A flagellum-specific chaperone facilitates assembly of the core type III export apparatus of the bacterial flagellum

Structural basis for assembly and function of the Salmonella flagellar MS-ring with three different symmetries

The flagellar MS-ring is the initial template for flagellar assembly and houses the flagellar protein export complex. The MS-ring has three parts of different symmetries within the ring structure by assembly of FliF subunits in two different conformations with distinct arrangements of three ring-building motifs, RBM1, RBM2, and RBM3. However, it remains unknown how these symmetries are generated. A combination of cryoEM structure and structure-based mutational analyses demonstrates that the well-conserved DQxGxxL motif in the RBM2-RBM3 hinge loop allows RBM2 to take two different orientations relative to RBM3. Of 34 FliF subunits of the MS-ring in the basal body, 23 RBM2 domains form an inner ring with a central pore that accommodates the flagellar protein export complex, and the remaining 11 RBM2 domains form 11 cog-like structures together with RBM1 domains just outside the inner RBM2-ring. We propose that a dimer of FliF with two different conformations initiates MS-ring assembly.

Genomic diversity of Campylobacter jejuni and Campylobacter coli isolated from the Ethiopian dairy supply chain

Campylobacteriosis outbreaks have previously been linked to dairy foods. While the genetic diversity of Campylobacter is well understood in high-income countries, it is largely unknown in low-income countries, such as Ethiopia. This study therefore aimed to conduct the first genomic characterization of Campylobacter isolates from the Ethiopian dairy supply chain to aid in future epidemiological studies. Fourteen C. jejuni and four C. coli isolates were whole genome sequenced using an Illumina platform. Sequences were analyzed using the bioinformatics tools in the GalaxyTrakr platform to identify MLST types, and single nucleotide polymorphisms, and infer phylogenetic relationships among the studied isolates. Assembled genomes were further screened to detect antimicrobial resistance and virulence gene sequences. Among 14 C. jejuni, ST 2084 and ST 51, which belong to the clonal complexes ST-353 and ST-443, respectively, were identified. Among the 4 sequenced C. coli isolates, two isolates belonged to ST 1628 and two to ST 830 from the clonal complex ST-828. The isolates of C. jejuni ST 2084 and ST 51 carried β-lactam resistance gene blaOXA-605, a fluoroquinolone resistance-associated mutation T86I in the gryA gene, and a macrolide resistance-associated mutation A103V in 50S L22. Only ST 2084 isolates carried the tetracycline resistance gene tetO. Conversely, all four C. coli ST 830 and ST 1628 isolates carried tetO, but only ST 1628 isolates also carried blaOXA-605. Lastly, C. jejuni ST 2084 isolates carried a total of 89 virulence genes, and ST 51 isolates carried up to 88 virulence genes. Among C. coli, ST 830 isolates carried 71 genes involved in virulence, whereas two ST 1628 isolates carried up to 82 genes involved in virulence. Isolates from all identified STs have previously been isolated from human clinical cases, demonstrating a potential food safety concern. This finding warrants further monitoring of Campylobacter in dairy foods in Ethiopia to better understand and manage the risks associated with Campylobacter contamination and transmission.

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.

Quantifying Substrate Protein Secretion via the Type III Secretion System of the Bacterial Flagellum

Protein transport across the cytoplasmic membrane is coupled to energy derived from ATP hydrolysis or the proton motive force. A sophisticated, multi-component type III secretion system (T3SS) exports substrate proteins of both the bacterial flagellum and virulence-associated injectisome system of many Gram-negative pathogens. The T3SS is primarily a proton motive force-driven protein exporter. Here, we describe a method to investigate the export of substrate proteins of the flagellar T3SS into the culture supernatant under conditions that manipulate the proton motive force. Further, we describe methods to precisely quantify flagellar protein export into the culture supernatant using a split NanoLuc luciferase, and how fluorescence labeling of the extracellular flagellar filament can bring insights into the protein export rate of individual flagellar T3SS.

Structure, Assembly, and Function of Flagella Responsible for Bacterial Locomotion

Many motile bacteria use flagella for locomotion under a variety of environmental conditions. Because bacterial flagella are under the control of sensory signal transduction pathways, each cell is able to autonomously control its flagellum-driven locomotion and move to an environment favorable for survival. The flagellum of Salmonella enterica serovar Typhimurium is a supramolecular assembly consisting of at least three distinct functional parts: a basal body that acts as a bidirectional rotary motor together with multiple force generators, each of which serves as a transmembrane proton channel to couple the proton flow through the channel with torque generation; a filament that functions as a helical propeller that produces propulsion; and a hook that works as a universal joint that transmits the torque produced by the rotary motor to the helical propeller. At the base of the flagellum is a type III secretion system that transports flagellar structural subunits from the cytoplasm to the distal end of the growing flagellar structure, where assembly takes place. In recent years, high-resolution cryo-electron microscopy (cryoEM) image analysis has revealed the overall structure of the flagellum, and this structural information has made it possible to discuss flagellar assembly and function at the atomic level. In this article, we describe what is known about the structure, assembly, and function of Salmonella flagella.

