Mechanisms of type III protein export for bacterial flagellar assembly

FlhE functions as a chaperone to prevent formation of periplasmic flagella in Gram-negative bacteria

The bacterial flagellum, which facilitates motility, is composed of ~20 structural proteins organized into a long extracellular filament connected to a cytoplasmic rotor-stator complex via a periplasmic rod. Flagellum assembly is regulated by multiple checkpoints that ensure an ordered gene expression pattern coupled to the assembly of the various building blocks. Here, we use epifluorescence, super-resolution, and transmission electron microscopy to show that the absence of a periplasmic protein (FlhE) prevents proper flagellar morphogenesis and results in the formation of periplasmic flagella in Salmonella enterica. The periplasmic flagella disrupt cell wall synthesis, leading to a loss of normal cell morphology resulting in cell lysis. We propose that FlhE functions as a periplasmic chaperone to control assembly of the periplasmic rod, thus preventing formation of periplasmic flagella.

FlhE functions as a chaperone to prevent formation of periplasmic flagella in Gram-negative bacteria

The bacterial flagellum is an organelle utilized by many Gram-negative bacteria to facilitate motility. The flagellum is composed of a several µm long, extracellular filament that is connected to a cytoplasmic rotor-stator complex via a periplasmic rod. Composed of ∼20 structural proteins, ranging from a few subunits to several thousand building blocks, the flagellum is a paradigm of a complex macromolecular structure that utilizes a highly regulated assembly process. This process is governed by multiple checkpoints that ensure an ordered gene expression pattern coupled to the assembly of the various flagellar building blocks in order to produce a functional flagellum. Using epifluorescence, super-resolution STED and transmission electron microscopy, we discovered that in Salmonella , the absence of one periplasmic protein, FlhE, prevents proper flagellar morphogenesis and results in the formation of periplasmic flagella. The periplasmic flagella disrupt cell wall synthesis, leading to a loss of the standard cell morphology resulting in cell lysis. We propose a model where FlhE functions as a periplasmic chaperone to control assembly of the periplasmic rod to prevent formation of periplasmic flagella. Our results highlight that bacteria evolved sophisticated regulatory mechanisms to control proper flagellar assembly and minor deviations from this highly regulated process can cause dramatic physiological consequences.

Twists and turns: 40 years of investigating how and why bacteria swim

Fifty years of research has transformed our understanding of bacterial movement from one of description, based on a limited number of electron micrographs and some low-magnification studies of cells moving towards or away from chemical effectors, to probably the best understood behavioural system in biology. We have a molecular understanding of how bacteria sense and respond to changes in their environment and detailed structural insights into the workings of one of the most complex motor structures we know of. Thanks to advances in genomics we also understand how, through evolution, different species have tuned and adapted a core shared system to optimize behaviour in their specific environment. In this review, I will highlight some of the unexpected findings we made during my over 40-year career, how those findings changed some of our understanding of bacterial behaviour and biochemistry and some of the battles to have those observations accepted.

Comparative secretome analysis reveals cross-talk between type III secretion system and flagella assembly in Pseudomonas plecoglossicida

The Gram-negative bacterium Pseudomonas plecoglossicida has caused visceral granulomas disease in several farmed fish species, including large yellow croaker (Larimichthys crocea), which results in severe economic losses. Type III secretion systems (T3SS) are protein secretion and translocation nanomachines widely employed by many Gram-negative bacterial pathogens for infection and pathogenicity. However, the exact role of T3SS in the pathogenesis of P. plecoglossicida infection is still unclear. In this study, a T3SS translocators deletion strain (△popBD) of P. plecoglossicida was constructed to investigate the function of T3SS. Then comparative secretome analysis of the P. plecoglossicida wild-type (WT) and △popBD mutant strains was conducted by label-free quantitation (LFQ) mass spectrometry. The results show that knockout of T3SS translocators popB and popD has an adverse effect on the effector protein ExoU secretion, flagella assembly, and biofilm formation. Further experimental validations also confirmed that popB-popD deletion could affect the P. plecoglossicida flagella morphology/formation, adherence, mobility, and biofilm formation. These data indicate that a cross-talk exists between the P. plecoglossicida T3SS and the flagella system. Our results, therefore, will facilitate the further under-standing of the pathogenic mechanisms leading to visceral granulomas disease caused by P. plecoglossicida.

