Modification of α-bungarotoxin and cholinergic ligand-binding properties of Torpedo acetylcholine receptor by a monoclonal anti-acetylcholine receptor antibody

The interaction between acetylcholine receptor (AcChR) and monoclonal antibody (mab) 247G - whose binding is blocked by the presence of α-bungarotoxin (αBgTx) - leads, in the absence of αBgTx, to a maximum binding of 0.5 mabs/αBgTx-binding site and, in turn, produces a maximum of 50% inhibition of αBgTx binding. For the solubilized AcChR, this inhibition is the result of blockade by mab 247G of the kinetically resolved slow component of αBgTx binding. The presence of cholinergic ligands does not significantly inhibit mab binding to the AcChR. AcChR·mab 247G complexes bind d-[³H]tubocurarine and carbamyl[³H]choline with the same stoichiometry as for free AcChR. However, while the binding isotherms for the agonist remain unaltered, the dissociation constant of the antagonist for its high-affinity site increases at least 3 times and there is a decrease in the total number of high-affinity sites and a concomitant increase in the total number of low-affinity sites. These results indicate that the binding of mab 247G to the AcChR stabilizes a new conformational state of the molecule capable of binding cholinergic ligands and confirm previous reports indicating that the cholinergic binding site can be viewed as a region of overlapping cholinergic binding subsites.