Identification of a brain acetylcholine receptor alpha subunit able to bind alpha-bungarotoxin

Peptides corresponding to sequence segments homologous to an alpha-bungarotoxin (alpha-BGT) binding region on the alpha subunit of the Torpedo nicotinic cholinergic receptor (nAChR) were synthesized for each identified nAChR alpha subunit of the rat nervous system (alpha 1, which is expressed in muscle, and alpha 2, alpha 3, alpha 4, and alpha 5, which are expressed by neurons). The peptides were tested for their ability to directly bind 125I-alpha-BGT and to compete for 125I-alpha-BGT with Torpedo nAChR and with the alpha-BGT-binding component expressed by PC12, a sympathetic neuronal cell line. In addition to peptides of the muscle alpha 1 subunit, peptides corresponding to the sequence of a neuronal subunit, alpha 5, were able to bind 125I-alpha-BGT. Peptides containing the sequence segments 182-201 of the alpha 1 subunit and 180-199 of the alpha 5 subunit competed with Torpedo nAChR for 125I-alpha-BGT binding with IC50 values of 0.5 and 3.5 microM, respectively. Both of these peptides were also able to compete for 125I-alpha-BGT binding with native Torpedo nAChR and with the alpha-BGT-binding protein(s) expressed on PC12 cells. To determine if other sequence segments contribute to form the neuronal alpha-BGT-binding site, overlapping peptides corresponding to the putative extracellular domain of the alpha 5 subunit were synthesized and used both in direct binding assays and in competition experiments. Peptides corresponding to amino acids 16-35 and 180-199 of the alpha 5 subunit directly bound 125I-alpha-BGT and inhibited the binding of toxin to both Torpedo nAChR and PC12 cells. The results of these studies strongly support identification of the alpha 5 subunit as a component of a neuronal alpha-BGT-binding nAChR.