Department of Plant Physiology, Institute of Biology, Humboldt University Berlin, Berlin, Germany.
Plant Cell. 2011 Dec;23(12):4476-91. doi: 10.1105/tpc.111.086421. Epub 2011 Dec 16.
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.
5-氨基乙酰丙酸（ALA）是四吡咯生物合成的通用前体，在植物中通过三个酶促步骤合成：谷氨酸（Glu）与 tRNA（Glu）的连接由谷氨酰-tRNA 合成酶完成，谷氨酸还原酶（GluTR）将活化的 Glu 还原为 Glu-1-半醛，然后由 Glu 1-半醛氨基转移酶将其转化为 ALA。ALA 的形成控制着四吡咯生物合成途径中的代谢流。GluTR 被提议为关键调节酶，在转录和翻译后水平受到严格控制。我们鉴定了一种 GluTR 结合蛋白（GluTRBP；以前称为 PROTON GRADIENT REGULATION7），它定位于叶绿体中，是类囊体膜中 300-kD 蛋白复合物的一部分。尽管该蛋白不会调节 ALA 合成的活性，但 GluTRBP 的敲除在拟南芥中是致命的，而表达水平降低的 GluTRBP 的突变体则含有较少的血红素。GluTRBP 的表达与血红素生物合成的功能相关。推测 GluTRBP 通过将一部分 GluTR 维持在质膜上，将 ALA 导入血红素生物合成途径，从而有助于ALA 生物合成的亚区室化。这些关于 GluTRBP 的结果支持了植物 ALA 合成的模型，该模型在叶绿体中组织为两个独立的 ALA 池，以提供适量的底物，用于平衡血红素和叶绿素的合成。
