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

Interaction of the C-terminal tail of FliF with FliG from the Na+-driven flagellar motor of Vibrio alginolyticus

Rotation of the polar flagellum of Vibrio alginolyticus is driven by a Na(+)-type flagellar motor. FliG, one of the essential rotor proteins located at the upper rim of the C ring, binds to the membrane-embedded MS ring. The MS ring is composed of a single membrane protein, FliF, and serves as a foundation for flagellar assembly. Unexpectedly, about half of the Vibrio FliF protein produced at high levels in Escherichia coli was found in the soluble fraction. Soluble FliF purifies as an oligomer of ∼700 kDa, as judged by analytical size exclusion chromatography. By using fluorescence correlation spectroscopy, an interaction between a soluble FliF multimer and FliG was detected. This binding was weakened by a series of deletions at the C-terminal end of FliF and was nearly eliminated by a 24-residue deletion or a point mutation at a highly conserved tryptophan residue (W575). Mutations in FliF that caused a defect in FliF-FliG binding abolish flagellation and therefore confer a nonmotile phenotype. As data from in vitro binding assays using the soluble FliF multimer correlate with data from in vivo functional analyses, we conclude that the C-terminal region of the soluble form of FliF retains the ability to bind FliG. Our study confirms that the C-terminal tail of FliF provides the binding site for FliG and is thus required for flagellation in Vibrio, as reported for other species. This is the first report of detection of the FliF-FliG interaction in the Na(+)-driven flagellar motor, both in vivo and in vitro.

The structure and regulation of flagella in Bacillus subtilis

Bacterial flagellar motility is among the most extensively studied physiological systems in biology, but most research has been restricted to using the highly similar Gram-negative species Escherichia coli and Salmonella enterica. Here, we review the recent advances in the study of flagellar structure and regulation of the distantly related and genetically tractable Gram-positive bacterium Bacillus subtilis. B. subtilis has a thicker layer of peptidoglycan and lacks the outer membrane of the Gram-negative bacteria; thus, not only phylogenetic separation but also differences in fundamental cell architecture contribute to deviations in flagellar structure and regulation. We speculate that a large number of flagella and the absence of a periplasm make B. subtilis a premier organism for the study of the earliest events in flagellar morphogenesis and the type III secretion system. Furthermore, B. subtilis has been instrumental in the study of heterogeneous gene transcription in subpopulations and of flagellar regulation at the translational and functional level.

Conformational change in the periplamic region of the flagellar stator coupled with the assembly around the rotor

The torque of the bacterial flagellum is generated by the rotor-stator interaction coupled with the ion flow through the channel in the stator. Anchoring the stator unit to the peptidoglycan layer with proper orientation around the rotor is believed to be essential for smooth rotation of the flagellar motor. The stator unit of the sodium-driven flagellar motor of Vibrio is composed of PomA and PomB, and is thought to be fixed to the peptidoglycan layer and the T-ring by the C-terminal periplasmic region of PomB. Here, we report the crystal structure of a C-terminal fragment of PomB (PomBC) at 2.0-Å resolution, and the structure suggests a conformational change in the N-terminal region of PomBC for anchoring the stator. On the basis of the structure, we designed double-Cys replaced mutants of PomB for in vivo disulfide cross-linking experiments and examined their motility. The motility can be controlled reproducibly by reducing reagent. The results of these experiments suggest that the N-terminal disordered region (121-153) and following the N-terminal two-thirds of α1(154-164) in PomBC changes its conformation to form a functional stator around the rotor. The cross-linking did not affect the localization of the stator nor the ion conductivity, suggesting that the conformational change occurs in the final step of the stator assembly around the rotor.

Molecular architecture of the bacterial flagellar motor in cells

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.

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.

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.

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 tale of two machines: a review of the BLAST meeting, Tucson, AZ, 20-24 January 2013

Since its inception, Bacterial Locomotion and Signal Transduction (BLAST) meetings have been the place to exchange and share the latest developments in the field of bacterial signal transduction and motility. At the 12th BLAST meeting, held last January in Tucson, AZ, researchers from all over the world met to report and discuss progress in diverse aspects of the field. The majority of these advances, however, came at the level of atomic level structures and their associated mechanisms. This was especially true of the biological machines that sense and respond to environmental changes.

