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

Jorge Alegre-Cebollada ^*, Álvaro Martínez del Pozo ^*, José G. Gavilanes ^* and Erik Goormaghtigh 

^* Departamento de Bioquímica y Biología Molecular I, Facultad de Ciencias Químicas, Universidad Complutense, 28040 Madrid, Spain; and Laboratory for the Structure and Function of Biological Membranes, Center for Structural Biology and Bioinformatics, CP 206/2, Free University of Brussels, B-1050 Brussels, Belgium

Correspondence: Address reprint requests to Jorge Alegre-Cebollada or José G. Gavilanes, Tel.: 34-91-394-41-58; Fax: 34-91-394-41-59; E-mails: alegre{at}bbm1.ucm.es or ppgf{at}bbm1.ucm.es.

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 -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.