Plant defensin | |||||||||
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Identifiers | |||||||||
Symbol | Plant defensin | ||||||||
Pfam | PF00304 | ||||||||
Pfam clan | CL0054 | ||||||||
InterPro | IPR008176 | ||||||||
PROSITE | PDOC00725 | ||||||||
SCOP2 | 1gps / SCOPe / SUPFAM | ||||||||
OPM superfamily | 58 | ||||||||
OPM protein | 1jkz | ||||||||
CDD | cd00107 | ||||||||
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Plant defensin | |||||||||
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Identifiers | |||||||||
Symbol | Plant defensin | ||||||||
Pfam | PF00304 | ||||||||
Pfam clan | CL0054 | ||||||||
InterPro | IPR008176 | ||||||||
PROSITE | PDOC00725 | ||||||||
SCOP2 | 1gps / SCOPe / SUPFAM | ||||||||
OPM superfamily | 58 | ||||||||
OPM protein | 1jkz | ||||||||
CDD | cd00107 | ||||||||
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Plant defensins (formerly gamma-thionins) are a family of primitive, highly stable, cysteine-rich defensins found in plants that function to defend them against pathogens and parasites. [1] Defensins are integral components of the innate immune system and belong to the ancient superfamily of antimicrobial peptides (AMPs). AMPs are also known as host defense peptides (HDPs), [2] and they are thought to have diverged about 1.4 billion years ago before the evolution of prokaryotes and eukaryotes. [3] [4] They are ubiquitous in almost all plant species, functionally diverse, and their primary structure varies significantly from one species to the next, except for a few cysteine residues, which stabilize the protein structure through disulfide bond formation. [1] Plant defensins usually have a net positive charge due to the abundance of cationic amino acids [5] and are generally divided into two classes. Those in the class II category contain a C-terminal pro-peptide domain of approximately 33 amino acids [5] and are targeted to the vacuole, [6] while the class I defensins lack this domain and mature in the cell wall. Unlike their class I counterparts, class II plant defensins are relatively smaller, and their acidic C-terminal prodomain is hypothesized to contribute to their vacuolar targeting. [7] The first plant defensins were discovered in barley and wheat in 1990 and were initially designated as γ-thionins. [8] [9] In 1995, the name was changed to 'plant defensin' when it was identified that they are evolutionarily unrelated to other thionins and were more similar to defensins from insects and mammals. [10] [11]
A large number of defensins were initially isolated from seeds, where they are linked to the defense of germinating seeds against fungal pathogens, [11] but recent advances in bioinformatics and molecular biology techniques have revealed that these peptides are present in other parts of the plant, including flowers and roots. [12] [3] Defensins can be expressed in two ways: constitutively or induced under certain stresses. For example, the defensin AtPDF2.2 from Arabidopsis thaliana is expressed constitutively, [13] while another defensin from the same plant is induced by methyl jasmonate and ethylene. [14]
Plant defensins are members of the protein superfamily called the cis-defensins or CSαβ fold. [15] This superfamily includes arthropod defensins and fungal defensins (but not defensins found in mammals). It also includes several families of proteins not involved in the immune system, including plant S-locus 11 proteins involved in self-incompatibility during reproduction and toxin proteins in scorpion venoms. [16] [17] Defensin proteins are produced as an amphipathic protein precursor with one or two pro-domains that are removed to make the final mature protein. In their mature form, they generally consist of about 45 to 50 amino acid residues. The folded globular structure is characterized by a well-defined 3-stranded anti-parallel beta-sheet and a short alpha-helix. [18] The structure of most plant defensins is cross-linked by four disulfide bridges: three in the core and one linking the N- and C-termini. Some plant defensins have only the core three disulfides, and a few have been found with an additional one (resulting in five total bridges). [19] Two of these bonds, those formed between the α-helix and the last β-strand, are arranged into the Cys-stabilized α-helix β-strand (CSαβ) motif, which play significant roles in their biological activities and stability. [20] [21] The globular structures of plant defensins make them resistant to degradation by proteolytic digestion and stable up to a pH and a temperature range of 10 and 90 degrees Celsius, respectively. [22] [23]
Plant defensins are a large component of the plant innate immune system. They are regarded as highly promiscuous molecules due to their diverse biological functions. A plant genome typically contains large numbers of different defensin genes [24] that vary in their efficacy against different pathogens and the amount they are expressed in different tissues. [25] In addition to their functions in the immune system, many of these low-molecular-weight peptides have developed additional roles in aiding reproduction and abiotic stress tolerance. [1]
