Role of charged amino acids conserved in the vasoactive intestinal polypeptide/secretin family of receptors on the secretin receptor functionality

The role of GPCRs in bone diseases and dysfunctions

The superfamily of G protein-coupled receptors (GPCRs) contains immense structural and functional diversity and mediates a myriad of biological processes upon activation by various extracellular signals. Critical roles of GPCRs have been established in bone development, remodeling, and disease. Multiple human GPCR mutations impair bone development or metabolism, resulting in osteopathologies. Here we summarize the disease phenotypes and dysfunctions caused by GPCR gene mutations in humans as well as by deletion in animals. To date, 92 receptors (5 glutamate family, 67 rhodopsin family, 5 adhesion, 4 frizzled/taste2 family, 5 secretin family, and 6 other 7TM receptors) have been associated with bone diseases and dysfunctions (36 in humans and 72 in animals). By analyzing data from these 92 GPCRs, we found that mutation or deletion of different individual GPCRs could induce similar bone diseases or dysfunctions, and the same individual GPCR mutation or deletion could induce different bone diseases or dysfunctions in different populations or animal models. Data from human diseases or dysfunctions identified 19 genes whose mutation was associated with human BMD: 9 genes each for human height and osteoporosis; 4 genes each for human osteoarthritis (OA) and fracture risk; and 2 genes each for adolescent idiopathic scoliosis (AIS), periodontitis, osteosarcoma growth, and tooth development. Reports from gene knockout animals found 40 GPCRs whose deficiency reduced bone mass, while deficiency of 22 GPCRs increased bone mass and BMD; deficiency of 8 GPCRs reduced body length, while 5 mice had reduced femur size upon GPCR deletion. Furthermore, deficiency in 6 GPCRs induced osteoporosis; 4 induced osteoarthritis; 3 delayed fracture healing; 3 reduced arthritis severity; and reduced bone strength, increased bone strength, and increased cortical thickness were each observed in 2 GPCR-deficiency models. The ever-expanding number of GPCR mutation-associated diseases warrants accelerated molecular analysis, population studies, and investigation of phenotype correlation with SNPs to elucidate GPCR function in human diseases.

Receptor Activity Modifying Proteins Have Limited Effects on the Class B G Protein-Coupled Receptor Calcitonin Receptor-Like Receptor Stalk

The calcitonin receptor-like receptor (CLR) is a class B G protein-coupled receptor (GPCR) that forms the basis of three pharmacologically distinct receptors, the calcitonin gene-related peptide (CGRP) receptor, and two adrenomedullin (AM) receptors. These three receptors are created by CLR interacting with three receptor activity-modifying proteins (RAMPs). Class B GPCRs have an N-terminal extracellular domain (ECD) and transmembrane bundle that are both important for binding endogenous ligands. These two domains are joined together by a stretch of amino acids that is referred to as the "stalk". Studies of other class B GPCRs suggest that the stalk may act as hinge, allowing the ECD to adopt multiple conformations. It is unclear what the role of the stalk is within CLR and whether RAMPs can influence its function. Therefore, this study investigated the role of this region using an alanine scan. Effects of mutations were measured with all three RAMPs through cell surface expression, cAMP production and, in select cases, radioligand binding and total cell expression assays. Most mutants did not affect expression or cAMP signaling. CLR C127A, N140A, F142A, and L144A impaired cell surface expression with all three RAMPs. T125A decreased the potency of all peptides at all receptors. N128A, V135A, and L139A showed ligand-dependent effects. While the stalk appears to play a role in CLR function, the effect of RAMPs on this region seems limited, in contrast to their effects on the structure of CLR in other receptor regions.

Key interactions by conserved polar amino acids located at the transmembrane helical boundaries in Class B GPCRs modulate activation, effector specificity and biased signalling in the glucagon-like peptide-1 receptor

