Bacterial Flagella: Flagellar Motor


The bacterial flagellar motor is a complex biological rotary molecular motor, which is situated in the cell envelopes of bacteria. Whereas most biological motors use adenosine triphosphate as their energy source, the rotation of the flagellar motor is driven by a flow of charged ions across the bacterial plasma membrane. The motor powers the rotation of helical flagellar filaments at speeds of up to several hundred hertz. These rotating filaments act like propellers, pushing the cells through their environment. The motors are regulated by one of the best‐characterised biological signalling pathways, the chemotaxis pathway. This pathway changes the swimming pattern of the bacteria in response to changes in the concentration of external chemicals so that they move into environments, which are optimal for their growth. Other pathways can regulate the flagellar motor and the motor itself can respond to changing conditions by adapting parts of its structure.

Key Concepts:

  • Many bacteria swim using a small biological rotary motor which is powered by the movement of ions (H+ or Na+) across the plasma membrane.

  • The bacterial flagellar motor consists of a rotor which rotates against stator units that are anchored to the peptidoglycan cell wall.

  • Torque is generated by the interaction of the stator units, MotA and MotB (or PomA and PomB for Na+‐driven motors), with FliG in the rotor.

  • Despite the fact that the driving ions always flow in one direction through the stator units, many flagellar motors can switch between clockwise and counterclockwise rotation.

  • The structure of the flagellar motor is highly dynamic, some of its components undergo rapid turnover while the motor is functioning in response to changing conditions.

  • A complex signalling pathway regulates the motor output in response to environmental signals ensuring that bacteria swim towards nutrient rich environments.

Keywords: motility; molecular motors; nanomachines; protein exchange; signalling

Figure 1.

The bacterial flagellum. The bacterial flagellar motor is a rotary motor that sits in the cell envelope of bacteria. It is driven by the flow of ions across the cytoplasmic (plasma) membrane, and its purpose is to rotate the long helical filaments that protrude fom the cell and propel swimming bacteria. The diagram on the left depicts a Gram‐negative envelope, typifying E. coli; the L‐ and P‐ rings are associated with the outer membrane and peptidoglycan layer. In Gram‐positive bacteria, flagellar basal bodies lack the L‐ and P‐ rings because of their thick cell walls. The image on the right is a three‐dimensional reconstruction of S. enterica flagellar motor obtained by cryo‐electron microscopy. CM, cytoplasmic membrane; OM, outer membrane; PG, peptidoglycan cell wall.

Figure 2.

Torque is defined as the cross product of the distance vector r and the force vector F: τ=r×F. (a) When a force is applied to a particle, only the perpendicular component F produces a torque for which the magnitude is given by the formula: τ=|r| |F| sinθ where θ is the angle between r and F. (b) In the bacterial flagellar motor, the force is applied by the stator units to the perimeter of the rotor, and the torque is equal to this force multiplied by the radius of the rotor.

Figure 3.

Proposed model for stator unit topology and torque‐generating interaction between rotor and stator in E. coli. The cytoplasmic loop of stator subunit MotA contains important charged residues that interact with charged residues of the rotor protein FliG. In this model, ions pass through a channel formed by MotB and and 2 MotA, bind to a conserved aspartate, leading to conformational changes in MotA that will in turn exert torque on FliG. CM, cytoplasmic membrane; PBD, peptidoglycan binding domain; PL, peptidoglycan layer.

Figure 4.

Swimming and flagellar rotation in E. coli are controlled by the chemotaxis pathway (a) cells of the bacterium E. coli swim in a series of ‘runs’ and ‘tumbles’. In a run, flagella rotate CCW and form a bundle that propels the cell. In a tumble, flagella rotate clockwise (CW) and the bundle flies apart, causing the cell to jiggle on the spot. (b) The chemotaxis pathway in E. coli. Bacteria modulate the probability direction of rotation of their flagellar motors in response to changes in the concentration of chemicals in the environment. This allows them to swim towards nutrients or away from harmful chemicals, a process known as chemotaxis. The biochemical pathway that controls this response consists of membrane‐bound receptors – also known as methyl‐accepting chemotaxis proteins (MCPs) – and the chemotaxis proteins CheW, A, Y, Z, B and R. The curved arrows represent phosphorylation and dephosphorylation reactions; the thin straight arrows represent transitions between alternative states of the MCPs or of the flagellar motor and the wide straight arrows represent positive influences of one element in the pathway upon a particular transition. The arrows in red highlight the sequence of events that generates an increase in the probability of clockwise (CW) rotation in response to a reduction in the amount of attractant bound to an MCP; CCW, counterclockwise.



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Delalez, Nicolas J(Aug 2014) Bacterial Flagella: Flagellar Motor. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000744.pub4]