Thionins: plant peptides that modify membrane permeability in cultured mammalian cells

Advances of peptides for antibacterial applications

In the past few decades, peptide antibacterial products with unique antibacterial mechanisms have attracted widespread interest. They can effectively reduce the probability of drug resistance of bacteria and are biocompatible, so they possess tremendous development prospects. This review provides recent research and analysis on the basic types of antimicrobial peptides (including poly (amino acid)s, short AMPs, and lipopeptides) and factors to optimize antimicrobial effects. It also summarizes the two most important modes of action of antimicrobial peptides and the latest developments in the application of AMPs, including antimicrobial agent, wound healing, preservative, antibacterial coating and others. Finally, we discuss the remaining challenges to improve the antibacterial peptides and propose prospects in the field.

[Präklinische Untersuchungen von Wechselwirkungen zwischen Mistel und Radio- oder Chemotherapie auf pädiatrische Tumorzellen]

Hintergrund: Mistelanwendungen werden als komplementäre Therapien häufig in der pädiatrischen Onkologie zusammen mit einer Radio- oder Chemotherapie verabreicht. Wechselwirkungen bei simultaner Applikation sind gerade in der Pädiatrie von großer Bedeutung, sie sind allerdings nach wie vor unzureichend untersucht. Material und Methoden: Zytotoxische Effekte des Mistelextraktes abnobaVISCUM Fraxini (aVF) auf LAN-1 Neuroblastomzellen und deren Etoposid- bzw. Cisplatin-resistente Subzelllinien wurden mittels Viabilitätstest untersucht, sowie mögliche Synergieeffekte zwischen aVF und den Chemotherapeutika durch die Softwareprodukte Combenefit und CompuSyn analysiert. Effekte einer Kombinationstherapie aus aVF und Bestrahlung auf SH-SY5Y Zellen wurden mittels Koloniebildungstest untersucht und Auswirkungen auf die Reparatureffizienz strahleninduzierter Doppelstrangbrüche mit Hilfe durchflusszytometrischer Quantifizierungen von γ-H2AX-Foci nach PI/FITC Doppelfärbung analysiert. Ergebnisse: Die Chemotherapie-resistenten LAN-1 Subzelllinien erwiesen sich als resistenter gegenüber der Mistelbehandlung als die Ursprungszelllinie. Auf Basis vier verschiedener Referenz-modelle konnten vor allem synergistisch/additive Effekte zwischen aVF und den Zytostatika Etoposid und Cisplatin berechnet werden. Die Kombination aus Mistelbehandlung und Bestrahlung führte zu einer Verringerung der Koloniebildung und zu einer Verzögerung der Reparaturgeschwindigkeit von strahleninduzierten Doppelstrangbrüchen. Schlussfolgerung: Die präklinischen Daten könnten darauf hinweisen, dass die Verwendung des Mistelextraktes, aVF, eine unterstützende Wirkung auf Radio- und Chemotherapien hat.

BACKGROUND

Mistletoe therapies belong to the field of complementary medicines and are often administered simultaneously or successive to conventional radio- or chemotherapy. Drug-herb interactions are of great significance, especially in pediatrics, but are still insufficiently investigated.

MATERIAL AND METHODS

Cytotoxic effects of the mistletoe extract, abnobaVISCUM Fraxini (aVF), on LAN-1 neuroblastoma cell line and their chemotherapy-resistant (cisplatin; etoposide) subclones were investigated by cell viability assays. Potential synergistic or antagonistic effects of the co-treatment of aVF and cisplatin or etoposide, respectively, were analyzed by Combenefit and CompuSyn software. Combinational effects of mistletoe and irradiation were assessed by colony formation assays and repair efficiency of irradiation-induced double strand breaks was investigated by flow cytometric analyses of γ-H2AX foci after PI/FITC double staining.

RESULTS

Chemotherapy-resistant subclones were more resistant to mistletoe therapy than the parental cells. Based on four different reference models, primarily synergistic/additive effects between aVF and the cytostatic drugs could be calculated. Simultaneous application of mistletoe extract and irradiation led to a delay of irradiation-induced double strand break repair in neuroblastoma cells and a decreased colony formation compared to irradiation monotherapy.

CONCLUSION

The preclinical data may indicate that the use of aVF has a supportive effect on radio- and chemotherapies.

Nigellothionins from Black Cumin (Nigella sativa L.) Seeds Demonstrate Strong Antifungal and Cytotoxic Activity

High-cationic biologically active peptides of the thionins family were isolated from black cumin (Nigella sativa L.) seeds. According to their physicochemical characteristics, they were classified as representatives of the class I thionin subfamily. Novel peptides were called “Nigellothionins”, so-called because of their source plant. Thionins are described as components of plant innate immunity to environmental stress factors. Nine nigellothionins were identified in the plant in different amounts. Complete amino acid sequences were determined for three of them, and a high degree of similarity was detected. Three nigellothionins were examined for antifungal properties against collection strains. The dominant peptide, NsW2, was also examined for activity against clinical isolates of fungi. Cytotoxic activity was determined for NsW2. Nigellothionins activity against all collection strains and clinical isolates varied from absence to a value comparable to amphotericin B, which can be explained by the presence of amino acid substitutions in their sequences. Cytotoxic activity in vitro for NsW2 was detected at sub-micromolar concentrations. This has allowed us to propose an alteration of the molecular mechanism of action at different concentrations. The results obtained suggest that nigellothionins are natural compounds that can be used as antimycotic and anti-proliferative agents.

Analysis of structures, functions, and transgenicity of phytopeptides defensin and thionin: a review

Background

Antimicrobial peptides are very primitive innate defense molecules of almost all organisms, from microbes to mammalians and vascular seed-bearing plants. Antimicrobial peptides of plants categorized into cysteine-rich peptides (CRPs) and others and most of the antimicrobial peptides belong to CRPs group. These peptides reported showing the great extent of protecting property against bacteria, fungi, viruses, insect, nematode, and another kind of microbes. To develop a resistant plant against pathogenic fungi, there have been several studies executed to understand the efficiency of transgenicity of these antimicrobial peptides.

