Tissue-specific expression of head-to-tail cyclized miniproteins in Violaceae and structure determination of the root cyclotide Viola hederacea root cyclotide1

Characterization of cyclotides Mra30 and cycloviolacin O17 derived from Viola dalatensis Gadnep

Bacteria threaten human and animal health, and standard antibiotics no longer effective. Antibiotic-resistant microorganisms can make infection treatment challenging and perhaps fail. Investigating the attributes of cyclotide, a peptide with promising antibacterial properties that holds great potential in the field of antibiotic research. The structure of these cyclic peptides involves six conserved cysteine residues that form three disulfide bonds, resulting in a cyclic cystine knot (CCK). This feature guarantees their durability when exposed to changes in temperature, chemicals, and enzymatic degradation. The two cyclotides, cycloviolacin O17 and mra30, were obtained from Viola dalatensis Gadnep through a series of techniques including the use of a 50% acetonitrile/49% miliQ water/1% formic acid solution for extraction, ammonium salt precipitation, RP-HPLC purification and sequence identification by LC-MS/MS. These cyclotides exhibit antibacterial effects on specific strains of bacteria like Staphylococcus aureus, Bacillus subtilis, and Pseudomonas aeruginosa at a concentration of 0.2 mg/mL, leading to inhibition zones ranging from 10 to 14 mm. In addition, the disulfide bonds play a crucial role in the antibacterial function of cyclotides. Disrupting the disulfide bonds through ankylation reaction results in the loss of antibacterial properties in the cyclotides (cyO17 and mra30). The minimum inhibitory concentration (MIC) values of mra30 and cyO17 are significantly low, ranging from 0.1 to 0.6 µM. These values are approximately three times lower than the MIC values observed in salt precipitation samples.

Plant proteins, peptides, and non-protein amino acids: Toxicity, sources, and analysis

Plants have evolved various mechanisms to synthesize diverse range of substances that contribute to their survival against pests, pathogens, predators, and adverse environmental conditions. Although several plant metabolites possess therapeutic potential, some can be potentially harmful to human and animal health when consumed in large proportion. Proteins, peptides, and non-protein amino acids are products of plant biochemical pathways with proven beneficial and nutritional effects. Despite these benefits, the in vivo toxicities associated with certain plant-derived proteins, peptides, and non-protein amino acids pose a significant risk to humans and animals. Symptoms of poisoning include nausea, vomiting, diarrhea, hair and weight loss, goiter, cataracts, and infertility. Even though plant processing methods such as soaking and drying can reduce the amount of toxin contained in plants, complete riddance is often impossible. As such, food regulatory bodies need to prevent uncontrolled consumption of the listed and many other toxin-containing plant species to keep the public safe. For this purpose, this review collates crucial insights into the sources, and in vivo toxicity associated with certain plant-derived proteins, peptides, and non-protein amino acids that have the clear potential to adversely affect human health. Additionally, this review provides information on analytical methods suitable for the detection of these substances in plants.

A comprehensive review of phytochemistry, pharmacology and quality control of plants from the genus Viola

OBJECTIVES

The genus Viola belongs to the family Violaceae, and the plants from this genus are essential folk herb medicine extensively used in many areas. The plants from the genus Viola are used to treat various diseases and exert a significant role in protecting people's health. This review summarized the genus Viola plants' phytochemistry, pharmacology, and quality control methods.

KEY FINDINGS

The information on chemical constituents and pharmacological effects of the genus Viola was obtained by searching the Web of Science, Pubmed, CNKI, and other databases. A total of 208 valuable articles were selected and analyzed in this review. The main chemical components of plants from the genus Viola consist of flavonoids, coumarins, alkaloids, lignans, sesquiterpenes, cyclotides, etc. The active chemical components of medicinal plants from this genus exert antibacterial, antiviral, antioxidant, anti-inflammatory, anti-tumor, neuroprotective, hepatoprotective activities, and so on. The quality control of these plants is not sufficient and needs further research.

SUMMARY

The chemical constituents, pharmacological effects, and quality control of plants from the genus Viola were systematically summarized in this paper, and this review provides a literature basis for the further research of plants from this genus.

Two dimensional materials are non-nanotoxic and biocompatible towards cyclotides: evidence from classical molecular dynamics simulations

Cyclotides are backbone-cyclized peptides of plant origin enriched with disulfide bonds, having exceptional stability towards thermal denaturation and proteolytic degradation. They have a plethora of activities like antibacterial, antifungal, anti-tumor and anti-HIV properties predominantly owing to their selective interaction with certain phospholipids, thereby leading to the disruption of cellular membranes. On the other hand, low-dimensional materials like graphene and hexagonal boron nitride (h-BN) are also known to show membrane-proliferating activities through lipid extraction. A plausible and more effective antibacterial, anti-tumor and antifungal agent would be a composite of these 2D materials and cyclotides, provided the structures of the peptides remain unperturbed upon adsorption and interaction. In this study, classical molecular dynamics simulations are performed to understand the nature of adsorption of cyclotides belonging to different families on graphene and h-BN and analyze the resulting structural changes. It is revealed that, due to their exceptional structural stability, cyclotides maintain their structural integrity upon adsorption on the 2D materials. In addition, the aggregated states of the cyclotides, which are ubiquitous in plant organs, are also not disrupted upon adsorption. Extensive free energy calculations show that the adsorption strength of the cyclotides is moderate in comparison to those of other similar-sized biomolecules, and the larger the size of the aggregates, the weaker the binding of individual peptides with the 2D materials, thereby leading to their lower release times from the materials. It is predicted that graphene and h-BN may safely be used for the preparation of composites with cyclotides, which in turn may be envisaged to be probable candidates for manufacturing next-generation bionano agents for agricultural, antibacterial and therapeutic applications.

Protocols for measuring the stability and cytotoxicity of cyclotides

Cyclotides are plant host-defense peptides that have a wide range of biological activities and have diverse potential applications in medicine and agriculture. These 27-37 amino acid peptides have a head-to-tail cyclic backbone and are built around a cystine knot core, which makes them exceptionally stable. This stability and their amenability to sequence modifications has made cyclotides attractive scaffolds in drug design, and many synthetic cyclotides have now been designed and synthesized to test their efficacy as leads for a wide range of diseases, including infectious disease, cancer, pain and multiple sclerosis. Additionally, some natural cyclotides are selectively toxic to certain cancer cell lines, opening their potential as anticancer agents, and others have insecticidal activity, with applications in crop protection. With these applications in mind, there is a need to be able to measure cyclotides in pharmaceutical or agrichemical formulations and in biological media such as blood serum, as well as to assess their potential persistence in the environment when used as agrichemical agents. This chapter describes protocols for quantifying cyclotides in biological fluids, measuring their stability, and assessing their relative cytotoxicity on various types of cells.

