Structural basis for activation and potentiation in a human α5β3 GABA _A receptor

General anesthetics like etomidate mediate amnesic as well as sedative effects through different populations of gamma amino butyric acid receptors (GABA _A Rs) in the central nervous system. The amnesic effects have largely been attributed to α5-subunit-containing receptors in the hippocampus, indicating a clear role in learning and memory for these receptors. The α5 subunit is thought to primarily coassemble with β3 and, in some cases, γ2 subunits, generating a variety of receptor subtypes with differential functional and pharmacological properties. However, the stoichiometry, structure, and gating mechanisms of these different subpopulations are not well understood. Here we report structures of human α5β3 GABA _A Rs with various modulators, assembled in two stoichiometries. Our cryo-EM structures, combined with electrophysiology in Xenopus oocytes, support a primary assembly of 2:3 α:β subunits, though a minority population of 1:4 α:β indicates multiple assemblies are possible. Differential glycosylation of the α5 and β3 subunits enabled reconstruction of the heteromeric complex even in the absence of added fiducials. In the resting state of the receptor, Zn ²⁺ binds to histidine residues at the M2-17′ position in the β3 subunit, blocking ion passage. In the activated state, GABA binding to the orthosteric site is associated with global rearrangements propagating to unbinding of the 17′ Zn ²⁺ atoms and opening of the 9′ hydrophobic gate. Unlike the α1β3 receptor where GABA is effectively a partial agonist, saturating GABA binding to α5β3 appears to drive activation of nearly all receptors, resulting in a single desensitized state observed under the cryo-EM conditions. In agreement with this high GABA efficacy, the GABA-bound structure is virtually unaffected by further addition of the anesthetic etomidate at the β-α transmembrane domain interface. These structures offer new detailed models for gating and modulation of a GABA _A R subtype critical to learning and memory, including prospective templates for structure-based drug discovery.