Identification of two amino acids of the human cholecystokinin-A receptor that interact with the N-terminal moiety of cholecystokinin

The Human Orexin/Hypocretin Receptor Crystal Structures

The human orexin/hypocretin receptors (hOX1R and hOX2R) are G protein-coupled receptors (GPCRs) that mediate the diverse functions of the orexin/hypocretin neuropeptides. Orexins/hypocretins produced by neurons in the lateral hypothalamus stimulate their cognate GPCRs in multiple regions of the central nervous system to control sleep and arousal, circadian rhythms, metabolism, reward pathways, and other behaviors. Dysfunction of orexin/hypocretin signaling is associated with human disease, and the receptors are active targets in a number of therapeutic areas. To better understand the molecular mechanism of the orexin/hypocretin neuropeptides, high-resolution three-dimensional structures of hOX1R and hOX2R are critical. We have solved high-resolution crystal structures of both human orexin/hypocretin receptors bound to high-affinity antagonists. These atomic structures have elucidated how different small molecule antagonists bind with high potency and selectivity, and have also provided clues as to how the native ligands may associate with their receptors. The orexin/hypocretin receptor coordinates, now available to the broader academic and drug discovery community, will facilitate rational design of new therapeutics that modulate orexin/hypocretin signaling in humans.

Milestones in the Activation of a G Protein-Coupled Receptor. Insights from Molecular-Dynamics Simulations into the Human Cholecystokinin Receptor-1

Activation of G protein-coupled receptors (GPCRs) obeys an allosteric mechanism triggered by ligand binding. To understand how the signal is transduced in the cell, identification of the milestones paving the pathway between the active and the inactive states of the receptor is necessary. A model of the human cholecystokinin receptor-1 (CCK1R) has been proposed recently. The complex formed by CCK1R and an agonist ligand will serve as a paradigm of an active conformation to capture milestones in GPCR activation. To reach this goal, assuming microreversibility, the initial step toward the inactivation of CCK1R was modeled using free energy calculations, whereby the ligand is removed from the binding pocket. However accurate the reproduction of the experimental affinity constant, this simulation only represents an embryonic stage of the inactivation process. Starting from the apo receptor, an unprecedented 0.1-μs molecular dynamics trajectory was generated, bereft of experimental biases, bringing into the light key events in the inactivation of CCK1R, chief among which the hydration of its internal cavity, concomitant with the spatial rearrangement of the transmembrane helical segments. Hydration is intimately related to the isomerization of the highly conserved residue W326 of helix VI, acting as a two-state toggle switch, and of residue M121 of helix III. In the active state, the former residue obstructs the crevice, thereby preventing water leakage, which would otherwise trigger the disruption of an ionic lock between helices II and III involving the signature E/DRY motif ubiquitous to GPCRs.

Flexibility and extracellular opening determine the interaction between ligands and insect sulfakinin receptors

Despite their fundamental importance for growth, the mechanisms that regulate food intake are poorly understood. Our previous work demonstrated that insect sulfakinin (SK) signaling is involved in inhibiting feeding in an important model and pest insect, the red flour beetle Tribolium castaneum. Because the interaction of SK peptide and SK receptors (SKR) initiates the SK signaling, we have special interest on the structural factors that influence the SK-SKR interaction. First, the three-dimensional structures of the two T. castaneum SKRs (TcSKR1 and TcSKR2) were generated from molecular modeling and they displayed significance in terms of the outer opening of the cavity and protein flexibility. TcSKR1 contained a larger outer opening of the cavity than that in TcSKR2, which allows ligands a deep access into the cavity through cell membrane. Second, normal mode analysis revealed that TcSKR1 was more flexible than TcSKR2 during receptor-ligand interaction. Third, the sulfated SK (sSK) and sSK-related peptides were more potent than the nonsulfated SK, suggesting the importance of the sulfate moiety.

Cholecystokinin receptors in Atlantic salmon: molecular cloning, gene expression, and structural basis

The peptide hormone cholecystokinin (CCK) exerts a wide range of digestive and CNS-related physiological signaling via CCK receptors in brain and gut. There is very limited information available on these receptors in Atlantic salmon. The aim of this study was to characterize CCK receptors in gut and brain of salmon. We have identified and cloned one CCK-1 receptor and duplicates of CCK-2 receptor in salmon. The phylogenetic analysis indicates the existence of one common ancestor gene for all CCK receptors. CCK-1R mRNA is highly expressed in pancreas followed by midgut, hindgut, gallbladder, and stomach indicating an involvement in pancreatic regulation and gallbladder contractions. CCK-2R1/gastrin mRNA is expressed at high levels in midgut and at relatively low levels in stomach, gallbladder, and pancreas. We postulate CCK-2R1/gastrin receptor to have gastrin-related functions because of its distribution and abundance in gastro-intestinal (GI) tissues. CCK-2R2 is relatively abundant in brain but has low expression levels in gut tissues supporting the hypothesis for involvement in the gut-brain signaling. Major functional motifs and ligand interaction sites in salmon are conserved with that of mammals. This information will be instrumental for comparative studies and further targeting receptor activation and selectivity of biological responses of CCK in salmon.

The CCK(-like) receptor in the animal kingdom: functions, evolution and structures

In this review, the cholecystokinin (CCK)(-like) receptors throughout the animal kingdom are compared on the level of physiological functions, evolutionary basis and molecular structure. In vertebrates, the CCK receptor is an important member of the G-protein coupled receptors as it is involved in the regulation of many physiological functions like satiety, gastrointestinal motility, gastric acid secretion, gall bladder contraction, pancreatic secretion, panic, anxiety and memory and learning processes. A homolog for this receptor is also found in nematodes and arthropods, called CK receptor and sulfakinin (SK) receptor, respectively. These receptors seem to have evolved from a common ancestor which is probably still closely related to the nematode CK receptor. The SK receptor is more closely related to the CCK receptor and seems to have similar functions. A molecular 3D-model for the CCK receptor type 1 has been built together with the docking of the natural ligands for the CCK and SK receptors in the CCK receptor type 1. These molecular models can help to study ligand-receptor interactions, that can in turn be useful in the development of new CCK(-like) receptor agonists and antagonists with beneficial health effects in humans or potential for pest control.

Use of multidimensional fluorescence resonance energy transfer to establish the orientation of cholecystokinin docked at the type A cholecystokinin receptor

Fluorescence resonance energy transfer (FRET) represents a powerful tool to establish relative distances between donor and acceptor fluorophores. By utilizing several donors situated in distinct positions within a docked full agonist ligand and several acceptors distributed at distinct sites within its receptor, multiple interdependent dimensions can be determined. These can provide a unique method to establish or confirm three-dimensional structure of the molecular complex. In this work, we have utilized full agonist analogues of cholecystokinin (CCK) with Aladan distributed throughout the pharmacophore in positions 24, 29, and 33, along with receptor constructs derivatized with Alexa (546) at positions 94, 102, 204, and 341 in the helical bundle and first, second, and third extracellular loops, respectively. These provided 12 FRET distances to overlay on working models of the CCK-occupied receptor. These established that the carboxyl terminus of CCK resides at the external surface of the lipid bilayer, adjacent to the receptor amino-terminal tail, rather than being inserted into the helical bundle. They also provide important experimentally derived constraints for understanding spatial relationships between the docked ligand and the flexible extracellular loop regions. Multidimensional FRET provides a new independent method to establish and refine structural insights into ligand-receptor complexes.

