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AKR2A-mediated import of chloroplast outer membrane proteins is essential for chloroplast biogenesis

Nature Cell Biology volume 10, pages 220227 (2008) | Download Citation



In plant cells, chloroplasts have essential roles in many biochemical reactions and physiological responses1. Chloroplasts require numerous protein components, but only a fraction of these proteins are encoded by the chloroplast genome. Instead, most are encoded by the nuclear genome and imported into chloroplasts from the cytoplasm post-translationally2,3,4,5. Membrane proteins located in the chloroplast outer envelope membrane (OEM) have a critical function in the import of proteins into the chloroplast. However, the biogenesis of chloroplast OEM proteins remains poorly understood. Here, we report that an Arabidopsis ankyrin repeat protein, AKR2A, plays an essential role in the biogenesis of the chloroplast OEM proteins. AKR2A binds to chloroplast OEM protein targeting signals, as well as to chloroplasts. It also displays chaperone activity towards chloroplast OEM proteins, and facilitates the targeting of OEP7 to chloroplasts in vitro. AKR2A RNAi in plants with an akr2b knockout background showed greatly reduced levels of chloroplast proteins, including OEM proteins, and chloroplast biogenesis was also defective. Thus, AKR2A functions as a cytosolic mediator for sorting and targeting of nascent chloroplast OEM proteins to the chloroplast.

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We thank H.-M. Li (Institute of Molecular Biology, Academia Sinica, Taiwan) for the anti-OEP7, anti-Toc33, anti-Toc34, anti-Toc75 and anti-Toc159 antibodies. The akr2b mutant (SALK T-DNA insertion line) was obtained from the Arabidopsis Biological Resources Center. This work was supported in part by a grant from Crop Functional Genomics Research Center (M107KG010003-07K0701-00330), Systems Biodynamics-National Core Research Center (R15-2004-033-05002-0) and the National Creative Research Initiatives program funded by the Ministry of Science and Technology, Korea and Core research fund of POSTECH.

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    • Yong Jik Lee
    •  & Eun Ju Sohn

    Current address: Department of Cell Biology, University of California-Riverside, CA 92521, USA.

    • Wonsil Bae
    •  & Yong Jik Lee

    These authors contributed equally to this work.


  1. Division of Molecular and Life Sciences, Pohang University of Science and Technology, Pohang, 790-784, Korea.

    • Wonsil Bae
    • , Yong Jik Lee
    • , Dae Heon Kim
    • , Junho Lee
    • , Soojin Kim
    • , Eun Ju Sohn
    •  & Inhwan Hwang


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The authors declare no competing financial interests.

Corresponding author

Correspondence to Inhwan Hwang.

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    Supplementary Figures S1, S2, S3, S4, S5, s6, s7 and S8, Supplementary Table S1 and Supplementary Methods

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