Flagellum

A flagellum (; plural: flagella) is a lash-like appendage that protrudes from the cell body of certain fascicle". Other bacteria, such as most Spirochetes, have two or more specialized flagella (endoflagella) arising from opposite poles of the cell, which together constitute the so-called "axial filament" that is located within the periplasmic space between the flexible cell wall and an outer sheath. The rotation of the axial filament relative to the cell body causes the entire bacterium to move forward in a corkscrew-like motion, even through material viscous enough to prevent the passage of normally flagellated bacteria.

Counterclockwise rotation of a monotrichous polar flagellum pushes the cell forward with the flagellum trailing behind, much like a corkscrew moving inside cork. Indeed water on the microscopic scale is highly viscous, very different from our daily experience of water.

Flagella are left-handed helices, and bundle and rotate together only when rotating counterclockwise. When some of the rotors reverse direction, the flagella unwind and the cell starts "tumbling". It has also been suggested that even if all flagella would rotate clockwise, they will not form a bundle, due to geometrical as well as hydrodynamic reasons.^[28][29] Such "tumbling" may happen occasionally, leading to the cell seemingly thrashing about in place, resulting in the reorientation of the cell. The clockwise rotation of a flagellum is suppressed by chemical compounds favorable to the cell (e.g. food), but the motor is highly adaptive to this. Therefore, when moving in a favorable direction, the concentration of the chemical attractant increases and "tumbles" are continually suppressed; however, when the cell's direction of motion is unfavorable (e.g., away from a chemical attractant), tumbles are no longer suppressed and occur much more often, with the chance that the cell will be thus reoriented in the correct direction.

In some Vibrio spp. (particularly Vibrio parahemolyticus^[30]) and related proteobacteria such as Aeromonas, two flagellar systems co-exist, using different sets of genes and different ion gradients for energy. The polar flagella are constitutively expressed and provide motility in bulk fluid, while the lateral flagella are expressed when the polar flagella meet too much resistance to turn.^{[31][32][33][34][35][36]} These provide swarming motility on surfaces or in viscous fluids.

Archaeal

The archaeal flagellum (Archaellum) is superficially similar to the bacterial (or eubacterial) flagellum; in the 1980s they were thought to be homologous on the basis of gross morphology and behavior.^[37] Both flagella and archaella consist of filaments extending outside the cell, and rotate to propel the cell. Archaeal flagella have a unique structure which lacks a central channel. Similar to bacterial type IV pilins, the archaeal flagellins (archaellins) are made with class 3 signal peptides and they are processed by a type IV prepilin peptidase-like enzyme. The archaellins are typically modified by the addition of N-linked glycans which are necessary for proper assembly and/or function.^[4]

Discoveries in the 1990s revealed numerous detailed differences between the archaeal and bacterial flagella; these include:

• Bacterial flagella are motorized by a flow of H⁺ ions (or occasionally Na⁺ ions); archaeal flagella are almost certainly powered by ATP. The torque-generating motor that powers rotation of the archaeal flagellum has not been identified.
• While bacterial cells often have many flagellar filaments, each of which rotates independently, the archaeal flagellum is composed of a bundle of many filaments that rotate as a single assembly.
• Bacterial flagella grow by the addition of flagellin subunits at the tip; archaeal flagella grow by the addition of subunits to the base.
• Bacterial flagella are thicker than archaella [Singular Archaellum], and the bacterial filament has a large enough hollow "tube" inside that the flagellin subunits can flow up the inside of the filament and get added at the tip; the archaellum is too thin (12-15 nm) to allow this.^[38]
• Many components of bacterial flagella share sequence similarity to components of the type III secretion systems, but the components of bacterial flagella and archaella share no sequence similarity. Instead, some components of archaella share sequence and morphological similarity with components of type IV pili, which are assembled through the action of type II secretion systems (the nomenclature of pili and protein secretion systems is not consistent).^[38]

These differences could mean that the bacterial flagella and archaella could be a classic case of biological analogy, or convergent evolution, rather than homology. However, in comparison to the decades of well-publicized study of bacterial flagella (e.g. by Howard Berg),^[39] archaella have only recently begun to get serious scientific attention. Therefore, many assume erroneously that there is only one basic kind of prokaryotic flagellum, and that archaella are homologous to it. For example, Cavalier-Smith (2002)^[37] is aware of the differences between bacterial flagellins and archaeal flagellins (archaellins).

