Solution structure of Pisum sativum defensin 1 by high resolution NMR: plant defensins, identical backbone with different mechanisms of action

Identification of the Plant Defensin (MsPDF) Gene Family in Medicago sativa and Analysis of Expression Patterns Under Abiotic Stress

Medicago sativa L. (alfalfa) is a major forage crop due to its high yield and stress resilience. However, its growth and productivity are often compromised by abiotic stresses, including cold, drought, and salinity. The plant defensin (PDF) gene family plays a crucial role in resistance to abiotic stress. In this study, a total of 11 MsPDF gene family members were identified in the alfalfa genome and classified into three groups. Phylogenetic and conserved motif analyses revealed that the MsPDF genes are highly conserved. Promoter analysis, gene regulatory network analysis (GRN), and gene ontology (GO)-enrichment analyses were used to infer the potential functions of MsPDF genes. The results showed that the gene actively responds to abiotic stress, participates in phytohormonal responses, and regulates plant growth and development through gene interactions. Transcriptome and qRT-PCR analyses showed that most of the MsPDF genes were significantly up-regulated under cold, drought, and salinity stresses. Among them, the MsPDF03 exhibited superior performance under cold stress. The MsPDF04, MsPDF08, and MsPDF09 genes were able to respond positively to drought and salt stresses. Finally, the monomeric, dimeric, and tetrameric structures of the proteins encoded by the MsPDF genes were predicted using AlphaFold 2 software. This study lays the foundation for the identification and evolutionary relationship analysis of the MsPDF gene family, and provides a new reference for subsequent research on abiotic stress resistance.

Plant peptides - redefining an area of ribosomally synthesized and post-translationally modified peptides

Covering 1965 to February 2024Plants are prolific peptide chemists and are known to make thousands of different peptidic molecules. These peptides vary dramatically in their size, chemistry, and bioactivity. Despite their differences, all plant peptides to date are biosynthesized as ribosomally synthesized and post-translationally modified peptides (RiPPs). Decades of research in plant RiPP biosynthesis have extended the definition and scope of RiPPs from microbial sources, establishing paradigms and discovering new families of biosynthetic enzymes. The discovery and elucidation of plant peptide pathways is challenging due to repurposing and evolution of housekeeping genes as both precursor peptides and biosynthetic enzymes and due to the low rates of gene clustering in plants. In this review, we highlight the chemistry, biosynthesis, and function of the known RiPP classes from plants and recommend a nomenclature for the recent addition of BURP-domain-derived RiPPs termed burpitides. Burpitides are an emerging family of cyclic plant RiPPs characterized by macrocyclic crosslinks between tyrosine or tryptophan side chains and other amino acid side chains or their peptide backbone that are formed by copper-dependent BURP-domain-containing proteins termed burpitide cyclases. Finally, we review the discovery of plant RiPPs through bioactivity-guided, structure-guided, and gene-guided approaches.

Fighting pathogenic yeasts with plant defensins and anti-fungal proteins from fungi

Fungal infections represent a significant health risk worldwide. Opportunistic infections caused by yeasts, particularly by Candida spp. and their virulent emerging isolates, have become a major threat to humans, with an increase in fatal cases of infections attributed to the lack of effective anti-yeast therapies and the emergence of fungal resistance to the currently applied drugs. In this regard, the need for novel anti-fungal agents with modes of action different from those currently available is undeniable. Anti-microbial peptides (AMPs) are promising candidates for the development of novel anti-fungal biomolecules to be applied in clinic. A class of AMPs that is of particular interest is the small cysteine-rich proteins (CRPs). Among CRPs, plant defensins and anti-fungal proteins (AFPs) of fungal origin constitute two of the largest and most promising groups of CRPs showing anti-fungal properties, including activity against multi-resistant pathogenic yeasts. In this review, we update and compare the sequence, structure, and properties of plant defensins and AFPs with anti-yeast activity, along with their in vitro and in vivo potency. We focus on the current knowledge about their mechanism of action that may lead the way to new anti-fungals, as well as on the developments for their effective biotechnological production. KEY POINTS: • Plant defensins and fungal AFPs are alternative anti-yeast agents • Their multi-faceted mode of action makes occurrence of resistance rather improbable • Safe and cost-effective biofactories remain crucial for clinical application.

Antimicrobial Peptides with Anti-Candida Activity

Mycoses are accountable for millions of infections yearly worldwide. Invasive candidiasis is the most usual, presenting a high morbidity and mortality. Candida albicans remains the prevalent etiologic agent, but the incidence of other species such as Candida parapsilosis, Candida glabrata and Candida auris keeps increasing. These pathogens frequently show a reduced susceptibility to commonly used antifungal drugs, including polyenes, triazoles and echinocandins, and the incidence of emerging multi-drug-resistant strains of these species continues to increase. Therefore, the need to search for new molecules that target these pathogenic species in a different manner is now more urgent than ever. Nature is an almost endless source of interesting new molecules that could meet this need. Among these molecules, antimicrobial peptides, present in different sources in nature, possess some advantages over conventional antifungal agents, even with their own drawbacks, and are considered as a promising pharmacological option against a wide range of microbial infections. In this review, we describe 20 antimicrobial peptides from different origins that possess an activity against Candida.

Use of Defensins to Develop Eco-Friendly Alternatives to Synthetic Fungicides to Control Phytopathogenic Fungi and Their Mycotoxins

Crops are threatened by numerous fungal diseases that can adversely affect the availability and quality of agricultural commodities. In addition, some of these fungal phytopathogens have the capacity to produce mycotoxins that pose a serious health threat to humans and livestock. To facilitate the transition towards sustainable environmentally friendly agriculture, there is an urgent need to develop innovative methods allowing a reduced use of synthetic fungicides while guaranteeing optimal yields and the safety of the harvests. Several defensins have been reported to display antifungal and even-despite being under-studied-antimycotoxin activities and could be promising natural molecules for the development of control strategies. This review analyses pioneering and recent work addressing the bioactivity of defensins towards fungal phytopathogens; the details of approximately 100 active defensins and defensin-like peptides occurring in plants, mammals, fungi and invertebrates are listed. Moreover, the multi-faceted mechanism of action employed by defensins, the opportunity to optimize large-scale production procedures such as their solubility, stability and toxicity to plants and mammals are discussed. Overall, the knowledge gathered within the present review strongly supports the bright future held by defensin-based plant protection solutions while pointing out the obstacles that still need to be overcome to translate defensin-based in vitro research findings into commercial products.

