Assembly Properties of Modified Subunits in the Glycinin Subunit Family

Abstract

Soybeans provide an interesting experimental system to study for a variety of reasons. They are an extremely rich but underutilized source of dietary protein that could be used directly for human nutrition. In order to make soybean protein more acceptable, efforts will need to be directed toward correcting an imbalance in the essential amino acids, improving the digestibility of the major seed storage proteins, and reducing the tendency of soybean proteins to elicit serious antigenic responses in animals that consume them. The soybean seed storage proteins are also an interesting biological system to study. Two types of proteins, glycinin and s-conglycinin, account for about 70% of the total seed protein, and these are synthesized during an intensive burst of metabolic activity in a carefully orchestrated developmental sequence. This developmental process is characterized not only by temporal and spatial control of gene expression, but also by a complicated assembly process that involves at least three subcellular compartments and several proteolytic cleavages. These latter events occur as the storage proteins make their way through the endomembrane system of the cell to their final site of deposition in the seed. The purpose of this communication is to review some of the key elements in the pathway that leads to the deposition of the glycinin in the protein bodies of soybean cotyledons.