Properties of chimeric secretin and VIP receptor proteins indicate the importance of the N-terminal domain for ligand discrimination.

Two chimeras were obtained by substituting the DNA sequence encoding the N-terminal extracellular domain of the VIP and secretin receptors by the homologous DNA sequence encoding the secretin (N-Sn/VIP.r) and VIP receptor (N-VIP/Sn.r), respectively. These chimeric receptors were transfected and stably expressed in CHO cells. Their pharmacological properties were then compared to the corresponding recombinant "wild type" receptors, expressed in the same cell line. Binding data were obtained for the wild types and the N-VIP/Sn.r but not for the N-Sn/VIP receptor. Functional data (adenylate cyclase activation) were obtained in all cases. In order to minimize the effects of an excess of receptors and thus, to compare validly binding and functional data, we determined agonists EC50 values after down regulation of the receptors (i.e. after a pretreatment of the cells for 24 h with VIP or secretin). The order of potency of the peptides for receptor occupancy and adenylate cyclase activation indicated that the N-terminal extracellular domain of each receptor was the key element for discrimination between secretin and VIP.