NMR structures of the human α7 nAChR transmembrane domain and associated anesthetic binding sites

The alpha 7 nicotinic acetylcholine receptor (nAChR), assembled as homomeric pentameric ligand-gated ion channels, is one of the most abundant nAChR subtypes in the brain. Despite its importance in memory, learning and cognition, no structure has been determined for the alpha 7 nAChR TM domain, a target for allosteric modulators. Using solution state NMR, we determined the structure of the human alpha 7 nAChR TM domain (PDB ID: 2MAW) and demonstrated that the alpha 7 TM domain formed functional channels in Xenopus oocytes. We identified the associated binding sites for the anesthetics halothane and ketamine; the former cannot sensitively inhibit alpha 7 function, but the latter can. The alpha 7 TM domain folds into the expected four-helical bundle motif, but the intra-subunit cavity at the extracellular end of the alpha 7 TM domain is smaller than the equivalent cavity in the alpha 4 beta 2 nAChRs (PDB IDs: 2LLY: 2LM2). Neither drug binds to the extracellular end of the alpha 7 TM domain, but two halothane molecules or one ketamine molecule binds to the intracellular end of the alpha 7 TM domain. Halothane and ketamine binding sites are partially overlapped. Ketamine, but not halothane, perturbed the alpha 7 channel-gate residue L9'. Furthermore, halothane did not induce profound dynamics changes in the alpha 7 channel as observed in alpha 7. The study offers a novel high-resolution structure for the human alpha 7 nAChR TM domain that is invaluable for developing alpha 7-specific therapeutics. It also provides evidence to support the hypothesis: only when anesthetic binding perturbs the channel pore or alters the channel motion, can binding generate functional consequences. (C) 2014 Elsevier B.V. All rights reserved.