The function of a highly-conserved arginine residue in activation of the muscarinic M1 receptor

Tyrosine 7.43 is important for mu-opioid receptor downstream signaling pathways activated by fentanyl

G protein–coupled receptors can signal through both G proteins and ß-arrestin2. For the µ-opioid receptor (MOR), early experimental evidence from a single study suggested that G protein signaling mediates analgesia and sedation, whereas ß-arrestin signaling mediates respiratory depression and constipation. Then, receptor mutations were used to clarify which residues interact with ligands to selectively regulate signals in a ligand-specific manner. However, there is no systematic study on how to determine these residues and clarify the molecular mechanism of their influence on signal pathways. We have therefore used molecular docking to predict the amino acid sites that affect the binding of ligands and MOR. Then, the corresponding sites were mutated to determine the effect of the structural determinant of MOR on G_i/o protein and ß-arrestin pathways. The pharmacological and animal behavioral experiments in combination with molecular dynamics simulations were used to elucidate the molecular mechanism of key residues governing the signaling. Without affecting ligand binding to MOR, MOR^Y7.43A attenuated the activation of both G_i/o protein and ß-arrestin signaling pathways stimulated by fentanyl, whereas it did not change these two pathways stimulated by morphine. Likewise, the activation peak time of extracellular regulated protein kinases was significantly prolonged at MOR^Y7.43A compared with that at MOR^wildtype stimulated by fentanyl, but there was no difference stimulated by morphine. In addition, MOR^Y7.43A significantly enhanced analgesia by fentanyl but not by morphine in the mice behavioral experiment. Furthermore, the molecular dynamics simulations showed that H6 moves toward the cellular membrane. H6 of the fentanyl–Y7.43A system moved outward more than that in the morphine–Y7.43A system. Y7.43 mutation disrupted hydrophobic interactions between W6.48 and Y7.43 in the fentanyl–Y7.43A system but not in the morphine–Y7.43A system. Our results have disclosed novel mechanisms of Y7.43 mutation affecting MOR signaling pathways. Y7.43 mutation reduced the activation of the G_i/o protein pathway and blocked the ß-arrestin2 recruitment, increased the H6 outward movement of MOR, and disrupted hydrophobic interactions. This may be responsible for the enhanced fentanyl analgesia. These findings are conducive to designing new drugs from the perspective of ligand and receptor binding, and Y7.43 is also expected to be a key site to structure optimization of synthesized compounds.

Structure and function of serotonin G protein-coupled receptors

Serotonin receptors are prevalent throughout the nervous system and the periphery, and remain one of the most lucrative and promising drug discovery targets for disorders ranging from migraine headaches to neuropsychiatric disorders such as schizophrenia and depression. There are 14 distinct serotonin receptors, of which 13 are G protein-coupled receptors (GPCRs), which are targets for approximately 40% of the approved medicines. Recent crystallographic and biochemical evidence has provided a converging understanding of the basic structure and functional mechanics of GPCR activation. Currently, two GPCR crystal structures exist for the serotonin family, the 5-HT1B and 5-HT2B receptor, with the antimigraine and valvulopathic drug ergotamine bound. The first serotonin crystal structures not only provide the first evidence of serotonin receptor topography but also provide mechanistic explanations into functional selectivity or biased agonism. This review will detail the findings of these crystal structures from a molecular and mutagenesis perspective for driving rational drug design for novel therapeutics incorporating biased signaling.

Unifying family A GPCR theories of activation

Several new pairs of active and inactive GPCR structures have recently been solved enabling detailed structural insight into the activation process, not only of rhodopsin but now also of the β2 adrenergic, M2 muscarinic and adenosine A2A receptors. Combined with structural analyses they have enabled us to examine the different recent theories proposed for GPCR activation and show that they are all indeed parts of the same process, and are intrinsically related through their effect on the central hydrophobic core of GPCRs. This new unifying general process of activation is consistent with the identification of known constitutively active mutants and an in-depth conservational analysis of significant residues implicated in the process.

GPCR activation: a mutagenic spotlight on crystal structures

The crystal structures of antagonist and agonist complexes of isolated β(2) and β(1) adrenoceptors have recently been supplemented by antagonist structures of M(2) and M(3) muscarinic acetylcholine receptors. Importantly, a structure of an agonist-ligated β(2) adrenoceptor complexed with its cognate G protein has provided the first view of a ternary complex representing the transition state in agonist-mediated G protein activation. This review interprets these G-protein-coupled receptor (GPCR) structures through the focus provided by extensive mutagenesis studies on muscarinic receptors, revealing an activation mechanism that is both modular and dynamic. Specific motifs, based around highly conserved residues, functionalise the seven-transmembrane architecture of these receptors. While exploiting conserved motifs, the ligand binding and signal transduction pathways work around and through water-containing cavities, an emerging feature of GPCR structures. These cavities may have undergone evolutionary selection to adapt GPCRs to particular signalling niches, and may provide targeting opportunities to enhance drug selectivity.

Structure-function studies of muscarinic acetylcholine receptors

There has been great interest in the structure-function relationships of the muscarinic acetylcholine receptors (mAChRs) because these prototypical Family A/class 1 G protein-coupled receptors (GPCRs) are attractive therapeutic targets for both peripheral and central nervous system disorders. A multitude of drugs that act at the mAChRs have been identified over the years, but many of these show minimal selectivity for any one of the five mAChR subtypes over the others, which has hampered their development into therapeutics due to adverse side effects. The lack of drug specificity is primarily due to high sequence similarity in this family of receptor, especially in the orthosteric binding pocket. Thus, there remains an ongoing need for a molecular understanding of how mAChRs bind their ligands, and how selectivity in binding and activation can be achieved. Unfortunately, there remains a paucity of solved high-resolution structures of GPCRs, including the mAChRs, and thus most of our knowledge of structure-function mechanisms related to this receptor family to date has been obtained indirectly through approaches such as mutagenesis. Nonetheless, such studies have revealed a wealth of information that has led to novel insights and may be used to guide future rational drug design campaigns.

Helix 8 of the M1 muscarinic acetylcholine receptor: scanning mutagenesis delineates a G protein recognition site

We have used alanine-scanning mutagenesis followed by functional expression and molecular modeling to analyze the roles of the 14 residues, Asn422 to Cys435, C-terminal to transmembrane (TM) helix 7 of the M(1) muscarinic acetylcholine receptor. The results suggest that they form an eighth (H8) helix, associated with the cytoplasmic surface of the cell membrane in the active state of the receptor. We suggest that the amide side chain of Asn422 may act as a cap to the C terminus of TM7, stabilizing its junction with H8, whereas the side chain of Phe429 may restrict the relative movements of H8 and the C terminus of TM7 in the inactive ground state of the receptor. We have identified four residues, Phe425, Arg426, Thr428, and Leu432, which are important for G protein binding and signaling. These may form a docking site for the C-terminal helix of the G protein α subunit, and collaborate with G protein recognition residues elsewhere in the cytoplasmic domain of the receptor to form a coherent surface for G protein binding in the activated state of the receptor.

An evolutionarily conserved arginine is essential for Tre1 G protein-coupled receptor function during germ cell migration in Drosophila melanogaster

BACKGROUND

G protein-coupled receptors (GPCRs) play central roles in mediating cellular responses to environmental signals leading to changes in cell physiology and behaviors, including cell migration. Numerous clinical pathologies including metastasis, an invasive form of cell migration, have been linked to abnormal GPCR signaling. While the structures of some GPCRs have been defined, the in vivo roles of conserved amino acid residues and their relationships to receptor function are not fully understood. Trapped in endoderm 1 (Tre1) is an orphan receptor of the rhodopsin class that is necessary for primordial germ cell migration in Drosophila melanogaster embryos. In this study, we employ molecular genetic approaches to identify residues in Tre1 that are critical to its functions in germ cell migration.

