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

Secretin Receptor as a Target in Gastrointestinal Cancer: Expression Analysis and Ligand Development

Secretin was originally discovered as a gastrointestinal peptide that stimulates fluid secretion from the pancreas and liver and delays gastric emptying. In disease, a secretin receptor (SCTR) was found to occur as a splice variant in gastrinoma and pancreatic adenocarcinoma. Overexpression of SCTR has been described for gastrinomas, carcinoid tumors of the lung and cholangiocarcinoma. SCTR therefore is considered a candidate target for molecular tumor imaging as well as for peptide receptor radioligand therapy (PRRT) in a number of oncological indications. The aim of this study was to characterize SCTR expression in esophageal and pancreatic cancer, demonstrating for the first time high SCTR overexpression in these tumor types. In total, 65 of 70 pancreatic ductal adenocarcinoma tissues stained strongly positive for SCTR in immunohistochemistry, as did most of the 151 esophageal cancer samples, with minor influence of grading in both entities. In addition, the aim of this study was to further delineate residues in human secretin that are critical for binding to and activation of human SCTR. For a potential development of short and metabolically stable analogs for clinical use, it was intended to probe the peptide for its capacity to incorporate deletions and substitutions without losing its affinity to SCTR. In a systematic approach, a library of 146 secretin variants containing single amino acid substitutions as well as truncations on either end was tested in β-arrestin2-GFP translocation and fluorescent ligand internalization assays employing high-content analysis, in cAMP assays which run in agonist and antagonist mode, and in radioligand binding. The main structural determinants of SCTR binding and activation were localized to the N-terminus, with His1, Asp3 being among the most sensitive positions, followed by Phe6, Thr7 and Leu10. Aminoterminal truncation caused a rapid decline in receptor activity and most of these variants proved to be partial agonists showing antagonistic properties. In this study, the most potent novel antagonist showed an IC50 of 309 ± 74 nM in the β-arrestin2-GFP translocation assay on human SCTR while remaining a weak partial agonist. Future studies will have to demonstrate the utility of further enhanced secretin analogues as tracers for in vivo imaging and therapy.

Use of Cysteine Trapping to Map Spatial Approximations between Residues Contributing to the Helix N-capping Motif of Secretin and Distinct Residues within Each of the Extracellular Loops of Its Receptor

Amino-terminal regions of secretin-family peptides contain key determinants for biological activity and binding specificity, although the nature of interactions with receptors is unclear. A helix N-capping motif within this region has been postulated to directly contribute to agonist activity while also stabilizing formation of a helix extending toward the peptide carboxyl terminus and docking within the receptor amino terminus. We used cysteine trapping to systematically explore spatial approximations between cysteines replacing each residue in this motif of secretin (sec), Phe(6), Thr(7), and Leu(10), and cysteines incorporated into the extracellular face of the receptor. Each peptide was a full agonist for cAMP, but had a lower binding affinity than natural hormone. These bound to COS cells expressing 61 receptor constructs incorporating cysteines in every position along each extracellular loop (ECL) and adjacent parts of transmembrane (TM) segments. Patterns of covalent labeling were distinct for each probe, with Cys(6)-sec labeling multiple residues in the carboxyl-terminal half of ECL2 and throughout ECL3, Cys(7)-sec predominantly labeling only single residues in the carboxyl-terminal end of ECL2 and the amino-terminal end of ECL3, and Cys(10)-sec not efficiently labeling any of these residues. These spatial constraints were used to refine our model of secretin bound to its receptor, now bringing ECL3 above the amino terminus of the ligand and revealing possible charge-charge interactions between this part of secretin and receptor residues in TM5, TM6, ECL2, and ECL3, which can orient and stabilize the peptide-receptor complex. This was validated by testing predicted approximations by mutagenesis and residue-residue complementation studies.

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

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

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.

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

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

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.

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.

