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 alpha-helix content increases in similar to 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 degrees tilt with respect to the membrane normal. The orientation of the central beta-sandwich was also estimated. In addition, it was detected that StnII alters the orientation of the lipid acyl chains. H-1/H-2 exchange experiments sustain a mainly superficial interaction between StnII and the membrane, with no protection of the beta-sandwich. The implications of the results in the mechanism of pore formation are discussed.