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

A Helicobacter pylori flagellar motor accessory is needed to maintain the barrier function of the outer membrane during flagellar rotation

The Helicobacter pylori flagellar motor contains several accessory structures that are not found in the archetypal Escherichia coli and Salmonella enterica motors. H. pylori hp0838 encodes a previously uncharacterized lipoprotein and is in an operon with flgP, which encodes a motor accessory protein. Deletion analysis of hp0838 in H. pylori B128 showed that the gene is not required for motility in soft agar medium, but the mutant displayed a reduced growth rate and an increased sensitivity to bacitracin, which is an antibiotic that is normally excluded by the outer membrane. Introducing a plasmid-borne copy of hp0838 into the H. pylori Δhp0838 mutant suppressed the fitness defect and antibiotic sensitivity of the strain. A variant of the Δhp0838 mutant containing a frameshift mutation in pflA, which resulted in paralyzed flagella, displayed wild-type growth rate and resistance to bacitracin, suggesting the fitness defect and antibiotic sensitivity of the Δhp0838 mutant are dependent on flagellar rotation. Comparative analysis of in-situ structures of the wild type and Δhp0838 mutant motors revealed the Δhp0838 mutant motor lacked a previously undescribed ring structure with 18-fold symmetry located near the outer membrane. Given its role in formation of the motor outer ring, HP0838 was designated FapH (flagellar accessory protein in Helicobacter pylori) and the motor accessory formed the protein was named the FapH ring. Our data suggest that the FapH ring helps to preserve outer membrane barrier function during flagellar rotation. Given that FapH homologs are present in many members of the phylum Campylobacterota, they may have similar roles in protecting the outer membrane from damage due to flagellar rotation in these bacteria.

Molecular model of a bacterial flagellar motor in situ reveals a "parts-list" of protein adaptations to increase torque

One hurdle to understanding how molecular machines work, and how they evolve, is our inability to see their structures in situ. Here we describe a minicell system that enables in situ cryogenic electron microscopy imaging and single particle analysis to investigate the structure of an iconic molecular machine, the bacterial flagellar motor, which spins a helical propeller for propulsion. We determine the structure of the high-torque Campylobacter jejuni motor in situ, including the subnanometre-resolution structure of the periplasmic scaffold, an adaptation essential to high torque. Our structure enables identification of new proteins, and interpretation with molecular models highlights origins of new components, reveals modifications of the conserved motor core, and explain how these structures both template a wider ring of motor proteins, and buttress the motor during swimming reversals. We also acquire insights into universal principles of flagellar torque generation. This approach is broadly applicable to other membrane-residing bacterial molecular machines complexes.

Viscosity-dependent determinants of Campylobacter jejuni impacting the velocity of flagellar motility

IMPORTANCE

Bacteria can adapt flagellar motor output in response to the load that the extracellular milieu imparts on the flagellar filament to enable propulsion. Bacteria can adapt flagellar motor output in response to the load that the extracellular milieu imparts on the flagellar filament to enable propulsion through diverse environments. These changes may involve increasing power and torque in high-viscosity environments or reducing power and flagellar rotation upon contact with a surface. C. jejuni swimming velocity in low-viscosity environments is comparable to other bacterial flagellates and increases significantly as external viscosity increases. In this work, we provide evidence that the mechanics of the C. jejuni flagellar motor has evolved to naturally promote high swimming velocity in high-viscosity environments. We found that C. jejuni produces VidA and VidB as auxiliary proteins to specifically affect flagellar motor activity in low viscosity to reduce swimming velocity. Our findings provide some of the first insights into different mechanisms that exist in bacteria to alter the mechanics of a flagellar motor, depending on the viscosity of extracellular environments.

The Vibrio Polar Flagellum: Structure and Regulation

Here we discuss the structure and regulation of the Vibrio flagellum and its role in the virulence of pathogenic species. We will cover some of the novel insights into the structure of this nanomachine that have recently been enabled by cryoelectron tomography. We will also highlight the recent genetic studies that have increased our understanding in flagellar synthesis specifically at the bacterial cell pole, temporal regulation of flagellar genes, and how the flagellum enables directional motility through Run-Reverse-Flick cycles.

