Department Biology I, Botany, Ludwig-Maximilians-University Munich, Großhaderner Straße 2-4, 82152 Martinsried, Germany.
Center for Plant Molecular Biology, Plant Physiology, University of Tuebingen, Auf der Morgenstelle 32, 72076 Tuebingen, Germany.
Mol Plant. 2017 Apr 3;10(4):575-589. doi: 10.1016/j.molp.2016.12.012. Epub 2017 Jan 3.
The translocon on the outer membrane of mitochondria (TOM) facilitates the import of nuclear-encoded proteins. The principal machinery of mitochondrial protein transport seems conserved in eukaryotes; however, divergence in the composition and structure of TOM components has been observed between mammals, yeast, and plants. TOM9, the plant homolog of yeast Tom22, is significantly smaller due to a truncation in the cytosolic receptor domain, and its precise function is not understood. Here we provide evidence showing that TOM9.2 from Arabidopsis thaliana is involved in the formation of mature TOM complex, most likely by influencing the assembly of the pore-forming subunit TOM40. Dexamethasone-induced RNAi gene silencing of TOM9.2 results in a severe reduction in the mature TOM complex, and the assembly of newly imported TOM40 into the complex is impaired. Nevertheless, mutant plants are fully viable and no obvious downstream effects of the loss of TOM complex, i.e., on mitochondrial import capacity, were observed. Furthermore, we found that TOM9.2 can bind calmodulin (CaM) in vitro and that CaM impairs the assembly of TOM complex in the isolated wild-type mitochondria, suggesting a regulatory role of TOM9.2 and a possible integration of TOM assembly into the cellular calcium signaling network.
线粒体膜外腔转位蛋白复合体（TOM）促进核编码蛋白的输入。线粒体蛋白转运的主要机制在真核生物中似乎是保守的；然而，在哺乳动物、酵母和植物之间，TOM 组件的组成和结构已经观察到了分歧。由于胞质受体结构域的截断，TOM9 是酵母 Tom22 的植物同源物，其大小显著变小，其确切功能尚不清楚。在这里，我们提供的证据表明，拟南芥的 TOM9.2 参与了成熟 TOM 复合体的形成，很可能通过影响孔形成亚基 TOM40 的组装来实现。用地塞米松诱导的 TOM9.2 RNAi 基因沉默导致成熟 TOM 复合体严重减少，并且新输入的 TOM40 进入复合体的组装受损。然而，突变体植物仍然是完全有活力的，并且没有观察到 TOM 复合体缺失的明显下游效应，即线粒体输入能力。此外，我们发现 TOM9.2 可以在体外结合钙调蛋白（CaM），并且 CaM 会损害分离的野生型线粒体中 TOM 复合体的组装，这表明 TOM9.2 具有调节作用，并且 TOM 组装可能整合到细胞内钙信号网络中。
