Insights into the impact of phenolic residue incorporation at each position along secretin for receptor binding and biological activity

Understanding of the structural importance of each position along a peptide ligand can provide important insights into the molecular basis for its receptor binding and biological activity. This has typically been evaluated using serial replacement of each natural residue with an alanine. In the current report, we have further complemented alanine scanning data with the serial replacement of each residue within secretin-27, the natural ligand for the prototypic class B G protein-coupled secretin receptor, using a photolabile phenolic residue. This not only provided the opportunity to probe spatial approximations between positions along a docked ligand with its receptor, but also provided structure-activity insights when compared with tolerance for alanine replacement of the same residues. The pattern of sensitivity to phenolic residue replacement was periodic within the carboxyl-terminal region of this peptide ligand, corresponding with alanine replacements in that region. This was supportive of the alpha-helical conformation of the peptide in that region and its docking within a groove in the receptor amino-terminal domain. In contrast, the pattern of sensitivity to phenolic residue replacement was almost continuous in the amino-terminal region of this peptide ligand, again similar to alanine replacements, however, there were key positions in which either the phenolic residue or alanine was differentially preferred. This provided insights into the receptor environment of the portion of this ligand most critical for its biological activity. As the structure of the intact receptor is elucidated, these data will provide a guide for ligand docking to the core domain and to help clarify the molecular basis of receptor activation.

Mapping spatial approximations between the amino terminus of secretin and each of the extracellular loops of its receptor using cysteine trapping

While it is evident that the carboxyl-terminal region of natural peptide ligands bind to the amino-terminal domain of class B GPCRs, how their biologically critical amino-terminal regions dock to the receptor is unclear. We utilize cysteine trapping to systematically explore spatial approximations among residues in the first five positions of secretin and in every position within the receptor extracellular loops (ECLs). Only Cys(2) and Cys(5) secretin analogues exhibited full activity and retained moderate binding affinity (IC(50): 92±4 and 83±1 nM, respectively). When these peptides probed 61 human secretin receptor cysteine-replacement mutants, a broad network of receptor residues could form disulfide bonds consistent with a dynamic ligand-receptor interface. Two distinct patterns of disulfide bond formation were observed: Cys(2) predominantly labeled residues in the amino terminus of ECL2 and ECL3 (relative labeling intensity: Ser(340), 94±7%; Pro(341), 84±9%; Phe(258), 73±5%; Trp(274) 62±8%), and Cys(5) labeled those in the carboxyl terminus of ECL2 and ECL3 (Gln(348), 100%; Ile(347), 73±12%; Glu(342), 59±10%; Phe(351), 58±11%). These constraints were utilized in molecular modeling, providing improved understanding of the structure of the transmembrane bundle and interconnecting loops, the orientation between receptor domains, and the molecular basis of ligand docking. Key spatial approximations between peptide and receptor predicted by this model (H(1)-W(274), D(3)-N(268), G(4)-F(258)) were supported by mutagenesis and residue-residue complementation studies.

Site of action of a pentapeptide agonist at the glucagon-like peptide-1 receptor. Insight into a small molecule agonist-binding pocket

The development of small molecule agonists for class B G protein-coupled receptors (GPCRs) has been quite challenging. With proof-of-concept that exenatide, the parenterally administered peptide agonist of the glucagon-like peptide-1 (GLP1) receptor, is an effective treatment for patients with diabetes mellitus, the development of small molecule agonists could have substantial advantages. We previously reported a lead for small molecule GLP1 receptor agonist development representing the pentapeptide NRTFD. In this work, we have prepared an NRTFD derivative incorporating a photolabile benzoylphenylalanine and used it to define its site of action. This peptide probe was a full agonist with potency similar to NRTFD, which bound specifically and saturably to a single, distinct site within the GLP1 receptor. Peptide mapping using cyanogen bromide and endoproteinase Lys-C cleavage of labeled wild type and M397L mutant receptor constructs identified the site of covalent attachment of NRTFD within the third extracellular loop above the sixth transmembrane segment (TM6). This region is the same as that identified using an analogous photolabile probe based on secretin receptor sequences, and has been shown in mutagenesis studies to be important for natural agonist action of several members of this family. While these observations suggest that small molecule ligands can act at a site bordering the third extracellular loop to activate this class B GPCR, the relationship of this site to the site of action of the amino-terminal end of the natural agonist peptide is unclear.

Lactam constraints provide insights into the receptor-bound conformation of secretin and stabilize a receptor antagonist

The natural ligands for family B G protein-coupled receptors are moderate-length linear peptides having diffuse pharmacophores. The amino-terminal regions of these ligands are critical for biological activity, with their amino-terminal truncation leading to production of orthosteric antagonists. The carboxyl-terminal regions of these peptides are thought to occupy a ligand-binding cleft within the disulfide-bonded amino-terminal domains of these receptors, with the peptides in amphipathic helical conformations. In this work, we have characterized the binding and activity of a series of 11 truncated and lactam-constrained secretin(5-27) analogues at the prototypic member of this family, the secretin receptor. One peptide in this series with lactam connecting residues 16 and 20 [c[E(16),K(20)][Y(10)]sec(5-27)] improved the binding affinity of its unconstrained parental peptide 22-fold while retaining the absence of endogenous biological activity and competitive antagonist characteristics. Homology modeling with molecular mechanics and molecular dynamics simulations established that this constrained peptide occupies the ligand-binding cleft in an orientation similar to that of natural full-length secretin and provided insights into why this peptide was more effective than other truncated conformationally constrained peptides in the series. This lactam bridge is believed to stabilize an extended α-helical conformation of this peptide while in solution and not to interfere with critical residue-residue approximations while docked to the receptor.

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.

Importance of each residue within secretin for receptor binding and biological activity

Secretin is a linear 27-residue peptide hormone that stimulates pancreatic and biliary ductular bicarbonate and water secretion by acting at its family B G protein-coupled receptor. While, like other family members, the carboxyl-terminal region of secretin is most important for high affinity binding and its amino-terminal region is most important for receptor selectivity and receptor activation, determinants for these activities are distributed throughout the entire length of this peptide. In this work, we have systematically investigated changing each residue within secretin to alanine and evaluating the impact on receptor binding and biological activity. The residues most critical for receptor binding were His1, Asp3, Gly4, Phe6, Thr7, Ser8, Leu10, Asp15, Leu19, and Leu23. The residues most critical for biological activity included His1, Gly4, Thr7, Ser8, Glu9, Leu10, Leu19, Leu22, and Leu23, with Asp3, Phe6, Ser11, Leu13, Asp15, Leu26, and Val27 also contributing. While the importance of residues in positions analogous to His1, Asp3, Phe6, Thr7, and Leu23 is conserved for several closely related members of this family, Leu19 is uniquely important for secretin. We, therefore, have further studied this residue by molecular modeling and molecular dynamics simulations. Indeed, the molecular dynamics simulations showed that mutation of Leu19 to alanine was destabilizing, with this effect greater than that observed for the analogous position in the other close family members. This could reflect reduced contact with the receptor or an increase in the solvent-accessible surface area of the hydrophobic residues in the carboxyl terminus of secretin as bound to its receptor.