Primary architecture and energy requirements of Type III and Type IV secretion systems

Many pathogens use Type III and Type IV protein secretion systems to secrete virulence factors from the bacterial cytosol into host cells. These systems operate through a one-step mechanism. The secreted substrates (protein or nucleo-protein complexes in the case of Type IV conjugative systems) are guided to the base of the secretion channel, where they are directly delivered into the host cell in an ATP-dependent unfolded state. Despite the numerous disparities between these secretion systems, here we have focused on the structural and functional similarities between both systems. In particular, on the structural similarity shared by one of the main ATPases (EscN and VirD4 in Type III and Type IV secretion systems, respectively). Interestingly, these ATPases also exhibit a structural resemblance to F1-ATPases, which suggests a common mechanism for substrate secretion. The correlation between structure and function of essential components in both systems can provide significant insights into the molecular mechanisms involved. This approach is of great interest in the pursuit of identifying inhibitors that can effectively target these systems.

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.

Surface-modified bacteria: synthesis, functionalization and biomedical applications

The past decade has witnessed a great leap forward in bacteria-based living agents, including imageable probes, diagnostic reagents, and therapeutics, by virtue of their unique characteristics, such as genetic manipulation, rapid proliferation, colonization capability, and disease site targeting specificity. However, successful translation of bacterial bioagents to clinical applications remains challenging, due largely to their inherent susceptibility to environmental insults, unavoidable toxic side effects, and limited accumulation at the sites of interest. Cell surface components, which play critical roles in shaping bacterial behaviors, provide an opportunity to chemically modify bacteria and introduce different exogenous functions that are naturally unachievable. With the help of surface modification, a wide range of functionalized bacteria have been prepared over the past years and exhibit great potential in various biomedical applications. In this article, we mainly review the synthesis, functionalization, and biomedical applications of surface-modified bacteria. We first introduce the approaches of chemical modification based on the bacterial surface structure and then highlight several advanced functions achieved by modifying specific components on the surface. We also summarize the advantages as well as limitations of surface chemically modified bacteria in the applications of bioimaging, diagnosis, and therapy and further discuss the current challenges and possible solutions in the future. This work will inspire innovative design thinking for the development of chemical strategies for preparing next-generation biomedical bacterial agents.

An overview of the structure and function of the flagellar hook FlgE protein

The flagellum is an important organelle for the survival of bacteria and consists of a basal body, hook, and filament. The FlgE protein is the subunit of the hook that connects the basal body and the filament and determines the motility of bacteria. Also, flgE gene plays an essential role in flagellar biosynthesis, swimming ability and biofilm formation. Although the intact flagella and the major component filament have been extensively studied, so far, little is known about the comprehensive understanding of flagellar hook and FlgE. Here in this review, we summarize the structures of flagellar hook and its subunit FlgE in various species and physiological functions of FlgE, including the hook assembly, the structural characteristics of flagellar hook, the mechanical properties of hook, and the similarities and differences between FlgE (hook) and FlgG (distal rod), with special attention on the interaction of FlgE with other molecules, the antigenicity and pro-inflammatory effect of FlgE, and cross-linking of FlgE in spirochetes. We hope our summary of this review could provide a better understanding of the FlgE protein and provide some useful information for developing new effective antibacterial drugs in the future work.

In Vitro Flagellar Type III Protein Transport Assay Using Inverted Membrane Vesicles

The flagellar axial proteins are transported across the cytoplasmic membrane into the central channel of the growing flagellum via the flagellar protein export apparatus, a member of the type III secretion system (T3SS). To reveal the molecular mechanism of protein transport by the T3SS, accurate measurement of protein transport under various conditions is essential. In this chapter, we describe an in vitro method for flagellar protein transport assay using inverted membrane vesicles (IMVs) prepared from Salmonella cells. This method can easily and precisely control the condition around the T3SS and be applied to other T3SSs.

