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STARCH-EXCESS4 Is a Laforin-Like Phosphoglucan Phosphatase Required for Starch Degradation in Arabidopsis thaliana^[W],[OA]

^a Institute of Plant Sciences, ETH Zurich, 8092 Zurich, Switzerland
^b Plant Physiology, Institute of Biochemistry and Biology, University of Potsdam, 14476 Potsdam-Golm, Germany
^c Department of Molecular and Cellular Biochemistry, University of Kentucky, Lexington, Kentucky 40536-0509
^d Department of Metabolic Biology, John Innes Centre, Norwich NR4 7UH, United Kingdom
^e Institute of Molecular Biology, Academia Sinica, Taipei 115, Taiwan

² Address correspondence to koliver{at}ethz.ch.

Starch is the major storage carbohydrate in plants. It is comprised of glucans that form semicrystalline granules. Glucan phosphorylation is a prerequisite for normal starch breakdown, but phosphoglucan metabolism is not understood. A putative protein phosphatase encoded at the Starch Excess 4 (SEX4) locus of Arabidopsis thaliana was recently shown to be required for normal starch breakdown. Here, we show that SEX4 is a phosphoglucan phosphatase in vivo and define its role within the starch degradation pathway. SEX4 dephosphorylates both the starch granule surface and soluble phosphoglucans in vitro, and sex4 null mutants accumulate phosphorylated intermediates of starch breakdown. These compounds are linear -1,4-glucans esterified with one or two phosphate groups. They are released from starch granules by the glucan hydrolases -amylase and isoamylase. In vitro experiments show that the rate of starch granule degradation is increased upon simultaneous phosphorylation and dephosphorylation of starch. We propose that glucan phosphorylating enzymes and phosphoglucan phosphatases work in synergy with glucan hydrolases to mediate efficient starch catabolism.

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