Structural insights into flagellar stator-rotor interactions

Structural basis of the bacterial flagellar motor rotational switching

The bacterial flagellar motor is a huge bidirectional rotary nanomachine that drives rotation of the flagellum for bacterial motility. The cytoplasmic C ring of the flagellar motor functions as the switch complex for the rotational direction switching from counterclockwise to clockwise. However, the structural basis of the rotational switching and how the C ring is assembled have long remained elusive. Here, we present two high-resolution cryo-electron microscopy structures of the C ring-containing flagellar basal body-hook complex from Salmonella Typhimurium, which are in the default counterclockwise state and in a constitutively active CheY mutant-induced clockwise state, respectively. In both complexes, the C ring consists of four subrings, but is in two different conformations. The CheY proteins are bound into an open groove between two adjacent protomers on the surface of the middle subring of the C ring and interact with the FliG and FliM subunits. The binding of the CheY protein induces a significant upward shift of the C ring towards the MS ring and inward movements of its protomers towards the motor center, which eventually remodels the structures of the FliG subunits and reverses the orientations and surface electrostatic potential of the αtorque helices to trigger the counterclockwise-to-clockwise rotational switching. The conformational changes of the FliG subunits reveal that the stator units on the motor require a relocation process in the inner membrane during the rotational switching. This study provides unprecedented molecular insights into the rotational switching mechanism and a detailed overall structural view of the bacterial flagellar motors.

Flagellar protein FliL: A many-splendored thing

FliL is a bacterial flagellar protein demonstrated to associate with, and regulate ion flow through, the stator complex in a diverse array of bacterial species. FliL is also implicated in additional functions such as stabilizing the flagellar rod, modulating rotor bias, sensing the surface, and regulating gene expression. How can one protein do so many things? Its location is paramount to understanding its numerous functions. This review will look at the evidence, attempt to resolve some conflicting findings, and offer new thoughts on FliL.

DnaA modulates gene expression and morphologyof the Lyme Disease spirochete

All bacteria encode a multifunctional DNA-binding protein, DnaA, which initiates chromosomal replication. Despite having the most complex, segmented bacterial genome, little is known about Borrelia burgdorferi DnaA and its role in maintaining the spirochete's physiology. We utilized inducible CRISPR-interference and overexpression to modulate cellular levels of DnaA to better understand this essential protein. Dysregulation of DnaA, either up or down, significantly slowed replication and increased or decreased cell lengths. Using fluorescent microscopy, we found the DnaA CRISPRi mutants had increased numbers of chromosomes with irregular spacing patterns. The DnaA-depleted spirochetes also exhibited a significant defect in helical morphology. RNA-seq of the conditional mutants showed significant changes in the levels of transcripts involved with flagellar synthesis, elongation, cell division, virulence, and other functions. These findings demonstrate that the DnaA plays a commanding role in maintaining borrelial growth dynamics and protein expression, which are essential for the survival of the Lyme disease spirochete.

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.

FlgV forms a flagellar motor ring that is required for optimal motility of Helicobacter pylori

Flagella-driven motility is essential for Helicobacter pylori to colonize the human stomach, where it causes a variety of diseases, including chronic gastritis, peptic ulcer disease, and gastric cancer. H. pylori has evolved a high-torque-generating flagellar motor that possesses several accessories not found in the archetypical Escherichia coli motor. FlgV was one of the first flagellar accessory proteins identified in Campylobacter jejuni, but its structure and function remain poorly understood. Here, we confirm that deletion of flgV in H. pylori B128 and a highly motile variant of H. pylori G27 (G27M) results in reduced motility in soft agar medium. Comparative analyses of in-situ flagellar motor structures of wild-type, ΔflgV, and a strain expressing FlgV-YFP showed that FlgV forms a ring-like structure closely associated with the junction of two highly conserved flagellar components: the MS and C rings. The results of our studies suggest that the FlgV ring has adapted specifically in Campylobacterota to support the assembly and efficient function of the high-torque-generating motors.

