The nicotinic acetylcholine receptor from Torpedo electroplax

Similarity of acetylcholine receptors of denervated, innervated and embryonic chicken muscles. 2. Subunit compositions

The native (9-S) form of the acetylcholine receptor, purified from chick embryonic or adult innervated or denervated chicken muscles, was shown in each case to contain three subunits of Mr about 40000 (predominant), 50000 and 54000. The 40000-Mr subunit (termed alpha) appears to exist in two forms; the minor variant form of it, with Mr about 41000, may have a different carbohydrate content. Bromo[3H]acetylcholine, an affinity alkylating reagent for this receptor, labels the 40000-Mr subunit and the accompanying 41000-Mr variant. The apparent ratio of the subunits varies with the several methods of detection used (protein staining, radio-iodination or tritiation) but the alpha subunit always predominates. The alpha subunit was separated in urea gel isoelectric focusing; its isoelectric point was identical there for the three receptors from innervated, denervated and embryonic muscles. Their respective peptide maps for alpha subunit were identical, and likewise for the 54000-Mr subunit. All of the evidence obtained shows that the subunit structures of the junctional, extra-junctional and embryonic chick muscle receptors are identical. Although the alpha subunit in the receptor from Torpedo electric organ, cat muscle and chicken muscles is distinguishable in all three cases in dodecylsulphate gel electrophoresis, it is affinity-labelled in each case. The peptide map of this subunit after limited proteolysis is also very similar in all three cases. It is concluded that the alpha subunits from these three diverse sources are closely related structurally.

Distribution of cytochemically detectable cholesterol in the electric organ of Torpedo marmorata

A cytochemical probe for cholesterol, the polyene antibiotic filipin, was applied to aldehyde-fixed samples of the electric organ of Torpedo marmorata to identify filipin-binding sites in the various membrane components of the organ and, hence, the probable cholesterol content at these levels. In both thin-sectioned and freeze-fractured samples, filipin-cholesterol complexes appeared numerous and homogeneously distributed on the Schwann cell plasma membrane. On the presynaptic membrane, filipin-cholesterol complexes occurred in patches alternating with unlabeled membrane segments. The postsynaptic, acetylcholine receptor-rich plasma membrane of the electroplax showed no or few filipin-cholesterol complexes in the flat region and upper part of the invaginations (both areas characterized by a lattice of small intramembrane particles); however, the membrane of the bottom part of the postsynaptic invaginations contained several complexes. The ventral, noninnervated plasma membrane of the electroplax showed a moderate, homogeneous filipin labeling. These data suggest that the distribution of cholesterol among membranes of the electroplax is not homogeneous and that the acetylcholine receptor-rich region of the postsynaptic membrane (as characterized by the lattice of small intramembrane particles) may contain little cholesterol.