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.

Load-dependent assembly of the bacterial flagellar motor

It is becoming clear that the bacterial flagellar motor output is important not only for bacterial locomotion but also for mediating the transition from liquid to surface living. The output of the flagellar motor changes with the mechanical load placed on it by the external environment: at a higher load, the motor runs more slowly and produces higher torque. Here we show that the number of torque-generating units bound to the flagellar motor also depends on the external mechanical load, with fewer stators at lower loads. Stalled motors contained at least as many stators as rotating motors at high load, indicating that rotation is unnecessary for stator binding. Mutant stators incapable of generating torque could not be detected around the motor. We speculate that a component of the bacterial flagellar motor senses external load and mediates the strength of stator binding to the rest of the motor.

The transition between liquid living and surface living is important in the life cycles of many bacteria. In this paper, we describe how the flagellar motor, used by bacteria for locomotion through liquid media and across solid surfaces, is capable of adjusting the number of bound stator units to better suit the external load conditions. By stalling motors using external magnetic fields, we also show that rotation is not required for maintenance of stators around the motor; instead, torque production is the essential factor for motor stability. These new results, in addition to previous data, lead us to hypothesize that the motor stators function as mechanosensors as well as functioning as torque-generating units.

Mutation in the a-subunit of F(1)F(O)-ATPase causes an increased motility phenotype through the sodium-driven flagella of Vibrio

Bacterial flagellar motors exploit the electrochemical potential gradient of a coupling ion as energy source and are composed of stator and rotor proteins. Vibrio alginolyticus has a Na(+)-driven motor and its stator is composed of PomA and PomB. Recently, we isolated increased motility strains (sp1-sp4) from the PomA-N194D/PomB-D24N mutant whose motility was quite weak. To detect the responsible mutation, we have used a next-generation sequencer and determined the entire genome sequences of the sp1 and sp2 strains. Candidate mutations were identified in the gene encoding the a-subunit of F1Fo-ATPase (uncB). To confirm this, we constructed a deletion strain, which gave the increased motility phenotype. The amount of membrane-bound ATPase was reduced in the sp2 and ΔuncB mutants. From these results, we conclude that a mutation in the uncB gene causes the increased motility phenotype in V. alginolyticus. They confer faster motility in low concentrations of sodium than in the parental strain and this phenotype is suppressed in the presence of KCN. Those results may suggest that the proton gradient generated by the respiratory chain is increased by the uncB mutation, consequently the sodium motive force is increased and causes the increased motility phenotype.

Insight into the assembly mechanism in the supramolecular rings of the sodium-driven Vibrio flagellar motor from the structure of FlgT

Flagellar motility is a key factor for bacterial survival and growth in fluctuating environments. The polar flagellum of a marine bacterium, Vibrio alginolyticus, is driven by sodium ion influx and rotates approximately six times faster than the proton-driven motor of Escherichia coli. The basal body of the sodium motor has two unique ring structures, the T ring and the H ring. These structures are essential for proper assembly of the stator unit into the basal body and to stabilize the motor. FlgT, which is a flagellar protein specific for Vibrio sp., is required to form and stabilize both ring structures. Here, we report the crystal structure of FlgT at 2.0-Å resolution. FlgT is composed of three domains, the N-terminal domain (FlgT-N), the middle domain (FlgT-M), and the C-terminal domain (FlgT-C). FlgT-M is similar to the N-terminal domain of TolB, and FlgT-C resembles the N-terminal domain of FliI and the α/β subunits of F1-ATPase. To elucidate the role of each domain, we prepared domain deletion mutants of FlgT and analyzed their effects on the basal-body ring formation. The results suggest that FlgT-N contributes to the construction of the H-ring structure, and FlgT-M mediates the T-ring association on the LP ring. FlgT-C is not essential but stabilizes the H-ring structure. On the basis of these results, we propose an assembly mechanism for the basal-body rings and the stator units of the sodium-driven flagellar motor.