Brucella and Its Hidden Flagellar System

Brucella, a Gram-negative bacterium with a high infective capacity and a wide spectrum of hosts in the animal world, is found in terrestrial and marine mammals, as well as amphibians. This broad spectrum of hosts is closely related to the non-classical virulence factors that allow this pathogen to establish its replicative niche, colonizing epithelial and immune system cells, evading the host's defenses and defensive response. While motility is the primary role of the flagellum in most bacteria, in Brucella, the flagellum is involved in virulence, infectivity, cell growth, and biofilm formation, all of which are very important facts in a bacterium that to date has been described as a non-motile organism. Evidence of the expression of these flagellar proteins that are present in Brucella makes it possible to hypothesize certain evolutionary aspects as to where a free-living bacterium eventually acquired genetic material from environmental microorganisms, including flagellar genes, conferring on it the ability to reach other hosts (mammals), and, under selective pressure from the environment, can express these genes, helping it to evade the immune response. This review summarizes relevant aspects of the presence of flagellar proteins and puts into context their relevance in certain functions associated with the infective process. The study of these flagellar genes gives the genus Brucella a very high infectious versatility, placing it among the main organisms in urgent need of study, as it is linked to human health by direct contact with farm animals and by eventual transmission to the general population, where flagellar genes and proteins are of great relevance.

Structure of the bacterial flagellar hook cap provides insights into a hook assembly mechanism

Assembly of bacterial flagellar hook requires FlgD, a protein known to form the hook cap. Symmetry mismatch between the hook and the hook cap is believed to drive efficient assembly of the hook in a way similar to the filament cap helping filament assembly. However, the hook cap dependent mechanism of hook assembly has remained poorly understood. Here, we report the crystal structure of the hook cap composed of five subunits of FlgD from Salmonella enterica at 3.3 Å resolution. The pentameric structure of the hook cap is divided into two parts: a stalk region composed of five N-terminal domains; and a petal region containing five C-terminal domains. Biochemical and genetic analyses show that the N-terminal domains of the hook cap is essential for the hook-capping function, and the structure now clearly reveals why. A plausible hook assembly mechanism promoted by the hook cap is proposed based on the structure.

Role of DegQ in differential stability of flagellin subunits in Vibrio vulnificus

Biofilm formation of Vibrio vulnificus is initiated by adherence of flagellated cells to surfaces, and then flagellum-driven motility is not necessary during biofilm maturation. Once matured biofilms are constructed, cells become flagellated and swim to disperse from biofilms. As a consequence, timely regulations of the flagellar components’ expression are crucial to complete a biofilm life-cycle. In this study, we demonstrated that flagellins’ production is regulated in a biofilm stage-specific manner, via activities of a protease DegQ and a chaperone FlaJ. Among four flagellin subunits for V. vulnificus filament, FlaC had the highest affinities to hook-associated proteins, and is critical for maturating flagellum, showed the least susceptibility to DegQ due to the presence of methionine residues in its DegQ-sensitive domains, ND1 and CD0. Therefore, differential regulation by DegQ and FlaJ controls the cytoplasmic stability of flagellins, which further determines the motility-dependent, stage-specific development of biofilms.

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.

Insights into the structure of Escherichia coli outer membrane as the target for engineering microbial cell factories

Escherichia coli is generally used as model bacteria to define microbial cell factories for many products and to investigate regulation mechanisms. E. coli exhibits phospholipids, lipopolysaccharides, colanic acid, flagella and type I fimbriae on the outer membrane which is a self-protective barrier and closely related to cellular morphology, growth, phenotypes and stress adaptation. However, these outer membrane associated molecules could also lead to potential contamination and insecurity for fermentation products and consume lots of nutrients and energy sources. Therefore, understanding critical insights of these membrane associated molecules is necessary for building better microbial producers. Here the biosynthesis, function, influences, and current membrane engineering applications of these outer membrane associated molecules were reviewed from the perspective of synthetic biology, and the potential and effective engineering strategies on the outer membrane to improve fermentation features for microbial cell factories were suggested.

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.

