Receptor-Gα fusion proteins as a tool for ligand screening

Interaction of the muscarinic acetylcholine receptor M₂ subtype with G protein Gα(i/o) isotypes and Gβγ subunits as studied with the maltose-binding protein-M₂-Gα(i/o) fusion proteins expressed in Escherichia coli

We expressed the fusion proteins of the muscarinic acetylcholine receptor M2 subtype (M2 receptor) with a maltose-binding protein (MBP) and various G protein α subunits (Gα(i1-i3/o)) at its N- and C-terminals, respectively (MBP-M2-Gα(i/o)), in Escherichia coli, and examined the effect of G protein βγ subunits (Gβγ) on the receptor-Gα interaction as assessed by agonist- and GDP-dependent [(35)S]GTPγS binding of the fusion proteins. We found that (i) Gβγ promoted both the agonist-dependent and -independent [(35)S]GTPγS binding with little effect on the guanine nucleotide-sensitive high-affinity agonist binding, (ii) the specific [(35)S]GTPγS binding activity was much greater for MBP-M2-Gα(oA) than for MBP-M2-Gα(i1-i3) in the absence of Gβγ, whereas Gβγ preferentially promoted the agonist-dependent decrease in the affinity for GDP of MBP-M2-Gα(i1-i3) rather than of MBP-M2-Gα(oA), and (iii) the proportion of agonist-dependent [(35)S]GTPγS binding was roughly 50% irrespective of species of Gα and the presence or absence of Gβγ. These results demonstrate that receptor-Gα fusion proteins expressed in E. coli could be useful for studies of receptor-G interaction.

Cleavage-resistant fusion proteins of the M(2) muscarinic receptor and Gα(i1). Homotropic and heterotropic effects in the binding of ligands

BACKGROUND

G protein-coupled receptors fused to a Gα-subunit are functionally similar to their unfused counterparts. They offer an intriguing view into the nature of the receptor-G protein complex, but their usefulness depends upon the stability of the fusion.

METHODS

Fusion proteins of the M(2) muscarinic receptor and the α-subunit of G(i1) were expressed in CHO and Sf9 cells, extracted in digitonin-cholate, and examined for their binding properties and their electrophoretic mobility on western blots.

RESULTS

Receptor fused to native α(i1) underwent proteolysis near the point of fusion to release a fragment with the mobility of α(i1). The cleavage was prevented by truncation of the α-subunit at position 18. Binding of the agonist oxotremorine-M to the stable fusion protein from Sf9 cells was biphasic, and guanylylimidodiphosphate promoted an apparent interconversion of sites from higher to lower affinity. With receptor from CHO cells, the apparent capacity for N-[(3)H]methylscopolamine was 60% of that for [(3)H]quinuclidinylbenzilate; binding at saturating concentrations of the latter was inhibited in a noncompetitive manner at low concentrations of unlabeled N-methylscopolamine.

CONCLUSIONS

A stable fusion protein of the M(2) receptor and truncated α(i1) resembles the native receptor-G protein complex with respect to the guanyl nucleotide-sensitive binding of agonists and the noncompetitive binding of antagonists.

GENERAL SIGNIFICANCE

Release of the α-subunit is likely to occur with other such fusion proteins, rendering the data ambiguous or misleading. The properties of a chemically stable fusion protein support the notion that signaling proceeds via a stable multimeric complex of receptor and G protein.

Fusion proteins as model systems for the analysis of constitutive GPCR activity

In many cases, the coexpression of GPCRs with G-proteins and/or regulators of G-protein signaling (RGS-proteins) allows a successful reconstitution of high-affinity agonist binding and functional responses. However, in some cases, coexpressed GPCRs and G-proteins interact inefficiently, resulting in weak [³⁵S]GTPγS- and steady-state GTPase assay signals. This may be, for example, caused by a rapid dissociation of the G-protein from the plasma membrane, as has been reported for Gα(s). Moreover, for a detailed characterization of GPCR/G-protein interactions, it may be required to work with a defined GPCR/G-protein stoichiometry and to avoid cross-interaction with endogenous G-proteins. Cross-talk to endogenous G-proteins has been shown to play a role in some mammalian expression systems. These problems can be addressed by the generation of GPCR-Gα fusion proteins and their expression in Sf9 insect cells. When the C-terminus of the receptor is fused to the N-terminus of the G-protein, a 1:1 stoichiometry of both proteins is achieved. In addition, the close proximity of GPCR and G-protein in fusion proteins leads to enhanced interaction efficiency, resulting in increased functional signals. This approach can also be extended to fusion proteins of GPCRs with RGS-proteins, specifically when steady-state GTP hydrolysis is used as read-out. GPCR-RGS fusion proteins optimize the interaction of RGS-proteins with coexpressed Gα subunits, since the location of the RGS-protein is close to the site of receptor-mediated G-protein activation. Moreover, in contrast to coexpression systems, GPCR-Gα and GPCR-RGS fusion proteins provide a possibility to imitate physiologically occurring interactions, for example, the precoupling of receptors and G-proteins or the formation of complexes between GPCRs, G-proteins and RGS-proteins (transducisomes). In this chapter, we describe the technique for the generation of fusion proteins and show the application of this approach for the characterization of constitutively active receptors.