The Periplasmic Domain of the Ion-Conducting Stator of Bacterial Flagella Regulates Force Generation

The bacterial flagellar stator is a unique ion-conducting membrane protein complex composed of two kinds of proteins, the A subunit and the B subunit. The stator couples the ion-motive force across the membrane into rotational force. The stator becomes active only when it is incorporated into the flagellar motor. The periplasmic region of the B subunit positions the stator by using the peptidoglycan-binding (PGB) motif in its periplasmic C-terminal domain to attach to the cell wall. Functional studies based on the crystal structures of the C-terminal domain of the B subunit (MotB_C or PomB_C) reveal that a dramatic conformational change in a characteristic α-helix allows the stator to conduct ions efficiently and bind to the PG layer. The plug and the following linker region between the transmembrane (TM) and PG-binding domains of the B subunit function in regulating the ion conductance. In Vibrio spp., the transmembrane protein FliL and the periplasmic MotX and MotY proteins also contribute to the motor function. In this review, we describe the functional and structural changes which the stator units undergo to regulate the activity of the stator to drive flagellar rotation.

Hoop-like role of the cytosolic interface helix in Vibrio PomA, an ion-conducting membrane protein, in the bacterial flagellar motor

Vibrio has a polar flagellum driven by sodium ions for swimming. The force-generating stator unit consists of PomA and PomB. PomA contains four-transmembrane regions and a cytoplasmic domain of approximately 100 residues which interacts with the rotor protein, FliG, to be important for the force generation of rotation. The three-dimensional structure of the stator shows that the cytosolic interface (CI) helix of PomA is located parallel to the inner membrane. In this study, we investigated the function of CI helix and its role as stator. Systematic proline mutagenesis showed that residues K64, F66, and M67 were important for this function. The mutant stators did not assemble around the rotor. Moreover, the growth defect caused by PomB plug deletion was suppressed by these mutations. We speculate that the mutations affect the structure of the helices extending from TM3 and TM4 and reduce the structural stability of the stator complex. This study suggests that the helices parallel to the inner membrane play important roles in various processes, such as the hoop-like function in securing the stability of the stator complex and the ion conduction pathway, which may lead to the elucidation of the ion permeation and assembly mechanism of the stator.

Structure and Assembly of the Bacterial Flagellum

The bacterial flagellum is a large macromolecular assembly that acts as propeller, providing motility through the rotation of a long extracellular filament. It is composed of over 20 different proteins, many of them highly oligomeric. Accordingly, it has attracted a huge amount of interest amongst researchers and the wider public alike. Nonetheless, most of its molecular details had long remained elusive.This however has changed recently, with the emergence of cryo-EM to determine the structure of protein assemblies at near-atomic resolution. Within a few years, the atomic details of most of the flagellar components have been elucidated, revealing not only its overall architecture but also the molecular details of its rotation mechanism. However, many questions remained unaddressed, notably on the complexity of the assembly of such an intricate machinery.In this chapter, we review the current state of our understanding of the bacterial flagellum structure, focusing on the recent development from cryo-EM. We also highlight the various elements that still remain to be fully characterized. Finally, we summarize the existing model for flagellum assembly and discuss some of the outstanding questions that are still pending in our understanding of the diversity of assembly pathways.

Achievements in bacterial flagellar research with focus on Vibrio species

In 1980's, the most genes involved in the bacterial flagellar function and formation had been isolated though many of their functions or roles were not clarified. Bacterial flagella are the primary locomotive organ and are not necessary for growing in vitro but are probably essential for living in natural condition and are involved in the pathogenicity. In vitro, the flagella-deficient strains can grow at rates similar to wild-type strains. More than 50 genes are responsible for flagellar function, and the flagellum is constructed by more than 20 structural proteins. The maintenance cost of flagellum is high as several genes are required for its development. The fact that it evolved as a motor organ even with such the high cost shows that the motility is indispensable to survive under the harsh environment of Earth. In this review, we focus on flagella-related research conducted by the authors for about 40 years and flagellar research focused on Vibrio spp. This article is protected by copyright. All rights reserved.

Structure, Assembly, and Function of Tripartite Efflux and Type 1 Secretion Systems in Gram-Negative Bacteria

Tripartite efflux pumps and the related type 1 secretion systems (T1SSs) in Gram-negative organisms are diverse in function, energization, and structural organization. They form continuous conduits spanning both the inner and the outer membrane and are composed of three principal components-the energized inner membrane transporters (belonging to ABC, RND, and MFS families), the outer membrane factor channel-like proteins, and linking the two, the periplasmic adaptor proteins (PAPs), also known as the membrane fusion proteins (MFPs). In this review we summarize the recent advances in understanding of structural biology, function, and regulation of these systems, highlighting the previously undescribed role of PAPs in providing a common architectural scaffold across diverse families of transporters. Despite being built from a limited number of basic structural domains, these complexes present a staggering variety of architectures. While key insights have been derived from the RND transporter systems, a closer inspection of the operation and structural organization of different tripartite systems reveals unexpected analogies between them, including those formed around MFS- and ATP-driven transporters, suggesting that they operate around basic common principles. Based on that we are proposing a new integrated model of PAP-mediated communication within the conformational cycling of tripartite systems, which could be expanded to other types of assemblies.

