Differential determinants for peptide and non-peptidyl ligand binding to the motilin receptor. Critical role of second extracellular loop for peptide binding and action

Structural basis for motilin and erythromycin recognition by motilin receptor

Motilin is an endogenous peptide hormone almost exclusively expressed in the human gastrointestinal (GI) tract. It activates the motilin receptor (MTLR), a class A G protein-coupled receptor (GPCR), and stimulates GI motility. To our knowledge, MTLR is the first GPCR reported to be activated by macrolide antibiotics, such as erythromycin. It has attracted extensive attention as a potential drug target for GI disorders. We report two structures of G_q-coupled human MTLR bound to motilin and erythromycin. Our structures reveal the recognition mechanism of both ligands and explain the specificity of motilin and ghrelin, a related gut peptide hormone, for their respective receptors. These structures also provide the basis for understanding the different recognition modes of erythromycin by MTLR and ribosome. These findings provide a framework for understanding the physiological regulation of MTLR and guiding drug design targeting MTLR for the treatment of GI motility disorders.

Identification, expression analysis, and functional characterization of motilin and its receptor in spotted sea bass (Lateolabrax maculatus)

Motilin (MLN), an interdigestive hormone secreted by endocrine cells of the intestinal mucosa, binds to a G protein-coupled receptor to exert its biological function of regulating gastrointestinal motility. In the present study, we identified the prepromotilin and mln receptor (mlnr) from the spotted sea bass, Lateolabrax maculatus. Mln consisted of an ORF of 336 nucleotides encoding 111 amino acids. The precursor protein contained a 17-amino-acid mature peptide. Mlnr had an ORF of 1089 bp encoding a protein of 362 amino acids. Seven transmembrane domains were predicted with TMHMM analysis. The phylogenetic analysis of mln and mlnr showed that they fell into the same clade with respective counterpart of selected fishes before clustering with other detected vertebrates. Both mln and mlnr genes were highly expressed in intestine of spotted sea bass using quantitative real-time PCR. In situ hybridization indicated that mln and mlnr mRNA were both localized in the lamina propria and the epithelial cell of intestinal villus. The expressions of both genes were regulated under short-term starvation in a time-dependent manner. In vitro experiments indicated that the expressions of ghrelin (ghrl), gastrin (gas) and cholecystokinin (cck) were enhanced by MLN after 3-h treatment, but the effect was absent after 6 or 12-h incubation. Taken together, the MLN and its receptor might play important roles in regulating intestinal motility in spotted sea bass.

Motilin: from gastric motility stimulation to hunger signalling

After the discovery of motilin in 1972, motilin and the motilin receptor were studied intensely for their role in the control of gastrointestinal motility and as targets for treating hypomotility disorders. The genetic revolution - with the use of knockout models - sparked novel insights into the role of multiple peptides but contributed to a decline in interest in motilin, as this peptide and its receptor exist only as pseudogenes in rodents. The past 5 years have seen a major surge in interest in motilin, as a series of studies have shown its relevance in the control of hunger and regulation of food intake in humans in both health and disease. Luminal stimuli, such as bitter tastants, have been identified as modulators of motilin release, with effects on hunger and food intake. The current state of knowledge and potential implications for therapy are summarized in this Review.

Effects of Motilin Receptor Agonists and Ghrelin in Human motilin receptor Transgenic Mice

Gastrointestinal motility is regulated by neural factors and humoral factors. Both motilin and ghrelin improve gastrointestinal motility, but many issues remain unclear. We prepared human motilin receptor transgenic (Tg) mice and performed experiments evaluating the effects of motilin, erythromycin (EM), and ghrelin. EM and ghrelin promoted gastric emptying (GE) when administered either peripherally or centrally to Tg mice. Atropine (a muscarinic receptor antagonist) counteracted GE induced by centrally administered EM, but not that induced by peripherally administered EM. The administration of EM in this model promoted the effect of mosapride (a selective serotonin 5-hydroxytryptamine 4 (5-HT4) receptor agonist), and improved loperamide (a μ-opioid receptor agonist)-induced gastroparesis. The level of acyl-ghrelin was significantly attenuated by EM administration. Thus, we have established an animal model appropriate for the evaluation of motilin receptor agonists. These data and the model are expected to facilitate the identification of novel compounds with clinical potential for relieving symptoms of dyspepsia and gastroparesis.

