Interaction of PomB with the third transmembrane segment of PomA in the Na+-driven polar flagellum of Vibrio alginolyticus

Putative Spanner Function of the Vibrio PomB Plug Region in the Stator Rotation Model for Flagellar Motor

Bacterial flagella are the best-known rotational organelles in the biological world. The spiral-shaped flagellar filaments that extend from the cell surface rotate like a screw to create a propulsive force. At the base of the flagellar filament lies a protein motor that consists of a stator and a rotor embedded in the membrane. The stator is composed of two types of membrane subunits, PomA (similar to MotA in Escherichia coli) and PomB (similar to MotB in E. coli), which are energy converters that assemble around the rotor to couple rotation with the ion flow. Recently, stator structures, where two MotB molecules are inserted into the center of a ring made of five MotA molecules, were reported. This structure inspired a model in which the MotA ring rotates around the MotB dimer in response to ion influx. Here, we focus on the Vibrio PomB plug region, which is involved in flagellar motor activation. We investigated the plug region using site-directed photo-cross-linking and disulfide cross-linking experiments. Our results demonstrated that the plug interacts with the extracellular short loop region of PomA, which is located between transmembrane helices 3 and 4. Although the motor stopped rotating after cross-linking, its function recovered after treatment with a reducing reagent that disrupted the disulfide bond. Our results support the hypothesis, which has been inferred from the stator structure, that the plug region terminates the ion influx by blocking the rotation of the rotor as a spanner. IMPORTANCE The biological flagellar motor resembles a mechanical motor. It is composed of a stator and a rotor. The force is transmitted to the rotor by the gear-like stator movements. It has been proposed that the pentamer of MotA subunits revolves around the axis of the B subunit dimer in response to ion flow. The plug region of the B subunit regulates the ion flow. Here, we demonstrated that the ion flow was terminated by cross-linking the plug region of PomB with PomA. These findings support the rotation hypothesis and explain the role of the plug region in blocking the rotation of the stator unit.

Essential ion binding residues for Na+ flow in stator complex of the Vibrio flagellar motor

The bacterial flagellar motor is a unique supramolecular complex which converts ion flow into rotational force. Many biological devices mainly use two types of ions, proton and sodium ion. This is probably because of the fact that life originated in seawater, which is rich in protons and sodium ions. The polar flagellar motor in Vibrio is coupled with sodium ion and the energy converting unit of the motor is composed of two membrane proteins, PomA and PomB. It has been shown that the ion binding residue essential for ion transduction is the conserved aspartic acid residue (PomB-D24) in the PomB transmembrane region. To reveal the mechanism of ion selectivity, we identified essential residues, PomA-T158 and PomA-T186, other than PomB-D24, in the Na⁺-driven flagellar motor. It has been shown that the side chain of threonine contacts Na⁺ in Na⁺-coupled transporters. We monitored the Na⁺-binding specific structural changes using ATR-FTIR spectroscopy. The signals were abolished in PomA-T158A and -T186A, as well as in PomB-D24N. Molecular dynamics simulations further confirmed the strong binding of Na⁺ to D24 and showed that T158A and T186A hindered the Na⁺ binding and transportation. The data indicate that two threonine residues (PomA-T158 and PomA-T186), together with PomB-D24, are important for Na⁺ conduction in the Vibrio flagellar motor. The results contribute to clarify the mechanism of ion recognition and conversion of ion flow into mechanical force.

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.

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.

A quinazoline-2,4-diamino analog suppresses Vibrio cholerae flagellar motility by interacting with motor protein PomB and induces envelope stress

Vibrio cholerae strains of serogroups O1 and O139, the causative agents of the diarrheal illness cholera, express a single polar flagellum powered by sodium motive force and require motility to colonize and spread along the small intestine. In a previous study, we described a high-throughput assay for screening for small molecules that selectively inhibit bacterial motility and identified a family of quinazoline-2,4-diamino analogs (Q24DAs) that (i) paralyzed the sodium-driven polar flagellum of Vibrios and (ii) diminished cholera toxin secreted by El Tor biotype V. cholerae. In this study, we provide evidence that a Q24DA paralyzes the polar flagellum by interacting with the motor protein PomB. Inhibition of motility with the Q24DA enhanced the transcription of the cholera toxin genes in both biotypes. We also show that the Q24DA interacts with outer membrane protein OmpU and other porins to induce envelope stress and expression of the extracellular RNA polymerase sigma factor σ(E). We suggest that Q24DA-induced envelope stress could affect the correct folding, assembly, and secretion of pentameric cholera toxin in El Tor biotype V. cholerae independently of its effect on motility.

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.

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.

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.

Aeromonas hydrophila motY is essential for polar flagellum function, and requires coordinate expression of motX and Pom proteins

By the analysis of the Aeromonas hydrophila ATCC7966(T) genome we identified A. hydrophila AH-3 MotY. A. hydrophila MotY, like MotX, is essential for the polar flagellum function energized by an electrochemical potential of Na(+) as coupling ion, but is not involved in lateral flagella function energized by the proton motive force. Thus, the A. hydrophila polar flagellum stator is a complex integrated by two essential proteins, MotX and MotY, which interact with one of two redundant pairs of proteins, PomAB and PomA(2)B(2). In an A. hydrophila motX mutant, polar flagellum motility is restored by motX complementation, but the ability of the A. hydrophila motY mutant to swim is not restored by introduction of the wild-type motY alone. However, its polar flagellum motility is restored when motX and -Y are expressed together from the same plasmid promoter. Finally, even though both the redundant A. hydrophila polar flagellum stators, PomAB and PomA(2)B(2), are energized by the Na(+) ion, they cannot be exchanged. Furthermore, Vibrio parahaemolyticus PomAB and Pseudomonas aeruginosa MotAB or MotCD are unable to restore swimming motility in A. hydrophila polar flagellum stator mutants.