Main text

Apart from the intrinsic property of the higher organism for identifying and activating microbial attack defense device, it also involves innate defense mechanism and molecules. In the current review article, apart from the structural and functional characterization of peptides defensin and thionin, we have attempted to provide a succinct overview of the transgenic development of these defense peptides, that are expressed in a constitutive and or over-expressive manner when biotic and abiotic stress inflicted. Transgenic of different peptides show different competence in plants. Most of the transgenic studies made for defensin and thionin revealed the effective transgenic capacity of these peptides.

Conclusion

There have been several studies reported successful development of transgenic plants based on peptides defensin and thionin and observed diverse level of resistance-conferring potency in different plants against phytopathogenic fungi. But due to long regulatory process, there has not been marketed any antimicrobial peptides based transgenic plants yet. However, success report state that possibly in near future transgenic plants of AMPs would be released with devoid of harmful effect, with good efficiency, reproducibility, stability, and least production cost.

In Silico Analyses of Rice Thionin Genes and the Antimicrobial Activity of OsTHION15 Against Phytopathogens

Thionins are a family of antimicrobial peptides. We performed in silico expression analyses of the 44 rice (Oryza sativa) thionins (OsTHIONs). Modulated expression levels of OsTHIONs under different treatments suggest their involvement in many processes, including biotic, abiotic, and nutritional stress responses, and in hormone signaling. OsTHION15 (LOC_Os06g32600) was selected for further characterization based on several in silico analyses. OsTHION15 in O. sativa subsp. indica 'KDML 105' was expressed in all of the tissues and organs examined, including germinating seed, leaves, and roots of seedlings and mature plants, and inflorescences. To investigate the antimicrobial activity of OsTHION15, we produced a recombinant peptide in Escherichia coli Rosetta-gami (DE3). The recombinant OsTHION15 exhibited inhibitory activities toward rice-pathogenic bacteria such as Xanthomonas oryzae pv. oryzae and Pectobacterium carotovorum pv. atroseptica, with minimum inhibitory concentrations of 112.6 and 14.1 µg ml^-1, respectively. A significant hyphal growth inhibition was also observed toward Fusarium oxysporum f. sp. cubense and Helminthosporium oryzae. In addition, we demonstrated the in planta antibacterial activity of this peptide in Nicotiana benthamiana against X. campestris pv. glycines. These activities suggest the possible application of OsTHION15 in plant disease control.

Plant thionins: structure, biological functions and potential use in biotechnology

Antimicrobial peptides (AMPs) are important components of defense system in both plants and animals. They represent an ancient mechanism of innate immunity providing rapid first line of defense against pathogens. Plant AMPs are classified into several families: thionins, defensins, nonspecific lipid-transfer proteins, hevein- and knottin-type peptides, hairpinins and macrocyclic peptides (cyclotides). The review focuses on the thionin family. Thionins comprise a plant-specific AMP family that consists of short (~5 kDA) cysteine-rich peptides containing 6 or 8 cysteine residues with antimicrobial and toxic properties. Based on similarity in amino acid sequences and the arrangement of disulphide bonds, five structural classes of thionins are discriminated. The three-dimensional structures of a number of thionins were determined. The amphipathic thionin molecule resembles the Greek letter Г, in which the long arm is formed by two antiparallel α-helices, while the short one, by two parallel β-strands. The residues responsible for the antimicrobial activity of thionins were identified. Thionins are synthesized as precursor proteins consisting of a signal peptide, the mature peptide region and the C-terminal prodomain. Thionins protect plants from pathogenic bacteria and fungi acting directly on the membranes of microorganisms at micromolar concentrations, although their precise mode of action remains unclear. In addition to plant pathogens, thionins inhibit growth of a number of human pathogens and opportunistic microorganisms, such as Candida spp., Saccharomyces cerevisiae, Fusarium solani, Staphylococcus aureus and Escherichia coli. Thionins are toxic to different types of cells including mammalian cancer cell lines. Transgenic plants expressing thionin genes display enhanced resistance to pathogens. A wide range of biological activities makes thionins promising candidates for practical application in agriculture and medicine.

Metal ions provide structural stability and compactness to tetrameric purothionin

Metal ions impart structural stability to the purothionin tetramer.

Recent perspectives on the anticancer properties of aqueous extracts of Nigerian Vernonia amygdalina

Innovative developments are necessary for treating and defeating cancer, an oftentimes deadly group of diseases characterized by the uncontrolled growth and spread of abnormal cells. Breast cancer (BC) is the second leading cause of cancer-related deaths of women in the USA, and prostate cancer (PC) is the second leading cause of cancer-related deaths of American men. Although some efficacious BC drugs are pharmaceutically marketed, they affect the quality of life for some patients because they are toxic in that their usages have been accompanied by side effects such as stroke, thrombosis, slow heart rate, seizure, increased blood pressure, nausea, emesis, and more. Therefore, there is an urgent need for the discovery of molecular markers for early detection of this disease and discovery of targets for the development of novel, less toxic therapeutics. A botanical plant Vernonia amygdalina has been widely used in Nigerian and other Central and West African cultures for centuries as an herbal medicine. Mounting evidence suggests that treatment with low concentrations of aqueous leaf extracts of the edible Nigerian V. amygdalina plant (Niger-VA) arrests the proliferative activities and induces apoptosis in estrogen receptor-positive, estrogen receptor-negative, and triple-negative human breast cancerous cells and in androgen-independent human PC-3. Also, in athymic mice, Niger-VA potentiates increased efficacies and optimizes treatment outcomes when given as a cotreatment with conventional chemotherapy drugs. Evidence of its noticeable cytostatic activities ranging from changes in DNA synthesis to growth inhibition, mechanisms of inducing apoptosis in different cancer cell lines, and in vivo antitumorigenic activities and chemopreventive efficacy reinforce the idea that Niger-VA deserves increased attention for further development as a phytoceutical, anticancer drug entity. Hence, the present review article highlights impactful published literature on the anticancer effects of Niger-VA in multiple cancerous cell lines and in a nude mouse model, supporting its potential usefulness as a natural product, chemotherapeutic medicine for treatment of both BC and PC.

