Cyclotide proteins and precursors from the genus Gloeospermum: Filling a blank spot in the cyclotide map of Violaceae |
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Authors: | Robert Burman Kristina Rizzardi Anders Herrmann Mahabir P Gupta |
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Institution: | a Division of Pharmacognosy, Department of Medicinal Chemistry, Uppsala University, Biomedical Centre, Box 574, SE-751 23 Uppsala, Sweden b Institute for Molecular Bioscience, The University of Queensland, Brisbane, Queensland 4072, Australia c Department of Pharmacognosy, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria d Centre for Pharmacognostic Research on Panamanian Flora (CIFLORPAN), College of Pharmacy, Panama University, Panama |
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Abstract: | Cyclotides are disulfide-rich plant proteins that are exceptional in their cyclic structure; their N and C termini are joined by a peptide bond, forming a continuous circular backbone, which is reinforced by three interlocked disulfide bonds. Cyclotides have been found mainly in the coffee (Rubiaceae) and violet (Violaceae) plant families. Within the Violaceae, cyclotides seem to be widely distributed, but the cyclotide complements of the vast majority of Violaceae species have not yet been explored. This study provides insight into cyclotide occurrence, diversity and biosynthesis in the Violaceae, by identifying mature cyclotide proteins, their precursors and enzymes putatively involved in their biosynthesis in the tribe Rinoreeae and the genus Gloeospermum. Twelve cyclotides from two Panamanian species, Gloeospermum pauciflorum Hekking and Gloeospermum blakeanum (Standl.) Hekking (designated Glopa A-E and Globa A-G, respectively) were characterised through cDNA screening and protein isolation. Screening of cDNA for the oxidative folding enzymes protein-disulfide isomerase (PDI) and thioredoxin (TRX) resulted in positive hits in both species. These enzymes have demonstrated roles in oxidative folding of cyclotides in Rubiaceae, and results presented here indicate that Violaceae plants have evolved similar mechanisms of cyclotide biosynthesis. We also describe PDI and TRX sequences from a third cyclotide-expressing Violaceae species, Viola biflora L., which further support this hypothesis. |
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Keywords: | CCK cyclic cystine knot ER endoplasmic reticulum NTR N-terminal repeat PDI protein-disulfide isomerase TRX thioredoxin |
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