The Limiting Speed of the Bacterial Flagellar Motor.
Ontology highlight
ABSTRACT:
SUBMITTER: Nirody JA
PROVIDER: S-EPMC4982938 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:Many swimming bacteria are propelled by flagellar filaments driven by a rotary motor. Each of these tiny motors can generate an impressive torque. The motor torque vs. speed relationship is considered one of the most important measurable characteristics of the motor and therefore is a major criterion for judging models proposed for the working mechanism. Here we give an explicit explanation for this torque-speed curve. The same physics also can explain certain puzzling properties of other motors.
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Project description:Physiological properties of the flagellar rotary motor have been taken to indicate a tightly coupled mechanism in which each revolution is driven by a fixed number of energizing ions. Measurements that would directly test the tight-coupling hypothesis have not been made. Energizing ions flow through membrane-bound complexes formed from the proteins MotA and MotB, which are anchored to the cell wall and constitute the stator. Genetic and biochemical evidence points to a "power stroke" mechanism in which the ions interact with an aspartate residue of MotB to drive conformational changes in MotA that are transmitted to the rotor protein FliG. Each stator complex contains two separate ion-binding sites, raising the question of whether the power stroke is driven by one, two, or either number of ions. Here, we describe simulations of a model in which the conformational change can be driven by either one or two ions. This loosely coupled model can account for the observed physiological properties of the motor, including those that have been taken to indicate tight coupling; it also accords with recent measurements of motor torque at high load that are harder to explain in tight-coupling models. Under loads relevant to a swimming cell, the loosely coupled motor would perform about as well as a two-proton motor and significantly better than a one-proton motor. The loosely coupled motor is predicted to be especially advantageous under conditions of diminished energy supply, or of reduced temperature, turning faster than an obligatorily two-proton motor while using fewer ions.
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Project description:The bacterial flagellum is a supramolecular motility machine consisting of the basal body as a rotary motor, the hook as a universal joint, and the filament as a helical propeller. Intact structures of the bacterial flagella have been observed for different bacterial species by electron cryotomography and subtomogram averaging. The core structures of the basal body consisting of the C ring, the MS ring, the rod and the protein export apparatus, and their organization are well conserved, but novel and divergent structures have also been visualized to surround the conserved structure of the basal body. This suggests that the flagellar motors have adapted to function in various environments where bacteria live and survive. In this review, we will summarize our current findings on the divergent structures of the bacterial flagellar motor.
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Project description:Mechanosensing by flagella is thought to trigger bacterial swarmer-cell differentiation, an important step in pathogenesis. How flagellar motors sense mechanical stimuli is not known. To study this problem, we suddenly increased the viscous drag on motors by a large factor, from very low loads experienced by motors driving hooks or hooks with short filament stubs, to high loads, experienced by motors driving tethered cells or 1-?m latex beads. From the initial speed (after the load change), we inferred that motors running at very low loads are driven by one or at most two force-generating units. Following the load change, motors gradually adapted by increasing their speeds in a stepwise manner (over a period of a few minutes). Motors initially spun exclusively counterclockwise, but then increased the fraction of time that they spun clockwise over a time span similar to that observed for adaptation in speed. Single-motor total internal reflection fluorescence imaging of YFP-MotB (part of a stator force-generating unit) confirmed that the response to sudden increments in load occurred by the addition of new force-generating units. We estimate that 6-11 force-generating units drive motors at high loads. Wild-type motors and motors locked in the clockwise or counterclockwise state behaved in a similar manner, as did motors in cells deleted for the motor protein gene fliL or for genes in the chemotaxis signaling pathway. Thus, it appears that stators themselves act as dynamic mechanosensors. They change their structure in response to changes in external load. How such changes might impact cellular functions other than motility remains an interesting question.
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Project description:Switching of flagellar motor rotation sense dictates bacterial chemotaxis. Multi-subunit FliM-FliG rotor rings couple signal protein binding in FliM with reversal of a distant FliG C-terminal (FliGC) helix involved in stator contacts. Subunit dynamics were examined in conformer ensembles generated by molecular simulations from the X-ray structures. Principal component analysis extracted collective motions. Interfacial loop immobilization by complex formation coupled elastic fluctuations of the FliM middle (FliMM) and FliG middle (FliGM) domains. Coevolved mutations captured interfacial dynamics as well as contacts. FliGM rotation was amplified via two central hinges to the FliGC helix. Intrinsic flexibility, reported by the FliGMC ensembles, reconciled conformers with opposite FliGC helix orientations. FliG domain stacking deformed the inter-domain linker and reduced flexibility; but conformational changes were not triggered by engineered linker deletions that cause a rotation-locked phenotype. These facts suggest that binary rotation states arise from conformational selection by stacking interactions.