Changes in the hydrophobic network of the FliGMC domain induce rotational switching of the flagellar motor

The FliG protein plays a pivotal role in switching the rotational direction of the flagellar motor between clockwise and counterclockwise. Although we previously showed that mutations in the Gly-Gly linker of FliG induce a defect in switching rotational direction, the detailed molecular mechanism was not elucidated. Here, we studied the structural changes in the FliG fragment containing the middle and C-terminal regions, named FliG_MC, and the switch-defective FliG_MC-G215A, using nuclear magnetic resonance (NMR) and molecular dynamics simulations. NMR analysis revealed multiple conformations of FliG_MC, and the exchange process between these conformations was suppressed by the G215A residue substitution. Furthermore, changes in the intradomain orientation of FliG were induced by changes in hydrophobic interaction networks throughout FliG. Our finding applies to FliG in a ring complex in the flagellar basal body, and clarifies the switching mechanism of the flagellar motor.

Ion selectivity and rotor coupling of the Vibrio flagellar sodium-driven stator unit

Bacteria swim using a flagellar motor that is powered by stator units. Vibrio spp. are highly motile bacteria responsible for various human diseases, the polar flagella of which are exclusively driven by sodium-dependent stator units (PomAB). However, how ion selectivity is attained, how ion transport triggers the directional rotation of the stator unit, and how the stator unit is incorporated into the flagellar rotor remained largely unclear. Here, we have determined by cryo-electron microscopy the structure of Vibrio PomAB. The electrostatic potential map uncovers sodium binding sites, which together with functional experiments and molecular dynamics simulations, reveal a mechanism for ion translocation and selectivity. Bulky hydrophobic residues from PomA prime PomA for clockwise rotation. We propose that a dynamic helical motif in PomA regulates the distance between PomA subunit cytoplasmic domains, stator unit activation, and torque transmission. Together, our study provides mechanistic insights for understanding ion selectivity and rotor incorporation of the stator unit of the bacterial flagellum.

Revealing bacterial cell biology using cryo-electron tomography

Visualizing macromolecules inside bacteria at a high spatial resolution has remained a challenge owing to their small size and limited resolution of optical microscopy techniques. Recent advances in cryo-electron tomography (cryo-ET) imaging methods have revealed the spatial and temporal assemblies of many macromolecules involved in different cellular processes in bacteria at a resolution of a few nanometers in their native milieu. Specifically, the application of cryo-focused ion beam (cryo-FIB) milling to thin bacterial specimens makes them amenable for high-resolution cryo-ET data collection. In this review, we highlight recent research in three emerging areas of bacterial cell biology that have benefited from the cryo-FIB-ET technology - cytoskeletal filament assembly, intracellular organelles, and multicellularity.

The Bacterial Flagellar Motor: Insights Into Torque Generation, Rotational Switching, and Mechanosensing

The flagellar motor is a bidirectional rotary nanomachine used by many bacteria to sense and move through environments of varying complexity. The bidirectional rotation of the motor is governed by interactions between the inner membrane-associated stator units and the C-ring in the cytoplasm. In this review, we take a structural biology perspective to discuss the distinct conformations of the stator complex and the C-ring that regulate bacterial motility by switching rotational direction between the clockwise (CW) and counterclockwise (CCW) senses. We further contextualize recent in situ structural insights into the modulation of the stator units by accessory proteins, such as FliL, to generate full torque. The dynamic structural remodeling of the C-ring and stator complexes as well as their association with signaling and accessory molecules provide a mechanistic basis for how bacteria adjust motility to sense, move through, and survive in specific niches both outside and within host cells and tissues.