Structural basis of cholecystokinin receptor binding and regulation

Two structurally-related guanine nucleotide-binding protein-coupled receptors for two related peptides, cholecystokinin (CCK) and gastrin, have evolved to exhibit substantial diversity in specificity of ligand recognition, in their molecular basis of binding these ligands, and in their mechanisms of biochemical and cellular regulation. Consistent with this, the CCK1 and CCK2 receptors also play unique and distinct roles in physiology and pathophysiology. The paradigms for ligand recognition and receptor regulation and function are reviewed in this article, and should be broadly applicable to many members of this remarkable receptor superfamily. This degree of specialization is instructive and provides an encouraging basis for the diversity of potential drugs targeting these receptors and their actions that can be developed.

Cholecystokinin 1 receptor modulates the MEKK1-induced c-Jun trans-activation: structural requirements of the receptor

In cells overexpressing active MEKK1 to enhance c-Jun trans-activation, expression of rat cholecystokinin 1 receptor increased the activity of c-Jun while in the same experimental conditions overexpression of mouse cholecystokinin 1 receptor repressed it. This differential trans-activation is specific, since it was not observed for either the other overexpressed kinases (MEK, PKA) or for other transcription factors (ATF2, ELK-1, CREB). This differential behaviour was also detected in a human colon adenocarcinoma cell-line naturally producing high levels of endogenous MEKK1. This differential behaviour between the two receptors on the MEKK1-induced c-Jun trans-activation was independent of the activation state of JNK, of the phosphorylation level of c-Jun and of its ability to bind its specific DNA responsive elements. Two amino acids (Val43 and Phe50 in the mouse cholecystokinin 1 receptor, replaced by Leu43 and Ileu50 in the rat cholecystokinin 1 receptor) localized in the first transmembrane domain were found to play a crucial role in this differential behaviour. MEKK1 probably activates a transcriptional partner of c-Jun whose activity is maintained or increased in the presence of the rat cholecystokinin 1 receptor but repressed in the presence of the mouse cholecystokinin 1 receptor.

Fluorescent Indicators Distributed throughout the Pharmacophore of Cholecystokinin Provide Insights into Distinct Modes of Binding and Activation of Type A and B Cholecystokinin Receptors

Ligand probes with fluorescent indicators positioned throughout the pharmacophoric domain can provide important insights into the molecular basis of receptor binding and activation as reflected in the microenvironment of each indicator while docked at a receptor. We developed three cholecystokinin-like probes with Aladan situated at the N terminus, in the mid-region, and at the C terminus (positions 24, 29, and 33, respectively). These were studied in solution and docked at type A and B cholecystokinin receptors. This study demonstrated clear differences in mechanisms of cholecystokinin binding and activation of these structurally related receptors with distinct agonist structure-activity relationships. The fluorescence characteristics of Aladan are highly sensitive to the polarity of its microenvironment. The mid-region probe was least accessible to the aqueous milieu as determined by fluorescence emission spectra and iodide quenching, which was not altered by changes in conformation from active to inactive. Accessibility of the N- and C-terminal probes was affected by receptor conformation. The position 24 probe was more easily quenched in the active than in the G protein-uncoupled conformation for both receptors. However, the position 33 probe docked at the type A cholecystokinin receptor was more easily quenched in the active conformation, whereas the same probe docked at the type B cholecystokinin receptor was more easily quenched in the inactive conformation. Fluorescence anisotropy and red edge excitation shift determinations confirmed these observations and supported the proposed movements. Although both type A and B cholecystokinin receptors bind cholecystokinin with high affinity, resulting in fully efficacious biological responses, these receptors utilize distinct molecular modes of binding.

Cholecystokinin and gastrin receptors

Cholecystokinin and gastrin receptors (CCK1R and CCK2R) are G protein-coupled receptors that have been the subject of intensive research in the last 10 years with corresponding advances in the understanding of their functioning and physiology. In this review, we first describe general properties of the receptors, such as the different signaling pathways used to exert short- and long-term effects and the structural data that explain their binding properties, activation, and regulation. We then focus on peripheral cholecystokinin receptors by describing their tissue distribution and physiological actions. Finally, pathophysiological peripheral actions of cholecystokinin receptors and their relevance in clinical disorders are reviewed.

Recognition of GPCRs by Peptide Ligands and Membrane Compartments theory: Structural Studies of Endogenous Peptide Hormones in Membrane Environment

One of the largest family of cell surface proteins, G-protein coupled receptors (GPCRs) regulate virtually all known physiological processes in mammals. With seven transmembrane segments, they respond to diverse range of extracellular stimuli and represent a major class of drug targets. Peptidergic GPCRs use endogenous peptides as ligands. To understand the mechanism of GPCR activation and rational drug design, knowledge of three-dimensional structure of receptor–ligand complex is important. The endogenous peptide hormones are often short, flexible and completely disordered in aqueous solution. According to “Membrane Compartments Theory”, the flexible peptide binds to the membrane in the first step before it recognizes its receptor and the membrane-induced conformation is postulated to bind to the receptor in the second step. Structures of several peptide hormones have been determined in membrane-mimetic medium. In these studies, micelles, reverse micelles and bicelles have been used to mimic the cell membrane environment. Recently, conformations of two peptide hormones have also been studied in receptor-bound form. Membrane environment induces stable secondary structures in flexible peptide ligands and membrane-induced peptide structures have been correlated with their bioactivity. Results of site-directed mutagenesis, spectroscopy and other experimental studies along with the conformations determined in membrane medium have been used to interpret the role of individual residues in the peptide ligand. Structural differences of membrane-bound peptides that belong to the same family but differ in selectivity are likely to explain the mechanism of receptor selectivity and specificity of the ligands. Knowledge of peptide 3D structures in membrane environment has potential applications in rational drug design.

Probing a model of a GPCR/ligand complex in an explicit membrane environment: the human cholecystokinin-1 receptor

A three-dimensional model structure of a complex formed by a G-protein-coupled receptor (GPCR) and an agonist ligand is probed and refined using molecular-dynamics simulations and free energy calculations in a realistic environment. The model of the human receptor of cholecystokinin associated to agonist ligand CCK9 was obtained from a synergistic procedure combining site-directed mutagenesis experiments and in silico modeling. The 31-ns molecular-dynamics simulation in an explicit membrane environment indicates that both the structure of the receptor and its interactions with the ligand are robust. Whereas the secondary structure of the alpha-helix bundle is well preserved, the region of the intracellular loops exhibits a significant flexibility likely to be ascribed to the absence of G-protein subunits in the model. New insight into the structural features of the binding pocket is gained, in particular, the interplay of the ligand with both the receptor and internal water molecules. Water-mediated interactions are shown to participate in the binding, hence, suggesting additional site-directed mutagenesis experiments. Accurate free energy calculations on mutated ligands provide differences in the receptor-ligand binding affinity, thus offering a direct, quantitative comparison to experiment. We propose that this detailed consistency-checking procedure be used as a routine refinement step of in vacuo GPCR models, before further investigation and application to structure-based drug design.