The involvement of cyclotides in mutual interactions of violets and the two-spotted spider mite

Plants employ different chemicals to protect themselves from herbivory. These defenses may be constitutive or triggered by stress. The chemicals can be toxic, act as repellents, phagosuppressants and/or phago-deterrents. The two-spotted spider mite (Tetranychus urticae) is a generalist arthropod herbivorous pest and its feeding causes extensive damage both to crops and wild plants. Cyclotides are cyclic peptides involved in host-plant defenses. A single Viola sp. can produce more than a hundred cyclotides with different biological activities and roles. The organ and tissue specific cyclotide patterns change over the seasons and/or with environment, but the role of biotic/abiotic stress in shaping them remains unclear. Here, we demonstrate the involvement of cyclotides in mutual interactions between violets and mites. We used immunohistochemistry and mass spectrometry imaging to show the ingested cyclotides in T. urticae and assess the Viola odorata response to mite feeding. Moreover, to assess how mites are affected by feeding on violets, acceptance and reproductive performance was compared between Viola uliginosa, V. odorata and Phaseolus vulgaris. We demonstrate that cyclotides had been taken in by mites feeding on the violets. The ingested peptides were found in contact with epithelial cells of the mite digestive system, in the fecal matter, feces, ovary and eggs. Mites preferred common bean plants (P. vulgaris) to any of the violet species; the latter affected their reproductive performance. The production of particular cyclotides in V. odorata (denoted by molecular weights: 2979, 3001, 3017, 3068, 3084, 3123) was activated by mite feeding and their levels were significantly elevated compared to the control after 5 and 21 days of infestation. Specific cyclotides may affect mites by being indigestible or through direct interaction with cells in the mite digestive tract and reproductive organs. A group of particular peptides in V. odorata appears to be involved in defense response against herbivores.

Mass Spectrometric Identification of Antimicrobial Peptides from Medicinal Seeds

Traditional medicinal plants contain a variety of bioactive natural products including cysteine-rich (Cys-rich) antimicrobial peptides (AMPs). Cys-rich AMPs are often crosslinked by multiple disulfide bonds which increase their resistance to chemical and enzymatic degradation. However, this class of molecules is relatively underexplored. Herein, in silico analysis predicted 80–100 Cys-rich AMPs per species from three edible traditional medicinal plants: Linum usitatissimum (flax), Trifolium pratense (red clover), and Sesamum indicum (sesame). Bottom-up proteomic analysis of seed peptide extracts revealed direct evidence for the translation of 3–10 Cys-rich AMPs per species, including lipid transfer proteins, defensins, α-hairpinins, and snakins. Negative activity revealed by antibacterial screening highlights the importance of employing a multi-pronged approach for AMP discovery. Further, this study demonstrates that flax, red clover, and sesame are promising sources for further AMP discovery and characterization.

Cyclotide host-defense tailored for species and environments in violets from the Canary Islands

Cyclotides are cyclic peptides produced by plants. Due to their insecticidal properties, they are thought to be involved in host defense. Violets produce complex mixtures of cyclotides, that are characteristic for each species and variable in different environments. Herein, we utilized mass spectrometry (LC–MS, MALDI-MS), transcriptomics and biological assays to investigate the diversity, differences in cyclotide expression based on species and different environment, and antimicrobial activity of cyclotides found in violets from the Canary Islands. A wide range of different habitats can be found on these islands, from subtropical forests to dry volcano peaks at high altitudes. The islands are inhabited by the endemic Viola palmensis, V. cheiranthifolia, V. anagae and the common V. odorata. The number of cyclotides produced by a given species varied in plants from different environments. The highest diversity was noted in V. anagae which resides in subtropical forest and the lowest in V. cheiranthifolia from the Teide volcano. Transcriptome sequencing and LC–MS were used to identify 23 cyclotide sequences from V. anagae. Cyclotide extracts exhibited antifungal activities with the lowest minimal inhibitory concentrations noted for V. anagae (15.62 μg/ml against Fusarium culmorum). The analysis of the relative abundance of 30 selected cyclotides revealed patterns characteristic to both species and populations, which can be the result of genetic variability or environmental conditions in different habitats. The current study exemplifies how plants tailor their host defense peptides for various habitats, and the usefulness of cyclotides as markers for chemosystematics.

Molecular Dynamics Simulation Study on Interactions of Cycloviolacin with Different Phospholipids

Cyclotides are disulfide-rich cyclic peptides isolated from plants, which are extremely stable against thermal and proteolytic degradation, with a variety of biological activities including antibacterial, hemolytic, anti-HIV, and anti-tumor. Most of these bioactivities are related to their preference for binding to certain types of phospholipids and subsequently disrupt lipid membranes. In the present study, we use a cyclotide, cycloviolacin O2 (cyO2), as a model system to investigate its interactions with three lipid bilayers 1-palmitoyl-2-oleoylphosphatidylethanolamine (POPE), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG)-doped POPE, and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), to help understand its potential mechanism of action toward the membranes at the molecular level using molecular dynamics simulations. In our simulations, cyO2 repeatedly forms stable binding complexes with the POPE-containing bilayers, while within the same simulation time scale, it "jumps" back and forth on the surface of the POPC bilayer without a strong binding. Detailed analyses reveal that the electrostatic attraction is the main driving force for the initial bindings between cyO2 and the lipids, but with strikingly different strengths in different bilayers. For the POPE-containing bilayers, the charged residues of cyO2 attract both POPE amino and phosphate head groups favorably; meanwhile, its hydrophobic residues are deeply inserted into the lipid hydrophobic tails (core) of the membrane, thus forming stable binding complexes. In contrast, POPC lipids with three methyl groups on the amino head group create a steric hindrance when interacting with cyO2, thus resulting in a relatively difficult binding of cyO2 on POPC compared to POPE. Our current findings provide additional insights for a better understanding of how cyO2 binds to the POPE-containing membrane, which should shed light on the future cyclotide-based antibacterial agent design.

Cyclotides from Brazilian Palicourea sessilis and Their Effects on Human Lymphocytes

Cyclotides are plant-derived peptides found within five families of flowering plants (Violaceae, Rubiaceae, Fabaceae, Solanaceae, and Poaceae) that have a cyclic backbone and six conserved cysteine residues linked by disulfide bonds. Their presence within the Violaceae species seems ubiquitous, yet not all members of other families produce these macrocyclic peptides. The genus Palicourea Aubl. (Rubiaceae) contains hundreds of neotropical species of shrubs and small trees; however, only a few cyclotides have been discovered hitherto. Herein, five previously uncharacterized Möbius cyclotides within Palicourea sessilis and their pharmacological activities are described. Cyclotides were isolated from leaves and stems of this plant and identified as pase A-E, as well as the known peptide kalata S. Cyclotides were de novo sequenced by MALDI-TOF/TOF mass spectrometry, and their structures were solved by NMR spectroscopy. Because some cyclotides have been reported to modulate immune cells, pase A-D were assayed for cell proliferation of human primary activated T lymphocytes, and the results showed a dose-dependent antiproliferative function. The toxicity on other nonimmune cells was also assessed. This study reveals that pase cyclotides have potential for applications as immunosuppressants and in immune-related disorders.

