cDNA sequence analysis and mutagenesis studies on the A chain of beta-bungarotoxin from Taiwan banded krait

The cDNA encoding the A chain of beta-bungarotoxin (beta-Bgt) was constructed from the cellular RNA isolated from the venom glands of Bungarus multicinctus (Taiwan banded krait). The deduced amino acid sequence encoding the A chain revealed that the determined chain was different from the known A chains (A1, A2, A3, A4, and A5). Nevertheless, the amino acid sequence and the cDNA sequence of the novel A chain were highly homologous with those of other A chains. The gene encoding the A chain of beta-Bgt was subjected to mutagenesis, and the Tyr-11, Cys-15, and Leu-72 of the A chain were substituted by Cys-11, Ser-15, and Cys-72, respectively. Instead of the six disulfide bonds observed with the A chain, the resulting mutant contained seven disulfide linkages in its molecular structure which simulated those of presynaptic PLA2 neurotoxins and PLA2 enzymes. However, the mutant did not exhibit a higher phospholipase activity than that noted with the recombinant A chain. These results seem to suggest that, in the absence of the B chain, the six pairs of disulfide bonds in the recombinant A-chain molecule are enough to maintain its active conformation for exerting the phospholipase activity.