Visualization at the mouse neuromuscular junction of a submembrane structure in common with Torpedo postsynaptic membranes.

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.