Design of novel histone-derived antimicrobial peptides

From inside to outside: exploring extracellular antimicrobial histone-derived peptides as multi-talented molecules

The emergence of bacterial resistance to antibiotics poses a global health threat, necessitating innovative solutions. The contemporary challenge lies in bacterial resistance, impacting morbidity, mortality, and global economies. Antimicrobial peptides (AMPs) offer a promising avenue for addressing antibiotic resistance. The Antimicrobial Peptide Database catalogs 3569 peptides from various organisms, representing a rich resource for drug development. Histones, traditionally recognized for their role in nucleosome structures, have gained attention for their extracellular functions, including antimicrobial and immunomodulatory properties. This review aims to thoroughly investigate antimicrobial peptides derived from histones in various organisms, elucidating their mechanisms. In addition, it gives us clues about how extracellular histones might be used in drug delivery systems to fight bacterial infections. This comprehensive analysis emphasizes the importance of histone-derived peptides in developing innovative therapeutic strategies for evolving bacterial challenges.

Histone derived antimicrobial peptides identified from Mytilus coruscus serum by peptidomics

Histones and their N-terminal or C-terminal derived peptides have been studied in vertebrates and presented as potential antimicrobial agents playing important roles in the innate immune defenses. Although histones and their derived peptides had been reported as components of innate immunity in invertebrates, the knowledge about the histone derived antimicrobial peptides (HDAPs) in invertebrates are still limited. Using a peptidomic technique, a set of peptide fragments derived from the histones was identified in this study from the serum of microbes challenged Mytilus coruscus. Among the 85 identified histone-derived-peptides with high confidence, 5 HDAPs were chemically synthesized and the antimicrobial activities were verified, showing strong growth inhibition against Gram-positive bacteria, Gram-negative bacteria, and fungus. The gene expression level of the precursor histones matched by representative HDAPs were further tested using q-PCR, and the results showed a significant upregulation of the histone gene expression levels in hemocytes, gill, and mantle of the mussel after immune stress. In addition, three identified HDAPs were selected for preparation of specific antibodies, and the corresponding histones and their derived C-terminal fragments were detected by Western blotting in the blood cell and serum of immune challenged mussel, respectively, indicating the existence of HDAPs in M. coruscus. Our findings revealed the immune function of histones in Mytilus, and confirmed the existence of HDAPs in the mussel. The identified Mytilus HDAPs represent a new source of immune effector with antimicrobial function in the innate immune system, and thus provide promising candidates for the treatment of microbial infections in aquaculture and medicine.

Dual Roles of Extracellular Histone H3 in Host Defense: Its Differential Regions Responsible for Antimicrobial and Cytotoxic Properties and Their Modes of Action

Extracellular histones play a dual role—antimicrobial and cytotoxic—in host defense. In this study, we evaluated the antimicrobial and cytotoxic activities of histone H3 and identified the responsible molecular regions for these properties. Broth microdilution assays indicated that histone H3 exhibits growth inhibitory activity against not only Gram-negative and -positive bacteria but also fungi. Observations under scanning electron microscopy (SEM) revealed that histone H3 induced morphological abnormalities on the cell surface of a wide range of reference pathogens. MTT assays and SEM observations indicated that histone H3 has strong cytotoxic and cell lytic effects on mammalian normal, immortal, and tumor cell lines. Assays using synthetic peptides corresponding to fragments 1–34 (H3DP1), 35–68 (H3DP2), 69–102 (H3DP3), and 103–135 (H3DP4) of histone H3 molecule demonstrated that its antimicrobial activity and cytotoxicity are elicited by the H3DP2 and H3DP3 protein regions, respectively. Enzyme-linked endotoxin binding assays indicated that histones H3 and H3DP1, H3DP2, and H3DP4, but not H3DP3, exhibited high affinities toward lipopolysaccharide and lipoteichoic acid. Our findings are expected to contribute to the development of new histone H3-based peptide antibiotics that are not cytotoxic.

