Histidine7.36(305) in the conserved peptide receptor activation domain of the gonadotropin releasing hormone receptor couples peptide binding and receptor activation

Transmembrane helix seven residues of G protein-coupled receptors (GPCRs) couple agonist binding to a conserved receptor activation mechanism. Amino-terminal residues of the GnRH peptide determine agonist activity. We investigated GnRH interactions with the His7.36(305) residue of the GnRH receptor, using functional and computational analysis of modified GnRH receptors and peptides. Non-polar His7.36(305) substitutions decreased receptor affinity for GnRH four- to forty-fold, whereas GnRH signaling potency was more decreased (~150-fold). Uncharged polar His7.36(305) substitutions decreased GnRH potency, but not affinity. [2-Nal3]-GnRH retained high affinity at receptors with non-polar His7.36(305) substitutions, supporting a role for His7.36(305) in recognizing Trp3 of GnRH. Compared with GnRH, [2-Nal3]-GnRH potency was lower at the wild type GnRH receptor, but unchanged or higher at mutant receptors. Results suggest that His7.36(305) of the GnRH receptor forms two distinct interactions that determine binding to Trp3 and couple agonist binding to the conserved transmembrane domain network that activates GPCRs.