Molecular Mechanism of Mitoshondrial Protein Import in Higher Animals

大多数线粒体蛋白在细胞质溶胶中合成，并通过外膜上的Tom复合体转运到线粒体中。我们用体外导入系统和培养细胞系统分析了Tom20的作用。Tom20可溶性结构域强烈抑制前鸟氨酸转氨基酰基酶（pOTC）、前天冬氨酸转氨酶（pAAT）和前丝氨酸丙酮酸转氨酶（pSPT）的输入。这些前体的输入也被一种针对Tom20的抗体所抑制。然而，抑制程度因前体而异。当pOTC在COS-7细胞中表达时，pOTC首先出现，然后导入线粒体，加工成成熟形态。当Tom20与pOTC共表达时，pOTC的线粒体输入和加工被延缓。另一方面，可溶性Tom20共表达影响不大。这些结果表明，Tom20参与线粒体蛋白的输入，并且不同前体蛋白对Tom20的需求不同。

Most mitochondrial proteins are synthesized in the cytosol and transported into the mitochondria via Tom complex on the outer membrane. We analyzed the roles of Tom20 using in vitro import system and cultured cell system. Import of pre-ornithine transcarbamylase (pOTC), pre-aspartate aminotransferase (pAAT) and pre-serine : pyruvate aminotransferase (pSPT) was strongly inhibited by soluble domain of Tom20. Import of these precursors was also inhibited by an antibody against Tom20. However, the degree of inhibition differed from one precursor to another. When pOTC is expressed in COS-7 cells, pOTC appeared first and then imported into the mitochondria and processed to the mature form. When Tom20 was co-expressed with pOTC,mitochondrial import and processing of pOTC was retarded. On the other hand, co-expression of soluble Tom20 had little effect. All these results indicate that Tom20 is involved in mitochondrial protein import and that the requirement of Tom20 differs among the precursor proteins.