The chloroplastic protein translocation channel Toc75 and its paralog OEP80 represent two distinct protein families and are targeted to the chloroplastic outer envelope by different mechanisms
The chloroplastic protein translocation channel Toc75 and its paralog OEP80 represent two distinct protein families and are targeted to the chloroplastic outer envelope by different mechanisms
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DOI：
10.1111/j.1365-313x.2004.02135.x
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发表时间：
2004-08-01
期刊：
影响因子：
7.2
通讯作者：
Potter, D
中科院分区：
文献类型：
--
作者：
Inoue, K;Potter, D
Toc 75被认为是叶绿体外被膜蛋白质转运通道的形成者。与Toc 75同源的蛋白质存在于广泛的生物体中，最接近的同源物出现在蓝藻中。因此，Toc 75的内共生起源已被假定。最近，在拟南芥中发现了一个Toc 75的同源基因，并将其命名为atToc 75-V。本研究对这一新的Toc 75同源基因进行了表征，并对高等植物和细菌中Toc 75同源基因之间的关系进行了深入的研究，以期了解叶绿体蛋白转运通道的进化起源。首先，我们发现atToc 75-V的天然分子量为80 kDa，并将其重新命名为（AtOEP 80）80 kDa的拟南芥外膜蛋白。其次，我们发现AtOEP 80和Toc 75利用不同的机制来靶向叶绿体包膜。Toc 75通过一个可裂解的二分转运肽部分地通过一般输入途径定向，而AtOEP 80在其成熟序列内包含靶向信息，并且其靶向独立于一般途径。第三，我们进行了Toc 75同源物从各种生物的系统发育分析，发现Toc 75和OEP 80代表两个独立的基因家族，最有可能来自蓝藻序列。我们的研究结果表明，Toc 75和OEP 80分歧的质体从它们的共同祖先与现代蓝藻的进化早期。
Toc75 is postulated to form the protein translocation channel in the chloroplastic outer envelope membrane. Proteins homologous to Toc75 are present in a wide range of organisms, with the closest homologs occurring in cyanobacteria. Therefore, an endosymbiotic origin of Toc75 has been postulated. Recently, a gene encoding a paralog to Toc75 was identified in Arabidopsis and its product was named atToc75-V. In the present study, we characterized this new Toc75 paralog, and investigated extensively the relationships among Toc75 homologs from higher plants and bacteria in order to gain insights into the evolutionary origin of the chloroplastic protein translocation channel. First, we found that the native molecular weight of atToc75-V is 80 kDa and renamed it (AtOEP80) Arabidopsis thaliana outer envelope protein of 80kDa. Second, we found that AtOEP80 and Toc75 utilize different mechanisms for their targeting to the chloroplastic envelope. Toc75 is directed with a cleavable bipartite transit peptide partly via the general import pathway, whereas AtOEP80 contains the targeting information within its mature sequence, and its targeting is independent of the general pathway. Third, we undertook phylogenetic analyses of Toc75 homologs from various organisms, and found that Toc75 and OEP80 represent two independent gene families that are most likely derived from cyanobacterial sequences. Our results suggest that Toc75 and OEP80 diverged early in the evolution of plastids from their common ancestor with modern cyanobacteria.