Lipid-induced conformation of helix 7 from the pore-forming domain of the Bacillus thuringiensis Cry4Ba toxin: Implications for toxicity mechanism

Helix 7 in the Cry4Ba-pore-forming domain contains conserved Tyr249 and Phe264 that are crucially involved in mosquito-larvicidal activity. We have now characterized lipid-induced conformation of a 27-residue Cry4Ba-α7 peptide in phospholipid membranes using ATR-FTIR and hydrogen/deuterium (H+/D+) exchange experiments. ATR-FTIR results showed that conformation of this peptide is influenced by lipid composition and peptide–lipid ratio. For zwitterionic membranes, 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) or 1,2-didecanoyl-sn-glycero-3-phosphocholine, the peptide adopted both α-helix and α-structure, but only α-helical conformation was observed in anionic membranes (1,2-dimyristoyl-sn-glycero-3-phosphoglycerol). H+/D+ exchange results showed protection of ∼90% in DMPC for β-form, while α-helical form was found preferentially on membrane surface with both critical aromatic residues pointing towards bilayers. Analysis of 10-ns simulations of Cry4Ba-α7 in DMPC supports the stability of α-helical and β-conformations for membrane-associated and membrane-inserted states, respectively. We suggest that this lipid-induced conformational change of α7 is conceivably related to pore-forming mechanism as structural requirement for efficient membrane insertion.