Abstract
Lactic acid bacteria produce diverse antimicrobial peptides called bacteriocins. Most bacteriocins target sensitive bacteria by binding to specific receptors. Although a plethora of bacteriocins have been identified, for only a few of them the receptors they recognize are known. Here, we identified permease IIC and surface protein IID, two membrane subunits of the mannose-specific quaternary phosphotransferase system (Man-PTS), as a receptor for BacSJ, a subclass IId bacteriocin produced by Lactobacillus paracasei subsp. paracasei BGSJ2-8. BacSJ shares 45% identity with another Man-PTS binding bacteriocin, garvicin Q (GarQ). Similarly to GarQ, BacSJ has a relatively broad activity spectrum acting against several Gram-positive bacteria, such as Lactococcus lactis and Listeria monocytogenes, harboring fairly similar Man-PTSs, but not against Lactococcus garvieae. To identify specific Man-PTS amino acids responsible for the L. lactis sensitivity to BacSJ, and thus likely involved in the interaction with this bacteriocin, we generated eight independent BacSJ resistant L. lactis mutants harboring five distinct missense mutations in the ptnC or ptnD genes encoding the IIC and IID subunits. Concurrently with the resistance to BacSJ, the mutants efficiently utilized mannose as a carbon source, which indicated functionality of their mutated Man-PTS. The amino acid substitutions in the mutants localized to the intracellular region of the IIC permease or to the extracellular parts of IID. This localization coincides with regions targeted by GarQ and some other Man-PTS-binding garvicins, pointing to similarities between all these bacteriocins in the mechanism of their interaction with Man-PTS. During the attack by these bacteriocins, subunits IID and IIC are assumed to function sequentially as a docking and an entry module allowing the toxic peptide to bind the cell and then open the pore. However, since not all of the BacSJ-resistant mutants exhibited cross-resistance to GarQ, we propose that BacSJ interacts with Man-PTS in a manner slightly different from that of GarQ.
Highlights
Synthesized antimicrobial peptides or proteins, categorized as bacteriocins, provide the bacterial strains producing them with an advantage over susceptible bacterial strains competing for nutrients or habitats in the respective ecological niches [1]
BacSJ is synthesized as a 68-amino acid prebacteriocin with the GG-motif and after processing gives a mature form of 50 amino acid (Figure 1)
It shares similarity with 86 peptides encoded on plasmids or chromosomes of several species of the families Enterococcaceae, Lactobacillaceae, Leuconostocaceae, and Streptococcaceae, with the majority of representatives among Lactobacillus and Streptococcus spp. (Figure S1). Most of these peptides are annotated as hypothetical proteins or putative pheromones/bacteriocins; only two
Summary
Synthesized antimicrobial peptides or proteins, categorized as bacteriocins, provide the bacterial strains producing them with an advantage over susceptible bacterial strains competing for nutrients or habitats in the respective ecological niches [1]. Bacteriocins can kill or inhibit the growth of bacterial strains from closely related species (narrow-spectrum bacteriocins) or from distant genera (broad-spectrum bacteriocins). They have no activity against the producers’ cells which are protected by specific immunity proteins [1]. Bacteriocin-producing probiotic strains may be used with antibiotic therapy to promote a desirable intestinal microbiota and help eliminate pathogenic bacterial strains [5]. Some bacteriocins have been shown to exhibit significantly higher apoptotic and necrotic activity against cancer cells, in comparison to normal cells, which makes them promising novel anticancer therapeutic agents [6]
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