Proline residue 280 in the second extracellular loop (EC2) of the VPAC(2) receptor is essential for the receptor structure

Inspection of the amino acid sequence of the human VPAC(1) and the VPAC(2) receptors after alignment of the conserved residues indicates that the seco nd extracellular loop (EC2) is one amino acid shorter in the VPAC, receptor due to the lack of a proline residue in position 294. We hypothesized that this could be of importance for receptor structure and/or for ligand recog nition. Insertion by directed mutagenesis of a proline in that position (< Pro > 294 VPAC(1)) had little consequence on the binding of several agonist s but reduced the affinity for the VPAC(1) antagonist. Coupling of the < Pr o > 294 VPAC(1) receptor to adenylate cyclase was improved, as demonstrated by an increased affinity for VIP and other agonists. and by a shift of the VPAC, antagonist to partial agonist behavior, Deletion of the proline 280 (Delta Pro280 VPAC(2)) in the VPAC(2) receptor markedly reduced the apparen t affinity for all the agonists tested. Replacement of the proline by a gly cine residue had a smaller effect on the ligands affinities. The proline re sidue in the VPAC, receptor EC2 is thus essential for the receptor structur e, and the EC2 domain is involved in ligand recognition and receptor functi onality, (C) 2001 Elsevier Science Inc. All rights reserved.