Use of probes with fluorescence indicator distributed throughout the pharmacophore to examine the peptide agonist-binding environment of the family B G protein-coupled secretin receptor

Impact of secretin receptor homo-dimerization on natural ligand binding

Class B G protein-coupled receptors can form dimeric complexes important for high potency biological effects. Here, we apply pharmacological, biochemical, and biophysical techniques to cells and membranes expressing the prototypic secretin receptor (SecR) to gain insights into secretin binding to homo-dimeric and monomeric SecR. Spatial proximity between peptide and receptor residues, probed by disulfide bond formation, demonstrates that the secretin N-terminus moves from adjacent to extracellular loop 3 (ECL3) at wild type SecR toward ECL2 in non-dimerizing mutants. Analysis of fluorescent secretin analogs demonstrates stable engagement of the secretin C-terminal region within the receptor extracellular domain (ECD) for both dimeric and monomeric receptors, while the mid-region exhibits lower mobility while docked at the monomer. Moreover, decoupling of G protein interaction reduces mobility of the peptide mid-region at wild type receptor to levels similar to the mutant, whereas it has no further impact on the monomer. These data support a model of peptide engagement whereby the ability of SecR to dimerize promotes higher conformational dynamics of the peptide-bound receptor ECD and ECLs that likely facilitates more efficient G protein recruitment and activation, consistent with the higher observed functional potency of secretin at wild type SecR relative to the monomeric mutant receptor.

Design, synthesis and characterization of a fluorescently labeled functional analog of full-length human ghrelin

Human ghrelin receptor (GHSR) is a recognized prospective target in the diagnosis and therapy of multiple cancer types. To gain a better understanding of this receptor signaling system, we have synthesized a novel full-length ghrelin analog that is fluorescently labeled at the side-chain of a C-terminal cysteine extension. This analog exhibited nanomolar affinity and potency for the ghrelin receptor. It shows comparable efficacy with that of endogenous ghrelin. The fluorescently-labeled ghrelin analog is a valuable tool for in vitro imaging of cell lines that express ghrelin receptor.

Probing the pharmacology of G protein-coupled receptors with fluorescent ligands

G protein-coupled receptors control a wide range of physiological processes and are the target for many clinically used drugs. Understanding the way in which receptors bind agonists and antagonists, their organisation in the membrane and their regulation after agonist binding are important properties which are key to developing new drugs. One way to achieve this knowledge is through the use of fluorescent ligands, which have been used to study the expression and function of receptors in endogenously expressing systems. Fluorescent ligands with appropriate imaging properties can be used in conjunction with confocal microscopy to investigate the regulation of receptors after activation. Alternatively, through the use of single molecule microscopy, they can probe the spatial organisation of receptors within the membrane. This review focuses on the techniques in which fluorescent ligands have been used and the novel aspects of G protein-coupled receptor pharmacology which have been uncovered. This article is part of the Special Issue entitled 'Fluorescent Tools in Neuropharmacology'.

Fluorescent approaches for understanding interactions of ligands with G protein coupled receptors

G protein coupled receptors are responsible for a wide variety of signaling responses in diverse cell types. Despite major advances in the determination of structures of this class of receptors, the underlying mechanisms by which binding of different types of ligands specifically elicits particular signaling responses remain unclear. The use of fluorescence spectroscopy can provide important information about the process of ligand binding and ligand dependent conformational changes in receptors, especially kinetic aspects of these processes that can be difficult to extract from X-ray structures. We present an overview of the extensive array of fluorescent ligands that have been used in studies of G protein coupled receptors and describe spectroscopic approaches for assaying binding and probing the environment of receptor-bound ligands with particular attention to examples involving yeast pheromone receptors. In addition, we discuss the use of fluorescence spectroscopy for detecting and characterizing conformational changes in receptors induced by the binding of ligands. Such studies have provided strong evidence for diversity of receptor conformations elicited by different ligands, consistent with the idea that GPCRs are not simple on and off switches. This diversity of states constitutes an underlying mechanistic basis for biased agonism, the observation that different stimuli can produce different responses from a single receptor. It is likely that continued technical advances will allow fluorescence spectroscopy to play an important role in continued probing of structural transitions in G protein coupled receptors. This article is part of a Special Issue entitled: Structural and biophysical characterisation of membrane protein-ligand binding.

