The Journal of Cell Biology, Vol 92, 333-342, Copyright © 1982 by The Rockefeller University Press

ARTICLES

Nicotinic postsynaptic membranes from Torpedo: sidedness, permeability to macromolecules, and topography of major polypeptides

PA St. John, SC Froehner, DA Goodenough and JB Cohen

Experiments were conducted to examine the topographic arrangement of the polypeptides of the acetylcholine receptor (AcChR) and the nonreceptor Mr 43,000 protein in postsynaptic membranes isolated from Torpedo electric organ. When examined by electron microscopy, greater than 85% of vesicles were not permeable to ferritin or lactoperoxidase (LPO). Exposure to saponin was identified as a suitable procedure to permeabilize the vesicles to macromolecules with minimal alteration of vesicle size or ultrastructure. The sidedness of vesicles was examined morphologically and biochemically. Comparison of the distribution of intramembrane particles on freeze-fractured vesicles and the distribution found in situ indicated that greater than 85% of the vesicles were extracellular-side out. Vesicles labeled with alpha- bungarotoxin (alpha-Bgtx) were reacted with antibodies against alpha- BgTx or against purified AcChR of Torpedo. Bound antibodies were detected by the use of ferritin-conjugated goat anti-rabbit antibody and were located on the outside of greater than 99% of labeled vesicles. Similar results were obtained for normal vesicles or vesicles exposed to saponin. Quantification of the amount of [3H]-alpha-BgTx bound to vesicles before and after they were made permeable with saponin indicated that less than 5% of alpha-BgTx binding sites were cryptic in normal vesicles. It was concluded that greater than 95% of postsynaptic membranes were oriented extracellular-side out. LPO- catalyzed radioiodinations were performed on normal and saponin-treated vesicles and on vesicles from which the Mr (relative molecular mass) 43,000 protein had been removed by alkaline extraction. In normal vesicles, polypeptides of the AcChR were iodinated while the Mr 43,000 protein was not. In vesicles made permeable with saponin, the pattern of labeling of AcChR polypeptides was unchanged, but the Mr 43,000 protein was heavily iodinated. The relative iodination of AcChR polypeptides was unchanged in membranes equilibrated with agonist or with alpha-BgTx or after alkaline-extraction. It was concluded that the Mr 43,000 protein is present on the intracellular surface of the postsynaptic membrane and that AcChR polypeptides are exposed on the extracellular surface.
CiteULike    Complore    Connotea    Del.icio.us    Digg    Reddit    Technorati    What's this?

This article has been cited by other articles:

• Cartaud, A, Jasmin, B., Changeux, J., Cartaud, J (1995). Direct involvement of a lamin-B-related (54 kDa) protein in the association of intermediate filaments with the postsynaptic membrane of the Torpedo marmorata electrocyte. J. Cell Sci. 108: 153-160 [Abstract]  
• Changeux, J.P., Babinet, C., Bessereau, J.L., Bessis, A., Cartaud, A., Cartaud, J., Daubas, P., Devillers-Thiery, A., Duclert, A., Hill, J.A., Jasmin, B., Klarsfeld, A., Laufer, R., Nghiem, H.O., Piette, J., Roa, M., Salmon, A.M. (1990). Compartmentalization of Acetylcholine Receptor Gene Expression during Development of the Neuromuscular Junction. Cold Spring Harb Symp Quant Biol 55: 381-396 [Abstract]  
• Changeux, J., Devillers-Thiery, A, Chemouilli, P (1984). Acetylcholine receptor: an allosteric protein. Science 225: 1335-1345 [Abstract]  
• Changeux, J.-P., Bon, F., Cartaud, J., Devillers-Thiery, A., Giraudat, J., Heidmann, T., Holton, B., Nghiem, H.-O., Popot, J.-L., Van Rapenbusch, R., Tzartos, S. (1983). Allosteric Properties of the Acetylcholine Receptor Protein from Torpedo marmorata. Cold Spring Harb Symp Quant Biol 48: 35-52 [Abstract]  

Home | Help | Feedback | Subscriptions | Archive | Search | Table of Contents