Refinement of glucagon-like peptide 1 docking to its intact receptor using mid-region photolabile probes and molecular modeling

The glucagon-like peptide 1 (GLP1) receptor is an important drug target within the B family of G protein-coupled receptors. Its natural agonist ligand, GLP1, has incretin-like actions and the receptor is a recognized target for management of type 2 diabetes mellitus. Despite recent solution of the structure of the amino terminus of the GLP1 receptor and several close family members, the molecular basis for GLP1 binding to and activation of the intact receptor remains unclear. We previously demonstrated molecular approximations between amino- and carboxyl-terminal residues of GLP1 and its receptor. In this work, we study spatial approximations with the mid-region of this peptide to gain insights into the orientation of the intact receptor and the ligand-receptor complex. We have prepared two new photolabile probes incorporating a p-benzoyl-l-phenylalanine into positions 16 and 20 of GLP1(7-36). Both probes bound to the GLP1 receptor specifically and with high affinity. These were each fully efficacious agonists, stimulating cAMP accumulation in receptor-bearing CHO cells in a concentration-dependent manner. Each probe specifically labeled a single receptor site. Protease cleavage and radiochemical sequencing identified receptor residue Leu(141) above transmembrane segment one as its site of labeling for the position 16 probe, whereas the position 20 probe labeled receptor residue Trp(297) within the second extracellular loop. Establishing ligand residue approximation with this loop region is unique among family members and may help to orient the receptor amino-terminal domain relative to its helical bundle region.

Elucidation of the active conformation of the amino terminus of receptor-bound secretin using intramolecular disulfide bond constraints

Family B G protein-coupled receptors include several potentially important drug targets, yet our understanding of the molecular basis of ligand binding to and activation of these receptors is incomplete. While NMR and crystal structures exist for peptide ligand-associated amino-terminal domains of several family members, these only provide insights into the conformation of the carboxyl-terminal region of the peptides. The amino-terminal region of these peptides, critical for biological activity, is believed to interact with the helical bundle domain, and is, therefore, unconstrained in these structures. The aim of the current study was to provide insights into the conformation of the amino terminus of secretin as bound to its receptor. We prepared a series of conformationally constrained secretin peptides containing intramolecular disulfide bonds that were predicted by molecular modeling to approximate the conformation of the analogous region of PACAP bound to its receptor that had been determined using transfer-NOE NMR techniques. Secretin peptides with pairs of cysteine residues in positions 2-7, 3-5, 3-6, 4-7, 7-9, and 4-10 were studied as linear and disulfide-bonded forms. The analog with a disulfide bond connecting positions 7-9 had binding affinity and biological activity similar to natural secretin, supporting the relevance of this constraint to its active conformation. While this feature is shared between secretin and PACAP, absence of activity in other constrained peptides in this series also suggest that there are differences between these receptor-bound conformations. It will be critical to extend similar studies to other family members to learn what structural elements might be most conserved in this family.

Spatial approximations between residues 6 and 12 in the amino-terminal region of glucagon-like peptide 1 and its receptor: a region critical for biological activity

Understanding the molecular basis of natural ligand binding and activation of the glucagon-like peptide 1 (GLP1) receptor may facilitate the development of agonist drugs useful for the management of type 2 diabetes mellitus. We previously reported molecular approximations between carboxyl-terminal residues 24 and 35 within GLP1 and its receptor. In this work, we have focused on the amino-terminal region of GLP1, known to be critical for receptor activation. We developed two high-affinity, full agonist photolabile GLP1 probes having sites of covalent attachment in positions 6 and 12 of the 30-residue peptide (GLP1(7-36)). Both probes bound to the receptor specifically and covalently labeled single distinct sites. Chemical and protease cleavage of the labeled receptor identified the juxtamembrane region of its amino-terminal domain as the region of covalent attachment of the position 12 probe, whereas the region of labeling by the position 6 probe was localized to the first extracellular loop. Radiochemical sequencing identified receptor residue Tyr(145), adjacent to the first transmembrane segment, as the site of labeling by the position 12 probe, and receptor residue Tyr(205), within the first extracellular loop, as the site of labeling by the position 6 probe. These data provide support for a common mechanism for natural ligand binding and activation of family B G protein-coupled receptors. This region of interaction of peptide amino-terminal domains with the receptor may provide a pocket that can be targeted by small molecule agonists.

Refinement of the pharmacophore of an agonist ligand of the secretin receptor using conformationally constrained cyclic hexapeptides

There is a compelling need for the development of small molecule agonists acting at family B G protein-coupled receptors. A possible lead for the development of such drugs was reported when it was recognized that sequences endogenous to the amino terminus of the secretin receptor and certain other receptors in this family possess weak full agonist activity (Dong et al. Mol Pharmacol 2006;70:206-213). In the current report, we extended those observations by building the active dipeptide motif found in the secretin receptor (WD) into each position around a conformationally constrained d-amino acid-containing cyclic hexapeptide, and determining the biological activity of each peptide at the secretin receptor. Indeed, only two positions for WD around this constrained ring resulted in biological activity at the receptor, providing further insights into the structural specificity of this phenomenon. Molecular modeling supported the presence of a unique WD backbone conformation shared only by these active peptides, and provided a more constrained template for future receptor-active agonist drug development.

Secretin occupies a single protomer of the homodimeric secretin receptor complex: insights from photoaffinity labeling studies using dual sites of covalent attachment

The secretin receptor, a prototypic family B G protein-coupled receptor, forms a constitutive homodimeric complex that is stable even in the presence of hormone. Recently, a model of this agonist-bound receptor was built based on high resolution structures reported for amino-terminal domains of other family members. Although this model provided the best solution for all extant data, including 10 photoaffinity labeling constraints, a new such constraint now obtained with a position 16 photolabile probe was inconsistent with this model. As the secretin receptor forms constitutive homodimers, we explored whether secretin might dock across both protomers of the complex, an observation that could also contribute to the negative cooperativity observed. To directly explore this, we prepared six secretin analogue probes that simultaneously incorporated two photolabile benzoylphenylalanines as sites of covalent attachment, in positions known to label distinct receptor subdomains. Each bifunctional probe was a full agonist that labeled the receptor specifically and saturably, with electrophoretic migration consistent with labeling a single protomer of the homodimeric secretin receptor. No band representing radiolabeled receptor dimer was observed with any bifunctional probe. The labeled monomeric receptor bands were cleaved with cyanogen bromide to demonstrate that both of the photolabile benzoylphenylalanines within a single probe had established covalent adducts with a single receptor in the complex. These data are consistent with a model of secretin occupying a single secretin receptor protomer within the homodimeric receptor complex. A new molecular model accommodating all constraints is now proposed.

