Location of a region of the muscarinic acetylcholine receptor involved in selective effector coupling

Signal transduction through Cys-loop receptors is mediated by the nonspecific bumping of closely apposed domains

One of the most fundamental questions in the field of Cys-loop receptors (pentameric ligand-gated ion channels, pLGICs) is how the affinity for neurotransmitters and the conductive/nonconductive state of the transmembrane pore are correlated despite the ∼60-Å distance between the corresponding domains. Proposed mechanisms differ, but they all converge into the idea that interactions between wild-type side chains across the extracellular-transmembrane-domain (ECD-TMD) interface are crucial for this phenomenon. Indeed, the successful design of fully functional chimeras that combine intact ECD and TMD modules from different wild-type pLGICs has commonly been ascribed to the residual conservation of sequence that exists at the level of the interfacial loops even between evolutionarily distant parent channels. Here, using mutagenesis, patch-clamp electrophysiology, and radiolabeled-ligand binding experiments, we studied the effect of eliminating this residual conservation of sequence on ion-channel function and cell-surface expression. From our results, we conclude that proper state interconversion ("gating") does not require conservation of sequence-or even physicochemical properties-across the ECD-TMD interface. Wild-type ECD and TMD side chains undoubtedly interact with their surroundings, but the interactions between them-straddling the interface-do not seem to be more important for gating than those occurring elsewhere in the protein. We propose that gating of pLGICs requires, instead, that the overall structure of the interfacial loops be conserved, and that their relative orientation and distance be the appropriate ones for changes in one side to result in changes in the other, in a phenomenon akin to the nonspecific "bumping" of closely apposed domains.

G protein-coupled receptor-G protein interactions: a single-molecule perspective

G protein-coupled receptors (GPCRs) regulate many cellular and physiological processes, responding to a diverse range of extracellular stimuli including hormones, neurotransmitters, odorants, and light. Decades of biochemical and pharmacological studies have provided fundamental insights into the mechanisms of GPCR signaling. Thanks to recent advances in structural biology, we now possess an atomistic understanding of receptor activation and G protein coupling. However, how GPCRs and G proteins interact in living cells to confer signaling efficiency and specificity remains insufficiently understood. The development of advanced optical methods, including single-molecule microscopy, has provided the means to study receptors and G proteins in living cells with unprecedented spatio-temporal resolution. The results of these studies reveal an unexpected level of complexity, whereby GPCRs undergo transient interactions among themselves as well as with G proteins and structural elements of the plasma membrane to form short-lived signaling nanodomains that likely confer both rapidity and specificity to GPCR signaling. These findings may provide new strategies to pharmaceutically modulate GPCR function, which might eventually pave the way to innovative drugs for common diseases such as diabetes or heart failure.

Structures of the M1 and M2 muscarinic acetylcholine receptor/G-protein complexes

Muscarinic acetylcholine receptors are G protein-coupled receptors that respond to acetylcholine and play important signaling roles in the nervous system. There are five muscarinic receptor subtypes (M1R to M5R), which, despite sharing a high degree of sequence identity in the transmembrane region, couple to different heterotrimeric GTP-binding proteins (G proteins) to transmit signals. M1R, M3R, and M5R couple to the G_{q/ 11} family, whereas M2R and M4R couple to the G_{i/ o} family. Here, we present and compare the cryo-electron microscopy structures of M1R in complex with G₁₁ and M2R in complex with G_oA The M1R-G₁₁ complex exhibits distinct features, including an extended transmembrane helix 5 and carboxyl-terminal receptor tail that interacts with G protein. Detailed analysis of these structures provides a framework for understanding the molecular determinants of G-protein coupling selectivity.

Gi/o-coupled muscarinic receptors co-localize with GIRK channel for efficient channel activation

G protein-gated inwardly rectifying K+ (GIRK) channel regulates cellular excitability upon activation of Gi/o-coupled receptors. In Gi/o-coupled muscarinic M2R, the intracellular third loop (i3) is known as a key domain for Gi/o coupling, because replacement of i3 of Gq-coupled muscarinic M1R with that of M2R enables the chimeric receptor (MC9) to activate the GIRK channel. In the present study, we showed that MC9, but not M1R, co-localizes with the GIRK channel and Gαi1 by Förster resonance energy transfer (FRET) analysis. When M1R was forced to stay adjacent to the channel through ligation with short linkers, M1R activated the GIRK channel. FRET analysis further suggested that the efficacy of channel activation is correlated with the linker length between M1R and the GIRK channel. The results show that co-localization is an important factor for activating the GIRK channel. In contrast, for MC9 and M2R, the GIRK channel was activated even when they were connected by long linkers, suggesting the formation of a molecular complex even in the absence of a linker. We also observed that replacement of 13 amino acid residues at the N-terminal end of i3 of MC9 with those of M1R impaired the co-localization with the GIRK channel as well as channel activation. These results show that localization of the receptor near the GIRK channel is a key factor in efficiently activating the channel and that the N-terminal end of i3 of M2R plays an important role in co-localization.

Control of neurotransmitter release: From Ca2+ to voltage dependent G-protein coupled receptors

This review discusses two theories that try to explain mechanisms of control of neurotransmitter release in fast synapses: the Ca(2+) hypothesis and the Ca(2+) voltage hypothesis. The review summarizes experimental results that are incompatible with predictions from the Ca(2+) hypothesis and concludes that Ca(2+) is involved in the control of the amount of release but not in the control of the time course of evoked release, i.e., initiation and termination of evoked release. Results summarizing direct effects of changes in membrane potential on the release machinery are then presented. These changes in membrane potential affect the affinity (for the transmitter) of presynaptic autoinhibitory G-protein coupled receptors (GPCRs). The voltage dependence of these GPCRs and their pivotal role in determining the time course of evoked release is discussed.

Behavioral impact of neurotransmitter-activated G-protein-coupled receptors: muscarinic and GABAB receptors regulate Caenorhabditis elegans locomotion

Neurotransmitter released from presynaptic terminals activates both ligand-gated ion channels (ionotropic receptors) and a variety of G-protein-coupled receptors (GPCRs). These neurotransmitter receptors are expressed on both presynaptic and postsynaptic cells. Thus, each neurotransmitter acts on multiple receptor classes, generating a large repertoire of physiological responses. The impact of many ionotropic receptors on neuronal activity and behavior has been clearly elucidated; however, much less is known about how neurotransmitter-gated GPCRs regulate neurons and circuits. In Caenorhabditis elegans, both acetylcholine (ACh) and GABA are released in the nerve cord and mediate fast neuromuscular excitation and inhibition during locomotion. Here we identify a muscarinic receptor (GAR-2) and the GABA(B) receptor dimer (GBB-1/2) that detect synaptically released ACh and GABA, respectively. Both GAR-2 and GBB-1/2 inhibited cholinergic motor neurons when ACh and GABA levels were enhanced. Loss of either GPCR resulted in movement defects, suggesting that these receptors are activated during locomotion. When the negative feedback provided by GAR-2 was replaced with positive feedback, animals became highly sensitive to ACh levels and locomotion was severely impaired. Thus, conserved GPCRs act in the nematode motor circuit to provide negative feedback and to regulate locomotory behaviors that underlie navigation.

