Architecture of a flagellar apparatus in the fast-swimming magnetotactic bacterium MO-1

Abstract

The bacterial flagellum is a motility organelle that consists of a rotary motor and a helical propeller. The flagella usually work individually or by forming a loose bundle to produce thrust. However, the flagellar apparatus of marine bacterium MO-1 is a tight bundle of seven flagellar filaments enveloped in a sheath, and it has been a mystery as to how the flagella rotate smoothly in coordination. Here we have used electron cryotomography to visualize the 3D architecture of the sheathed flagella. The seven filaments are enveloped with 24 fibrils in the sheath, and their basal bodies are arranged in an intertwined hexagonal array similar to the thick and thin filaments of vertebrate skeletal muscles. This complex and exquisite architecture strongly suggests that the fibrils counter-rotate between flagella in direct contact to minimize the friction of high-speed rotation of individual flagella in the tight bundle within the sheath to enable MO-1 cells to swim at about 300 µm/s.

Fig. 1.

CryoEM observation of an MO-1 cell and its flagellar apparatus. (A) CryoEM image of a vitrified MO-1 cell showing two flagellar apparatus (arrows). A phosphorous oxygen-rich granule is labeled P, and magnetosome crystals are labeled M. (Scale bar, 500 nm.) (B) A slice of a tomogram showing the distal end of a flagellar bundle where flagellar filaments and fibrils are exposed out of the sheath. (Scale bar, 100 nm.) (C–E) Three slices of a tomogram showing the structure of the sheath and the bundle of flagella and fibrils within it. The slice distance is 30.7 nm from C to D and 16.5 nm from D to E. (C) Lower surface of the sheath. (D) Middle section of the bundle. (E) Upper surface of the sheath. Arrowheads in C and E indicate the helical lines of the sheath. (Scale bars, 100 nm.) (F) Negatively stained EM image of an isolated filament-hook-basal body complex. The L, P, and MS rings are indicated. (Scale bar, 100 nm.) The tomogram was reconstructed from a tilt-series of cryoEM images over a range of ± 62°, with an increment of 2°.

Fig. 2.

Architecture of the flagellar apparatus detached from the cell body. (A–E) Five selected slices of a tomogram from lower to upper positions. The slice distance is 18.9 nm from A to B, and 9.44 nm from B to C, C to D, and D to E. The flagellar apparatus is viewed from outside of the cell. (Scale bar, 100 nm.) (A) Slice showing seven flagellar basal bodies in the hexagonal array. Each basal body shows the L and P ring and the rod within the rings. The upper five are clear, but the bottom two, indicated by red arrowheads, are only partially visible in this slice. The edge of the base platform is indicated by a white arrowhead. (B) Slice showing the proximal end of flagella in the hexagonal array. Two arrowheads indicate the ends from which two flagella are extend upward. (C and D) Slices showing the flagella and fibrils. Fibrils are indicated by red arrowheads. (E) Slice showing the upper surface of the sheath. White arrowheads indicate the helical lines shown in Fig. 1E. (F) Solid surface rendered by 3D segmentation of the seven flagella, part of the sheath, and the edge of the base platform. The tomogram was reconstructed from a tilt-series of cryoEM images from −50° to 70° with an increment of 2°.

Fig. 3.

Side view of an intact flagellar apparatus on the cell envelope of a vitrified MO-1 cell. (A–E) Five selected slices of a tomogram from lower to higher positions. The slice distance is 40.1 nm from A to B, 16.5 nm from B to C, 26.0 nm from C to D, and 47.2 nm from D to E. In A, the cytoplasmic membrane, outer membrane, and S-layer are labeled IM, OM, and S, respectively. The points at which the sheath attaches to the disk-like platform are indicated by black arrows in B and C. Slice through the S-layer is shown in E, with its Fourier transform in the Inset. (Scale bar, 100 nm.) (F) Solid surface rendered by 3D segmentation of the base platform and hexagonal array of seven flagella viewed from outside of the cell. Arrows labeled A, B, and D indicate the position of slices shown in panels A, B, and D, respectively. The tomogram was reconstructed from a tilt-series of cryoEM images from −62° to 58° with an increment of 2°.

Fig. 4.

Structure of the flagellar basal body around the cytoplasmic membrane of isolated base platform. (A) Slice showing seven large rings, each with a central core density. (Scale bar, 50 nm.) (B) Solid surface rendered by 3D segmentation of the tomogram. The tomogram was reconstructed from a tilt-series of cryoEM images in a range of ± 62° with an increment of 2°.

Fig. 5.

Intertwined hexagonal array of the basal bodies of 7 flagella and 24 fibrils. (A) Negatively stained EM image of a detergent-solubilized base platform. The outer membrane is indicated by arrows. (Scale bar, 50 nm.) (B) The same image as A with large brown and small yellow-green circles overlaid on the flagellar and fibril basal bodies, respectively. (C) The same as A but treated with a basic solution of pH 11. (Scale bar, 50 nm.) (D) The same image as C with the same overlays as B.

Fig. 6.

Schematic diagram showing a hypothetical mechanism for how the MO-1 flagellar apparatus might work efficiently with a tight bundle of 7 flagella and 24 fibrils encased in a sheath. The flagella are represented as large brown gears, the fibrils are represented as small blue-green gears, and the sheath is represented as the pale-blue surrounding circle. The flagella and fibrils rotate counterclockwise and clockwise, respectively, as indicated by arrows, to minimize the friction at high-speed rotation.