Plant defensins

Thionins are cystine-rich, cationic small peptides ( 5 kDa) found in monocots and eudicots. They are divided into the families of o //3-thionins and 7-thionins. As is now generally accepted practice, we will refer to 7-thionins as plant defensins, as they are structurally more closely related to mammalian and insect defensins... 

Plant defensins are cystine-rich, cationic peptides ranging in size from 45 to 54 amino acids, of which eight are cysteine. They were first discovered in wheat and barley ° and were proposed to form a novel subclass of thionins, the 7-thionins. As it became clear that they closely resemble mammalian and insect defensins in primary and secondary structure, the term plant defensins was introduced to describe these peptides. It is generally assumed that all plants express plant defensins " and that they are expressed in a wide range of plant tissue, that is, leaves, floral tissue,tubers,bark, root, pods, and seeds,with seeds in particular being from where most plant defensins have been isolated. ... 

The three-dimensional structural architecture of plant defensins is exemplified by the structure of Rs-AFP, ° which comprises an N-terminal /3-strand followed by an ct-helix and two /3-strands (/3a/3/3 configuration). The /3-strands form a triple-stranded antiparallel /3-sheet. The three-dimensional structure is stabilized by three disulfide bonds. In general, in plant defensins two disulfide bonds form between the ct-helix and the central /3-strand. A third disulfide bond stabilizes the structure by linking the /3-strand after the helix to the coiled part after the ct-helix. This motif is called the cysteine-stabilized a/3-motif (CSa/3)" and also occurs in toxins isolated from insects, spiders, and scorpions.The fourth disulfide bond links the C-terminal end of the peptide with the N-terminal /3-strand. Two plant defensins, PhDl and PhD2, feature a fifth disulfide bond and have been proposed to be the prototypes of a new subclass within plant defensins." As a result of these structural features the global structure of plant defensins is notably different from o //3-thionins, which is one of the reasons for the different nomenclature. The structures of plant defensins Rs-AFP ° and NaDf are shown in Figure 6, where they are compared to the thionin /3-purothionin and the structurally more related drosomycin and charybdotoxin. ... 

Plant defensins have a range of biological activities and target a wide range of plant pests, including fungi, bacteria, and insects. Additionally, their modes of action are more diverse than those observed for thionins. Plant defensin expression is often induced upon infection with plant pathogens. ... 

Figures Comparison of the plant defensin structures Rs-AFP((a), gray, 1ayj)and NaDI ((b), green, 1mr4) with/3-purothionin ((c), magenta, 1 bhp) reveals the structural differences between plant defensins and a//3-thionins. The architecture resembles that of insect defensins, for example, drosomycin ((d), pink, 1 myn) or the scorpion toxin charybdotoxin ((e), yellow, 2crd). The structural similarities become clear in the overlay of Rs-AFP, NaDI, and drosomycin ((f), colors as before).

Several plant defensins have also been found to be inhibitors of various enzymes in plant pests. The plant defensins Slctl, SIo 2, and SIa3 were the first plant defensins where inhibition of a-amylase was shown at low concentration whereas purothionins inhibited a-amylase activity only at high concentrations. Blal and BIa2 isolated from barley are two more representatives of proteinaceous a-amylase inhibitors in the plant defensin family. ... 

Inhibition of trypsin is another mechanism of activity recently discovered in plant defensins. CfDl and CfD2 from Cassia fistula were the first plant defensins to be identified as trypsin inhibitors. Cp-thionin from cowpea was more recently discovered to have inhibitory potency against trypsin. Searches of protein sequence databases have yielded a number of other plant proteins annotated as trypsin inhibitors or potential trypsin inhibitors. These annotations were most likely made on the basis of sequence similarities with other known trypsin inhibitors, namely the Bowman—Birk trypsin inhibitor. Since the actual framework of the disulfide bonds is not known, it is possible that structure and therefore activity differ from this prototype framework. ... 

