Mapping the binding site of arginine vasopressin to V1a and V1b vasopressin receptors

Starting from the 2.8-angstrom resolution x-ray structure of bovine rhodopsin, three-dimensional molecular models of the complexes between arginine vasopressin and two receptor subtypes (V-1a, V-1b) have been built. Amino acid sequence alignment and docking studies suggest that four key residues (1.35, 2.65, 4.61, and 5.35) fine tune the binding of vasopressin and related peptide agonists to both receptor subtypes. To validate these predictions, a series of single or double mutants were engineered at V-1a and V-1b receptor subtypes and tested for their binding and functional properties. Two negatively charged amino acids at positions 1.35 and 2.65 are key anchoring residues to the Arg8 residue of arginine vasopressin. Moreover, two amino acids (V-4.61 and P-5.35) delineating a hydrophobic subsite at the human V-1b receptor are responsible for the recognition of V-1b selective peptide agonists. Last, one of the latter positions (5.35) is hypothesized to explain the pharmacological species differences between rat and human vasopressin receptors for a V-1b peptide agonist. Altogether these refined three-dimensional models of V-1a and V-1b human receptors should enable the identification of further new selective V-1a and V-1b agonists as pharmacological but also therapeutic tools.