Abstract

Large protein complexes assemble spontaneously, yet their subunits do not prematurely form unwanted aggregates. This paradox is epitomized in the bacterial flagellar motor, a sophisticated rotary motor and sensory switch consisting of hundreds of subunits. Here we demonstrate that Escherichia coli FliG, one of the earliest-assembling flagellar motor proteins, forms ordered ring structures via domain-swap polymerization, which in other proteins has been associated with uncontrolled and deleterious protein aggregation. Solution structural data, in combination with in vivo biochemical cross-linking experiments and evolutionary covariance analysis, revealed that FliG exists predominantly as a monomer in solution but only as domain-swapped polymers in assembled flagellar motors. We propose a general structural and thermodynamic model for self-assembly, in which a structural template controls assembly and shapes polymer formation into rings.

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Acknowledgements

SAXS data were collected on the SAXS/WAXS beamline at the Australian Synchrotron, Victoria, Australia. This work was supported by the Australian Research Council (grant DP130102219 to L.K.L.) and the Human Frontiers Science Program (grant RGP0030/2013 to L.K.L., K.N., R.M.B. and A.J.T.). L.K.L. is supported by an Australian Research Council Discovery Early Career Research Award (grant DE140100262).

Author information

Author notes

    • Cy M Jeffries

    Present address: European Molecular Biology Laboratory, Hamburg, Germany.

Affiliations

  1. Structural and Computational Biology Division, Victor Chang Cardiac Research Institute, Darlinghurst, New South Wales, Australia.

    • Matthew A B Baker
    • , Robert M G Hynson
    • , Lorraine A Ganuelas
    • , Nasim Shah Mohammadi
    • , Chu Wai Liew
    • , Anthony A Rey
    • , Daniela Stock
    •  & Lawrence K Lee
  2. European Molecular Biology Laboratory Australia Node for Single Molecule Science, School of Medical Sciences, University of New South Wales, Sydney, New South Wales, Australia.

    • Matthew A B Baker
    •  & Lawrence K Lee
  3. Australian Nuclear and Science Technology Organisation, Lucas Heights, New South Wales, Australia.

    • Anthony P Duff
    • , Andrew E Whitten
    •  & Cy M Jeffries
  4. Department of Biochemistry, Oxford University, Oxford, UK.

    • Nicolas J Delalez
    •  & Judith P Armitage
  5. Graduate School of Frontier Biosciences, Osaka University, Osaka, Japan.

    • Yusuke V Morimoto
    •  & Keiichi Namba
  6. Department of Physics, Oxford University, Oxford, UK.

    • Andrew J Turberfield
    •  & Richard M Berry

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Contributions

M.A.B.B. analyzed SAXS data, performed covariance and thermodynamic analysis, conceived experiments and wrote the manuscript. R.M.G.H. collected SAXS data and performed cross-linking assays. L.A.G. expressed and purified protein and collected SAXS data. N.S.M. performed cross-linking assays and in vivo functional assays. C.W.L. expressed and purified protein and collected SAXS data. A.A.R. expressed and purified protein. A.P.D. collected and analyzed SAXS data. A.E.W. and C.M.J. analyzed SAXS data. N.J.D. and J.P.A. constructed cell lines for this study. Y.V.M. performed fluorescence imaging experiments. D.S. conceived experiments and contributed to cross-linking experiments. A.J.T. and R.M.B. performed thermodynamic analysis, conceived experiments and wrote the manuscript. K.N. performed fluorescence imaging experiments, conceived experiments and wrote the manuscript. L.K.L. conceived experiments, performed cross-linking assays, collected and analyzed SAXS data and wrote the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Lawrence K Lee.

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https://doi.org/10.1038/nsmb.3172

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