An insight into biological activities of native cyclotides for potential applications in agriculture and pharmaceutics

Cyclotides are plant-derived mini-proteins of 28 - 37 amino acids. They have a characteristic head-to-tail cyclic backbone and three disulfide cross-linkages formed by six highly conserved cysteine residues, creating a unique knotted ring structure, known as a cyclic cystine knot (CCK) motif. The CCK topology confers immense stability to cyclotides with resistance to thermal and enzymatic degradation. Native cyclotides are of interest due to their multiple biological activities with several potential applications in agricultural (e.g. biopesticides, antifungal) and pharmaceutical (e.g. anti-HIV, cytotoxic to tumor cells) sectors. The most recent application of insecticidal activity of cyclotides is the commercially available biopesticidal spray known as 'Sero X' for cotton crops. Cyclotides have a general mode of action and their potency of bioactivity is determined through their binding ability, pore formation and disruption of the target biological membranes. Keeping in view the important potential applications of biological activities of cyclotides and the lack of an extensive and analytical compilation of bioactive cyclotides, the present review systematically describes eight major biological activities of the native cyclotides from four angiosperm families viz. Fabaceae, Poaceae, Rubiaceae, Violaceae. The bioactivities of 94 cytotoxic, 57 antibacterial, 44 hemolytic, 25 antifungal, 21 anti-HIV, 20 nematocidal, 10 insecticidal and 5 molluscicidal cyclotides have been comprehensively elaborated. Further, their distribution in angiosperm families, mode of action and future prospects have also been discussed.

The life cycle of cyclotides: biosynthesis and turnover in plant cells

Turnover rates have implications for understanding cyclotide biology and improving plant cell culture-based production systems. Cyclotides are a family of polypeptides recognized for a broad spectrum of bioactivities. The cyclic, cystine knot structural motif imparts these peptides with resistance to temperature, chemicals and proteolysis. Cyclotides are found widely distributed across the Violaceae and in five other plant families, where their presumed biological role is host defense. Violets produce mixtures of different cyclotides that vary depending on the organ, tissue or influence of environmental factors. In the present study, we investigated the biosynthesis and turnover of cyclotides in plant cells. Viola uliginosa suspension cultures were grown in media where all nitrogen containing salts were replaced with their ¹⁵N counterparts. This approach combined with LC-MS analysis allowed to separately observe the production of ¹⁵N-labelled peptides and decomposition of ¹⁴N cyclotides present in the cells when switching the media. Additionally, we investigated changes in cyclotide content in V. odorata germinating seeds. In the suspension cultures, the degradation rates varied for individual cyclotides and the highest was noted for cyO13. Rapid increase in production of ¹⁵N peptides was observed until day 19 and subsequently, a plateau of production, indicating an equilibrium between biosynthesis and turnover. The developing seedling appeared to consume cyclotides present in the seed endosperm. We show that degradation processes shape the cyclotide pattern present in different tissues and environments. The results indicate that individual cyclotides play different roles-some in defense and others as storage proteins. The turnover of cyclotides should be accounted to improve cell culture production systems.

Cysteine-rich antimicrobial peptides from plants: The future of antimicrobial therapy

There has been a spurt in the spread of microbial resistance to antibiotics due to indiscriminate use of antimicrobial agents in human medicine, agriculture, and animal husbandry. It has been realized that conventional antibiotic therapy would be less effective in the coming decades and more emphasis should be given for the development of novel antiinfective therapies. Cysteine rich peptides (CRPs) are broad-spectrum antimicrobial agents that modulate the innate immune system of different life forms such as bacteria, protozoans, fungi, plants, insects, and animals. These are also expressed in several plant tissues in response to invasion by pathogens, and play a crucial role in the regulation of plant growth and development. The present work explores the importance of CRPs as potent antimicrobial agents, which can supplement and/or replace the conventional antibiotics. Different plant parts of diverse plant species showed the presence of antimicrobial peptides (AMPs), which had significant structural and functional diversity. The plant-derived AMPs exhibited potent activity toward a range of plant and animal pathogens, protozoans, insects, and even against cancer cells. The cysteine-rich AMPs have opened new avenues for the use of plants as biofactories for the production of antimicrobials and can be considered as promising antimicrobial drugs in biotherapeutics.

Exploring the Sequence Diversity of Cyclotides from Vietnamese Viola Species

Viola is the largest genus in the Violaceae plant family and is known for its ubiquitous natural production of cyclotides. Many Viola species are used as medicinal herbs across Asia and are often consumed by humans in teas for the treatment of diseases, including ulcers and asthma. Previous studies reported the isolation of cyclotides from Viola species in many countries in the hope of discovering novel compounds with anti-cancer activities; however, Viola species from Vietnam have not been investigated to date. Here, the discovery of cyclotides from three Viola species (V. arcuata, V. tonkinensis, and V. austrosinensis) collected in the northern mountainous region of Vietnam is reported. Ten cyclotides were isolated from these three Viola species: four are novel and six were previously reported to be expressed in other plants. The structures of three of the new bracelet cyclotides are similar to that of cycloviolacin O2. Because cycloviolacin O2 has previously been shown to have potent activity against a wide range of cancer cell lines including HeLa (human cervical cancer cells) and PC-3 (human prostate cancer cells), the cancer cytotoxicity of the cyclotides isolated from V. arcuata was assessed. All tested cyclotides were cytotoxic against cancer cells, albeit to varying degrees. The sequences discovered in this study significantly expand the understanding of cyclotide diversity, especially in comparison with other cyclotides found in plants from the Asian region.

Behavioral and physiological effects of Viola spp. cyclotides on Myzus persicae (Sulz.)

Cyclotides are defense peptides produced by several plant families. Viola spp. (Violaceae) produce an array of cyclotides with varying biological activities. The peach potato aphid Myzus persicae (Sulz.) (Hemiptera: Aphididae) is a generalist that feeds on the secondary hosts of over 40 plant families, including Violaceae. The present work aimed to evaluate the activities of cycloviolacins from Viola odorata L. and V. ulignosa Besser (cyO2, cyO3, cyO13, cyO19) against M. persicae. To investigate the peptides' influence on aphid feeding behavior, we used 20% sucrose diets supplemented with cyclotides and measured the effects with electrical penetration graph (EPG) technique. We also applied anti-cyclotide antibodies and immunohistochemistry to track the peptides in the digestive systems of the aphids. Our study shows that cyclotides affect aphid probing and feeding behavior and limit their diet sap uptake. The cycloviolacin cyclotides: cyO13 (100 µM) and cyO19 (50 µM) most strongly impeded aphid ingestion activities when applied in sucrose diet. Sustained ingestion of the diet was blocked by 100 µM cyO13, and no aphid showed ingestion of the diet for longer than 10 min. Cyclotides were detected in the pharynx, in contact with the epipharyngeal gustatory organ, in the stomach (midgut) and upper intestine. The present study shows the deterrent activity of cycloviolacins on M. persicae. This activity may be related to the peptides' effects on epithelial cells and gustatory organs along the aphid digestive system. We demonstrate that cyclotides may play an important role in plant-aphid interactions.

