IONIC CHANNELS INDUCED BY SURFACTIN IN PLANAR LIPID BILAYER-MEMBRANES

Surfactin is a lipopeptide produced by certain strains of Bacillus subtilis and has potent surface activity. Here, we present the first results showing that ion-conducting pores can be formed by surfactin in artificial lipid membranes. With a low aqueous concentration of surfactin (1-mu-M) and a restricted membrane area (5 . 10(-5) cm2) we observed conductance jumps that indicate the formation of individual ionic channels in the presence of K+, Rb+, Cs+, Na+ or Li+ chlorides. Although for every salt concentration (C(i)), the distribution in amplitude of the conductance steps (LAMBDA-i) may be rather broad, there is always a step amplitude which is more frequent than the others. In addition, the channels corresponding to this most frequent step amplitude are the longest in duration. For C(i) = 1 M, the cationic selectivity sequence deduced from these most frequent events is K+ > Rb+ > Na+ > Cs+ = Li+ with respective values for LAMBDA-i(M): 130, 110, 80 and 30 pS. In KCl solutions LAMBDA-KCl(M) increases as a function of C(i) for low C(i), and shows a plateau for C(i) > 0.5 M. When measured on larger area membranes (10-2 cm2) with 1 M solutions of the monovalent salts KCl, NaCl, RbCl and CsCl or the divalent salt CaCl2, the macroscopic low voltage conductance (G(o)) increases with a slope of 2 on a log-log plot as a function of surfactin concentration. These results demonstrate that surfactin produces selective cationic channels in lipid bilayer membranes and suggest that at higher salt concentration, a dimer is involved in this functional channel-forming process.