Molecular basis of glucagon-like peptide 1 docking to its intact receptor studied with carboxyl-terminal photolabile probes

The glucagon-like peptide 1 (GLP1) receptor is a member of Family B G protein-coupled receptors and represents an important drug target for type 2 diabetes. Despite recent solution of the structure of the amino-terminal domain of this receptor and that of several close family members, understanding of the molecular basis of natural ligand GLP1 binding to its intact receptor remains limited. The goal of this study was to explore spatial approximations between specific receptor residues within the carboxyl terminus of GLP1 and its receptor as normally docked. Therefore, we developed and characterized two high affinity, full-agonist photolabile GLP1 probes having sites for covalent attachment in positions 24 and 35. Both probes labeled the receptor specifically and saturably. Subsequent peptide mapping using chemical and proteinase cleavages of purified wild-type and mutant GLP1 receptor identified that the Arg(131)-Lys(136) segment at the juxtamembrane region of the receptor amino terminus contained the site of labeling for the position 24 probe, and the specific receptor residue labeled by this probe was identified as Glu(133) by radiochemical sequencing. Similarly, nearby residue Glu(125) within the same region of the receptor amino-terminal domain was identified as the site of labeling by the position 35 probe. These data represent the first direct demonstration of spatial approximation between GLP1 and its intact receptor as docked, providing two important constraints for the modeling of this interaction. This should expand our understanding of the molecular basis of natural agonist ligand binding to the GLP1 receptor and may be relevant to other family members.

Elucidation of the molecular basis of cholecystokinin Peptide docking to its receptor using site-specific intrinsic photoaffinity labeling and molecular modeling

G protein-coupled receptors represent the largest family of receptors and the major target of current drug development efforts. Understanding of the mechanisms of ligand binding and activation of these receptors remains limited, despite recent advances in structural determination of family members. This work focuses on the use of photoaffinity labeling and molecular modeling to elucidate the structural basis of binding a natural peptide ligand to a family A G protein-coupled receptor, the type 1 cholecystokinin receptor. Two photolabile cholecystokinin analogues were developed and characterized as representing high-affinity, fully biologically active probes with sites of covalent attachment at positions 28 and 31. The sites of receptor labeling were identified by purification, proteolytic peptide mapping, and radiochemical sequencing of labeled wild-type and mutant cholecystokinin receptors. The position 28 probe labeled second extracellular loop residue Leu(199), while the position 31 probe labeled first extracellular loop residue Phe(107). Along with five additional spatial approximation constraints coming from previous photoaffinity labeling studies and 12 distance restraints from fluorescence resonance energy transfer studies, these were built into two homology models of the cholecystokinin receptor, based on the recent crystal structures of the beta2-adrenergic receptor and A2a-adenosine receptor. The resultant agonist ligand-occupied receptor models fully accommodate all existing experimental data and represent the best refined models of a peptide hormone receptor in this important family.

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.

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.

Insights into the structural basis of endogenous agonist activation of family B G protein-coupled receptors

Agonist drugs targeting the glucagon-like peptide-1 (GLP1) receptor represent important additions to the clinical management of patients with diabetes mellitus. In the current report, we have explored whether the recently described concept of a receptor-active endogenous agonist sequence within the amino terminus of the secretin receptor may also be applicable to the GLP1 receptor. If so, this could provide a lead for the development of additional small molecule agonists targeting this and other important family members. Indeed, the region of the GLP1 receptor analogous to that containing the active WDN within the secretin receptor was found to possess full agonist activity at the GLP1 receptor. The minimal fragment within this region that had full agonist activity was NRTFD. Despite having no primary sequence identity with the WDN, it was also active at the secretin receptor, where it had similar potency and efficacy to WDN, suggesting common structural features. Molecular modeling demonstrated that an intradomain salt bridge between the side chains of arginine and aspartate could yield similarities in structure with cyclic WDN. This directly supports the relevance of the endogenous agonist concept to the GLP1 receptor and provides new insights into the rational development and refinement of new types of drugs activating this important receptor.

Transmembrane segment IV contributes a functionally important interface for oligomerization of the Class II G protein-coupled secretin receptor

Oligomerization of the Class II G protein-coupled secretin receptor has been reported, but the molecular basis for this and its functional significance have not been determined. In the current work, we have examined the possible contribution of each of the transmembrane (TM) segments of this receptor to its homo-oligomerization, using the method of competitive disruption screening for inhibition of receptor bioluminescence resonance energy transfer signal. TM IV was the only segment that was found to disrupt receptor bioluminescence resonance energy transfer. Evaluation of predicted interhelical and lipid-exposed faces of this TM segment demonstrated that its lipid-exposed face represented the determinant for oligomerization. This was further confirmed by mutagenesis of the intact secretin receptor. Morphological FRET was utilized to demonstrate that secretin receptor oligomerization occurred at the cell surface and that this oligomerization was disrupted by mutating Gly(243) and Ile(247), key residues within the lipid-exposed face of TM IV. Although disruption of the receptor oligomerization interface had no effect on secretin binding parameters, it reduced the ability of secretin to stimulate intracellular cAMP. This supports a clear functional effect of oligomerization of this receptor. Such an effect might be particularly relevant to clinical situations in which this receptor is overexpressed, such as in certain neoplasms.

