Effect of FliG three amino acids deletion in Vibrio polar-flagellar rotation and formation

Role of the N- and C-terminal regions of FliF, the MS ring component in Vibrio flagellar basal body

The MS ring is a part of the flagellar basal body and formed by 34 subunits of FliF, which consists of a large periplasmic region and two transmembrane segments connected to the N- and C-terminal regions facing the cytoplasm. A cytoplasmic protein, FlhF, which determines the position and number of the basal body, supports MS ring formation in the membrane in Vibrio species. In this study, we constructed FliF deletion mutants that lack 30 or 50 residues from the N-terminus (ΔN30 and ΔN50), and 83 (ΔC83) or 110 residues (ΔC110) at the C-terminus. The N-terminal deletions were functional and conferred motility of Vibrio cells, whereas the C-terminal deletions were nonfunctional. The mutants were expressed in Escherichia coli to determine whether an MS ring could still be assembled. When co-expressing ΔN30FliF or ΔN50FliF with FlhF, fewer MS rings were observed than with the expression of wild-type FliF, in the MS ring fraction, suggesting that the N-terminus interacts with FlhF. MS ring formation is probably inefficient without FlhF. The deletion of the C-terminal cytoplasmic region did not affect the ability of FliF to form an MS ring because a similar number of MS rings were observed for ΔC83FliF as with wild-type FliF, although further deletion of the second transmembrane segment (ΔC110FliF) abolished it. These results suggest that the terminal regions of FliF have distinct roles; the N-terminal region for efficient MS ring formation and the C-terminal region for MS ring function. The second transmembrane segment is indispensable for MS ring assembly.ImportanceThe bacterial flagellum is a supramolecular architecture involved in cell motility. At the base of the flagella, a rotary motor that begins to construct an MS ring in the cytoplasmic membrane comprises 34 transmembrane proteins (FliF). Here, we investigated the roles of the N and C terminal regions of FliF, which are MS rings. Unexpectedly, the cytoplasmic regions of FliF are not indispensable for the formation of the MS ring, but the N-terminus appears to assist in ring formation through recruitment of FlhF, which is essential for flagellar formation. The C-terminus is essential for motor formation or function.

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.

Organization of the Flagellar Switch Complex of Bacillus subtilis

While the protein complex responsible for controlling the direction (clockwise [CW] or counterclockwise [CCW]) of flagellar rotation has been fairly well studied in Escherichia coli and Salmonella, less is known about the switch complex in Bacillus subtilis or other Gram-positive species. Two component proteins (FliG and FliM) are shared between E. coli and B. subtilis, but in place of the protein FliN found in E. coli, the B. subtilis complex contains the larger protein FliY. Notably, in B. subtilis the signaling protein CheY-phosphate induces a switch from CW to CCW rotation, opposite to its action in E. coli Here, we have examined the architecture and function of the switch complex in B. subtilis using targeted cross-linking, bacterial two-hybrid protein interaction experiments, and characterization of mutant phenotypes. In major respects, the B. subtilis switch complex appears to be organized similarly to that in E. coli The complex is organized around a ring built from the large middle domain of FliM; this ring supports an array of FliG subunits organized in a similar way to that of E. coli, with the FliG C-terminal domain functioning in the generation of torque via conserved charged residues. Key differences from E. coli involve the middle domain of FliY, which forms an additional, more outboard array, and the C-terminal domains of FliM and FliY, which are organized into both FliY homodimers and FliM heterodimers. Together, the results suggest that the CW and CCW conformational states are similar in the Gram-negative and Gram-positive switches but that CheY-phosphate drives oppositely directed movements in the two cases.IMPORTANCE Flagellar motility plays key roles in the survival of many bacteria and in the harmful action of many pathogens. Bacterial flagella rotate; the direction of flagellar rotation is controlled by a multisubunit protein complex termed the switch complex. This complex has been extensively studied in Gram-negative model species, but little is known about the complex in Bacillus subtilis or other Gram-positive species. Notably, the switch complex in Gram-positive species responds to its effector CheY-phosphate (CheY-P) by switching to CCW rotation, whereas in E. coli or Salmonella CheY-P acts in the opposite way, promoting CW rotation. In the work here, the architecture of the B. subtilis switch complex has been probed using cross-linking, protein interaction measurements, and mutational approaches. The results cast light on the organization of the complex and provide a framework for understanding the mechanism of flagellar direction control in B. subtilis and other Gram-positive species.