Chemical Diversity in the G Protein-Coupled Receptor Superfamily

G protein-coupled receptors (GPCRs) are the largest family of cell signaling transmembrane proteins that can be modulated by a plethora of chemical compounds. Systematic cheminformatics analysis of structurally and pharmacologically characterized GPCR ligands shows that cocrystallized GPCR ligands cover a significant part of chemical ligand space, despite their limited number. Many GPCR ligands and substructures interact with multiple receptors, providing a basis for polypharmacological ligand design. Experimentally determined GPCR structures represent a variety of binding sites and receptor-ligand interactions that can be translated to chemically similar ligands for which structural data are lacking. This integration of structural, pharmacological, and chemical information on GPCR-ligand interactions enables the extension of the structural GPCR-ligand interactome and the structure-based design of novel modulators of GPCR function.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Molecular identification of GHS-R and GPR38 in Suncus murinus

We previously identified ghrelin and motilin genes in Suncus murinus (suncus), and also revealed that motilin induces phase III-like strong contractions in the suncus stomach in vivo, as observed in humans and dogs. Moreover, repeated migrating motor complexes were found in the gastrointestinal tract of suncus at regular 120-min intervals. We therefore proposed suncus as a small laboratory animal model for the study of gastrointestinal motility. In the present study, we identified growth hormone secretagogue receptor (GHS-R) and motilin receptor (GPR38) genes in the suncus. We also examined their tissue distribution throughout the body. The amino acids of suncus GHS-R and GPR38 showed high homology with those of other mammals and shared 42% amino acid identity. RT-PCR showed that both the receptors were expressed in the hypothalamus, medulla oblongata, pituitary gland and the nodose ganglion in the central nervous system. In addition, GHS-R mRNA expressions were detected throughout the stomach and intestine, whereas GPR38 was expressed in the gastric muscle layer, lower intestine, lungs, heart, and pituitary gland. These results suggest that ghrelin and motilin affect gut motility and energy metabolism via specific receptors expressed in the gastrointestinal tract and/or in the central nervous system of suncus.

Food intake and interdigestive gastrointestinal motility in ghrelin receptor mutant rats

BACKGROUND

Ghrelin is the endogenous ligand for the growth hormone secretagogue receptor (GHSR). Ghrelin regulates feeding activity and interdigestive contractions of the stomach in rodents. To investigate the role of endogenous ghrelin in the digestive system, we have developed GHSR-mutant rats, named FHH-Ghsr(m1Mcwi), using the Fawn-Hooded Hypertensive (FHH) parental strain.

METHODS

N-ethyl-N-nitrosourea (ENU) was used as a mutagen. Genomic DNA prepared from a tail clip was analyzed using the targeting induced local lesions in genomes (TILLING) approach. The non-synonymous mutation in position 343 (NM_032075) led to the generation of a premature stop codon, causing deletion of the last 22 amino acids at the C-terminal of ghrelin receptor protein. Spontaneous and ghrelin-stimulated food intake was measured in wild-type (WT) FHH and FHH-Ghsr(m1Mcwi) rats. For interdigestive motility recording, two strain gauge transducers were sutured on the antrum and duodenum. Spontaneous gastroduodenal contractions were recorded in freely moving conscious rats.

