Roles of the intramolecular disulfide bridge in MotX and MotY, the specific proteins for sodium-driven motors in Vibrio spp

The cyclic lipopeptides suppress the motility of Vibrio alginolyticus via targeting the Na+ -driven flagellar motor component MotX

In our previous study, we found that pumilacidin-like cyclic lipopeptides (CLPs) derived from marine bacterium Bacillus sp. strain 176 significantly suppressed the mobile capability and virulence of Vibrio alginolyticus. Here, to further disclose the mechanism of CLPs inhibiting the motility of V. alginolyticus, we first applied transcriptomic analysis to V. alginolyticus treated with or without CLPs. The transcriptomic results showed that the expression of several important components of the Na⁺ -driven flagellar motor closely related to bacterial motility were markedly suppressed, suggesting that the structure and function of Na⁺ -driven flagellar motor might be disabled by CLPs. The transcriptomic data were further analysed by the protein-protein interaction network, and the results supported that MotX, one of the essential components of Na⁺ -driven flagellar motor was most likely the action target of CLPs. In combination of gene knockout, electrophoretic mobility shift assay and immunoblotting techniques, CLPs were demonstrated to affect the rotation of flagella of Vibrio alginolyticus via direct interacting with the Na⁺ -driven flagellar motor component MotX, which eventually inhibited the bacterial motility. Interestingly, homologues of MotX were found broadly distributed and highly conserved in different pathogenic species, which extends the application range of CLPs as an antibacterial drug targeting bacterial motility in many pathogens.

Mutational analysis and overproduction effects of MotX, an essential component for motor function of Na+-driven polar flagella of Vibrio

The bacterial flagellar motor is a rotary motor complex composed of various proteins. The motor contains a central rod, multiple ring-like structures and stators. The Na+-driven polar flagellar motor of the marine bacterium Vibrio alginolyticus has a specific ring, called the ‘T-ring’, which consists of two periplasmic proteins, MotX and MotY. The T-ring is essential for assembly of the torque-generating unit, the PomA/PomB stator complex, into the motor. To investigate the role of the T-ring for motor function, we performed random mutagenesis of the motX gene on a plasmid. The isolated MotX mutants showed nonmotile, slow-motile, and up-motile phenotypes by the expression from the plasmid. Deletion analysis indicated that the C-terminal region and the signal peptide in MotX are not always essential for flagellar motor function. We also found that overproduction of MotX caused the delay of growth and aberrant cell shape. MotX might have unexpected roles not only in flagellar motor function but also in cell morphology control.

The FlgT Protein Is Involved in Aeromonas hydrophila Polar Flagella Stability and Not Affects Anchorage of Lateral Flagella

Aeromonas hydrophila sodium-driven polar flagellum has a complex stator-motor. Consist of two sets of redundant and non-exchangeable proteins (PomA/PomB and PomA₂/PomB₂), which are homologs to other sodium-conducting polar flagellum stator motors; and also two essential proteins (MotX and MotY), that they interact with one of those two redundant pairs of proteins and form the T-ring. In this work, we described an essential protein for polar flagellum stability and rotation which is orthologs to Vibrio spp. FlgT and it is encoded outside of the A. hydrophila polar flagellum regions. The flgT was present in all mesophilic Aeromonas strains tested and also in the non-motile Aeromonas salmonicida. The A. hydrophila ΔflgT mutant is able to assemble the polar flagellum but is more unstable and released into the culture supernatant from the cell upon completion assembly. Presence of FlgT in purified polar hook-basal bodies (HBB) of wild-type strain was confirmed by Western blotting and electron microscopy observations showed an outer ring of the T-ring (H-ring) which is not present in the ΔflgT mutant. Anchoring and motility of proton-driven lateral flagella was not affected in the ΔflgT mutant and specific antibodies did not detect FlgT in purified lateral HBB of wild type strain.

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.

