Structural and functional studies of a synthetic peptide mimicking a proposed membrane inserting region of aBacillus thuringiensisδ-endotoxin

In order to study the mechanism of action of Bacillus thuringiensis delta-endotoxins, a synthetic 31-mer peptide corresponding to the sequence of a putative pore-forming segment of the CrylA(c) toxin was characterized structurally and functionally. The peptide maps onto the central helix (alpha 5) of the six-helix bundle of domain I of the crystal structure of the CryIIIA toxin. CD and NMR spectroscopic studies indicated that the peptide exists as an alpha-helix in methanol and a random coil in water. The peptide associated with liposomes at pH 4.7 and formed discrete, characterizable channels in planar lipid bilayers at low pHs. These channels had a conductance value of 60 picosiemens (pS). It is possible that this helix is a component of the transmembrane pore formed by B. thuringiensis delta-endotoxins in vivo.