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Crystal structure of pea Toc34, a novel GTPase of the chloroplast protein translocon

Nature Structural Biology volume 9, pages 95–100 (2002)Cite this article

Abstract

Toc34, a 34-kDa integral membrane protein, is a member of the Toc (translocon at the outer-envelope membrane of chloroplasts) complex, which associates with precursor proteins during protein transport across the chloroplast outer membrane. Here we report the 2.0 Å resolution crystal structure of the cytosolic part of pea Toc34 in complex with GDP and Mg2+. In the crystal, Toc34 molecules exist as dimers with features resembling those found in a small GTPase in complex with a GTPase activating protein (GAP). However, gel filtration experiments revealed that dimeric and monomeric forms of Toc34 coexisted in phosphate saline buffer solution at pH 7.2. Mutation of Arg 128, an essential residue for dimerization, to an Ala residue led to the formation of an exclusively monomeric species whose GTPase activity is significantly reduced compared to that of wild type Toc34. These results, together with a number of structural features unique to Toc34, suggest that each monomer acts as a GAP on the other interacting monomer.

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Figure 1: Structure of the GDP-bound form of the Toc34 dimer and its interface.
Figure 2: Stereo view of the superposition of the Cα and the G motifs of Toc34 and H-ras-p21.
Figure 3: Sequence alignment of the Toc34/Toc159-G subfamily.
Figure 4: The GTP-binding region of Toc34.

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Acknowledgements

We thank N. Sakabe and the staff for helping in synchrotron data collection; T.-C. Wu for assisting in figure preparation; C.-F. Chang for the technical assistance in 19F NMR; C. Lim for comments on the manuscript; and the Pharmacia http://rice-research.org program for access to the Monsanto Rice Genome Database. This work was supported in part by National Science Council and Academia Sinica (C.D.H. and H.M.L.), Taiwan, Republic of China.

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  1. Yuh-Ju Sun, Farhad Forouhar and Chinpan Chen: These authors contributed equally to this work.

Authors and Affiliations

  1. Institute of Molecular Biology, Academia Sinica, Taipei, 115, ROC, Taiwan

    Yuh-Ju Sun, Farhad Forouhar, Hsou-min Li, Shuh-Long Tu, Yi-Hong Yeh, Sen Kao, Hui-Lin Shr, Chia-Cheng Chou & Chwan-Deng Hsiao

  2. Department of Life Science, National Tsing Hua University, Hsinchu, 300, ROC, Taiwan

    Yuh-Ju Sun

  3. Institute of Biomedical Sciences, Academia Sinica, Taipei, 115, ROC, Taiwan

    Chinpan Chen

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Sun, YJ., Forouhar, F., Li, Hm. et al. Crystal structure of pea Toc34, a novel GTPase of the chloroplast protein translocon. Nat Struct Mol Biol 9, 95–100 (2002). https://doi.org/10.1038/nsb744

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