METHODOLOGY/PRINCIPAL FINDINGS

First, we show that the previously reported scattershot mutation is an allele of tre1. The scattershot allele results in an in-frame deletion of 8 amino acids at the junction of the third transmembrane domain and the second intracellular loop of Tre1 that dramatically impairs the function of this GPCR in germ cell migration. To further refine the molecular basis for this phenotype, we assayed the effects of single amino acid substitutions in transgenic animals and determined that the arginine within the evolutionarily conserved E/N/DRY motif is critical for receptor function in mediating germ cell migration within an intact developing embryo.

CONCLUSIONS/SIGNIFICANCE

These structure-function studies of GPCR signaling in native contexts will inform future studies into the basic biology of this large and clinically important family of receptors.

Forced unbinding of GPR17 ligands from wild type and R255I mutant receptor models through a computational approach

Background

GPR17 is a hybrid G-protein-coupled receptor (GPCR) activated by two unrelated ligand families, extracellular nucleotides and cysteinyl-leukotrienes (cysteinyl-LTs), and involved in brain damage and repair. Its exploitment as a target for novel neuro-reparative strategies depends on the elucidation of the molecular determinants driving binding of purinergic and leukotrienic ligands. Here, we applied docking and molecular dynamics simulations (MD) to analyse the binding and the forced unbinding of two GPR17 ligands (the endogenous purinergic agonist UDP and the leukotriene receptor antagonist pranlukast from both the wild-type (WT) receptor and a mutant model, where a basic residue hypothesized to be crucial for nucleotide binding had been mutated (R255I) to Ile.

Results

MD suggested that GPR17 nucleotide binding pocket is enclosed between the helical bundle and extracellular loop (EL) 2. The driving interaction involves R255 and the UDP phosphate moiety. To support this hypothesis, steered MD experiments showed that the energy required to unbind UDP is higher for the WT receptor than for R255I. Three potential binding sites for pranlukast where instead found and analysed. In one of its preferential docking conformations, pranlukast tetrazole group is close to R255 and phenyl rings are placed into a subpocket highly conserved among GPCRs. Pulling forces developed to break polar and aromatic interactions of pranlukast were comparable. No differences between the WT receptor and the R255I receptor were found for the unbinding of pranlukast.

Conclusions

These data thus suggest that, in contrast to which has been hypothesized for nucleotides, the lack of the R255 residue doesn't affect the binding of pranlukast a crucial role for R255 in binding of nucleotides to GPR17. Aromatic interactions are instead likely to play a predominant role in the recognition of pranlukast, suggesting that two different binding subsites are present on GPR17.

Characterization of intracellular signaling mediated by human somatostatin receptor 5: role of the DRY motif and the third intracellular loop

Somatostatin (SST) exerts inhibitory effects on hormone secretion and cell proliferation by interacting with five different receptors (SST1-SST5) linked to multiple cellular effectors. The receptor structural domains involved in these effects have been only partially elucidated. The aim of the study was to investigate the molecular determinants mediating the interaction of the human SST5 with intracellular signaling in the pituitary cell line GH3, focusing on the BBXXB domain in the third intracellular loop and the DRY motif in the second intracellular loop. We analyzed the effects of the SST5 agonist BIM23206 on cAMP accumulation, intracellular calcium, GH secretion, cell proliferation, and ERK1/2 phosphorylation in cells expressing either wild-type SST5 or mutant receptors, in particular the naturally occurring mutant R240W in the BBXXB domain and the D136A and R137A mutants in the DRY motif. We found that residues D136 and R137 were critical for SST5 signaling because their substitutions abolished all the intracellular responses. Conversely, third intracellular loop mutations resulted in receptor that inhibited intracellular cAMP levels similar to the wild-type (50 +/- 9 vs. 53 +/- 12% inhibition) but failed to mediate the other responses elicited by wild-type SST5, i.e. reduction of intracellular calcium levels as well as inhibition of ERK1/2. These events resulted in an absent inhibition of GH release and an impaired reduction of cell proliferation (38 +/- 7 vs. 76 +/- 6% inhibition in wild type, P < 0.05). These data indicate that different regions of SST5 are required for the activation of different signaling pathways.

Identification of an Ascaris G protein-coupled acetylcholine receptor with atypical muscarinic pharmacology

Acetylcholine (ACh) is a neurotransmitter/neuromodulator in the nematode nervous system and induces its effects through interaction with both ligand-gated ion channels (LGICs) and G protein-coupled receptors (GPCRs). The structure, pharmacology and physiological importance of LGICs have been appreciably elucidated in model nematodes, including parasitic species where they are targets for anthelmintic drugs. Significantly less, however, is understood about nematode ACh GPCRs, termed GARs (G protein-linked ACh receptors). What is known comes from the free-living Caenorhabditis elegans as no GARs have been characterized from parasitic species. Here we clone a putative GAR from the pig gastrointestinal nematode Ascaris suum with high structural homology to the C. elegans receptor GAR-1. Our GPCR, dubbed AsGAR-1, is alternatively spliced and expressed in the head and tail of adult worms but not in dorsal or ventral body wall muscle, or the ovijector. ACh activated AsGAR-1 in a concentration-dependent manner but the receptor was not activated by other small neurotransmitters. The classical muscarinic agonists carbachol, arecoline, oxotremorine M and bethanechol were also AsGAR-1 agonists but pilocarpine was ineffective. AsGAR-1 activation by ACh was partially antagonized by the muscarinic blocker atropine but pirenzepine and scopolamine were largely ineffective. Certain biogenic amine GPCR antagonists were also found to block AsGAR-1. Our conclusion is that Ascaris possesses G protein-coupled ACh receptors that are homologous in structure to those present in C. elegans, and that although they have some sequence homology to vertebrate muscarinic receptors, their pharmacology is atypically muscarinic.

The majority of adrenocorticotropin receptor (melanocortin 2 receptor) mutations found in familial glucocorticoid deficiency type 1 lead to defective trafficking of the receptor to the cell surface

CONTEXT

There are at least 24 missense, nonconservative mutations found in the ACTH receptor [melanocortin 2 receptor (MC2R)] that have been associated with the autosomal recessive disease familial glucocorticoid deficiency (FGD) type 1. The characterization of these mutations has been hindered by difficulties in establishing a functional heterologous cell transfection system for MC2R. Recently, the melanocortin 2 receptor accessory protein (MRAP) was identified as essential for the trafficking of MC2R to the cell surface; therefore, a functional characterization of MC2R mutations is now possible.

OBJECTIVE

Our objective was to elucidate the molecular mechanisms responsible for defective MC2R function in FGD.

METHODS

Stable cell lines expressing human MRAPalpha were established and transiently transfected with wild-type or mutant MC2R. Functional characterization of mutant MC2R was performed using a cell surface expression assay, a cAMP reporter assay, confocal microscopy, and coimmunoprecipitation of MRAPalpha.

RESULTS

Two thirds of all MC2R mutations had a significant reduction in cell surface trafficking, even though MRAPalpha interacted with all mutants. Analysis of those mutant receptors that reached the cell surface indicated that four of six failed to signal, after stimulation with ACTH.

