The [alpha]5(H105R) mutation impairs [alpha]5 selective binding properties by altered positioning of the [alpha]5 subunit in GABAA receptors containing two distinct types of [alpha] subunits

GABAA receptors are pentameric ligand-gated ion channels that are major mediators of fast inhibitory neurotransmission. Clinically relevant GABAA receptor subtypes are assembled from [alpha]5(1-3, 5), [beta]1-3 and the [gamma]2 subunit. They exhibit a stoichiometry of two [alpha], two [beta] and one [gamma] subunit, with two GABA binding sites located at the [alpha]/[beta] and one benzodiazepine binding site located at the [alpha]/[gamma] subunit interface. Introduction of the H105R point mutation into the [alpha]5 subunit, to render [alpha]5 subunit-containing receptors insensitive to the clinically important benzodiazepine site agonist diazepam, unexpectedly resulted in a reduced level of [alpha]5 subunit protein in [alpha]5(H105R) mice. In this study, we show that the [alpha]5(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 [alpha]5-selective ligand binding. Immunoprecipitation studies suggest that the diminished [alpha]5-selective binding is presumably due to a repositioning of the [alpha]5(H105R) subunit in GABAA receptor complexes containing two different [alpha] subunits. These findings imply an important role of histidine 105 in determining the position of the [alpha]5 subunit within the receptor complex by determining the affinity for assembly with the [gamma]2 subunit. [PUBLICATION ABSTRACT]