The Dynamic Ion Motive Force Powering the Bacterial Flagellar Motor

The bacterial flagellar motor (BFM) is a rotary molecular motor embedded in the cell membrane of numerous bacteria. It turns a flagellum which acts as a propeller, enabling bacterial motility and chemotaxis. The BFM is rotated by stator units, inner membrane protein complexes that stochastically associate to and dissociate from individual motors at a rate which depends on the mechanical and electrochemical environment. Stator units consume the ion motive force (IMF), the electrochemical gradient across the inner membrane that results from cellular respiration, converting the electrochemical energy of translocated ions into mechanical energy, imparted to the rotor. Here, we review some of the main results that form the base of our current understanding of the relationship between the IMF and the functioning of the flagellar motor. We examine a series of studies that establish a linear proportionality between IMF and motor speed, and we discuss more recent evidence that the stator units sense the IMF, altering their rates of dynamic assembly. This, in turn, raises the question of to what degree the classical dependence of motor speed on IMF is due to stator dynamics vs. the rate of ion flow through the stators. Finally, while long assumed to be static and homogeneous, there is mounting evidence that the IMF is dynamic, and that its fluctuations control important phenomena such as cell-to-cell signaling and mechanotransduction. Within the growing toolbox of single cell bacterial electrophysiology, one of the best tools to probe IMF fluctuations may, ironically, be the motor that consumes it. Perfecting our incomplete understanding of how the BFM employs the energy of ion flow will help decipher the dynamical behavior of the bacterial IMF.

The Structure, Composition, and Role of Periplasmic Stator Scaffolds in Polar Bacterial Flagellar Motors

In the bacterial flagellar motor, the cell-wall-anchored stator uses an electrochemical gradient across the cytoplasmic membrane to generate a turning force that is applied to the rotor connected to the flagellar filament. Existing theoretical concepts for the stator function are based on the assumption that it anchors around the rotor perimeter by binding to peptidoglycan (P). The existence of another anchoring region on the motor itself has been speculated upon, but is yet to be supported by binding studies. Due to the recent advances in electron cryotomography, evidence has emerged that polar flagellar motors contain substantial proteinaceous periplasmic structures next to the stator, without which the stator does not assemble and the motor does not function. These structures have a morphology of disks, as is the case with Vibrio spp., or a round cage, as is the case with Helicobacter pylori. It is now recognized that such additional periplasmic components are a common feature of polar flagellar motors, which sustain higher torque and greater swimming speeds compared to peritrichous bacteria such as Escherichia coli and Salmonella enterica. This review summarizes the data available on the structure, composition, and role of the periplasmic scaffold in polar bacterial flagellar motors and discusses the new paradigm for how such motors assemble and function.

[Flagellar related genes and functions in Vibrio]

Bacteria can move or swim by flagella. On the other hand, the motile ability is not necessary to live at all. In laboratory, the flagella-deficient strains can grow just like the wild-type strains. The flagellum is assembled from more than 20 structural proteins and there are more than 50 genes including the structural genes to regulate or support the flagellar formation. The cost to construct the flagellum is so expensive. The fact that it evolved as a motor organ means even at such the large cost shows that the flagellum is essential for survival in natural condition. In this review, we would like to focus on the flagella-related researches conducted by the authors and the flagellar research on Vibrio spp.

CryoEM map of Pseudomonas aeruginosa PilQ enables structural characterization of TsaP

The type IV pilus machinery is a multi-protein complex that polymerizes and depolymerizes a pilus fiber used for attachment, twitching motility, phage adsorption, natural competence, protein secretion, and surface-sensing. An outer membrane secretin pore is required for passage of the pilus fiber out of the cell. Herein, the structure of the tetradecameric secretin, PilQ, from the Pseudomonas aeruginosa type IVa pilus system was determined to 4.3 Å and 4.4 Å resolution in the presence and absence of C₇ symmetric spikes, respectively. The heptameric spikes were found to be two tandem C-terminal domains of TsaP. TsaP forms a belt around PilQ and while it is not essential for twitching motility, overexpression of TsaP triggers a signal cascade upstream of PilY1 leading to cyclic di-GMP up-regulation. These results resolve the identity of the spikes identified with Proteobacterial PilQ homologs and may reveal a new component of the surface-sensing cyclic di-GMP signal cascade.

