Cobrotoxin: structure and function.

Cobrotoxin is the main neurotoxic protein isolated from the venom of Taiwan cobra Naja naja atra. It is a small, basic protein consisting of a single polypeptide chain of 62 amino acids, cross-linked by four disulfide bonds. The disulfide bonds and Tyr-25 which is buried in the molecule form a central core to maintain and stabilize the active conformation of the toxin. Selective and stepwise chemical modifications of cobrotoxin indicate that at least two cationic groups, an epsilon-amino group of Lys-47 and a guanidino group of Arg-33, both of which are common to all known postsynaptic neurotoxins, held at a certain critical distance in the molecule, are functionally important for its neuromuscular blocking activity. The cDNA encoding cobrotoxin was constructed from the cellular RNA isolated from the venom glands of Naja naja atra by reverse transcription polymerase chain reaction. Sequencing several clones containing about 0.5 Kb DNA inserts contained a complete and full-length reading frame of 249 base pairs covering a precursor of cobrotoxin gene with a deduced mature protein sequence of 62 amino acids which are identical to the amino acid sequence of cobrotoxin and a 21 amino acid segment of signal peptide. Expression of cobrotoxin in E. coli vector generated a polypeptide which can cross-react with the antisera against the native cobrotoxin.