Gene-guided identifications of a structure-chimeric cyclotide viphi I from Viola philippica: Potential functions against cadmium and nematodes

Abstract

The cyclic peptides, cyclotides, are identified mostly with 29–31-aa (amino acid residues) but rarely with ≥ 34-aa in plants. Viola philippica is a well-known medicinal plant but a rare metallophyte with cyclotides. A hypothesis was hence raised that the potential novel 34-aa cyclotide of Viola philippica would clearly broaden the structural and functional diversities of plant cyclotides. After homology-cloning the cyclotide precursor gene of VpCP5, a 34-aa cyclotide (viphi I) was identified to be larger than 22 other known cyclotides in V. philippica. It had a chimeric primary structure, due to its unusual loop structures (8 residues in loop 2 and 6 residues in loop 5) and aa composition (3 E and 5 R), by using phylogenetic analyses and an in-house cyclotide analysis tool, CyExcel_V1. A plasmid pCYC-viphi_I and a lab-used recombinant process were specially constructed for preparing viphi I. Typically, 0.12 or 0.25 mg ml−1 co-exposed viphi I could significantly remain cell activities with elevating Cd2+-exposed doses from 10−8 to 10−6 mol l−1 in MCF7 cells. In the model nematode Caenorhabditis elegans, IC50 values of viphi I to inhibit adult ratios and to induce death ratios, were 184.7 and 585.9 µg ml−1, respectively; the median lifespan of adult worms decreased from 14 to 2 d at viphi I doses ranging from 0.05 to 2 mg ml−1. Taken together, the newly identified viphi I exhibits functional potentials against cadmium and nematodes, providing new insights into structural and functional diversity of chimeric cyclotides in plants.