Decoding microbial genomes to understand their functional roles in human complex diseases

Complex diseases such as cardiovascular disease (CVD), obesity, inflammatory bowel disease (IBD), kidney disease, type 2 diabetes (T2D), and cancer have become a major burden to public health and affect more than 20% of the population worldwide. The etiology of complex diseases is not yet clear, but they are traditionally thought to be caused by genetics and environmental factors (e.g., dietary habits), and by their interactions. Besides this, increasing pieces of evidence now highlight that the intestinal microbiota may contribute substantially to the health and disease of the human host via their metabolic molecules. Therefore, decoding the microbial genomes has been an important strategy to shed light on their functional potential. In this review, we summarize the roles of the gut microbiome in complex diseases from its functional perspective. We further introduce artificial tools in decoding microbial genomes to profile their functionalities. Finally, state-of-the-art techniques have been highlighted which may contribute to a mechanistic understanding of the gut microbiome in human complex diseases and promote the development of the gut microbiome-based personalized medicine.

Dynamic Hybrid Flagellar Motors-Fuel Switch and More

Flagellar motors are intricate rotating nanomachines that are powered by transmembrane ion gradients. The stator complexes are the powerhouses of the flagellar motor: They convert a transmembrane ion gradient, mainly of H⁺ or Na⁺, into rotation of the helical flagellar filament. They are thus essential for motor function. The number of stators synchronously engaged in the motor is surprisingly dynamic and depends on the load and the environmental concentration of the corresponding coupling ion. Thus, the rotor-stator interactions determine an important part of the properties of the motor. Numerous bacteria have been identified as possessing more than one set of stators, and some species have been demonstrated to use these different stators in various configurations to modify motor functions by dynamic in-flight swapping. Here, we review knowledge of the properties, the functions, and the evolution of these hybrid motors and discuss questions that remain unsolved.

A physical mechanism underlying the torque generation of the bacterial flagellar motor

This study proposes a physical mechanism underlying the torque generation of a bacterial flagellar motor (BFM), in which the torque for the rotation of the rotor can be generated from impulsive forces resulting from collisions between the randomly moving stator and rotor. The torque required for the rotation of the rotor may be generated through two coexisting mechanisms: mechanism (A), in which the stator collides with the rotor, whose rotation axis fluctuates asymmetrically, generating a torque in the direction of the rod, and mechanism (B), in which physical collisions between the stator and the asymmetric [Formula: see text]and [Formula: see text] generate the torque in the direction of the rod. Mechanism (A) might be related the bidirectional rotation and the tumbling of the motion of the cell. Mechanism (B) might be related to occurrence of the steps in the time traces of the rotational angle, backward stepping or switching of the rotational direction, and the knee-shaped [Formula: see text]-[Formula: see text] relation. The above-mentioned characteristics of the rotation of BFM are reproduced in a model device designed to confirm the applicability of the proposed concept to real BFM. Moreover, a prediction of the disappearance of the knee-shaped [Formula: see text]-[Formula: see text] relation of the actual BFM at a high temperature is proposed.

FliL ring enhances the function of periplasmic flagella

SignificanceHow flagella sense complex environments and control bacterial motility remain fascinating questions. Here, we deploy cryo-electron tomography to determine in situ structures of the flagellar motor in wild-type and mutant cells of Borrelia burgdorferi, revealing that three flagellar proteins (FliL, MotA, and MotB) form a unique supramolecular complex in situ. Importantly, FliL not only enhances motor function by forming a ring around the stator complex MotA/MotB in its extended, active conformation but also facilitates assembly of the stator complex around the motor. Our in situ data provide insights into how cooperative remodeling of the FliL-stator supramolecular complex helps regulate the collective ion flux and establishes the optimal function of the flagellar motor to guide bacterial motility in various environments.

