Agonist binding to peptide hormone receptors

A fundamental issue in molecular pharmacology is to define how agonist–receptor interaction differs from that of antagonist–receptor interaction. The V_1a vasopressin receptor (V_1aR) is a member of a family of related G-protein-coupled receptors (GPCRs) that are activated by vasopressin, oxytocin (OT) and related peptides. A segment of the N-terminus that was required for agonist binding, but not antagonist binding, was identified by characterizing truncated V_1aR constructs. Site-directed mutagenesis revealed that a single residue (Arg⁴⁶) was critical for agonist binding and receptor activation. The N-terminus of the related OT receptor (OTR) could recover agonist binding in a chimaeric OTR^N–V_1aR construct. Furthermore, Arg³⁴ of the human OTR, which corresponds to Arg⁴⁶ of the rat V_1aR, provided agonist-specific binding epitopes in the OTR, indicating a conserved function of this locus throughout this GPCR subfamily. Mutation of Arg⁴⁶ revealed that high-affinity agonist binding had an absolute requirement for arginine at this position.