Bacterially expressed human serotonin receptor 3A is functionally reconstituted in proteoliposomes

A nondestructive membrane engineering method using an amphiphilic polymer

The cellular signaling process or ion transport is mediated by membrane proteins (MPs) located on the cell surface, and functional studies of MPs have mainly been conducted using cells endogenously or transiently expressing target proteins. Reconstitution of purified MPs in the surface of live cells would have advantages of short manipulation time and ability to target cells in which gene transfection is difficult. However, direct reconstitution of MPs in live cells has not been established. The traditional detergent-mediated reconstitution method of MPs into a lipid bilayer cannot be applied to live cells because this disrupts and reforms the lipid bilayer structure, which is detrimental to cell viability. In this study, we demonstrated that GPCRs (prostaglandin E2 receptor 4 [EP4] and glucagon-like peptide-1 receptor [GLP1R]) or serotonin receptor 3A (5HT3A), a ligand-gated ion channel, stabilized with amphiphilic poly-γ-glutamate (APG), can be reconstituted into mammalian cell plasma membranes without affecting cell viability. Furthermore, 5HT3A reconstituted in mammalian cells showed ligand-dependent Ca2+ ion transport activity. APG-mediated reconstitution of GPCR in synthetic liposomes showed that electrostatic interaction between APG and membrane surface charge contributed to the reconstitution process. This APG-mediated membrane engineering method could be applied to the functional modification of cell membranes with MPs in live cells.

Functional expression of human prostaglandin E2 receptor 4 (EP4) in E. coli and characterization of the binding property of EP4 with Gα proteins

Human prostaglandin E2 receptor 4 (EP4) is one of the four subtypes of prostaglandin E₂ (PGE₂) receptors and belongs to the rhodopsin-type G protein-coupled receptor (GPCR) family. Particularly, EP4 is expressed in various cancer cells and is involved in cancer-cell proliferation by a G protein signaling cascade. To prepare an active form of EP4 for biochemical characterization and pharmaceutical application, this study designed a recombinant protein comprising human EP4 fused to the P9 protein (a major envelope protein of phi6 phage) and overexpressed the P9-EP4 fusion protein in the membrane fraction of E. coli. The solubilized P9-EP4 with sarkosyl (a strong anionic detergent) was purified by affinity chromatography. The purified protein was stabilized with amphiphilic polymers derived from poly-γ-glutamate. The polymer-stabilized P9-EP4 showed specific interaction with the alpha subunits of G_s or G_i proteins, and a high content of α-helical structure by a circular dichroism spectroscopy. Furthermore, the polymer-stabilized P9-EP4 showed strong heat resistance compared with P9-EP4 in detergents. The functional preparation of EP4 and its stabilization with amphiphilic polymers could facilitate both the biochemical characterization and pharmacological applications targeting EP4.

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Prostaglandin E2 receptor 4 (EP4) was overexpressed as P9-fusion protein in E. coli.

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The APG-stabilized P9-EP4 showed specific interaction with the alpha subunits and its ligands.

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The APG-stabilized P9-EP4 showed strong heat resistance compared with P9-EP4 in detergents.

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The binding kinetics of P9-EP4 with both antagonists and agonists were analyzed.

Half a Century of Research on Membrane-Containing Bacteriophages: Bringing New Concepts to Modern Virology

Half a century of research on membrane-containing phages has had a major impact on virology, providing new insights into virus diversity, evolution and ecological importance. The recent revolutionary technical advances in imaging, sequencing and lipid analysis have significantly boosted the depth and volume of knowledge on these viruses. This has resulted in new concepts of virus assembly, understanding of virion stability and dynamics, and the description of novel processes for viral genome packaging and membrane-driven genome delivery to the host. The detailed analyses of such processes have given novel insights into DNA transport across the protein-rich lipid bilayer and the transformation of spherical membrane structures into tubular nanotubes, resulting in the description of unexpectedly dynamic functions of the membrane structures. Membrane-containing phages have provided a framework for understanding virus evolution. The original observation on membrane-containing bacteriophage PRD1 and human pathogenic adenovirus has been fundamental in delineating the concept of "viral lineages", postulating that the fold of the major capsid protein can be used as an evolutionary fingerprint to trace long-distance evolutionary relationships that are unrecognizable from the primary sequences. This has brought the early evolutionary paths of certain eukaryotic, bacterial, and archaeal viruses together, and potentially enables the reorganization of the nearly immeasurable virus population (~1 × 1031) on Earth into a reasonably low number of groups representing different architectural principles. In addition, the research on membrane-containing phages can support the development of novel tools and strategies for human therapy and crop protection.