Cryo-electron Tomography Reveals the Roles of FliY in Helicobacter pylori Flagellar Motor Assembly

Helicobacter pylori plays a causative role in gastric diseases. The pathogenicity of H. pylori depends on its ability to colonize the stomach guided by motility. FliY is a unique flagellar motor switch component coexisting with the classical FliG, FliM, and FliN switch proteins in some bacteria and has been shown to be essential for flagellation. However, the functional importance of FliY in H. pylori flagellar motor assembly is not well understood. Here, we applied cryo-electron tomography and subtomogram averaging to analyze the in situ structures of flagellar motors from wild-type strain, fliY-null mutant and complementation mutants expressing the N-terminal or C-terminal domain of FliY. Loss of full-length FliY or its C-terminal domain interrupted the formation of an intact C ring and soluble export apparatus, as well as the hook and flagellar filaments. Complementation with FliY C-terminal domain restored all these missing components of flagellar motor. Taken together, these results provide structural insights into the roles of FliY, especially its C-terminal domain in flagellar motor assembly in H. pylori.

IMPORTANCEHelicobacter pylori is the major risk factor related with gastric diseases. Flagellar motor is one of the most important virulence factors in H. pylori. However, the assembly mechanism of H. pylori flagellar motor is not fully understood yet. Previous report mainly described the overall structures of flagellum but had not focused on its specific components. Here, we focus on H. pylori flagellar C-ring protein FliY. We directly visualize the flagellar structures of H. pylori wild-type and FliY N-/C-terminal complementary strains by cryo-electron tomography and subtomogram averaging. Our results show that deletion of FliY or its C-terminal domain causes the loss of C ring, whereas deletion of FliY N-terminal does not affect C-ring assembly and flagellar structures. Our results provide direct evidence that C-ring protein FliY, especially its C-terminal domain, plays an indispensable role in H. pylori motor assembly and flagellar formation. This study will deepen our understanding about H. pylori pathogenesis.

Candidatus Liberibacter: From Movement, Host Responses, to Symptom Development of Citrus Huanglongbing

Candidatus Liberibacter spp. are fastidious α-proteobacteria that cause multiple diseases on plant hosts of economic importance, including the most devastating citrus disease: Huanglongbing (HLB). HLB was reported in Asia a century ago but has since spread worldwide. Understanding the pathogenesis of Candidatus Liberibacter spp. remains challenging as they are yet to be cultured in artificial media and infect the phloem, a sophisticated environment that is difficult to manipulate. Despite those challenges, tremendous progress has been made on Ca. Liberibacter pathosystems. Here, we first reviewed recent studies on genetic information of flagellar and type IV pili biosynthesis, their expression profiles, and movement of Ca. Liberibacter spp. inside the plant and insect hosts. Next, we reviewed the transcriptomic, proteomic, and metabolomic studies of susceptible and tolerant plant genotypes to Ca. Liberibacter spp. infection and how Ca. Liberibacter spp. adapt in plants. Analyses of the interactions between plants and Ca. Liberibacter spp. imply the involvement of immune response in the Ca. Liberibacter pathosystems. Lastly, we reviewed how Ca. Liberibacter spp. movement inside and interactions with plants lead to symptom development.

Characterization of the Flagellar Collar Reveals Structural Plasticity Essential for Spirochete Motility

Spirochetes are a remarkable group of bacteria with distinct morphology and periplasmic flagella that enable motility in viscous environments, such as host connective tissues. The collar, a spirochete-specific complex of the periplasmic flagellum, is required for this unique spirochete motility, yet it has not been clear how the collar assembles and enables spirochetes to transit between complex host environments. Here, we characterize the collar complex in the Lyme disease spirochete Borrelia burgdorferi. We discover as well as delineate the distinct functions of two novel collar proteins, FlcB and FlcC, by combining subtractive bioinformatic, genetic, and cryo-electron tomography approaches. Our high-resolution in situ structures reveal that the multiprotein collar has a remarkable structural plasticity essential not only for assembly of flagellar motors in the highly curved membrane of spirochetes but also for generation of the high torque necessary for spirochete motility.