Criteria for the design and biological characterization of radiolabeled peptide-based pharmaceuticals

Radiolabeled peptide-based formulations are being evaluated for their application in oncological imaging and therapy using nuclear medicine techniques. A major breakthrough in the field was the discovery and identification of the G-protein coupled receptor superfamily that are overexpressed in a variety of human cancers. These receptors act as targets for endogenous compounds, often of peptidic nature, which can be radiolabeled and, therefore, could potentially be utilized as radiopharmaceuticals. This general strategy has proven successful for application in humans in only a few cases thus far. However, the use of more sophisticated structural methodology to enhance our understanding of the interactions between the receptor and the endogenous peptide or its analogs, and a more efficient preclinical evaluation process, may help to single out the most promising compounds for further development and eventual use in the clinical application of radiopharmaceuticals. This review analyzes current methods of approaching these key points. The rational process for developing peptide-based radiopharmaceuticals is presented, from the structural analysis of the peptide-receptor interaction for the identification and modeling of the peptide analogs to the synthesis, with an appropriate metal carrier, of compounds that mimic endogenous peptides. Finally, the in vitro and in vivo biological testing and evaluation in preclinical animal models is described. To render the entire process successful, expertise in different areas of drug development is indispensable.

Differential spatial approximation between cholecystokinin residue 30 and receptor residues in active and inactive conformations

Understanding the structures of active and inactive agonist- and antagonist-bound receptor complexes is of great interest. In this work, we focus on position 30 of cholecystokinin (CCK) and its spatial approximation with the type A CCK receptor. For this, we developed two photoaffinity labeling probes, replacing the naturally occurring tryptophan with p-benzoyl-l-phenylalanine (Bpa) or p-nitro-phenylalanine (NO(2)-Phe). The Bpa probe was shown to represent an antagonist, whereas the NO(2)-Phe probe stimulated intracellular calcium as a fully efficacious agonist (EC(50) = 81 +/- 15 nM). Both ligands bound to the receptor specifically, although with lower affinity than CCK (K(i) values: Bpa probe, 270 +/- 72 nM; NO(2)-Phe probe, 180 +/- 40 nM). Both probes covalently labeled the receptor in an efficient manner. The Bpa antagonist labeled the receptor in two distinct regions as identified by cyanogen bromide cleavage, with labeled bands migrating at M(r) = 25,000 and 4500. The former represented the glycosylated amino-terminal fragment, with the site of attachment further localized by endoproteinase Lys-C cleavage to the region between Asn(10) and Lys(37). The latter was shown to represent the first extracellular loop using further cleavage and sequencing of the wild-type and a mutant receptor. Following the same approach, the NO(2)-Phe agonist probe was shown to also label the first extracellular loop region. Radiochemical sequencing identified that the Bpa antagonist probe labeled receptor residue Lys(105), whereas the NO(2)-Phe agonist probe labeled residue Leu(99). These data extend our understanding of the molecular basis of binding and the conformational states of this important receptor.

Cross-interactions of two p38 mitogen-activated protein (MAP) kinase inhibitors and two cholecystokinin (CCK) receptor antagonists with the CCK1 receptor and p38 MAP kinase

Although SB202190 and SB203580 are described as specific p38 MAP kinase inhibitors, several reports have indicated that other enzymes are also sensitive to SB203580. Using a pharmacological approach, we report for the first time that compounds SB202190 and SB203580 were able to directly and selectively interact with a G-protein-coupled receptor, namely the cholecystokinin receptor subtype CCK1, but not with the CCK2 receptor. We demonstrated that these compounds were non-competitive antagonists of the CCK1 receptor at concentrations typically used to inhibit protein kinases. By chimeric construction of the CCK2 receptor, we determined the involvement of two CCK1 receptor intracellular loops in the binding of SB202190 and SB203580. We also showed that two CCK antagonists, L364,718 and L365,260, were able to regulate p38 mitogen-activated protein (MAP) kinase activity. Using a reporter gene strategy and immunoblotting experiments, we demonstrated that both CCK antagonists inhibited selectively the enzymatic activity of p38 MAP kinase. Kinase assays suggested that this inhibition resulted from a direct interaction with both CCK antagonists. Molecular modeling simulations suggested that this interaction occurs in the ATP binding pocket of p38 MAP kinase. These results suggest that SB202190 and SB203580 bind to the CCK1 receptor and, as such, these compounds should be used with caution in models that express this receptor. We also found that L364,718 and L365,260, two CCK receptor antagonists, directly interacted with p38 MAP kinase and inhibited its activity. These findings suggest that the CCK1 receptor shares structural analogies with the p38 MAP kinase ATP binding site. They open the way to potential design of either a new family of MAP kinase inhibitors from CCK1 receptor ligand structures or new CCK1 receptor ligands based on p38 MAP kinase inhibitor structures.

Distinct molecular mechanisms for agonist peptide binding to types A and B cholecystokinin receptors demonstrated using fluorescence spectroscopy

Fluorescence spectroscopy provides a direct method for evaluating the environment of a fluorescent ligand bound to its receptor. We utilized this methodology to determine the environment of Alexa within a cholecystokinin (CCK)-like probe (Alexa488-Gly-[(Nle(28,31))CCK-26-33]; CCK-8 probe) bound to the type A CCK receptor (Harikumar, K. G., Pinon, D. L., Wessels, W. S., Prendergast, F. G., and Miller, L. J. (2002) J. Biol. Chem. 277, 18552-18560). Here, we study this probe at the type B CCK receptor and develop another probe with its fluorophore closer to the carboxyl-terminal pharmacophore of type B receptor ligands (Alexa488-Trp-Nle-Asp-Phe-NH2; CCK-4 probe). Both probes bound to type B CCK receptors in a saturable and specific manner and represented full agonists. Similar to the type A receptor, at the type B receptor these probes exhibited shorter lifetimes and lower anisotropy when the receptor was in the active conformation than when it was shifted to its inactive, G protein-uncoupled state using guanosine 5'-[beta,gamma-imido]-triphosphate trisodium salt. Absolute values for lifetime and anisotropy were lower for the CCK-8 probe bound to the type B receptor than for this probe bound to the type A receptor, and Alexa fluorescence was more easily quenched by iodide at the type B receptor. This represents the first direct evidence that, despite having identical affinities for binding and potencies for activating type A and B receptors, CCK is docked via distinct mechanisms, with the amino terminus more exposed to the aqueous milieu when bound to the type B CCK receptor than to the type A CCK receptor. Of interest, despite this difference in binding, activation of both receptors results in analogous direction of movement of the fluorescent indicator probes.

Structure-activity function for binding and signaling in CHO-K1 and COS-7 cells expressing the cholecystokinin A receptor

Key amino acids of the cholecystokinin (CCK) peptide for receptor binding are sulfated Y27, W30, D32, and F33-NH(2). Three-dimensional modeling showed that the CCK-A receptor (CCK-AR) antagonist devazepide penetrated into the transmembrane (TM) domains, whereas CCK was placed on the surface of the CCK-AR. Four types of rat CCK-AR cDNAs were transfected into CHO-K1 and COS-7 cells: normal CCK-AR cDNA transfected cells (wild type, WT); K120 substituted with V; K130V; and R352V. Binding of [3H]CCK-8 was observed in WT and K130V, but not in K120V and R352V. CCK caused Ca(2+) spiking in WT and K130V, whereas K120V and R352V had no effect. Three chimeras including the CCK-AR/3ibeta2 adrenergic receptor (beta2AR), 3Nibeta2AR, and 3Cibeta2AR were constructed. Two groups of point mutations in the CCK-AR3i were also made: Y252V, S274V, S281V, and S289V (non-phospho-acceptor Y or S); S260V, S264V, S271V, and S275V (phospho-acceptor S). WT and CCK-AR/3Cibeta2AR increased [Ca(2+)](i) in response to CCK; 3Nibeta2AR was vice versa. CCK failed to increase [IP(3)] in phospho-acceptor S to V without affecting binding. Non-phospho-acceptor S or Y to V showed normal response. Thus, Lys120 outside the TM2 and Arg352 outside the TM6 of the CCK-AR are amino acids interacting with Tyr[SO(3)H]27 and Asp32 of the CCK peptide for binding. Phospho-acceptor Ser groups in the CCK-AR 3Ni are amino acids for initiating cell signaling.

