A noncholinergic site-directed monoclonal antibody can impair agonist-induced ion flux in Torpedo californica acetylcholine receptor

We have employed several monoclonal antibodies (mAbs) directed against several of the acetylcholine receptor (AcChoR) to assist in the determination of the antigenic structure of this multisubunit glycoprotein and to better understand molecular events involved in the impairment of neuromuscular transmission in the autoimmune disease myasthenia gravis. Among three mAbs shown to block agonist-induced ion fluxes, mAb 371A is a putative probe of an ion channel domain(s) of the AcChoR. It appears to bind to an antigenic determinant whose structure is maintained upon treatment with sodium dodecyl sulfate, the stoichiometry of binding being of one mAb per α-bungarotoxin binding site. Binding of mAb 371A to the AcChoR does not affect binding of cholinergic agonists (carbamoylcholine and d-tubocurarine) or neurotoxins (α-bungarotoxin) or the ability of membrane-bound AcChoR to undergo reversible sensitization-desensitization affinity transitions. However, this mAb inhibits agonist-induced thallium (TI⁺) influx into AcChoR-rich membrane vesicles, as measured on a millisecond time scale by means of a rapid kinetics 'stopped-flow/fluorescence quenching' technique. The stoichiometry of inhibition by bound mAb 371A coincides with that for maximal binding.