April 22, 2008
Cell Motility: The Flagella, mostly
Though not universal, one of the oft recognized characteristics of life is that an entity is able to move under its own power. Indeed, such an ability is often necessary for organisms as they need to be able to relocate to areas of greater nutrient concentration, or lower predator concentration.
Microorganisms are no different. Despite their tiny size, microorganisms have developed a number of distinct strategies for controlling their locomotion, which will be the subject of today’s post. By far the most common means of locomotion in microorganisms is the flagella, a long whip like structure that allows microorganisms to propel themselves through the expense of their chemiosmotic gradient. Additionally there is the mysterious movement through a process known as *gliding*, and vertical motion through control of gas vacuoles. While other forms of cell motility exist, these are the three that I shall touch on today, and most of my time will be spent discussing the flagella.
Flagella are thin strands of flagellin protein that are used to create motion. Three different forms of flagella have been described, corresponding to the three branches of the tree of life. Eukaryotic flagella are whip like, and propel their cells in explosive movements akin to sculling in a boat (For those who did not grow up in an oceanside community, sculling a highly efficient technique whereby a dinghy is propelled not by the traditional means of double oared rowing, but instead by standing in the back of the boat and moving an single oar side to side in the water. The motion is similar to the undulations of a fish as it swims through the water. Traditional rowing is more closely related to the motions of cilia. )
In contrast, Bacterial flagella are comparatively stiff helical structures that rotate, generating propulsion in a manner akin to propellers on most powered ships. Archaeal flagella are superficially similar to Bacterial flagella, however have numerous differences that shall be noted in the discussion below. I will leave discussion of Eukaryotic flagella for another time, and focus on Archaeal and Bacterial flagella.
There are several different gross morphologies of Bacterial flagella, based upon the number and placement of individual flagella. While the actual mechanism of each flagella unit is fairly common across species, the placement and structure varies, making it a common feature by which to classify bacterial species.
The first mechanism of classification is to classify how flagella are distributed. Some bacterial cells only have one flagella (Below, A). These species are considered monotrichous, while other cells have two flagella on opposite sides of a cell (Below, C) and are considered amphitrichous. Both of these arrangements are also considered to be polar arrangements. Sometimes Flagella are present in a single clump, an arrangement that is termed lophotrichous. These units often wrap together to make what is effectively a single larger screw drive. Peritrichous flagella are arranged all over the cell (Below, D). This image below, from Wikimedia Commons shows examples of these arrangements of flagella.
Thank you Mike Jones
The other major means of differentiation comes within the characterization of the helix that is formed by the filament section of the flagella unit. These filaments will have a characteristic undulation that described as a wavelength. This wavelength can be used to determine the species of bacteria that is being examined. (Note that in this respect, the images of flagella above are not correct.)
However, while there are differences between flagella in these respects, the basic structure and mechanism of flagella function remains similar. The flagella consists of three main parts: The long helical filament which provides the interface transform rotational motion into linear motion; the hook, which connects the filament to the motor complex; and the motor complex, which provides the rotational motion.
Of these three, the motor complex shows the most variation. Since it is a transmembrane protein, it needs to transverse all layers which surround the cell, thus it is significantly different in gram-negative proteins and gram-positive proteins.
In gram positive bacteria, there are three structures that allow a central rod to bypass the two layers that separate the extracellular environment from the cytoplasm. The central rod acts like an axle to transfer the rotation to the outside of the cell. In the cytoplasmic membrane, the two structures are the MS ring and the C ring. The C ring is located in the cytoplasm, and is connected to the MS ring via the central rod. It is on the cytoplasmic layer that the turning force is generated. In Bacteria, it is generated via the discharging of the chemiosmotic gradient. A concentration of protons in the periplasmic or extracellular are allowed to flow back in by forcing the rotor to turn. The protons are thought to be brought back in via the Mot proteins, while the Fli proteins act as on and off switches. In Archaea, the same basic system is used, except the rotary motion is not generated by the chemiosmotic gradient, but instead by the hydrolysis of ATP.
The P ring is the structure that allows the rotary motion to be translated through the peptidoglycan layer. It is embedded in the peptidoglyan and does not rotate along with the C ring and the MS ring. It acts as a cuff that allows the rod to revolve (and thus not form strong bonds with the surrounding layer) while not allowing extracellular solutes to come into the cell. It is essentially a similar structure to the oil cuff in boats that allows an engine within a ship to turn a propeller outside the ship. In Gram Negative bacteria, there is yet another layer to bypass, so another cuff is needed, this one is called the L ring. Thus the P and L rings are essentially low friction cuffs that allow the rod to rotate, the C ring, MS ring and the rod are the rotor and the Mot Proteins are the sator. This is diagrammed in the picture below. For an alternate rendering of this system, take a look at the Wikipedia image.
