Purification and Characterization of the Methionine-Rich 2S Seed Proteins from the Brazil Nut Family (Lecythidaceae)

2S seed proteins were purified from paradise nut ( Lecythis zabucajo ) and cannonball ( Couroupita quianensis ), two members of the Brazil nut family (Lecythidaceae). The amino acid composition, subunit structure, partial protein sequence, immunoreactivity, and processing pattern of these proteins were determined. The 2S proteins from both paradise nut (PN2S) and cannonball (CB2S) are rich in the sulfur amino acids, especially methionine (Met): 14.0% Met and 4.3% cysteine (Cys) in PN2S; 19.9% Met and 4.5% Cys in CB2S. Both 2S proteins are synthesized as a precursor polypeptide of 18 kDa and processed stepwise to form the 9- and 3-kDa mature subunit polypeptides. These properties are similar to those of the Brazil nut ( Bertholletia excelsa H.K.B.) 2S protein (BN2S). However, while the 18-kDa precursor polypeptide of PN2S, like that of BN2S, undergoes three sequential cleavages to form the two mature subunits, only two stepwise cleavages occur during the processing of CB2S.