Mutational analysis of the class IIa bacteriocin curvacin A and its orientation in target cell membranes

Pediocin-Like Antimicrobial Peptides of Bacteria

Bacteriocins are bacterial antimicrobial peptides that, unlike classical peptide antibiotics, are products of ribosomal synthesis and usually have a narrow spectrum of antibacterial activity against species closely related to the producers. Pediocin-like bacteriocins (PLBs) belong to the class IIa of the bacteriocins of Gram-positive bacteria. PLBs possess high activity against pathogenic bacteria from Listeria and Enterococcus genera. Molecular target for PLBs is a membrane protein complex - bacterial mannose-phosphotransferase. PLBs can be synthesized by components of symbiotic microflora and participate in the maintenance of homeostasis in various compartments of the digestive tract and on the surface of epithelial tissues contacting the external environment. PLBs could give a rise to a new group of antibiotics of narrow spectrum of activity.

Bacteria Hunt Bacteria through an Intriguing Cyclic Peptide

In the last few decades, peptides have been victorious over small molecules as therapeutics due to their broad range of applications, high biological activity, and high specificity. However, the main challenges to overcome if peptides are to become effective drugs is their low oral bioavailability and instability under physiological conditions. Cyclic peptides play a vital role in this context because they show higher stability under physiological conditions, higher membrane permeability, and greater oral bioavailability than that of their corresponding linear analogues. In this regard, cyclic antimicrobial peptides (AMPs) have gained considerable attention in the field of novel antibiotic development. Bacterial strains produce cyclic AMPs through two pathways: ribosomal and nonribosomal. This review provides an overview of the chemical classification of cyclic AMPs isolated from bacteria, and provides a description of their biological activity and mode of action.

Bacteriocins: perspective for the development of novel anticancer drugs

Antimicrobial peptides (AMPs) from prokaryotic source also known as bacteriocins are ribosomally synthesized by bacteria belonging to different eubacterial taxonomic branches. Most of these AMPs are low molecular weight cationic membrane active peptides that disrupt membrane by forming pores in target cell membranes resulting in cell death. While these peptides known to exhibit broad-spectrum antimicrobial activity, including antibacterial and antifungal, they displayed minimal cytotoxicity to the host cells. Their antimicrobial efficacy has been demonstrated in vivo using diverse animal infection models. Therefore, we have discussed some of the promising peptides for their ability towards potential therapeutic applications. Further, some of these bacteriocins have also been reported to exhibit significant biological activity against various types of cancer cells in different experimental studies. In fact, differential cytotoxicity towards cancer cells as compared to normal cells by certain bacteriocins directs for a much focused research to utilize these compounds as novel therapeutic agents. In this review, bacteriocins that demonstrated antitumor activity against diverse cancer cell lines have been discussed emphasizing their biochemical features, selectivity against extra targets and molecular mechanisms of action.

Synthesis, antimicrobial activity and conformational analysis of the class IIa bacteriocin pediocin PA-1 and analogs thereof

The antimicrobial peptide pediocin PA-1 is a class IIa bacteriocin that inhibits several clinically relevant pathogens including Listeria spp. Here we report the synthesis and characterization of whole pediocin PA-1 and novel analogs thereof using a combination of solid- and solution-phase strategies to overcome difficulties due to instability and undesired reactions. Pediocin PA-1 thus synthesized was a potent inhibitor of Listeria monocytogenes (MIC = 6.8 nM), similar to the bacteriocin produced naturally by Pediococcus acidilactici. Of particular interest is that linear analogs lacking both of the disulfide bridges characterizing pediocin PA-1 were as potent. One linear analog was also a strong inhibitor of Clostridium perfringens, another important food-borne pathogen. These results are discussed in light of conformational information derived from circular dichroism, solution NMR spectroscopy and structure-activity relationship studies.