Fluorescence imaging of GFP-fused periplasmic components of Na+-driven flagellar motor using Tat pathway in Vibrio alginolyticus

The twin-arginine translocation (Tat) system works to export folded proteins across the cytoplasmic membrane via specific signal peptides harbouring a twin-arginine motif. In Escherichia coli, a functional GFP is exported to the periplasm through the Tat pathway by fusion of the signal peptide of TorA, which is one of the periplasmic proteins exported by the Tat pathway. In this study, we fused the signal peptide of Vibrio alginolyticus TorA (TorASP) to GFP and demonstrate the export of functional GFP to the periplasm of V. alginolyticus. We also made fusions of TorASP-GFP with MotX, MotY and FlgT, which are periplasmic components of the Na(+)-driven flagellar motor. Those fusion proteins were localized to the flagellar motor independent of the Na(+) concentration in the environment.

Characterization of the flagellar motor composed of functional GFP-fusion derivatives of FliG in the Na+-driven polar flagellum of Vibrio alginolyticus

The polar flagellum of Vibrio alginolyticus is driven by sodium ion flux via a stator complex, composed of PomA and PomB, across the cell membrane. The interaction between PomA and the rotor component FliG is believed to generate torque required for flagellar rotation. Previous research reported that a GFP-fused FliG retained function in the Vibrio flagellar motor. In this study, we found that N-terminal or C-terminal fusion of GFP has different effects on both torque generation and the switching frequency of the direction of flagellar motor rotation. We could detect the GFP-fused FliG in the basal-body (rotor) fraction although its association with the basal body was less stable than that of intact FliG. Furthermore, the fusion of GFP to the C-terminus of FliG, which is believed to be directly involved in torque generation, resulted in very slow motility and prohibited the directional change of motor rotation. On the other hand, the fusion of GFP to the N-terminus of FliG conferred almost the same swimming speed as intact FliG. These results are consistent with the premise that the C-terminal domain of FliG is directly involved in torque generation and the GFP fusions are useful to analyze the functions of various domains of FliG.

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.

The flagellar basal body-associated protein FlgT is essential for a novel ring structure in the sodium-driven Vibrio motor

In Vibrio alginolyticus, the flagellar motor can rotate at a remarkably high speed, ca. three to four times faster than the Escherichia coli or Salmonella motor. Here, we found a Vibrio-specific protein, FlgT, in the purified flagellar basal body fraction. Defects of FlgT resulted in partial Fla⁻ and Mot⁻ phenotypes, suggesting that FlgT is involved in formation of the flagellar structure and generating flagellar rotation. Electron microscopic observation of the basal body of ΔflgT cells revealed a smaller LP ring structure compared to the wild type, and most of the T ring was lost. His₆-tagged FlgT could be coisolated with MotY, the T-ring component, suggesting that FlgT may interact with the T ring composed of MotX and MotY. From these lines of evidence, we conclude that FlgT associates with the basal body and is responsible to form an outer ring of the LP ring, named the H ring, which can be distinguished from the LP ring formed by FlgH and FlgI. Vibrio-specific structures, e.g., the T ring and H ring might contribute the more robust motor structure compared to that of E. coli and Salmonella.

Physiological and transcriptomic characterization of a fliA mutant of Pseudomonas putida KT2440

Pseudomonas putida KT2440 encodes 23 alternative sigma factors. The fliA gene, which encodes σ(28) , is in a cluster with other genes involved in flagella biosynthesis and chemotaxis. Reverse transcriptase-PCR revealed that this cluster is comprised of four independent transcriptional units: flhAF, fleNfliA, cheYZA and cheBmotAB. We generated a nonpolar fliA mutant by homologous recombination and tested its motility, adhesion to biotic and abiotic surfaces, and responses to various stress conditions. The mutant strain was nonmotile and exhibited decreased capacity to bind to corn seeds, although its ability to colonize the rhizosphere of plants was unaffected. The mutant was also affected in binding to abiotic surfaces and its ability to form biofilms decreased by almost threefold. In the fliA mutant background expression of 25 genes was affected: two genes were upregulated and 23 genes were downregulated. In addition to a number of motility and chemotaxis genes, the fliA gene product is also necessary for the expression of some genes potentially involved in amino acid utilization or stress responses; however, we were unable to assign specific phenotypes linked to these genes since the fliA mutant used the same range of amino acids as the parental strain, and was as tolerant as the wild type to stress imposed by heat, antibiotics, NaCl, sodium dodecyl sulfate, H2 O2 and benzoate. Based on the sequence alignment of promoters recognized by FliA and genome in silico analysis, we propose that P. putidaσ(28) recognizes a TCAAG-t-N12 -GCCGATA consensus sequence located between -34 and -8 and that this sequence is preferentially associated with an AT-rich upstream region.