Determination of the endothelin-1 recognition sites of endothelin receptor type A by the directed-degeneration method

G-protein coupled receptors (GPCRs) play indispensable physiological roles in cell proliferation, differentiation, and migration; therefore, identifying the mechanisms by which ligands bind to GPCRs is crucial for developing GPCR-targeting pharmaceutics and for understanding critical biological functions. Although some structural information is available regarding the interactions between GPCRs and their small molecule ligands, knowledge of how GPCRs interact with their corresponding macromolecule ligands, such as peptides and proteins, remains elusive. In this study, we have developed a novel strategy to investigate the precise ligand recognition mechanisms involved in the interaction of endothelin receptor type A (ET_A) with its ligand, endothelin-1 (ET-1); we call this method “directed degeneration” method. Through flow cytometric screening of a randomized ET_A library, statistical analysis of the identified sequences, and biochemical studies, the ligand interaction map was successfully obtained.

Preparation of functional human lysophosphatidic acid receptor 2 using a P9∗ expression system and an amphipathic polymer and investigation of its in vitro binding preference to Gα proteins

Human lysophosphatidic acid receptor 2 (LPA₂), a member of the G-protein coupled receptor family, mediates lysophosphatidic acid (LPA)-dependent signaling by recruiting various G proteins. Particularly, it is directly implicated in the progression of colorectal and ovarian cancer through G protein signaling cascades. To investigate the biochemical binding properties of LPA₂ against various alpha subunits of G protein (G_α), a functional recombinant LPA₂ was overexpressed in E. coli membrane with a P9^∗ expression system, and the purified protein was stabilized with an amphipathic polymer that had been synthesized by coupling octylamine, glucosamine, and diethyl aminoproylamine at the carboxylic groups of poly-γ-glutamic acid. The purified LPA₂ stabilized with the amphipathic polymer showed selective binding activity to the various G_α proteins as well as agonist-dependent dissociation from G_αi3. Understanding the binding properties of LPA₂ against various G_α proteins advances the understanding of downstream signaling cascades of LPA₂. The functional LPA₂ prepared using a P9^∗ expression system and an amphipathic polymer could also facilitate the development of LPA₂-targeting drugs.

Expression, Biochemistry, and Stabilization with Camel Antibodies of Membrane Proteins: Case Study of the Mouse 5-HT3 Receptor

There is growing interest in the use of mammalian protein expression systems, and in the use of antibody-derived chaperones, for structural studies. Here, we describe protocols ranging from the production of recombinant membrane proteins in stable inducible cell lines to biophysical characterization of purified membrane proteins in complex with llama antibody domains. These protocols were used to solve the structure of the mouse 5-HT3 serotonin receptor but are of broad applicability for crystallization or cryo-electron microscopy projects.

High yield and efficient expression and purification of the human 5-HT3A receptor

AIM

To establish a method for efficient expression and purification of the human serotonin type 3A receptor (5-HT3A) that is suitable for structural studies.

METHODS

Codon-optimized cDNA of human 5-HT3A was inserted into a modified BacMam vector, which contained an IgG leader sequence, an 8×His tag linked with two-Maltose Binding Proteins (MBP), and a TEV protease cleavage site. The BacMam construct was used to generate baculoviruses for expression of 5-HT3A in HEK293F cells. The proteins were solubilized from the membrane with the detergent C12E 9, and purified using MBP affinity chromatography. The affinity tag was removed by TEV protease treatment and immobilized metal ion affinity chromatography. The receptors were further purified by size-exclusion chromatography (SEC). Western blot and SDS-PAGE were used to detect 5-HT3A during purification. The purified receptor was used in crystallization and analyzed with negative stain electron microscopy (EM).

RESULTS

The BacMam system yielded 0.5 milligram of the human 5-HT3A receptor per liter of cells. MBP affinity purification resulted in good yields with high purity and homogeneity. SEC profiles indicated that the purified receptors were pentameric. No protein crystals were obtained; however, a reconstructed 3D density map generated from the negative stain EM data fitted well with the mouse 5-HT3A structure.

CONCLUSION

With the BacMam system, robust expression of the human 5-HT3A receptor is obtained, which is monodisperse, therefore enabling 3D reconstruction of an EM map. This method is suitable for high-throughput screening of different constructs, thus facilitating structural and biochemical studies of the 5-HT3A receptor.

