Mutational analysis of extracellular cysteine residues of rat secretin receptor shows that disulfide bridges are essential for receptor function

Understanding VPAC receptor family peptide binding and selectivity

The vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) receptors are key regulators of neurological processes. Despite recent structural data, a comprehensive understanding of peptide binding and selectivity among different subfamily receptors is lacking. Here, we determine structures of active, Gs-coupled, VIP-VPAC1R, PACAP27-VPAC1R, and PACAP27-PAC1R complexes. Cryo-EM structural analyses and molecular dynamics simulations (MDSs) reveal fewer stable interactions between VPAC1R and VIP than for PACAP27, more extensive dynamics of VIP interaction with extracellular loop 3, and receptor-dependent differences in interactions of conserved N-terminal peptide residues with the receptor core. MD of VIP modelled into PAC1R predicts more transient VIP-PAC1R interactions in the receptor core, compared to VIP-VPAC1R, which may underlie the selectivity of VIP for VPAC1R over PAC1R. Collectively, our work improves molecular understanding of peptide engagement with the PAC1R and VPAC1R that may benefit the development of novel selective agonists.

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.

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.

Enhancement of plasmid DNA transformation efficiencies in early stationary-phase yeast cell cultures

Chemical-based methods have been developed for transformation of DNA into log-phase cells of the budding yeast Saccharomyces cerevisiae with high efficiency. Transformation of early stationary-phase cells, e.g. cells grown in overnight liquid cultures or as colonies on plates, is less efficient than log-phase cells but is simpler and more adaptable to high-throughput projects. In this study we have tested different approaches for transformation of early stationary-phase cell cultures and identified a method utilizing polyethylene glycol (PEG), lithium acetate and dimethyl sulphoxide (DMSO) as the most efficient. Plasmid DNA transformations using this method could be improved modestly by allowing cells to recover from the chemical treatment in rich broth before plating to selective media. Strong increases in transformation efficiencies were observed when cells were treated briefly with dithiothreitol (DTT). Tests using several different yeast strain backgrounds indicated that DTT treatment could enhance transformation efficiencies by up to 40-fold. Evaluation of multiple parameters affecting the efficiency of the method led to development of an optimized protocol achieving > 50 000 transformants/µg DNA in most backgrounds tested.

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.

Functional coupling of Cys-226 and Cys-296 in the glucagon-like peptide-1 (GLP-1) receptor indicates a disulfide bond that is close to the activation pocket

G protein-coupled receptors (GPCRs) are seven transmembrane α-helical (7TM) integral membrane proteins that play a central role in both cell signaling and in the action of many pharmaceuticals. The crystal structures of several Family A GPCRs have shown the presence of a disulfide bond linking transmembrane helix 3 (TM3) to the second extracellular loop (ECL2), enabling ECL2 to stabilize and contribute to the ligand binding pocket. Family B GPCRs share no significant sequence identity with those in Family A but nevertheless share two conserved cysteines in topologically equivalent positions. Since there are no available crystal structures for the 7TM domain of any Family B GPCR, we used mutagenesis alongside pharmacological analysis to investigate the role of ECL2 and the conserved cysteine residues. We mutated Cys-226, at the extracellular end of TM3 of the glucagon-like peptide-1 (GLP-1) receptor, to alanine and observed a 38-fold reduction in GLP-1 potency. Interestingly, this potency loss was restored by the additional substitution of Cys-296 in ECL2 to alanine. Alongside the complete conservation of these cysteine residues in Family B GPCRs, this functional coupling suggested the presence of a disulfide bond. Further mutagenesis demonstrated that the low potency observed at the C226A mutant, compared with the C226A-C296A double mutant, was the result of the bulky nature of the released Cys-296 side chain. Since this suggested that ECL2 was in close proximity to the agonist activation pocket, an alanine scan of ECL2 was carried out which confirmed the important role of this loop in agonist-induced receptor activation.

Mutational and cysteine scanning analysis of the glucagon receptor N-terminal domain

The glucagon receptor belongs to the B family of G-protein coupled receptors. Little structural information is available about this receptor and its association with glucagon. We used the substituted cysteine accessibility method and three-dimensional molecular modeling based on the gastrointestinal insulinotropic peptide and glucagon-like peptide 1 receptor structures to study the N-terminal domain of this receptor, a central element for ligand binding and specificity. Our results showed that Asp(63), Arg(116), and Lys(98) are essential for the receptor structure and/or ligand binding because mutations of these three residues completely disrupted or markedly impaired the receptor function. In agreement with these data, our models revealed that Asp(63) and Arg(116) form a salt bridge, whereas Lys(98) is engaged in cation-π interactions with the conserved tryptophans 68 and 106. The native receptor could not be labeled by hydrophilic cysteine biotinylation reagents, but treatment of intact cells with [2-(trimethylammonium)ethyl]methanethiosulfonate increased the glucagon binding site density. This result suggested that an unidentified protein with at least one free cysteine associated with the receptor prevented glucagon recognition and that [2-(trimethylammonium)ethyl]methanethiosulfonate treatment relieved this inhibition. The substituted cysteine accessibility method was also performed on 15 residues selected using the three-dimensional models. Several receptor mutants, despite a relatively high predicted cysteine accessibility, could not be labeled by specific reagents. The three-dimensional models show that these mutated residues are located on one face of the protein. This could be part of the interface between the receptor and the unidentified inhibitory protein, making these residues inaccessible to biotinylation compounds.

