An extra-plastidial alpha-glucan, water dikinase from Arabidopsis phosphorylates amylopectin in vitro and is not necessary for transient starch degradation.
Starch phosphorylation catalysed by the alpha-glucan, water dikinases (GWD) has profound effects on starch degradation in plants. The Arabidopsis thaliana genome encodes three isoforms of GWD, two of which are localized in the chloroplast and are involved in the degradation of transient starch. The third isoform, termed AtGWD2 (At4g24450), was heterologously expressed and purified and shown to have a substrate preference similar to potato GWD. Analyses of AtGWD2 null mutants did not reveal any differences in growth or starch and sugar levels, when compared to the wild type. Subcellular localization studies in Arabidopsis leaves and in vitro chloroplast import assays indicated that AtGWD2 was not targeted to the chloroplasts. The AtGWD2 promoter showed a highly restricted pattern of activity, both spatially and temporally. High activity was observed in the companion cells of the phloem, with expression appearing just before the onset of senescence. Taken together, these data indicate that, although AtGWD2 is capable of phosphorylating alpha-glucans in vitro, it is not directly involved in transient starch degradation.
Glaring
Mikkel A
MA
Plant Biochemistry Laboratory, Center for Molecular Plant Physiology (PlaCe), Department of Plant Biology, University of Copenhagen, 40 Thorvaldsensvej, 1871 Frederiksberg C, Copenhagen, Denmark. mig@life.ku.dk
Zygadlo
Agnieszka
A
Thorneycroft
David
D
Schulz
Alexander
A
Smith
Steven M
SM
Blennow
Andreas
A
Baunsgaard
Lone
L
D000687:Amylopectin / Q000378:metabolism*; D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D002736:Chloroplasts; D018506:Gene Expression Regulation, Plant; D010766:Phosphorylation; D017866:Phosphotransferases (Paired Acceptors) / Q000235:genetics / Q000378:metabolism*; D010802:Phylogeny; D018087:Plastids / Q000378:metabolism / Q000648:ultrastructure; D013213:Starch / Q000378:metabolism*; D013379:Substrate Specificity
D029681:Arabidopsis Proteins; D013213:Starch; D000687:Amylopectin; D017866:Phosphotransferases (Paired Acceptors); C500668:alpha-glucan, water dikinase, Arabidopsis