Membrane cholesterol affects stimulus-activity coupling in type 1, but not type 2, CCK receptors: use of cell lines with elevated cholesterol

The lipid microenvironment of membrane proteins can affect their structure, function, and regulation. We recently described differential effects of acute modification of membrane cholesterol on the function of type 1 and 2 cholecystokinin (CCK) receptors. We now explore the regulatory impact of chronic cholesterol modification on these receptors using novel receptor-bearing cell lines with elevated membrane cholesterol. Stable CCK1R and CCK2R expression was established in clonal lines of 25RA cells having gain-of-function in SCAP [sterol regulatory element binding protein (SREBP) cleavage-activating protein] and SRD15 cells having deficiencies in Insig-1 and Insig-2 enzymes affecting HMG CoA reductase and SREBP. Increased cholesterol in the plasma membrane of these cells was directly demonstrated, and receptor binding and signaling characteristics were shown to reflect predicted effects on receptor function. In both environments, both types of CCK receptors were internalized and recycled normally in response to agonist occupation. No differences in receptor distribution within the membrane were appreciated at the light microscopic level in these CHO-derived cell lines. Fluorescence anisotropy was studied for these receptors occupied by fluorescent agonist and antagonist, as well as when tagged with YFP. These studies demonstrated increased anisotropy of the agonist ligand occupying the active state of the CCK1R in a cholesterol-enriched environment, mimicking fluorescence of the uncoupled, inactive state of this receptor, while there was no effect of increasing cholesterol on fluorescence at the CCK2R. These cell lines should be quite useful for examining the functional characteristics of potential drugs that might be used in an abnormal lipid environment.

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.

Cholesterol-induced conformational changes in the oxytocin receptor

Recent studies suggest that cholesterol binding is widespread among GPCRs (G-protein-coupled receptors). In the present study, we analysed putative cholesterol-induced changes in the OTR [OT (oxytocin) receptor], a prototype of cholesterol-interacting GPCRs. For this purpose, we have created recombinant OTRs that are able to bind two small-sized fluorescence-labelled ligands simultaneously. An OTR antagonist was chosen as one of the ligands. To create a second ligand-binding site, a small-sized α-BTB (bungarotoxin binding) site was inserted at the N-terminus or within the third extracellular loop of the OTR. All receptor constructs were functionally active and bound both ligands with high affinity in the nanomolar range. Measurements of the quenching behaviour, fluorescence anisotropy and energy transfer of both receptor-bound ligands were performed to monitor receptor states at various cholesterol concentrations. The quenching studies suggested no major changes in the molecular environment of the fluorophores in response to cholesterol. The fluorescence anisotropy data indicated that cholesterol affects the dynamics or orientation of the antagonist. The energy transfer efficiency between both ligands clearly increased with increasing cholesterol. Overall, cholesterol induced both a changed orientation and a decreased distance of the receptor-bound ligands, suggesting a more compact receptor state in association with cholesterol.

Molecular basis of secretin docking to its intact receptor using multiple photolabile probes distributed throughout the pharmacophore

The molecular basis of ligand binding and activation of family B G protein-coupled receptors is not yet clear due to the lack of insight into the structure of intact receptors. Although NMR and crystal structures of amino-terminal domains of several family members support consistency in general structural motifs that include a peptide-binding cleft, there are variations in the details of docking of the carboxyl terminus of peptide ligands within this cleft, and there is no information about siting of the amino terminus of these peptides. There are also no empirical data to orient the receptor amino terminus relative to the core helical bundle domain. Here, we prepared a series of five new probes, incorporating photolabile moieties into positions 2, 15, 20, 24, and 25 of full agonist secretin analogues. Each bound specifically to the receptor and covalently labeled single distinct receptor residues. Peptide mapping of labeled wild-type and mutant receptors identified that the position 15, 20, and 25 probes labeled residues within the distal amino terminus of the receptor, whereas the position 24 probe labeled the amino terminus adjacent to TM1. Of note, the position 2 probe labeled a residue within the first extracellular loop of the receptor, a region not previously labeled, providing an important new constraint for docking the amino-terminal region of secretin to its receptor core. These additional experimentally derived constraints help to refine our understanding of the structure of the secretin-intact receptor complex and provide new insights into understanding the molecular mechanism for activation of family B G protein-coupled receptors.