Structural Conservation and Adaptation of the Bacterial Flagella Motor

Many bacteria require flagella for the ability to move, survive, and cause infection. The flagellum is a complex nanomachine that has evolved to increase the fitness of each bacterium to diverse environments. Over several decades, molecular, biochemical, and structural insights into the flagella have led to a comprehensive understanding of the structure and function of this fascinating nanomachine. Notably, X-ray crystallography, cryo-electron microscopy (cryo-EM), and cryo-electron tomography (cryo-ET) have elucidated the flagella and their components to unprecedented resolution, gleaning insights into their structural conservation and adaptation. In this review, we focus on recent structural studies that have led to a mechanistic understanding of flagellar assembly, function, and evolution.

Propulsive nanomachines: the convergent evolution of archaella, flagella and cilia

Echoing the repeated convergent evolution of flight and vision in large eukaryotes, propulsive swimming motility has evolved independently in microbes in each of the three domains of life. Filamentous appendages – archaella in Archaea, flagella in Bacteria and cilia in Eukaryotes – wave, whip or rotate to propel microbes, overcoming diffusion and enabling colonization of new environments. The implementations of the three propulsive nanomachines are distinct, however: archaella and flagella rotate, while cilia beat or wave; flagella and cilia assemble at their tips, while archaella assemble at their base; archaella and cilia use ATP for motility, while flagella use ion-motive force. These underlying differences reflect the tinkering required to evolve a molecular machine, in which pre-existing machines in the appropriate contexts were iteratively co-opted for new functions and whose origins are reflected in their resultant mechanisms. Contemporary homologies suggest that archaella evolved from a non-rotary pilus, flagella from a non-rotary appendage or secretion system, and cilia from a passive sensory structure. Here, we review the structure, assembly, mechanism and homologies of the three distinct solutions as a foundation to better understand how propulsive nanomachines evolved three times independently and to highlight principles of molecular evolution.

In Situ Structure of the Vibrio Polar Flagellum Reveals a Distinct Outer Membrane Complex and Its Specific Interaction with the Stator

The bacterial flagellum is a biological nanomachine that rotates to allow bacteria to swim. For flagellar rotation, torque is generated by interactions between a rotor and a stator. The stator, which is composed of MotA and MotB subunit proteins in the membrane, is thought to bind to the peptidoglycan (PG) layer, which anchors the stator around the rotor. Detailed information on the stator and its interactions with the rotor remains unclear. Here, we deployed cryo-electron tomography and genetic analysis to characterize in situ structure of the bacterial flagellar motor in Vibrio alginolyticus, which is best known for its polar sheathed flagellum and high-speed rotation. We determined in situ structure of the motor at unprecedented resolution and revealed the unique protein-protein interactions among Vibrio-specific features, namely the H ring and T ring. Specifically, the H ring is composed of 26 copies of FlgT and FlgO, and the T ring consists of 26 copies of a MotX-MotY heterodimer. We revealed for the first time a specific interaction between the T ring and the stator PomB subunit, providing direct evidence that the stator unit undergoes a large conformational change from a compact form to an extended form. The T ring facilitates the recruitment of the extended stator units for the high-speed motility in Vibrio species.IMPORTANCE The torque of flagellar rotation is generated by interactions between a rotor and a stator; however, detailed structural information is lacking. Here, we utilized cryo-electron tomography and advanced imaging analysis to obtain a high-resolution in situ flagellar basal body structure in Vibrio alginolyticus, which is a Gram-negative marine bacterium. Our high-resolution motor structure not only revealed detailed protein-protein interactions among unique Vibrio-specific features, the T ring and H ring, but also provided the first structural evidence that the T ring interacts directly with the periplasmic domain of the stator. Docking atomic structures of key components into the in situ motor map allowed us to visualize the pseudoatomic architecture of the polar sheathed flagellum in Vibrio spp. and provides novel insight into its assembly and function.

Evolution of the Stator Elements of Rotary Prokaryote Motors

The bacterial flagellar motor is driven by an ion flux that is converted to torque by motor-attendant complexes known as stators. The dynamics of stator assembly around the motor in response to external stimuli have been the subject of much recent research, but less is known about the evolutionary origins of stator complexes and how they select for specific ions. Here, we review the latest structural and biochemical data for the stator complexes and compare these with other ion transporters and microbial motors to examine possible evolutionary origins of the stator complex.