Molecular Approximations between Residues 21 and 23 of Secretin and Its Receptor: Development of a Model for Peptide Docking with the Amino Terminus of the Secretin Receptor

The structurally unique amino-terminal domain of class II G protein-coupled receptors is critically important for ligand binding and receptor activation. Understanding the precise role it plays requires detailed insights into the molecular basis of its ligand interactions and the conformation of the ligand-receptor complex. In this work, we used two high-affinity, full-agonist, secretin-like photolabile probes having sites for covalent attachment in positions 21 and 23 and used sequential proteolysis and sequencing of the labeled region of the receptor to identify two new spatial approximation constraints. The position 21 probe labeled receptor residue Arg(15), whereas the position 23 probe labeled receptor residue Arg(21). A homology model of the amino-terminal domain of the secretin receptor was developed using the NMR structure of the analogous domain of the corticotropin-releasing factor receptor. This was attached to a homology model of the secretin receptor transmembrane bundle, with the two domains oriented relative to each other based on continuity of the peptide backbone and by imposing a distance restraint recently identified between the amino-terminal WDN sequence and the region of the helical bundle above transmembrane segment six. Secretin was docked to this model using seven sets of spatial approximation constraints identified in previous photoaffinity labeling studies. This model was found to fully accommodate all existing constraints, as well as the two new approximations identified in this work.

Transmembrane segment peptides can disrupt cholecystokinin receptor oligomerization without affecting receptor function

Oligomerization of the G protein-coupled cholecystokinin (CCK) receptor has been demonstrated, but its molecular basis and functional importance are not clear. We now examine contributions of transmembrane (TM) segments to oligomerization of this receptor using a peptide competitive inhibition strategy. Oligomerization of CCK receptors tagged at the carboxyl terminus with Renilla luciferase or yellow fluorescent protein was quantified using bioluminescence resonance energy transfer (BRET). Synthetic peptides representing TM I, II, V, VI, and VII of the CCK receptor were utilized as competitors. Of these, only TM VI and VII peptides disrupted receptor BRET. Control studies established that the beta2-adrenergic receptor TM VI peptide that disrupts oligomerization of that receptor had no effect on CCK receptor BRET. Notably, disruption of CCK receptor oligomerization had no effect on agonist binding, biological activity, or receptor internalization. To gain insight into the face of TM VI contributing to oligomerization, we utilized analogous peptides with alanines in positions 315, 319, and 323 (interhelical face) or 317, 321, and 325 (external lipid-exposed face). The Ala317,321,325 peptide eliminated the disruptive effect on CCK receptor BRET, whereas the other mutant peptide behaved like wild-type TM VI. This suggests that the lipid-exposed face of the CCK receptor TM VI most contributes to oligomerization and supports external contact dimerization of helical bundles, rather than domain-swapped dimerization. Fluorescent CCK receptor mutants with residues 317, 321, and 325 replaced with alanines were also prepared and failed to yield significant resonance transfer signals using either BRET or a morphological FRET assay, further supporting this interpretation.

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.

Differential Spatial Approximation between Secretin and Its Receptor Residues in Active and Inactive Conformations Demonstrated by Photoaffinity Labeling

Understanding of the conformational changes in G protein-coupled receptors associated with activation and inactivation is of great interest. We previously used photoaffinity labeling to elucidate spatial approximations between photolabile residues situated throughout the pharmacophore of secretin agonist probes and this receptor. The aim of the current work was to develop analogous photolabile secretin antagonist probes and to explore their spatial approximations. The most potent secretin antagonist reported is a pseudopeptide ([ψ4, 5]secretin) in which the peptide bond between residues 4 and 5 was replaced by a ψ(CH2-NH) peptide bond isostere. We have developed a series of [ψ4, 5]secretin analogs incorporating photolabile benzoyl phenylalanine residues in positions 6, 22, and 26. Each bound to the secretin receptor saturably and specifically, with affinity similar to their parental peptide. At concentrations with no measurable agonist activity, each probe covalently labeled the secretin receptor. Peptide mapping using proteolytic cleavage, immunoprecipitation, and radiochemical sequencing identified that each of these three probes labeled the amino terminus of the secretin receptor. Whereas the position 22 probe labeled the same residue as its analogous agonist probe and the position 6 probe labeled a residue within two residues of that labeled by its analogous agonist probe, the position 26 probe labeled a site 16 residues away from that labeled by its analogous agonist probe. Thus, whereas structurally related agonist and antagonist probes dock in the same general region of this receptor, conformational differences in active and inactive states result in substantial differences in spatial approximation at the carboxyl-terminal end of secretin analogs.

Fluorescence Characteristics of Hydrophobic Partial Agonist Probes of the Cholecystokinin Receptor

Fluorescence spectroscopic studies are powerful tools for the evaluation of receptor structure and the dynamic changes associated with receptor activation. Here, we have developed two chemically distinct fluorescent probes of the cholecystokinin (CCK) receptor by attaching acrylodan or a nitrobenzoxadiazole moiety to the amino terminus of a partial agonist CCK analogue. These two probes were able to bind to the CCK receptor specifically and with high affinity, and were able to elicit only submaximal intracellular calcium responses typical of partial agonists. The fluorescence characteristics of these probes were compared with those previously reported for structurally-related full agonist and antagonist probes. Like the previous probes, the partial agonist probes exhibited longer fluorescence lifetimes and increased anisotropy when bound to the receptor than when free in solution. The receptor-bound probes were not easily quenched by potassium iodide, suggesting that the fluorophores were protected from the extracellular aqueous milieu. The fluorescence characteristics of the partial agonist probes were quite similar to those of the analogous full agonist probes and quite distinct from the analogous antagonist probes. These data suggest that the partially activated conformational state of this receptor is more closely related to its fully active state than to its inactive state.

Possible endogenous agonist mechanism for the activation of secretin family G protein-coupled receptors

The class B family of G protein-coupled receptors contains several potentially important drug targets, yet our understanding of the molecular basis of ligand binding and receptor activation remains incomplete. Although a key role is recognized for the cysteine-rich, disulfide-bonded amino-terminal domain of these receptors, detailed insights into ligand docking and resultant conformational changes are not clear. We postulate that binding natural ligands to this domain results in a conformational change that exposes an endogenous ligand which interacts with the body of the receptor to activate it. In this work, we examined whether a synthetic peptide corresponding to a candidate region between the first and third conserved cysteines could act as an agonist. Indeed, this peptide was a weakly potent but fully efficacious agonist, stimulating a concentration-dependent cAMP response in secretin receptor-bearing cells. This effect was maintained as the peptide length was reduced from 30 to 5, and ultimately, three residues focused on the conserved residue Asp49. The agonist potency was enhanced by cyclization through a diaminopropionic acid linker and by amino-terminal fatty acid acylation. Both ends of the cyclic peptide were shown to interact with the top of transmembrane segment 6 of the receptor, using probes with a photolabile benzoyl-phenylalanine on each end. Analogous observations were also made for two other members of this family, the vasoactive intestinal polypeptide type 1 and calcitonin receptors. These data may provide a unique molecular mechanism and novel leads for the development of small-molecule agonists acting at potential drug targets within this physiologically important receptor family.