CONCLUSION

The majority of MC2R mutations found in FGD fail to function because they fail to traffic to the cell surface.

[Functional arginine-containing amino acid sequences in peptides and proteins]

L-arginine is a source of nitrogen oxide and plays a great role in a number of other biochemical processes. Functions and prospects for practical application of five groups of arginine-containing amino acid sequences and synthetic polyarginine sequences are considered. The physiological characteristics of well-known arginine-containing peptides, such as RGD peptides, kyotorphin, and tuftsin, are described in detail. The English version of the paper: Russian Journal of Bioorganic Chemistry, 2008, vol. 34, no. 2; see also http://www.maik.ru

Roof and floor of the muscarinic binding pocket: variations in the binding modes of orthosteric ligands

Alanine substitution mutagenesis has been used to investigate residues that make up the roof and floor of the muscarinic binding pocket and regulate ligand access. We mutated the amino acids in the second extracellular loop of the M1 muscarinic acetylcholine receptor that are homologous to the cis-retinal contact residues in rhodopsin, the disulfide-bonded Cys178 and Cys98 that anchor the loop to transmembrane helix 3, the adjoining acidic residue Asp99, and the conserved aromatic residues Phe197 and Trp378 in the transmembrane domain. The effects on ligand binding, kinetics, and receptor function suggest that the second extracellular loop does not provide primary contacts for orthosteric ligands, including acetylcholine, but that it does contribute to microdomains that are important for the conformational changes that accompany receptor activation. Kinetic studies suggest that the disulfide bond between Cys98 and Cys178 may contribute to structures that regulate the access of positively charged ligands such as N-methyl scopolamine to the binding pocket. Asp99 may act as a gatekeeper residue to this channel. In contrast, the bulkier lipophilic ligand 3-quinuclidinyl benzilate may require breathing motions of the receptor to access the binding site. Trp378 is a key residue for receptor activation as well as binding, whereas Phe197 represents the floor of the N-methyl scopolamine binding pocket but does not interact with acetylcholine or 3-quinuclidinyl benzilate. Differences between the binding modes of N-methyl scopolamine, 3-quinuclidinyl benzilate, and acetylcholine have been modeled. Although the head groups of these ligands occupy overlapping volumes within the binding site, their side chains may follow significantly different directions.

Multiple facets of follicle-stimulating hormone receptor function

Follicle-stimulating hormone (FSH) is a glycoprotein hormone produced by the anterior pituitary gland. This gonadotropin plays an essential role in reproduction. Its receptor (FSHR) belongs to the superfamily of G protein-coupled receptors (GPCR), specifically the family of rhodopsin-like receptors. Agonist binding to the FSHR triggers the rapid activation of multiple signaling cascades, mainly the cAMP-adenylyl cyclase-protein kinase A cascade, that impact diverse biological effects of FSH in the gonads. As in other G protein-coupled receptors, the several cytoplasmic domains of the FSHR are involved in signal transduction and termination of the FSH signal. Here we summarize some recent information on the signaling cascades activated by FSH as well as on the role of the intracytoplasmic domains of the FSHR in coupling to membrane and cytosolic proteins linked to key biological functions regulated by the FSH-FSHR system.

Phenotypic classification of mutants: a tool for understanding ligand binding and activation of muscarinic acetylcholine receptors

GPCRs (G-protein-coupled receptors) such as the M(1) muscarinic receptor have so far proved recalcitrant to direct structure determination. Nevertheless systematic mutagenesis, particularly alanine scanning, has advanced our understanding of their structure-function relationships. GPCRs exhibit multiple conformational states with different affinities for and abilities to activate their cognate G-proteins. Ligand binding alters these conformational equilibria, thus promoting or inhibiting signalling. Alanine-scanning mutagenesis probes the relative contributions of a particular amino acid side chain to the stability of the ground and activated states of the receptor and its complexes. These determine the phenotype of the mutant receptor. Classification of the phenotypes suggests functional roles for particular amino acid side chains, allowing us to group them accordingly. From a rhodopsin-based homology model of the M(1) mAChR, a coherent view emerges of how these clusters of residues function in ligand anchoring, transduction of binding energy, global structural stabilization and selective stabilization of the ground state or the activated state of the receptor. We can identify differences in ligand-binding modes, and suggest inter- and intra-molecular interactions that are weakened or broken, or formed or intensified during acetylcholine-induced activation. In due course, we may be able to extend these insights to activation by unconventional agonists.

Functional analysis of transmembrane domain 2 of the M1 muscarinic acetylcholine receptor

Ala substitution scanning mutagenesis has been used to probe the functional role of amino acids in transmembrane (TM) domain 2 of the M1 muscarinic acetylcholine receptor, and of the highly conserved Asn43 in TM1. The mutation of Asn43, Asn61, and Leu64 caused an enhanced ACh affinity phenotype. Interpreted using a rhodopsin-based homology model, these results suggest the presence of a network of specific contacts between this group of residues and Pro415 and Tyr418 in the highly conserved NPXXY motif in TM7 that exhibit a similar mutagenic phenotype. These contacts may be rearranged or broken when ACh binds. D71A, like N414A, was devoid of signaling activity. We suggest that formation of a direct hydrogen bond between the highly conserved side chains of Asp71 and Asn414 may be a critical feature stabilizing the activated state of the M1 receptor. Mutation of Leu67, Ala70, and Ile74 also reduced the signaling efficacy of the ACh-receptor complex. The side chains of these residues are modeled as an extended surface that may help to orient and insulate the proposed hydrogen bond between Asp71 and Asn414. Mutation of Leu72, Gly75, and Met79 in the outer half of TM2 primarily reduced the expression of functional receptor binding sites. These residues may mediate contacts with TM1 and TM7 that are preserved throughout the receptor activation cycle. Thermal inactivation measurements confirmed that a reduction in structural stability followed the mutation of Met79 as well as Asp71.

Common structural requirements for heptahelical domain function in class A and class C G protein-coupled receptors

G protein-coupled receptors (GPCRs) are key players in cell communication. Several classes of such receptors have been identified. Although all GPCRs possess a heptahelical domain directly activating G proteins, important structural and sequence differences within receptors from different classes suggested distinct activation mechanisms. Here we show that highly conserved charged residues likely involved in an interaction network between transmembrane domains (TM) 3 and 6 at the cytoplasmic side of class C GPCRs are critical for activation of the gamma-aminobutyric acid type B receptor. Indeed, the loss of function resulting from the mutation of the conserved lysine residue into aspartate or glutamate in the TM3 of gamma-aminobutyric acid type B(2) can be partly rescued by mutating the conserved acidic residue of TM6 into either lysine or arginine. In addition, mutation of the conserved lysine into an acidic residue leads to a nonfunctional receptor that displays a high agonist affinity. This is reminiscent of a similar ionic network that constitutes a lock stabilizing the inactive state of many class A rhodopsin-like GPCRs. These data reveal that despite their original structure, class C GPCRs share with class A receptors at least some common structural feature controlling G protein activation.