Class B GPCRs can activate multiple signalling effectors with the potential to exhibit biased agonism in response to ligand stimulation. Previously, we highlighted key TM domain polar amino acids that were crucial for the function of the GLP-1 receptor, a key therapeutic target for diabetes and obesity. Using a combination of mutagenesis, pharmacological characterisation, mathematical and computational molecular modelling, this study identifies additional highly conserved polar residues located towards the TM helical boundaries of Class B GPCRs that are important for GLP-1 receptor stability and/or controlling signalling specificity and biased agonism. This includes (i) three positively charged residues (R3.30227, K4.64288, R5.40310) located at the extracellular boundaries of TMs 3, 4 and 5 that are predicted in molecular models to stabilise extracellular loop 2, a crucial domain for ligand affinity and receptor activation; (ii) a predicted hydrogen bond network between residues located in TMs 2 (R2.46176), 6 (R6.37348) and 7 (N7.61406 and E7.63408) at the cytoplasmic face of the receptor that is important for stabilising the inactive receptor and directing signalling specificity, (iii) residues at the bottom of TM 5 (R5.56326) and TM6 (K6.35346 and K6.40351) that are crucial for receptor activation and downstream signalling; (iv) residues predicted to be involved in stabilisation of TM4 (N2.52182 and Y3.52250) that also influence cell signalling. Collectively, this work expands our understanding of peptide-mediated signalling by the GLP-1 receptor.

The peptide agonist-binding site of the glucagon-like peptide-1 (GLP-1) receptor based on site-directed mutagenesis and knowledge-based modelling

Mutagenesis and molecular pharmacological analysis of the glucagon-like peptide-1 (GLP-1) receptor highlighted several residues involved in peptide agonist recognition. Coupled with a new molecular model of the full-length agonist-docked receptor, the binding site and a pharmacophore for agonist peptides are described.

Glucagon-like peptide-1 (7–36)amide (GLP-1) plays a central role in regulating blood sugar levels and its receptor, GLP-1R, is a target for anti-diabetic agents such as the peptide agonist drugs exenatide and liraglutide. In order to understand the molecular nature of the peptide–receptor interaction, we used site-directed mutagenesis and pharmacological profiling to highlight nine sites as being important for peptide agonist binding and/or activation. Using a knowledge-based approach, we constructed a 3D model of agonist-bound GLP-1R, basing the conformation of the N-terminal region on that of the receptor-bound NMR structure of the related peptide pituitary adenylate cyclase-activating protein (PACAP21). The relative position of the extracellular to the transmembrane (TM) domain, as well as the molecular details of the agonist-binding site itself, were found to be different from the model that was published alongside the crystal structure of the TM domain of the glucagon receptor, but were nevertheless more compatible with published mutagenesis data. Furthermore, the NMR-determined structure of a high-potency cyclic conformationally-constrained 11-residue analogue of GLP-1 was also docked into the receptor-binding site. Despite having a different main chain conformation to that seen in the PACAP21 structure, four conserved residues (equivalent to His-7, Glu-9, Ser-14 and Asp-15 in GLP-1) could be structurally aligned and made similar interactions with the receptor as their equivalents in the GLP-1-docked model, suggesting the basis of a pharmacophore for GLP-1R peptide agonists. In this way, the model not only explains current mutagenesis and molecular pharmacological data but also provides a basis for further experimental design.

Fish genomes provide novel insights into the evolution of vertebrate secretin receptors and their ligand

The secretin receptor (SCTR) is a member of Class 2 subfamily B1 GPCRs and part of the PAC1/VPAC receptor subfamily. This receptor has long been known in mammals but has only recently been identified in other vertebrates including teleosts, from which it was previously considered to be absent. The ligand for SCTR in mammals is secretin (SCT), an important gastrointestinal peptide, which in teleosts has not yet been isolated, or the gene identified. This study revises the evolutionary model previously proposed for the secretin-GPCRs in metazoan by analysing in detail the fishes, the most successful of the extant vertebrates. All the Actinopterygii genomes analysed and the Chondrichthyes and Sarcopterygii fish possess a SCTR gene that shares conserved sequence, structure and synteny with the tetrapod homologue. Phylogenetic clustering and gene environment comparisons revealed that fish and tetrapod SCTR shared a common origin and diverged early from the PAC1/VPAC subfamily group. In teleosts SCTR duplicated as a result of the fish specific whole genome duplication but in all the teleost genomes analysed, with the exception of tilapia (Oreochromis niloticus), one of the duplicates was lost. The function of SCTR in teleosts is unknown but quantitative PCR revealed that in both sea bass (Dicentrarchus labrax) and tilapia (Oreochromis mossambicus) transcript abundance is high in the gastrointestinal tract suggesting it may intervene in similar processes to those in mammals. In contrast, no gene encoding the ligand SCT was identified in the ray-finned fishes (Actinopterygii) although it was present in the coelacanth (lobe finned fish, Sarcopterygii) and in the elephant shark (holocephalian). The genes in linkage with SCT in tetrapods and coelacanth were also identified in ray-finned fishes supporting the idea that it was lost from their genome. At present SCTR remains an orphan receptor in ray-finned fishes and it will be of interest in the future to establish why SCT was lost and which ligand substitutes for it so that full characterization of the receptor can occur.

