Differential roles for disulfide bonds in the structural integrity and biological activity of κ-bungarotoxin, a neuronal nicotinic acetylcholine receptor antagonist

κ-Bungarotoxin, a κ-neurotoxin derived from the venom of the banded Krait, Bungarus multicinctus, is a homodimeric protein composed of subunits of 66 amino acid residues containing five disulfide bonds. κ-Bungarotoxin is a potent, selective, and slowly reversible antagonist of α3β2 neuronal nicotinic acetylcholine receptors. κ-Bungarotoxin is structurally related to the α-neurotoxins, such as α-bungarotoxin derived from the same snake, which are monomeric in solution and which effectively antagonize muscle type receptors (α1β1γδ) and the homopentameric neuronal type receptors (α7, α8, and α9). Like the κ-neurotoxins, the long α-neurotoxins contain the same five conserved disulfide bonds, while the short α-neurotoxins only contain four of the five. Systematic removal of single disulfide bonds in κ- bungarotoxin by site-specific mutagenesis reveals a differential role for each of the disulfide bonds. Removal of either of the two disulfides connecting elements of the carboxy terminal loop of this toxin (Cys 46-Cys 58 and Cys 59-Cys 64) interferes with the ability of the toxin to fold. In contrast, removal of each of the other three disulfides does not interfere with the general folding of the toxin and yields molecules with biological activity. In fact, when either C3-C21 or C14-C42 are removed individually, no loss in biological activity is seen. However, removing both produces a polypeptide chain which fails to fold properly. Removal of the C27-C31 disulfide only reduces the activity of the toxin 46.6-fold. This disulfide may play a role in specific interaction of the toxin with specific neuronal receptors.