Molecular Mechanism of Mitoshondrial Protein Import in Higher Animals

大多数线粒体蛋白质在胞质中合成，并通过外膜上的Tom复合物转运到线粒体中。我们利用体外导入系统和细胞培养系统分析了Tom 20的作用。Tom 20的可溶性结构域强烈抑制前鸟氨酸转氨甲酰酶（pOTC）、前天冬氨酸氨基转移酶（pAAT）和前丝氨酸：丙酮酸氨基转移酶（pSPT）的输入。这些前体的输入也被针对Tom 20的抗体抑制。然而，抑制的程度不同，从一个前体到另一个。当pOTC在COS-7细胞中表达时，pOTC首先出现，然后输入线粒体并加工成成熟形式。当Tom 20与pOTC共表达时，pOTC的线粒体输入和加工被延迟。另一方面，可溶性Tom 20的共表达几乎没有影响。所有这些结果表明Tom 20参与线粒体蛋白质的输入，并且Tom 20的需求在前体蛋白之间是不同的。

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.