Na+ conductivity of the Na+-driven flagellar motor complex composed of unplugged wild-type or mutant PomB with PomA

PomA and PomB form the stator complex, which functions as a Na(+) channel, in the Na(+)-driven flagellar motor of Vibrio alginolyticus. The plug region of PomB is thought to regulate the Na(+) flow and to suppress massive ion influx through the stator channel. In this study, in order to measure the Na(+) conductivity of the unplugged stator, we over-produced a plug-deleted stator of the Na(+)-driven flagellar motor in Escherichia coli. The over-production of the plug-deleted stator in E. coli cells caused more severe growth inhibition than in Vibrio cells and that growth inhibition depended on the Na(+) concentration in the growth medium. Measurement of intracellular Na(+) concentration by flame photometry and fluorescent analysis with a Na(+) indicator, Sodium Green, revealed that over-production of the plug-deleted stator increased the Na(+) concentration in cell. Some mutations in the channel region of PomB or in the cytoplasmic region of PomA suppressed both the growth inhibition and the increase in intracellular Na(+) concentration. These results suggest that the level of growth inhibition correlates with the intracellular Na(+) concentration, probably due to the Na(+) conductivity through the stator due to the mutations.

Design, purification and characterization of a soluble variant of the integral membrane protein MotB for structural studies

The bacterial flagellar motor is an intricate nanomachine powered by a transmembrane electrochemical gradient. Rotation is driven by the cumulative action of several peptidoglycan-anchored stator complexes on the rotor. In proton-motive force-driven motors, the stator complex is composed of a motility protein B (MotB) dimer surrounded by four copies of MotA, where both MotA and MotB are integral membrane proteins. The lack of full-length MotA and MotB structures hinders understanding of the mechanism of torque generation. Given the low levels of expression and low stability of detergent-solubilized MotB, a soluble chimaeric variant was engineered, where the two transmembrane helices of the MotB dimer were replaced by a leucine zipper. The biochemical and biophysical analysis of the resultant protein showed that it was properly folded, stable, behaved as a monodisperse dimer at low pH, had molecular dimensions close to those expected for native MotB and yielded reproducible crystals. The chimaeric protein is, therefore, a good candidate for structural studies. This 'solubilization by design' approach may be generally applicable to the production of soluble forms of other dimeric, trimeric and tetrameric single-span membrane proteins for functional and structural studies.

Expression, purification and biochemical characterization of the cytoplasmic loop of PomA, a stator component of the Na(+) driven flagellar motor

Flagellar motors embedded in bacterial membranes are molecular machines powered by specific ion flows. Each motor is composed of a stator and a rotor and the interactions of those components are believed to generate the torque. Na⁺ influx through the PomA/PomB stator complex of Vibrio alginolyticus is coupled to torque generation and is speculated to trigger structural changes in the cytoplasmic domain of PomA that interacts with a rotor protein in the C-ring, FliG, to drive the rotation. In this study, we tried to overproduce the cytoplasmic loop of PomA (PomA-Loop), but it was insoluble. Thus, we made a fusion protein with a small soluble tag (GB1) which allowed us to express and characterize the recombinant protein. The structure of the PomA-Loop seems to be very elongated or has a loose tertiary structure. When the PomA-Loop protein was produced in E. coli, a slight dominant effect was observed on motility. We conclude that the cytoplasmic loop alone retains a certain function.

Purification, crystallization and preliminary X-ray crystallographic analysis of diaminopimelate epimerase from Acinetobacter baumannii

The meso isomer of diaminopimelate (meso-DAP) is a biosynthetic precursor of L-lysine in bacteria and plants, and is a key component of the peptidoglycan layer in the cell walls of Gram-negative and some Gram-positive bacteria. Diaminopimelate epimerase (DapF) is a pyridoxal-5'-phosphate-independent racemase which catalyses the interconversion of (6S,2S)-2,6-diaminopimelic acid (LL-DAP) and meso-DAP. In this study, DapF from Acinetobacter baumannii was overexpressed in Escherichia coli strain SoluBL21, purified and crystallized using a vapour-diffusion method. A native crystal diffracted to a resolution of 1.9 Å and belonged to space group P3(1) or P3(2), with unit-cell parameters a = b = 74.91, c = 113.35 Å, α = β = 90, γ = 120°. There were two molecules in the asymmetric unit.

