Bacterial Flagella

Abstract

Bacteria propel themselves through liquid or over semisolid media using rotation of a propeller‐like organelle, the flagellum. The membrane‐embedded basal body is connected via a flexible coupling structure, the hook, to the rigid, external filament. The flagellum is a sophisticated, molecular nanomachine, and to build a flagellum, several thousand copies of more than two dozen proteins assemble in an ordered process. Flagellar gene expression is temporally coupled to its assembly state, and complex self‐assembly mechanisms control the accurate size and subunit composition of flagellar substructures. A specialised protein export machine, termed type‐III secretion system, transports flagellar substrates across the inner membrane. Secreted substrates diffuse through a narrow channel within the flagellum and self‐assemble at the distal end. Intermittent secretion of a molecular ruler protein controls the hook length, while diffusion of flagellin proteins intrinsically limits the length of the flagellar filament.

Key Concepts

  • Bacteria swim through their environment by rotating an extracellular appendage, the flagellum.
  • Rotation of the flagellum is energised by influx of ions through stator proteins that are attached to the cell body, which is called the ion motive force.
  • The bacterial flagellum consists of three major parts: (1) the basal body as the engine, (2) a flexible joint structure, the hook, that connects the engine to (3) the long external filament.
  • Assembly of the flagellum involves dozens of proteins and the coordination of gene expression to the assembly state of the flagellum.
  • A specialised protein export system, a type‐III secretion apparatus, is responsible for the export of flagellar secretion substrates, including the majority of the external structures of the flagellum.
  • Protein export via the flagellar‐specific type‐III secretion system is dependent on the proton motive force.
  • The length of the hook structure is regulated to an optimal length of 55 nm in Salmonella. A molecular ruler controls the final length and catalyses a switch in secretion specificity from early to late substrate secretion mode.
  • After proton motive force‐dependent injection, flagellin molecules, the building blocks of the filament, diffuse through a narrow channel and self‐assemble at the distal tip of the structure.

Keywords: rotary motor; protein export; chemiosmotic device; supramolecular assembly; molecular ruler; proton motive force; type‐III secretion

Figure 1. Morphology of motile bacteria. Bar, 10 μm. Khan . Reproduced with permission of Elsevier.
Figure 2. (a) Schematic representation of the flagellum of Salmonella (left) and structure of the export pore complex within the basal body complex (right). The MS (membrane/supramembrane), L (lipopolysaccharide) and P (periplasma) rings anchor the structure into the cell envelope. The MotAB motor‐force generators, which drive flagellar rotation, are omitted in this scheme. The C (cytoplasmic)‐ring complex is involved in controlling the direction of flagellar rotation and in providing an affinity site for substrate docking. The flagellar‐specific type‐III secretion apparatus embedded within the MS‐ring at the core of the basal body exports most building blocks of the flagellum. Adapted from Erhardt et al. and Johnson et al. . (b–d) Basal body diversity in different bacterial species as shown by tomographic slices (left) and their subtomogram averages (right). Adapted from Beeby et al. .
Figure 3. An infrequent molecular ruler determines hook length. The molecular ruler FliK is secreted throughout the hook–basal body assembly process (a). The N‐terminus of FliK interacts with the hook subunits (FlgE), as well as the hook cap (FlgD). During a measurement of hook length by FliK, hook polymerisation temporarily halts. If the hook is too short, FliK is secreted (b) and hook polymerisation continues until another FliK molecule measures hook length again (c). When the hook has reached its physiological length of 55 nm in Salmonella, the C‐terminus of FliK is in proximity with the FlhB component of the type‐III secretion apparatus for a productive interaction that catalyses the switch in secretion specificity from rod‐hook‐type (early) to filament‐type (late) substrate secretion mode (d). Adapted from Erhardt et al. .
Figure 4. In situ multicolour labelling of flagellar filaments. Bacteria are grown in the presence of different dyes (dye 1–3; green, red and orange) for a specified time (e.g. 30 min), resulting in differentially labelled fragments of the filament (F1–F6; 1–6) as shown in the fluorescence microscopy image. The growth rate of individual filaments can be determined from the length of the different fragments. Blue, DNA staining; Bar, 2 μm. Adapted from Renault et al. .
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Kühne, Caroline, Guse, Alina, Hüsing, Svenja, and Erhardt, Marc(Feb 2020) Bacterial Flagella. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000301.pub3]