A Rotary Motor Drives Bacterial Motion

IV. Responding to Environmental Changes
34. Molecular Motors
34.3. Kinesin and Dynein Move Along Microtubules
Figure 34.27. Motion in Ncd. The replacement of ATP for ADP releases the α-helical region from the head domain,
moving the other head domain toward the minus end of the microtubule.
IV. Responding to Environmental Changes
34. Molecular Motors
34.4. A Rotary Motor Drives Bacterial Motion
In one second, a motile bacterium can move approximately 25 µm, or about 10 body lengths. A human being sprinting at
a proportional rate would complete the 100-meter dash in slightly more than 5 seconds. The motors that power this
impressive motion are strikingly different from the eukaryotic motors that we have seen so far. In the bacterial motor, an
element spins around a central axis rather than moving along a polymeric track. The direction of rotation can change
rapidly, a feature that is central to chemotaxis, the process by which bacteria swim preferentially toward an increasing
concentration of certain useful compounds and away from potentially harmful ones.
34.4.1. Bacteria Swim by Rotating Their Flagella
Bacteria such as Eschericia coli and Salmonella typhimurium swim by rotating flagella that lie on their surfaces (Figure
34.28). When the flagella rotate in a counterclockwise direction (viewed from outside the bacterium), the separate
flagella form a bundle that very efficiently propels the bacterium through solution.
Bacterial flagella are polymers approximately 15 nm in diameter and as much as 15 µm in length, composed of 53-kd
subunits of a protein called flagellin (Figure 34.29). These subunits associate into a helical structure that has 5.5 subunits
per turn, giving the appearance of 11 protofilaments. Each flagellum has a hollow core. Remarkably, flagella form not by
growing at the base adjacent to the cell body but, instead, by the addition of new subunits that pass through the hollow
core and add to the free end. Each flagellum is intrinsically twisted in a left-handed sense. At its base, each flagellum has
a rotory motor.
34.4.2. Proton Flow Drives Bacterial Flagellar Rotation
Early experiments by Julius Adler demonstrated that ATP is not required for flagellar motion. What powers these rotary
motors? The necessary free energy is derived from the proton gradient that exists across the plasma membrane. The
flagellar motor is quite complex, containing as many as 40 distinct proteins (Figure 34.30). Five components particularly
crucial to motor function have been identified through genetic studies. MotA is a membrane protein that appears to have
four transmembrane helices as well as a cytoplasmic domain. MotB is another membrane protein with a single
transmembrane helix and a large periplasmic domain. Approximately 11 MotA - MotB pairs form a ring around the base
of the flagellum. The proteins FliG, FliM, and FliN are part of a disc-like structure called the MS (membrane and
supramembrane) ring, with approximately 30 FliG subunits coming together to form the ring. The three-dimensional
structure of the carboxyl-terminal half of FliG reveals a wedge-shaped domain with a set of charged amino acids,
conserved among many species, lying along the thick edge of the wedge (Figure 34.31).
The MotA - MotB pair and FliG combine to create a proton channel that drives rotation of the flagellum. How can
proton flow across a membrane drive mechanical rotation? We have seen such a process earlier in regard to ATP
synthase (Section 18.4.4). Recall that the key to driving the rotation of the γ subunit of ATP synthase is the a subunit of
the F0 fragment. This subunit appears to have two half-channels; protons can move across the membrane only by moving
into the half-channel from the side of the membrane with the higher local proton concentration, binding to a disc-like
structure formed by the c subunits, riding on this structure as it rotates to the opening of the other half-channel, and
exiting to the side with the lower local proton concentration. Could a similar mechanism apply to flagellar rotation?
Indeed, such a mechanism was first proposed by Howard Berg to explain flagellar rotation before the rotary mechanism
of ATP synthase was elucidated. Each MotA - MotB pair is conjectured to form a structure that has two half-channels;
FliG serves as the rotating proton carrier, perhaps with the participation of some of the charged resides identified in
crystallographic studies (Figure 34.32). In this scenario, a proton from the periplasmic space passes into the outer halfchannel and is transferred to an FliG subunit. The MS ring rotates, rotating the flagellum with it and allowing the proton
to pass into the inner half-channel and into the cell. Ongoing structural and mutagenesis studies are testing and refining
this hypothesis.
34.4.3. Bacterial Chemotaxis Depends on Reversal of the Direction of Flagellar
Rotation
Many species of bacteria respond to changes in their environments by adjusting their swimming behavior. Examination
of the paths taken is highly revealing (Figure 34.33). The bacteria swim in one direction for some length of time
(typically about a second), tumble briefly, and then set off in a new direction. The tumbling is caused by a brief reversal
in the direction of the flagellar motor. When the flagella rotate counterclockwise, the helical filaments form a coherent
bundle favored by the intrinsic shape of each filament, and the bacterium swims smoothly. When the rotation reverses,
the bundle flies apart because the screw sense of the helical flagella does not match the direction of rotation (Figure
34.34). Each flagellum then pulls in a different direction and the cell tumbles.
