Identification and isolation of methionine-cysteine rich proteins in soybean seed.

We recently developed a method to identify methionine-containing proteins and quantitate their methionine contents. We applied this method to soybeans and identified relatively methionine-rich proteins (MRP) among the albumins. By acidic methanol extraction of the albumins, we obtained a group of low molecular weight methionine-cysteine rich proteins (MCRP) that analyzed 4.0% methionine and 8.8% cysteine. MCRP made up 1-2% of the total protein in soybeans. Reversed-phase HPLC purification of MCRP yielded a protein peak that exhibited a single major band on denaturing polyacrylamide gel electrophoresis, had a molecular weight of 16 kD and contained 6.2% methionine and 18.8% cysteine. We are cloning the gene for this protein. Increasing its level through genetic engineering could increase the methionine-cysteine content of soybeans.