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 Asp ⁴⁹. 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.