The β-bungarotoxin-binding protein from chick brain: binding sites for different neuronal K channel ligands co-fractionate upon partial purification

β-Bungarotoxin (β-Butx) is a presynaptically active neurotoxin which blocks neuronal A-type K⁺ channels. Here, the efficient solubilisation and about 300-fold purification of the β-Butx-binding protein from chick brain were achieved by detergent extraction at high ionic strength followed by chromatography on DEAE Affigel Blue, β-Butx Affigel 102 and wheat germ agglutinin Sepharose. Binding of ¹²⁵I-labelled β-Butx to the purified protein was inhibited by two other K⁺ channel ligands, dendrotoxin I and mast cell-degranulating peptide. It is concluded that the β-Butx-binding protein is a member of a family of voltage-gated K⁺ channels which exhibit varying affinities for different polypeptide ligands.