5.3.3
Conclusion
In conclusion, these data indicate striking differences in the processing and
secretion mechanism of lantibiotic class IA I and class IA II and non-lantibiotic
class II bacteriocins. Secretion and processing of class IA I lantibiotics is mediated by two different proteins. Processing occurs extracellularly, except for class
IA I lantibiotics that are N-terminally modified prior to export. Class IA II lantibiotics and class II non-lantibiotic bacteriocins are both characterized by an
ABC transporter containing a conserved proteolytic processing domain in its
N-terminal tail. Processing occurs at the cytoplasmic side of the cellular membrane followed by secretion of the mature bacteriocin. Although its function
and occurrence in other class IA II lantibiotics remains to be investigated, an additional proteolytic enzyme was detected in the cytolysin operon. The accessory
factor protein, also conserved in Gram-negative secretion systems, was characterized as an unique feature of non-lantibiotic class II bacteriocin secretion
systems. Based on its divergent leader peptide, containing an ELS box but missing a double-glycine type processing site, an alternative model is expected to
emerge for lactocin S.
6
Role of the Leader Peptide
The bacteriocin leader peptides differ from the N-terminal signal sequences for
export of proteins secreted by the sec-dependent pathway in that they lack a
hydrophobic membrane-spanning stretch of 10 or more residues as well as the
typical proteolytic processing site [82, 120]. Class IA I leaders are slightly
positively or negatively charged and contain conserved residues such as a
proline residue at position –2, a serine residue at position –6 and the so-called
-F-N-L-D-V-box (Fig. 7 [40]). Site-directed mutagenesis has shown that the
very conserved proline residue is not essential for processing, but that a positive
charge at position –1 and a small hydrophobic residue at position –4 are essential [102]. Mutation of the phenylalanine, leucine or aspartate residue in the
-F-N-L-D-V-box or the serine residue at position –6 prevented biosynthesis of
nisin, not even the precursor could be detected [102]. The results so far obtained with site-directed mutagenesis clearly demonstrate the importance of
particular residues in the leader peptide for biosynthesis. Meanwhile, there is
evidence that processing is the last step in lantibiotic maturation, so that
modification reactions are made at the prepeptide stage, and that the precursor
molecule is inactive [102]. Therefore, the leader peptide might have an essential
role in biosynthesis, either in that it could contain a specific recognition motif
which would direct the precursor towards biosynthetic enzymes and/or in that
the leader peptide may interact with the propeptide region to stabilize a conformation which is essential for correct modification [39]. In contrast, the class
IA II lantibiotics contain a double-glycine type processing site and a leader
peptide that matches the consensus of the non-lantibiotic class II bacteriocins
[21, 40] (Fig. 7). However, putative modifying enzymes for the class IA II
Antimicrobial Peptides of Lactic Acid Bacteria: Mode of Action, Genetics and Biosynthesis
45
Conclusion
In conclusion, these data indicate striking differences in the processing and
secretion mechanism of lantibiotic class IA I and class IA II and non-lantibiotic
class II bacteriocins. Secretion and processing of class IA I lantibiotics is mediated by two different proteins. Processing occurs extracellularly, except for class
IA I lantibiotics that are N-terminally modified prior to export. Class IA II lantibiotics and class II non-lantibiotic bacteriocins are both characterized by an
ABC transporter containing a conserved proteolytic processing domain in its
N-terminal tail. Processing occurs at the cytoplasmic side of the cellular membrane followed by secretion of the mature bacteriocin. Although its function
and occurrence in other class IA II lantibiotics remains to be investigated, an additional proteolytic enzyme was detected in the cytolysin operon. The accessory
factor protein, also conserved in Gram-negative secretion systems, was characterized as an unique feature of non-lantibiotic class II bacteriocin secretion
systems. Based on its divergent leader peptide, containing an ELS box but missing a double-glycine type processing site, an alternative model is expected to
emerge for lactocin S.
6
Role of the Leader Peptide
The bacteriocin leader peptides differ from the N-terminal signal sequences for
export of proteins secreted by the sec-dependent pathway in that they lack a
hydrophobic membrane-spanning stretch of 10 or more residues as well as the
typical proteolytic processing site [82, 120]. Class IA I leaders are slightly
positively or negatively charged and contain conserved residues such as a
proline residue at position –2, a serine residue at position –6 and the so-called
-F-N-L-D-V-box (Fig. 7 [40]). Site-directed mutagenesis has shown that the
very conserved proline residue is not essential for processing, but that a positive
charge at position –1 and a small hydrophobic residue at position –4 are essential [102]. Mutation of the phenylalanine, leucine or aspartate residue in the
-F-N-L-D-V-box or the serine residue at position –6 prevented biosynthesis of
nisin, not even the precursor could be detected [102]. The results so far obtained with site-directed mutagenesis clearly demonstrate the importance of
particular residues in the leader peptide for biosynthesis. Meanwhile, there is
evidence that processing is the last step in lantibiotic maturation, so that
modification reactions are made at the prepeptide stage, and that the precursor
molecule is inactive [102]. Therefore, the leader peptide might have an essential
role in biosynthesis, either in that it could contain a specific recognition motif
which would direct the precursor towards biosynthetic enzymes and/or in that
the leader peptide may interact with the propeptide region to stabilize a conformation which is essential for correct modification [39]. In contrast, the class
IA II lantibiotics contain a double-glycine type processing site and a leader
peptide that matches the consensus of the non-lantibiotic class II bacteriocins
[21, 40] (Fig. 7). However, putative modifying enzymes for the class IA II
Antimicrobial Peptides of Lactic Acid Bacteria: Mode of Action, Genetics and Biosynthesis
45