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Abstract

Chimeric vasopressin V₂/OT receptors were constructed and investigated to identify receptor regions involved in ligand binding or G protein coupling. The fusion sites for one series of hybrid receptors were either located at the C-terminal end of the third extracellular domain or in the centre of the third transmembrane helix, respectively. In each pair of the resulting symmetrical hybrids only one receptor was able to bind arginine vasopressin (AVP) and/or oxytocin (OT). In both cases a major part of the vasopressin V₂, receptor (V₂R) was needed for ligand binding. A chimeric OT/V₂ receptor including Of receptor (OTR) sequences from its N-terminus to the middle of transmembrane region three showed both high-affinity OT binding (K_i = 3 nM) and activation of the adenylyl cyclase. In contrast, a hybrid containing OTR sequences reaching from transmembrane helix five to its C-terminus showed the V₂ receptors ligand binding profile and was unable to couple to Gα_s. These results indicate (i) that the third and/or the fourth intracellular domain of the V₂R are involved in G protein coupling and (ii) for high-affinity OT binding the N-terminal third of the OTR plays an important role.

By detailed binding studies on a second series of chimeric V₂/OT receptors with AVP, OT and the two hybrid hormone derivatives arginine vasotocin and oxypressin it was further demonstrated that the first two extracellular domains of the OTR are involved in binding to the C-terminal tripeptide of OT. Moreover, the third extracellular domain of the OTR is able to contact the cyclic part of OT and the fourth outer domain does not interact with the two variable amino acid residues of AVP and OT. Thus, the first three extracellu­lar domains of the OTR provide an essential part of the OT binding site. The other part is most probably contributed by the OTRs transmembrane helices 3 and 4.

Photoaffinity labeling and ligand binding studies demonstrated that the binding site for the OT antagonist d(CH₂)₅[Tyr(Me)², Thr⁴, Orn⁸, Tyr⁹]vasotocin is located in the heli­ces 1, 2 and 7. Our results provide evidence for the existence of separate domains of a peptide hormone receptor involved in binding and selectivity for agonists and peptide an­tagonists.