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

Fluorescence techniques can provide insight into the environment of fluorescence indicators situated at distinct sites within a ligand as it is bound to its receptor. Here, we have developed a series of analogues of the 27-amino acid hormone, secretin, that incorporate a fluorescent Alexa Fluor 488 into the amino terminus, the carboxyl terminus, and positions 13 and 22. Each probe bound with high affinity and was biologically active, stimulating full cAMP responses in receptor-bearing Chinese hamster ovary-SecR cells. Treatment with 10 mum guanosine 5'-(beta,gamma-imido)triphosphate (GppNHp) shifted the agonist-bound receptor into a G protein-uncoupled low affinity state. Fluorescence spectra for the probes in solution and bound to the receptor demonstrated maximal emission at 521 nm after excitation at 481 nm. Collisional quenching of fluorescence with potassium iodide revealed that Alexa at the amino terminus of secretin was more accessible than at the other three positions within the probes. Of note, quenching constants for each probe were higher when bound in the active state than in the G protein-uncoupled, low affinity state of the receptor, with the most marked changes occurring for the two midregion probes. Anisotropy values and fluorescence lifetimes confirmed this, with higher anisotropy and longer lifetimes observed for position 13 and 22 probes bound to the receptor in its uncoupled state than in its active state. These observations suggest that the amino terminus of secretin as docked to the receptor is most exposed to the hydrophilic aqueous milieu, and that the major changes in conformation and exposure to the medium occur in the midregion of secretin. Photoaffinity labeling studies have demonstrated approximation of each of these ligand residues with distinct receptor residues. Combining the fluorescence data with photoaffinity labeling data provides insights into the conformation and dynamics of a natural peptide ligand docked to a Family B G protein-coupled receptor.

Differential docking of high-affinity peptide ligands to type A and B cholecystokinin receptors demonstrated by photoaffinity labeling

Type A and B cholecystokinin (CCK) receptors are highly homologous members of the class-I family of G protein-coupled receptors that bind CCK with high affinity. However, they have divergent structural specificities, with the type A receptor requiring seven carboxyl-terminal residues including a sulfated tyrosine and the type B receptor requiring only the carboxyl-terminal tetrapeptide. The aim of this work was to utilize affinity labeling to determine spatial approximations with photolabile p-benzoyl-l-phenylalanine (Bpa) residues sited at each end of CCK as docked at the type B CCK receptor, contrasting this with analogous work using similar probes docked at the type A receptor. Both probes were fully efficacious, potent agonists that stimulated intracellular calcium in receptor-bearing CHO-CCKBR cells (EC(50) values: Bpa(24) probe, 41 +/- 9 pM; Bpa(33) probe, 15 +/- 3.3 pM). They bound specifically, with high affinity (K(i) values: Bpa(24) probe, 0.60 +/- 0.17 nM; Bpa(33) probe, 0.58 +/- 0.11 nM). Cyanogen bromide cleavage of the covalently labeled receptor suggested the first extracellular loop as the region of labeling by each probe, distinct from the type A CCK receptor regions labeled using the same probes (third loop and amino-terminal tail, respectively). This was confirmed by subsequent enzymatic and chemical cleavage of labeled wild-type and mutant receptors. Sequential cycles of Edman degradation of labeled receptor fragments identified the specific residues within loop one labeled by each probe (Bpa(24) probe labeled Phe(122); Bpa(33) probe labeled Thr(119)). This provides a direct demonstration of distinct modes of docking the same high-affinity ligand to highly homologous receptors.

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.

Insights into the structure and molecular basis of ligand docking to the G protein-coupled secretin receptor using charge-modified amino-terminal agonist probes

The amino terminus and third loop regions of class B G protein-coupled receptors play critical roles in ligand docking and action. For the prototypic secretin receptor, the hormone amino terminus is spatially approximated with receptor region high in transmembrane segment 6 (TM6), whereas residues ranging from position 6 through 26 label the amino terminus. Here, we focus on the role of charge of the secretin amino terminus, using a series of full-agonist, acetylated probes. Sites of covalent labeling were examined using sequential purification, chemical and enzymatic cleavage, and Edman degradation. High-affinity amino-terminally-blocked probes labeled the distal amino-terminal tail, rather than TM6, while adding a basic residue, again labeled TM6. These data suggest that the secretin amino terminus docks between the amino terminus and TM6 of the receptor, with this region of secretin likely interacting with an acidic residue within the receptor TM6 and the third extracellular loop. To explore this, candidate acidic residues were mutated to Ala (E341A, D342A, E345A, E351A). The E351A mutant markedly interfered with binding, biological activity, and internalization, whereas all others bound secretin and signaled and internalized normally. This supports the possibility that there is a charge-charge interaction between this residue and the amino terminus of secretin that is critical to its normal docking.

Demonstration of a Specific Site of Covalent Labeling of the Human Motilin Receptor Using a Biologically Active Photolabile Motilin Analog

The motilin receptor belongs to a group of class I G protein-coupled receptors that also includes the growth hormone secretagogue and ghrelin receptors. These represent clinically useful targets for pharmacotherapy. Their potentially unique structures and the molecular basis of their binding are not yet clear. We previously reported the initial affinity labeling of a region within this receptor (a cyanogen bromide fragment extending from the first to the second extracellular loop) using a position 1 photolabile motilin analog. To extend our understanding of the molecular basis of motilin binding, we have developed an additional radioiodinatable motilin analog probe having site of covalent attachment in position 5. This was a full agonist that bound to the motilin receptor specifically and with high affinity, and that efficiently established a single covalent bond to its receptor. Sequential chemical and enzymatic cleavage of labeled wild-type and mutant motilin receptor constructs established that the region of labeling was within the third extracellular loop. This was further localized to Phe(332) using radiochemical Edman degradation sequencing. These data provide the first spatial approximation constraint that can be used in the docking of this peptide ligand to its receptor. We hope that a series of such constraints can be determined to provide adequate structural information to begin to elucidate the conformation of this agonist-bound receptor and to ultimately be useful in the rational design of drugs acting at this important target.

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.