Role of the intracellular domains of the human FSH receptor in G(alphaS) protein coupling and receptor expression

The human (h) follicle-stimulating hormone receptor (FSHR) belongs to the superfamily of G protein-coupled receptors (GPCRs). This receptor consists of 695 amino acid residues and is preferentially coupled to the G(s) protein. This receptor is highly conserved among species (overall homology, 85%), with a 25-69% homology drop when compared to the human LH and TSH receptors. Although studies in prototypical rhodopsin/beta-adrenergic receptors suggest that multiple domains in the intracellular loops (iL) and the carboxyl-terminus (Ctail) of these receptors contribute to G protein coupling and receptor expression, there is a paucity of structure/function data on the role of these domains in FSHR function. Employing point mutations we have found that several residues present in the iL2 of the hFSHR are important for both coupling the receptor to the G(s) protein and maintaining the receptor molecule in an inactive conformation. In fact, HEK-293 cells expressing several hFSHR mutants with substitutions at R(450) (central to the highly conserved ERW triplet motif) and T(453) (a potential target for phosphorylation) failed to mediate ligand-provoked G(s) protein activation but not agonist binding, whereas substitutions at the hydrophobic L(460) (a conserved residue present in all glycoprotein hormone receptors) conferred elevated basal cAMP to the transfected cells. Thus, this particular loop apparently acts as a conformational switch for allowing the receptor to adopt an active conformation upon agonist stimulation. Residues in both ends of the iL3 are important for signal transduction in a number of GPCRs, including the FSHR. We have recently explored the importance of the reversed BBXXB motif (BXXBB; where B represents a basic residue and X a non-basic residue) present in the juxtamembrane region of the hFSHR iL3. A hFSHR mutant with all basic amino acids present in the iL3 BXXBB motif replaced by alanine failed to bind agonist and activate effector, and was expressed as an immature < or =62kDa form of the receptor. Individual substitutions of basic residues resulted in mutants that bound agonist normally but failed to activate effector when replaced at R(552) or R(556). Triple mutations in the same motif located in the NH(2)-end of the Ctail resulted in a complete inability of the receptor to bind agonist and activate effector, whereas individual substitutions resulted in decreased or virtually abolished agonist binding and cAMP accumulation, with both functions correlating with the detected levels of mature (80kDa) forms of the receptor. Thus, the BXXBB motif at the iL3 of the FSHR is essential for coupling the activated receptor to the G(s) protein, whereas the same motif in the Ctail is apparently more important for membrane expression. The role of cysteine residues present in the Ctail of the FSHR is an enigma since there are no conserved cysteines amongst LHR, FSHR and TSHR. C(629) and C(655) are conserved in the gonadotropin receptors but not in the TSHR. Alanine replacement of C(627) had no effect on hFSHR expression and function, whereas the same mutation at C(629) altered membrane expression and signal transduction. Serine or threonine substitutions of C(655) did not modify any of the parameters analyzed. In the hFSHR, C(629) may be a target for palmitoylation, and apparently it is the only cysteine residue in the Ctail domain that might play an important role in receptor function.

Functional consequences of naturally occurring DRY motif variants in the mammalian chemoattractant receptor GPR33

Most members of the large family of rhodopsin-like G-protein-coupled receptors possess an evolutionarily conserved Asp-Arg-Tyr (DRY) motif in the C-terminal region of the third transmembrane domain. Mutations of residues within this motif usually abolish receptor function and, when they occur naturally, can even cause human diseases. By analyzing over 100 mammalian orthologs of the chemoattractant receptor GPR33 we identified several polymorphic and fixed sequence variations within the DRY motif. Unexpectedly, the naturally occurring mutation of Arg(3.50) to His in mouse GPR33 showed no difference from the wild-type receptor in several functional tests. Sequence analysis of GPR33 from Asian house mice revealed the polymorphic existence of Arg(3.50) and His(3.50) alleles in wild-trapped populations, further supporting the functional equivalence of both allelic variants. In contrast, the Arg(3.50) to Gly mutation found in hamster GPR33 inactivates the receptor and may have contributed to pseudogenization of this gene in this species. Functional data with GPR33 variants indicate different receptor- and context-specific consequences of DRY mutations. Our study also reveals GPR33 as a new example illustrating missense mutations as a first step in the pseudogenization process.

Constitutive activity of muscarinic acetylcholine receptors

We review the literature describing constitutive activity of the five muscarinic acetylcholine receptors in native and recombinant systems and discuss the effect of constitutive activity on muscarinic pharmacology in the context of modern models of receptor activation. We include a summary of mutations found to cause constitutive activity and discuss the implications of these data for the structure, function, and activation mechanism of muscarinic receptors. Finally, we discuss the possible physiological significance of constitutive activity of muscarinic receptors, incorporating information provided by targeted deletion of each of the muscarinic subtypes.

Charged residues of the conserved DRY triplet of the vasopressin V1a receptor provide molecular determinants for cell surface delivery and internalization

The highly conserved "Asp-Arg-Tyr" triplet in the distal region of the third transmembrane region of most G-protein-coupled receptors is implicated in their activation process and mediation of G-protein signaling. The aim of this study was to determine whether specific features at this locus are important for the vasopressin V(1a) receptor (V(1a)R) by performing site-directed mutagenesis. In transfected HEK 293T cells, mutation of Asp (D148A) resulted in a misfolded receptor that was nonfunctional, localized intracellularly, and not constitutively active. Nonconservative (D148R) substitution was not expressed, whereas asparagine (D148N) partially restored cell surface expression, although no specific ligand-binding or inositol phosphate signaling was detected. In contrast, conservative (D148E) substitution was expressed moderately higher, bound ligands, and signaled similarly to a hemagglutinin epitope-tagged wild-type receptor. However, D148E showed a greater tendency to be internalized once it was delivered to the membrane. Individual replacements of the conserved arginine and tyrosine (R149A, Y150A) led to decreased signal transduction without affecting surface expression, agonist affinity, or internalization or increasing basal signaling activity. Incorporation of aspartate (R149D) or reversal of charges (D148R/R149D) were nonfunctional, localized intracellularly, and indicated the absence of an ionic interaction between Asp-148 and Arg-149. It is noteworthy that an important role of arginine was identified for regulating agonist-mediated internalization when a histidine (R149H) was present. This mutant was expressed on the cell surface but was rapidly internalized after agonist treatment. This study highlights the importance of specific charged residues within this motif that provide important determinants for cell surface delivery, internalization and for normal V(1a)R function.

Random mutagenesis of the M3 muscarinic acetylcholine receptor expressed in yeast: identification of second-site mutations that restore function to a coupling-deficient mutant M3 receptor

The M(3) muscarinic receptor is a prototypical member of the class A family of G protein-coupled receptors (GPCRs). To gain insight into the structural mechanisms governing agonist-mediated M(3) receptor activation, we recently developed a genetically modified yeast strain (Saccharomyces cerevisiae) which allows the efficient screening of large libraries of mutant M(3) receptors to identify mutant receptors with altered/novel functional properties. Class A GPCRs contain a highly conserved Asp residue located in transmembrane domain II (TM II; corresponding to Asp-113 in the rat M(3) muscarinic receptor) which is of fundamental importance for receptor activation. As observed previously with other GPCRs analyzed in mammalian expression systems, the D113N point mutation abolished agonist-induced receptor/G protein coupling in yeast. We then subjected the D113N mutant M(3) receptor to PCR-based random mutagenesis followed by a yeast genetic screen to recover point mutations that can restore G protein coupling to the D113N mutant receptor. A large scale screening effort led to the identification of three such second-site suppressor mutations, R165W, R165M, and Y250D. When expressed in the wild-type receptor background, these three point mutations did not lead to an increase in basal activity and reduced the efficiency of receptor/G protein coupling. Similar results were obtained when the various mutant receptors were expressed and analyzed in transfected mammalian cells (COS-7 cells). Interestingly, like Asp-113, Arg-165 and Tyr-250, which are located at the cytoplasmic ends of TM III and TM V, respectively, are also highly conserved among class A GPCRs. Our data suggest a conformational link between the highly conserved Asp-113, Arg-165, and Tyr-250 residues which is critical for receptor activation.