Fluorescence polarization screening for allosteric small molecule ligands of the cholecystokinin receptor

The success in screening for drug candidates is highly dependent on the power of the strategy implemented. In this work, we report and characterize a novel fluorescent benzodiazepine antagonist of the type 1 cholecystokinin receptor (3-(3-(7-fluoro-1-(2-isopropyl(4-methoxyphenyl)amino)-2-oxoethyl)-2,4-dioxo-5-phenyl-2,3,4,5-tetrahydro-1H-benzo[b][1,4]-diazepin-3-yl)ureido)benzoic acid) that can be used as a receptor ligand in a fluorescence polarization assay, which is ideally suited for the identification of small molecule allosteric modulators of this physiologically important receptor. By binding directly to the small molecule-docking region within the helical bundle of this receptor, this indicator can be displaced by many small molecule candidate drugs, even those that might not affect the binding of an orthosteric cholecystokinin-like peptide ligand. The biological, pharmacological, and fluorescence properties of this reagent are described, and proof-of-concept is provided in a fluorescence polarization assay utilizing this fluorescent benzodiazepine ligand.

Design and use of fluorescent ligands to study ligand-receptor interactions in single living cells

The interaction of ligands with G protein-coupled receptors (GPCRs) has been traditionally studied using radiolabelled variants of receptor ligands. More recently, increased knowledge about the way in which GPCRs exist in a highly organised membrane environment has led to an interest in investigating receptor-ligand interactions in single cells. In addition, substantial improvements in imaging technology and an increase in the expense of radioactive waste disposal have resulted in an expansion in the use of fluorescent technologies. One major requirement for these methods is a suitable fluorescent ligand for the receptor of interest. The design of fluorescent ligands for GPCRs is complex, and has to take into account their pharmacological, photophysical, and also physicochemical properties. Here, we describe some basic considerations in the design of fluorescent GPCR ligands, including choice of pharmacological template, linker, and fluorophore. In addition, we describe basic protocols for determining the photophysical properties of the ligand and determining the cellular localisation of their interaction with the target receptors. Finally, we provide a basic protocol for using fluorescent GPCR ligands to quantify the number and diffusion of receptor-ligand complexes in small areas of the cell membrane.

Importance of lipid-exposed residues in transmembrane segment four for family B calcitonin receptor homo-dimerization

Dimerization of the prototypic family B G protein-coupled secretin receptor is determined by the lipid-exposed face of transmembrane segment four (TM4), and has substantial functional importance, facilitating G protein coupling. Recently, we demonstrated that the human secretin receptor elicits an inter-receptor bioluminescence resonance energy transfer (BRET) signal with most other human family B peptide receptors, except for the calcitonin receptor. In this study we have explored the occurrence and importance of calcitonin receptor oligomerization. Static and saturation receptor BRET were utilized to demonstrate that, unlike the human calcitonin receptor that does not yield a significant homomeric BRET signal, the rabbit calcitonin receptor exhibits strong resonance energy transfer. Within the lipid-exposed face of TM4, rabbit and human calcitonin receptors differ by a single amino acid (Arg236 in human; His in rabbit), while Thr253 that occurs in human and rabbit calcitonin receptors is unique across family B receptors. Mutating Arg236 or Thr253 of the human calcitonin receptor to residues found in the rabbit calcitonin receptor or the human secretin receptor (R236H, R236Y and T253A) resulted in generation of significant BRET signals. Similarly, mutation of Val250 of the human calcitonin receptor to another key lipid-facing residue found in the secretin receptor (V250I) also increased the receptor BRET signal. These data support the consistent theme of lipid-exposed residues of TM4 being important for the dimerization of the calcitonin receptor. However, rabbit and human calcitonin receptor constructs bound calcitonin and stimulated cAMP similarly, suggesting that differences in BRET could reflect differences in orientation or in the stability of homo-dimeric receptor complexes, which were nevertheless similarly effective in eliciting the functions attributed to that complex. The likelihood of human calcitonin receptor dimerization, even in the absence of a significant BRET signal, was further supported by data demonstrating that the peptide representing TM4 of this receptor that disrupts the rabbit receptor BRET signal, produced a right shift in the cAMP concentration-response curves for both rabbit and human receptors.