RESULTS

Ghrelin (40 μg/kg) failed to stimulate food intake in the mutant rats, while spontaneous food intake was not significantly different between the WT rats and FHH-Ghsr(m1Mcwi) rats. Phase III-like contractions were observed in stomach and duodenum both in the WT and FHH-Ghsr(m1Mcwi) rats. In the WT rats, ghrelin (12 μg/kg) administration enhanced spontaneous phase III-like contractions, and a GHSR antagonist, (D-lys3)GHRP-6 (0.28 mg/kg), abolished the spontaneous phase III-like contractions. In FHH-Ghsr(m1Mcwi) rats, ghrelin and (D-lys3)GHRP-6 did not affect phase III-like contractions.

CONCLUSIONS

It is suggested that the intact GHSR structure is essential for the ghrelin-dependent regulation of interdigestive motility and feeding behavior. Even in FHH-Ghsr(m1Mcwi) rats, spontaneous gastric phase III-like contractions were still observed, suggesting the development of a compensatory mechanism to maintain these contractions.

Mutational analysis of predicted extracellular domains of human growth hormone secretagogue receptor 1a

The Class A family of guanine nucleotide-binding protein (G protein)-coupled receptors that includes receptors for motilin, ghrelin, and growth hormone secretagogue (GHS) has substantial potential importance as drug targets. Understanding of the molecular basis of hormone binding and receptor activation should provide insights helpful in the development of such drugs. We previously reported that Cys residues and the perimembranous residues in the extracellular loops and amino-terminal tail of the motilin receptor are critical for peptide ligand, motilin, binding and biological activity. In the current work, we focused on the predicted extracellular domains of the human GHS receptor 1a, and identified functionally important residues by using sequential deletions ranging from one to twelve amino acid residues and site-directed replacement mutagenesis approach. Each construct was transiently expressed in COS cells, and characterized for ghrelin- and growth hormone releasing peptide (GHRP)-6-stimulated intracellular calcium responses and ghrelin radioligand binding. Cys residues in positions 116 and 198 in the first and second extracellular loops and the perimembranous Glu¹⁸⁷ residue in the second extracellular loop were critical for ghrelin and GHRP-6 biological activity. These results suggest that Cys residues in the extracellular domains in this family of Class A G protein-coupled receptor is likely involved in the highly conserved and functionally important disulfide bond, and that the perimembranous residues contribute peptide ligand binding and signaling.

Primary structure, tissue distribution, and biological activity of chicken motilin receptor

Motilin is a peptide hormone involved in gastrointestinal motility. GPR38, initially cloned as an orphan receptor, is now considered a specific receptor for motilin. Previously, molecular characterization of the motilin receptor had only been performed in mammalian and fish species. In this study, we cloned cDNA for chicken motilin receptor from the duodenum and characterized its primary structure, tissue distribution, and biological activity. The cDNA encoded 349 amino acids showing significant overall sequence identity to mammalian motilin receptors. Chicken motilin increased intracellular Ca2+ concentration in human embryonic kidney (HEK) 293 cells transiently expressing the recombinant chicken motilin receptor. Comparison of the cDNA sequence with the genomic sequence of chicken motilin receptor revealed that the chicken motilin receptor gene consists of two exons separated by an intron. Real-time PCR analysis showed that chicken motilin receptor mRNA is expressed in a wide range of tissues in 21-day-old chickens, with markedly high levels in the proventriculus, duodenum, and oviduct. The expression levels of the mRNA in the proventriculus and duodenum were highest just before hatching and rapidly decreased during post-hatch development. These results suggest that chicken motilin receptor is largely involved in gastrointestinal functions at pre- and post-hatch periods through an intracellular signaling pathway accompanied by an increase in Ca2+ levels.