LsbB Bacteriocin Interacts with the Third Transmembrane Domain of the YvjB Receptor

The Zn-dependent membrane-located protease YvjB has previously been shown to serve as a target receptor for LsbB, a class II leaderless lactococcal bacteriocin. Although yvjB is highly conserved in the genus Lactococcus, the bacteriocin appears to be active only against the subspecies L. lactis subsp. lactis Comparative analysis of the YvjB proteins of a sensitive strain (YvjBMN) and a resistant strain (YvjBMG) showed that they differ from each other in 31 positions. In this study, we applied site-directed mutagenesis and performed directed binding studies to provide biochemical evidence that LsbB interacts with the third transmembrane helix of YvjB in susceptible cells. The site-directed mutagenesis of LsbB and YvjB proteins showed that certain amino acids and the length of LsbB are responsible for the bacteriocin activity, most probably through adequate interaction of these two proteins; the essential amino acids in LsbB responsible for the activity are tryptophan (Trp(25)) and terminal alanine (Ala(30)). It was also shown that the distance between Trp(25) and terminal alanine is crucial for LsbB activity. The crucial region in YvjB for the interaction with LsbB is the beginning of the third transmembrane helix, particularly amino acids tyrosine (Tyr(356)) and alanine (Ala(353)). In vitro experiments showed that LsbB could interact with both YvjBMN and YvjBMG, but the strength of interaction is significantly less with YvjBMG In vivo experiments with immunofluorescently labeled antibody demonstrated that LsbB specifically interacts only with cells carrying YvjBMN IMPORTANCE: The antimicrobial activity of LsbB bacteriocin depends on the correct interaction with the corresponding receptor in the bacterial membrane of sensitive cells. Membrane-located bacteriocin receptors have essential primary functions, such as cell wall synthesis or sugar transport, and it seems that interaction with bacteriocins is suicidal for cells. This study showed that the C-terminal part of LsbB is crucial for the bacteriocin activity, most probably through adequate interaction with the third transmembrane domain of the YvjB receptor. The conserved Tyr(356) and Ala(353) residues of YvjB are essential for the function of this Zn-dependent membrane-located protease as a bacteriocin receptor.

Bacteriocins of lactic acid bacteria: extending the family

Lactic acid bacteria (LAB) constitute a heterogeneous group of microorganisms that produce lactic acid as the major product during the fermentation process. LAB are Gram-positive bacteria with great biotechnological potential in the food industry. They can produce bacteriocins, which are proteinaceous antimicrobial molecules with a diverse genetic origin, posttranslationally modified or not, that can help the producer organism to outcompete other bacterial species. In this review, we focus on the various types of bacteriocins that can be found in LAB and the organization and regulation of the gene clusters responsible for their production and biosynthesis, and consider the food applications of the prototype bacteriocins from LAB. Furthermore, we propose a revised classification of bacteriocins that can accommodate the increasing number of classes reported over the last years.

Chemical and genetic characterization of bacteriocins: antimicrobial peptides for food safety

Antimicrobial peptides are produced across all domains of life. Among these diverse compounds, those produced by bacteria have been most successfully applied as agents of biocontrol in food and agriculture. Bacteriocins are ribosomally synthesized, proteinaceous compounds that inhibit the growth of closely related bacteria. Even within the subcategory of bacteriocins, the peptides vary significantly in terms of the gene cluster responsible for expression, and chemical and structural composition. The polycistronic gene cluster generally includes a structural gene and various combinations of immunity, secretion, and regulatory genes and modifying enzymes. Chemical variation can exist in amino acid identity, chain length, secondary and tertiary structural features, as well as specificity of active sites. This diversity posits bacteriocins as potential antimicrobial agents with a range of functions and applications. Those produced by food-grade bacteria and applied in normally occurring concentrations can be used as GRAS-status food additives. However, successful application requires thorough characterization.

