1.C.25 The Lactococcin G (Lactococcin G) Family
Many organisms synthesize proteins (or peptides) which are degraded to relatively small hydrophobic or amphipathic, bioactive peptides. These peptides exhibit antibiotic, fungicidal, virucidal, hemolytic and/or tumoricidal activities by interacting with membranes and forming transmembrane channels that allow the free flow of electrolytes, metabolites and water across the phospholipid bilayers. Most of these peptides appear to function in biological warfare. There are many designations given to these bioactive peptides. They include the magainins, cecropins, melittins, defensins, bacteriocidins, etc. The proteins in each family within this functional superfamily are homologous, but they exhibit little or no significant sequence similarity with members of the other families. Thus, each family may have evolved independently. However, certain common structural features observed between members of distinct families suggest that at least some of these families share a common ancestry.
The generalized transport reaction catalyzed by channel-forming amphipathic peptides is:
small solutes, electrolytes and water (in) small solutes, electrolytes and water (out).
Bacteriocins are bacterially produced peptide antibiotics with the ability to kill a limited range of bacteria, usually but not always those that are closely related to the producer bacterium. Many of them exhibit structural features typical of members of the eukaryotic channel-forming amphipathic peptides. That is, they are usually synthesized as small precursor proteins or peptides which are processed with proteolytic elimination of their N-terminal leader sequences, and the resultant mature peptides form one, two or more putative amphipathic transmembrane α-helical spanners (TMSs). For those with two TMSs, a characteristic hinge region that separates the two putative transmembrane segments is usually observed. A similar structural arrangement occurs in the two-TMS Cecropin A proteins (TC #1.C.17).
Many bacteriocins are encoded in operons that also encode an immunity protein and an ABC transport system (TC #3.A.1) with a protease domain at the N-terminus. The ABC systems export the bacteriocins while the protease domains cleave the N-terminal leader sequence. A few bacteriocins are exported by the type II general secretory pathway rather than by ABC-type export systems. In some cases, expression of the bacteriocin-encoding operon is induced by a bacteriocin-like peptide which acts in conjunction with a two component sensor kinase-response regulator to effect induction.
Class II non lanthionine-containing heat-stable bacteriocins are small membrane active peptides of less than 10 kDa characterized by a Gly-Gly-1 Xaa+1 processing site in the bacteriocin precursor, processed by the protease domain linked to the ABC-type bacteriocin export permease (e.g., TC #3.A.1.42.2). The mature bacteriocins are predicted to form amphipathic helices with varying amounts of hydrophobicity. Subgroups in Class II bacteriocins include:
IIb, Poration complexes requiring two peptides for activity.
Many bacteriocins have been identified in addition to those tabulated in the TC system, but those listed are among the best characterized, with respect to evidence for channel formation in target bacterial membranes. Class III and IV bacteriocins (Klaenhammer, 1993) are large heat-labile proteins that function by mechanisms unrelated to those of the bacteriocins listed here.
The membrane-permeabilizing two-peptide bacteriocin, lactococcin G, consists of two different peptides, LcnG-α and LcnG-β. The bacteriocin contains several tryptophan and tyrosine residues and three putative helix-helix interacting GxxxG-motifs: G7xxxG11 and G18xxxG22 in LcnG-α and G18xxxG22 in LcnG-β. Possibly, the G7xxxG11-motif in LcnG-α and the G18xxxG22-motif in LcnG-β interact and allow the two peptides to form a parallel transmembrane helix−helix structure, with the tryptophan-rich N-terminal part of LcnG-β positioned in the outer membrane interface and the cationic C-terminal end of LcnG-α inside the cell (Oppegard et al., 2008).