Measurement of Intermolecular Distances for the Natural Agonist Peptide Docked at the Cholecystokinin Receptor Expressed in Situ Using Fluorescence Resonance Energy Transfer

Fluorescence resonance energy transfer is a powerful biophysical technique used to analyze the structure of membrane proteins. Here, we used this tool to determine the distances between a distinct position within a docked agonist and a series of distinct sites within the intramembranous confluence of helices and extracellular loops of the cholecystokinin (CCK) receptor. Pseudo-wild-type CCK receptor constructs having single reactive cysteine residues inserted into each of these sites were developed. The experimental strategy included the use of the full agonist, Alexa488-CCK, bound to these receptors as donor, with Alexa568 covalently bound to the specific sites within the CCK receptor as acceptor. Site-labeling was achieved by derivatization of intact cells with a novel fluorescent methanethiosulfonate reagent. A high degree of spectral overlap was observed between receptor-bound donor and receptor-derivatized acceptors, with no transfer observed for a series of controls representing saturation of the receptor binding site with nonfluorescent ligand and use of a null-reactive CCK receptor construct. The measured distances between the fluorophore within the docked agonist and the sites within the first (residue 102) and third (residue 341) extracellular loops of the receptor were shorter than those directed to the second loop (residue 204) or to intramembranous helix two (residue 94). These distances were accommodated well within a refined molecular model of the CCK-occupied receptor that is fully consistent with all existing structure-activity and photoaffinity-labeling studies. This approach provides the initial insights into the conformation of extracellular loop regions of this receptor and establishes clear differences from analogous loops in the rhodopsin crystal structure.

Determination of ligand-receptor interactions of cholecystokinin by nuclear magnetic resonance

To date high resolution structural studies of G protein coupled receptors, with the exception of rhodopsin, have not been feasible using conventional spectroscopic techniques. To overcome these difficulties, the structural features of partial or intact domains of GPCRs have been studied by nuclear magnetic resonance spectroscopy and X-ray crystallography. Here, we describe the structural characterization of receptor domains from the cholecystokinin 1 and 2 receptors and the elucidation of intermolecular interactions between the extracellular receptor domains and CCK-8 by solution state nmr.

Identification of tyrosine 189 and asparagine 358 of the cholecystokinin 2 receptor in direct interaction with the crucial C-terminal amide of cholecystokinin by molecular modeling, site-directed mutagenesis, and structure/affinity studies

The cholecystokinin (CCK) receptors CCK1R and CCK2R exert important central and peripheral functions by binding the neuropeptide cholecystokinin. Because these receptors are potential therapeutic targets, great interest has been devoted to the identification of efficient ligands that selectively activate or inhibit these receptors. A complete mapping of the CCK binding site in these receptors would help to design new CCK ligands and to optimize their properties. In this view, a molecular model of the CCK2R occupied by CCK was built to identify CCK2R residues that interact with CCK functional groups. No such study has yet been reported for the CCK2R. Docking of CCK in the receptor was performed by taking into account our previous mutagenesis data and by using, as constraint, the direct interaction that we demonstrated between His207 in the CCK2R and Asp8 of CCK (Mol Pharmacol 54:364-371, 1998; J Biol Chem 274:23191-23197, 1999). Two residues that had not been revealed in our previous mutagenesis studies, Tyr189 (Y4.60) and Asn358 (N6.55), were identified in interaction via hydrogen bonds with the C-terminal amide of CCK, a crucial functional group of the peptide. Mutagenesis of Tyr189 (Y4.60) and Asn358 (N6.55) as well as structure-affinity studies with modified CCK analogs validated these interactions and the involvement of both residues in the CCK binding site. These results indicate that the present molecular model is an important tool to identify direct contact points between CCK and the CCK2R and to rapidly progress in mapping of the CCK2R binding site. Moreover, comparison of the present CCK2R.CCK molecular model with that of CCK1R.CCK, which we have previously published and validated, clearly argues that the positioning of CCK in these receptors is different.

The role of segment 32-47 of cholecystokinin receptor type A in CCK8 binding: synthesis, nuclear magnetic resonance, circular dichroism and fluorescence studies

The segment 32-47 of the N-terminal extracellular domain of the type A cholecystokinn receptor, CCK(A)-R(32-47), was synthesized and structurally characterized in a membrane mimicking environment by CD, NMR and molecular dynamics calculations. The region of CCK(A)-R(32-47) encompassing residues 39-46 adopted a well-defined secondary structure in the presence of DPC micelles, whereas the conformation of the N-terminal region (segment 32-37) could not be uniquely defined by the NOE derived distance constraints because of local flexibility. The conformation of the binding domain of CCK(A)-R(32-47) was different from that found for the Intact N-terminal receptor tail, CCK(A)-R(1-47). To assess whether CCK(A)-R(32-47) was still able to bind the nonsulfated cholecystokinin C-terminal octapeptide, CCK8, a series of titrations was carried out in SDS and DPC micelles, and the binding interaction was followed by fluorescence spectroscopy. These titrations gave no evidence for complex formation, whereas a high binding affinity was found between CCK(A)-R(1-47) and CCK8. The different affinities for the ligand shown by CCK(A)-R(32-47) and CCK(A)-R(1-47) were paralleled by different interaction modes between the receptor segments and the micelles.The interaction of CCK(A)-R(32-47) with DPC micelles was much weaker than that of CCK(A)-R(1-47), because the former receptor segment lacks proper stabilizing contacts with the micelle surface. In the case of SDS micelles CCK(A)-R(32-47] was found to form non-micellar adducts with the detergent that prevented the onset of a functionally significant Interaction between the receptor segment and the micelle. It is concluded that tertiary structure interactions brought about by the 1-31 segment play a key role in the stabilization of the membrane bound, biologically active conformation of the N-terminal extracellular tail of the CCKA receptor.

Rhodopsin crystal: new template yielding realistic models of G-protein-coupled receptors?

Molecular modelling is of major help to understand structure-function data on G-protein-coupled receptors (GPCRs). Since the first determination of the structure of rhodopsin, at high resolution, the view has emerged that it will be now easy to automatically obtain realistic models for any GPCR by homology modeling. Our experience on cholecystokinin CCK(1) receptor modelling together with available data on other GPCRs leads us to rule out this opinion. We believe that construction of realistic models of certain GPCRs still remains time-consuming and requires many refinements of the models in close association with experiments. This conclusion has important consequences for modelling orphan GPCRs.

Moleular models for cholecystokinin-A receptor

Numerous techniques have been used to elucidate the structural basis for interaction of cholecystokinin (CCK)-related peptides with their hormone-binding receptor, the CCK-A receptor (CCK-AR), including structure-activity relationship studies, site-directed mutagenesis, photoaffinity-labeling, and solution NMR analysis of both CCK peptide ligands and peptide fragments derived from the CCK-A receptor. Different structural models have been developed for the peptide-receptor complexes using various subsets of the available experimental data (Giragossian & Mierke 2001; Ding et al. 2002; Escrieut et al. 2002). Here, we review details of the various models and evaluate the impact of selected experimental data sets on model development.

