A structural role for lipids in coupling ligand binding to channel gating in a neurotransmitter receptor

Abstract

A simple structure-based mechanism explaining how membrane lipid composition influences the ability of the nicotinic acetylcholine receptor (nAChR) to convert agonist binding into opening of its transmembrane ion channel is proposed. This mechanism is based both on a recent atomic model of the nAChR, as well as extensive characterization of affinity purified nAChRs reconstituted into a number of different model membranes. Biophysical characterization of the nAChR upon reconstitution into membranes of defined lipid composition identifies the receptor's specific lipid requirements, and highlights the structural consequences of its reconstitution into membranes lacking these essential lipids. Infrared measurements show that while membrane lipid composition has no effect on nAChR secondary structure, it influences the ability of the nAChR to undergo agonist induced conformational change. In the absence of specific lipids the nAChR appears locked in a non-conducting conformation in which the binding of agonist fails to trigger a response. Hydrogen-deuterium exchange studies show that these non-functional nAChRs also exhibit increased 1H/2H exchange kinetics, while thermal denaturation data shows that they display reduced thermal stability. Interpreted in light of the new nAChR atomic model (PDB ID: 2BG9), these data provide insight into the lipid-dependent structural rearrangements resulting in an uncoupling of the nAChR's ligand binding and channel gating functions.