Role of the transmembrane domain 4/extracellular loop 2 junction of the human gonadotropin-releasing hormone receptor in ligand binding and receptor conformational selection

Recent crystal structures of G protein-coupled receptors (GPCRs) show the remarkable structural diversity of extracellular loop 2 (ECL2), implying its potential role in ligand binding and ligand-induced receptor conformational selectivity. Here we have applied molecular modeling and mutagenesis studies to the TM4/ECL2 junction (residues Pro ^174(4.59)-Met ^180(4.66)) of the human gonadotropin-releasing hormone (GnRH) receptor, which uniquely has one functional type of receptor but two endogenous ligands in humans. We suggest that the above residues assume an α-helical extension of TM4 in which the side chains of Gln ^174(4.60) and Phe ^178(4.64) face toward the central ligand binding pocket to make H-bond and aromatic contacts with pGlu ¹ and Trp ³ of both GnRH I and GnRH II, respectively. The interaction between the side chains of Phe ^178(4.64) of the receptor and Trp ³ of the GnRHs was supported by reciprocal mutations of the interacting residues. Interestingly, alanine mutations of Leu ^175(4.61), Ile ^177(4.63), and Met ^180(4.66) decreased mutant receptor affinity for GnRH I but, in contrast, increased affinity for GnRH II. This suggests that these residues make intramolecular or intermolecular contacts with residues of transmembrane (TM) domain 3, TM5, or the phospholipid bilayer, which couple the ligand structure to specific receptor conformational switches. The marked decrease in signaling efficacy of I177A and F178A also indicates that IIe ^177(4.63) and Phe ^178(4.64) are important in stabilizing receptor-active conformations. These findings suggest that the TM4/ECL2 junction is crucial for peptide ligand binding and, consequently, for ligand-induced receptor conformational selection.