The a5(H105R) mutation impairs a5 selective binding properties by altered positioning of the a5 subunit in GABAA receptors containing two distinct types of a subunits

AbstractGABAA receptors are pentameric ligand-gated ion channels that are major mediators of fast inhibitory neurotransmission. Clinically relevant GABAA receptor subtypes are assembled from a5(1-3, 5), b1-3 and the g2 subunit. They exhibit a stoichiometry of two a, two b and one g subunit, with two GABA binding sites located at the a-b and one benzodiazepine binding site located at the a-g subunit interface. Introduction of the H105R point mutation into the a5 subunit, to render a5 subunit-containing receptors insensitive to the clinically important benzodiazepine site agonist diazepam, unexpectedly resulted in a reduced level of a5 subunit protein in a5(H105R) mice. In this study, we show that the a5(H105R) mutation did not affect cell surface expression and targeting of the receptors or their assembly into macromolecular receptor complexes but resulted in a severe reduction of a5-selective ligand binding. Immunoprecipitation studies suggest that the diminished a5-selective binding is presumably due to a repositioning of the a5(H105R) subunit in GABAA receptor complexes containing two different a subunits. These findings imply an important role of histidine 105 in determining the position of the a5 subunit within the receptor complex by determining the affinity for assembly with the g2 subunit.