Cryo-EM structure of the human α1β3γ2 GABA_A receptor in a lipid bilayer.

Abstract

Type A γ-aminobutyric acid (GABA_A) receptors are pentameric ligand-gated ion channels and the main drivers of fast inhibitory neurotransmission in the vertebrate nervous system^1,2. Their dysfunction is implicated in a range of neurological disorders, including depression, epilepsy and schizophrenia^3,4. Among the numerous assemblies that are theoretically possible, the most prevalent in the brain are the α1β2/3γ2 GABA_A receptors⁵. The β3 subunit has an important role in maintaining inhibitory tone, and the expression of this subunit alone is sufficient to rescue inhibitory synaptic transmission in β1-β3 triple knockout neurons⁶. So far, efforts to generate accurate structural models for heteromeric GABA_A receptors have been hampered by the use of engineered receptors and the presence of detergents^7-9. Notably, some recent cryo-electron microscopy reconstructions have reported 'collapsed' conformations^8,9; however, these disagree with the structure of the prototypical pentameric ligand-gated ion channel the Torpedo nicotinic acetylcholine receptor^10,11, the large body of structural work on homologous homopentameric receptor variants¹² and the logic of an ion-channel architecture. Here we present a high-resolution cryo-electron microscopy structure of the full-length human α1β3γ2L-a major synaptic GABA_A receptor isoform-that is functionally reconstituted in lipid nanodiscs. The receptor is bound to a positive allosteric modulator 'megabody' and is in a desensitized conformation. Each GABA_A receptor pentamer contains two phosphatidylinositol-4,5-bisphosphate molecules, the head groups of which occupy positively charged pockets in the intracellular juxtamembrane regions of α1 subunits. Beyond this level, the intracellular M3-M4 loops are largely disordered, possibly because interacting post-synaptic proteins are not present. This structure illustrates the molecular principles of heteromeric GABA_A receptor organization and provides a reference framework for future mechanistic investigations of GABAergic signalling and pharmacology.