Studies on purothionin by chemical modifications

Plant Antimicrobial Peptides: Insights into Structure-Function Relationships for Practical Applications

Antimicrobial peptides (AMPs) are short polypeptide molecules produced by multicellular organisms that are involved in host defense and microbiome preservation. In recent years, AMPs have attracted attention as novel drug candidates. However, their successful use requires detailed knowledge of the mode of action and identification of the determinants of biological activity. In this review, we focused on structure-function relationships in the thionins, α-hairpinins, hevein-like peptides, and the unique Ib-AMP peptides isolated from Impatiens balsamina. We summarized the available data on the amino acid sequences and 3D structure of peptides, their biosynthesis, and their biological activity. Special attention was paid to the determination of residues that play a key role in the activity and the identification of the minimal active cores. We have shown that even subtle changes in amino acid sequences can affect the biological activity of AMPs, which opens up the possibility of creating molecules with improved properties, better therapeutic efficacy, and cheaper large-scale production.

Cationic Amphiphilic Peptides: Synthetic Antimicrobial Agents Inspired by Nature

Antimicrobial peptides are ubiquitous in multicellular organisms and have served as defense mechanisms for their successful evolution and throughout their life cycle. These peptides are short cationic amphiphilic polypeptides of fewer than 50 amino acids containing either a few disulfide-linked cysteine residues with a characteristic β-sheet-rich structure or linear α-helical conformations with hydrophilic side chains at one side of the helix and hydrophobic side chains on the other side. Antimicrobial peptides cause bacterial cell lysis either by direct cell-surface damage via electrostatic interactions between the cationic side chains of the peptide and the negatively charged cell surface, or by indirect modulation of the host defense systems. Electrostatic interactions lead to bacterial cell membrane disruption followed by leakage of cellular components and finally bacterial cell death. Because of their unusual mechanism of cell damage, antimicrobial peptides are effective against drug-resistant bacteria and may therefore prove more effective than classical antibiotics in certain cases. Currently, around 3000 natural antimicrobial peptides from six kingdoms (bacteria, archaea, protists, fungi, plants, and animals) have been isolated and sequenced. However, only a few of them are under clinical trials and/or in the commercial development stage for the treatment of bacterial infections caused by antibiotic-resistant bacteria. Moreover, high structural complexity, poor pharmacokinetic properties, and low antibacterial activity of natural antimicrobial peptides hinder their progress in drug development. To overcome these hurdles, researchers have become increasingly interested in modification and nature-inspired synthetic antimicrobial peptides. This review discusses some of the recent studies reported on antimicrobial peptides.

Plant Antimicrobial Peptides: State of the Art, In Silico Prediction and Perspectives in the Omics Era

Even before the perception or interaction with pathogens, plants rely on constitutively guardian molecules, often specific to tissue or stage, with further expression after contact with the pathogen. These guardians include small molecules as antimicrobial peptides (AMPs), generally cysteine-rich, functioning to prevent pathogen establishment. Some of these AMPs are shared among eukaryotes (eg, defensins and cyclotides), others are plant specific (eg, snakins), while some are specific to certain plant families (such as heveins). When compared with other organisms, plants tend to present a higher amount of AMP isoforms due to gene duplications or polyploidy, an occurrence possibly also associated with the sessile habit of plants, which prevents them from evading biotic and environmental stresses. Therefore, plants arise as a rich resource for new AMPs. As these molecules are difficult to retrieve from databases using simple sequence alignments, a description of their characteristics and in silico (bioinformatics) approaches used to retrieve them is provided, considering resources and databases available. The possibilities and applications based on tools versus database approaches are considerable and have been so far underestimated.

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.

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.

