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 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 canter 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 he broad diversity-of cyclotides in Violaceae species.