title: Intramembrane aromatic interactions influence the lipid sensitivities of pentameric ligand-gated ion channels.
abstract: Although the Torpedo nicotinic acetylcholine receptor (nAChR) reconstituted into phosphatidylcholine (PC) membranes lacking cholesterol and anionic lipids adopts a conformation where agonist binding is uncoupled from channel gating, the underlying mechanism remains to be defined. Here, we examine the mechanism behind lipid-dependent uncoupling by comparing the propensities of two prokaryotic homologs, Gloebacter and Erwinia ligand-gated ion channel (GLIC and ELIC, respectively), to adopt a similar uncoupled conformation. Membrane-reconstituted GLIC and ELIC both exhibit folded structures in the minimal PC membranes that stabilize an uncoupled nAChR. GLIC, with a large number of aromatic interactions at the interface between the outermost transmembrane α-helix, M4, and the adjacent transmembrane α-helices, M1 and M3, retains the ability to flux cations in this uncoupling PC membrane environment. In contrast, ELIC, with a level of aromatic interactions intermediate between that of the nAChR and GLIC, does not undergo agonist-induced channel gating, although it does not exhibit the expected biophysical characteristics of the uncoupled state. Engineering new aromatic interactions at the M4-M1/M3 interface to promote effective M4 interactions with M1/M3, however, increases the stability of the transmembrane domain to restore channel function. Our data provide direct evidence that M4 interactions with M1/M3 are modulated during lipid sensing. Aromatic residues strengthen M4 interactions with M1/M3 to reduce the sensitivities of pentameric ligand-gated ion channels to their surrounding membrane environment.
authors.affiliation: From the Department of Biochemistry, Microbiology, and Immunology, University of Ottawa, Ottawa Ontario, K1H 8M5, Canada.
authors.forename: Casey L
authors.initials: CL
authors.lastname: Carswell
authors.affiliation: From the Department of Biochemistry, Microbiology, and Immunology, University of Ottawa, Ottawa Ontario, K1H 8M5, Canada.
authors.forename: Jiayin
authors.initials: J
authors.lastname: Sun
authors.affiliation: From the Department of Biochemistry, Microbiology, and Immunology, University of Ottawa, Ottawa Ontario, K1H 8M5, Canada John.Baenziger@uottawa.ca.
authors.forename: John E
authors.initials: JE
authors.lastname: Baenziger
mesh_terms: D000818:Animals; D001426:Bacterial Proteins / Q000737:chemistry*; D002412:Cations; D002462:Cell Membrane / Q000378:metabolism; D018360:Crystallography, X-Ray; D000458:Cyanobacteria / Q000737:chemistry*; D004885:Erwinia / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D058446:Ligand-Gated Ion Channels / Q000737:chemistry*; D008051:Lipid Bilayers; D008055:Lipids / Q000737:chemistry*; D009865:Oocytes / Q000166:cytology; D011485:Protein Binding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D011510:Proteolipids / Q000737:chemistry; D011978:Receptors, Nicotinic / Q000737:chemistry; D013055:Spectrophotometry, Infrared; D017550:Spectroscopy, Fourier Transform Infrared; D014101:Torpedo
chemical_list: D001426:Bacterial Proteins; D002412:Cations; D058446:Ligand-Gated Ion Channels; D008051:Lipid Bilayers; D008055:Lipids; D011510:Proteolipids; D011978:Receptors, Nicotinic; C020485:proteoliposomes
keywords: Electrophysiology; Infrared Spectroscopy (IR Spectroscopy); Ion Channel; Lipid Bilayer; Membrane Biophysics; Membrane Lipid; Membrane Protein; Membrane Reconstitution; Nicotinic Acetylcholine Receptor (nAChR)