Abstract
Channel functions of the neuronal alpha 4 beta 2 nicotinic acetylcholine receptor (nAChR). one of the most widely expressed subtypes in the brain, can be inhibited by volatile anesthetics. Our Na+ flux experiments confirmed that the second transmembrane domains (TM2) of alpha 4 and beta 2 in 2:3 stoichiometry, (alpha 4)(2)(beta 2)(3), could form pentameric channels, whereas the alpha 4 TM2 alone could not. The structure, topology, and dynamics of the alpha 4 TM2 and (alpha 4)(2)(beta 2)(3) TM2 in magnetically aligned phospholipid bicelles were investigated using solid-state NMR spectroscopy in the absence and presence of halothane and isoflurane, two clinically used volatile anesthetics. H-2 NMR demonstrated that anesthetics increased lipid conformational heterogeneity. Such anesthetic effects on lipids became more profound in the presence of transmembrane proteins. PISEMA experiments on the selectively N-15-labeled alpha 4 TM2 showed that the TM2 formed transmembrane helices with tilt angles of 12 degrees +/- 1 degrees and 16 degrees +/- 1 degrees relative to the bicelle normal for the alpha 4 and (alpha 4)(2)(beta 2)(3) samples, respectively. Anesthetics changed the tilt angle of the alpha 4 TM2 from 12 degrees +/- 1 degrees to 14 degrees +/- 1 degrees, but had only a subtle effect on the tilt angle of the (alpha 4)(2)(beta 2)(3) TM2. A small degree of wobbling motion of the helix axis occurred in the (alpha 4)(2)(beta 2)(3) TM2. In addition, a subset of the (alpha 4)(2)(beta 2)(3) TM2 exhibited counterclockwise rotational motion around the helix axis on a time scale slower than 10(-4) s in the presence of anesthetics. Both helical tilting and rotational motions have been identified computationally as critical elements for ion channel functions. This study suggested that anesthetics could alter these motions to modulate channel functions. (c) 2009 Elsevier B.V. All rights reserved.