Thioredoxin Reduction Alters the Solubility of Proteins of Wheat Starchy Endosperm: An Early Event in Cereal Germination

A KCl-soluble, albumin/globulin fraction of wheat (Triticum aestivum L.) starchy endosperm was further separated into a methanol-insoluble fraction that contained metabolic proteins and a methanol-soluble fraction that contained "chloroform-methanol" or CM-like proteins. Reduction of the disulfide bonds of the CM proteins with thioredoxin or dithiothreitol altered their properties so that, like the metabolic proteins, they were insoluble in methanol. Glutathione had little effect, indicating dithiol specificity. Proteomic analysis of the CM protein fraction revealed the presence of isoforms of low molecular weight disulfide proteins (alpha-amylase, alpha-amylase/trypsin and WCI proteinase inhibitors, lipid transfer proteins, gamma-thionins), stress enzymes (Cu-Zn superoxide dismutase and peroxidase), storage proteins (alpha-, gamma- and omega-gliadins, low molecular weight glutenin subunits and globulins of the avenin N9 type), and a component of protein degradation (polyubiquitin). These findings support the view that, in addition to modifying activity and increasing protease sensitivity, reduction by thioredoxin alters protein solubility, thereby promoting processes of the grain starchy endosperm, notably the mobilization of reserves during germination and seedling development.