Plant defensins elicit diverse antimicrobial properties, including antibacterial, [2] and antifungal [26] activities. The modes of action of different defensins depend on the type of organism and specific molecular targets, [27] [2] although their exact mechanisms of action vary. For instance, their antifungal activities, which are their best-characterized property, are attributed to their ability to interact with lipid structures on pathogenic fungi surfaces. These include sphingolipids, [28] glucosyceramide, [29] and phosphatidic acid [30] Apart from their capacity to attack and damage fungal membranes, these peptides have also been extensively researched for their capacity to trigger apoptosis and target other intracellular structures and biomolecules. [31] Plant defensins can spread their lethality by interfering with important developmental and/or regulatory processes, such as the cell cycle, when they perturb or disrupt the membrane of the fungus they target. [32] On the other hand, their ability to induce apoptosis has been linked to the bioaccumulation of reactive oxygen species [33] and the recruitment of specific caspases and caspase-associated proteins/ [34] In mediating their antibacterial mechanisms, plant defensin has been shown to cause loss of cell viability by inducing an unfavorable morphological change in the bacterial target via membrane targeting and permeation. [35] This defensin-membrane interaction has been linked to the presence of the cationic amino acid residues arginine, lysine, and histidine. [36] Furthermore, studies have shown that plant defensin inhibits in vitro protein synthesis in a cell-free system, [37] and their interactions with the DNA of bacterial pathogens have also been documented, hinting that they might have a lethal effect on DNA replication or transcription. [35]
Some plant defensins have also been identified as enzyme inhibitors of α-amylase or trypsin. [38] [39] [40] It is believed that these are antifeedant activities to deter insects. [39] Typically, molecular modeling analysis of defensin expressed in Vigna unguiculata revealed that defensin inhibits α-amylase in the weevils Acanthoscelides obtectus and Zabrotes subfasciatus by binding via its N-terminal to the active site of the enzyme. [38] Defensins with alpha-amylase-inhibitory activity have also been identified in Sorghum bicolor, [39] [41] suggesting defensins might interfere with carbohydrate metabolism in insect targets. Beyond their ability to inhibit alpha-amylases, defensins also demonstrate inhibitory properties toward trypsin and chymotrypsin. For instance, two defensins from the seeds of Cassia fistula have been documented to inhibit the activity of trypsin, [42] [40] and Capsicum annuum (CanDef-20) defensin has been reported to alter insect metabolism and retard growth in a number of ways, such as upregulation of lipase, serine endopeptidase, glutathione S-transferase, cadherin, alkaline phosphatase, and aminopeptidases and triggering transposon mobilization in Helicoverpa armigera. [43]
An additional promiscuous activity of some plant defensins is stopping the growth or disrupting the membranes of cancer cells in in vitro experiments. [44] [45] This interaction is basically facilitated and made stable due to the negatively charged membrane components on cancer cells relative to the positive charge of defensin. [46] [47] Typically, in addition to reducing the viability of melanoma and leukemia cells, Nicotiana alata defensin 1 (NaD1) reportedly induces the death of tumor cells within 30 minutes of contact. [48] This necrotic-like cell killing was facilitated by the binding of NaD1 to the plasma membrane lipid, phosphatidylinositol 4,5-bisphosphate (PIP2), which resulted in subsequent cell lysis. Defensins from plant origins have also shown potent toxicity towards colon and breast cancer. [49]
Plant defensins are expressed in diverse organelles and tissues in plants, and exposure of plants to specific environmental stresses has been associated with increased expression of defensin, suggesting their function in abiotic stress defense. [50] By means of endoplasmic reticulum adaptive activity, plant defensins AhPDF1.1 and AhPDF1.2 were recently found to exhibit metal (Zn) tolerance in yeast and plants. [51] Also, a defensin from paddy has been documented to sequester cadmium in rice, preventing its intracellular distribution. [7] Overexpression of chickpea defensin gene also confers tolerance to water-deficit stress in Arabidopsis thaliana. [52]
The following plant proteins belong to this family:
A database for antimicrobial peptides, including defensins is available: PhytAMP (http://phytamp.hammamilab.org). [57]
Lactoferrin (LF), also known as lactotransferrin (LTF), is a multifunctional protein of the transferrin family. Lactoferrin is a globular glycoprotein with a molecular mass of about 80 kDa that is widely represented in various secretory fluids, such as milk, saliva, tears, and nasal secretions. Lactoferrin is also present in secondary granules of PMNs and is secreted by some acinar cells. Lactoferrin can be purified from milk or produced recombinantly. Human colostrum has the highest concentration, followed by human milk, then cow milk (150 mg/L).