Structure of cholecystokinin receptor binding sites and mechanism of activation/inactivation by agonists/antagonists

Delineation of CCK receptor binding sites is a prerequisite for the understanding of the molecular basis for ligand recognition, partial agonism, ligand-induced traffiking of receptor signalling. In the current paper, we illustrate how, in the past 5 years, studies from our laboratory and others have provided new data on the molecular basis of the pharmacology and functioning of CCK1 and CCK2 receptors. Available data on CCK1 and CCK2R binding sites indicate that 1) homologous regions of the two receptors are involved in the binding site of CCK, however, positioning of CCK slightly differs; 2) binding sites of non-peptide agonists/antagonist are buried in the pocket formed by transmembrane helices and overlap that of CCK and 3) residues of the binding sites as well as of conserved motifs such as E/DRY, NPXXY are crucial for receptor activation.

Molecular basis of agonist binding to the type A cholecystokinin receptor

The receptors for cholecystokinin (CCK) peptides are guanine nucleotide-binding protein-coupled receptors in the rhodopsin/beta-adrenergic receptor family. The molecular basis of natural ligand binding to the type A CCK receptor has been studied using ligand structure-activity series, receptor mutagenesis, and photoaffinity labeling studies. These have focused attention on the extracellular loop and tail domains, with the most direct insights coming from intrinsic photoaffinity labeling studies. A model of the binding of CCK to this receptor is consistent with all these studies. This model places the carboxyl terminus of CCK adjacent to the amino-terminal tail outside of transmembrane segment 1, the mid-region of the peptide adjacent to the third extracellular loop outside of transmembrane segment 7, and includes a charge-charge interaction between peptide residue tyrosine-sulfate 27 and the arginine residue in the second extracellular loop of the receptor in position 197.

Insights into the molecular basis of ligand binding by the cholecystokinin receptor

The receptor for the peptide hormone, cholecystokinin, is a G-protein-coupled receptor in the rhodopsin/beta-adrenergic receptor family. A number of methodological approaches have been utilized to gain insights into the molecular basis for natural peptide ligand binding and activation of this physiologically important receptor. Insights into this have come from sequence analysis, ligand and receptor structure-activity data, receptor mutagenesis, conformational analysis of ligand and receptor fragments, and photoaffinity labeling. In this work, we review the contributions of each of these complementary approaches and provide a current integrated view of the active complex of cholecystokinin bound to its receptor.

CCK8 peptide derivatized with diphenylphosphine for rhenium labelling: synthesis and molecular mechanics calculations

A novel CCK8 derivative bearing a chelating agent at its N- end and its oxo-rhenium(V) complex have been synthesized and characterized. The chelating agent N-[N-13-(diphenylphosphino)propionyl]glycyl]cysteine (PN2S) ligand, the coordination set of which is made by the phosphorus atom of phosphine, the nitrogen atoms of the two amido groups and the sulphur atom of cysteine, has been used due to its high affinity towards the oxo-rhenium(V) moiety. Molecular modelling studies indicate that the CCK8 peptide adopts the right conformation for cholecystokinin receptor binding, and that modifications on the N-terminal side of CCK8 obtained by introducing chelating agents and its metal complexes should not affect the interaction with CCK(A) receptor.

Environment and mobility of a series of fluorescent reporters at the amino terminus of structurally related peptide agonists and antagonists bound to the cholecystokinin receptor

Fluorescence is a powerful biophysical tool for the analysis of the structure and dynamics of proteins. Here, we have developed two series of new fluorescent probes of the cholecystokinin (CCK) receptor, representing structurally related peptide agonists and antagonists. Each ligand had one of three distinct fluorophores (Alexa(488), nitrobenzoxadiazolyl, or acrylodan) incorporated in analogous positions at the amino terminus just outside the hormone's pharmacophore. All of the probes bound to the CCK receptor specifically and with high affinity, and intracellular calcium signaling studies showed the chemically modified peptides to be fully biologically active. Quenching by iodide and measurement of fluorescence spectra, anisotropy, and lifetimes were used to characterize the response of the fluorescence of the probe in the peptide-receptor complex for agonists and antagonists. All three fluorescence indicators provided the same insights into differences in the environment of the same indicator in the analogous position for agonist and antagonist peptides bound to the CCK receptor. Each agonist had its fluorescence quenched more easily and showed lower anisotropy (higher mobility of the probe) and shorter lifetime than the analogous antagonist. Treatment of agonist-occupied receptors with a non-hydrolyzable GTP analogue shifted the receptor into its inactive low affinity state and increased probe fluorescence lifetimes toward values observed with antagonist probes. These data are consistent with a molecular conformational change associated with receptor activation that causes the amino terminus of the ligand (situated above transmembrane segment six) to move away from its somewhat protected environment and toward the aqueous milieu.

Conformational and molecular modeling studies of sulfated cholecystokinin-15

Conformational features of the C-terminal carboxyamidated pentadecapeptide of CCK (S(19)HRISDRD[SO(4)]-YMGWMDF(33)-NH(2)) were determined by NMR spectroscopy in a zwitterionic membrane-mimetic solvent system, composed of DPC micelles. The C-terminal octapeptide consisted of a well-defined pseudohelix that was nearly identical to the structure previously reported for nonsulfated CCK-8 in the same solvent system. N-terminal amino acids of CCK-15 were highly disordered, with no clear conformational preference. Extensive NOE-restrained molecular dynamics simulations of the CCK-15/CCK(1)-R complex suggested that almost all the experimentally determined intermolecular contact points provided by NMR, site-directed mutagenesis, and photoaffinity labeling could be simultaneously satisfied, when the N-terminus of the ligand is placed in close spatial proximity to the N-terminus of the receptor.

NMR studies of CCK-8/CCK1 complex support membrane-associated pathway for ligand-receptor interaction

The interaction of peptide ligands with their associated G-protein-coupled receptors has been examined by a number of different experimental approaches over the years. We have been developing an approach utilizing high-resolution NMR to determine the structural features of the peptide ligand, well-designed fragments of the receptor, and the ligand-receptor complexes formed upon titration of the peptide hormone. The results from these investigations provide evidence for a membrane-associated pathway for the initial interaction of peptide ligands with the receptor. Here, our results from the investigation of the interaction of CCK-8 with the CCK1 receptor are described. Our spectroscopic results clearly show that both CCK-8 and the regions of CCK1 with which it interacts are closely associated with the zwitterionic interface of the lipids utilized in our solution spectroscopic studies.

