A novel methionine-rich protein in soybean seed: identification, amino acid composition, and N-terminal sequence

Using the powerful resolution obtainable with two-dimensional gel electrophoresis and an in vitro labeling method we developed for detecting methionine-containing proteins, we identified a new methionine-rich protein (MRP) in soybean seed, confirmed its methionine content by amino acid microanalysis, and sequenced its N terminus without using the usual protein purification procedures. The complete amino acid composition and N-terminal sequence of the first 20 amino acids are presented. The MRP has a methionine content of 12.1%, constitutes 0.6% of the total protein, and has a molecular weight of 10 800. This value is exceptionally high considering that the average overall methionine content of soybean seed protein is 1.4%. This is the first report of an exceptionally methionine-rich protein in soybean seed that is distinct from the methionine-cysteine protein we reported earlier and from the cysteine-rich proteins reported previously by other workers. The MRP presents a genetic engineering strategy to improve the nutritional quality of soybean protein and raises the question of its possible biological role in the seed