Discovery of Cyclotides from Australasian Plants

This article is part of a special issue celebrating the contributions of Professor Paul Alewood to peptide science. We begin by providing a summary of collaborative projects between the Alewood and Craik groups at The University of Queensland and highlighting the impacts of some of these studies. In particular, studies on the discovery, synthesis, structures, and bioactivities of disulfide-rich toxins from animal venoms have led to a greater understanding of the biology of ion channels and to applications of these bioactive peptides in drug design. The second part of the article focuses on plant-derived disulfide-rich cyclic peptides, known as cyclotides, and includes an analysis of the geographical distribution of Australasian plant species that contain cyclotides as well as an analysis of the diversity of cyclotide sequences found in Australasian plants. This should provide a useful resource for researchers to access native cyclotides and explore their chemistry and biology.

Cyclotides: From Structure to Function

This Review explores the class of plant-derived macrocyclic peptides called cyclotides. We include an account of their discovery, characterization, and distribution in the plant kingdom as well as a detailed analysis of their sequences and structures, biosynthesis and chemical synthesis, biological functions, and applications. These macrocyclic peptides are around 30 amino acids in size and are characterized by their head-to-tail cyclic backbone and cystine knot motif, which render them to be exceptionally stable, with resistance to thermal or enzymatic degradation. Routes to their chemical synthesis have been developed over the past two decades, and this capability has facilitated a wide range of mutagenesis and structure-activity relationship studies. In turn, these studies have both led to an increased understanding of their mechanisms of action as well as facilitated a range of applications in agriculture and medicine, as ecofriendly crop protection agents, and as drug leads or scaffolds for pharmaceutical design. Our overall objective in this Review is to provide readers with a comprehensive overview of cyclotides that we hope will stimulate further work on this fascinating family of peptides.

Pharmaceutical applications of cyclotides

Cyclotides are cyclic peptides, present in several plant families, that show diverse biological properties. Structurally, cyclotides share a distinctive head-to-tail circular knotted topology of three disulfide bonds. This framework provides cyclotides with extraordinary resistance to thermal and chemical denaturation. There is increasing interest in the therapeutic potential of cyclotides, which combine several promising pharmaceutical properties, including binding affinity, target selectivity, and low toxicity towards healthy mammalian cells. Recently, cyclotides have been reported to be orally bioavailable and have proved to be amenable to modifications. Here, we provide an overview of the structure, properties, and pharmaceutical applications of cyclotides.

Discovery and Characterization of Cyclotides from Rinorea Species

Cyclotides are macrocyclic cystine-knotted peptides most commonly found in the Violaceae plant family. Although Rinorea is the second-largest genera within the Violaceae family, few studies have examined whether or not they contain cyclotides. To further our understanding of cyclotide diversity and evolution, we examined the cyclotide content of two Rinorea species found in Southeast Asia: R. virgata and R. bengalensis. Seven cyclotides were isolated from R. virgata (named Rivi1-7), and a known cyclotide (cT10) was found in R. bengalensis. Loops 2, 5, and 6 of Rivi1-4 contained sequences not previously seen in corresponding loops of known cyclotides, thereby expanding our understanding of the diversity of cyclotides. In addition, the sequence of loop 2 of Rivi3 and Rivi4 were identical to some related noncyclic "acyclotides" from the Poaceae plant family. As only acyclotides, but not cyclotides, have been reported in monocotyledons thus far, our findings support an evolutionary link between monocotyledon-derived ancestral cyclotide precursors and dicotyledon-derived cyclotides. Furthermore, Rivi2 and Rivi3 had comparable cytotoxic activities to the most cytotoxic cyclotide known to date: cycloviolacin O2 from Viola odorata; yet, unlike cycloviolacin O2, they did not show hemolytic activity. Therefore, these cyclotides represent novel scaffolds for use in future anticancer drug design.

How Does the Sweet Violet (Viola odorata L.) Fight Pathogens and Pests - Cyclotides as a Comprehensive Plant Host Defense System

Cyclotides are cyclic plant polypeptides of 27-37 amino acid residues. They have been extensively studied in bioengineering and drug development contexts. However, less is known about the relevance of cyclotides for the plants producing them. The anti-insect larvae effects of kB1 and antibacterial activity of cyO2 suggest that cyclotides are a part of plant host defense. The sweet violet (Viola odorata L.) produces a wide array of cyclotides, including kB1 (kalata B1) and cyO2 (cycloviolacin O2), with distinct presumed biological roles. Here, we evaluate V. odorata cyclotides' potency against plant pathogens and their mode of action using bioassays, liposome experiments and immunogold labeling for transmission electron microscopy (TEM). We explore the link between the biological activity and distribution in plant generative, vegetative tissues and seeds, depicted by immunohistochemistry and matrix assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI). Cyclotides cyO2, cyO3, cyO13, and cyO19 are shown to have potent activity against model fungal plant pathogens (Fusarium oxysporum, F. graminearum, F. culmorum, Mycosphaerella fragariae, Botrytis cinerea) and fungi isolated from violets (Colletotrichum utrechtense and Alternaria alternata), with minimal inhibitory concentrations (MICs) ranging from 0.8 μM to 25 μM. Inhibition of phytopathogenic bacteria - Pseudomonas syringae pv. syringae, Dickeya dadantii and Pectobacterium atrosepticum - is also observed with MIC = 25-100 μM. A membrane-disrupting antifungal mode of action is shown. Finding cyO2 inside the fungal spore cells in TEM images may indicate that other, intracellular targets may be involved in the mechanism of toxicity. Fungi can not break down cyclotides in the course of days. varv A (kalata S) and kB1 show little potency against pathogenic fungi when compared with the tested cycloviolacins. cyO2, cyO3, cyO19 and kB1 are differentially distributed and found in tissues vulnerable to pathogen (epidermis, rizodermis, vascular bundles, protodermis, procambium, ovary walls, outer integuments) and pest (ground tissues of leaf and petiole) attacks, respectively, indicating a link between the cyclotides' sites of accumulation and biological role. Cyclotides emerge as a comprehensive defense system in V. odorata, in which different types of peptides have specific targets that determine their distribution in plant tissues.

Inhibition of Breast Cancer Cell Migration by Cyclotides Isolated from Pombalia calceolaria

Two new bracelet cyclotides from roots of Pombalia calceolaria with potential anticancer activity have been characterized in this work. The cyclotides Poca A and B (1 and 2) and the previously known CyO4 (3) were de novo sequenced by MALDI-TOF/TOF mass spectrometry (MS). The MS² spectra were examined and the amino acid sequences were determined. The purified peptides were tested for their cytotoxicity and effects on cell migration of MDA-MB-231, a triple-negative breast cancer cell line. The isolated cyclotides reduced the number of cancer cells by more than 80% at 20 μM, and the concentration-related cytotoxic responses were observed with IC₅₀ values of 1.8, 2.7, and 9.8 μM for Poca A (1), Poca B (2), and CyO4 (3), respectively. Additionally, the inhibition of cell migration (wound-healing assay) exhibited that CyO4 (3) presents an interesting activity profile, in being able to inhibit cell migration (50%) at a subtoxic concentration (2 μM). The distribution of these cyclotides in the roots was analyzed by MALDI imaging, demonstrating that all three compounds are present in the phloem and cortical parenchyma regions.

