Membrane lysis by the antibacterial peptides cecropins B1 and B3: A spin-label electron spin resonance study on phospholipid bilayers

A Review of Bioactive Peptides: Chemical Modification, Structural Characterization and Therapeutic Applications

In recent years, the development and applications of protein drugs have attracted extensive attention from researchers. However, the shortcomings of protein drugs also limit their further development. Therefore, bioactive peptides isolated or simulated from protein polymers have broad application prospects in food, medicine, biotechnology, and other industries. Such peptides have a molecular weight distribution between 180 and 1000 Da. As a small molecule substance, bioactive peptide is usually degraded by various enzymes in the organism and have a short half-life. At the same time, such substances have poor stability and are difficult to produce and store. Therefore, these active peptides may be modified through phosphorylation, glycosylation, and acylation. Compared with other protein drugs, the modified active peptides are more easily absorbed by the body, have longer half-life, stronger targeting, and fewer side effects in addition to higher bioavailability. In the light of their functions, bioactive peptide can be divided into antimicrobial, anti-tumour, anti-angiogenic, antioxidant, anti-fatigue, and anti-hypertensive peptides. This article mainly focuses on the introduction of several promising biologically active peptides functioning as antimicrobial, anti-tumour, antiangiogenic, and antioxidant peptides from the three aspects modification, structural characteristics and mechanism of action.

Conservation and divergence in NaChBac and NaV1.7 pharmacology reveals novel drug interaction mechanisms

Voltage-gated Na⁺ (Na_V) channels regulate homeostasis in bacteria and control membrane electrical excitability in mammals. Compared to their mammalian counterparts, bacterial Na_V channels possess a simpler, fourfold symmetric structure and have facilitated studies of the structural basis of channel gating. However, the pharmacology of bacterial Na_V remains largely unexplored. Here we systematically screened 39 Na_V modulators on a bacterial channel (NaChBac) and characterized a selection of compounds on NaChBac and a mammalian channel (human Na_V1.7). We found that while many compounds interact with both channels, they exhibit distinct functional effects. For example, the local anesthetics ambroxol and lidocaine block both Na_V1.7 and NaChBac but affect activation and inactivation of the two channels to different extents. The voltage-sensing domain targeting toxin BDS-I increases Na_V1.7 but decreases NaChBac peak currents. The pore binding toxins aconitine and veratridine block peak currents of Na_V1.7 and shift activation (aconitine) and inactivation (veratridine) respectively. In NaChBac, they block the peak current by binding to the pore residue F224. Nonetheless, aconitine has no effect on activation or inactivation, while veratridine only modulates activation of NaChBac. The conservation and divergence in the pharmacology of bacterial and mammalian Na_V channels provide insights into the molecular basis of channel gating and will facilitate organism-specific drug discovery.

Synthetic macromolecules as therapeutics that overcome resistance in cancer and microbial infection

Synthetic macromolecular antimicrobials have shown efficacy in the treatment of multidrug resistant (MDR) pathogens. These synthetic macromolecules, inspired by Nature's antimicrobial peptides (AMPs), mitigate resistance by disrupting microbial cell membrane or targeting multiple intracellular proteins or genes. Unlike AMPs, these polymers are less prone to degradation by proteases and are easier to synthesize on a large scale. Recently, various studies have revealed that cancer cell membrane, like that of microbes, is negatively charged, and AMPs can be used as anticancer agents. Nevertheless, efforts in developing polymers as anticancer agents has remained limited. This review highlights the recent advancement in the development of synthetic biodegradable antimicrobial polymers (e.g. polycarbonates, polyesters and polypeptides) and anticancer macromolecules including peptides and polymers. Additionally, strategies to improve their in vivo bioavailability and selectivity towards bacteria and cancer cells are examined. Lastly, future perspectives, including use of artificial intelligence or machine learning, in the development of antimicrobial and anticancer macromolecules are discussed.