Eukaryotic

Eukaryotic flagella. 1–axoneme, 2–cell membrane, 3–IFT (IntraFlagellar Transport), 4–Basal body, 5–Cross section of flagella, 6–Triplets of microtubules of basal body

Cross section of an axoneme

Longitudinal section through the flagella area in Chlamydomonas reinhardtii. In the cell apex is the basal body that is the anchoring site for a flagellum. Basal bodies originate from and have a substructure similar to that of centrioles, with nine peripheral microtubule triplets (see structure at bottom center of image).

The "9+2" structure is visible in this cross-section micrograph of axoneme.

Along with undulipodia.^[40]

Structure

A eukaryotic flagellum is a bundle of nine fused pairs of centrioles. The flagellum is encased within the cell's plasma membrane, so that the interior of the flagellum is accessible to the cell's cytoplasm.

Mechanism

Each of the outer nine doublet microtubules extends a pair of dynein arms (an "inner" and an "outer" arm) to the adjacent microtubule; these dynein arms are responsible for flagellar beating, as the force produced by the arms causes the microtubule doublets to slide against each other and the flagellum as a whole to bend. These dynein arms produce force through ATP hydrolysis. The flagellar axoneme also contains radial spokes, polypeptide complexes extending from each of the outer nine microtubule doublets towards the central pair, with the "head" of the spoke facing inwards. The radial spoke is thought to be involved in the regulation of flagellar motion, although its exact function and method of action are not yet understood.

Flagella vs cilia

Difference of beating pattern of flagellum and cilia

The regular beat patterns of eukaryotic cilia and flagella generate motion on a cellular level. Examples range from the propulsion of single cells such as the swimming of spermatozoa to the transport of fluid along a stationary layer of cells such as in the respiratory tract. Though eukaryotic flagella and motile cilia are ultrastructurally identical, the beating pattern of the two organelles can be different. In the case of flagella, the motion is often planar and wave-like, whereas the motile cilia often perform a more complicated three-dimensional motion with a power and recovery stroke.

Intraflagellar transport

Intraflagellar transport (IFT), the process by which axonemal subunits, transmembrane receptors, and other proteins are moved up and down the length of the flagellum, is essential for proper functioning of the flagellum, in both motility and signal transduction.^[41]

For information on biologists' ideas about how the various flagella may have evolved, see evolution of flagella.

See also

• Archaellum
• Cilium
• Evolution of flagella
• Genetic ciliopathy
• Rotating locomotion in living systems
• Undulipodia

