title: The chloroplastic 2-oxoglutarate/malate transporter has dual function as the malate valve and in carbon/nitrogen metabolism.
abstract: Transport of dicarboxylates across the chloroplast envelope plays an important role in transferring carbon skeletons to the nitrogen assimilation pathway and exporting reducing equivalent to the cytosol to prevent photo-inhibition (the malate valve). It was previously shown that the Arabidopsis plastidic 2-oxoglutarate/malate transporter (AtpOMT1) and the general dicarboxylate transporter (AtpDCT1) play crucial roles at the interface between carbon and nitrogen metabolism. However, based on the in vitro transport properties of the recombinant transporters, it was hypothesized that AtpOMT1 might play a dual role, also functioning as an oxaloacetate/malate transporter, which is a crucial but currently unidentified component of the chloroplast malate valve. Here, we test this hypothesis using Arabidopsis T-DNA insertional mutants of AtpOMT1. Transport studies revealed a dramatically reduced rate of oxaloacetate uptake into chloroplasts isolated from the knockout plant. CO(2) -dependent O(2) evolution assays showed that cytosolic oxaloacetate is efficiently transported into chloroplasts mainly by AtpOMT1, and supported the absence of additional oxaloacetate transporters. These findings strongly indicate that the high-affinity oxaloacetate transporter in Arabidopsis chloroplasts is AtpOMT1. Further, the knockout plants showed enhanced photo-inhibition under high light due to greater accumulation of reducing equivalents in the stroma, indicating malfunction of the malate valve in the knockout plants. The knockout mutant showed a phenotype consistent with reductions in 2-oxoglutarate transport, glutamine synthetase/glutamate synthase activity, subsequent amino acid biosynthesis and photorespiration. Our results demonstrate that AtpOMT1 acts bi-functionally as an oxaloacetate/malate transporter in the malate valve and as a 2-oxoglutarate/malate transporter mediating carbon/nitrogen metabolism.
authors.affiliation: Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, Aichi 464-8601, JapanFaculty of Agriculture and Life Science, Hirosaki University, Hirosaki, Aomori 036-8561, JapanRIKEN Plant Science Center, Yokohama, Kanagawa 230-0045, JapanInstitute for Plant Biochemistry, Heinrich Heine University, Universitätsstraße 1, D-40225 Düsseldorf, Germany.
authors.forename: Hiromu
authors.initials: H
authors.lastname: Kinoshita
authors.affiliation: Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, Aichi 464-8601, JapanFaculty of Agriculture and Life Science, Hirosaki University, Hirosaki, Aomori 036-8561, JapanRIKEN Plant Science Center, Yokohama, Kanagawa 230-0045, JapanInstitute for Plant Biochemistry, Heinrich Heine University, Universitätsstraße 1, D-40225 Düsseldorf, Germany.
authors.forename: Junko
authors.initials: J
authors.lastname: Nagasaki
authors.affiliation: Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, Aichi 464-8601, JapanFaculty of Agriculture and Life Science, Hirosaki University, Hirosaki, Aomori 036-8561, JapanRIKEN Plant Science Center, Yokohama, Kanagawa 230-0045, JapanInstitute for Plant Biochemistry, Heinrich Heine University, Universitätsstraße 1, D-40225 Düsseldorf, Germany.
authors.forename: Nanako
authors.initials: N
authors.lastname: Yoshikawa
authors.affiliation: Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, Aichi 464-8601, JapanFaculty of Agriculture and Life Science, Hirosaki University, Hirosaki, Aomori 036-8561, JapanRIKEN Plant Science Center, Yokohama, Kanagawa 230-0045, JapanInstitute for Plant Biochemistry, Heinrich Heine University, Universitätsstraße 1, D-40225 Düsseldorf, Germany.
authors.forename: Aya
authors.initials: A
authors.lastname: Yamamoto
authors.affiliation: Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, Aichi 464-8601, JapanFaculty of Agriculture and Life Science, Hirosaki University, Hirosaki, Aomori 036-8561, JapanRIKEN Plant Science Center, Yokohama, Kanagawa 230-0045, JapanInstitute for Plant Biochemistry, Heinrich Heine University, Universitätsstraße 1, D-40225 Düsseldorf, Germany.
authors.forename: Shizuka
authors.initials: S
authors.lastname: Takito
authors.affiliation: Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, Aichi 464-8601, JapanFaculty of Agriculture and Life Science, Hirosaki University, Hirosaki, Aomori 036-8561, JapanRIKEN Plant Science Center, Yokohama, Kanagawa 230-0045, JapanInstitute for Plant Biochemistry, Heinrich Heine University, Universitätsstraße 1, D-40225 Düsseldorf, Germany.
authors.forename: Michio
authors.initials: M
authors.lastname: Kawasaki
authors.affiliation: Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, Aichi 464-8601, JapanFaculty of Agriculture and Life Science, Hirosaki University, Hirosaki, Aomori 036-8561, JapanRIKEN Plant Science Center, Yokohama, Kanagawa 230-0045, JapanInstitute for Plant Biochemistry, Heinrich Heine University, Universitätsstraße 1, D-40225 Düsseldorf, Germany.
authors.forename: Tatsuo
authors.initials: T
authors.lastname: Sugiyama
authors.affiliation: Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, Aichi 464-8601, JapanFaculty of Agriculture and Life Science, Hirosaki University, Hirosaki, Aomori 036-8561, JapanRIKEN Plant Science Center, Yokohama, Kanagawa 230-0045, JapanInstitute for Plant Biochemistry, Heinrich Heine University, Universitätsstraße 1, D-40225 Düsseldorf, Germany.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Miyake
authors.affiliation: Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, Aichi 464-8601, JapanFaculty of Agriculture and Life Science, Hirosaki University, Hirosaki, Aomori 036-8561, JapanRIKEN Plant Science Center, Yokohama, Kanagawa 230-0045, JapanInstitute for Plant Biochemistry, Heinrich Heine University, Universitätsstraße 1, D-40225 Düsseldorf, Germany.
authors.forename: Andreas P M
authors.initials: APM
authors.lastname: Weber
authors.affiliation: Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, Aichi 464-8601, JapanFaculty of Agriculture and Life Science, Hirosaki University, Hirosaki, Aomori 036-8561, JapanRIKEN Plant Science Center, Yokohama, Kanagawa 230-0045, JapanInstitute for Plant Biochemistry, Heinrich Heine University, Universitätsstraße 1, D-40225 Düsseldorf, Germany.
authors.forename: Mitsutaka
authors.initials: M
authors.lastname: Taniguchi
mesh_terms: D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000378:metabolism*; D002244:Carbon / Q000378:metabolism*; D002734:Chlorophyll / Q000378:metabolism; D002736:Chloroplasts / Q000378:metabolism*; D029382:Dicarboxylic Acid Transporters / Q000378:metabolism*; D008293:Malates / Q000378:metabolism*; D009584:Nitrogen / Q000378:metabolism*; D030821:Plants, Genetically Modified / Q000235:genetics / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction
chemical_list: D029681:Arabidopsis Proteins; D029382:Dicarboxylic Acid Transporters; D008293:Malates; D002734:Chlorophyll; D002244:Carbon; C030298:malic acid; D009584:Nitrogen