Journal
PLANT CELL
Volume 21, Issue 1, Pages 334-346Publisher
AMER SOC PLANT BIOLOGISTS
DOI: 10.1105/tpc.108.064360
Keywords
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Funding
- National Institute of Health [5R00NS061803]
- University of Kentucky [BBS/B/11877, BB/C513542/1]
- John Innes Centre by the Biotechnology and Biological Sciences Research Council
- National Science Council, Taiwan [NSC-96-2311-B-001-004]
- NATIONAL CENTER FOR RESEARCH RESOURCES [P20RR020171] Funding Source: NIH RePORTER
- NATIONAL INSTITUTE OF NEUROLOGICAL DISORDERS AND STROKE [R00NS061803] Funding Source: NIH RePORTER
- Biotechnology and Biological Sciences Research Council [BBS/E/J/00000020, BBS/B/11877, BB/C513542/1] Funding Source: researchfish
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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 alpha-1,4-glucans esterified with one or two phosphate groups. They are released from starch granules by the glucan hydrolases alpha-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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