Effects of pH and temperature on photoaffinity labeling of Family B G protein-coupled receptors

The efficiency of covalent labeling of a receptor by a photolabile analogue of its natural ligand is dependent on the spatial approximation of the probe and its target. Systematic application of intrinsic photoaffinity labeling to the secretin receptor, a prototypic Family B G protein-coupled receptor, demonstrated reduced efficiency of labeling for amino-terminal and mid-region sites of labeling relative to carboxyl-terminal sites. Reduction of pH from 7.4 to 5.5 and reduction of temperature from 25 degrees C to 4 degrees C improved the efficiency of covalent labeling of the receptor with these probes. This correlated with sites of labeling at the interface between the receptor amino terminus and the receptor core, a region containing histidine residues that have their ionization affected in this pH range. Application to the calcitonin receptor, another Family B G protein-coupled receptor, yielded analogous results. These results support the consistent mode of docking peptide ligands to this group of receptors.

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

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

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.

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

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

A novel secretin receptor splice variant potentially useful for early diagnosis of pancreatic carcinoma

BACKGROUND & AIMS

Pancreatic and bile duct carcinomas represent highly aggressive malignancies that evolve from secretin receptor-rich ductular cells. With premessenger RNA splicing abnormalities common in cancer, we evaluated whether an abnormal secretin receptor spliceoform were present, characterized it, and developed a serum assay for it.

METHODS

Cancer cell lines and healthy and neoplastic tissue were studied by nested reverse-transcription polymerase chain reaction and sequencing. A promising spliceoform was isolated and characterized, and monoclonal antibodies were raised to 2 distinct regions. A dual antibody enzyme-linked immunosorbent assay was developed and applied to blinded serum samples from 26 patients with pancreatic carcinoma, 10 patients with chronic pancreatitis, and 14 controls.

RESULTS

Each of 9 pancreatic cancer specimens and no normal tissue expressed a secretin receptor variant with exons 3 and 4 deleted. This encoded a 111-residue peptide with its first 43 residues identical to wild-type receptor, but, subsequent to a shift in coding frame and early truncation, the next 68 residues were unique in the transcriptome/proteome. This nonfunctional soluble protein did not bind or signal in response to secretin and was secreted from transfected MiaPaCa-2 cells. Elevated serum levels of this variant were present in 69% of pancreatic cancer patients, 60% of chronic pancreatitis patients, and 1 of 14 controls.

CONCLUSIONS

We identified a novel abnormal spliceoform of the secretin receptor in pancreatic and bile duct cancers and developed a dual antibody sandwich enzyme-linked immunosorbent assay to measure it in the circulation. Initial application of this assay in patients with pancreatic cancer and chronic pancreatitis was promising, but additional validation will be required to evaluate its clinical utility.

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

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

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.

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.

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.

Determining the Environment of the Ligand Binding Pocket of the Human Angiotensin II Type I (hAT1) Receptor Using the Methionine Proximity Assay

The peptide hormone angiotensin II (AngII) binds to the AT0 (angiotensin type 1) receptor within the transmembrane domains in an extended conformation, and its C-terminal residue interacts with transmembrane domain VII at Phe-293/Asn-294. The molecular environment of this binding pocket remains to be elucidated. The preferential binding of benzophenone photolabels to methionine residues in the target structure has enabled us to design an experimental approach called the methionine proximity assay, which is based on systematic mutagenesis and photolabeling to determine the molecular environment of this binding pocket. A series of 44 transmembrane domain III, VI, and VII X --> Met mutants photolabeled either with 125I-[Sar1,p'-benzoyl-L-Phe8]AngII or with 125I-[Sar1,p''-methoxy-p'-benzoyl-L-Phe8]AngII were purified and digested with cyanogen bromide. Several mutants produced digestion patterns different from that observed with wild type human AT1, indicating that they had a new receptor contact with position 8 of AngII. The following residues form this binding pocket: L112M and Y113M in transmembrane domain (TMD) III; F249M, W253M, H256M, and T260M in TMD VI; and F293M, N294M, N295M, C296M, and L297M in TMD VII. Homology modeling and incorporation of these contacts allowed us to develop an evidence-based molecular model of interactions with human AT1 that is very similar to the rhodopsin-retinal interaction.