Secretin: Should we revisit its metabolic outcomes?

Metabolic pathologies such as Type 2 Diabetes have become a major health problem for worldwide populations. Unfortunately, efforts to cure and especially to prevent these significant global problems have so far been met with disappointment. Recently, the involvement of the gut-derived hormonal dysregulation in the development of obesity-related disturbances has been intensively studied. For instance, studies of gut-derived peptides such as peptide YY 3-36, glucagon-like peptide-1, oxyntomodulin and, more recently, ghrelin have significantly improved our understanding of mechanisms underlying weight and metabolic regulation. Even though early reports of the existence of secretin, the first peptide hormone to be described, date back as far as 1825, so much and yet so little is still known about its physiological role in mammals, including humans. However, recent years have provided a better understanding of how the release of secretin is regulated by enteral secretagogues. On the other hand, most basic questions about its role in the post-prandial regulation of metabolic functions in normal and pathophysiological conditions remain to be elucidated. The present work intends to review the physiology of secretin along with its central and peripheral outcomes on metabolic functions.

Signaling mechanisms of secretin receptor

Secretin, a 27-amino acid gastrointestinal peptide, was initially discovered based on its activities in stimulating pancreatic juice. In the past 20 years, secretin was demonstrated to exhibit pleiotropic functions in many different tissues and more importantly, its role as a neuropeptide was substantiated. To carry out its activities in the central nervous system and in peripheral organs, secretin interacts specifically with one known receptor. Secretin receptor, a member of guanine nucleotide-binding protein (G protein)-coupled receptor (GPCR) in the secretin/VIP/glucagon subfamily, possesses the characteristics of GPCR with seven conserved transmembrane domains, a relatively large amino-terminal extracellular domain and an intracellular carboxyl terminus. The structural features and signal transduction pathways of the secretin receptor in various tissues are reviewed in this article.

Combinatorial activity of Flamingo proteins directs convergence and extension within the early zebrafish embryo via the planar cell polarity pathway

The seven-transmembrane protocadherin, Flamingo, functions in a number of processes during Drosophila development, including planar cell polarity (PCP). To assess the role(s) of Flamingo1/Celsr1 (Fmi1) during vertebrate embryogenesis we have exploited the zebrafish system, identifying two Fmi1 orthologues (zFmi1a and zFmi1b) and employing morpholinos to induce mis-splicing of zebrafish fmi1 mRNAs, to both imitate mutations identified in Drosophila flamingo and generate novel aberrant Flamingo proteins. We demonstrate that in the zebrafish gastrula, Fmi1 proteins function in concert with each other and with the vertebrate PCP proteins, Wnt11 and Strabismus, to mediate convergence and extension during gastrulation, without altering early dorso-ventral patterning. We show that zebrafish Fmi1a promotes extension of the entire antero-posterior axis of the zebrafish gastrula including prechordal plate and ventral diencephalic precursors. However, while we show that control over axial extension is autonomous, we find that Fmi1a is not required within lateral cells undergoing dorsal convergence.

Paired cysteine mutagenesis to establish the pattern of disulfide bonds in the functional intact secretin receptor

The amino-terminal domain of class B G protein-coupled receptors contains six conserved cysteine residues involved in structurally and functionally critical disulfide bonds. The mapping of these bonds has been unclear, with one pattern based on biochemical and NMR structural characterizations of refolded, nonglycosylated amino-terminal fragments, and another pattern derived from functional characterizations of intact receptors having paired cysteine mutations. In the present study, we determined the disulfide bonding pattern of the prototypic class B secretin receptor by applying the same paired cysteine mutagenesis approach and confirming the predicted bonding pattern with proteolytic cleavage of intact functional receptor. As expected, systematic mutation to serine of the six conserved cysteine residues within this region of the secretin receptor singly and in pairs resulted in loss of function of most constructs. Notable exceptions were single mutations of the 4th and 6th cysteine residues and paired mutations involving the 1st and 3rd, 2nd and 5th, and 4th and 6th conserved cysteines, with secretin eliciting statistically significant cAMP responses above basal levels of activation for each of these constructs. Immunofluorescence microscopy confirmed similar levels of plasma membrane expression for each of the mutated receptors. Furthermore, cyanogen bromide cleaved a series of wild type and mutant secretin receptors, yielding patterns that agreed with our paired cysteine mutagenesis results. In conclusion, these data suggest the same pattern of disulfide bonding as that predicted previously by NMR and thus support a consistent pattern of amino-terminal disulfide bonds in class B G protein-coupled receptors.