Molecular identification and characterization of the dog motilin receptor

Motilin, a 22-amino acid peptide hormone secreted by endocrine cells of the intestinal mucosa, plays an important role in the regulation of gastrointestinal motility. The actions of motilin agonists have been extensively investigated in dogs due to physiological similarities between the dog and human alimentary tracts. The amino acid sequence of the dog motilin receptor, however, was previously unknown. We have cloned a cDNA from dog stomach corresponding to the motilin receptor. The deduced protein shared 71% and 72% sequence identity with the human and rabbit motilin receptors, respectively. Expression of the dog motilin receptor in CHO cells promoted the typical cellular responses to the agonists, motilin and erythromycin. The rank order of potency determined for these agonists was similar to that found for the human motilin receptor, with motilin being more potent than erythromycin. Immunohistochemistry of the dog stomach revealed that the motilin receptor was localized in neuronal cell bodies and fibers. This is the first study detailing the cloning, expression, and functional characterization of the dog motilin receptor. Determination of the full sequence and functional properties of the dog motilin receptor will provide useful information enabling us to interpret previous and future studies of motilin agonists in dogs.

Mutational analysis of predicted intracellular loop domains of human motilin receptor

Motilin is an important endogenous regulator of gastrointestinal motor function, mediated by the class I G protein-coupled motilin receptor. Motilin and erythromycin, two chemically distinct full agonists of the motilin receptor, are known to bind to distinct regions of this receptor, based on previous systematic mutagenesis of extracellular regions that dissociated the effects on these two agents. In the present work, we examined the predicted intracellular loop regions of this receptor for effects on motilin- and erythromycin-stimulated activity. We prepared motilin receptor constructs that included sequential deletions throughout the predicted first, second, and third intracellular loops, as well as replacing the residues in key regions with alanine, phenylalanine, or histidine. Each construct was transiently expressed in COS cells and characterized for motilin- and erythromycin-stimulated intracellular calcium responses and for motilin binding. Deletions of receptor residues 63-66, 135-137, and 296-301 each resulted in substantial loss of intracellular calcium responses to stimulation by both motilin and erythromycin. Constructs with mutations of residues Tyr66, Arg136, and Val299 were responsible for the negative impact on biological activity stimulated by both agonists. These data suggest that action by different chemical classes of agonists that are known to interact with distinct regions of the motilin receptor likely yield a common activation state of the cytosolic face of this receptor that is responsible for interaction with its G protein. The identification of functionally important residues in the predicted cytosolic face provides strong candidates for playing roles in receptor-G protein interaction.

Endogenous acyl ghrelin is involved in mediating spontaneous phase III-like contractions of the rat stomach

In humans and dogs, it is known that motilin regulates phase III contractions of migrating motor complex (MMC) in the fasted state. In rats, however, motilin and its receptor have not been found, and administration of motilin failed to induce any phase III-like contractions. Ghrelin was discovered as the endogenous ligand for the growth hormone secretagogue receptor (GHS-R) from the rat stomach. Ghrelin promotes gastric premature phase III (phase III-like contractions) in the fasted state in rats. We hypothesized that endogenous ghrelin regulates spontaneous phase III-like contractions in rats. Strain gauge transducer was sutured on the antrum and a catheter was inserted into the jugular vein. We studied the effects of i.v. administration of ghrelin and a GHS-R antagonist on gastric phase III-like contractions in conscious rats. Plasma level of ghrelin was measured by a radioimmunoassay. Ghrelin augmented spontaneous phase III-like contractions and a GHS-R antagonist significantly attenuated the occurrence of spontaneous phase III-like contractions. During the phase I period, plasma ghrelin level increased to its peak then returned to basal level, subsequently phase III-like contractions were observed. These results suggest that endogenous ghrelin regulates gastric phase III-like contractions in rats.