Refinement of the conformation of a critical region of charge-charge interaction between cholecystokinin and its receptor

Insight into the molecular basis of cholecystokinin (CCK) binding to its receptor has come from receptor mutagenesis and photoaffinity labeling studies, with both contributing to the current hypothesis that the acidic Tyr-sulfate-27 residue within the peptide is situated adjacent to basic Arg(197) in the second loop of the receptor. Here, we refine our understanding of this region of interaction by examining a structure-activity series of these positions within both ligand and receptor and by performing three-dimensional molecular modeling of key pairs of modified ligand and receptor constructs. The important roles of Arg(197) and Tyr-sulfate-27 were supported by the marked negative impact on binding and biological response with their natural partner molecule when the receptor residue was replaced by acidic Asp or Glu and when the peptide residue was replaced by basic Arg, Lys, p-amino-Phe, p-guanidino-Phe, or p-methylamino-Phe. Complementary ligand-receptor charge-exchange experiments were unable to regain the lost function. This was supported by the molecular modeling, which demonstrated that the charge-reversed double mutants could not form a good interaction without extensive rearrangement of receptor conformation. The models further predicted that R197D and R197E mutations would lead to conformational changes in the extracellular domain, and this was experimentally supported by data showing that these mutations decreased peptide agonist and antagonist binding and increased nonpeptidyl antagonist binding. These receptor constructs also had increased susceptibility to trypsin degradation relative to the wild-type receptor. In contrast, the relatively conservative R197K mutation had modest negative impact on peptide agonist binding, again consistent with the modeling demonstration of loss of a series of stabilizing inter- and intramolecular bonds. The strong correlation between predicted and experimental results support the reported refinement in the three-dimensional structure of the CCK-occupied receptor.

The biologically crucial C terminus of cholecystokinin and the non-peptide agonist SR-146,131 share a common binding site in the human CCK1 receptor. Evidence for a crucial role of Met-121 in the activation process

The cholecystokinin (CCK) receptor-1 (CCK1R) is a G protein-coupled receptor, which mediates important central and peripheral cholecystokinin actions. Our aim was to progress in mapping of the CCK1R binding site by identifying residues that interact with the methionine and phenylalanine residues of the C-terminal moiety of CCK because these are crucial for its binding and biological activity, and to determine whether CCK and the selective non-peptide agonist, SR-146,131, share a common binding site. Identification of putative amino acids of the CCK1R binding site was achieved by dynamics-based docking of the ligand CCK in a refined three-dimensional model of the CCK1R using, as constraints, previous results that identified contact points between residues of CCK and CCK1R (Kennedy, K., Gigoux, V., Escrieut, C., Maigret, B., Martinez, J., Moroder, L., Frehel, D., Gully, D., Vaysse, N., and Fourmy, D. (1997) J. Biol. Chem. 272, 2920-2926 and Gigoux, V., Escrieut, C., Fehrentz, J. A., Poirot, S., Maigret, B., Moroder, L., Gully, D., Martinez, J., Vaysse, N., and Fourmy, D. (1999) J. Biol. Chem. 274, 20457-20464). By this approach, a series of residues forming connected hydrophobic clusters were identified. Pharmacological and functional analysis of mutated receptors indicated that a network of hydrophobic residues including Cys-94, Met-121, Val-125, Phe-218, Ile-329, Phe-330, Trp-326, Ile-352, Leu-356, and Tyr-360, is involved in the binding site for CCK and in the activation process of the CCK1R. Within this hydrophobic network, the physico-chemical nature of residue 121 seems to be essential for CCK1R functioning. Finally, the biological properties of mutants together with dynamic docking of SR-146,131 in the CCK1R binding site demonstrated that SR-146,131 occupies a region of CCK1R binding site which interacts with the C-terminal amidated tripeptide of CCK, i.e. Met-Asp-Phe-NH(2). These new and important insights will serve to better understand the activation process of CCK1R and to design or optimize ligands.

Peptide/benzodiazepine hybrids as ligands of CCK(A) and CCK(B) receptors

The (neuro)hormones gastrin and cholecystokinin (CCK) share a common C-terminal tetrapeptide amide sequence that has been recognized as the message portion while the N-terminal extensions are responsible for the CCK(A) and CCK(B) receptor subtype selectivity and avidity. 1,4-Benzodiazepine derivatives are potent and selective antagonists of these receptors, and according to comparative molecular field analysis, the structures of these nonpeptidic compounds could well mimic the message sequence of the peptide agonists at least in terms of spatial array of the aromatic residues. Docking of a larger series of low molecular weight nonpeptide antagonists to a homology modeling derived CCK(B) receptor structure revealed a consensus binding mode that is further validated by data from site-directed mutagenesis studies of the receptors. Whether this putative binding pocket of the nonpeptide antagonists is identical to that of the message portion of the peptide agonists, or whether it is distinct and spatially separated, or overlapping, but with distinct interaction sites, is still object of debate. Using a 1,4-benzodiazepine core amino-functionalized at the C3 position, related tryptophanyl derivatives were synthesized as mimics of the tetrapeptide and subsequently extended N-terminally with gastrin and CCK address sequences. All hybrid constructs were recognized as antagonists by the CCK(A) and CCK(B) receptors, but their address portions were incapable of enhancing in significant manner selectivity and avidity. Consequently, the binding of the peptide/benzodiazepine hybrids has to be dictated mainly by the benzodiazepine moiety, which apparently prevents optimal interactions of the address peptides with extracellular receptor subdomains. These findings would strongly support the view of distinct binding sites for the message portion of the peptide agonists and the benzodiazepine-based nonpeptide antagonists.

Peptide hormone binding to G-protein-coupled receptors: structural characterization via NMR techniques

G-protein-coupled receptors (GPCRs) allow cells to respond to calcium, hormones, and neurotransmitters. Not surprisingly, they currently make up the largest family of validated drug targets. Rational drug design for molecular regulators targeting GPCRs has been limited to theoretical-based computational approaches. X-ray crystallography of intact GPCRs has provided the topological orientation of the seven transmembrane helices, but limited structural information of the extracellular and intracellular loops and protein termini. In this review we detail an NMR-based approach which provides the high-resolution structural features on the extracellular domains of GPCRs and the ligand/receptor complexes formed upon titration of the peptide hormone. The results provide important contact points and a high-resolution description of the ligand/receptor interactions, which may be useful for the rational design of therapeutic agents targeting GPCRs. Recent results from our investigation of the cholecystokinin peptide hormone system are used to highlight this approach.

Characterization of the type A cholecystokinin receptor hormone-binding domain: use of contrasting and complementary methodologies

Insights into the molecular basis of binding of the peptide hormone, cholecystokinin, to its G protein-coupled receptor is of substantial interest and may contribute to the successful production and refinement of receptor-active drugs. A number of methodological approaches provide complementary data to contribute to these insights. These include receptor mutagenesis, ligand structure-activity data, conformational analysis of ligand and receptor fragments, and photoaffinity labeling. In this work, we compare and contrast each of these methods and provide our current view of the cumulative impact of the current data on molecular conformational models of the agonist-occupied type A cholecystokinin receptor. These support the key roles played by extracellular loop and tail regions of this receptor for binding its natural peptide ligand.

Synthesis and solution characterization of a porphyrin-CCK8 conjugate

In this paper we report the synthesis and a detailed NMR solution characterization of a new CCK8 analogue and its indium(III) complex, PK-CCK8 and In-PK-CCK8. The new compounds contain a porphyrin moiety covalently bound through an amide bond to the side chain of a Lys residue introduced at the N-terminus of CCK8. A molecular dynamics simulation, based on the NMR structure of the complex between CCK8 and the N-terminal extracellular arm of the CCK(A) receptor, is also reported. Both the NMR study and the molecular dynamics simulation indicate that the porphyrin-peptide conjugate might be able to bind to the CCK(A) receptor model. The results of the molecular dynamics calculations show that the conformational features of the CCK8/CCK(A) receptor model complex and of the PK-CCK8/CCK(A) receptor-model complex are similar. This evidence supports the view that the introduction of the porphyrin-Lys moiety does not influence the mode of ligand binding to the CCK(A) receptor model. The NMR structure of PK-CCK8 in DMSO consists of a well defined pseudo-helical N-terminal region, while the C-terminal region is flexible. Moreover, the absence of NOE contacts between the porphyrin and the peptide indicates that the macrocyclic ring is directed away from the peptide region involved in the binding with the receptor.