Molecular mechanisms of ligand binding, signaling, and regulation within the superfamily of G-protein-coupled receptors: molecular modeling and mutagenesis approaches to receptor structure and function

The superfamily of G-protein-coupled receptors (GPCRs) could be subclassified into 7 families (A, B, large N-terminal family B-7 transmembrane helix, C, Frizzled/Smoothened, taste 2, and vomeronasal 1 receptors) among mammalian species. Cloning and functional studies of GPCRs have revealed that the superfamily of GPCRs comprises receptors for chemically diverse native ligands including (1) endogenous compounds like amines, peptides, and Wnt proteins (i.e., secreted proteins activating Frizzled receptors); (2) endogenous cell surface adhesion molecules; and (3) photons and exogenous compounds like odorants. The combined use of site-directed mutagenesis and molecular modeling approaches have provided detailed insight into molecular mechanisms of ligand binding, receptor folding, receptor activation, G-protein coupling, and regulation of GPCRs. The vast majority of family A, B, C, vomeronasal 1, and taste 2 receptors are able to transduce signals into cells through G-protein coupling. However, G-protein-independent signaling mechanisms have also been reported for many GPCRs. Specific interaction motifs in the intracellular parts of these receptors allow them to interact with scaffold proteins. Protein engineering techniques have provided information on molecular mechanisms of GPCR-accessory protein, GPCR-GPCR, and GPCR-scaffold protein interactions. Site-directed mutagenesis and molecular dynamics simulations have revealed that the inactive state conformations are stabilized by specific interhelical and intrahelical salt bridge interactions and hydrophobic-type interactions. Constitutively activating mutations or agonist binding disrupts such constraining interactions leading to receptor conformations that associates with and activate G-proteins.

VIGR - a novel inducible adhesion family G-protein coupled receptor in endothelial cells

Using a signal sequence trap for selection of differentially expressed secretory and membrane proteins, we identified a novel member of the adhesion family of G-protein coupled receptors (GPCRs), termed vascular inducible GPCR (VIGR). VIGR contains C1r-C1s, Uegf and Bmp1 (CUB) and pentraxin (PTX)-like modules and a mucin-like spacer, followed by seven transmembrane domains. By surface biotinylation as well as by immunofluorescence analysis we demonstrate that endogenous, highly glycosylated VIGR is expressed on the cell surface of endothelial cells (ECs) upon LPS or thrombin treatment, and inducible expression is mediated by MAP kinases, but not NF-kappaB. We show that VIGR is selectively expressed in ECs derived from larger vessels, but not from microvessels. In summary, VIGR represents a novel GPCR of the adhesion family, which is unique in its long extra-cellular domain comprising CUB and PTX-like modules and in its inducibility by LPS and thrombin in a subset of ECs, suggesting an important function in cell-adhesion and potentially links inflammation and coagulation.

Spatial proximity between a photolabile residue in position 19 of salmon calcitonin and the amino terminus of the human calcitonin receptor

Calcitonins are 32-amino acid peptide hormones with both peripheral and central actions mediated via specific cell surface receptors, which belong to the class II subfamily of G protein-coupled receptors. Understanding receptor function, particularly in terms of ligand recognition by calcitonin receptors, may aid in the rational design of calcitonin analogs with increased potency and improved selectivity. To directly identify sites of proximity between calcitonin and its receptor, we carried out photoaffinity labeling studies followed by protein digestion and mapping of the radiolabeled photoconjugated receptor. A fully active salmon calcitonin analog [Arg(11,18),Bpa19]sCT, incorporating a photolabile p-benzoyl-L-phenylalanine into position 19 of the ligand, has been used to demonstrate spatial proximity between residue 19 of the peptide and the amino-terminal extracellular domain of the receptor. Cyanogen bromide cleavage together with endoproteinase Asp-N digestion indicated that binding was predominantly to the region delimited by receptor residues Cys134 and Met187. Binding to this fragment was supported further by cyanogen bromide-digestion of receptors that were mutated to remove the predicted cleavage site at Met133 (M133A, M133L). Binding within the 54-amino acid fragment was refined further by digestion with endoproteinase Lys-C to the 8-amino acid region corresponding to Cys134-Lys141. These results provide the first direct demonstration of a contact domain between salmon calcitonin and its receptor and will contribute toward modeling of the calcitonin-receptor interface.

