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ScienceWeek
CELL BIOLOGY: ON THE COORDINATION OF FLAGELLA
The following points are made by K.A. Wemmer and W.F. Marshall (Current Biology 2004 14:R992):
1) Flagella are microtubule-based structures that propel biological cells through the surrounding fluid. The internal structure of a flagellum consists of nine parallel doublet microtubules arranged around a central pair of singlet microtubules. Force for propulsion is provided by thousands of dynein motors anchored in rows along one side of each doublet, which can walk along the microtubule of the adjacent doublet. In order to produce coordinated bending of the flagellum, these dynein motors -- organized into multi-headed complexes called the inner and outer dynein arms -- must produce their power strokes in synchrony, like the oarsmen on an ancient Mediterranean war-galley. But whereas oar-strokes were coordinated by a continuous drum-beat, it is much less clear how flagellar dynein motors are synchronized.
2) One possibility is that the dynein motors can synchronize themselves. An extensive theory exists for spontaneous entrainment of coupled oscillators. This theory shows that if a system of oscillators are connected such that the phase of an oscillator shifts according to the phase difference between itself and the other oscillators, the entire system will spontaneously synchronize under a wide range of coupling parameters, resulting in coherent behavior [1]. This emergent property of coupled-oscillator systems is thought to explain the synchronized flashing of fire-flies and the beating of the heart.
3) Motor proteins are oscillators which undergo coupled cycles of movement and ATP hydrolysis, and it is known that mechanical forces applied to a motor can affect the rate of progress through the ATP hydrolysis cycle. For example, force-dependent oscillatory behavior has been shown for flagellar dyneins [2]. A system of dynein motors mechanically connected via the flagellar microtubules could, therefore, potentially undergo spontaneous entrainment leading to coherent bending movement. This type of model predicts that bending forces applied to a flagellum should alter the activity of dynein arms.
4) In a recent study, Morita and Shingyoji [3] directly applied bending forces to sea urchin sperm flagella. They used the dynein-driven fragmentation of elastase-treated sea urchin flagella as a reporter for dynein arm activity. Wherever dynein arms were activated, the doublets slid apart, providing a visual indicator of which dynein rows were active. Using this assay, these workers found that an applied bending force activated dynein arms that were inactive in unbent flagella. Their observations support the general idea that force-feedback alters dynein motor activity.
5) But how is a bending force transduced to the dynein arms in a flagellum? One possibility, mentioned above, is that the transduction involves direct mechanical feedback, whereby the dynein arms can sense the applied force. There is substantial evidence, however, that the central pair may play a role in controlling dynein arm activity, which raises the possibility that this structure might regulate dynein in response to bending.[4,5]
References (abridged):
1. Strogatz, S.H. (1994). Nonlinear dynamics and chaos: with applications in physics, biology, chemistry, and engineering. (New York: Addison Wesley)
2. Shingyoji, C., Higuchi, H., Yoshimura, M., Katayama, E. and Yanagida, T. (1998). Dynein arms are oscillating force generators. Nature 393, 711-714
3. Morita, Y. and Shingyoji, C. (2004). Effects of imposed bending on microtubule sliding in sperm flagella. Curr. Biol. 14:2113
4. Nakano, I., Kobayashi, T., Yoshimura, M. and Shingyoji, C. (2003). Central-pair-linked regulation of microtubule sliding by calcium in flagellar axonemes. J. Cell Sci. 116, 1627-1636
5. Wargo, M.J. and Smith, E.F. (2003). Asymmetry of the central apparatus defines the location of active microtubule sliding in Chlamydomonas flagella. Proc. Natl. Acad. Sci. U.S.A. 100, 137-142
Current Biology http://www.current-biology.com
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Related Material:
MICROBIOLOGY: STRUCTURE OF THE BACTERIAL FLAGELLAR FILAMENT
Notes by ScienceWeek:
Eukaryotes and prokaryotes (bacteria) both use flagella for movement, but the microstructures of the flagella are not the same, and the molecular motor mechanisms are different.
The following points are made by Koji Yonekura (Nature 2003 424:643):
1) Bacteria swim by rotating helical flagellar filaments, which grow as long as 15 microns, but the diameter is only 120-250 angstroms. The rotary motor at the base of the filament drives the rotation of this helical propeller(1,2) at hundreds of revolutions per second(3,4).
2) For chemotaxis and thermotaxis, the swimming pattern of bacteria such as Salmonella and Escherichia coli alternates between "run" and "tumble"; a run lasts for a few seconds and a tumble for a fraction of second. During a run, the motor rotates anti-clockwise (as it is viewed from outside the cell), and several flagellar filaments with a left-handed helical shape form a bundle and propel the cell. A tumble is caused by quick reversal of the motor to clockwise rotation(5), which produces a twisting force that transforms the left-handed helical form of the filament into a right-handed one, causing the bundle to fall apart rapidly. The separated filaments act in an uncoordinated way to generate forces that change the orientation of the cell. Thus, the structure of the flagellar filament and its dynamic properties have an essential role in bacterial taxis.