Intragenic suppressor of a plug deletion nonmotility mutation in PotB, a chimeric stator protein of sodium-driven flagella

The torque of bacterial flagellar motors is generated by interactions between the rotor and the stator and is coupled to the influx of H(+) or Na(+) through the stator. A chimeric protein, PotB, in which the N-terminal region of Vibrio alginolyticus PomB was fused to the C-terminal region of Escherichia coli MotB, can function with PomA as a Na(+)-driven stator in E. coli. Here, we constructed a deletion variant of PotB (with a deletion of residues 41 to 91 [Δ41-91], called PotBΔL), which lacks the periplasmic linker region including the segment that works as a "plug" to inhibit premature ion influx. This variant did not confer motile ability, but we isolated a Na(+)-driven, spontaneous suppressor mutant, which has a point mutation (R109P) in the MotB/PomB-specific α-helix that connects the transmembrane and peptidoglycan binding domains of PotBΔL in the region of MotB. Overproduction of the PomA/PotBΔL(R109P) stator inhibited the growth of E. coli cells, suggesting that this stator has high Na(+)-conducting activity. Mutational analyses of Arg109 and nearby residues suggest that the structural alteration in this α-helix optimizes PotBΔL conformation and restores the proper arrangement of transmembrane helices to form a functional channel pore. We speculate that this α-helix plays a key role in assembly-coupled stator activation.

Peptidoglycan maturation enzymes affect flagellar functionality in bacteria

The flagellar machinery is a highly complex organelle composed of a free rotating flagellum and a fixed stator that converts energy into movement. The assembly of the flagella and the stator requires interactions with the peptidoglycan layer through which the organelle has to pass for externalization. Lytic transglycosylases are peptidoglycan degrading enzymes that cleave the sugar backbone of peptidoglycan layer. We show that an endogenous lytic transglycosylase is required for full motility of Helicobacter pylori and colonization of the gastric mucosa. Deficiency of motility resulted from a paralysed phenotype implying an altered ability to generate flagellar rotation. Similarly, another Gram-negative pathogen Salmonella typhimurium and the Gram-positive pathogen Listeria monocytogenes required the activity of lytic transglycosylases, Slt or MltC, and a glucosaminidase (Auto), respectively, for full motility. Furthermore, we show that in absence of the appropriate lytic transglycosylase, the flagellar motor protein MotB from H. pylori does not localize properly to the bacterial pole. We present a new model involving the maturation of the surrounding peptidoglycan for the proper anchoring and functionality of the flagellar motor.

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.

Structural studies of Mycobacterium tuberculosis Rv0899 reveal a monomeric membrane-anchoring protein with two separate domains

Rv0899 from Mycobacterium tuberculosis belongs to the OmpA (outer membrane protein A) family of outer membrane proteins. It functions as a pore-forming protein; the deletion of this gene impairs the uptake of some water-soluble substances, such as serine, glucose, and glycerol. Rv0899 has also been shown to play a part in low-pH environment adaption, which may play a part in pathogenic mycobacteria overcoming the host's defense mechanisms. Based on many bacterial physiological data and recent structural studies, it was proposed that Rv0899 forms an oligomeric channel to carry out such functions. In this work, biochemical and structural data obtained from solution NMR and EPR spectroscopy indicated that Rv0899 is a monomeric membrane-anchoring protein with two separate domains, rather than an oligomeric pore. Using NMR chemical shift perturbation and isothermal calorimetric titration assays, we show that Rv0899 was able to interact with Zn(2+) ions, which may indicate a role for Rv0899 in the process of Zn(2+) acquisition.