Unrecognized risk of perfluorooctane sulfonate in promoting conjugative transfers of bacterial antibiotic resistance genes

Antibiotic resistance is a major global health crisis facing humanity, with horizontal gene transfer (HGT) as a principal dissemination mechanism in the natural and clinical environments. Perfluoroalkyl substances (PFASs) are emerging contaminants of global concern due to their high persistence in the environment and adverse effects on humans. However, it is unknown whether PFASs affect the HGT of bacterial antibiotic resistance. Using a genetically engineered Escherichia coli MG1655 as the donor of plasmid-encoded antibiotic resistance genes (ARGs), E. coli J53 and soil bacterial community as two different recipients, this study demonstrated that the conjugation frequency of ARGs between two E. coli strains was (1.45 ± 0.17) × 10-5 and perfluorooctane sulfonate (PFOS) at environmentally relevant concentrations (2-50 μg L-1) increased conjugation transfer between E. coli strains by up to 3.25-fold. Increases in reactive oxygen species production, cell membrane permeability, biofilm formation capacity, and cell contact in two E. coli strains were proposed as major promotion mechanisms from PFOS exposure. Weighted gene co-expression network analysis of transcriptome data identified a series of candidate genes whose expression changes could contribute to the increase in conjugation transfer induced by PFOS. Furthermore, PFOS also generally increased the ARG transfer into the studied soil bacterial community, although the uptake ability of different community members of the plasmid either increased or decreased upon PFOS exposure depending on specific bacterial taxa. Overall, this study reveals an unrecognized risk of PFOS in accelerating the dissemination of antibiotic resistance. IMPORTANCE Perfluoroalkyl substances (PFASs) are emerging contaminants of global concern due to their high persistence in the environment and adverse health effects. Although the influence of environmental pollutants on the spread of antibiotic resistance, one of the biggest threats to global health, has attracted increasing attention in recent years, it is unknown whether environmental residues of PFASs affect the dissemination of bacterial antibiotic resistance. Considering PFASs, often called "forever" compounds, have significantly higher environmental persistence than most emerging organic contaminants, exploring the effect of PFASs on the spread of antibiotic resistance is more environmentally relevant and has essential ecological and health significance. By systematically examining the influence of perfluorooctane sulfonate on the antibiotic resistance gene conjugative transfer, not only at the single-strain level but also at the community level, this study has uncovered an unrecognized risk of PFASs in promoting conjugative transfers of bacterial antibiotic resistance genes, which could be incorporated into the risk assessment framework of PFASs.

Purification of the Transmembrane Polypeptide Channel Complex of the Salmonella Flagellar Type III Secretion System

Many motile bacteria employ the flagellar type III secretion system (fT3SS) to build the flagellum on the cell surface. The fT3SS consists of a transmembrane export gate complex, which acts as a proton/protein antiporter that couples proton flow with flagellar protein export, and a cytoplasmic ATPase ring complex, which works as an activator of the export gate complex. Three transmembrane proteins, FliP, FliQ, and FliR, form a core structure of the export gate complex, and this core complex serves as a polypeptide channel that allows flagellar structural subunits to be translocated across the cytoplasmic membrane. Here, we describe the methods for overproduction, solubilization, and purification of the Salmonella FliP/FliQ/FliR complex.

Activation mechanism of the bacterial flagellar dual-fuel protein export engine

Bacteria employ the flagellar type III secretion system (fT3SS) to construct flagellum, which acts as a supramolecular motility machine. The fT3SS of Salmonella enterica serovar Typhimurium is composed of a transmembrane export gate complex and a cytoplasmic ATPase ring complex. The transmembrane export gate complex is fueled by proton motive force across the cytoplasmic membrane and is divided into four distinct functional parts: a dual-fuel export engine; a polypeptide channel; a membrane voltage sensor; and a docking platform. ATP hydrolysis by the cytoplasmic ATPase complex converts the export gate complex into a highly efficient proton (H⁺)/protein antiporter that couples inward-directed H⁺ flow with outward-directed protein export. When the ATPase ring complex does not work well in a given environment, the export gate complex will remain inactive. However, when the electric potential difference, which is defined as membrane voltage, rises above a certain threshold value, the export gate complex becomes an active H⁺/protein antiporter to a considerable degree, suggesting that the export gate complex has a voltage-gated activation mechanism. Furthermore, the export gate complex also has a sodium ion (Na⁺) channel to couple Na⁺ influx with flagellar protein export. In this article, we review our current understanding of the activation mechanism of the dual-fuel protein export engine of the fT3SS. This review article is an extended version of a Japanese article, Membrane voltage-dependent activation of the transmembrane export gate complex in the bacterial flagellar type III secretion system, published in SEIBUTSU BUTSURI Vol. 62, p165-169 (2022).

Discovery of a Novel Inner Membrane-Associated Bacterial Structure Related to the Flagellar Type III Secretion System

The bacterial flagellar type III secretion system (fT3SS) is a suite of membrane-embedded and cytoplasmic proteins responsible for building the flagellar motility machinery. Homologous nonflagellar (NF-T3SS) proteins form the injectisome machinery that bacteria use to deliver effector proteins into eukaryotic cells, and other family members were recently reported to be involved in the formation of membrane nanotubes. Here, we describe a novel, evolutionarily widespread, hat-shaped structure embedded in the inner membranes of bacteria, of yet-unidentified function, that is present in species containing fT3SS. Mutant analysis suggests a relationship between this novel structure and the fT3SS, but not the NF-T3SS. While the function of this novel structure remains unknown, we hypothesize that either some of the fT3SS proteins assemble within the hat-like structure, perhaps including the fT3SS core complex, or that fT3SS components regulate other proteins that form part of this novel structure.

IMPORTANCE The type III secretion system (T3SS) is a fascinating suite of proteins involved in building diverse macromolecular systems, including the bacterial flagellar motility machine, the injectisome machinery that bacteria use to inject effector proteins into host cells, and probably membrane nanotubes which connect bacterial cells. Here, we accidentally discovered a novel inner membrane-associated complex related to the flagellar T3SS. Examining our lab database, which is comprised of more than 40,000 cryo-tomograms of dozens of species, we discovered that this novel structure is both ubiquitous and ancient, being present in highly divergent classes of bacteria. Discovering a novel, widespread structure related to what are among the best-studied molecular machines in bacteria will open new venues for research aiming at understanding the function and evolution of T3SS proteins.

