Dynamics of mechanosensing in the bacterial flagellar 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.

Altered motility of Caulobacter Crescentus in viscous and viscoelastic media

BACKGROUND

Motility of flagellated bacteria depends crucially on their organelles such as flagella and pili, as well as physical properties of the external medium, such as viscosity and matrix elasticity. We studied the motility of wild-type and two mutant strains of Caulobacter crescentus swarmer cells in two different types of media: a viscous and hyperosmotic glycerol-growth medium mixture and a viscoelastic growth medium, containing polyethylene glycol or polyethylene oxide of different defined sizes.

RESULTS

For all three strains in the medium containing glycerol, we found linear drops in percentage of motile cells and decreases in speed of those that remained motile to be inversely proportional to viscosity. The majority of immobilized cells lost viability, evidenced by their membrane leakage. In the viscoelastic media, we found less loss of motility and attenuated decrease of swimming speed at shear viscosity values comparable to the viscous medium. In both types of media, we found more severe loss in percentage of motile cells of wild-type than the mutants without pili, indicating that the interference of pili with flagellated motility is aggravated by increased viscosity. However, we found no difference in swimming speed among all three strains under all test conditions for the cells that remained motile. Finally, the viscoelastic medium caused no significant change in intervals between flagellar motor switches unless the motor stalled.

CONCLUSION

Hyperosmotic effect causes loss of motility and cell death. Addition of polymers into the cell medium also causes loss of motility due to increased shear viscosity, but the majority of immobilized bacteria remain viable. Both viscous and viscoelastic media alter the motility of flagellated bacteria without affecting the internal regulation of their motor switching behavior.

Fifty ways to inhibit motility via cyclic di-GMP: the emerging Pseudomonas aeruginosa swarming story

There are numerous ways by which cyclic dimeric GMP (c-di-GMP) inhibits motility. Kuchma et al. (S. L. Kuchma, N. J. Delalez, L. M. Filkins, E. A. Snavely, J. P. Armitage, and G. A. O'Toole, J. Bacteriol. 197:420-430, 2015, http://dx.doi.org/10.1128/JB.02130-14) offer a new, previously unseen way of swarming motility inhibition in Pseudomonas aeruginosa PA14. This bacterium possesses a single flagellum with one rotor and two sets of stators, only one of which can provide torque for swarming. The researchers discovered that elevated levels of c-di-GMP inhibit swarming by skewing stator selection in favor of the nonfunctional, "bad" stators.

Surface attachment induces Pseudomonas aeruginosa virulence

Pseudomonas aeruginosa infects every type of host that has been examined by deploying multiple virulence factors. Previous studies of virulence regulation have largely focused on chemical cues, but P. aeruginosa may also respond to mechanical cues. Using a rapid imaging-based virulence assay, we demonstrate that P. aeruginosa activates virulence in response to attachment to a range of chemically distinct surfaces, suggesting that this bacterial species responds to mechanical properties of its substrates. Surface-activated virulence requires quorum sensing, but activating quorum sensing does not induce virulence without surface attachment. The activation of virulence by surfaces also requires the surface-exposed protein PilY1, which has a domain homologous to a eukaryotic mechanosensor. Specific mutation of the putative PilY1 mechanosensory domain is sufficient to induce virulence in non-surface-attached cells, suggesting that PilY1 mediates surface mechanotransduction. Triggering virulence only when cells are both at high density and attached to a surface—two host-nonspecific cues—explains how P. aeruginosa precisely regulates virulence while maintaining broad host specificity.

Cyclic di-GMP-mediated repression of swarming motility by Pseudomonas aeruginosa PA14 requires the MotAB stator

The second messenger cyclic diguanylate (c-di-GMP) plays a critical role in the regulation of motility. In Pseudomonas aeruginosa PA14, c-di-GMP inversely controls biofilm formation and surface swarming motility, with high levels of this dinucleotide signal stimulating biofilm formation and repressing swarming. P. aeruginosa encodes two stator complexes, MotAB and MotCD, that participate in the function of its single polar flagellum. Here we show that the repression of swarming motility requires a functional MotAB stator complex. Mutating the motAB genes restores swarming motility to a strain with artificially elevated levels of c-di-GMP as well as stimulates swarming in the wild-type strain, while overexpression of MotA from a plasmid represses swarming motility. Using point mutations in MotA and the FliG rotor protein of the motor supports the conclusion that MotA-FliG interactions are critical for c-di-GMP-mediated swarming inhibition. Finally, we show that high c-di-GMP levels affect the localization of a green fluorescent protein (GFP)-MotD fusion, indicating a mechanism whereby this second messenger has an impact on MotCD function. We propose that when c-di-GMP level is high, the MotAB stator can displace MotCD from the motor, thereby affecting motor function. Our data suggest a newly identified means of c-di-GMP-mediated control of surface motility, perhaps conserved among Pseudomonas, Xanthomonas, and other organisms that encode two stator systems.