Importance of amino acids of the central portion of the second intracellular loop of the gastrin-releasing Peptide receptor for phospholipase C activation, internalization, and chronic down-regulation

Little is known about the function of the central portion of the second intracellular loop (i2 loop) of peptide receptors in activation of downstream pathways and receptor modulatory processes such as receptor internalization or chronic down-regulation (DR). Recent data suggest a role for i2 loop hydrophobic amino acids in these processes. We used site-directed mutagenesis to address these issues with the gastrin-releasing peptide receptor (GRP-R). Each i2 loop residue from 142 to 148 was mutated and the receptors were expressed in Balb 3T3 cells. Two mutants showed a minimal (<2-fold) decrease in affinity. Five mutants showed decreased efficacy for activating phospholipase C (PLC). Two double mutants (IM143.147AA and VM144.147AA) showed a minimal decrease in affinity but had a decreased ability to fully activate PLC. Only the IM double mutation had decreased maximal internalization, whereas the R145A single mutant showed an increase, suggesting a tonic inhibitory role for Arg-145 in internalization. Three single and both double mutants showed decreases in receptor DR. There was a weak correlation between the extent of GRP-R internalization and the maximal PLC activation, whereas changes in the maximal PLC activation were significantly (p = 0.008) coupled to receptor DR. This study shows that amino acids of the i2 loop of the GRP-R are important in activation of PLC, internalization and down-regulation, but not for affinity. Our results support the proposal that internalization and chronic down-regulation have differing dependence on PLC and are largely independent processes, because some mutants showed no changes in internalization, but significant alterations in down-regulation.

The role of a conserved region of the second intracellular loop in AT1 angiotensin receptor activation and signaling

The pleiotropic actions of angiotensin II are mediated by the primarily G(q) protein-coupled type 1 angiotensin (AT(1)) receptor. In this study a mutational analysis of the function of the conserved DRYXXV/IXXPL domain in the second intracellular loop of the rat AT(1A) receptor was performed in COS7 cells. Alanine substitution studies showed that single replacement of the highly conserved Asp(125) and Arg(126), but not Tyr(127), moderately impaired angiotensin II-induced inositol phosphate signaling. However, concomitant substitution of both Asp(125) and Arg(126) caused marked reduction of both inositol phosphate signaling and receptor internalization. Alanine scanning of the adjacent residues showed that substitution of Ile(130), His(132), and Pro(133) reduced agonist-induced inositol phosphate signal generation, whereas mutations of Met(134) also impaired receptor internalization. Expression of the D125A mutant AT(1A) receptor in COS7 cells endowed the receptor with moderate constitutive activity, as indicated by its enhanced basal Elk1 promoter activity and inositol phosphate response to partial agonists. Angiotensin II-induced stimulation of the Elk1 promoter showed parallel impairment with inositol phosphate signal generation in receptors containing mutations in this region of the AT(1A) receptor. These data confirm that Ca(2+) signal generation is required for the nuclear effects of angiotensin II-induced ERK activation. They are also consistent with the role of the conserved DRY sequence of the AT(1A) receptor in receptor activation, and of Asp(125) in constraining the receptor in its inactive conformation. Furthermore, in the cytoplasmic helical extension of the third helix, an apolar surface that includes Ile(130) and Met(134) appears to have a direct role in G protein coupling.

The role of extracellular regulated kinases I/II in late-phase long-term potentiation

Extracellular regulated kinases (ERKI/II), members of the mitogen-activated protein kinase family, play a role in long-term memory and long-term potentiation (LTP). ERKI/II is required for the induction of the early phase of LTP, and we show that it is also required for the late phase of LTP in area CA1 in vitro, induced by a protocol of brief, repeated 100 Hz trains. We also show that ERKI/II is necessary for the upregulation of the proteins encoded by the immediate early genes Zif268 and Homer after the induction of LTP in the dentate gyrus by tetanic stimulation of the perforant path in vivo or by BDNF stimulation of primary cortical cultures. To test whether the induction of persistent synaptic plasticity by stimuli such as BDNF is associated with nuclear translocation of ERKI/II, we expressed enhanced green fluorescent protein (EGFP)-ERKII in PC12 cell lines and primary cortical cultures. In both preparations, we observed translocation of EGFP-ERKII from the cytoplasm to the nucleus in cells exposed to neurotrophic factors. Our results suggest that the induction of late LTP involves translocation of ERKI/II to the nucleus in which it activates the transcription of immediate early genes. The ability to visualize the cellular redistribution of ERKII after induction of long-term synaptic plasticity may provide a method for visualizing neuronal circuits underlying information storage in the brain in vivo.

Mutagenesis and peptide analysis of the DRY motif in the alpha2A adrenergic receptor: evidence for alternate mechanisms in G protein-coupled receptors

In G protein-coupled receptors (GPCRs), a conserved aspartic acid in the DRY motif at the cytoplasmic end of helix 3 regulates the transition to the active state, while the adjacent arginine is crucial for G protein activation. To examine the functions of these two residues, we made D130I and R131Q mutations in the alpha2A adrenergic receptor (AR). We demonstrate that, unlike other GPCRs, the alpha2A AR is not constitutively activated by the D130I mutation, although the mutation increases agonist affinity. While the R131Q mutation severely disrupts function, it decreases rather than increasing agonist affinity as seen in other GPCRs. We then investigated the molecular effects of the same mutations in a peptide model and showed that Arg131 is not required for peptide-mediated G protein activation. These results indicate that the alpha2A AR does not follow the conventional GPCR mechanistic paradigm with respect to the function of the DRY motif.

Structural determinants in the second intracellular loop of the human follicle-stimulating hormone receptor are involved in G(s) protein activation

In the present study, we analyzed the structural determinants present in the second intracellular loop (IL-2) of the human follicle-stimulating hormone (FSH) receptor (R) involved in G(s) protein-mediated signal transduction. Human embryonic kidney 293 (HEK-293) cells, stably expressing wild-type (Wt) human FSHR (HEK-293((+))), were transiently transfected with plasmids containing cDNAs encoding the entire IL-2 or several IL-2 sequences mutated in R467 (a residue located at the center of the conserved ERW motif in the glycoprotein hormone receptors), T470 (a potential site for phosphorylation by protein kinase-A and -C) or L477 (a residue conserved in all glycoprotein hormone receptors). Expression of the IL-2 Wt in HEK-293((+)) cells reduced the maximum FSH-stimulated cAMP production significantly by approximately 40%; similar results were observed with the R467A and R467K IL-2 mutants. The IL-2(R467H), IL-2(T470A), the triple R467A/T470A/L477A IL-2 mutant and the IL-2 of the oxytocin receptor (G(q/11)-coupled) had no effects on Wt FSHR-mediated intracellular signaling whereas the L477A mutation provoked a higher ( approximately 55%) inhibition of FSH-stimulated cAMP than the free, Wt IL-2. These results suggested a specific role of IL-2 residues in FSHR function. Site directed mutagenesis of the FSHR and the expression of resulting mutants in HEK-293 cells were performed in order to corroborate the effects of these substitutions. Expression of FSHR(R467H), FSHR(R467A) and FSHR(T470A) failed to mediate ligand-provoked G(s) protein activation, whereas the R467K mutant behaved as the Wt receptor. Interestingly, the expression of L477A, L477D and L477P FSHR mutants conferred elevated basal cAMP levels to HEK-293 cells. This study indicates that the IL-2 of the human FSHR possesses amino acid residues that are important for both coupling the receptor to the G(s) protein (R467 and T470) and maintaining the receptor molecule in an inactive conformation (L477). It appears that this particular intracellular domain may act as a conformational switch to produce the activation of G proteins as has been reported for the IL-2 of other G protein-coupled receptors.

