Suppressor analysis of the MotB(D33E) mutation to probe bacterial flagellar motor dynamics coupled with proton translocation

The bacterial flagellum as an object for optical trapping

This letter considers the possibility of using the optical trap to study the structure and function of the microbial flagellum. The structure of the flagellum of a typical gram-negative bacterium is described in brief. A standard mathematical model based on the principle of superposition is used to describe the movement of an ellipsoidal microbial cell in a liquid medium. The basic principles of optical trapping based on the combined action of the light pressure and the gradient force are briefly clarified. Several problems related to thermal damage of living microscopic objects when the latter gets to the focus of a laser beam are shortly discussed. It is shown that the probability of cell damage depends nonlinearly on the wavelength of laser radiation. Finally, the model systems that would make it possible to study flagella of the free bacteria and the ones anchored or tethered on the surface of a solid material are discussed in detail.

Bidirectional Optical Control of Proton Motive Force in Escherichia coli Using Microbial Rhodopsins

Proton (H+) motive force (PMF) serves as the energy source for the flagellar motor rotation, crucial for microbial motility. Here, to control PMF using light, we introduced light-driven inward and outward proton pump rhodopsins, RmXeR and AR3, into Escherichia coli. The motility of E. coli cells expressing RmXeR and AR3 significantly decreased and increased upon illumination, respectively. Tethered cell experiments revealed that, upon illumination, the torque of the flagellar motor decreased to nearly zero (28 pN nm) with RmXeR, while it increased to 1170 pN nm with AR3. These alterations in PMF correspond to +146 mV (RmXeR) and -140 mV (AR3), respectively. Thus, bidirectional optical control of PMF in E. coli was successfully achieved by using proton pump rhodopsins. This system holds a potential for enhancing our understanding of the roles of PMF in various biological functions.

Rotation of the Fla2 flagella of Cereibacter sphaeroides requires the periplasmic proteins MotK and MotE that interact with the flagellar stator protein MotB2

The bacterial flagellum is a complex structure formed by more than 25 different proteins, this appendage comprises three conserved structures: the basal body, the hook and filament. The basal body, embedded in the cell envelope, is the most complex structure and houses the export apparatus and the motor. In situ images of the flagellar motor in different species have revealed a huge diversity of structures that surround the well-conserved periplasmic components of the basal body. The identity of the proteins that form these novel structures in many cases has been elucidated genetically and biochemically, but in others they remain to be identified or characterized. In this work, we report that in the alpha proteobacteria Cereibacter sphaeroides the novel protein MotK along with MotE are essential for flagellar rotation. We show evidence that these periplasmic proteins interact with each other and with MotB2. Moreover, these proteins localize to the flagellated pole and MotK localization is dependent on MotB2 and MotA2. These results together suggest that the role of MotK and MotE is to activate or recruit the flagellar stators to the flagellar structure.

Structure, Assembly, and Function of Flagella Responsible for Bacterial Locomotion

Many motile bacteria use flagella for locomotion under a variety of environmental conditions. Because bacterial flagella are under the control of sensory signal transduction pathways, each cell is able to autonomously control its flagellum-driven locomotion and move to an environment favorable for survival. The flagellum of Salmonella enterica serovar Typhimurium is a supramolecular assembly consisting of at least three distinct functional parts: a basal body that acts as a bidirectional rotary motor together with multiple force generators, each of which serves as a transmembrane proton channel to couple the proton flow through the channel with torque generation; a filament that functions as a helical propeller that produces propulsion; and a hook that works as a universal joint that transmits the torque produced by the rotary motor to the helical propeller. At the base of the flagellum is a type III secretion system that transports flagellar structural subunits from the cytoplasm to the distal end of the growing flagellar structure, where assembly takes place. In recent years, high-resolution cryo-electron microscopy (cryoEM) image analysis has revealed the overall structure of the flagellum, and this structural information has made it possible to discuss flagellar assembly and function at the atomic level. In this article, we describe what is known about the structure, assembly, and function of Salmonella flagella.

Measurements of the Ion Channel Activity of the Transmembrane Stator Complex in the Bacterial Flagellar Motor

The bacterial flagellum is driven by a rotational motor located at the base of the flagellum. The stator unit complex conducts cations such as protons (H⁺) and sodium ions (Na⁺) along the electrochemical potential across the cytoplasmic membrane and interacts with the rotor to generate the rotational force. Escherichia coli and Salmonella have the H⁺-type stator complex, which serves as a transmembrane H⁺ channel that couples H⁺ flow through an ion channel to torque generation whereas Vibrio and some Bacillus species have the Na⁺-type stator complex. In this chapter, we describe how to measure the ion conductivity of the transmembrane stator complex over-expressed in E. coli cells using fluorescent indicators. Intensity measurements of fluorescent indicators using either a fluorescence spectrophotometer or microscope allow quantitative detection of changes in the intracellular ion concentrations due to the ion channel activity of the transmembrane protein complex.

High-Resolution Rotation Assay of the Bacterial Flagellar Motor Near Zero Loads Using a Mutant Having a Rod-Like Straight Hook

The bacterial flagellar motor is embedded within the cell envelop and rotates the long helical filament, which acts as a molecular screw to propel the bacterium. The flagellar motor comprises a rotor and a dozen stator units, converting ion flux through the stator unit into torque. However, the energy coupling mechanism has not been fully understood. Various methods for rotation measurement have been developed to understand the rotation mechanism of the flagellar motor, but the most preferred method in recent studies is a bead assay, which tracks the rotation of a micron to submicron bead attached to the partially sheared flagellar filament at high temporal and spatial resolutions. The bead assay allows us to assess the motor rotation over a wide range of external load, but the elasticity of the axial parts of the flagellum, such as the hook and filament, limits the spatiotemporal resolution. In this chapter, we describe a bead assay optimized for the analysis of the flagellar motor dynamics at near zero load.