A Chaperone for the Stator Units of a Bacterial Flagellum

The bacterial flagellum is a reversible rotating motor powered by ion transport through stator units, which also exert torque on the rotor component to turn the flagellum for motility. Species-specific adaptations to flagellar motors impact stator function to meet the demands of each species to sufficiently power flagellar rotation. We identified another evolutionary adaptation by discovering that FlgX of Campylobacter jejuni preserves the integrity of stator units by functioning as a chaperone to protect stator proteins from degradation by the FtsH protease complex due to the physiology of the bacterium. FlgX is required to maintain a level of stator units sufficient to power the naturally high-torque flagellar motor of C. jejuni for motility in intestinal mucosal layers to colonize hosts. Our work continues to identify an increasing number of adaptations to flagellar motors across bacterial species that provide the mechanics necessary for producing an effective rotating nanomachine for motility.

The stator units of the flagellum supply power to the flagellar motor via ion transport across the cytoplasmic membrane and generate torque on the rotor for rotation. Flagellar motors across bacterial species have evolved adaptations that impact and enhance stator function to meet the demands of each species, including producing stator units using different fuel types or various stator units for different motility modalities. Campylobacter jejuni produces one of the most complex and powerful flagellar motors by positioning 17 stator units at a greater radial distance than in most other bacteria to increase power and torque for high velocity of motility. We report another evolutionary adaptation impacting flagellar stators by identifying FlgX as a chaperone for C. jejuni stator units to ensure sufficient power and torque for flagellar rotation and motility. We discovered that FlgX maintains MotA and MotB stator protein integrity likely through a direct interaction with MotA that prevents their degradation. Suppressor analysis suggested that the physiology of C. jejuni drives the requirement for FlgX to protect stator units from proteolysis by the FtsH protease complex. C. jejuni ΔflgX was strongly attenuated for colonization of the natural avian host, but colonization capacity was greatly restored by a single mutation in MotA. These findings suggest that the likely sole function of FlgX is to preserve stator unit integrity for the motility required for host interactions. Our findings demonstrate another evolved adaptation in motile bacteria to ensure the equipment of the flagellar motor with sufficient power to generate torque for motility.

Polar landmark protein HubP recruits flagella assembly protein FapA under glucose limitation in Vibrio vulnificus

How motile bacteria recognize their environment and decide whether to stay or navigate toward more favorable location is a fundamental issue in survival. The flagellum is an elaborate molecular device responsible for bacterial locomotion, and the flagellum-driven motility allows bacteria to move themselves to the appropriate location at the right time. Here, we identify the polar landmark protein HubP as a modulator of polar flagellation that recruits the flagellar assembly protein FapA to the old cell pole, thereby controlling its activity for the early events of flagellar assembly in Vibrio vulnificus. We show that dephosphorylated EIIA^Glc of the PEP-dependent sugar transporting phosphotransferase system sequesters FapA from HubP in response to glucose and hence inhibits FapA-mediated flagellation. Thus, flagellar assembly and motility is governed by spatiotemporal control of FapA, which is orchestrated by the competition between dephosphorylated EIIA^Glc and HubP, in the human pathogen V. vulnificus.

γ-proteobacteria eject their polar flagella under nutrient depletion, retaining flagellar motor relic structures

Bacteria switch only intermittently to motile planktonic lifestyles under favorable conditions. Under chronic nutrient deprivation, however, bacteria orchestrate a switch to stationary phase, conserving energy by altering metabolism and stopping motility. About two-thirds of bacteria use flagella to swim, but how bacteria deactivate this large molecular machine remains unclear. Here, we describe the previously unreported ejection of polar motors by γ-proteobacteria. We show that these bacteria eject their flagella at the base of the flagellar hook when nutrients are depleted, leaving a relic of a former flagellar motor in the outer membrane. Subtomogram averages of the full motor and relic reveal that this is an active process, as a plug protein appears in the relic, likely to prevent leakage across their outer membrane; furthermore, we show that ejection is triggered only under nutritional depletion and is independent of the filament as a possible mechanosensor. We show that filament ejection is a widespread phenomenon demonstrated by the appearance of relic structures in diverse γ-proteobacteria including Plesiomonas shigelloides, Vibrio cholerae, Vibrio fischeri, Shewanella putrefaciens, and Pseudomonas aeruginosa. While the molecular details remain to be determined, our results demonstrate a novel mechanism for bacteria to halt costly motility when nutrients become scarce.

