PORE FORMATION BY S. AUREUS ALPHA -TOXIN IN LIPOSOMES AND PLANAR LIPID BILAYERS : EFFECTS OF NONELECTROLYTES

Nonelectrolytes such as polyethylene glycols (PEG) and dextrans (i) promote the association of S. aureus α-toxin with liposomes (shown by Coomassie staining) and (ii) enhance the rate and extent of calcein leakage from calcein-loaded liposomes; such leakage is inhibited by H+, Zn2+ and Ca2+ to the same extent as that of nonPEG-treated liposomes. Incubation of liposomes treated with α-toxin in the presence of PEG with the hydrophobic photo-affinity probe 3-(trifluoromethyl)-3-m-[125I]iodophenyl)diazirine(125I-TID) labels monomeric and—predominantly—hexameric forms of liposome-associated α-toxin; in the absence of PEG little labeling is apparent. At high concentrations of H+ and Zn2+ but not of Ca2+—all of which inhibit calcein leakage—the distribution of label between hexamer and monomer is perturbed in favor of the latter. In α-toxin-treated planar lipid bilayers from which excess toxin has been washed away, PEGs and dextrans strongly promote the appearance of ion-conducting pores. The properties of such pores are similar in most regards to pores induced in the absence of nonelectrolytes; they differ only in being more sensitive to ``closure'' by voltage (as are pores induced in cells). In both systems, the stimulation by nonelectrolytes increases with concentration and with molecular mass up to a maximum around 2,000 Da. We conclude (i) that most of the α toxin that becomes associated with liposome or planar lipid bilayers does not form active pores and (ii) that the properties of α-toxin-induced pores in lipid bilayers can be modulated to resemble those in cells.