Isolation of basal bodies with C-ring components from the Na+-driven flagellar motor of Vibrio alginolyticus

To investigate the Na(+)-driven flagellar motor of Vibrio alginolyticus, we attempted to isolate its C-ring structure. FliG but not FliM copurified with the basal bodies. FliM proteins may be easily dissociated from the basal body. We could detect FliG on the MS ring surface of the basal bodies.

Heterologous expression and molecular characterization of the NAD(P)H:acceptor oxidoreductase (FerB) of Paracoccus denitrificans

FerB is a flavoenzyme capable of reducing quinones, ferric complexes and chromate. Its expression in Escherichia coli as a hexahistidine fusion resulted in a functional product only when the tag was placed on the C-terminus. The molecular mass values estimated by gel permeation chromatography were compatible with the existence of either dimer or trimer, whereas the light scattering data, together with cross-linking experiments that yielded exclusively monomer and dimer bands on dodecyl sulfate-polyacrylamide gels, strongly supported a dimeric nature of both native and tagged form of FerB. These two proteins also exhibited almost identical secondary structure as judged by Fourier transform infra red spectrometry. The presence of tag, however, shifted the temperature of thermal inactivation as well as the thermal denaturation curve towards lower temperatures. Despite somewhat lower thermal stability, the fusion protein is considered a better candidate for crystallization than the wild-type one due to a more negative value of its second optical viral coefficient.

MotX and MotY are required for flagellar rotation in Shewanella oneidensis MR-1

The single polar flagellum of Shewanella oneidensis MR-1 is powered by two different stator complexes, the sodium-dependent PomAB and the proton-driven MotAB. In addition, Shewanella harbors two genes with homology to motX and motY of Vibrio species. In Vibrio, the products of these genes are crucial for sodium-dependent flagellar rotation. Resequencing of S. oneidensis MR-1 motY revealed that the gene does not harbor an authentic frameshift as was originally reported. Mutational analysis demonstrated that both MotX and MotY are critical for flagellar rotation of S. oneidensis MR-1 for both sodium- and proton-dependent stator systems but do not affect assembly of the flagellar filament. Fluorescence tagging of MotX and MotY to mCherry revealed that both proteins localize to the flagellated cell pole depending on the presence of the basal flagellar structure. Functional localization of MotX requires MotY, whereas MotY localizes independently of MotX. In contrast to the case in Vibrio, neither protein is crucial for the recruitment of the PomAB or MotAB stator complexes to the flagellated cell pole, nor do they play a major role in the stator selection process. Thus, MotX and MotY are not exclusive features of sodium-dependent flagellar systems. Furthermore, MotX and MotY in Shewanella, and possibly also in other genera, must have functions beyond the recruitment of the stator complexes.

