Complete atomic model of the bacterial flagellar filament by electron cryomicroscopy


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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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.

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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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