closely resembles that of the non-lantibiotics. In agreement with this model, an
ABC transporter including the N-terminal proteolytic domain was encoded in
the lactococcin DR (LctT) and cytolysin (CylT) operons, indicating that they
share a common mechanism of export with the non-lantibiotic, double-glycine
type leader peptide bacteriocins [118, 122]. However, in contradiction to the
class II non-lantibiotic bacteriocins, downstream from the 714-amino acid
residue translocator CylT, a 412 amino acid residue protease is encoded [122].
Functional analysis studies and newly obtained information residing from
other class IA II bacteriocin operons will have to resolve this apparently conflicting data.
Recently, a new lantibiotic, lactocin S has been characterized. Its leader
peptide is typical for the class IA II lantibiotics but lacks a double-glycine
processing site, suggesting an alternative proteolytic cleavage [82, 121]. This
hypothesis is reflected in the lactocin S operon which encodes an ABC transporter of 535 amino acids, a protease of 266 residues as well as several open
reading frames with unknown functions [82, 121].
5.3.2
Accessory Proteins of the Class II Non-Lantibiotic Bacteriocins
Additionally, all non-lantibiotic class II bacteriocin operons, studied so far, were
shown to encode the homologous counterpart of HlyD, the accessory protein of
the haemo1ysin A secretion apparatus of E. coli [49, 124, 127, 128, 130–133].
These so-called accessory proteins are encoded adjacent to the structural ABC
exporter gene and characterized by a unique, N-terminal transmembrane domain [49, 124, 127, 128, 130–133]. They are predicted as integral proteins of the
cytoplasmic membrane speculated to facilitate signal sequence independent
secretion. It has been shown that the biosynthesis of non-lantibiotic bacteriocins requires both the ABC transporter and the accessory factor [105, 126, 133].
These accessory proteins belong to a recently identified, novel class of export
proteins designated as the membrane fusion protein (MPF) family [200].
Proteins of the MPF family have been hypothesized to cause local interaction of
the bacterial membranes, allowing direct exchange of substrates between the
two membranes of the Gram-negative envelope [201]. For some members of the
HlyD protein family, including HlyD, evidence has been presented that ten
residues of their periplasmic carboxy-terminus mediate association with the
outer membrane [202]. Based on these data, the long carboxy-terminal tail of
the accessory proteins for the secretion of non-lantibiotic class II bacteriocins
of Gram-positive bacteria could hence be expected at the extracellular face of
the cytoplasmic membrane since Gram-positive bacteria do not have an outer
membrane. The function of such MPF in the cytoplasmic membrane of Grampositive bacteria, however, is not clear. Counterparts of these accessory proteins
in either class IA I or IA II lantibiotic operons have not yet been reported.
44
E. Sablon et al.
ABC transporter including the N-terminal proteolytic domain was encoded in
the lactococcin DR (LctT) and cytolysin (CylT) operons, indicating that they
share a common mechanism of export with the non-lantibiotic, double-glycine
type leader peptide bacteriocins [118, 122]. However, in contradiction to the
class II non-lantibiotic bacteriocins, downstream from the 714-amino acid
residue translocator CylT, a 412 amino acid residue protease is encoded [122].
Functional analysis studies and newly obtained information residing from
other class IA II bacteriocin operons will have to resolve this apparently conflicting data.
Recently, a new lantibiotic, lactocin S has been characterized. Its leader
peptide is typical for the class IA II lantibiotics but lacks a double-glycine
processing site, suggesting an alternative proteolytic cleavage [82, 121]. This
hypothesis is reflected in the lactocin S operon which encodes an ABC transporter of 535 amino acids, a protease of 266 residues as well as several open
reading frames with unknown functions [82, 121].
5.3.2
Accessory Proteins of the Class II Non-Lantibiotic Bacteriocins
Additionally, all non-lantibiotic class II bacteriocin operons, studied so far, were
shown to encode the homologous counterpart of HlyD, the accessory protein of
the haemo1ysin A secretion apparatus of E. coli [49, 124, 127, 128, 130–133].
These so-called accessory proteins are encoded adjacent to the structural ABC
exporter gene and characterized by a unique, N-terminal transmembrane domain [49, 124, 127, 128, 130–133]. They are predicted as integral proteins of the
cytoplasmic membrane speculated to facilitate signal sequence independent
secretion. It has been shown that the biosynthesis of non-lantibiotic bacteriocins requires both the ABC transporter and the accessory factor [105, 126, 133].
These accessory proteins belong to a recently identified, novel class of export
proteins designated as the membrane fusion protein (MPF) family [200].
Proteins of the MPF family have been hypothesized to cause local interaction of
the bacterial membranes, allowing direct exchange of substrates between the
two membranes of the Gram-negative envelope [201]. For some members of the
HlyD protein family, including HlyD, evidence has been presented that ten
residues of their periplasmic carboxy-terminus mediate association with the
outer membrane [202]. Based on these data, the long carboxy-terminal tail of
the accessory proteins for the secretion of non-lantibiotic class II bacteriocins
of Gram-positive bacteria could hence be expected at the extracellular face of
the cytoplasmic membrane since Gram-positive bacteria do not have an outer
membrane. The function of such MPF in the cytoplasmic membrane of Grampositive bacteria, however, is not clear. Counterparts of these accessory proteins
in either class IA I or IA II lantibiotic operons have not yet been reported.
44
E. Sablon et al.