Molecular Characterization of a Purified 5-HT4 Receptor

5-hydroxytryptamine 4 receptor activation of the extracellular signal-regulated kinase pathway depends on Src activation but not on G protein or beta-arrestin signaling

The 5-hydroxytryptamine(4) (5-HT(4)) receptors have recently emerged as key modulators of learning, memory, and cognitive processes. In neurons, 5-hydroxytryptamine(4) receptors (5-HT(4)Rs) activate cAMP production and protein kinase A (PKA); however, nothing is known about their ability to activate another key signaling pathway involved in learning and memory: the extracellular signal-regulated kinase (ERK) pathway. Here, we show that 5-HT(4)R stimulation, in primary neurons, produced a potent but transient activation of the ERK pathway. Surprisingly, this activation was mostly PKA independent. Similarly, using pharmacological, genetic, and molecular tools, we observed that 5-HT(4)Rs in human embryonic kidney 293 cells, activated the ERK pathway in a G(s)/cAMP/PKA-independent manner. We also demonstrated that other classical G proteins (G(q)/G(i)/G(o)) and associated downstream messengers were not implicated in the 5-HT(4)R-activated ERK pathway. The 5-HT(4)R-mediated ERK activation seemed to be dependent on Src tyrosine kinase and yet totally independent of beta-arrestin. Immunocytofluorescence revealed that ERK activation by 5-HT(4)R was restrained to the plasma membrane, whereas p-Src colocalized with the receptor and carried on even after endocytosis. This phenomenon may result from a tight interaction between 5-HT(4)R and p-Src detected by coimmunoprecipitation. Finally, we confirmed that the main route by which 5-HT(4)Rs activate ERKs in neurons was Src dependent. Thus, in addition to classical cAMP/PKA signaling pathways, 5-HT(4)Rs may use ERK pathways to control memory process.

Two transmembrane Cys residues are involved in 5-HT4 receptor dimerization

The 5-HT(4) receptor (5-HT(4)R) belongs to the G-protein-coupled receptor (GPCR) family and is of considerable interest for the development of new drugs to treat gastrointestinal diseases and memory disorders. The 5-HT(4)R exists as a constitutive dimer but its molecular determinants are still unknown. Using co-immunoprecipitation and Bioluminescence Resonance Energy Transfer (BRET) techniques, we show here that 5-HT(4)R homodimerization but not 5-HT(4)R-beta(2) adrenergic receptor (beta(2)AR) heterodimerization is largely decreased under reducing conditions suggesting the participation of disulfide bonds in 5-HT(4)R dimerization. Molecular modeling and protein docking experiments identified four cysteine (Cys) residues potentially involved in the dimer interface through intramolecular or intermolecular disulfide bonds. We show that disulfide bridges between Cys112 and Cys145 located within TM3 and TM4, respectively, are of critical importance for 5-HT(4)R dimer formation. Our data suggest that two disulfide bridges between two transmembrane Cys residues are involved in the dimerization interface of a GPCR.

Characterization of monoclonal antibodies directed against the rat neurotensin receptor NTS1

G protein-coupled receptors (GPCRs) are integral membrane proteins that mediate cellular responses to a variety of ligands and represent major drug targets. Despite their medical importance, detailed structural information is limited because only one GPCR has been crystallized and its structure determined. To develop tools to aid in the formation of well-ordered crystals, we generated monoclonal antibodies with high affinity to the rat neurotensin receptor. All antibodies bound to the C-terminus of the receptor, which may reflect the selection strategy used to identify high-affinity binders. Further characterization revealed that some antibodies bound to the receptor in a sodium chloride sensitive manner, but others did not. Epitope mapping revealed distinct antigenic regions within the receptor C-terminus. Tight binding of Fab fragments to the receptor was verified by size exclusion chromatography.

Amino-pyrrolidine tricarboxylic acids give new insight into group III metabotropic glutamate receptor activation mechanism

Like most class C G-protein-coupled receptors, metabotropic glutamate (mGlu) receptors possess a large extracellular domain where orthosteric ligands bind. Crystal structures revealed that this domain, called Venus FlyTrap (VFT), adopts a closed or open conformation upon agonist or antagonist binding, respectively. We have described amino-pyrrolidine tricarboxylic acids (APTCs), including (2S,4S)-4-amino-1-[(E)-3-carboxyacryloyl]pyrrolidine-2,4-dicarboxylic acid (FP0429), as new selective group III mGlu agonists. Whereas FP0429 is an almost full mGlu4 agonist, it is a weak and partial agonist of the closely related mGlu8 subtype. To get more insight into the activation mechanism of mGlu receptors, we aimed to elucidate why FP0429 behaves differently at these two highly homologous receptors by focusing on two residues within the binding site that differ between mGlu4 and mGlu8. Site-directed mutagenesis of Ser157 and Gly158 of mGlu4 into their mGlu8 homologs (Ala) turned FP0429 into a weak partial agonist. Conversely, introduction of Ser and Gly residues into mGlu8 increased FP0429 efficacy. Docking of FP0429 in mGlu4 VFT 3D model helped us characterize the role of each residue. Indeed, mGlu4 Ser157 seems to have an important role in FP0429 binding, whereas Gly158 may allow a deeper positioning of this agonist in the cavity of lobe I, thereby ensuring optimal interactions with lobe II residues in the fully closed state of the VFT. In contrast, the presence of a methyl group in mGlu8 (Ala instead of Gly) weakens the interactions with the lobe II residues. This probably results in a less stable or a partially closed form of the mGlu8 VFT, leading to partial receptor activation.