Defensins are small cysteine-rich cationic proteins across cellular life, including vertebrate and invertebrate animals, plants, and fungi. They are host defense peptides, with members displaying either direct antimicrobial activity, immune signaling activities, or both. They are variously active against bacteria, fungi and many enveloped and nonenveloped viruses. They are typically 18-45 amino acids in length, with three or four highly conserved disulphide bonds.
Antimicrobial peptides (AMPs), also called host defence peptides (HDPs) are part of the innate immune response found among all classes of life. Fundamental differences exist between prokaryotic and eukaryotic cells that may represent targets for antimicrobial peptides. These peptides are potent, broad spectrum antimicrobials which demonstrate potential as novel therapeutic agents. Antimicrobial peptides have been demonstrated to kill Gram negative and Gram positive bacteria, enveloped viruses, fungi and even transformed or cancerous cells. Unlike the majority of conventional antibiotics it appears that antimicrobial peptides frequently destabilize biological membranes, can form transmembrane channels, and may also have the ability to enhance immunity by functioning as immunomodulators.
Paneth cells are cells in the small intestine epithelium, alongside goblet cells, enterocytes, and enteroendocrine cells. Some can also be found in the cecum and appendix. They are located below the intestinal stem cells in the intestinal glands and the large eosinophilic refractile granules that occupy most of their cytoplasm.
Systemin is a plant peptide hormone involved in the wound response in the family Solanaceae. It was the first plant hormone that was proven to be a peptide having been isolated from tomato leaves in 1991 by a group led by Clarence A. Ryan. Since then, other peptides with similar functions have been identified in tomato and outside of the Solanaceae. Hydroxyproline-rich glycopeptides were found in tobacco in 2001 and AtPeps were found in Arabidopsis thaliana in 2006. Their precursors are found both in the cytoplasm and cell walls of plant cells, upon insect damage, the precursors are processed to produce one or more mature peptides. The receptor for systemin was first thought to be the same as the brassinolide receptor but this is now uncertain. The signal transduction processes that occur after the peptides bind are similar to the cytokine-mediated inflammatory immune response in animals. Early experiments showed that systemin travelled around the plant after insects had damaged the plant, activating systemic acquired resistance, now it is thought that it increases the production of jasmonic acid causing the same result. The main function of systemins is to coordinate defensive responses against insect herbivores but they also affect plant development. Systemin induces the production of protease inhibitors which protect against insect herbivores, other peptides activate defensins and modify root growth. They have also been shown to affect plants' responses to salt stress and UV radiation. AtPEPs have been shown to affect resistance against oomycetes and may allow A. thaliana to distinguish between different pathogens. In Nicotiana attenuata, some of the peptides have stopped being involved in defensive roles and instead affect flower morphology.
Cathelicidin antimicrobial peptide (CAMP) is a polypeptide that is primarily stored in the lysosomes of macrophages and polymorphonuclear leukocytes (PMNs); in humans, the CAMP gene encodes the peptide precursor CAP-18, which is processed by proteinase 3-mediated extracellular cleavage into the active form LL-37. LL-37 is the only peptide in the Cathelicidin family found in the human body.
Beta-defensin 2 (BD-2) also known as skin-antimicrobial peptide 1 (SAP1) is a peptide that in humans is encoded by the DEFB4 gene.
Alpha defensins are a family of mammalian defensin peptides of the alpha subfamily. In mammals they are also known as cryptdins and are produced within the small bowel. Cryptdin is a portmanteau of crypt and defensin.
Beta defensins are a family of vertebrate defensins. The beta defensins are antimicrobial peptides implicated in the resistance of epithelial surfaces to microbial colonization.
Defensin, alpha 1 also known as human alpha defensin 1, human neutrophil peptide 1 (HNP-1) or neutrophil defensin 1 is a human protein that is encoded by the DEFA1 gene. Human alpha defensin 1 belongs to the alpha defensin family of antimicrobial peptides.
