Phosphorylation increases alpha-bungarotoxin binding to acetylcholine receptor-enriched membrane preparations

Acetylcholine receptor (AChR)-enriched membrane preparations from Torpedo electroplax bind alpha-bungarotoxin with a KD of 8.4 nM and the concentration of binding sites is 16 pmoles/mg protein. When, however, the alpha-toxin is incubated with receptor samples, phosphorylated in the presence of Mn2+ (10 mM) and ATP (0.5 mM), the concentration of binding sites is increased to 94 pmoles/mg protein, although there is no marked change in the KD (11 nM). Binding of acetylcholine to AChR-enriched membrane preparations occurs at a receptor which has two different conformational states with KD's of 8.4 nM and 520 nM. The concentration of binding sites for the high affinity component is 1.35 pmoles/mg protein and for the low affinity component is 12 pmoles/mg protein. The effect of phosphorylation of the AChR on agonist binding could not be determined.