Direct visualization of ion-channel gating in a native environment

Abstract

Pentameric ligand-gated ion channels (pLGICs), also known as Cys-loop receptors, are localized primarily in the postsynaptic membranes, and mediate fast chemical transmission in the central and peripheral nervous systems. Binding of neurotransmitter activates these receptors, causing changes in postsynaptic membrane potential and consequently modulation of neuronal or muscle activity. pLGIC functions are altered by a variety of drugs, making them significant pharmaceutical targets. Indeed, the rise in pLGIC crystal and cryoelectron microscopy structures underscores the magnitude of research efforts that have gone into unraveling the molecular details of channel function. Current structure determination methods of pLGICs capture stationary images and, most often, in nonnative environments. As a result, gaps remain in our understanding of the dynamic properties, intermediate conformational states, and the energetics of gating transitions. In PNAS, Ruan et al. (1) seek to probe some of these missing links using high-speed atomic force microscopy (HS-AFM) by directly visualizing the pLGIC gating process in a membrane environment and under conditions that mimic physiological buffer, temperature, and pressure. The Cys-loop family in vertebrates includes the cationic acetylcholine receptor (nAChR) and serotonin receptor (5HT3AR) and the anionic γ-aminobutyric acid receptor (GABAAR) and glycine receptor (GlyR). Homologs of pLGICs include invertebrate members (2), as well as a growing number of prokaryotic members, including the Erwinia chrysanthemi ligand-gated ion channel (ELIC) and Gloeobacter violaceus ligand-gated ion channel (GLIC) (3). In general, pLGICs share a conserved architecture (despite low sequence identity) where five identical or homologous subunits are pseudosymmetrically arranged around an ion-conducting pore. Each subunit has an N-terminal extracellular domain (ECD) that binds neurotransmitters, a transmembrane domain (TMD) with four membrane-spanning helices (M1–M4) with M2 helices lining the central pore, and a cytosolic intracellular domain in eukaryotic channels formed by the M3–M4 loop (4). Interestingly, high-resolution pLGIC structures solved … [↵][1]1To whom correspondence should be addressed. Email: sudha.chakrapani{at}case.edu. [1]: #xref-corresp-1-1