INTERACTION OF THE LANTIBIOTIC NISIN WITH MEMBRANES REVEALED BY FLUORESCENCE QUENCHING OF AN INTRODUCED TRYPTOPHAN

Nisin is a lantibiotic produced by strains of Lactococcus lactis subsp . lactis. The target for nisin action is the cytoplasmic membrane of g ram-positive bacteria, To aid understanding of its mode of action, the interaction of nisin with vesicles of differing phospholipid composit ion were investigated by fluorescence techniques, using a variant of n isin in which the isoleucine at position 30 was replaced by a tryptoph an residue, Activity of the site-directed variant containing tryptopha n was established to be similar to that of the wild-type peptide. Fluo rescence experiments showed a blue shift of the emission wavelength ma ximum in the presence of lipid vesicles, indicating that the tryptopha n residue enters a more hydrophobic environment. Quenching experiments with aqueous and membrane-restricted quenchers (iodide and spin-label led lipids, respectively) both confirmed a non-aqueous environment for the Trp30 residue, and implied that the residue resides between 0.35 nm and 0.52 nm from the centre of the membrane, depending on the lipid identity. The results clearly demonstrate that nisin interacts strong ly with the hydrophobic phase of lipid vesicles, This interaction is s tronger in the presence of negatively charged lipids suggesting their importance in the functional interaction of nisin with membranes.