Architecture and Assembly of the Bacterial Flagellar Motor Complex

One of the central systems responsible for bacterial motility is the flagellum. The bacterial flagellum is a macromolecular protein complex that is more than five times the cell length. Flagella-driven motility is coordinated via a chemosensory signal transduction pathway, and so bacterial cells sense changes in the environment and migrate towards more desirable locations. The flagellum of Salmonella enterica serovar Typhimurium is composed of a bi-directional rotary motor, a universal joint and a helical propeller. The flagellar motor, which structurally resembles an artificial motor, is embedded within the cell envelop and spins at several hundred revolutions per second. In contrast to an artificial motor, the energy utilized for high-speed flagellar motor rotation is the inward-directed proton flow through a transmembrane proton channel of the stator unit of the flagellar motor. The flagellar motor realizes efficient chemotaxis while performing high-speed movement by an ingenious directional switching mechanism of the motor rotation. To build the universal joint and helical propeller structures outside the cell body, the flagellar motor contains its own protein transporter called a type III protein export apparatus. In this chapter we summarize the structure and assembly of the Salmonella flagellar motor complex.

GFP Fusion to the N-Terminus of MotB Affects the Proton Channel Activity of the Bacterial Flagellar Motor in Salmonella

The bacterial flagellar motor converts the energy of proton flow through the MotA/MotB complex into mechanical works required for motor rotation. The rotational force is generated by electrostatic interactions between the stator protein MotA and the rotor protein FliG. The Arg-90 and Glu-98 from MotA interact with Asp-289 and Arg-281 of FliG, respectively. An increase in the expression level of the wild-type MotA/MotB complex inhibits motility of the gfp-motBfliG(R281V) mutant but not the fliG(R281V) mutant, suggesting that the MotA/GFP-MotB complex cannot work together with wild-type MotA/MotB in the presence of the fliG(R281V) mutation. However, it remains unknown why. Here, we investigated the effect of the GFP fusion to MotB at its N-terminus on the MotA/MotB function. Over-expression of wild-type MotA/MotB significantly reduced the growth rate of the gfp-motBfliG(R281V) mutant. The over-expression of the MotA/GFP-MotB complex caused an excessive proton leakage through its proton channel, thereby inhibiting cell growth. These results suggest that the GFP tag on the MotB N-terminus affects well-regulated proton translocation through the MotA/MotB proton channel. Therefore, we propose that the N-terminal cytoplasmic tail of MotB couples the gating of the proton channel with the MotA-FliG interaction responsible for torque generation.

Mechanisms and Dynamics of the Bacterial Flagellar Motor

Many bacteria are able to actively propel themselves through their complex environment, in search of resources and suitable niches. The source of this propulsion is the Bacterial Flagellar Motor (BFM), a molecular complex embedded in the bacterial membrane which rotates a flagellum. In this chapter we review the known physical mechanisms at work in the motor. The BFM shows a highly dynamic behavior in its power output, its structure, and in the stoichiometry of its components. Changes in speed, rotation direction, constituent protein conformations, and the number of constituent subunits are dynamically controlled in accordance to external chemical and mechanical cues. The mechano-sensitivity of the motor is likely related to the surface-sensing ability of bacteria, relevant in the initial stage of biofilm formation.

Structural insights into flagellar stator-rotor interactions

The bacterial flagellar motor is a molecular machine that can rotate the flagellar filament at high speed. The rotation is generated by the stator–rotor interaction, coupled with an ion flux through the torque-generating stator. Here we employed cryo-electron tomography to visualize the intact flagellar motor in the Lyme disease spirochete, Borrelia burgdorferi. By analyzing the motor structures of wild-type and stator-deletion mutants, we not only localized the stator complex in situ, but also revealed the stator–rotor interaction at an unprecedented detail. Importantly, the stator–rotor interaction induces a conformational change in the flagella C-ring. Given our observation that a non-motile mutant, in which proton flux is blocked, cannot generate the similar conformational change, we propose that the proton-driven torque is responsible for the conformational change required for flagellar rotation.

Flagella-Driven Motility of Bacteria

The bacterial flagellum is a helical filamentous organelle responsible for motility. In bacterial species possessing flagella at the cell exterior, the long helical flagellar filament acts as a molecular screw to generate thrust. Meanwhile, the flagella of spirochetes reside within the periplasmic space and not only act as a cytoskeleton to determine the helicity of the cell body, but also rotate or undulate the helical cell body for propulsion. Despite structural diversity of the flagella among bacterial species, flagellated bacteria share a common rotary nanomachine, namely the flagellar motor, which is located at the base of the filament. The flagellar motor is composed of a rotor ring complex and multiple transmembrane stator units and converts the ion flux through an ion channel of each stator unit into the mechanical work required for motor rotation. Intracellular chemotactic signaling pathways regulate the direction of flagella-driven motility in response to changes in the environments, allowing bacteria to migrate towards more desirable environments for their survival. Recent experimental and theoretical studies have been deepening our understanding of the molecular mechanisms of the flagellar motor. In this review article, we describe the current understanding of the structure and dynamics of the bacterial flagellum.

Evaluation of the Duty Ratio of the Bacterial Flagellar Motor by Dynamic Load Control

The bacterial flagellar motor is one of the most complex and sophisticated nanomachineries in nature. A duty ratio D is a fraction of time that the stator and the rotor interact and is a fundamental property to characterize the motor but remains to be determined. It is known that the stator units of the motor bind to and dissociate from the motor dynamically to control the motor torque depending on the load on the motor. At low load, at which the kinetics such as proton translocation speed limits the rotation rate, the dependency of the rotation rate on the number of stator units N implies D: the dependency becomes larger for smaller D. Contradicting observations supporting both the small and large D have been reported. A dilemma is that it is difficult to explore a broad range of N at low load because the stator units easily dissociate, and N is limited to one or two at vanishing load. Here, we develop an electrorotation method to dynamically control the load on the flagellar motor of Salmonella with a calibrated magnitude of the torque. By instantly reducing the load for keeping N high, we observed that the speed at low load depends on N, implying a small duty ratio. We recovered the torque-speed curves of individual motors and evaluated the duty ratio to be 0.14 ± 0.04 from the correlation between the torque at high load and the rotation rate at low load.