Comparative study of the ion flux pathway in stator units of proton- and sodium-driven flagellar motors

Flagellar motor proteins, MotA/B and PomA/B, are essential for the motility of Escherichia coli and Vibrio alginolyticus, respectively. Those complexes work as a H⁺ and a Na⁺ channel, respectively and play important roles in torque generation as the stators of the flagellar motors. Although Asp32 of MotB and Asp24 of PomB are believed to function as ion binding site(s), the ion flux pathway from the periplasm to the cytoplasm is still unclear. Conserved residues, Ala39 of MotB and Cys31 of PomB, are located on the same sides as Asp32 of MotB and Asp24 of PomB, respectively, in a helical wheel diagram. In this study, a series of mutations were introduced into the Ala39 residue of MotB and the Cys31 residue of PomB. The motility of mutant cells were markedly decreased as the volume of the side chain increased. The loss of function due to the MotB(A39V) and PomB(L28A/C31A) mutations was suppressed by mutations of MotA(M206S) and PomA(L183F), respectively, and the increase in the volume caused by the MotB(A39V) mutation was close to the decrease in the volume caused by the MotA(M206S) mutation. These results demonstrate that Ala39 of MotB and Cys31 of PomB form part of the ion flux pathway and pore with Met206 of MotA and Leu183 of PomA in the MotA/B and PomA/B stator units, respectively.

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.

Cell-free synthesis of the torque-generating membrane proteins, PomA and PomB, of the Na+-driven flagellar motor in Vibrio alginolyticus

Flagellar motor proteins, PomA and PomB, are essential for converting the sodium motive force into rotational energy in the Na(+)-driven flagella motor of Vibrio alginolyticus. PomA and PomB, which are cytoplasmic membrane proteins, together comprise the stator complex of the motor and form a Na(+) channel. We tried to synthesize PomA and PomB by using the cell-free protein synthesis system, PURESYSTEM. We succeeded in doing so in the presence of liposomes, and showed an interaction between them using the pull-down assay. It seems likely that the proteins are inserted into liposomes and assembled spontaneously. The N-terminal region of in vitro synthesized PomB appeared to be lost, but this problem was suppressed by fusing GFP to the N-terminus of PomB or by mutagenesis at Pro-11 or Pro-12. A structural change of the N-terminal region of PomB by these modifications may prevent cleavage during protein synthesis in PURESYSTEM. The mutations did not affect the functioning of the motor. Using this system, biochemical analysis of PomA and PomB can be performed easily and efficiently.

Flagellar motility in bacteria structure and function of flagellar motor

Bacterial flagella are filamentous organelles that drive cell locomotion. They thrust cells in liquids (swimming) or on surfaces (swarming) so that cells can move toward favorable environments. At the base of each flagellum, a reversible rotary motor, which is powered by the proton- or the sodium-motive force, is embedded in the cell envelope. The motor consists of two parts: the rotating part, or rotor, that is connected to the hook and the filament, and the nonrotating part, or stator, that conducts coupling ion and is responsible for energy conversion. Intensive genetic and biochemical studies of the flagellum have been conducted in Salmonella typhimurium and Escherichia coli, and more than 50 gene products are known to be involved in flagellar assembly and function. The energy-coupling mechanism, however, is still not known. In this chapter, we survey our current knowledge of the flagellar system, based mostly on studies from Salmonella, E. coli, and marine species Vibrio alginolyticus, supplemented with distinct aspects of other bacterial species revealed by recent studies.

Mutational Analyses Define Helix Organization and Key Residues of a Bacterial Membrane Energy-transducing Complex

In Gram-negative bacteria, many biological processes are coupled to inner membrane ion gradients. Ions transit at the interface of helices of integral membrane proteins, generating mechanical energy to drive energetic processes. To better understand how ions transit through these channels, we used a model system involved in two different processes, one of which depends on inner membrane energy. The Tol machinery of the Escherichia coli cell envelope is dedicated to maintaining outer membrane stability, a process driven by the proton-motive force. The Tol system is parasitized by bacterial toxins called colicins, which are imported through the outer membrane using an energy-independent process. Herein, we mutated TolQ and TolR transmembrane residues, and we analyzed the mutants for outer membrane stability, colicin import and protein complex formation. We identified residues involved in the assembly of the complex, and a new class of discriminative mutations that conferred outer membrane destabilization identical to a tol deletion mutant, but which remained fully sensitive to colicins. Further genetic approaches revealed transmembrane helix interactions and organization in the bilayer, and suggested that most of the discriminative residues are located in a putative aqueous ion channel. We discuss a model for the function of related bacterial molecular motors.

Deletion analysis of the carboxyl-terminal region of the PomB component of the vibrio alginolyticus polar flagellar motor

The stator of the sodium-driven flagellar motor of Vibrio alginolyticus is a membrane protein complex composed of four PomA and two PomB subunits. PomB has a peptidoglycan-binding motif in the C-terminal region. In this study, four kinds of PomB deletions in the C terminus were constructed. None of the deletion proteins restored motility of the DeltapomB strain. The PomA protein was coisolated with all of the PomB derivatives under detergent-solubilized conditions. Homotypic disulfide cross-linking of all of the deletion derivatives through naturally occurring Cys residues was detected. We conclude that the C-terminal region of PomB is essential for motor function but not for oligomerization of PomB with itself or PomA.