Membrane Clustering and Bungarotoxin Binding by the Nicotinic Acetylcholine Receptor: Role of the β Subunit

Abstract: Nicotinic acetylcholine receptors (nAChRs) are localised at morphologically distinct regions of the postsynaptic membrane by interactions between the receptor subunits and cytoskeletal proteins, such as the 43-kDa protein. We have used Xenopus oocytes to examine the localisation and pharmacological properties of muscle nAChRs associated with 43-kDa protein and to compare them with hybrid muscle nAChRs containing a β subunit derived from a neuronal source. Receptors expressed on the oocyte outer membrane were visualised using confocal scanning laser microscopy. Coexpression of mouse muscle subunit α₁β₁γδ and 43-kDa protein transcripts produced discrete receptor aggregates with a diameter of 1–5 µm whose function was partially blocked by application of neuronal bungarotoxin (NBT) at 100 nM. Substitution of the β₁ subunit by the neuronal β₂ protein produced a functioning receptor that did not aggregate in the presence of 43-kDa protein and was substantially blocked by the same concentration of NBT. Hybrid α₁β₄γδ receptors exhibited a combination of characteristics in that they clustered like normal muscle subunits in the presence of 43-kDa protein, but showed a sensitivity to NBT intermediate between that of muscle receptors and that of hybrids containing β₂. These results suggest that the β subunit is an important determinant in receptor localisation and sensitivity to NBT.