Movement of 'gating charge' is coupled to ligand binding in a G-protein-coupled receptor

Activation by agonist binding of G-protein-coupled receptors (GPCRs) controls most signal transduction processes. Although these receptors span the cell membrane, they are not considered to be voltage sensitive. Recently it was shown that both the activity of GPCRs and their affinity towards agonists are regulated by membrane potential. However, it remains unclear whether GPCRs intrinsically respond to changes in membrane potential. Here we show that two prototypical GPCRs, the m2 and m1 muscarinic receptors (m2R and m1R), display charge-movement-associated currents analogous to 'gating currents' of voltage-gated channels. The gating charge-voltage relationship of m2R correlates well with the voltage dependence of the affinity of the receptor for acetylcholine. The loop that couples m2R and m1R to their G protein has a crucial function in coupling voltage sensing to agonist-binding affinity. Our data strongly indicate that GPCRs serve as sensors for both transmembrane potential and external chemical signals.

Molecular interaction of serotonin 5-HT2A receptor residues Phe339(6.51) and Phe340(6.52) with superpotent N-benzyl phenethylamine agonists

Experiments were conducted to examine the molecular basis for the high affinity and potency of a new class of 5-HT(2A) receptor agonists, N-benzyl phenethylamines. Competition binding assays at several serotonin receptors confirmed that an N-arylmethyl substitution was necessary for affinity increases up to 300-fold over simple N-alkyl homologs, as well as enhanced selectivity for 5-HT(2A) versus 5-HT(2C) and 5-HT(1A) receptors. PI hydrolysis functional assays confirmed that these N-benzyl phenethylamines are potent and highly efficacious agonists at the rat 5-HT(2A) receptor. Virtual docking of these compounds into a human 5-HT(2A) receptor homology model indicated that the N-benzyl moiety might be interacting with Phe339((6.51)), whereas the phenethylamine portion was likely to be interacting with Phe340((6.52)). Experiments in h5-HT(2A) receptors with Phe339((6.51))L and Phe340((6.52))L mutations seem to support this hypothesis. Dramatic detrimental effects on affinity, potency, and intrinsic activity were observed with the Phe339((6.51))L mutation for all N-benzyl analogs, whereas most N-unsubstituted phenethylamines and traditional agonists were only weakly affected, if at all. Consistent with other published studies, the Phe340((6.52))L mutation detrimentally affected affinity, potency, and intrinsic activity of nearly all compounds tested, although a strong change in intrinsic activity was not seen with most N-aryl analogs. These data further validate the topology of our h5-HT(2A) receptor homology model. It is noteworthy that this study is the first to identify a hitherto unrecognized role for residue 6.51 in agonist activation of a serotonin G protein-coupled receptor (GPCR), whereas most previous reports have suggested a varied and sometimes contradictory role in homologous GPCRs.

Probing receptor structure/function with chimeric G-protein-coupled receptors

Owing its name to an image borrowed from Greek mythology, a chimera is seen to represent a new entity created as a composite from existing creatures or, in this case, molecules. Making use of various combinations of three basic domains of the receptors (i.e., exofacial, transmembrane, and cytoplasmic segments) that couple agonist binding into activation of effectors through heterotrimeric G-proteins, molecular pharmacology has probed the basic organization, structure/function relationships of this superfamily of heptahelical receptors. Chimeric G-protein-coupled receptors obviate the need for a particular agonist ligand when the ligand is resistant to purification or, in the case of orphan receptors, is not known. Chimeric receptors created from distant members of the heptahelical receptors enable new strategies in understanding how these receptors transduce agonist binding into receptor activation and may be able to offer insights into the evolution of G-protein-coupled receptors from yeast to humans.

Mutational analysis of the OA1 gene in ocular albinism

Ocular albinism type 1 (OA1) is an X-linked disorder, mainly characterized by a severe reduction in visual acuity, foveal hypoplasia, nystagmus, hypopigmentation of the retina, the presence of macromelanosomes in the skin and eyes, and the misrouting of optic pathways, resulting in the loss of stereoscopic vision. We screened the OA1 gene for mutations in three unrelated Canadian and French families and in two isolated patients with OA1. We found three different missense mutations and two different nonsense mutations, three of which were novel. To date, 41 mutations (including missense mutations, insertions, and deletions) have been reported in the OA1 gene. Mutation and polymorphism data for this gene are available from the international albinism center albinism database website: http://www.cbc.umn.edu/tad/oa1map.htm.

Polymethylene tetraamine backbone as template for the development of biologically active polyamines

The concept that polyamines may represent a universal template in the receptor recognition process is embodied in the design of ligands for different biological targets. As a matter of fact, the insertion of different pharmacophores onto the polymethylene tetraamine backbone can tune both affinity and selectivity for any given receptor. The application of this approach provided a prospect of modifying benextramine (1). structure to achieve specific recognition of muscarinic receptors that led to the discovery of methoctramine (2). which is widely used as a pharmacological tool for muscarinic receptor characterization. In turn, appropriate structural modifications performed on the structure of methoctramine led to the discovery of new polyamines endowed with high affinity and selectivity for (a). muscarinic receptor subtypes, (b). G(i) proteins, and (c). muscle-type nicotinic receptors. Thus, polyamines tripitramine (9) and spirotramine (33), among others, were designed, which were shown to be highly selective for muscarinic M(2) and M(1) receptors, respectively. Several polyamines have been discovered, which inhibit noncompetitively a closed state of the nicotinic receptor. These ligands, such as 66, resulted in important tools for elucidating the mode and site of interaction of polyamines with the ion channel. It was discovered that reducing the flexibility of the diaminohexane spacer of methoctramine led to polyamines, such as 70, which are endowed with a biological profile significantly different from that of the prototype. Most likely, tetraamine (70) is a potent activator of G(i) proteins. Finally, the universal template approach formed the basis for modifying benextramine (1) structure to the design of ligands, which display affinity for acetylcholinesterase and muscarinic M(2) receptors. Thus, these polyamines, such as caproctamine (78), could have potential in the investigation of Alzheimer disease.

