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) pr omote the association of S. aureus alpha-toxin with liposomes (shown b y 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 alpha-toxin in the presence of PEG with the hydrophobic photo-affinity probe methyl)-3-m-[I-125]iodop henyl)diazirine(I-125-TID) labels monomeric and-predominantly-hexameri c forms of liposome-associated alpha-toxin; in the absence of PEG litt le labeling is apparent. At high concentrations of H+ and Zn2+ but not of Ca2+-all of which inhibit calcein leakage-the distribution of labe l between hexamer and monomer is perturbed in favor of the latter. In alpha-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 mos t regards to pores induced in the absence of nonelectrolytes; they dif fer only in being more sensitive to ''closure'' by voltage (as are por es induced in cells). In both systems, the stimulation by nonelectroly tes increases with concentration and with molecular mass up to a maxim um around 2,000 Da. We conclude (i) that most of the a toxin that beco mes associated with liposome or planar lipid bilayers does not form ac tive pores and (ii) that the properties of alpha-toxin-induced pores i n lipid bilayers can be modulated to resemble those in cells.