The secreted antifungal protein thionin 2.4 in Arabidopsis thaliana suppresses the toxicity of a fungal fruit body lectin from Fusarium graminearum

Plants possess active defense systems and can protect themselves from pathogenic invasion by secretion of a variety of small antimicrobial or antifungal proteins such as thionins. The antibacterial and antifungal properties of thionins are derived from their ability to induce open pore formation on cell membranes of phytopathogens, resulting in release of potassium and calcium ions from the cell. Wheat thionin also accumulates in the cell walls of Fusarium-inoculated plants, suggesting that it may have a role in blocking pathogen infection at the plant cell walls. Here we developed an anti-thionin 2.4 (Thi2.4) antibody and used it to show that Thi2.4 is localized in the cell walls of Arabidopsis and cell membranes of F. graminearum, when flowers are inoculated with F. graminearum. The Thi2.4 protein had an antifungal effect on F. graminearum. Next, we purified the Thi2.4 protein, conjugated it with glutathione-S-transferase (GST) and coupled the proteins to an NHS-activated column. Total protein from F. graminearum was applied to GST-Thi2.4 or Thi2.4-binding columns, and the fungal fruit body lectin (FFBL) of F. graminearum was identified as a Thi2.4-interacting protein. This interaction was confirmed by a yeast two-hybrid analysis. To investigate the biological function of FFBL, we infiltrated the lectin into Arabidopsis leaves and observed that it induced cell death in the leaves. Application of FFBL at the same time as inoculation with F. graminearum significantly enhanced the virulence of the pathogen. By contrast, FFBL-induced host cell death was effectively suppressed in transgenic plants that overexpressed Thi2.4. We found that a 15 kD Thi2.4 protein was specifically expressed in flowers and flower buds and suggest that it acts not only as an antifungal peptide, but also as a suppressor of the FFBL toxicity. Secreted thionin proteins are involved in this dual defense mechanism against pathogen invasion at the plant-pathogen interface.

The role of protein hydrophobicity in thionin-phospholipid interactions: a comparison of α1 and α2-purothionin adsorbed anionic phospholipid monolayers

The plant defence proteins α1- and α2-purothionin (Pth) are type 1 thionins from common wheat (Triticum aestivum). These highly homologous proteins possess characteristics common amongst antimicrobial peptides and proteins, that is, cationic charge, amphiphilicity and hydrophobicity. Both α1- and α2-Pth possess the same net charge, but differ in relative hydrophobicity as determined by C18 reversed phase HPLC. Brewster angle microscopy, X-ray and neutron reflectometry, external reflection FTIR and associated surface pressure measurements demonstrated that α1 and α2-Pth interact strongly with condensed phase 1,2-dipalmitoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DPPG) monolayers at the air/liquid interface. Both thionins disrupted the in-plane structure of the anionic phospholipid monolayers, removing lipid during this process and both penetrated the lipid monolayer in addition to adsorbing as a single protein layer to the lipid head-group. However, analysis of the interfacial structures revealed that the α2-Pth showed faster disruption of the lipid film and removed more phospholipid (12%) from the interface than α1-Pth. Correlating the protein properties and lipid binding activity suggests that hydrophobicity plays a key role in the membrane lipid removal activity of thionins.

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.

Mechanism of β-purothionin antimicrobial peptide inhibition by metal ions: Molecular dynamics simulation study

Wheat beta-purothionin is a highly potent antimicrobial peptide which, however, is inactivated by metal ions. The key structural properties and mechanisms of inhibition of beta-purothionin were investigated for the first time using unconstrained molecular dynamics simulations in explicit water. A series of simulations were performed to determine effects of temperature and the metal ions. Analyses of the unconstrained simulations allowed the experimentally unavailable structural and dynamic details to be unambiguously examined. The global fold and the alpha1 helix of beta-purothionin are thermally stable and not affected by metal ions. In contrast, the alpha2 helix unfolds with shift of temperature from 300 K and in the presence of metal ions. The network of conserved residues including Arg30 and Lys5 is sensitive to environmental changes and triggers unfolding. Loop regions display high flexibility and elevated dynamics, but are affected by metal ions. Our study provides insights into the mechanism of metal ion-based inhibition.

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.