Proteomics profiles of Cronobacter sakazakii and a fliF mutant: Adherence and invasion in mouse neuroblastoma cells

Cronobacter sakazakii is an opportunistic foodborne pathogen associated with necrotizing enterocolitis, bacteremia, and meningitis in infants. A comparative proteomic study of C. sakazakii ATCC BAA-894 (CS WT) and a fliF::Tn5 mutant was performed, including the ability of both strains to adhere to and invade N1E-115 cells. To achieve this goal, a nonmotile C. sakazakii‬ ATCC BAA-894 fliF::Tn5 (CS fliF::Tn5) strain was generated using an EZ-Tn5 <KAN-2>Tnp Transposome kit. Analysis of differential protein expression showed that 81.49% (361/443) of the proteins were expressed in both strains, 8.35% (37/443) were exclusively expressed in the CS WT strain, and 10.16% (45/443) were exclusively expressed in the CS fliF::Tn5 strain. The main exclusively expressed proteins in the CS WT strain were classified into the "cell motility" and "signal transduction mechanisms" subcategories. The proteins exclusively expressed in the CS fliF::Tn5 strain were classified into the following subcategories: "intracellular trafficking, secretion, and vesicular transport", "replication, recombination, and repair", "nucleotide transport and metabolism", "carbohydrate transport and metabolism", "coenzyme transport and metabolism", and "lipid transport and metabolism". Expression of the Cpa protein was detected in both strains, but Cpa was more abundant in the CS WT strain than in the CS fliF::Tn5 strain. A significant increase (p = 0.0001) in adherence to N1E-115 cells was observed in the nonmotile CS fliF::Tn5 strain (31.3 × 10⁶ CFU/mL) compared to the CS WT strain (14.5 × 10⁶ CFU/mL). Additionally, the CS WT strain showed a 0.17% invasion frequency in N1E-115 cells, which was significantly higher (p = 0.01) than that of the nonmotile CS fliF::Tn5 strain. In conclusion, the proteins involved in the motility were mainly identified by proteomic analysis in the CS WT strain compared to the CS fliF::Tn5 strain. Our data indicate that flagella are required to promote the invasion of N1E-115 cells and that the absence of flagella significantly increases the adherence to N1E-115 cells.

A structure-function study of C-terminal residues predicted to line the export channel in Salmonella Flagellin

BACKGROUND

Structural studies of a Salmonella Typhimurium flagellin protein indicated that four polar or charged C-terminal amino acid residues line the inner channel of the flagellum. The hydrophilic character of these putative channel-lining residues was predicted to be essential to facilitate the transport of unfolded flagellin monomers during flagellar assembly. The structure-function relationship of these putative channel-lining residues was investigated by site-directed mutagenesis to examine effects of side chain polarity and size on flagella assembly and function.

METHODS

Channel-lining residue variants were generated using site-directed mutagenesis to substitute alanine and other residues to examine the effects of altered side-chain polarity on export and assembly. The export, in vivo motility function, and flagellar structure of variants was characterized by agar motility, video microscopy, immunofluorescence, and SDS-PAGE.

RESULTS

Alanine substitution yielded decreased motility and flagellar assembly for three of the four residues. However, alanine substitution of residue Arg 494 did not alter export, although substitution with negatively charged glutamate decreased motility and flagellar filament length. Furthermore, many of the C-terminal mutations affected flagellar filament morphology and stability, often resulting in more tightly coiled and/or more brittle flagella than the wild type.

CONCLUSIONS

The four channel-lining C-terminal residues may facilitate monomer protein transport but also have structural roles in determining the stability and morphology of the flagellum.

GENERAL SIGNIFICANCE

These results provide further insight into the complex process of bacterial flagellin export and flagellar assembly and provide evidence of previously unknown structural functions for the four putative channel-lining residues.

The cryo-EM structure of the bacterial flagellum cap complex suggests a molecular mechanism for filament elongation

The bacterial flagellum is a remarkable molecular motor, whose primary function in bacteria is to facilitate motility through the rotation of a filament protruding from the bacterial cell. A cap complex, consisting of an oligomer of the protein FliD, is localized at the tip of the flagellum, and is essential for filament assembly, as well as adherence to surfaces in some bacteria. However, the structure of the intact cap complex, and the molecular basis for its interaction with the filament, remains elusive. Here we report the cryo-EM structure of the Campylobacter jejuni cap complex, which reveals that FliD is pentameric, with the N-terminal region of the protomer forming an extensive set of contacts across several subunits, that contribute to FliD oligomerization. We also demonstrate that the native C. jejuni flagellum filament is 11-stranded, contrary to a previously published cryo-EM structure, and propose a molecular model for the filament-cap interaction.

FliD forms a cap complex at the tip of bacterial flagella and is essential for flagellum filament assembly. Here, the authors present the cryo-EM structure of the Campylobacter jejuni cap complex, revealing a pentameric assembly of FliD and further show that the C. jejuni flagellum filament is 11-stranded.