Functional characterization and purification of the secretin receptor expressed in baculovirus-infected insect cells

Structural insights into Class II G protein-coupled receptors have been limited by the absence of a plentiful and highly enrichable source such as rhodopsin in the Class I family. With structural differences predicted to exist between these families, and with the key importance of an intact, disulfide-bonded amino-terminal domain for the Class II receptors, an overproduction and purification scheme is critically important. In this work, we have established and characterized a baculoviral expression and purification system for the secretin receptor. Hemagglutinin epitope-tagged wild-type rat secretin receptor construct was expressed using the recombinant baculovirus/Sf9 insect cell-based system, achieving a level of expression substantially higher than that previously achieved in Chinese hamster ovary (CHO-SecR) cells. Receptor expressed in Sf9 cells had similar affinity for secretin (Ki=1.4+/-0.2 nM) and similar potency to stimulate intracellular cAMP in response to this hormone (EC50=194+/-45 pM) as did wild-type receptor expressed in CHO cells. Receptors from Sf9 cells were also affinity labeled saturably and specifically by a photolabile secretin analogue. The receptors were purified to homogeneity by solubilization with sodium deoxycholate, selective ammonium sulfate precipitation, gel filtration and immunoaffinity purification. This expression system should facilitate the structural characterization of this receptor and its important amino-terminal domain.

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.

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

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

Presence of a N-terminal signal peptide in class II G protein-coupled receptors: crucial role for expression of the human VPAC1 receptor

The hVPAC1 receptor for vasoactive intestinal peptide (VIP) and pituitary adenylyl cyclase activating peptide (PACAP) has an N-terminal signal peptide like all other class II G protein-coupled receptors (GPCRs). We determined the role of the signal peptide in expression of human VPAC1 receptor in transfected CHO cells. Three constructs were transfected: Flag30-hVPAC1, a receptor containing an inserted FLAG sequence between Ala30 and Ala31 and fused in the C-terminal position to GFP; Flag30-[delta1-30]-hVPAC1, the same construct as Flag30-hVPAC1 but lacking the 1-30 putative signal peptide (SP) sequence; Flag0-hVPAC1, a receptor containing an N-terminal FLAG sequence and fused in the C-terminal position to GFP. For each construct, we determined 125I-VIP binding, VIP-induced cAMP production, GFP fluorescence and indirect immunofluorescence on nonpermeabilized cells incubated with mouse monoclonal anti-Flag antibodies. The data were consistent with a crucial role of the signal peptide for expression of functional VPAC1 receptors at the cell surface and suggested that the signal peptide is cleaved during the translocation of the receptor to the plasma membrane, probably in the endoplasmic reticulum.

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.

Identification of a contact site for residue 19 of parathyroid hormone (PTH) and PTH-related protein analogs in transmembrane domain two of the type 1 PTH receptor

Recent functional studies have suggested that position 19 in PTH interacts with the portion of the PTH-1 receptor (P1R) that contains the extracellular loops and seven transmembrance helices (TMs) (the J domain). We tested this hypothesis using the photoaffinity cross-linking approach. A PTHrP(1-36) analog and a conformationally constrained PTH(1-21) analog, each containing para-benzoyl-l-phenylalanine (Bpa) at position 19, each cross-linked efficiently to the P1R expressed in COS-7 cells, and digestive mapping analysis localized the cross-linked site to the interval (Leu232-Lys240) at the extracellular end of TM2. Point mutation analysis identified Ala234, Val235, and Lys240 as determinants of cross-linking efficiency, and the Lys240-->Ala mutation selectively impaired the binding of PTH(1-21) and PTH(1-19) analogs, relative to that of PTH(1-15) analogs. The findings support the hypothesis that residue 19 of the receptor-bound ligand contacts, or is close to, the P1R J domain-specifically, Lys240 at the extracellular end of TM2. The findings also support a molecular model in which the 1-21 region of PTH binds to the extracellular face of the P1R J domain as an alpha-helix.

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

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

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.

