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

The emerging role of cysteine-rich peptides in pollen-pistil interactions

Unlike early land plants, flowering plants have evolved a pollen tube that transports a pair of non-motile sperm cells to the female gametophyte. This process, known as siphonogamy, was first observed in gymnosperms and later became prevalent in angiosperms. However, the precise molecular mechanisms underlying the male-female interactions remain enigmatic. From the landing of the pollen grain on the stigma to gamete fusion, the male part needs to pass various tests: how does the stigma distinguish between compatible and incompatible pollen? what mechanisms guide the pollen tube towards the ovule? what factors trigger pollen tube rupture? how is polyspermy prevented? and how does the sperm cell ultimately reach the egg? Successful male-female communication is essential for surmounting these challenges, with cysteine-rich peptides (CRPs) playing a pivotal role in this dialogue. In this review, we summarize the characteristics of four distinct classes of CRPs, systematically review recent progress in the role of CRPs in four crucial stages of pollination and fertilization, consider potential applications of this knowledge in crop breeding, and conclude by suggesting avenues for future research.

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.

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.

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.

Deciphering a mechanism of membrane permeabilization by α-hordothionin peptide

α-Hordothionin (αHTH) belongs to thionins, the plant antimicrobial peptides with membrane-permeabilizing activity which is associated with broad-range antimicrobial activity. Experimental data have revealed a phospholipid-binding site and indicated formation of ion channels as well as membrane disruption activity of thionin. However, the mechanism of membrane permeabilization by thionin remained unknown. Here it is shown that thionin is a small water-selective channel. Unbiased high-precision molecular modeling revealed formation of a water-selective pore running through the αHTH double α-helix core when the peptide interacted with anions. Anion-induced unfolding of the C-end of the α2-helix opened a pore mouth. The pore started at the α2 C-end between the hydrophilic and the hydrophobic regions of the peptide surface and ended in the middle of the unique hydrophobic region at the C-end of the α1-helix. Highly conserved residues including cysteines and tyrosine lined the pore walls. A large positive electrostatic potential accumulated inside the pore. The narrow pore was, nonetheless, sufficient to accommodate at least one water molecule along the channel except for two constriction sites. Both constriction sites were formed by residues participating in the phospholipid-binding site. The channel properties resembled that of aquaporins with two selectivity filters, one at the entrance, inside the α2 C-end cavity, and a second in the middle of the channel. It is proposed that the αHTH water channel delivers water molecules to the bilayer center that leads to local membrane disruption. The proposed mechanism of membrane permeabilization by thionins explains seemingly controversial experimental data.

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.

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.

The role of tryptophan residues in the hemolytic activity of stonustoxin,a lethal factor from stonefish (Synanceja horrida) venom

Stonustoxin (SNTX) is a pore-forming cytolytic lethal factor, isolated from the venom of the stonefish Synanceja horrida, that has potent hemolytic activity. The role of tryptophan residues in the hemolytic activity of SNTX was investigated. Oxidation of tryptophan residues of SNTX with N-bromosuccinimide (NBS) resulted in loss of hemolytic activity. Binding of 8-anilino-1-naphthalenesulphonate (ANS) to SNTX resulted in occlusion of tryptophan residues that resulted in loss of hemolytic activity. Circular dichroism and fluorescence studies indicated that ANS binding resulted in a conformational change of SNTX, in particular, a relocation of surface tryptophan residues to the hydrophobic interior. NBS-modification resulted in oxidised surface tryptophan residues that did not relocate to the hydrophobic interior. These results suggest that native surface tryptophan residues play a pivotal role in the hemolytic activity of STNX, possibly by being an essential component of a hydrophobic surface necessary for pore-formation. This study is the first report on the essentiality of tryptophan residues in the activity of a lytic and lethal factor from a fish venom.

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.

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.

Pyrularia thionin binding to and the role of tryptophan-8 in the enhancement of phosphatidylserine domains in erythrocyte membranes

Pyrularia thionin is a small, strongly basic peptide which interacts readily with cellular and synthetic membranes. With cells it induces hemolysis, depolarizes the cellular membrane with an accompanying influx of Ca2+, and activates an endogenous phospholipase A2. Evidence points toward a binding site involving phosphatidylserine (PS). This study shows that addition of the peptide to erythrocyte membranes as well as to vesicles formed from phospholipids isolated from erythrocyte membranes causes an enhancement of phospholipid domains which are made visible by the use of fluorescence digital imaging microscopy with fluorescent derivatives of PS (NBD-PS) and phosphatidylcholine (NBD-PC). Addition of thionin caused a large increase in NBD-PS domains, with an accompanying enrichment of NBD-PC in another separate domain. Double-labeling experiments performed with a Texas Red derivative of thionin show that the peptide binds to the domain enriched in NBD-PS. P thionin inactivated by modification of Trp-8 with N-bromosuccinimide lost the ability to enhance PS domains, although it bound to the membrane with the same affinity as native P thionin. This shows that binding to the membrane is not in itself sufficient to cause the NBD-PS and NBD-PC redistribution into domains.