Structure of the torque ring of the flagellar motor and the molecular basis for rotational switching


The flagellar motor drives the rotation of flagellar filaments at hundreds of revolutions per second1,2, efficiently propelling bacteria through viscous media3. The motor uses the potential energy from an electrochemical gradient of cations4,5 across the cytoplasmic membrane to generate torque. A rapid switch from anticlockwise to clockwise rotation determines whether a bacterium runs smoothly forward or tumbles to change its trajectory6,7. A protein called FliG forms a ring in the rotor of the flagellar motor that is involved in the generation of torque8,9,10,11,12,13 through an interaction with the cation-channel-forming stator subunit MotA12. FliG has been suggested to adopt distinct conformations that induce switching but these structural changes and the molecular mechanism of switching are unknown. Here we report the molecular structure of the full-length FliG protein, identify conformational changes that are involved in rotational switching and uncover the structural basis for the formation of the FliG torque ring. This allows us to propose a model of the complete ring and switching mechanism in which conformational changes in FliG reverse the electrostatic charges involved in torque generation.

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Figure 1: Structural overview of the full-length FliG monomer.
Figure 2: Conformational differences between A. aeolicus FliGFL and T. maritima FliGMC.
Figure 3: Structural basis for the formation of FliG multimers.
Figure 4: Molecular basis of rotational switching.

Accession codes

Primary accessions

Protein Data Bank

Data deposits

The coordinates and structure factors of the A. aeolicus FliG monomer have been deposited in the Protein Data Bank under accession code 3HJL.

Change history

  • 19 August 2010

    Three mistakes were corrected for the print issue.


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We thank the staff at beamline ID14-1 at the ESRF, and at beamlines 14-ID (BioCARS) and 23-ID (General Medicine and Cancer Institutes Collaborative Access Team (GM/CA-CAT)) at the APS for their support. Use of the APS was supported by the US Department of Energy, Basic Energy Sciences, Office of Science (contract DE-AC02-06CH11357), and use of the BioCARS Sector 14 was supported by the National Institutes of Health, National Center for Research Resources (grant RR007707). GM/CA-CAT was funded in whole or in part with federal funds from the National Cancer Institute (Y1-CO-1020) and the National Institute of General Medical Science (Y1-GM-1104). This work was supported by the Australian Synchrotron Research Program of the Australian Nuclear Science Technology Organization. D. Thomas is acknowledged for providing us with the electron microscopy density map of the clockwise-locked S. typhimurium mutant and O. Perisic and H. Huber are acknowledged for cloning vectors (pOPT-GST) and A. aeolicus genomic DNA, respectively. D.S. was initially funded by an MRC Career development award (UK) and C.C. by an MRC predoctoral fellowship (UK).

Author information




L.K.L. collected synchrotron data, solved and analysed the structure, wrote software to generate FliG rings and switch morphs, and wrote the manuscript, M.D. collected synchrotron data, M.D. and M.A.G. purified and crystallized selenomethionine-containing protein, C.C. cloned, purified and crystallized the native protein, D.S. conceived and coordinated project, collected synchrotron data and wrote the manuscript.

Corresponding author

Correspondence to Daniela Stock.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Tables 1-2, Supplementary Figures 1-11 with legends, full legends for Supplementary Movies 1-6 and References. (PDF 3066 kb)

Supplementary Movie 1

This animated movie shows the formation of the FliG ring. (MOV 30576 kb)

Supplementary Movie 2

This movie shows the Interpolation of the M236 phi and F237 phi angles between the C-terminal conformations of FliGFL (CterFL) and FliGMC (CterMC). (MOV 908 kb)

Supplementary Movie 3

This movie shows a rotating FliG monomer displaying the equivalent residues at the location of all known rotationally biased mutants. (MOV 1526 kb)

Supplementary Movie 4

This movie shows parallels between the ARMC – ARMM+1 right-handed superhelix in the FliG multimer and a eukaryotic ARM superhelix in β-catenin. (MOV 3948 kb)

Supplementary Movie 5

This movie shows the structure of the FliGUNIT. (MOV 978 kb)

Supplementary Movie 6

This movie shows the reversal of the torque generating charges in the FliG ring. (MOV 1952 kb)

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Lee, L., Ginsburg, M., Crovace, C. et al. Structure of the torque ring of the flagellar motor and the molecular basis for rotational switching. Nature 466, 996–1000 (2010).

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