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Project description:The high-resolution structures of nearly all the proteins that comprise the bacterial flagellar motor switch complex have been solved; yet a clear picture of the switching mechanism has not emerged. Here, we used NMR to characterize the interaction modes and solution properties of a number of these proteins, including several soluble fragments of the flagellar motor proteins FliM and FliG, and the response-regulator CheY. We find that activated CheY, the switch signal, binds to a previously unidentified region of FliM, adjacent to the FliM-FliM interface. We also find that activated CheY and FliG bind with mutual exclusivity to this site on FliM, because their respective binding surfaces partially overlap. These data support a model of CheY-driven motor switching wherein the binding of activated CheY to FliM displaces the carboxy-terminal domain of FliG (FliGC) from FliM, modulating the FliGC-MotA interaction, and causing the motor to switch rotational sense as required for chemotaxis.
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Project description:Multisubunit protein complexes are ubiquitous in biology and perform a plethora of essential functions. Most of the scientific literature treats such assemblies as static: their function is assumed to be independent of their manner of assembly, and their structure is assumed to remain intact until they are degraded. Recent observations of the bacterial flagellar motor, among others, bring these notions into question. The torque-generating stator units of the motor assemble and disassemble in response to changes in load. Here, we used electrorotation to drive tethered cells forward, which decreases motor load, and measured the resulting stator dynamics. No disassembly occurred while the torque remained high, but all of the stator units were released when the motor was spun near the zero-torque speed. When the electrorotation was turned off, so that the load was again high, stator units were recruited, increasing motor speed in a stepwise fashion. A model in which speed affects the binding rate and torque affects the free energy of bound stator units captures the observed torque-dependent stator assembly dynamics, providing a quantitative framework for the environmentally regulated self-assembly of a major macromolecular machine.
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Project description:Bacteria sense temporal changes in extracellular stimuli via sensory signal transducers and move by rotating flagella towards into a favorable environment for their survival. Each flagellum is a supramolecular motility machine consisting of a bi-directional rotary motor, a universal joint and a helical propeller. The signal transducers transmit environmental signals to the flagellar motor through a cytoplasmic chemotactic signaling pathway. The flagellar motor is composed of a rotor and multiple stator units, each of which acts as a transmembrane proton channel to conduct protons and exert force on the rotor. FliG, FliM and FliN form the C ring on the cytoplasmic face of the basal body MS ring made of the transmembrane protein FliF and act as the rotor. The C ring also serves as a switching device that enables the motor to spin in both counterclockwise (CCW) and clockwise (CW) directions. The phosphorylated form of the chemotactic signaling protein CheY binds to FliM and FliN to induce conformational changes of the C ring responsible for switching the direction of flagellar motor rotation from CCW to CW. In this mini-review, we will describe current understanding of the switching mechanism of the bacterial flagellar motor.
| S-EPMC6700473 | biostudies-literature
Project description:Many bacteria are propelled by flagellar motors that stochastically switch between the clockwise and counterclockwise rotation direction. Although the switching dynamics is one of their most important characteristics, the mechanisms that control it are poorly understood. We present a statistical-mechanical model of the bacterial flagellar motor. At its heart is the assumption that the rotor protein complex, which is connected to the flagellum, can exist in two conformational states and that switching between these states depends on the interactions with the stator proteins, which drive the rotor. This couples switching to rotation, making the switch sensitive to torque and speed. Another key element is that after a switch, it takes time for the load to build up, due to conformational transitions of the flagellum. This slow relaxation dynamics of the filament leads, in combination with the load dependence of the switching frequency, to a characteristic switching time, as recently observed. Hence, our model predicts that the switching dynamics is not only controlled by the chemotaxis-signaling network, but also by mechanical feedback of the flagellum.
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Project description:The flagellum is one of the most sophisticated self-assembling molecular machines in bacteria. Powered by the proton-motive force, the flagellum rapidly rotates in either a clockwise or counterclockwise direction, which ultimately controls bacterial motility and behavior. Escherichia coli and Salmonella enterica have served as important model systems for extensive genetic, biochemical, and structural analysis of the flagellum, providing unparalleled insights into its structure, function, and gene regulation. Despite these advances, our understanding of flagellar assembly and rotational mechanisms remains incomplete, in part because of the limited structural information available regarding the intact rotor-stator complex and secretion apparatus. Cryo-electron tomography (cryo-ET) has become a valuable imaging technique capable of visualizing the intact flagellar motor in cells at molecular resolution. Because the resolution that can be achieved by cryo-ET with large bacteria (such as E. coli and S. enterica) is limited, analysis of small-diameter bacteria (including Borrelia burgdorferi and Campylobacter jejuni) can provide additional insights into the in situ structure of the flagellar motor and other cellular components. This review is focused on the application of cryo-ET, in combination with genetic and biophysical approaches, to the study of flagellar structures and its potential for improving the understanding of rotor-stator interactions, the rotational switching mechanism, and the secretion and assembly of flagellar components.
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