Molecular mapping of epitopes involved in ligand activation of the human receptor for the neuropeptide, VIP, based on hybrids with the human secretinreceptor

Receptors for the neurotransmitter and neuroendocrine peptides, vasoactive intesinal peptide (VIP) and secretin, both belong to the Type B subfamily o f G-protein-coupled receptors. This group is evolutionally as well as struc turally distinct from the much larger Type A, or rhodopsin-type, subfamily. We have mapped the ligand-activating epitopes of the human VIP1 receptor b y the use of hybrid receptor constructs with the human secretin receptor. T welve chimeras were synthesized by successively replacing portions of the f ormer receptor with corresponding portions of the latter receptor, or by in terchanging the first extracellular loops. Each of the different chimeric r eceptor DNAs were then expressed in murine reporter cells, and their abilit y to activate cAMP production was investigated on stimulation with the resp ective natural peptide ligands. We stimulated the reporter cells with secre tin or VIP following transient expression of the receptor chimeras. The exp eriments indicated that there are two molecular domains of importance for t he recognition and activation of these peptides, namely, the inner portion of the extracellular tail and the first extracellular loop of the two recep tors.