Key Differences in Molecular Complexes of the Cholecystokinin Receptor with Structurally Related Peptide Agonist, Partial Agonist, and Antagonist

The molecular basis of docking of receptor ligands having differences in biological activity and their subsequent effects on receptor conformation represent areas of great interest. In this work, we focus on the sulfated tyrosyl residue in position 27 of cholecystokinin (CCK) and its spatial approximation with the type A CCK receptor residue Arg(197) that has been predicted from mutagenesis experiments. We have examined the requirement for sulfation of this residue in a series of structurally related peptide agonists, partial agonists, and antagonists using assays of receptor binding and biological activity. Whereas sulfation of CCK position 27 was critical for affinity and potency of a full agonist, it had progressively less effect as the biological activity of the ligand was reduced. It had an intermediate effect on the partial agonist and no effect on the antagonist. In addition, photoaffinity labeling was used to determine the spatial approximations between the receptor and residue 27 of the agonist and antagonist in this series. Direct photoaffinity labeling with a full agonist probe confirmed the spatial approximation of ligand residue 27 and receptor residue Arg(197) in the active complex. Of note, the analogous antagonist probe labeled a distinct region within the receptor amino terminus, confirming a key structural difference in active and inactive complexes.

Molecular approximation between a residue in the amino-terminal region of calcitonin and the third extracellular loop of the class B G protein-coupled calcitonin receptor

The calcitonin receptor is a member of the class B family of G protein-coupled receptors, which contains numerous potentially important drug targets. Delineation of themes for agonist binding and activation of these receptors will facilitate the rational design of receptor-active drugs. We reported previously that a photolabile residue within the carboxyl-terminal half (residue 26) and mid-region (residue 16) of calcitonin covalently label the extracellular amino-terminal domain of this receptor (Dong, M., Pinon, D. I., Cox, R. F., and Miller, L. J. (2004) J. Biol. Chem. 279, 1167-1175). Chimeric receptor studies support the importance of this region and suggest important contributions of extracellular loop domains. To examine whether other parts of the ligand may contact those loops, we developed another probe that has its photolabile site of labeling within the amino-terminal half in position 8 of the ligand. This probe was a full agonist (EC(50) = 563 +/- 67 pm), stimulating cAMP accumulation in receptor-bearing human embryonic kidney 293 cells in a concentration-dependent manner. It bound specifically and saturably (K(i) = 14.3 +/- 1.9 nm) and was able to efficiently label the calcitonin receptor. By purification, specific cleavage, and sequencing of labeled wild-type and mutant calcitonin receptors, the site of attachment was identified as residue Leu(368) within the third extracellular loop of the receptor, a domain distinct from that labeled by previous probes. These data are consistent with a common ligand binding mechanism for receptors in this important family.

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.

Spatial approximation between the amino terminus of a peptide agonist and the top of the sixth transmembrane segment of the secretin receptor

Distinct spatial approximations between residues within the secretin pharmacophore and its receptor can provide important constraints for modeling this agonist-receptor complex. We previously used a series of probes incorporating photolabile residues into positions 6, 12, 13, 14, 18, 22, and 26 of the 27-residue peptide and demonstrated that each covalently labeled a site within the receptor amino terminus. Although supporting a critical role of this domain for ligand binding, it does not explain the molecular mechanism of receptor activation. Here, we developed probes having photolabile residues at the amino terminus of secretin to explore possible approximations with a different receptor domain. The first probe incorporated a photolabile p-benzoyl-l-phenylalanine into the position of His(1) of rat secretin ([Bpa(1),Tyr(10)]secretin-27). Because His(1) is critical for function, we also positioned a photolabile Bpa as an amino-terminal extension, in positions -1 (rat [Bpa(-1),Tyr(10)]secretin-27) and -2 (rat [Bpa(-2),Gly(-1),Tyr(10)]secretin-27). Each analog was shown to be a full agonist, stimulating cAMP accumulation in receptor-bearing Chinese hamster ovary-SecR cells in a concentration-dependent manner, with the position -2 probe being most potent. They bound specifically and saturably, although the position 1 analog had lowest affinity, and all were able to label the receptor efficiently. Sequential specific cleavage, purification, and sequencing demonstrated that the sites of covalent attachment for each probe were high within the sixth transmembrane segment. This suggests that secretin binding may exert tension between the receptor amino terminus and the transmembrane domain to elicit a conformational change effecting receptor activation.

Importance of the amino terminus in secretin family G protein-coupled receptors. Intrinsic photoaffinity labeling establishes initial docking constraints for the calcitonin receptor

The calcitonin receptor is a member of the class B family of G protein-coupled receptors, closely related to secretin and parathyroid hormone receptors. Although mechanisms of ligand binding have been directly explored for those receptors, current knowledge of the molecular basis of calcitonin binding to its receptor is based only on receptor mutagenesis. In this work we have utilized the more direct approach of photoaffinity labeling to explore spatial approximations between distinct residues within calcitonin and its receptor. For this we have developed two human calcitonin analogues incorporating a photolabile p-benzoyl-l-phenylalanine residue in the mid-region and carboxyl-terminal half of the peptide in positions 16 and 26, respectively. Both probes specifically bound to the human calcitonin receptor with high affinity and were potent stimulants of cAMP accumulation in calcitonin receptor-bearing human embryonic kidney 293 cells. They covalently labeled the calcitonin receptor in a saturable and specific manner. Further purification, deglycosylation, specific chemical and enzymatic cleavage, and sequencing of labeled wild type and mutant calcitonin receptors identified the sites of labeling for the position 16 and 26 probes as receptor residues Phe137 and Thr30, respectively. Both were within the extracellular amino terminus of the calcitonin receptor, with the former adjacent to the first transmembrane segment and the latter within the distal amino-terminal tail of the receptor. These data are consistent with affinity labeling of other members of the class B G protein-coupled receptors using analogous probes and may suggest a common ligand binding mechanism for this family.

Spatial approximation between two residues in the mid-region of secretin and the amino terminus of its receptor. Incorporation of seven sets of such constraints into a three-dimensional model of the agonist-bound secretin receptor

Photoaffinity labeling of receptors by bound agonists can provide important spatial constraints for molecular modeling of activated receptor complexes. Secretin is a 27-residue peptide hormone with a diffuse pharmacophoric domain that binds to the secretin receptor, a prototypic member of the Class B family of G protein-coupled receptors. In this work, we have developed, characterized, and applied two new photolabile probes for this receptor, with sites for covalent attachment in peptide positions 12 and 14, surrounding the previously most informative site of affinity labeling of this receptor. The [Tyr10,(BzBz)Lys12]rat secretin-27 probe covalently labeled receptor residue Val6, whereas the [Tyr10,(BzBz)Lys14]rat secretin-27 probe labeled receptor residue Pro38. When combined with previous photoaffinity labeling data, there are now seven independent sets of constraints distributed throughout the peptide and receptor amino-terminal domain that can be used together to generate a new molecular model of the ligand-occupied secretin receptor. The amino-terminal domain of this receptor presented a stable platform for peptide ligand interaction, with the amino terminus of the peptide hormone extended toward the transmembrane helix domain of the receptor. This provides clear insights into the molecular basis of natural ligand binding and supplies testable hypotheses regarding the molecular basis of activation of this receptor.