In the presence of a gradient of certain substances such as glucose, bacteria swim preferentially toward the direction of
the higher concentration of the substance. Such compounds are referred to as chemoattractants. Bacteria also swim
preferentially away from potentially harmful compounds such as phenol, a chemorepellant. The process of moving in
specific directions in response to environmental cues is called chemotaxis. In the presence of a gradient of a
chemoattractant, bacteria swim for longer periods of time without tumbling when moving toward higher concentrations
of chemoattractant. In contrast, they tumble more frequently when moving toward lower concentrations of
chemoattractant. This behavior is reversed for chemorepellants. The result of these actions is a biased random walk that
facilitates net motion toward conditions more favorable to the bacterium.
Chemotaxis depends on a signaling pathway that terminates at the flagellar motor. The signaling pathway begins with
the binding of molecules to receptors in the plasma membrane (Figure 34.35). In their unoccupied forms, these receptors
initiate a pathway leading eventually to the phosphorylation of a specific aspartate residue on a soluble protein called
CheY. In its phosphorylated form, CheY binds to the base on the flagellar motor. When bound to phosphorylated CheY,
the flagellar motor rotates in a clockwise rather than a counterclockwise direction, causing tumbling.
The binding of a chemoattractant to a surface receptor blocks the signaling pathway leading to CheY phosphorylation.
Phosphorylated CheY spontaneously hydrolyzes and releases its phosphate group in a process accelerated by another
protein, CheZ. The concentration of phosphorylated CheY drops, and the flagella are less likely to rotate in a clockwise
direction. Under these conditions, bacteria swim smoothly without tumbling. Thus, the reversible rotary flagellar motor
and a phosphorylation-based signaling pathway work together to generate an effective means for responding to
environmental conditions.
Bacteria sense spatial gradients of chemoattractants by measurements separated in time. A bacterium sets off in a
random direction and, if the concentration of the chemoattractant has increased after the bacterium has been swimming
for a period of time, the likelihood of tumbling decreases and the bacterium continues in roughly the same direction. If
the concentration has decreased, the tumbling frequency increases and the bacterium tests other random directions. The
success of this mechanism once again reveals the power of evolutionary problem solving
tried at random, and those that are beneficial are selected and exploited.
IV. Responding to Environmental Changes
34. Molecular Motors
many possible solutions are
34.4. A Rotary Motor Drives Bacterial Motion
Figure 34.28. Bacterial Flagella. Electron micrograph of S. typhimurium shows flagella in a bundle. [Courtesy of Dr.
Daniel Koshland, Jr.]
IV. Responding to Environmental Changes
34. Molecular Motors
34.4. A Rotary Motor Drives Bacterial Motion
Figure 34.29. Structure of Flagellin. A bacterial flagellum is a helical polymer of the protein flagellin.
IV. Responding to Environmental Changes
34. Molecular Motors
34.4. A Rotary Motor Drives Bacterial Motion
Figure 34.30. Flagellar Motor. A schematic view of the flagellar motor, a complex structure containing as many as 40
distinct types of protein. The approximate positions of the proteins MotA and MotB (red), FliG (orange), FliN (yellow),
and FliM (green) are shown.
IV. Responding to Environmental Changes
34. Molecular Motors
34.4. A Rotary Motor Drives Bacterial Motion
Figure 34.31. Flagellar Motor Components. Approximately 30 subunits of FliG assemble to form part of the MS ring.
The ring is surrounded by approximately 11 structures consisting of MotA and MotB. The carboxyl-terminal
domain of FliG includes a ridge lined with charged residues that may participate in proton transport.
IV. Responding to Environmental Changes
34. Molecular Motors
34.4. A Rotary Motor Drives Bacterial Motion
Figure 34.32. Proton Transport-Coupled Rotation of the Flagellum. (A) MotA-MotB may form a structure having
two half-channels. (B) One model for the mechanism of coupling rotation to a proton gradient requires protons to be
taken up into the outer half-channel and transferred to the MS ring. The MS ring rotates in a counterclockwise direction,
and the protons are released into the inner half-channel. The flagellum is linked to the MS ring and so the flagellum
rotates as well.
IV. Responding to Environmental Changes
34. Molecular Motors
34.4. A Rotary Motor Drives Bacterial Motion
Figure 34.33. Charting a Course. This projection of the track of an E. coli bacterium was obtained with a microscope
that automatically follows bacterial motion in three dimensions. The points show the locations of the bacterium at 80-ms
intervals. [After H. C. Berg. Nature 254(1975):390.]
IV. Responding to Environmental Changes
34. Molecular Motors
34.4. A Rotary Motor Drives Bacterial Motion
Figure 34.34. Changing Direction. Tumbling is caused by an abrupt reversal of the flagellar motor, which disperses the
flagellar bundle. A second reversal of the motor restores smooth swimming, almost always in a different direction. [After
a drawing kindly provided by Dr. Daniel Koshland, Jr.]
IV. Responding to Environmental Changes
34. Molecular Motors
34.4. A Rotary Motor Drives Bacterial Motion