Variations in cyclotide expression in Viola species

Cyclotides, a family of approximately 50 mini-proteins isolated from various Violaceae and Rubiaceae plants, are characterized by their circular peptide backbone and six conserved cysteine residues arranged in a cystine knot motif. Cyclotides show a wide range of biological activities, making them interesting targets for both pharmaceutical and agrochemical research, but little is known about their natural function and the events that trigger their expression. An investigation of the geographical and seasonal variations of cyclotide profiles has been performed, using the native Australian violet, Viola hederacea, and the Swedish sweet violet, Viola odorata, as model plants. The results showed that in the Australian violet the relative peptide levels of some cyclotides remained almost constant throughout the year, while other cyclotides were present only at certain times of the year. Therefore, it appears that V. hederacea expresses a basic armory of cyclotides as well as special add-ons whose levels are influenced by external factors. In the Swedish violet, cyclotide levels were increased up to 14 times during the warmest period of the year. The larger variation in expression levels of the Swedish plants may be a reflection of a greater climatic variation.

Isolation and characterization of novel cyclotides from Viola hederaceae - Solution structure and anti-HIV activity of vhl-1, a leaf-specific expressed cyclotide

Based on a newly established sequencing strategy featured by its efficiency, simplicity, and easy manipulation, the sequences of four novel cyclotides (macrocyclic knotted proteins) isolated from an Australian plant Viola hederaceae were determined. The three-dimensional solution structure of V. hederaceae leaf cyclotide-1 ( vhl-1), a leaf-specific expressed 31-residue cyclotide, has been determined using two-dimensional H-1 NMR spectroscopy. vhl-1 adopts a compact and well defined structure including a distorted triple-stranded β- sheet, a short 310 helical segment and several turns. It is stabilized by three disulfide bonds, which, together with backbone segments, form a cyclic cystine knot motif. The three-disulfide bonds are almost completely buried into the protein core, and the six cysteines contribute only 3.8% to the molecular surface. A pH titration experiment revealed that the folding of vhl-1 shows little pH dependence and allowed the pK(a) of 3.0 for Glu(3) and ∼ 5.0 for Glu(14) to be determined. Met(7) was found to be oxidized in the native form, consistent with the fact that its side chain protrudes into the solvent, occupying 7.5% of the molecular surface. vhl-1 shows anti-HIV activity with an EC50 value of 0.87 μ m.

Cycloviolacin H4, a hydrophobic cyclotide from Viola hederaceae

Cycloviolacin H4, a new macrocyclic miniprotein comprising 30 amino acid residues, was isolated from the underground parts of the Australian native violet Viola hederaceae. Its sequence, cyclo-(CAESCVWIPCTVTALLGCSCSNNVCYNGIP), was determined by nanospray tandem mass spectrometry and quantitative amino acid analysis. A knotted disuffide arrangement, which was designated as a cyclic cystine knot motif and characteristic to all known cyclotides, is proposed for stabilizing the molecular structure and folding. The cyclotide is classified in the bracelet subfamily of cyclotides due to the absence of a cis-Pro peptide bond in the circular peptide backbone. A model of its three-dimensional structure was derived based on the template of the homologous cyclotide vhr1 (Trabi et al. Plant Cell 2004, 16, 2204-2216). Cycloviolacin H4 exhibits the most potent hemolytic activity in cyclotides reported so far, and this activity correlates with the size of a surface-exposed hydrophobic patch. This work has thus provided insight into the factors that modulate the cytotoxic properties of cyclotides.

Discovery and characterization of a linear cyclotide from Viola odorata: Implications for the processing of circular proteins

Cyclotides are mini-proteins of 28-37 amino acid residues that have the unusual feature of a head-to-tail cyclic backbone surrounding a cystine knot. This molecular architecture gives the cyclotides heightened resistance to thermal, chemical and enzymatic degradation and has prompted investigations into their use as scaffolds in peptide therapeutics. There are now more than 80 reported cyclotide sequences from plants in the families Rubiaceae, Violaceae and Cucurbitaceae, with a wide variety of biological activities observed. However, potentially limiting the development of cyclotide-based therapeutics is a lack of understanding of the mechanism by which these peptides are cyclized in vivo. Until now, no linear versions of cyclotides have been reported, limiting our understanding of the cyclization mechanism. This study reports the discovery of a naturally occurring linear cyclotide, violacin A, from the plant Viola odorata and discusses the implications for in vivo cyclization of peptides. The elucidation of the cDNA clone of violacin A revealed a point mutation that introduces a stop codon, which inhibits the translation of a key Asn residue that is thought to be required for cyclization. The three-dimensional solution structure of violacin A was determined and found to adopt the cystine knot fold of native cyclotides. Enzymatic stability assays on violacin A indicate that despite an increase in the flexibility of the structure relative to cyclic counterparts, the cystine knot preserves the overall stability of the molecule. (c) 2006 Elsevier Ltd. All rights reserved.

A novel suite of cyclotides from Viola odorata: sequence variation and the implications for structure, function and stability

Cyclotides are a fascinating family of plant-derived peptides characterized by their head-to-tail cyclized backbone and knotted arrangement of three disulfide bonds. This conserved structural architecture, termed the CCK (cyclic cystine knot), is responsible for their exceptional resistance to thermal, chemical and enzymatic degradation. Cyclotides have a variety of biological activities, but their insecticidal activities suggest that their primary function is in plant defence. In the present study, we determined the cyclotide content of the sweet violet Viola odorata, a member of the Violaceae family. We identified 30 cyclotides from the aerial parts and roots of this plant, 13 of which are novel sequences. The new sequences provide information about the natural diversity of cyclotides and the role of particular residues in defining structure and function. As many of the biological activities of cyclotides appear to be associated with membrane interactions, we used haemolytic activity as a marker of bioactivity for a selection of the new cyclotides. The new cyclotides were tested for their ability to resist proteolysis by a range of enzymes and, in common with other cyclotides, were completely resistant to trypsin, pepsin and thermolysin. The results show that while biological activity varies with the sequence, the proteolytic stability of the framework does not, and appears to be an inherent feature of the cyclotide framework. The structure of one of the new cyclotides, cycloviolacin O14, was determined and shown to contain the CCK motif. This study confirms that cyclotides may be regarded as a natural combinatorial template that displays a variety of peptide epitopes most likely targeted to a range of plant pests and pathogens.