Zufferey Mónica, Montandon Cyrille, Douet Véronique, Demarsy Emilie, Agne Birgit, Baginsky Sacha, Kessler Felix
From the Laboratory of Plant Physiology, University of Neuchâtel, 2000 Neuchâtel, Switzerland.
the College of Agriculture and Life Sciences, Cornell University, Ithaca, New York 14853.
J Biol Chem. 2017 Apr 28;292(17):6952-6964. doi: 10.1074/jbc.M117.776468. Epub 2017 Mar 10.
The biogenesis and maintenance of cell organelles such as mitochondria and chloroplasts require the import of many proteins from the cytosol, a process that is controlled by phosphorylation. In the case of chloroplasts, the import of hundreds of different proteins depends on translocons at the outer and inner chloroplast membrane (TOC and TIC, respectively) complexes. The essential protein TOC159 functions thereby as an import receptor. It has an N-terminal acidic (A-) domain that extends into the cytosol, controls receptor specificity, and is highly phosphorylated However, kinases that phosphorylate the TOC159 A-domain to enable protein import have remained elusive. Here, using co-purification with TOC159 from , we discovered a novel component of the chloroplast import machinery, the regulatory kinase at the outer chloroplast membrane 1 (KOC1). We found that KOC1 is an integral membrane protein facing the cytosol and stably associates with TOC. Moreover, KOC1 phosphorylated the A-domain of TOC159 , and in mutant chloroplasts, preprotein import efficiency was diminished. seedlings had reduced survival rates after transfer from the dark to the light in which protein import into plastids is required to rapidly complete chloroplast biogenesis. In summary, our data indicate that KOC1 is a functional component of the TOC machinery that phosphorylates import receptors, supports preprotein import, and contributes to efficient chloroplast biogenesis.
线粒体和叶绿体等细胞器的生物发生和维持需要从细胞质中导入许多蛋白质，这一过程受磷酸化作用控制。就叶绿体而言，数百种不同蛋白质的导入依赖于叶绿体膜外和膜内的转运体（分别为TOC和TIC复合体）。必需蛋白TOC159在此作为一种导入受体发挥作用。它有一个延伸到细胞质中的N端酸性（A-）结构域，控制受体特异性，且高度磷酸化。然而，使TOC159 A结构域磷酸化以实现蛋白质导入的激酶一直未被发现。在这里，通过与来自……的TOC159共同纯化，我们发现了叶绿体导入机制的一个新组分，即叶绿体膜外调节激酶1（KOC1）。我们发现KOC1是一种面向细胞质的整合膜蛋白，并与TOC稳定结合。此外，KOC1使TOC159的A结构域磷酸化，在突变的叶绿体中，前体蛋白的导入效率降低。在从黑暗转移到光照后，……幼苗的存活率降低，在这种情况下，蛋白质导入质体对于快速完成叶绿体生物发生是必需的。总之，我们的数据表明KOC1是TOC机制的一个功能组分，它使导入受体磷酸化，支持前体蛋白导入，并有助于高效的叶绿体生物发生。
