Symmetry and Dimensions of Membrane-Bound Nicotinic Acetylcholine Receptors from Torpedo californica Electric Tissue: Rapid Rearrangement to Two-Dimensional Ordered Lattices
AbstractComputer-aided image-averaging methods are applied to different preparations of membrane-bound nicotinic acetylcholine receptor. Circular harmonic averaging (CHA), a novel, reference-independent averaging method developed by W. Kunath and H. Sack-Kongehl ((1989) Ultramicroscopy 27:171–184) allows analyzing images of single molecules of the receptor in its native membrane-bound state. The five subunits of the receptor are clearly resolved. At the resolution obtained (∼20 Å) no differences were observed with resting and agonist-desensitized receptors. A method is proposed for rapidly arranging the acetylcholine receptors to ordered lattices. Depending on the conditions, tetragonal or hexagonal, two-dimensional lattices can be obtained within 2 to 6 days at 4 °C. Analysis by CHA shows that the receptor molecules preserve their gross structure and dimensions in these membranes, but that they are randomly oriented. Both lattices, therefore, do not represent true two-dimensional crystals.