Trafficking of Starch Synthase from the Cytosol to Functional Sites in Younger and Older Proplastids in Developing Stolons of Potato

• Published in: Journal of plant physiology 1999-03, Vol.154 (3), p.310-318
• Main Authors: Sagisaka, Shonosuke, Akita, Tomoko, Akase, Tomohisa, Matsumoto, Keiko, Nozaki, Koichi, Matsui, Hirokazu, Ito, Hiroyuki, Honma, Mamoru
• Format: Article
• Language: English
• Subjects: Biological and medical sciences; Fundamental and applied biological sciences. Psychology; Metabolism; Photosynthesis, respiration. Anabolism, catabolism; Plant physiology and development; Potato stolon; Proplastid; Solanum tuberosum L; Starch synthase (location); Starch synthase precursor (accumulation)
• ISSN: 0176-1617; 1618-1328
• DOI: 10.1016/S0176-1617(99)80173-4

Trafficking of starch synthase and of a precursor to starch synthase in younger and older proplastids and in the cytosol of cells in apices of developing stolons of potato was analyzed by immunogold staining and electron microscopy. Since granule-bound starch synthase I is known to be the major active form of the synthase in potato, we used antibodies raised against the 57-kD granule-bound isoform I of starch synthase. The amount of immunoreactive proteins in the cytosol was low. In contrast, the precursor protein was abundant in regions less than 80 nm from the surface of older proplastids, suggesting the presence of undefined recognition sites for transit into the proplastids. During the translocation across the envelope membranes, and soon after the end of this process, no accumulation of immunoreactive proteins was detected, indicating that the pool size of the polypeptides is low in areas close to the transit machinery. Inside older proplastids, starch synthase was found in immature starch granules, at their surface and in the stroma. In proplastids that contained large starch granules, almost all the starch synthase was located in the large starch granules that were increasing in size, and no starch synthase was detected in the stroma. Localization of immunogold particles on the surface of older proplastids indicates that the import is biased to one side of the organelles. Essentially no immunogold staining was detected in younger proplastids both on the surface and in the stroma.