Monoclonal antibodies to Torpedo acetylcholine receptor. Characterisation of antigenic determinants within the cholinergic binding site.

Thirteen monoclonal antibodies (mAb) to the acetylcholine receptor (AChR) from Torpedo marmorata showed high avidity for the receptor but none exhibited binding to muscle AChR solubilised from seven other animal species. Five mAb and Fab monomer fragments prepared from two of them, inhibited alpha-bungarotoxin (alpha BuTx) binding to receptor by a maximum of 50%. In the presence of excess mAb the 125I-alpha BuTx bound could be precipitated by anti-IgG indicating that the mAb bound to only one of the two alpha BuTx binding sites on each AChR monomer. This site appeared to have a lower affinity for d-tubocurarine and decamethonium than the non-mAb site. Binding of five anti-site mAb was mutually competitive and four of them (AS2-AS5) were inhibited by other cholinergic ligands and influenced by four non-toxin binding site antibodies. One (AS1) bound within the toxin binding site yet outside the main neurotransmitter binding region. It is concluded that these five mAb distinguish between the two alpha BuTx binding sites on the Torpedo AChR, and bind only to the site which displays lower affinity for d-tubocurarine and other competitive ligands.