ACPred: A Computational Tool for the Prediction and Analysis of Anticancer Peptides

Anticancer peptides (ACPs) have emerged as a new class of therapeutic agent for cancer treatment due to their lower toxicity as well as greater efficacy, selectivity and specificity when compared to conventional small molecule drugs. However, the experimental identification of ACPs still remains a time-consuming and expensive endeavor. Therefore, it is desirable to develop and improve upon existing computational models for predicting and characterizing ACPs. In this study, we present a bioinformatics tool called the ACPred, which is an interpretable tool for the prediction and characterization of the anticancer activities of peptides. ACPred was developed by utilizing powerful machine learning models (support vector machine and random forest) and various classes of peptide features. It was observed by a jackknife cross-validation test that ACPred can achieve an overall accuracy of 95.61% in identifying ACPs. In addition, analysis revealed the following distinguishing characteristics that ACPs possess: (i) hydrophobic residue enhances the cationic properties of α-helical ACPs resulting in better cell penetration; (ii) the amphipathic nature of the α-helical structure plays a crucial role in its mechanism of cytotoxicity; and (iii) the formation of disulfide bridges on β-sheets is vital for structural maintenance which correlates with its ability to kill cancer cells. Finally, for the convenience of experimental scientists, the ACPred web server was established and made freely available online.

Cooperative Modes of Action of Antimicrobial Peptides Characterized with Atomistic Simulations: A Study on Cecropin B

Antimicrobial peptides (AMPs) are widely occurring host defense agents of interest as one route for addressing the growing problem of multidrug-resistant pathogens. Understanding the mechanisms behind their antipathogen activity is instrumental in designing new AMPs. Herein, we present an all-atom molecular dynamics and free energy study on cecropin B (CB) and its constituent domains. We find a cooperative mechanism in which CB inserts into an anionic model membrane with its amphipathic N-terminal segment, supported by the hydrophobic C-terminal segment of a second peptide. The two peptides interact via a Glu···Lys salt bridge and together sustain a pore in the membrane. Using a modified membrane composition, we demonstrate that when the lower leaflet is overall neutral, insertion of the cationic segment is retarded and thus this mode of action is membrane specific. The observed mode of action utilizes a flexible hinge, a common structural motif among AMPs, which allows CB to insert into the membrane using either or both termini. Data from both unbiased trajectories and enhanced sampling simulations indicate that a requirement for CB to be an effective AMP is the interaction of its hydrophobic C-terminal segment with the membrane. Simulations of these segments in isolation reveal their aggregation in the membrane and a different mechanism of supporting pore formation. Together, our results show the complex interaction of different structural motifs of AMPs and, in particular, a potential role for electronegative side chains in an overall cationic AMP.

Novel peptide motifs from lysozyme suppress pro-inflammatory cytokines in macrophages by antagonizing toll-like receptor and LPS-scavenging action

Lysozyme is commonly found in spots where bacterial infections are most likely to enter the body. Earlier we found that lysozyme possesses five antimicrobial peptide motifs in its N-terminal region which can be generated by newborn pepsin. In this study, we explore the role of these peptides in the anti-inflammatory activity of lysozyme. The five peptides, helix1 (H1), helix2 (H2), H1 and H2 connected with a loop (HLH), H2 extended with either 2 β-strands (H2-S12) or 3 β-strands (H2-S13), were synthesized and examined for anti-inflammatory action. The five peptides dose-dependently decreased, to different degrees, expression of pro-inflammatory cytokines, TNF-α, IL-6 and IL-1β, in lipopolysaccharide (LPS)- or interferon-gamma (INF-γ)-stimulated mouse macrophage cells (RAW264.7). The HLH peptide and its individual helices (H1 and H2) were markedly the most potent anti-inflammatory. When macrophage cells were stimulated with live bacteria (E. coli), H1 peptide was the most powerful suppressor of TNF-α and IL-6 expression, providing evidence that the peptide is able to antagonize the pathogen-induced inflammatory response. Receptor binding assay and docking simulation provided evidence that H1 peptide bind specifically to the pocket for endotoxin binding of the toll-like receptor 4 (TLR-4) of macrophage. The results demonstrate, for the first time, the molecular basis of anti-inflammatory action of lysozyme that N-terminal helical peptides are the main contributors. This exciting finding offers new classes of therapeutic peptides with potential in the treatment of infection-induced inflammatory diseases.