Insight Into Distinct Functional Roles of the Flagellar ATPase Complex for Flagellar Assembly in Salmonella

Most motile bacteria utilize the flagellar type III secretion system (fT3SS) to construct the flagellum, which is a supramolecular motility machine consisting of basal body rings and an axial structure. Each axial protein is translocated via the fT3SS across the cytoplasmic membrane, diffuses down the central channel of the growing flagellar structure and assembles at the distal end. The fT3SS consists of a transmembrane export complex and a cytoplasmic ATPase ring complex with a stoichiometry of 12 FliH, 6 FliI and 1 FliJ. This complex is structurally similar to the cytoplasmic part of the F_OF₁ ATP synthase. The export complex requires the FliH₁₂-FliI₆-FliJ₁ ring complex to serve as an active protein transporter. The FliI₆ ring has six catalytic sites and hydrolyzes ATP at an interface between FliI subunits. FliJ binds to the center of the FliI₆ ring and acts as the central stalk to activate the export complex. The FliH dimer binds to the N-terminal domain of each of the six FliI subunits and anchors the FliI₆-FliJ₁ ring to the base of the flagellum. In addition, FliI exists as a hetero-trimer with the FliH dimer in the cytoplasm. The rapid association-dissociation cycle of this hetero-trimer with the docking platform of the export complex promotes sequential transfer of export substrates from the cytoplasm to the export gate for high-speed protein transport. In this article, we review our current understanding of multiple roles played by the flagellar cytoplasmic ATPase complex during efficient flagellar assembly.

Novel transient cytoplasmic rings stabilize assembling bacterial flagellar motors

The process by which bacterial cells build their intricate flagellar motility apparatuses has long fascinated scientists. Our understanding of this process comes mainly from studies of purified flagella from two species, Escherichia coli and Salmonella enterica. Here, we used electron cryo-tomography (cryo-ET) to image the assembly of the flagellar motor in situ in diverse Proteobacteria: Hylemonella gracilis, Helicobacter pylori, Campylobacter jejuni, Pseudomonas aeruginosa, Pseudomonas fluorescens, and Shewanella oneidensis. Our results reveal the in situ structures of flagellar intermediates, beginning with the earliest flagellar type III secretion system core complex (fT3SScc) and MS-ring. In high-torque motors of Beta-, Gamma-, and Epsilon-proteobacteria, we discovered novel cytoplasmic rings that interact with the cytoplasmic torque ring formed by FliG. These rings, associated with the MS-ring, assemble very early and persist until the stators are recruited into their periplasmic ring; in their absence the stator ring does not assemble. By imaging mutants in Helicobacter pylori, we found that the fT3SScc proteins FliO and FliQ are required for the assembly of these novel cytoplasmic rings. Our results show that rather than a simple accretion of components, flagellar motor assembly is a dynamic process in which accessory components interact transiently to assist in building the complex nanomachine.

Type III secretion by Yersinia pseudotuberculosis is reliant upon an authentic N-terminal YscX secretor domain

YscX was discovered as an essential part of the Yersinia type III secretion system about 20 years ago. It is required for substrate secretion and is exported itself. Despite this central role, its precise function and mode of action remains unknown. In order to address this knowledge gap, this present study refocused attention on YscX to build on the recent advances in the understanding of YscX function. Our experiments identified a N-terminal secretion domain in YscX promoting its secretion, with the first five codons constituting a minimal signal capable of promoting secretion of the signalless β-lactamase reporter. Replacing the extreme YscX N-terminus with known secretion signals of other Ysc-Yop substrates revealed that the YscX N-terminal segment contains non-redundant information needed for YscX function. Further, both in cis deletion of the YscX N-terminus in the virulence plasmid and ectopic expression of epitope tagged YscX variants again lead to stable YscX production but not type III secretion of Yop effector proteins. Mislocalisation of the needle components, SctI and SctF, accompanied this general defect in Yops secretion. Hence, a coupling exists between YscX secretion permissiveness and the assembly of an operational secretion system.

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.

Interactions and substrate selectivity within the SctRST complex of the type III secretion system of enteropathogenic Escherichia coli

Many bacterial pathogens employ a protein complex, termed the type III secretion system (T3SS), to inject bacterial effectors into host cells. These effectors manipulate various cellular processes to promote bacterial growth and survival. The T3SS complex adopts a nano-syringe shape that is assembled across the bacterial membranes, with an extracellular needle extending toward the host cell membrane. The assembly of the T3SS is initiated by the association of three proteins, known as SctR, SctS, and SctT, which create an entry portal to the translocation channel within the bacterial inner membrane. Using the T3SS of enteropathogenic Escherichia coli, we investigated, by mutational and functional analyses, the role of two structural construction sites formed within the SctRST complex and revealed that they are mutation-resistant components that are likely to act as seals preventing leakage of ions and metabolites rather than as substrate gates. In addition, we identified two residues in the SctS protein, Pro23, and Lys54, that are critical for the proper activity of the T3SS. We propose that Pro23 is critical for the physical orientation of the SctS transmembrane domains that create the tip of the SctRST complex and for their positioning with regard to other T3SS substructures. Surprisingly, we found that SctS Lys54, which was previously suggested to mediate the SctS self-oligomerization, is critical for T3SS activity due to its essential role in SctS-SctT hetero-interactions.

