Plant gamma-thionins: novel insights on the mechanism of action of a multi-functional class of defense proteins

Molecular identification of four different alpha-amylase inhibitors from baru (Dipteryx alata) seeds with activity toward insect enzymes

The endophytic bruchid pest Callosobruchus maculatus causes severe damage to storage cowpea seeds, leading to economical losses. For this reason the use of alpha-amylase inhibitors to interfere with the pest digestion process has been an interesting alternative to control bruchids. With this aim, alpha-amylase inhibitors from baru seeds (Dipteryx alata) were isolated by affinity chromatographic procedures, causing enhanced inhibition of C. maculatus and Anthonomus grandis alpha-amylases. To attempt further purification, this fraction was applied onto a reversed-phase HPLC column, generating four peaks with remarkable inhibition toward C. maculatus alpha-amylases. SDS-PAGE and MALDI-ToF analysis identified major proteins of approximately 5.0, 11.0, 20.0 and 55 kDa that showed alpha-amylase inhibition. Results of in vivo bioassays using artificial seeds containing 1.0% (w/w) of baru crude extract revealed 40% cowpea weevil larvae mortality. These results provide evidence that several alpha-amylase inhibitors classes, with biotechnological potential, can be isolated from a single plant species.

Structure-based protein engineering for alpha-amylase inhibitory activity of plant defensin

The structure of a novel plant defensin isolated from the seeds of the mung bean, Vigna radiate, has been determined by (1)H nuclear magnetic resonance spectroscopy. The three-dimensional structure of VrD2, the V. radiate plant defensin 2 protein, comprises an alpha-helix and one triple-stranded anti-parallel beta-sheet stabilized by four disulfide bonds. This protein exhibits neither insecticidal activity nor alpha-amylase inhibitory activity in spite of showing a similar global fold to that of VrD1, an insecticidal plant defensin that has been suggested to function by inhibiting insect alpha-amylase. Our previous study proposed that loop L3 of plant defensins is important for this inhibition. Structural analyses and surface charge comparisons of VrD1 and VrD2 revealed that the charged residues of L3 correlate with the observed difference in inhibitory activities of these proteins. A VrD2 chimera that was produced by transferring the proposed functional loop of VrD1 onto the structurally equivalent loop of VrD2 supported this hypothesis. The VrD2 chimera, which differs by only five residues compared with VrD2, showed obvious activity against Tenebrio molitor alpha-amylase. These results clarify the mode of alpha-amylase inhibition of plant defensins and also represent a possible approach for engineering novel alpha-amylase inhibitors. Plant defensins are important constituents of the innate immune system of plants, and thus the application of protein engineering to this protein family may provide an efficient method for protecting against crop losses.

How rhizobial symbionts invade plants: the Sinorhizobium-Medicago model

Nitrogen-fixing rhizobial bacteria and leguminous plants have evolved complex signal exchange mechanisms that allow a specific bacterial species to induce its host plant to form invasion structures through which the bacteria can enter the plant root. Once the bacteria have been endocytosed within a host-membrane-bound compartment by root cells, the bacteria differentiate into a new form that can convert atmospheric nitrogen into ammonia. Bacterial differentiation and nitrogen fixation are dependent on the microaerobic environment and other support factors provided by the plant. In return, the plant receives nitrogen from the bacteria, which allows it to grow in the absence of an external nitrogen source. Here, we review recent discoveries about the mutual recognition process that allows the model rhizobial symbiont Sinorhizobium meliloti to invade and differentiate inside its host plant alfalfa (Medicago sativa) and the model host plant barrel medic (Medicago truncatula).