Assembly mechanism of a supramolecular MS-ring complex to initiate bacterial flagellar biogenesis in Vibrio species

The bacterial flagellum is an organelle responsible for motility and has a rotary motor comprising the rotor and the stator. Flagellar biogenesis is initiated by the assembly of the MS-ring, a supramolecular complex embedded in the cytoplasmic membrane. The MS-ring consists of a few dozen copies of the transmembrane FliF protein, and is an essential core structure which is a part of the rotor. The number and location of the flagella are controlled by the FlhF and FlhG proteins in some species. However, there is no clarity on the factors initiating MS-ring assembly, and contribution of FlhF/FlhG to this process. Here, we show that FlhF and a C-ring component FliG facilitate Vibrio MS-ring formation. When Vibrio FliF alone was expressed in Escherichia coli cells, MS-ring formation rarely occurred, indicating the requirement of other factors for MS-ring assembly. Consequently, we investigated if FlhF aided FliF in MS-ring assembly. We found that FlhF allowed GFP-fused FliF to localize at the cell pole in a Vibrio cell, suggesting that it increases local concentration of FliF at the pole. When FliF was co-expressed with FlhF in E. coli cells, the MS-ring was effectively formed, indicating that FlhF somehow contributes to MS-ring formation. The isolated MS-ring structure was similar to the MS-ring formed by Salmonella FliF. Interestingly, FliG facilitates MS-ring formation, suggesting that FliF and FliG assist in each other's assembly into the MS-ring and C-ring. This study aids in understanding the mechanism behind MS-ring assembly using appropriate spatial/temporal regulations.Importance Flagellar formation is initiated by the assembly of the FliF protein into the MS-ring complex, embedded in the cytoplasmic membrane. The appropriate spatial/temporal control of MS-ring formation is important for the morphogenesis of the bacterial flagellum. Here, we focus on the assembly mechanism of Vibrio FliF into the MS-ring. FlhF, a positive regulator of the number and location of flagella, recruits the FliF molecules at the cell pole and facilitates MS-ring formation. FliG also facilitates MS-ring formation. Our study showed that these factors control flagellar biogenesis in Vibrio, by initiating the MS-ring assembly. Furthermore, it also implies that flagellar biogenesis is a sophisticated system linked with the expression of certain genes, protein localization and a supramolecular complex assembly.

Type three secretion system in Salmonella Typhimurium: the key to infection

BACKGROUND

Type Three Secretion Systems (T3SS) are nanomachine complexes, which display the ability to inject effector proteins directly into host cells. This skill allows for gram-negative bacteria to modulate several host cell responses, such as cytoskeleton rearrangement, signal transduction, and cytokine production, which in turn increase the pathogenicity of these bacteria. The Salmonella enterica subsp. enterica serovar Typhimurium (ST) T3SS has been the most characterized so far. Among gram-negative bacterium, ST is one of enterica groups predicted to have two T3SSs activated during different phases of infection.

OBJECTIVE

To comprise current information about ST T3SS structure and function as well as an overview of its assembly and hierarchical regulation.

METHODS

With a brief and straightforward reading, this review summarized aspects of both ST T3SS, such as its structure and function. That was possible due to the development of novel techniques, such as X-ray crystallography, cryoelectron microscopy, and nano-gold labelling, which also elucidated the mechanisms behind T3SS assembly and regulation, which was addressed in this review.

CONCLUSION

This paper provided fundamental overview of ST T3SS assembly and regulation, besides summarized the structure and function of this complex. Due to T3SS relevance in ST pathogenicity, this complex could become a potential target in therapeutic studies as this nanomachine modulates the infection process.

The flexible linker of the secreted FliK ruler is required for export switching of the flagellar protein export apparatus

The hook length of the flagellum is controlled to about 55 nm in Salmonella. The flagellar type III protein export apparatus secretes FliK to determine hook length during hook assembly and changes its substrate specificity from the hook protein to the filament protein when the hook length has reached about 55 nm. Salmonella FliK consists of an N-terminal domain (FliK_N, residues 1–207), a C-terminal domain (FliK_C, residues 268–405) and a flexible linker (FliK_L, residues 208–267) connecting these two domains. FliK_N is a ruler to measure hook length. FliK_C binds to a transmembrane export gate protein FlhB to undergo the export switching. FliK_L not only acts as part of the ruler but also contributes to this switching event, but it remains unknown how. Here we report that FliK_L is required for efficient interaction of FliK_C with FlhB. Deletions in FliK_L not only shortened hook length according to the size of deletions but also caused a loose length control. Deletion of residues 206–265 significantly reduced the binding affinity of FliK_C for FlhB, thereby producing much longer hooks. We propose that an appropriate length of FliK_L is required for efficient interaction of FliK_C with FlhB.