A histone H2A-derived antimicrobial peptide, Hipposin from mangrove whip ray, Himantura walga: Molecular and functional characterisation

Antimicrobial peptides (AMPs) are biologically dynamic molecules produced by all type of organisms as a fundamental component of their innate immune system. The present study deals with the identification of a histone H2A-derived antimicrobial peptide, Hipposin from mangrove whip ray, Himantura walga. A 243 base pair fragment encoding 81 amino acid residues amplified from complementary DNA was identified as Hipposin and termed as Hw-Hip. Homologous analysis showed that Hw-Hip belongs to the Histone H2A superfamily and shares sequence identity with other histone-derived AMPs from fishes. Phylogenetic analysis of Hw-Hip displayed clustering with the fish H2A histones. Secondary structure analysis showed the presence of three α-helices and four random coils with a prominent proline hinge. The physicochemical properties of Hw-Hip are in agreement with the properties of antimicrobial peptides. A 39-mer active peptide sequence was released by proteolytic cleavage in silico. Functional characterisation of active peptide in silico revealed antibacterial, anticancer and antibiofilm activities making Hw-Hip a promising candidate for further exploration.

Antibacterial activity and mechanism of the cell-penetrating peptide CF-14 on the gram-negative bacteria, Escherichia coli

In the present study, we characterized CF-14, a novel antimicrobial peptide derived from the catfish skin mucus. The objective of this study was to explore the antimicrobial mechanism of CF-14 against Escherichia coli. The agar-diffusion assay and the microdilution method were used to evaluate the antimicrobial activity and the minimum inhibitory concentration (MIC) of CF-14 against E. coli, respectively. In addition, the absorbance of the bacterial suspension filtrate at 260 nm was measured to quantify the leakage of bacterial cytoplasmic components. The bacterial morphological changes were observed by scanning electron microscopy, while confocal microscopy was used to investigate the localization site of CF-14 in E.coli. The DNA binding ability of CF-14 was evaluated using gel retardation assay and the binding of CF-14 to DnaK was evaluated using Discovery Studio. The results demonstrated that CF-14 exhibited strong antimicrobial activity against E.coli with an MIC of 31.3 μg/mL. Unlike common cationic anti-microbial peptides (AMPs) that target the cellmembrane, CF-14 penetrated the E.coli cell membrane and induced only minormembrane perturbations. Furthermore, the antimicrobial mechanism of CF-14 against E.coli involved DNA binding and competitive inhibition of bacterial DnaK. Finally, by deleting or replacing the amino acid sequence, the antibacterial activity of CF-14 was affected, which helped the optimization of amino acid sequence. Therefore, CF-14 can be a potential antimicrobial peptide.

Using fluorescence microscopy to shed light on the mechanisms of antimicrobial peptides

Antimicrobial peptides (AMPs) are promising in the fight against increasing bacterial resistance, but the development of AMPs with enhanced activity requires a thorough understanding of their mechanisms of action. Fluorescence microscopy is one of the most flexible and effective tools to characterize AMPs, particularly in its ability to measure the membrane interactions and cellular localization of peptides. Recent advances have increased the scope of research questions that can be addressed via microscopy through improving spatial and temporal resolution. Unique combinations of fluorescent labels and dyes can simultaneously consider different aspects of peptide-membrane interaction mechanisms. This review emphasizes the central role that fluorescence microscopy will continue to play in the interrogation of AMP structure-function relationships and the engineering of more potent peptides.

Synthesis and biological activity of Ub2 derived peptides as potential host-directed antitubercular therapy

The correlation of mycobactericidal property of macrophages with its potential to deliver bacteria to hydrolytic lysosomes, augmented with ubiquitin-derived peptides (Ub2), activates the process of autophagy. This leads to the formation of phagolysosomes supported by factor involving increased cationic charges which regulate the acidic pH causing elimination of Mycobacterium. To better understand this interaction of cationic-rich ubiquitin-derived peptides with mycobacteria and to identify putative mycobacterial intrinsic resistance mechanisms for phagolysosome formation, we have synthesized a new series of Ub2 peptides, wherein the Gly residues are replaced with azaGly with the aim to improve metabolic stability. In addition to that a new methodology is reported for the synthesis of heteroaryl tethered peptides using azaGly as a linker. We have demonstrated that positive puncta (directly proportional to the acidification of lysosome) in cytosol was significantly increased after 6 hours on the treatment of macrophage with Ub2 peptide derivatives (1, 6, 10, and 11) causing the higher intensity of lysosome observed through LysoTracker Red Dye. The circular dichroism spectral studies are carried out in water and water:TFE mixture and demonstrated that the Ub2 peptides have helix-forming tendency in the presence of TFE. The recognizable intracellular killing of Mycobacterium tuberculosis by Ub2 peptides provides a new approach for host-directed therapy.