Structural and functional insights into the juxtamembranous amino-terminal tail and extracellular loop regions of class B GPCRs

Class B guanine nucleotide-binding protein GPCRs share heptahelical topology and signalling via coupling with heterotrimeric G proteins typical of the entire superfamily of GPCRs. However, they also exhibit substantial structural differences from the more extensively studied class A GPCRs. Even their helical bundle region, most conserved across the superfamily, is predicted to differ from that of class A GPCRs. Much is now known about the conserved structure of the amino-terminal domain of class B GPCRs, coming from isolated NMR and crystal structures, but the orientation of that domain relative to the helical bundle is unknown, and even less is understood about the conformations of the juxtamembranous amino-terminal tail or of the extracellular loops linking the transmembrane segments. We now review what is known about the structure and function of these regions of class B GPCRs. This comes from indirect analysis of structure-function relationships elucidated by mutagenesis and/or ligand modification and from the more direct analysis of spatial approximation coming from photoaffinity labelling and cysteine trapping studies. Also reviewed are the limited studies of structure of some of these regions. No dominant theme was recognized for the structures or functional roles of distinct regions of these juxtamembranous portions of the class B GPCRs. Therefore, it is likely that a variety of molecular strategies can be engaged for docking of agonist ligands and for initiation of conformational changes in these receptors that would be expected to converge to a common molecular mechanism for activation of intracellular signalling cascades.

The role of ECL2 in CGRP receptor activation: a combined modelling and experimental approach

The calcitonin gene-related peptide (CGRP) receptor is a complex of a calcitonin receptor-like receptor (CLR), which is a family B G-protein-coupled receptor (GPCR) and receptor activity modifying protein 1. The role of the second extracellular loop (ECL2) of CLR in binding CGRP and coupling to Gs was investigated using a combination of mutagenesis and modelling. An alanine scan of residues 271–294 of CLR showed that the ability of CGRP to produce cAMP was impaired by point mutations at 13 residues; most of these also impaired the response to adrenomedullin (AM). These data were used to select probable ECL2-modelled conformations that are involved in agonist binding, allowing the identification of the likely contacts between the peptide and receptor. The implications of the most likely structures for receptor activation are discussed.

Structure of the human glucagon class B G-protein-coupled receptor

Binding of the glucagon peptide to the glucagon receptor (GCGR) triggers the release of glucose from the liver during fasting; thus GCGR plays an important role in glucose homeostasis. Here we report the crystal structure of the seven transmembrane helical domain of human GCGR at 3.4 Å resolution, complemented by extensive site-specific mutagenesis, and a hybrid model of glucagon bound to GCGR to understand the molecular recognition of the receptor for its native ligand. Beyond the shared seven transmembrane fold, the GCGR transmembrane domain deviates from class A G-protein-coupled receptors with a large ligand-binding pocket and the first transmembrane helix having a 'stalk' region that extends three alpha-helical turns above the plane of the membrane. The stalk positions the extracellular domain (~12 kilodaltons) relative to the membrane to form the glucagon-binding site that captures the peptide and facilitates the insertion of glucagon's amino terminus into the seven transmembrane domain.

Modulation of the CXC chemokine receptor 4 agonist activity of ubiquitin through C-terminal protein modification

