Membrane clustering and bungarotoxin binding by the nicotinic acetylcholine receptor: role of the beta subunit

Nicotinic acetylcholine receptors (nAChRs) are localised at morphologically distinct regions of the post-synaptic 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 beta 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 alpha 1 beta 1 gamma delta and 43-kDa protein transcripts produced discrete receptor aggregates with a diameter of 1-5 microns whose function was partially blocked by application of neuronal bungarotoxin (NBT) at 100 nM. Substitution of the beta 1 subunit by the neuronal beta 2 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 alpha 1 beta 4 gamma delta 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 beta 2. These results suggest that the beta subunit is an important determinant in receptor localisation and sensitivity to NBT.