Molecular determinants of glucagon receptor signaling

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

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

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

International Union of Pharmacology. XXXV. The glucagon receptor family

Peptide hormones within the secretin-glucagon family are expressed in endocrine cells of the pancreas and gastrointestinal epithelium and in specialized neurons in the brain, and subserve multiple biological functions, including regulation of growth, nutrient intake, and transit within the gut, and digestion, energy absorption, and energy assimilation. Glucagon, glucagon-like peptide-1, glucagon-like peptide-2, glucose-dependent insulinotropic peptide, growth hormone-releasing hormone and secretin are structurally related peptides that exert their actions through unique members of a structurally related G protein-coupled receptor class 2 family. This review discusses advances in our understanding of how these peptides exert their biological activities, with a focus on the biological actions and structural features of the cognate receptors. The receptors have been named after their parent and only physiologically relevant ligand, in line with the recommendations of the International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC-IUPHAR).

Cgamma H2 of Met174 side chain is the site of covalent attachment of a substance P analog photoactivable in position 5

Analogs of substance P (H-RPKPQQFFGLM-NH(2)) incorporating a photoreactive para-benzoyl-l-phenylalanine (p-Bzl)Phe at position 4, 5, 6, 9, or 10 of the sequence have been synthesized and pharmacologically characterized previously as full NK-1 receptor agonists. In this study we show that all analogs, [BAPA(0), (p-Bzl)Phe(x), Met(O(2))(11)]SP also display high yields (40-70%) of NK-1 receptor photolabeling. To identify the site of photoinsertion in the receptor, covalent ligand/receptor complexes were digested with enzymes or chemically cleaved with cyanogen bromide and purified with streptavidin-coated magnetic beads before matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry analysis. Only the analog photoreactive at position 5 gave irreversible, reproducible, and unequivocal covalent linkage. Sequential digestions of the covalent complex, substance P analog photoreactive at position 5/NK-1 receptor, with trypsin, endo-GluC and carboxypeptidase Y, led to the identification of the tripeptide (173)TMP(175) in the second extracellular loop of the hNK-1 receptor as the site of photoinsertion. Reaction of cyanogen bromide on the pentapeptide TMPSR did not yield the expected cleavage on the carboxylic side of methionine. The high precision of mass spectrometry analysis on the mass measured led us to determine that C(gamma)H(2) of Met(174) was the site of covalent linkage of the photoreactive substance P analog. Such an insertion (photolinked ligand) on its C(gamma)H(2) renders methionine refractory to CNBr cleavage.

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.

Involvement of the second extracellular loop (E2) of the neurokinin-1 receptor in the binding of substance P. Photoaffinity labeling and modeling studies

Substance P (SP) interacts with the neurokinin-1 (NK-1) G-protein-coupled receptor, which has been cloned in several species. In the present study, the domains of the NK-1 receptor involved in the binding of SP and SP-(7-11) C-terminal fragment have been analyzed using two peptide analogs containing the photoreactive amino acid para-benzoylphenylalanine ((p-Bz)Phe) in position 8 of their sequence. This study was carried out with [BAPA-Lys(6),(p-Bz)Phe(8),Pro(9),Met(O(2))(11)]SP-(7-11) and [BAPA(0),(p-Bz)Phe(8)]SP on both rat and human NK-1 receptors expressed in CHO cells. Combined trypsin and endo-GluC enzymatic complete digestions and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis led to the identification of the same domain of covalent interaction, (173)TMPSR(177), for the two photoactivatable peptides. Further digestion of this fragment with carboxypeptidase Y led to the identification of (173)TMP(175) in the second extracellular loop (E2) of the NK-1 receptor as the site of covalent attachment. Models of the conformation of this E2 loop in the human NK-1 receptor were generated using two different strategies, one based on homology with bovine rhodopsin and the other based on the solution conformation preferences of a synthetic peptide corresponding to the E2 loop.

