Several members of three plants families - the Rubiaceae, Violaceae and Cucurbitaceae- produce cyclotides, cyclic mini-peptides made up of 28-37 amino acids arranged in a circular configuration. These compounds are very stable and have attracted the attention of pharmaceutical companies. Because these peptides lack free amino and carboxyl ends, they cannot be broken down by proteases. The compounds appear to act primarily as insecticides. In an article published in the January 29 issue of PNAS, Barbara Barbeta and colleagues investigated the role of these compounds on the larvae of lepidopterans (butterflies and moths). The compounds damaged the cells of the midgut of Helicoverpa armigera larvae, which severely stunted their growth (image from Wikipedia; see license details).
Barbeta, B.L., Marshall, A.T., Gillon, A.D., Craik, D.J., and Anderson, M.A. 2008. Plant cyclotides disrupt epithelial cells in the midgut of lepidopteran larvae. Proceedings of the National Academy of Sciences USA 105(4):1221-1225 DOI:10.1073/pnas.0710338104