Collaboration of FlhF and FlhG to regulate polar-flagella number and localization in Vibrio alginolyticus

Precise regulation of the number and placement of flagella is critical for the mono-polar-flagellated bacterium Vibrio alginolyticus to swim efficiently. We have shown previously that the number of polar flagella is positively regulated by FlhF and negatively regulated by FlhG. We now show that DeltaflhF cells are non-flagellated as are most DeltaflhFG cells; however, some of the DeltaflhFG cells have several flagella at lateral positions. We found that FlhF-GFP was localized at the flagellated pole, and its polar localization was seen more intensely in DeltaflhFG cells. On the other hand, most of the FlhG-GFP was diffused throughout the cytoplasm, although some was localized at the pole. To investigate the FlhF-FlhG interaction, immunoprecipitation was performed by using an anti-FlhF antibody, and FlhG co-precipitated with FlhF. From these results we propose a model in which FlhF localization at the pole determines polar location and production of a flagellum, FlhG interacts with FlhF to prevent FlhF from localizing at the pole, and thus FlhG negatively regulates flagellar number in V. alginolyticus cells.

Insights into the stator assembly of the Vibrio flagellar motor from the crystal structure of MotY

Rotation of the sodium-driven polar flagellum of Vibrio alginolyticus requires four motor proteins: PomA, PomB, MotX, and MotY. PomA and PomB form a sodium-ion channel in the cytoplasmic membrane that functions as a stator complex to couple sodium-ion flux with torque generation. MotX and MotY are components of the T-ring, which is located beneath the P-ring of the polar flagellar basal body and is involved in incorporation of the PomA/PomB complex into the motor. Here, we describe the determination of the crystal structure of MotY at 2.9 A resolution. The structure shows two distinct domains: an N-terminal domain (MotY-N) and a C-terminal domain (MotY-C). MotY-N has a unique structure. MotY-C contains a putative peptidoglycan-binding motif that is remarkably similar to those of peptidoglycan-binding proteins, such as Pal and RmpM, but this region is disordered in MotY. Motility assay of cells producing either of the MotY-N and MotY-C fragments and subsequent biochemical analyses indicate that MotY-N is essential for association of the stator units around the rotor, whereas MotY-C stabilizes the association by binding to the peptidoglycan layer. Based on these observations, we propose a model for the mechanism of stator assembly around the rotor.

Crystallization and preliminary X-ray analysis of MotY, a stator component of the Vibrio alginolyticus polar flagellar motor

The polar flagellum of Vibrio alginolyticus is rotated by the sodium motor. The stator unit of the sodium motor consists of four different proteins: PomA, PomB, MotX and MotY. MotX and MotY, which are unique components of the sodium motor, form the T-ring structure attached to the LP ring in the periplasmic space. MotY has a putative peptidoglycan-binding motif in its C-terminal region and MotX is suggested to interact with PomB. Thus, MotX and MotY are thought to be required for incorporation and stabilization of the PomA/B complex. In this study, mature MotY composed of 272 amino-acid residues and its SeMet derivative were expressed with a C-terminal hexahistidine-tag sequence, purified and crystallized. Native crystals were grown in the hexagonal space group P6(1)22/P6(5)22, with unit-cell parameters a = b = 104.1, c = 132.6 A. SeMet-derivative crystals belonged to the same space group with the same unit-cell parameters as the native crystals. Anomalous difference Patterson maps of the SeMet derivative showed significant peaks in their Harker sections, indicating that the derivatives are suitable for structure determination.

The Vibrio motor proteins, MotX and MotY, are associated with the basal body of Na-driven flagella and required for stator formation

The four motor proteins PomA, PomB, MotX and MotY, which are believed to be stator proteins, are essential for motility by the Na(+)-driven flagella of Vibrio alginolyticus. When we purified the flagellar basal bodies, MotX and MotY were detected in the basal body, which is the supramolecular complex comprised of the rotor and the bushing, but PomA and PomB were not. By antibody labelling, MotX and MotY were detected around the LP ring. These results indicate that MotX and MotY associate with the basal body. The basal body had a new ring structure beneath the LP ring, which was named the T ring. This structure was changed or lost in the basal body from a DeltamotX or DeltamotY strain. The T ring probably comprises MotX and MotY. In the absence of MotX or MotY, we demonstrated that PomA and PomB were not localized to a cell pole. From the above results, we suggest that MotX and MotY of the T ring are involved in the incorporation and/or stabilization of the PomA/PomB complex in the motor.