Circular proteins from Melicytus (Violaceae) refine the conserved protein and gene architecture of cyclotides

Abstract

Cyclotides are cyclic disulfide rich mini-proteins found in various Rubiaceae (coffee family), Violaceae (violet family) and Cucurbitaceae (squash family) plant species. Within the Violaceae, cyclotides have been found in numerous species of the genus Viola as well as species from two other genera, namely Hybanthus and Leonia. This is the first in-depth report of cyclotides in the genus Melicytus (Violaceae). We present the chromatographic profiles of extracts of eight Melicytus species and one Melicytus hybrid that were found to contain these circular peptides. We isolated and characterised five novel cyclotides (mra1 to mra5) from the aerial parts of a common New Zealand tree, Melicytus ramiflorus. All five peptides show the characteristics of the bracelet subfamily of cyclotides. Furthermore, we isolated 17 non-redundant cDNA clones from the leaves of Melicytus ramiflorus encoding cyclotide prepropeptides. This detailed report on the presence of cyclotides in several species of the genus Melicytus further strengthens our hypothesis that cyclotides are ubiquitous in Violaceae family plants and provides additional insight into the biochemical processing mechanisms that produce the cyclic protein backbone of this unique family of ultra-stable plant proteins.