In this study, acetylcholine receptor-rich postsynaptic membranes from electric tissues of the electric rays Narcine brasiliensis and Torpedo californica are negatively contrasted for thin-section electron microscopy through the use of tannic acid. Both outer (extracellular) and inner (cytoplasmic) membrane surfaces are negatively contrasted, and can be studied together in transverse sections. The hydrophobic portion of the membrane appears as a thin (approximately 2 nm), strongly contrasted band. This band is the only image given by membrane regions which are devoid of acetylcholine receptor. In regions of high receptor density, however, both surfaces of the membrane are seen to bear or be associated with material which extends approximately 6.5 nm beyond the center of the bilayer. The material on the outer surface can be identified with the well-known extracellular portion of the receptor molecule. A major portion of the inner surface image is eliminated by extraction of the membranes at pH 11 to remove peripheral membrane proteins, principally the 43,000 Mr (43K) protein. The images thus suggest a cytoplasmic localization of the 43K protein, with its distribution being coextensive with that of the receptor. They also suggest that the 43K protein extends farther from the cytoplasmic surface than does the receptor.

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