Structural basis for activation and potentiation in a human α5β3 GABAA receptor

Neuromodulatory drugs, including anesthetics and anticonvulsants, have been shown to mediate inhibitory and excitatory effects through different populations of type-A ɣ-aminobutyric acid receptors (GABAARs) in the central nervous system. GABAARs in the hippocampus containing α5 subunits have been particularly implicated in learning and memory. 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 of these subpopulations are not well understood. Here we report cryogenic electron microscopy structures of human α5β3 GABAARs in context of various membrane mimetics, stoichiometries and ligands. Combined with electrophysiology in Xenopus oocytes, our data support a predominant assembly of 2:3 α:β subunits, though a minority population of 1:4 α:β indicates multiple assemblies are possible. Differential glycosylation of α5 and β3 enabled reconstruction of the heteromeric complex even in the absence of protein fiducials. In a resting-like state, ion conduction is blocked at a central conserved activation gate, and by Zn2+ coordinated at histidine residues on the β3 subunits. In activated receptors, GABA binding to an extracellular orthosteric site is associated with global rearrangements that propagate to release Zn2+ and open the activation gate. In contrast to the lower-efficacy α1β3 subtype, saturating GABA appears to drive activation of nearly all receptors. The GABA-bound structure is virtually unaffected by binding of the anesthetic etomidate or anticonvulsant topiramate, supporting both high GABA efficacy and a conformational selection mechanism of positive allosteric modulation. This work thus reveals the assembly, activation and modulation of a GABAAR subtype critical to cognition, including prospective templates for structure-based drug discovery.