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• W2008298900 abstract "What does the motor do? It rotates a thin helical filament (a propeller) that powers swimming motility. There are one or several motors per cell, each with its own filament. The filaments extend out from the surface of the cell into the external medium (in common bacteria) or run beneath the cell's outer membrane (in spirochetes). As the helical axis of the filament generally runs at an angle different from that of the motor axis, the two are coupled by a flexible coupling or universal joint (the hook). What does the motor look like? In electron micrographs, it appears as a series of rings mounted on a rod surrounded by an array of studs embedded in the inner cell membrane (Figure 1). These sub-assemblies are built from about 20 different kinds of parts (proteins), each with a distinct name (given in Figure 1). The names (also applied to the genes that encode the proteins) were devised by bacterial geneticists according to the defects that resulted from null mutations, for example, fla (no flagellum) or mot (flagellated but non-motile). Genes with similar mutant phenotypes were labeled A, B, C, and so on. Eventually, fla became flg, flh, fli, and flj, because there turned out to be more fla genes than letters in the alphabet. The third letter (g, h, i, j) denotes different clusters of fla genes on the chromosome. How is the motor assembled? The motor is built from the inside out, component-by-component. The MS- and C-rings are assembled, a transport apparatus is added that controls the export of the axial structures, and then these are assembled in the order: rod, hook, hook-associated proteins, cap and filament. Genes encoding these components are expressed in a similar sequence. P- and L-ring proteins transit the inner membrane by a different export pathway. There are a number of checks and balances in this process, the most dramatic of which involves an antibody-like factor that blocks expression of late genes, which encode the filament protein FliC, the Mot proteins A and B, and the various components of the chemotaxis pathway. When motor assembly reaches the level of the hook, this factor is pumped out of the cell by the flagellar transport apparatus, relieving suppression of late-gene transcription. At about the same time, the export apparatus switches from transport of components of the rod and hook to the hook-associated proteins and filament. Ingenious mechanisms are involved in supplying raw material at the base of the motor, in rod and hook-length control, and in pumping hook and filament subunits through a 2 nm pore along the motor axis. In Escherichia coli and Salmonella, the energy required for this export is supplied by an electrochemical proton gradient (protonmotive force). Remarkably, the filament grows at its distal end. Does the motor turn only one way? No, it spins either clockwise (CW) or counterclockwise (CCW) at about the same speed. These are the directions that you see when looking down at the drive shaft as it emerges from the cell wall. In the best-studied system (the gram-negative bacterium E. coli), the motor changes direction on average about once per second (in the absence of an external stimulus.) The intervals between reversals are exponentially distributed: there is a constant probability of reversal per unit time. How does the filament generate thrust? The viscous drag on a thin stick in water is about twice as large when it moves sideways than when it moves lengthwise. Propulsion depends upon this asymmetry. In common bacteria, the normal filament is long and thin: the pitch of the helix is about five times its diameter. Think of this helix as a series of sticks moving slantwise in the aqueous medium, rotate the helix about its long axis, and add-up the forces due to the viscous drag on each stick. You will find that the helix generates both thrust and torque (axial force and circumferential twist). Suppose you hold such a helix in front of you and turn it CW: it will push you backwards and try to twist you CCW. An observer looking at you from the far end of the helix will see the filament rotate CCW, your body rotate more slowly CW, and the pair of you move off into the distance. But to do this experiment in a manner relevant to a bacterium, you would have to immerse yourself in a medium much more viscous than water, such as molasses. Why molasses? On the scale of a bacterium, inertial forces (required to accelerate masses) are roughly a million times smaller than viscous forces (required to shear fluids — to move adjacent layers of a fluid at different speeds). A number that characterizes the relative importance of these forces (the Reynolds number, R) is the product of cell size, cell speed and fluid density, divided by viscosity. For swimming E. coli, R ∼ 10−5. To do an experiment at low Reynolds number when size and speed are both large, the viscosity must be large: hence, molasses. An important corollary: the flagellar motor knows nothing about inertia; it does not have a flywheel. If it puts in the clutch, it coasts to a stop within about a millionth of a revolution. Is the propeller of variable pitch? Yes, the filament, a polymer of a single protein (called flagellin, or FliC), can change from one helical form to another. These changes, called polymorphic transformations, rapidly propagate from one end of the filament to the other. When the motor switches from CW to CCW, the change in torsion unwinds the normal filament, which is left-handed (pitch ∼2.5 μm, diameter ∼0.5 μm), into a form called semi-coiled, which is right-handed (roughly half the normal pitch but normal amplitude), and then into a form called curly, which also is right-handed (roughly half the normal pitch and also half the normal amplitude). When the motor switches back to CCW, the curly filament relaxes back to normal. Why does the direction of rotation change? So that the cell can alter course and move toward regions that it deems more favorable. Cells respond to temporal changes in chemical composition, osmotic stress, intensity of light, temperature, and so on. Response to chemicals (towards attractants or away from repellents) is the best-understood sensory modality, and is called chemotaxis. This term is a misnomer because cells do not steer towards a source of attractant (or away from a source of repellent), they simply back up or try new directions at random. Then they decide whether life is getting better or worse. If it's getting better, they tend to keep going in the same direction. If it's