Sf9 cells: a versatile model system to investigate the pharmacological properties of G protein-coupled receptors

The Sf9 cell/baculovirus expression system is widely used for high-level protein expression, often with the purpose of purification. However, proteins may also be functionally expressed in the defined Sf9 cell environment. According to the literature, the pharmacology of G-protein-coupled receptors (GPCRs) functionally reconstituted in Sf9 cells is similar to the receptor properties in mammalian cells. Sf9 cells express both recombinant GPCRs and G-proteins at much higher levels than mammalian cells. Sf9 cells can be grown in suspension culture, providing an inexpensive way of obtaining large protein amounts. Co-infection with various baculoviruses allows free combination of GPCRs with different G-proteins. The absence of constitutively active receptors in Sf9 cells provides an excellent signal-to background ratio in functional assays, allowing the detection of agonist-independent receptor activity and of small ligand-induced signals including partial agonistic and inverse agonistic effects. Insect cell Gα(i)-like proteins mostly do not couple productively to mammalian GPCRs. Thus, unlike in mammalian cells, Sf9 cells do not require pertussis toxin treatment to obtain a Gα(i)-free environment. Co-expression of GPCRs with Gα(i1), Gα(i2), Gα(i3) or Gα(o) in Sf9 cells allows the generation of a selectivity profile for these Gα(i/o)-isoforms. Additionally, GPCR-G-protein combinations can be compared with defined 1:1 stoichiometry by expressing GPCR-Gα fusion proteins. Sf9 cells can also be employed for ligand screening in medicinal chemistry programs, using radioligand binding assays or functional assays, like the steady-state GTPase- or [(35)S]GTPγS binding assay. This review shows that Sf9 cells are a versatile model system to investigate the pharmacological properties of GPCRs.

Ligand screening system using fusion proteins of G protein-coupled receptors with G protein alpha subunits

G protein-coupled receptors (GPCRs) constitute one of the largest families of genes in the human genome, and are the largest targets for drug development. Although a large number of GPCR genes have recently been identified, ligands have not yet been identified for many of them. Various assay systems have been employed to identify ligands for orphan GPCRs, but there is still no simple and general method to screen for ligands of such GPCRs, particularly of G(i)-coupled receptors. We have examined whether fusion proteins of GPCRs with G protein alpha subunit (Galpha) could be utilized for ligand screening and showed that the fusion proteins provide an effective method for the purpose. This article focuses on the followings: (1) characterization of GPCR genes and GPCRs, (2) identification of ligands for orphan GPCRs, (3) characterization of GPCR-Galpha fusion proteins, and (4) identification of ligands for orphan GPCRs using GPCR-Galpha fusion proteins.

Structural Genomics

Drug discovery based on structural knowledge has proven useful as several structure-based medicines are already on the market. Structural genomics aims at studying a large number of gene products including whole genomes, topologically similar proteins, protein families and protein subtypes in parallel. Particularly, therapeutically relevant targets have been selected for structural genomics initiatives. In this context, integral membrane proteins, which represent 60–70% of the current drug targets, have been of major interest. Paradoxically, membrane proteins present the last frontier to conquer in structural biology as some 100 high resolution structures among the 30,000 entries in public structural databases are available. The modest success rate on membrane proteins relates to the difficulties in their expression, purification and crystallography. To facilitate technology development large networks providing expertise in molecular biology, protein biochemistry and structural biology have been established. The privately funded MePNet program has studied 100 G protein-coupled receptors, which resulted in high level expression of a large number of receptors at structural biology compatible levels. Currently, selected GPCRs have been purified and subjected to crystallization attempts

Identification of endogenous surrogate ligands for human P2Y12 receptors by in silico and in vitro methods

Endogenous ligands acting on a human P2Y12 receptor, one of the G-protein coupled receptors, were searched by in silico screening against our own database, which contains more than 500 animal metabolites. The in silico screening using the docking software AutoDock resulted in selection of cysteinylleukotrienes (CysLTs) and 5-phosphoribosyl 1-pyrophosphate (PRPP), with high free energy changes, in addition to the known P2Y12 ligands such as 2MeSADP and ADP. These candidates were subjected to an in vitro Ca2+ assay using the CHO cells stably expressing P2Y12-G16alpha fusion proteins. We found that CysLTE4 and PRPP acted on the P2Y12 receptor as agonists with the EC50 values of 1.3 and 7.8 nM, respectively. Furthermore, we analyzed the phylogenetic relationship of the P2Y, P2Y-like, and CysLT receptors based on sequence alignment followed by evolutionary analyses. The analyses showed that the P2Y12, P2Y13, P2Y14, GPR87, CysLT-1, and CysLT-2 receptors formed a P2Y-related receptor subfamily with common sequence motifs in the transmembrane regions.