Extracellular ubiquitin has recently been described as a CXC chemokine receptor (CXCR) 4 agonist. Studies on the structure-function relationship suggested that the C-terminus of ubiquitin facilitates CXCR4 activation. It remains unknown, however, whether C-terminal processing of ubiquitin could be biologically relevant and whether modifications of the ubiquitin C-terminus can modulate CXCR4 activation. We show that C-terminal truncated ubiquitin antagonizes ubiquitin and stromal cell-derived factor (SDF)-1α induced effects on cell signaling and function. Reduction of cell surface expression of insulin degrading enzyme (IDE), which cleaves the C-terminal di-Gly of ubiquitin, enhances ubiquitin induced reduction of cAMP levels in BV2 and THP-1 cells, but does not influence changes in cAMP levels in response to SDF-1α. Reduction of cell surface IDE expression in THP-1 cells also increases the chemotactic activity of ubiquitin. As compared with native ubiquitin, C-terminal Tyr extension of ubiquitin results in reduced CXCR4 mediated effects on cellular cAMP levels and abolishes chemotactic activity. Replacement of C-terminal di-Gly of ubiquitin with di-Val or di-Arg enhances CXCR4 mediated effects on cAMP levels and the di-Arg substitution exerts increased chemotactic activity, when compared with wild type ubiquitin. The chemotactic activities of the di-Val and di-Arg mutants and their effects on cAMP levels can be antagonized with C-terminal truncated ubiquitin. These data suggest that the development of CXCR4 ligands with enhanced agonist activities is possible and that C-terminal processing of ubiquitin could constitute a biological mechanism, which regulates termination of receptor signaling.

Calcitonin and calcitonin receptor-like receptors: common themes with family B GPCRs?

The calcitonin receptor (CTR) and calcitonin receptor-like receptor (CLR) are two of the 15 human family B (or Secretin-like) GPCRs. CTR and CLR are of considerable biological interest as their pharmacology is moulded by interactions with receptor activity-modifying proteins. They also have therapeutic relevance for many conditions, such as osteoporosis, diabetes, obesity, lymphatic insufficiency, migraine and cardiovascular disease. In light of recent advances in understanding ligand docking and receptor activation in both the family as a whole and in CLR and CTR specifically, this review reflects how applicable general family B GPCR themes are to these two idiosyncratic receptors. We review the main functional domains of the receptors; the N-terminal extracellular domain, the juxtamembrane domain and ligand interface, the transmembrane domain and the intracellular C-terminal domain. Structural and functional findings from the CLR and CTR along with other family B GPCRs are critically appraised to gain insight into how these domains may function. The ability for CTR and CLR to interact with receptor activity-modifying proteins adds another level of sophistication to these receptor systems but means careful consideration is needed when trying to apply generic GPCR principles. This review encapsulates current thinking in the realm of family B GPCR research by highlighting both conflicting and recurring themes and how such findings relate to two unusual but important receptors, CTR and CLR.

Ligand binding and activation of the secretin receptor, a prototypic family B G protein-coupled receptor

The secretin receptor is a prototypic member of family B G protein-coupled receptors that binds and responds to a linear 27-residue peptide natural ligand. The carboxyl-terminal region of this peptide assumes a helical conformation that occupies the peptide-binding cleft within the structurally complex disulphide-bonded amino-terminal domain of this receptor. The amino terminus of secretin is directed toward the core helical bundle domain of this receptor that seems to be structurally distinct from the analogous region of family A G protein-coupled receptors. This amino-terminal region of secretin is critical for its biological activity, to stimulate Gs coupling and the agonist-induced cAMP response. While the natural peptide ligand is known to span the two key receptor domains, with multiple residue-residue approximation constraints well established, the orientation of the receptor amino terminus relative to the receptor core helical bundle domain is still unclear. Fluorescence studies have established that the mid-region and carboxyl-terminal end of secretin are protected by the receptor peptide-binding cleft and the amino terminus of secretin is most exposed to the aqueous milieu as it is directed toward the receptor core, with the mid-region of the peptide becoming more exposed upon receptor activation. Like other family B peptide hormone receptors, the secretin receptor is constitutively present in a structurally specific homo-dimeric complex built around the lipid-exposed face of transmembrane segment four. This complex is important for facilitating G protein association and achieving the high affinity state of this receptor.

Extracellular loops 1 and 3 and their associated transmembrane regions of the calcitonin receptor-like receptor are needed for CGRP receptor function

