This description of bacterial locomotion is well known, but the mechanisms that allow the flagella to shift gears from counterclockwise to clockwise have proven difficult to identify. Now, in a new study in this issue of PLoS Biology , Katsumi Imada, Tohru Minamino and colleagues bring us closer to answering this fundamental question and propose a new model describing how flagella manage this switch.
Filaments in the flagella are powered by rotary motors that span the cell membrane. The rotor shifts from the forward-propelling counterclockwise to the tumble-inducing clockwise when chemical gradients tell bacteria they've gone astray, for example, away from food.
This activates a cytoplasmic signaling protein that binds proteins in the rotor switch, changing the orientation of another switch protein called FliG and thereby reversing the rotor's spin to clockwise. The details of the switch mechanism had been hypothesized but were as yet unproven. Previous X-ray crystallography studies of a FliG fragment had shown that two of its domains FliG M and FliG C are connected by a helical linker called helix E, and the 3-D structure of a FliG protein predicted from its DNA sequence suggested that helix E might be flexible enough to make a good molecular switch.
This suggestion was further supported by a report that compared the structure of a full-length FliG to the fragment: helix E was tightly packed in closed conformation in the full-length structure, but was in open conformation and dissociated from FliG M in the fragment. To find out if helix E is indeed the molecular switch that sets the direction of rotor spin, the researchers compared wild-type and mutant FliG fragments containing the two domains linked by helix E.
The wild-type motors were set to spin counterclockwise by experimental conditions, and the mutant had a type of amino acid deletion that sets the rotor spin to clockwise. Cells use flagella for locomotion to look for food and to escape danger. The whiplike flagella can be rotated to promote motion via a corkscrew effect, or they can act like oars to row cells through liquids. Flagella are found in bacteria and in some eukaryotes, but those two types of flagella have a different structure.
A bacterial flagellum helps beneficial bacteria move through the organism and helps disease-causing bacteria to spread during infections. They can move to where they can multiply, and they can avoid some of the attacks from the immune system of the organism.
For advanced animals, cells such as sperm move with the aid of a flagellum. Flagella for prokaryotes such as bacteria are made up of three parts:. The flagellar filament is created by transporting the protein flagellin from cell ribosomes through the hollow core to the tip where the flagellin attaches and makes the filament grow.
The basal body forms the motor of the flagellum, and the hook gives the rotation a corkscrew effect. The motion of eukaryotic flagella and those of prokaryotic cells is similar, but the structure of the filament and the mechanism for rotation are different. The basal body of eukaryotic flagella is anchored to the cell body, but the flagellum lacks a rod and disks.
Instead, the filament is solid and is made up of pairs of microtubules. The tubules are made up of linear protein strings around a hollow center. The double tubes share a common wall while the central tubes are independent.
Protein spokes, axes and links join the microtubules along the length of the filament. Instead of a motion created at the base by rotating rings, the flagellum motion comes from interaction of the microtubules. Although bacterial flagella and those of eukaryotic cells have a different structure, they both work through a rotational movement of the filament to propel the cell or move fluids past the cell. Shorter filaments will tend to move back and forth while longer filaments will have a circular spiral motion.
In bacterial flagella, the hook at the bottom of the filament rotates where it is anchored to the cell wall and plasma membrane. The rotation of the hook results in a propeller-like motion of the flagella. In eukaryotic flagella, the rotational motion is due to the sequential bending of the filament. Under the hook of bacterial flagella, the the base of the flagellum is attached to the cell wall and the cell's plasma membrane by a series of rings surrounded by protein chains.
A proton pump creates a proton gradient across the lowest of the rings, and the electrochemical gradient powers rotation through a proton motive force. Bacterial flagella filaments are long, hollow, rotating polymers of flagellin protein that bacteria use to swim to new habitats. In many species, flagella are also involved in initial attachment, but the mechanisms and specific factors involved are not fully understood.
We spoke with the co-author Eliza Wolfson to find out more:. I am based at the University of Bristol as a visiting postdoctoral researcher, but I'm mostly a freelance scientific illustrator , which I have been doing for the last five years.
The work was largely carried out at The Roslin Institute, though, this paper has been a slow burner.
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