The β subunit determines the ion selectivity of the GABAA receptor

The gamma-aminobutyric acid, type A (GABA(A)) receptor is a chloride-conducting receptor composed of alpha, beta, and gamma subunits assembled in a pentameric structure forming a central pore. Each subunit has a large extracellular agonist binding domain and four transmembrane domains (M1-M4), with the second transmembrane (M2) domain lining the pore. Mutation of five amino acids in the M1-M2 loop of the beta(3) subunit to the corresponding amino acids of the alpha(7) nicotinic acetylcholine subunit rendered the GABA(A) receptor cation-selective upon co-expression with wild type alpha2 and gamma2 subunits. Similar mutations in the a2 or gamma2 subunits did not lead to such a change in ion selectivity. This suggests a unique role for the beta(3) subunit in determining the ion selectivity of the GABA(A) receptor. The pharmacology of the mutated GABA(A) receptor is similar to that of the wild type receptor, with respect to muscimol binding, Zn2+ and bicuculline sensitivity, flumazenil binding, and potentiation of GABA-evoked currents by diazepam. There was, however, an increase in GABA sensitivity (EC50 = 1.3 mum) compared with the wild type receptor (EC50 = 6.4 mum) and a loss of desensitization to GABA of the mutant receptor.