Genome-wide expression profiling of the Arabidopsis female gametophyte identifies families of small, secreted proteins

The female gametophyte of flowering plants, the embryo sac, develops within the diploid (sporophytic) tissue of the ovule. While embryo sac–expressed genes are known to be required at multiple stages of the fertilization process, the set of embryo sac–expressed genes has remained poorly defined. In particular, the set of genes responsible for mediating intracellular communication between the embryo sac and the male gametophyte, the pollen grain, is unknown. We used high-throughput cDNA sequencing and whole-genome tiling arrays to compare gene expression in wild-type ovules to that in dif1 ovules, which entirely lack embryo sacs, and myb98 ovules, which are impaired in pollen tube attraction. We identified nearly 400 genes that are downregulated in dif1 ovules. Seventy-eight percent of these embryo sac–dependent genes were predicted to encode for secreted proteins, and 60% belonged to multigenic families. Our results define a large number of candidate extracellular signaling molecules that may act during embryo sac development or fertilization; less than half of these are represented on the widely used ATH1 expression array. In particular, we found that 37 out of 40 genes encoding Domain of Unknown Function 784 (DUF784) domains require the synergid-specific transcription factor MYB98 for expression. Several DUF784 genes were transcribed in synergid cells of the embryo sac, implicating the DUF784 gene family in mediating late stages of embryo sac development or interactions with pollen tubes. The coexpression of highly similar proteins suggests a high degree of functional redundancy among embryo sac genes.

During the sexual reproduction of flowering plants, a pollen tube delivers sperm cells to a specialized group of cells known as the embryo sac, which contains the egg cell. It is known that embryo sacs are active participants in guiding the growth of pollen tubes, in facilitating fertilization, and in initiating seed development. However, the genes responsible for the complex biology of embryo sacs are poorly understood. The authors use two recently developed technologies, whole-genome tiling microarrays and high-throughput cDNA sequencing, to identify hundreds of genes expressed in embryo sacs of Arabidopsis thaliana. Most embryo sac–dependent genes have no known function, and include entire families of related genes that are only expressed in embryo sacs. Furthermore, most embryo sac–dependent genes encode small proteins that are potentially secreted from their cells of origin, suggesting that they may act as intracellular signals or to modify the extracellular matrix during fertilization or embryo sac development. These results illustrate the extent to which our understanding of plant sexual reproduction is limited and identifies hundreds of candidate genes for future studies investigating the molecular biology of the embryo sac.

Plant cyclotides: an unusual class of defense compounds

Plant cyclotides are unusual peptides with low molecular masses and a three-dimensional structure characterized by the presence of a cyclic fold. Synthetic peptides can adopt this circular conformation, but it is not a common feature for most members of other peptide groups. Cyclotides present a wide range of functions, such as the ability to induce stronger contractions during childbirth and anti-tumor activity. Additionally, some cyclotides present anti-viral, insecticidal or proteinase inhibitory activity. In this paper, we describe the structural and functional characteristics of plant cyclotides, their most conserved features and the development of these peptides for human health and biotechnological applications.

Susceptibility of Human Pathogenic Bacteria to Antimicrobial Peptides from Sesame Kernels

Hospital infection caused by Gram-negative bacteria is a serious and common problem, especially in developing countries. Aiming to reduce these infections, this report focuses on the identification and characterization of novel antimicrobial peptides from sesame (Sesamum indicum) kernel meals. Thus, sesame flour was extracted and precipitated with ammonium sulfate (100%). After dialysis, a rich fraction was applied to affinity red-Sepharose CL-6B chromatography, followed by reversed-phase high-performance liquid chromatography. Mass spectrometry analysis indicated the presence of a major peptide with molecular mass of approximately 5.8 kDa in both cultivars. The bactericidal activities of antimicrobial peptides were evaluated against several human pathogens that had been effective only against Klebsiella sp., a Gram-negative bacterium responsible for human urinary infection. These data indicate the biotechnological potential of sesame peptides as an alternative method for hospital infection control and also the decrease of bacterial resistance to synthetic antibiotics.