Antimicrobial Peptides from Plants

Plant antimicrobial peptides (AMPs) have evolved differently from AMPs from other life forms. They are generally rich in cysteine residues which form multiple disulfides. In turn, the disulfides cross-braced plant AMPs as cystine-rich peptides to confer them with extraordinary high chemical, thermal and proteolytic stability. The cystine-rich or commonly known as cysteine-rich peptides (CRPs) of plant AMPs are classified into families based on their sequence similarity, cysteine motifs that determine their distinctive disulfide bond patterns and tertiary structure fold. Cystine-rich plant AMP families include thionins, defensins, hevein-like peptides, knottin-type peptides (linear and cyclic), lipid transfer proteins, α-hairpinin and snakins family. In addition, there are AMPs which are rich in other amino acids. The ability of plant AMPs to organize into specific families with conserved structural folds that enable sequence variation of non-Cys residues encased in the same scaffold within a particular family to play multiple functions. Furthermore, the ability of plant AMPs to tolerate hypervariable sequences using a conserved scaffold provides diversity to recognize different targets by varying the sequence of the non-cysteine residues. These properties bode well for developing plant AMPs as potential therapeutics and for protection of crops through transgenic methods. This review provides an overview of the major families of plant AMPs, including their structures, functions, and putative mechanisms.

Toxic proteins in plants

Plants have evolved to synthesize a variety of noxious compounds to cope with unfavorable circumstances, among which a large group of toxic proteins that play a critical role in plant defense against predators and microbes. Up to now, a wide range of harmful proteins have been discovered in different plants, including lectins, ribosome-inactivating proteins, protease inhibitors, ureases, arcelins, antimicrobial peptides and pore-forming toxins. To fulfill their role in plant defense, these proteins exhibit various degrees of toxicity towards animals, insects, bacteria or fungi. Numerous studies have been carried out to investigate the toxic effects and mode of action of these plant proteins in order to explore their possible applications. Indeed, because of their biological activities, toxic plant proteins are also considered as potentially useful tools in crop protection and in biomedical applications, such as cancer treatment. Genes encoding toxic plant proteins have been introduced into crop genomes using genetic engineering technology in order to increase the plant's resistance against pathogens and diseases. Despite the availability of ample information on toxic plant proteins, very few publications have attempted to summarize the research progress made during the last decades. This review focuses on the diversity of toxic plant proteins in view of their toxicity as well as their mode of action. Furthermore, an outlook towards the biological role(s) of these proteins and their potential applications is discussed.

Plant antimicrobial peptides as potential anticancer agents

Antimicrobial peptides (AMPs) are part of the innate immune defense mechanism of many organisms and are promising candidates to treat infections caused by pathogenic bacteria to animals and humans. AMPs also display anticancer activities because of their ability to inactivate a wide range of cancer cells. Cancer remains a cause of high morbidity and mortality worldwide. Therefore, the development of methods for its control is desirable. Attractive alternatives include plant AMP thionins, defensins, and cyclotides, which have anticancer activities. Here, we provide an overview of plant AMPs anticancer activities, with an emphasis on their mode of action, their selectivity, and their efficacy.

Plant peptides in defense and signaling

This review focuses on plant peptides involved in defense against pathogen infection and those involved in the regulation of growth and development. Defense peptides, defensins, cyclotides and anti-microbial peptides are compared and contrasted. Signaling peptides are classified according to their major sites of activity. Finally, a network approach to creating an interactomic peptide map is described.

Properties and mechanisms of action of naturally occurring antifungal peptides

Antimicrobial peptides are a vital component of the innate immune system of all eukaryotic organisms and many of these peptides have potent antifungal activity. They have potential application in the control of fungal pathogens that are a serious threat to both human health and food security. Development of antifungal peptides as therapeutics requires an understanding of their mechanism of action on fungal cells. To date, most research on antimicrobial peptides has focused on their activity against bacteria. Several antimicrobial peptides specifically target fungal cells and are not active against bacteria. Others with broader specificity often have different mechanisms of action against bacteria and fungi. This review focuses on the mechanism of action of naturally occurring antifungal peptides from a diverse range of sources including plants, mammals, amphibians, insects, crabs, spiders, and fungi. While antimicrobial peptides were originally proposed to act via membrane permeabilization, the mechanism of antifungal activity for these peptides is generally more complex and often involves entry of the peptide into the cell.

Biological control of bacterial wilt in Arabidopsis thaliana involves abscissic acid signalling

Means to control bacterial wilt caused by the phytopathogenic root bacteria Ralstonia solanacearum are limited. Mutants in a large cluster of genes (hrp) involved in the pathogenicity of R. solanacearum were successfully used in a previous study as endophytic biocontrol agents in challenge inoculation experiments on tomato. However, the molecular mechanisms controlling this resistance remained unknown. We developed a protection assay using Arabidopsis thaliana as a model plant and analyzed the events underlying the biological control by genetic, transcriptomic and molecular approaches. High protection rates associated with a significant decrease in the multiplication of R. solanacearum were observed in plants pre-inoculated with a ΔhrpB mutant strain. Neither salicylic acid, nor jasmonic acid/ethylene played a role in the establishment of this resistance. Microarray analysis showed that 26% of the up-regulated genes in protected plants are involved in the biosynthesis and signalling of abscissic acid (ABA). In addition 21% of these genes are constitutively expressed in the irregular xylem cellulose synthase mutants (irx), which present a high level of resistance to R. solanacearum. We propose that inoculation with the ΔhrpB mutant strain generates a hostile environment for subsequent plant colonization by a virulent strain of R. solanacearum.

Crambin in phospholipid vesicles: Circular dichroism analysis of crystal structure relevance

Crambin, a hydrophobic plant seed protein that exhibits sequence homology to membrane-active plant toxins, was incorporated into phospholipid vesicles. Circular dichroism spectroscopy indicates that its structure in vesicles is nearly identical to its structure in 60% ethanol solution, the solvent from which the protein was crystallized. The secondary structure predicted from the circular dichroism data of the ethanol solution closely resembles that determined by x-ray diffraction of the crystals. This agreement suggests that the x-ray structure may form a useful basis for modeling the structure and behavior of lipophilic plant toxins. Finally, because the structure of crambin has been determined in an organic solvent medium, it provides a protein standard for examination of the effect of solvent dipole moment on the circular dichroism spectra of proteins, which may be important for interpretation of data for membrane proteins.