Novel Insights into Conformational Rearrangements of the Bacterial Flagellar Switch Complex

The flagellar motor can spin in both counterclockwise (CCW) and clockwise (CW) directions. The flagellar motor consists of a rotor and multiple stator units, which act as a proton channel. The rotor is composed of the transmembrane MS ring made of FliF and the cytoplasmic C ring consisting of FliG, FliM, and FliN. The C ring is directly involved in rotation and directional switching. The Salmonella FliF-FliG deletion fusion motor missing 56 residues from the C terminus of FliF and 94 residues from the N terminus of FliG keeps a domain responsible for the interaction with the stator intact, but its motor function is reduced significantly. Here, we report the structure and function of the FliF-FliG deletion fusion motor. The FliF-FliG deletion fusion not only resulted in a strong CW switch bias but also affected rotor-stator interactions coupled with proton translocation through the proton channel of the stator unit. The energy coupling efficiency of the deletion fusion motor was the same as that of the wild-type motor. Extragenic suppressor mutations in FliG, FliM, or FliN not only relieved the strong CW switch bias but also increased the motor speed at low load. The FliF-FliG deletion fusion made intersubunit interactions between C ring proteins tighter compared to the wild-type motor, whereas the suppressor mutations affect such tighter intersubunit interactions. We propose that a change of intersubunit interactions between the C ring proteins may be required for high-speed motor rotation as well as direction switching.IMPORTANCE The bacterial flagellar motor is a bidirectional rotary motor for motility and chemotaxis, which often plays an important role in infection. The motor is a large transmembrane protein complex composed of a rotor and multiple stator units, which also act as a proton channel. Motor torque is generated through their cyclic association and dissociation coupled with proton translocation through the proton channel. A large cytoplasmic ring of the motor, called C ring, is responsible for rotation and switching by interacting with the stator, but the mechanism remains unknown. By analyzing the structure and function of the wild-type motor and a mutant motor missing part of the C ring connecting itself with the transmembrane rotor ring while keeping a stator-interacting domain for bidirectional torque generation intact, we found interesting clues to the change in the C ring conformation for the switching and rotation involving loose and tight intersubunit interactions.

Effect of the MotA(M206I) Mutation on Torque Generation and Stator Assembly in the Salmonella H+-Driven Flagellar Motor

The bacterial flagellar motor is composed of a rotor and a dozen stators and converts the ion flux through the stator into torque. Each stator unit alternates in its attachment to and detachment from the rotor even during rotation. In some species, stator assembly depends on the input energy, but it remains unclear how an electrochemical potential across the membrane (e.g., proton motive force [PMF]) or ion flux is involved in stator assembly dynamics. Here, we focused on pH dependence of a slow motile MotA(M206I) mutant of Salmonella The MotA(M206I) motor produces torque comparable to that of the wild-type motor near stall, but its rotation rate is considerably decreased as the external load is reduced. Rotation assays of flagella labeled with 1-μm beads showed that the rotation rate of the MotA(M206I) motor is increased by lowering the external pH whereas that of the wild-type motor is not. Measurements of the speed produced by a single stator unit using 1-μm beads showed that the unit speed of the MotA(M206I) is about 60% of that of the wild-type and that a decrease in external pH did not affect the MotA(M206I) unit speed. Analysis of the subcellular stator localization revealed that the number of functional stators is restored by lowering the external pH. The pH-dependent improvement of stator assembly was observed even when the PMF was collapsed and proton transfer was inhibited. These results suggest that MotA-Met206 is responsible for not only load-dependent energy coupling between the proton influx and rotation but also pH-dependent stator assembly.IMPORTANCE The bacterial flagellar motor is a rotary nanomachine driven by the electrochemical transmembrane potential (ion motive force). About 10 stators (MotA/MotB complexes) are docked around a rotor, and the stator recruitment depends on the load, ion motive force, and coupling ion flux. The MotA(M206I) mutation slows motor rotation and decreases the number of docked stators in Salmonella We show that lowering the external pH improves the assembly of the mutant stators. Neither the collapse of the ion motive force nor a mutation mimicking the proton-binding state inhibited stator localization to the motor. These results suggest that MotA-Met206 is involved in torque generation and proton translocation and that stator assembly is stabilized by protonation of the stator.

Na+-induced structural transition of MotPS for stator assembly of the Bacillus flagellar motor

The bacterial flagellar motor consists of a rotor and a dozen stator units and regulates the number of active stator units around the rotor in response to changes in the environment. The MotPS complex is a Na⁺-type stator unit in the Bacillus subtilis flagellar motor and binds to the peptidoglycan layer through the peptidoglycan-binding (PGB) domain of MotS to act as the stator. The MotPS complex is activated in response to an increase in the Na⁺ concentration in the environment, but the mechanism of this activation has remained unknown. We report that activation occurs by a Na⁺-induced folding and dimer formation of the PGB domain of MotS, as revealed in real-time imaging by high-speed atomic force microscopy. The MotPS complex showed two distinct ellipsoid domains connected by a flexible linker. A smaller domain, corresponding to the PGB domain, became structured and unstructured in the presence and absence of 150 mM NaCl, respectively. When the amino-terminal portion of the PGB domain adopted a partially stretched conformation in the presence of NaCl, the center-to-center distance between these two domains increased by up to 5 nm, allowing the PGB domain to reach and bind to the peptidoglycan layer. We propose that assembly of the MotPS complex into a motor proceeds by means of Na⁺-induced structural transitions of its PGB domain.

Load- and polysaccharide-dependent activation of the Na+-type MotPS stator in the Bacillus subtilis flagellar motor

The flagellar motor of Bacillus subtilis possesses two distinct H⁺-type MotAB and Na⁺-type MotPS stators. In contrast to the MotAB motor, the MotPS motor functions efficiently at elevated viscosity in the presence of 200 mM NaCl. Here, we analyzed the torque-speed relationship of the Bacillus MotAB and MotPS motors over a wide range of external loads. The stall torque of the MotAB and MotPS motors at high load was about 2,200 pN nm and 220 pN nm, respectively. The number of active stators in the MotAB and MotPS motors was estimated to be about ten and one, respectively. However, the number of functional stators in the MotPS motor was increased up to ten with an increase in the concentration of a polysaccharide, Ficoll 400, as well as in the load. The maximum speeds of the MotAB and MotPS motors at low load were about 200 Hz and 50 Hz, respectively, indicating that the rate of the torque-generation cycle of the MotPS motor is 4-fold slower than that of the MotAB motor. Domain exchange experiments showed that the C-terminal periplasmic domain of MotS directly controls the assembly and disassembly dynamics of the MotPS stator in a load- and polysaccharide-dependent manner.