Identification of an alpha-amylase inhibitor from Pterodon pubescens with ability to inhibit cowpea weevil digestive enzymes

Cowpea seeds (Vigna ungiculata) are widely cultivated by poor farmers in Latin America and Africa and are often severely damaged by the cowpea weevil Callosobruchus maculatus. A proteinaceous inhibitor of cowpea weevil digestive enzymes, PpAI, was purified from white sucupira seeds (Pterodon pubescens) and biochemically characterized in this study. Proteins were extracted from seeds and precipitated with ammonium sulfate at 100% saturation. This fraction was applied onto a Red-sepharose CL-6B column, and the retained peak showed 70% inhibitory activity toward larval C. maculatus digestive alpha-amylases. The retained peak was then purified using an analytical reversed-phase HPLC column. Purified PpAI showed 65% inhibitory activity against larval C. maculatus enzymes. Enzymatic assays also showed that the purified P. pubescens inhibitor was unable to reduce the activity of mammalian alpha-amylases, suggesting specificity toward insect enzymes. Moreover, artificial seeds containing PpAI were able to reduce larval weight by 36% and cause 55% mortality. Mass spectrometry and SDS-PAGE analyses indicated that PpAI showed a molecular mass of approximately 5.0 kDa. This alpha-amylase inhibitor, coming from a native Cerrado plant, could be used to construct a genetically engineered cowpea with enhanced resistance against weevil pests.

Novel families of toxin-like peptides in insects and mammals: a computational approach

Most animal toxins are short proteins that appear in venom and vary in sequence, structure and function. A common characteristic of many such toxins is their apparent structural stability. Sporadic instances of endogenous toxin-like proteins that function in non-venom context have been reported. We have utilized machine learning methodology, based on sequence-derived features and guided by the notion of structural stability, in order to conduct a large-scale search for toxin and toxin-like proteins. Application of the method to insect and mammalian sequences revealed novel families of toxin-like proteins. One of these proteins shows significant similarity to ion channel inhibitors that are expressed in cone snail and assassin bug venom, and is surprisingly expressed in the bee brain. A toxicity assay in which the protein was injected to fish induced a strong yet reversible paralytic effect. We suggest that the protein may function as an endogenous modulator of voltage-gated Ca(2+) channels. Additionally, we have identified a novel mammalian cluster of toxin-like proteins that are expressed in the testis. We suggest that these proteins might be involved in regulation of nicotinic acetylcholine receptors that affect the acrosome reaction and sperm motility. Finally, we highlight a possible evolutionary link between venom toxins and antibacterial proteins. We expect our methodology to enhance the discovery of additional novel protein families.

A putative novel role for plant defensins: a defensin from the zinc hyper-accumulating plant, Arabidopsis halleri, confers zinc tolerance

The metal tolerance of metal hyper-accumulating plants is a poorly understood mechanism. In order to unravel the molecular basis of zinc (Zn) tolerance in the Zn hyper-accumulating plant Arabidopsis halleri ssp. halleri, we carried out a functional screening of an A. halleri cDNA library in the yeast Saccharomyces cerevisiae to search for genes conferring Zn tolerance to yeast cells. The screening revealed four A. halleri defensin genes (AhPDFs), which induced Zn but not cadmium (Cd) tolerance in yeast. The expression of AhPDF1.1 under the control of the 35S promoter in A. thaliana made the transgenic plants more tolerant to Zn than wild-type plants, but did not change the tolerance to Cd, copper (Cu), cobalt (Co), iron (Fe) or sodium (Na). Thus, AhPDF1.1 is able to confer Zn tolerance both to yeast and plants. In A. halleri, defensins are constitutively accumulated at a higher level in shoots than in A. thaliana. A. halleri defensin pools are Zn-responsive, both at the mRNA and protein levels. In A. thaliana, some but not all defensin genes are induced by ZnCl2 treatment, and these genes are not induced by NaCl treatment. Defensins, found in a very large number of organisms, are known to be involved in the innate immune system but have never been found to play any role in metal physiology. Our results support the proposition that defensins could be involved in Zn tolerance in A. halleri, and that a role for plant defensins in metal physiology should be considered.