Structural Dissection of a Highly Knotted Peptide Reveals Minimal Motif with Antimicrobial Activity

The increasing occurrence of bacterial resistance to antibiotics is driving a renewed interest on antimicrobial peptides, in the hope that understanding the structural features responsible for their activity will provide leads into new anti-infective drug candidates. Most chemical studies in this field have focused on linear peptides of various eukaryotic origins, rather than on structures with complex folding patterns found also in nature. We have undertaken the structural dissection of a highly knotted, cysteine-rich plant thionin, with the aim of defining a minimal, synthetically accessible, structure that preserves the bioactive properties of the parent peptide. Using efficient strategies for directed disulfide bond formation, we have prepared a substantially simplified (45% size reduction) version with undiminished antimicrobial activity against a representative panel of pathogens. Analysis by circular dichroism shows that the downsized peptide preserves the central double alpha-helix of the parent form as an essential bioactive motif. Membrane permeability and surface plasmon resonance studies confirm that the mechanism of action remains unchanged.

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.

Stimulation of prothrombinase activity by the nonapeptide Thr-Trp-Ala-Arg-Asn-Ser-Tyr-Asn-Val, a segment of a plant thionin

Pyrularia thionin (PT) is a basic 47 amino acid peptide isolated from the nuts of Pyrularia pubera. Its structure and properties have been studied in some detail. Its receptor site is a domain of membrane phosphatidyl serine (PS), where it binds with a relatively high specificity. A segment of its covalent structure, the nonapeptide Thr-Trp-Ala-Arg-Asn-Ser-Tyr-Asn-Val, designated serine nonapeptide (SNP), corresponds to amino acids 7-15 of the thionin, except for the position 12 (Ser), which substitutes for Cys, to give stability. This peptide represents what we consider to be the active site of the thionin, and it also binds to PS domains, but less tightly than thionin does. The peptide has an effect on the prothrombinase assay using the chromophore S2238 to measure the thrombin produced by the prothrombinase complex. It is shown that SNP stimulates the prothrombinase complex activity, instead of inhibiting it, as would be expected if it simply covered the PS sites on the membrane of erythrocyte ghosts, used in the prothrombinase assay. SNP appears to substitute for Va in the prothrombinase complex reaction, in a Ca(2+) independent manner, being even more effective in the absence than in the presence of ghosts. In the clotting system, SNP can also substitute for Factor Va.

Structural characterization of hellethionins from Helleborus purpurascens

Thionins are relatively small-sized multiple-cystine peptides that are probably involved in the plant defense against pathogens. As such, these peptides constitute promising candidates for engineered plant resistance in the agricultural industry. More recently, thionins have been proposed as potential immunotoxins in tumor therapy. In the search for pharmacologically active natural products, a new family of thionins was recently discovered in the roots of Helleborus purpurascens that accordingly were termed hellethionins. The structural characterization by NMR of one representative member of this family, i.e., of hellethionin D, clearly reveals that thionins from different sources share a highly conserved overall fold. In fact, the well-defined 3D structure of hellethionin D is very similar to those reported so far for viscotoxins, purothionins, or crambin, although distinct differences could be detected in the C-terminal portion, especially for loop 36-39. These differences may derive from the unusual distribution of charged residues in the C-terminal half of the peptide sequence compared to other thionins and from the uncommon occurrence of four contiguous threonine residues in loop 36-39. As expected, reduction of the disulfide bonds in hellethionin D leads to complete unfolding, but upon oxidative refolding by air oxygen in the presence of glutathione the correct isomer is recovered in high yields, confirming the very robust fold of this class of bioactive cystine peptides.

Modes of membrane interaction of a natural cysteine-rich peptide: viscotoxin A3

Among the very homologous family of alpha- and beta-thionins, known for their antimicrobial activity, the viscotoxin subfamily differs from other members because it is cytotoxic against tumoral cells but weakly hemolytic. We studied the interactions between the most active of these toxins, viscotoxin A3 (VA3), and model membranes made of phosphatidylcholine and phosphatidylserine (PS), the major zwitterionic and acidic phospholipids found in eukaryotic cells. Monolayer studies showed that electrostatic forces are essential for the interaction and are mainly involved in modulating the embedding of the toxin in the PS head group region. This in turn induces membrane stiffening, as shown by fluorescence polarization assays with 1,6-diphenyl-1,3,5-hexatriene and its derivatives. Moreover, vesicle permeabilization analyses showed that there are two modes of interaction, which are directly related to the stiffening effect and depend on the amount of VA3 bound to the surface of the vesicles. We propose an interaction model in which the embedding of VA3 in the membrane induces membrane defects leading to the gradual release of encapsulated dye. When the surfaces of the vesicles are saturated with the viscotoxin, complete vesicle destabilization is induced which leads to bilayer disruption, all-or-none encapsulated dye release and rearrangement of the vesicles.

