The electron microscopy of the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor is a glycoprotein occurring in the electric tissue of the electric ray Torpedo sp. and the electric eel Electrophorus electricus in postsynaptic membranes in high densities. Since these membranes can be easily prepared they have been, since their discovery, a favourable object for electron microscopists. The receptor protein appears in negatively stained membranes as a ring with a diameter of about 75 A. With improved techniques of preparing membranes which contain the receptor molecules in two-dimensional crystalline arrays and especially with computer-aided image processing, the ring appeared as an arrangement of five maxima (representing probably the five receptor subunits) with a five-fold axis of pseudosymmetry perpendicular to the membrane plane. At present the resolution obtained is better than 20 A, enough to depict the receptor's overall shape and dimensions but not enough to resolve functional moieties, as for example the selectivity filter and the gating device of the ion channel, which is an integral part of the receptor complex. The receptor-rich membranes turned out to be models for developing and comparing image processing methods. In this article some of these methods, especially the Circular Harmonic Averaging method, are critically reviewed.