Studies on molecular structure of soybean glycinin by protein engineering.

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One of the major objectives of the food industries is the enrichment of the functional properties and nutritional value of soybean storage proteins. Of the storage proteins, glycinin contains more methionines, which is limiting amino acid of soybean proteins. Therefore, glycinin is targeted to improve the nutritional and functional properties of soybean proteins. To attain this goal theoretically, molecular structure of glycinin should be elucidated. In this research, the object is the elucidation of glycinin molecular structure by means of protein engineering.1. The author attempted to express cDNAs encoding glycinin subunits in microorganisms (Escherichia coli and Saccharomyces cerevisiae). It was necessary for the expression of glycinin cDNA in E. coil to delete nucleotide sequences corresponding to the signal sequence. Then, a high-level expression system was established by controlling the cultivation conditions of E. coli cells harboring the expression plasmids. In Saccharomyces cerevisiae, the signal sequence of the expressed Iycinin subunit was el cleaved at the right processing site.2. The expressed protein from glycinin cDNA in E. coil was purified to homogeneity. The purified protein self-assembled and its secondary structure and fundamental properties were similar to those of the native glycinin. This indicated that the E. coli expression system of glycinin cDNA may be employed for the studies of glycinin molecular structure.3. Expression plasmids carrying the glycinin cDNAs modified to change the molecular structure of glycinin were constructed. At present, the modified proteins are being purified. The ability of the self-assembly of the modified proteins and their higher structures will be investigated to elucidate the molecular structure of glycinin.