The binding site of neuropeptide vasopressin V1a receptor. Evidence for a major localization within transmembrane regions.

To identify receptor functional domains underlying binding of the neurohypophysial hormones vasopressin (AVP) and oxytocin (OT), we have constructed a three-dimensional (3D) model of the V1a vasopressin receptor subtype and docked the endogenous ligand AVP. To verify and to refine the 3D model, residues likely to be involved in agonist binding were selected for site-directed mutagenesis. Our experimental results suggest that AVP, which is characterized by a cyclic structure, could be completely buried into a 15-20-A deep cleft defined by the transmembrane helices of the receptor and interact with amino acids located within this region. Moreover, the AVP-binding site is situated in a position equivalent to that described for the cationic neurotransmitters. Since all mutated residues are highly conserved in AVP and OT receptors, we propose that the same agonist-binding site is shared by all members of this receptor family. In contrast, the affinity for the antagonists tested, including those with a structure closely related to AVP, is not affected by mutations. This indicates a different binding mode for agonists and antagonists in the vasopressin receptor.