Energy utilization in fluctuating biological energy converters

We have argued previously [Szoke et al., FEBS Lett. 553, 18-20 (2003); Curr. Chem. Biol. 1, 53-57 (2007)] that energy utilization and evolution are emergent properties based on a small number of established laws of physics and chemistry. The relevant laws constitute a framework for biology on a level intermediate between quantum chemistry and cell biology. There are legitimate questions whether these concepts are valid at the mesoscopic level. Such systems fluctuate appreciably, so it is not clear what their efficiency is. Advances in fluctuation theorems allow the description of such systems on a molecular level. We attempt to clarify this topic and bridge the biochemical and physical descriptions of mesoscopic systems.

Genetic analysis of revertants isolated from the rod-fragile fliF mutant of Salmonella

FliF is the protein comprising the MS-ring of the bacterial flagellar basal body, which is the base for the assembly of flagellar axial structures. From a fliF mutant that easily releases the rod-hook-filament in viscous environments, more than 400 revertants that recovered their swarming ability in viscous conditions, were isolated. The second-site mutations were determined for approximately 70% of them. There were three regions where the mutations were localized: two in Region I, 112 in Region II, and 71 in Region III including the true reversion. In Region I, second-site mutations were found in FlgC and FlgF of the proximal rod, suggesting that they affect the interaction between the MS-ring and the rod. In Region II, there were 69 and 42 mutations in MotA and MotB, respectively, suggesting that the second-site mutations in MotA and MotB may decrease the rotational speed of the flagellar motor to reduce the probability of releasing the rod under this condition. One exception is a mutation in FlhC that caused a down regulation of the flagellar proteins production but it may directly affect transcription or translation of motA and motB. In Region III, there were 44, 24, and 3 mutations in FliG, FliM, and FliF, respectively. There were no second-site mutations identified in FliN although it is involved in torque generation as a component of the C-ring. Many of the mutations were involved in the motor rotation, and it is suggested that such reduced speeds result in stabilizing the filament attachment to the motor.

Suppressor mutants from MotB-D24E and MotS-D30E in the flagellar stator complex of Bacillus subtilis

The bacterial flagellar motor is mainly energized by either a proton (H(+)) or sodium ion (Na(+)) motive force and the motor torque is generated by interaction at the rotor-stator interface. MotA/MotB-type stators use H(+) as the coupling ion, whereas MotP/MotS- and PomA/PomB-type stators use Na(+). Bacillus subtilis employs both H(+)-coupled MotA/MotB and Na(+)-coupled MotP/MotS stators, which contribute to the torque required for flagellar rotation. In Escherichia coli, there is a universally conserved Asp-32 residue of MotB that is critical for motility and is a predicted H(+)-binding site. In B. subtilis, the conserved aspartic acid residue corresponds to Asp-24 of MotB (MotB-D24) and Asp-30 of MotS (MotS-D30). Here we report the isolation of two mutants, MotB-D24E and MotS-D30E, which showed a non-motile and poorly motile phenotype, respectively. Up-motile mutants were spontaneously isolated from each mutant. We identified a suppressor mutation at MotB-T181A and MotP-L172P, respectively. Mutants MotB-T181A and MotP-L172P showed about 50% motility and a poorly motile phenotype compared to each wild type strain. These suppressor sites were suggested to indirectly affect the structure of the ion influx pathway.

Serine 26 in the PomB subunit of the flagellar motor is essential for hypermotility of Vibrio cholerae

Vibrio cholerae is motile by means of its single polar flagellum which is driven by the sodium-motive force. In the motor driving rotation of the flagellar filament, a stator complex consisting of subunits PomA and PomB converts the electrochemical sodium ion gradient into torque. Charged or polar residues within the membrane part of PomB could act as ligands for Na⁺, or stabilize a hydrogen bond network by interacting with water within the putative channel between PomA and PomB. By analyzing a large data set of individual tracks of swimming cells, we show that S26 located within the transmembrane helix of PomB is required to promote very fast swimming of V. cholerae. Loss of hypermotility was observed with the S26T variant of PomB at pH 7.0, but fast swimming was restored by decreasing the H⁺ concentration of the external medium. Our study identifies S26 as a second important residue besides D23 in the PomB channel. It is proposed that S26, together with D23 located in close proximity, is important to perturb the hydration shell of Na⁺ before its passage through a constriction within the stator channel.

A new player at the flagellar motor: FliL controls both motor output and bias

The bacterial flagellum is driven by a bidirectional rotary motor, which propels bacteria to swim through liquids or swarm over surfaces. While the functions of the major structural and regulatory components of the flagellum are known, the function of the well-conserved FliL protein is not. In Salmonella and Escherichia coli, the absence of FliL leads to a small defect in swimming but complete elimination of swarming. Here, we tracked single motors of these bacteria and found that absence of FliL decreases their speed as well as switching frequency. We demonstrate that FliL interacts strongly with itself, with the MS ring protein FliF, and with the stator proteins MotA and MotB and weakly with the rotor switch protein FliG. These and other experiments show that FliL increases motor output either by recruiting or stabilizing the stators or by increasing their efficiency and contributes additionally to torque generation at higher motor loads. The increased torque enabled by FliL explains why this protein is essential for swarming on an agar surface expected to offer increased resistance to bacterial movement.

FliL is a well-conserved bacterial flagellar protein whose absence leads to a variety of motility defects, ranging from moderate to complete inhibition of swimming in some bacterial species, inhibition of swarming in others, structural defects that break the flagellar rod during swarming in E. coli and Salmonella, and failure to eject the flagellar filament during the developmental transition of a swimmer to a stalk cell in Caulobacter crescentus. Despite these many phenotypes, a specific function for FliL has remained elusive. Here, we established a central role for FliL at the Salmonella and E. coli motors, where it interacts with both rotor and stator proteins, increases motor output, and contributes to the normal rotational bias of the motor.

The bacterial flagellar motor and its structural diversity

The bacterial flagellum is a reversible rotary motor powered by an electrochemical-potential difference of specific ions across the cytoplasmic membrane. The H(+)-driven motor of Salmonella spins at ∼300 Hz, whereas the Na(+)-driven motor of marine Vibrio spp. can rotate much faster, up to 1700 Hz. A highly conserved motor structure consists of the MS ring, C ring, rod, and export apparatus. The C ring and the export apparatus show dynamic properties for exerting their functional activities. Various additional structures surrounding the conserved motor structure are observed in different bacterial species. In this review we summarize our current understanding of the structure, function, and assembly of the flagellar motor in Salmonella and marine Vibrio.

