Role Of Phosphoglucan Phosphatases In Regulating Starch Degradation In Plants
Date
2013-05-10
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Abstract
Starch is the major carbohydrate reserve in plants. Sugars are assimilated into storage granules during photosynthesis and released for continual growth at night from the chloroplast. Breakdown of leaf starch is initiated through reversible glucan phosphorylation by novel dikinases and phosphatases at the granule surface. This phosphorylation disrupts the semi-crystalline structure of starch, and removal of these phosphates provides access for β- amylases to release maltose. The phosphoglucan phosphatases are members of the dual- specificity protein phosphatase (DSP) family, which includes starch excess 4 (SEX4) and like-SEX4 1 (LSF1). SEX4 is required for proper glucan dephosphorylation; however, whether LSF1 participates in starch degradation is unknown. Moreover, how the activity of these phosphatases regulate reversible glucan phosphorylation is unclear.
SEX4 and LSF1 both contain phosphatase and carbohydrate-binding domains, but LSF1 also possesses a protein-protein interaction PDZ domain. I show that LSF1 is chloroplastic and necessary for proper starch breakdown, as lsf1 mutants accumulate starch at the end of night. LSF1 lacks phosphatase activity and contains an irregular catalytic motif from known DSPs. The PDZ domain of LSF1 alone forms dimers and can disrupt protein complexes with β-amylases that include LSF1. In the end, these results indicate LSF1 acts as an inactive, scaffold protein that associates with starch degradative enzymes at the granule surface of starch.
SEX4 activity is sensitive to oxidation, suggesting this phosphatase might undergo reversible oxidation like other mammalian DSPs. I show that SEX4 phosphatase activity can be modulated between reduced (active) and oxidized (inactive) states, and the endogenous SEX4 can exist in these forms. Oxidation of SEX4 promotes the formation of a disulfide linkage between the catalytic cysteine 198 (Cys198) and Cys130 within the phosphatase domain, and mutation of the latter residue renders SEX4 redox-impaired. Not only does this disulfide bridge protect Cys198 from irreversible oxidation, these data provide the first biochemical evidence for the redox-dependent structural switch that regulates SEX4 activity.
Altogether, these results expand our understanding of the protein biology of leaf starch degradation and provide new insights into manipulating the phosphorylation state of starch in planta for industrial applications.
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Keywords
Bioinformatics, Plant Physiology, Biochemistry
Citation
Silver, D. (2013). Role Of Phosphoglucan Phosphatases In Regulating Starch Degradation In Plants (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/27683