Expression of G protein coupled receptors in a cell-free translational system using detergents and thioredoxin-fusion vectors

In Escherichia coli and other cell-based expression systems, there are critical difficulties in synthesizing membrane proteins, such as the low protein expression levels and the formation of insoluble aggregates. However, structure determinations by X-ray crystallography require the purification of milligram quantities of membrane proteins. In this study, we tried to solve these problems by using cell-free protein expression with an E. coli S30 extract, with G protein coupled receptors (GPCRs) as the target integral membrane proteins. In this system, the thioredoxin-fusion vector induced high protein expression levels as compared with the non-fusion and hexa-histidine-tagged proteins. Two detergents, Brij35 and digitonin, effectively solubilized the produced GPCRs, with little or no effect on the protein yields. The synthesized proteins were detected by Coomassie brilliant blue staining within 1h of reaction initiation, and were easily reconstituted within phospholipid vesicles. Surprisingly, the unpurified, reconstituted thioredoxin-fused receptor proteins had functional activity, in that a specific affinity binding value of an antagonist was obtained for the receptor. This cell-free translation system (about 1mg/ml of reaction volume for 6-8 h) has biophysical and biochemical advantages for the synthesis of integral membrane proteins.

A pentylenetetrazole-induced generalized seizure in early life enhances the efficacy of muscarinic receptor coupling to G-protein in hippocampus and neocortex of adult rat

We have previously shown that exposure to the anti-cholinesterase eserine provokes interictal-like discharges in the CA3 area of hippocampal slices from adult rats in which a generalized seizure has been induced by pentylenetetrazole (PTZ) when immature (at 20 days). Such increased responsiveness to acetylcholine (ACh) was not associated with any change in hippocampal acetylcholine or gamma-aminobutyric acid (GABA) content, GABAergic inhibition or density of ACh innervation, but was blocked by the muscarinic receptor antagonist atropine. We therefore turned to quantitative radioligand binding autoradiography, in situ hybridization and the [35S]GTPgammaS method to assess the properties of hippocampal and neocortical muscarinic receptors in adult rats having experienced a PTZ seizure at P20. The densities of M1 and M2 receptor binding sites, respectively labeled with [3H]pirenzepine and [3H]AFDX-384, as well as the amount of m1, m2 and m3 receptor mRNAs, did not differ from control in the hippocampus and neocortex of these rats. In contrast, in PTZ rats, both brain regions displayed a marked increase in [35S]GTPgammaS incorporation stimulated by ACh, bethanechol and particularly oxotremorine. This finding indicates that a generalized seizure in immature rat can entail a long-term and presumably permanent increase in the efficacy of G-protein coupling to muscarinic receptors in the hippocampus and neocortex of the adult. By analogy, such a mechanism could account for the susceptibility to epilepsy of human adults having suffered from prolonged convulsions in early life.

The [35S]GTPgammaS binding assay: approaches and applications in pharmacology

Receptors of the of seven transmembrane spanning, heterotrimeric G protein coupled family (GPCR) play crucial roles in regulating physiological functions and consequently are targets for the action of many classes of drugs. Activation of receptor by agonist leads to the dissociation of GDP from Galpha of the Galphabetagamma heterotrimer, followed by the binding of GTP to Galpha and subsequent modulation of downstream effectors. The G protein heterotrimer is reformed by GTPase activity of the Galpha subunit, forming Galpha-GDP and so allowing Galpha and Gbetagamma to recombine. The [35S]GTPgammaS assay measures the level of G protein activation following agonist occupation of a GPCR, by determining the binding of the non-hydrolyzable analog [35S]GTPgammaS to Galpha subunits. Thus, the assay measures a functional consequence of receptor occupancy at one of the earliest receptor-mediated events. The assay allows for traditional pharmacological parameters of potency, efficacy and antagonist affinity, with the advantage that agonist measures are not subjected to amplification or other modulation that may occur when analyzing parameters further downstream of the receptor. In general the assay is experimentally more feasible for receptors coupled to the abundant G(i/o) proteins. Nevertheless, [35S]GTPgammaS binding assays are used with GPCRs that couple to the G(s) and G(q) families of G proteins, especially in artificial expression systems, or using receptor-Galpha constructs or immunoprecipitation of [35S]GTPgammaS-labeled Galpha. The relative simplicity of the assay has made it very popular and its use is providing insights into contemporary pharmacological topics including the roles of accessory proteins in signaling, constitutive activity of receptors and agonist specific signaling.