getting worse, they don't worry about it; they go back to what they were doing in the absence of a stimulus. In E. coli, which is pushed by several normal filaments rotating in parallel, the change in course occurs as one or more motors switch from CCW to CW and their respective filaments transform from normal to semi-coiled. What triggers changes in the direction of rotation? Switching is essentially a thermal isomerization. Imagine two energy wells, one for the CW state and the other for the CCW state, separated by a barrier. The system hops across this barrier from one state to the other about once per second. The switching is influenced by binding of various ligands, including the phosphorylated chemotaxis signaling molecule CheY–P. When this protein binds to FliM and FliN, the CW state is stabilized. When the cell swims up a spatial gradient of a chemical attractant, less CheY–P is made, less binds to FliM and FliN, and the motor spends more time spinning CCW, which is the direction that promotes smooth swimming. So the cell tends to continue in a favorable direction. Motor reversals are rapid, approximately all-or-none events — there can be pauses — so it is unlikely that the several force generators are controlled independently. Evidently, the rotor snaps into a different conformation, like a pop-up toy, changing the orientation of FliG relative to MotA in such a way that the force exerted by MotA on FliG changes sign. What drives motor rotation? An ion flux, either of protons (as in E. coli) or of sodium ions (as in the polar flagella of Vibrio species). As noted in the Figure 1 legend, there are a number of independent force-generating elements, each comprising four copies of MotA and two copies of MotB, bolted to the rigid framework of the cell wall (via the carboxyl terminus of MotB). These elements exert force at the periphery of the rotor via an interaction between the cytoplasmic part of MotA and the rotor component FliG. No one knows precisely how this works yet, because we do not have atomic structures of MotA and MotB. But for E. coli it is clear that protonation and deprotonation of an essential aspartate residue near the cytoplasmic end of each proton channel drives conformational changes that cause MotA to move from one FliG subunit to the next (of which there are ∼26). This is rather like myosin or kinesin ratcheting along an actin filament or a microtubule (driven by the hydrolysis of ATP). Is the motor a thermal ratchet? No, the torque-speed relationship, which is flat out to relatively high speeds and then declines sharply, is what one expects for a mechanism in which energy is extracted from the ion gradient continuously as an ion moves down its electrochemical gradient, rather than by a sudden jump between states that are populated (or depopulated) through the motion of ions down an electrochemical gradient. In particular, it is possible to drive the motor backwards by applying torque externally. There is no barrier to backwards rotation, a barrier that one would expect were the motor waiting for proton motion to disengage a ratchet. How fast does the motor spin? At room temperature, E. coli drives its flagellar bundle ∼100 Hz. The cell body counter-rotates ∼25 Hz, so the motors spin ∼125 Hz. The motor can drive a 60 nm gold sphere ∼300 Hz (when linked to a hook in the absence of the filament). Motors driven by sodium ions spin about five times faster. But we know that the mechanisms used by protons and sodium ions are similar: if an E. coli motor is constructed in which all of MotA and MotB (except for the part of MotB that anchors the force generator to the peptidoglycan) is replaced by PomA and PomB (homologous components from a Vibrio species), then the hybrid motor runs on sodium ions rather than protons. Does the motor rotate continuously or take discrete steps? Steps of ∼26 per revolution have been measured in hybrid motors driven by a single force generator at very low torque and speed. The motion is much smoother with a full complement of force generators. How much power does the motor generate? With E. coli at room temperature, the torque is about 1300 pN nm and the filaments spin about 125 Hz (2π × 125 radians/s). The power output is the product of the two, ∼106 pN nm/s = 10−15 J/s or, if you will, ∼1.3 × 10−18 horsepower. That sounds negligible, but the motor is very small (shaped like a cylinder about 50 nm in diameter by 50 nm long), with a volume ∼105 nm3. Given a protein density of 1.3 gm/cm3, that works out to about 1.3 × 10−16 gm, or 2.9 × 10−19 lb. So the power output is nearly 5 hp/lb. That's roughly the power per pound generated by a turboprop airplane engine. If you work out the force generated by each MotA–MotB complex, you will find that it is quite small, roughly the force between two electrons in a medium of dielectric constant 20 about 1 nm apart. So almost any kind of chemical interaction will do. Does the motor get hot? No, the motor is water-cooled and thermal diffusion is very efficient over small distances, so its temperature remains very close to ambient. Is the motor good for anything? If you are a bacterium, a great deal: a lot of energy is expended in building such a machine so that a cell can find essential nutrients. For humans, very little so far, except to illustrate how extraordinary nanotechnology can be. For some bacteria, motility plays an important role in pathogenesis. In such cases, the motor might be a good drug target. Is the flagellar motor unique? Yes and no. As a device that powers flagellar rotation, yes. As a device composed of rings, rods, and external filaments, no. There is a homologous structure, called the needle structure, assembled by the same kind of transport apparatus, used by pathogenic species (such as Salmonella) to inject virulence factors into eukaryotic cells. Some argue that the flagellar rotary motor evolved from the needle structure, but it was probably the other way around, since flagellated bacteria existed long before their eukaryotic targets. Perhaps they evolved from a common ancestor. What was the rotary motor doing before the helical propeller was invented, if indeed that was the order of events? Serving as a secretory apparatus that acquired the ability to spin? Packaging polynucleic acids into virus heads? Food for thought." @default.
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• W2008298900 date "2008-08-01" @default.
• W2008298900 modified "2023-10-12" @default.
• W2008298900 title "Bacterial flagellar motor" @default.
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