Membrane-Interacting Antifungal Peptides

The incidence of invasive fungal infections is increasing worldwide, resulting in more than 1.6 million deaths every year. Due to growing antifungal drug resistance and the limited number of currently used antimycotics, there is a clear need for novel antifungal strategies. In this context, great potential is attributed to antimicrobial peptides (AMPs) that are part of the innate immune system of organisms. These peptides are known for their broad-spectrum activity that can be directed toward bacteria, fungi, viruses, and/or even cancer cells. Some AMPs act via rapid physical disruption of microbial cell membranes at high concentrations causing cell leakage and cell death. However, more complex mechanisms are also observed, such as interaction with specific lipids, production of reactive oxygen species, programmed cell death, and autophagy. This review summarizes the structure and mode of action of antifungal AMPs, thereby focusing on their interaction with fungal membranes.

Amino acid-derived defense metabolites from plants: A potential source to facilitate novel antimicrobial development

For millennia, humanity has relied on plants for its medicines, and modern pharmacology continues to reexamine and mine plant metabolites for novel compounds and to guide improvements in biological activity, bioavailability, and chemical stability. The critical problem of antibiotic resistance and increasing exposure to viral and parasitic diseases has spurred renewed interest into drug treatments for infectious diseases. In this context, an urgent revival of natural product discovery is globally underway with special attention directed toward the numerous and chemically diverse plant defensive compounds such as phytoalexins and phytoanticipins that combat herbivores, microbial pathogens, or competing plants. Moreover, advancements in “omics,” chemistry, and heterologous expression systems have facilitated the purification and characterization of plant metabolites and the identification of possible therapeutic targets. In this review, we describe several important amino acid–derived classes of plant defensive compounds, including antimicrobial peptides (e.g., defensins, thionins, and knottins), alkaloids, nonproteogenic amino acids, and phenylpropanoids as potential drug leads, examining their mechanisms of action, therapeutic targets, and structure–function relationships. Given their potent antibacterial, antifungal, antiparasitic, and antiviral properties, which can be superior to existing drugs, phytoalexins and phytoanticipins are an excellent resource to facilitate the rational design and development of antimicrobial drugs.

A systematic structural comparison of all solved small proteins deposited in PDB. The effect of disulfide bonds in protein fold

Defensins are small proteins, usually ranging from 3 to 6 kDa, amphipathic, disulfide-rich, and with a small or even absent hydrophobic core. Since a hydrophobic core is generally found in globular proteins that fold in an aqueous solvent, the peculiar fold of defensins can challenge tertiary protein structure predictors. We performed a Protein Data Bank survey of small proteins (3–6 kDa) to understand the similarities of defensins with other small disulfide-rich proteins. We found no differences when we compared defensins with non-defensins regarding the proportion of apolar, polar and charged residues and their exposure to the solvent. Then we divided all small proteins (3–6 kDa) in the Protein Data Bank into two groups, one group with at least one disulfide bond (bonded, defensins included) and another group without any disulfide bond (unbonded). The group of bonded proteins contained apolar residues more exposed to the solvent than the unbonded group. The ab initio algorithm for tertiary protein structure prediction Robetta was more accurate at predicting unbonded than bonded proteins. On the other hand, the trRosetta algorithm, which uses artificial intelligence, improved the prediction of most bonded proteins, while for the unbonded group no improvement was obtained. Our work highlights one more layer of complexity for the prediction of protein tertiary structure: The ability of small disulfide-rich proteins to fold even with a poorly hydrophobic core.

Cytotoxicity of Snake Venoms and Cytotoxins From Two Southeast Asian Cobras (Naja sumatrana, Naja kaouthia): Exploration of Anticancer Potential, Selectivity, and Cell Death Mechanism

Venoms of cobras (Naja spp.) contain high abundances of cytotoxins, which contribute to tissue necrosis in cobra envenomation. The tissue-necrotizing activity of cobra cytotoxins, nevertheless, indicates anticancer potentials. This study set to explore the anticancer properties of the venoms and cytotoxins from Naja sumatrana (equatorial spitting cobra) and Naja kaouthia (monocled cobra), two highly venomous species in Southeast Asia. The cytotoxicity, selectivity, and cell death mechanisms of their venoms and cytotoxins (NS-CTX from N. sumatrana: NS-CTX; N. kaouthia: NK-CTX) were elucidated in human lung (A549), prostate (PC-3), and breast (MCF-7) cancer cell lines. Cytotoxins were purified through a sequential fractionation approach using cation-exchange chromatography, followed by C₁₈ reverse-phase high-performance liquid chromatography (HPLC) to homogeneity validated with sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and identified by liquid chromatography-tandem mass spectrometry (LCMS/MS). The cobra venoms and their respective cytotoxins exhibited concentration-dependent growth inhibitory effects in all cell lines tested, with the cytotoxins being more potent compared to the corresponding whole venoms. NS-CTX and NK-CTX are, respectively, P-type and S-type isoforms of cytotoxin, based on the amino acid sequences as per LCMS/MS analysis. Both cytotoxins exhibited differential cytotoxic effects in the cell lines tested, with NS-CTX (P-type cytotoxin) being significantly more potent in inhibiting the growth of the cancer cells. Both cytotoxins demonstrated promising selectivity only for the A549 lung cancer cell line (selectivity index = 2.17 and 2.26, respectively) but not in prostate (PC-3) and breast (MCF-7) cancer cell lines (selectivity index < 1). Flow cytometry revealed that the A549 lung cancer cells treated with NS-CTX and NK-CTX underwent necrosis predominantly. Meanwhile, the cytotoxins induced mainly caspase-independent late apoptosis in the prostate (PC-3) and breast (MCF-7) cancer cells lines but lacked selectivity. The findings revealed the limitations and challenges that could be faced during the development of new cancer therapy from cobra cytotoxins, notwithstanding their potent anticancer effects. Further studies should aim to overcome these impediments to unleash the anticancer potentials of the cytotoxins.

