Histidine^7.36(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 His^7.36(305) residue of the GnRH receptor, using functional and computational analysis of modified GnRH receptors and peptides. Non-polar His^7.36(305) substitutions decreased receptor affinity for GnRH four- to forty-fold, whereas GnRH signaling potency was more decreased (~150-fold). Uncharged polar His^7.36(305) substitutions decreased GnRH potency, but not affinity. [2-Nal³]-GnRH retained high affinity at receptors with non-polar His^7.36(305) substitutions, supporting a role for His^7.36(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 His^7.36(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.