Another interesting activity of plant defensins has been described by Kushmerick et al. for two defensins called 7I- and 72-zeathionin from Zea mays. These two peptides showed fast and reversible inhibition of sodium channels in rat tumor cell lines. Owing to an overall conservation of ion channels in eukaryotic cells it is feasible to assume that sodium channels in plant pests may be targets for these plant defensins, although their activities have not been determined in vivo. ... 

There are also some peptides exhibiting a-amylase inhibitory potency. The plant defensins Slal, SIa2, and SIa3 from Sorghum bicolor and Blal and BIa2 from barley have been mentioned before. The knottin-type a-amylase inhibitor peptide AAI from Amaranthus hypochondtiacus is the smallest peptide ct-amylase inhibitor known to date. ... 

Figure 31-7 Ribbon structures of some defensins. (A) Structure of a human fl-defensin showing the three disulfide bonds. From Bauer et al.134 Courtesy of Heinrich Sticht. (B) Comparison of the folding patterns of four types of defensins. Mammalian a- and P-defensins are all P sheets with somewhat different arrangements of disulfide bridges. Insect and plant defensins have an a helix joined to the P sheet. Mammalian and insect defensins have three disulfide bridges, while plant defensins have four. From Hoffmann et al.12 Courtesy of Jules A. Hoffmann.

The plant was traditionally boiled in water to produce a medicinal tea that was used to accelerate childbirth. Reports of this use stimulated interest in characterising the bioactive component, which was subsequently found to be a 29 amino acid peptide referred to as kalata Bl.105 Cyclotides have also been reported to possess antimicrobial activity,106 and, for example, synthetic versions of the cyclotides circulin B and cyclopsychotride A are active against both Grampositive and Gram-negative bacteria.106 Impaired antimicrobial activity of cyclotides is observed at high salt concentration in the assay buffer and the phenomenon of activity linked to ionic strength has also been described for thionins and plant defensins and is an indicator of activity at membranes. 

In addition to the plant defensins mentioned in Section 4, defensins are also widespread in vertebrates. These antimicrobial peptides are involved in the innate immune response, and are classified into three subfamilies that present... 

Several cyclic peptides have between two and eight cysteine residues. They adopt a triple-stranded 3-sheet structure e.g. vertebrate defensins) or a P-hairpin-like structure (e.g thanatin, androctonin, gomesin, and tachyplesin from arthropods and protegrin from vertebrate) or a mixed a-helix/p-sheet conformation (e.g. invertebrate and plant defensins, including some vertebrate defensins). Several reviews have been published in the past years discussing the structure and the mode of action of cyclic AMPs from animals. The reader is referred to the reviews written by Powers and Hancock [108], Bulet et al. [4], Ganz [8], and Yount [88]. In this chapter, only cyclic peptides with a P-hairpin-like structure will be discussed. 

Thomma, B.P., Cammue, B.P., Thevissen, K. Mode of action of plant defensins suggests therapeutic potential. Curr. Drug Targets Infect. Disord. 2003, 3, 1—8... 

Broekaert, W. F., Terras, F.R.G., Cammue, B.P.A., Osborn, R.W. Plant defensins novel antimicrobial peptides as components of the host defense system. Plant Physiol. 1995, 108, 1353-1358... 

Wu, Y., Woerner, B.M., Stark, D.M., Shah, D.M., Liang, J., Rommens, C.M.T. Fungal pathogen protection in potato by expression of a plant defensin peptide. Nat. Biotechnol. 

Plant defensins, antifungal peptides which are not related to mammalian or insect defensins. The plant cc-defensins are present in seeds or leaves and are characterized by complex structures. They contain disulfide-linked cysteines in a triple-stranded antiparallel /3-sheet with only one cf-helix, and are homologous to those produced in human and rabbit neutrophils. Plant Q -defensins are not capable of forming ion-permeable pores in artificial membranes, but they may act through a receptor-mediated mechanism. They are considered PR proteins and are classified as members of the PR-12 group [K. Thevissen et al., J. Biol. Chem. 1996, 271,15018 A. J. De Lucca etal.. Can.]. Microbiol. 2005, 51,1001]. 

Plant Defensins have Specific Binding Sites on Fungal Membranes... 

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