Design, synthesis, and conformational study of biologically active photolabeled analogues of the main immunogenic region of the acetylcholine receptor

Photoaffinity labeling is a powerful tool for the characterization of the molecular basis of ligand binding to acceptor molecules, which provides important insights for mapping the bimolecular interfaces. The autoimmune disease myasthenia gravis is caused by autoantibodies against the acetylcholine receptor (AChR). The majority of the anti-AChR antibodies bind to the "main immunogenic region" (MIR) of the AChR. To identify the contact points between the complementarity determining regions of the anti-MIR antibodies that recognize the MIR contact sites of the AChR, we present here three photoreactive dodecapeptide MIR analogues containing the photolabel p-benzoyl-L-phenylalanine (Bpa) moiety, either in position 1 or 11. The structure of the produced 12-mers was analyzed using two-dimensional (1)H-NMR spectroscopy, whereas their binding to anti-MIR monoclonal antibodies (mAbs) was determined by immunochemical assays. In all cases the modifications resulted in conservation of the beta-turn conformation of the N-terminus, which has been proved essential for antibody recognition and increased anti-MIR binding relative to the MIR decapeptide.

Dominant negative action of an abnormal secretin receptor arising from mRNA missplicing in a gastrinoma

BACKGROUND & AIMS

The provocative secretin-stimulation test has an important role in the diagnosis and management of gastrin-secreting neuroendocrine tumors. The aim of the present study was to explore the molecular basis for positive and false-negative secretin-stimulation test results in patients with these tumors.

METHODS

One of the rare patients with this histologically proven tumor who had a normal serum gastrin level and a negative secretin-stimulation test result, and 2 more typical patients with this syndrome were investigated using immunohistochemistry, reverse-transcription polymerase chain reaction, receptor binding, and signaling assays.

RESULTS

We confirmed the molecular nature of the secretin receptor in the gastrinomas with a positive provocative test result and identified a novel mechanism for a false-negative result. Tumor expression of the class B G protein-coupled secretin receptor mediates a positive result. The false-negative result was explained by messenger RNA missplicing, resulting in a receptor variant missing exon 3 that encodes residues 44-79 in the amino-terminal tail of the mature receptor. This variant with an in-frame deletion was shown to be synthesized and to traffic to the cell surface normally, where it could neither bind secretin nor mediate a secretin-stimulated adenosine 3',5'-cyclic monophosphate response. It was able to act as a dominant negative inhibitor of wild-type secretin receptor function.

CONCLUSIONS

These data may explain some of the atypical presentations of this syndrome and provide important insights into basic mechanisms of disease.

Receptor biology and signal transduction

This year has witnessed substantial advances in receptor biology and signal transduction that are relevant to the function and regulation of the healthy pancreas and to the pathogenesis and potential therapy of pancreatitis and pancreatic carcinoma. There has been an expansion in the cast of pancreatic regulatory molecules, now including protease-activated receptors, chemokines, and chemokine receptors. There have been new insights into the cellular distribution and signaling initiated at the classic pancreatic receptors. There have also been dramatic advances in insights into the structure of G protein-coupled receptors, with the first solution of a crystal structure of a member of this superfamily, and into the molecular basis of ligand binding and activation of these important molecules. This will clearly improve the opportunities for the rational design and refinement of receptor-active drugs. In addition to these fundamental advances, there has been renewed attention to the expression, function, and regulation of receptors and signaling pathways in pancreatic cells present in the setting of pancreatitis and pancreatic carcinoma. It is hoped that this will contribute toward earlier diagnosis, more successful therapy, and new chemopreventive strategies for these illnesses.

Tritiated photoactivatable analogs of the native human thrombin receptor (PAR-1) agonist peptide, SFLLRN-NH2

Six photoactivatable analogs of the human thrombin receptor activating peptide (TRAP), SFLLRN-NH2, were synthesized by substituting the photoactive amino acid, p-benzoylphenylalanine (Bpa), into each position of the peptide sequence. Platelet aggregation assays indicated that the peptides with Bpa substitutions at positions 3 to 6 retained agonist activity. These peptides were prepared in tritiated form as potential thrombin receptor photoaffinity labels. The [3H]Bpa-containing analogs were constructed by resynthesizing the peptides with the amino acid, 4-benzoyl-2',5'-dibromophenylalanine (Br2Bpa), and subjecting the purified peptides to Pd-catalyzed tritiodebromination. The radiochemical yields for the reductive tritiation were < 2% for peptides with [3H]Bpa in the third and fourth positions, and between 7 and 16% for the peptides with substitutions at the fifth and sixth positions. The low yields were due to over-reduction of the Bpa carbonyl group and nonspecific degradation during reductive tritiation. This report describes the first use of Br2Bpa for the preparation of tritiated photoactivatable peptides.

