An Arabidopsis GluTR Binding Protein Mediates Spatial Separation of 5-Aminolevulinic Acid Synthesis in Chloroplasts
An Arabidopsis GluTR Binding Protein Mediates Spatial Separation of 5-Aminolevulinic Acid Synthesis in Chloroplasts
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DOI：
10.1105/tpc.111.086421
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发表时间：
2011-12-01
期刊：
影响因子：
11.6
通讯作者：
Grimm, Bernhard
中科院分区：
文献类型：
--
作者：
Czarnecki, Olaf;Hedtke, Boris;Grimm, Bernhard
5-氨基乙酰丙酸（ALA）是四吡啶生物合成的普遍前体，在植物中通过三个酶促步骤合成：谷氨酰胺-tRNA合成酶将谷氨酸（Glu）连接到tRNA(Glu)，谷氨酰胺-tRNA还原酶（GluTR）将活化的谷氨酸还原为谷氨酸-1-半醛，谷氨酸-1-半醛转氨酶将谷氨酸转氨酶转化为ALA。ALA的形成控制着进入四吡咯生物合成途径的代谢流。GluTR被认为是在转录和翻译后水平受到严格控制的关键调控酶。我们发现了一种GluTR结合蛋白（GluTRBP，以前称为质子梯度调节7），它定位于叶绿体和类囊体膜中300-kD蛋白复合物的一部分。虽然该蛋白不调节ALA合成活性，但在拟南芥中，敲除GluTRBP是致命的，而表达低水平GluTRBP的突变体含有较少的血红素。GluTRBP的表达与血红素生物合成的功能相关。据推测，GluTRBP通过维持质体膜上的一部分GluTR，将ALA输送到血红素生物合成途径，从而有助于亚区隔化ALA的生物合成。这些关于GluTRBP的结果支持植物ALA合成模型，该模型由叶绿体中两个独立的ALA池组织，为血红素和叶绿素的平衡合成提供适当的底物量。
5-Aminolevulinic acid (ALA) is the universal precursor for tetrapyrrole biosynthesis and is synthesized in plants in three enzymatic steps: ligation of glutamate (Glu) to tRNA(Glu) by glutamyl-tRNA synthetase, reduction of activated Glu to Glu-1-semialdehyde by glutamyl-tRNA reductase (GluTR), and transamination to ALA by Glu 1-semialdehyde aminotransferase. ALA formation controls the metabolic flow into the tetrapyrrole biosynthetic pathway. GluTR is proposed to be the key regulatory enzyme that is tightly controlled at transcriptional and posttranslational levels. We identified a GluTR binding protein (GluTRBP; previously called PROTON GRADIENT REGULATION7) that is localized in chloroplasts and part of a 300-kD protein complex in the thylakoid membrane. Although the protein does not modulate activity of ALA synthesis, the knockout of GluTRBP is lethal in Arabidopsis thaliana, whereas mutants expressing reduced levels of GluTRBP contain less heme. GluTRBP expression correlates with a function in heme biosynthesis. It is postulated that GluTRBP contributes to subcompartmentalized ALA biosynthesis by maintaining a portion of GluTR at the plastid membrane that funnels ALA into the heme biosynthetic pathway. These results regarding GluTRBP support a model of plant ALA synthesis that is organized in two separate ALA pools in the chloroplast to provide appropriate substrate amounts for balanced synthesis of heme and chlorophyll.