Reinbothe Steffen, Gray John, Rustgi Sachin, von Wettstein Diter, Reinbothe Christiane
Biologie Environnementale et Systémique (BEeSy), Université Joseph Fourier, F-38041 Grenoble Cedex 9, France;
Department of Biological Sciences, University of Toledo, Toledo, OH 43606; and.
Proc Natl Acad Sci U S A. 2015 May 5;112(18):5838-43. doi: 10.1073/pnas.1506339112. Epub 2015 Apr 21.
Tetrapyrroles such as chlorophyll, heme, and bacteriochlorophyll play fundamental roles in the energy absorption and transduction of all photosynthetic organisms. They are synthesized via a complex pathway taking place in chloroplasts. Chlorophyll biosynthesis in angiosperms involves 16 steps of which only one is light-requiring and driven by the NADPH:protochlorophyllide oxidoreductase (POR). Three POR isoforms have been identified in Arabidopsis thaliana--designated PORA, PORB, and PORC--that are differentially expressed in etiolated, light-exposed, and light-adapted plants. All three isoforms are encoded by nuclear genes, are synthesized as larger precursors in the cytosol (pPORs), and are imported posttranslationally into the plastid compartment. Import of the precursor to the dark-specific isoform PORA (pPORA) is protochlorophyllide (Pchlide)-dependent and due to the operation of a unique translocon complex dubbed PTC (Pchlide-dependent translocon complex) in the plastid envelope. Here, we identified a ∼30-kDa protein that participates in pPORA import. The ∼30-kDa protein is identical to the previously identified CELL GROWTH DEFECT FACTOR 1 (CDF1) in Arabidopsis that is conserved in higher plants and Synechocystis. CDF1 operates in pPORA import and stabilization and hereby acts as a chaperone for PORA protein translocation. CDF1 permits tight interactions between Pchlide synthesized in the plastid envelope and the importing PORA polypeptide chain such that no photoexcitative damage occurs through the generation of singlet oxygen operating as a cell death inducer. Together, our results identify an ancient mechanism dating back to the endosymbiotic origin of chloroplasts as a key element of Pchlide-dependent pPORA import.
叶绿素、血红素和细菌叶绿素等四吡咯在所有光合生物的能量吸收和转换过程中发挥着重要作用。它们通过叶绿体中发生的复杂途径合成。被子植物中的叶绿素生物合成涉及16个步骤，其中只有一步需要光，并由NADPH：原叶绿素酸酯氧化还原酶（POR）驱动。在拟南芥中已鉴定出三种POR同工型——分别命名为PORA、PORB和PORC——它们在黄化、光照和光适应植物中差异表达。所有这三种同工型均由核基因编码，在细胞质中作为较大的前体（pPORs）合成，并在翻译后导入质体区室。黑暗特异性同工型PORA（pPORA）的前体导入是原叶绿素酸酯（Pchlide）依赖性的，这是由于质体包膜中一种独特的转运体复合物（称为PTC，即Pchlide依赖性转运体复合物）的作用。在此，我们鉴定出一种参与pPORA导入的约30 kDa蛋白。该约30 kDa蛋白与拟南芥中先前鉴定的细胞生长缺陷因子1（CDF1）相同，CDF1在高等植物和集胞藻中保守。CDF1在pPORA导入和稳定过程中起作用，从而作为PORA蛋白转运的伴侣。CDF1允许在质体包膜中合成的Pchlide与导入的PORA多肽链之间紧密相互作用，从而不会因作为细胞死亡诱导剂的单线态氧的产生而发生光激发损伤。总之，我们的结果确定了一种可追溯到叶绿体内共生起源的古老机制，它是Pchlide依赖性pPORA导入的关键要素。
