PURIFICATION, CHARACTERIZATION, AND INTRACELLULAR-LOCALIZATION OF GLYCOSYLATED PROTEIN DISULFIDE-ISOMERASE FROM WHEAT GRAINS

Wheat (Triticum aestivum) storage proteins fold and assemble into comp lexes that are linked by intra- and intermolecular disulfide bonds, bu t it is not yet clear whether these processes are spontaneous or requi re the assistance of endoplasmic reticulum (ER)-resident enzymes and m olecular chaperones. Aiming to unravel these processes, we have purifi ed and characterized the enzyme protein disulfide isomerase (PDI) from wheat endosperm, as well as studied its developmental expression and intracellular localization. This ER-resident enzyme was previously sho wn to be involved in the formation of disulfide bonds in secretory pro teins. Wheat PDI appears as a 60-kD glycoprotein and is among the most abundant proteins within the ER of developing grains. PDI is notably upregulated in developing endosperm in comparison to embryos, leaves, and roots. In addition, the increase in PDI expression in grains appea rs at relatively early stages of development, preceding the onset of s torage protein accumulation by several days. Subcellular localization analysis and immunogold labeling of electron micrographs showed that P DI is not only present in the lumen of the ER but is also co-localized with the storage proteins in the dense protein bodies. These observat ions are consistent with the hypothesis that PDI is involved in the as sembly of wheat storage proteins within the ER.