Role of the intracellular domains of CXCR4 in SDF-1-mediated signaling

The CXCR4 chemokine receptor is a G(i) protein-coupled receptor that triggers multiple intracellular signals in response to stromal cell-derived factor 1 (SDF-1), including calcium mobilization and p44/42 extracellular signal-regulated kinases (ERK1/2). Transduced signals lead to cell chemotaxis and are terminated through receptor internalization depending on phosphorylation of the C terminus part of CXCR4. Receptor endocytosis is also required for some receptors to stimulate ERK1/2 and to migrate through a chemokine gradient. In this study, we explored the role played by the 3 intracellular loops (ICL1-3) and the C terminus domain of CXCR4 in SDF-1-mediated signaling by using human embryonic kidney (HEK)-293 cells stably expressing wild-type or mutated forms of CXCR4. ICL3 of CXCR4 is specifically involved in G(i)-dependent signals such as calcium mobilization and ERK activation, but does not trigger CXCR4 internalization after SDF-1 binding, indicating that ERK phosphorylation is independent of CXCR4 endocytosis. Surprisingly, ICL2, with or without the aspartic acid, arginine, and tyrosine (DRY) motif, is dispensable for G(i) signaling. However, ICL2 and ICL3, as well as the C terminus part of CXCR4, are needed to transduce SDF-1-mediated chemotaxis, suggesting that this event involves multiple activation pathways and/or cooperation of several cytoplasmic domains of CXCR4.

Zebrafish M2 muscarinic acetylcholine receptor: cloning, pharmacological characterization, expression patterns and roles in embryonic bradycardia

1. A zebrafish M2 muscarinic acetylcholine receptor (mAChR) gene was cloned. It encodes 495 amino acids in a single exon. The derived amino acid sequence is 73.5% identical to its human homologue. 2. Competitive binding studies of the zebrafish M2 receptor and [(3)H]-NMS gave negative log dissociation constants (pK(i)) for each antagonist as follows: atropine (9.16)>himbacine (8.05)>/=4-DAMP (7.83)>AF-DX 116 (7.26)>/=pirenzepine (7.18)>/=tropicamide (6.97)>/=methoctramine (6.82)>/=p-F-HHSiD (6.67)>carbachol (5.20). The antagonist affinity profile correlated with the profile of the human M2 receptor, except for pirenzepine. 3. Reverse transcription polymerase chain reaction and Southern blotting analysis demonstrated that the M2 mAChR mRNA levels increased during the segmentation period (12 h post-fertilization; h.p.f.) in zebrafish. By whole-mount in situ hybridization, the M2 mAChR was first detectable in the heart, vagus motor ganglion, and vagus sensory ganglion at 30, 48 and 60 h.p.f., respectively. 4. The muscarinic receptor that mediates carbachol (CCh)-induced bradycardia was functionally mature at 72 h.p.f. The effect of CCh-induced bradycardia was antagonized by several muscarinic receptor antagonists with the order of potency (pIC(50) values): atropine (6.76)>methoctramine (6.47)>himbacine (6.10)>4-DAMP (5.72)>AF-DX 116 (4.77), however, not by pirenzepine, p-F-HHSiD, or tropicamide (<10 micro M). 5. The effect of CCh-induced bradycardia was abolished completely before 56 h.p.f. by M2 RNA interference, and the bradycardia effect gradually recovered after 72 h.p.f. The basal heart rate was increased in embryos injected with M2 mAChR morpholino antisense oligonucleotide (M2 MO) and the effect of CCh-induced bradycardia was abolished by M2 MO in a dose-dependent manner. In conclusion, the results suggest that the M2 mAChR inhibit basal heart rate in zebrafish embryo and the M2 mAChR mediates the CCh-induced bradycardia.

Site-directed mutagenesis implicates a threonine residue in TM6 in the subtype selectivities of UH-AH 37 and pirenzepine at muscarinic receptors

The structural basis for the selectivity of the antagonist UH-AH 37 at human muscarinic acetylcholine receptors was investigated by expressing mutant receptors in COS-7 cells. Previous studies have demonstrated that the interaction between UH-AH 37 and [(3)H]N-methylscopolamine in equilibrium assays is competitive and that the high affinity of UH-AH 37 for the M(5) subtype, compared to M(2), is due to an epitope in the sixth transmembrane domain (TM6) or the third outer loop of the receptor. By mutating each nonconserved residue in this region of M(2) and M(5) to its counterpart in the other receptor, we identified a threonine residue in the middle of TM6 uniquely responsible for the higher affinity of the M(5) receptor (M(1), M(3), and M(4) receptors also carry a threonine at that location and also have high affinity for UH-AH 37). The mutant receptor in which the corresponding alanine of the M(2) receptor was replaced by threonine, M(2)(401)ala --> thr, expressed enhanced affinity for pirenzepine as well as for UH-AH 37. The chick M(2) receptor, which expresses anomalously high affinity for pirenzepine, differs from its mammalian counterparts by the presence of a threonine at this position. Affinities of AF-DX 116 and 4-DAMP, as well as the allosteric potency of UH-AH 37, were not sensitive to the M(2)(401) ala --> thr mutation.

Angiotensin II type 1 receptor-function affected by mutations in cytoplasmic loop CD

To explore peptide hormone-induced conformational changes, we attempted to engineer a metal-ion binding site between the cytoplasmic loops CD and EF in the angiotensin II type 1 (AT(1)) receptor. We constructed 12 double and six triple histidine mutant receptors, and tested the ability of each mutant and the wild-type to activate inositol phosphate (IP) production with and without ZnCl(2). Inhibition by ZnCl(2) in the double and triple His mutant receptors was not significant, but these mutations directly decreased the IP production. Systematic analysis of single His mutants demonstrated that the loop CD-mutants displayed 52-74% inhibition of IP production, whereas the loop EF-mutants did not affect IP production. These results indicate that the cytoplasmic loop CD-segment from Tyr(127) to Ile(130) is important for G(q/11) activation by the AT(1) receptor.

Extended life-span and stress resistance in the Drosophila mutant methuselah

Toward a genetic dissection of the processes involved in aging, a screen for gene mutations that extend life-span in Drosophila melanogaster was performed. The mutant line methuselah (mth) displayed approximately 35 percent increase in average life-span and enhanced resistance to various forms of stress, including starvation, high temperature, and dietary paraquat, a free-radical generator. The mth gene predicted a protein with homology to several guanosine triphosphate-binding protein-coupled seven-transmembrane domain receptors. Thus, the organism may use signal transduction pathways to modulate stress response and life-span.

