title: Nature of the periplastidial pathway of starch synthesis in the cryptophyte Guillardia theta.
abstract: The nature of the periplastidial pathway of starch biosynthesis was investigated with the model cryptophyte Guillardia theta. The storage polysaccharide granules were shown to be composed of both amylose and amylopectin fractions with a chain length distribution and crystalline organization very similar to those of starch from green algae and land plants. Most starch granules displayed a shape consistent with biosynthesis occurring around the pyrenoid through the rhodoplast membranes. A protein with significant similarity to the amylose-synthesizing granule-bound starch synthase 1 from green plants was found as the major polypeptide bound to the polysaccharide matrix. N-terminal sequencing of the mature protein proved that the precursor protein carries a nonfunctional transit peptide in its bipartite topogenic signal sequence which is cleaved without yielding transport of the enzyme across the two inner plastid membranes. The enzyme was shown to display similar affinities for ADP and UDP-glucose, while the V(max) measured with UDP-glucose was twofold higher. The granule-bound starch synthase from Guillardia theta was demonstrated to be responsible for the synthesis of long glucan chains and therefore to be the functional equivalent of the amylose-synthesizing enzyme of green plants. Preliminary characterization of the starch pathway suggests that Guillardia theta utilizes a UDP-glucose-based pathway to synthesize starch.
authors.affiliation: CNRS, UMR8576, Cité Scientifique, 59655 Villeneuve d'Ascq, France.
authors.forename: Philippe
authors.initials: P
authors.lastname: Deschamps
authors.forename: Ilka
authors.initials: I
authors.lastname: Haferkamp
authors.forename: David
authors.initials: D
authors.lastname: Dauvillée
authors.forename: Sophie
authors.initials: S
authors.lastname: Haebel
authors.forename: Martin
authors.initials: M
authors.lastname: Steup
authors.forename: Alain
authors.initials: A
authors.lastname: Buléon
authors.forename: Jean-Luc
authors.initials: JL
authors.lastname: Putaux
authors.forename: Christophe
authors.initials: C
authors.lastname: Colleoni
authors.forename: Christophe
authors.initials: C
authors.lastname: d'Hulst
authors.forename: Charlotte
authors.initials: C
authors.lastname: Plancke
authors.forename: Sven
authors.initials: S
authors.lastname: Gould
authors.forename: Uwe
authors.initials: U
authors.lastname: Maier
authors.forename: H Ekkehard
authors.initials: HE
authors.lastname: Neuhaus
authors.forename: Steven
authors.initials: S
authors.lastname: Ball
mesh_terms: D000595:Amino Acid Sequence; D000687:Amylopectin / Q000378:metabolism; D000688:Amylose / Q000378:metabolism; D044785:Cryptophyta / Q000378:metabolism* / Q000648:ultrastructure; D003594:Cytoplasmic Granules / Q000737:chemistry / Q000648:ultrastructure; D005964:Glucosyltransferases / Q000378:metabolism; D008969:Molecular Sequence Data; D010802:Phylogeny; D018087:Plastids / Q000737:chemistry; D013213:Starch / Q000096:biosynthesis* / Q000737:chemistry; D013214:Starch Synthase / Q000737:chemistry / Q000378:metabolism*
chemical_list: D013213:Starch; D000688:Amylose; D000687:Amylopectin; D005964:Glucosyltransferases; C028142:UDP-glucose-specific starch synthetase; C434607:granule-bound starch synthase I; D013214:Starch Synthase