Molecular determinants of glucagon receptor signaling

A 29-amino acid polypeptide hormone, glucagon has been one of the most prolific models in the study of hormone action. The key biologic function of glucagon is to counterbalance the actions of insulin and maintain a normal level of serum glucose. Diabetes mellitus can thus be considered a bihormonal disorder with an excess of glucagon contributing to the hyperglycemic state. The effects of glucagon are mediated by the glucagon receptor, which is itself a prototypical member of a distinct category called family B receptors within the G protein-coupled superfamily of seven-helical transmembrane receptors (GPCRs). At the structural level, the peptide ligands of family B receptors are highly homologous, in particular in the N-terminal region of the molecules. The mechanism by which highly homologous peptide ligands selectively recognize their receptors involves distinct molecular interactions that are gradually being elucidated. This review focuses on structural determinants of the glucagon receptor that are important for its activity with respect to interaction with its ligand and G proteins. Information about the glucagon receptor is presented within the context of what is known about other members of the family B GPCRs.

Spatial approximation between a photolabile residue in position 13 of secretin and the amino terminus of the secretin receptor

The amino-terminal domain of class B G protein-coupled receptors is critically important for natural peptide agonist binding and action. The precise role it plays and the molecular basis of the interaction between ligand and this domain are not well understood. In the current work, we have developed a new probe for affinity labeling the secretin receptor through a photolabile benzoyl-phenylalanine residue in position 13. This represented a high affinity ligand (K(i) = 56 +/- 8 nM) that was a potent full agonist to stimulate cellular cAMP (EC(50) = 236 +/- 22 pM). It covalently labeled the secretin receptor saturably in a single site. This was localized to the amino-terminal domain near the first transmembrane segment using a series of chemical and enzymatic digestions. Edman degradation sequencing of radiolabeled cyanogen bromide and skatole digestion products that were attached to glass beads and further cleaved with endoproteinase Asp-N demonstrated that the labeled residue represented Val(103). This is in contrast with previous photoaffinity labeling through positions 6, 18, 22, and 26 of secretin that all labeled the distal end of the amino terminus of this receptor. Together, these five pairs of residue-residue approximations provide important constraints to better understand the molecular conformation of the agonist-bound receptor.

Ab initio modelling of the N-terminal domain of the secretin receptors

G protein coupled receptors of the secretin family are activated by peptide hormones of about 30 residues in length. There is considerable sequence homology within both the hormone and receptor families. The receptors possess in addition to the integral membrane domain a characteristic extracellular domain of about 120 residues in length, having conserved cysteine residues, which are involved in disulphide bridge formation, and tryptophanes, which have been shown to be critical for hormone binding. This extracellular domain does not have detectable homology to any known protein fold. In order to be able to propose a structure for this domain we have used ab initio prediction methods combined with constraints based on experimental results for the disulphide connectivity. The results of computational tools for predicting secondary structure and accessibility, together with ligand binding and mutational data and other structural considerations were used in the ab initio protein folding programs DRAGON and GADGET and also the simpler program RAMBLE, which was able to explore different permutations of disulphide bond connectivity, tryptophan side chain orientation and chain topology. The methods generated a limited number of plausible models but no single unique solution was found under the constraints. One of these was refined into a full atomic model that contained a possible peptide binding site comprising the most conserved residues.

Functional and protein chemical characterization of the N-terminal domain of the rat corticotropin-releasing factor receptor 1

Rat corticotropin-releasing factor receptor 1 (rCRFR1) was produced either in transfected HEK 293 cells as a complex glycosylated protein or in the presence of the mannosidase I inhibitor kifunensine as a high mannose glycosylated protein. The altered glycosylation did not influence the biological function of rCRFR1 as demonstrated by competitive binding of rat urocortin (rUcn) or human/rat corticotropin-releasing factor (h/rCRF) and agonist-induced cAMP accumulation. The low production rate of the N-terminal domain of rCRFR1 (rCRFR1-NT) by transfected HEK 293 cells, was increased by a factor of 100 in the presence of kifunensine. The product, rCRFR1-NT-Kif, bound rUcn specifically (K(D) = 27 nM) and astressin (K(I) = 60 nM). This affinity was 10-fold lower than the affinity of full length rCRFR1. However, it was sufficiently high for rCRFR1-NT-Kif to serve as a model for the N-terminal domain of rCRFR1. With protein fragmentation, Edman degradation, and mass spectrometric analysis, evidence was found for the signal peptide cleavage site C-terminally to Thr(23) and three disulfide bridges between precursor residues 30 and 54, 44 and 87, and 68 and 102. Of all putative N-glycosylation sites in positions 32, 38, 45, 78, 90, and 98, all Asn residues except for Asn(32) were glycosylated to a significant extent. No O-glycosylation was observed.

