Role of the conserved lysine residue in the middle of the predicted extracellular loop between M2 and M3 in the GABAA receptor

In alpha 1, beta 2, and gamma 2 subunits of the gamma-aminobutyric acid A (GABA(A)) receptor, a conserved lysine residue occupies the position in the middle of the predicted extracellular loop between the transmembrane M2 and M3 regions. In all three subunits, this residue was mutated to alanine, Whereas the mutation in alpha 1 and beta 2 subunits resulted each in about a sixfold shift of the concentration-response curve for GABA to higher concentrations, no significant effect by mutation in the gamma subunit was detected. The affinity for the competitive inhibitor bicuculline methiodide was not affected by the mutations in either the alpha 1 subunit or the beta 2 subunit. Concentration-response curves for channel activation by pentobarbital were also shifted to higher concentrations by the mutation in the alpha and beta subunits. Binding of [H-3]Ro 15-1788 was unaffected by the mutation in the alpha subunit, whereas the binding of [H-3]muscimol was shifted to lower affinity. Mutation of the residue in the alpha 1 subunit to E, Q, or R resulted in an about eight-, 10-, or fivefold shift, respectively, to higher concentrations of the concentration-response curve for GABA. From these observations, it is concluded that the corresponding residues on the al and beta 2 subunits are involved more likely in the gating of the channel by GABA than in the binding of GABA or benzodiazepines.