Infrared Spectroscopy Study on the Conformational Changes Leading to Pore Formation of the Toxin Sticholysin II

The structure of the actinoporin sticholysin II (StnII) in the pore state was investigated by Fourier transform infrared spectroscopy in the attenuated total reflection configuration. 1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine/cholesterol unilamellar vesicles were employed. The α-helix content increases in ∼30% upon lipid binding, which agrees with an extension of eight or nine residues at the N-terminal helix. Furthermore, analyses of dichroic spectra show that the extended N-terminal helix would have a 31° tilt with respect to the membrane normal. The orientation of the central β-sandwich was also estimated. In addition, it was detected that StnII alters the orientation of the lipid acyl chains. 1H/2H exchange experiments sustain a mainly superficial interaction between StnII and the membrane, with no protection of the β-sandwich. The implications of the results in the mechanism of pore formation are discussed.