Plant pathogenesis-related (PR) proteins: a focus on PR peptides

The novel classes of plant pathogenesis-related (PR) proteins identified during the last decade also include novel peptide families. This review specifically focuses on these pathogenesis-related peptides, including proteinase inhibitors (PR-6 family), plant defensins (PR-12 family), thionins (PR-13 family) and lipid transfer proteins (PR-14 family). For each family of PR peptides, the general features concerning occurrence, expression and possible functions of their members are described. Next, more specifically the occurrence of each PR peptide family in the model plant Arabidopsis thaliana is discussed. Single-gene studies performed on particular gene members of a PR peptide family are reported. In addition, expression data of yet undescribed gene members of that particular PR peptide family are presented by consultation of publicly available micro-array databases. Finally an update is provided on the potential role of these PR peptides in A. thaliana, with a focus on their possible involvement in plant defense.

Sophisticated Functions for a Simple Molecule: The Role of Glucosylceramides in Fungal Cells

It is well known that mammalian glycosphingolipids (GSL) play key roles in different physiological and pathophysiological processes. The simplest GSL, glucosylceramide (GlcCer), is formed through the enzymatic transfer of glucose to a ceramide moiety. In mammalian cells this molecule is the building block for the synthesis of lactosylceramides and many other complex GSLs. In fungal cells GlcCer is a major neutral GSL that has been considered during decades merely as a structural component of cell membranes. The recent literature, however, describes the participation of fungal GlcCer in vital processes such as secretion, cell wall assembly, recognition by the immune system and regulation of virulence. In this review we discuss the most recent information regarding fungal GlcCer, including (i) new aspects of GlcCer metabolism, (ii) the involvement of these molecules in virulence mechanisms, (iii) their role as targets of new antifungal drugs and immunotherapeutic agents and, finally, (v) their potential participation on cellular signaling in response to different stimuli.

Studies on solution NMR structure of brazzein : Secondary structure and molecular scaffold

Brazzein is a sweet-tasting protein isolated from the fruit of West African plantPentadiplandra brazzeana Baillon. It is the smallest and the most water-soluble sweet protein discovered so far and is highly thermostable. The proton NMR study of brazzein at 600 MHz (pH 3.5, 300 K) is presented. The complete sequence specific assignments of the individual backbone and sidechain proton resonances were achieved using through-bond and through-space connectivities obtained from standard two-dimensional NMR techniques. The secondary structure of brazzein contains one alpha-helix (residues 21-29), one short 3(10)-helix (residues 14-17), two strands of antiparallel beta-sheet (residues 34-39, 44-50) and probably a third strand (residues 5-7) near the N-terminus. A comparative analysis found that brazzein shares a so-called 'cysteine-stabilized alpha-beta' (CSalphabeta) motif with scorpion neurotoxins, insect defensins and plant gamma - thionins. The significance of this multi-function motif, the possible active sites and the structural basis of themostability were discussed.

Plant peptides and peptidomics

Extracellular plant peptides perform a large variety of functions, including signalling and defence. Intracellular peptides often have physiological functions or may merely be the products of general proteolysis. Plant peptides have been identified and, in part, functionally characterized through biochemical and genetic studies, which are lengthy and in some cases impractical. Peptidomics is a branch of proteomics that has been developed over the last 5 years, and has been used mainly to study neuropeptides in animals and the degradome of proteases. Peptidomics is a fast, efficient methodology that can detect minute and transient amounts of peptides and identify their post-translational modifications. This review describes known plant peptides and introduces the use of peptidomics for the detection of novel plant peptides.

Is there a difference in metal ion-based inhibition between members of thionin family: Molecular dynamics simulation study

Thionins have a considerable potential as antimicrobial compounds although their application may be restricted by metal ion-based inhibition of membrane permeabilizing activity. We previously reported the properties associated with the proposed mechanism of metal ion-based inhibition of beta-purothionin. In this study, we investigated the effects of metal ions on alpha-hordothionin which differs from beta-purothionin by eight out of 45 residues. Three of the differing residues are thought to be involved in the mechanism of metal ion-based inhibition in beta-purothionin. The structure and dynamics of alpha-hordothionin were explored using unconstrained molecular dynamics (MD) simulations in explicit water as a function of metal ions. Although the global fold is almost identical to that of beta-purothionin, alpha-hordothionin displays reduced fluctuating motions. Moreover, alpha-hordothionin is more resistant to the presence of metal ions than beta-purothionin. Mg(+2) ions do not affect alpha-hordothionin, whereas K(+) ions induce perturbations in the alpha2 helix, modify dynamics and electrostatic properties. Nevertheless, these changes are considerably smaller than those in beta-purothionin. The proposed mechanism of metal ion-based inhibition involves the hydrogen bonding network of Arg5-Arg30-Gly27, which regulates dynamic unfolding of the alpha2 C-end which is similar to beta-purothionin response. The key residues responsible for the increased resistance for alpha-hordothionin are Gly27 and Gly42 which replace Asn27 and Asp42 involved into the mechanism of metal ion-based inhibition in beta-purothionin. Comparison of MD simulations of alpha-hordothionin with beta-purothionin reveals dynamic properties which we believe are intrinsic properties of thionins with four disulphide bonds.