Can functional regions of proteins be predicted from their coding sequences? The case study of G-protein coupled receptors

A filter based on a set of unsupervised neural networks trained with a winner-take-all strategy discloses signals along the coding sequences of G-protein coupled receptors. By comparing with the existing experimental data it appears that these signals correlate with putative functional domains of the proteins. After protein alignment within subfamilies, signals cluster in protein regions which, according to the presently available experimental results, are described as possible functional domains of the folded proteins. The mapping procedure reveals characteristic regions in the coding sequences common and/or characteristic of the receptor subtype. This is particularly noticeable for the third cytoplasmic loop, which is likely to be involved in the molecular coupling of all the subfamilies with G-proteins. The results indicate that our mapping can highlight intrinsic representative features of the coding sequences which, in the case of G-protein coupled receptors, are characteristic of protein functional regions and suggest a possible application of the filter for predicting functional determinants in proteins starting from the coding sequence.

Mediation of cyclic AMP signaling by the first intracellular loop of the gonadotropin-releasing hormone receptor

The gonadotropin-releasing hormone (GnRH) receptor, which is a unique G protein-coupled receptor without a C-terminal cytoplasmic domain, activates both inositol phosphate (InsP) and cAMP signaling responses. The function of the highly basic first intracellular (1i) loop of the GnRH receptor in signal transduction was evaluated by mutating selected residues located in its N and C termini. Replacements of Leu58, Lys59, Gln61, and Lys62 at the N terminus, and Leu73, Ser74, and Leu80 at the C terminus, caused no change in binding affinity. The agonist-induced InsP and cAMP responses of the Q61E and K59Q,K62Q receptors were also unaffected, but the L58A receptor showed a normal InsP response and an 80% decrease in cAMP production. At the C terminus, the InsP response of the L73R receptor was normal, but cAMP production was reduced by 80%. The EC50 for GnRH-induced InsP responses of the S74E and L80A receptors was increased by about one order of magnitude, and the cAMP responses were essentially abolished. These findings indicate that cAMP signaling from the GnRH receptor is dependent on specific residues in the 1i loop that are not essential for activation of the phosphoinositide signaling pathway.

Structural correlates for down-regulation of m1 and m2 muscarinic receptor subtypes

Three chimeric receptors stably expressed in murine fibroblast (B82) cells were used to examine how different parts of the rat muscarinic m1 and m2 receptors contribute to the down-regulation process. The MCH7 chimeric m2 receptor contained a fragment between VIth TM and C-terminal end derived from the m1 receptor. The MCH3 and MCH5 receptors have exchanged N-terminal and third intracellular loop regions of the MCH7 receptor. Fibroblast cells stably expressing individual muscarinic wild type (m1, m2) or chimeric (MCH3, MCH5, or MCH7) receptors were treated with plain medium (control) or medium containing carbachol for 24 h. Receptor density changes were measured by [3H](-)1-N-methyl-3-quinuclidinyl benzilate ([3H](-)MQNB) saturation binding studies. There was a significant loss of receptor density, different for each receptor studied, following carbachol treatment relative to control cells. We related this loss of [3H](-)MQNB binding to the number of amino acids derived from m1 or m2 receptors for each constructed chimera and to the affinity of carbachol to the receptors studied. We demonstrate that: 1) the region from the VIth TMD to the end of C-terminal controls the extent of m1 and m2 receptor down-regulation; 2) the overall receptor conformation and the interaction between intracellular portions of the receptor influence the extent of receptor down-regulation; and 3) resistance to down-regulation by carbachol correlates with the affinity of carbachol to the muscarinic receptor construct.

The second intracellular loop of the m5 muscarinic receptor is the switch which enables G-protein coupling

We have completed a systematic search of the intracellular loops of a muscarinic acetylcholine receptor for domains that govern G-protein coupling. A unique feature of the second intracellular (i2) loop was an ordered cluster of residues where diverse substitutions cause constitutive activation. A second group of residues in i2 was identified where mutations compromised receptor/G-protein coupling. The residues of each group alternate and are spaced three to four positions apart, suggesting an alpha-helical structure where these groups form opposing faces of the helix. We propose that the constitutively activating face normally constrains the receptor in the "off-state," while the other face couples G-proteins in the "on-state." Therefore, the i2 loop functions as the switch enabling G-protein activation.

Identification of a ligand-dependent switch within a muscarinic receptor

G-protein-coupled receptors spontaneously switch between active and inactive conformations. Agonists stabilize the active conformation, whereas antagonists stabilize the inactive conformation. In a systematic search for residues that participate in receptor function, several regions of the m5 muscarinic receptor were randomly mutated and tested for their functional properties. Mutations spanning one face of transmembrane 6 (TM6) were found to induce high levels of receptor activity in the absence of agonists (constitutive activity). The same face of TM6 contained several residues crucial for receptor activation by agonists and one residue identified as a contact site for both agonists and antagonists. In addition, one mutation induced agonist-like responses from the receptor when exposed to classical antagonists. These results suggest that TM6 is a switch that defines the activation state of the receptor, and that ligand interactions with TM6 stabilize the receptor in either an active or an inactive conformation.

Antagonist binding profile of the split chimeric muscarinic m2-trunc/m3-tail receptor

Recent evidence suggests that G-protein-coupled receptors can behave as multiple subunit receptors, and can be split into parts, maintaining their binding ability. Transfection of a truncated muscarinic m2 receptor (containing transmembrane domains I-V, named m2-trunc) with a gene fragment coding for the carboxyl-terminal receptor portion of the muscarinic m3 receptor (containing transmembrane domains VI and VII, named m3-tail) results in the formation of a binding site with a high affinity for the muscarinic ligand N-[3H]methylscopolamine. In this paper we analyse the antagonist binding profile of this chimeric m2-trunc/m3-tail receptor in comparison with the wild-type muscarinic m2 and m3 receptors. While many of the substances tested had an intermediate affinity for the chimeric m2-trunc/m3-tail receptor compared with m2 and m3, some compounds were able to distinguish between the chimeric m2-trunc/m3-tail receptor on the one hand and the m2 or the m3 receptor on the other. Among them, tripitramine (a high-affinity M2 receptor antagonist) bound to the m2-trunc/m3-tail receptor with the same affinity as m2, but it bound to the m3 receptor with a 103-fold lower affinity; pirenzepine (a selective muscarinic M1 receptor antagonist) bound to the chimeric receptor with an affinity that was 12- and 3-fold higher than that of m2 and m3, respectively. The results of this study demonstrate that the chimeric m2-trunc/m3-tail receptor has a pharmacological profile distinct from that of the originating muscarinic m2 and m3 receptors.

