The continuing story of class IIa bacteriocins

Abstract

Many bacteria produce antimicrobial peptides, which are also referred to as peptide bacteriocins. The class IIa bacteriocins, often designated pediocin-like bacteriocins, constitute the most dominant group of antimicrobial peptides produced by lactic acid bacteria. The bacteriocins that belong to this class are structurally related and kill target cells by membrane permeabilization. Despite their structural similarity, class IIa bacteriocins display different target cell specificities. In the search for new antibiotic substances, the class IIa bacteriocins have been identified as promising new candidates and have thus received much attention. They kill some pathogenic bacteria (e.g., Listeria) with high efficiency, and they constitute a good model system for structure-function analyses of antimicrobial peptides in general. This review focuses on class IIa bacteriocins, especially on their structure, function, mode of action, biosynthesis, bacteriocin immunity, and current food applications. The genetics and biosynthesis of class IIa bacteriocins are well understood. The bacteriocins are ribosomally synthesized with an N-terminal leader sequence, which is cleaved off upon secretion. After externalization, the class IIa bacteriocins attach to potential target cells and, through electrostatic and hydrophobic interactions, subsequently permeabilize the cell membrane of sensitive cells. Recent observations suggest that a chiral interaction and possibly the presence of a mannose permease protein on the target cell surface are required for a bacteria to be sensitive to class IIa bacteriocins. There is also substantial evidence that the C-terminal half penetrates into the target cell membrane, and it plays an important role in determining the target cell specificity of these bacteriocins. Immunity proteins protect the bacteriocin producer from the bacteriocin it secretes. The three-dimensional structures of two class IIa immunity proteins have been determined, and it has been shown that the C-terminal halves of these cytosolic four-helix bundle proteins specify which class IIa bacteriocin they protect against.

FIG. 1.

Sequence alignment of mature class IIa bacteriocins. Underlined cysteine residues are those involved in disulfide bond formation.

FIG. 2.

Schematic presentation of the four class IIa bacteriocins for which 3D structures have been determined by NMR, i.e., leucocin A (67), carnobacteriocin B2 (189), sakacin P (176), and curvacin A (79). Due to sequence similarities among these class IIa bacteriocins, it is assumed that the 3D structures of their N-terminal beta-sheet-like structures are relatively similar despite the fact that their NMR structures display some variation in this region. It is also assumed that the C-terminal tails following the central alpha helices of leucocin A and carnobacteriocin B2 most likely form a hairpin-like structure together with the alpha helix, despite the fact that this could not be judged from the NMR structural analyses. This hairpin-like structure has been seen only for the structurally stabilized sakacin P variant (176).

FIG. 3.

Schematic presentation of the biosynthesis of class IIa bacteriocins. The ribosomally produced prebacteriocin and preinducer peptide are matured and secreted through the ABC transporter. Mature inducer peptide interacts with its receptor (the histidine kinase [HK]), which is autophosphorylated at the cytosolic side. The phosphate group is subsequently transferred to the response regulator (RR), which then becomes active, enabling it to function as a transcriptional activator for bacteriocin-related genes.

FIG. 4.

Sequence alignment and grouping of putative immunity proteins of class IIa bacteriocins. Regions of sequence similarity are indicated by black and gray boxes. The following amino acids were considered similar: D and E, F and Y, V and L, N and Q, K and R, and S and T. Group A consists of the following (putative) immunity proteins: leucocin A (leuA-im), mesentericin Y105 (mesY-im), divercin V41 (divI-im), enterocin A (entA-im), an open reading frame in the sakacin P locus with no corresponding bacteriocin (orfY-im), pediocin PA-1 (ped-im), and coagulin (coa-im). Group B consists of the following (putative) immunity proteins: an open reading frame in the carnobacteriocin locus with no corresponding bacteriocin (orfβ 3-im), an open reading frame in Lactobacillus sakei LB790 (GenBank accession no. CAF25009) with no corresponding bacteriocin (orf285-im), piscicolin 126 (pisc-im), sakacin 5x (sakX-im), sakacin P (sakP-im), mundticin KS (munKS-im), enterocin CRL35 (entCL35-im), listeriocin 743A (lisA-im), and an open reading frame in the divercin V41 locus with no corresponding bacteriocin (divT2-im). Group C consists of the following (putative) immunity proteins: carnobacteriocin B1 (cbnBM1-im), curvacin A (curA-im), enterocin P (entP-im), bacteriocin 31 (bac31-im), and carnobacteriocin B2 (cbnB2). See the text for references.

FIG. 5.

A model showing a class IIa bacteriocin on the extracellular side of the cell membrane and the cognate four-helix bundle immunity protein on the cytosolic side. The bacteriocin (here sakacin P) has an oblique orientation which is caused in part by two Trp residues (Trp18 and Trp41) located in the water-membrane interface (62). By unknown mechanisms the C-terminal part of the immunity protein recognizes the bacteriocin and protects against it.

FIG. 6.

Schematic illustration of the two steps involved in the putative mode of action of class IIa bacteriocins.