title: The novel chloroplast outer membrane kinase KOC1 is a required component of the plastid protein import machinery.
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 phosphorylatedin vivoHowever, kinases that phosphorylate the TOC159 A-domain to enable protein import have remained elusive. Here, using co-purification with TOC159 fromArabidopsis, 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 TOC159in vitro, and in mutantkoc1chloroplasts, preprotein import efficiency was diminished.koc1 Arabidopsisseedlings 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.
authors.affiliation: From the Laboratory of Plant Physiology, University of Neuchâtel, 2000 Neuchâtel, Switzerland.
authors.forename: Mónica
authors.initials: M
authors.lastname: Zufferey
authors.affiliation: the College of Agriculture and Life Sciences, Cornell University, Ithaca, New York 14853.
authors.forename: Cyrille
authors.initials: C
authors.lastname: Montandon
authors.affiliation: From the Laboratory of Plant Physiology, University of Neuchâtel, 2000 Neuchâtel, Switzerland.
authors.forename: Véronique
authors.initials: V
authors.lastname: Douet
authors.affiliation: the Department of Botany and Plant Biology, University of Geneva, 1211 Geneva, Switzerland, and.
authors.forename: Emilie
authors.initials: E
authors.lastname: Demarsy
authors.affiliation: Institut für Biochemie und Biotechnologie, Martin-Luther-Universität Halle-Wittenberg, 06099 Halle (Saale), Germany.
authors.forename: Birgit
authors.initials: B
authors.lastname: Agne
authors.affiliation: Institut für Biochemie und Biotechnologie, Martin-Luther-Universität Halle-Wittenberg, 06099 Halle (Saale), Germany.
authors.forename: Sacha
authors.initials: S
authors.lastname: Baginsky
authors.affiliation: From the Laboratory of Plant Physiology, University of Neuchâtel, 2000 Neuchâtel, Switzerland, felix.kessler@unine.ch.
authors.forename: Felix
authors.initials: F
authors.lastname: Kessler
mesh_terms: D029681:Arabidopsis Proteins / Q000378:metabolism* / Q000502:physiology; D060365:Chloroplast Proteins / Q000378:metabolism* / Q000502:physiology; D002736:Chloroplasts / Q000378:metabolism*; D002846:Chromatography, Affinity; D003600:Cytosol / Q000378:metabolism; D004926:Escherichia coli / Q000378:metabolism; D013058:Mass Spectrometry; D009154:Mutation; D010766:Phosphorylation; D000072417:Protein Domains; D021381:Protein Transport; D011994:Recombinant Proteins / Q000378:metabolism
chemical_list: D029681:Arabidopsis Proteins; D060365:Chloroplast Proteins; C000621131:KOC1 protein, Arabidopsis; D011994:Recombinant Proteins
keywords: Arabidopsis; TOC complex; chloroplast; integral membrane kinase; phosphorylation; protein import; receptor