The presence and absence of periplasmic rings in bacterial flagellar motors correlates with stator type

The bacterial flagellar motor, a cell-envelope-embedded macromolecular machine that functions as a cellular propeller, exhibits significant structural variability between species. Different torque-generating stator modules allow motors to operate in different pH, salt or viscosity levels. How such diversity evolved is unknown. Here, we use electron cryo-tomography to determine the in situ macromolecular structures of three Gammaproteobacteria motors: Legionella pneumophila, Pseudomonas aeruginosa, and Shewanella oneidensis, providing the first views of intact motors with dual stator systems. Complementing our imaging with bioinformatics analysis, we find a correlation between the motor’s stator system and its structural elaboration. Motors with a single H⁺-driven stator have only the core periplasmic P- and L-rings; those with dual H⁺-driven stators have an elaborated P-ring; and motors with Na⁺ or Na⁺/H⁺-driven stators have both their P- and L-rings embellished. Our results suggest an evolution of structural elaboration that may have enabled pathogenic bacteria to colonize higher-viscosity environments in animal hosts.

Bacteria are so small that for them, making their way through water is like swimming in roofing tar for us. In response, these organisms have evolved a molecular machine that helps them move in their environment. Named the bacterial flagellum, this complex assemblage of molecules is formed of three main parts: a motor that spans the inner and outer membranes of the cell, and then a ‘hook’ that connects to a long filament which extends outside the bacterium. More precisely, the motor is formed of the stator, an ion pump that stays still, and of a rotor that can spin. Different rings can also be present in the space between the inner and outer membranes (the periplasm) and surround these components. The stator uses ions to generate the energy that makes the rotor whirl. In turn, this movement sets the filament in motion, propelling the bacterium. Depending on where the bacteria live, the stator can use different types of ions. In addition, while many species have a single stator system per motor, some may have several stator systems for one motor: this may help the microorganisms move in different conditions.

As microbes colonize environments with a different pH or viscosity, they constantly evolve new versions of the motor which are more suitable to their new surroundings. However, a part of the motor remains the same across species. Overall, it is still unclear how bacterial flagella evolve, but examining the structure of new motors can shed light upon this process. Here, Kaplan et al. combine a bioinformatics approach with an imaging technique known as electron cryo-tomography to dissect the structure of the flagellar motor of three species of bacteria with different stator systems, and compare these to known motors of the same class.

The results reveal a correlation between the nature of the stator system and the presence of certain elements. Stators that use sodium ions, or both sodium and hydrogen ions, are associated with two periplasmic rings surrounding the conserved motor structure. These rings do not exist in motors with single hydrogen-driven stators. Motors with dual hydrogen-driven stators are, to some extent, an ‘intermediate state’, with only one of those rings present. As all the studied species currently exist, it is difficult to know which version of the motor is the most ancient, and which one has evolved more recently.

Capturing the diversity of bacterial motors gives us insight into the evolutionary forces that shape complex molecular structures, which is essential to understand how life evolved on Earth. More practically, this knowledge may also help us design better nanomachines to power microscopic robots.

The Vibrio H-Ring Facilitates the Outer Membrane Penetration of the Polar Sheathed Flagellum

The bacterial flagellum has evolved as one of the most remarkable nanomachines in nature. It provides swimming and swarming motilities that are often essential for the bacterial life cycle and pathogenesis. Many bacteria such as Salmonella and Vibrio species use flagella as an external propeller to move to favorable environments, whereas spirochetes utilize internal periplasmic flagella to drive a serpentine movement of the cell bodies through tissues. Here, we use cryo-electron tomography to visualize the polar sheathed flagellum of Vibrio alginolyticus with particular focus on a Vibrio-specific feature, the H-ring. We characterized the H-ring by identifying its two components FlgT and FlgO. We found that the majority of flagella are located within the periplasmic space in the absence of the H-ring, which are different from those of external flagella in wild-type cells. Our results not only indicate the H-ring has a novel function in facilitating the penetration of the outer membrane and the assembly of the external sheathed flagella but also are consistent with the notion that the flagella have evolved to adapt highly diverse needs by receiving or removing accessary genes.IMPORTANCE Flagellum is the major organelle for motility in many bacterial species. While most bacteria possess external flagella, such as the multiple peritrichous flagella found in Escherichia coli and Salmonella enterica or the single polar sheathed flagellum in Vibrio spp., spirochetes uniquely assemble periplasmic flagella, which are embedded between their inner and outer membranes. Here, we show for the first time that the external flagella in Vibrio alginolyticus can be changed as periplasmic flagella by deleting two flagellar genes. The discovery here may provide new insights into the molecular basis underlying assembly, diversity, and evolution of flagella.

Structural differences in the bacterial flagellar motor among bacterial species

The bacterial flagellum is a supramolecular motility machine consisting of the basal body as a rotary motor, the hook as a universal joint, and the filament as a helical propeller. Intact structures of the bacterial flagella have been observed for different bacterial species by electron cryotomography and subtomogram averaging. The core structures of the basal body consisting of the C ring, the MS ring, the rod and the protein export apparatus, and their organization are well conserved, but novel and divergent structures have also been visualized to surround the conserved structure of the basal body. This suggests that the flagellar motors have adapted to function in various environments where bacteria live and survive. In this review, we will summarize our current findings on the divergent structures of the bacterial flagellar motor.

