Interaction of .theta.-Toxin (Perfringolysin O), a Cholesterol-Binding Cytolysin, with Liposomal Membranes: Change in the Aromatic Side Chains upon Binding and Insertion

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Abstract
To understand the mechanism of membrane lysis by theta-toxin (perfringolysin O) from Clostridium perfringens, a cholesterol-binding, pore-forming cytolysin, we undertook a spectroscopic analysis of the structural changes that occur during the lytic process using lipid vesicles. In particular, the spectra were compared with those obtained using a modified theta-toxin, MC theta, that binds membrane cholesterol without forming oligomeric pores, thus bypassing the oligomerization step. The interaction of theta-toxin with liposomes composed of cholesterol and phosphatidylcholine but not with cholesterol-free liposomes caused a remarkable increase in the intensity of the tryptophan fluorescence emission spectra and ellipticity changes in the near- and far-UV CD peaks. A CD peak shift from 292 to 300 nm was specific for theta-toxin, suggesting oligomerization-specific changes occurring around tryptophan residues. Structural changes in the aromatic side chains were detected in the near-UV CD and fluorescence spectra upon MC theta-liposome interaction, although the far-UV CD spectra indicate that the beta-rich secondary structure of MC theta is well-conserved after membrane binding. Quenching of the intrinsic tryptophan fluorescence of MC theta by brominated lecithin/cholesterol liposomes suggests that theta-toxin inserts at least partly into membranes in the absence of oligomerization. These results indicate that regardless of oligomerization, the binding of theta-toxin to cholesterol induces partial membrane insertion and triggers conformational changes accompanied by aromatic side chain rearrangement with retention of secondary structure.(ABSTRACT TRUNCATED AT 250 WORDS)

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