Abstract
GABA(A) receptors mediate fast inhibitory synaptic transmission. The transmembrane ion channel is lined by a ring of five α helices, M2 segments, one from each subunit. An outer ring of helices comprising the alternating M1, M3, and M4 segments from each subunit surrounds the inner ring and forms the interface with the lipid bilayer. The structural rearrangements that follow agonist binding and culminate in opening of the ion pore remain incompletely characterized. Propofol and other intravenous general anesthetics bind at the βM3-αM1 subunit interface. We sought to determine whether this region undergoes conformational changes during GABA activation. We measured the reaction rate of p-chloromercuribenzenesulfonate (pCMBS) with cysteines substituted in the GABA(A) receptor α1M1 and β2M3 segments. In the presence of GABA, the pCMBS reaction rate increased significantly in a cluster of residues in the extracellular third of the α1M1 segment facing the β2M3 segment. Mutation of the β2M2 segment 19' position, R269Q, altered the pCMBS reaction rate with several α1M1 Cys, some only in the resting state and others only in the GABA-activated state. Thus, β2R269 is charged in both states. GABA activation induced disulfide bond formation between β2R269C and α1I228C. The experiments demonstrate that α1M1 moves in relationship to β2M2R269 during gating. Thus, channel gating does not involve rigid body movements of the entire transmembrane domain. Channel gating causes changes in the relative position of transmembrane segments both within a single subunit and relative to the neighboring subunits.