Characterization and gene organization of Taiwan banded krait (Bungarus multicinctus) gamma-bungarotoxin

gamma-Bungarotoxin was isolated from Bungarus multicinctus (Taiwan banded krait) venom using a combination of chromatography on a SP-Sephadex C-25 column and a reverse-phase high-performance liquid chromatography column. Circular dichroism (CD) measurement revealed that its secondary structure was dominant with beta-sheet structure as is that of snake venom alpha-neurotoxins and cardiotoxins. gamma-Bungarotoxin exhibits activity on inhibiting the binding of [3H]quinuclidinyl benzilate to the M2 muscarinic acetylcholine receptor subtype, and competes weakly with radioiodinated alpha-bungarotoxin for binding to the Torpedo nicotinic acetylcholine receptor. Moreover, the toxin inhibits collagen-induced platelet aggregation, with an IC50 of approximately 200 nM. The genomic DNA encoding the gamma-bungarotoxin precursor is amplified by polymerase chain reaction (PCR). The gene is organized with three exons separated by two introns, and shares virtually identical overall organization with those reported for alpha-neurotoxin and cardiotoxin genes, including similar intron insertions. The intron sequences of these genes share sequence identity up to 85%, but the exon sequences are highly variable. These observations suggest that gamma-bungarotoxin, alpha-neurotoxins, and cardiotoxins originate from a common ancestor, and the evolution of these genes shows a tendency to diversify the functions of snake venom proteins.