The amino-terminal fragment of the adenylate cyclase activating polypeptide (PACAP) receptor functions as a high affinity PACAP binding domain.

The PACAP receptor represents a member of a novel subfamily of G-protein coupled receptors with a common structurally conserved extracellular domain of about 150 amino acids. We have addressed the question whether this extracellular amino-terminus of the PACAP type I receptor can solely function as a PACAP binding domain. For that purpose a cDNA was constructed that encodes the membrane-anchored amino-terminus of the rat PACAP receptor including the decapeptide epitope EQKLISEEDL for immunodetection. COS-7 cells were transfected with this cDNA and a comparable construct of the wild-type receptor. Binding analysis showed that the amino-terminal fragment of the PACAP receptor bound PACAP with high-affinity (Kd = 3.8 nM; Bmax = 12.8 pmol/mg protein). In comparison to the full-length receptor (Kd = 0.2 nM; Bmax = 1.96 pmol/mg protein) its affinity was reduced by a factor of about 20. The results suggest that the amino-terminus of the PACAP receptor functions as the major binding site for its ligand.