Characterization of a G protein coupling "YL" motif of the human VPAC1 receptor, equivalent to the first two amino acids in the "DRY" motif of the rhodopsin family

The conserved residues Y239 and L240 of human VPAC1 receptor are predicted to be at the same location as the asparagine and arginine in the "DRY" motif in the Rhodopsin family of G protein-coupled receptors. By comparing vasoactive intestinal peptide (VIP) binding with or without the presence of GTP-gamma-S, it was found that the deltadelta G(o) for the endogenous G-protein coupling was 1.5 kJ/mol, 0.95 kJ/mol, and 3.4 kJ/mol for theY239A, L240A, and wild-type receptor, respectively. VIP-induced cAMP production in whole cells support the results of the binding studies, as Y239A had a moderate and L240A a pronounced impaired ability to produce cAMP. The mutants had a minor influence on the intrinsic "low affinity to high affinity equilibrium," suggesting that the dominating effect of these mutants is a perturbation of the G protein-binding site. Thus, the highly diverged chemical properties of the hydrophobic "YL" motif and charged "DR(Y)" motif could be a crucial difference between the Secretin Receptor Family and the Rhodopsin Family with respect to receptor activation and G-protein coupling.

Transmembrane domains 4 and 7 of the M(1) muscarinic acetylcholine receptor are critical for ligand binding and the receptor activation switch

Activation of the muscarinic acetylcholine receptors requires agonist binding followed by a conformational change, but the ligand binding and conformation-switching residues have not been completely identified. Systematic alanine-scanning mutagenesis has been used to assess residues 142-164 in transmembrane helix 4 and 402-421 in transmembrane helix 7 of the M(1) muscarinic acetylcholine receptor. Several inward-facing amino acid side chains in the exofacial parts of transmembrane helices 4 and 7 contribute to acetylcholine binding. Alanine substitution of the aromatic residues in this group reduced signaling efficacy, suggesting that they may form part of a charge-stabilized aromatic cage, which triggers rotation and movement of the transmembrane helices. The mutation of adjacent residues modulated receptor activation, either reducing signaling or causing constitutive activation. In the buried endofacial section of transmembrane helix 7, alanine substitution mutants of the conserved NSXXNPXXY motif displayed strongly reduced signaling efficacy, despite having increased or unchanged acetylcholine affinity. These residues may have dual functions, forming intramolecular contacts that stabilize the receptor in the inactive ground state, but that are broken, allowing them to form new intramolecular bonds in the activated state. This conformational rearrangement is critical to produce a G protein binding site and may represent a key mechanism of receptor activation.

beta -Amyloid peptide-induced apoptosis regulated by a novel protein containing a g protein activation module

Degeneration of neurons in Alzheimer's disease is mediated by beta-amyloid peptide by diverse mechanisms, which include a putative apoptotic component stimulated by unidentified signaling events. This report describes a novel beta-amyloid peptide-binding protein (denoted BBP) containing a G protein-coupling module. BBP is one member of a family of three proteins containing this conserved structure. The BBP subtype bound human beta-amyloid peptide in vitro with high affinity and specificity. Expression of BBP in cell culture induced caspase-dependent vulnerability to beta-amyloid peptide toxicity. Expression of a signaling-deficient dominant negative BBP mutant suppressed sensitivity of human Ntera-2 neurons to beta-amyloid peptide mediated toxicity. These findings suggest that BBP is a target of neurotoxic beta-amyloid peptide and provide new insight into the molecular pathophysiology of Alzheimer's disease.

A network of conserved intramolecular contacts defines the off-state of the transmembrane switch mechanism in a seven-transmembrane receptor

Activation of the rhodopsin-like 7-transmembrane (7-TM) receptors requires switching interhelical constraints that stabilize the inactive state to a new set of contacts in the activated state, which binds the cognate G-protein. The free energy to drive this is provided by agonist binding, which has higher affinity to the active than to the inactive conformation. We have sought specific interhelical constraint contacts, using the M(1) muscarinic acetylcholine receptor as a model. Histidine substitutions of particular groups of amino acids, in transmembrane domains 3, 6, and 7, created high-affinity Zn(2+) binding sites, demonstrating the close proximity of their side chains in the inactive state. Alanine point substitutions have shown the effect of weakening the individual intramolecular contacts. In each case, the acetylcholine affinity was increased, implying promotion of the activated state. These amino acids are highly conserved throughout the 7-TM receptor superfamily. We propose that they form an important part of a network of conserved interhelical contacts that defines the off-state of a general transmembrane switch mechanism.

Role of the highly conserved Asp-Arg-Tyr motif in signal transduction of the CB2 cannabinoid receptor

The DRY motif, at the junction of transmembrane helix 3 and intracellular loop 2 of G protein-coupled receptors, is highly conserved. Mutations were introduced into the CB2 cannabinoid receptor to study the role of this motif in CB2 signaling. D mutations (DRY130-132AAA and D130A) markedly reduced binding of cannabinoid agonists, while no significant reduction was observed with R131A or Y132A. Mutating R (R131A) only partially reduced, and mutating Y (Y132A) more efficiently reduced the cannabinoid-induced inhibition of adenylyl cyclase. Thus, in CB2, D130 is involved in agonist binding, whereas Y seems to have a role in receptor downstream signaling.

Alanine-scanning mutagenesis of transmembrane domain 6 of the M(1) muscarinic acetylcholine receptor suggests that Tyr381 plays key roles in receptor function

Transmembrane domain 6 of the muscarinic acetylcholine (ACh) receptors is important in ligand binding and in the conformational transitions of the receptor but the roles of individual residues are poorly understood. We have carried out a systematic alanine-scanning mutagenesis study on residues Tyr381 to Val387 within the binding domain of the M(1) muscarinic ACh receptor. The seven mutations were then analyzed to define the effects on receptor expression, agonist and antagonist binding, and signaling efficacy. Tyr381Ala produced a 40-fold reduction in ACh affinity and a 50-fold reduction in ACh-signaling efficacy. Leu386Ala had similar but smaller effects. Asn382Ala caused the largest inhibition of antagonist binding. The roles of the hydroxyl group and benzene ring of Tyr381 were probed further by comparative analysis of the Tyr381Phe and Tyr381Ala mutants using three series of ligands: ACh analogs, azanorbornane- and quinuclidine-based ligands, and atropine analogs. These data suggested that the hydroxyl group of Tyr381 is primarily involved in forming hydrogen bond interactions with the oxygen atoms present in the side chain of ACh. We propose that this interaction is established in the ground state and preserved in the activated state of the receptor. In contrast, the Tyr381 benzene ring may form a cation-pi interaction with the positively charged head group of ACh that contributes to the activated state of the receptor but not the ground state. However, the hydroxyl group and benzene ring of Tyr381 both participate in interactions with azanorbornane- and quinuclidine-based ligands and atropine analogs in the ground state as well as the activated state of the receptor.