Selective Membrane Disruption Mechanism of an Antibacterial γ-AApeptide Defined by EPR Spectroscopy

γ-AApeptides are a new class of antibacterial peptidomimetics that are not prone to antibiotic resistance and are highly resistant to protease degradation. It is not clear how γ-AApeptides interact with bacterial membranes and alter lipid assembly, but such information is essential to understanding their antimicrobial activities and guiding future design of more potent and specific antimicrobial agents. Using electron paramagnetic resonance techniques, we characterized the membrane interaction and destabilizing mechanism of a lipo-cyclic-γ-AApeptide (AA1), which has broad-spectrum antibacterial activities. The analyses revealed that AA1 binding increases the membrane permeability of POPC/POPG liposomes, which mimic negatively charged bacterial membranes. AA1 binding also inhibits membrane fluidity and reduces solvent accessibility around the lipid headgroup region. Moreover, AA1 interacts strongly with POPC/POPG liposomes, inducing significant lipid lateral-ordering and membrane thinning. In contrast, minimal membrane property changes were observed upon AA1 binding for liposomes mimicking mammalian cell membranes, which consist of neutral lipids and cholesterol. Our findings suggest that AA1 interacts and disrupts bacterial membranes through a carpet-like mechanism. The results showed that the intrinsic features of γ-AApeptides are important for their ability to disrupt bacterial membranes selectively, the implications of which extend to developing new antibacterial biomaterials.

Selective cytotoxicity of the antibacterial peptide ABP-dHC-Cecropin A and its analog towards leukemia cells

Some cationic antibacterial peptides, with typical amphiphilic α-helical conformations in a membrane-mimicking environment, exhibit anticancer properties as a result of a similar mechanism of action towards both bacteria and cancer cells. We previously reported the cDNA sequence of the antimicrobial peptide ABP-dHC-Cecropin A precursor cloned from drury (Hyphantria cunea) (dHC). In the present study, we synthesized and structurally characterized ABP-dHC-Cecropin A and its analog, ABP-dHC-Cecropin A-K(24). Circular dichroism spectroscopy showed that ABP-dHC-Cecropin A and its analog adopt a well-defined α-helical structure in a 50% trifluorethanol solution. The cytotoxicity and cell selectivity of these peptides were further examined in three leukemia cell lines and two non-cancerous cell lines. The MTT assay indicated both of these peptides have a concentration-dependent cytotoxic effect in leukemia cells, although the observed cytotoxicity was greater with ABP-dHC-Cecropin A-K(24) treatment, whereas they were not cytotoxic towards the non-cancerous cell lines. Moreover, ABP-dHC-Cecropin A and its analog had a lower hemolytic effect in human red blood cells. Together, these results suggest the peptides are selectively cytotoxic towards leukemia cells. Confocal laser scanning microscopy determined that the peptides were concentrated at the surface of the leukemia cells, and changes in the cell membrane were determined with a permeability assay, which suggested that the anticancer activity of ABP-dHC-Cecropin A and its analog is a result of its presence at the leukemia cell membrane. ABP-dHC-Cecropin A and its analog may represent a novel anticancer agent for leukemia therapy, considering its cancer cell selectivity and relatively low cytotoxicity in normal cells.

Skin-safe photothermal therapy enabled by responsive release of acid-activated membrane-disruptive polymer from polydopamine nanoparticle upon very low laser irradiation

How to ablate tumor without damaging skin is a challenge for photothermal therapy.

Bioactive compounds from crocodile (Crocodylus siamensis) white blood cells induced apoptotic cell death in hela cells

Crocodile (Crocodylus siamensis) white blood cell extracts (WBCex) were examined for anticancer activity in HeLa cell lines using the MTT assay. The percentage viability of HeLa cells significantly deceased after treatment with WBCex in a dose- and time-dependent manner. The IC50 dose was suggested to be approximately 225 μg/mL protein. Apoptotic cell death occurred in a time-dependent manner based on investigation by flow cytometry using annexin V-FITC and PI staining. DAPI nucleic acid staining indicated increased chromatin condensation. Caspase-3, -8 and -9 activities also increased, suggesting the induction of the caspase-dependent apoptotic pathway. Furthermore, the mitochondrial membrane potential (ΔΨm ) of HeLa cells was lost as a result of increasing levels of Bax and reduced levels of Bcl-2, Bcl-XL, Bcl-Xs, and XIAP. The decreased ΔΨm led to the release of cytochrome c and the activation of caspase-9 and -3. Apoptosis-inducing factor translocated into the nuclei, and endonuclease G (Endo G) was released from the mitochondria. These results suggest that anticancer agents in WBCex can induce apoptosis in HeLa cells via both caspase-dependent and -independent pathways. © 2015 Wiley Periodicals, Inc. Environ Toxicol 31: 986-997, 2016.

