, such as the early diagnosis of diseases, assessment of food quality, and drug and fragrance development. Significant efforts have been made to develop the biosensor using GPCRs; nevertheless, they remain a challenge. This chapter describes an attractive methodology for the development of a stable bioelectronic nose using OR-embedded nanodiscs. The ORs were produced in Escherichia coli (E. coli), purified with column chromatography, reconstituted into nanodiscs and applied to a carbon nanotube-field effect transistor (CNT-FET) with floating electrodes. The nanodisc-based bioelectronic nose exhibits high-performance in terms of sensitivity, selectivity and stability. This strategy can be used as a practical method for the receptor-based sensing approach, which represents significant progress in nano-bio technology toward a practical biosensor.

Nanodisc-Based Bioelectronic Nose Using Olfactory Receptor Produced in Escherichia coli for the Assessment of the Death-Associated Odor Cadaverine

Cadaverine (CV), a death-associated odor, is an important target molecule for various sensor applications, including the evaluation of food spoilage. In this study, we developed an oriented nanodisc (ND)-functionalized bioelectronic nose (ONBN), based on carbon nanotube transistors and nanodiscs embedded with an olfactory receptor produced in Escherichia coli (E. coli) for detection of CV. To fabricate ONBN devices, a trace-amine-associated receptor 13c (TAAR13c) binding to CV was produced in E. coli, purified, reconstituted into NDs, and assembled, in the desired orientation, onto a carbon- nanotube-based field-effect transistor with floating electrodes. The ONBN showed high performance in terms of sensitivity and selectivity. Moreover, the ONBN was used to measure CV in diverse real-food samples for the determination of food freshness. These results indicate ONBN devices can be utilized to evaluate the quality of food samples quantitatively, which should enable versatile practical applications such as food safety and preservative development. Moreover, the ONBN could provide a useful tool for detection of corpses, which could be practically used in disaster responses.

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.

Expression and refolding of the protective antigen of Bacillus anthracis: A model for high-throughput screening of antigenic recombinant protein refolding

Bacillus anthracis protective antigen (PA) is a well known and relevant immunogenic protein that is the basis for both anthrax vaccines and diagnostic methods. Properly folded antigenic PA is necessary for these applications. In this study a high level of PA was obtained in recombinant Escherichia coli. The protein was initially accumulated in inclusion bodies, which facilitated its efficient purification by simple washing steps; however, it could not be recognized by specific antibodies. Refolding conditions were subsequently analyzed in a high-throughput manner that enabled nearly a hundred different conditions to be tested simultaneously. The recovery of the ability of PA to be recognized by antibodies was screened by dot blot using a coefficient that provided a measure of properly refolded protein levels with a high degree of discrimination. The best refolding conditions resulted in a tenfold increase in the intensity of the dot blot compared to the control. The only refolding additive that consistently yielded good results was L-arginine. The statistical analysis identified both cooperative and negative interactions between the different refolding additives. The high-throughput approach described in this study that enabled overproduction, purification and refolding of PA in a simple and straightforward manner, can be potentially useful for the rapid screening of adequate refolding conditions for other overexpressed antigenic proteins.

Membranes Do Not Tell Proteins How To Fold

Which properties of the membrane environment are essential for the folding and oligomerization of transmembrane proteins? Because the lipids that surround membrane proteins in situ spontaneously organize into bilayers, it may seem intuitive that interactions with the bilayer provide both hydrophobic and topological constraints that help the protein to achieve a stable and functional three-dimensional structure. However, one may wonder whether folding is actually driven by the membrane environment or whether the folded state just reflects an adaptation of integral proteins to the medium in which they function. Also, apart from the overall transmembrane orientation, might the asymmetry inherent in biosynthesis processes cause proteins to fold to out-of-equilibrium, metastable topologies? Which of the features of a bilayer are essential for membrane protein folding, and which are not? To which extent do translocons dictate transmembrane topologies? Recent data show that many membrane proteins fold and oligomerize very efficiently in media that bear little similarity to a membrane, casting doubt on the essentiality of many bilayer constraints. In the following discussion, we argue that some of the features of bilayers may contribute to protein folding, stability and regulation, but they are not required for the basic three-dimensional structure to be achieved. This idea, if correct, would imply that evolution has steered membrane proteins toward an accommodation to biosynthetic pathways and a good fit into their environment, but that their folding is not driven by the latter or dictated by insertion apparatuses. In other words, the three-dimensional structure of membrane proteins is essentially determined by intramolecular interactions and not by bilayer constraints and insertion pathways. Implications are discussed.