References

1. ^ Wang, Qingfeng; Suzuki, Asaka; Mariconda, Susana; Porwollik, Steffen; Harshey, Rasika M (2005). "Sensing wetness: A new role for the bacterial flagellum". The EMBO Journal 24 (11): 2034–42.  
2. ^ Bardy SL, Ng SY, Jarrell KF (February 2003). "Prokaryotic motility structures". Microbiology (Reading, Engl.) 149 (Pt 2): 295–304.  
3. ^ Lefebvre PA; Lefebvre, PA (2001). "Assembly and Motility of Eukaryotic Cilia and Flagella. Lessons from Chlamydomonas reinhardtii". Plant Physiol. 127 (4): 1500–1507.  
4. ^ ^{a b} Jarrell, K (editor) (2009). Pili and Flagella: Current Research and Future Trends. Caister Academic Press.  
5. ^ Lacy BE, Rosemore J (October 2001). "Helicobacter pylori: ulcers and more: the beginning of an era" (abstract page). J. Nutr. 131 (10): 2789S–2793S.  
6. ^ Malo AF, Gomendio M, Garde J, Lang-Lenton B, Soler AJ, Roldan ER (June 2006). "Sperm design and sperm function". Biol. Lett. 2 (2): 246–9.  
7. ^ Haimo LT, Rosenbaum JL (December 1981). "Cilia, flagella, and microtubules". J. Cell Biol. 91 (3 Pt 2): 125s–130s.  
8. ^ Silverman M, Simon M (1974). "Flagellar rotation and the mechanism of bacterial motility". Nature 249 (452): 73–74.  
9. ^ Meister GLM, Berg HC (1987). "Rapid rotation of flagellar bundles in swimming bacteria". Nature 325 (6105): 637–640.  
10. ^ Berg HC, Anderson RA (1973). "Bacteria Swim by Rotating their Flagellar Filaments". Nature 245 (5425): 380–382.  
11. ^ Jahn TL, Bovee EC (1965). "Movement and Locomotion of Microorganisms". Annual Review of Microbiology 19: 21–58.  
12. ^ Harshey RM (2003). "Bacterial Motility on a Surface: Many Ways to a Common Goal". Annual Review of Microbiology 57: 249–273.  
13. ^ Ng SY, Chaban B, Jarrell KF (2006). "Archaeal flagella, bacterial flagella and type IV pili: a comparison of genes and posttranslational modifications". J. Mol. Microbiol. Biotechnol. 11 (3–5): 167–91.  
14. ^ Metlina AL (2004). "Bacterial and archaeal flagella as prokaryotic motility organelles". Biochemistry Mosc. 69 (11): 1203–12.  
15. ^ Jarrell (2009). "Archaeal Flagella and Pili". Pili and Flagella: Current Research and Future Trends. Caister Academic Press.  
16. ^ A Dictionary of Biology, 2004, accessed 2011-01-01.
17. ^ Macnab RM (2003). "How bacteria assemble flagella". Annu. Rev. Microbiol. 57: 77–100.  
18. ^ Diószeghy Z, Závodszky P, Namba K, Vonderviszt F (2004). "Stabilization of flagellar filaments by HAP2 capping". FEBS Lett. 568 (1–3): 105–9.  
19. ^ Galkin VE, Yu X, Bielnicki J, Heuser J, Ewing CP, Guerry P, Egelman EH. (2008). "Divergence of quaternary structures among bacterial flagellar filaments". Science 320 (5874): 382–5.  
20. ^ Atsumi T, McCarter L, Imae Y. (1992). "Polar and lateral flagellar motors of marine Vibrio are driven by different ion-motive forces". Nature 355 (6356): 182–4.  
21. ^ Dean, Tim. "Inside nature’s most efficient motor: the flagellar", Australian Life Scientist, 2 August 2010. Retrieved on 2013-08-28.
22. ^ Unlocking the secrets of nature's nanomotor Nikkei Asian Review, 2014.
23. ^ Dusenbery DB (2009). "Chapter 13". Living at Micro Scale: The Unexpected Physics of Being Small. Cambridge: Harvard University Press.  
24. ^ Hildebrand, Milton (November 1959). "Motions of the running Cheetah and Horse".   Although according to Cheetah, Luke Hunter and Dave Hamman, (Struik Publishers, 2003), pp. 37–38, the cheetah's fastest recorded speed was 110 km/h (68 mph).
25. ^ Minamino T, Imada K, Namba K. (2008). "Mechanisms of type III protein export for bacterial flagellar assembly". Mol. Biosyst. 4 (11): 1105–15.  
26. ^ Asakura S, Eguchi G, Iino T. (1964). "Reconstitution of Bacterial Flagella in Vitro". J. Mol. Biol. 10: 42–56.  
27. ^ Rajagopala, S. V.; Titz, B. R.; Goll, J.; Parrish, J. R.; Wohlbold, K.; McKevitt, M. T.; Palzkill, T.; Mori, H.; Finley Jr, R. L.; Uetz, P. (2007). "The protein network of bacterial motility". Molecular Systems Biology 3: 128.  
28. ^ Kim M, Bird JC, Van Parys AJ, Breuer KS, Powers TR (December 2003). "A macroscopic scale model of bacterial flagellar bundling". Proc. Natl. Acad. Sci. U.S.A. 100 (26): 15481–5.  
29. ^ Macnab RM (January 1977). "Bacterial flagella rotating in bundles: a study in helical geometry". Proc. Natl. Acad. Sci. U.S.A. 74 (1): 221–5.  
30. ^ Kim YK, McCarter LL (2000). "Analysis of the Polar Flagellar Gene System of Vibrio parahaemolyticus". Journal of Bacteriology 182 (13): 3693–3704.  
31. ^ Atsumi T, Maekawa Y, Yamada T, Kawagishi I, Imae Y, Homma M (1 August 1996). "Effect of viscosity on swimming by the lateral and polar flagella of Vibrio alginolyticus". Journal of Bacteriology 178 (16): 5024–5026.  
32. ^ McCarter LL (2004). "Dual Flagellar Systems Enable Motility under Different Circumstances". Journal of Molecular Microbiology and Biotechnology 7 (1–2): 18–29.  
33. ^ Merino S, Shaw JG, Tomás JM. (2006). "Bacterial lateral flagella: an inducible flagella system". FEMS Microbiol Lett 263 (2): 127–35.  
34. ^ Belas R, Simon M, Silverman M. (1986). "Regulation of lateral flagella gene transcription in Vibrio parahaemolyticus". J Bacteriol 167 (1): 210–8.  
35. ^ Canals R, Altarriba M, Vilches S, Horsburgh G, Shaw JG, Tomás JM, Merino S (2006). "Analysis of the Lateral Flagellar Gene System of Aeromonas hydrophila AH-3". Journal of Bacteriology 188 (3): 852–862.  
36. ^ Canals R, Ramirez S, Vilches S, Horsburgh G, Shaw JG, Tomás JM, Merino S (January 2006). "Polar Flagellum Biogenesis in Aeromonas hydrophila". J. Bacteriol. 188 (2): 542–55.  
37. ^ ^{a b} Cavalier-Smith T (1987). "The origin of eukaryotic and archaebacterial cells". Ann. N. Y. Acad. Sci. 503 (1): 17–54.  
38. ^ ^{a b} Ghosh A, Albers SV (January 2011). "Assembly and function of the archaeal flagellum". Biochem. Soc. Trans. 39 (1): 64–9.  
39. ^ Berg, Howard C. (2003). E. coli in motion (1. Aufl. ed.). New York: Springer.  
40. ^ Satir P, Christensen ST (June 2008). "Structure and function of mammalian cilia". Histochem. Cell Biol. 129 (6): 687–93.  
41. ^ Pazour GJ (October 2004). "Intraflagellar transport and cilia-dependent renal disease: the ciliary hypothesis of polycystic kidney disease". J. Am. Soc. Nephrol. 15 (10): 2528–36.  