Molecular characterization of a c-type lysozyme from the desert locust, Schistocerca gregaria (Orthoptera: Acrididae)

Lysozymes are bacteriolytic peptides that are implicated in the insect nonspecific innate immune responses. In this study, a full-length cDNA encoding a c-type lysozyme from Schistocerca gregaria (SgLys) has been cloned and characterized from the fat body of immune-challenged 5(th) instar. The deduced mature lysozyme is 119 amino acid residues in length, has a calculated molecular mass of 13.4 kDa and an isoelectric point (Ip) of 9.2. SgLys showed high identities with other insect lysozymes, ranging from 41.5% to 93.3% by BLASTp search in NCBI. Eukaryotic in vitro expression of the SgLys ORF (rSgLys) with an apparent molecular mass of ∼16 kDa under SDS-PAGE is close to the calculated molecular weight of the full-length protein. rSgLys displayed growth inhibitory activity against Gram-negative and Gram-positive bacteria. 3D structure modeling of SgLys, based on comparison with that of silkworm lysozyme, and sequence comparison with the helix-loop-helix (α-hairpin) structure of hen egg white lysozyme (HEWL) were employed to interpret the antibacterial potencies. Phylogenetic alignments indicate that SgLys aligns well with insect c-type lysozymes that expressed principally in fat body and hemocytes and whose role has been defined as immune-related. Western blot analysis showed that SgLys expression was highest at 6-12 h post-bacterial challenge and subsequently decreased with time. Transcriptional profiles of SgLys were determined by semi-quantitative RT-PCR analysis. SgLys transcript was upregulated at the highest level in fat body, hemocytes, salivary gland, thoracic muscles, and epidermal tissue. It was expressed in all developmental stages from egg to adult. These data indicate that SgLys is a predominant acute-phase protein that is expressed and upregulated upon immune challenge.

Acid-Responsive Therapeutic Polymer for Prolonging Nanoparticle Circulation Lifetime and Destroying Drug-Resistant Tumors

How to destroy drug-resistant tumor cells remains an ongoing challenge for cancer treatment. We herein report on a therapeutic nanoparticle, aHLP-PDA, which has an acid-activated hemolytic polymer (aHLP) grafted onto photothermal polydopamine (PDA) nanosphere via boronate ester bond, in efforts to ablate drug-resistant tumors. Upon exposure to oxidative stress and/or near-infrared laser irradiation, aHLP-PDA nanoparticle responsively releases aHLP, likely via responsive cleavage of boronate ester bond, and thus responsively exhibits acid-facilitated mammalian-membrane-disruptive activity. In vitro cell studies with drug-resistant and/or thermo-tolerant cancer cells show that the aHLP-PDA nanoparticle demonstrates preferential cytotoxicity at acidic pH over physiological pH. When administered intravenously, the aHLP-PDA nanoparticle exhibits significantly prolonged blood circulation lifetime and enhanced tumor uptake compared to bare PDA nanosphere, likely owing to aHLP's stealth effects conferred by its zwitterionic nature at blood pH. As a result, the aHLP-PDA nanoparticle effectively ablates drug-resistant tumors, leading to 100% mouse survival even on the 32nd day after suspension of photothermal treatment, as demonstrated with the mouse model. This work suggests that a combination of nanotechnology with lessons learned in bacterial antibiotic resistance may offer a feasible and effective strategy for treating drug-resistant cancers often found in relapsing patients.

Oncolytic activities of host defense peptides

Cancer continues to be a leading source of morbidity and mortality worldwide in spite of progress in oncolytic therapies. In addition, the incidence of cancers affecting the breast, kidney, prostate and skin among others continue to rise. Chemotherapeutic drugs are widely used in cancer treatment but have the serious drawback of nonspecific toxicity because these agents target any rapidly dividing cell without discriminating between healthy and malignant cells. In addition, many neoplasms eventually become resistant to conventional chemotherapy due to selection for multidrug-resistant variants. The limitations associated with existing chemotherapeutic drugs have stimulated the search for new oncolytic therapies. Host defense peptides (HDPs) may represent a novel family of oncolytic agents that can avoid the shortcomings of conventional chemotherapy because they exhibit selective cytotoxicity against a broad spectrum of malignant human cells, including multi-drug-resistant neoplastic cells. Oncolytic activity by HDPs is usually via necrosis due to cell membrane lysis, but some HDPs can trigger apoptosis in cancer cells via mitochondrial membrane disruption. In addition, certain HDPs are anti-angiogenic which may inhibit cancer progression. This paper reviews oncolytic HDP studies in order to address the suitability of selected HDPs as oncolytic therapies.

