Insertion of atToc34 into the chloroplastic outer membrane is assisted by at least two proteinaceous components in the import system
Insertion of atToc34 into the chloroplastic outer membrane is assisted by at least two proteinaceous components in the import system
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DOI：
10.1074/jbc.274.26.18735
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发表时间：
1999-06-25
影响因子：
4.8
通讯作者：
Li, HM
中科院分区：
文献类型：
--
作者：
Tsai, LY;Tu, SL;Li, HM
Toc34是外膜转运复合体的成员，介导蛋白质进入叶绿体的初始阶段。Toc34，像大多数外膜蛋白一样，是在胞浆中合成的，没有可切割的传递肽。大多数外膜蛋白不需要叶绿体表面的热溶素敏感成分或ATP就能插入外膜。然而，关于Toc34插入外膜所需的因素，我们得到了不同的结果。利用豌豆Toc34的拟南芥同源物atToc34，我们发现，无论是蛋白酶消化还是碱性提取，叶绿体热溶素预处理都大大减少了atToc34的插入。插入也依赖于ATP或GTP的存在。缺失gtp结合域的atToc34突变体仍需要ATP才能实现最优插入，这表明在导入系统中，ATP被其他蛋白质组分所使用。即使在热溶酶预处理的叶绿体中也观察到atp支持的插入，这表明负责atp刺激插入的蛋白质成分与协助atToc34插入的热溶酶敏感成分是不同的蛋白质。
Toc34 is a member of the outer membrane translocon complex that mediates the initial stage of protein import into chloroplasts. Toc34, like most outer membrane proteins, is synthesized in the cytosol at its mature size without a cleavable transit peptide. The majority of outer membrane proteins do not require thermolysin-sensitive components on the chloroplastic surface or ATP for their insertion into the outer membrane. However, different results have been obtained concerning the factors required for Toc34 insertion into the outer membrane, Using an Arabidopsis homologue of pea Toc34, atToc34, we show that the insertion of atToc34 was greatly reduced by thermolysin pretreatment of chloroplasts as assayed either by protease digestion or by alkaline extraction. The insertion was also dependent on the presence of ATP or GTP. A mutant of atToc34 with the GTP-binding domain deleted still required ATP for optimal insertion, indicating that ATP was used by other protein components in the import system. The ATP-supported insertion was observed even in thermolysin-pretreated chloroplasts, suggesting that the protein component responsible for ATP-stimulated insertion is a different protein from the thermolysin-sensitive component that assists atToc34 insertion.