Histone H5 is a potent Antimicrobial Agent and a template for novel Antimicrobial Peptides

Modern medicine is challenged continuously by the increasing prevalence of antibiotic resistant bacteria. Cationic antimicrobial peptides and their derivatives are interesting potential alternatives to antibiotics due to their rapid action, broad-spectrum of antimicrobial activity and limited emergence of bacterial resistance. This study reports the novel antimicrobial properties of histone H5, purified from chicken erythrocytes, and histone H5-derived synthetic peptides. Broth microdilution assays revealed that histone H5 has potent broad-spectrum antimicrobial activity against Gram-positive and Gram-negative planktonic bacteria (MIC range: 1.9 ± 1.8 to 4.9 ± 1.5 µg/mL), including vancomycin-resistant Enterococcus (VRE) and methicillin-resistant Staphylococcus aureus (MRSA). Moreover, histone H5 displayed anti-biofilm activity against established Listeria monocytogenes and Pseudomonas aeruginosa biofilms. Scanning electron microscopy demonstrated bacterial membrane damage after histone H5 treatment, while a hemolytic assay revealed that histone H5 is non-toxic towards mammalian erythrocytes, even at a concentration of 1 mg/mL. Although the predicted H5-derived antimicrobial peptides tested in this study were located within the antimicrobial domain of histone H5, their synthetic versions did not possess more potent antimicrobial activity than the full length protein. Overall, this study demonstrates that histone H5 is a potent antimicrobial and therefore a promising template for the development of novel histone H5-derived antimicrobial peptides.

Investigating the nucleic acid interactions of histone-derived antimicrobial peptides

While many antimicrobial peptides (AMPs) disrupt bacterial membranes, some translocate into bacteria and interfere with intracellular processes. Buforin II and DesHDAP1 are thought to kill bacteria by interacting with nucleic acids. Here, molecular modeling and experimental measurements are used to show that neither nucleic acid binding peptide selectively binds DNA sequences. Simulations and experiments also show that changing lysines to arginines enhances DNA binding, suggesting that including additional guanidinium groups is a potential strategy to engineer more potent AMPs. Moreover, the lack of binding specificity may make it more difficult for bacteria to evolve resistance to these and other similar AMPs.

Role of arginine and lysine in the antimicrobial mechanism of histone-derived antimicrobial peptides

Translocation of cell-penetrating peptides is often promoted by increased content of arginine or other guanidinium groups. However, relatively little research has considered the role of these functional groups on antimicrobial peptide activity. This study compared the activity of three histone-derived antimicrobial peptides-buforin II, DesHDAP1, and parasin-with variants that contain only lysine or arginine cationic residues. These peptides operate via different mechanisms as parasin causes membrane permeabilization while buforin II and DesHDAP1 translocate into bacteria. For all peptides, antibacterial activity increased with increased arginine content. Higher arginine content increased permeabilization for parasin while it improved translocation for buforin II and DesHDAP1. These observations provide insight into the relative importance of arginine and lysine in these antimicrobial peptides.

EST-based in silico identification and in vitro test of antimicrobial peptides in Brassica napus

BACKGROUND

Brassica napus is the third leading source of vegetable oil in the world after soybean and oil palm. The accumulation of gene sequences, especially expressed sequence tags (ESTs) from plant cDNA libraries, has provided a rich resource for genes discovery including potential antimicrobial peptides (AMPs). In this study, we used ESTs including those generated from B. napus cDNA libraries of seeds, pathogen-challenged leaves and deposited in the public databases, as a model, to perform in silico identification and consequently in vitro confirmation of putative AMP activities through a highly efficient system of recombinant AMP prokaryotic expression.

RESULTS

In total, 35,788 were generated from cDNA libraries of pathogen-challenged leaves and 187,272 ESTs from seeds of B. napus, and the 644,998 ESTs of B. napus were downloaded from the EST database of PlantGDB. They formed 201,200 unigenes. First, all the known AMPs from the AMP databank (APD2 database) were individually queried against all the unigenes using the BLASTX program. A total of 972 unigenes that matched the 27 known AMP sequences in APD2 database were extracted and annotated using Blast2GO program. Among these unigenes, 237 unigenes from B. napus pathogen-challenged leaves had the highest ratio (1.15 %) in this unigene dataset, which is 13 times that of the unigene datasets of B. napus seeds (0.09 %) and 2.3 times that of the public EST dataset. About 87 % of each EST library was lipid-transfer protein (LTP) (32 % of total unigenes), defensin, histone, endochitinase, and gibberellin-regulated proteins. The most abundant unigenes in the leaf library were endochitinase and defensin, and LTP and histone in the pub EST library. After masking of the repeat sequence, 606 peptides that were orthologous matched to different AMP families were found. The phylogeny and conserved structural motifs of seven AMPs families were also analysed. To investigate the antimicrobial activities of the predicted peptides, 31 potential AMP genes belonging to different AMP families were selected to test their antimicrobial activities after bioinformatics identification. The AMP genes were all optimized according to Escherichia coli codon usage and synthetized through one-step polymerase chain reaction method. The results showed that 28 recombinant AMPs displayed expected antimicrobial activities against E. coli and Micrococcus luteus and Sclerotinia sclerotiorum strains.