Molecular and Cell Biological Analysis of SwrB in Bacillus subtilis

Swarming motility is flagellum-mediated movement over a solid surface, and Bacillus subtilis cells require an increase in flagellar density to swarm. SwrB is a protein of unknown function required for swarming that is necessary to increase the number of flagellar hooks but not basal bodies. Previous work suggested that SwrB activates flagellar type III secretion, but the mechanism by which it might perform this function is unknown. Here, we show that SwrB likely acts substoichiometrically as it localizes as puncta at the membrane in numbers fewer than those of flagellar basal bodies. Moreover, the action of SwrB is likely transient as puncta of SwrB were not dependent on the presence of the basal bodies and rarely colocalized with flagellar hooks. Random mutagenesis of the SwrB sequence found that a histidine within the transmembrane segment was conditionally required for activity and punctate localization. Finally, three hydrophobic residues that precede a cytoplasmic domain of poor conservation abolished SwrB activity when mutated and caused aberrant migration during electrophoresis. Our data are consistent with a model in which SwrB interacts with the flagellum, changes conformation to activate type III secretion, and departs. IMPORTANCE Type III secretion systems (T3SSs) are elaborate nanomachines that form the core of the bacterial flagellum and injectisome of pathogens. The machines not only secrete proteins like virulence factors but also secrete the structural components for their own assembly. Moreover, proper construction requires complex regulation to ensure that the parts are roughly secreted in the order in which they are assembled. Here, we explore a poorly understood activator of the flagellar T3SS activation in Bacillus subtilis called SwrB. To aid mechanistic understanding, we determine the rules for subcellular punctate localization, the topology with respect to the membrane, and critical residues required for SwrB function.

Structural basis of torque generation in the bi-directional bacterial flagellar motor

The flagellar stator unit is an oligomeric complex of two membrane proteins (MotA₅B₂) that powers bi-directional rotation of the bacterial flagellum. Harnessing the ion motive force across the cytoplasmic membrane, the stator unit operates as a miniature rotary motor itself to provide torque for rotation of the flagellum. Recent cryo-electron microscopic (cryo-EM) structures of the stator unit provided novel insights into its assembly, function, and subunit stoichiometry, revealing the ion flux pathway and the torque generation mechanism. Furthermore, in situ cryo-electron tomography (cryo-ET) studies revealed unprecedented details of the interactions between stator unit and rotor. In this review, we summarize recent advances in our understanding of the structure and function of the flagellar stator unit, torque generation, and directional switching of the motor.

The peptidoglycan-associated protein NapA plays an important role in the envelope integrity and in the pathogenesis of the lyme disease spirochete

The bacterial pathogen responsible for causing Lyme disease, Borrelia burgdorferi, is an atypical Gram-negative spirochete that is transmitted to humans via the bite of an infected Ixodes tick. In diderms, peptidoglycan (PG) is sandwiched between the inner and outer membrane of the cell envelope. In many other Gram-negative bacteria, PG is bound by protein(s), which provide both structural integrity and continuity between envelope layers. Here, we present evidence of a peptidoglycan-associated protein (PAP) in B. burgdorferi. Using an unbiased proteomics approach, we identified Neutrophil Attracting Protein A (NapA) as a PAP. Interestingly, NapA is a Dps homologue, which typically functions to bind and protect cellular DNA from damage during times of stress. While B. burgdorferi NapA is known to be involved in the oxidative stress response, it lacks the critical residues necessary for DNA binding. Biochemical and cellular studies demonstrate that NapA is localized to the B. burgdorferi periplasm and is indeed a PAP. Cryo-electron microscopy indicates that mutant bacteria, unable to produce NapA, have structural abnormalities. Defects in cell-wall integrity impact growth rate and cause the napA mutant to be more susceptible to osmotic and PG-specific stresses. NapA-linked PG is secreted in outer membrane vesicles and augments IL-17 production, relative to PG alone. Using microfluidics, we demonstrate that NapA acts as a molecular beacon-exacerbating the pathogenic properties of B. burgdorferi PG. These studies further our understanding of the B. burgdorferi cell envelope, provide critical information that underlies its pathogenesis, and highlight how a highly conserved bacterial protein can evolve mechanistically, while maintaining biological function.