Identification of a cowpea gamma-thionin with bactericidal activity

Antimicrobial peptides are an abundant group of proteinaceous compounds widely produced in the plant kingdom. Among them, the gamma-thionin family, also known as plant defensins, represents one typical family and comprises low molecular mass cysteine-rich proteins, usually cationic and distributed in different plant tissues. Here, we report the purification and characterization of a novel gamma-thionin from cowpea seeds (Vigna unguiculata), named Cp-thionin II, with bactericidal activity against Gram-positive and Gram-negative bacteria. Once the primary structure was elucidated, molecular modelling experiments were used to investigate the multimerization and mechanism of action of plant gamma-thionins. Furthermore, Cp-thionin II was also localized in different tissues in cowpea seedlings during germination in contrasting conditions, to better understand the plant protection processes. The use of plant defensins in the construction of transgenic plants and also in the production of novel drugs with activity against human pathogens is discussed.

Fungicidal and cytotoxic activity of a Capsicum chinense defensin expressed by endothelial cells

Plant defensins are antimicrobial peptides that exhibit mainly antifungal activity against a broad range of plant fungal pathogens. However, their actions against Candida albicans have not been extensively studied. The mRNA for gamma-thionin, a defensin from Capsicum chinense, has been expressed in bovine endothelial cells. The conditioned medium of these cells showed antifungal activity on germ tube formation (60-70% of inhibition) and on the viability of C. albicans (70-80% of inhibition). Additionally, C. albicans was not able to penetrate transfected cells. Conditioned medium from these cells also inhibited the viability (80%) of the human tumor cell line, HeLa.

An antifungal peptide from passion fruit (Passiflora edulis) seeds with similarities to 2S albumin proteins

An actual worldwide problem consists of an expressive increase of economic losses and health problems caused by fungi. In order to solve this problem, several studies have been concentrating on the screening of novel plant defence peptides with antifungal activities. These peptides are commonly characterized by having low molecular masses and cationic charges. This present work reports on the purification and characterization of a novel plant peptide of 5.0 kDa, Pe-AFP1, purified from the seeds of passion fruit (Passiflora edulis). Purification was achieved using a Red-Sepharose Cl-6B affinity column followed by reversed-phase chromatography on Vydac C18-TP column. In vitro assays indicated that Pe-AFP1 was able of inhibiting the development of the filamentous fungi Trichoderma harzianum, Fusarium oxysporum, and Aspergillus fumigatus with IC50 values of 32, 34, and 40 microg ml(-1), respectively, but not of Rhyzoctonia solani, Paracoccidioides brasiliensis and Candida albicans. This protein was also subjected to automated N-terminal amino acid sequence, showing high degree of similarities to storage 2S albumins, adding a new member to this protein-defence family. The discovery of Pe-AFP1 could contribute, in a near future, to the development of biotechnological products as antifungal drugs and transgenic plants with enhanced resistance to pathogenic fungi.

Identification of a novel bean α-amylase inhibitor with chitinolytic activity

Zabrotes subfasciatus is a devastating starch-dependent storage bean pest. In this study, we attempted to identify novel alpha-amylase inhibitors from wild bean seeds, with efficiency toward pest alpha-amylases. An inhibitor named Phaseolus vulgaris chitinolytic alpha-amylase inhibitor (PvCAI) was purified and mass spectrometry analyses showed a protein with 33330 Da with the ability to form dimers. Purified PvCAI showed significant inhibitory activity against larval Z. subfasciatus alpha-amylases with no activity against mammalian enzymes. N-terminal sequence analyses showed an unexpected high identity to plant chitinases from the glycoside hydrolase family 18. Furthermore, their chitinolytic activity was also detected. Our data provides compelling evidence that PvCAI also possessed chitinolytic activity, indicating the emergence of a novel alpha-amylase inhibitor class.