Analysis of parathyroid hormone (PTH)/secretin receptor chimeras differentiates the role of functional domains in the pth/ pth-related peptide (PTHrP) receptor on hormone binding and receptor activation

The type 1 parathyroid hormore receptor (PTH1r) belongs to the class II family of G protein-coupled receptors. To delineate the sites in the PTH1r's N-terminal region, and the carboxy-core domain (transmembrane segments + extracellular loops) involved in PTH binding, we have evaluated the functional properties of 27 PTH1-secretin chimeras receptors stably expressed in HEK-293 cells. The wild type and chimeric receptors were analyzed for cell surface expression, binding for PTH and secretin, and functional responsiveness (cAMP induction) toward secretin and PTH. The expression levels of the chimeric receptors were comparable to that of the PTH1r (60-100%). The N-terminal region of PTH1r was divided into three segments that were replaced either singly or in various combinations with the homologous region of the secretin receptor (SECr). Substitution of the carboxy-terminal half (residues 105-186) of the N-terminal region of PTH1r for a SECr homologous segment did not reduced affinity for PTH but abolished signaling in response to PTH. This data indicate that receptor activation is dissociable from high affinity hormone binding in the PTH1r, and that the N-terminal region might play a critical role in the activation process. Further segment replacements in the N-termini focus on residues 105-186 and particularly residues 146-186 of PTH1r as providing critical segments for receptor activation. The data obtained suggest the existence of two distinct PTH binding sites in the PTH1r's N-terminal region: one site in the amino-terminal half (residues 1-62) (site 1) that participates in high-affinity PTH binding; and a second site of lower affinity constituted by amino acid residues scattered throughout the carboxy-terminal half (residues 105-186) (site 2). In the absence of PTH binding to site 1, higher concentrations of hormone are required to promote receptor activation. In addition, elimination of the interaction of PTH with site 2 results in a loss of signal transduction without loss of high-affinity PTH binding. Divers substitutions of the extracellular loops of the PTH1r highlight the differential role of the first- and third extracellular loop in the process of PTH1r activation after hormone binding. A chimera containing the entire extracellular domains of the PTH1r and the transmembrane + cytoplasmic domains of SECr had very low PTH binding affinity and did not signal in response to PTH. Further substitution of helix 5 of PTH1r in this chimera increased affinity for PTH that is close to the PTH affinity for the wild-type PTH1r but surprisingly, did not mediate signaling response. Additional substitutions of PTH1r's helices in various combinations emphasize the fundamental role of helix 3 and helix 6 on the activation process of the PTH1r. Overall, our studies demonstrated that several PTH1r domains contribute differentially to PTH binding affinity and signal transduction mechanism and highlight the role of the N-terminal domain and helix 3 and helix 6 on receptor activation.

Dual contacts between peptide agonist ligands and the secretin receptor directly established by photoaffinity labeling

Structural analysis of secretin in solution has demonstrated extended helical domains within both amino- and carboxyl-terminal halves, with a possible turn in between. However, the conformation of this peptide as it resides in its binding site within the receptor has not been established. In the work reported here, we performed affinity labeling of the secretin receptor with radioiodinated secretin analogues having photolabile benzoyl-phenylalanine residues positioned in each half of the peptide. The probes had sites of covalent attachment in positions 6 and 22, and have been recently synthesized and characterized to represent high affinity agonist ligands. Both covalently labeled the secretin receptor in a saturable, specific, and efficient manner. After purification of the labeled receptor, we used a series of chemical and enzymatic cleavage techniques to define the domain of labeling. We complemented this by receptor mutagenesis, followed by additional cleavage and Edman degradation sequencing to refine our insights into the labeled residues. This has allowed us to demonstrate that sites of attachment were both within the extracellular aminoterminal domain of the receptor. Of particular interest, both probes labeled residues within the amino-terminal thirty residues at the distal end of the receptor. It will be particularly interesting to use these molecular approximations to model the binding domain of this important receptor.

Isolation of peptides from porcine intestinal tissue that induce extracellular acidification in CHO cells: identification of the active peptide as IGF-I and characterization of a fragment of calponin H1 processed at a dibasic site