Structural basis of bacterial flagellar motor rotation and switching

The bacterial flagellar motor, a remarkable rotary machine, can rapidly switch between counterclockwise (CCW) and clockwise (CW) rotational directions to control the migration behavior of the bacterial cell. The flagellar motor consists of a bidirectional spinning rotor surrounded by torque-generating stator units. Recent high-resolution in vitro and in situ structural studies have revealed stunning details of the individual components of the flagellar motor and their interactions in both the CCW and CW senses. In this review, we discuss these structures and their implications for understanding the molecular mechanisms underlying flagellar rotation and switching.

Mechanosensitive remodeling of the bacterial flagellar motor is independent of direction of rotation

Motility is important for the survival and dispersal of many bacteria, and it often plays a role during infections. Regulation of bacterial motility by chemical stimuli is well studied, but recent work has added a new dimension to the problem of motility control. The bidirectional flagellar motor of the bacterium Escherichia coli recruits or releases torque-generating units (stator units) in response to changes in load. Here, we show that this mechanosensitive remodeling of the flagellar motor is independent of direction of rotation. Remodeling rate constants in clockwise rotating motors and in counterclockwise rotating motors, measured previously, fall on the same curve if plotted against torque. Increased torque decreases the off rate of stator units from the motor, thereby increasing the number of active stator units at steady state. A simple mathematical model based on observed dynamics provides quantitative insight into the underlying molecular interactions. The torque-dependent remodeling mechanism represents a robust strategy to quickly regulate output (torque) in response to changes in demand (load).

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.

Molecular mechanism for rotational switching of the bacterial flagellar motor

The bacterial flagellar motor can rotate in counterclockwise (CCW) or clockwise (CW) senses, and transitions are controlled by the phosphorylated form of the response regulator CheY (CheY-P). To dissect the mechanism underlying flagellar rotational switching, we use Borrelia burgdorferi as a model system to determine high-resolution in situ motor structures in cheX and cheY3 mutants, in which motors are locked in either CCW or CW rotation. The structures showed that CheY3-P interacts directly with a switch protein, FliM, inducing a major remodeling of another switch protein, FliG2, and altering its interaction with the torque generator. Our findings lead to a model in which the torque generator rotates in response to an inward flow of H⁺ driven by the proton motive force, and conformational changes in FliG2 driven by CheY3-P allow the switch complex to interact with opposite sides of the rotating torque generator, facilitating rotational switching.

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.

Exploring the bacterial nano-universe

Since the days of the first acknowledged microscopist, Antonie van Leeuwenhoek, the 'animalcules', that is, bacteria and other microbes have been subject to increasingly detailed visualization. With the currently most sophisticated molecular imaging method; cryo electron tomography (Cryo-ET), we are reaching the milestone of being able to image an entire organism in a single dataset at nanometer resolution. Cryo-ET will enable the next revolution in our understanding of bacterial cells, their ultra-structure and intricate molecular nanomachines. Here, we highlight recent research discoveries based on constantly progressing technology developments. We discuss advantages and challenges of using Cryo-ET to visualize spatial structure of microorganisms and macromolecular complexes in their native environment.

The Architectural Dynamics of the Bacterial Flagellar Motor Switch

The rotary bacterial flagellar motor is remarkable in biochemistry for its highly synchronized operation and amplification during switching of rotation sense. The motor is part of the flagellar basal body, a complex multi-protein assembly. Sensory and energy transduction depends on a core of six proteins that are adapted in different species to adjust torque and produce diverse switches. Motor response to chemotactic and environmental stimuli is driven by interactions of the core with small signal proteins. The initial protein interactions are propagated across a multi-subunit cytoplasmic ring to switch torque. Torque reversal triggers structural transitions in the flagellar filament to change motile behavior. Subtle variations in the core components invert or block switch operation. The mechanics of the flagellar switch have been studied with multiple approaches, from protein dynamics to single molecule and cell biophysics. The architecture, driven by recent advances in electron cryo-microscopy, is available for several species. Computational methods have correlated structure with genetic and biochemical databases. The design principles underlying the basis of switch ultra-sensitivity and its dependence on motor torque remain elusive, but tantalizing clues have emerged. This review aims to consolidate recent knowledge into a unified platform that can inspire new research strategies.

