ProtStruc Seminar: "Structure-Function Analysis of the Two-Peptide Bacteriocin Lactococcin G and its Immunity Protein" by Camilla Oppegård

Antimicrobial peptides produced by lactic acid bacteria are often termed bacteriocins. Bacteriocins kill bacteria by permeabilizing their membrane. In order for the bacteriocin-producing strain to be resistant to its own bacteriocin, it also produces a cognate immunity protein. The two-peptide bacteriocins, are unique in that they consists of two different peptides, both of which are required for antimicrobial activity. Lactococcin G was the first two-peptide bacteriocin to be characterized and it consists of LcnG-α and LcnG-β. The two peptides are proposed to interact trough helix-helix mediating GxxxG-motifs, and thereby forming a transmembrane helix-helix structure. In the same operon as the genes encoding the lactococcin G peptides, one also finds the gene encoding the lactococcin G immunity protein. The immunity protein protects the lactococcin G-producing strain against lactococcin G and enterocin 1071 (a two-peptide bacteriocin that is homologous to lactococcin G). The functionality of the lactococcin G immunity protein depends on a cellular component. Regions in both LcnG-α and LcnG-β, corresponding to the N-terminal part of the transmembrane helix-helix structure in the proposed model, seem to be important for the recognition by the immunity protein. It appears that the regions in lactococcin G that are important for the recognition by the immunity protein are also important in determining the target cell specificity. One might speculate that lactococcin G and its immunity protein function in an analogous manner as the pediocin-like bacteriocins and their immunity proteins. This would entail that lactococcin G recognizes and binds a membrane-embedded receptor in target cells in a manner that leads to membrane leakage and eventually cell death. Membrane-permeabilization is prevented by binding of the lactococcin G immunity protein to the bacteriocin-receptor complex. The functionality of the immunity protein would thus depend on a cellular component - the bacteriocin receptor.