Switching dynamics of the bacterial flagellar motor near zero load

Switching dynamics of flagellar motors of Escherichia coli is commonly observed through markers attached to the flagellar filaments. To eliminate possible complications resulting from the conformational transitions of these filaments and to look at the output of motors more directly, we monitored motor rotation by attaching nanogold spheres to the hooks of cells lacking filaments. We observed exponentially distributed counterclockwise (CCW) and clockwise (CW) intervals and Lorentzian power spectra of the switching time series consistent with models that treat motor switching as a two-state Poisson process.

Loss of FliL alters Proteus mirabilis surface sensing and temperature-dependent swarming

Proteus mirabilis is a dimorphic motile bacterium well known for its flagellum-dependent swarming motility over surfaces. In liquid, P. mirabilis cells are 1.5- to 2.0-μm swimmer cells with 4 to 6 flagella. When P. mirabilis encounters a solid surface, where flagellar rotation is limited, swimmer cells differentiate into elongated (10- to 80-μm), highly flagellated swarmer cells. In order for P. mirabilis to swarm, it first needs to detect a surface. The ubiquitous but functionally enigmatic flagellar basal body protein FliL is involved in P. mirabilis surface sensing. Previous studies have suggested that FliL is essential for swarming through its involvement in viscosity-dependent monitoring of flagellar rotation. In this study, we constructed and characterized ΔfliL mutants of P. mirabilis and Escherichia coli. Unexpectedly and unlike other fliL mutants, both P. mirabilis and E. coli ΔfliL cells swarm (Swr(+)). Further analysis revealed that P. mirabilis ΔfliL cells also exhibit an alteration in their ability to sense a surface: e.g., ΔfliL P. mirabilis cells swarm precociously over surfaces with low viscosity that normally impede wild-type swarming. Precocious swarming is due to an increase in the number of elongated swarmer cells in the population. Loss of fliL also results in an inhibition of swarming at <30°C. E. coli ΔfliL cells also exhibit temperature-sensitive swarming. These results suggest an involvement of FliL in the energetics and function of the flagellar motor.

Stoichiometry and turnover of the bacterial flagellar switch protein FliN

Some proteins in biological complexes exchange with pools of free proteins while the complex is functioning. Evidence is emerging that protein exchange can be part of an adaptive mechanism. The bacterial flagellar motor is one of the most complex biological machines and is an ideal model system to study protein dynamics in large multimeric complexes. Recent studies showed that the copy number of FliM in the switch complex and the fraction of FliM that exchanges vary with the direction of flagellar rotation. Here, we investigated the stoichiometry and turnover of another switch complex component, FliN, labeled with the fluorescent protein CyPet, in Escherichia coli. Our results confirm that, in vivo, FliM and FliN form a complex with stoichiometry of 1:4 and function as a unit. We estimated that wild-type motors contained 120 ± 26 FliN molecules. Motors that rotated only clockwise (CW) or counterclockwise (CCW) contained 114 ± 17 and 144 ± 26 FliN molecules, respectively. The ratio of CCW-to-CW FliN copy numbers was 1.26, very close to that of 1.29 reported previously for FliM. We also measured the exchange of FliN molecules, which had a time scale and dependence upon rotation direction similar to those of FliM, consistent with an exchange of FliM-FliN as a unit. Our work confirms the highly dynamic nature of multimeric protein complexes and indicates that, under physiological conditions, these machines might not be the stable, complete structures suggested by averaged fixed methodologies but, rather, incomplete rings that can respond and adapt to changing environments.

The flagellum is one of the most complex structures in a bacterial cell, with the core motor proteins conserved across species. Evidence is now emerging that turnover of some of these motor proteins depends on motor activity, suggesting that turnover is important for function. The switch complex transmits the chemosensory signal to the rotor, and we show, by using single-cell measurement, that both the copy number and the fraction of exchanging molecules vary with the rotational bias of the rotor. When the motor is locked in counterclockwise rotation, the copy number is similar to that determined by averaged, fixed methodologies, but when locked in a clockwise direction, the number is much lower, suggesting that that the switch complex ring is incomplete. Our results suggest that motor remodeling is an important component in tuning responses and adaptation at the motor.

Building a flagellum outside the bacterial cell

Flagella, the helical propellers that extend from the bacterial surface, are a paradigm for how complex molecular machines can be built outside the living cell. Their assembly requires ordered export of thousands of structural subunits across the cell membrane and this is achieved by a type III export machinery located at the flagellum base, after which subunits transit through a narrow channel at the core of the flagellum to reach the assembly site at the tip of the nascent structure, up to 20μm from the cell surface. Here we review recent findings that provide new insights into flagellar export and assembly, and a new and unanticipated mechanism for constant rate flagellum growth.

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.

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.