Taming the flagellar motor of pseudomonads with a nucleotide messenger

Pseudomonads rely on the flagellar motor to rotate a polar flagellum for swimming and swarming, and to sense surfaces for initiating the motile-to-sessile transition to adopt a surface-dwelling lifestyle. Deciphering the function and regulation of the flagellar motor is of paramount importance for understanding the behaviours of environmental and pathogenic pseudomonads. Recent studies disclosed the preeminent role played by the messenger c-di-GMP in controlling the real-time performance of the flagellar motor in pseudomonads. The studies revealed that c-di-GMP controls the dynamic exchange of flagellar stator units to regulate motor torque/speed and modulates the frequency of flagellar motor switching via the chemosensory signalling pathways. Apart from being a rotary motor, the flagellar motor is emerging as a mechanosensor that transduces surface-induced mechanical signals into an increase of cellular c-di-GMP concentration to initiate the cellular programs required for long-term colonization. Collectively, the studies generate long-awaited mechanistic insights into how c-di-GMP regulates bacterial motility and the motile-to-sessile transition. The new findings also raise the fundamental questions of how cellular c-di-GMP concentrations are dynamically coupled to flagellar output and the proton-motive force, and how c-di-GMP signalling is coordinated spatiotemporally to fine-tune flagellar response and the behaviour of pseudomonads in solutions and on surfaces.

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.

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.

BB0326 is responsible for the formation of periplasmic flagellar collar and assembly of the stator complex in Borrelia burgdorferi

Borrelia burgdorferi is a highly motile spirochete due to its periplasmic flagella. Unlike flagella of other bacteria, spirochetes' periplasmic flagella possess a complex structure called the collar, about which little is known in terms of function and composition. Using various approaches, we have identified a novel protein, BB0326, as a key component of the collar. We show that a peripheral portion of the collar is diminished in the Δbb0326 mutant and restored in the complemented bb0326+ cells, leading us to rename BB0326 as periplasmic flagellar collar protein A or FlcA. The ΔflcA mutant cells produced fewer, abnormally tilted and shorter flagella, as well as diminished stators, suggesting that FlcA is crucial for flagellar and stator assemblies. We provide further evidence that FlcA interacts with the stator and that this collar-stator interaction is essential for the high torque needed to power the spirochete's periplasmic flagellar motors. These observations suggest that the collar provides various important functions to the spirochete's periplasmic flagellar assembly and rotation.

Load-dependent adaptation near zero load in the bacterial flagellar motor

The bacterial flagellar motor is an ion-powered transmembrane protein complex which drives swimming in many bacterial species. The motor consists of a cytoplasmic 'rotor' ring and a number of 'stator' units, which are bound to the cell wall of the bacterium. Recently, it has been shown that the number of functional torque-generating stator units in the motor depends on the external load, and suggested that mechanosensing in the flagellar motor is driven via a 'catch bond' mechanism in the motor's stator units. We present a method that allows us to measure-on a single motor-stator unit dynamics across a large range of external loads, including near the zero-torque limit. By attaching superparamagnetic beads to the flagellar hook, we can control the motor's speed via a rotating magnetic field. We manipulate the motor to four different speed levels in two different ion-motive force (IMF) conditions. This framework allows for a deeper exploration into the mechanism behind load-dependent remodelling by separating out motor properties, such as rotation speed and energy availability in the form of IMF, that affect the motor torque.