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.

Salmonella Pathogenicity Island 1 (SPI-1) and Its Complex Regulatory Network

Salmonella species can infect a diverse range of birds, reptiles, and mammals, including humans. The type III protein secretion system (T3SS) encoded by Salmonella pathogenicity island 1 (SPI-1) delivers effector proteins required for intestinal invasion and the production of enteritis. The T3SS is regarded as the most important virulence factor of Salmonella. SPI-1 encodes transcription factors that regulate the expression of some virulence factors of Salmonella, while other transcription factors encoded outside SPI-1 participate in the expression of SPI-1-encoded genes. SPI-1 genes are responsible for the invasion of host cells, regulation of the host immune response, e.g., the host inflammatory response, immune cell recruitment and apoptosis, and biofilm formation. The regulatory network of SPI-1 is very complex and crucial. Here, we review the function, effectors, and regulation of SPI-1 genes and their contribution to the pathogenicity of Salmonella.

Mutational analysis of the C-terminal cytoplasmic domain of FlhB, a transmembrane component of the flagellar type III protein export apparatus in Salmonella

The flagellar protein export apparatus switches its substrate specificity when hook length has reached approximately 55 nm in Salmonella. The C-terminal cytoplasmic domain of FlhB (FlhB_C ) is involved in this switching process. FlhB_C consists of FlhB_CN and FlhB_CC polypeptides. FlhB_CC has a flexible C-terminal tail (FlhB_CCT ). FlhB_CC is involved in substrate recognition, and conformational rearrangements of FlhB_CN -FlhB_CC boundary are postulated to be required for the export switching. However, it remains unknown how it occurs. To clarify this question, we carried out mutational analysis of highly conserved residues in FlhB_C . The flhB(E230A) mutation reduced the FlhB function. The flhB(E11S) mutation restored the protein transport activity of the flhB(E230A) mutant to the wild-type level, suggesting that the interaction of FlhB_CN with the extreme N-terminal region of FlhB is required for flagellar protein export. The flhB(R320A) mutation affected hydrophobic interaction networks in FlhB_CC , thereby increasing insolubility of FlhB_C . The R320A mutation also affected the export switching, thereby producing longer hooks with the filament attached. C-terminal truncations of FlhB_CCT induced a conformational change of FlhB_CN -FlhB_CC boundary, resulting in a loose hook length control. We propose that FlhB_CCT may control conformational arrangements of FlhB_CN -FlhB_CC boundary through the hydrophobic interaction networks of FlhB_CC .

Identification of in vivo Essential Genes of Vibrio vulnificus for Establishment of Wound Infection by Signature-Tagged Mutagenesis

Vibrio vulnificus can cause severe necrotic lesions within a short time. Recently, it has been reported that the numbers of wound infection cases in healthy hosts are increasing, for which surgical procedures are essential in many instances to eliminate the pathogen owing to its rapid proliferation. However, the mechanisms by which V. vulnificus can achieve wound infection in healthy hosts have not been elucidated. Here, we advance a systematic understanding of V. vulnificus wound infection through genome-wide identification of the relevant genes. Signature-tagged mutagenesis (STM) has been developed to identify functions required for the establishment of infection including colonization, rapid proliferation, and pathogenicity. Previously, STM had been regarded to be unsuitable for negative selection to detect the virulence genes of V. vulnificus owing to the low colonization and proliferation ability of this pathogen in the intestinal tract and systemic circulation. Alternatively, we successfully identified the virulence genes by applying STM to a murine model of wound infection. We examined a total of 5418 independent transposon insertion mutants by signature-tagged transposon mutagenesis and detected 71 clones as attenuated mutants consequent to disruption of genes by the insertion of a transposon. This is the first report demonstrating that the pathogenicity of V. vulnificus during wound infection is highly dependent on its characteristics: flagellar-based motility, siderophore-mediated iron acquisition system, capsular polysaccharide, lipopolysaccharide, and rapid chromosome partitioning. In particular, these functions during the wound infection process and are indispensable for proliferation in healthy hosts. Our results may thus allow the potential development of new strategies and reagents to control the proliferation of V. vulnificus and prevent human infections.