Identification of New Antimicrobial Peptides from Mediterranean Medical Plant Charybdis pancration (Steinh.) Speta

The present work was designed to identify and characterize novel antimicrobial peptides (AMPs) from Charybdis pancration (Steinh.) Speta, previously named Urginea maritima, is a Mediterranean plant, well-known for its biological properties in traditional medicine. Polypeptide-enriched extracts from different parts of the plant (roots, leaves and bulb), never studied before, were tested against two relevant pathogens, Staphylococcus aureus and Pseudomonas aeruginosa. With the aim of identifying novel natural AMPs, peptide fraction displaying antimicrobial activity (the bulb) that showed minimum inhibitory concentration (MICs) equal to 30 µg/mL against the above mentioned strains, was analysed by high-resolution mass spectrometry and database search. Seventeen peptides, related to seven proteins present in the investigated database, were described. Furthermore, we focused on three peptides, which due to their net positive charge, have a better chance to be AMPs and they were investigated by molecular modelling approaches, in order to shed light on the solution properties of their equilibrium structures. Some of new detected peptides could represent a good platform for the development of new antimicrobials in the fight against antibiotic resistance phenomenon.

Synthesis, Structure, and Activity of the Antifungal Plant Defensin PvD1

Available treatments for invasive fungal infections have limitations, including toxicity and the emergence of resistant strains. Therefore, there is an urgent need for alternative solutions. Because of their unique mode of action and high selectivity, plant defensins (PDs) are worthy therapeutic candidates. Chemical synthesis remains a preferred method for the production of many peptide-based therapeutics. Given the relatively long sequence of PDs, as well as their complicated posttranslational modifications, the synthetic route can be considered challenging. Here, we describe a total synthesis of PvD₁, the defensin from the common bean Phaseolus vulgaris. Analytical, structural, and functional characterization revealed that both natural and synthetic peptides fold into a canonical CSαβ motif stabilized by conserved disulfide bonds. Moreover, synthetic PvD₁ retained the biological activity against four different Candida species and showed no toxicity in vivo. Adding the high resistance of synthetic PvD₁ to proteolytic degradation, we claim that conditions are now met to consider PDs druggable biologicals.

Plant Defensins from a Structural Perspective

Plant defensins form a family of proteins with a broad spectrum of protective activities against fungi, bacteria, and insects. Furthermore, some plant defensins have revealed anticancer activity. In general, plant defensins are non-toxic to plant and mammalian cells, and interest in using them for biotechnological and medicinal purposes is growing. Recent studies provided significant insights into the mechanisms of action of plant defensins. In this review, we focus on structural and dynamics aspects and discuss structure-dynamics-function relations of plant defensins.

Pisum sativum Defensin 1 Eradicates Mouse Metastatic Lung Nodules from B16F10 Melanoma Cells

Psd1 is a pea plant defensin which can be actively expressed in Pichia pastoris and shows broad antifungal activity. This activity is dependent on fungal membrane glucosylceramide (GlcCer), which is also important for its internalization, nuclear localization, and endoreduplication. Certain cancer cells present a lipid metabolism imbalance resulting in the overexpression of GlcCer in their membrane. In this work, in vitroassays using B16F10 cells showed that labeled fluorescein isothiocyanate FITC-Psd1 internalized into live cultured cells and targeted the nucleus, which underwent fragmentation, exhibiting approximately 60% of cells in the sub-G0/G1 stage. This phenomenon was dependent on GlcCer, and the participation of cyclin-F was suggested. In a murine lung metastatic melanoma model, intravenous injection of Psd1 together with B16F10 cells drastically reduced the number of nodules at concentrations above 0.5 mg/kg. Additionally, the administration of 1 mg/kg Psd1 decreased the number of lung inflammatory cells to near zero without weight loss, unlike animals that received melanoma cells only. It is worth noting that 1 mg/kg Psd1 alone did not provoke inflammation in lung tissue or weight or vital signal losses over 21 days, inferring no whole animal cytotoxicity. These results suggest that Psd1 could be a promising prototype for human lung anti-metastatic melanoma therapy.

Characterization, Expression Profiling, and Functional Analysis of PtDef, a Defensin-Encoding Gene From Populus trichocarpa

PtDef cloned from Populus trichocarpa contained eight cysteine domains specific to defensins. Quantitative reverse-transcription polymerase chain reaction (qRT-PCR) analysis showed that PtDef was expressed in all tissues tested, with lower expression in leaves and higher expression in petioles, stems, and roots. Purified fused PtDef inhibited Aspergillus niger, Alternaria Nees, Mucor corymbifer, Marssonina populi, Rhizopus sp., and Neurospora crassa. PtDef also inhibited the growth of Escherichia coli by triggering autolysis. PtDef overexpression in Nanlin895 poplar (Populus × euramericana cv. Nanlin895) enhanced the level of resistance to Septotinia populiperda. qRT-PCR analysis also showed that the expression of 13 genes related to salicylic acid (SA) and jasmonic acid (JA) signal transduction differed between transgenic and wild-type (WT) poplars before and after inoculation, and that PR1-1 (12–72 h), NPR1-2, TGA1, and MYC2-1 expression was higher in transgenic poplars than in WT. During the hypersensitivity response (HR), large amounts of H₂O₂ were produced by the poplar lines, particularly 12–24 h after inoculation; the rate and magnitude of the H₂O₂ concentration increase were greater in transgenic lines than in WT. Overall, our findings suggest that PtDef, a defensin-encoding gene of P. trichocarpa, could be used for genetic engineering of woody plants for enhanced disease resistance.

Characterization, expression profiling, and functional analysis of a Populus trichocarpa defensin gene and its potential as an anti-Agrobacterium rooting medium additive

The diverse antimicrobial properties of defensins have attracted wide scientific interest in recent years. Also, antimicrobial peptides (AMPs), including cecropins, histatins, defensins, and cathelicidins, have recently become an antimicrobial research hotspot for their broad-spectrum antibacterial and antifungal activities. In addition, defensins play important roles in plant growth, development, and physiological metabolism, and demonstrate tissue specificity and regulation in response to pathogen attack or abiotic stress. In this study, we performed molecular cloning, characterization, expression profiling, and functional analysis of a defensin from Populus trichocarpa. The PtDef protein was highly expressed in the prokaryotic Escherichia coli system as a fusion protein (TrxA–PtDef). The purified protein exhibited strong antibacterial and antifungal functions. We then applied PtDef to rooting culture medium as an alternative exogenous additive to cefotaxime. PtDef expression levels increased significantly following both biotic and abiotic treatment. The degree of leaf damage observed in wild-type (WT) and transgenic poplars indicates that transgenic poplars that overexpress the PtDef gene gain enhanced disease resistance to Septotis populiperda. To further study the salicylic acid (SA) and jasmonic acid (JA) signal transduction pathways, SA- and JA-related and pathogenesis-related genes were analyzed using quantitative reverse-transcription polymerase chain reaction; there were significant differences in these pathways between transgenic and WT poplars. The defensin from Populus trichocarpa showed significant activity of anti-bacteria and anti-fungi. According to the results of qRT-PCR and physiological relevant indicators, the applied PtDef to rooting culture medium was chosen as an alternative exogenous additive to cefotaxime. Overexpressing the PtDef gene in poplar improve the disease resistance to Septotis populiperda.

