Lactococcins:Mode of action, immunity and secretion

Lactococcus lactis subsp. cremoris 9B4 produces three small (around 5kDa), heat-stable, non-lanthionine containing, membrane active bacteriocins. Amino acid uptake experiments and proton motive force measurements have indicated that these peptides most probably form pores in the cytoplasmic membrane of sensitive cells. For bacteriocin activity a membrane protein (receptor) is necessary. The bacteriocin producing strain protects itself against the deleterious action of the bacteriocin by producing an immunity protein expressed from the same operon. The immunity protein works by blocking the receptor, thereby preventing the bacteriocin to form pores.The producer cell most probably secretes these peptides by a sec-independent transport machinery, homologous to the E. coli haemolysin secretion system. Two membrane-located proteins (LcnC and LcnD) are necessary for extracellular, active bacteriocin. By Phoa/LacZ fusion studies the topology of LcnD has been determined. The protein contains one transmembrane alpha-helix near its N-terminus. The N-terminus is located inside, the C-terminus is one the outside of the cell. LcnC most probably contains six transmembrane helices. The present model suggests that boyh membrane proteins are necessary for export of the bacteriocins, forming a dedicated transport-machinery of the ATP-binding cassette family.