CONCLUSION

The study not only significantly expanded the number of known/predicted peptides, but also contributed to long-term plant genetic improvement for increased resistance to diverse pathogens of B.napus. These results proved that the high-throughput method developed that combined an in silico procedure with a recombinant AMP prokaryotic expression system is considerably efficient for identification of new AMPs from genome or EST sequence databases.

Arginine-rich histones have strong antiviral activity for influenza A viruses

While histones are best known for DNA binding and transcription-regulating properties, they also have antimicrobial activity against a broad range of potentially pathogenic organisms. Histones are abundant in neutrophil extracellular traps, where they play an important role in NET-mediated antimicrobial killing. Here, we show anti-influenza activity of histones against both seasonal H3N2 and H1N1, but not pandemic H1N1. The arginine rich histones, H3 and H4, had greater neutralizing and viral aggregating activity than the lysine rich histones, H2A and H2B. Of all core histones, histone H4 is most potent in neutralizing IAV, and incubation with IAV with histone H4 results in a decrease in uptake and viral replication by epithelial cells when measured by qRT-PCR. The antiviral activity of histone H4 is mediated principally by direct effects on viral particles. Histone H4 binds to IAV as assessed by ELISA and co-sedimentation of H4 with IAV. H4 also induces aggregation, as assessed by confocal microscopy and light transmission assays. Despite strong antiviral activity against the seasonal IAV strains, H4 was inactive against pandemic H1N1. These findings indicate a possible role for histones in the innate immune response against IAV.

Modular analysis of hipposin, a histone-derived antimicrobial peptide consisting of membrane translocating and membrane permeabilizing fragments

Antimicrobial peptides continue to garner attention as potential alternatives to conventional antibiotics. Hipposin is a histone-derived antimicrobial peptide (HDAP) previously isolated from Atlantic halibut. Though potent against bacteria, its antibacterial mechanism had not been characterized. The mechanism of this peptide is particularly interesting to consider since the full hipposin sequence contains the sequences of parasin and buforin II (BF2), two other known antimicrobial peptides that act via different antibacterial mechanisms. While parasin kills bacteria by inducing membrane permeabilization, buforin II enters cells without causing significant membrane disruption, harming bacteria through interactions with intracellular nucleic acids. In this study, we used a modular approach to characterize hipposin and determine the role of the parasin and buforin II fragments in the overall hipposin mechanism. Our results show that hipposin kills bacteria by inducing membrane permeabilization, and this membrane permeabilization is promoted by the presence of the N-terminal domain. Portions of hipposin lacking the N-terminal sequence do not cause membrane permeabilization and function more similarly to buforin II. We also determined that the C-terminal portion of hipposin, HipC, is a cell-penetrating peptide that readily enters bacterial cells but has no measurable antimicrobial activity. HipC is the first membrane active histone fragment identified that does not kill bacterial or eukaryotic cells. Together, these results characterize hipposin and provide a useful starting point for considering the activity of chimeric peptides made by combining peptides with different antimicrobial mechanisms. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.

Differential mode of antimicrobial actions of arginine-rich and lysine-rich histones against Gram-positive Staphylococcus aureus

We previously reported the activities and modes of action of arginine (Arg)-rich histones H3 and H4 against Gram-negative bacteria. In the present study, we investigated the properties of the Arg-rich histones against Gram-positive bacteria in comparison with those of lysine (Lys)-rich histone H2B. In a standard microdilution assay, calf thymus histones H2B, H3, and H4 showed growth inhibitory activity against Staphylococcus aureus with minimum effective concentration values of 4.0, 4.0, and 5.6 μM, respectively. Laser confocal microscopic analyses revealed that both the Arg-rich and Lys-rich histones associated with the surface of S. aureus. However, while the morphology of S. aureus treated with histone H2B appeared intact, those treated with the histones H3 and H4 closely resembled each other, and the cells were blurred. Electrophoretic mobility shift assay results revealed these histones have binding affinity to lipoteichoic acid (LTA), one of major cell surface components of Gram-positive bacteria. Scanning electron microscopic analyses demonstrated that while histone H2B elicited no obvious changes in cell morphology, histones H3 and H4 disrupted the cell membrane structure with bleb formation in a manner similar to general antimicrobial peptides. Consequently, our results suggest that bacterial cell surface LTA initially attracts both the Arg- and Lys-rich histones, but the modes of antimicrobial action of these histones are different; the former involves cell membrane disruption and the latter involves the cell integrity disruption.