Mutations targeting the C-terminal domain of FliG can disrupt motor assembly in the Na(+)-driven flagella of Vibrio alginolyticus

The torque of the bacterial flagellar motor is generated by the rotor-stator interaction coupled with specific ion translocation through the stator channel. To produce a fully functional motor, multiple stator units must be properly incorporated around the rotor by an as yet unknown mechanism to engage the rotor-stator interactions. Here, we investigated stator assembly using a mutational approach of the Na(+)-driven polar flagellar motor of Vibrio alginolyticus, whose stator is localized at the flagellated cell pole. We mutated a rotor protein, FliG, which is located at the C ring of the basal body and closely participates in torque generation, and found that point mutation L259Q, L270R or L271P completely abolishes both motility and polar localization of the stator without affecting flagellation. Likewise, mutations V274E and L279P severely affected motility and stator assembly. Those residues are localized at the core of the globular C-terminal domain of FliG when mapped onto the crystal structure of FliG from Thermotoga maritima, which suggests that those mutations induce quite large structural alterations at the interface responsible for the rotor-stator interaction. These results show that the C-terminal domain of FliG is critical for the proper assembly of PomA/PomB stator complexes around the rotor and probably functions as the target of the stator at the rotor side.

Mechanism of anchoring of OmpA protein to the cell wall peptidoglycan of the gram-negative bacterial outer membrane

The outer membrane protein A (OmpA) plays important roles in anchoring of the outer membrane to the bacterial cell wall. The C-terminal periplasmic domain of OmpA (OmpA-like domain) associates with the peptidoglycan (PGN) layer noncovalently. However, there is a paucity of information on the structural aspects of the mechanism of PGN recognition by OmpA-like domains. To elucidate this molecular recognition process, we solved the high-resolution crystal structure of an OmpA-like domain from Acinetobacter baumannii bound to diaminopimelate (DAP), a unique bacterial amino acid from the PGN. The structure clearly illustrates that two absolutely conserved Asp271 and Arg286 residues are the key to the binding to DAP of PGN. Identification of DAP as the central anchoring site of PGN to OmpA is further supported by isothermal titration calorimetry and a pulldown assay with PGN. An NMR-based computational model for complexation between the PGN and OmpA emerged, and this model is validated by determining the crystal structure in complex with a synthetic PGN fragment. These structural data provide a detailed glimpse of how the anchoring of OmpA to the cell wall of gram-negative bacteria takes place in a DAP-dependent manner.

Protein dynamics and mechanisms controlling the rotational behaviour of the bacterial flagellar motor

The proteins that make up the bacterial flagellar rotary motor have recently been shown to be more dynamic than previously thought, with some key proteins exchanging with pools of proteins in the membrane/cytoplasm. It has also become clear that in addition to simply switching in response to chemosensory signals, the rotation of the bacterial flagellar motor can be slowed or stopped, using a clutch or a brake, by signals from metabolism and growth state.

Sodium-driven motor of the polar flagellum in marine bacteria Vibrio

The Na(+) -driven bacterial flagellar motor is a molecular machine powered by an electrochemical potential gradient of sodium ions across the cytoplasmic membrane. The marine bacterium Vibrio alginolyticus has a single polar flagellum that enables it to swim in liquid. The flagellar motor contains a basal body and a stator complexes, which are composed of several proteins. PomA, PomB, MotX, and MotY are thought to be essential components of the stator that are required to generate the torque of the rotation. Several mutations have been investigated to understand the characteristics and function of the ion channel in the stator and the mechanism of its assembly around the rotor to complete the motor. In this review, we summarize recent results of the Na(+) -driven motor in the polar flagellum of Vibrio.

A conserved residue, PomB-F22, in the transmembrane segment of the flagellar stator complex, has a critical role in conducting ions and generating torque

Bacterial flagellar motors exploit the electrochemical potential gradient of a coupling ion (H+ or Na+) as their energy source, and are composed of stator and rotor proteins. Sodium-driven and proton-driven motors have the stator proteins PomA and PomB or MotA and MotB, respectively, which interact with each other in their transmembrane (TM) regions to form an ion channel. The single TM region of PomB or MotB, which forms the ion-conduction pathway together with TM3 and TM4 of PomA or MotA, respectively, has a highly conserved aspartate residue that is the ion binding site and is essential for rotation. To investigate the ion conductivity and selectivity of the Na+-driven PomA/PomB stator complex, we replaced conserved residues predicted to be near the conserved aspartate with H+-type residues, PomA-N194Y, PomB-F22Y and/or PomB-S27T. Motility analysis revealed that the ion specificity was not changed by either of the PomB mutations. PomB-F22Y required a higher concentration of Na+ to exhibit swimming, but this effect was suppressed by additional mutations, PomA-N194Y or PomB-S27T. Moreover, the motility of the PomB-F22Y mutant was resistant to phenamil, a specific inhibitor for the Na+ channel. When PomB-F22 was changed to other amino acids and the effects on swimming ability were investigated, replacement with a hydrophilic residue decreased the maximum swimming speed and conferred strong resistance to phenamil. From these results, we speculate that the Na+ flux is reduced by the PomB-F22Y mutation, and that PomB-F22 is important for the effective release of Na+ from PomB-D24.

