is encoded by the pep5 operon [85, 156, 157]. The mechanism of immunity
conferred by the NisI protein remains very speculative. The lipoprotein NisI
could, when attached to the exterior of the cellular membrane by lipid moieties,
confer immunity by direct interaction with extracellular nisin or by disturbing
the association of nisin aggregates, thus preventing channel formation [107].
For all class II bacteriocins genetically studied until now, a protein conferring immunity to the producer organism was encoded in the 3¢ end of the
bacteriocin operon, for example lactococcin A [46], lactococcin B [136], lactococcin M [46], lactococcin G [128], pediocin PA-1 and AcH [130, 131], mesentericin Y105 [124], carnobacteriocin B2 and BM1 [158], leucocin UAL-187 [42],
plantaricin A [49] and sakacin A [133]. These immunity proteins have a high pI
[49]. Furthermore, those associated with two-peptide bacteriocins consist of
110 to 154 amino acids containing several transmembrane domains [65, 77,
128], while those of the one-peptide bacteriocins are generally smaller (51 to
113 residues) and contain few (one or two) or no putative transmembrane
helices [65, 130, 133, 136, 158, 159]. Recently, a new class of immunity proteins
was reported consisting of 247 to 257 residues spanning the cytoplasmic membrane seven times [49].
Based on these findings, it seems that an important group of the immunity
proteins exert their activity at the cytoplasmic membrane, although lciA is the
only immunity protein studied in detail. The lactococcin A immunity factor was
purified and shown to interact with the cell membrane, whereas the presence of
free intracellular lciA is considered as a reservoir of immunity factor protein
[161]. LciA may span the membrane once by virtue of an a-amphiphilic helix
between residues 29 and 47 [162]. Topological studies showed that the carboxyterminus of LciA was orientated at the outside of the cytoplasmic membrane
[162]. LcnA acts on intact cells or membrane vesicles, but not on liposomes suggesting that a specific membrane receptor is required for LcnA recognition and
action [65, 162]. Membrane vesicles are protected from LcnA action if they are
derived from cells expressing LcnA immunity. Exposing lactococcin A-sensitive
cells to excess of the immunity protein did not affect the LcnA-induced killing
of the cells, indicating that the immunity protein does not protect cells by
simply binding to lactococcin A, or to externally exposed domains of the cell
surface [161]. Comparable results were reported for carnobacteriocin immunity
factors [158]. This suggests that LcnA immunity occurs at the cytoplasmic
membrane via a mechanism that either blocks a receptor, prevents LcnA channel formation, or inactivates the bacteriocin [62, 65]. The cell localization and
mode of action of immunity proteins without apparent potential membranespanning helices is not yet known, although membrane association of such
proteins can not be excluded. Interestingly, two such proteins MesI and PedB
display an almost identical hydrophobicity plot, suggesting a common mode of
action.
Recently, three additional open reading frames, nisF, nisE and nisG, were
revealed adjacent to nisK [104]. A comparable gene cluster, epiFEG has been
described for the lantibiotic epidermin, produced by Staphylococcus epidermidis [163]. The NisE/EpiE and NisG/EpiG proteins are both predominantly
hydrophobic with six transmembrane domains [104, 163]. The NisF/EpiF comAntimicrobial Peptides of Lactic Acid Bacteria: Mode of Action, Genetics and Biosynthesis
37
conferred by the NisI protein remains very speculative. The lipoprotein NisI
could, when attached to the exterior of the cellular membrane by lipid moieties,
confer immunity by direct interaction with extracellular nisin or by disturbing
the association of nisin aggregates, thus preventing channel formation [107].
For all class II bacteriocins genetically studied until now, a protein conferring immunity to the producer organism was encoded in the 3¢ end of the
bacteriocin operon, for example lactococcin A [46], lactococcin B [136], lactococcin M [46], lactococcin G [128], pediocin PA-1 and AcH [130, 131], mesentericin Y105 [124], carnobacteriocin B2 and BM1 [158], leucocin UAL-187 [42],
plantaricin A [49] and sakacin A [133]. These immunity proteins have a high pI
[49]. Furthermore, those associated with two-peptide bacteriocins consist of
110 to 154 amino acids containing several transmembrane domains [65, 77,
128], while those of the one-peptide bacteriocins are generally smaller (51 to
113 residues) and contain few (one or two) or no putative transmembrane
helices [65, 130, 133, 136, 158, 159]. Recently, a new class of immunity proteins
was reported consisting of 247 to 257 residues spanning the cytoplasmic membrane seven times [49].
Based on these findings, it seems that an important group of the immunity
proteins exert their activity at the cytoplasmic membrane, although lciA is the
only immunity protein studied in detail. The lactococcin A immunity factor was
purified and shown to interact with the cell membrane, whereas the presence of
free intracellular lciA is considered as a reservoir of immunity factor protein
[161]. LciA may span the membrane once by virtue of an a-amphiphilic helix
between residues 29 and 47 [162]. Topological studies showed that the carboxyterminus of LciA was orientated at the outside of the cytoplasmic membrane
[162]. LcnA acts on intact cells or membrane vesicles, but not on liposomes suggesting that a specific membrane receptor is required for LcnA recognition and
action [65, 162]. Membrane vesicles are protected from LcnA action if they are
derived from cells expressing LcnA immunity. Exposing lactococcin A-sensitive
cells to excess of the immunity protein did not affect the LcnA-induced killing
of the cells, indicating that the immunity protein does not protect cells by
simply binding to lactococcin A, or to externally exposed domains of the cell
surface [161]. Comparable results were reported for carnobacteriocin immunity
factors [158]. This suggests that LcnA immunity occurs at the cytoplasmic
membrane via a mechanism that either blocks a receptor, prevents LcnA channel formation, or inactivates the bacteriocin [62, 65]. The cell localization and
mode of action of immunity proteins without apparent potential membranespanning helices is not yet known, although membrane association of such
proteins can not be excluded. Interestingly, two such proteins MesI and PedB
display an almost identical hydrophobicity plot, suggesting a common mode of
action.
Recently, three additional open reading frames, nisF, nisE and nisG, were
revealed adjacent to nisK [104]. A comparable gene cluster, epiFEG has been
described for the lantibiotic epidermin, produced by Staphylococcus epidermidis [163]. The NisE/EpiE and NisG/EpiG proteins are both predominantly
hydrophobic with six transmembrane domains [104, 163]. The NisF/EpiF comAntimicrobial Peptides of Lactic Acid Bacteria: Mode of Action, Genetics and Biosynthesis
37