Biochemistry and Molecular Biology of Snake Neurotoxin

A review of research on snake neurotoxin is presented, with emphasis on the chemical modification studies and molecular cloning of postsynaptic and presynaptic neurotoxins from Naja naja atra (Taiwan cobra) (Fig. 1a) and Bungarus multicinctus (Taiwan banded krait) (Fig. 1b). Cobrotoxin and α-bungarotoxin are the primary postsynaptic neurotoxins isolated from the venom of N. naja atra and B. multicinctus, respectively. Although they share a common three-dimensional structure, the functional elements essential for the manifestation of their toxicity are different. Selective and stepwise chemical modification of cobrotoxin indicate that at least two cationic groups, an ?-amino group of Lys-47 and a guanidino group of Arg-33 common to all known postsynaptic neurotoxins, are functionally important for its neuromuscular blocking activity. However, for α-bungarotoxin, the side chains of several basic amino acid residues are involved in the multipoint contact between the toxin and acetylcholine receptor. Moreover, the conserved Trp residue is essential for the neurotoxicity of cobrotoxin, but not for α-bungarotoxin. The cDNAs encoding cobrotoxin and α-bungarotoxin was constructed from the cellular RNA isolated from the venom glands of N. naja atra and B. multicinctus by polymerase chain reaction. The sequence of their 3′-untranslational region, signal peptide and 5′-untranslational region share a high degree of homology, suggesting that they are evolutionarily related. Expression of both neurotoxic protein in E. coli generated polypeptide chains for reactions with the antisera against the native neurotoxins. Presynaptic neurotoxins constitute a different group of neurotoxic proteins in snake venom proteins. These presynaptic neurotoxins are either basic phospholipase A₂ (PLA₂) per se or contain basic PLA₂ as an indispensible part of their structures. Thus, the presynaptic neurotoxins usually show both PLA₂ activity and presynaptic neurotoxicity. β-Bungarotoxin (β-Bgt), the main presynaptic PLA₂ neurotoxin isolated from the venom of B. multicinctus (Taiwan banded krait), consists of two dissimilar polypeptide chains, a PLA₂ subunit (A chain) and potassium channel recognition subunit (B chain). Chemical modification studies show that the toxin might possess two functional sites, one responsible for the catalytic activity and the other for its pharmacological properties. Molecular cloning and expression of the A chain and B chain of β-Bgt reveal that the A chain of β-Bgt is an active subunit with PLA₂ activity, and that the B chain is involved in voltage-gated potassium channel blocking action observed with β-Bgt.