Baldwin Amy, Wardle Anthony, Patel Ramesh, Dudley Penny, Park Soon Ki, Twell David, Inoue Kentaro, Jarvis Paul
Department of Biology, University of Leicester, Leicester LE1 7RH, United Kingdom.
Plant Physiol. 2005 Jun;138(2):715-33. doi: 10.1104/pp.105.063289. Epub 2005 May 20.
Toc75 (translocon at the outer envelope membrane of chloroplasts, 75 kD) is the protein translocation channel at the outer envelope membrane of plastids and was first identified in pea (Pisum sativum) using biochemical approaches. The Arabidopsis (Arabidopsis thaliana) genome contains three Toc75-related sequences, termed atTOC75-I, atTOC75-III, and atTOC75-IV, which we studied using a range of molecular, genetic, and biochemical techniques. Expression of atTOC75-III is strongly regulated and at its highest level in young, rapidly expanding tissues. By contrast, atTOC75-IV is expressed uniformly throughout development and at a much lower level than atTOC75-III. The third sequence, atTOC75-I, is a pseudogene that is not expressed due to a gypsy/Ty3 transposon insertion in exon 1, and numerous nonsense, frame-shift, and splice-junction mutations. The expressed genes, atTOC75-III and atTOC75-IV, both encode integral envelope membrane proteins. Unlike atToc75-III, the smaller atToc75-IV protein is not processed upon targeting to the envelope, and its insertion does not require ATP at high concentrations. The atTOC75-III gene is essential for viability, since homozygous atToc75-III knockout mutants (termed toc75-III) could not be identified, and aborted seeds were observed at a frequency of approximately 25% in the siliques of self-pollinated toc75-III heterozygotes. Homozygous toc75-III embryos were found to abort at the two-cell stage. Homozygous atToc75-IV knockout plants (termed toc75-IV) displayed no obvious visible phenotypes. However, structural abnormalities were observed in the etioplasts of toc75-IV seedlings and atTOC75-IV overexpressing lines, and toc75-IV plants were less efficient at deetiolation than wild type. These results suggest some role for atToc75-IV during growth in the dark.
Toc75（叶绿体外被膜转位子，75千道尔顿）是质体外被膜上的蛋白质转运通道，最初是在豌豆（Pisum sativum）中通过生化方法鉴定出来的。拟南芥（Arabidopsis thaliana）基因组包含三个与Toc75相关的序列，分别称为atTOC75-I、atTOC75-III和atTOC75-IV，我们使用了一系列分子、遗传和生化技术对其进行了研究。atTOC75-III的表达受到强烈调控，在幼嫩、快速生长的组织中表达水平最高。相比之下，atTOC75-IV在整个发育过程中均一表达，且表达水平远低于atTOC75-III。第三个序列atTOC75-I是一个假基因，由于第1外显子中插入了一个gypsy/Ty3转座子以及众多无义、移码和剪接连接突变而不表达。表达的基因atTOC75-III和atTOC75-IV均编码外被膜整合蛋白。与atToc75-III不同，较小的atToc75-IV蛋白在靶向外被膜时不会被加工，其插入不需要高浓度的ATP。atTOC75-III基因对于生存力至关重要，因为无法鉴定出纯合的atToc75-III敲除突变体（称为toc75-III），并且在自花授粉的toc75-III杂合子的角果中观察到约25%的败育种子。发现纯合的toc75-III胚胎在二细胞阶段败育。纯合的atToc75-IV敲除植物（称为toc75-IV）没有明显的可见表型。然而，在toc75-IV幼苗和atTOC75-IV过表达系的黄化质体中观察到结构异常，并且toc75-IV植物在去黄化过程中的效率低于野生型。这些结果表明atToc75-IV在黑暗生长过程中发挥了一些作用。