Effect of the MotB(D33N) mutation on stator assembly and rotation of the proton-driven bacterial flagellar motor

The bacterial flagellar motor generates torque by converting the energy of proton translocation through the transmembrane proton channel of the stator complex formed by MotA and MotB. The MotA/B complex is thought to be anchored to the peptidoglycan (PG) layer through the PG-binding domain of MotB to act as the stator. The stator units dynamically associate with and dissociate from the motor during flagellar motor rotation, and an electrostatic interaction between MotA and a rotor protein FliG is required for efficient stator assembly. However, the association and dissociation mechanism of the stator units still remains unclear. In this study, we analyzed the speed fluctuation of the flagellar motor of Salmonella enterica wild-type cells carrying a plasmid encoding a nonfunctional stator complex, MotA/B(D33N), which lost the proton conductivity. The wild-type motor rotated stably but the motor speed fluctuated considerably when the expression level of MotA/B(D33N) was relatively high compared to MotA/B. Rapid accelerations and decelerations were frequently observed. A quantitative analysis of the speed fluctuation and a model simulation suggested that the MotA/B(D33N) stator retains the ability to associate with the motor at a low affinity but dissociates more rapidly than the MotA/B stator. We propose that the stator dissociation process depends on proton translocation through the proton channel.

Populational heterogeneity vs. temporal fluctuation in Escherichia coli flagellar motor switching

The dynamic switching of the bacterial flagellar motor regulates cell motility in bacterial chemotaxis. It has been reported under physiological conditions that the switching bias of the flagellar motor undergoes large temporal fluctuations, which reflects noise propagating in the chemotactic signaling network. On the other hand, nongenetic heterogeneity is also observed in flagellar motor switching, as a large group of switching motors show different switching bias and frequency under the same physiological condition. In this work, we present simultaneous measurement of groups of Escherichia coli flagellar motor switching and compare them to long time recording of single switching motors. Consistent with previous studies, we observed temporal fluctuations in switching bias in long time recording experiments. However, the variability in switching bias at the populational level showed much higher volatility than its temporal fluctuation. These results suggested stable individuality in E. coli motor switching. We speculate that uneven expression of key regulatory proteins with amplification by the ultrasensitive response of the motor can account for the observed populational heterogeneity and temporal fluctuations.

Structure and function of the bi-directional bacterial flagellar motor

The bacterial flagellum is a locomotive organelle that propels the bacterial cell body in liquid environments. The flagellum is a supramolecular complex composed of about 30 different proteins and consists of at least three parts: a rotary motor, a universal joint, and a helical filament. The flagellar motor of Escherichia coli and Salmonella enterica is powered by an inward-directed electrochemical potential difference of protons across the cytoplasmic membrane. The flagellar motor consists of a rotor made of FliF, FliG, FliM and FliN and a dozen stators consisting of MotA and MotB. FliG, FliM and FliN also act as a molecular switch, enabling the motor to spin in both counterclockwise and clockwise directions. Each stator is anchored to the peptidoglycan layer through the C-terminal periplasmic domain of MotB and acts as a proton channel to couple the proton flow through the channel with torque generation. Highly conserved charged residues at the rotor–stator interface are required not only for torque generation but also for stator assembly around the rotor. In this review, we will summarize our current understanding of the structure and function of the proton-driven bacterial flagellar motor.

The C-terminal periplasmic domain of MotB is responsible for load-dependent control of the number of stators of the bacterial flagellar motor

The bacterial flagellar motor is made of a rotor and stators. In Salmonella it is thought that about a dozen MotA/B complexes are anchored to the peptidoglycan layer around the motor through the C-terminal peptidoglycan-binding domain of MotB to become active stators as well as proton channels. MotB consists of 309 residues, forming a single transmembrane helix (30–50), a stalk (51–100) and a C-terminal peptidoglycan-binding domain (101–309). Although the stalk is dispensable for torque generation by the motor, it is required for efficient motor performance. Residues 51 to 72 prevent premature proton leakage through the proton channel prior to stator assembly into the motor. However, the role of residues 72–100 remains unknown. Here, we analyzed the torque-speed relationship of the MotB(Δ72–100) motor. At a low speed near stall, this mutant motor produced torque at the wild-type level. Unlike the wild-type motor, however, torque dropped off drastically by slight decrease in external load and then showed a slow exponential decay over a wide range of load by its further reduction. Since it is known that the stator is a mechano-sensor and that the number of active stators changes in a load-dependent manner, we interpreted this unusual torque-speed relationship as anomaly in load-dependent control of the number of active stators. The results suggest that residues 72–100 of MotB is required for proper load-dependent control of the number of active stators around the rotor.

Load-sensitive coupling of proton translocation and torque generation in the bacterial flagellar motor

The Salmonella flagellar motor consists of a rotor and about a dozen stator elements. Each stator element, consisting of MotA and MotB, acts as a proton channel to couple proton flow with torque generation. A highly conserved Asp33 residue of MotB is directly involved in the energy coupling mechanism, but it remains unknown how it carries out this function. Here, we show that the MotB(D33E) mutation dramatically alters motor performance in response to changes in external load. Rotation speeds of the MotA/B(D33E) and MotA(V35F)/B(D33E) motors were markedly slower than the wild-type motor and fluctuated considerably at low load but not at high load, whereas the rotation rate of the wild-type motor was stable at any load. At low load, pausing events were frequently observed in both mutant motors. The proton conductivities of these mutant stator channels in their 'unplugged' forms were only half of the conductivity of the wild-type channel. These results suggest that the D33E mutation induces a load-dependent inactivation of the MotA/B complex. We propose that the stator element is a load-sensitive proton channel that efficiently couples proton translocation with torque generation and that Asp33 of MotB is critical for this co-ordinated proton translocation.