Mutational analysis of positively charged residues in the N-terminal region of the class IIa bacteriocin pediocin PA-1

The significance of positively charged residues for the target cell binding of pediocin PA-1 bacteriocins was studied by site-directed mutagenesis. Most of the charged residues are located in the N-terminal half of the peptide, which is thought to mediate the initial binding of these bacteriocins to their target cells through electrostatic interactions. Mutated peptides in which the positively charged residues were substituted or increased in number were constructed, and some of these peptides exhibited a twofold increase in the bacteriostatic activity. The greatest enhancement was achieved by introduced the positive charges at position 13, their results show the benefits of introducing an additional cationic residue within this patch in the middle of the N-terminal half of pediocin PA-1 bacteriocins. Thus, the presence of additional cationic residues in the N-terminal half influenced the electrostatic binding of this bacteriocin to its target cells and increased the potency of the peptide on the potency of Micrococcus luteus and Staphylococcus aureus.

SIGNIFICANCE AND IMPACT OF THE STUDY

No previous work has systematically examined the N-terminal cationic residues of the pediocin PA-1 for their functional importance or redundancy. In this study, we examined the structure-function relationships of pediocin PA-1 by site-directed mutagenesis. Mutated peptides in which the positively charged residues were substituted and increased in number exhibited a twofold increase in the bacteriostatic activity. This study demonstrated the importance of the cationic patch in the N-terminal half of pediocin PA-1. The cationic residues influenced the electrostatic binding of the bacteriocin to the target cells and had a greater effect on the potency of the peptide towards Micrococcus luteus and Staphylococcus aureus.

Target recognition, resistance, immunity and genome mining of class II bacteriocins from Gram-positive bacteria

Due to their very potent antimicrobial activity against diverse food-spoiling bacteria and pathogens and their favourable biochemical properties, peptide bacteriocins from Gram-positive bacteria have long been considered promising for applications in food preservation or medical treatment. To take advantage of bacteriocins in different applications, it is crucial to have detailed knowledge on the molecular mechanisms by which these peptides recognize and kill target cells, how producer cells protect themselves from their own bacteriocin (self-immunity) and how target cells may develop resistance. In this review we discuss some important recent progress in these areas for the non-lantibiotic (class II) bacteriocins. We also discuss some examples of how the current wealth of genome sequences provides an invaluable source in the search for novel class II bacteriocins.

Mutational analysis of residues in the helical region of the class IIa bacteriocin pediocin PA-1

A 15-mer fragment that is derived from the helical region in the C-terminal half of pediocin PA-1 inhibited the activity of pediocin PA-1. Of 13 other pediocin-like (hybrid) bacteriocins, only the hybrid bacteriocin Sak/Ped was markedly inhibited by the 15-mer fragment. Sak/Ped was the only one of these bacteriocins that had a sequence (in the C-terminal helix-containing half) identical to that of the 15-mer fragment, indicating that the fragment inhibits pediocin-like bacteriocins in a sequence-dependent manner. By replacing (one at a time) all 15 residues in the fragment with Ala or Leu, five residues (K1, A2, T4, N8, and A15) were identified as being especially important for the inhibitory action of the fragment. The results suggest that the corresponding residues (K20, A21, T23, N27, and A34, respectively) in pediocin PA-1 might be involved in interactions between pediocin PA-1 and its receptor. To characterize the environment surrounding these five residues when pediocin PA-1 interacts with target cells, these residues were replaced (one at a time) with a hydrophobic large (Leu) residue, a hydrophilic charged (Asp or Arg) residue, and a small (Ala or Gly) residue. The results revealed that residues A21 and A34 are in a spatially constrained environment, since the replacement with a small (Gly) residue was the only substitution that did not markedly reduce the bacteriocin activity. The positive charge in K20 and the polar amide group in N27 appeared to interact with electronegative groups, since the replacement of these two residues with a positive (Arg) residue was well tolerated, while replacement with a negative (Asp) residue was detrimental to the bacteriocin activity. K20 was in a less constrained environment than N27, since the replacement of K20 with a large hydrophobic (Leu) residue was tolerated fairly well and to a greater extent than N27. T23 seemed to be in an environment that was not restricted with respect to size, polarity, and charge, since replacements with large (Leu) and small (Ala) hydrophobic residues and a hydrophilic negative (Asp) residue were tolerated fairly well (2- to 6-fold reduction in activity). Moreover, the replacement of T23 with a large positive (Arg) residue resulted in wild-type or better-than-wild-type activity.