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

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

Development of a biologically active secretin analogue incorporating a radioiodinatable photolabile p-(4-hydroxybenzoyl)phenylalanine in position 10

Photoaffinity labeling is a powerful approach for direct elucidation of residue-residue approximations as a ligand is bound to its receptor, providing important constraints for molecular modeling. Probes utilized for this need to incorporate photolabile sites of covalent attachment and an indicator, such as a radiolabel. Radioiodine provides a particularly useful high specific radioactivity label, but due to its size, can only be accommodated in limited positions within a peptide ligand. In this work, we attempted to develop a probe for the secretin receptor that would directly provide spatial approximation data for position 10 of secretin, its site of radiolabeling. This was achieved by incorporation into a secretin analogue of the radioiodinatable and photolabile benzophenone moiety, p-(4-hydroxybenzoyl)phenylalanine (OH-Bpa). An unintended additional modification of secretin in synthesizing this probe was the elimination of Gly(4). This probe was shown to bind to the secretin receptor specifically and saturably (K(i)=25.3+/-6.0 nM). It represented a full agonist, stimulating intracellular cAMP in a concentration-dependent manner (EC(50)=4.2+/-0.7 nM). It was also able to affinity label the secretin receptor in a specific and efficient manner. This probe should provide the opportunity to identify the region of the secretin receptor in spatial approximation with position 10, within the pharmacophore of secretin, leading to refinement of molecular conformational models of this agonist-bound receptor.

Interaction among four residues distributed through the secretin pharmacophore and a focused region of the secretin receptor amino terminus

The amino terminus of the secretin receptor (SecR) is known to be critical for natural agonist action, although the role it plays is still unclear. We have demonstrated that photolabile residues within both the amino-terminal (position 6) and carboxyl-terminal (positions 22 and 26) halves of secretin each covalently label receptor amino-terminal tail residues [Dong et al., J Biol Chem, 274:19161-19167 (1999), 274:903-909 (1999), and 275:26032-26039 (2000)]. Here, we extend this series of studies with an additional probe having its site of covalent attachment in a distinct region of the peptide, between amino- and carboxyl-terminal helical domains. This probe incorporated a photolabile (epsilon-p-benzoylbenzoyl)lysine in position 18 and a site for oxidative radioiodination [(tyrosine(10),(benzoyl-benzoyl)lysine(18))rat secretin-27]. This analog represented a full agonist, stimulating cAMP accumulation in Chinese hamster ovary-SecR cells in a concentration-dependent manner. It bound to the SecR specifically and saturably, and was able to efficiently label that molecule within its amino terminus. Sequential specific cleavage, purification, and sequencing demonstrated that this probe labeled receptor residue arginine(14), in the same subdomain as that labeled by previous probes. Consistent with the importance of this residue, alanine replacement mutagenesis (R14A) resulted in substantial reductions in the potency (127-fold) and binding affinity (400-fold) of secretin relative to its action at the wild-type receptor. We have been able to accommodate all four extant pairs of residue-residue approximations between divergent regions of the secretin pharmacophore and the first forty residues of the SecR into a credible molecular model of this interaction. Additional experimentally derived constraints will be necessary to determine the spatial positioning of this complex with the remainder of the SecR.

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.

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.

Identification of peptide ligand-binding domains within the human motilin receptor using photoaffinity labeling

The cDNA encoding the human motilin receptor was recently cloned and found to represent a G protein-coupled receptor that is structurally related to the growth hormone secretagogue receptors. Together, these represent a new Class I receptor family. Our aim in the present work is to gain insight into the molecular basis of binding of motilin to its receptor using photoaffinity labeling. To achieve this, we developed a Chinese hamster ovary cell line that overexpressed functional motilin receptor (CHO-MtlR; 175,000 sites per cell, with K(i) = 2.3 +/- 0.4 nm motilin and EC(50) = 0.3 +/- 0.1 nm motilin) and a radioiodinatable peptide analogue of human motilin that incorporated a photolabile p-benzoyl-l-phenylalanine (Bpa) residue into its pharmacophoric domain. This probe, [Bpa(1),Ile(13)]motilin, was a full agonist at the motilin receptor that increased intracellular calcium in a concentration-dependent manner (EC(50) = 1.5 +/- 0.4 nm). This photolabile ligand bound specifically and with high affinity to the motilin receptor (K(i) = 12.4 +/- 1.0 nm), and covalently labeled that molecule within its M(r) = 45,000 deglycosylated core. Cyanogen bromide cleavage demonstrated its covalent attachment to fragments of the receptor having apparent M(r) = 6,000 and M(r) = 31,000. These were demonstrated to represent fragments that included both the first and the large second extracellular loop domains, with the latter representing a unique structural feature of this receptor. The spatial approximation of the pharmacophoric domain of motilin with these receptor domains support their functional importance as well.

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.

Cross-chimeric analysis of selectivity of secretin and VPAC(1) receptor activation

Agonist action at receptors is highly specific, affected by the structure of both ligand and receptor. Chimeric constructs of structurally related receptors and/or ligands that have biological differences provide an opportunity to correlate a specific structural domain with function. In this work, we have used a cross-chimeric approach to explore the structural basis for rat secretin and vasoactive intestinal polypeptide action at their closely related secretin and VPAC(1) receptors, belonging to class II of the G protein-coupled receptor superfamily. Multiple domains of both ligands and receptors contributed toward their selectivity, with differing combinations of such domains able to support high-potency interactions. The amino-terminal 15 residues of secretin were most critical for potent stimulation of secretin receptors, whereas either the amino- or carboxyl-terminal halves of vasoactive intestinal polypeptide, when complemented by Lys(15), provided potent stimulation of the VPAC(1) receptor. The amino terminus of the VPAC(1) receptor was most critical for potent response to vasoactive intestinal polypeptide, whereas the amino terminus of the secretin receptor was important, but not adequate, requiring the complementation of an extracellular loop domain for potent response to secretin. Differences in the distribution of these determinants within these receptors provided an opportunity to produce a more "universal" receptor that contained the first extracellular loop of the secretin receptor and the remainder of the VPAC(1) receptor. This cross-chimeric approach should be applied to other members of this receptor family to test the emerging themes and to expand these insights as broadly as possible.

