An arginyl in the N-terminus of the V1a vasopressin receptor is part of the conformational switch controlling activation by agonist.

Defining how the agonist–receptor interaction differs from that of the antagonist–receptor and understanding the mechanisms of receptor activation are fundamental issues in cell signalling. The V_1a vasopressin receptor (V_1aR) is a member of a family of related G-protein coupled receptors that are activated by neurohypophysial peptide hormones, including vasopressin (AVP). It has recently been reported that an arginyl in the distal N-terminus of the V_1aR is critical for binding agonists but not antagonists. To determine specific features required at this locus to support high affinity agonist binding and second messenger generation, Arg46 was substituted by all other 19 encoded amino acids. Our data establish that there is an absolute requirement for arginyl, as none of the [R46X]V_1aR mutant constructs supported high affinity agonist binding and all 19 had defective signalling. In contrast, all of the mutant receptors possessed wildtype binding for both peptide and nonpeptide antagonists. The ratio of K_i to EC₅₀, an indicator of efficacy, was increased for all substitutions. Consequently, although [R46X]V_1aR constructs have a lower affinity for agonist, once AVP has bound all 19 are more likely than the wildtype V_1aR to become activated. Therefore, in the wildtype V_1aR, Arg46 constrains the inactive conformation of the receptor. On binding AVP this constraint is alleviated, promoting the transition to active V_1aR. Our findings explain why arginyl is conserved at this locus throughout the evolutionary lineage of the neurohypophysial peptide hormone receptor family of G-protein coupled receptors. [ABSTRACT FROM AUTHOR]