Transmembrane domain orientation and neurotransmitter binding in the nicotinic acetylcholine receptor

Abstract

The orientation of the hydrogen-deuterium exchange resistant "core" of the of the intact nicotinic acetylcholine receptor from Torpedo was studied using linear dichroism attenuated total reflectance infrared spectroscopy both in the presence and absence of the agonist Carb. Our results show that the hydrogen exchange resistant alpha-helical peptide hydrogens in the nAChR are preferentially oriented parallel to the bilayer normal, therefore, providing evidence for a predominantly alpha-helical transmembrane domain, and that there are no detectable net changes in orientation upon agonist-induced desensitization. We also examined the linear dichroism of lipid bands in spectra recorded from membranes both with and without the nAChR in order to accurately assess the effects of membrane film mosaic spread on the interpretation of the linear dichroism data. Our data also show that the mosaic spread of the reconstituted membrane films is greater than the mosaic spread observed with pure lipid films. Our work illustrates the importance of mosaic spread characterization when interpreting experimental order parameters in terms of molecular structure orientation. (Abstract shortened by UMI.)