The glucagon-like peptide-1 receptor (GLP-1R) is a family B G proteincoupled receptor and an important drug target for the treatment of type II diabetes, with activation of pancreatic GLP-1Rs eliciting glucose-dependent insulin secretion. Currently, approved therapeutics acting at this receptor are peptide based, and there is substantial interest in small molecule modulators for the GLP-1R. Using a variety of resonance energy transfer techniques, we demonstrate that the GLP-1R forms homodimers and that transmembrane helix 4 (TM4) provides the primary dimerization interface. We show that disruption of dimerization using a TM4 peptide, a minigene construct encoding TM4, or by mutation of TM4, eliminates G protein-dependent highaffinity binding to GLP-1(7-36)NH 2 but has selective effects on receptor signaling. There was <10-fold decrease in potency in cAMP accumulation or ERK1/2 phosphorylation assays but marked loss of intracellular calcium mobilization by peptide agonists. In contrast, there was near-complete abrogation of the cAMP response to an allosteric agonist, compound 2, but preservation of ERK phosphorylation. Collectively, this indicates that GLP-1R dimerization is important for control of signal bias. Furthermore, we reveal that two small molecule ligands are unaltered in their ability to allosterically modulate signaling from peptide ligands, demonstrating that these modulators act in cis within a single receptor protomer, and this has important implications for small molecule drug design.allosteric modulation | biased signaling | G protein-coupled receptors | family B GPCRs G protein-coupled receptors (GPCRs) are the largest superfamily of cell surface proteins and play crucial roles in virtually every physiological process. Their widespread abundance, yet selective distribution, and ability to couple to a variety of signaling and effector systems make them extremely attractive targets for drug development (1). Recently, there has been increasing interest in the stoichiometry of receptors involved in GPCR signaling complexes and how this may impact on receptor function and drug discovery (2-4).With the exception of family C GPCRs, where obligate dimerization can occur (4), the role of oligomerization in GPCR function has remained controversial (2-8), and this has been the subject of a number of recent reviews (9-11). Although there is an increasing body of evidence supporting dimerization of GPCRs as a widespread feature of GPCR biology, including numerous studies on family A GPCRs, whether these are stable, transient, constitutive, or ligand dependent, and how they impact on receptor function and drug discovery are less clear, and general rules for oligomeric behavior are not evident (9-16). Even where effects on signaling are studied, these are generally linked to a single pathway and the role of dimerization in the control of receptor engagement and preference for distinct intracellular signaling intermediates (i.e., signal bias) is virtually unstudied.For family B GPCRs, which encompass many ther...

Background: Allosteric ligands targeting cholecystokinin receptors are needed. Results: Stereochemically distinct iodinated 1,4-benzodiazepine antagonists of type 1 and 2 cholecystokinin receptors dock to analogous intramembranous pockets that have distinct shape and molecular determinants. Conclusion: The geometry of the binding pockets and specific residue interactions are unique for each receptor. Significance: The predictive power of these insights should be useful in the discovery of lead compounds and in their refinement.

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

Dimerization of the prototypic family B G protein-coupled secretin receptor is determined by the lipid-exposed face of transmembrane segment four (TM4), and has substantial functional importance, facilitating G protein coupling. Recently, we demonstrated that the human secretin receptor elicits an inter-receptor bioluminescence resonance energy transfer (BRET) signal with most other human family B peptide receptors, except for the calcitonin receptor. In this study we have explored the occurrence and importance of calcitonin receptor oligomerization. Static and saturation receptor BRET were utilized to demonstrate that, unlike the human calcitonin receptor that does not yield a significant homomeric BRET signal, the rabbit calcitonin receptor exhibits strong resonance energy transfer. Within the lipid-exposed face of TM4, rabbit and human calcitonin receptors differ by a single amino acid (Arg 236 in human; His in rabbit), while Thr 253 that occurs in human and rabbit calcitonin receptors is unique across family B receptors. Mutating Arg 236 or Thr 253 of the human calcitonin receptor to residues found in the rabbit calcitonin receptor or the human secretin receptor (R236H, R236Y and T253A) resulted in generation of significant BRET signals. Similarly, mutation of Val 250 of the human calcitonin receptor to another key lipid-facing residue found in the secretin receptor (V250I) also increased the receptor BRET signal. These data support the consistent theme of lipid-exposed residues of TM4 being important for the dimerization of the calcitonin receptor. However, rabbit and human calcitonin receptor constructs bound calcitonin and stimulated cAMP similarly, suggesting that differences in BRET could reflect differences in orientation or in the stability of homo-dimeric receptor complexes, which were nevertheless similarly effective in eliciting the functions attributed to that complex. The likelihood of human calcitonin receptor dimerization, even in the absence of a significant BRET signal, was further supported by data demonstrating that the peptide representing TM4 of this receptor that disrupts the rabbit receptor BRET signal, produced a right shift in the cAMP concentration-response curves for both rabbit and human receptors.

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