Cyclotides, a versatile ultrastable micro-protein scaffold for biotechnological applications

Cyclotides are fascinating microproteins (≈30-40 residues long) with a unique head-to-tail cyclized backbone, stabilized by three disulfide bonds forming a cystine knot. This unique topology makes them exceptionally stable to chemical, thermal and biological degradation compared to other peptides of similar size. Cyclotides have been also found to be highly tolerant to sequence variability, aside from the conserved residues forming the cystine knot, able to cross cellular membranes and modulate intracellular protein-protein interactions both in vitro and in vivo. These properties make them ideal scaffolds for many biotechnological applications. This article provides and overview of the properties of cyclotides and their applications as molecular imaging agents and peptide-based therapeutics.

Cyclotides: Overview and Biotechnological Applications

Cyclotides are globular microproteins with a unique head-to-tail cyclized backbone, stabilized by three disulfide bonds forming a cystine knot. This unique circular backbone topology and knotted arrangement of three disulfide bonds makes them exceptionally stable to chemical, thermal, and biological degradation compared to other peptides of similar size. In addition, cyclotides have been shown to be highly tolerant to sequence variability, aside from the conserved residues forming the cystine knot. Cyclotides can also cross cellular membranes and are able to modulate intracellular protein-protein interactions, both in vitro and in vivo. All of these features make cyclotides highly promising as leads or frameworks for the design of peptide-based diagnostic and therapeutic tools. This article provides an overview on cyclotides and their applications as molecular imaging agents and peptide-based therapeutics.

Improved method for quantitative analysis of the cyclotide kalata B1 in plasma and brain homogenate

This study provides a new method for quantifying the cyclotide kalata B1 in both plasma and brain homogenate. Cyclotides are ultra-stable peptides with three disulfide bonds that are interesting from a drug development perspective as they can be used as scaffolds. In this study we describe a new validated LC-MS/MS method with high sensitivity and specificity for kalata B1. The limit of quantification was 2 ng/mL in plasma and 5 ng/gmL in brain homogenate. The method was linear in the range 2-10,000 ng/mL for plasma and 5-2000 ng/g for brain. Liquid Chromatographic separation was performed on a HyPurity C18 column, 50 × 4.6 mm, 3 µm particle size. The method had inter- and intra-day precision and accuracy levels <15% and 12% respectively. Applying the method to in vivo plasma samples and brain homogenate samples from equilibrium dialysis yielded satisfying results and was able to describe the plasma pharmacokinetics and brain tissue binding of kalata B1. The described method is quick, reproducible and well suited to quantifying kalata B1 in biological matrices.

Cloning and characterization of novel cyclotides genes from South American plants

Cyclotides are multifunctional plant cyclic peptides containing 28-37 amino acid residues and a pattern of three disulfide bridges, forming a motif known as the cyclic cystine knot. Due to their high biotechnological potential, the sequencing and characterization of cyclotide genes are crucial not only for cloning and establishing heterologous expression strategies, but also to understand local plant evolution in the context of host-pathogen relationships. Here, two species from the Brazilian Cerrado, Palicourea rigida (Rubiaceae) and Pombalia lanata (A.St.-Hil.) Paula-Souza (Violaceae), were used for cloning and characterizing novel cyclotide genes. Using 3' and 5' RACE PCR and sequencing, two full cDNAs, named parigidin-br2 (P. rigida) and hyla-br1 (P. lanata), were isolated and shown to have similar genetic structures to other cyclotides. Both contained the conserved ER-signal domain, N-terminal prodomain, mature cyclotide domain and a C-terminal region. Genomic sequencing of parigidin-br2 revealed two different gene copies: one intronless allele and one presenting a rare 131-bp intron. In contrast, genomic sequencing of hyla-br1 revealed an intronless gene-a common characteristic of members of the Violaceae family. Parigidin-br2 5' and 3' UTRs showed the presence of 12 putative candidate sites for binding of regulatory proteins, suggesting that the flanking and intronic regions of the parigidin-br2 gene must play important roles in transcriptional rates and in the regulation of temporal and spatial gene expression. The high degree of genetic similarity and structural organization among the cyclotide genes isolated in the present study from the Brazilian Cerrado and other well-characterized plant cyclotides may contribute to a better understanding of cyclotide evolution.

Review seed biopharmaceutical cyclic peptides: From discovery to applications

Mini-proteins (or peptides) with disulfide bond/s and a cyclic backbone offer exciting opportunities for applications in medicine, as these ribosomally synthesized and posttranslationally modified peptides are exceptionally stable and amenable to grafting epitopes with desirable activities. Here I discuss important aspects of the discovery and applications of disulfide-bonded cyclic peptides from seeds, i.e., the trypsin inhibitor cyclotides and the preproalbumin with sunflower trypsin inhibitor-derived peptides, focusing on bioanalytical methods for and insights generated from their discovery as well as their potential use as engineering scaffolds for peptide-based drug design. The recent discovery of their precursors and processing enzymes could potentially enable in planta production of designer disulfide-bonded cyclic peptides, preferably in edible seeds, and address the demand for new biopharmaceutical peptides in a cost-effective manner.

Immunolocalization of cyclotides in plant cells, tissues and organ supports their role in host defense

The distribution of cyclotides was visualized in plant cells, tissues and organs using immunohistochemistry. Finding of cyclotides in tissues potentially vulnerable to pathogen attacks supports their role as defense molecules. The cyclotide family of plant peptides is characterized by the cyclic cystine knot motif and its diverse biological activities. Given their insecticidal and antimicrobial properties, the role of cyclotides in planta is probably associated with host defense. Our current understanding of the cellular compartmentalization of cyclotides in the vacuole is based on indirect studies on transgenic model plants that do not express cyclotides naturally. Matrix-assisted laser desorption ionization (MALDI) imaging has also been used to study the distribution of cyclotides, but the technique's resolution was insufficient to determine their tissue or cell distribution. To avoid the limitations of these approaches, immunohistochemical visualization methods were used. Antibodies were raised in rabbits using cycloviolacin O2 (cyO2), and their specificity was determined by Western and dot blot experiments. Slides for immunohistochemical analysis were prepared from leaf, petiole and root fragments of Viola odorata and Viola uliginosa, and specimens were visualized using indirect epifluorescence microscopy. The antibodies against cyclotides were specific against selected bracelet cyclotides with high similarity (cyO2, cyO3, cyO8, cyO13) and suitable for immunohistochemistry. The tissue distribution of the cyclotides visualized in this way is consistent with their proposed role in host defense-relatively large quantities were observed in the leaf and petiole epidermis in both Viola species. Cyclotides were also found in vascular tissue in all the assessed plant organs. The vacuole storage of cyclotides was directly shown.

