The alpha-bungarotoxin receptor purified from a human neuroblastoma cell line: biochemical and immunological characterization.

The pharmacological and electrophysiological characteristics of the alpha-bungarotoxin receptor present on the human neuroblastoma cell line IMR-32 indicate that this receptor is not associated with an acetylcholine-operated ionic channel. In this paper we report its biochemical purification and immunological characterization. This molecule has a standard sedimentation coefficient of 10S and sodium dodecyl-sulphate gel electrophoresis shows that it is made up of three polypeptide chains of molecular weights of 67,000, 60,000 and 52,000. Ligand binding to blots of purified receptor revealed that only the polypeptide of molecular weight 52,000 is bound by [125I]alpha-bungarotoxin. The purified alpha-bungarotoxin receptor was bound by polyclonal antibodies raised against purified fetal calf, Torpedo and chick optic lobe nicotinic receptors and by the sera of myasthenic patients. Furthermore, despite the fact that a number of different immunological techniques were used, it was impossible to label this alpha-bungarotoxin receptor with mAb 35, a monoclonal antibody which binds some neuronal nicotinic receptors. Rabbit antisera against the purified alpha-bungarotoxin receptor were used to compare this protein with other known nicotinic receptors and, once again, it was demonstrated that there is some immunological cross-reactivity between the alpha-bungarotoxin receptor present on neuroblastoma cells and Torpedo, fetal calf and chick optic lobe nicotinic receptors. All these immunological data, together with previously published pharmacological and molecular biology data, demonstrate that the alpha-bungarotoxin receptor present in nerve cells is neither a muscular nor a neuronal nicotinic receptor, although it has similarities with both.