Mechanism of targeting and transport of mitochondrial precursor proteins to mitochondria

尽管未检测到孔蛋白前体与胰酶处理的大鼠肝脏线粒体的结合，但在一定程度上观察到它与这些线粒体的整合，可能是通过首次在真菌线粒体中发现的旁路-输入系统。对这种旁路-进口系统的拓扑学研究表明，孔蛋白的进口发生在外膜和内膜之间的接触点。此外，抗29 KDa外膜蛋白的抗体抑制了孔蛋白的整合，表明29 KDa蛋白参与了大多数线粒体前体蛋白的输入。当大鼠肝内膜制备的倒置小泡与前大鼠血清白蛋白孵育时，通过离心法重新分离倒置小泡，回收了相当数量的前白蛋白和前白蛋白。前白蛋白对胰酶有抵抗力，但前白蛋白被胰酶完全消化，说明前白蛋白被转运到囊泡中，同时转化为前白蛋白。这一转运过程需要ATP，但不需要膜电位。这些结果表明，在线粒体膜内存在一种N-末端氨基酸序列与分泌蛋白信号序列相似的蛋白质的输出机制。

Although no binding of porin-precursor to trypsin-pretreated rat liver mitochondria was detectable, its integration into these mitochondria was observed to some extent, possibly by the bypass-import system first demonstrated in fungus mitochondria. A topographical study of this bypass-import system demonstrated that import of porin occurs at contact sites between the outer and inner membrane. Furthermore, antibodies to 29 KDa outer membrane protein, a component of the import machinery in contact sites for precursors of ornithine aminotransferase and sulfite oxidase, inhibited the integration of porin, suggesting that the 29 KDa protein is involved in the imports of most mitochondrial precursor proteins.When inverted vesicles prepared from the inner membrane of rat liver mitochondria were incubated with prepro-rat serum albumin, considerable amounts of prepro-albumin and pro-albumin were recovered with the inverted vesicles re-isolated by centrifugation. Pro-albumin was resistant to trypsin, but prepro-albumin was completely digested by trypsin, indicating that prepro-albumin was transported into the vesicles and concomitantly converted to pro-albumin. This transport process required ATP, but not a membrane potential. These results suggest that some export machinery for a protein having an amino acid sequence in its N-terminal portion similar to the signal sequence of secretory protein exists in the inner mitochondrial membrane.