Intramembrane Aromatic Interactions Influence the Lipid Sensitivities of Pentameric Ligand-gated Ion Channels |
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Authors: | Casey L Carswell Jiayin Sun John E Baenziger |
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Institution: | From the Department of Biochemistry, Microbiology, and Immunology, University of Ottawa, Ottawa Ontario, K1H 8M5, Canada |
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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. |
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Keywords: | Electrophysiology Infrared Spectroscopy (IR Spectroscopy) Ion Channel Lipid Bilayer Membrane Biophysics Membrane Lipid Membrane Protein Membrane Reconstitution Nicotinic Acetylcholine Receptor (nAChR) |
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