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J. Biol. Chem., Vol. 280, Issue 33, 29796-29803, August 19, 2005

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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^*

Zhi-Liang Lu, Ryan Gallagher, Robin Sellar, Marla Coetsee, and Robert P. Millar

From the Medical Research Council Human Reproductive Sciences Unit, Centre for Reproductive Biology, Edinburgh EH16 4SB, Scotland, United Kingdom

The human gonadotropin-releasing hormone (GnRH) receptor is evolutionarily configured for high affinity binding of GnRH I ([Tyr⁵,Leu⁷,Arg⁸]GnRH) but at lower affinity for GnRH II ([His⁵,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 (Met¹³²), 5 (Met²²⁷), 6 (Phe²⁷² and Phe²⁷⁶), 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.

Received for publication, December 1, 2004 , and in revised form, June 20, 2005.

^* This work was supported by the Medical Research Council, UK. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains the homology model of the 7-TM domains.

To whom correspondence should be addressed: Medical Research Council Human Reproductive Sciences Unit, Centre for Reproductive Biology, The Chancellor's Bldg., 49 Little France Crescent, Edinburgh EH16 4SB, Scotland, UK. Tel.: 44-131-242-6218; Fax: 44-131-242-6231; E-mail: z.lu{at}hrsu.mrc.ac.uk.

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