Functional expression and site-directed mutagenesis of a synthetic gene for alpha-bungarotoxin

In order to explore the structure-function relationships of the curare mimetic alpha-neurotoxins we have constructed and cloned a synthetic gene for Bungarus multicinctus alpha-bungarotoxin which is expressed in Escherichia coli. The recombinant alpha-bungarotoxin is expressed as a fusion protein with alpha-bungarotoxin linked to the COOH-terminal end of the T7 Gene 9-encoded coat protein. After treatment of the fusion protein with Factor Xa protease, a recombinant alpha-bungarotoxin is released that co-migrates with authentic alpha-bungarotoxin upon reverse-phase high performance liquid chromatography and ion-exchange chromatography. Final yields of active recombinant alpha-bungarotoxin were about 0.4 mg/liter of starting bacterial culture. The recombinant alpha-bungarotoxin contains 10 additional residues linked to the NH2-terminal Ile of the alpha-bungarotoxin sequence due apparently to the inaccessibility of the engineered cleavage site to Factor Xa. Nevertheless, the recombinant alpha-bungarotoxin is capable of binding to the nicotinic acetylcholine receptor with an apparent affinity that is only decreased approximately 1.7-fold from that of authentic alpha-bungarotoxin. Alanine substitution of a residue, Asp30, highly conserved among alpha-neurotoxins and previously suggested to play a key role in receptor recognition, resulted in a recombinant alpha-bungarotoxin whose receptor binding activity is indistinguishable from authentic alpha-bungarotoxin.