The Net Orientation of Nicotinic Receptor Transmembrane α-Helices in the Resting and Desensitized States

The net orientation of nicotinic acetylcholine receptor transmembrane α-helices has been probed in both the activatable resting and nonactivatable desensitized states using linear dichroism Fourier-transform infrared spectroscopy. Infrared spectra recorded from reconstituted nicotinic acetylcholine receptor membranes after 72 h exposure to 2H2O exhibit an intense amide I component band near 1655 cm−1 that is due predominantly to hydrogen-exchange-resistant transmembrane peptides in an α-helical conformation. The measured dichroism of this band is 2.37, suggesting a net tilt of the transmembrane α-helices of roughly 40° from the bilayer normal, although this value overestimates the tilt angle because the measured dichroism at 1655 cm−1 also reflects the dichroism of overlapping amide I component bands. Significantly, no change in the net orientation of the transmembrane α-helices is observed upon agonist binding. In fact, the main changes in structure and orientation detected upon desensitization involve highly solvent accessible regions of the polypeptide backbone. Our data are consistent with a capping of the ligand binding site by the solvent accessible C-loop with little change in the structure of the transmembrane domain in the desensitized state. Changes in structure at the interface between the ligand-binding and transmembrane domains may uncouple binding from gating.