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Crystal structure of the carnitine transporter and insights into the antiport mechanism

Nature Structural & Molecular Biology volume 17, pages 492–496 (2010)Cite this article

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Abstract

CaiT is a membrane antiporter that catalyzes the exchange of l-carnitine with γ-butyrobetaine across the Escherichia coli membrane. To obtain structural insights into the antiport mechanism, we solved the crystal structure of CaiT at a resolution of 3.15 Å. We crystallized CaiT as a homotrimer complex, in which each protomer contained 12 transmembrane helices and 4 l-carnitine molecules outlining the transport pathway across the membrane. Mutagenesis studies revealed a primary binding site at the center of the protein and a secondary substrate-binding site at the bottom of the intracellular vestibule. These results, together with the insights obtained from structural comparison with structurally homologous transporters, provide mechanistic insights into the association between substrate translocation and the conformational changes of CaiT.

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Figure 1: The overall structure of CaiT.
Figure 2: The substrate-binding sites.
Figure 3: Comparison of the structures of CaiT and LeuT.
Figure 4: Characterization of the antiport activity for wild-type (WT) and mutant CaiT.
Figure 5: Structural superposition of CaiT (orange) with BetP (green).

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Acknowledgements

We thank the staff at the European Synchrotron Radiation Facility in France, the Spring-8 (beamline BL41XU) in Japan and the Swiss Light Source (beamline X06SA) in Switzerland for help with data collection, J. Tang and Y.-B. Wang for technical support and J. Chen for discussions. This research was financially supported by the National Key Basic Research Program (grant numbers 2009CB918600, 2009CB918803), the National Natural Science Foundation of China (grant number 30721003), the National High Technology Research and Development Program of China (grant number 2006AA02A319) and the Knowledge Innovation Program of the Chinese Academy of Sciences (grant number KSCX2-YW-R-123).

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  1. Lin Tang and Lin Bai: These authors contributed equally to this work.

Authors and Affiliations

  1. National Laboratory of Biomacromolecules, Institute of Biophysics, Chinese Academy of Sciences, Beijing, China

    Lin Tang, Lin Bai, Wen-hua Wang & Tao Jiang

  2. Graduate University of Chinese Academy of Sciences, Beijing, China

    Lin Bai

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Contributions

L.T. performed expression, purification, crystallization and the crystal structure determination; L.B. and W.-h.W. contributed to vector construction and protein expression; L.B. performed the mutagenesis studies and transport activity measurements; T.J. and L.T. designed the research; T.J. supervised the work; L.T. and T.J. analyzed the results and wrote the paper.

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Correspondence to Tao Jiang.

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Tang, L., Bai, L., Wang, Wh. et al. Crystal structure of the carnitine transporter and insights into the antiport mechanism. Nat Struct Mol Biol 17, 492–496 (2010). https://doi.org/10.1038/nsmb.1788

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