PROTEIN STORAGE VACUOLES OF SOYBEAN ALEURONE CELLS ACCUMULATE A UNIQUE GLYCOPROTEIN

The single cell-layer of aleurone cells exhibits structural evidence of high levels of Golgi apparatus mediated secretory activity [1]. We have used an antiserum directed against beta-1,2-xyloglycan side chains of proteins as a probe of protein trafficking and Golgi activity. Although the aleurone contains many distinct xyloglycoproteins the primary xyloglycoprotein accumulated in the aleurone is a 42-kDa polypeptide. The 42-kDa xyloglycoprotein is accumulated in protein storage vacuoles within the aleurone cells. Comparison of anti-beta-xyloglycoprotein immunoblots of the other embryonic tissues, cotyledon and axis, indicated that while numerous xyloglycoproteins appeared to be present in these tissues the 42-kDa xyloglycoprotein was absent. The identity of the protein storage vacuoles was confirmed by the presence of alpha-TIP (alpha-tonoplast integral protein) in the limiting membrane of the vacuoles. The aleurone also accumulates a variety of embryo-specific proteins including conglycinin, soybean agglutinin, and the oil body protein 24-kDa oleosin. The xyloglycoproteins appear to traffic through the Golgi apparatus. The anti-beta-xyloglycoprotein serum labeled xyloglycoproteins in the medial-trans Golgi cisternae suggesting that the xylosylation of N-linked glycan was a medial-late processing event initiated in the medial Golgi cisternae. We interpret our results to indicate soybean aleurone cells accumulate a specific abundant xyloglycoprotein of unknown function in the cell's protein storage vacuole that appears to be unique to this single-cell layer of embryonic cells.