The first and third extracellular loops (ECL) of G protein-coupled receptors (GPCRs) have been implicated in ligand binding and receptor function. This study describes the results of an alanine/leucine scan of ECLs 1 and 3 and loop-associated transmembrane (TM) domains of the secretin-like GPCR calcitonin receptor-like receptor which associates with receptor activity modifying protein 1 to form the CGRP receptor. Leu195Ala, Val198Ala and Ala199Leu at the top of TM2 all reduced αCGRP-mediated cAMP production and internalization; Leu195Ala and Ala199Leu also reduced αCGRP binding. These residues form a hydrophobic cluster within an area defined as the “minor groove” of rhodopsin-like GPCRs. Within ECL1, Ala203Leu and Ala206Leu influenced the ability of αCGRP to stimulate adenylate cyclase. In TM3, His219Ala, Leu220Ala and Leu222Ala have influences on αCGRP binding and cAMP production; they are likely to indirectly influence the binding site for αCGRP as well as having an involvement in signal transduction. On the exofacial surfaces of TMs 6 and 7, a number of residues were identified that reduced cell surface receptor expression, most noticeably Leu351Ala and Glu357Ala in TM6. The residues may contribute to the RAMP1 binding interface. Ile360Ala impaired αCGRP-mediated cAMP production. Ile360 is predicted to be located close to ECL2 and may facilitate receptor activation. Identification of several crucial functional loci gives further insight into the activation mechanism of this complex receptor system and may aid rational drug design.

Identification of determinants of glucose-dependent insulinotropic polypeptide receptor that interact with N-terminal biologically active region of the natural ligand

Glucose-dependent insulinotropic polypeptide receptor (GIPR), a member of family B of the G-protein coupled receptors, is a potential therapeutic target for which discovery of nonpeptide ligands is highly desirable. Structure-activity relationship studies indicated that the N-terminal part of glucose-dependent insulinotropic polypeptide (GIP) is crucial for biological activity. Here, we aimed at identification of residues in the GIPR involved in functional interaction with N-terminal moiety of GIP. A homology model of the transmembrane core of GIPR was constructed, whereas a three-dimensional model of the complex formed between GIP and the N-terminal extracellular domain of GIPR was taken from the crystal structure. The latter complex was docked to the transmembrane domains of GIPR, allowing in silico identification of putative residues of the agonist binding/activation site. All mutants were expressed at the surface of human embryonic kidney 293 cells as indicated by flow cytometry and confocal microscopy analysis of fluorescent GIP binding. Mutation of residues Arg183, Arg190, Arg300, and Phe357 caused shifts of 76-, 71-, 42-, and 16-fold in the potency to induce cAMP formation, respectively. Further characterization of these mutants, including tests with alanine-substituted GIP analogs, were in agreement with interaction of Glu3 in GIP with Arg183 in GIPR. Furthermore, they strongly supported a binding mode of GIP to GIPR in which the N-terminal moiety of GIP was sited within transmembrane helices (TMH) 2, 3, 5, and 6 with biologically crucial Tyr1 interacting with Gln224 (TMH3), Arg300 (TMH5), and Phe357 (TMH6). These data represent an important step toward understanding activation of GIPR by GIP, which should facilitate the rational design of therapeutic agents.

Structural diversity of G protein-coupled receptors and significance for drug discovery

G protein-coupled receptors (GPCRs) are the largest family of membrane-bound receptors and also the targets of many drugs. Understanding of the functional significance of the wide structural diversity of GPCRs has been aided considerably in recent years by the sequencing of the human genome and by structural studies, and has important implications for the future therapeutic potential of targeting this receptor family. This article aims to provide a comprehensive overview of the five main human GPCR families--Rhodopsin, Secretin, Adhesion, Glutamate and Frizzled/Taste2--with a focus on gene repertoire, general ligand preference, common and unique structural features, and the potential for future drug discovery.

Identification of the hydrophobic ligand binding pocket of the S1P1 receptor

Sphingosine 1-phosphate (S1P), a naturally occurring sphingolipid mediator and also a second messenger with growth factor-like actions in almost every cell type, is an endogenous ligand of five G protein-coupled receptors (GPCRs) in the endothelial differentiation gene family. The lack of GPCR crystal structures sets serious limitations to rational drug design and in silico searches for subtype-selective ligands. Here we report on the experimental validation of a computational model of the ligand binding pocket of the S1P1 GPCR surrounding the aliphatic portion of S1P. The extensive mutagenesis-based validation confirmed 18 residues lining the hydrophobic ligand binding pocket, which, combined with the previously validated three head group-interacting residues, now complete the mapping of the S1P ligand recognition site. We identified six mutants (L3.43G/L3.44G, L3.43E/L3.44E, L5.52A, F5.48G, V6.40L, and F6.44G) that maintained wild type [32P]S1P binding with abolished ligand-dependent activation by S1P. These data suggest a role for these amino acids in the conformational transition of S1P1 to its activated state. Three aromatic mutations (F5.48Y, F6.44G, and W6.48A) result in differential activation, by S1P or SEW2871, indicating that structural differences between the two agonists can partially compensate for differences in the amino acid side chain. The now validated ligand binding pocket provided us with a pharmacophore model, which was used for in silico screening of the NCI, National Institutes of Health, Developmental Therapeutics chemical library, leading to the identification of two novel nonlipid agonists of S1P1.