Motilin activates neurons in the rat amygdala and increases gastric motility

Motilin and motilin receptors have been found in most regions of the brain, including the amygdala, one of the most important parts of the limbic system. Our previous study found that administration of motilin in the hippocampus stimulates gastric motility. We now explore the effect of motilin in the amygdala on gastric motility. In conscious rats, gastric motility was recorded after microinjection of motilin, motilin receptor antagonist (GM-109) or a mixture of the two into the basomedial amygdala nucleus (BMA). In anesthetized rats the changes of spontaneous discharges of gastric distention sensitive neurons (GDSN) in the BMA were recorded after intracerebroventricular (i.c.v.) microinjection of motilin or GM-109. In conscious rats the amplitude of gastric contractions increased dose-dependently after microinjection of motilin in the BMA, and decreased after microinjection of GM-109. The excitatory or inhibitory effects induced by motilin or GM-109 alone, were weakened by microinjection of a mixture solution of both. The spontaneous discharge frequency of gastric distention excitatory neuron (GDEN) was mainly inhibited by i.c.v. microinjection of motilin but excited by GM-109. In contrast, the spontaneous discharge frequency of gastric distention inhibitory neuron (GDIN) was mainly excited by motilin, but inhibited by GM-109. Our findings suggest that motilin may regulate gastric motility by modulating neural pathways in the BMA.

Differences between the abilities of tegaserod and motilin receptor agonists to stimulate gastric motility in vitro

BACKGROUND AND PURPOSE

Motilin or 5-HT4 receptor agonists stimulate gastrointestinal motility. Differences in activity are suggested but direct comparisons are few. A method was devised to directly compare the gastric prokinetic activities of motilin, the motilin receptor agonist, erythromycin, and the 5-HT4 receptor agonist, tegaserod.

EXPERIMENTAL APPROACH

Gastric prokinetic-like activity was assessed by measuring the ability to facilitate cholinergically-mediated contractions evoked by electrical field stimulation (EFS) in rabbit isolated stomach. Comparisons were made between potency, maximal activity and duration of responses.

KEY RESULTS

Rabbit motilin (r.motilin) 0.003-0.3 microM, [Nle13]motilin 0.003-0.3 microM, erythromycin 0.3-10 microM and tegaserod 0.1-10 microM caused concentration - dependent potentiation of EFS-evoked contractions. The potency ranking was r.motilin = [Nle13]motilin > tegaserod > erythromycin. The Emax ranking was r.motilin = [Nle13]motilin = erythromycin > tegaserod. Responses to r.motilin and [Nle13]motilin faded rapidly (t1/2 9 and 11 min, respectively) whereas those to erythromycin and tegaserod were maintained longer (t1/2 24 and 28 min). The difference did not appear to be due to peptide degradation. A second application of [Nle13]motilin was excitatory after 60 min contact and fade of the initial response (responses to 0.03 and 0.1 microM [Nle13]motilin were not different from those caused by the first application).

CONCLUSIONS AND IMPLICATIONS

Prokinetic-like activities of the 5-HT4 agonist tegaserod and the motilin receptor agonists were compared by measuring changes in cholinergically-mediated contractions. This novel approach highlighted important differences between classes (greater Emax of motilin, compared with tegaserod) and for the first time, within each class (short t1/2 for motilin, compared with erythromycin).

Molecular characterization and distribution of motilin family receptors in the human gastrointestinal tract

BACKGROUND

Motilin and ghrelin have been recognized as important endogenous regulators of gastrointestinal motor function in mammals, mediated respectively by the motilin receptor and by the closely related ghrelin receptor. The aims of this study were to explore the distribution of motilin and ghrelin receptors along the human gastrointestinal tract and to establish the molecular nature of the human motilin receptor.

METHODS

Post mortem and surgical human tissue specimens with no hemorrhage, necrosis, or tumor were obtained from various parts of the gastrointestinal tract. We analyzed levels of expression of mRNA for motilin and ghrelin receptors and examined their molecular identities. Portions of some specimens were also studied by immunohistochemistry for expression of the motilin and ghrelin receptor.

RESULTS

The long form of the motilin receptor, but not the short form, was expressed in all parts of the gastrointestinal tract, and expressed at higher levels in muscle than in mucosa. Motilin receptor immunoreactivity was present in muscle cells and the myenteric plexus, but not in mucosal or submucosal cells. In contrast, ghrelin receptor mRNA was expressed equally in all parts of the gastrointestinal tract, with similar levels of expression in mucosal and muscle layers.