The cytotoxic plant protein, beta-purothionin, forms ion channels in lipid membranes

Thionins are small cysteine-containing, amphipathic plant proteins found in seeds and vegetative tissues of a number of plant genera. Many of them have been shown to be toxic to microorganisms such as fungi, yeast, and bacteria and also to mammalian cells. It has been suggested that thionins are present in seeds to protect them, and the germinating seedling, from attack by phytopathogenic microorganisms, but the mechanism by which they kill cells remains unclear. Using electrophysiological measurements, we have shown that beta-purothionin from wheat flour can form cation-selective ion channels in artificial lipid bilayer membranes and in the plasmalemma of rat hippocampal neurons. We suggest that the generalized toxicity of thionins is due to their ability to generate ion channels in cell membranes, resulting in the dissipation of ion concentration gradients essential for the maintenance of cellular homeostasis.

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.

Organ-specific expression of highly divergent thionin variants that are distinct from the seed-specific crambin in the crucifer Crambe abyssinica

Most thionins of higher plants are toxic to various bacteria, fungi, and animal and plant cells. The only known exception is the seed-specific thionin, crambin, of the crucifer Crambe abyssinica. Crambin has no net charge, is very hydrophobic and exhibits no toxicity. In the present work, the organization of the crambin precursor polypeptide was deduced from cD-NA sequences. The precursor shows a domain structure similar to that of the preproprotein of other thionins, which contains a signal peptide, a thionin domain and a C-terminal amino acid extension. Unlike the thionin precursors studied thus far, both the thionin domain and the C-terminal amino acid extension of the crambin precursor have no net charge and are hydrophobic, thus facilitating their interaction, by analogy to that proposed for the corresponding domains of other thionin precursors that have positive and negative charges. The existence of a large number of novel and highly variable thionin variants in Crambe abyssinica has been deduced from cDNA sequences that were amplified by the polymerase chain reaction (PCR) from RNA of seeds, leaves and cotyledons. While the deduced amino acid sequences of the thionin domains of most of these thionin precursor molecules are highly divergent, the two other domains are conserved. Most of the predicted thionin variants are positively charged. The presence of positively charged residues in the thionin domains consistently correlates with the presence of a negatively charged residue in the C-terminal amino acid extension of the various thionin precursors. The different thionin variants are encoded by distinct sets of genes and are expressed in an organ-specific manner.

Thionins: properties, possible biological roles and mechanisms of action

Thionins are low-molecular-weight proteins (M(r) ca. 5000) occurring in seeds, stems, roots and leaves of a number of plant species. The different members of this family of plant proteins show both sequence and structural homology, and are toxic to bacteria, fungi, yeasts and various naked cells in vitro. Toxicity requires an electrostatic interaction of the positively charged thionin with the negatively charged phospholipids making up the membrane, followed by either pore formation or a specific interaction with a certain lipid domain. This domain might be composed of phosphoinositides, which mediate transduction of environmental signals in eukaryotes. Their in vitro toxicity to plant pathogenic bacteria and fungi could reflect a direct role in plant defence, although, in view of the many divergent activities displayed by thionins both in vitro and in vivo, a biological role other than inhibition of microbial growth is equally plausible.