Molecular architecture of the sheathed polar flagellum in Vibrio alginolyticus

Vibrio species are Gram-negative rod-shaped bacteria that are ubiquitous and often highly motile in aqueous environments. Vibrio swimming motility is driven by a polar flagellum covered with a membranous sheath, but this sheathed flagellum is not well understood at the molecular level because of limited structural information. Here, we use Vibrio alginolyticus as a model system to study the sheathed flagellum in intact cells by combining cryoelectron tomography (cryo-ET) and subtomogram analysis with a genetic approach. We reveal striking differences between sheathed and unsheathed flagella in V. alginolyticus cells, including a novel ring-like structure at the bottom of the hook that is associated with major remodeling of the outer membrane and sheath formation. Using mutants defective in flagellar motor components, we defined a Vibrio-specific feature (also known as the T ring) as a distinctive periplasmic structure with 13-fold symmetry. The unique architecture of the T ring provides a static platform to recruit the PomA/B complexes, which are required to generate higher torques for rotation of the sheathed flagellum and fast motility of Vibrio cells. Furthermore, the Vibrio flagellar motor exhibits an intrinsic length variation between the inner and the outer membrane bound complexes, suggesting the outer membrane bound complex can shift slightly along the axial rod during flagellar rotation. Together, our detailed analyses of the polar flagella in intact cells provide insights into unique aspects of the sheathed flagellum and the distinct motility of Vibrio species.

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.

Oligomeric lipoprotein PelC guides Pel polysaccharide export across the outer membrane of Pseudomonas aeruginosa

Secreted polysaccharides are important functional and structural components of bacterial biofilms. The opportunistic pathogen Pseudomonas aeruginosa produces the cationic exopolysaccharide Pel, which protects bacteria from aminoglycoside antibiotics and contributes to biofilm architecture through ionic interactions with extracellular DNA. A bioinformatics analysis of genome databases suggests that gene clusters for Pel biosynthesis are present in >125 bacterial species, yet little is known about how this biofilm exopolysaccharide is synthesized and exported from the cell. In this work, we characterize PelC, an outer membrane lipoprotein essential for Pel production. Crystal structures of PelC from Geobacter metallireducens and Paraburkholderia phytofirmans coupled with structure-guided disulfide cross-linking in P. aeruginosa suggest that PelC assembles into a 12- subunit ring-shaped oligomer. In this arrangement, an aromatic belt in proximity to its lipidation site positions the highly electronegative surface of PelC toward the periplasm. PelC is structurally similar to the Escherichia coli amyloid exporter CsgG; however, unlike CsgG, PelC does not possess membrane-spanning segments required for polymer export across the outer membrane. We show that the multidomain protein PelB with a predicted C-terminal β-barrel porin localizes to the outer membrane, and propose that PelC functions as an electronegative funnel to guide the positively charged Pel polysaccharide toward an exit channel formed by PelB. Together, our findings provide insight into the unique molecular architecture and export mechanism of the Pel apparatus, a widespread exopolysaccharide secretion system found in environmental and pathogenic bacteria.

Studies on the mechanism of bacterial flagellar rotation and the flagellar number regulation

Many motile bacteria have the motility organ, the flagellum. It rotates by the rotary motor driven by the ion-motive force and is embedded in the cell surface at the base of each flagellar filament. Many researchers have been studying its rotary mechanism for years, but most of the energy conversion processes have been remained in mystery. We focused on the flagellar stator, which works at the core process of energy conversion, and found that the periplasmic region of the stator changes its conformation to be activated only when the stator units are incorporated into the motor and anchored at the cell wall. Meanwhile, the physiologically important supramolecular complex is localized in the cell at the right place and the right time with a proper amount. How the cell achieves such a proper localization is the fundamental question for life science, and we undertake this problem by analyzing the mechanism for biogenesis of a single polar flagellum of Vibrio alginolyticus. Here I describe the molecular mechanism of how the flagellum is generated at the specific place with a proper number, and also how the flagellar stator is incorporated into the motor to complete the functional motor assembly, based on our studies.

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.

