ANALYSIS OF EXPOSED REGIONS ON THE MAIN EXTRACELLULAR DOMAIN OF MOUSEACETYLCHOLINE-RECEPTOR ALPHA-SUBUNIT IN LIVE MUSCLE-CELLS BY BINDING PROFILES OF ANTIPEPTIDE ANTIBODIES

To study the structural organization of the main extracellular domain of the nicotinic acetylcholine receptor (AChR) alpha subunit in live m uscle cells, we examined the native membrane-bound receptors in cultur ed mouse skeletal muscle cells for their ability to bind a panel of an tibodies against uniform-sized overlapping synthetic peptides which co llectively represent this entire domain. The binding profile indicated that the regions alpha 23-49, alpha 78-126, alpha 146-174, and (alpha 182-210 are accessible to binding with antibody. Residues alpha 23-49 , alpha 78-126, and alpha 194-210 contain binding regions for alpha-ne urotoxin and some myasthenia gravis autoantibodies. A comparison of th is binding profile with the profile obtained for membrane-bound Torped o californica AChR in isolated membrane fractions showed some similari ties as well as significant differences between the subunit organizati on in the isolated membrane fraction and that in the membrane of live muscle cells. Regions alpha 89-104 and alpha 158-174, which are expose d in the isolated membrane fraction, are also exposed in the live cell . On the other hand, regions (alpha 23-49, and alpha 182-210, which ar e exposed in the live cell, are not accessible in the isolated membran e and, furthermore, the region alpha 1-16, which has marginal accessib ility in the cell, becomes highly accessible in the membrane isolates. The exposed regions defined by this study may be the primary targets for the initial autoimmune attack on the receptors in experimental aut oimmune myasthenia gravis.