In situ imaging of bacterial outer membrane projections and associated protein complexes using electron cryo-tomography

The ability to produce outer membrane projections in the form of tubular membrane extensions (MEs) and membrane vesicles (MVs) is a widespread phenomenon among diderm bacteria. Despite this, our knowledge of the ultrastructure of these extensions and their associated protein complexes remains limited. Here, we surveyed the ultrastructure and formation of MEs and MVs, and their associated protein complexes, in tens of thousands of electron cryo-tomograms of ~90 bacterial species that we have collected for various projects over the past 15 years (Jensen lab database), in addition to data generated in the Briegel lab. We identified outer MEs and MVs in 13 diderm bacterial species and classified several major ultrastructures: (1) tubes with a uniform diameter (with or without an internal scaffold), (2) tubes with irregular diameter, (3) tubes with a vesicular dilation at their tip, (4) pearling tubes, (5) connected chains of vesicles (with or without neck-like connectors), (6) budding vesicles and nanopods. We also identified several protein complexes associated with these MEs and MVs which were distributed either randomly or exclusively at the tip. These complexes include a secretin-like structure and a novel crown-shaped structure observed primarily in vesicles from lysed cells. In total, this work helps to characterize the diversity of bacterial membrane projections and lays the groundwork for future research in this field.

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.

Control of membrane barrier during bacterial type-III protein secretion

Type-III secretion systems (T3SSs) of the bacterial flagellum and the evolutionarily related injectisome are capable of translocating proteins with a remarkable speed of several thousand amino acids per second. Here, we investigate how T3SSs are able to transport proteins at such a high rate while preventing the leakage of small molecules. Our mutational and evolutionary analyses demonstrate that an ensemble of conserved methionine residues at the cytoplasmic side of the T3SS channel create a deformable gasket (M-gasket) around fast-moving substrates undergoing export. The unique physicochemical features of the M-gasket are crucial to preserve the membrane barrier, to accommodate local conformational changes during active secretion, and to maintain stability of the secretion pore in cooperation with a plug domain (R-plug) and a network of salt-bridges. The conservation of the M-gasket, R-plug, and salt-bridge network suggests a universal mechanism by which the membrane integrity is maintained during high-speed protein translocation in all T3SSs.

Type-III secretion systems (T3SSs) are capable of translocating proteins with high speed while maintaining the membrane barrier for small molecules. Here, a structure-function analysis of the T3SS pore complex elucidates the precise mechanisms enabling the gating and the conformational changes required for protein substrate secretion.

A positive charge region of Salmonella FliI is required for ATPase formation and efficient flagellar protein export

The FliH₂FliI complex is thought to pilot flagellar subunit proteins from the cytoplasm to the transmembrane export gate complex for flagellar assembly in Salmonella enterica. FliI also forms a homo-hexamer to hydrolyze ATP, thereby activating the export gate complex to become an active protein transporter. However, it remains unknown how this activation occurs. Here we report the role of a positively charged cluster formed by Arg-26, Arg-27, Arg-33, Arg-76 and Arg-93 of FliI in flagellar protein export. We show that Arg-33 and Arg-76 are involved in FliI ring formation and that the fliI(R26A/R27A/R33A/R76A/R93A) mutant requires the presence of FliH to fully exert its export function. We observed that gain-of-function mutations in FlhB increased the probability of substrate entry into the export gate complex, thereby restoring the export function of the ∆fliH fliI(R26A/R27A/R33A/R76A/R93A) mutant. We suggest that the positive charge cluster of FliI is responsible not only for well-regulated hexamer assembly but also for substrate entry into the gate complex.

Kinoshita, Namba and Minamino show that a cluster of positively-charged arginines in the Salmonella FliI is necessary for formation of the FliI homo-hexamer ATPase. Through loss- and gain-of-function experiments, they demonstrate that hexamer assembly is also responsible for efficient export of flagellar proteins during flagellar assembly.

Architecture and Assembly of the Bacterial Flagellar Motor Complex

One of the central systems responsible for bacterial motility is the flagellum. The bacterial flagellum is a macromolecular protein complex that is more than five times the cell length. Flagella-driven motility is coordinated via a chemosensory signal transduction pathway, and so bacterial cells sense changes in the environment and migrate towards more desirable locations. The flagellum of Salmonella enterica serovar Typhimurium is composed of a bi-directional rotary motor, a universal joint and a helical propeller. The flagellar motor, which structurally resembles an artificial motor, is embedded within the cell envelop and spins at several hundred revolutions per second. In contrast to an artificial motor, the energy utilized for high-speed flagellar motor rotation is the inward-directed proton flow through a transmembrane proton channel of the stator unit of the flagellar motor. The flagellar motor realizes efficient chemotaxis while performing high-speed movement by an ingenious directional switching mechanism of the motor rotation. To build the universal joint and helical propeller structures outside the cell body, the flagellar motor contains its own protein transporter called a type III protein export apparatus. In this chapter we summarize the structure and assembly of the Salmonella flagellar motor complex.

Protein Export via the Type III Secretion System of the Bacterial Flagellum

The bacterial flagellum and the related virulence-associated injectisome system of pathogenic bacteria utilize a type III secretion system (T3SS) to export substrate proteins across the inner membrane in a proton motive force-dependent manner. The T3SS is composed of an export gate (FliPQR/FlhA/FlhB) located in the flagellar basal body and an associated soluble ATPase complex in the cytoplasm (FliHIJ). Here, we summarise recent insights into the structure, assembly and protein secretion mechanisms of the T3SS with a focus on energy transduction and protein transport across the cytoplasmic membrane.

Adaptivity and dynamics in type III secretion systems

The type III secretion system is the common core of two bacterial molecular machines: the flagellum and the injectisome. The flagellum is the most widely distributed prokaryotic locomotion device, whereas the injectisome is a syringe-like apparatus for inter-kingdom protein translocation, which is essential for virulence in important human pathogens. The successful concept of the type III secretion system has been modified for different bacterial needs. It can be adapted to changing conditions, and was found to be a dynamic complex constantly exchanging components. In this review, we highlight the flexibility, adaptivity, and dynamic nature of the type III secretion system.

