Institute of Molecular Biology, Academia Sinica, Taipei, Taiwan.
Department of Plant Sciences, University of California, Davis, CA, USA.
Nature. 2018 Dec;564(7734):125-129. doi: 10.1038/s41586-018-0713-y. Epub 2018 Nov 21.
The two-membrane envelope is a defining feature of chloroplasts. Chloroplasts evolved from a Gram-negative cyanobacterial endosymbiont. During evolution, genes of the endosymbiont have been transferred to the host nuclear genome. Most chloroplast proteins are synthesized in the cytosol as higher-molecular-mass preproteins with an N-terminal transit peptide. Preproteins are transported into chloroplasts by the TOC and TIC (translocons at the outer- and inner-envelope membranes of chloroplasts, respectively) machineries, but how TOC and TIC are assembled together is unknown. Here we report the identification of the TIC component TIC236; TIC236 is an integral inner-membrane protein that projects a 230-kDa domain into the intermembrane space, which binds directly to the outer-membrane channel TOC75. The knockout mutation of TIC236 is embryonically lethal. In TIC236-knockdown mutants, a smaller amount of the inner-membrane channel TIC20 was associated with TOC75; the amount of TOC-TIC supercomplexes was also reduced. This resulted in a reduced import rate into the stroma, though outer-membrane protein insertion was unaffected. The size and the essential nature of TIC236 indicate that-unlike in mitochondria, in which the outer- and inner-membrane translocons exist as separate complexes and a supercomplex is only transiently assembled during preprotein translocation-a long and stable protein bridge in the intermembrane space is required for protein translocation into chloroplasts. Furthermore, TIC236 and TOC75 are homologues of bacterial inner-membrane TamB and outer-membrane BamA, respectively. Our evolutionary analyses show that, similar to TOC75, TIC236 is preserved only in plants and has co-evolved with TOC75 throughout the plant lineage. This suggests that the backbone of the chloroplast protein-import machinery evolved from the bacterial TamB-BamA protein-secretion system.
双层膜包膜是叶绿体的一个定义特征。叶绿体是由革兰氏阴性蓝细菌内共生体进化而来的。在进化过程中，内共生体的基因已转移到宿主核基因组中。大多数叶绿体蛋白在细胞质中作为高分子质量的前体蛋白合成，其 N 端具有转运肽。前体蛋白通过 TOC 和 TIC（叶绿体外膜和内膜转位体）机器转运到叶绿体中，但 TOC 和 TIC 如何组装在一起尚不清楚。在这里，我们报告了 TIC 成分 TIC236 的鉴定；TIC236 是一种完整的内膜蛋白，其在膜间空间中突出一个 230kDa 的结构域，直接与外膜通道 TOC75 结合。TIC236 的敲除突变是胚胎致死的。在 TIC236 敲除突变体中，与 TOC75 结合的内膜通道 TIC20 的量较少；TOC-TIC 超复合物的量也减少了。这导致导入基质的速度降低，尽管外膜蛋白插入不受影响。TIC236 的大小和重要性表明——与线粒体不同，在线粒体中，外膜和内膜转位体作为独立的复合物存在，并且只有在蛋白质易位过程中才会短暂组装超复合物——在膜间空间中需要一个长而稳定的蛋白质桥才能将蛋白质转运到叶绿体中。此外，TIC236 和 TOC75 分别是细菌内膜 TamB 和外膜 BamA 的同源物。我们的进化分析表明，与 TOC75 相似，TIC236 仅在植物中保留，并且在整个植物谱系中与 TOC75 共同进化。这表明叶绿体蛋白导入机制的骨干是从细菌 TamB-BamA 蛋白分泌系统进化而来的。
