Lipid-induced pore formation of the Bacillus thuringiensis Cry1Aa insecticidal toxin

After activation, Bacillus thuringiensis (Bt) insecticidal toxin forms pore s in larval midgut epithelial cell membranes, leading to host death. Althou gh the crystal structure of the soluble form of Cry1Aa has been determined, the conformation of the pores and the mechanism of toxin interaction with and insertion into membranes are still not clear. Here we show that Cry1Aa spontaneously inserts into lipid mono- and bilayer membranes of appropriate compositions. Fourier Transform InfraRed spectroscopy (FTIR) indicates tha t insertion is accompanied by conformational changes characterized mainly b y an unfolding of the beta -sheet domains. Moreover, Atomic Force Microscop y (AFM) imaging strongly suggests that the pores are composed of four subun its surrounding a 1.5 nm diameter central depression.