Production of monoclonal antibodies against GPCR using cell-free synthesized GPCR antigen and biotinylated liposome-based interaction assay

G-protein-coupled receptors (GPCRs) are one of the most important drug targets, and anti-GPCR monoclonal antibody (mAb) is an essential tool for functional analysis of GPCRs. However, it is very difficult to develop GPCR-specific mAbs due to difficulties in production of recombinant GPCR antigens, and lack of efficient mAb screening method. Here we describe a novel approach for the production of mAbs against GPCR using two original methods, bilayer-dialysis method and biotinylated liposome-based interaction assay (BiLIA), both of which are developed using wheat cell-free protein synthesis system and liposome technology. Using bilayer-dialysis method, various GPCRs were successfully synthesized with quality and quantity sufficient for immunization. For selection of specific mAb, we designed BiLIA that detects interaction between antibody and membrane protein on liposome. BiLIA prevented denaturation of GPCR, and then preferably selected conformation-sensitive antibodies. Using this approach, we successfully obtained mAbs against DRD1, GHSR, PTGER1 and T1R1. With respect to DRD1 mAb, 36 mouse mAbs and 6 rabbit mAbs were obtained which specifically recognized native DRD1 with high affinity. Among them, half of the mAbs were conformation-sensitive mAb, and two mAbs recognized extracellular loop 2 of DRD1. These results indicated that this approach is useful for GPCR mAb production.

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.

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.

Expression, stabilization and purification of membrane proteins via diverse protein synthesis systems and detergents involving cell-free associated with self-assembly peptide surfactants

G-protein coupled receptors (GPCRs) are involved in regulating most of physiological actions and metabolism in the bodies, which have become most frequently addressed therapeutic targets for various disorders and diseases. Purified GPCR-based drug discoveries have become routine that approaches to structural study, novel biophysical and biochemical function analyses. However, several bottlenecks that GPCR-directed drugs need to conquer the problems including overexpression, solubilization, and purification as well as stabilization. The breakthroughs are to obtain efficient protein yield and stabilize their functional conformation which are both urgently requiring of effective protein synthesis system methods and optimal surfactants. Cell-free protein synthesis system is superior to the high yields and post-translation modifications, and early signs of self-assembly peptide detergents also emerged to superiority in purification of membrane proteins. We herein focus several predominant protein synthesis systems and surfactants involving the novel peptide detergents, and uncover the advantages of cell-free protein synthesis system with self-assembling peptide detergents in purification of functional GPCRs. This review is useful to further study in membrane proteins as well as the new drug exploration.

Expression and functional characterization of membrane-integrated mammalian corticotropin releasing factor receptors 1 and 2 in Escherichia coli

Corticotropin-Releasing Factor Receptors (CRFRs) are class B1 G-protein-coupled receptors, which bind peptides of the corticotropin releasing factor family and are key mediators in the stress response. In order to dissect the receptors' binding specificity and enable structural studies, full-length human CRFR1α and mouse CRFR2β as well as fragments lacking the N-terminal extracellular domain, were overproduced in E. coli. The characteristics of different CRFR2β-PhoA gene fusion products expressed in bacteria were found to be in agreement with the predicted ones in the hepta-helical membrane topology model. Recombinant histidine-tagged CRFR1α and CRFR2β expression levels and bacterial subcellular localization were evaluated by cell fractionation and Western blot analysis. Protein expression parameters were assessed, including the influence of E. coli bacterial hosts, culture media and the impact of either PelB or DsbA signal peptide. In general, the large majority of receptor proteins became inserted in the bacterial membrane. Across all experimental conditions significantly more CRFR2β product was obtained in comparison to CRFR1α. Following a detergent screen analysis, bacterial membranes containing CRFR1α and CRFR2β were best solubilized with the zwitterionic detergent FC-14. Binding of different peptide ligands to CRFR1α and CRFR2β membrane fractions were similar, in part, to the complex pharmacology observed in eukaryotic cells. We suggest that our E. coli expression system producing functional CRFRs will be useful for large-scale expression of these receptors for structural studies.

Retinal proteins as model systems for membrane protein folding

Experimental folding studies of membrane proteins are more challenging than water-soluble proteins because of the higher hydrophobicity content of membrane embedded sequences and the need to provide a hydrophobic milieu for the transmembrane regions. The first challenge is their denaturation: due to the thermodynamic instability of polar groups in the membrane, secondary structures in membrane proteins are more difficult to disrupt than in soluble proteins. The second challenge is to refold from the denatured states. Successful refolding of membrane proteins has almost always been from very subtly denatured states. Therefore, it can be useful to analyze membrane protein folding using computational methods, and we will provide results obtained with simulated unfolding of membrane protein structures using the Floppy Inclusions and Rigid Substructure Topography (FIRST) method. Computational methods have the advantage that they allow a direct comparison between diverse membrane proteins. We will review here both, experimental and FIRST studies of the retinal binding proteins bacteriorhodopsin and mammalian rhodopsin, and discuss the extension of the findings to deriving hypotheses on the mechanisms of folding of membrane proteins in general. This article is part of a Special Issue entitled: Retinal Proteins-You can teach an old dog new tricks.