Hordothionins inhibit protein synthesis at the level of initiation in the wheat-germ system

The inhibitory effect of pure alpha and beta hordothionins on protein synthesis directed by pea mRNA has been studied in the wheat-germ translation system. It is demonstrated that a component of the wheat germ counteracts the thionin effect. Formation of polysomes in vitro in the presence of thionin was inhibited to the same extent as the total translation system while run-off translation of isolated polysomes from pea plants was not affected by thionin. These data are consistent with an effect of thionin on the initiation reaction. Analyses of the formation of initiation complexes in the presence and absence of mRNA support this view and show that thionin interferes with the formation of the 43S complex. In accordance with this observation and in contrast to earlier studies no evidence has been obtained for a direct interaction between mRNAs and thionins. The analysis of the translation products also gave no indication for preferential translation of individual mRNAs by the thionin-inhibited translation system. Compared to translation in vitro, exposure of barley protoplasts to thionins showed a less dramatic effect on protein synthesis as measured by incorporation of [35S]methionine into proteins. These data are discussed with respect to the effects of thionins on the plasma membranes as shown previously with animal cell cultures. It is concluded that at least in barley such effects would need higher concentrations of thionins than are required for the inhibition of protein synthesis.

Role of Tyr and Trp in membrane responses of Pyrularia thionin determined by optical and NMR spectra following Tyr iodination and Trp modification

Pyrularia thionin is a strongly basic and bioactive 47 amino acid peptide which contains two Tyr residues at positions 13 and 45 and one Trp at position 8. Limited iodination does not have a significant effect, but prolonged iodination of the peptide leads to progressive inactivation for all known cellular responses (Evans, J. et al. (1989) Proc. natn. Acad. Sci. U.S.A. 86, 5849-5853). 1H NMR spectra of the native Pyrularia thionin show four Tyr bands, two arising from each Tyr residue. One resonance band for the epsilon hydrogens of Tyr 45 disappears early during limited iodination and the band from the delta hydrogens shifts to low field. The two bands corresponding to Tyr 13 remain during limited iodination, but both decrease in intensity during prolonged iodination, with the epsilon hydrogen band decreasing somewhat more. The resonance bands arising from Trp disappear during prolonged iodination. This sequence of reactions is verified by the optical absorbance properties of two small peptide fragments obtained by Staphylococcal V8 protease hydrolysis of thionin which had been iodinated to varying degrees. Limited iodination did not significantly inhibit the thionin's biological activity, yet the fragment from the -COOH terminus showed the conversion of Tyr 45 to diiodoTyr. This treatment did not significantly modify the Tyr 13 or Trp 8 located in the -NH2 terminal fragment. More extensive iodination resulted in a disappearance of Trp 8 absorbance with an accompanying conversion of Tyr 13 to the monoiodo form. Extensive iodination yielded two atoms of iodine in the Tyr 45-containing fragment, and only one atom in the Tyr 13 fragment. The data indicate that Tyr 45 of the native thionin is more readily iodinated, proceeding to the diiodo form without significant loss of activity. Prolonged iodination does not lead to the formation of any diiodoTyr 13, but does lead to modification of Trp 8 and probably formation of monoiodoTyr 13. Modification of Trp 8 with N-bromosuccinimide inhibits the hemolytic activity of the thionin, showing that Trp 8 is necessary for Pyrularia thionin activity. It is most likely Trp 8 modification during prolonged iodination which results in the loss of biological activity.

Crystal structure of a protein-toxin alpha 1-purothionin at 2.5A and a comparison with predicted models

Alpha 1-Purothionin (alpha 1-P), a wheatgerm protein and lytic toxin, has a secondary and tertiary structure similar to that of crambin as revealed by CD and NMR studies. alpha 1-P crystallizes in the tetragonal space group 1422 with unit cell dimensions: a = b = 53.59 and c = 69.79 A. X-ray diffraction data have been measured to 2.5 A Bragg spacing. The crystal structure has been determined by molecular replacement methods, using an energy-minimized alpha 1-P model structure derived from crambin (Whitlow and Teeter: Journal of Biomolecular Structure and Dynamics 2:831-848, 1985, Journal of the American Chemical Society 108:7163-7172, 1986). The energy-minimized model gives a slightly cleaner rotation solution and better refinement against the x-ray data than do the crambin or unminimized alpha 1-P structures. The final crystallographic residual with the data in the 10-2.5 A resolution range is 0.216. The refined alpha 1-P structure has a backbone rms difference of 0.74 A from crambin and 0.55 A from the energy-minimized alpha 1-P model. A low resolution NMR model of alpha 1-P calculated from metric matrix distance geometry and restrained molecular dynamics differs from crambin's backbone by 2.3 A rms deviation (Clore et al.: EMBO Journal 5:2729-2735, 1986). Backbone dihedral angles for our predicted model differ from the refined alpha 1-P structure in only one region (at a turn where there is a deletion relative to crambin). The NMR model had differences in four regions.

