Maltose is exported from the Arabidopsis chloroplast as the main product of starch degradation at night. To investigate its fate in the cytosol, we characterised plants with mutations in a gene encoding a putative glucanotransferase (disproportionating enzyme; DPE2), a protein similar to the maltase Q (MalQ) gene product involved in maltose metabolism in bacteria. Use of a DPE2 antiserum revealed that the DPE2 protein is cytosolic. Four independent mutant lines lacked this protein and displayed a decreased capacity for both starch synthesis and starch degradation in leaves. They contained exceptionally high levels of maltose, and elevated levels of glucose, fructose and other malto-oligosaccharides. Sucrose levels were lower than those in wild-type plants, especially at the start of the dark period. A glucosyltransferase activity, capable of transferring one of the glucosyl units of maltose to glycogen or amylopectin and releasing the other, was identified in leaves of wild-type plants. Its activity was sufficient to account for the rate of starch degradation. This activity was absent from dpe2 mutant plants. Based on these results, we suggest that DPE2 is an essential component of the pathway from starch to sucrose and cellular metabolism in leaves at night. Its role is probably to metabolise maltose exported from the chloroplast. We propose a pathway for the conversion of starch to sucrose in an Arabidopsis leaf.
glucosyltransferase, Arabidopsis, maltose metabolism, maltose transporter, starch degradation, starch mutants
Publication DOI: 10.1111/j.1365-313X.2003.02012.xJournal NLM ID: 9207397Publisher: Oxford: Blackwell Scientific Publishers and BIOS Scientific Publishers for the Society for Experimental Biology
Correspondence: alison.smith@bbrsc.ac.uk
Institutions: Institute of Cell and Molecular Biology, University of Edinburgh, Mayfield Road, Edinburgh EH9 3JH, UK, Department of Metabolic Biology, John Innes Centre, Colney Lane, Norwich NR4 7UH, UK, Institute of Plant Sciences, University of Bern, Altenbergrain 21, CH-3013 Bern, Switzerland, Institute of Molecular Biology, Academia Sinica, Taipei, Taiwan, China
Methods: biological assays, HPAEC-PAD, PAGE, extraction, enzymatic assay, genetic manipulations
Found