Expression of Viola cyclotides by liquid chromatography-mass spectrometry and tandem mass spectrometry sequencing of intercysteine loops after introduction of charges and cleavage sites by aminoethylation.

The expression of cyclotides-macrocyclic plant peptides-was profiled in six violets, Viola cotyledon, V. biflora, V. arvensis, V. tricolor, V. riviniana, and V. odorata, by LC-MS. All were found to express notably complex mixtures, with single species containing >50 cyclotides. To facilitate their sequencing by MS-MS, an analytical strategy is presented involving aminoethylation of cysteines. This overcomes a number of problems intimately associated with the cyclotide core structure-that is, their joined N and C termini, disulfide knot, and low or clustered content of positively charged amino acids and enzymatic cleavage sites. As a result, charges as well as cleavage sites are introduced at the most conserved part of their sequence, the cysteines. Combined with tryptic digestion, all intercysteine loops are then of suitable size and charge for MS-MS sequencing. The utility of this strategy is shown by the sequencing of two novel cyclotides isolated from V. cotyledon; vico A (cyclo-(AESCVYIPCFTGIAGCSCKNKVCYYNGSIPC)) and vico B (cyclo-(AESCVYIPCITGIAGCSCKNKVCYYNGSIPC)); their complete sequence could be determined by nanospray MS-MS. The strategy for converting conserved cysteines to enzymatic cleavage sites might also benefit the study of other peptides and proteins displaying similar structural problems for MS analysis.