Isolation and characterisation of the antifungal activity of the cowpea defensin Cp-thionin II

As a result of the rapidly growing human population, reducing post-harvest crop losses of cereals due to microbial pests has major importance. Plant defensins have the potential to fulfil these demands, being highly specific and efficient antimicrobial agents. Hence, this study aimed to extract and characterise a peptide from cowpea seeds and investigate its antifungal performance. After extraction and partial purification, N-terminal sequencing was used to identify the primary peptide in the extract as cowpea-thionin II. Antifungal activity in vitro was found against Fusarium culmorum (MIC = 50 μg/mL), but Aspergillus niger and Penecillium expansum showed an MIC > 500 μg/mL. The extract was resistant against heat treatment (100 °C, 15 min) but lost its antifungal activity in presence of cations (Na⁺, K⁺, Ca²⁺ and Mg²⁺, respectively). Membrane permeabilization of fungal hyphae was evident at 25 μg/mL, while induction of oxidative stress only had minor contribution to the antifungal performance. The extract did not induce haemolysis at all concentrations tested (up to 200 μg/mL). Finally, it was successfully used to protect stored wheat grains from fungal spoilage (determined via ergosterol content) when applied at 100 μg/mL. In conclusion, the defensin Cp-thionin II showed the potential for future application as food bio-preservative.

Medicinal Potentialities of Plant Defensins: A Review with Applied Perspectives

Plant-based secondary metabolites with medicinal potentialities such as defensins are small, cysteine-rich peptides that represent an imperative aspect of the inherent defense system. Plant defensins possess broad-spectrum biological activities, e.g., bactericidal and insecticidal actions, as well as antifungal, antiviral, and anticancer activities. The unique structural and functional attributes provide a nonspecific and versatile means of combating a variety of microbial pathogens, i.e., fungi, bacteria, protozoa, and enveloped viruses. Some defensins in plants involved in other functions include the development of metal tolerance and the role in sexual reproduction, while most of the defensins make up the innate immune system of the plants. Defensins are structurally and functionally linked and have been characterized in various eukaryotic microorganisms, mammals, plants, gulls, teleost species of fish, mollusks, insect pests, arachnidan, and crustaceans. This defense mechanism has been improved biotechnologically as it helps to protect plants from fungal attacks in genetically modified organisms (GMO). Herein, we review plant defensins as secondary metabolites with medicinal potentialities. The first half of the review elaborates the origin, structural variations, and mechanism of actions of plant defensins. In the second part, the role of defensins in plant defense, stress response, and reproduction are discussed with suitable examples. Lastly, the biological applications of plant defensins as potential antimicrobial and anticancer agents are also deliberated. In summary, plant defensins may open a new prospect in medicine, human health, and agriculture.

Overexpression of Chickpea Defensin Gene Confers Tolerance to Water-Deficit Stress in Arabidopsis thaliana

Plant defensins are mainly known for their antifungal activity. However, limited information is available regarding their function in abiotic stresses. In this study, a defensin gene, Ca-AFP, from Cicer arietinum, commonly known as chickpea, was cloned and transformed in Arabidopsis thaliana for its functional characterization under simulated water-deficit conditions. Under simulated water-deficit conditions (mannitol and polyethylene glycol-6000 induced), the transgenic A. thaliana plants had higher accumulation of the Ca-AFP transcript compared to that under non-stress condition and showed higher germination rate, root length, and biomass than the wild-type (WT) plants. To get further insights into the role of Ca-AFP in conferring tolerance to water-deficit stress, we determined various physiological parameters and found significant reduction in the transpiration rate and stomatal conductance whereas the net photosynthesis and water use efficiency was increased in the transgenic plants compared to that in the WT plants under water deficit conditions. The transgenic plants showed enhanced superoxide dismutase, ascorbate peroxidase, and catalase activities, had higher proline, chlorophyll, and relative water content, and exhibited reduced ion leakage and malondialdehyde content under water-deficit conditions. Overall, our results indicate that overexpression of Ca-AFP could be an efficient approach for conferring tolerance to water-deficit stress in plants.

Conformational Dynamics of a Cysteine-Stabilized Plant Defensin Reveals an Evolutionary Mechanism to Expose Hydrophobic Residues

Sugar cane defensin 5 (Sd5) is a small antifungal protein, whose structure is held together by four conserved disulfide bridges. Sd5 and other proteins sharing a cysteine-stabilized α-β (CSαβ) fold lack a regular hydrophobic core. Instead, they are stabilized by tertiary contacts formed by surface-exposed hydrophilic and hydrophobic residues. Despite excessive cross-links, Sd5 exhibits complex millisecond conformational dynamics involving all secondary structure elements. We used Carr-Purcell-Meiboom-Gill (CPMG) NMR relaxation dispersion (RD) measurements performed at different temperatures and denaturant concentrations to probe brief excursions of Sd5 to a sparsely populated "excited" state. Temperature-dependent CPMG RD experiments reveal that the excited state is enthalpically unfavorable, suggesting a rearrangement of stabilizing contacts formed by surface-exposed side chains and/or secondary structure, while the experiments performed at different denaturant concentrations suggest a decrease in accessible surface area of Sd5 in the excited state. The measured backbone 15N chemical shift changes point to a global conformational rearrangement such as a potential α- to β-transition of the Sd5 α-helix or other major secondary structure reorganization and concomitant conformational changes in other parts of the protein. Overall, the emerging picture of Sd5 dynamics suggests this protein can populate two alternative well-ordered conformational states, with the excited conformer being more compact than the native state and having a distinct secondary structure and side-chain arrangements. The observation of an energetically unfavorable yet more compact excited state reveals a remarkable evolution of the CSαβ fold to expose and reorganize hydrophobic residues, which enables the creation of versatile binding sites.