Human lysozyme possesses novel antimicrobial peptides within its N-terminal domain that target bacterial respiration

Human milk lysozyme is thought to be a key defense factor in protecting the gastrointestinal tract of newborns against bacterial infection. Recently, evidence was found that pepsin, under conditions relevant to the newborn stomach, cleaves chicken lysozyme (cLZ) at specific loops to generate five antimicrobial peptide motifs. This study explores the antimicrobial role of the corresponding peptides of human lysozyme (hLZ), the actual protein in breast milk. Five peptide motifs of hLZ, one helix-loop-helix (HLH), its two helices (H1 and H2), and two helix-sheet motifs, H2-β-strands 1-2 (H2-S12) or H2-β-strands 1-3 (H2-S13), were synthesized and examined for antimicrobial action. The five peptides of hLZ exhibit microbicidal activity to various degrees against several bacterial strains. The HLH peptide and its N-terminal helix (H1) were significantly the most potent bactericidal to Gram-positive and Gram-negative bacteria and the fungus Candida albicans . Outer and inner membrane permeabilization studies, as well as measurements of transmembrane electrochemical potentials, provided evidence that HLH peptide and its N-terminal helix (H1) kill bacteria by crossing the outer membrane of Gram-negative bacteria via self-promoted uptake and are able to dissipate the membrane potential-dependent respiration of Gram-positive bacteria. This finding is the first to describe that hLZ possesses multiple antimicrobial peptide motifs within its N-terminal domain, providing insight into new classes of antibiotic peptides with potential use in the treatment of infectious diseases.

Cationic amphiphilic peptides with cancer-selective toxicity

During the last two decades cationic amphiphilic peptides and peptide sequences (CAPs) with cancer-selective toxicity have appeared. Based on their spectrum of anticancer activity CAPs can be divided into two major classes. The first class includes peptides that are highly potent against both bacteria and cancer cells, but not against mammalian cells. The second class includes peptides that are toxic to bacteria, and both mammalian cancer and non-cancer cells. Most antimicrobial and anticancer CAPs share a common membranolytic mode of action that results either in the selective disruption of the cancer cell membrane or permeation and swelling of mitochondria. The electrostatic attraction between the negatively charged membrane components of bacterial and cancer cells and CAPs is believed to play a crucial role in the disruption of bacterial and cancer cell membranes. This mode of action appears to bypass established resistance mechanisms. However, it is currently unclear as to why some CAPs kill cancer cells when others do not. In addition, non-membranolytic mode of actions of CAPs is increasingly recognized to contribute significantly to the anticancer activity of certain CAPs. The development of CAP-based chemotherapeutics is complicated due to the traditionally poor pharmacokinetic properties and high manufacturing costs of peptides and the low intrinsic selectivity for cancer cells. Peptidomimetic approaches combined with novel selective delivery devices show promise in overcoming some of these obstacles. Furthermore, the ability of CAPs to bypass established resistance mechanisms provides an attractive strategy to develop novel lead structures for cancer treatment.

Studies on anticancer activities of antimicrobial peptides

In spite of great advances in cancer therapy, there is considerable current interest in developing anticancer agents with a new mode of action because of the development of resistance by cancer cells towards current anticancer drugs. A growing number of studies have shown that some of the cationic antimicrobial peptides (AMPs), which are toxic to bacteria but not to normal mammalian cells, exhibit a broad spectrum of cytotoxic activity against cancer cells. Such studies have considerably enhanced the significance of AMPs, both synthetic and from natural sources, which have been of importance both for an increased understanding of the immune system and for their potential as clinical antibiotics. The electrostatic attraction between the negatively charged components of bacterial and cancer cells and the positively charged AMPs is believed to play a major role in the strong binding and selective disruption of bacterial and cancer cell membranes, respectively. However, it is unclear why some host defense peptides are able to kill cancer cells when others do not. In addition, it is not clear whether the molecular mechanism(s) underlying the antibacterial and anticancer activities of AMPs are the same or different. In this article, we review various studies on different AMPs that exhibit cytotoxic activity against cancer cells. The suitability of cancer cell-targeting AMPs as cancer therapeutics is also discussed.

