Assembly of the alpha-toxin-hexamer of Staphylococcus aureus in the liposome membrane

It has been shown that the access of the alpha-toxin of Staphylococcus aureus to the target membrane and assembly of the hexamer can be monitored independently by respectively measuring the fluorescence energy transfer from the tryptophan residue(s) of the toxin to the dansylated phosphatidylethanolamine in the liposome membrane and the fluorescence increment of the toxin at 336 nm (Ikigai, H., and Nakae, T., (1987) J. Biol. Chem. 262, 2150-2155). Measurement of these parameters under various conditions showed the following results: when phosphatidylcholine (PC) liposomes composed of saturated fatty acids were mixed with the toxin, the fluorescence energy transfer occurred below, at, and above the transition temperature of the lipid, but the change of fluorescence at 336 nm was never detectable; when PC-liposomes containing unsaturated fatty acids were used, both the fluorescence energy transfer and the fluorescence increment of 336 nm were observed. These results suggested that the toxin-membrane interaction occurs in PC-membranes containing saturated and/or unsaturated fatty acids and that the oligomerization occurs only in the presence of PC containing unsaturated fatty acid(s). This conclusion was supported by the results of quantitative determination of the toxin-hexamer assembly and leakage of carboxyfluorescein from PC-liposomes under conditions similar to the above.