Molecular Mechanism of Mitoshondrial Protein Import in Higher Animals

大多数线粒体蛋白是在胞浆中合成的，并通过外膜上的TOM复合体运输到线粒体中。我们利用体外导入系统和培养细胞系统分析了Tom20的作用。前鸟氨酸转氨酶(POTC)、前天冬氨酸氨基转移酶(PAAT)和前丝氨酸：丙酮酸转氨酶(PSPT)的进口被Tom20的可溶性区强烈抑制。这些前体的进口也被抗Tom20的抗体抑制。然而，抑制程度因前体不同而不同。当POTC在COS-7细胞中表达时，POTC首先出现，然后导入线粒体，加工成成熟的形式。当Tom20与POTC共表达时，POTC的线粒体输入和加工受到抑制。另一方面，共表达可溶性Tom20的效果不明显。所有这些结果表明，Tom20参与了线粒体蛋白的输入，并且对Tom20的需求在不同的前体蛋白中有所不同。

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.