A distant homologue of the FlgT protein interacts with MotB and FliL and is essential for flagellar rotation in Rhodobacter sphaeroides

In this work, we describe a periplasmic protein that is essential for flagellar rotation in Rhodobacter sphaeroides. This protein is encoded upstream of flgA, and its expression is dependent on the flagellar master regulator FleQ and on the class III flagellar activator FleT. Sequence comparisons suggest that this protein is a distant homologue of FlgT. We show evidence that in R. sphaeroides, FlgT interacts with the periplasmic regions of MotB and FliL and with the flagellar protein MotF, which was recently characterized as a membrane component of the flagellum in this bacterium. In addition, the localization of green fluorescent protein (GFP)-MotF is completely dependent on FlgT. The Mot(-) phenotype of flgT cells was weakly suppressed by point mutants of MotB that presumably keep the proton channel open and efficiently suppress the Mot(-) phenotype of motF and fliL cells, indicating that FlgT could play an additional role beyond the opening of the proton channel. The presence of FlgT in purified filament-hook-basal bodies of the wild-type strain was confirmed by Western blotting, and the observation of these structures under an electron microscope showed that the basal bodies from flgT cells had lost the ring that covers the LP ring in the wild-type structure. Moreover, MotF was detected by immunoblotting in the basal bodies obtained from the wild-type strain but not from flgT cells. From these results, we suggest that FlgT forms a ring around the LP ring, which anchors MotF and stabilizes the stator complex of the flagellar motor.

Mechanism and kinetics of a sodium-driven bacterial flagellar motor

The bacterial flagellar motor is a large rotary molecular machine that propels swimming bacteria, powered by a transmembrane electrochemical potential difference. It consists of an ∼50-nm rotor and up to ∼10 independent stators anchored to the cell wall. We measured torque-speed relationships of single-stator motors under 25 different combinations of electrical and chemical potential. All 25 torque-speed curves had the same concave-down shape as fully energized wild-type motors, and each stator passes at least 37 ± 2 ions per revolution. We used the results to explore the 25-dimensional parameter space of generalized kinetic models for the motor mechanism, finding 830 parameter sets consistent with the data. Analysis of these sets showed that the motor mechanism has a "powerstroke" in either ion binding or transit; ion transit is channel-like rather than carrier-like; and the rate-limiting step in the motor cycle is ion binding at low concentration, ion transit, or release at high concentration.

Distinct roles of highly conserved charged residues at the MotA-FliG interface in bacterial flagellar motor rotation

Electrostatic interactions between the stator protein MotA and the rotor protein FliG are important for bacterial flagellar motor rotation. Arg90 and Glu98 of MotA are required not only for torque generation but also for stator assembly around the rotor, but their actual roles remain unknown. Here we analyzed the roles of functionally important charged residues at the MotA-FliG interface in motor performance. About 75% of the motA(R90E) cells and 45% of the motA(E98K) cells showed no fluorescent spots of green fluorescent protein (GFP)-MotB, indicating reduced efficiency of stator assembly around the rotor. The FliG(D289K) and FliG(R281V) mutations, which restore the motility of the motA(R90E) and motA(E98K) mutants, respectively, showed reduced numbers and intensity of GFP-MotB spots as well. The FliG(D289K) mutation significantly recovered the localization of GFP-MotB to the motor in the motA(R90E) mutant, whereas the FliG(R281V) mutation did not recover the GFP-MotB localization in the motA(E98K) mutant. These results suggest that the MotA-Arg90-FliG-Asp289 interaction is critical for the proper positioning of the stators around the rotor, whereas the MotA-Glu98-FliG-Arg281 interaction is more important for torque generation.

Microscopic analysis of bacterial motility at high pressure

The bacterial flagellar motor is a molecular machine that converts an ion flux to the rotation of a helical flagellar filament. Counterclockwise rotation of the filaments allows them to join in a bundle and propel the cell forward. Loss of motility can be caused by environmental factors such as temperature, pH, and solvation. Hydrostatic pressure is also a physical inhibitor of bacterial motility, but the detailed mechanism of this inhibition is still unknown. Here, we developed a high-pressure microscope that enables us to acquire high-resolution microscopic images, regardless of applied pressures. We also characterized the pressure dependence of the motility of swimming Escherichia coli cells and the rotation of single flagellar motors. The fraction and speed of swimming cells decreased with increased pressure. At 80 MPa, all cells stopped swimming and simply diffused in solution. After the release of pressure, most cells immediately recovered their initial motility. Direct observation of the motility of single flagellar motors revealed that at 80 MPa, the motors generate torque that should be sufficient to join rotating filaments in a bundle. The discrepancy in the behavior of free swimming cells and individual motors could be due to the applied pressure inhibiting the formation of rotating filament bundles that can propel the cell body in an aqueous environment.

A Bacillus flagellar motor that can use both Na+ and K+ as a coupling ion is converted by a single mutation to use only Na+

In bacteria, the sodium ion (Na(+)) cycle plays a critical role in negotiating the challenges of an extremely alkaline and sodium-rich environment. Alkaliphilic bacteria that grow optimally at high pH values use Na(+) for solute uptake and flagellar rotation because the proton (H(+)) motive force is insufficient for use at extremely alkaline pH. Only three types of electrically driven rotary motors exist in nature: the F-type ATPase, the V-type ATPase, and the bacterial flagellar motor. Until now, only H(+) and Na(+) have been reported as coupling ions for these motors. Here, we report that the alkaliphilic bacterium Bacillus alcalophilus Vedder 1934 can grow not only under a Na(+)-rich and potassium ion (K(+))-poor condition but also under the opposite condition in an extremely alkaline environment. In this organism, swimming performance depends on concentrations of Na(+), K(+) or Rb(+). In the absence of Na(+), swimming behavior is clearly K(+)- dependent. This pattern was confirmed in swimming assays of stator-less Bacillus subtilis and Escherichia coli mutants expressing MotPS from B. alcalophilus (BA-MotPS). Furthermore, a single mutation in BA-MotS was identified that converted the naturally bi-functional BA-MotPS to stators that cannot use K(+) or Rb(+). This is the first report that describes a flagellar motor that can use K(+) and Rb(+) as coupling ions. The finding will affect the understanding of the operating principles of flagellar motors and the molecular mechanisms of ion selectivity, the field of the evolution of environmental changes and stresses, and areas of nanotechnology.