Calcium-dependent ligand binding and G-protein signaling of family B GPCR parathyroid hormone 1 receptor purified in nanodiscs

GPCRs mediate intracellular signaling upon external stimuli, making them ideal drug targets. However, little is known about their activation mechanisms due to the difficulty in purification. Here, we introduce a method to purify GPCRs in nanodiscs, which incorporates GPCRs into lipid bilayers immediately after membrane solubilization, followed by single-step purification. Using this approach, we purified a family B GPCR, parathyroid hormone 1 receptor (PTH1R), which regulates calcium and phosphate homeostasis and is a drug target for osteoporosis. We demonstrated that the purified PTH1R in nanodiscs can bind to PTH(1-34) and activate G protein. We also observed that Ca(2+) is a weak agonist of PTH1R, and Ca(2+) in millimolar concentration can switch PTH(1-34) from an inverse agonist to an agonist. Hence, our results show that nanodiscs are a viable vehicle for GPCR purification, enabling studies of GPCRs under precise experimental conditions without interference from other cellular or membrane components.

A key agonist-induced conformational change in the cannabinoid receptor CB1 is blocked by the allosteric ligand Org 27569

Allosteric ligands that modulate how G protein-coupled receptors respond to traditional orthosteric drugs are an exciting and rapidly expanding field of pharmacology. An allosteric ligand for the cannabinoid receptor CB1, Org 27569, exhibits an intriguing effect; it increases agonist binding, yet blocks agonist-induced CB1 signaling. Here we explored the mechanism behind this behavior, using a site-directed fluorescence labeling approach. Our results show that Org 27569 blocks conformational changes in CB1 that accompany G protein binding and/or activation, and thus inhibit formation of a fully active CB1 structure. The underlying mechanism behind this behavior is that simultaneous binding of Org 27569 produces a unique agonist-bound conformation, one that may resemble an intermediate structure formed on the pathway to full receptor activation.

Ultrasensitive and selective recognition of peptide hormone using close-packed arrays of hPTHR-conjugated polymer nanoparticles

Recognition of diverse hormones in the human body is a highly significant challenge because numerous diseases can be affected by hormonal imbalances. However, the methodologies reported to date for detecting hormones have exhibited limited performance. Therefore, development of innovative methods is still a major concern in hormone-sensing applications. In this study, we report an immobilization-based approach to facilitate formation of close-packed arrays of carboxylated polypyrrole nanoparticles (CPPyNPs) and their integration with human parathyroid hormone receptor (hPTHR), which is a B-class family of G-protein-coupled receptors (GPCRs). Our devices enabled use of an electrically controllable liquid-ion-gated field-effect transistor by using the surrounding phosphate-buffered saline solution (pH 7.4) as electrolyte solution. Field-induced signals from the peptide hormone sensors were observed and provided highly sensitive and selective recognition of target molecules at unprecedentedly low concentrations (ca. 48 fM). This hormone sensor also showed long-term stability and excellent selectivity in fetal bovine serum. Importantly, the hormone receptor attached on the surface of CPPyNPs enabled GPCR functional studies; synergistic effects corresponding to increased hPTH peptide length were monitored. These results demonstrate that close-packed CPPyNP arrays are a promising approach for high-performance biosensing devices.

Biosynthesis and spectroscopic characterization of 2-TM fragments encompassing the sequence of a human GPCR, the Y4 receptor

