Sam Seeman - Protein Targeting to STarch: A new class of proteins that bring enzymes and substrates together

  • Date: Feb 1, 2017
  • Time: 02:00 PM - 03:30 PM (Local Time Germany)
  • Speaker: Sam Zeeman
  • ETH Zürich, Switzerland
  • Location: Central Building
  • Room: Seminar Room
  • Host: John Lunn
Starch is a vital plant product, being the major nutritive component of our staple crops and an important feedstock for industry. Starch takes the form of insoluble, semi-crystalline granules composed of two glucose polymers: branched amylopectin and near-linear amylose. Amylopectin, the major component, is responsible for the semi-crystalline nature of starch. It is made by a set of enzymes; starch synthases, branching enzymes and debranching enzymes. In contrast, amylose is made within the amylopectin matrix by a single enzyme – Granule-Bound Starch Synthase (GBSS) – that becomes trapped as amylopectin crystalizes about it. There is much that we still do not understand about starch biosynthesis, such as how the enzyme activities are coordinated and how starch granules are first initiated. We recently discovered that a new class of proteins is required to localize some of the Starch Synthase activities. We called these proteins PTST (for Protein Targeting to Starch). Arabidopsis has three PTSTs. PTST1 binds GBSS and delivers it to the starch granule surface, whereupon it dissociates and leaves GBSS to synthesise amylose. In the absence of PTST1, GBSS fails to localize to the granule and the starch is composed solely of amylopectin. In contrast, PTST2 and PTST3 bind to another starch synthase, SS4, which is implicated in starch granule initiation. Defects in SS4 or in PTST2/3 significantly alter the number, size and shape of starch granules without necessarily affecting the structure or ratio of the constituent polymers. We believe that these PTSTs bind low-abundance precursors from a pool of malto-oligosaccharides and deliver them to SS4 for elaboration into starch granule initials. In this talk I will summarize these and other results which shed new light onto how plants make starch.
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