Bacterial Flagellar Motor: Overview
- 138 Downloads
The bacterial flagellar motor is an ion-driven rotary motor embedded in the cell membrane that drives a long helical filament (5–10 μm long with a diameter of ∼15 nm) enabling the cell to swim. The motor is connected to the filament by a short flexible hook about 100 nm long. Hook and filament form the flagellum. The flagellum/motor complex self-assembles and is the result of the coordinated, sequential expression of over 50 genes. The final structure is composed of at least 13 different proteins, all present in different copy numbers, ranging from a few to hundreds of them.
In peritrichously flagellated bacteria such as Escherichia coli, hooks allow multiple filaments to come together to form a bundle that propels the cell forward. This happens when all the motors driving these filaments rotate in the same direction (counterclockwise). Flagellar motors are also able to rotate in the reverse direction, causing the bundle to dissociate which sends the cell into a tumble. When rotation resumes in the original rotational direction, the cell swims in a new random direction. The frequency at which these switching events occur is controlled by the chemotaxis sensory pathway, which can sense changes in the concentration of nutrients/toxins in the cell environment. By terminating runs in the “wrong” direction (i.e., a tumbling event), cells execute a biased random walk; subsequently, cells accumulate in areas favorable for their survival.
Bacterial Flagellar Motor
The flagellar motor consists of two main parts: a rotor spanning the entire cell membrane which is surrounded by independent stator units anchored to the peptidoglycan layer of the cell wall. In E. coli, the rotor is about 45 nm in diameter at its base (the cytoplasmic part of the motor) and is surrounded by 11–13 stator units. The stators allow ions (either protons or sodium depending on the bacterial species) to flow down the electrochemical ion gradient from the periplasm to the cytoplasm. The interaction with a conserved aspartate causes electrostatic interactions at the rotor/stator interface to drive rotation of the rotor, with ∼26 steps per revolution (this coincides with the number of FliG proteins estimated to be present in the rotor). The electrochemical gradient of ions is generally maintained by electron transport chains (respiration or photosynthesis). It is estimated that in E. coli, approximately 36 ions are transmitted through each stator unit per revolution. Estimates for the torque of the proton-driven motor typically range from ∼1,300 to ∼2,000 pN.nm, while sodium-driven motor torques have been measured between ∼2,000 and 4,000 pN.nm.
Variations Between Bacterial Species
Different bacterial species have evolved different ways of swimming to best fit their environmental conditions. Some species have additional rings, while others are wider and incorporate larger numbers of stators around the rotor, correlating with the torque output needed by that species in, often, more viscous environments. A few examples are outlined below:
E. coli: It has four to eight randomly located proton-driven flagella; motors rotate at ∼100 Hz; cells swim at ∼ 25 μm/s; and motors can switch direction. Rotors are ~22 nm in radius with 11 surrounding stators at high load.
Campylobacter jejuni: It has a polar flagellum, and rotor is 26 nm in radius with 17 stators. Two additional disks in periplasm are thought to allow high torque to move through viscous environment.
Vibrio alginolyticus: It has one sodium-driven polar flagellum; motor rotates at ∼400 Hz; cells swim at ∼60 μm/s; and motor can switch direction. Multiple lateral proton-driven motors are expressed to allow the cell to swarm over hard surfaces.
Rhodobacter sphaeroides: It has one proton-driven flagellum; it has similar rotation/swimming speeds to E. coli, but the motor stops to achieve a cell tumble instead of reversing its direction.
Shewanella oneidensis: One single motor is able to recruit sodium-driven stators or proton-driven stators depending on the availability of ions.
Bacillus clausii: Its stator units are able to utilize protons or sodium ions depending upon availability.
Paenibacillus sp. TCA20: Its stator units are coupled to divalent cations (Ca2+ and Mg2+).
Helicobacter pylori: Proton-driven motors are capable of generating a torque of approximately 3,600 pN.nm.