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J Biol Chem. 2017 Apr 28;292(17):6952-6964. doi: 10.1074/jbc.M117.776468. Epub 2017 Mar 10.

The novel chloroplast outer membrane kinase KOC1 is a required component of the plastid protein import machinery.

Author information

1
From the Laboratory of Plant Physiology, University of Neuchâtel, 2000 Neuchâtel, Switzerland.
2
the College of Agriculture and Life Sciences, Cornell University, Ithaca, New York 14853.
3
the Department of Botany and Plant Biology, University of Geneva, 1211 Geneva, Switzerland, and.
4
Institut für Biochemie und Biotechnologie, Martin-Luther-Universität Halle-Wittenberg, 06099 Halle (Saale), Germany.
5
From the Laboratory of Plant Physiology, University of Neuchâtel, 2000 Neuchâtel, Switzerland, felix.kessler@unine.ch.

Abstract

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 in vivo However, kinases that phosphorylate the TOC159 A-domain to enable protein import have remained elusive. Here, using co-purification with TOC159 from Arabidopsis, 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 in vitro, and in mutant koc1 chloroplasts, preprotein import efficiency was diminished. koc1 Arabidopsis 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.

KEYWORDS:

Arabidopsis; TOC complex; chloroplast; integral membrane kinase; phosphorylation; protein import; receptor

PMID:
28283569
PMCID:
PMC5409464
DOI:
10.1074/jbc.M117.776468
[Indexed for MEDLINE]
Free PMC Article

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