κ-Bungarotoxin is a 66 residue polypeptide found in the venom of the Taiwanese banded krait, Bungarus multicinctus. It binds tightly to neuronal nicotinic acetylcholine receptors and inhibits nerve transmission mediated by these postsynaptic receptors. It is related, by similarity in amino acid sequence, to α-bungarotoxin and other α-neurotoxins, but differs sharply in physiologic action. The α-neurotoxins inhibit nerve transmission in nicotinic acetylcholine receptors associated with vertebrate skeletal muscle and fish electric organs. The κ-neurotoxins inhibit nerve transmission in neuronal nicotinic acetylcholine receptors such as those found in chick ciliary ganglia. The κ-neurotoxins display a low level of interaction with receptors that are strongly affected by α-neurotoxins, but α-neurotoxins are completely without effect on receptors that are affected by κ-bungarotoxin. The structural basis for this physiologic differentiation is not known. Crystals of κ-bungarotoxin have now been obtained that diffract to at least 2.3 Å. These crystals are hexagonal, space group P6, and have dimensions of a = b = 80.2 A ̊, c = 39.6 A ̊, and angles of α = β = 90° and γ = 120°. Each unit cell contains 12 molecules of the 66 residue protein or two molecules per asymmetric unit. Comparison of the structure of κ-bungarotoxin, which will result from further diffraction analysis of these crystals, with the structures of the α-neurotoxins that have been determined may provide information on the structural basis of physiologic action in these acetylcholine receptor antagonists.
- Bungarus multicinctus