Puroindoline-a and alpha1-purothionin form ion channels in giant liposomes but exert different toxic actions on murine cells

Puroindoline-a (PIN-a) and alpha1-purothionin (alpha1-PTH), isolated from wheat endosperm of Triticum aestivum sp., have been suggested to play a role in plant defence mechanisms against phytopathogenic organisms. We investigated their ability to form pores when incorporated into giant liposomes using the patch-clamp technique. PIN-a formed cationic channels (approximately 15 pS) with the following selectivity K(+) > Na(+) >> Cl(-). Also, alpha1-PTH formed channels of approximately 46 pS and 125 pS at +100 mV, the selectivity of which was Ca(2+) > Na(+) approximately K(+) >> Cl(-) and Cl(-) >> Na(+), respectively. In isolated mouse neuromuscular preparations, alpha1-PTH induced muscle membrane depolarization, leading to blockade of synaptic transmission and directly elicited muscle twitches. Also, alpha1-PTH caused swelling of differentiated neuroblastoma NG108-15 cells, membrane bleb formation, and disorganization of F-actin. In contrast, similar concentrations of PIN-a had no detectable effects. The cytotoxic actions of alpha1-PTH on mammalian cells may be explained by its ability to induce cationic-selective channels.

Aqueous Vernomia amygdalina extracts alter MCF-7 cell membrane permeability and efflux

Breast cancer is the second leading cause of cancer related deaths of women in the United States. Several treatment strategies have been developed over the past decade to reduce cancer morbidity and mortality rates. While mortality rates have declined in some ethnic populations, the overall cancer incidence continues to grow. Hence, chemotherapeutic agents are needed to improve cancer treatment outcome. Previous studies show that low concentrations (microgram/ml) of water-soluble leaf extracts of a Nigerian edible plant, V. amygdalina (VA), potently retard the proliferative activities of estrogen receptor positive (ER+) human breast cancerous cells (MCF-7) cells in vitro in a concentration-dependent fashion. The anti-proliferative activities of VA are extracellular signal-regulated kinases (1/2) (ERKs (1/2))-dependent. Cell culture and animal model studies, conducted by other investigators using other plant extracts, have also revealed that plant extract components called thionins may be responsible for their anticancer activities. These thionins are believed to interact with the cells in ways that compromise membrane potential/permeability resulting in the alteration of efflux, cytosolic activities, and subsequent cell death. Therefore, we hypothesized that VA exposure may compromise cell membrane as another mode of action to elicit its anticancer activities in MCF-7 cells. The exposure of cells to VA decreased [3H]thymidine uptake in a concentration-dependent (0, 30, and 100 mug/ml VA) manner (p < 0.05) but increased [3H]thymidine release, expressed as percent of [3H]thymidine incorporated, into the medium (p < 0.05). The amount of [3H]thymidine released into the medium was 1.7, 7.4, and 11.0 % for 0, 30, and 100 mug/ml VA respectively. Thus suggesting the membranes in VA-treated cells were compromised in a concentration-dependent fashion.

Antifungal effects and mechanism of action of viscotoxin A3

Viscotoxins are cationic proteins, isolated from different mistletoe species, that belong to the group of thionins, a group of basic cysteine-rich peptides of approximately 5 kDa. They have been shown to be cytotoxic to different types of cell, including animal, bacterial and fungal. The aim of this study was to obtain information on the cell targets and the mechanism of action of viscotoxin isoform A3 (VtA3). We describe a detailed study of viscotoxin interaction with fungal-derived model membranes, its location inside spores of Fusarium solani, as well as their induced spore death. We show that VtA3 induces the appearance of ion-channel-like activity, the generation of H2O2, and an increase in cytoplasmic free Ca2+. Moreover, we show that Ca2+ is involved in VtA3-induced spore death and increased H2O2 concentration. The data presented here strongly support the notion that the antifungal activity of VtA3 is due to membrane binding and channel formation, leading to destabilization and disruption of the plasma membrane, thereby supporting a direct role for viscotoxins in the plant defence mechanism.

Crystal structure of α-hordothionin at 1.9 Å resolution

Crystal structure of ubiquitous toxin from barley alpha-hordothionin (alpha-HT) has been determined at 1.9A resolution by X-ray crystallography. The primary sequence as well as the NMR solution structure of alpha-HT firmly established that alpha-HT belongs to a family of membrane active plant toxins-thionins. Since alpha-HT crystallized in a space group (P4(1)2(1)2) that is different from the space group (I422) of previously determined alpha(1)- and beta-purothionins, and visocotoxin A3, therefore, it provided independent information on protein-protein interactions that may be relevant to the toxin mechanism. The structure of alpha-HT not only confirms overall architectural features (crambin fold) but also provides an additional confirmation of the role for crucial solute molecules, that were postulated to be directly involved in the mechanism of toxicity for thionins.

Structure of beta-purothionin in membranes: a two-dimensional infrared correlation spectroscopy study

Two-dimensional infrared correlation spectroscopy has been used to investigate the structure of beta-purothionin, a small basic protein found in the endosperm of wheat seeds, in the absence and presence of dimyristoylphosphatidylglycerol (DMPG) membranes. To generate the two-dimensional synchronous and asynchronous maps, hydrogen-deuterium exchange of the protein amide protons has been used as an external perturbation. This method has allowed us to separate the different secondary structure elements and side chain contributions in the regions of amide I, II, and II' bands to determine that the relative order of deuteration of the beta-purothionin protons is as follows: turns, asparagines, and lysines > unordered structure and tyrosine > beta-sheet > alpha-helices and arginines. The results also indicate that the protein undergoes significant changes both in secondary structure and in deuteration in the presence of DMPG bilayers. The helical content of beta-purothionin is higher in the presence of the lipid, and the relative order of deuteration is as follows: lysines and arginines > asparagines and beta-sheet > unordered structure and alpha-helices. The inversion in the deuteration order of the arginine residues is assigned to a change of the degree of association of the protein in the membrane. In addition, the results reveal that the part of the protein containing the tyrosine residue interacts with the lipid membrane. Our results combined with those previously published suggest that the toxicity of beta-purothionin is more associated with the formation of functional channels in cell membranes rather than with a lytic phenomenon.