Conformational energy analysis of the pentapeptide Ac-Arg-Asn-Cys-Tyr-Asn-NMA from alpha 1-purothionin

Conformational energy analyses were carried out on the pentapeptide RNCYN (Ac-Arg-Asn-Cys-Tyr-Asn-NMA) and on related peptides. RNCYN is a highly conserved amino acid sequence in thionins and viscotoxins. The conformation of the pentapeptide was calculated to be an amphipathic alpha-helix, with the tyrosine and cysteine on the nonpolar side of the helix and the arginine and both asparagines on the polar side. Our results are inconsistent with the conformation determined using the Chou-Fasman prediction method, but are consistent with the conformation determined experimentally (using n.m.r.) for this pentapeptide sequence in alpha 1-purothionin.

Purothionin from wheat endosperm reversibly blocks myogenic differentiation of chick embryonic muscle cells in culture

Purothionin from wheat endosperm is a cysteine-rich, basic polypeptide of about 5000 Da, which modifies membrane permeability of cultured mammalian cells. This peptide was found to block fusion of chick embryonic muscle cells in culture but allows proliferation and alignment. A purothionin concentration of 6 micrograms/ml (1.2 microM) was necessary for the complete prevention of myotube formation. Under similar conditions, incorporation of [35S]methionine occurred normally but the synthesis of muscle-specific proteins including creatine kinase and acetylcholine receptor was strongly inhibited. In addition, purothionin blocked the uptake of 86Rb+, immediately after its addition to the cultured myoblasts. No such effects were found with the purothionin chemically modified with acetic or succinic anhydride. Thus, the basic residues in purothionin appear to be associated with the inhibition of myogenic differentiation. These results suggest that purothionin exerts its regulatory effect on the transition from proliferative to differentiative myoblasts by interfering with membrane permeability or intercellular contact and recognition, which are necessary for the initiation of muscle differentiation.

Action of a thionin isolated from nuts of Pyrularia pubera on human erythrocytes

Pyrularia thionin is a strongly basic peptide of 47 amino acids isolated from Pyrularia pubera. This peptide, a member of the thionin family, is hemolytic, cytotoxic and neurotoxic. The characteristics of the hemolytic activity on human erythrocytes are as follows: (1) the peptide does not itself have any phospholipase activity in a micellar assay system with egg yolk phosphatidylcholine, as evidenced by a lack of pH change or uptake of oxygen in the presence of lipoxidase; (2) erythrocyte membranes treated with thionin, however, show a low level of oxygen uptake in the presence of lipoxidase as a consequence of fatty acid release, and this activity is synergistic with that of bee venom phospholipase A2; (3) hemolysis caused by thionin is synergistic with added bee venom phospholipase A2; (4) kinetic analysis of the hemolytic assay reveals that the reaction follows Michaelis-Menten kinetics, being saturable with thionin with a Km of 1.6 microM; (5) binding studies with 125I-thionin show by Scatchard analysis a Kd value of 2.1 microM; (6) although iodinated thionin is inactive in the hemolysis assay, it acts as a competitive inhibitor to native thionin in the hemolytic assay; the inhibitor constant, Ki, for this reaction is 7.0 microM; and (7) Ca2+ above 1 mM inhibits the reaction. All the data are consistent with thionin binding to a receptor, most likely a protein, on the erythrocyte membrane, leading to the release of free fatty acids, most likely by activation of phospholipase A2. The release of fatty acids is itself not sufficient to explain the hemolytic reaction.