Immunolocalization of cyclotides in plant cells, tissues and organ supports their role in host defense

The distribution of cyclotides was visualized in plant cells, tissues and organs using immunohistochemistry. Finding of cyclotides in tissues potentially vulnerable to pathogen attacks supports their role as defense molecules. The cyclotide family of plant peptides is characterized by the cyclic cystine knot motif and its diverse biological activities. Given their insecticidal and antimicrobial properties, the role of cyclotides in planta is probably associated with host defense. Our current understanding of the cellular compartmentalization of cyclotides in the vacuole is based on indirect studies on transgenic model plants that do not express cyclotides naturally. Matrix-assisted laser desorption ionization (MALDI) imaging has also been used to study the distribution of cyclotides, but the technique's resolution was insufficient to determine their tissue or cell distribution. To avoid the limitations of these approaches, immunohistochemical visualization methods were used. Antibodies were raised in rabbits using cycloviolacin O2 (cyO2), and their specificity was determined by Western and dot blot experiments. Slides for immunohistochemical analysis were prepared from leaf, petiole and root fragments of Viola odorata and Viola uliginosa, and specimens were visualized using indirect epifluorescence microscopy. The antibodies against cyclotides were specific against selected bracelet cyclotides with high similarity (cyO2, cyO3, cyO8, cyO13) and suitable for immunohistochemistry. The tissue distribution of the cyclotides visualized in this way is consistent with their proposed role in host defense-relatively large quantities were observed in the leaf and petiole epidermis in both Viola species. Cyclotides were also found in vascular tissue in all the assessed plant organs. The vacuole storage of cyclotides was directly shown.

Discovery, structure, function, and applications of cyclotides: circular proteins from plants

Cyclotides are plant-derived cyclic peptides that have a head-to-tail cyclic backbone and three conserved disulphide bonds that form a cyclic cystine knot motif. They occur in plants from the Violaceae, Rubiaceae, Cucurbitaceae, Fabaceae, and Solanaceae families, typically with 10-100 cyclotides in a given plant species, in a wide range of tissues, including flowers, leaves, stems, and roots. Some cyclotides are expressed in large amounts (up to 1g kg(-1) wet plant weight) and their natural function appears to be to protect plants from pests or pathogens. This article provides a brief overview of their discovery, distribution in plants, and applications. In particular, their exceptional stability has led to their use as peptide-based scaffolds in drug design applications. They also have potential as natural 'ecofriendly' insecticides, and as protein engineering frameworks.

Lysine-rich Cyclotides: A New Subclass of Circular Knotted Proteins from Violaceae

Cyclotides are macrocyclic proteins produced by plants for host defense. Although they occur sparsely in other plant families, cyclotides have been detected in every Violaceae plant species so far screened. Many of the Violaceae species examined until now have been from closely related geographical regions or habitats. To test the hypothesis that cyclotides are ubiquitous in this family, two geographically isolated (and critically endangered) species of Australasian Violaceae, namely Melicytus chathamicus and M. latifolius, were examined. Surprisingly, we discovered a suite of cyclotides possessing novel sequence features, including a lysine-rich nature, distinguishing them from "conventional" cyclotides and suggesting that they might have different physiological activities in plants to those reported to date. The newly discovered cyclotides were found to bind to lipid membranes and were cytotoxic against cancer cell lines but had low toxicity against red blood cells, which is advantageous for potential therapeutic applications. This suite of novel Lys-rich cyclotides emphasizes the broad diversity of cyclotides in Violaceae species.

Peptidomics of Circular Cysteine-Rich Plant Peptides: Analysis of the Diversity of Cyclotides from Viola tricolor by Transcriptome and Proteome Mining

Cyclotides are plant-derived mini proteins. They are genetically encoded as precursor proteins that become post-translationally modified to yield circular cystine-knotted molecules. Because of this structural topology cyclotides resist enzymatic degradation in biological fluids, and hence they are considered as promising lead molecules for pharmaceutical applications. Despite ongoing efforts to discover novel cyclotides and analyze their biodiversity, it is not clear how many individual peptides a single plant specimen can express. Therefore, we investigated the transcriptome and cyclotide peptidome of Viola tricolor. Transcriptome mining enabled the characterization of cyclotide precursor architecture and processing sites important for biosynthesis of mature peptides. The cyclotide peptidome was explored by mass spectrometry and bottom-up proteomics using the extracted peptide sequences as queries for database searching. In total 164 cyclotides were discovered by nucleic acid and peptide analysis in V. tricolor. Therefore, violaceous plants at a global scale may be the source to as many as 150 000 individual cyclotides. Encompassing the diversity of V. tricolor as a combinatorial library of bioactive peptides, this commercially available medicinal herb may be a suitable starting point for future bioactivity-guided screening studies.

Exogenous plant hormones and cyclotide expression in Viola uliginosa (Violaceae)

Plants from Violaceae produce cyclotides, peptides characterized by a circular peptide backbone and a cystine knot. This signature motif gives stability that can harness a wide spectrum of biological activities, with implications in plant defense and with applications in medicine and biotechnology. In the current work, cyclotide expressing in vitro cultures were established from Viola uliginosa. These cultures are useful models for studying biosynthesis of cyclotides and can also be used in their production. The cyclotide expression pattern is shown to be dependent on exogenous plant growth regulators, both on peptide and gene expression levels. The highest yields of cyclotides were obtained on media containing only a cytokinin and were correlated with storage material accumulation. Exposure to auxins decreased cyclotide production and caused shifting of the biosynthesis pattern to root specific cyclotides. The response to stimuli in terms of cyclotide expression pattern appears to be developmental, and related to polar auxin transportation and the auxin/cytokinin ratio regulating tissue differentiation. By the use of whole transcriptome shotgun sequencing (WTSS) and peptidomics, 20 cyclotide sequences from V. uliginosa (including 12 new) and 12 complete precursor proteins could be identified. The most abundant cyclotides were cycloviolacin O3 (CyO3), CyO8 and CyO13. A suspension culture was obtained that grew exponentially with a doubling time of approximately 3 days. After ten days of growth, the culture provided a yield of more than 4 mg CyO13 per gram dry mass.

Transcriptomic screening for cyclotides and other cysteine-rich proteins in the metallophyte Viola baoshanensis

Cysteine (Cys)-rich proteins (CRPs) are frequently associated with plant defense and stress resistance. Viola baoshanensis is a cadmium (Cd) hyper-accumulating plant whose CRPs-based defense systems are so far poorly understood. Next generation sequencing (NGS) techniques and a specialist searching tool, CrpExcel, were employed for identifying CRPs in V. baoshanensis. The transcriptome sequences of V. baoshanensis were assembled primarily from 454FLX/Hiseq2000 reads of plant cDNA sequencing libraries. CrpExcel was then used to search the ORFs and 9687 CRPs were identified, and included zinc finger (ZF) proteins, lipid transfer proteins, thaumatins and cyclotide precursors. Real-time PCR results showed that all CRP genes tested are constitutively expressed, but the genes of defensive peptides showed greater up-regulated expression than those of ZF-proteins in Cd- and/or wounding (Wd) treatments of V. baoshanensis seedlings. The NGS-derived sequences of cyclotide precursor genes were verified by RT-PCR and ABI3730 sequencing studies, and 32 novel cyclotides were identified in V. baoshanensis. In general, the metal-binding sites of ZF-containing CRPs also represented the potential vulnerable targets of toxic metals. This study provides broad insights into CRPs-based defense systems and stress-vulnerable targets in V. baoshanensis. It now brings the number of cyclotide sequences in V. baoshanensis to 53 and based on projections from this work, the number of cyclotides in the Violaceae is now conservatively estimated to be >30000.

