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J. Biol. Chem., Vol. 276, Issue 13, 10110-10118, March 30, 2001

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Construction of a High Affinity Zinc Binding Site in the Metabotropic Glutamate Receptor mGluR1
NONCOMPETITIVE ANTAGONISM ORIGINATING FROM THE AMINO-TERMINAL DOMAIN OF A FAMILY C G-PROTEIN-COUPLED RECEPTOR^*

Anders A. Jensen, Paul O. Sheppard^§, Liselotte B. Jensen^¶, Patrick J. O'Hara^§, and Hans Bräuner-Osborne

From the NeuroScience PharmaBiotec Research Centre, Departments of  Medicinal Chemistry and ^¶ Pharmacology, The Royal Danish School of Pharmacy, 2 Universitetsparken, DK-2100 Copenhagen, Denmark, and ^§ ZymoGenetics Inc., Seattle, Washington 98102

The metabotropic glutamate receptors (mGluRs) belong to family C of the G-protein-coupled receptor (GPCR) superfamily. The receptors are characterized by having unusually long amino-terminal domains (ATDs), to which agonist binding has been shown to take place. Previously, we have constructed a molecular model of the ATD of mGluR1 based on a weak amino acid sequence similarity with a bacterial periplasmic binding protein. The ATD consists of two globular lobes, which are speculated to contract from an "open" to a "closed" conformation following agonist binding. In the present study, we have created a Zn²⁺ binding site in mGluR1b by mutating the residue Lys²⁶⁰ to a histidine. Zinc acts as a noncompetitive antagonist of agonist-induced IP accumulation on the K260H mutant with an IC₅₀ value of 2 µM. Alanine mutations of three potential "zinc coligands" in proximity to the introduced histidine in K260H knock out the ability of Zn²⁺ to antagonize the agonist-induced response. Zn²⁺ binding to K260H does not appear to affect the dimerization of the receptor. Instead, we propose that binding of zinc has introduced a structural constraint in the ATD lobe, preventing the formation of a "closed" conformation, and thus stabilizing a more or less inactive "open" form of the ATD. This study presents the first metal ion site constructed in a family C GPCR. Furthermore, it is the first time a metal ion site has been created in a region outside of the seven transmembrane regions of a GPCR and the loops connecting these. The findings offer valuable insight into the mechanism of ATD closure and family C receptor activation. Furthermore, the findings demonstrate that ATD regions other than those participating in agonist binding could be potential targets for new generations of ligands for this family of receptors.

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