Structural basis of G protein-coupled receptor function

The vast majority of extracellular signaling molecules, like hormones and neurotransmitters, interact with a class of membranous receptors characterized by a uniform molecular architecture of seven transmembrane alpha-helices linked by extra- and intracelluar peptide loops. In a reversible manner, binding of diverse agonists to heptahelical receptors leads to activation of a limited repertoire of heterotrimeric guanine nucleotide-binding proteins (G proteins) forwarding the signal to intracellular effectors such as enzymes and ion channels. Proper functioning of a G protein-coupled receptor is based on a complex interplay of structural determinants which are ultimately responsible for receptor folding, trafficking and transmembrane signaling. Applying novel biochemical and molecular biological methods interesting insights into receptor structure/function relationships became available. These studies have a significant impact on our understanding of the molecular basis of human diseases and may eventually lead to novel therapeutic strategies.

The functional topography of transmembrane domain 3 of the M1 muscarinic acetylcholine receptor, revealed by scanning mutagenesis

Alanine-scanning mutagenesis has been applied to residues 100-121 in transmembrane domain 3 of the M1 muscarinic acetylcholine receptor. This study complements a previous investigation of the triad Asp122-Arg123-Tyr124 (Lu, Z-L., Curtis, C. A., Jones, P. G., Pavia, J., and Hulme, E. C. (1997) Mol. Pharmacol. 51, 234-241). The results demonstrate the alpha-helical secondary structure of the domain and suggest its orientation with respect to the other transmembrane domains. The C-terminal part of the helix appears to be largely buried within the receptor structure. On its surface, there is a patch of three residues, Val113, Leu116, and Ser120, which may form intramolecular contacts that help to stabilize the inactive ground state of the receptor. Mutagenic disruption of these increased agonist affinity and signaling efficacy. In two cases (L116A and S120A), this led to constitutive activation of the receptor. Parallel to the helix axis and spanning the whole transmembrane region, a distinct strip of residues on one face of transmembrane domain 3 forms intermolecular (acetylcholine-receptor, receptor-G protein) or intrareceptor bonds that contribute to the activated state. The binding of acetylcholine may destabilize the first set of contacts while favoring the formation of the second.

Multiple activation steps of the N-formyl peptide receptor

The human N-formyl peptide receptor (FPR) is representative of a growing family of G protein-coupled receptors (GPCR) that respond to chemokines and chemoattractants. Despite the importance of this receptor class to immune function, relatively little is known about the molecular mechanisms involved in their activation. To reveal steps required for the activation of GPCR receptors, we utilized mutants of the FPR which have previously been shown to be incapable of binding and activating G proteins. For this study, the FPR mutants were expressed in human myeloid U937 cells and characterized for functions in addition to G protein coupling, such as receptor phosphorylation and ligand-induced receptor internalization. The results demonstrated that one of the mutants, R123G, though being unable to activate G protein, was capable of undergoing ligand-induced phosphorylation as well as internalization. Receptor internalization was monitored by following the fate of the ligand as well as by directly monitoring the fate of the receptor. The results with the R123G mutant were in contrast to those obtained for mutants D71A and R309G/E310A/R311G which, though being expressed at the cell surface and binding ligand, were incapable of being phosphorylated or internalized upon agonist stimulation. These results suggest that following ligand binding at least two "steps" are required for full activation of the wild-type FPR. That these observations may be of more general importance in GPCR-mediated signaling is suggested by the highly conserved nature of the mutants studied: D71, R123, and the site represented by amino acids 309-311 are very highly conserved throughout the entire superfamily of G protein-coupled receptors. Models of receptor activation based on the observed results are discussed.

Role of the third intracellular loop for the activation of gonadotropin receptors

Hyperfunctional endocrine thyroid and testicular disorders can frequently be traced back to gainof-function mutations in glycoprotein hormone receptor genes. Deletion mutations in the third intracellular (i3) loop of the TSH receptor have recently been identified as a cause of constitutive receptor activity. To examine whether the underlying mechanism of receptor activation applies to all glycoprotein hormone receptors, we created deletion mutations in the LH and FSH receptors. In analogy to the situation with the TSH receptor, a deletion of nine amino acids resulted in constitutive activity irrespective of the location of deletions within the i3 loop of the LH receptor. In contrast, only one (delta563-566) of four different 4-amino acid deletion mutants displayed agonist-independent activity. Systematic examination of the structural requirements for this effect in the delta563-566 mutant revealed that only deletions including D564 resulted in constitutive receptor activity. Replacement of D564 by G, K, and N led to agonist-independent cAMP formation while introduction of a negatively charged E silenced constitutive receptor activity, indicating that an anionic amino acid at this position may be required to maintain an inactive receptor conformation. Insertion of A residues up- and downstream of D564 did not perturb receptor quiescence, showing that a certain degree of spatial freedom of the negatively charged amino acid within the context of the i3 loop is well tolerated. In contrast to the results obtained with the LH receptor, deletion of the corresponding D567 from the i3 loop of the FSH receptor did not cause constitutive receptor activation, highlighting significant differences in the activation mechanism of gonadotropin receptors.

Double mutant cycle analysis of aspartate 69, 97, and 103 to asparagine mutants in the m2 muscarinic acetylcholine receptor

Double mutant cycles provide a method for analyzing the effects of a mutation at a defined position in the protein structure on the properties of an amino acid at a second site. This approach was used to map potential interactions between aspartates 69, 97, and 103 in the m2 muscarinic acetylcholine receptor transmembrane helices 2 and 3. Receptors containing single and double aspartate to asparagine mutants were expressed in Chinese hamster ovary cells and their effects on ligand binding, signal transduction, and thermal stability determined. Analysis of the double mutant cycles showed that the mutations had approximately additive effects on ligand binding, signal transduction, and thermal stability. Ligand binding and thermal inactivation results support the conclusion that aspartate-103 is the ligand amine counterion. Effector coupling properties of the mutant receptors showed that aspartate-103 was also required for signal transduction activity. The mutation of aspartate-69 to asparagine completely eliminated signal transduction by the agonists acetylcholine, carbachol, and pilocarpine but not oxotremorine M, which caused reduced but significant inhibition of adenylyl cyclase and stimulation of phospholipase C. In contrast, adenylyl cyclase stimulation by the asparagine-69 mutant was elicited only by acetylcholine and carbachol but not by oxotremorine M. The variation in agonist-dependent effector coupling properties provides evidence that the asparagine-69 mutant can exist in activated receptor states that are different from the wild-type m2 muscarinic receptor.

Molecular basis of receptor/G-protein-coupling selectivity

Molecular cloning studies have shown that G-protein-coupled receptors form one of the largest protein families found in nature, and it is estimated that approximately 1000 different such receptors exist in mammals. Characteristically, when activated by the appropriate ligand, an individual receptor can recognize and activate only a limited set of the many structurally closely related heterotrimeric G-proteins expressed within a cell. To understand how this selectivity is achieved at a molecular level has become the focus of an ever increasing number of laboratories. This review provides an overview of recent structural, molecular genetic, biochemical, and biophysical studies that have led to novel insights into the molecular mechanisms governing receptor-mediated G-protein activation and receptor/G-protein coupling selectivity.

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.

