The chloroplastic 2-oxoglutarate/malate transporter has dual function as the malate valve and in carbon/nitrogen metabolism
The chloroplastic 2-oxoglutarate/malate transporter has dual function as the malate valve and in carbon/nitrogen metabolism
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DOI：
10.1111/j.1365-313x.2010.04397.x
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发表时间：
2011-01-01
期刊：
影响因子：
7.2
通讯作者：
Taniguchi, Mitsutaka
中科院分区：
文献类型：
--
作者：
Kinoshita, Hiromu;Nagasaki, Junko;Taniguchi, Mitsutaka
二羧酸盐在叶绿体包膜上的运输在将碳骨架转移到氮同化途径和向细胞质输出还原物以防止光抑制（苹果酸阀）方面起着重要作用。先前的研究表明，拟南芥可塑性2-氧葡萄糖酸盐/苹果酸转运蛋白（AtpOMT1）和一般二羧酸转运蛋白（AtpDCT1）在碳氮代谢的界面上起着至关重要的作用。然而，基于重组转运体的体外转运特性，我们假设AtpOMT1可能具有双重作用，也可作为草酰乙酸/苹果酸转运体，这是叶绿体苹果酸阀的关键成分，但目前尚未确定。在这里，我们使用拟南芥T-DNA插入突变体AtpOMT1来验证这一假设。运输研究表明，从基因敲除植物中分离出的叶绿体对草酰乙酸的吸收速率显著降低。二氧化碳依赖的O-2进化实验表明，细胞内的草酰乙酸主要通过AtpOMT1有效地转运到叶绿体中，这支持了没有其他草酰乙酸转运体的存在。这些发现有力地表明拟南芥叶绿体中高亲和的草酰乙酸转运蛋白是AtpOMT1。此外，基因敲除植物在强光下表现出增强的光抑制，这是由于基质中还原性当量的积累更多，这表明基因敲除植物的苹果酸阀出现了故障。基因敲除突变体的表型与2-氧戊二酸转运、谷氨酰胺合成酶/谷氨酸合成酶活性、随后的氨基酸生物合成和光呼吸的减少一致。我们的研究结果表明，AtpOMT1在苹果酸阀中作为草酰乙酸/苹果酸转运体和作为2-氧戊二酸/苹果酸转运体介导碳/氮代谢的双重功能。
P>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. CO2-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.