Diverse high-torque bacterial flagellar motors assemble wider stator rings using a conserved protein scaffold

Although it is known that diverse bacterial flagellar motors produce different torques, the mechanism underlying torque variation is unknown. To understand this difference better, we combined genetic analyses with electron cryo-tomography subtomogram averaging to determine in situ structures of flagellar motors that produce different torques, from Campylobacter and Vibrio species. For the first time, to our knowledge, our results unambiguously locate the torque-generating stator complexes and show that diverse high-torque motors use variants of an ancestrally related family of structures to scaffold incorporation of additional stator complexes at wider radii from the axial driveshaft than in the model enteric motor. We identify the protein components of these additional scaffold structures and elucidate their sequential assembly, demonstrating that they are required for stator-complex incorporation. These proteins are widespread, suggesting that different bacteria have tailored torques to specific environments by scaffolding alternative stator placement and number. Our results quantitatively account for different motor torques, complete the assignment of the locations of the major flagellar components, and provide crucial constraints for understanding mechanisms of torque generation and the evolution of multiprotein complexes.

Architecture of the type IVa pilus machine

Type IVa pili are filamentous cell surface structures observed in many bacteria. They pull cells forward by extending, adhering to surfaces, and then retracting. We used cryo-electron tomography of intact Myxococcus xanthus cells to visualize type IVa pili and the protein machine that assembles and retracts them (the type IVa pilus machine, or T4PM) in situ, in both the piliated and nonpiliated states, at a resolution of 3 to 4 nanometers. We found that T4PM comprises an outer membrane pore, four interconnected ring structures in the periplasm and cytoplasm, a cytoplasmic disc and dome, and a periplasmic stem. By systematically imaging mutants lacking defined T4PM proteins or with individual proteins fused to tags, we mapped the locations of all 10 T4PM core components and the minor pilins, thereby providing insights into pilus assembly, structure, and function.

Undiscovered regions on the molecular landscape of flagellar assembly

The bacterial flagellum is a motility structure and one of the most complicated motors in the biosphere. A flagellum consists of several dozens of building blocks in different stoichiometries and extends from the cytoplasm to the extracellular space. Flagellar biogenesis follows a strict spatio-temporal regime that is guided by a plethora of flagellar assembly factors and chaperones. The goal of this review is to summarize our current structural and mechanistic knowledge of this intricate process and to identify the undiscovered regions on the molecular landscape of flagellar assembly.

Architecture and roles of periplasmic adaptor proteins in tripartite efflux assemblies

Recent years have seen major advances in the structural understanding of the different components of tripartite efflux assemblies, which encompass the multidrug efflux (MDR) pumps and type I secretion systems. The majority of these investigations have focused on the role played by the inner membrane transporters and the outer membrane factor (OMF), leaving the third component of the system – the Periplasmic Adaptor Proteins (PAPs) – relatively understudied. Here we review the current state of knowledge of these versatile proteins which, far from being passive linkers between the OMF and the transporter, emerge as active architects of tripartite assemblies, and play diverse roles in the transport process. Recognition between the PAPs and OMFs is essential for pump assembly and function, and targeting this interaction may provide a novel avenue for combating multidrug resistance. With the recent advances elucidating the drug efflux and energetics of the tripartite assemblies, the understanding of the interaction between the OMFs and PAPs is the last piece remaining in the complete structure of the tripartite pump assembly puzzle.

The bacterial flagellar motor and its structural diversity

The bacterial flagellum is a reversible rotary motor powered by an electrochemical-potential difference of specific ions across the cytoplasmic membrane. The H(+)-driven motor of Salmonella spins at ∼300 Hz, whereas the Na(+)-driven motor of marine Vibrio spp. can rotate much faster, up to 1700 Hz. A highly conserved motor structure consists of the MS ring, C ring, rod, and export apparatus. The C ring and the export apparatus show dynamic properties for exerting their functional activities. Various additional structures surrounding the conserved motor structure are observed in different bacterial species. In this review we summarize our current understanding of the structure, function, and assembly of the flagellar motor in Salmonella and marine Vibrio.

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.

Structural insights into the lipoprotein outer membrane regulator of penicillin-binding protein 1B

In bacteria, the synthesis of the protective peptidoglycan sacculus is a dynamic process that is tightly regulated at multiple levels. Recently, the lipoprotein co-factor LpoB has been found essential for the in vivo function of the major peptidoglycan synthase PBP1b in Enterobacteriaceae. Here, we reveal the crystal structures of Salmonella enterica and Escherichia coli LpoB. The LpoB protein can be modeled as a ball and tether, consisting of a disordered N-terminal region followed by a compact globular C-terminal domain. Taken together, our structural data allow us to propose new insights into LpoB-mediated regulation of peptidoglycan synthesis.

Structure, gene regulation and environmental response of flagella in Vibrio

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

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.