Department of Biochemistry and Molecular Biology, University of Massachusetts, Amherst MA, USA.
Department of Biology, Wilfrid Laurier University, Waterloo ON, Canada.
Front Plant Sci. 2014 Jun 11;5:269. doi: 10.3389/fpls.2014.00269. eCollection 2014.
The translocon at the outer envelope membrane of chloroplasts (TOC) initiates the import of thousands of nuclear encoded preproteins required for chloroplast biogenesis and function. The multimeric TOC complex contains two GTP-regulated receptors, Toc34 and Toc159, which recognize the transit peptides of preproteins and initiate protein import through a β-barrel membrane channel, Toc75. Different isoforms of Toc34 and Toc159 assemble with Toc75 to form structurally and functionally diverse translocons, and the composition and levels of TOC translocons is required for the import of specific subsets of coordinately expressed proteins during plant growth and development. Consequently, the proper assembly of the TOC complexes is key to ensuring organelle homeostasis. This review will focus on our current knowledge of the targeting and assembly of TOC components to form functional translocons at the outer membrane. Our analyses reveal that the targeting of TOC components involves elements common to the targeting of other outer membrane proteins, but also include unique features that appear to have evolved to specifically facilitate assembly of the import apparatus.
叶绿体外膜上的转位酶（TOC）起始数千种核编码前体蛋白的输入，这些蛋白是叶绿体发生和功能所必需的。多聚体 TOC 复合物包含两个 GTP 调节受体，Toc34 和 Toc159，它们识别前体蛋白的转运肽，并通过β-桶膜通道 Toc75 起始蛋白输入。Toc34 和 Toc159 的不同同工型与 Toc75 组装形成结构和功能不同的转位酶，并且 TOC 转位酶的组成和水平是在植物生长和发育过程中导入特定协调表达蛋白亚群所必需的。因此，TOC 复合物的正确组装对于确保细胞器的内稳态至关重要。这篇综述将重点介绍我们目前对 TOC 组件的靶向和组装以在外膜上形成功能转位酶的认识。我们的分析表明，TOC 组件的靶向涉及到其他外膜蛋白靶向的共同元素，但也包括似乎为专门促进导入装置组装而进化出的独特特征。