Chinese hamster ovary (CHO) cells are widely used as hosts for receptor expression and pharmacological studies. However, several endogenous receptor populations are present on these cells. Intestinal tissue extracts were found to induce strong extracellular acidification responses (ECAR) in CHO cells, yet several pure hormonal peptides, such as VIP, secretin, CCK, GIP, and galanin were ineffective. It is not known, which are the active compounds in the extracts that can stimulate the extracellular acidification in CHO cells. These active substances may be ligands for yet unknown receptors that are present natively in this cell type. We therefore decided to identify the active compound(s) by isolation from intestinal extract and structural characterization. Using chromatographic separations in combination with microphysiometry we have purified and characterized one such bioactive ligand. Structural analysis indicated that the isolated peptide was identical to insulin-like growth factor I (IGF-I). In the intestine, IGF-I is present in low amounts and has previously been detected only with radioimmunoassays. The results indicate that CHO cells express functional receptors for IGF-I. Among the peptides extracted from the intestine IGF-I is probably the strongest stimulator of ECAR in CHO cells. Moreover, IGF-I acts synergistically with other factors present in the crude tissue extract. Additionally, a fragment of calponin H1 (residues 1-43), previously not described at the protein level, was identified in the IGF-I containing fractions. The fragment was characterized by mass spectrometry and found to be N-terminally modified by acetylation suggesting that the whole protein bears the same posttranslational modification.

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.

Serine and alanine mutagenesis of the nine native cysteine residues of the human A(1) adenosine receptor

To examine the importance of the nine native cysteine residues in the human A(1) adenosine receptor, each cysteine was individually mutated to both serine and alanine. Saturation binding with the A(1) selective antagonist [(3)H]DPCPX [8-cyclopentyl-1,3-di(2, 3-(3)H-propyl)xanthine] resulted in a wild-type K(d) value of 0.92 nM. All serine and alanine mutants had similar K(d) values with the exception of serine/alanine mutations at Cys80 and Cys169. These two cysteine residues, which are highly conserved in G protein-coupled receptors and hypothesized to be linked through a disulfide bridge, demonstrated no detectable binding with [(3)H]DPCPX. Both serine and alanine mutations at residues Cys80 and Cys169 resulted in receptors that were not detectable at the cell surface, as visualized by immunostaining. The serine/alanine mutants that did bind [(3)H]DPCPX were characterized further through competition binding with the antagonist theophylline and the agonists NECA (5'-N-ethylcarboxamidoadenosine) and R-PIA [(R)N(6)-phenylisopropyl adenosine]. The wild-type theophylline K(i) value was 2.41 microM, with the serine/alanine mutants having similar values. Wild-type NECA and R-PIA K(i) values were 0.74 microM and 97.0 nM, respectively. All mutants had K(i) values similar to wild-type with the exception of the Cys85Ser mutant, which had NECA and R-PIA values of 9.30 microM and 387.3 nM, respectively. These data show that Cys80 and Cys169 are absolutely required for delivery of the receptor to the plasma membrane. The Cys85Ser data indicate that although a cysteine is not required at this position, this residue may have an important role in ligand binding or for the structure of the receptor.

Structural insights into the amino-terminus of the secretin receptor: I. Status of cysteine and cystine residues

The secretin receptor is prototypic of the class II family of G protein-coupled receptors, with a long extracellular amino-terminal domain containing six highly conserved Cys residues and one Cys residue (Cys(11)) that is present only in the most closely related family members. This domain is critical for function, with some component Cys residues believed to be involved in key disulfide bonds, although these have never been directly demonstrated. Here, we examine the functional importance of each of these residues and determine their involvement in disulfide bonds. Secretin binding was markedly diminished after treating cells with cell-impermeant reducing reagents, supporting the presence of important extracellular disulfide bonds. To determine whether the amino-terminal domain was covalently attached to the receptor body by disulfide linkage, a strategy was implemented that involved introduction of an acid-labile Asp-Pro sequence to enable specific cleavage at the boundary of these domains. Under nonreducing conditions, the amino terminus was released from the receptor body, supporting the absence of covalent association between these domains. Quantitative [(14)C]iodoacetamide incorporation into the isolated amino-terminal domain of the receptor in the absence and presence of chemical reduction established the ratio of free to total Cys residues as 1:7, consistent with three disulfide bonds. Mutagenesis of each of the amino-terminal Cys residues to Ala was tolerated only for Cys(11), suggesting that these bonds linked the conserved Cys residues. This was further supported by treatment of intact cells expressing wild-type or C11A mutant secretin receptor with a cell-impermeant sulfhydryl-reactive reagent. Thus, the functionally important amino terminus of the secretin receptor represents a structurally independent, highly folded, and disulfide-bonded domain, with a pattern that is likely critical and conserved throughout this receptor family.