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

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

Identification of an interaction between residue 6 of the natural peptide ligand and a distinct residue within the amino-terminal tail of the secretin receptor

Photoaffinity labeling is a powerful tool for the characterization of the molecular basis of ligand binding. We recently used this technique to demonstrate the proximity between a residue within the carboxyl-terminal half of a secretin-like ligand and the amino-terminal domain of the secretin receptor (Dong, M., Wang, Y., Pinon, D. I., Hadac, E. M., and Miller, L. J. (1999) J. Biol. Chem. 274, 903-909). In this work, we have developed another novel radioiodinatable secretin analogue ([Bpa6,Tyr10]rat secretin-27) that incorporates a photolabile p-benzoyl-L-phenylalanine (Bpa) residue into position 6 of the amino-terminal half of the ligand and used this to identify a specific receptor residue proximate to it. This probe specifically bound to the secretin receptor with high affinity (IC50 = 13.2 +/- 2.5 nM) and was a potent stimulant of cAMP accumulation in secretin receptor-bearing Chinese hamster ovary-SecR cells (EC50 = 720 +/- 230 pM). It covalently labeled the secretin receptor in a saturable and specific manner. Cyanogen bromide cleavage of this molecule yielded a single labeled fragment that migrated on an SDS-polyacrylamide gel at Mr = 19,000 that shifted to 10 after deglycosylation, most consistent with either of two glycosylated fragments within the amino-terminal tail. By immunoprecipitation with antibody directed to epitope tags incorporated into each of the two candidate fragments, the most distal fragment at the amino terminus was identified as the domain of labeling. The labeled domain was further refined to the first 16 residues by endoproteinase Lys-C cleavage and by cyanogen bromide cleavage of another receptor construct in which Val16 was mutated to Met. Radiochemical sequencing of photoaffinity-labeled secretin receptor fragments established that Val4 was the specific site of covalent attachment. This provides the first residue-residue contact between a secretin ligand and its receptor and will contribute substantially to the molecular understanding of this interaction.

A peptide agonist acts by occupation of a monomeric G protein-coupled receptor: dual sites of covalent attachment to domains near TM1 and TM7 of the same molecule make biologically significant domain-swapped dimerization unlikely

Membrane receptor dimerization is a well-established event for initiation of signaling at growth factor receptors and has been postulated to exist for G protein-coupled receptors, based on correction of nonfunctional truncated, mutant, or chimeric constructs by coexpression of appropriate normal complementary receptor domains. In this work, we have directly explored the molecular composition of the minimal functional unit of an agonist ligand and the wild-type G protein-coupled cholecystokinin (CCK) receptor, using photoaffinity labeling with a CCK analogue probe incorporating dual photolabile benzoylphenylalanine (Bpa) residues as sites of covalent attachment. This probe, 125I-D-Tyr-Gly-[(Nle28, 31, Bpa29,33)CCK-26-33], was shown to represent a full agonist and to specifically label the CCK receptor. Like probes incorporating individual photolabile residues in these positions,1,2 the two Bpa residues in the dual photoprobe covalently labeled receptor domains in the amino-terminal tail outside TM1 and in the third extracellular loop outside TM7. Absence of demonstrable receptor dimerization after the establishment of dual sites of covalent attachment supports the presence of these two domains within a single receptor molecule. Demonstration of the covalent adduct of a single probe molecule with the two cyanogen bromide fragments of the CCK receptor representing the expected domains further supports this interpretation. Thus, while domain-swapped dimerization of G protein-coupled receptors may be possible as a mechanism of rescue for nonfunctional molecules, it is not necessary for ligand binding and initiation of signaling at a wild-type receptor in this superfamily. The functional unit for CCK action is normally a ligand-receptor monomer.

Regulation of lateral mobility and cellular trafficking of the CCK receptor by a partial agonist

Partial agonists are effective tools for advancing development of highly selective drugs and providing insights into molecular regulation of cellular functions. Here, we explore the impact of a partial agonist on key aspects of cholecystokinin (CCK) receptor regulation, its lateral mobility and cellular trafficking, in native pancreatic acinar cells and Chinese hamster ovary cells expressing CCK receptor (CHO-CCKR). We developed and characterized a novel fluorescent partial agonist, rhodamine-Gly-[(Nle28, 31)CCK-26-32]-phenethyl ester, that binds specifically and with high affinity to CCK receptors. Such analogs are fully efficacious pancreatic acinar cell secretagogues without supramaximal inhibition that mobilize intracellular calcium with little or no increase in phospholipase C (PLC) activity. Despite minimal phosphorylation of CCK receptors in response to this partial agonist, receptor trafficking was the same as that observed with full agonist (CCK). This included normal internalization via clathrin-dependent endocytosis in CHO-CCKR cells and insulation on the surface of pancreatic acinar cells. Also, as with CCK-occupied receptor, fluorescence recovery after photobleaching of partial agonist-occupied receptor on the acinar cell surface demonstrated a marked temperature-dependent slowing of its rate of diffusion. This was similarly associated with resistance to acid-induced dissociation of ligand. Thus some key molecular regulatory mechanisms for CCK receptor internalization and insulation may be initiated by cellular signaling cascades that are not dependent on PLC activation or receptor phosphorylation.

Structurally related peptide agonist, partial agonist, and antagonist occupy a similar binding pocket within the cholecystokinin receptor. Rapid analysis using fluorescent photoaffinity labeling probes and capillary electrophoresis

The molecular basis of ligand binding to receptors provides important insights for drug development. Here, we explore domains of the cholecystokinin (CCK) receptor that are critical for ligand binding, using a novel series of fluorescent photolabile probes, receptor proteolysis, and rapid high resolution separation of peptide fragments by capillary electrophoresis. Each probe incorporated the same fluorophore and a photolabile p-benzoylphenylalanine at the amino terminus of the pharmacophoric domain (residue 24 of CCK-33) of CCK analogues representing full agonist, partial agonist, and antagonist of this receptor. Each was used to label the CCK receptor expressed on Chinese hamster ovary-CCKR cells, with the labeled domain then released by cyanogen bromide cleavage. Capillary electrophoresis with laser-induced fluorescence detection achieved an on-capillary mass sensitivity of 1.6 attomoles (10(-18) mol), with an excellent signal-to-noise ratio. Each of the biologically divergent, but structurally similar probes saturably and specifically labeled the same receptor domain, consistent with conservation of "docking" determinants. This had an apparent mass of 2.9 kDa, most consistent with the first extracellular loop domain. An additional probe having its site of covalent attachment in a different region of the probe (residue 29 of CCK-33) labeled a distinct receptor fragment with differential migration on capillary electrophoresis (third extracellular loop). Identification of the specific receptor residue(s) covalently linked to the amino-terminal probes must await further fragmentation and sequence analysis.

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

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