Structural and functional characterization of mycobactericidal ubiquitin-derived peptides in model and bacterial membranes

The mycobactericidal properties of macrophages include the delivery of bacteria to a hydrolytic lysosome enriched in bactericidal ubiquitin-derived peptides (Ub-peptides). To improve our understanding of interactions of ubiquitin-derived peptides with mycobacteria, we further characterized the structure and function of bactericidal Ub-peptide Ub2. We found that Ub2 adopts a β-sheet conformation in the context of sodium dodecyl sulfate micelles and phospholipid (1:1 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine) vesicles that was dependent upon the primary sequence of the peptide. Point mutations in Ub2 that reduced the net charge of the peptide decreased Ub2 bactericidal activity. We investigated Ub-peptide function in the context of model membranes and intact bacteria. Differential scanning calorimetry analysis demonstrated that Ub2 inserts into and perturbs model phospholipid vesicles. In addition, we demonstrate that Ub2 disrupts the integrity of the mycobacterial membrane, equilibrates the transmembrane potential, and is localized within both the mycobacterial membrane and cytoplasm of treated bacteria. Finally, we identified additional bactericidal Ub-peptides and characterized their activity and structure. This study provides new insight into the mycobactericidal mechanisms of Ub-peptides.

Selectivity for and destruction of Salmonella typhimurium via a membrane damage mechanism of a cell-penetrating peptide ppTG20 analogue

P7, an analogue of the cell-penetrating peptide (CPP) ppTG20, was derived by replacing Phe and Trp with Arg based on the structure-activity relationships of CPPs and antimicrobial peptides (AMPs). P7 showed antimicrobial activity against Salmonella typhimurium at 4 μM and possessed broad antimicrobial activity greater than its parent peptide. P7 displayed good selectivity, with low haemolysis below its minimum inhibitory concentration range, but displayed cytotoxic activity against the HT29 and MDA-MB231 mammalian cell lines. Studies of calcein leakage from egg yolk L-α-phosphatidylcholine/egg yolk L-α-phosphatidyl-DL-glycerol (EYPC/EYPG) (bacterial membrane mimic) and EYPC/cholesterol (eukaryotic membrane mimic) vesicles also demonstrated that P7 exhibited high selectivity and caused pore formation in the bacterial membrane. Circular dichroism experiments suggested that the conformation of P7 underwent transitions from a random coil in sodium phosphate buffer to an α-helical conformation in bacterial model membranes. P7 induced influx of the membrane fluorescent probe 1-N-phenylnaphthylamine (NPN) and the nucleic acid fluorescent probe SYTOX(®) Green by increasing live S. typhimurium cell outer membrane and plasma membrane permeability, respectively. P7 also induced ion channel formation in the cell plasma membrane causing leakage of potassium ions. Flow cytometric analysis demonstrated that S. typhimurium cell membrane integrity was destroyed following incubation with P7. These results indicated that P7 exhibited good bacterial selectivity and exerted its antibacterial activity by a membrane damage mechanism. Furthermore, these results suggested that CPPs may represent a source of templates for AMP design.

Novel histone-derived antimicrobial peptides use different antimicrobial mechanisms

The increase in multidrug resistant bacteria has sparked an interest in the development of novel antibiotics. Antimicrobial peptides that operate by crossing the cell membrane may also have the potential to deliver drugs to intracellular targets. Buforin 2 (BF2) is an antimicrobial peptide that shares sequence identity with a fragment of histone subunit H2A and whose bactericidal mechanism depends on membrane translocation and DNA binding. Previously, novel histone-derived antimicrobial peptides (HDAPs) were designed based on properties of BF2, and DesHDAP1 and DesHDAP3 showed significant antibacterial activity. In this study, their DNA binding, permeabilization, and translocation abilities were assessed independently and compared to antibacterial activity to determine whether they share a mechanism with BF2. To investigate the importance of proline in determining the peptides' mechanisms of action, proline to alanine mutants of the novel peptides were generated. DesHDAP1, which shows significant similarities to BF2 in terms of secondary structure, translocates effectively across lipid vesicle and bacterial membranes, while the DesHDAP1 proline mutant shows reduced translocation abilities and antimicrobial potency. In contrast, both DesHDAP3 and its proline mutant translocate poorly, though the DesHDAP3 proline mutant is more potent. Our findings suggest that a proline hinge can promote membrane translocation in some peptides, but that the extent of its effect on permeabilization depends on the peptide's amphipathic properties. Our results also highlight the different antimicrobial mechanisms exhibited by histone-derived peptides and suggest that histones may serve as a source of novel antimicrobial peptides with varied properties.

