Abstract
To test for submembrane structures common to postsynaptic membranes of muscle and electric tissue, mouse sternomastoid muscles were fixed in the presence of tannic acid under conditions such that a heavy layer of densely staining material was built up on plasma membranes. In this section electron micrographs of such muscles, the hydrophobic portion of the bilayer and some membrane-associated structures appear in strong negative contrast. At the endplate, acetylcholine receptor-rich membrane regions appear as a broad (14–15-nm) image approximately bisected by the thin (approximately 2-nm) image of the bilayer. The extracellular half of the image arises from the receptor protein. The cytoplasmic half contains a distinct bar of contrasted material which is precisely co-extensive with the receptor image Quantitative and direct visual comparisons show that the image is almost indistinguishable from that obtained by a similar method applied to isolated postsynaptic membranes from electric tissue of Torpedo californica (Sealock, R. (1982) J. Cell Biol. 92:514–522). In the latter case, the bar probably arises from the 43,000-dalton protein, a major peripheral membrane protein component of the isolated membranes. The results thus suggest that the 43,000-dalton protein in muscle and electric tissue forms similar structures and, conversely, that the structure in the isolated membranes retains the essential features of its in situ state.