Gaegurin 4, a peptide antibiotic of frog skin, forms voltage-dependent channels in planar lipid bilayers

Gaegurin 4 (GGN4) is a cationic peptide of 37 amino acids (MW 3748) isolate d from the skin of Rana rugosa. It has shown a broad spectrum antimicrobial activity in vitro against Gram-negative and -positive bacteria, fungi and protozoa. To understand its mechanism of antimicrobial action, we examined the effect of GGN4 on the membrane conductance and the electrical propertie s of GGN4-induced pores in planar lipid bilayers under voltage clamp. Natur al and synthetic GGN4 (0.01-1 mu g/mL) increased the membrane conductance i n a concentration-dependent manner, but GGN4 (1-23), an N-terminal fragment of the peptide with little antimicrobial activity, failed to increase the conductance. At symmetrical 100 mM KCl, unitary conductances of about 120 p S were frequently observed. Their current-voltage relations were linear and open state probabilities were close to 1, but longer closing events were s een more frequently at negative voltages. In addition, GGN4-induced pores w ere selective for cation over anion, the permeability ratio of K+ to Cl- be ing 6.1 in neutral and 7:1 in acidic lipid bilayers. In conclusion, our res ults indicate that GGN4 forms voltage-dependent and cation-selective pores in planar lipid bilayers. The ionophoric property of GGN4 is likely to cont ribute to its antimicrobial activity.