3) The filament is a helical assembly of flagellin with roughly 11 subunits per two turns of the 1-start helix; it can also be described as a tubular structure comprising 11 protofilaments, which are nearly longitudinal helical arrays of subunits. Left-and right-handed helical forms are produced by supercoiling caused by a mixture of two distinct protofilament conformations, L- and R-type. When all 11 protofilaments are of the same type, two types of straight filaments with distinct helical symmetries are formed: the longitudinal 11-start helix is left-handed in the L-type and right-handed in the R-type straight filament.
4) Electron cryomicroscopy and X-ray fibre diffraction have revealed the domain organization of flagellin and subunit packing in these two straight filaments at about 10 resolution; however, higher resolution is needed to understand the structural basis of the filament formation and supercoiling in atomic detail. Flagellin has a strong tendency to polymerize into filaments and this has prevented its crystallization.
5) In summary: The bacterial flagellar filament is a helical propeller for bacterial locomotion. It is a helical assembly of a single protein, flagellin, and its tubular structure is formed by 11 protofilaments in two distinct conformations, L- and R-type, for supercoiling. The X-ray crystal structure of a flagellin fragment lacking about 100 terminal residues has revealed the protofilament structure, but the full filament structure is still essential for understanding the mechanism of supercoiling and polymerization. The authors report a complete atomic model of the R-type filament by electron cryomicroscopy. A density map obtained from image data up to 4 angstroms resolution shows the feature of alpha-helical backbone and some large side chains. The atomic model built on the map reveals intricate molecular packing and an alpha-helical coiled coil formed by the terminal chains in the inner core of the filament, with its intersubunit hydrophobic interactions having an important role in stabilizing the filament.
1. Berg, H. C. & Anderson, R. A. Bacteria swim by rotating their flagellar filaments. Nature 245, 380-382 (1973)
2. Silverman, M. & Simon, M. Flagellar rotation and the mechanism of bacterial motility. Nature 249, 73-74 (1974)
3. Kudo, S., Magariyama, Y. & Aizawa, S.-I. Abrupt changes in flagellar rotation observed by laser darkfield microscopy. Nature 346, 677-680 (1990)
4. Ryu, W. S., Berry, R. M. & Berg, H. C. Torque-generating units of the flagellar motor of Escherichia coli have a high duty ratio. Nature 403, 444-447 (2000)
5. Larsen, S. H., Reader, R. W., Kort, E. N., Tso, W. W. & Adler, J. Change in direction of flagellar rotation is the basis of the chemotactic response in Escherichia coli. Nature 249, 74-77 (1974)
Nature http://www.nature.com/nature
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Related Material:
CRYSTALLOGRAPHIC STRUCTURE OF FLAGELLIN
The following points are made by F.A. Samatey et al (Nature 2001 410:331):
1) Bacteria swim by rotating helical flagellar filaments, which are up to 15 microns long, but only 120 to 250 angstroms in diameter. The rotary motor at the base of the filament drives the rotation of this helical propeller at hundreds of revolutions per second. For chemotaxis and thermotaxis, the swimming pattern of bacteria such as Salmonella and Escherichia coli alternates between "run" and "tumble": a run lasts for a few seconds and a tumble for a fraction of a second. During a run, the motor rotates counterclockwise (as it is viewed from outside the cell), and several flagellar filaments with a left-handed helical shape form a bundle and propel the cell. A tumble is caused by quick reversal of the motor to clockwise rotation, which produces a twisting force that transforms the left-handed helical form of the filament into a right-handed one, causing the bundle to rapidly fall apart.
2) The separated filaments act in an uncoordinated way to generate forces that change the orientation of the bacterial cell. Thus, the structure of the flagellar filament and its dynamic properties have an essential role in bacterial taxis. The flagellar filament is constructed from 11 protofilaments of a single protein, flagellin. The filament switches between left-and right-handed supercoiled forms when bacteria switch their swimming mode between running and tumbling. Supercoiling is produced by two different packing interactions of flagellin called L and R. In switching from L to R, the intersubunit distance (approximately 52 angstroms) along the protofilament decreases by 0.8 angstroms. Changes in the number of L and R protofilaments govern supercoiling of the filament. The authors report crystallographic evidence for identification of possible switch regions responsible for the bi-stable mechanical switch that generates the 0.8 angstrom difference in repeat distance.
Nature http://www.nature.com/nature
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