Molecular pharmacology of the secretin receptor.

The secretin receptor was the first member of the Class II family of G protein-coupled receptors to be cloned. It is prototypic of this family in its structure, function, and regulation. The extended amino-terminal tail domain includes a series of six conserved Cys residues that contribute three intradomain disulfide bonds. This region of the receptor has been shown by mutagenesis and photo-affinity labeling to be particularly important in secretin binding and stimulation of signaling activity. There is clear evidence for the direct interaction of the natural agonist peptide with this receptor domain. Mutagenesis has also identified important contributions of extracellular loop domains, although their specific roles remain unclear. This receptor is regulated by agonist-stimulated phosphorylation and internalization, with details dependent on the cellular environment.