Refinement of the structure of the ligand-occupied cholecystokinin receptor using a photolabile amino-terminal probe

Affinity labeling is a powerful tool to establish spatial approximations between photolabile residues within a ligand and its receptor. Here, we have utilized a cholecystokinin (CCK) analogue with a photolabile benzoylphenylalanine (Bpa) sited in position 24, adjacent to the pharmacophoric domain of this hormone (positions 27-33). This probe was a fully efficacious agonist that bound to the CCK receptor saturably and with high affinity (K(i) = 8.9 +/- 1.1 nm). It covalently labeled the CCK receptor either within the amino terminus (between Asn(10) and Lys(37)) or within the third extracellular loop (Glu(345)), as demonstrated by proteolytic peptide mapping, deglycosylation, micropurification, and Edman degradation sequencing. Truncation of the receptor to eliminate residues 1-30 had no detrimental effect on CCK binding, stimulated signaling, or affinity labeling through a residue within the pharmacophore (Bpa(29)) but resulted in elimination of the covalent attachment of the Bpa(24) probe to the receptor. Thus, the distal amino terminus of the CCK receptor resides above the docked ligand, compressing the portion of the peptide extending beyond its pharmacophore toward the receptor core. Exposure of wild type and truncated receptor constructs to extracellular trypsin damaged the truncated construct but not the wild type receptor, suggesting that this domain also may play a protective role. Use of these additional insights into molecular approximations provided key constraints for molecular modeling of the peptide-receptor complex, supporting the counterclockwise organization of the transmembrane helical domains.

Minireview: Insights into G protein-coupled receptor function using molecular models

G protein-coupled receptors (GPCRs) represent the largest family of signal-transducing molecules known. They convey signals for light and many extracellular regulatory molecules. GPCRs have been found to be dysfunctional/dysregulated in a growing number of human diseases and have been estimated to be the targets of more than 30% of the drugs used in clinical medicine today. Thus, understanding how GPCRs function at the molecular level is an important goal of biological research. In order to understand function at this level, it is necessary to delineate the 3D structure of these receptors. Recently, the 3D structure of rhodopsin has been resolved, but in the absence of experimentally determined 3D structures of other GPCRs, a powerful approach is to construct a theoretical model for the receptor and refine it based on experimental results. Computer-generated models for many GPCRs have been constructed. In this article, we will review these studies. We will place the greatest emphasis on an iterative, bi-directional approach in which models are used to generate hypotheses that are tested by experimentation and the experimental findings are, in turn, used to refine the model. The success of this approach is due to the synergistic interaction between theory and experiment.

Structural characterization of peptide hormone/receptor interactions by NMR spectroscopy

The structural characterization of peptide hormones and their interaction with G-protein (guanine nucleotide-binding regulatory protein) coupled receptors by high-resolution nmr is described. The general approaches utilized can be categorized into three different classes based on their target: the ligand, the receptor, and the ligand/receptor complex. Examples of these different approaches, aimed at facilitating the rational design of peptides and peptidomimetics with improved pharmacological profiles, based on work carried out in our own laboratory, are given. In the ligand-based approach, the high-resolution structures of bradykinin analogues allowing for the development of a structure-activity relationship for activation of the B1 receptor are described. Studies targeting the receptor are to a large extent theoretical, based on computational molecular modeling. However, experimentally based structural features provided by high-resolution nmr can be used to great advantage, providing insight into the mechanism of receptor function, as illustrated here with results from parathyroid hormone. A similar combination of theoretical methods, supplemented by high-resolution structures from nmr has been utilized to probe the formation and stabilization of the ligand/receptor complex both for parathyroid hormone and cholecystokinin. In each of these three approaches, the importance of well-designed peptide mimetics and accurate structural analysis by high-resolution nmr, will be highlighted.

Mutation of Asn-391 within the conserved NPXXY motif of the cholecystokinin B receptor abolishes Gq protein activation without affecting its association with the receptor

Among the most conserved regions in the G-protein-coupled receptors is the (N/D)PX(2-3)Y motif of the seventh transmembrane domain (X represents any amino acid). The mutation of the Asn/Asp residue of this motif in different G-protein-coupled receptors was shown to affect the activation of either adenylyl cyclase or phospholipase C. We have mutated the Asn residue (Asn-391) of the NPXXY motif in the CCKBR to Ala and determined the effects of the mutation on binding, signaling, and G-proteins coupling after expression of the mutated receptor in COS cells. The mutated receptor displayed similar expression levels and high affinity CCK binding compared with the wild type CCKBR. However, unlike the wild type CCKBR, the mutated receptor was completely unable to mediate activation of either phospholipase C and protein kinase C-dependent and -independent mitogen-activated protein kinase pathways, indicating an essential role of Asn-391 in CCKBR signaling. Coimmunoprecipitation experiments allowed us to show that the inactive mutant retains an intact capacity to form stable complexes with G(q)alpha subunits in response to CCK. These results indicate that the formation of high affinity CCK-receptor-G(q) protein complexes is not sufficient to activate G(q) and suggest that Asn-391 is specifically involved in G(q) proteins activation.

Essential role of extracellular charged residues of the human CCK(1) receptor for interactions with SR 146131, SR 27897 and CCK-8S

We hypothesized that charge-charge interactions may be important for the binding of the human cholecystokinin type 1 (CCK(1)) receptor-specific non-peptide full agonist SR 146131, (2-[4-(4-chloro-2, 5-dimethoxyphenyl)-5-(2-cyclohexyl-ethyl)-thiazol-2-ylcarbamoyl ]-5, 7-dimethyl-indol-1-yl-1-acetic acid), the competitive antagonist SR 27897, (1-[2-(4-(2-chlorophenyl)thiazol-2-yl) aminocarbonyl indoyl] acetic acid) and the natural octapeptide CCK-8S to the CCK(1) receptor. Alanine replacement studies of positively charged residues in the extracellular domains of the receptor showed that only the R336A mutation affected SR 146131 potency of mutated receptors transiently expressed in monkey kidney epithelial COS-7 cells. Two residues, Lys(115) and Lys(187), were implicated in SR 27897 binding. Only the replacement of Lys(115), Arg(197) and Arg(336) significantly affected CCK-8S binding or activity. These results clearly indicated the importance of certain charged residues, but not others, in SR 146131, SR 27897 and CCK-8S binding. Furthermore, although these molecules probably occupy different binding sites on the CCK(1) receptor, we show that a small non-peptide agonist, SR 146131, can stimulate the dual signaling pathways mediated by the CCK(1) receptor.

Uncovering molecular mechanisms involved in activation of G protein-coupled receptors

G protein-coupled, seven-transmembrane segment receptors (GPCRs or 7TM receptors), with more than 1000 different members, comprise the largest superfamily of proteins in the body. Since the cloning of the first receptors more than a decade ago, extensive experimental work has uncovered multiple aspects of their function and challenged many traditional paradigms. However, it is only recently that we are beginning to gain insight into some of the most fundamental questions in the molecular function of this class of receptors. How can, for example, so many chemically diverse hormones, neurotransmitters, and other signaling molecules activate receptors believed to share a similar overall tertiary structure? What is the nature of the physical changes linking agonist binding to receptor activation and subsequent transduction of the signal to the associated G protein on the cytoplasmic side of the membrane and to other putative signaling pathways? The goal of the present review is to specifically address these questions as well as to depict the current awareness about GPCR structure-function relationships in general.