External links

• "Molecular Machines Museum Index". Access Research Network. 2001. Retrieved 2008-05-18. 
• Berg, Howard C. (January 2000). "Motile Behavior of Bacteria". Physics Today 53 (1): 24.  
• Charles Lindemann (2008-04-04). "Mechanisms of sperm motility". Oakland University. Retrieved 2008-05-18. 
• Purcell, E.M. (1977). "Life at Low Reynolds Number" (PDF). American Journal of Physics 45 (1): 3–11.  
• N. J. Matzke (2003-11-10). "Evolution in (Brownian) space: a model for the origin of the bacterial flagellum". www.talkdesign.org. 
• "What is flagella". 

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• Cell movement
• Organelles
• Protein complexes

• Monotrichous bacteria have a single flagellum (e.g., Vibrio cholerae).
• Lophotrichous bacteria have multiple flagella located at the same spot on the bacteria's surfaces which act in concert to drive the bacteria in a single direction. In many cases, the bases of multiple flagella are surrounded by a specialized region of the cell membrane, the so-called polar organelle.
• Amphitrichous bacteria have a single flagellum on each of two opposite ends (only one flagellum operates at a time, allowing the bacteria to reverse course rapidly by switching which flagellum is active).
• Peritrichous bacteria have flagella projecting in all directions (e.g., E. coli).

Different species of bacteria have different numbers and arrangements of flagella.

Examples of bacterial flagella arrangement schemes. A-Monotrichous; B-Lophotrichous; C-Amphitrichous; D-Peritrichous.

Flagellar arrangement schemes

Evolution. The evolution of bacterial flagella has been used as an argument against evolution by creationists. They argue that complex structures like flagella cannot evolve from simple structures. In other words, flagella are "irreducibly complex" and need all of their protein components to function. However, it has been shown by numerous studies that a large number of proteins can be deleted without (complete) loss of function.^[27] Moreover, it is generally accepted now that bacterial flagella have evolved from much simpler secretion systems, such as the Type III secretion system.

Assembly. During flagellar assembly, components of the flagellum pass through the hollow cores of the basal body and the nascent filament. During assembly, protein components are added at the flagellar tip rather than at the base.^[25] In vitro, flagellar filaments assemble spontaneously in a solution containing purified flagellin as the sole protein.^[26]

Through use of their flagella, E. coli are able to move rapidly towards attractants and away from repellents. They do this by means of a biased random walk, with 'runs' and 'tumbles' brought about by rotating the flagellum counterclockwise and clockwise respectively.

