Abstract
Starch contains phosphate covalently bound to the C6-position (70 to 80% of total bound phosphate) and the C3-position (20 to 30%) of the glucosyl residues of the amylopectin fraction. In plants, the transient phosphorylation of starch renders the granule surface more accessible to glucan hydrolyzing enzymes and is required for proper starch degradation. Phosphate also confers desired properties to starch-derived pastes for industrial applications. In Arabidopsis thaliana, the removal of phosphate by the glucan phosphatase Starch Excess4 (SEX4) is essential for starch breakdown. We identified a homolog of SEX4, LSF2 (Like Sex Four2), as a novel enzyme involved in starch metabolism in Arabidopsis chloroplasts. Unlike SEX4, LSF2 does not have a carbohydrate binding module. Nevertheless, it binds to starch and specifically hydrolyzes phosphate from the C3-position. As a consequence, lsf2 mutant starch has elevated levels of C3-bound phosphate. SEX4 can release phosphate from both the C6- and the C3-positions, resulting in partial functional overlap with LSF2. However, compared with sex4 single mutants, the lsf2 sex4 double mutants have a more severe starch-excess phenotype, impaired growth, and a further change in the proportion of C3- and C6-bound phosphate. These findings significantly advance our understanding of the metabolism of phosphate in starch and provide innovative options for tailoring novel starches with improved functionality for industry.
Highlights
Starch is the major storage carbohydrate in higher plants and a key resource for humankind both as the main component of our staple crops and as a renewable industrial material
LSF2 does not possess the carbohydrate binding module (CBM) (Figure 1A) located between the DSP and C-terminal domain (CT) domains in both Starch Excess4 (SEX4) and Like Sex Four1 (LSF1) nor does it possess the PDZ-like putative protein–protein interaction domain identified in LSF1 (Figure 1A; Fordham-Skelton et al, 2002)
This study identifies LSF2 as an enzyme involved in the transitory starch metabolism in Arabidopsis
Summary
Starch is the major storage carbohydrate in higher plants and a key resource for humankind both as the main component of our staple crops and as a renewable industrial material. Amylopectin (a polymer comprised of a-1,4-linked glucan chains, branched via a-1,6-bonds) is the major constituent of starch, accounting for 70% or more of the granule. Starch is composed of amylose, which is essentially a linear 1,4-linked Glc polymer interspersed between the amylopectin molecules. Many industrial applications require chemical modification of native starches (e.g., oxidation and esterification) to stabilize the constituent glucan polymers during processing or to introduce required functional groups (BeMiller, 1997). The extent of phosphorylation varies from a relatively high level in potato (Solanum tuberosum) tuber starch (0.5% of glucosyl units) to almost undetectable amounts in the cereal
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