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.