Two Blast-independent tools, CyPerl and CyExcel, for harvesting hundreds of novel cyclotides and analogues from plant genomes and protein databases

Two high-throughput tools harvest hundreds of novel cyclotides and analogues in plants. Cyclotides are gene-encoded backbone-cyclized polypeptides displaying a diverse range of bioactivities associated with plant defense. However, genome-scale or database-scale evaluations of cyclotides have been rare so far. Here, a novel time-efficient Perl program, CyPerl, was developed for searching cyclotides from predicted ORFs of 34 available plant genomes and existing plant protein sequences from Genbank databases. CyPerl-isolated sequences were further analyzed by removing repeats, evaluating their cysteine-distributed regions (CDRs) and comparing with CyBase-collected cyclotides in a user-friendly Excel (Microsoft Office) template, CyExcel. After genome-screening, 186 ORFs containing 145 unique cyclotide analogues were identified by CyPerl and CyExcel from 30 plant genomes tested from 10 plant families. Phaseolus vulgaris and Zea mays were the richest two species containing cyclotide analogues in the plants tested. After screening protein databases, 266 unique cyclotides and analogues were identified from seven plant families. By merging with 288 unique CyBase-listed cyclotides, 510 unique cyclotides and analogues were obtained from 13 plant families. In total, seven novel plant families containing cyclotide analogues and 202 novel cyclotide analogues were identified in this study. This study has established two Blast-independent tools for screening cyclotides from plant genomes and protein databases, and has also significantly widened the plant distribution and sequence diversity of cyclotides and their analogues.

Cyclotide structure-activity relationships: qualitative and quantitative approaches linking cytotoxic and anthelmintic activity to the clustering of physicochemical forces

Cyclotides are a family of plant-derived proteins that are characterized by a cyclic backbone and a knotted disulfide topology. Their cyclic cystine knot (CCK) motif makes them exceptionally resistant to thermal, chemical, and enzymatic degradation. Cyclotides exert much of their biological activity via interactions with cell membranes. In this work, we qualitatively and quantitatively analyze the cytotoxic and anthelmintic membrane activities of cyclotides. The qualitative and quantitative models describe the potency of cyclotides using four simple physicochemical terms relevant to membrane contact. Specifically, surface areas of the cyclotides representing lipophilic and hydrogen bond donating properties were quantified and their distribution across the molecular surface was determined. The resulting quantitative structure-activity relation (QSAR) models suggest that the activity of the cyclotides is proportional to their lipophilic and positively charged surface areas, provided that the distribution of these surfaces is asymmetric. In addition, we qualitatively analyzed the physicochemical differences between the various cyclotide subfamilies and their effects on the cyclotides' orientation on the membrane and membrane activity.

A comparative study of extraction methods reveals preferred solvents for cystine knot peptide isolation from Momordica cochinchinensis seeds

MCoTI-I and MCoTI-II (short for Momordica cochinchinensis Trypsin Inhibitor-I and -II, respectively) are attractive candidates for developing novel intracellular-targeting drugs because both are exceptionally stable and can internalize into cells. These seed-derived cystine knot peptides are examples of how natural product discovery efforts can lead to biomedical applications. However, discovery efforts are sometimes hampered by the limited availability of seed materials, highlighting the need for efficient extraction methods. In this study, we assessed five extraction methods using M. cochinchinensis seeds, a source of well-characterized cystine knot peptides. The most efficient extraction of nine known cystine knot peptides was achieved by a method based on acetonitrile/water/formic acid (25:24:1), followed by methods based on sodium acetate (20 mM, pH 5.0), ammonium bicarbonate (5 mM, pH 8.0), and boiling water. On average, the yields obtained by these four methods were more than 250-fold higher than that obtained using dichloromethane/methanol (1:1) extraction, a previously applied standard method. Extraction using acetonitrile/water/formic acid (25:24:1) yielded the highest number of reconstructed masses within the majority of plant-derived cystine knot peptide mass range but only accounted for around 50% of the total number of masses, indicating that any single method may result in under-sampling. Applying acetonitrile/water/formic acid (25:24:1), boiling water, and ammonium bicarbonate (5 mM, pH 8.0) extractions either successively or discretely significantly increased the sampling number. Overall, acetonitrile/water/formic acid (25:24:1) can facilitate efficient extraction of cystine-knot peptides from M. cochinchinensis seeds but for discovery purposes the use of a combination of extraction methods is recommended where practical.

Cyclotide discovery in Gentianales revisited--identification and characterization of cyclic cystine-knot peptides and their phylogenetic distribution in Rubiaceae plants

Cyclotides are a unique class of ribosomally synthesized cysteine-rich miniproteins characterized by a head-to-tail cyclized backbone and three conserved disulfide-bonds in a knotted arrangement. Originally they were discovered in the coffee-family plant Oldenlandia affinis (Rubiaceae) and have since been identified in several species of the violet, cucurbit, pea, potato, and grass families. However, the identification of novel cyclotide-containing plant species still is a major challenge due to the lack of a rapid and accurate analytical workflow in particular for large sampling numbers. As a consequence, their phylogeny in the plant kingdom remains unclear. To gain further insight into the distribution and evolution of plant cyclotides, we analyzed ∼300 species of >40 different families, with special emphasis on plants from the order Gentianales. For this purpose, we have developed a refined screening methodology combining chemical analysis of plant extracts and bioinformatic analysis of transcript databases. Using mass spectrometry and transcriptome-mining, we identified nine novel cyclotide-containing species and their related cyclotide precursor genes in the tribe Palicoureeae. The characterization of novel peptide sequences underlines the high variability and plasticity of the cyclotide framework, and a comparison of novel precursor proteins from Carapichea ipecacuanha illustrated their typical cyclotide gene architectures. Phylogenetic analysis of their distribution within the Psychotria alliance revealed cyclotides to be restricted to Palicourea, Margaritopsis, Notopleura, Carapichea, Chassalia, and Geophila. In line with previous reports, our findings confirm cyclotides to be one of the largest peptide families within the plant kingdom and suggest that their total number may exceed tens of thousands.