Aspartate(69) of the calcitonin-like receptor is required for its functional expression together with receptor-activity-modifying proteins 1 and -2

The calcitonin-like receptor (CLR) associated with receptor-activity-modifying proteins (RAMP) 1 or -2 recognizes calcitonin gene-related peptide (CGRP) and adrenomedullin (AM), respectively. The amino acid sequence CNRTWDGWLCW corresponding to residues 64-74 in the extracellular N-terminus of the CLR is conserved. The Asp(69) (D(69)) is present in all family B1 G-protein-coupled receptors. Here the D(69) of a V5-tagged mouse CLR has been mutated to Ala (A), Glu (E), and Asn (N). The function of the intact and the mutant CLR was investigated in COS-7 cells coexpressing myc-tagged mouse RAMP1 or -2. In CLR/RAMP1 and -2 expressing cells CGRP and AM stimulated cAMP formation with an EC(50) of 0.17 and 0.50 nM, respectively. The expression of the D69A, D69E, and D69N mutants at the cell surface was comparable to that of the intact CLR. cAMP stimulation by CGRP and AM was abolished in the D69A mutant. With the D69E mutant the EC(50) of CGRP and AM were 1000-fold higher than those with the intact CLR. With the D69N mutant the EC(50) of CGRP was 0.48 nM and that of AM 0.44 nM, but the maximal cAMP formation was reduced to 24% and to 12% of cells with the intact CLR. Co-immunoprecipitation of RAMP1 with the CLR, indicating complex formation, was reduced with the D69A, D69N, and D69E mutants. RAMP2 co-precipitated with the mutant receptors indistinguishable from the intact CLR. In conclusion, mutation of D69 to N, E or A in the CLR did not affect its expression at the cell surface, but impaired or abolished the CGRP and AM receptor function in the presence of RAMP1 and -2, respectively.

Lysine 195 and aspartate 196 in the first extracellular loop of the VPAC1 receptor are essential for high affinity binding of agonists but not of antagonists

The role in ligand recognition and receptor activation of two adjacent charged residues (lysine 195 and aspartate 196) in the first extracellular loop of the human VPAC(1) receptor was investigated in stably transfected CHO cells expressing the wild type or point mutated receptors.Replacement of lysine 195 by glutamine or of aspartate 196 by asparagine reduced the agonists' ability to stimulate adenylate cyclase activity; VIP behaved like a partial agonist and a partial agonist behaved as an antagonist. The receptor's capacity to recognize agonists was reduced but antagonists' affinity was unaffected. Both results suggesting that the two charged residues are essential for VPAC(1) receptor activation. On the other hand, the double mutant was less severely affected than single mutants suggesting that hydrogen bonds may partially compensate the loss of charged residues. But the inversion of the residues affected receptor recognition and activation more markedly suggesting that the two charged residues do not interact directly.

The glucagon-like peptide-1 receptor binding site for the N-terminus of GLP-1 requires polarity at Asp198 rather than negative charge

The mutation of Asp198 to Asn in the receptor for glucagon-like peptide-1(7-36)amide (GLP-1) had no effect upon GLP-1 affinity whereas substitution with Ala greatly reduced affinity, demonstrating the importance of polarity rather than negative charge at Asp198. However, the Asp198-Ala mutation had less effect upon the affinity of Exendin-4, a peptide agonist that has been shown previously not to require its N-terminus for high affinity. Moreover, the affinity of a truncated GLP-1 analogue lacking the first eight residues was not affected by the Asp198-Ala mutation, demonstrating that Asp198 is required for maintaining the binding site of the N-terminal region of GLP-1.