CONCLUSIONS

Both the motilin and ghrelin receptors are expressed along the human gastrointestinal tract, but they have clearly distinct distributions in regard to both level and layer. The diffuse muscle expression of the motilin receptor, at both the levels of the gene and the protein product, along the entire gastrointestinal tract makes it a useful potential target for motilide drugs for dysmotility.

Differential Contributions of Motilin Receptor Extracellular Domains for Peptide and Non-peptidyl Agonist Binding and Activity

The family of G protein-coupled receptors that includes receptors for motilin, ghrelin, and growth hormone secretagogue has substantial potential importance as drug targets. Understanding of the molecular basis of hormone binding and receptor activation should provide insights that are helpful in the development of such drugs. We previously examined the unique second extracellular loop domain of the motilin receptor, identifying key epitopes in perimembranous locations at each end of this long loop (Matsuura, B., Dong, M., and Miller, L. J. (2002) J. Biol. Chem. 277, 9834-9839). Here, we have extended that work, examining the other predicted extracellular domains of the motilin receptor by using sequential deletions of segments ranging from one to six amino acid residues and site-directed alanine replacement mutagenesis approaches. Each construct was transiently expressed in COS cells, and characterized for motilin- and erythromycin-stimulated intracellular calcium responses and motilin radioligand binding. Only those receptor segments that included key Cys residues in positions 25, 30, and 111 or perimembranous regions at the ends of the amino terminus and the first and third extracellular loops disrupted motilin biological activity. Each of these Cys deletions also disrupted action of erythromycin. Alanine replacements for each of the potentially important amino acid residues in the perimembranous segments revealed that residues Gly36, Pro103, Leu109, and Phe332 were responsible for the selective negative impact on motilin biological activity, while responding normally to erythromycin. These results support the presence of functionally important disulfide bonds in the motilin receptor ectodomain and demonstrate that the structural determinants for binding and biological activity of peptide and non-peptidyl agonist ligands are distinct, with a broad extracellular perimembranous base contributing to normal motilin binding.

Molecular characterization and distribution of motilin family receptors in the human gastrointestinal tract

BACKGROUND

Motilin and ghrelin have been recognized as important endogenous regulators of gastrointestinal motor function in mammals, mediated respectively by the motilin receptor and by the closely related ghrelin receptor. The aims of this study were to explore the distribution of motilin and ghrelin receptors along the human gastrointestinal tract and to establish the molecular nature of the human motilin receptor.

METHODS

Post mortem and surgical human tissue specimens with no hemorrhage, necrosis, or tumor were obtained from various parts of the gastrointestinal tract. We analyzed levels of expression of mRNA for motilin and ghrelin receptors and examined their molecular identities. Portions of some specimens were also studied by immunohistochemistry for expression of the motilin and ghrelin receptor.

RESULTS

The long form of the motilin receptor, but not the short form, was expressed in all parts of the gastrointestinal tract, and expressed at higher levels in muscle than in mucosa. Motilin receptor immunoreactivity was present in muscle cells and the myenteric plexus, but not in mucosal or submucosal cells. In contrast, ghrelin receptor mRNA was expressed equally in all parts of the gastrointestinal tract, with similar levels of expression in mucosal and muscle layers.

CONCLUSIONS

Both the motilin and ghrelin receptors are expressed along the human gastrointestinal tract, but they have clearly distinct distributions in regard to both level and layer. The diffuse muscle expression of the motilin receptor, at both the levels of the gene and the protein product, along the entire gastrointestinal tract makes it a useful potential target for motilide drugs for dysmotility.