Structure of the flagellar motor protein complex PomAB: implications for the torque-generating conformation

The bacterial flagellar motor is driven by an ion flux through a channel called MotAB in Escherichia coli or Salmonella and PomAB in Vibrio alginolyticus. PomAB is composed of two transmembrane (TM) components, PomA and PomB, and converts a sodium ion flux to rotation of the flagellum. Its homolog, MotAB, utilizes protons instead of sodium ions. PomB/MotB has a peptidoglycan (PG)-binding motif in the periplasmic domain, allowing it to function as the stator by being anchored to the PG layer. To generate torque, PomAB/MotAB is thought to undergo a conformational change triggered by the ion flux and to interact directly with FliG, a component of the rotor. Here, we present the first three-dimensional structure of this torque-generating stator unit analyzed by electron microscopy. The structure of PomAB revealed two arm domains, which contain the PG-binding site, connected to a large base made of the TM and cytoplasmic domains. The arms lean downward to the membrane surface, likely representing a "plugged" conformation, which would prevent ions leaking through the channel. We propose a model for how PomAB units are placed around the flagellar basal body to function as torque generators.

Characterization of the periplasmic region of PomB, a Na+-driven flagellar stator protein in Vibrio alginolyticus

The stator proteins PomA and PomB form a complex that couples Na(+) influx to torque generation in the polar flagellar motor of Vibrio alginolyticus. This stator complex is anchored to an appropriate place around the rotor through a putative peptidoglycan-binding (PGB) domain in the periplasmic region of PomB (PomB(C)). To investigate the function of PomB(C), a series of N-terminally-truncated and in-frame mutants with deletions between the transmembrane (TM) segment and the PGB domain of PomB was constructed. A PomB(C) fragment consisting of residues 135 to 315 (PomB(C₅) formed a stable homodimer and significantly inhibited the motility of wild-type cells when overexpressed in the periplasm. A fragment with an in-frame deletion (PomB(ΔL)) of up to 80 residues retained function, and its overexpression with PomA impaired cell growth. This inhibitory effect was suppressed by a mutation at the functionally critical Asp (D24N) in the TM segment of PomB, suggesting that a high level of Na(+) influx through the mutant stator causes the growth impairment. The overproduction of functional PomA/PomB(ΔL) stators also reduced the motile fractions of the cells. That effect could be slightly relieved by a mutation (L168P) in the putative N-terminal α-helix that connects to the PGB domain without affecting the growth inhibition, suggesting that a conformational change of the region including the PGB domain affects stator assembly. Our results reveal common features of the periplasmic region of PomB/MotB and demonstrate that a flexible linker that contains a "plug" segment is important for the control of Na(+) influx through the stator complex as well as for stator assembly.

Crystallographic and molecular dynamics analysis of loop motions unmasking the peptidoglycan-binding site in stator protein MotB of flagellar motor

Background

The C-terminal domain of MotB (MotB-C) shows high sequence similarity to outer membrane protein A and related peptidoglycan (PG)-binding proteins. It is believed to anchor the power-generating MotA/MotB stator unit of the bacterial flagellar motor to the peptidoglycan layer of the cell wall. We previously reported the first crystal structure of this domain and made a puzzling observation that all conserved residues that are thought to be essential for PG recognition are buried and inaccessible in the crystal structure. In this study, we tested a hypothesis that peptidoglycan binding is preceded by, or accompanied by, some structural reorganization that exposes the key conserved residues.