Hybrid-fuel bacterial flagellar motors in Escherichia coli

The bacterial flagellar motor rotates driven by an electrochemical ion gradient across the cytoplasmic membrane, either H(+) or Na(+) ions. The motor consists of a rotor ∼50 nm in diameter surrounded by multiple torque-generating ion-conducting stator units. Stator units exchange spontaneously between the motor and a pool in the cytoplasmic membrane on a timescale of minutes, and their stability in the motor is dependent upon the ion gradient. We report a genetically engineered hybrid-fuel flagellar motor in Escherichia coli that contains both H(+)- and Na(+)-driven stator components and runs on both types of ion gradient. We controlled the number of each type of stator unit in the motor by protein expression levels and Na(+) concentration ([Na(+)]), using speed changes of single motors driving 1-μm polystyrene beads to determine stator unit numbers. De-energized motors changed from locked to freely rotating on a timescale similar to that of spontaneous stator unit exchange. Hybrid motor speed is simply the sum of speeds attributable to individual stator units of each type. With Na(+) and H(+) stator components expressed at high and medium levels, respectively, Na(+) stator units dominate at high [Na(+)] and are replaced by H(+) units when Na(+) is removed. Thus, competition between stator units for spaces in a motor and sensitivity of each type to its own ion gradient combine to allow hybrid motors to adapt to the prevailing ion gradient. We speculate that a similar process may occur in species that naturally express both H(+) and Na(+) stator components sharing a common rotor.

Defects in the flagellar motor increase synthesis of poly-γ-glutamate in Bacillus subtilis

Bacillus subtilis swims in liquid media and swarms over solid surfaces, and it encodes two sets of flagellar stator homologs. Here, we show that B. subtilis requires only the MotA/MotB stator during swarming motility and that the residues required for stator force generation are highly conserved from the Proteobacteria to the Firmicutes. We further find that mutants that abolish stator function also result in an overproduction of the extracellular polymer poly-γ-glutamate (PGA) to confer a mucoid colony phenotype. PGA overproduction appeared to be the result of an increase in the expression of the pgs operon that encodes genes for PGA synthesis. Transposon mutagenesis was conducted to identify insertions that abolished colony mucoidy and disruptions in known transcriptional regulators of PGA synthesis (Com and Deg two-component systems) as well as mutants defective in transcription-coupled DNA repair (Mfd)-reduced expression of the pgs operon. A final class of insertions disrupted proteins involved in the assembly of the flagellar filament (FliD, FliT, and FlgL), and these mutants did not reduce expression of the pgs operon, suggesting a second mechanism of PGA control.

ATP synthase: the right size base model for nanomotors in nanomedicine

Nanomedicine results from nanotechnology where molecular scale minute precise nanomotors can be used to treat disease conditions. Many such biological nanomotors are found and operate in living systems which could be used for therapeutic purposes. The question is how to build nanomachines that are compatible with living systems and can safely operate inside the body? Here we propose that it is of paramount importance to have a workable base model for the development of nanomotors in nanomedicine usage. The base model must placate not only the basic requirements of size, number, and speed but also must have the provisions of molecular modulations. Universal occurrence and catalytic site molecular modulation capabilities are of vital importance for being a perfect base model. In this review we will provide a detailed discussion on ATP synthase as one of the most suitable base models in the development of nanomotors. We will also describe how the capabilities of molecular modulation can improve catalytic and motor function of the enzyme to generate a catalytically improved and controllable ATP synthase which in turn will help in building a superior nanomotor. For comparison, several other biological nanomotors will be described as well as their applications for nanotechnology.

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.

Structure, gene regulation and environmental response of flagella in Vibrio

Vibrio species are Gram-negative, rod-shaped bacteria that live in aqueous environments. Several species, such as V. harveyi, V. alginotyticus, and V. splendidus, are associated with diseases in fish or shellfish. In addition, a few species, such as V. cholerae and V. parahaemolyticus, are risky for humans due to infections from eating raw shellfish infected with these bacteria or from exposure of wounds to the marine environment. Bacterial flagella are not essential to live in a culture medium. However, most Vibrio species are motile and have rotating flagella which allow them to move into favorable environments or to escape from unfavorable environments. This review summarizes recent studies about the flagellar structure, function, and regulation of Vibrio species, especially focused on the Na⁺-driven polar flagella that are principally responsible for motility and sensing the surrounding environment, and discusses the relationship between flagella and pathogenicity.

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 mechanical signal transmitted by the flagellum controls signalling in Bacillus subtilis

In the natural environment bacteria predominantly live adhered to a surface as part of a biofilm. While many of the components needed for biofilm assembly are known, the mechanism by which microbes sense and respond to contact with a surface is poorly understood. Bacillus subtilis is a Gram-positive model for biofilm formation. The DegS–DegU two-component system controls several multicellular behaviours in B. subtilis, including biofilm formation. Here we identify the B. subtilis flagellum as a mechanosensor that activates the DegS–DegU regulatory pathway. Inhibition of flagellar rotation by deletion or mutation of the flagellar stator gene, motB, results in an increase in both degU transcription and DegU∼P driven processes, namely exoprotease production and poly-γ-dl-glutamic acid biosynthesis. Similarly, inhibition of flagellar rotation by engaging the flagellar clutch or by tethering the flagella with antibodies also promotes an increase in degU transcription that is reflective of increased DegU∼P levels in the cell. Collectively, these findings strongly indicate that inhibition of flagellar rotation acts as a mechanical trigger to activate the DegS–DegU two-component signal transduction system. We postulate that inhibition of flagellar rotation could function as a mechanical trigger to activate bacterial signal transduction cascades in many motile bacteria upon contact with a surface.