κ-Bungarotoxin. Self-association of a neuronal nicotinic receptor probe

κ-Bungarotoxin is a postsynaptic neurotoxin purified from the venom of the elapid snake Bungarus multicinctus. The amino acid sequence of this basic polypeptide reveals a single chain containing 66 amino acids having a M(r) of 7,313. κ-Bungarotoxin is a potent antagonist of nicotinic cholinergic transmission in avian and murine automatic ganglia, a characteristic which distinguishes the toxin from other postsynaptic neurotoxins isolated from snake venoms. The self-association of κ-bungarotoxin has now been examined using molecular sizing columns, sedimentation velocity, and sedimentation equilibrium. The results demonstrate that, under physiological solvent conditions, κ-bungarotoxin exists as a dimer (M(r) = 14,000 ± 3,000) of identical subunits. κ-Bungarotoxin monomers are not observed at toxin concentrations typically used in electrophysiological experiments (0.5-22 μg/ml), indicating that the dimer may be physiologically active. Denaturation with sodium dodecyl sulfate or urea dissociates κ-bungarotoxin dimers into monomers. Significant amounts of monomers are also produced under nondenaturing conditions of high ionic strength and high pH. However, complete reassociation of nondenatured monomers occurs following return to a physiological buffer. The unique pharmacological spectrum of κ-bungarotoxin may be due to its strong tendency to self-associate.