Structure/function relationships of a G-protein coupling pocket formed by the third intracellular loop of the m5 muscarinic receptor

Using random saturation mutagenesis, we have previously identified the amino acids K439, A440, and A441 in the C-terminus of the third intracellular loop (Ci3) of the m5 muscarinic receptor as being critical for G-protein coupling [Burstein, E. S., Spalding, T. A., Hill-Eubanks, D., and Brann, M. R. (1995) J. Biol. Chem. 270, 3141-3146]. In the present study, we have constructed a series of point mutants at each of these residues and characterized their functional phenotypes in order to define the structure/function relationships of each of these residues for G-protein coupling. Although a wide variety of substitutions were tolerated at K439, most caused significant increases in the EC50 of carbachol and decreases in the maximum response (Rmax). Only other basic residues were well tolerated (<10-fold increase in EC50, >70% of wild type). Acidic substitutions had the largest effects, reducing Rmax to under 20% of wild type. At A440, only the conservative substitution threonine was well tolerated. Substitutions by hydrophobic, polar, and basic residues caused 10-80-fold increases in EC50 values and in many cases also significantly reduced Rmax (<70% of wild type). In contrast, at A441 mutations selectively affected EC50 but not Rmax values. Previously we identified I216, Y217, T220, and R223 as the residues in the N-terminus of the i3 loop of m5 (Ni3) that are critical for G-protein coupling [Burstein, E. S., Spalding, T. S., and Brann, M. R. (1996) J. Biol. Chem. 271, 2882-2885]. To investigate whether there were additive contributions of Ni3 and Ci3 to G-protein coupling, the functional responses of two double mutants, R223E/K439E and Y217S/A441T, were evaluated. Though these mutations were tolerated individually, both double mutant receptors produced almost indetectable responses. Little or no changes in expression levels or ligand binding properties were detected, suggesting the observed effects were caused primarily by changes receptor/G-protein coupling. We conclude that K439 participates in G-protein activation through an ionic mechanism, that A440 fulfills a structural role forming part of the G-protein coupling pocket, and that A441 contributes to receptor affinity for G-proteins. We propose that the third intracellular loop forms a G-protein coupling pocket comprised of a positively charged "lip" and a hydrophobic core.

The thrombin receptor second cytoplasmic loop confers coupling to Gq-like G proteins in chimeric receptors. Additional evidence for a common transmembrane signaling and G protein coupling mechanism in G protein-coupled receptors

Thrombin activates human platelets and other cells in part by cleaving an unusual G protein-coupled receptor. Thrombin cleavage of this receptor's amino-terminal exodomain unmasks a new amino terminus. This then binds intramolecularly to the body of the receptor to trigger transmembrane signaling and activation of Gi- and Gq-like G proteins. Toward identifying the domains responsible for thrombin receptor-G protein interactions, we examined the signaling properties of chimeric receptors in which thrombin receptor cytoplasmic sequences replaced the cognate sequences in the Gs-coupled beta2-adrenergic receptor (beta2AR) or the Gi-coupled dopamine D2 receptor (D2R). In Xenopus oocytes, a chimeric beta2AR bearing the thrombin receptor second cytoplasmic (C2) loop gained the ability to trigger intracellular Ca2+ release in response to adrenergic agonist, whereas a beta2AR bearing the cognate C2 loop from the D2R did not. Similarly, in COS-7 cells, a chimeric D2R bearing the thrombin receptor C2 loop gained the ability to trigger phosphoinositide hydrolysis in response to dopaminergic agonist, apparently by coupling to a Gq-like G protein. No detectable Gs coupling was seen. Thus, the thrombin receptor C2 loop was able to confer Gq-like coupling in several different receptor contexts. These observations suggest that the thrombin receptor C2 loop specifies Gq coupling by directly contacting Gq or by contributing to a structure required for Gq coupling. The ability of the thrombin receptor C2 loop to function in the context of the D2R and beta2AR strongly suggests that the transmembrane switching and G protein activation strategies used by the thrombin receptor must be very similar to those used by the D2R and beta2AR despite the thrombin receptor's strikingly different liganding mechanism.

Functional analysis of the human D2 dopamine receptor missense variants

The human dopamine D2 receptor gene (DRD2) has three polymorphic variants that predict the amino acid substitutions Val96 --> Ala, Pro310 --> Ser, and Ser311 --> Cys in the receptor protein. We have investigated the ligand binding and signal transduction properties of these human D2 receptor variants by stably expressing them in cultured mammalian cells. The Cys311 and Ser310 variants of the human D2 receptor, which involve substitutions located in the third cytoplasmic loop, were markedly less effective in inhibiting cAMP synthesis than the most prevalent form (Pro310, Ser311). Despite this difference, the Cys311 and Ser310 variants couple to G proteins in CHO-K1 (Chinese hamster ovary) cells. The impairment of the Cys311 and Ser310 variants to inhibit cAMP levels thus appears to result from a reduced ability of those variant receptors to activate the appropriate Gi-like protein. The demonstration of substantial functional differences between DRD2 gene variants found in the human population might have important pharmacological implications given the widespread use of D2 receptor blocking drugs in the treatment of schizophrenia and other psychiatric disorders.

Constitutive activation of chimeric m2/m5 muscarinic receptors and delineation of G-protein coupling selectivity domains

To derive structure/function relationships for muscarinic receptor/G-protein coupling, the m2 and m5 muscarinic receptors and a series of m2/m5 chimeras were tested for agonist binding and functional responses in a cellular proliferation/transformation assay. m5, which mediates stimulation of phosphatidylinositol turnover, displayed robust activity in the proliferation assay, whereas m2, which mediates inhibition of adenylyl cyclase, was inactive in the proliferation assay. Chimeras that contained m2 sequences in the i2 or i3 loops had impaired activity or were inactive, respectively. Chimeras that contained m2 segments reaching from the N-terminus to TM2, or from TM6 to the C-terminus, had enhanced activity relative to m5, and a chimera with both of these elements was constitutively activated.