Bacteriophage membrane protein P9 as a fusion partner for the efficient expression of membrane proteins in Escherichia coli

Despite their important roles and economic values, studies of membrane proteins have been hampered by the difficulties associated with obtaining sufficient amounts of protein. Here, we report a novel membrane protein expression system that uses the major envelope protein (P9) of phage φ6 as an N-terminal fusion partner. Phage membrane protein P9 facilitated the synthesis of target proteins and their integration into the Escherichia coli cell membrane. This system was used to produce various multi-pass transmembrane proteins, including G-protein-coupled receptors, transporters, and ion channels of human origin. Green fluorescent protein fusion was used to confirm the correct folding of the expressed proteins. Of the 14 membrane proteins tested, eight were highly expressed, three were moderately expressed, and three were barely expressed in E. coli. Seven of the eight highly expressed proteins could be purified after extraction with the mild detergent lauryldimethylamine-oxide. Although a few proteins have previously been developed as fusion partners to augment membrane protein production, we believe that the major envelope protein P9 described here is better suited to the efficient expression of eukaryotic transmembrane proteins in E. coli.

Characterizing new fluorescent tools for studying 5-HT₃ receptor pharmacology

The pharmacological characterization of ligands depends upon the ability to accurately measure their binding properties. Fluorescence provides an alternative to more traditional approaches such as radioligand binding. Here we describe the binding and spectroscopic properties of eight fluorescent 5-HT3 receptor ligands. These were tested on purified receptors, expressed receptors on live cells, or in vivo. All compounds had nanomolar affinities with fluorescent properties extending from blue to near infra-red emission. A fluorescein-derivative had the highest affinity as measured by fluorescence polarization (FP; 1.14 nM), flow cytometry (FC; 3.23 nM) and radioligand binding (RB; 1.90 nM). Competition binding with unlabeled 5-HT3 receptor agonists (5-HT, mCPBG, quipazine) and antagonists (granisetron, palonosetron, tropisetron) yielded similar affinities in all three assays. When cysteine substitutions were introduced into the 5-HT3 receptor binding site the same changes in binding affinity were seen for both granisetron and the fluorescein-derivative, suggesting that they both adopt orientations that are consistent with co-crystal structures of granisetron with a homologous protein (5HTBP). As expected, in vivo live imaging in anaesthetized mice revealed staining in the abdominal cavity in intestines, but also in salivary glands. The unexpected presence of 5-HT3 receptors in mouse salivary glands was confirmed by Western blots. Overall, these results demonstrate the wide utility of our new high-affinity fluorescently-labeled 5-HT3 receptor probes, ranging from in vitro receptor pharmacology, including FC and FP ligand competition, to live imaging of 5-HT3 expressing tissues.

The oligomeric states of the purified sigma-1 receptor are stabilized by ligands

Sigma-1 receptor (S1R) is a mammalian member of the ERG2 and sigma-1 receptor-like protein family (pfam04622). It has been implicated in drug addiction and many human neurological disorders, including Alzheimer and Parkinson diseases and amyotrophic lateral sclerosis. A broad range of synthetic small molecules, including cocaine, (+)-pentazocine, haloperidol, and small endogenous molecules such as N,N-dimethyltryptamine, sphingosine, and steroids, have been identified as regulators of S1R. However, the mechanism of activation of S1R remains obscure. Here, we provide evidence in vitro that S1R has ligand binding activity only in an oligomeric state. The oligomeric state is prone to decay into an apparent monomeric form when exposed to elevated temperature, with loss of ligand binding activity. This decay is suppressed in the presence of the known S1R ligands such as haloperidol, BD-1047, and sphingosine. S1R has a GXXXG motif in its second transmembrane region, and these motifs are often involved in oligomerization of membrane proteins. Disrupting mutations within the GXXXG motif shifted the fraction of the higher oligomeric states toward smaller states and resulted in a significant decrease in specific (+)-[(3)H]pentazocine binding. Results presented here support the proposal that S1R function may be regulated by its oligomeric state. Possible mechanisms of molecular regulation of interacting protein partners by S1R in the presence of small molecule ligands are discussed.

Atomistic insights into human Cys-loop receptors by solution NMR

Cys-loop receptors are pentameric ligand-gated ion channels (pLGICs) mediating fast neurotransmission in the central and peripheral nervous systems. They are important targets for many currently used clinical drugs, such as general anesthetics, and for allosteric modulators with potential therapeutic applications. Here, we provide an overview of advances in the use of solution NMR in structural and dynamic characterization of ion channels, particularly human Cys-loop receptors. We present challenges to overcome and realistic solutions for achieving high-resolution structural information for this family of receptors. We discuss how subtle structural differences among homologous channels define unique channel pharmacological properties and advocate the necessity to determine high-resolution structures for individual receptor subtypes. Finally, we describe drug binding to the TMDs of Cys-loop receptors identified by solution NMR and the associated dynamics changes relevant to channel functions.