Cyclotides associate with leaf vasculature and are the products of a novel precursor in petunia (Solanaceae)

Cyclotides are a large family of plant peptides that are structurally defined by their cyclic backbone and a trifecta of disulfide bonds, collectively known as the cyclic cystine knot (CCK) motif. Structurally similar cyclotides have been isolated from plants within the Rubiaceae, Violaceae, and Fabaceae families and share the CCK motif with trypsin-inhibitory knottins from a plant in the Cucurbitaceae family. Cyclotides have previously been reported to be encoded by dedicated genes or as a domain within a knottin-encoding PA1-albumin-like gene. Here we report the discovery of cyclotides and related non-cyclic peptides we called "acyclotides" from petunia of the agronomically important Solanaceae plant family. Transcripts for petunia cyclotides and acyclotides encode the shortest known cyclotide precursors. Despite having a different precursor structure, their sequences suggest that petunia cyclotides mature via the same biosynthetic route as other cyclotides. We assessed the spatial distribution of cyclotides within a petunia leaf section by MALDI imaging and observed that the major cyclotide component Phyb A was non-uniformly distributed. Dissected leaf midvein extracts contained significantly higher concentrations of this cyclotide compared with the lamina and outer margins of leaves. This is the third distinct type of cyclotide precursor, and Solanaceae is the fourth phylogenetically disparate plant family to produce these structurally conserved cyclopeptides, suggesting either convergent evolution upon the CCK structure or movement of cyclotide-encoding sequences within the plant kingdom.

Host-defense activities of cyclotides

Cyclotides are plant mini-proteins whose natural function is thought to be to protect plants from pest or pathogens, particularly insect pests. They are approximately 30 amino acids in size and are characterized by a cyclic peptide backbone and a cystine knot arrangement of three conserved disulfide bonds. This article provides an overview of the reported pesticidal or toxic activities of cyclotides, discusses a possible common mechanism of action involving disruption of biological membranes in pest species, and describes methods that can be used to produce cyclotides for potential applications as novel pesticidal agents.

Identification and structural characterization of novel cyclotide with activity against an insect pest of sugar cane

Cyclotides are a family of plant-derived cyclic peptides comprising six conserved cysteine residues connected by three intermolecular disulfide bonds that form a knotted structure known as a cyclic cystine knot (CCK). This structural motif is responsible for the pronounced stability of cyclotides against chemical, thermal, or proteolytic degradation and has sparked growing interest in this family of peptides. Here, we isolated and characterized a novel cyclotide from Palicourea rigida (Rubiaceae), which was named parigidin-br1. The sequence indicated that this peptide is a member of the bracelet subfamily of cyclotides. Parigidin-br1 showed potent insecticidal activity against neonate larvae of Lepidoptera (Diatraea saccharalis), causing 60% mortality at a concentration of 1 μm but had no detectable antibacterial effects. A decrease in the in vitro viability of the insect cell line from Spodoptera frugiperda (SF-9) was observed in the presence of parigidin-br1, consistent with in vivo insecticidal activity. Transmission electron microscopy and fluorescence microscopy of SF-9 cells after incubation with parigidin-br1 or parigidin-br1-fluorescein isothiocyanate, respectively, revealed extensive cell lysis and swelling of cells, consistent with an insecticidal mechanism involving membrane disruption. This hypothesis was supported by in silico analyses, which suggested that parigidin-br1 is able to complex with cell lipids. Overall, the results suggest promise for the development of parigidin-br1 as a novel biopesticide.

Cyclotides

In recent years, a number of peptides containing a cyclic structural fold have been described. Among them, the cyclotides family was widely reported in different plant tissues, being composed of small cyclic peptides containing 6 conserved cysteine residues connected by disulfide bonds and forming a cysteine-binding cyclic structure known as a cyclic cysteine knot. This structural scaffold is responsible for an enhanced structural stability against chemical, thermal, and proteolytic degradation. Because of the observed stability and multifunctionality, including insecticidal, antimicrobial, and anti-HIV (human immunodeficiency virus) action, much effort has gone into trying to elucidate the structural-function relations of cyclotide compounds. This review focuses on the novelties involving gene structure, precursor formation and processing, and protein folding of the cyclotide family, shedding some light on molecular mechanisms of cyclotide production. Because cyclotides are clear targets for drug development and also biotechnology applications, their chemical synthesis, heterologous systems production, and protein grafting are also addressed.

Processing and maturation of the pilin of the type IV secretion system encoded within the gonococcal genetic island

The type IV secretion system (T4SS) encoded within the gonococcal genetic island (GGI) of Neisseria gonorrhoeae has homology to the T4SS encoded on the F plasmid. The GGI encodes the putative pilin protein TraA and a serine protease TrbI, which is homologous to the TraF protein of the RP4 plasmid involved in circularization of pilin subunits of P-type pili. TraA was processed to a 68-amino acid long circular peptide by leader peptidase and TrbI. Processing occurred after co-translational membrane insertion and was independent of other proteins. Circularization occurred after removal of three C-terminal amino acids. Mutational analysis of TraA revealed limited flexibility at the cleavage and joining sites. Mutagenesis of TrbI showed that the conserved Lys-93 and Asp-155 are essential, whereas mutagenesis of Ser-52, the putative catalytic serine did not influence circularization. Further mutagenesis of other serine residues did not identify a catalytic serine, indicating that TrbI either contains redundant catalytic serine residues or does not function via a serine-lysine dyad mechanism. In vitro studies revealed that circularization occurs via a covalent intermediate between the C terminus of TraA and TrbI. The intermediate is processed to the circular form after cleavage of the N-terminal signal sequence. This is the first demonstration of a covalent intermediate in the circularization mechanism of conjugative pili.

Discovery of an unusual biosynthetic origin for circular proteins in legumes

Cyclotides are plant-derived proteins that have a unique cyclic cystine knot topology and are remarkably stable. Their natural function is host defense, but they have a diverse range of pharmaceutically important activities, including uterotonic activity and anti-HIV activity, and have also attracted recent interest as templates in drug design. Here we report an unusual biosynthetic origin of a precursor protein of a cyclotide from the butterfly pea, Clitoria ternatea, a representative member of the Fabaceae plant family. Unlike all previously reported cyclotides, the domain corresponding to the mature cyclotide from this Fabaceae plant is embedded within an albumin precursor protein. We confirmed the expression and correct processing of the cyclotide encoded by the Cter M precursor gene transcript following extraction from C. ternatea leaf and sequencing by tandem mass spectrometry. The sequence was verified by direct chemical synthesis and the peptide was found to adopt a classic knotted cyclotide fold as determined by NMR spectroscopy. Seven additional cyclotide sequences were also identified from C. ternatea leaf and flower, five of which were unique. Cter M displayed insecticidal activity against the cotton budworm Helicoverpa armigera and bound to phospholipid membranes, suggesting its activity is modulated by membrane disruption. The Fabaceae is the third largest family of flowering plants and many Fabaceous plants are of huge significance for human nutrition. Knowledge of Fabaceae cyclotide gene transcripts should enable the production of modified cyclotides in crop plants for a variety of agricultural or pharmaceutical applications, including plant-produced designer peptide drugs.