Environmentally sensitive fluorescent sensors based on synthetic peptides

Biosensors allow the direct detection of molecular analytes, by associating a biological receptor with a transducer able to convert the analyte-receptor recognition event into a measurable signal. We review recent work aimed at developing synthetic fluorescent molecular sensors for a variety of analytes, based on peptidic receptors labeled with environmentally sensitive fluorophores. Fluorescent indicators based on synthetic peptides are highly interesting alternatives to protein-based sensors, since they can be synthesized chemically, are stable, and can be easily modified in a site-specific manner for fluorophore coupling and for immobilization on solid supports.

Molecular basis of association of receptor activity-modifying protein 3 with the family B G protein-coupled secretin receptor

The three receptor activity-modifying proteins (RAMPs) have been recognized as being important for the trafficking and function of a subset of family B G protein-coupled receptors, although the structural basis for this has not been well established. In the current work, we use morphological fluorescence techniques, bioluminescence resonance energy transfer, and bimolecular fluorescence complementation to demonstrate that the secretin receptor associates specifically with RAMP3, but not with RAMP1 or RAMP2. We use truncation constructs, peptide competition experiments, and chimeric secretin-GLP1 receptor constructs to establish that this association is structurally specific, dependent on the intramembranous region of the RAMP and TM6 and TM7 of this receptor. There were no observed changes in secretin-stimulated cAMP, intracellular calcium, ERK1/2 phosphorylation, or receptor internalization in receptor-bearing COS or CHO-K1 cells in the presence or absence of exogenous RAMP transfection, although the secretin receptor trafficks normally to the cell surface in these cells in a RAMP-independent manner, resulting in both free and RAMP-associated receptor on the cell surface. RAMP3 association with this receptor was shown to be capable of rescuing a receptor mutant (G241C) that is normally trapped intracellularly in the biosynthetic machinery. Similarly, secretin receptor expression had functional effects on adrenomedullin activity, with increasing secretin receptor expression competing for RAMP3 association with the calcitonin receptor-like receptor to yield a functional adrenomedullin receptor. These data provide important new insights into the structural basis for RAMP3 interaction with a family B G protein-coupled receptor, potentially providing a highly selective target for drug action. This may be representative of similar interactions between other members of this receptor family and RAMP proteins.

Use of fluorescence indicators in receptor ligands

Fluorescence techniques can provide insights into the environment of fluorescent indicators incorporated within a ligand as it is bound to its receptor. Fluorescent indicators of different sizes and chemical characteristics ranging from hydrophilic to hydrophobic that are incorporated into a receptor ligand can provide insights into the nature of the binding environment, the surrounding structures, and even into conformational changes associated with receptor activation. Methods for determining fluorescence spectral analysis, fluorescence quenching, fluorescence anisotropy, and fluorescence lifetimes of the ligand probes are described. The applications of these techniques to the CCK(1) receptor occupied by Alexa(488)-CCK are utilized as examples. These methods represent powerful tools to expand our understanding of the structure and function of G protein-coupled receptors.

Functional importance of a structurally distinct homodimeric complex of the family B G protein-coupled secretin receptor

Oligomerization of G protein-coupled receptors has been described, but its structural basis and functional importance have been inconsistent. Here, we demonstrate that the agonist occupied wild-type secretin receptor is predominantly in a guanine nucleotide-sensitive high-affinity state and exhibits negative cooperativity, whereas the monomeric receptor is primarily in a guanine nucleotide-insensitive lower affinity state. We previously demonstrated constitutive homodimerization of this receptor through the lipid-exposed face of transmembrane (TM) IV. We now use cysteine-scanning mutagenesis of 14 TM IV residues, bioluminescence resonance energy transfer (BRET), and functional analysis to map spatial approximations and functional importance of specific residues in this complex. All, except for three helix-facing mutants, trafficked to the cell surface, where secretin was shown to bind and elicit cAMP production. Cells expressing complementary-tagged receptors were treated with cuprous phenanthroline to establish disulfide bonds between spatially approximated cysteines. BRET was measured as an indication of receptor oligomerization and was repeated after competitive disruption of oligomers with TM IV peptide to distinguish covalent from noncovalent associations. Although all constructs generated a significant BRET signal, this was disrupted by peptide in all except for single-site mutants replacing five residues with cysteine. Of these, covalent stabilization of receptor homodimers through positions of Gly(243), Ile(247), and Ala(250) resulted in a GTP-sensitive high-affinity state of the receptor, whereas the same procedure with Ala(246) and Phe(240) mutants resulted in a GTP-insensitive lower affinity state. We propose the existence of a functionally important, structurally specific high-affinity dimeric state of the secretin receptor, which may be typical of family B G protein-coupled receptors.