I created this image, and you are free to use it. It is modeled off a diagram in Brock’s Biology of Microorganisms
Through the center of the whole complex runs a hollow rod. It is through this rod that flagellin proteins travel so that they can travel to the point of flagella synthesis. In Bacteria flagella are synthesized at the tip, not at the base. This makes the growth of flagella more alike the growth of branches on trees than the growth of hairs. However, in Archaea flagella grow from the base. Furthermore, the size of the filament in Archaea is much smaller than in Bacteria, and is in fact too small to allow flagellin to flow through the tube.
Movement by flagella is very fast. Bacteria powered by flagella are able to propel themselves at speeds of 60 cell lengths a second. While this is still only 0.00017 km/h, the cheetah, moving at a maximum speed of 110 km/h, is only moving at 25 body lengths per second. Furthermore, some flagella are capable of rotating in both directions, and thus creating movement in both directions.
Youtube has a bunch of cool videos documenting flagellar action.Note that in these videos, in order to see the flagella they are using a special stain which actually increases the diameter of the flagella. Unstained flagella are too small to resolve with an optical microscope. It shows a good diversity of the different arrangements of flagella. If you want to see how the flagella is assembled, take a look here. This video shows how the rotary force is generated in the flagella. This video shows both how the proton motive force drives the flagella to rotate, and how the flagella is created. It is less spiffy than the previous two, but maybe a little more clear. I
Gliding is a mysterious form of motion, and is not well understood. It is much slower than flagellar movement, and is only seen as a method of moving along surfaces, not though a liquid medium. Cyanobacteria are some of the most well known gliders.
It is thought that numberous mechanisms for gliding exist, but only a few are currently understood at any level. This method is effected by the secretion of a polysaccharide solution that binds to the solid surface. As the secreted goo, which binds to both the cell and to the surface the cell is gliding along, binds to the solid substrate the cell tends to get pulled in one direction or another. By controlling where the goo is secreted, the cell can control in which direction it moves.
A second mechanism to explain gliding involves the motion of large protein complexes that are attached through the different layers of cell. These proteins bind to the substrate, and then are driven to the “back” of the cell. Continuing our boat analogy, this is akin to how canal boats were propelled. Boatmen would drive long poles into the mud, securing them in place. They would then “walk” to the rear of the boat, while holding onto the pole. Since the pole stays in the same geographic location, the boat would be propelled forward.
By controlling their density, cells can control their position in the water column. Seawater becomes more dense the deeper it gets, both by compression and by varying concentrations of solutes. Unicellular organisms use this to their advantage by adjusting their net density. Gas filled vesicles are in many unicellular organisms, and adjustment of the size of these vesicles will adjust the net density of the organism. Many microorganisms in the ocean undergo a daily migration to lower depths in the day and a return in the evening to avoid predators.
So to summarize:
- The flagella is an important means of motility in microbes, and it works in different ways in the three major branches of the tree of life.
- There are many ways that flagella can be organized on the surface of a cell, including polar arrangements, patchy arrangements and peritrichous arrangements.
- In Bacteria the flagella drives a cell forward by rotating a helical filament.
- In Bacteria the circular motion of the flagella is driven by the chemiosmotic gradient, while in Archaea the circular motion is driven by ATP.
- The flagella is composed of the filament, the hook, the rotor, one or two cuffs and a stator. The sator provides the torque on the rotor, the cuffs alow it to rotate through the peptidoglycan layer and outer membrane, if present. The hook connects the rotor to the filament, which is the interface between the environment and the motor which allows the motor to do work.
- The cuffs are the L ring and the P ring, for the LPS layer and the Peptidoglycan layer respectively. The C ring is located in the cytoplasm, while the MS ring is located in the cytoplasmic membrane, these rings interact with the Mot proteins to create the turning motion. The Fli proteins turn the complex on and off.
- Gliding motility is poorly understood but does not involve flagella. It only occurs along surfaces.
- Gas vesicles can be used to change the position of a microbe in the water column. This is effected by changing the organisms density, so that it either comes to the surface or sinks to the mixing layer.
The information in this post was attained from Brocks Biology of Microorganisms and from Wikipedia articles on the subject, particularly here.
Here is a video of Ken Miller talking about the use of the bacterial flagellum by intelligent design folks as an argument for intelligent design. He shows how it is not an instance of ”irreducible complexity” and can be evolved. His argument is simply that contrary to the ID argument, precursors to the flagella are actually functional, or homologous to proteins that have purpose. Thus the argument goes: they could be evolved.
Science wins again.