Structural and biological features of a novel plant defensin from Brugmansia x candida

Data from both the laboratory and clinic in the last decade indicate that antimicrobial peptides (AMPs) are widely regarded as potential sources of future antibiotics owing to their broad-spectrum activities, rapid killing, potentially low-resistance rate and multidirectional mechanisms of action compared to conventional antibiotics. Defensins, a prominent family of AMPs, have been found in a wide range of organisms including plants. Thailand is a rich source of plants including medicinal plants used therapeutically, however there is no report of defensin from among these plants. In this study, a novel plant defensin gene, BcDef, was successfully cloned from Brugmansia x candida (Bc). BcDef cDNA was 237 bp in length, encoding 78 amino acids with a putative 31-amino acid residue signal peptide at the N-terminal followed by the mature sequence. BcDef shared high sequence identity (78–85%) with Solanaceae defensins and belonged to the class I plant defensins. From homology modeling, BcDef shared a conserved triple stranded β-sheet (β1-β3) and one α-helix (α1) connected by a loop (L1-L3). BcDef1 peptide, designed from the γ-core motifs of BcDef located in loop 3, showed antibacterial activity against both Gram-positive and Gram-negative pathogens with the lowest MIC (15.70 μM) against Staphylococcus epidermidis. This peptide affected cell membrane potential and permeability, and caused cell membrane disruption. Moreover, BcDef1 also exhibited antioxidant activity and showed low cytotoxicity against mouse fibroblast L929 cells. These findings may provide an opportunity for developing a promising antibacterial agent for medical application in the future.

Disease Resistance Mechanisms in Plants

Plants have developed a complex defense system against diverse pests and pathogens. Once pathogens overcome mechanical barriers to infection, plant receptors initiate signaling pathways driving the expression of defense response genes. Plant immune systems rely on their ability to recognize enemy molecules, carry out signal transduction, and respond defensively through pathways involving many genes and their products. Pathogens actively attempt to evade and interfere with response pathways, selecting for a decentralized, multicomponent immune system. Recent advances in molecular techniques have greatly expanded our understanding of plant immunity, largely driven by potential application to agricultural systems. Here, we review the major plant immune system components, state of the art knowledge, and future direction of research on plant⁻pathogen interactions. In our review, we will discuss how the decentralization of plant immune systems have provided both increased evolutionary opportunity for pathogen resistance, as well as additional mechanisms for pathogen inhibition of such defense responses. We conclude that the rapid advances in bioinformatics and molecular biology are driving an explosion of information that will advance agricultural production and illustrate how complex molecular interactions evolve.

The evolution, function and mechanisms of action for plant defensins

Plant defensins are an extensive family of small cysteine rich proteins characterised by a conserved cysteine stabilised alpha beta protein fold which resembles the structure of insect and vertebrate defensins. However, secondary structure and disulphide topology indicates two independent superfamilies of defensins with similar structures that have arisen via an extreme case of convergent evolution. Defensins from plants and insects belong to the cis-defensin superfamily whereas mammalian defensins belong to the trans-defensin superfamily. Plant defensins are produced by all species of plants and although the structure is highly conserved, the amino acid sequences are highly variable with the exception of the cysteine residues that form the stabilising disulphide bonds and a few other conserved residues. The majority of plant defensins are components of the plant innate immune system but others have evolved additional functions ranging from roles in sexual reproduction and development to metal tolerance. This review focuses on the antifungal mechanisms of plant defensins. The activity of plant defensins is not limited to plant pathogens and many of the described mechanisms have been elucidated using yeast models. These mechanisms are more complex than simple membrane permeabilisation induced by many small antimicrobial peptides. Common themes that run through the characterised mechanisms are interactions with specific lipids, production of reactive oxygen species and induction of cell wall stress. Links between sequence motifs and functions are highlighted where appropriate. The complexity of the interactions between plant defensins and fungi helps explain why this protein superfamily is ubiquitous in plant innate immunity.

Psd1 Effects on Candida albicans Planktonic Cells and Biofilms

Candida albicans is an important human pathogen, causing opportunistic infections. The adhesion of planktonic cells to a substrate is the first step for biofilm development. The antimicrobial peptide (AMP) Psd1 is a defensin isolated from Pisum sativum seeds. We tested the effects of this AMP on C. albicans biofilms and planktonic cells, comparing its activity with amphotericin B and fluconazole. Three C. albicans variants were studied, one of them a mutant deficient in glucosylceramide synthase, conferring resistance to Psd1 antifungal action. Atomic force microscopy (AFM) was used to assess morphological and biomechanical changes on fungal cells. Surface alterations, with membrane disruption and leakage of cellular contents, were observed. Cytometry assays and confocal microscopy imaging showed that Psd1 causes cell death, in a time and concentration-dependent manner. These results demonstrate Psd1 pleiotropic action against a relevant fungal human pathogen, suggesting its use as natural antimycotic agent.

Antifungal plant defensins: increased insight in their mode of action as a basis for their use to combat fungal infections

Plant defensins are small, cationic peptides with a highly conserved 3D structure. They have been studied extensively in the past decades. Various biological activities have been attributed to plant defensins, such as anti-insect and antimicrobial activities, but they are also known to affect ion channels and display antitumor activity. This review focuses on the structure, biological activity and antifungal mode of action of some well-characterized plant defensins, with particular attention to their fungal membrane target(s), their induced cell death mechanisms as well as their antibiofilm activity. As plant defensins are, in general, not toxic to human cells, show in vivo efficacy and have low frequencies of resistance occurrence, they are of particular interest in the fight against fungal infections.