Application of fluorescence resonance energy transfer techniques to establish ligand-receptor orientation

Fluorescence resonance energy transfer (FRET) has been utilized to determine distances between a fluorescence donor and a fluorescence acceptor having appropriately overlapping spectra. In this chapter, we utilize this approach to establish distances between a fluorescence donor situated in a distinct position within a docked ligand and a fluorescence acceptor situated in a distinct position within its receptor. This technique is applicable to receptor expressed in the environment of an intact cell containing the full complement of signaling and regulatory proteins. A number of controls are necessary, including those establishing the normal function of the modified ligand and receptor, the absence of energy transfer to non-receptor proteins, and the specificity of transfer between the donor of interest and the acceptor of interest. We have utilized the example of FRET between a secretin peptide incorporating Alexa(488) and a secretin receptor construct derivatized with Alexa(568). The latter was prepared by the derivatization of a mono-cysteine-reactive receptor construct with a fluorescent methanethiosulfonate reagent. This approach can provide important spatial information that can be useful in the meaningful docking of a ligand at its receptor.

Dimerization in the absence of higher-order oligomerization of the G protein-coupled secretin receptor

Oligomerization of G protein-coupled receptors has been proposed to affect receptor function and regulation; however, little is known about the molecular nature of such complexes. We previously utilized bioluminescence resonance energy transfer (BRET) to demonstrate that the prototypic Family B secretin receptor can form oligomers. We now explore the order of oligomerization present utilizing unique bimolecular fluorescence complementation and energy transfer techniques. The non-fluorescent carboxyl-terminal and amino-terminal halves of yellow fluorescent protein (YFP) were fused to the carboxyl terminus of the secretin receptor. These constructs bound secretin normally and signaled in response to secretin like wild type receptor. When co-expressed on COS cells, these constructs physically interacted to yield typical YFP fluorescence in biosynthetic compartments and at the plasma membrane, reflecting receptor homo-dimerization. However, the addition of another potential partner in form of Rlu- or CFP-tagged secretin receptor yielded no significant BRET or FRET signal, respectively, under conditions in which intact YFP-tagged secretin receptor yielded such a signal. Absence of higher-order receptor oligomers was further confirmed using saturation BRET techniques. Absence of significant resonance transfer to the secretin receptor homo-dimer was true for carboxyl-terminally-tagged secretin receptor, as well as for receptor incorporating the transfer partner into each of the three distinct intracellular loop domains. These results suggest that the secretin receptor can exist only as a structurally-specific homo-dimer, without being present as higher-order oligomers.

Spatial approximation between secretin residue five and the third extracellular loop of its receptor provides new insight into the molecular basis of natural agonist binding

The amino terminus of class II G protein-coupled receptors plays an important role in ligand binding and receptor activation. Understanding of the conformation of the amino-terminal domain of these receptors has been substantially advanced with the solution of nuclear magnetic resonance and crystal structures of this region of receptors for corticotrophin-releasing factor, pituitary adenylate cyclase-activating polypeptide, and gastric inhibitory polypeptide. However, the orientation of the amino terminus relative to the receptor core and how the receptor gets activated upon ligand binding remain unclear. In this work, we have used photoaffinity labeling to identify a critical spatial approximation between residue five of secretin and a residue within the proposed third extracellular loop of the secretin receptor. This was achieved by purification, deglycosylation, cyanogen bromide cleavage, and sequencing of labeled wild-type and mutant secretin receptors. This constraint has been used to refine our evolving molecular model of secretin docked at the intact receptor, which for the first time includes refined helical bundle and loop regions and reflects a peptide-binding groove within the receptor amino terminus that directs the amino terminus of the peptide toward the receptor body. This model is fully consistent with the endogenous agonist mechanism for class II G protein-coupled receptor activation, where ligand binding promotes the interaction of a portion of the receptor amino terminus with the receptor body to activate it.