Antimicrobial properties of arginine- and lysine-rich histones and involvement of bacterial outer membrane protease T in their differential mode of actions

There is growing evidence of the antimicrobial properties of histones and histone-derived peptides; however, most of them are specific to lysine (Lys)-rich histones (H1, H2A, and H2B). In the present study, we focused on arginine (Arg)-rich histones (H3 and H4) and investigated their antimicrobial properties in comparison with those of histone H2B. In a standard microdilution assay, calf thymus histones H2B, H3, and H4 showed growth inhibitory activity against the bacterial outer membrane protease T (OmpT) gene-expressing Escherichia coli strain JCM5491 with calculated 50% growth inhibitory concentrations of 3.8, 10, and 12.7 μM, respectively. A lysate prepared from the JCM5491 cells was capable of strongly, moderately, and slightly fragmenting histones H2B, H3, and H4, respectively. While the lysate prepared from the cells of the ompT-deleted E. coli strain BL21(DE3) did not digest these histones, the ompT-transformed BL21(DE3), termed BL21/OmpT(+), cell lysate digested the histones more strongly than the JCM5491 cell lysate. Laser confocal and scanning electron microscopic analyses demonstrated that while histone H2B penetrated the cell membrane of JCM5491 or BL21/OmpT(+) cells, histones H3 and H4 remained on the cell surface and subsequently disrupted the cell membrane structure with bleb formation in a manner similar to general antimicrobial peptides. The BL21(DE3) cells treated with each histone showed no bleb formation, but cell integrity was affected and the cell surface was corrugated. Consequently, it is suggested that OmpT is involved in the antimicrobial properties of Arg- and Lys-rich histones and that the modes of antimicrobial action of these histones are different.

Effect of proline position on the antimicrobial mechanism of buforin II

Buforin II (BF2) is a histone-derived antimicrobial peptide that causes cell death by translocating across membranes and interacting with nucleic acids. It contains one proline residue critical for its function. Previous research found that mutations replacing proline lead to decreased membrane translocation and antimicrobial activity as well as increased membrane permeabilization. This study further investigates the role of proline in BF2's antimicrobial mechanism by considering the effect of changing proline position on membrane translocation, membrane permeabilization, and antimicrobial activity. For this purpose, four mutants were made with proline substitution (P11A) or relocation (P11A/G7P, P11A/V12P, P11A/V15P). These mutations altered the amount of helical content. Although antimicrobial activity correlated with the α-helical content for the peptides containing proline, membrane translocation did not. This observation suggests that factors in BF2's bactericidal mechanism other than translocation must be altered by these mutations. To better explain these trends we also measured the nucleic acid binding and membrane permeabilization of the mutant peptides. A comparison of mutant and wild type BF2 activity revealed that BF2 relies principally on membrane translocation and nucleic acid binding for antimicrobial activity, although membrane permeabilization may play a secondary role for some BF2 variants. A better understanding of the role of proline in the BF2 antimicrobial mechanism will contribute to the further design and development of BF2 analogs. Moreover, since proline residues are prevalent among other antimicrobial peptides, this systematic characterization of BF2 provides general insights that can promote our understanding of other systems.

Conventional and unconventional antimicrobials from fish, marine invertebrates and micro-algae

All eukaryotic organisms, single-celled or multi-cellular, produce a diverse array of natural anti-infective agents that, in addition to conventional antimicrobial peptides, also include proteins and other molecules often not regarded as part of the innate defences. Examples range from histones, fatty acids, and other structural components of cells to pigments and regulatory proteins. These probably represent very ancient defence factors that have been re-used in new ways during evolution. This review discusses the nature, biological role in host protection and potential biotechnological uses of some of these compounds, focusing on those from fish, marine invertebrates and marine micro-algae.