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-Bungarotoxin Binding to Acetylcholine Receptor Membranes Studied by Low Angle X-Ray Diffraction

Department of Biochemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2H7; and ^* Biomolecular Structure Analysis Center & Department of Biochemistry, University of Connecticut Health Center, Farmington, Connecticut 06030-2017

Correspondence: Address reprint requests to Dr. Howard S. Young, Tel.: 780-492-3931; E-mail: hyoung{at}biochem.ualberta.ca.

The nicotinic acetylcholine receptor (nAChR) carries two binding sites for snake venom neurotoxins. -Bungarotoxin from the Southeast Asian banded krait, Bungarus multicinctus, is a long neurotoxin which competitively blocks the nAChR at the acetylcholine binding sites in a relatively irreversible manner. Low angle x-ray diffraction was used to generate electron density profile structures at 14-Å resolution for Torpedo californica nAChR membranes in the absence and presence of -bungarotoxin. Analysis of the lamellar diffraction data indicated a 452-Å lattice spacing between stacked nAChR membrane pairs. In the presence of -bungarotoxin, the quality of the diffraction data and the lamellar lattice spacing were unchanged. In the plane of the membrane, the nAChRs packed together with a nearest neighbor distance of 80 Å, and this distance increased to 85 Å in the presence of toxin. Electron density profile structures were calculated in the absence and presence of -bungarotoxin, revealing a location for the toxin binding sites. In native, fully-hydrated nAChR membranes, -bungarotoxin binds to the nAChR outer vestibule and contacts the surface of the membrane bilayer.