Scanning mutagenesis of transmembrane domain 3 of the M1 muscarinic acetylcholine receptor

Scanning mutagenesis of transmembrane domain 3 of the M1 muscarinic acetylcholine receptor has revealed a highly-differentiated alpha-helical structure. Lipid-facing residues are distinguished from a patch of residues which selectively stabilise the ground state of the receptor, and from a band of amino acids extending the full length of the helix, which contribute to the active agonist-receptor-G protein complex. The most important residues are strongly conserved in the GPCR superfamily.

Development of a three-dimensional CysLT1 (LTD4) antagonist model with an incorporated amino acid residue from the receptor

This paper describes the molecular modeling of leukotriene CysLT1 (or LTD4) receptor antagonists. Several different structural classes of CysLT1 antagonists were superimposed onto the new and highly rigid CysLT1 antagonist 8-carboxy-3'-[2-(2-quinolinyl)ethenyl]flavone (1, VUF 5017) to generate a common pharmacophoric arrangement. On the basis of known structure-activity relationships of CysLT1 antagonists, the quinoline nitrogen (or a bioisosteric equivalent thereof) and an acidic function were taken as the matching points. In order to optimize the fitting of acidic moieties of all antagonists, an arginine residue from the receptor was proposed as the interaction site for the acidic moieties. Incorporation of this amino acid residue into the model revealed additional interactions between the guanidine group and the nitrogen atoms of quinoline-containing CysLT1 antagonists. In some cases, the arginine may even interact with pi-clouds of phenyl residues of CysLT1 antagonists. The alignment of Montelukast (MK-476) suggests the presence of an additional pocket in the binding site for CysLT1 antagonists. The derived model should be useful for a better understanding of the molecular recognition of the leukotriene CysLT1 receptor.

Rhodopsin arginine-135 mutants are phosphorylated by rhodopsin kinase and bind arrestin in the absence of 11-cis-retinal

Arginine-135, located at the border between the third transmembrane domain and the second cytoplasmic loop of rhodopsin, is one of the most highly conserved amino acids in the family of G protein-coupled receptors. The effect of mutation at Arg-135 on the ability of rhodopsin to undergo desensitization was investigated. Four mutants, R135K, R135Q, R135A, and R135L, were examined for their ability to be phosphorylated by rhodopsin kinase, to bind arrestin, and to activate the rod cell G protein, transducin (Gt). All of the mutants were phosphorylated, bound arrestin, and were able to activate Gt when reconstituted with 11-cis-retinal. Surprisingly, several of the mutants could be phosphorylated by rhodopsin kinase and could bind arrestin in the absence of 11-cis-retinal but were not able to activate Gt. These observations represent the first demonstration of a mutant G protein-coupled receptor that assumes a conformation able to interact with its G protein-coupled receptor kinase and arrestin, but not with its G protein, in the absence of ligand.

Review: amino acid domains involved in constitutive activation of G-protein-coupled receptors

Guanine nucleotide-binding protein-coupled receptors may attain an active conformation in the absence of agonist by spontaneous isomerization and thus yield constitutive, agonist-independent, activity. This has mainly been demonstrated for isolated membranes and recombinant wild-type receptors, and mutant receptors. They generally show remarkable increases in the sensitivity of a biological response. The location of activating mutations both within a single receptor and across receptors is widespread, with changes reported in the seven-transmembrane domains, the second and third intracellular loop. For most of these receptors, examples of ligands defined as inverse agonists have been documented. Regulation of these receptors by inverse agonists opposite to that observed by agonists, and the therapeutic potential of inverse agonists is underlined.

The role of the aspartate-arginine-tyrosine triad in the m1 muscarinic receptor: mutations of aspartate 122 and tyrosine 124 decrease receptor expression but do not abolish signaling

An Asp-Arg-Tyr triad occurs in a majority of rhodopsin-like G protein-coupled receptors. The fully conserved Arg is critical for G protein activation, but the function of the flanking residues is not well understood. We expressed in COS-7 cells m1 muscarinic receptors that were mutated at Asp122 and Tyr124. Most mutations at either position strongly attenuated or prevented the expression of binding sites for the antagonist [3H]N-methylscopolamine. However, sites that were expressed displayed unaltered affinity for the antagonist. Receptor protein, visualized with a carboxyl-terminally directed antibody, was reduced but never completely abolished. The effects of these mutations were partially reversed by the deletion of 129 amino acids from the third intracellular loop of the receptor. In several cases, comparison of immunocytochemistry with binding measurements suggested the presence of substantial amounts of inactive, presumably misfolded, receptor protein. Some of the variants that bound [3H]N-methylscopolamine underwent small changes in their affinities for acetylcholine. All retained nearly normal abilities to mediate an acetylcholine-induced phosphoinositide response. We propose that Asp122 and Tyr124 make intramolecular contacts whose integrity is important for efficient receptor folding but that they do not participate directly in signaling. The role of these residues is completely distinct from that of Arg123, whose mutation abolishes signaling without diminishing receptor expression.

The affinity of adenosine for the high- and low-affinity states of the human adenosine A1 receptor

The affinity of adenosine for the human adenosine A1 receptor expressed on Chinese hamster ovary cell membranes has been measured in the presence and absence of GTP. The competitive effect of endogenous adenosine on the binding properties of adenosine A1 receptors was estimated from differences in the binding of N6-cyclohexyladenosine measured in the absence and presence of adenosine deaminase. From these data, the affinity of adenosine for the high- and low-affinity states of the human adenosine A1 receptor (7 x 10(7) and 1.3 x 10(5) M-1, respectively) was calculated.

The effects of saponin on the binding and functional properties of the human adenosine A1 receptor

1. Experiments with adenosine deaminase suggest that adenosine is present in membrane preparations from CHO cells bearing adenosine A1 receptors. 2. Pretreatment of the membranes (ca 0.6 mg protein ml-1) with the permeabilizing agent saponin (100 micrograms ml-1) or addition of saponin (10 micrograms ml-1) to the membranes (0.02-0.08 mg protein ml-1) in the assay, generates homogeneous low affinity agonist binding curves in the presence of GTP and an increased function, assessed by agonist stimulation of [35S]-GTP gamma S binding. The affinity constants for the binding of an agonist and an antagonist are not affected by this saponin treatment. Saponin facilitates the interaction of guanine nucleotides with receptor G-protein complexes, possibly by removing a permeability barrier to access of G-proteins by GTP. However, adenosine is still present in the binding assays after saponin treatment. 3. The agonist binding properties of the human A1 receptor have been characterized. In saponin pretreated membranes, 80-90% of the A1 receptors are capable of forming agonist-receptor-G protein complexes in the absence of GTP. These complexes have a 300-600 fold higher affinity than uncoupled receptors for N6-cyclohexyladenosine. 4. A very slow component is observed in the association and dissociation kinetics of the agonist [3H]-N6-cyclohexyladenosine ([3H]-CHA) and in the association but not dissociation kinetics of the antagonist [3H]-8-cyclopentyl-1,3-dipropylxanthine ([3H]-DPCPX). The slow association component of [3H]-DPCPX is essentially absent when incubations are carried out in the presence of GTP. The slow dissociation component of [3H]-CHA binding is rapidly disrupted by GTP. 5. It is hypothesized that long-lasting adenosine-receptor-G protein complexes are present in the CHO membrane preparations. The existence of these complexes, resistant to the action of adenosine deaminase but sensitive to GTP, may rationalize the observed kinetics and the increase in 3H-antagonist binding produced by GTP which has been observed in essentially all studies of A1 receptors and has been ascribed previously to precoupling of A1 receptors to G-proteins in the absence of agonists.