Structure analysis of the two-peptide bacteriocin lactococcin G by introducing D-amino acid residues

The importance of 3D structuring in the N- and C-terminal ends of the two peptides (39-mer LcnG-alpha and 35-mer LcnG-beta) that constitute the two-peptide bacteriocin lactococcin G was analysed by replacing residues in the end regions with the corresponding D-isomeric residues. When assayed for antibacterial activity in combination with the complementary wild-type peptide, LcnG-alpha with four D-residues in its C-terminal region and LcnG-beta with four d-residues in either its N- or its C-terminal region were relatively active (two- to 20-fold reduction in activity). 3D structuring of the C-terminal region in LcnG-alpha and the C- and N-terminal regions in LcnG-beta is thus not particularly critical for retaining antibacterial activity, indicating that the 3D structure of these regions is not vital for interpeptide interactions or for interactions between the peptides and cellular components. The 3D structure of the N-terminal region in LcnG-alpha may be more important, as LcnG-alpha with four N-terminal D-residues was the least active of these four peptides (10- to 100-fold reduction in activity). The results are consistent with a proposed structural model of lactococcin G in which LcnG-alpha and -beta form a transmembrane parallel helix-helix structure involving approximately 20 residues in each peptide, starting near the N terminus of LcnG-alpha and at about residue 13 in LcnG-beta. Upon expressing the lactococcin G immunity protein, sensitive target cells became resistant to all of these D-residue-containing peptides. The end regions of the two lactococcin G peptides are consequently not involved in essential structure-dependent interactions with the immunity protein. The relatively high activity of most of the D-residue-containing peptides suggests that bacteriocins with increased resistance to exopeptidases may be generated by replacing their N- and C-terminal residues with d-residues.