This paper presents a divide-and-conquer approach towards obtaining solution structures of G protein-coupled receptors. The human Y4 receptor was dissected into two to three transmembrane helix fragments, which were individually studied by solution NMR. We systematically compared various biosynthetic routes for the expression of the fragments in Escherichia coli and discuss purification strategies. In particular, we have compared the production of transmembrane (TM) fragments as inclusion bodies by using the ΔTrp leader sequence, with membrane-directed expression by using Mistic as the fusion partner, and developed methods for enzymatic cleavage. In addition, direct expression of two-TM fragments into inclusion bodies is a successful route in some cases. With the exception of TM13, we could produce all fragments in isotope-labeled form in quantities sufficient for NMR studies. Almost complete backbone resonance assignment was obtained for the first two helices, as well as for helices 5 and 7, and a high degree was obtained for TM6, while conformational exchange processes resulted in the disappearance of many signals from TM4. In addition, complete assignments were obtained for all residues of the N-terminal domain, as well as the extracellular and cytosolic loops (with the exception of an undecapeptide segment in the second extracellular loop, EC2) and for the complete cytosolic C-terminal tail. In total, backbone resonances of 78 % of all residues were assigned for the Y4 receptor. Predictions of secondary structure based on backbone chemical shifts indicate that most residues from the TM regions adopt helical conformations, with exception of those around polar residues or prolines. However, the domain boundaries differ slightly from those predicted for homology models. We suggest that the obtained chemical shifts might be useful in assigning the full-length receptor.

Mistic and TarCF as fusion protein partners for functional expression of the cannabinoid receptor 2 in Escherichia coli

G protein coupled receptors (GPCRs) are key players in signal recognition and cellular communication making them important therapeutic targets. Large-scale production of these membrane proteins in their native form is crucial for understanding their mechanism of action and target-based drug design. Here we report the overexpression system for a GPCR, the cannabinoid receptor subtype 2 (CB2), in Escherichia coli C43(DE3) facilitated by two fusion partners: Mistic, an integral membrane protein expression enhancer at the N-terminal, and TarCF, a C-terminal fragment of the bacterial chemosensory transducer Tar at the C-terminal of the CB2 open reading frame region. Multiple histidine tags were added on both ends of the fusion protein to facilitate purification. Using individual and combined fusion partners, we found that CB2 fusion protein expression was maximized only when both partners were used. Variable growth and induction conditions were conducted to determine and optimize protein expression. More importantly, this fusion protein Mistic-CB2-TarCF can localize into the E. coli membrane and exhibit functional binding activities with known CB2 ligands including CP55,940, WIN55,212-2 and SR144,528. These results indicate that this novel expression and purification system provides us with a promising strategy for the preparation of biologically active GPCRs, as well as general application for the preparation of membrane-bound proteins using the two new fusion partners described.

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.

Evaluation of the Pichia pastoris expression system for the production of GPCRs for structural analysis

Background

Various protein expression systems, such as Escherichia coli (E. coli), Saccharomyces cerevisiae (S. cerevisiae), Pichia pastoris (P. pastoris), insect cells and mammalian cell lines, have been developed for the synthesis of G protein-coupled receptors (GPCRs) for structural studies. Recently, the crystal structures of four recombinant human GPCRs, namely β₂ adrenergic receptor, adenosine A_2a receptor, CXCR4 and dopamine D3 receptor, were successfully determined using an insect cell expression system. GPCRs expressed in insect cells are believed to undergo mammalian-like posttranscriptional modifications and have similar functional properties than in mammals. Crystal structures of GPCRs have not yet been solved using yeast expression systems. In the present study, P. pastoris and insect cell expression systems for the human muscarinic acetylcholine receptor M2 subtype (CHRM2) were developed and the quantity and quality of CHRM2 synthesized by both expression systems were compared for the application in structural studies.

Results

The ideal conditions for the expression of CHRM2 in P. pastoris were 60 hr at 20°C in a buffer of pH 7.0. The specific activity of the expressed CHRM2 was 28.9 pmol/mg of membrane protein as determined by binding assays using [³H]-quinuclidinyl benzilate (QNB). Although the specific activity of the protein produced by P. pastoris was lower than that of Sf9 insect cells, CHRM2 yield in P. pastoris was 2-fold higher than in Sf9 insect cells because P. pastoris was cultured at high cell density. The dissociation constant (Kd) for QNB in P. pastoris was 101.14 ± 15.07 pM, which was similar to that in Sf9 insect cells (86.23 ± 8.57 pM). There were no differences in the binding affinity of CHRM2 for QNB between P. pastoris and Sf9 insect cells.

Conclusion

Compared to insect cells, P. pastoris is easier to handle, can be grown at lower cost, and can be expressed quicker at a large scale. Yeast, P. pastoris, and insect cells are all effective expression systems for GPCRs. The results of the present study strongly suggested that protein expression in P. pastoris can be applied to the structural and biochemical studies of GPCRs.

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.

Recent advances in electronic and bioelectronic noses and their biomedical applications

Significant effort has been made in the development of an artificial nose system for various applications. Advances in sensor technology have facilitated the development of high-performance electronic and bioelectronic noses. Numerous articles describe the advantages of artificial nose systems for biomedical applications. Recent advances in the development of electronic and bioelectronic noses and their biomedical applications are reviewed in this article.