After fixation with glutaraldehyde and impregnation with tannic acid, the membrane that underlies the nerve terminals in Torpedo marmorata electroplaque presents a typical asymmetric triple-layered structure with an unusual thickness; in addition, it is coated with electron-dense material on its inner, cytoplasmic face. Filamentous structures are frequently found attached to these "subsynaptic densities." The organization of the subsynaptic membrane is partly preserved after homogenization of the electric organ and purification of acetylcholine-receptor (AchR)-rich membrane fragments. In vitro treatment at pH 11 and 4 degrees C of these AchR-rich membranes releases an extrinsic protein of 43,000 mol wt and at the same time causes the complete disappearance of the cytoplasmic condensations. Freeze-etching of native membrane fragments discloses remnants of the ribbonlike organization of the AchR rosettes. This organization disappears ater alkaline treatment and is replaced by a network which is not observed after rapid freezing and, therefore, most likely results from the lateral redistribution of the AchR rosettes during condition of slow freezing. A dispersion of the AchR rosettes in the plane of the membrane also occurs after fusion of the pH 11-treated fragments with phospholipid vesicles. These results are interpreted in terms of a structural stabilization and immobilization of the AchR by the 43,000-Mr protein binding to the inner face of the subsynaptic membrane.

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