Construction and Loss of Bacterial Flagellar Filaments

The bacterial flagellar filament is an extracellular tubular protein structure that acts as a propeller for bacterial swimming motility. It is connected to the membrane-anchored rotary bacterial flagellar motor through a short hook. The bacterial flagellar filament consists of approximately 20,000 flagellins and can be several micrometers long. In this article, we reviewed the experimental works and models of flagellar filament construction and the recent findings of flagellar filament ejection during the cell cycle. The length-dependent decay of flagellar filament growth data supports the injection-diffusion model. The decay of flagellar growth rate is due to reduced transportation of long-distance diffusion and jamming. However, the filament is not a permeant structure. Several bacterial species actively abandon their flagella under starvation. Flagellum is disassembled when the rod is broken, resulting in an ejection of the filament with a partial rod and hook. The inner membrane component is then diffused on the membrane before further breakdown. These new findings open a new field of bacterial macro-molecule assembly, disassembly, and signal transduction.

The Homologous Components of Flagellar Type III Protein Apparatus Have Acquired a Novel Function to Control Twitching Motility in a Non-Flagellated Biocontrol Bacterium

The bacterial flagellum is one of the best-studied surface-attached appendages in bacteria. Flagellar assembly in vivo is promoted by its own protein export apparatus, a type III secretion system (T3SS) in pathogenic bacteria. Lysobacter enzymogenes OH11 is a non-flagellated soil bacterium that utilizes type IV pilus (T4P)-driven twitching motility to prey upon nearby fungi for food. Interestingly, the strain OH11 encodes components homologous to the flagellar type III protein apparatus (FT3SS) on its genome, but it remains unknown whether this FT3SS-like system is functional. Here, we report that, despite the absence of flagella, the FT3SS homologous genes are responsible not only for the export of the heterologous flagellin in strain OH11 but also for twitching motility. Blocking the FT3SS-like system by in-frame deletion mutations in either flhB or fliI abolished the secretion of heterologous flagellin molecules into the culture medium, indicating that the FT3SS is functional in strain OH11. A deletion of flhA, flhB, fliI, or fliR inhibited T4P-driven twitching motility, whereas neither that of fliP nor fliQ did, suggesting that FlhA, FlhB, FliI, and FliR may obtain a novel function to modulate the twitching motility. The flagellar FliI ATPase was required for the secretion of the major pilus subunit, PilA, suggesting that FliI would have evolved to act as a PilB-like pilus ATPase. These observations lead to a plausible hypothesis that the non-flagellated L. enzymogenes OH11 could preserve FT3SS-like genes for acquiring a distinct function to regulate twitching motility associated with its predatory behavior.

Bacterial flagellar motor PL-ring disassembly subcomplexes are widespread and ancient

In order to understand the evolution of complex biological machines like the bacterial flagellar motor, it is crucial to know what each component does and when it arose. Here, we show that a subcomplex of the motor thought to act as a bushing for the spinning motor likely also serves another function—it plugs the hole in the outer membrane left when the flagellum disassembles. Moreover, this component and function is ancient, since it appears in diverse phyla without evidence of recent gene transfer.

The bacterial flagellum is an amazing nanomachine. Understanding how such complex structures arose is crucial to our understanding of cellular evolution. We and others recently reported that in several Gammaproteobacterial species, a relic subcomplex comprising the decorated P and L rings persists in the outer membrane after flagellum disassembly. Imaging nine additional species with cryo-electron tomography, here, we show that this subcomplex persists after flagellum disassembly in other phyla as well. Bioinformatic analyses fail to show evidence of any recent horizontal transfers of the P- and L-ring genes, suggesting that this subcomplex and its persistence is an ancient and conserved feature of the flagellar motor. We hypothesize that one function of the P and L rings is to seal the outer membrane after motor disassembly.

The KdpD/KdpE two-component system contributes to the motility and virulence of avian pathogenic Escherichia coli

Two-component systems (TCSs) are widespread regulatory systems which can help bacteria to control their cellular functions and respond to a diverse range of stimuli. The KdpD/KdpE system had been well studied for regulating potassium transport and identified as an adaptive regulator involved in the virulence of some pathogenic bacteria, but its role in avian pathogenic Escherichia coli (APEC) was still unknown. In this study, the mutant strain AE17ΔKdpDE was obtained successfully of a clinical APEC isolation AE17 using the lambda Red recombinase system and performed the transcriptional sequencing of the wild type strain AE17 and the mutant strain AE17ΔKdpDE. The transcriptional sequencing results revealed that the KdpD/KdpE two-component system mainly influenced the expression of the genes covering metabolic pathways, flagellar assembly, global transcription regulator. The expression of some flagellar-related genes detecting by quantitative real-time PCR was consistent with the results of transcriptional sequencing. Importantly, fewer flagellum of the mutant strain AE17ΔKdpDE was observed than AE17 using the transmission electron microscope and a decreased motility circle of AE17ΔKdpDE appeared in the semisolid medium. In addition, the serum bactericidal assay was carried out with the specific-pathogen-free chicken in different dilution and the survival ability in the serum of AE17ΔKdpDE was also obviously lower than that of AE17. These results suggested that in APEC, the KdpD/KdpE two-component system mainly influenced the expression of flagella-related genes, the flagellum formation, the motility and antiserum bactericidal activity.

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.

A Polar Flagellar Transcriptional Program Mediated by Diverse Two-Component Signal Transduction Systems and Basal Flagellar Proteins Is Broadly Conserved in Polar Flagellates

Relative to peritrichous bacteria, polar flagellates possess regulatory systems that order flagellar gene transcription differently and produce flagella in specific numbers only at poles. How transcriptional and flagellar biogenesis regulatory systems are interlinked to promote the correct synthesis of polar flagella in diverse species has largely been unexplored. We found evidence for many Gram-negative polar flagellates encoding two-component signal transduction systems with activity linked to the formation of flagellar type III secretion systems to enable production of flagellar rod and hook proteins at a discrete, subsequent stage during flagellar assembly. This polar flagellar transcriptional program assists, in some manner, the FlhF/FlhG flagellar biogenesis regulatory system, which forms specific flagellation patterns in polar flagellates in maintaining flagellation and motility when activity of FlhF or FlhG might be altered. Our work provides insight into the multiple regulatory processes required for polar flagellation.