Structural basis for specific self-incompatibility response in Brassica

Self-incompatibility (SI) is a widespread mechanism in flowering plants which prevents self-fertilization and inbreeding. In Brassica, recognition of the highly polymorphic S-locus cysteine-rich protein (SCR; or S-locus protein 11) by the similarly polymorphic S-locus receptor kinase (SRK) dictates the SI specificity. Here, we report the crystal structure of the extracellular domain of SRK9 (eSRK9) in complex with SCR9 from Brassica rapa. SCR9 binding induces eSRK9 homodimerization, forming a 2:2 eSRK:SCR heterotetramer with a shape like the letter "A". Specific recognition of SCR9 is mediated through three hyper-variable (hv) regions of eSRK9. Each SCR9 simultaneously interacts with hvI and one-half of hvII from one eSRK9 monomer and the other half of hvII from the second eSRK9 monomer, playing a major role in mediating SRK9 homodimerization without involving interaction between the two SCR9 molecules. Single mutations of residues critical for the eSRK9-SCR9 interaction disrupt their binding in vitro. Our study rationalizes a body of data on specific recognition of SCR by SRK and provides a structural template for understanding the co-evolution between SRK and SCR.

NMR structure and conformational dynamics of AtPDFL2.1, a defensin-like peptide from Arabidopsis thaliana

Plant defensins constitute the innate immune response against pathogens such as fungi and bacteria. Typical plant defensins are small, basic peptides that possess a characteristic three-dimensional fold stabilized by three or four disulfide bridges. In addition to known defensin genes, the Arabidopsis genome comprises >300 defensin-like genes coding for small cysteine-rich peptides. One of such genes encodes for AtPDFL2.1, a putative antifungal peptide of 55 amino acids, with six cysteine residues in its primary sequence. To understand the functional role of AtPDFL2.1, we carried out antifungal activity assays and determined its high-resolution three-dimensional structure using multidimensional solution NMR spectroscopy. We found that AtPDFL2.1 displays a strong inhibitory effect against Fusarium graminearum (IC₅₀≈4μM). This peptide folds in the canonical cysteine-stabilized αβ (CSαβ) motif, consisting of one α-helix and one triple-stranded antiparallel β-sheet stabilized by three disulfide bridges and a hydrophobic cluster of residues within its core where the α-helix packs tightly against the β-sheets. Nuclear spin relaxation measurements show that the structure of AtPDFL2.1 is essentially rigid, with the L3 loop located between β-strands 2 and 3 being more flexible and displaying conformational exchange. Interestingly, the dynamic features of loop L3 are conserved among defensins and are probably correlated to the antifungal and receptor binding activities.

The antifungal plant defensin AtPDF2.3 from Arabidopsis thaliana blocks potassium channels

Scorpion toxins that block potassium channels and antimicrobial plant defensins share a common structural CSαβ-motif. These toxins contain a toxin signature (K-C₄-X-N) in their amino acid sequence, and based on in silico analysis of 18 plant defensin sequences, we noted the presence of a toxin signature (K-C₅-R-G) in the amino acid sequence of the Arabidopsis thaliana defensin AtPDF2.3. We found that recombinant (r)AtPDF2.3 blocks K_v1.2 and K_v1.6 potassium channels, akin to the interaction between scorpion toxins and potassium channels. Moreover, rAtPDF2.3[G36N], a variant with a KCXN toxin signature (K-C₅-R-N), is more potent in blocking K_v1.2 and K_v1.6 channels than rAtPDF2.3, whereas rAtPDF2.3[K33A], devoid of the toxin signature, is characterized by reduced K_v channel blocking activity. These findings highlight the importance of the KCXN scorpion toxin signature in the plant defensin sequence for blocking potassium channels. In addition, we found that rAtPDF2.3 inhibits the growth of Saccharomyces cerevisiae and that pathways regulating potassium transport and/or homeostasis confer tolerance of this yeast to rAtPDF2.3, indicating a role for potassium homeostasis in the fungal defence response towards rAtPDF2.3. Nevertheless, no differences in antifungal potency were observed between the rAtPDF2.3 variants, suggesting that antifungal activity and K_v channel inhibitory function are not linked.

Why cellular communication during plant reproduction is particularly mediated by CRP signalling

Secreted cysteine-rich peptides (CRPs) represent one of the main classes of signalling peptides in plants. Whereas post-translationally modified small non-CRP peptides (psNCRPs) are mostly involved in signalling events during vegetative development and interactions with the environment, CRPs are overrepresented in reproductive processes including pollen germination and growth, self-incompatibility, gamete activation and fusion as well as seed development. In this opinion paper we compare the involvement of both types of peptides in vegetative and reproductive phases of the plant lifecycle. Besides their conserved cysteine pattern defining structural features, CRPs exhibit hypervariable primary sequences and a rapid evolution rate. As a result, CRPs represent a pool of highly polymorphic signalling peptides involved in species-specific functions during reproduction and thus likely represent key players to trigger speciation in plants by supporting reproductive isolation. In contrast, precursers of psNCRPs are proteolytically processed into small functional domains with high sequence conservation and act in more general processes. We discuss parallels in downstream processes of CRP signalling in both reproduction and defence against pathogenic fungi and alien pollen tubes, with special emphasis on the role of ROS and ion channels. In conclusion we suggest that CRP signalling during reproduction in plants has evolved from ancient defence mechanisms.

Psd1 binding affinity toward fungal membrane components as assessed by SPR: The role of glucosylceramide in fungal recognition and entry

Psd1 is a plant defensin that has antifungal activity against several pathogenic and nonpathogenic fungi. Previous analysis of Psd1 chemical shift perturbations by nuclear magnetic resonance (NMR) spectroscopy demonstrated that this defensin interacts with phospholipids and the sphingolipid glucosylceramide isolated from Fusarium solani (GlcCer(Fusarium solani)). In this study, these interactions were evaluated by real-time surface plasmon resonance (SPR) analysis. The data obtained demonstrated that Psd1 could bind more strongly to small unilamellar vesicles (SUV)-containing GlcCer(Fusarium solani) than to SUV that was composed of phosphatidylcholine (PC) alone or was enriched with GlcCer that had been isolated from soybeans. An increase in the SPR response after cholesterol or ergosterol incorporation in PC-SUV was detected; however, SUV composed of PC:Erg (7:3; molar:molar) became unstable in the presence of Psd1, suggesting membrane destabilization. We also observed a lack of Psd1 internalization in Candida albicans strains that were deficient in the glucosyl ceramide synthase gene. Together, these data indicate that GlcCer is essential for Psd1 anchoring in the fungal plasma membrane as well as internalization.

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.

