Li H m, Chen L J
Institute of Molecular Biology, Academia Sinica, Nankang, Taipei 11529, Taiwan, Republic of China.
J Biol Chem. 1997 Apr 18;272(16):10968-74.
Several components in the machinery mediating the import of nuclear-encoded chloroplastic precursor proteins have been identified. One of the components, OEP34, is an outer membrane protein and is synthesized at its mature size in the cytosol without a distinguishable chloroplast-targeting signal. To address the question of how components in the transport machinery are imported to chloroplasts themselves, we first identified the chloroplastic outer membrane-targeting signal of OEP34. Using an Arabidopsis homologue of the originally isolated pea OEP34, we show that the outer membrane-targeting signal of OEP34 is located within a 10-amino acid hydrophobic core of the C-terminal membrane anchor. Interestingly, this signal can target a passenger protein to the chloroplastic outer membrane no matter whether it is placed at the N or C terminus of a passenger protein. Proper insertion of fusion proteins into the outer membrane requires in addition the C-terminal hydrophilic region following the hydrophobic core. Furthermore, passenger proteins fused to the C terminus of the targeting/insertion signal were most likely imported into the intermembrane space of the envelope.
介导核编码叶绿体前体蛋白输入的机制中的几个组分已被鉴定出来。其中一个组分OEP34是一种外膜蛋白，它在细胞质中以成熟大小合成，没有可识别的叶绿体靶向信号。为了解决运输机制中的组分如何自身输入到叶绿体的问题，我们首先鉴定了OEP34的叶绿体外膜靶向信号。利用最初分离的豌豆OEP34的拟南芥同源物，我们表明OEP34的外膜靶向信号位于C末端膜锚定的10个氨基酸的疏水核心内。有趣的是，无论该信号置于乘客蛋白的N端还是C端，它都能将乘客蛋白靶向到叶绿体外膜。融合蛋白正确插入外膜还需要疏水核心之后的C末端亲水区域。此外，与靶向/插入信号的C末端融合的乘客蛋白最有可能输入到被膜的膜间隙中。
