title: Characterization of ADP-glucose transport across the cereal endosperm amyloplast envelope.
abstract: Most of the carbon used for starch biosynthesis in cereal endosperms is derived from ADP-glucose (ADP-Glc) synthesized by extra-plastidial AGPase activity, and imported directly across the amyloplast envelope. The properties of the wheat endosperm amyloplast ADP-Glc transporter were analysed with respect to substrate kinetics and specificities using reconstituted amyloplast envelope proteins in a proteoliposome-based assay system, as well as with isolated intact organelles. Experiments with liposomes showed that ADP-Glc transport was dependent on counter-exchange with other adenylates. Rates of ADP-Glc transport were highest with ADP and AMP as counter-exchange substrates, and kinetic analysis revealed that the transport system has a similar affinity for ADP and AMP. Measurement of ADP and AMP efflux from intact amyloplasts showed that, under conditions of ADP-Glc-dependent starch biosynthesis, ADP is exported from the plastid at a rate equal to that of ADP-Glc utilization by starch synthases. Photo-affinity labelling of amyloplast membranes with the substrate analogue 8-azido-[alpha-32P]ADP-Glc showed that the polypeptide involved in substrate binding is an integral membrane protein of 38 kDa. This study shows that the ADP-Glc transporter in cereal endosperm amyloplasts imports ADP-Glc in exchange for ADP which is produced as a by-product of the starch synthase reaction inside the plastid.
authors.affiliation: Faculty of Life Sciences, 3.614 Stopford Building, University of Manchester, Oxford Road, Manchester M13 9PT, UK.
authors.forename: Caroline G
authors.initials: CG
authors.lastname: Bowsher
authors.forename: Edward F A L
authors.initials: EF
authors.lastname: Scrase-Field
authors.forename: Sergio
authors.initials: S
authors.lastname: Esposito
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Emes
authors.forename: Ian J
authors.initials: IJ
authors.lastname: Tetlow
mesh_terms: D000245:Adenosine Diphosphate Glucose / Q000378:metabolism*; D001692:Biological Transport; D051599:Glucose-1-Phosphate Adenylyltransferase / Q000378:metabolism; D008081:Liposomes / Q000378:metabolism; D014908:Triticum / Q000201:enzymology / Q000254:growth & development / Q000378:metabolism
chemical_list: D008081:Liposomes; D000245:Adenosine Diphosphate Glucose; D051599:Glucose-1-Phosphate Adenylyltransferase