InterPro domain: IPR005535

Cyclotides (cyclo peptides) are plant peptides of ~30 amino acids with a head to-tail cyclic backbone and six cysteine residues involved in three disulphide bonds. The cyclotides are extremely resistant to proteolysis and are remarkably stable. Cyclotides display a diverse range of biological activities, including uterotonic activity, inhibition of neurotensin binding, hemolytic, anti-HIV and anti-microbial activity. This range of biological activities makes cyclotides amenable to potential pharmaceutical and agricultural applications. Although their precise role in plants has not yet been reported, it appears that they are most likely present as defence molecules [ 1 , 2 , 3 , 4 ].

The three-dimensional structure of cyclotides is compact and contains a number of beta-turns, three beta strands arranged in a distorted triple-stranded beta-sheet, a short helical segment, and a network of disulphide bonds which form a cystine knot. The cystine knot consists of an embedded ring in the structure, formed by two disulphide bonds and their connecting backbone segments is threaded by a third disulphide bond. Although the cystine knot motif is now well known in a wide variety of proteins, the cyclotides remain as the only example in which a cystine knot is embedded within a circular protein backbone, a motif that is referred to as the cyclic cystine knot (CCK) [ 5 , 5 , 5 , 5 ].

Cyclotides can be separated into two sub-families, one of which tends to contain a larger number of positively charged residues and has a bracelet-like circularisation of the backbone. The second subfamily contains a backbone twist due to a cis-Pro peptide bond and may conceptually be regarded as a molecular Moebius strip [ 5 , 5 ]. Bracelet and Moebius families of cyclotides possess a Knottin scaffold. The cyclotide family of proteins is abundant in plants from the Rubiaceae and Violaceae families and includes:

• Kalata B1.
• Circulins.
• Cyclopsychotride A.
• Cycloviolacin O1.

Also included in this entry are cliotides from the leguminous plant Clitoria ternatea. These are cyclotides with known medicinal properties that have antimicrobial activities against Escherichia coli and are cytotoxic to HeLa cells [ 5 ].

This entry also includes chassatides. There are 18 chassatides peptides: 14 new cyclotides and 4 uncyclotides from the Rubiaceae family. Uncyclotides are the most potent chassatides for antimicrobial, cytotoxic, and hemolytic activities. All uncyclotides belong to the bracelet subfamily, all lacking the Asn/Asp residue at their C termini, which is crucial for backbone cyclization. Genetic characterization of novel cyclotides revealed that their precursors are highly shortened. They consist of five bracelet, two Möbius, and two hybrid cyclotides. Two Met-oxidized derivatives of chassatide C2 and C11 have been isolated, while we know that oxydation of methionine to methionine sulfoxide (MetO) causes a complete loss of biological activities [ 6 ].

1. Plant cyclotides: A unique family of cyclic and knotted proteins that defines the cyclic cystine knot structural motif. J. Mol. Biol. 294, 1327-36
2. Disulfide folding pathways of cystine knot proteins. Tying the knot within the circular backbone of the cyclotides. J. Biol. Chem. 278, 6314-22
3. Twists, knots, and rings in proteins. Structural definition of the cyclotide framework. J. Biol. Chem. 278, 8606-16
4. Primary and 3-D modelled structures of two cyclotides from Viola odorata. Phytochemistry 64, 135-42
5. Discovery and characterization of novel cyclotides originated from chimeric precursors consisting of albumin-1 chain a and cyclotide domains in the Fabaceae family. J. Biol. Chem. 286, 24275-87
6. Novel cyclotides and uncyclotides with highly shortened precursors from Chassalia chartacea and effects of methionine oxidation on bioactivities. J. Biol. Chem. 287, 17598-607