Mutations remote from the human gonadotropin-releasing hormone (GnRH) receptor-binding sites specifically increase binding affinity for GnRH II but not GnRH I: Evidence for ligand-selective, receptor-active conformations

The human gonadotropin-releasing hormone (GnRH) receptor is evolutionarily configured for high affinity binding of GnRH I ([Tyr⁵,Leu ⁷,Arg⁸GnRH) but at lower affinity for GnRH II ([His ⁵,Trp⁷,Tyr⁸]GnRH). GnRH I is more potent in the activation of the G_q/11 protein in the gonadotrope; however, GnRH II is more potent in the stimulation of apoptosis and antiproliferative effects through activating G_i protein-mediated signaling, implying that GnRH I and II selectively stabilize different receptor-active conformations that preferentially couple to different signaling pathways. Receptor activation involves ligand induction or conformational selection, but the molecular basis of the communication between ligand-binding sites and receptor allosteric sites remains unclear. We have sought conformational coupling between receptor-ligand intermolecular interactions and intramolecular interaction networks in the human GnRH receptor by mutating remote residues that induce differential ligand binding affinity shifts for GnRH I and II. We have demonstrated that certain Ala mutations in the intracellular segments of transmembrane domains 3 (Met ¹³²), 5 (Met²²⁷), 6 (Phe²⁷² and Phe ²⁷⁶), and 7 (Ile³²² and Tyr³²³) of the human GnRH receptor allosterically increased ligand binding affinity for GnRH II but had little effect on GnRH I binding affinity. We examined the role of the three amino acids that differ in these two ligands, and we found that Tyr⁸ in GnRH II plays a dominant role for the increased affinity of the receptor mutants for GnRH II. We propose that creation of a high affinity binding site for GnRH II accompanies receptor conformational changes, i.e. "induced fit" or "conformational selection," mainly determined by the intermolecular interactions between Tyr⁸ and the receptor contact residues, which can be facilitated by disruption of particular sets of receptor-stabilizing intramolecular interactions. The findings suggest that GnRH I and II binding may selectively stabilize different receptor-active conformations and therefore different ligand-induced selective signaling described previously for these ligands.