Identification of surrogate ligands for orphan G protein-coupled receptors

We prepared fusion proteins with an alpha subunit of G protein Gi (Gi1alpha) of 26 orphan G protein-coupled receptors (GPCRs) and with Gsalpha of 10 orphan GPCRs, most of which had been identified from the human genome previously [FEBS Lett 520 (2002) 97]. Ligands for these fusion proteins were screened from a library consisting of approximately 1000 authentic compounds by measuring their effect on [35S]GTPgammaS binding to membrane preparations of insect Sf9 cells expressing these fusion proteins. Eleven compounds were found to act as surrogate agonists for a GPCR-Gsalpha and four GPCR-Gialpha fusion proteins, a compound as an inverse agonist for two GPCR-Gsalpha fusion proteins, and a compound as an endogenous agonist for a GPCR-Gialpha fusion protein.

G protein-coupled receptor overexpression with the baculovirus-insect cell system: a tool for structural and functional studies

G protein-coupled receptors, whose topology shows seven transmembrane domains, form the largest known family of receptors involved in higher organism signal transduction. These receptors are generally of low natural abundance and overexpression is usually a prerequisite to their structural or functional characterisation. The baculovirus-insect cell system constitutes a versatile tool for the maximal production of receptors. This heterologous expression system also provides interesting alternatives for receptor functional studies in a well-controlled cellular context.

Conformation of ligands bound to the muscarinic acetylcholine receptor

Many biogenic amines evoke a variety of physiological responses by acting on G protein-coupled receptors. We have determined the conformation of two acetylcholine analogs, (S)-methacholine and (2S,4R,5S)-muscarine, bound to the M(2) muscarinic acetylcholine receptor (M(2) mAChR) by NMR spectroscopy. The analysis of the transferred nuclear Overhauser effect indicated that the receptor selectively recognized the conformers of (S)-methacholine and (2S,4R,5S)-muscarine with the gauche O-C2-C1-N dihedral angle at +60 degrees. This is distinct from the predominant conformations of these ligands in solution with O-C2-C1-N dihedral angle (+80 to approximately 85 degrees ) in the absence of the M(2) mAChR, as assessed by analyses of the coupling constants and nuclear Overhauser effect spectroscopy. We have also built a molecular model of the M(2) mAChR-(S)-methacholine complex, based on the X-ray crystallographic structure of rhodopsin. This model indicated that the conformation with the gauche O-C2-C1-N dihedral angle at +55.5 degrees, which is similar to the one determined by NMR measurement, is energetically favored in the binding of (S)-methacholine to the receptor. We suggest that this conformation represents the binding of the agonist to the M(2) mAChR in the absence of G protein.

Enhanced detection of receptor constitutive activity in the presence of regulators of G protein signaling: applications to the detection and analysis of inverse agonists and low-efficacy partial agonists

Fusion proteins between the human 5-hydroxytryptamine (5-HT)(1A) receptor and either wild type or certain pertussis toxin-resistant forms of G(o1)alpha and G(i1)alpha display constitutive GTPase activity that can be inhibited by the inverse agonist spiperone. Addition of recombinant regulator of G protein signaling (RGS) 1 or RGS16 to membranes expressing these fusion proteins resulted in elevation of this constitutive GTPase activity without significantly altering the binding affinity of antagonist/inverse agonist ligands. For a 5-HT(1A) receptor-(Cys(351)Ile)G(o1)alpha fusion protein the increase in basal GTPase activity was greater than 4-fold. Enzyme kinetic analysis demonstrated that the effect of RGS1 was as a GTPase-activating protein for the fusion construct. In the presence of the RGS proteins, both agonists and inverse agonists produced much more robust regulation of high-affinity GTPase activity than in their absence. This allowed detection of the partial agonist nature of WAY100635, which has been described previously as a neutral antagonist at the 5-HT(1A) receptor. Of a range of ligands studied, only haloperidol functioned as a neutral ligand in the presence of RGS1. These studies show that addition of a recombinant RGS protein provides a simple and novel means to elevate the fraction of basal membrane GTPase activity contributed by the constitutive activity of a receptor. By so doing, it also greatly enhances the ability to detect and analyze the effects of inverse agonists and to discriminate between neutral ligands and those with low levels of positive intrinsic efficacy.