Structure of the native supercoiled flagellar hook as a universal joint

Abstract

Bacteria swim in viscous liquid environments by using the flagellum1–3. The flagellum is composed of about 30 different proteins and can be roughly divided into three parts: the basal body, the hook and the filament. The basal body acts as a rotary motor powered by ion motive force across the cytoplasmic membrane as well as a protein export apparatus to construct the axial structure of the flagellum. The filament is as a helical propeller, and it is a supercoiled form of a helical tubular assembly consisting of a few tens of thousands of flagellin molecules4. The hook is a relatively short axial segment working as a universal joint connecting the basal body and the filament for smooth transmission of motor torque to the filament5,6. The structure of hook has been studied by combining X-ray crystal structure of a core fragment of hook protein FlgE and electron cryomicroscopy (cryoEM) helical image analysis of the polyhook in the straight form and has given a deep insight into the universal joint mechanism7. However, the supercoiled structure of the hook was an approximate model based on the atomic model of the straight hook without its inner core domain7 and EM observations of supercoiled polyhook by freeze-dry and Pt/Pd shadow cast8. Here we report the native supercoiled hook structure at 3.1 Å resolution by cryoEM single particle image analysis of the polyhook. The atomic model built on the three-dimensional (3D) density map show the actual changes in subunit conformation and intersubunit interactions upon compression and extension of the 11 protofilaments that occur during their smoke ring-like rotation and allow the hook to function as a dynamic molecular universal joint with high bending flexibility and twisting rigidity.

An archaellum filament composed of two alternating subunits

“…The flexibility of the M. villosus archaellum filament supports recent insights into the motion of the flagellar hook from the bacterium S. enterica 63,64 . Similar to archaellins, the three domains of the flagellar hook subunits (D1, D2, D3) behave as rigid bodies connected by two flexible hinges.…”

Cryo‐EM structure of the CENP‐A nucleosome in complex with phosphorylated CENP‐C

“…Motion correction was carried out by MotionCor2 (Zheng et al, 2017) to align all CENP-A nucleosome complex micrographs, and the CTF parameters were estimated by Gctf (Zhang, 2016). Both programs were performed via the pipeline program Gwatch (https://github.com/FumiakiMakino/Gwatch) (Kato et al, 2019). All CENP-A complexes were automatically selected by Auto-picking using the Laplacian and Gaussian in RELION 3.0 (Zivanov et al, 2018), and they were extracted into a box of 192 × 192 pixels.…”

Direct observation of speed fluctuations of flagellar motor rotation at extremely low load close to zero

“…In Figure a, this predicted value is shown by a gray circle (the error bar is the standard deviation), and 3 σ from the mean value are shown by horizontal gray lines. Because the rotations of the gold beads with rotation radii of about 100 nm (Figure a) were also stable (Figure b,c), their hooks are most likely rigid against bending and twisting but may still form a slightly curved tube due to possible intermolecular interactions of the D2 domains of FlgE on the surface of the hook (Fujii et al, ; Kato et al, ; Samatey et al, ).…”

“…FlgE consists of four domains, D0, Dc, D1 and D2 (Fujii, Kato, & Namba, ; Horváth, Kato, Miyata, & Namba, ), but FlgG is smaller than FlgE by lacking domain D2 (Chevance et al, ; Fujii et al, ). Intermolecular axial packing interactions between the D2 domains are responsible for hook supercoiling (Fujii, Matsunami, Inoue, & Namba, ; Kato, Makino, Miyata, Horváth, & Namba, ; Samatey et al, ), but those between Domain D2 and the triangular loop of domain D1 are dispensable for bending flexibility of the hook albeit important for the structural stability of the hook (Sakai, Inoue, Terahara, Namba, & Minamino, ). Both domain D2 and the triangular loop are absent in FlgG.…”