The first extracellular loop of the Saccharomyces cerevisiae G protein-coupled receptor Ste2p undergoes a conformational change upon ligand binding

In this study of the Saccharomyces cerevisiae G protein-coupled receptor Ste2p, we present data indicating that the first extracellular loop (EL1) of the alpha-factor receptor has tertiary structure that limits solvent accessibility and that its conformation changes in a ligand-dependent manner. The substituted cysteine accessibility method was used to probe the solvent exposure of single cysteine residues engineered to replace residues Tyr(101) through Gln(135) of EL1 in the presence and absence of the tridecapeptide alpha-factor and a receptor antagonist. Surprisingly, many residues, especially those at the N-terminal region, were not solvent-accessible, including residues of the binding-competent yet signal transduction-deficient mutants L102C, N105C, S108C, Y111C, and T114C. In striking contrast, two N-terminal residues, Y101C and Y106C, were readily solvent-accessible, but upon incubation with alpha-factor labeling was reduced, suggesting a pheromone-dependent conformational change limiting solvent accessibility had occurred. Labeling in the presence of the antagonist, which binds Ste2p but does not initiate signal transduction, did not significantly alter reactivity with the Y101C and Y106C receptors, suggesting that the alpha-factor-dependent decrease in solvent accessibility was not because of steric hindrance that prevented the labeling reagent access to these residues. Based on these and previous observations, we propose a model in which the N terminus of EL1 is structured such that parts of the loop are buried in a solvent-inaccessible environment interacting with the extracellular part of the transmembrane domain bundle. This study highlights the essential role of an extracellular loop in activation of a G protein-coupled receptor upon ligand binding.

Multiple residues in the second extracellular loop are critical for M3 muscarinic acetylcholine receptor activation

Recent studies suggest that the second extracellular loop (o2 loop) of bovine rhodopsin and other class I G protein-coupled receptors (GPCRs) targeted by biogenic amine ligands folds deeply into the transmembrane receptor core where the binding of cis-retinal and biogenic amine ligands is known to occur. In the past, the potential role of the o2 loop in agonist-dependent activation of biogenic amine GPCRs has not been studied systematically. To address this issue, we used the M(3) muscarinic acetylcholine receptor (M3R), a prototypic class I GPCR, as a model system. Specifically, we subjected the o2 loop of the M3R to random mutagenesis and subsequently applied a novel yeast genetic screen to identity single amino acid substitutions that interfered with M3R function. This screen led to the recovery of about 20 mutant M3Rs containing single amino acid changes in the o2 loop that were inactive in yeast. In contrast, application of the same strategy to the extracellular N-terminal domain of the M3R did not yield any single point mutations that disrupted M3R function. Pharmacological characterization of many of the recovered mutant M3Rs in mammalian cells, complemented by site-directed mutagenesis studies, indicated that the presence of several o2 loop residues is important for efficient agonist-induced M3R activation. Besides the highly conserved Cys(220) residue, Gln(207), Gly(211), Arg(213), Gly(218), Ile(222), Phe(224), Leu(225), and Pro(228) were found to be of particular functional importance. In general, mutational modification of these residues had little effect on agonist binding affinities. Our findings are therefore consistent with a model in which multiple o2 loop residues are involved in stabilizing the active state of the M3R. Given the high degree of structural homology found among all biogenic amine GPCRs, our findings should be of considerable general relevance.

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

Asymmetric conformational changes in a GPCR dimer controlled by G-proteins

G-protein-coupled receptors (GPCRs) are key players in cell communication. Although long considered as monomeric, it now appears that these heptahelical proteins can form homo- or heterodimers. Here, we analyzed the conformational changes in each subunit of a receptor dimer resulting from agonist binding to either one or both subunits by measuring the fluorescent properties of a leukotriene B(4) receptor dimer with a single 5-hydroxytryptophan-labeled protomer. We show that a receptor dimer with only a single agonist-occupied subunit can trigger G-protein activation. We also show that the two subunits of the receptor dimer in the G-protein-coupled state differ in their conformation, even when both are liganded by the agonist. No such asymmetric conformational changes are observed in the absence of G-protein, indicating that the interaction of the G-protein with the receptor dimer brings specific constraints that prevent a symmetric functioning of this dimer. These data open new options for the differential signaling properties of GPCR dimers.

Membrane proteins shape up: understanding in vitro folding

Progress in structural biology has begun to reveal the precise architecture of integral membrane proteins. However, the manner in which these complex structures are achieved remains unclear. Recent developments are starting to shed light on the unfolding and folding of a small but growing number of membrane proteins. Mechanistic details derived from kinetic and thermodynamic experiments now enable comparison of the folding of different membrane proteins and their water-soluble cousins. This work also has important implications for other structural and functional studies of membrane proteins in vitro.

