Structural Superimposition

Abstact

Anesthetics and anticonvulsants act, in part, through diverse populations of type-A ɣ-aminobutyric acid receptors (GABA(A)Rs) formed from a pool of 19 subunits. In the hippocampus, alpha5 subunits primarily coassemble with beta3 and, in some cases, gamma2, generating numerous subtypes with differential functional and pharmacological properties critical in learning and memory. The stoichiometry, structure, and gating of these subpopulations are poorly understood. Here we show using cryogenic electron microscopy and electrophysiology that the human alpha5beta3 GABA(A)R predominantly assembles with 2alpha:3beta stoichiometry, though a minority population of 1alpha:4beta indicates multiple assemblies are possible. In a resting-like state, a conserved activation gate and Zn(2+)-coordination at histidines on beta3 block ion conduction. Upon GABA binding, global rearrangements release Zn(2+) and open the activation gate in nearly all receptors. The activated receptor is unaffected upon binding the anesthetic etomidate or anticonvulsant topiramate, supporting a conformational selection mechanism of action. This work thus reveals the assembly, activation, and modulation of a GABA(A)R subtype critical to cognition, providing templates for structure-based drug discovery.