Structural details of membrane-bound acetylcholine receptor from Tropedo marmorata

A projection, at 15- to 20-A resolution, is presented of the structure of the membrane-bound acetylcholine receptor protein from Torpedo marmorata. The projection has its axis perpendicular to the membrane plane; its main contribution originates from a hydrated portion of the protein, which extends from the membrane into the aqueous medium. The structure is distinctly asymmetric, with individual morphological subunits barely resolvable. These results have been obtained by noncrystallographic averaging, using correlation functions, applied to electron micrographs of receptor-rich membrane fragments. The micrographs had been taken with minimal beam exposure in a scanning transmission electron microscope.