The uptake of newly synthesized nuclear-encoded mitochondrial proteins from the cytosol is mediated by a complex of mitochondrial outer membrane proteins comprising a central pore-forming component and associated receptor proteins. Distinct fractions of proteins initially bind to the receptor proteins and are subsequently transferred to the pore-forming component for import. The aim of this study was the identification of the decisive elements of this machinery that determine the specific selection of the proteins that should be imported. We identified the essential internal targeting signal of the members of the mitochondrial metabolite carrier proteins, the largest protein family of the mitochondria, and we investigated the specific recognition of this signal by the protein import machinery at the mitochondrial outer surface. We found that the outer membrane import receptors facilitated the uptake of these proteins, and we identified the corresponding binding site, marked by cysteine C141 in the receptor protein Tom70. However, in tests both in vivo and in vitro, the import receptors were neither necessary nor sufficient for specific recognition of the targeting signals. Although these signals are unrelated to the amino-terminal presequences that mediate the targeting of other mitochondrial preproteins, they were found to resemble presequences in their strict dependence on a content of positively charged residues as a prerequisite of interactions with the import pore. The general import pore of the mitochondrial outer membrane appears to represent not only the central channel of protein translocation but also to form the decisive general selectivity filter in the uptake of the newly synthesized mitochondrial proteins.

中文翻译：

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线粒体蛋白质进口机器的选择性过滤器

从细胞质中摄取新合成的核编码线粒体蛋白是由线粒体外膜蛋白复合物介导的，该复合物包含中央的成孔成分和相关的受体蛋白。蛋白质的不同部分最初会与受体蛋白质结合，然后转移到成孔成分中进行导入。这项研究的目的是确定该机器的决定性因素，这些决定因素决定了应输入蛋白质的特定选择。我们确定了线粒体代谢产物载体蛋白（线粒体的最大蛋白家族）成员的基本内部靶向信号，并且我们通过蛋白质导入机制在线粒体外表面研究了该信号的特异性识别。我们发现外膜导入受体促进了这些蛋白质的摄取，并且我们确定了相应的结合位点，该结合位点由受体蛋白Tom70中的半胱氨酸C141标记。然而，在体内和体外测试中，输入受体对于特异性识别靶向信号既不是必需的也不是足够的。尽管这些信号与介导其他线粒体前蛋白靶向的氨基末端序列无关，但发现它们在严格依赖带正电荷的残基含量方面类似于序列，这是与导入孔相互作用的先决条件。