THE PRIMARY STRUCTURE OF INHIBITOR OF CYSTEINE PROTEINASES FROM POTATO

The complete amino acid sequence of the cysteine proteinase inhibitor from potato tubers was determined. The inhibitor is a single-chain protein having 180 amino acid residues. Its primary structure was elucidated by automatic degradation of the intact protein and sequence analysis of peptides generated by CNBr, trypsin and glycyl endopeptidase. A search through the protein sequence database showed homology to other plant proteinase inhibitors of different specificities and non-inhibitory proteins of M(r) around 20,000. On the basis of sequence homology, prediction of secondary structure and fold compatibility, based on a 3D-1D score to the three-dimensional profile of Erythrina caffra trypsin inhibitor, we suggest that the potato cysteine proteinase inhibitor belongs to the superfamily of proteins that have the same pattern of three-dimensional structure as soybean trypsin inhibitor. This superfamily would therefore include proteins that inhibit three different classes of proteinases - serine, cysteine and aspartic proteinases.