Bacterial flagella are rotating nanomachines required for motility. Flagellar gene expression and protein secretion are coordinated for efficient flagellar biogenesis. Polar flagellates, unlike peritrichous bacteria, commonly order flagellar rod and hook gene transcription as a separate step after production of the MS ring, C ring, and flagellar type III secretion system (fT3SS) core proteins that form a competent fT3SS. Conserved regulatory mechanisms in diverse polar flagellates to create this polar flagellar transcriptional program have not been thoroughly assimilated. Using in silico and genetic analyses and our previous findings in Campylobacter jejuni as a foundation, we observed a large subset of Gram-negative bacteria with the FlhF/FlhG regulatory system for polar flagellation to possess flagellum-associated two-component signal transduction systems (TCSs). We present data supporting a general theme in polar flagellates whereby MS ring, rotor, and fT3SS proteins contribute to a regulatory checkpoint during polar flagellar biogenesis. We demonstrate that Vibrio cholerae and Pseudomonas aeruginosa require the formation of this regulatory checkpoint for the TCSs to directly activate subsequent rod and hook gene transcription, which are hallmarks of the polar flagellar transcriptional program. By reprogramming transcription in V. cholerae to more closely follow the peritrichous flagellar transcriptional program, we discovered a link between the polar flagellar transcription program and the activity of FlhF/FlhG flagellar biogenesis regulators in which the transcriptional program allows polar flagellates to continue to produce flagella for motility when FlhF or FlhG activity may be altered. Our findings integrate flagellar transcriptional and biogenesis regulatory processes involved in polar flagellation in many species.

The Role of the Small Export Apparatus Protein, SctS, in the Activity of the Type III Secretion System

Many gram-negative pathogens utilize a protein complex, termed the type III secretion system (T3SS), to inject virulence factors from their cytoplasm directly into the host cell. An export apparatus that is formed by five putative integral membrane proteins (SctR/S/T/U/V), resides at the center of the T3SS complex. In this study, we characterized the smallest export apparatus protein, SctS, which contains two putative transmembrane domains (PTMD) that dynamically extract from the inner membrane and adopt a helix-turn-helix structure upon assembly of the T3SS. Replacement of each SctS PTMD with an alternative hydrophobic sequence resulted in abolishment of the T3SS activity, yet SctS self- and hetero-interactions as well as the overall assembly of the T3SS complex were unaffected. Our findings suggest that SctS PTMDs are not crucial for the interactions or the assembly of the T3SS base complex but rather that they are involved in adjusting the orientation of the export apparatus relative to additional T3SS sub-structures, such as the cytoplasmic- and the inner-membrane rings. This ensures the fittings between the dynamic and static components of the T3SS and supports the functionality of the T3SS complex.

In situ imaging of the bacterial flagellar motor disassembly and assembly processes

The self-assembly of cellular macromolecular machines such as the bacterial flagellar motor requires the spatio-temporal synchronization of gene expression with proper protein localization and association of dozens of protein components. In Salmonella and Escherichia coli, a sequential, outward assembly mechanism has been proposed for the flagellar motor starting from the inner membrane, with the addition of each new component stabilizing the previous one. However, very little is known about flagellar disassembly. Here, using electron cryo-tomography and sub-tomogram averaging of intact Legionella pneumophila, Pseudomonas aeruginosa, and Shewanella oneidensis cells, we study flagellar motor disassembly and assembly in situ. We first show that motor disassembly results in stable outer membrane-embedded sub-complexes. These sub-complexes consist of the periplasmic embellished P- and L-rings, and bend the membrane inward while it remains apparently sealed. Additionally, we also observe various intermediates of the assembly process including an inner-membrane sub-complex consisting of the C-ring, MS-ring, and export apparatus. Finally, we show that the L-ring is responsible for reshaping the outer membrane, a crucial step in the flagellar assembly process.

Flagella-Driven Motility of Bacteria

The bacterial flagellum is a helical filamentous organelle responsible for motility. In bacterial species possessing flagella at the cell exterior, the long helical flagellar filament acts as a molecular screw to generate thrust. Meanwhile, the flagella of spirochetes reside within the periplasmic space and not only act as a cytoskeleton to determine the helicity of the cell body, but also rotate or undulate the helical cell body for propulsion. Despite structural diversity of the flagella among bacterial species, flagellated bacteria share a common rotary nanomachine, namely the flagellar motor, which is located at the base of the filament. The flagellar motor is composed of a rotor ring complex and multiple transmembrane stator units and converts the ion flux through an ion channel of each stator unit into the mechanical work required for motor rotation. Intracellular chemotactic signaling pathways regulate the direction of flagella-driven motility in response to changes in the environments, allowing bacteria to migrate towards more desirable environments for their survival. Recent experimental and theoretical studies have been deepening our understanding of the molecular mechanisms of the flagellar motor. In this review article, we describe the current understanding of the structure and dynamics of the bacterial flagellum.

Visualizing the inner life of microbes: practices of multi-color single-molecule localization microscopy in microbiology

In this review, we discuss multi-color single-molecule imaging and tracking strategies for studying microbial cell biology. We first summarize and compare the methods in a detailed literature review of published studies conducted in bacteria and fungi. We then introduce a guideline on which factors and parameters should be evaluated when designing a new experiment, from fluorophore and labeling choices to imaging routines and data analysis. Finally, we give some insight into some of the recent and promising applications and developments of these techniques and discuss the outlook for this field.

