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Alternate Use of Distinct Intersubunit Contacts Controls GABAA Receptor Assembly and Stoichiometry

GABAA receptors are the major inhibitory transmitter receptors in the CNS. Recombinant GABAA receptors composed of α1β3γ2 subunits have been demonstrated to assemble as pentamers consisting of two α1, two β3, and one γ2 subunit. Using truncated and chimeric α1subunits, we identified the α1(80–100) sequence as a major binding site for γ2 subunits. In addition, we demonstrated its direct interaction with γ2(91–104), a sequence that previously has been identified to form the contact to α1 subunits. The observation that the amino acid residues α1P96 and α1H101, which can be photolabeled by [3H]flunitrazepam, are located within or adjacent to the α1(80–100) sequence, indicates that the benzodiazepine binding site of GABAA receptors is located close to this intersubunit contact. The observation that α1(80–100) interacts with γ2 but not with β3 subunits indicates the existence of an additional β3 binding site on α1 subunits. The preferred alternate use of the γ2 and β3binding sites in two different α1 subunits of the same receptor ensures the incorporation of only a single γ2subunit and thus, determines subunit stoichiometry of α1β3γ2 receptors. Distinct binding sites and their alternate use can therefore explain how subunits of hetero-oligomeric transmembrane proteins assemble into a defined protein complex.