Extracellular loop 3 (EL3) and EL3-proximal transmembrane helix 7 of the mammalian type I and type II gonadotropin-releasing hormone (GnRH) receptors determine differential ligand selectivity to GnRH-I and GnRH-II.
Mammalian type I and II gonadotropin-releasing hormone (GnRH) receptors (GnRHRs) show differential ligand preference for GnRH-I and GnRH-II, respectively. Using a variety of chimeric receptors based on green monkey GnRHR-2 (gmGnRHR-2), a representative type II GnRHR, and rat GnRHR, a representative type I GnRHR, this study elucidated specific domains responsible for this ligand selectivity. A chimeric gmGnRHR-2 with the extracellular loop 3 (EL3) and EL3-proximal transmembrane helix 7 (TMH7) of rat GnRHR showed a great increase in ligand sensitivity to GnRH-I but not to GnRH-II. Point-mutation studies indicate that four amino acids, Leu/Phe(7.38), Leu/Phe(7.43), Ala/Pro(7.46), and Pro/Cys(7.47) in TMH7 are critical for ligand selectivity as well as receptor conformation. Furthermore, a combinatory mutation (Pro(7.31)-Pro(7.32)-Ser(7.33) motif to Ser-Glu-Pro in EL3 and Leu(7.38), Leu(7.43), Ala(7.46), and Pro(7.47) to those of rat GnRHR) in gmGnRH-2 exhibited an approximately 500-fold increased sensitivity to GnRH-I, indicating that these residues are critical for discriminating GnRH-II from GnRH-I. [Trp(7)]GnRH-I and [Trp(8)]GnRH-I but not [His(5)]GnRH-I exhibit a higher potency in activating wild-type gmGnRHR-2 than native GnRH-I, indicating that amino acids at positions 7 and 8 of GnRHs are more important than position 5 for differential recognition by type I and type II GnRHRs. As a whole, these data suggest a molecular coevolution of ligands and their receptors and facilitate the understanding of the molecular interaction between GnRHs and their cognate receptors.
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amino acids amino acids Leu Phe Leu Phe Ala Pro Pro Cys Pro Pro Ser Ser-Glu-Pro Leu Leu Ala Pro Mammalian type I and II gonadotropin-releasing hormone (GnRH) receptors GnRH-I GnRH-II GnRH-I GnRH-I GnRH-I GnRH-I GnRH-I gmGnRHR-2 GnRH-I GnRHs GnRH-II type I and type II GnRHRs GnRHs green monkey GnRHR-2 gmGnRHR-2 type II GnRHR rat GnRHR type I GnRHR gmGnRHR-2 extracellular loop 3 EL3 EL3-proximal transmembrane helix 7 TMH7 rat GnRHR GnRH-I GnRH-II TMH7 GnRHRs EL3 rat GnRHR gmGnRH-2 Extracellular loop 3 GnRH-I GnRH-II EL3 EL3-proximal transmembrane helix 7 mammalian type I and type II gonadotropin-releasing hormone (GnRH) receptors