Identification of two pairs of spatially approximated residues within the carboxyl terminus of secretin and its receptor

The carboxyl-terminal domains of secretin family peptides have been shown to contain key determinants for high affinity binding to their receptors. In this work, we have examined the interaction between carboxyl-terminal residues within secretin and the prototypic secretin receptor. We previously utilized photoaffinity labeling to demonstrate spatial approximation between secretin residue 22 and the receptor domain that includes the first 30 residues of the amino terminus (Dong, M., Wang, Y., Pinon, D. I., Hadac, E. M., and Miller, L. J. (1999) J. Biol. Chem. 274, 903-909). Here, we further refined the site of labeling with the p-benzoyl-phenylalanine (Bpa(22)) probe to receptor residue Leu(17) using progressive cleavage of wild type and mutant secretin receptors (V13M and V16M) and sequence analysis. We also developed a new probe incorporating a photolabile Bpa at position 26 of secretin, closer to its carboxyl terminus. This analogue was also a potent agonist (EC(50) = 72 +/- 6 pm) and bound to the secretin receptor specifically and with high affinity (K(i) = 10.3 +/- 2.4 nm). It covalently labeled the secretin receptor at a single site saturably and specifically. This was localized to the segment between residues Gly(34) and Ala(41) using chemical and enzymatic cleavage of labeled wild type and A41M mutant receptor constructs and immunoprecipitation of epitope-tagged receptor fragments. Radiochemical sequencing identified the site of covalent attachment as residue Leu(36). These new insights, along with our recent report of contact between residue 6 within the amino-terminal half of secretin and this same amino-terminal region of this receptor (Dong, M., Wang, Y., Hadac, E. M., Pinon, D. I., Holicky, E. L., and Miller, L. J. (1999) J. Biol. Chem. 274, 19161-19167), support a key role for this region, making the molecular details of this interaction of major interest.

Characterization of a novel VPAC(1) selective agonist and identification of the receptor domains implicated in the carboxyl-terminal peptide recognition

Vasoactive Intestinal Polypeptide (VIP) interacts with a high affinity to two subclasses of G protein coupled receptors named VPAC(1) and VPAC(2), and has a 3 - 10 fold preference for VPAC(1) over VPAC(2) receptors. Selective ligands for each receptor subclass were recently described. [R(16)]-PACAP (1 - 23) and [L(22)]-VIP are two selective VPAC(1) agonists. Chimaeric human VPAC(2)-VPAC(1) recombinant receptors expressed in CHO cells were used to identify the receptor domains implicated in these two selective ligands recognition. The VPAC(2) preference for [R(16)]-PACAP (1 - 27) over [R(16)]-PACAP (1 - 23) did not require the receptor's NH(2)-terminus domain but involved the whole transmembrane domain. In contrast, the selectivity of [L(22)]-VIP depended only on the presence of the NH(2) terminus and EC(2) domains of the VPAC(1) receptor. The present data support the idea that in the GPCR-B family of receptors the different selective ligands require different domains for their selectivity, and that the peptides carboxyl terminal sequence (amino acids 24 - 27) folds back on the transmembrane receptor domain, close to the peptides, aminoterminus.

LNB-TM7, a group of seven-transmembrane proteins related to family-B G-protein-coupled receptors

A number of unusual seven-transmembrane molecules have recently been characterized that have significant amino acid sequence similarity within the membrane-spanning hydrophobic regions and intervening loops to members of G-protein-coupled receptor family B. However, in contrast to the family-B G-protein-coupled receptors, these molecules have unusually large N-terminal extracellular domains that contain a number of well- characterized protein modules. The range of cell types expressing these complex molecules and their potential roles in cell adhesion and signalling have become a major focus of research in a number of biological systems.

Role of second extracellular loop in the function of human vasoactive intestinal polypeptide/pituitary adenylate cyclase activating polypeptide receptor 1 (hVPAC1R)

To elucidate the functional role of the second extracellular loop of human vasoactive intestinal polypeptide/pituitary adenylate cyclase-activating polypeptide (VIP/PACAP) receptor (hVPAC1R), surface expression, ligand binding, and receptor activation were analyzed. Amino acids in the entire second extracellular loop were individually substituted by alanine by site-directed mutagenesis. The mutant and wild-type receptors were transiently expressed in HEK293 cells and purified cell membranes were tested for the ability to bind VIP, while the receptor activity was measured as potency of cAMP production analysed on intact cells. Surface expression of the substituted conserved residues, W286A, I289A, W294A, and W295A, was evidently decreased to 20-30% compared to the wild-type expression. W286A also showed an significantly reduced potency of cAMP production. Substituted residues as F280A, E281A, and G284A showed a significant reduction in the potency of stimulated cAMP production amounting to 8-46-fold, compared to the wild-type with unaffected surface expression and VIP binding. These results indicate that some residues in the second extracellular loop of the human VPAC1R participate in the active mechanism of a ligand-mediated response without being directly involved in the binding of VIP.

