AMPHIPHILIC ALPHA-HELICES ARE IMPORTANT STRUCTURAL MOTIFS IN THE ALPHA-PEPTIDE AND BETA-PEPTIDE THAT CONSTITUTE THE BACTERIOCIN LACTOCOCCIN-G - ENHANCEMENT OF HELIX FORMATION UPON ALPHA-BETA INTERACTION

Lactococcin G (LcnG) is an antimicrobial substance (bacteriocin) consi sting of two peptides, LcnG-alpha and LcnG-beta. The structures of int act LcnG-alpha and LcnG-beta as well as various fragments of these pep tides were studied by circular dichroism (CD) under several conditions . All peptides had a non-structured conformation in aqueous solutions. In the presence of trifluoroethanol, dodecylphosphocholine micelles a nd (negatively charged) dioleoylglycerophosphoglycerol (Ole(2)GroPGro) liposomes, varying amounts of a-helical structure were induced. Compa risons of the various fragments showed that helicity was concentrated in those parts of LcnG-alpha and LcnG-beta that would become amphiphil ic if an ct-helical structure was adopted. In the presence of zwitteri onic dioleoylglycerophosphocholine (Ole(2)GroPCho) liposomes, the pept ides were much less (if at all) structured, suggesting that the excess of positive charge on the antimicrobial peptides needs to be compensa ted by an excess of negative charge on the membrane. The structuring o f LcnG-alpha and LcnG-beta in the presence of Ole(2)GroPGro liposomes was considerably enhanced when both peptides were presented simultaneo usly to the membranes. Consecutive addition of the two peptides to Ole (2)GroPGro liposomes did not give this additional structuring, indicat ing that the individual LcnG-alpha and LcnG-beta peptides associate wi th the membrane in a virtually irreversible manner that makes them ina ccessible for interaction with the complementary peptide. The results suggest that upon arrival at and interaction with the target membrane, LcnG-alpha and LcnG-beta form a complex that consists of approximatel y 50% amphiphilic alpha-helices.