An intimate link between antimicrobial peptide sequence diversity and binding to essential components of bacterial membranes

Antimicrobial peptides and proteins (AMPs) are widespread in the living kingdom. They are key effectors of defense reactions and mediators of competitions between organisms. They are often cationic and amphiphilic, which favors their interactions with the anionic membranes of microorganisms. Several AMP families do not directly alter membrane integrity but rather target conserved components of the bacterial membranes in a process that provides them with potent and specific antimicrobial activities. Thus, lipopolysaccharides (LPS), lipoteichoic acids (LTA) and the peptidoglycan precursor Lipid II are targeted by a broad series of AMPs. Studying the functional diversity of immune effectors tells us about the essential residues involved in AMP mechanism of action. Marine invertebrates have been found to produce a remarkable diversity of AMPs. Molluscan defensins and crustacean anti-LPS factors (ALF) are diverse in terms of amino acid sequence and show contrasted phenotypes in terms of antimicrobial activity. Their activity is directed essentially against Gram-positive or Gram-negative bacteria due to their specific interactions with Lipid II or Lipid A, respectively. Through those interesting examples, we discuss here how sequence diversity generated throughout evolution informs us on residues required for essential molecular interaction at the bacterial membranes and subsequent antibacterial activity. Through the analysis of molecular variants having lost antibacterial activity or shaped novel functions, we also discuss the molecular bases of functional divergence in AMPs. This article is part of a Special Issue entitled: Antimicrobial peptides edited by Karl Lohner and Kai Hilpert.

Cysteine-stabilized αβ defensins: From a common fold to antibacterial activity

Antimicrobial peptides (AMPs) seem to be promising alternatives to common antibiotics, which are facing increasing bacterial resistance. Among them are the cysteine-stabilized αβ defensins. These peptides are small, with a length ranging from 34 to 54 amino acid residues, cysteine-rich and extremely stable, normally composed of an α-helix and three β-strands stabilized by three or four disulfide bonds and commonly found in several organisms. Moreover, animal and plant CSαβ defensins present different specificities, the first being mainly active against bacteria and the second against fungi. The role of the CSαβ-motif remains unknown, but a common antibacterial mechanism of action, based on the inhibition of the cell-wall formation, has already been observed in some fungal and invertebrate defensins. In this context, the present work aims to group the data about CSαβ defensins, highlighting their evolution, conservation, structural characteristics, antibacterial activity and biotechnological perspectives.

Synergistic Activity of the Plant Defensin HsAFP1 and Caspofungin against Candida albicans Biofilms and Planktonic Cultures

Plant defensins are small, cysteine-rich peptides with antifungal activity against a broad range of yeast and fungi. In this study we investigated the antibiofilm activity of a plant defensin from coral bells (Heuchera sanguinea), i.e. HsAFP1. To this end, HsAFP1 was heterologously produced using Pichia pastoris as a host. The recombinant peptide rHsAFP1 showed a similar antifungal activity against the plant pathogen Fusarium culmorum as native HsAFP1 purified from seeds. NMR analysis revealed that rHsAFP1 consists of an α-helix and a triple-stranded antiparallel β-sheet stabilised by four intramolecular disulfide bonds. We found that rHsAFP1 can inhibit growth of the human pathogen Candida albicans as well as prevent C. albicans biofilm formation with a BIC50 (i.e. the minimum rHsAFP1 concentration required to inhibit biofilm formation by 50% as compared to control treatment) of 11.00 ± 1.70 μM. As such, this is the first report of a plant defensin exhibiting inhibitory activity against fungal biofilms. We further analysed the potential of rHsAFP1 to increase the activity of the conventional antimycotics caspofungin and amphotericin B towards C. albicans. Synergistic effects were observed between rHsAFP1 and these compounds against both planktonic C. albicans cells and biofilms. Most notably, concentrations of rHsAFP1 as low as 0.53 μM resulted in a synergistic activity with caspofungin against pre-grown C. albicans biofilms. rHsAFP1 was found non-toxic towards human HepG2 cells up to 40 μM, thereby supporting the lack of a general cytotoxic activity as previously reported for HsAFP1. A structure-function study with 24-mer synthetic peptides spanning the entire HsAFP1 sequence revealed the importance of the γ-core and its adjacent regions for HsAFP1 antibiofilm activity. These findings point towards broad applications of rHsAFP1 and its derivatives in the field of antifungal and antibiofilm drug development.

Antifungal plant defensins: mechanisms of action and production

Plant defensins are small, cysteine-rich peptides that possess biological activity towards a broad range of organisms. Their activity is primarily directed against fungi, but bactericidal and insecticidal actions have also been reported. The mode of action of various antifungal plant defensins has been studied extensively during the last decades and several of their fungal targets have been identified to date. This review summarizes the mechanism of action of well-characterized antifungal plant defensins, including RsAFP2, MsDef1, MtDef4, NaD1 and Psd1, and points out the variety by which antifungal plant defensins affect microbial cell viability. Furthermore, this review summarizes production routes for plant defensins, either via heterologous expression or chemical synthesis. As plant defensins are generally considered non-toxic for plant and mammalian cells, they are regarded as attractive candidates for further development into novel antimicrobial agents.

Overexpression of a defensin enhances resistance to a fruit-specific anthracnose fungus in pepper

Functional characterization of a defensin, J1-1, was conducted to evaluate its biotechnological potentiality in transgenic pepper plants against the causal agent of anthracnose disease, Colletotrichum gloeosporioides. To determine antifungal activity, J1-1 recombinant protein was generated and tested for the activity against C. gloeosporioides, resulting in 50% inhibition of fungal growth at a protein concentration of 0.1 mg·mL⁻¹. To develop transgenic pepper plants resistant to anthracnose disease, J1-1 cDNA under the control of 35S promoter was introduced into pepper via Agrobacterium-mediated genetic transformation method. Southern and Northern blot analyses confirmed that a single copy of the transgene in selected transgenic plants was normally expressed and also stably transmitted to subsequent generations. The insertion of T-DNA was further analyzed in three independent homozygous lines using inverse PCR, and confirmed the integration of transgene in non-coding region of genomic DNA. Immunoblot results showed that the level of J1-1 proteins, which was not normally accumulated in unripe fruits, accumulated high in transgenic plants but appeared to differ among transgenic lines. Moreover, the expression of jasmonic acid-biosynthetic genes and pathogenesis-related genes were up-regulated in the transgenic lines, which is co-related with the resistance of J1-1 transgenic plants to anthracnose disease. Consequently, the constitutive expression of J1-1 in transgenic pepper plants provided strong resistance to the anthracnose fungus that was associated with highly reduced lesion formation and fungal colonization. These results implied the significance of the antifungal protein, J1-1, as a useful agronomic trait to control fungal disease.