Membrane insertion and bilayer perturbation by antimicrobial peptide CM15

Antimicrobial peptides (AMPs) are an important component of innate immunity and have generated considerable interest as a potential new class of antibiotic. The biological activity of AMPs is strongly influenced by peptide-membrane interactions; however, for many of these peptides the molecular details of how they disrupt and/or translocate across target membranes are not known. CM15 is a linear, synthetic hybrid AMP composed of the first seven residues of the cecropin A and residues 2-9 of the bee venom peptide mellitin. Previous studies have shown that upon membrane binding CM15 folds into an alpha-helix with its helical axis aligned parallel to the bilayer surface and have implicated the formation of 2.2-3.8 nm pores in its bactericidal activity. Here we report site-directed spin labeling electron paramagnetic resonance studies examining the behavior of CM15 analogs labeled with a methanethiosulfonate spin label (MTSL) and a brominated MTSL as a function of increasing peptide concentration and utilize phospholipid-analog spin labels to assess the effects of CM15 binding and accumulation on the physical properties of membrane lipids. We find that as the concentration of membrane-bound CM15 is increased the N-terminal domain of the peptide becomes more deeply immersed in the lipid bilayer. Only minimal changes are observed in the rotational dynamics of membrane lipids, and changes in lipid dynamics are confined primarily to near the membrane surface. However, the accumulation of membrane-bound CM15 dramatically increases accessibility of lipid-analog spin labels to the polar relaxation agent, nickel (II) ethylenediaminediacetate, suggesting an increased permeability of the membrane to polar solutes. These results are discussed in relation to the molecular mechanism of membrane disruption by CM15.

Utilizing ESEEM spectroscopy to locate the position of specific regions of membrane-active peptides within model membranes

Membrane-active peptides participate in many cellular processes, and therefore knowledge of their mode of interaction with phospholipids is essential for understanding their biological function. Here we present a new methodology based on electron spin-echo envelope modulation to probe, at a relatively high resolution, the location of membrane-bound lytic peptides and to study their effect on the water concentration profile of the membrane. As a first example, we determined the location of the N-terminus of two membrane-active amphipathic peptides, the 26-mer bee venom melittin and a de novo designed 15-mer D,L-amino acid amphipathic peptide (5D-L9K6C), both of which are antimicrobial and bind and act similarly on negatively charged membranes. A nitroxide spin label was introduced to the N-terminus of the peptides and measurements were performed either in H2O solutions with deuterated model membranes or in D2O solutions with nondeuterated model membranes. The lipids used were dipalmitoyl phosphatidylcholine (DPPC) and phosphatidylglycerol (PG), (DPPC/PG (7:3 w/w)), egg phosphatidylcholine (PC) and PG (PC/PG (7:3 w/w)), and phosphatidylethanolamine (PE) and PG (PE/PG, 7:3w/w). The modulation induced by the 2H nuclei was determined and compared with a series of controls that produced a reference "ruler". Actual estimated distances were obtained from a quantitative analysis of the modulation depth based on a simple model of an electron spin situated at a certain distance from the bottom of a layer with homogeneously distributed deuterium nuclei. The N-terminus of both peptides was found to be in the solvent layer in both the DPPC/PG and PC/PG membranes. For PE/PG, a further displacement into the solvent was observed. The addition of the peptides was found to change the water distribution in the membrane, making it "flatter" and increasing the penetration depth into the hydrophobic region.

Low microwave-amplitude ESR spectroscopy: measuring spin-relaxation interactions of moderately immobilized spin labels in proteins

Electron spin resonance (ESR) spectroscopy in combination with site-directed spin labeling (SDSL) is a powerful tool for determining protein structure, dynamics and interactions. We report here a method for determining interactions between spin labels and paramagnetic relaxation agents, which is performed under subsaturating conditions. The low microwave-field amplitude employed (h(1)<0.36 G) only requires standard, commercially available ESR equipment. The effect of relaxation enhancement on the spin-spin-relaxation time, T(2e), is measured by this method, and compared to classical progressive power saturation performed on a free spin label, (1-oxyl-2,2,5,5-tetramethyl-Delta(3)-pyrroline-3-methyl)methanethiosulfonate (MTSL), and a spin-labeled protein (Thermomyces lanuginosa lipase, TLL-I252C), employing the water-soluble relaxation agent chromium(III) oxalate (Crox) in concentrations between 0-10 mM. The low-amplitude theory showed excellent agreement with that of classical power saturation in quantifying Crox-induced relaxation enhancement. Low-amplitude measurements were then performed using a standard resonator, with Crox, on 11 spin-labeled TLL mutants displaying rotational correlation times in the motional narrowing regime. All spin-labeled proteins exhibited significant changes in T(2e). We postulate that this novel method is especially suitable for studying moderately immobilized spin labels, such as those positioned at exposed sites in a protein. This method should prove useful for research groups with access to any ESR instrumentation.

