Aronsson Henrik, Boij Patrik, Patel Ramesh, Wardle Anthony, Töpel Mats, Jarvis Paul
Department of Plant and Environmental Sciences, Göteborg University, Box 461, SE-405 30 Göteborg, Sweden.
Plant J. 2007 Oct;52(1):53-68. doi: 10.1111/j.1365-313X.2007.03207.x. Epub 2007 Jul 26.
Toc64/OEP64 was identified biochemically in pea as a putative component of the chloroplast protein import apparatus. In Arabidopsis, three paralogous genes (atTOC64-III, atTOC64-V and atTOC64-I) encode Toc64-related proteins, and these have been reported to localize in chloroplasts, mitochondria and the cytosol, respectively. To assess the role of the atToc64-III protein in chloroplast protein import in an in vivo context, we identified and characterized Arabidopsis knockout mutants. The absence of detectable defects in toc64-III single mutants raised the possibility of redundancy, and prompted us to also identify toc64-V and toc64-I mutants, cross them to toc64-III, and generate double- and triple-mutant combinations. The toc64 mutants were analysed carefully with respect to a variety of criteria, including chlorophyll accumulation, photosynthetic performance, organellar ultrastructure and chloroplast protein accumulation. In each case, the mutant plants were indistinguishable from wild type. Furthermore, the efficiency of chloroplast protein import was not affected by the toc64 mutations, even when a putative substrate of the atToc64-III protein (wheatgerm-translated precursor of the 33 kDa subunit of the oxygen-evolving complex, OE33) was examined. Moreover, under various stress conditions (high light, osmotic stress and cold), the toc64 triple-mutant plants were not significantly different from wild type. These results demonstrate that Toc64/OEP64 is not essential for the efficient import of proteins into chloroplasts in Arabidopsis, and draw into question the functional significance of this component.
Toc64/OEP64在豌豆中通过生化方法被鉴定为叶绿体蛋白质输入装置的一个假定成分。在拟南芥中，三个同源基因（atTOC64-III、atTOC64-V和atTOC64-I）编码与Toc64相关的蛋白质，据报道这些蛋白质分别定位于叶绿体、线粒体和细胞质中。为了在体内环境中评估atToc64-III蛋白在叶绿体蛋白质输入中的作用，我们鉴定并表征了拟南芥敲除突变体。toc64-III单突变体中未检测到明显缺陷，这增加了冗余的可能性，并促使我们鉴定toc64-V和toc64-I突变体，将它们与toc64-III杂交，并产生双突变体和三突变体组合。我们根据包括叶绿素积累、光合性能、细胞器超微结构和叶绿体蛋白质积累等多种标准，仔细分析了toc64突变体。在每种情况下，突变体植株与野生型没有区别。此外，即使检测atToc64-III蛋白的一个假定底物（放氧复合体33 kDa亚基的小麦胚芽翻译前体，OE33），叶绿体蛋白质输入效率也不受toc64突变的影响。此外，在各种胁迫条件下（高光、渗透胁迫和低温），toc64三突变体植株与野生型没有显著差异。这些结果表明，Toc64/OEP64对于拟南芥中蛋白质高效输入叶绿体不是必需的，并对该成分的功能意义提出了质疑。