Motilin and erythromycin-A share a common binding site in the third transmembrane segment of the motilin receptor

The motilin receptor (MTLR) represents a clinically useful pharmacological target, as agonists binding to the MTLR have gastroprokinetic properties. In order to compare the molecular basis for interaction of the MTLR with motilin and with the non-peptide motilin agonist, erythromycin-A (EM-A), the negatively charged E119 located in the third transmembrane (TM3) region was mutated to D (E119D) and Q (E119Q), respectively, and changes in activity of the mutant receptors were verified.

METHODS

Each mutant receptor was stably transfected in CHO-cells containing the Ca2+ indicator apo-aequorin. Receptor activation in response to motilin, EM-A and their analogues was assessed by Ca2+-luminescense.

RESULTS

In the E119Q mutant, the Ca2+ response to motilin and EM-A was abolished while in the E119D mutant it was reduced with 62% (motilin) and 81% (EM-A). The pEC50 values were shifted from 9.65+/-0.03 to 7.41+/-0.09 (motilin) and from 6.63+/-0.12 to 4.60+/-0.07 (EM-A). Acetylation of the N-terminal amine group as in [N-acetyl-Phe]1 mot (1-14), decreased the potency 6.3-fold (WT-MTLR) and 148-fold (E119D). Acetylation of EM-A enol ether induced a more pronounced shift in potency: 7943-fold (WT-MTLR) and 1413-fold (E119D).

CONCLUSION

The comparable loss of affinity of the mutant receptors for motilin and EM-A indicate that these agonists both interact with the TM3 domain of the MTLR. The results with acetylated derivatives support an ionic interaction between E119 of the MTLR with the N+ of the desosamine sugar in EM-A, but not with the N+ of the free amine group in motilin.

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.

Mutational analysis of the large periplasmic loop 7-8 of the putrescine transporter PotE in Escherichia coli

The PotE protein is a putrescine-ornithine antiporter found in many gram-negative bacteria. It is a member of the APA family of transporters and has 12 predicted alpha-helical transmembrane spanning segments (TMS). While the substrate binding site has previously been mapped to a region near the surface of the cytoplasmic lipid layer, no structural feature within the periplasmic domains of PotE have been shown to be important for function. We examined the role of the only large outer loop, situated between transmembrane spanning segment 7 and 8, in putrescine uptake. Deletion of the highly conserved amino acids in the region closest to transmembrane spanning segment 7 produced a protein with little activity. Glycine-scanning mutagenesis of this region showed that Val(249) and Leu(254) were required for optimal transporter function. The V249G mutant transported putrescine at a lower maximal rate compared to wild-type (WT) but with the same substrate binding affinity. In contrast, the L254G mutant had a higher substrate affinity. A series of Val(249) mutants indicated that the hydrophobicity of this residue, which is located at or near the membrane surface, is important for PotE function. Secondary structure predictions of the large outer loop indicated the presence of a hydrophobic alpha-helix in the centre with a hydrophobic region at each end suggesting that the loop was not entirely exposed to the aqueous periplasmic space. The study shows that loop 7-8 is important for PotE function, possibly by forming a re-entrant loop in the channel of the transporter.

The motilin pharmacophore in CHO cells expressing the human motilin receptor

We performed a structure-activity study with the human motilin receptor, which was recently cloned from thyroid tissue. N-terminal fragments, Ala-analogs of motilin, and motilides were tested in a cell line that expresses the cloned human motilin receptor and apoaequorin. Full potency to induce calcium fluxes was obtained with N-terminal fragments of 14 amino acids. Motilin fragments 1-14 in which residues 1 (Phe), 4 (Ile), and 7 (Tyr) were replaced by Ala showed the largest reduction in potency. Only motilides with an enol configuration had markedly higher potencies compared to erythromycin A. The potencies to induce Ca(2+) fluxes correlated strongly with rabbit binding and contractility data, suggesting that the cloned receptor is indeed the motilin receptor, responsible for contractile effects. Conservation of the motilin pharmacophore in evolution indicates an important physiological role of motilin.