Insights into the functionality of the putative residues involved in enterocin AS-48 maturation

AS-48 is a 70-residue, α-helical, cationic bacteriocin produced by Enterococcus faecalis and is very singular in its circular structure and its broad antibacterial spectrum. The AS-48 preprotein consists of an N-terminal signal peptide (SP) (35 residues) followed by a proprotein moiety that undergoes posttranslational modifications to yield the mature and active circular protein. For the study of the specificity of the region of AS-48 that is responsible for maturation, three single mutants have been generated by site-directed mutagenesis in the as-48A structural gene. The substitutions were made just in the residues that are thought to constitute a recognition site for the SP cleavage enzyme (His-1, Met1) and in those involved in circularization (Met1, Trp70). Each derivative was expressed in the enterococcal JH2-2 strain containing the necessary native biosynthetic machinery for enterocin production. The importance of these derivatives in AS-48 processing has been evaluated on the basis of the production and structural characterization of the corresponding derivatives. Notably, only two of them (Trp70Ala and Met1Ala derivatives) could be purified in different forms and amounts and are characterized for their bactericidal activity and secondary structure. We could not detect any production of AS-48 in JH2-2(pAM401-81(His-1Ile)) by using the conventional chromatographic techniques, despite the high efficiency of the culture conditions applied to produce this enterocin. Our results underline the different important roles of the mutated residues in (i) the elimination of the SP, (ii) the production levels and antibacterial activity of the mature proteins, and (iii) protein circularization. Moreover, our findings suggest that His-1 is critically involved in cleavage site recognition, its substitution being responsible for the blockage of processing, thereby hampering the production of the specific protein in the cellular culture supernatant.

Synthesis of trypsin-resistant variants of the Listeria-active bacteriocin salivaricin P

Two-component salivaricin P-like bacteriocins have demonstrated potential as antimicrobials capable of controlling infections in the gastrointestinal tract (GIT). The anti-Listeria activity of salivaricin P is optimal when the individual peptides Sln1 and Sln2 are added in succession at a 1:1 ratio. However, as degradation by digestive proteases may compromise the functionality of these peptides within the GIT, we investigated the potential to create salivaricin variants with enhanced resistance to the intestinal protease trypsin. A total of 11 variants of the salivaricin P components, in which conservative modifications at the trypsin-specific cleavage sites were explored in order to protect the peptides from trypsin degradation while maintaining their potent antimicrobial activity, were generated. Analysis of these variants revealed that eight were resistant to trypsin digestion while retaining antimicrobial activity. Combining the complementary trypsin-resistant variants Sln1-5 and Sln2-3 resulted in a MIC(50) of 300 nM against Listeria monocytogenes, a 3.75-fold reduction in activity compared to the level for wild-type salivaricin P. This study demonstrates the potential of engineering bacteriocin variants which are resistant to specific protease action but which retain significant antimicrobial activity.

Helix insertion into bilayers and the evolution of membrane proteins

Polytopic alpha-helical membrane proteins cannot spontaneously insert into lipid bilayers without assistance from polytopic alpha-helical membrane proteins that already reside in the membrane. This raises the question of how these proteins evolved. Our current knowledge of the insertion of alpha-helices into natural and model membranes is reviewed with the goal of gaining insight into the evolution of membrane proteins. Topics include: translocon-dependent membrane protein insertion, antibiotic peptides and proteins, in vitro insertion of membrane proteins, chaperone-mediated insertion of transmembrane helices, and C-terminal tail-anchored (TA) proteins. Analysis of the E. coli genome reveals several predicted C-terminal TA proteins that may be descendents of proteins involved in pre-cellular membrane protein insertion. Mechanisms of pre-translocon polytopic alpha-helical membrane protein insertion are discussed.