Antifungal defensins and their role in plant defense

Since the beginning of the 90s lots of cationic plant, cysteine-rich antimicrobial peptides (AMP) have been studied. However, Broekaert et al. (1995) only coined the term “plant defensin,” after comparison of a new class of plant antifungal peptides with known insect defensins. From there, many plant defensins have been reported and studies on this class of peptides encompass its activity toward microorganisms and molecular features of the mechanism of action against bacteria and fungi. Plant defensins also have been tested as biotechnological tools to improve crop production through fungi resistance generation in organisms genetically modified (OGM). Its low effective concentration towards fungi, ranging from 0.1 to 10 μM and its safety to mammals and birds makes them a better choice, in place of chemicals, to control fungi infection on crop fields. Herein, is a review of the history of plant defensins since their discovery at the beginning of 90s, following the advances on its structure conformation and mechanism of action towards microorganisms is reported. This review also points out some important topics, including: (i) the most studied plant defensins and their fungal targets; (ii) the molecular features of plant defensins and their relation with antifungal activity; (iii) the possibility of using plant defensin(s) genes to generate fungi resistant GM crops and biofungicides; and (iv) a brief discussion about the absence of products in the market containing plant antifungal defensins.

Defensins: antifungal lessons from eukaryotes

Over the last years, antimicrobial peptides (AMPs) have been the focus of intense research toward the finding of a viable alternative to current antifungal drugs. Defensins are one of the major families of AMPs and the most represented among all eukaryotic groups, providing an important first line of host defense against pathogenic microorganisms. Several of these cysteine-stabilized peptides present a relevant effect against fungi. Defensins are the AMPs with the broader distribution across all eukaryotic kingdoms, namely, Fungi, Plantae, and Animalia, and were recently shown to have an ancestor in a bacterial organism. As a part of the host defense, defensins act as an important vehicle of information between innate and adaptive immune system and have a role in immunomodulation. This multidimensionality represents a powerful host shield, hard for microorganisms to overcome using single approach resistance strategies. Pathogenic fungi resistance to conventional antimycotic drugs is becoming a major problem. Defensins, as other AMPs, have shown to be an effective alternative to the current antimycotic therapies, demonstrating potential as novel therapeutic agents or drug leads. In this review, we summarize the current knowledge on some eukaryotic defensins with antifungal action. An overview of the main targets in the fungal cell and the mechanism of action of these AMPs (namely, the selectivity for some fungal membrane components) are presented. Additionally, recent works on antifungal defensins structure, activity, and cytotoxicity are also reviewed.

Structural and functional studies of a phosphatidic acid-binding antifungal plant defensin MtDef4: identification of an RGFRRR motif governing fungal cell entry

MtDef4 is a 47-amino acid cysteine-rich evolutionary conserved defensin from a model legume Medicago truncatula. It is an apoplast-localized plant defense protein that inhibits the growth of the ascomycetous fungal pathogen Fusarium graminearum in vitro at micromolar concentrations. Little is known about the mechanisms by which MtDef4 mediates its antifungal activity. In this study, we show that MtDef4 rapidly permeabilizes fungal plasma membrane and is internalized by the fungal cells where it accumulates in the cytoplasm. Furthermore, analysis of the structure of MtDef4 reveals the presence of a positively charged γ-core motif composed of β₂ and β₃ strands connected by a positively charged RGFRRR loop. Replacement of the RGFRRR sequence with AAAARR or RGFRAA abolishes the ability of MtDef4 to enter fungal cells, suggesting that the RGFRRR loop is a translocation signal required for the internalization of the protein. MtDef4 binds to phosphatidic acid (PA), a precursor for the biosynthesis of membrane phospholipids and a signaling lipid known to recruit cytosolic proteins to membranes. Amino acid substitutions in the RGFRRR sequence which abolish the ability of MtDef4 to enter fungal cells also impair its ability to bind PA. These findings suggest that MtDef4 is a novel antifungal plant defensin capable of entering into fungal cells and affecting intracellular targets and that these processes are mediated by the highly conserved cationic RGFRRR loop via its interaction with PA.

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.

Revealing the properties of plant defensins through dynamics

Defensins are potent, ancient natural antibiotics that are present in organisms ranging from lower organisms to humans. Although the structures of several defensins have been well characterized, the dynamics of only a few have been studied. This review discusses the diverse dynamics of two plant defensins for which the structure and dynamics have been characterized, both in the free state and in the presence of target membranes. Multiple motions are observed in loops and in secondary structure elements and may be related to twisting or breathing of the α-helix and β-sheet. This complex behavior is altered in the presence of an interface and is responsive to the presence of the putative target. The stages of membrane recognition and disruption can be mapped over a large time scale range, demonstrating that defensins in solution exist as an ensemble of different conformations, a subset of which is selected upon membrane binding. Therefore, studies on the dynamics have revealed that defensins interact with membranes through a mechanism of conformational selection.

Dimerization of plant defensin NaD1 enhances its antifungal activity

The plant defensin, NaD1, from the flowers of Nicotiana alata, is a member of a family of cationic peptides that displays growth inhibitory activity against several filamentous fungi, including Fusarium oxysporum. The antifungal activity of NaD1 has been attributed to its ability to permeabilize membranes; however, the molecular basis of this function remains poorly defined. In this study, we have solved the structure of NaD1 from two crystal forms to high resolution (1.4 and 1.58 Å, respectively), both of which contain NaD1 in a dimeric configuration. Using protein cross-linking experiments as well as small angle x-ray scattering analysis and analytical ultracentrifugation, we show that NaD1 forms dimers in solution. The structural studies identified Lys(4) as critical in formation of the NaD1 dimer. This was confirmed by site-directed mutagenesis of Lys(4) that resulted in substantially reduced dimer formation. Significantly, the reduced ability of the Lys(4) mutant to dimerize correlated with diminished antifungal activity. These data demonstrate the importance of dimerization in NaD1 function and have implications for the use of defensins in agribiotechnology applications such as enhancing plant crop protection against fungal pathogens.