X-ray structures of GluCl in apo states reveal a gating mechanism of Cys-loop receptors

Author(s): Thorsten Althoff [sup.1] [sup.4] , Ryan E. Hibbs [sup.1] [sup.4] , Surajit Banerjee [sup.2] , Eric Gouaux [sup.1] [sup.3] Author Affiliations: (1) Vollum Institute, Oregon Health & Science University, [...]
This study solved structures of the glutamate-gated chloride channel (GluCl), a Cys-loop receptor from C. elegans, in an apo, closed state and in a lipid-bound state -- comparison of these structures with a previously published structure of GluCl in an ivermectin-bound state reveals what conformational changes probably occur as this membrane protein transitions from the closed/resting state towards an open/activated state. Resting-to-activated state of a Cys-loop receptor Depending on their ligand and ion selectivity, Cys-loop receptors are neurotransmitter-activated ion channels that mediate either excitatory or inhibitory neurotransmission. In this paper the authors solve the structures of the glutamate-gated chloride channel (GluCl), a Cys-loop receptor from Caenorhabditis elegans, in an apo or closed state and in a lipid-bound state. Comparison of these structures with the previously published structure of GluCl in an ivermectin-bound state reveals the conformational changes involved as this membrane protein transitions between the closed/resting and an open/activated state. Cys-loop receptors are neurotransmitter-gated ion channels that are essential mediators of fast chemical neurotransmission and are associated with a large number of neurological diseases and disorders, as well as parasitic infections.sup.1,2,3,4. Members of this ion channel superfamily mediate excitatory or inhibitory neurotransmission depending on their ligand and ion selectivity. Structural information for Cys-loop receptors comes from several sources including electron microscopic studies of the nicotinic acetylcholine receptor.sup.5, high-resolution X-ray structures of extracellular domains.sup.6 and X-ray structures of bacterial orthologues.sup.7,8,9,10. In 2011 our group published structures of the Caenorhabditis elegans glutamate-gated chloride channel (GluCl) in complex with the allosteric partial agonist ivermectin, which provided insights into the structure of a possibly open state of a eukaryotic Cys-loop receptor, the basis for anion selectivity and channel block, and the mechanism by which ivermectin and related molecules stabilize the open state and potentiate neurotransmitter binding.sup.11. However, there remain unanswered questions about the mechanism of channel opening and closing, the location and nature of the shut ion channel gate, the transitions between the closed/resting, open/activated and closed/desensitized states, and the mechanism by which conformational changes are coupled between the extracellular, orthosteric agonist binding domain and the transmembrane, ion channel domain. Here we present two conformationally distinct structures of C. elegans GluCl in the absence of ivermectin. Structural comparisons reveal a quaternary activation mechanism arising from rigid-body movements between the extracellular and transmembrane domains and a mechanism for modulation of the receptor by phospholipids.