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 Tyr(249) and Phe(264) that are crucially involved in mosquito-larvicidal activity. We have now characterized lipid-induced conformation of a 27-residue Cry4Ba-alpha7 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 alpha-helix and alpha-structure, but only alpha-helical conformation was observed in anionic membranes (1,2-dimyristoyl-sn-glycero-3-phosphoglycerol). H(+)/D(+) exchange results showed protection of approximately 90% in DMPC for beta-form, while alpha-helical form was found preferentially on membrane surface with both critical aromatic residues pointing towards bilayers. Analysis of 10-ns simulations of Cry4Ba-alpha7 in DMPC supports the stability of alpha-helical and beta-conformations for membrane-associated and membrane-inserted states, respectively. We suggest that this lipid-induced conformational change of alpha7 is conceivably related to pore-forming mechanism as structural requirement for efficient membrane insertion.