A novel component of the Rhodobacter sphaeroides Fla1 flagellum is essential for motor rotation

Here we describe a novel component essential for flagellar rotation in Rhodobacter sphaeroides. This protein is encoded by motF (RSP_0067), the first gene of a predicted transcriptional unit which contains two hypothetical genes. Sequence analysis indicated that MotF is a bitopic membrane-spanning protein. Protease sensitivity assays and green fluorescent protein (GFP) fusions confirmed this prediction and allowed us to conclude that the C terminus of MotF is located in the periplasmic space. Wild-type cells expressing a functional GFP-MotF fusion show a single fluorescent focus per cell. The localization of this protein in different genetic backgrounds allowed us to determine that normal localization of MotF depends on the presence of FliL and MotB. Characterization of a ΔmotF pseudorevertant strain revealed that a single nucleotide change in motB suppresses the Mot(-) phenotype of the motF mutant. Additionally, we show that MotF also becomes dispensable when other mutant alleles of motB previously isolated as second-site suppressors of ΔfliL were expressed in the motF mutant strain. These results show that MotF is a new component of the Fla1 flagellum, which together with FliL is required to promote flagellar rotation, possibly through MotB.

Bacterial motility measured by a miniature chamber for high-pressure microscopy

Hydrostatic pressure is one of the physical stimuli that characterize the environment of living matter. Many microorganisms thrive under high pressure and may even physically or geochemically require this extreme environmental condition. In contrast, application of pressure is detrimental to most life on Earth; especially to living organisms under ambient pressure conditions. To study the mechanism of how living things adapt to high-pressure conditions, it is necessary to monitor directly the organism of interest under various pressure conditions. Here, we report a miniature chamber for high-pressure microscopy. The chamber was equipped with a built-in separator, in which water pressure was properly transduced to that of the sample solution. The apparatus developed could apply pressure up to 150 MPa, and enabled us to acquire bright-field and epifluorescence images at various pressures and temperatures. We demonstrated that the application of pressure acted directly and reversibly on the swimming motility of Escherichia coli cells. The present technique should be applicable to a wide range of dynamic biological processes that depend on applied pressures.

Mycoplasma mobile cells elongated by detergent and their pivoting movements in gliding

Mycoplasma mobile glides on solid surfaces by the repeated binding of leg structures to sialylated oligosaccharide fixed on a solid surface. To obtain information about the propulsion caused by the leg, we made elongated and stiff cells using a detergent. Within 30 min after the cells were treated with 0.1% Tween 60, the cells were elongated from 0.8 μm to 2.2 μm in length while maintaining their gliding activity. Fluorescence and electron microscopy showed that a part of the cytoskeletal structure was elongated, while the localization of proteins involved in the gliding was not modified significantly. The elongated cells glided with repeated pivoting around the cellular position of gliding machinery by 10 degrees of amplitude at a frequency of 2 to 3 times per second, suggesting that the propulsion in a line perpendicular to the cell axis can occur with different timings. The pivoting speed decreased as the cell length increased, probably from the load generated by the friction. The torque required to achieve the actual pivoting increased with the cell length without saturation, reaching 54.7 pN nm at 4.3 μm in cell length.

Structural insight into the rotational switching mechanism of the bacterial flagellar motor

Structural analysis of a clockwise-biased rotation mutant of the bacterial flagellar rotor protein FliG provides a new model for the arrangement of FliG subunits in the motor, and novel insights into rotation switching.

The bacterial flagellar motor can rotate either clockwise (CW) or counterclockwise (CCW). Three flagellar proteins, FliG, FliM, and FliN, are required for rapid switching between the CW and CCW directions. Switching is achieved by a conformational change in FliG induced by the binding of a chemotaxis signaling protein, phospho-CheY, to FliM and FliN. FliG consists of three domains, FliG_N, FliG_M, and FliG_C, and forms a ring on the cytoplasmic face of the MS ring of the flagellar basal body. Crystal structures have been reported for the FliG_MC domains of Thermotoga maritima, which consist of the FliG_M and FliG_C domains and a helix E that connects these two domains, and full-length FliG of Aquifex aeolicus. However, the basis for the switching mechanism is based only on previously obtained genetic data and is hence rather indirect. We characterized a CW-biased mutant (fliG(ΔPAA)) of Salmonella enterica by direct observation of rotation of a single motor at high temporal and spatial resolution. We also determined the crystal structure of the FliG_MC domains of an equivalent deletion mutant variant of T. maritima (fliG(ΔPEV)). The FliG(ΔPAA) motor produced torque at wild-type levels under a wide range of external load conditions. The wild-type motors rotated exclusively in the CCW direction under our experimental conditions, whereas the mutant motors rotated only in the CW direction. This result suggests that wild-type FliG is more stable in the CCW state than in the CW state, whereas FliG(ΔPAA) is more stable in the CW state than in the CCW state. The structure of the TM-FliG_MC(ΔPEV) revealed that extremely CW-biased rotation was caused by a conformational change in helix E. Although the arrangement of FliG_C relative to FliG_M in a single molecule was different among the three crystals, a conserved FliG_M-FliG_C unit was observed in all three of them. We suggest that the conserved FliG_M-FliG_C unit is the basic functional element in the rotor ring and that the PAA deletion induces a conformational change in a hinge-loop between FliG_M and helix E to achieve the CW state of the FliG ring. We also propose a novel model for the arrangement of FliG subunits within the motor. The model is in agreement with the previous mutational and cross-linking experiments and explains the cooperative switching mechanism of the flagellar motor.

The bacterial flagellum is a rotating organelle that governs cell motility. At the base of each flagellum is a motor powered by the electrochemical potential difference of specific ions across the cytoplasmic membrane. In response to environmental stimuli, rotation of the motor switches between counterclockwise and clockwise, with a corresponding effect on the swimming direction of the cell. Switching is triggered by the binding of the signaling protein phospho-CheY to FliM and FliN, and achieved by conformational changes in the rotor protein FliG. The actual switching mechanism, however, remains unclear. In this study, we characterized a fliG mutant of Salmonella that shows an extreme clockwise-biased rotation, and determined the structure of a fragment of FliG (FliG_MC) of the equivalent mutant variant of Thermotoga maritima. FliG_MC is composed of two domains and covers the regions essential for torque generation and FliM binding. We showed that the mutant structure has a conformational change in the helix connecting the two domains, leading to a domain orientation distinct from that of the wild-type FliG. On the basis of this structure, we propose a new model for the arrangement of FliG subunits in the rotor that is consistent with the previous mutational studies and explains how cooperative switching occurs in the motor.