Initial structural and dynamic characterization of the M2 protein transmembrane and amphipathic helices in lipid bilayers

Amphipathic helices in membrane proteins that interact with the hydrophobic/hydrophilic interface of the lipid bilayer have been difficult to structurally characterize. Here, the backbone structure and orientation of an amphipathic helix in the full-length M2 protein from influenza A virus has been characterized. The protein has been studied in hydrated DMPC/DMPG lipid bilayers above the gel to liquid-crystalline phase transition temperature by solid-state NMR spectroscopy. Characteristic PISA (Polar Index Slant Angle) wheels reflecting helical wheels have been observed in uniformly aligned bilayer preparations of both uniformly 15N labeled and amino acid specific labeled M2 samples. Hydrogen/deuterium exchange studies have shown the very slow exchange of some residues in the amphipathic helix and more rapid exchange for the transmembrane helix. These latter results clearly suggest the presence of an aqueous pore. A variation in exchange rate about the transmembrane helical axis provides additional support for this claim and suggests that motions occur about the helical axes in this tetramer to expose the entire backbone to the pore.

Transmission electron microscopic observations of membrane effects of antibiotic cecropin B on Escherichia coli

The pathway of cell membrane lysis by the peptide antibiotic cecropin B (CB), which contains both a hydrophobic and an amphipathic alpha-helix, was analysed by assessing the morphological changes of Escherichia coli following treatment with the peptide. Exposure of green fluorescent protein (GFP)-expressing E. coli to CB does not lead to an efflux of GFP. Moreover, transmission electron microscopic (TEM) examination of cecropin B-treated cells showed that severe swelling precedes cell death and that the outer membrane becomes distended away from the plasma membrane. Using immuno-gold staining and TEM of E. coli expressing the maltose-binding protein in the cytoplasm, it was apparent that the protein remains restricted to the cytoplasmic compartment. These observations suggest that CB causes gross disruption of the outer membrane of Gram-negative bacteria. Circular dichroism measurements of CB in the presence of cell membrane-mimicking liposomes showed that CB forms secondary structure dependent on the ratio of [lipid]/[peptide]. These observations from this study are important for the future design of custom antimicrobial peptides.

Distinguishing between different pathways of bilayer disruption by the related antimicrobial peptides cecropin B, B1 and B3

Different pathways of bilayer disruption by the structurally related antimicrobial peptides cecropin B, B1 and B3, revealed by surface plasma resonance analysis of immobilized liposomes, differential scanning calorimetry of peptide-large unilamellar vesicle interactions, and light microscopic analysis of peptide-treated giant unilamellar vesicles, have been identified in this study. Natural cecropin B (CB) has one amphipathic and one hydrophobic alpha-helix, whereas cecropins B1 (CB1) and B3 (CB3), which are custom-designed, chimaeric analogues of CB, possess either two amphipathic or two hydrophobic alpha-helices, respectively. Surface plasma resonance analysis of unilamellar vesicles immobilized through a biotin-avidin interaction showed that both CB and CB1 bind to the lipid bilayers at high concentration (>10 microm); in contrast, CB3 induces disintegration of the vesicles at all concentrations tested. Differential scanning calorimetry showed the concentration-dependent effect of bilayer disruption, based on the different thermotrophic phase behaviours and the shapes of the thermal phase-transition curves obtained. The kinetics of the lysis of giant unilamellar vesicles observed by microscopy demonstrated that both CB and CB1 effect a continuous process involving loss of integrity followed by coalescence and resolution into smaller vesicles, whereas CB3 induces rapid formation of irregular-shaped, nonlamellar structures which rapidly disintegrate into twisted, microtubule-containing debris before being completely destroyed. On the basis of these observations, models by which CB, CB1 and CB3 induce lysis of lipid bilayers are discussed.

