Asn 183 in α5 is essential for oligomerisation and toxicity of the Bacillus thuringiensis Cry4Ba toxin

Abstract

The proposed toxicity mechanism of the Bacillus thuringiensis Cry insecticidal proteins involves membrane penetration and lytic pore formation of the α4–α5 hairpins in the target larval midgut cell membranes. In this study, alanine substitutions of selected polar residues (Tyr 178, Gln 180, Asn 183, Asn 185, and Asn 195) in the hydrophobic helix-α5 of the Cry4Ba mosquito-larvicidal protein were initially conducted via PCR-based directed mutagenesis. Upon IPTG induction, all the 130-kDa mutant protoxins were highly expressed in Escherichia coli as cytoplasmic inclusions, with yields similar to the wild-type protoxin. When E. coli cells expressing each mutant toxin were tested against Stegomyia aegypti mosquito larvae, the larvicidal activity of the N183A mutant was almost completely abolished whereas the four other mutants showed only a small reduction in toxicity. Additionally, replacements of this critical residue with various amino acids revealed that the uncharged polar residue at position 183 in α5 is crucial for larvicidal activity. Further characterisation of the N183K bio-inactive mutant revealed that the 65-kDa activated toxin was unable to form oligomers in lipid vesicles and its ability to induce the release of entrapped calcein from liposomes was much weaker than that of the wild-type toxin. These results suggest that the highly conserved Asn 183 located in the middle of the transmembrane α5 of Cry4Ba plays a crucial role in toxicity and toxin oligomerisation in the lipid membranes.