title: Preamylopectin Processing: A Mandatory Step for Starch Biosynthesis in Plants.
abstract: It has been generally assumed that the [alpha]-(1->4)-linked and [alpha]-(1->6)-branched glucans of starch are generated by the coordinated action of elongation (starch synthases) and branching enzymes. We have identified a novel Chlamydomonas locus (STA7) that when defective leads to a wipeout of starch and its replacement by a small amount of glycogen-like material. Our efforts to understand the enzymological basis of this phenotype have led us to determine the selective disappearance of an 88-kD starch hydrolytic activity. We further demonstrate that this enzyme is a debranching enzyme. Cleavage of the [alpha]-(1->6) linkage in a branched precursor of amylopectin (preamylopectin) has provided us with the ground rules for understanding starch biosynthesis in plants. Therefore, we propose that amylopectin clusters are synthesized by a discontinuous mechanism involving a highly specific glucan trimming mechanism.
authors.affiliation: Laboratoire de Chimie Biologique, Unite Mixte de Recherche du Centre National de la Recherche Scientifique No. 111, Universite des Sciences et Technologie de Lille, 59655 Villeneuve d'Ascq Cedex, France.
authors.forename: G.
authors.initials: G
authors.lastname: Mouille
authors.forename: M. L.
authors.initials: ML
authors.lastname: Maddelein
authors.forename: N.
authors.initials: N
authors.lastname: Libessart
authors.forename: P.
authors.initials: P
authors.lastname: Talaga
authors.forename: A.
authors.initials: A
authors.lastname: Decq
authors.forename: B.
authors.initials: B
authors.lastname: Delrue
authors.forename: S.
authors.initials: S
authors.lastname: Ball