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

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

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

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

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

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

Direct identification of a peptide binding region in the opioid receptor-like 1 receptor by photoaffinity labeling with [Bpa(10),Tyr(14)]nociceptin

The heptadecapeptide nociceptin, also known as orphanin FQ, is the endogenous agonist of the opioid receptor-like 1 (ORL1) G protein-coupled receptor. An affinity labeling approach has been implemented to probe the interactions of the neuropeptide with the receptor using the photolabile nociceptin derivative, [p-benzoyl-l-Phe(10),Tyr(14)]nociceptin ([Bpa(10),Tyr(14)]noc). In recombinant Chinese hamster ovary cells expressing the human ORL1 receptor, [Bpa(10),Tyr(14)]noc binds the receptor with high affinity (K(i) approximately 0.7 nm) and is as potent as nociceptin in the inhibition of forskolin-induced cAMP synthesis (EC(50) approximately 0.5 nm). UV irradiation at 365 nm of the complex formed by the ORL1 receptor and radioiodinated [Bpa(10),Tyr(14)]noc results in the irreversible labeling of a glycoprotein of approximately 65 kDa, determined by SDS-polyacrylamide gel electrophoresis. Complete digestion of the partially purified 65-kDa complex with kallikrein generates a single labeled fragment (approximately 6.5 kDa) that is readily cleaved by endoproteinase Glu-C to yield a labeled fragment of approximately 3.2 kDa. Kallikrein treatment of the photoaffinity cross-linked Glu(295) --> Asp mutant receptor also yields a single labeled fragment of approximately 6.5 kDa but is resistant to further cleavage by endoproteinase Glu-C. Based upon the expected proteolytic fingerprint of the labeled receptor, the photoreactive region can be identified as ORL1-(296-302; residues Thr-Ala-Val-Ala-Ile-Leu-Arg) spanning the C terminus of extracellular loop 3 and the N terminus of transmembrane helix VII. Molecular modeling of the ORL1 receptor complex with [Bpa(10)]noc suggests that reaction of the Bpa carbonyl group may occur with the side chain of Ile(300) within the experimentally identified photoreactive region.

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.

mu Opioid receptor: role for the amino terminus as a determinant of ligand binding affinity

The importance of the amino-terminal domain of the mu opioid receptor (MOR) as a component of the high affinity ligand-binding pocket was evaluated. A deletion mutant lacking 64 amino acids from the amino-terminus of MOR (DeltaN64) was constructed and expressed in HEK 293 cells. The affinities of bremazocine and cyclazocine were similar for the truncated and full-length MORs. Affinities of the mu receptor antagonist, naloxone, and the mu receptor agonist, morphine, were decreased 3.5-fold and 6-fold, respectively, for the truncated receptor relative to the wild-type MOR. Similarly, the affinities of the opioid peptide agonists, DAMGO (Tyr-D-Ala-Gly-MePhe-Gly-ol), beta-endorphin, and DADL (Tyr-D-Ala-Gly-Phe-D-Leu), for the DeltaN64 receptor were decreased from 3- to 8-fold as a result of the deletion. In contrast, the affinities of the alkaloid agonists, methadone and fentanyl, and the peptide agonists, endomorphin 1 and endomorphin 2, for the truncated receptor relative to MOR were reduced dramatically by 20- to 60-fold. MOR is glycosylated when expressed in HEK 293 cells; however, analysis of N-glycosidase F-treated membranes indicated that N-glycan chains within the amino-terminal domain of MOR do not contribute significantly to ligand affinities. These results indicate that amino acid residues within the amino-terminal domain of MOR play a crucial role in the composition of the binding pocket for a select group of agonists.