Proposal for molecular mechanism of thionins deduced from physico-chemical studies of plant toxins

We propose a molecular model for phospholipid membrane lysis by the ubiquitous plant toxins called thionins. Membrane lysis constitutes the first major effect exerted by these toxins that initiates a cascade of cytoplasmic events leading to cell death. X-ray crystallography, solution nuclear magnetic resonance (NMR) studies, small angle X-ray scattering and fluorescence spectroscopy provide evidence for the mechanism of membrane lysis. In the crystal structures of two thionins in the family, alpha(1)- and beta-purothionins (MW: approximately 4.8 kDa), a phosphate ion and a glycerol molecule are modeled bound to the protein. (31)P NMR experiments on the desalted toxins confirm phosphate-ion binding in solution. Evidence also comes from phospholipid partition experiments with radiolabeled toxins and with fluorescent phospholipids. This data permit a model of the phospholipid-protein complex to be built. Further, NMR experiments, one-dimensional (1D)- and two-dimensional (2D)-total correlation spectroscopy (TOCSY), carried out on the model compounds glycerol-3-phosphate (G3P) and short chain phospholipids, supported the predicted mode of phospholipid binding. The toxins' high positive charge, which renders them extremely soluble (>300 mg/mL), and the phospholipid-binding specificity suggest the toxin-membrane interaction is mediated by binding to patches of negatively charged phospholipids [phosphatidic acid (PA) or phosphatidyl serine (PS)] and their subsequent withdrawal. The formation of proteolipid complexes causes solubilization of the membrane and its lysis. The model suggests that the oligomerization may play a role in toxin's activation process and provides insight into the structural principles of protein-membrane interactions.

Mistletoe viscotoxins induce membrane permeabilization and spore death in phytopathogenic fungi

Viscotoxins (Vts) are basic peptides expressed in mistletoe leaves, seeds and stems which have been shown to be cytotoxic to mammalian cells. The aim of this study was to analyse whether Vts were able to control and/or inhibit the growth of phytopathogenic fungi to obtain a clue to their biological function. Incubation of two Vt isoforms, VtA(3) and VtB, at a final concentration of 10 micro M resulted in a complete blockage of the germination of spores from three different pathogenic fungi. It was also shown that lower concentrations than 10 micro M of VtA(3) and VtB inhibit their mycelial growth in a dose-dependent manner. The protein dose required to inhibit the growth of Fusarium solani and Sclerotinia sclerotiorum to a 50% was between 1.5 and 3.75 micro M, which represents a potent activity. No significant differences in the antifungal potency for each Vt isoform, either VtA(3) and VtB, were observed, although they have been shown to exert differential cytotoxicity on mammalian cells. It was also demonstrated that Vts act as fungicidal compounds. To explore the basis of the antifungal activity the ability of VtA(3) to induce changes in membrane permeability and on the oxidative status of F. solani spores was analysed. By using a specific fluorescent probe on intact spores, it was demonstrated that VtA(3) produces rapid changes in fungal membrane permeability. It also induces H(2)O(2) production verified by a histochemical staining. The data presented in this study support a direct role of Vts in the plant defence determined by their lethal effect on fungal pathogens.

Puroindolines form ion channels in biological membranes

Wheat seeds contain different lipid binding proteins that are low molecular mass, basic and cystine-rich proteins. Among them, the recently characterized puroindolines have been shown to inhibit the growth of fungi in vitro and to enhance the fungal resistance of plants. Experimental data, using lipid vesicles, suggest that this antimicrobial activity is related to interactions with cellular membranes, but the underlying mechanisms are still unknown. This paper shows that extracellular application of puroindolines on voltage-clamped Xenopus laevis oocytes induced membrane permeabilization. Electrophysiological experiments, on oocytes and artificial planar lipid bilayers, suggest the formation, modulated by voltage, of cation channels with the following selectivity: Cs(+) > K(+) > Na(+) > Li(+) > choline = TEA. Furthermore, this channel activity was prevented by addition of Ca(2+) ions in the medium. Puroindolines were also able to decrease the long-term oocyte viability in a voltage-dependent manner. Taken together, these results indicate that channel formation is one of the mechanisms by which puroindolines exert their antimicrobial activity. Modulation of channel formation by voltage, Ca(2+), and lipids could introduce some selectivity in the action of puroindolines on natural membranes.

Interaction between beta-Purothionin and dimyristoylphosphatidylglycerol: a (31)P-NMR and infrared spectroscopic study

The interaction of beta-purothionin, a small basic and antimicrobial protein from the endosperm of wheat seeds, with multilamellar vesicles of dimyristoylphosphatidylglycerol (DMPG) was investigated by (31)P solid-state NMR and infrared spectroscopy. NMR was used to study the organization and dynamics of DMPG in the absence and presence of beta-purothionin. The results indicate that beta-purothionin does not induce the formation of nonlamellar phases in DMPG. Two-dimensional exchange spectroscopy shows that beta-purothionin decreases the lateral diffusion of DMPG in the fluid phase. Infrared spectroscopy was used to investigate the perturbations, induced by beta-purothionin, of the polar and nonpolar regions of the phospholipid bilayers. At low concentration of beta-purothionin, the temperature of the gel-to-fluid phase transition of DMPG increases from 24 degrees C to ~33 degrees C, in agreement with the formation of electrostatic interactions between the cationic protein and the anionic phospholipid. At higher protein concentration, the lipid transition is slightly shifted toward lower temperature and a second transition is observed below 20 degrees C, suggesting an insertion of the protein in the hydrophobic core of the lipid bilayer. The results also suggest that the presence of beta-purothionin significantly modifies the lipid packing at the surface of the bilayer to increase the accessibility of water molecules in the interfacial region. Finally, orientation measurements indicate that the alpha-helices and the beta-sheet of beta-purothionin have tilt angles of ~60 degrees and 30 degrees, respectively, relative to the normal of the ATR crystal.

Mistletoe viscotoxins increase natural killer cell-mediated cytotoxicity

Mistletoe extracts have immunomodulatory activity. We show that nontoxic concentrations of Viscum album extracts increase natural killer (NK) cell-mediated killing of tumor cells but spare nontarget cells from NK lysis. The compounds responsible for this bioactivity were isolated from mistletoe and characterized. They have low molecular mass and are thermostable and protease-resistant. After complete purification by HPLC, they were identified by tandem MS as viscotoxins A1, A2 and A3 (VTA1, VTA2 and VTA3, respectively). Whereas micromolar concentrations of these viscotoxins are cytotoxic to the targets, the bioactivity with respect to NK lysis is within the nanomolar range and differs between viscotoxin isoforms: VTA1 (85 nm), VTA2 (18 nm) and VTA3 (8 nm). Microphysiometry and assays of cell killing indicate that, within such nontoxic concentrations, viscotoxins do not activate NK cells, but act on cell conjugates to increase the resulting lysis.

