Four Amino Acids in the α Subunits Determine the γ-Aminobutyric Acid Sensitivities of GABAA Receptor Subtypes

Abstract GABAA receptors, mediators of fast inhibitory neurotransmission, are heteropentameric assemblies from a large array of subunits. Differences in the sensitivity of receptor subtypes to endogenous GABA may permit subunit-dependent finely tuned responsiveness to the same GABAergic inputs. Using both radioligand binding and electrophysiology combined with mutagenesis, we identified a domain of four amino acids within the α subunits that mediates the distinct sensitivities to GABA allowing their selective switch between αβ3γ2 combinations. Replacing this domain in α3 by the corresponding segments of α1–α5 resulted in mutant receptors displaying the GABA EC50 values of the respective wild-type receptors. Vice versa, the α3 motif forced the low sensitivity to GABA of α3 upon α1β3γ2, α4β3γ2, and α5β3γ2. Binding of the GABA agonist [3H]muscimol was not affected by the exchange of the motif between α1 and α3 subunits. Thus, the equilibrium binding pocket is maintained upon replacement of the four amino acids. Taken together our data suggest that the identified motifs contribute to a structure involved in the transduction of the binding signal rather than to the binding itself.