Structure and Activity of Circular Plant Proteins : Cytotoxic Effects of Viola Cyclotides

Abstract: Cyclotides are a family of small and macrocyclic proteins that have been found in Violacaee and Rubiaceae plant species. These proteins contain a cystine knot: two disulfides bonds together with their connecting peptide backbone form an embedded ring which is penetrated by a third disulfide bond. The cyclotides have been attributed a wide range of biological activities, which in combination with their chemical stability and structural plasticity have made them attractive tools for pharmaceutical applications. The sequence of eleven novel cyclotides, vibi A-K, from Viola biflora was determined by the use of both chemical (extraction and characterization) and molecular biology (cDNA analyses) approaches. A clear discrepancy in the results from the two methods was observed. Additionally, one novel cyclotide, vodo O, was isolated from Viola odorata. To correlate cytotoxic potency to sequence, vodo O and vibi D, E, G and H were tested on a lymphoma cell line. Based on the presence or absence of a cis-Pro bond, the cyclotides are divided into the Möbius and bracelet subfamilies. The bracelet proteins have a higher net charge and are more cytotoxic potent than the Möbius ones. To explore these differences, charged and hydrophobic residues in varv A (Möbius) and cycloviolacin O2 (bracelet) were chemically modified and tested for their cytotoxicity. The net-charge of the two proteins was not important for the potency. The Glu residue in cycloviolacin O2 was crucial, while this residue was of minor importance in varv A. Oxidation of the single Trp residue declined the potency significantly in both proteins. To evaluate how the surface properties correlate to the degree of cytotoxic potency, models of all cyclotides hitherto tested were constructed by homology modelling. Calculations showed that the membrane orientation of varv A and cycloviolacin O2 differed significantly, which might explain their difference in potency