Gonadotropin-Releasing Hormone (GnRH) Receptor Structure and GnRH Binding

Abstract

Gonadotropin-releasing hormone (GnRH) regulates reproduction. The human GnRH receptor lacks a cytoplasmic carboxy-terminal tail but has amino acid sequence motifs characteristic of rhodopsin-like, class A, G protein-coupled receptors (GPCRs). This review will consider how recent descriptions of X-ray crystallographic structures of GPCRs in inactive and active conformations may contribute to understanding GnRH receptor structure, mechanism of activation and ligand binding. The structures confirmed that ligands bind to variable extracellular surfaces, whereas the seven membrane-spanning α-helices convey the activation signal to the cytoplasmic receptor surface, which binds and activates heterotrimeric G proteins. Forty non-covalent interactions that bridge topologically equivalent residues in different transmembrane (TM) helices are conserved in class A GPCR structures, regardless of activation state. Conformation-independent interhelical contacts account for a conserved receptor protein structure and their importance in the GnRH receptor structure is supported by decreased expression of receptors with mutations of residues in the network. Many of the GnRH receptor mutations associated with congenital hypogonadotropic hypogonadism, including the Glu^2.53(90) Lys mutation, involve amino acids that constitute the conserved network. Half of the ~250 intramolecular interactions in GPCRs differ between inactive and active structures. Conformation-specific interhelical contacts depend on amino acids changing partners during activation. Conserved inactive conformation-specific contacts prevent receptor activation by stabilizing proximity of TM helices 3 and 6 and a closed G protein-binding site. Mutations of GnRH receptor residues involved in these interactions, such as Arg^3.50(139) of the DRY/S motif or Tyr^7.53(323) of the N/DPxxY motif, increase or decrease receptor expression and efficiency of receptor coupling to G protein signaling, consistent with the native residues stabilizing the inactive GnRH receptor structure. Active conformation-specific interhelical contacts stabilize an open G protein-binding site. Progress in defining the GnRH-binding site has recently slowed, with evidence that Tyr^6.58(290) contacts Tyr⁵ of GnRH, whereas other residues affect recognition of Trp³ and Gly¹⁰NH₂. The surprisingly consistent observations that GnRH receptor mutations that disrupt GnRH binding have less effect on "conformationally constrained" GnRH peptides may now be explained by crystal structures of agonist-bound peptide receptors. Analysis of GPCR structures provides insight into GnRH receptor function.

Keywords: G protein-coupled receptor; gonadotropin-releasing hormone receptor; ligand binding; receptor activation; receptor structure.

Figure 1

Homology model of the inactive human gonadotropin-releasing hormone (GnRH) receptor. The model was downloaded from the GPCRdb website (www.gpcrdb.org/structure/homology_models) (71) and viewed using the UCSF Chimera software package (72) to show the spatial positioning of Glu^2.53(90) (red) relative to the neighboring residues Ser^3.35(124) (light blue), Lys^3.32(121) (dark blue), Asp^2.61(98) (magenta), Met^3.36(125) (yellow), and Trp^6.48(280) (green). Chimera is developed by the Resource for Biocomputing, Visualization, and Informatics at the University of California, San Francisco (supported by NIGMS P41-GM103311).

Figure 2

Human gonadotropin-releasing hormone (GnRH) receptor residues topologically equivalent to G protein-coupled receptor (GPCR) residues that form conserved conformation-specific interhelical contacts. The snake diagram of the GnRH receptor amino acid sequence was downloaded from GPCRdb (www.gpcrdb.org/structure/homology_models) (71). GnRH receptor residues topologically equivalent to residues that form conserved conformation-specific contacts (17, 19) only in inactive GPCR structures (blue); only in active GPCR structures (green) and with different partners in inactive and active GPCR structures (purple) are shown.