Arthropod defensins are a family defensin proteins found in mollusks, insects, and arachnids. These cysteine-rich antibacterial peptides are primarily active against Gram-positive bacteria and fungi in vitro. However Drosophila fruit flies mutant for the fly defensin were more susceptible to infection by the Gram-negative bacteria Providencia burhodogranariea, and resisted infection against Gram-positive bacteria like wild-type flies. It remains to be seen how in vitro activity relates to in vivo function. Mutants for the defensin-like antimicrobial peptide Drosomycin were more susceptible to fungi, validating a role for defensin-like peptides in anti-fungal defence.
Histatins are histidine-rich (cationic) antimicrobial proteins found in saliva. Histatin's involvement in antimicrobial activities makes histatin part of the innate immune system.
Plant lipid transfer proteins, also known as plant LTPs or PLTPs, are a group of highly-conserved proteins of about 7-9kDa found in higher plant tissues. As its name implies, lipid transfer proteins facilitate the shuttling of phospholipids and other fatty acid groups between cell membranes. LTPs are divided into two structurally related subfamilies according to their molecular masses: LTP1s (9 kDa) and LTP2s (7 kDa). Various LTPs bind a wide range of ligands, including fatty acids with a C10–C18 chain length, acyl derivatives of coenzyme A, phospho- and galactolipids, prostaglandin B2, sterols, molecules of organic solvents, and some drugs.
Protegrins are small peptides containing 16-18 amino acid residues. Protegrins were first discovered in porcine leukocytes and were found to have antimicrobial activity against bacteria, fungi, and some enveloped viruses. The amino acid composition of protegrins contains six positively charged arginine residues and four cysteine residues. Their secondary structure is classified as cysteine-rich β-sheet antimicrobial peptides, AMPs, that display limited sequence similarity to certain defensins and tachyplesins. In solution, the peptides fold to form an anti-parallel β-strand with the structure stabilized by two cysteine bridges formed among the four cysteine residues. Recent studies suggest that protegrins can bind to lipopolysaccharide, a property that may help them to insert into the membranes of gram-negative bacteria and permeabilize them.
Thionins are a family of small proteins found solely in higher plants. Typically, a thionin consists of 45–48 amino acid residues. 6–8 of these are cysteine forming 3–4 disulfide bonds. They include phoratoxins and viscotoxins.
Theta-defensins are a family of mammalian antimicrobial peptides. They are found in non-human 'Old World' primates, but not in human, gorilla, bonobo, and chimpanzee.
Drosomycin is an antifungal peptide from Drosophila melanogaster and was the first antifungal peptide isolated from insects. Drosomycin is induced by infection by the Toll signalling pathway, while expression in surface epithelia like the respiratory tract is instead controlled by the immune deficiency pathway (Imd). This means that drosomycin, alongside other antimicrobial peptides (AMPs) such as cecropins, diptericin, drosocin, metchnikowin and attacin, serves as a first line defence upon septic injury. However drosomycin is also expressed constitutively to a lesser extent in different tissues and throughout development.
Leucine-rich repeat receptor like protein kinase are plant cell membrane localized Leucine-rich repeat (LRR) receptor kinase that play critical roles in plant innate immunity. Plants have evolved intricate immunity mechanism to combat against pathogen infection by recognizing Pathogen Associated Molecular Patterns (PAMP) and endogenous Damage Associated Molecular Patterns (DAMP). PEPR 1 considered as the first known DAMP receptor of Arabidopsis.
Cysteine-rich proteins are small proteins that contain a large number of cysteines. These cysteines either cross-link to form disulphide bonds, or bind metal ions by chelation, stabilising the protein's tertiary structure. CRPs include a highly conserved secretion peptide signal at the N-terminus and a cysteine-rich region at the C-terminus.
A proteolipid is a protein covalently linked to lipid molecules, which can be fatty acids, isoprenoids or sterols. The process of such a linkage is known as protein lipidation, and falls into the wider category of acylation and post-translational modification. Proteolipids are abundant in brain tissue, and are also present in many other animal and plant tissues. They include ghrelin, a peptide hormone associated with feeding. Many proteolipids are composed of proteins covalenently bound to fatty acid chains, often granting them an interface for interacting with biological membranes. They are not to be confused with lipoproteins, a kind of spherical assembly made up of many molecules of lipids and some apolipoproteins.