Rotational direction of flagellar motor from the conformation of FliG middle domain in marine Vibrio

FliG, which is composed of three distinctive domains, N-terminal (N), middle (M), and C-terminal (C), is an essential rotor component that generates torque and determines rotational direction. To determine the role of FliG in determining flagellar rotational direction, we prepared rotational biased mutants of fliG in Vibrio alginolyticus. The E144D mutant, whose residue is belonging to the EHPQR-motif in FliG_M, exhibited an increased number of switching events. This phenotype generated a response similar to the phenol-repellent response in chemotaxis. To clarify the effect of E144D mutation on the rotational switching, we combined the mutation with other che mutations (G214S, G215A and A282T) in FliG. Two of the double mutants suppressed the rotational biased phenotype. To gain structural insight into the mutations, we performed molecular dynamic simulations of the FliG_MC domain, based on the crystal structure of Thermotoga maritima FliG and nuclear magnetic resonance analysis. Furthermore, we examined the swimming behavior of the fliG mutants lacking CheY. The results suggested that the conformation of FliG in E144D mutant was similar to that in the wild type. However, that of G214S and G215A caused a steric hindrance in FliG. The conformational change in FliG_M triggered by binding CheY may lead to a rapid change of direction and may occur in both directional states.

Pumilacidin-Like Lipopeptides Derived from Marine Bacterium Bacillus sp. Strain 176 Suppress the Motility of Vibrio alginolyticus

Bacterial motility is a crucial factor during the invasion and colonization processes of pathogens, which makes it an attractive therapeutic drug target. Here, we isolated a marine bacterium (Vibrio alginolyticus strain 178) from a seamount in the tropical West Pacific that exhibits vigorous motility on agar plates and severe pathogenicity to zebrafish. We found that V. alginolyticus 178 motility was significantly suppressed by another marine bacterium, Bacillus sp. strain 176, isolated from the same niche. We isolated, purified, and characterized two different cyclic lipopeptides (CLPs) from Bacillus sp. 176 using high-performance liquid chromatography, mass spectrometry, and nuclear magnetic resonance spectroscopy. The two related CLPs have a pumilacidin-like structure and were both effective inhibitors of V. alginolyticus 178 motility. The CLPs differ by only one methylene group in their fatty acid chains. In addition to motility suppression, the CLPs also induced cell aggregation in the medium and reduced adherence of V. alginolyticus 178 to glass substrates. Notably, upon CLP treatment, the expression levels of two V. alginolyticus flagellar assembly genes (flgA and flgP) dropped dramatically. Moreover, the CLPs inhibited biofilm formation in several other strains of pathogenic bacteria without inducing cell death. This study indicates that CLPs from Bacillus sp. 176 show promise as antimicrobial lead compounds targeting bacterial motility and biofilm formation with a low potential for eliciting antibiotic resistance.IMPORTANCE Pathogenic bacteria often require motility to establish infections and subsequently spread within host organisms. Thus, motility is an attractive therapeutic target for the development of novel antibiotics. We found that cyclic lipopeptides (CLPs) produced by marine bacterium Bacillus sp. strain 176 dramatically suppress the motility of the pathogenic bacterium Vibrio alginolyticus strain 178, reduce biofilm formation, and promote cellular aggregation without inducing cell death. These findings suggest that CLPs hold great promise as potential drug candidates targeting bacterial motility and biofilm formation with a low overall potential for triggering antibiotic resistance.

Domain-based biophysical characterization of the structural and thermal stability of FliG, an essential rotor component of the Na+-driven flagellar motor

Many bacteria move using their flagellar motor, which generates torque through the interaction between the stator and rotor. The most important component of the rotor for torque generation is FliG. FliG consists of three domains: FliG_N, FliG_M, and FliG_C. FliG_C contains a site(s) that interacts with the stator. In this study, we examined the physical properties of three FliG constructs, FliG_Full, FliG_MC, and FliG_C, derived from sodium-driven polar flagella of marine Vibrio. Size exclusion chromatography revealed that FliG changes conformational states under two different pH conditions. Circular dichroism spectroscopy also revealed that the contents of α-helices in FliG slightly changed under these pH conditions. Furthermore, we examined the thermal stability of the FliG constructs using differential scanning calorimetry. Based on the results, we speculate that each domain of FliG denatures independently. This study provides basic information on the biophysical characteristics of FliG, a component of the flagellar motor.