@article{HejaziFettkeKoettingetal.2010,
  author    = {Hejazi, Mahdi and Fettke, J{\"o}rg and Koetting, Oliver and Zeeman, Samuel C. and Steup, Martin},
  title     = {The Laforin-like dual-specificity phosphatase SEX4 from Arabidopsis hydrolyzes both C6-and C3-phosphate esters introduced by starch-related dikinases and thereby affects phase transition of alpha-glucans},
  issn      = {0032-0889},
  doi       = {10.1104/pp.109.149914},
  year      = {2010},
  abstract  = {The biochemical function of the Laforin-like dual-specific phosphatase AtSEX4 (EC 3.1.3.48) has been studied. Crystalline maltodextrins representing the A- or the B-type allomorph were prephosphorylated using recombinant glucan, water dikinase (StGWD) or the successive action of both plastidial dikinases (StGWD and AtPWD). AtSEX4 hydrolyzed carbon 6-phosphate esters from both the prephosphorylated A- and B-type allomorphs and the kinetic constants are similar. The phosphatase also acted on prelabeled carbon-3 esters from both crystalline maltodextrins. Similarly, native starch granules prelabeled in either the carbon-6 or carbon-3 position were also dephosphorylated by AtSEX4. The phosphatase did also hydrolyze phosphate esters of both prephosphorylated maltodextrins when the (phospho)glucans had been solubilized by heat treatment. Submillimolar concentrations of nonphosphorylated maltodextrins inhibited AtSEX4 provided they possessed a minimum of length and had been solubilized. As opposed to the soluble phosphomaltodextrins, the AtSEX4- mediated dephosphorylation of the insoluble substrates was incomplete and at least 50\% of the phosphate esters were retained in the pelletable (phospho) glucans. The partial dephosphorylation of the insoluble glucans also strongly reduced the release of nonphosphorylated chains into solution. Presumably, this effect reflects fast structural changes that following dephosphorylation occur near the surface of the maltodextrin particles. A model is proposed defining distinct stages within the phosphorylation/dephosphorylation-dependent transition of alpha-glucans from the insoluble to the soluble state.},
  language  = {en}
}
@article{ComparotMossKoettingStettleretal.2010,
  author    = {Comparot-Moss, Sylviane and Koetting, Oliver and Stettler, Michaela and Edner, Christoph and Graf, Alexander and Weise, Sean E. and Streb, Sebastian and Lue, Wei-Ling and MacLean, Daniel and Mahlow, Sebastian and Ritte, Gerhard and Steup, Martin and Chen, Jychian and Zeeman, Samuel C. and Smith, Alison M.},
  title     = {A putative phosphatase, LSF1, is required for normal starch turnover in Arabidopsis leaves},
  issn      = {0032-0889},
  doi       = {10.1104/pp.109.148981},
  year      = {2010},
  abstract  = {A putative phosphatase, LSF1 (for LIKE SEX4; previously PTPKIS2), is closely related in sequence and structure to STARCH-EXCESS4 (SEX4), an enzyme necessary for the removal of phosphate groups from starch polymers during starch degradation in Arabidopsis (Arabidopsis thaliana) leaves at night. We show that LSF1 is also required for starch degradation: lsf1 mutants, like sex4 mutants, have substantially more starch in their leaves than wild-type plants throughout the diurnal cycle. LSF1 is chloroplastic and is located on the surface of starch granules. lsf1 and sex4 mutants show similar, extensive changes relative to wild-type plants in the expression of sugar-sensitive genes. However, although LSF1 and SEX4 are probably both involved in the early stages of starch degradation, we show that LSF1 neither catalyzes the same reaction as SEX4 nor mediates a sequential step in the pathway. Evidence includes the contents and metabolism of phosphorylated glucans in the single mutants. The sex4 mutant accumulates soluble phospho- oligosaccharides undetectable in wild-type plants and is deficient in a starch granule-dephosphorylating activity present in wild-type plants. The lsf1 mutant displays neither of these phenotypes. The phenotype of the lsf1/sex4 double mutant also differs from that of both single mutants in several respects. We discuss the possible role of the LSF1 protein in starch degradation.},
  language  = {en}
}
@article{KoettingSanteliaEdneretal.2009,
  author    = {Koetting, Oliver and Santelia, Diana and Edner, Christoph and Eicke, Simona and Marthaler, Tina and Gentry, Matthew S. and Comparot-Moss, Sylviane and Chen, Jychian and Smith, Alison M. and Steup, Martin and Ritte, Gerhard and Zeeman, Samuel C.},
  title     = {STARCH-EXCESS4 is a laforin-like phosphoglucan phosphatase required for starch degradation in Arabidopsis thaliana},
  issn      = {1040-4651},
  doi       = {10.1105/tpc.108.064360},
  year      = {2009},
  abstract  = {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.},
  language  = {en}
}
@article{YuKoflerHaeusleretal.2001,
  author    = {Yu, Tien-Shin and Kofler, Heike and H{\"a}usler, Rainer E. and Hille, Diana and Fl{\"u}gge, Ulf-Ingo and Zeeman, Samuel C. and Smith, Alison M. and Kossmann, Jens and Lloyd, James R. and Ritte, Gerhard and Steup, Martin and Lue, Wei-Ling and Chen, Jychian and Weber, Andreas P. M.},
  title     = {The Arabidopsis sex1 mutant is defective in the R1 protein, a general regulator of starch degradation in plants, and not in the chloroplast hexose transporter},
  issn      = {1040-4651},
  year      = {2001},
  language  = {en}
}