A transglucosidase necessary for starch degradation and maltose metabolism in leaves at night acts on cytosolic heteroglycans (SHG)

Authors

  • Joerg Fettke,

    1. Institute of Biochemistry and Biology, Department of Plant Physiology, University of Potsdam, Karl-Liebknecht-Str. 24-25, Building 20, 14476 Potsdam-Golm, Germany, and
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  • Tansy Chia,

    1. Department of Metabolic Biology, John Innes Centre, Colney Lane, Norwich NR4 7UH, UK
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    • Present address: Broom's Barn Research Station, Higham, Bury St Edmunds, Suffolk IP28 6NP, UK.

  • Nora Eckermann,

    1. Institute of Biochemistry and Biology, Department of Plant Physiology, University of Potsdam, Karl-Liebknecht-Str. 24-25, Building 20, 14476 Potsdam-Golm, Germany, and
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  • Alison Smith,

    1. Department of Metabolic Biology, John Innes Centre, Colney Lane, Norwich NR4 7UH, UK
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  • Martin Steup

    Corresponding author
    1. Institute of Biochemistry and Biology, Department of Plant Physiology, University of Potsdam, Karl-Liebknecht-Str. 24-25, Building 20, 14476 Potsdam-Golm, Germany, and
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*(fax +49 331 977 2512; e-mail msteup@uni-potsdam.de).

Summary

The recently characterized cytosolic transglucosidase DPE2 (EC 2.4.1.25) is essential for the cytosolic metabolism of maltose, an intermediate on the pathway by which starch is converted to sucrose at night. In in vitro assays, the enzyme utilizes glycogen as a glucosyl acceptor but the in vivo acceptor molecules remained unknown. In this communication we present evidence that DPE2 acts on the recently identified cytosolic water-soluble heteroglycans (SHG) as does the cytosolic phosphorylase (EC 2.4.1.1) isoform. By using in vitro two-step 14C labeling assays we demonstrate that the two transferases can utilize the same acceptor sites of the SHG. Cytosolic heteroglycans from a DPE2-deficient Arabidopsis mutant were characterized. Compared with the wild type the glucose content of the heteroglycans was increased. Most of the additional glucosyl residues were found in the outer chains of SHG that are released by an endo-α-arabinanase (EC 3.2.1.99). Additional starch-related mutants were characterized for further analysis of the increased glucosyl content. Based on these data, the cytosolic metabolism of starch-derived carbohydrates is discussed.

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