The rotational speed of flagella varies in response to the intensity of the proton motive force, thereby permitting certain forms of speed control, and also permitting some types of bacteria to attain remarkable speeds in proportion to their size; some achieve roughly 60 cell lengths / second. Although at such a speed it would take a bacterium about 245 days to cover a kilometre, and although that may seem slow, the perspective changes when the concept of scale is introduced. In comparison to macroscopic life forms it is very fast indeed when expressed in terms of number of body lengths per second. A cheetah for example, only achieves about 25 body lengths / sec.^[24]

The cylindrical shape of flagella is suited to locomotion of microscopic organisms; these organisms operate at a low Reynolds number, where the viscosity of the surrounding water is much more important than its mass or inertia.^[23]

Motor. The bacterial flagellum is driven by a rotary engine (the Mot complex) made up of protein, located at the flagellum's anchor point on the inner cell membrane. The engine is powered by proton motive force, i.e., by the flow of protons (hydrogen ions) across the bacterial cell membrane due to a concentration gradient set up by the cell's metabolism (in Vibrio species there are two kinds of flagella, lateral and polar, and some are driven by a sodium ion pump rather than a proton pump^[20]). The rotor transports protons across the membrane, and is turned in the process. The rotor alone can operate at 6,000 to 17,000 rpm, but with the flagellar filament attached usually only reaches 200 to 1000 rpm. The direction of rotation can be switched almost instantaneously, caused by a slight change in the position of a protein, FliG, in the rotor.^[21] The flagellum is highly energy efficient and uses very little energy.^[22]

The basal body has several traits in common with some types of secretory pores, such as the hollow rod-like "plug" in their centers extending out through the plasma membrane. Given the structural similarities between bacterial flagella and bacterial secretory systems, it is thought that bacterial flagella may have evolved from the type three secretion system; however, it is not known for certain whether these pores are derived from the bacterial flagella or the bacterial secretory system.

The flagellar filament is the long helical screw that propels the bacterium when rotated by the motor, through the hook. In most bacteria that have been studied, including the Gram negative Escherichia coli, Salmonella typhimurium, Caulobacter crescentus, and Vibrio alginolyticus, the filament is made up of eleven protofilaments approximately parallel to the filament axis. Each protofilament is a series of tandem protein chains. However in Campylobacter jejuni, there are seven protofilaments.^[19]

Structure and composition. The bacterial flagellum is made up of the L ring associates with the lipopolysaccharides, the P ring associates with peptidoglycan layer, the M ring is embedded in the plasma membrane, and the S ring is directly attached to the plasma membrane. The filament ends with a capping protein.^[17][18]

Physical model of a bacterial flagellum

Bacterial

• Bacterial flagella are helical filaments, each with a rotary motor at its base which can turn clockwise or counterclockwise.^[8][9][10] They provide two of several kinds of bacterial motility.^[11][12]
• Archaeal flagella (Archaella) are superficially similar to bacterial flagella, but are different in many details and considered non-homologous.^[13][14][15]
• Eukaryotic flagella - those of animal, plant, and protist cells - are complex cellular projections that lash back and forth. Eukaryotic flagella are classed along with eukaryotic motile cilia as undulipodia^[16] to emphasize their distinctive wavy appendage role in cellular function or motility. Primary cilia are immotile, and are not undulipodia; they have a structurally different 9+0 axoneme rather than the 9+2 axoneme found in both flagella and motile cilia undulipodia.

The main differences among these three types are summarized below:

Three types of flagella have so far been distinguished; bacterial, archaeal and eukaryotic.

Types

An example of a flagellate bacterium is the ulcer-causing Helicobacter pylori, which uses multiple flagella to propel itself through the mucus lining to reach the stomach epithelium.^[5] An example of a eukaryotic flagellate cell is the mammalian sperm cell, which uses its flagellum to propel itself through the female reproductive tract.^[6] Eukaryotic flagella are structurally identical to eukaryotic cilia, although distinctions are sometimes made according to function and/or length.^[7]

Flagella are organelles defined by function rather than structure. There are large differences between different types of flagella; the prokaryotic and eukaryotic flagella differ greatly in protein composition, structure, and mechanism of propulsion. However, both are used for swimming. ^[4][3][2][1]