Amino acid side chains that define muscarinic receptor/G-protein coupling. Studies of the third intracellular loop

Amino acids in the third intracellular loops of receptors play pivotal roles in G-protein coupling. To define their structural requirements, we have subjected the N- and C-terminal regions of this loop (Ni3 and Ci3, respectively) of the m5 muscarinic receptor to random saturation mutagenesis. (see Burstein, E. S., Spalding, T. A., Hill-Eubanks, D., and Brann, M. R. (1995) J. Biol. Chem. 270, 3141 3146 and Hill-Eubanks, D., Burstein, E. S., Spalding, T. A., Bräuner-Osborne, H., and Brann, M. R. (1996) J. Biol. Chem. 271, 3058 3065). In the present study, we have extended our analysis of Ni3 by constructing libraries of receptors with all possible amino acid substitutions at the residues we previously identified as functionally important and characterizing their functional phenotypes. Numerous hydrophobic substitutions were well tolerated at Ile216 and Thr220 and caused constitutive activation in two cases, establishing that hydrophobicity is structurally favored at these positions and that many amino acid side chains are compatible with this structural role. Similarly, hydrophobic and polar, but not charged, substitutions were observed at Tyr217, but in contrast to results for Thr220, most substitutions at Tyr217 substantially decreased maximum response and increased the EC50 for carbachol, demonstrating that the specific side chain of residue 217 participates in G-protein coupling. Arg223 allowed the widest range of substitutions of the residues tested, but only basic residues were well tolerated. All other substitutions significantly increased (up to 100-fold) the EC50 for carbachol without significantly affecting maximal response. There were no significant changes in the ligand binding properties of these mutant receptors. We conclude that Ile216 and Thr220 fulfill a structural role, forming the foundation of the G-protein-coupling pocket, whereas Tyr217 and Arg223 contact G-proteins through specific side chain interactions. We propose that G-proteins are recruited to receptors by ionic interactions and that hydrophobic residues participate in activation.

Structure of a G-protein-coupling domain of a muscarinic receptor predicted by random saturation mutagenesis

The third intracellular loop (i3) plays a critical role in the coupling of many receptors to G-proteins. In muscarinic receptor subtypes, the N- and C-terminal regions (Ni3 and Ci3) of this loop are sufficient to direct appropriate G-protein coupling. The relative functional contributions of all amino acids within Ni3 was evaluated by constructing libraries of m5 muscarinic receptors containing random mutations in Ni3 and screening them using high throughput assays based on ligand-dependent transformation of NIH 3T3 cells. In receptors that retained a wild type phenotype, the pattern of functionally tolerated substitutions is consistent with the presence of three turns of an alpha helix extending from the transmembrane domain. All of the amino acid positions that tolerate radical substitutions face away from a conserved hydrophobic face that ends with an arginine, and helix-disrupting proline substitutions were not observed. All of the mutant receptors with significantly compromised phenotypes had amino acid substitutions in residues predicted to form the hydrophobic face. Similar data from the Ci3 region (Burstein, E. S., Spalding, T. A., Hill-Eubanks, D., and Brann, M. R. (1995) J. Biol. Chem. 270, 3141-3146) are consistent with the presence of a single helical turn extending from the transmembrane domain, with an alanine that defines G-protein affinity. Functionally critical residues of Ni3 and Ci3 are predicted to be in close proximity where they form the G-protein-coupling domain.

Selective coupling of prostaglandin E receptor EP3D to Gi and Gs through interaction of alpha-carboxylic acid of agonist and arginine residue of seventh transmembrane domain

Prostaglandin (PG) E receptor EP3D is coupled to both Gi and Gs. To examine the roles of the interaction of alpha-carboxylic acid of PGE2 and its putative binding site, the arginine residue in the seventh transmembrane domain of EP3D, in receptor-G protein coupling, we have mutated the arginine residue to the noncharged glutamine. PGE2 with a negatively charged alpha-carboxylic acid and sulprostone, an EP3 agonist with a noncharged modified alpha-carboxylic acid, inhibited the forskolin-stimulated adenylate cyclase activity via Gi activation in the EP3D receptor in the same concentration-dependent manner. In contrast, the adenylate cyclase stimulation via Gs activation by sulprostone was much lower than that by PGE2. On the other hand, both PGE2 and sulprostone showed potent Gi activity but failed to show Gs activity in the mutant receptor. EP3D receptor showed a high affinity binding for PGE2 in the form coupled to either Gi or Gs. Although the mutant receptor showed high affinity binding when coupled to Gi, it lost high affinity binding in the condition of Gs coupling. Furthermore, sulprostone bound to the Gi-coupled EP3D receptor with higher affinity than the Gs-coupled receptor. Among various EP3 agonists, alpha-carboxylic acid-unmodified agonists showed both Gi and Gs activities, but the modified agonists showed only Gi activity. These findings suggest that the interaction between the alpha-carboxylic acid of PGE2 and the arginine residue of the receptor regulates the selectivity of the G protein coupling.

Structural basis of G protein specificity of human endothelin receptors. A study with endothelinA/B chimeras

The endothelin (ET) family of peptides acts via two subtypes of guanine nucleotide-binding regulatory protein (G protein)-coupled receptors termed ETA and ETB. ET-1 stimulated cAMP formation in Chinese hamster ovary (CHO) cells stably expressing human wild-type ETA (CHO/hETA cells) while it inhibited cAMP formation in CHO cells expressing human wild-type ETB (CHO/hETB cells), and pharmacological evidence indicated that the opposite effects were due to the selective coupling of each receptor subtype with G alpha s/G alpha i. To find out a receptor domain(s) that determined the selective coupling, a series of chimeric receptors between hETA and hETB was expressed on CHO cells, and the effect of ET-1 on cAMP formation in each cell line was tested. hETA with the replacement of second and/or third intracellular loop (ICLII and/or -III) to the corresponding region(s) of hETB failed to transmit the stimulatory effect of ET-1. hETB with the replacement of ICLIII to the corresponding region of hETA failed to transmit the inhibitory effect of ET-1. A chimeric receptor with ICLII of hETB and with ICLIII of hETA failed to transmit both effects. In cells expressing chimeric receptors with ICLII of hETA and with ICLIII of hETB, ET-1 inhibited cAMP formation while it stimulated cAMP formation when cells were pretreated with pertussis toxin. These results indicated the roles of ICLII and -III of hETR as a major determinant of the selective coupling of hETA and hETB with G alpha s/G alpha i, respectively. We also demonstrated that each receptor subtype expressed on the same cell could work independently, i.e. for hETA to activate G alpha s and for hETB to activate G alpha i, resulting in dose-dependent dual effects of ET-1 on cAMP formation.