Plant defense peptides

Eight families of antimicrobial peptides, ranging in size from 2 to 9 kD, have been identified in plants. These are thionins, defensins, so-called lipid transfer proteins, hevein- and knottin-like peptides, MBP1, IbAMP, and the recently reported snakins. All of them have compact structures that are stabilized by 2-6 disulfide bridges. They are part of both permanent and inducible defense barriers. Transgenic overexpression of the corresponding genes leads to enhanced tolerance to pathogens, and peptide-sensitive pathogen mutants have reduced virulence.

Permeabilization of fungal membranes by plant defensins inhibits fungal growth

We used an assay based on the uptake of SYTOX Green, an organic compound that fluoresces upon interaction with nucleic acids and penetrates cells with compromised plasma membranes, to investigate membrane permeabilization in fungi. Membrane permeabilization induced by plant defensins in Neurospora crassa was biphasic, depending on the plant defensin dose. At high defensin levels (10 to 40 microM), strong permeabilization was detected that could be strongly suppressed by cations in the medium. This permeabilization appears to rely on direct peptide-phospholipid interactions. At lower defensin levels (0.1 to 1 microM), a weaker, but more cation-resistant, permeabilization occurred at concentrations that correlated with the inhibition of fungal growth. Rs-AFP2(Y38G), an inactive variant of the plant defensin Rs-AFP2 from Raphanus sativus, failed to induce cation-resistant permeabilization in N. crassa. Dm-AMP1, a plant defensin from Dahlia merckii, induced cation-resistant membrane permeabilization in yeast (Saccharomyces cerevisiae) which correlated with its antifungal activity. However, Dm-AMP1 could not induce cation-resistant permeabilization in the Dm-AMP1-resistant S. cerevisiae mutant DM1, which has a drastically reduced capacity for binding Dm-AMP1. We think that cation-resistant permeabilization is binding site mediated and linked to the primary cause of fungal growth inhibition induced by plant defensins.

Induction of mitochondrial Apo2.7 molecules and generation of reactive oxygen-intermediates in cultured lymphocytes by the toxic proteins from Viscum album L

We analysed mitochondrial alterations in human lymphocytes incubated with toxins exerting RNA and/or protein synthesis/transport inhibitory activity. We found that all toxins known to affect macromolecule synthesis, such as ricin from Ricinus communis, mistletoe lectin I (ML I) from Viscum album, cycloheximide, actinomycin D, and brefeldin A but also the thionins from Viscum album (viscotoxins; VT) generated reactive oxygen intermediates (ROI) and induced expression of newly described mitochondrial membrane proteins Apo2.7, however, with different kinetics. Apart from a rapid permeabilisation of cell membranes by the VT with swelling of mitochondria, loss of their cristae and ROI generation within 2-4 h, the majority of the cells may have received a distinct 'death signal' resulting in an induction of Apo2.7 molecules within 24 h. In contrast, protein synthesis/transport inhibition may signal for apoptosis within 24 h by decreasing distinct 'survival promotors' which remain to be characterised.

Solution conformation of brazzein by 1H nuclear magnetic resonance: resonance assignment and secondary structure

Brazzein is a sweet-tasting protein isolated from the fruit of the West African plant Pentadiplandra brazzeana Baillon. It is the smallest and the most water-soluble sweet protein discovered so far, it is also highly thermostable. The proton NMR study of brazzein at 600 MHz (pH 3.5, 300K) is presented. Complete sequence specific assignment of the individual backbone and sidechain proton resonances were achieved using through-bond and through-space connectivities obtained from standard two-dimensional NMR techniques. The secondary structure of brazzein contains one alpha-helix (residues 21-29), one short 3(10)-helix (residues 14-17), two strands of antiparallel beta-sheet (residues 34-39, 44-50) and probably a third strand (residues 5-7) near the N-terminus.

Accidental cell death and generation of reactive oxygen intermediates in human lymphocytes induced by thionins from Viscum album L

The cytotoxic mechanisms of thionins from Viscum album L., the viscotoxins, were investigated in human granulocytes and lymphocytes. The time course of viscotoxin effects indicate accidental cell death, i.e. membrane permeabilization, degradation of cytoplasm and chromatin, swelling of mitochondria with loss of their cristae, and generation of reactive oxygen intermediates within 1-2 h, followed by secondary apoptosis-associated events. The viscotoxin homologue purothionin from whole-wheat flour and viscotoxin B, however, did not induce cell death in cultured lymphocytes. Cytotoxicity of cationic and amphipathic viscotoxin was prevented only by cleavage of its disulphide bridges.

Characterisation of granulocyte stimulation by thionins from European mistletoe and from wheat

Thionins are small basic peptides found in different plant species, which are known to exert cytotoxic properties. In addition, previous data indicated an activation of human granulocytes by thionins from European mistletoe (viscotoxins, VT). To extend these latter findings, we investigated the influence of VT and from thionins from wheat flour (purothionin) on human granulocytes by flow cytometry and tried to characterise the involved molecular structures and mechanisms. Phagocytosis was determined by incorporation of FITC-labelled Escherichia coli and respiratory burst by oxidation of dihydrorhodamine 123 to rhodamine 123. VT and purothionin significantly enhanced E. coli-stimulated phagocytosis and respiratory burst at 25 and 250 microgram/ml. Phagocytosis of damaged lymphocytes by granulocytes was detected by electron microscopy in the VT-stimulated (100 microgram/ml) but not in the control cultures. The poly-cationic structure of the intact molecule seems to be crucial, as evidenced by comparison of the burst and phagocytosis-enhancing effects induced by other poly-cationic (protamine sulphate, histone, poly-l-arginine, poly-l-lysine) and poly-anionic (poly-l-glutamic acid) peptides, while pore forming due to amphipathic properties seems to be less important. Ca2+ and Mg2+ could not inhibit VT-enhanced phagocytosis and, thus, could not inhibit binding of VT to granulocytes. In addition, verapamil at low concentrations inhibited VT activity, suggesting the involvement of Ca2+ channels for granulocyte activation by the VT. Similarly, thionins and histones in contrast to protamine sulphate induced cell death of granulocytes at 250 microgram/ml as demonstrated by an enhanced release of reactive oxygen intermediates in unstimulated granulocytes. From these data one may suggest that activity of VT is induced by strong unspecific ionic binding, probably followed by specific receptor binding, and thionins exhibit stimulatory and cytotoxic effects on immune cells, which have to be further characterised.

