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The Journal of Neuroscience, September 1, 2002, 22(17):7352-7361

No Ligand Binding in the GB2 Subunit of the GABA_B Receptor Is Required for Activation and Allosteric Interaction between the Subunits

Julie Kniazeff¹, Thierry Galvez¹, Gilles Labesse², and Jean-Philippe Pin¹

¹ Centre National de la Recherche Scientifique (CNRS)-Unité Propre de Recherche 9023, Mécanismes Moléculaires des Communications Cellulaires, 34094 Montpellier Cedex 5, France, and ² CNRS-Unité Mixte de Recherche 5048, Centre de Biochimie Structurale, Faculté de Pharmacie, 34060 Montpellier Cedex, France

The GABA_B receptor plays important roles in the tuning of many synapses. Although pharmacological differences have been observed between various GABA_B-mediated effects, a single GABA_B receptor composed of two subunits (GB1 and GB2) has been identified. Although GB1 binds GABA, GB2 plays a critical role in G-protein activation. Moreover, GB2 is required for the high agonist affinity of GB1. Like any other family 3 G-protein-coupled receptors, GB1 and GB2 are composed of a Venus Flytrap module (VFTM) that usually contains the agonist-binding site and a heptahelical domain. So far, there has been no direct demonstration that GB2 binds GABA or another endogenous ligand. Here, we have further refined the GABA-binding site of GB1 and characterized the putative-binding site in the VFTM of GB2. None of the residues important for GABA binding in GB1 appeared to be conserved in GB2. Moreover, mutation of 10 different residues, alone or in combination, within the possible binding pocket of GB2 affects neither GABA activation of the receptor nor the ability of GB2 to increase agonist affinity on GB1. These data indicate that ligand binding in the GB2 VFTM is not required for activation. Finally, although in either GB1 or the related metabotropic glutamate receptors most residues of the binding pocket are conserved from Caenorhabditis elegans to human, no such conservation is observed in GB2. This suggests that the GB2 VFTM does not constitute a binding site for a natural ligand.

Key words: ligand recognition; evolution; three-dimensional modeling; dimerization; GPCR; allostery; baclofen

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Copyright © 2002 Society for Neuroscience  0270-6474/02/22177352-10$05.00/0

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