Contrasting roles of leu(356) in the human CCK(1) receptor for antagonist SR 27897 and agonist SR 146131 binding

A new highly specific, potent non-peptide agonist for the cholecystokinin subtype 1 receptor (CCK(1)), SR 146131 (2-[4-(4-chloro-2, 5-dimethoxyphenyl)-5-(2-cyclohexyl-ethyl)-thiazol-2-ylcarbamoyl ]-5, 7-dimethyl-indol-1-yl-1-acetic acid) was recently described [Bignon, E., Bachy, A., Boigegrain, R., Brodin, R., Cottineau, M., Gully, D., Herbert, J.-M., Keane, P., Labie, C., Molimard, J.-C., Olliero, D., Oury-Donat, F., Petereau, C., Prabonneaud, V., Rockstroh, M.-P., Schaeffer, P., Servant, O.Thurneyssen, O., Soubrié, P., Pascal, M., Maffrand, J.-P., Le Fur, G., 1999. SR 146131: a new, potent, orally active and selective non-peptide cholecystokinin subtype I receptor agonist: I. In vitro studies. J. Pharmacol. Exp. Ther. 289, 742-751]. From binding and activity assays with chimeric constructs of human CCK(1) and the cholecystokinin subtype 2 receptor (CCK(2)) and receptors carrying point mutations, we show that Leu(356), situated in transmembrane domain seven in the CCK(1) receptor, is a putative contact point for SR 146131. In contrast, Leu(356) is probably not in contact with the CCK(1) receptor specific antagonist SR 27897 (1-[2-(4-(2-chlorophenyl)thiazol-2-yl)aminocarbonyl indoyl]acetic acid), a compound structurally related to SR 146131, since its replacement by alanine, histidine or asparagine gave receptors having wild-type CCK(1) receptor SR 27897 binding affinity. Previous mutational analysis of His(381), the cognate position in the rat CCK(2) receptor, had implicated it as being involved in subtype specificity for SR 27897, results which we confirm with corresponding mutations in the human CCK(2) receptor. Moreover, binding and activity assays with the natural CCK receptor agonist, CCK-8S, show that CCK-8S is more susceptible to the mutations in that position in the CCK(1) receptor than in the CCK(2) receptor. The results suggest different binding modes for SR 27897, SR 146131 and CCK-8S in each CCK receptor subtype.

Arginine 197 of the cholecystokinin-A receptor binding site interacts with the sulfate of the peptide agonist cholecystokinin

The knowledge of the binding sites of G protein-coupled cholecystokinin receptors represents important insights that may serve to understand their activation processes and to design or optimize ligands. Our aim was to identify the amino acid of the cholecystokinin-A receptor (CCK-AR) binding site in an interaction with the sulfate of CCK, which is crucial for CCK binding and activity. A three-dimensional model of the [CCK-AR-CCK] complex was built. In this model, Arg197 was the best candidate residue for a ionic interaction with the sulfate of CCK. Arg197 was exchanged for a methionine by site-directed mutagenesis. Wild-type and mutated CCK-AR were transiently expressed in COS-7 cells for pharmacological and functional analysis. The mutated receptor on Arg197 did not bind the agonist radioligand 125I-BH-[Thr, Nle]-CCK-9; however, it bound the nonpeptide antagonist [3H]-SR27,897 as the wild-type receptor. The mutant was approximately 1,470- and 3,200-fold less potent than the wild-type CCK-AR to activate G proteins and to induce inositol phosphate production, respectively. This is consistent with the 500-fold lower potency and 800-fold lower affinity of nonsulfated CCK relative to sulfated CCK on the wild-type receptor. These data, together with those showing that the mutated receptor failed to discriminate nonsulfated and sulfated CCK while it retained other pharmacological features of the CCK-AR, strongly support an interaction between Arg197 of the CCK-AR binding site and the sulfate of CCK. In addition, the mutated CCK-AR resembled the low affinity state of the wild-type CCK-AR, suggesting that Arg197-sulfate interaction regulates conformational changes of the CCK-AR that are required for its physiological activation.

Evidence for a direct interaction between the penultimate aspartic acid of cholecystokinin and histidine 207, located in the second extracellular loop of the cholecystokinin B receptor

Recently, we reported that the mutation of His(207) to Phe located in the second extracellular loop of the cholecystokinin B receptor strongly affected cholecystokinin (CCK) binding (Silvente-Poirot, S., Escrieut, C., and Wank, S. A. (1998) Mol. Pharmacol. 54, 364-371). To characterize the functional group in CCK that interacts with His(207), we first substituted His(207) to Ala. This mutation decreased the affinity and the potency of CCK to produce total inositol phosphates 302-fold and 456-fold without affecting the expression of the mutant receptor. The screening of L-alanine-modified CCK peptides to bind and activate the wild type and mutant receptors allowed the identification of the interaction of the C-terminal Asp(8) of CCK with His(207). The H207A-CCKBR mutant, unlike the wild type receptor, was insensitive to substitution of Asp(8) of CCK to other amino acid residues. This interaction was further confirmed by mutating His(207) to Asp. The affinity of CCK for the H207D-CCKBR mutant was 100-fold lower than for the H207A-CCKBR mutant, consistent with an electrostatic repulsion between the negative charges of the two interacting aspartic acids. Peptides with neutral amino acids in position eight of CCK reversed this effect and displayed a gain of affinity for the H207D mutant compared with CCK. To date, this is the first report concerning the identification of a direct contact point between the CCKB receptor and CCK.

Arginine 336 and asparagine 333 of the human cholecystokinin-A receptor binding site interact with the penultimate aspartic acid and the C-terminal amide of cholecystokinin

The cholecystokinin-A receptor (CCK-AR) is a G protein-coupled receptor that mediates important central and peripheral cholecystokinin actions. Residues of the CCK-AR binding site that interact with the C-terminal part of CCK that is endowed with biological activity are still unknown. Here we report on the identification of Arg-336 and Asn-333 of CCK-AR, which interact with the Asp-8 carboxylate and the C-terminal amide of CCK-9, respectively. Identification of the two amino acids was achieved by dynamics-based docking of CCK in a refined three-dimensional model of CCK-AR using, as constraints, previous results that demonstrated that Trp-39/Gln-40 and Met-195/Arg-197 interact with the N terminus and the sulfated tyrosine of CCK, respectively. Arg-336-Asp-8 and Asn-333-amide interactions were pharmacologically assessed by mutational exchange of Arg-336 and Asn-333 in the receptor or reciprocal elimination of the partner chemical functions in CCK. This study also allowed us to demonstrate that (i) the identified interactions are crucial for stabilizing the high affinity phospholipase C-coupled state of the CCK-AR.CCK complex, (ii) Arg-336 and Asn-333 are directly involved in interactions with nonpeptide antagonists SR-27,897 and L-364,718, and (iii) Arg-336 but not Asn-333 is directly involved in the binding of the peptide antagonist JMV 179 and the peptide partial agonist JMV 180. These data will be used to obtain an integrated dynamic view of the molecular processes that link agonist binding to receptor activation.