Fluorescence Resonance Energy Transfer Analysis of Secretin Docking to Its Receptor

Full structural characterization of G protein-coupled receptors has been limited to rhodopsin, with its uniquely stable structure and ability to be crystallized. For other members of this important superfamily, direct structural insights have been limited to NMR structures of soluble domains. Two members of the Class II family have recently had the structures of their isolated amino-terminal regions solved by NMR, yet it remains unclear how that domain is aligned with the heptahelical transmembrane bundle domain of those receptors. Indeed, three distinct orientations have been suggested for different members of this family. In the current work, we have utilized fluorescence resonance energy transfer to establish the distances between four residues distributed throughout fully biologically active, high affinity analogues of secretin and distinct residues in each of four extracellular regions of the intact secretin receptor. These 16 distance constraints were utilized along with nine photoaffinity labeling spatial approximation constraints to study the three proposed orientations of the peptide-binding amino terminus and helical bundle domains of this receptor. In the best model, the carboxyl terminus of secretin was found to bind in a groove above the beta-hairpin region of the receptor amino terminus, with its amino-terminal end adjacent to the third extracellular loop and top of transmembrane segment VI. This refined model of the intact receptor was also fully consistent with the spatial approximation of the Trp(48)-Asp(49)-Asn(50) endogenous agonist segment with the third extracellular loop region that it has been shown to photolabel. This provides strong evidence for the orientation of peptide-binding and signaling domains of a prototypic Class II G protein-coupled receptor.

Structural basis of natural ligand binding and activation of the Class II G-protein-coupled secretin receptor

The secretin receptor is prototypic of Class II GPCRs (G-protein-coupled receptors), based on its structural and functional characteristics and those of its natural agonist ligand. Secretin represents a linear 27-residue peptide with diffuse pharmacophoric domain. The secretin receptor includes the typical signature sequences for this receptor family within its predicted transmembrane segments and the highly conserved six cysteine residues contributing to three intradomain disulfide bonds within its long N-terminus. This domain is critical for secretin binding based on receptor mutagenesis and photoaffinity labelling studies. Full agonist analogues of secretin incorporating a photolabile moiety at various positions throughout the pharmacophore covalently label residues within this region, while only N-terminal probes have labelled the core helical bundle domain. Combining insights coming from receptor structural studies, peptide structure-activity relationship considerations, photoaffinity labelling, and application of fluorescence techniques has resulted in the development of a working model of the secretin-receptor complex. This supports the initial docking of the peptide agonist within a cleft in the receptor N-terminus, providing the opportunity for an endogenous sequence within that domain to interact with the core of the receptor. This interaction is believed to be key in the molecular basis of conformational change associated with activation of this receptor. The site of action of this endogenous agonist could also provide a possible target for small molecule agonists to act.

Mapping the architecture of secretin receptors with intramolecular fluorescence resonance energy transfer using acousto-optic tunable filter-based spectral imaging

The molecular structure and agonist-induced conformational changes of class II G protein-coupled receptors are poorly understood. In this work, we developed and characterized a series of dual cyan fluorescent protein (CFP)-tagged and yellow fluorescent protein (YFP)-tagged secretin receptor constructs for use in various functional and fluorescence analyses of receptor structural variants. CFP insertions within the first or second intracellular loop domains of this receptor were tolerated poorly or partially, respectively, in receptors tagged with a carboxyl-terminal yellow fluorescent protein that itself had no effect on secretin binding or cAMP production. A similar CFP insertion into the third intracellular loop resulted in a plasma membrane-localized receptor that bound secretin and signaled normally. This fully active third-loop variant exhibited a significant decrease in fluorescence resonance energy transfer signals that were recorded with an acousto-optic tunable filter microscope after exposure to secretin agonist but not to a receptor antagonist. These data demonstrate changes in the relative positions of intracellular structures that support a model for secretin receptor activation.