1.C.20 The Nisin (Nisin) 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.
Peptide bacteriocins produced by lactic acid bacteria are categorized into two different classes according to their biochemical and genetic properties (Drider et al., 2006; Nes et al., 2007). Class I peptides are the lantibiotics, which are small, posttranslationally modified peptides that contain unusual amino acids such as lanthionine (1.C.20). Class II includes unmodified bacteriocins which are subdivided into three subclasses, namely, class IIa (pediocin-like bacteriocins), class IIb (two-peptide bacteriocins), and IIc (other [i.e., non-pediocin-like], one-peptide bacteriocins).
Class I lantibiotic bacteriocins are small membrane-active channel-forming peptides of less than 5 kDa. They contain the unusual amino acids lanthionine and β-methyl lanthionine, as well as dehydrated residues. One member of family 1.C.22 (TC #1.C.22.1.2) is the thiol-activated peptide, Lactococcin B, included in Class IIc by Klaenhammer (1993).
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.
Nisin apparently forms channels in bacterial membranes using Lipid II, the prenyl chain-linked donor of the peptidoglycan building block, both as a receptor and as an intrinsic component of the pore (Breukink et al., 2003). The length of the prenyl chain of Lipid II plays an important role in maintaining pore stability. The interaction with Lipid II is required for pore formation, and the pores are stable for seconds. They have a pore diameter of 2-2.5 nm (Wiedemann et al., 2004).
Lantibiotics may kill bacteria by multiple mechanisms. These polycyclic peptides, containing unusual amino acids, have binding specificity for bacterial cells, targeting the bacterial cell wall component lipid II to form pores and thereby lyse the cells. Several members of these lipid II&150;targeted lantibiotics are too short to be able to span the lipid bilayer and cannot form pores, but they maintain their antibacterial efficacy. Hasper et al. (2006) have described an alternative mechanism by which members of the lantibiotic family kill Gram-positive bacteria. This mechanism involves removing lipid II from the cell division site (or septum), thus blocking cell wall synthesis.