The plastidic pentose phosphate translocator represents a link between the cytosolic and the plastidic pentose phosphate pathways in plants.
Plastids are the site of the reductive and the oxidative pentose phosphate pathways, which both generate pentose phosphates as intermediates. A plastidic transporter from Arabidopsis has been identified that is able to transport, in exchange with inorganic phosphate or triose phosphates, xylulose 5-phosphate (Xul-5-P) and, to a lesser extent, also ribulose 5-phosphate, but does not accept ribose 5-phosphate or hexose phosphates as substrates. Under physiological conditions, Xul-5-P would be the preferred substrate. Therefore, the translocator was named Xul-5-P/phosphate translocator (XPT). The XPT shares only approximately 35% to 40% sequence identity with members of both the triose phosphate translocator and the phosphoenolpyruvate/phosphate translocator classes, but a higher identity of approximately 50% to glucose 6-phosphate/phosphate translocators. Therefore, it represents a fourth group of plastidic phosphate translocators. Database analysis revealed that plant cells contain, in addition to enzymes of the oxidative branch of the oxidative pentose phosphate pathway, ribose 5-phosphate isomerase and ribulose 5-phosphate epimerase in both the cytosol and the plastids, whereas the transketolase and transaldolase converting the produced pentose phosphates to triose phosphates and hexose phosphates are probably solely confined to plastids. It is assumed that the XPT function is to provide the plastidic pentose phosphate pathways with cytosolic carbon skeletons in the form of Xul-5-P, especially under conditions of a high demand for intermediates of the cycles.
Eicks
Michael
M
Botanisches Institut der Universität zu Köln, Lehrstuhl II, Gyrhofstrasse 15, D-50931 Cologne, Germany.
Maurino
Verónica
V
Knappe
Silke
S
Flügge
Ulf-Ingo
UI
Fischer
Karsten
K
D019747:Aldose-Ketose Isomerases / Q000235:genetics / Q000378:metabolism; D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000235:genetics / Q000502:physiology*; D001692:Biological Transport; D002238:Carbohydrate Epimerases / Q000235:genetics / Q000378:metabolism; D003001:Cloning, Molecular; D003600:Cytosol / Q000502:physiology; D020224:Expressed Sequence Tags; D015870:Gene Expression; D008969:Molecular Sequence Data; D010427:Pentose Phosphate Pathway / Q000235:genetics / Q000502:physiology*; D010428:Pentosephosphates / Q000378:metabolism; D028061:Phosphate Transport Proteins / Q000235:genetics / Q000378:metabolism; D010802:Phylogeny; D018087:Plastids / Q000502:physiology*; D012274:Ribulosephosphates / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D014996:Xylulose / Q000378:metabolism
D010428:Pentosephosphates; D028061:Phosphate Transport Proteins; D012274:Ribulosephosphates; C031524:ribulose 5-phosphate; D014996:Xylulose; C031625:xylulose-5-phosphate; D002238:Carbohydrate Epimerases; C022554:L-ribulosephosphate 4-epimerase; D019747:Aldose-Ketose Isomerases; C022548:ribosephosphate isomerase