Expression and functional purification of a glycosylation deficient version of the human adenosine 2a receptor for structural studies

A glycosylation deficient (dG) version of the human adenosine 2a receptor (hA2aR) was made in Pichia pastoris strain SMD1163. Under optimal conditions, expression levels of between 8 and 12pmol receptor/mg membrane protein were obtained routinely. In a shake flask, this is equivalent to ca. 0.2mg of receptor per litre of culture. The level of functional receptor produced was essentially independent of the pH of the yeast media. In contrast to this, addition of the hA2aR antagonist theophylline to the culture media caused a twofold increase in receptor expression. A similar effect on dG hA2aR production was also observed when the induction temperature was reduced from 29 to 22 degrees C. In P. pastoris membranes, dG hA2aR had native-like pharmacological properties, binding antagonists with rank potency ZM241385>XAC>theophylline, as well as the agonist NECA. Furthermore, the receptor was made with its large (ca. 120 amino acid) C-terminal domain intact. dG hA2aR was purified to homogeneity in three steps, and its identity confirmed by electrospray tandem mass spectrometry following digestion with trypsin. The secondary structure of the entire receptor is largely (ca. 81%) alpha-helical. Purified dG hA2aR bound [(3)H]ZM241385 in a saturable manner with a B(max) of 18.1+/-0.5 nmol/mg protein, close to the theoretical B(max) value for pure protein (21.3 nmol/mg protein), showing that the receptor had retained its functionality during the purification process. Regular production of pure dG hA2aR in milligram quantities has enabled crystallisation trials to be started.

Human P2Y(6) receptor: molecular modeling leads to the rational design of a novel agonist based on a unique conformational preference

Combining molecular dynamics (MD) in a hydrated phospholipid (DOPC) bilayer, a Monte Carlo search, and synthesis of locked nucleotide analogues, we discovered that the Southern conformation of the ribose is preferred for ligand recognition by the P2Y(6) receptor. 2'-Deoxy-(S)-methanocarbaUDP was found to be a full agonist of the receptor and displayed a 10-fold higher potency than that for the corresponding flexible 2'-deoxyUDP. MD results also suggested a conformational change of the second extracellular loop consequent to agonist binding.

A transmembrane helix-bundle from G-protein coupled receptor CB2: Biosynthesis, purification, and NMR characterization

The cannabinoid receptor subtype 2 (CB2) is a member of the G-protein coupled receptor (GPCR) superfamily. As the relationship between structure and function for this receptor remains poorly understood, the present study was undertaken to characterize the structure of a segment including the first and second transmembrane helix (TM1 and TM2) domains of CB2. To accomplish this, a transmembrane double-helix bundle from this region was expressed, purified, and characterized by NMR. Milligrams of this hydrophobic fragment of the receptor were biosynthesized using a fusion protein overexpression strategy and purified by affinity chromatography combined with reverse phase HPLC. Chemical and enzymatic cleavage methods were implemented to remove the fusion tag. The resultant recombinant protein samples were analyzed and confirmed by HPLC, mass spectrometry, and circular dichroism (CD). The CD analyses of HPLC-purified protein in solution and in DPC micelle preparations suggested predominant alpha-helical structures under both conditions. The 13C/15N double-labeled protein CB2(27-101) was further verified and analyzed by NMR spectroscopy. Sequential assignment was accomplished for more than 80% of residues. The 15N HSQC NMR results show a clear chemical shift dispersion of the amide nitrogen-proton correlation indicative of a pure double-labeled polypeptide molecule. The results suggest that this method is capable of generating transmembrane helical bundles from GPCRs in quantity and purity sufficient for NMR and other biophysical studies. Therefore, the biosynthesis of GPCR transmembrane helix bundles represents a satisfactory alternative strategy to obtain and assemble NMR structures from recombinant "building blocks."

Overlapping, Nonidentical Binding Sites of Different Classes of Nonpeptide Antagonists for the Human Gonadotropin-Releasing Hormone Receptor

Peptide agonists and antagonists of the human gonadotropin-releasing hormone receptor (GnRH-R) are widely used to treat a range of reproductive hormone related diseases. Recently, nonpeptide, orally available GnRH-R antagonists have emerged from several chemical classes. To understand how a relatively large peptide-binding pocket can recognize numerous nonpeptide ligands, we undertook a systematic mapping of GnRH-R residues involved in the binding of three nonpeptide antagonists. A region composed of the extracellular portions of transmembrane helices 6 and 7, extracellular loop 3, and the N-terminal domain significantly contributed to nonpeptide antagonist binding. However, each molecule was affected by a different subset of residues in these regions, indicating that each appears to occupy distinct, partially overlapping subregions within the more extensive peptide-binding pocket. Moreover, the resulting receptor interaction maps provide a basis to begin to reconcile structure-activity relationships between various nonpeptide and peptide series and facilitate the design of improved therapeutic agents.