Methodology/Principal Findings

We determined the structure of a new crystalline form (Form B) of Helicobacter pylori MotB-C. Comparisons with the existing Form A revealed conformational variations in the petal-like loops around the carbohydrate binding site near one end of the β-sheet. These variations are thought to reflect natural flexibility at this site required for insertion into the peptidoglycan mesh. In order to understand the nature of this flexibility we have performed molecular dynamics simulations of the MotB-C dimer. The results are consistent with the crystallographic data and provide evidence that the three loops move in a concerted fashion, exposing conserved MotB residues that have previously been implicated in binding of the peptide moiety of peptidoglycan.

Conclusion/Significance

Our structural analysis provides a new insight into the mechanism by which MotB inserts into the peptidoglycan mesh, thus anchoring the power-generating complex to the cell wall.

Development of real-time PCR assays for detection and quantification of Bacillus cereus group species: differentiation of B. weihenstephanensis and rhizoid B. pseudomycoides isolates from milk

Quantitative real-time PCR (qRT-PCR) offers an alternative method for the detection of bacterial contamination in food. This method provides the quantitation and determination of the number of gene copies. In our study, we established an RT-PCR assay using the LightCycler system to detect and quantify the Bacillus cereus group species, which includes B. cereus, B. anthracis, B. thuringiensis, B. weihenstephanensis, B. mycoides, and B. pseudomycoides. A TaqMan assay was designed to detect a 285-bp fragment of the motB gene encoding the flagellar motor protein, which was specific for the detection of the B. cereus group species, excluding B. pseudomycoides, and the detection of a 217-bp gene fragment of a hypothetical protein specific only for B. pseudomycoides strains. Based on three hydrolysis probes (MotB-FAM-1, MotB-FAM-2, and Bpm-FAM-1), it was possible to differentiate B. weihenstephanensis from the B. cereus group species with nonrhizoid growth and B. pseudomycoides from the whole B. cereus group. The specificity of the assay was confirmed with 119 strains belonging to the Bacillus cereus group species and was performed against 27 other Bacillus and non-Bacillus bacteria. A detection limit was determined for each assay. The assays performed well not only with purified DNA but also with DNA extracted from milk samples artificially contaminated with bacteria that belong to the B. cereus group species. This technique represents an alternative approach to traditional culture methods for the differentiation of B. cereus group species and differentiates B. weihenstephanensis and B. pseudomycoides in one reaction.

Assembly of the type II secretion system: identification of ExeA residues critical for peptidoglycan binding and secretin multimerization

Aeromonas hydrophila secretes a number of protein toxins across the outer membrane via the type II secretion system (T2SS). Assembly of the secretion channel ExeD secretin into the outer membrane is dependent on the peptidoglycan binding domain of ExeA. In this study, the peptidoglycan binding domain PF01471 family members were divided into a prokaryotic group and a eukaryotic group. By comparison of their sequence conservation profiles and their representative crystal structures, we found the prokaryotic members to have a highly conserved pocket(s) that is not present in the eukaryotic members. Substitution mutations of nine amino acids of the pocket were constructed in ExeA. Five of the substitution derivatives showed greatly decreased lipase secretion, accompanied by defects in secretin assembly. In addition, using in vivo cross-linking and in vitro cosedimentation assays, we showed that these mutations decreased ExeA-peptidoglycan interactions. These results suggest that the highly conserved pocket in ExeA is the binding site for its peptidoglycan ligand and identify residues critical for this binding.

Tuning the flagellar motor

Many bacteria are motile by means of flagella, semi-rigid helical filaments rotated at the filament's base and energized by proton or sodium-ion gradients. Torque is created between the two major components of the flagellar motor: the rotating switch complex and the cell-wall-associated stators, which are arranged in a dynamic ring-like structure. Being motile provides a survival advantage to many bacteria, and thus the flagellar motor should work optimally under a wide range of environmental conditions. Recent studies have demonstrated that numerous species possess a single flagellar system but have two or more individual stator systems that contribute differentially to flagellar rotation. This review describes recent findings on rotor–stator interactions, on the role of different stators, and on how stator selection could be regulated. An emerging model suggests that bacterial flagellar motors are dynamic and can be tuned by stator swapping in response to different environmental conditions.