DIFFERENTIAL SURFACE ACCESSIBILITY OF ALPHA(187-199) IN THE TORPEDO ACETYLCHOLINE-RECEPTOR ALPHA-SUBUNITS

We have probed the surface accessibility of residues alpha 187 to alph a 199 of the Torpedo acetylcholine receptor with monoclonal antibody 3 83C, which binds uniquely to these residues. However, 383C binds to on ly one of the two alpha subunits in the membrane-bound receptor, neith er of the two subunits in carbamylcholine-desensitized receptor, and t o both alpha subunits in Triton X-100 solubilized receptor. The kineti cs of association and dissociation of 383C with the peptide alpha(183- 199) compared to those with the membrane-bound receptor suggest that a ll but a single hydrogen bond of affinity derives from contacts betwee n this peptide and the monoclonal antibody paratope. Inhibition of 383 C binding by alpha-bungarotoxin selectively directed to the alpha subu nit correlated with the high-affinity d-tubocurarine binding site, alo ng with a lack of inhibition by alpha-bungarotoxin directed to the alp ha subunit correlated with the low-affinity d-tubocurarine binding sit e, suggests that the 383C epitope on the membrane-bound receptor resid es on the alpha subunit associated with the high-affinity d-tubocurari ne binding site. The results presented here suggest a structural basis for the differences between the two receptor acetylcholine binding si tes. (C) 1998 Academic Press.