Complete atomic model of the bacterial flagellar filament by electron cryomicroscopy

Abstract

The bacterial flagellar filament is a helical propeller for bacterial locomotion. It is a helical assembly of a single protein, flagellin, and its tubular structure is formed by 11 protofilaments in two distinct conformations, L- and R-type, for supercoiling. The X-ray crystal structure of a flagellin fragment lacking about 100 terminal residues revealed the protofilament structure, but the full filament structure is still essential for understanding the mechanism of supercoiling and polymerization. Here we report a complete atomic model of the R-type filament by electron cryomicroscopy. A density map obtained from image data up to 4 Å resolution shows the feature of α-helical backbone and some large side chains. The atomic model built on the map reveals intricate molecular packing and an α-helical coiled coil formed by the terminal chains in the inner core of the filament, with its intersubunit hydrophobic interactions having an important role in stabilizing the filament.

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Figure 1: Density maps of the flagellar filament with the atomic model of full-length flagellin superimposed.
Figure 2: The Cα backbone trace, hydrophobic side-chain distribution and structural information of flagellin.
Figure 3: Ribbon diagram of the Cα backbone of the filament model in stereo view.
Figure 4: Comparison of the Cα backbone of flagellin in the filament with F41 in the crystal.
Figure 5: Intersubunit interactions in the inner and outer tubes of the filament in stereo.
Figure 6: Distribution of charged, polar and hydrophobic residues in space-filling representation.

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Acknowledgements

We thank Y. Fujiyoshi for technical advice on the use of the electron cryomicroscope. We also thank F. Oosawa, S. Asakura and D. L. D. Caspar for continuous support and encouragement.

Author information

Correspondence to Keiichi Namba.

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The authors declare that they have no competing financial interests.

Supplementary information

Supplementary Movie: Animation of a rotating flagellar filament model containing 8 protofilaments (3 removed to show inside). (MOV 6115 kb)

Supplementary Figure 1 (JPG 141 kb)

Supplementary Figure 2 (JPG 217 kb)

Supplementary Figure 3ab (JPG 267 kb)

Supplementary Figure 3cd (JPG 254 kb)

Supplementary Information (DOC 46 kb)

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