Vasoactive intestinal polypeptide VPAC1 and VPAC2 receptor chimeras identify domains responsible for the specificity of ligand binding and activation

In order to identify the receptor domains responsible for the VPAC1 selectivity of the VIP1 agonist, [Lys15, Arg16, Leu27] VIP (1-7)/GRF (8-27) and VIP1 antagonist, Ac His1 [D-Phe2, Lys15, Arg16, Leu27] VIP (3-7)/GRF (8-27), we evaluated their binding and functional properties on chimeric VPAC1/VPAC2 receptors. Our results suggest that the N-terminal extracellular domain is responsible for the selectivity of the VIP1 antagonist. Selective recognition of the VIP1 agonist was supported by a larger receptor area: in addition to the N-terminal domain, the first extracellular loop, as well as additional determinants in the distal part of the VPAC1 receptor were involved. Furthermore, these additional domains were critical for an efficient receptor activation, as replacement of EC1 in VPAC1 by its counter part in the VPAC2 receptor markedly reduced the maximal response.

Demonstration of a direct interaction between residue 22 in the carboxyl-terminal half of secretin and the amino-terminal tail of the secretin receptor using photoaffinity labeling

An understanding of the molecular basis of hormonal activation of receptors provides important insights for drug design. Toward this end, intrinsic photoaffinity labeling is a powerful tool to directly identify the ligand-binding domain. We have developed a new radioiodinatable agonist ligand of the secretin receptor that incorporates a photolabile p-benzoyl-L-phenylalanine (Bpa) into the position of Leu22 and have utilized this to identify the adjacent receptor domain. The rat [Tyr10,Bpa22]secretin-27 probe was a fully efficacious agonist, with a potency to stimulate cAMP accumulation by Chinese hamster ovary SecR cells similar to that of natural secretin (EC50 = 68 +/- 22 pM analogue and 95 +/- 25 pM secretin). It bound specifically and with high affinity (Ki = 5.0 +/- 1.1 nM) and covalently labeled the Mr = 57,000-62,000 secretin receptor. Cyanogen bromide cleavage of the receptor yielded a major labeled fragment of apparent Mr = 19,000 that shifted to Mr = 9,000 after deglycosylation. This was most consistent with either of two glycosylated domains within the amino-terminal tail of the receptor. Immunoprecipitation with antibody directed to epitope tags incorporated into each of the candidate domains established that the fragment at the amino terminus of the receptor was the site of labeling. This was further localized to the amino-terminal 30 residues of the receptor by additional proteolysis of this fragment with endoproteinase Lys-C. This provides the first direct demonstration of a contact between a secretin-like agonist and its receptor and will contribute a useful constraint to the modeling of this interaction.

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

The secretin receptor is a member of a large family of G-protein-coupled receptors that recognize polypeptide hormone and/or neuropeptides. Charged, conserved residues might play a key role in their function, either by interacting with the ligand or by stabilizing the receptor structure. Of the four charged amino acids that are conserved in the whole secretin receptor family, D49 and R83 (in the N-terminal domain) were probably important for the secretin receptor structure: replacement of D49 by H or R and of R83 by D severely reduced both the maximal response to secretin and its potency. No functional secretin receptor could be detected after replacement of R83 by L. Mutation of D49 to E, A, or N had no effect or reduced 5-fold the potency of secretin. The highly conserved positive charges found at the extracellular ends of TM III (K194) and IV (R255) were important for the secretin receptor function, as K194 mutation to A or Q and R255 mutation to Q or D decreased the secretin's affinity 15- to 1000-fold, respectively. Six extracellular charged residues are conserved in closely related receptors but not in the whole family. K121 (TM I) and R277 (TM V) were not important for functional secretin receptor expression. D174 (TM II) was necessary to stabilize the active receptor structure: the D174N mutant receptors were unable to stimulate normally the adenylate cyclase in response to secretin, and functional D174A receptors could not be found. Mutation of R255, E259 (second extracellular loop), and E351 (third extracellular loop) to uncharged residues reduced only 10- to 100-fold the secretin potency without changing its efficacy: these residues either stabilized the active receptor conformation or formed hydrogen rather than ionic bonds with secretin. Mutation of K121 (TM I) to Q or L and of R277 (TM V) to E or Q did not affect the receptor functional properties.

Importance of conserved cysteines in the extracellular loops of human PACAP/VIP1 receptor for ligand binding and stimulation of cAMP production

The importance of two highly conserved cysteines in the human pituitary adenylate cyclase activating polypeptide (PACAP)/vasoactive intestinal peptide 1 (VIP1) receptor was examined. Using site-directed mutagenesis, each Cys residue was converted into Ala or Ser. The mutant and wildtype genes were transfected into HEK293 cells and tested for the ability to bind VIP and to activate cAMP production. Cys215Ala/Ser and Cys285Ala/Ser showed at least a tenfold decrease in binding affinity and receptor potency when compared to the wildtype. In contradiction to the wildtype receptor, both mutations were insensitive to dithiothreitol (DTT). The results indicate the existence of a disulfide bond between Cys215 and Cys285, which is important for stabilizing the receptor in the correct conformation for ligand binding and activation.