INSECT DEFENSIN, AN INDUCIBLE ANTIBACTERIAL PEPTIDE, FORMS VOLTAGE-DEPENDENT CHANNELS IN MICROCOCCUS-LUTEUS

Insect defensins are cationic, cysteine-rich peptides (approximately 4 kDa) that appear after bacterial challenge or injury in the hemolymph of insects belonging to a large variety of orders. These peptides pos sess anti-Gram-positive activity and participate in the potent antibac terial defense reactions of insects. Using recombinant insect defensin and the strain Micrococcus luteus as a test organism, we have investi gated the mode of action of this peptide. We show that defensin disrup ts the permeability barrier of the cytoplasmic membrane of M. luteus, resulting in a loss of cytoplasmic potassium, a partial depolarization of the inner membrane, a decrease in cytoplasmic ATP, and an inhibiti on of respiration. Potassium loss is inhibited below the order-disorde r transition of the lipid hydrocarbon chains. It is also inhibited by divalent cations and by a decrease in the membrane potential below a t hreshold of 110 mV. We propose that these permeability changes reflect the formation of channels in the cytoplasmic membrane by defensin oli gomers. This proposal is supported by patch-clamp experiments that sho w that insect defensins form channels in giant liposomes.