Interaction of wheat alpha-thionin with large unilamellar vesicles

The interaction of the wheat antibacterial peptide alpha-thionin with large unilamellar vesicles has been investigated by means of fluorescence spectroscopy. Binding of the peptide to the vesicles is followed by the release of vesicle contents, vesicle aggregation, and lipid mixing. Vesicle fusion, i.e., mixing of the aqueous contents, was not observed. Peptide binding is governed by electrostatic interactions and shows no cooperativity. The amphipatic nature of wheat alpha-thionin seems to destabilize the membrane bilayer and trigger the aggregation of the vesicles and lipid mixing. The presence of distearoylphosphatidylethanolamine-poly(ethylene glycol 2000) (PEG-PE) within the membrane provides a steric barrier that inhibits vesicle aggregation and lipid mixing but does not prevent leakage. Vesicle leakage through discrete membrane channels is unlikely, because the release of encapsulated large fluorescent dextrans is very similar to that of 8-aminonaphthalene-1,3,6,trisulfonic acid (ANTS). A minimum number of 700 peptide molecules must bind to each vesicle to produce complete leakage, which suggests a mechanism in which the overall destabilization of the membrane is due to the formation of transient pores rather than discrete channels.

Mechanisms by which thionin induces susceptibility of S49 cell membranes to extracellular phospholipase A2

Whereas cells normally resist attack by PLA2, they become susceptible under certain pathological conditions. To ascertain the regulatory mechanisms that induce cellular susceptibility to PLA2, the effect of thionin on S49 cells was examined in the presence of PLA2. Thionin alone was unable to evoke hydrolysis of the lipid bilayer. Likewise, the addition of PLA2 alone caused production of only a minimal amount of free fatty acid. However, thionin and PLA2 together resulted in significant hydrolysis of the cell membrane. Thionin caused perturbation of the bilayer structure as suggested by the changes in the emission spectra of laurdan and the permeability of the membrane to propidium iodide. These changes correlated quantitatively with the susceptibility of the lipid bilayer to PLA2. Furthermore, thionin induced a modest increase in intracellular Ca2+. The source of this Ca2+ was the extracellular fluid since EDTA in the extracellular medium inhibited the Ca2+ influx. Moreover, cobalt chloride, a universal Ca2+ channel blocker, prevented the rise in intracellular Ca2+, the uptake of propidium iodide, and the susceptibility to PLA2 induced by thionin. In contrast, the changes in the laurdan emission caused by the thionin were not affected by the cobalt. Furthermore, incubation of the cells with the calcium ionophore A23187 also caused the cells to become susceptible to PLA2. We hypothesize that thionin causes S49 cell membranes to become susceptible to PLA2 by a Ca2+-dependent perturbation of the bilayer structure.

Mutants of Ralstonia (Pseudomonas) solanacearum sensitive to antimicrobial peptides are altered in their lipopolysaccharide structure and are avirulent in tobacco

Ralstonia solanacearum K60 was mutagenized with the transposon Tn5, and two mutants, M2 and M88, were isolated. Both mutants were selected based on their increased sensitivity to thionins, and they had the Tn5 insertion in the same gene, 34 bp apart. Sequence analysis of the interrupted gene showed clear homology with the rfaF gene from Escherichia coli and Salmonella typhimurium (66% similarity), which encodes a heptosyltransferase involved in the synthesis of the lipopolysaccharide (LPS) core. Mutants M2 and M88 had an altered LPS electrophoretic pattern, consistent with synthesis of incomplete LPS cores. For these reasons, the R. solanacearum gene was designated rfaF. The mutants were also sensitive to purified lipid transfer proteins (LTPs) and to an LTP-enriched, cell wall extract from tobacco leaves. Mutants M2 and M88 died rapidly in planta and failed to produce necrosis when infiltrated in tobacco leaves or to cause wilting when injected in tobacco stems. Complemented strains M2* and M88* were respectively obtained from mutants M2 and M88 by transformation with a DNA fragment harboring gene rfaF. They had a different degree of wild-type reconstituted phenotype. Both strains retained the rough phenotype of the mutants, and their LPS electrophoretic patterns were intermediate between those of the wild type and those of the mutants.

Amino acid sequence, S-S bridge arrangement and distribution in plant tissues of thionins from Viscum album

The complete primary structure of a cytotoxic 5 kDa polypeptide, viscotoxin A1, isolated from Viscum album L., has been determined by combining classical Edman degradation methodology with advanced mass spectrometric procedures. The same integrated approach allowed correction of the sequence of viscotoxin A2 and definition of the pattern of the disulfide bridges. The arrangement of the cysteine pairing was determined as Cys3-Cys40, Cys4-Cys32 and Cys16-Cys26. The primary structure of viscotoxin A1 shares a high degree of similarity with the known viscotoxins and more generally with the plant alpha- and beta-thionins. The pattern of S-S bridges determined for viscotoxin A2 and A1 is similar to that inferred by X-ray and NMR analysis in crambin and related to that present in alpha-purothionin and beta-hordothionin, thus indicating a highly conserved organization of the S-S pairings within the entire family. This arrangement of S-S bridges describes a peculiar structural motif, indicated as 'concentric motif', which is suggested to stabilize a common structure occurring in various small proteins able to interact with cell membranes. The distribution of the new variant toxin in different mistletoe subspecies was investigated. Viscotoxin A1 is abundant in the seeds of the three European subspecies of V. album whereas it represents a minor component in the shoots.