Mutational analysis of the glucagon receptor: similarities with the vasoactive intestinal peptide (VIP)/pituitary adenylate cyclase-activating peptide (PACAP)/secretin receptors for recognition of the ligand's third residue

Receptor recognition by the Asp(3) residues of vasoactive intestinal peptide and secretin requires the presence of a lysine residue close to the second transmembrane helix (TM2)/first extracellular loop junction and an ionic bond with an arginine residue in TM2. We tested whether the glucagon Gln(3) residue recognizes the equivalent positions in its receptor. Our data revealed that the binding and functional properties of the wild-type glucagon receptor and the K188R mutant were not significantly different, whereas all agonists had markedly lower potencies and affinities at the I195K mutated receptor. In contrast, glucagon was less potent and the Asp(3)-, Asn(3)- and Glu(3)-glucagon mutants were more potent and efficient at the double-mutated K188R/I195K receptor. Furthermore, these alterations were selective for position 3 of glucagon, as shown by the functional properties of the mutant Glu(9)- and Lys(15)-glucagon. Our results suggest that although the Gln(3) residue of glucagon did not interact with the equivalent binding pocket as the Asp(3) residue of vasoactive intestinal peptide or secretin, the Asp(3)-glucagon analogue was able to interact with position 188 of the K188R/I195K glucagon receptor. Nevertheless, the Gln(3) side chain of glucagon probably binds very close to this region in the wild-type receptor.

Mutational analysis of the human vasoactive intestinal peptide receptor subtype VPAC(2): role of basic residues in the second transmembrane helix

1. We investigated the role of two conserved basic residues in the second transmembrane helix arginine 172 (R172) and lysine 179 (K179) of the VPAC(2) receptor. 2. Vasoactive intestinal polypeptide (VIP) activated VPAC(2) receptors with an EC(50) value of 7 nM, as compared to 150, 190 and 4000 nM at R172L, R172Q and K179Q-VPAC(2) receptors, respectively. It was inactive at K179I mutated VPAC(2) receptors. These results suggested that both basic residues were probably implicated in receptor recognition and activation. 3. The VPAC(2)-selective VIP analogue, [hexanoyl-His(1)]-VIP (C(6)-VIP), had a higher affinity and efficacy as compared to VIP at the mutated receptors. 4. VIP, Asn(3)-VIP and Gln(3)-VIP activated adenylate cyclase through R172Q receptors with EC(50) values of 190, 2 and 2 nM, respectively, and through R172L receptors with EC(50) values of 150, 12 and 8 nM, respectively. Asn(3)-VIP and Gln(3)-VIP behaved as partial agonists at the wild type receptor, with E(max) values (in per cent of VIP) of 75 and 52%, respectively. In contrast, they were more efficient than VIP (E(max) values of 150 and 150% at the R172Q VPAC(2) receptors, and of 400 and 360% at the R172L receptors, respectively). These results suggested that the receptor's R172 and the ligand's aspartate 3 are brought in close proximity in the active ligand-receptor complex. 5. The K179I and K179Q mutated receptors had a lower affinity than the wild-type receptors for all the agonists tested in this work: we were unable to identify the VIP amino acid(s) that interact with K179.

Two basic residues of the h-VPAC1 receptor second transmembrane helix are essential for ligand binding and signal transduction

We mutated the vasoactive intestinal peptide (VIP) Asp(3) residue and two VPAC(1) receptor second transmembrane helix basic residues (Arg(188) and Lys(195)). VIP had a lower affinity for R188Q, R188L, K195Q, and K195I VPAC(1) receptors than for VPAC(1) receptors. [Asn(3)] VIP and [Gln(3)] VIP had lower affinities than VIP for VPAC(1) receptors but higher affinities for the mutant receptors; the two basic amino acids facilitated the introduction of the negatively charged aspartate inside the transmembrane domain. The resulting interaction was necessary for receptor activation. 1/[Asn(3)] VIP and [Gln(3)] VIP were partial agonists at VPAC(1) receptors; 2/VIP did not fully activate the K195Q, K195I, R188Q, and R188L VPAC(1) receptors; a VIP analogue ([Arg(16)] VIP) was more efficient than VIP at the four mutated receptors; and [Asn(3)] VIP and [Gln(3)] VIP were more efficient than VIP at the R188Q and R188L VPAC(1) receptors; 3/the [Asp(3)] negative charge did not contribute to the recognition of the VIP(1) antagonist, [AcHis(1),D-Phe(2),Lys(15),Arg(16),Leu(27)] VIP ()/growth hormone releasing factor (8-27). This is the first demonstration that, to activate the VPAC(1) receptor, the Asp(3) side chain of VIP must penetrate within the transmembrane domain, in close proximity to two highly conserved basic amino acids from transmembrane 2.