M1 muscarinic receptors heterologously expressed in cardiac myocytes mediate Ras-dependent changes in gene expression

Stimulation of alpha 1-adrenergic receptors in neonatal ventricular cardiomyocytes induces hypertrophic changes including activation of the atrial natriuretic factor (ANF) gene. This receptor couples to Gq to activate phospholipase C (PLC) and protein kinase C, which have been implicated as mediators of the hypertrophic response. To directly determine whether receptor coupling to Gq/PLC is sufficient to induce ANF expression, we expressed wild-type and chimeric muscarinic cholinergic receptors (mAChRs) with altered G-protein coupling properties in cardiac myocytes and examined their ability to activate an ANF promoter/luciferase reporter gene. The cholinergic agonist carbachol failed to induce transcriptional activation of the ANF reporter gene through endogenous Gi-linked M2mAChRs or in cells transfected with M2mAChRs. In contrast, in cells transfected with M1mAChRs, which effectively couple to Gq/PLC, carbachol increased ANF reporter gene expression 10-fold and also increased ANF protein, as determined by immunofluorescence. Carbachol-mediated ANF gene expression was inhibited by the mAChR antagonist pirenzepine with a Ki value characteristic of an M1mAChR. Studies using chimeric M1- and M2mAChRs demonstrated that the N-terminal 21 amino acids of the third intracellular loop of the M1mAChR were required for receptor coupling to ANF gene expression. This region, previously shown to specify receptor coupling to Gq/PLC, also conferred partial activity to a chimeric M2 receptor. We further demonstrated that M1mAChR coupling to ANF gene expression was Ras-dependent since co-expression of dominant-interfering Ala-15 Ras inhibited M1mAChR-induced ANF expression by 60%. In contrast, ANF expression induced by the chimeric M2 receptor was not blocked by dominant-interfering Ras. We suggest that receptor coupling to Gq/PLC is sufficient to induce ANF expression and that a Ras-dependent pathway contributes additional signals required for maximal M1mAChR-mediated ANF gene expression.

Alternative splicing of the dopamine D2 receptor directs specificity of coupling to G-proteins

Two isoforms of the dopamine D2 receptor have been characterized, D2L (long) and D2S (short), generated by alternative splicing from the same gene. They differ by an in-frame insert of 29 amino acids specific to D2L within the putative third intracytoplasmic loop of the receptor. We have previously demonstrated (Montmayeur, J.-P., Guiramand, J., and Borelli, E. (1993) Mol. Endocrinol. 7, 161-170) that D2S and D2L, although presenting very similar pharmacological profiles, couple differently to the alpha-subunit of guanine nucleotide-binding regulatory proteins (G-proteins). In particular, D2L, but not D2S, requires the presence of the alpha-subunit of the inhibitory G-protein (G alpha i2) to elicit greater inhibition of adenylyl cyclase activity. The insert present in D2L must therefore confer the specificity of interaction with G alpha i2. Thus, we introduced substitution mutations within the D2L insert. These mutant receptors were expressed in JEG3 cells, a G alpha i2-deficient cell line, scoring for those presenting an increased inhibition of adenylyl cyclase by dopamine. Our analysis identified two mutants, S259/262A and D249V, with these properties. These results clearly show that the insert present in D2L plays a critical role in the selectivity for the G-proteins interacting with the receptor.

Structure-function of muscarinic receptor coupling to G proteins. Random saturation mutagenesis identifies a critical determinant of receptor affinity for G proteins

To derive structure-function relationships for receptor-G protein coupling, libraries were created of human m5 muscarinic acetylcholine receptors (m5) randomly mutated in the C-terminal region of the third intracellular loop. Functional receptors were identified based on their ability to amplify NIH 3T3 cells in a ligand-dependent manner. These receptors either had wild-type phenotypes (Group 1) or were functionally impaired (Group 2). No "activated receptors" were identified. Tolerated substitutions in Group 2 receptors were randomly distributed and frequently included prolines and glycines. In contrast, tolerated substitutions in Group 1 receptors exhibited a periodicity proximal to transmembrane domain 6 were proline and glycine substitutions were not observed. These observations are consistent with a short alpha-helical extension of the C-terminal region of the third intracellular loop from transmembrane domain 6. Mutations at Ala-441 were most commonly associated with impaired function of Group 2 receptors. Twelve point mutations at Ala-441 were tested, and all caused marked increases in EC50 values with little effect on maximal response or agonist binding affinity. These results indicate that Ala-441 is a key determinant of m5 receptor affinity for G proteins and exists within the structural context of a short alpha-helix.

Characterization of the rat m3 muscarinic acetylcholine receptor produced in insect cells infected with recombinant baculovirus

The m3 muscarinic acetylcholine receptor from rat heterologously produced in insect cells after infection with a recombinant baculovirus has an apparent molecular mass of approximately 75 kDa. Polyclonal antibodies raised against a carboxy-terminal nonapeptide that is unique to the m3 subtype can detect the receptors produced in the insect cells by Western blot and can also immunoprecipitate solubilized receptor. Immunofluorescence microscopy as well as electron microscopy revealed that the receptor was located intracellularly, visualized as a ring around the nucleus of the infected insect cells. Solubilization of the receptor was accomplished with digitonin which was added in increments (over 10 min) to a final concentration of 0.8% (mass/vol). The solubilized receptor is unstable when the ligand-binding site is not protected by a ligand. Here the low-affinity ligand propylbenzilylcholine (approximately 10 nM) has demonstrable protective ability during solubilization, but the usefulness of this ligand is limited by a very slow off rate. From the behaviour of the solubilized receptor during DEAE-Sephacel chromatography and lectin-affinity chromatography it can be deduced that the receptor produced in insect cells is heterogeneously glycosylated in the producing insect cells.

Modulation of the delayed rectifier K+ current in neurons by an angiotensin II type 2 receptor fragment

Angiotensin II (ANG II) stimulates the delayed rectifier K+ current (IK) in neurons cultured from rat hypothalamus and brain stem via AT2 receptors, and this effect involves activation of a Gi protein and protein phosphatase 2A (PP2A). However, there was no evidence that the AT2 receptor involved in this response was the same as the recently cloned AT2 receptor. In the present study, intracellular injection of a 22-amino acid peptide (PEP-22) corresponding to the putative third intracellular loop of the cloned AT2 receptor elicited an increase in IK in cultured neurons that was similar to the effect produced by ANG II. Furthermore, this effect of PEP-22 was abolished by pertussis toxin (200 ng/ml, 24 h) pretreatment and also by superfusion of the PP2A inhibitor okadaic acid (10 nM), suggesting the involvement of Gi protein and PP2A, respectively. Intracellular injection of a random peptide or normal pipette solution did not affect neuronal IK. This is direct evidence to link the cloned AT2 receptor to a defined response elicited by ANG II.

The angiotensin type 1 and type 2 receptor families. Siblings or cousins?

The diverse actions of angiotensin II (AngII) are mediated by cell surface receptors. Molecular cloning techniques have identified two distinct subtypes of AngII receptors referred to as AT1 and AT2. It is now well accepted that multiple forms of the AT1 receptor exist, but similar diversity of the AT2 subtype has not been conclusively demonstrated. Nonetheless, several converging lines of evidence do suggest that multiple AT2 receptors may be present in brain and cultured neuron-like cells lines. For instance, some AT2 receptors are regulated by guanine nucleotides and sulfhydryl-reducing agents, whereas others are insensitive. AT2 receptor populations also exhibit differing pharmacological profiles particularly with respect to their affinity for peptidic and non-peptidic ligands. Moreover, a recently developed anti-AT2 polyclonal antisera reveals a unique pattern of immunohistochemical staining in brain and it does not immunoreact with the recently cloned AT2 receptor. Collectively, these results support the hypothesis of multiple AT2 receptors at least within the CNS. Future studies should reveal whether these putative AT2 receptor subtypes result from unique genes or cell-specific post-translational modifications of a single gene product.

