Neuronal α-bungarotoxin receptors are α7 subunit homomers

Nicotinic acetylcholine receptors in the nervous system are heterogeneous with distinct pharmacological and functional properties resulting from differences in post-translational processing and subunit composition. Because of nicotinic receptor diversity, receptor purification and biochemical characterization have been difficult, and the precise subunit composition of each receptor subtype is poorly characterized. Evidence is presented that alpha-bungarotoxin (Bgt)-binding nicotinic receptors found in pheochromocytoma 12 (PC12) cells are pentamers composed solely of alpha 7 subunits. Metabolically labeled, affinity-purified Bgt receptors (BgtRs) consisted of a single 55 kDa band on SDS gels, which was recognized by anti-alpha 7 antibodies on immunoblots. Isoelectric focusing separated the 55 kDa band into multiple spots, all recognized by anti-alpha 7 antibodies and, therefore, each a differentially processed alpha 7 subunit. Cell-surface BgtR subunits, cross-linked to each other and I-125-Bgt, migrated on gels as a ladder of five bands with each band a multiple of an alpha 7 subunit monomer. Similar characteristics of BgtRs from rat brain suggest that they, like PC12 BgtRs, are alpha 7 pentamers containing differentially processed alpha 7 subunits.