Preamylopectin Processing: A Mandatory Step for Starch Biosynthesis in Plants.
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.
Mouille
G.
G
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.
Maddelein
M. L.
ML
Libessart
N.
N
Talaga
P.
P
Decq
A.
A
Delrue
B.
B
Ball
S.
S