The C-terminus of the prostaglandin-E-receptor EP3 subtype is essential for activation of GTP-binding protein

Three isoforms of the mouse prostaglandin-E-receptor EP3 subtype (EP3), EP3 alpha, EP3 beta and EP3 gamma, with different C-termini, which are produced through alternative splicing, showed different efficiencies with respect to heterotrimeric GTP-binding protein activation and adenylate cyclase inhibition [Sugimoto, Y., Negishi, M., Hayashi, Y., Namba, T., Honda, A., Watabe, A., Hirata, M., Narumiya, S. & Ichikawa, A. (1993) J. Biol. Chem. 268, 2712-2718; Irie, A., Sugimoto, Y., Namba, T., Harazono, A., Honda, A., Watabe, A., Negishi, M., Narumiya, S. & Ichikawa, A. (1993) Eur. J. Biochem. 217, 313-318]. To assess the role of the C-terminus in GTP-binding protein coupling, we truncated the C-terminus of EP3 at an alternative splicing site and expressed the mutant receptor. The truncated receptor retained the ability to physically associate with Gi2, forming an agonist/receptor/Gi2 ternary complex, and to undergo the characteristic conversion of its agonist-binding affinity, mediated by a guanine nucleotide from a low-affinity state to a high-affinity state. However, sulprostone, an EP3 agonist, failed not only to inhibit the forskolin-induced cAMP accumulation in the mutant receptor-expressing cells but also to stimulate the GTPase activity in the mutant receptor-expressing cell membrane. These results indicated that the C-terminus of EP3 is essential for the activation of GTP-binding protein.

Antibodies inactivating mGluR1 metabotropic glutamate receptor block long-term depression in cultured Purkinje cells

Antibodies were raised against two distinct extracellular sequences of the rat mGluR1 metabotropic glutamate receptor expressed as bacterial fusion proteins. Both antibodies specifically reacted with mGluR1 in the rat cerebellum and inhibited the mGluR1 activity as assessed by the analysis of glutamate-stimulated inositol phosphate formation in CHO cells expressing mGluR1. Using these antibodies, we examined the role of mGluR1 in the induction of long-term depression in cultured Purkinje cells. In voltage-clamped Purkinje cells, current induced by iontophoretically applied glutamate was persistently depressed by depolarization of the Purkinje cells in conjunction with the glutamate application. The mGluR1 antibodies completely blocked the depression of glutamate-induced current. The results indicate that activation of mGluR1 is necessary for the induction of cerebellar long-term depression and that these mGluR1 antibodies can be used as selective antagonists.

Activation of Gi protein by peptide structures of the muscarinic M2 receptor second intracellular loop

The muscarinic M2 receptor that normally couples via Gi to inhibit adenylyl cyclase was made to couple to Gs by exchange of its third intracellular loop for the comparable domain of the beta 2-adrenoceptor. In HeLa cells transfected with the recombinant M2 beta i-3 cDNA, the chimaeric receptor showed carbachol-mediated activation of adenylyl cyclase (EC50 = 73 nM) that was blocked by atropine, but not by propranolol. The chimaeric receptor was shown to mediate a carbachol-stimulated, Bordetella pertussis toxin-sensitive GTPase activity in membranes of transfected HeLa cells. Interestingly, stimulation of adenylyl cyclase by carbachol was 2-fold higher in transfected cells that had been pretreated with pertussis toxin. These data suggested that the M2 beta i-3 receptor was able to couple to both Gi and Gs, and that the ability to recognise and stimulate Gi did not involve the third cytoplasmic loop of M2. We investigated peptide elements taken from the second intracellular loop of the M2 receptor for their ability to mediate activation of Gi and found that a nine amino acid peptide representing the C-terminal sequence, VKRTTKMAG-NH2 (V9G), was capable of inhibiting forskolin-stimulated adenylyl cyclase by up to 18% and could stimulate high affinity GTPase activity of rat brain membranes by 32%. Further, V9G was shown to cause a doubling of the initial rate of [35S]GTP gamma S binding to purified bovine brain Gi/Go in reconstituted phospholipid vesicles. These data identify a domain on the second intracellular loop of the muscarinic M2 receptor that is involved in the selection of a pertussis toxin-sensitive G protein.

Disruption of potential alpha-helix in the G loop of the guinea pig 5-hydroxytryptamine2 receptor does not prevent receptor coupling to phosphoinositide hydrolysis

Heterogeneity of the 5-hydroxytryptamine2 (5-HT2) receptor across species has been implicated in several pharmacological and physiological studies. Although 5-HT2 receptors in the rat have been linked to increases in phosphoinositide (PI) hydrolysis, little evidence exists to support the association of guinea pig 5-HT2 receptors with PI hydrolysis, the second messenger generally linked with 5-HT2 receptors. In the present study, we have taken a molecular and biochemical approach to determining whether species differences in brain 5-HT2 receptors exist between rat and guinea pig. First, we isolated partial cortical 5-HT2 receptor cDNA clones that encompassed the third intracellular loop, a receptor area putatively important in receptor-effector coupling. The amino acid sequences deduced from the cDNA clones for rat and guinea pig brain 5-HT2 receptor were 97% homologous. However, the guinea pig 5-HT2 receptor had two tandem substitutions that disrupted a potential alpha helix in the region of the third cytoplasmic loop, which theoretically could alter the intracellular coupling of the guinea pig cortical 5-HT2 receptor. Because of these molecular differences, we examined further the pharmacological activation of the brain 5-HT2 receptor from guinea pig. 5-HT and the 5-HT2 receptor agonist alpha-methyl-5-HT increased PI hydrolysis in guinea pig cortical slices whereas the 5-HT1C receptor agonist 5-methyltryptamine was significantly less potent. In addition, the 5-HT2 receptor antagonists LY53857, ketanserin, and spiperone blocked 5-HT-stimulated PI hydrolysis. These pharmacological data suggested that activation of the 5-HT2 receptor in guinea pig cortical slices was associated with PI hydrolysis. Thus, although areas of the guinea pig brain 5-HT2 receptor that influence receptor-effector coupling were different from the rat, such differences were not critical to receptor-effector coupling because, as in the rat, guinea pig brain 5-HT2 receptors were also coupled to PI hydrolysis.