From single bacterial cell imaging towards in vivo single-molecule biochemistry studies

Bacteria as single-cell organisms are important model systems to study cellular mechanisms and functions. In recent years and with the help of advanced fluorescence microscopy techniques, immense progress has been made in characterizing and quantifying the behavior of single bacterial cells on the basis of molecular interactions and assemblies in the complex environment of live cultures. Importantly, single-molecule imaging enables the in vivo determination of the stoichiometry and molecular architecture of subcellular structures, yielding detailed, quantitative, spatiotemporally resolved molecular maps and unraveling dynamic heterogeneities and subpopulations on the subcellular level. Nevertheless, open challenges remain. Here, we review the past and current status of the field, discuss example applications and give insights into future trends.

The Structure of an Injectisome Export Gate Demonstrates Conservation of Architecture in the Core Export Gate between Flagellar and Virulence Type III Secretion Systems

Export of proteins through type III secretion systems (T3SS) is critical for motility and virulence of many major bacterial pathogens. Proteins are exported through a genetically defined export gate complex consisting of three proteins. We have recently shown at 4.2 Å that the flagellar complex of these three putative membrane proteins (FliPQR in flagellar systems, SctRST in virulence systems) assembles into an extramembrane helical assembly that likely seeds correct assembly of the rod. Here we present the structure of an equivalent complex from the Shigella virulence system at 3.5 Å by cryo-electron microscopy. This higher-resolution structure yields a more precise description of the structure and confirms the prediction of structural conservation in this core complex. Analysis of particle heterogeneity also suggests how the SctS/FliQ subunits sequentially assemble in the complex.IMPORTANCE Although predicted on the basis of sequence conservation, the work presented here formally demonstrates that all classes of type III secretion systems, flagellar or virulence, share the same architecture at the level of the core structures. This absolute conservation of the unusual extramembrane structure of the core export gate complex now allows work to move to focusing on both mechanistic studies of type III but also on fundamental studies of how such a complex is assembled.

Self-Labeling Enzyme Tags for Analyses of Translocation of Type III Secretion System Effector Proteins

Type III secretion systems (T3SS) are molecular machines in Gram-negative pathogens that translocate effector proteins with central roles in virulence. The analyses of the translocation, subcellular localization, and mode of action of T3SS effector proteins are of central importance for the understanding of host-pathogen interaction and pathogenesis of bacterial infections. The analysis of translocation requires dedicated techniques to address the temporal and spatial dynamics of translocation. Here we describe a novel approach to deploy self-labeling enzymes (SLE) as universal tags for localization and tracking of translocated effector proteins. Effector-SLE fusion proteins allow live-cell imaging of translocation by T3SS, superresolution microscopy, and single-molecule tracking of effector motility in living host cells. We describe the application of the approach to T3SS effector proteins for invasion and intracellular lifestyle of Salmonella enterica serovar Typhimurium and to a T3SS effector of Yersinia enterocolitica The novel approach enables analyses of the role of T3SS in host-pathogen interaction at the highest temporal and spatial resolution, toward understanding the molecular mechanisms of their effector proteins.IMPORTANCE Type III secretion systems mediate translocation of effector proteins into mammalian cells. These proteins interfere with host cell functions, being main virulence factors of Gram-negative pathogens. Analyses of the process of translocation, the subcellular distribution, and the dynamics of effector proteins in host cells have been hampered by the lack of suitable tags and detection systems. Here we describe the use of self-labeling enzyme tags for generation of fusions with effector proteins that are translocated and functional in host cell manipulation. Self-labeling reactions with cell-permeable ligand dyes are possible prior to or after translocation. We applied the new approach to superresolution microscopy for effector protein translocation. For the first time, we show the dynamic properties of effector proteins in living host cells after translocation by intracellular bacteria. The new approach of self-labeling enzyme tags fusions will enable analyses of type III secretion system effector proteins with new dimensions of temporal and spatial resolution.

A Ubiquitous Platform for Bacterial Nanotube Biogenesis

We have previously described the existence of membranous nanotubes, bridging adjacent bacteria, facilitating intercellular trafficking of nutrients, cytoplasmic proteins, and even plasmids, yet components enabling their biogenesis remain elusive. Here we reveal the identity of a molecular apparatus providing a platform for nanotube biogenesis. Using Bacillus subtilis (Bs), we demonstrate that conserved components of the flagellar export apparatus (FliO, FliP, FliQ, FliR, FlhB, and FlhA), designated CORE, dually serve for flagellum and nanotube assembly. Mutants lacking CORE genes, but not other flagellar components, are deficient in both nanotube production and the associated intercellular molecular trafficking. In accord, CORE components are located at sites of nanotube emergence. Deleting COREs of distinct species established that CORE-mediated nanotube formation is widespread. Furthermore, exogenous COREs from diverse species could restore nanotube generation and functionality in Bs lacking endogenous CORE. Our results demonstrate that the CORE-derived nanotube is a ubiquitous organelle that facilitates intercellular molecular trade across the bacterial kingdom.

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Conserved flagellar CORE components dually serve for flagella and nanotube assembly

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CORE mutants are deficient in nanotube formation and intercellular molecular trade

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CORE-dependent nanotube production is conserved among distinct bacterial species

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The CORE-nanotube organelle can provide a common path for bacterial molecular trade

Cellular Response of Escherichia coli to Photocatalysis: Flagellar Assembly Variation and Beyond

Bacterial cells can be inactivated by external reactive oxygen species (ROS) produced by semiconductor photocatalysis. However, little is known about cellular responses to photocatalysis. For a better understanding of this issue, one strain of Escherichia coli ( E. coli, hereafter named as MT), which has an increased ability to metabolize carbon sources, was screened out from the wild-type (WT) E. coli K12 by repeated exposure to photocatalysis with palladium oxide modified nitrogen-doped titanium dioxide. In this study, transcriptome sequencing of the WT and MT strains that were exposed or unexposed to photocatalysis were carried out. Cellular responses to photocatalysis were inferred from the functions of genes whose transcripts were either increased or decreased. Upregulation of expression of bacterial flagellar assembly genes used for chemotaxis was detected in cells exposed to semilethal photocatalytic conditions of the WT E. coli. Increased capability to degrade superoxide radicals and decreased bacterial flagellar assembly and chemotaxis were observed in MT E. coli compared to WT cells. We conclude that the differences in motility and intracellular ROS between MT and WT are directly related to survivability of E. coli during exposure to photodisinfection.