A helix-loop-helix peptide at the upper lip of the active site cleft of lysozyme confers potent antimicrobial activity with membrane permeabilization action

Recently, we have found that partially unfolded lysozyme exerts broad spectrum antimicrobial action in vitro against Gram-negative and Gram-positive bacteria independent of its catalytic activity. In parallel, an internal peptide (residues 98-112) of hen egg white lysozyme, obtained after digestion with clostripain, possessed broad spectrum antimicrobial action in vitro. This internal peptide is part of a helix-loop-helix domain (87-114 sequence of hen lysozyme) located at the upper lip of the active site cleft of lysozyme. The helix-loop-helix (HLH) structures are known motifs commonly found in membrane-active and DNA-binding proteins. To evaluate the contribution of the HLH peptide to the antimicrobial properties of lysozyme, the HLH sequence and its secondary structure derivatives of chicken and human lysozyme were synthesized and tested for antimicrobial activity against several bacterial strains. We found that the full HLH peptide of both chicken and human lysozymes was potently microbicidal against both Gram-positive and Gram-negative bacteria and the fungus Candida albicans. The N-terminal helix of HLH was specifically bactericidal to Gram-positive bacteria, whereas the C-terminal helix was bactericidal to all tested strains. Outer and inner membrane permeabilization studies, as well as measurements of transmembrane electrochemical potentials, provided evidence that HLH peptide and its C-terminal helix domain kill Gram-negative bacteria by crossing the outer membrane via self-promoted uptake and causing damage to the inner membrane through channel formation. The results are discussed in terms of proposed mechanisms for the catalytically independent antimicrobial activity of lysozyme that offer a new strategy for the design of potential antimicrobial drugs in the treatment of infectious diseases.

Crumpled structure of the custom hydrophobic lytic peptide cecropin B3

The solution structure of a custom lytic peptide, cecropin B3 (CB3), having two identical hydrophobic segments on both the N- and C-termini, was investigated by two-dimensional NMR spectroscopy. The need to determine the structure of this peptide is rooted in its specific ability to lyse lipid layers that have a high content of anionic lipid. The lytic activities of CB3 on cell membranes including cancer cells and bacteria is found to be less than cecropin B1. The results show that CB3 has four discrete segments forming alpha helical structures. The crumpled structure of CB3 provides evidence for the lysis of the lipid layer being via a pathway that differs from pore formation. The results in this study provide strong clues towards a rational design for a potent antimicrobial and antitumor peptide.

Kinetics of membrane lysis by custom lytic peptides and peptide orientations in membrane

To aid the development of custom peptide antibiotics, a kinetic study of membrane lysis by cecropin B (CB) and its analogs, cecropin B1 (CB1) and cecropin B3 (CB3) was carried out to determine the mechanism by which these peptides disrupt the bilayer structure of liposomes of defined composition. Disruption of the phospholipid bilayer was determined by a fluorescence assay involving the use of dithionite to quench the fluorescence of lipids labeled with N-7-nitro-2,1,3-benzoxadiazol-4-yl. Lytic peptides caused the disruption of liposomes to occur in two kinetic steps. For liposomes composed of mixtures of phosphatidylcholine and phosphatidic acid, the time constants for each kinetic step were shorter for CB and CB1 than for CB3. Oriented circular dichroism experiments showed that the peptides could exist in at least two different membrane-associated states that differed primarily in the orientation of the helical segments with respect to the bilayer surface. The results are discussed in terms of kinetic mechanisms of membrane lysis. The mode of actions of these peptides used for the interpretation of their kinetic mechanisms were supported by surface plasmon resonance experiments including or excluding the pore-forming activities.

Conformational study of a custom antibacterial peptide cecropin B1: implications of the lytic activity

Cecropin B1 (CB1) with two amphipathic alpha-helical segments is a derivative of the natural antibacterial peptide, cecropin B. The assays of cell lysis show that, compared with cecropin A (CA), CB1 has a similar ability to lyse bacteria with a higher potency (two- to six-fold higher) in killing cancer cells. The difference may be due to the fact that the peptides possess different structures and sequences. In this study, the solution structure of CB1 in 20% hexafluoroisopropanol was determined by two-dimensional nuclear magnetic resonance (NMR) spectroscopy. The (1)H NMR resonances were assigned. A total of 350 inter-proton distances were used to calculate the solution structure of CB1. The final ensemble structures were well converged, showing the minimum root mean square deviation. The results indicate that CB1 has two stretches of helices spanning from residues 3 to 22 and from residues 26 to 33, which are connected by a hinge section formed by Gly-23 and Pro-24. Lys-25 is partially incorporated in the hinge region. The bent angle between two helical segments located in two planes was between 100 and 110 degrees. With comparisons of the known NMR structure of CA and its activities on bacteria and cancer cells, the structure-function relationship of the peptides is discussed.