The flagellar protein FliL is essential for swimming in Rhodobacter sphaeroides

In this work we characterize the function of the flagellar protein FliL in Rhodobacter sphaeroides. Our results show that FliL is essential for motility in this bacterium and that in its absence flagellar rotation is highly impaired. A green fluorescent protein (GFP)-FliL fusion forms polar and lateral fluorescent foci that show different spatial dynamics. The presence of these foci is dependent on the expression of the flagellar genes controlled by the master regulator FleQ, suggesting that additional components of the flagellar regulon are required for the proper localization of GFP-FliL. Eight independent pseudorevertants were isolated from the fliL mutant strain. In each of these strains a single nucleotide change in motB was identified. The eight mutations affected only three residues located on the periplasmic side of MotB. Swimming of the suppressor mutants was not affected by the presence of the wild-type fliL allele. Pulldown and yeast two-hybrid assays showed that that the periplasmic domain of FliL is able to interact with itself but not with the periplasmic domain of MotB. From these results we propose that FliL could participate in the coupling of MotB with the flagellar rotor in an indirect fashion.

Evidence for symmetry in the elementary process of bidirectional torque generation by the bacterial flagellar motor

The bacterial flagellar motor can rotate in both counterclockwise (CCW) and clockwise (CW) directions. It has been shown that the sodium ion-driven chimeric flagellar motor rotates with 26 steps per revolution, which corresponds to the number of FliG subunits that form part of the rotor ring, but the size of the backward step is smaller than the forward one. Here we report that the proton-driven flagellar motor of Salmonella also rotates with 26 steps per revolution but symmetrical in both CCW and CW directions with occasional smaller backward steps in both directions. Occasional shift in the stepping positions is also observed, suggesting the frequent exchange of stators in one of the 11-12 possible anchoring positions around the rotor. These observations indicate that the elementary process of torque generation by the cyclic association/dissociation of the stator with every FliG subunit along the circumference of the rotor is symmetric in CCW and CW rotation even though the structure of FliG is highly asymmetric and suggests a 180° rotation of a FliG domain for the rotor-stator interaction to reverse the direction of rotation.

Role of a conserved prolyl residue (Pro173) of MotA in the mechanochemical reaction cycle of the proton-driven flagellar motor of Salmonella

The MotA/B complex acts as the stator of the proton-driven bacterial flagellar motor. Proton translocation through the stator complex is efficiently coupled with torque generation by the stator-rotor interactions. In Salmonella enterica serovar Typhimurium, the highly conserved Pro173 residue of MotA is close to the absolutely conserved Asp33 residue of MotB, which is believed to be a proton-binding site. Pro173 is postulated to be involved in coupling proton influx to torque generation. However, it remains unknown what critical function Pro173 carries out. Here, we characterize the motility and the torque-speed relation of the flagellar motor of the slow motile motA(P173A) mutant of Salmonella. Stall torque produced by the mutant motor was at the wild-type level, indicating that neither the number of stators in the motor nor the rotor-stator interaction is affected by the P173A substitution. In agreement with this, the motA(P173A) allele exerted a strong dominant-negative effect on wild-type motility. In contrast, high-speed rotation at low load was significantly impaired by the mutation, suggesting that the maximum rate of torque generation cycle is severely limited. Simulation of the torque-speed curve by a simple kinetic model indicated that the mutation reduces the rate of conformational changes of the MotA/B complex that switches the exposure of Asp33 to the outside and the inside of the cell, thereby slowing down the mechanochemical reaction cycle. Based on these results, we propose that Pro173 plays an important role in facilitating the conformational dynamics of the stator complex for rapid proton translocation and torque generation cycle.

Stator assembly and activation mechanism of the flagellar motor by the periplasmic region of MotB

Torque generation in the Salmonella flagellar motor is coupled to translocation of H(+) ions through the proton-conducting channel of the Mot protein stator complex. The Mot complex is believed to be anchored to the peptidoglycan (PG) layer by the putative peptidoglycan-binding (PGB) domain of MotB. Proton translocation is activated only when the stator is installed into the motor. We report the crystal structure of a C-terminal periplasmic fragment of MotB (MotB(C)) that contains the PGB domain and includes the entire periplasmic region essential for motility. Structural and functional analyses indicate that the PGB domains must dimerize in order to form the proton-conducting channel. Drastic conformational changes in the N-terminal portion of MotB(C) are required both for PG binding and the proton channel activation.

Functional role of a conserved aspartic acid residue in the motor of the Na(+)-driven flagellum from Vibrio cholerae

The flagellar motor consists of a rotor and a stator and couples the flux of cations (H(+) or Na(+)) to the generation of the torque necessary to drive flagellum rotation. The inner membrane proteins PomA and PomB are stator components of the Na(+)-driven flagellar motor from Vibrio cholerae. Affinity-tagged variants of PomA and PomB were co-expressed in trans in the non-motile V. cholerae pomAB deletion strain to study the role of the conserved D23 in the transmembrane helix of PomB. At pH 9, the D23E variant restored motility to 100% of that observed with wild type PomB, whereas the D23N variant resulted in a non-motile phenotype, indicating that a carboxylic group at position 23 in PomB is important for flagellum rotation. Motility tests at decreasing pH revealed a pronounced decline of flagellar function with a motor complex containing the PomB-D23E variant. It is suggested that the protonation state of the glutamate residue at position 23 determines the performance of the flagellar motor by altering the affinity of Na(+) to PomB. The conserved aspartate residue in the transmembrane helix of PomB and its H(+)-dependent homologs might act as a ligand for the coupling cation in the flagellar motor.

Effect of intracellular pH on the torque-speed relationship of bacterial proton-driven flagellar motor

Bacterial flagella responsible for motility are driven by rotary motors powered by the electrochemical potential difference of specific ions across the cytoplasmic membrane. The stator of proton-driven flagellar motor converts proton influx into mechanical work. However, the energy conversion mechanism remains unclear. Here, we show that the motor is sensitive to intracellular proton concentration for high-speed rotation at low load, which was considerably impaired by lowering intracellular pH, while zero-speed torque was not affected. The change in extracellular pH did not show any effect. These results suggest that a high intracellular proton concentration decreases the rate of proton translocation and therefore that of the mechanochemical reaction cycle of the motor but not the actual torque generation step within the cycle by the stator-rotor interactions.