Molecular Organization and Assembly of the Export Apparatus of Flagellar Type III Secretion Systems

The bacterial flagellum is a supramolecular motility machine consisting of the basal body, the hook, and the filament. For construction of the flagellum beyond the cellular membranes, a type III protein export apparatus uses ATP and proton-motive force (PMF) across the cytoplasmic membrane as the energy sources to transport flagellar component proteins from the cytoplasm to the distal end of the growing flagellar structure. The protein export apparatus consists of a PMF-driven transmembrane export gate complex and a cytoplasmic ATPase complex. In addition, the basal body C ring acts as a sorting platform for the cytoplasmic ATPase complex that efficiently brings export substrates and type III export chaperone-substrate complexes from the cytoplasm to the export gate complex. In this book chapter, we will summarize our current understanding of molecular organization and assembly of the flagellar type III protein export apparatus.

Export Mechanisms and Energy Transduction in Type-III Secretion Machines

The remarkably complex architecture and organization of bacterial nanomachines originally raised the enigma to how they are assembled in a coordinated manner. Over the years, the assembly processes of the flagellum and evolutionary-related injectisome complexes have been deciphered and were shown to rely on a conserved protein secretion machine: the type-III secretion system. In this book chapter, we demonstrate how individually evolved mechanisms cooperate in highly versatile and robust secretion machinery to export and assemble the building blocks of those nanomachines.

The Third Transmembrane Domain of EscR Is Critical for Function of the Enteropathogenic Escherichia coli Type III Secretion System

Many Gram-negative bacterial pathogens that cause life-threatening diseases employ a type III secretion system (T3SS) for their virulence. The T3SS comprises several proteins that assemble into a syringe-like structure dedicated to the injection of bacterial virulence factors into the host cells. Although many T3SS proteins are transmembrane proteins, our knowledge of these proteins is limited mostly to their soluble domains. In this study, we found that the third transmembrane domain (TMD) of EscR, a central protein of the T3SS in enteropathogenic E. coli, contributes to protein self-oligomerization. Moreover, we demonstrated that a single aspartic acid residue, located at the core of this TMD, is critical for the activity of the full-length protein and the function of the entire T3SS, possibly due to its involvement in mediating TMD-TMD interactions. Our findings should encourage the mapping of the entire interactome of the T3SS components, including interactions mediated through their TMDs.

Many Gram-negative bacterial pathogens utilize a specialized protein delivery system, called the type III secretion system (T3SS), to translocate effector proteins into the host cells. The translocated effectors are crucial for bacterial infection and survival. The base of the T3SS transverses both bacterial membranes and contains an export apparatus that comprises five membrane proteins. Here, we study the export apparatus of enteropathogenic Escherichia coli (EPEC) and characterize its central component, called the EscR protein. We found that the third transmembrane domain (TMD) of EscR mediates strong self-oligomerization in an isolated genetic reporter system. Replacing this TMD sequence with an alternative hydrophobic sequence within the full-length protein resulted in a complete loss of function of the T3SS, further suggesting that the EscR TMD3 sequence has another functional role in addition to its role as a membrane anchor. Moreover, we found that an aspartic acid residue, located at the core of EscR TMD3, is important for the oligomerization propensity of TMD3 and that a point mutation of this residue within the full-length protein abolishes the T3SS activity and the ability of the bacteria to translocate effectors into host cells.

IMPORTANCE Many Gram-negative bacterial pathogens that cause life-threatening diseases employ a type III secretion system (T3SS) for their virulence. The T3SS comprises several proteins that assemble into a syringe-like structure dedicated to the injection of bacterial virulence factors into the host cells. Although many T3SS proteins are transmembrane proteins, our knowledge of these proteins is limited mostly to their soluble domains. In this study, we found that the third transmembrane domain (TMD) of EscR, a central protein of the T3SS in enteropathogenic E. coli, contributes to protein self-oligomerization. Moreover, we demonstrated that a single aspartic acid residue, located at the core of this TMD, is critical for the activity of the full-length protein and the function of the entire T3SS, possibly due to its involvement in mediating TMD-TMD interactions. Our findings should encourage the mapping of the entire interactome of the T3SS components, including interactions mediated through their TMDs.

Structure of the core of the type III secretion system export apparatus

Export of proteins through type III secretion systems is critical for motility and virulence of many major bacterial pathogens. Three putative integral membrane proteins (FliP, FliQ, FliR) are suggested to form the core of an export gate in the inner membrane, but their structure, assembly and location within the final nanomachine remain unclear. Here, we present the cryoelectron microscopy structure of the Salmonella Typhimurium FliP-FliQ-FliR complex at 4.2 Å. None of the subunits adopt canonical integral membrane protein topologies, and common helix-turn-helix structural elements allow them to form a helical assembly with 5:4:1 stoichiometry. Fitting of the structure into reconstructions of intact secretion systems, combined with cross-linking, localize the export gate as a core component of the periplasmic portion of the machinery. This study thereby identifies the export gate as a key element of the secretion channel and implies that it primes the helical architecture of the components assembling downstream.

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.