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.

Driving the expression of the Salmonella enterica sv Typhimurium flagellum using flhDC from Escherichia coli results in key regulatory and cellular differences

The flagellar systems of Escherichia coli and Salmonella enterica exhibit a significant level of genetic and functional synteny. Both systems are controlled by the flagellar specific master regulator FlhD₄C₂. Since the early days of genetic analyses of flagellar systems it has been known that E. coli flhDC can complement a ∆flhDC mutant in S. enterica. The genomic revolution has identified how genetic changes to transcription factors and/or DNA binding sites can impact the phenotypic outcome across related species. We were therefore interested in asking: using modern tools to interrogate flagellar gene expression and assembly, what would the impact be of replacing the flhDC coding sequences in S. enterica for the E. coli genes at the flhDC S. entercia chromosomal locus? We show that even though all strains created are motile, flagellar gene expression is measurably lower when flhDC_EC are present. These changes can be attributed to the impact of FlhD₄C₂ DNA recognition and the protein-protein interactions required to generate a stable FlhD₄C₂ complex. Furthermore, our data suggests that in E. coli the internal flagellar FliT regulatory feedback loop has a marked difference with respect to output of the flagellar systems. We argue due diligence is required in making assumptions based on heterologous expression of regulators and that even systems showing significant synteny may not behave in exactly the same manner.

Phytochemical derivatives targeting fliJ flagellar protein from Escherichia coli

Approximately 50 per cent of nosocomial infections are caused by the use of indwelling medical devices. The surfaces of devices are ideal sites of attachment for bacterial cells and an increase in biofilm formation. Biofilms have been a constant concern due to their complex extracellular matrix (ECM) resulting in multiple drug resistance. E. coli is known to associate with biofilms. Therefore it is of interest to identify the proteins associated to biofilm formation in Escherichia coli through literature survey, investigate their protein-protein interactions and identify indispensible proteins of biofilm formation. These proteins were further analyzed and fliJ was identified as the target, based on betweenness, centrality and radiality. 87 phytochemicals were found to be associated with the microbe in question and were docked with the target using Molegro Virtual Docker (MVD) 5.0. The results showed that geranyl pyrophosphate, ferulic acid 4-o-b-d-glucuronide, 5-8'-dehydrodiferulic acid and geranyl diphosphate showed maximum activity. A combinatorial library of 96 models was generated using the four phytochemicals binding with fliJ.

Novel insight into an energy transduction mechanism of the bacterial flagellar type III protein export

Type III secretion system (T3SS) is a protein translocator complex family including pathogenic injectisome or bacterial flagellum. The inejectisomal T3SS serves to deliver virulence proteins into host cell and the flagellar T3SS constructs the flagellar axial structure. Although earlier studies have provided many findings on the molecular mechanism of the Type III protein export, they were not sufficient to reveal energy transduction mechanism due to difficulties in controlling measurement conditions in vivo. Recently, we developed an in vitro flagellar Type III protein transport assay system using inverted membrane vesicles (IMVs), and analyzed protein export by using the in vitro method. We reproduced protein export of the flagellar T3SS, hook assembly and substrate specificity switch in IMV to a similar extent to what is seen in living cell. Furthermore, we demonstrated that ATP-hydrolysis energy can drive protein transport even in the absence of proton-motive force (PMF). In this mini-review, we will summarize our new in vitro Type III transport assay method and our findings on the molecular mechanism of Type III protein export.

In Vitro Reconstitution of Functional Type III Protein Export and Insights into Flagellar Assembly

The type III secretion system (T3SS) forms the functional core of injectisomes, protein transporters that allow bacteria to deliver virulence factors into their hosts for infection, and flagella, which are critical for many pathogens to reach the site of infection. In spite of intensive genetic and biochemical studies, the T3SS protein export mechanism remains unclear due to the difficulty of accurate measurement of protein export in vivo . Here, we developed an in vitro flagellar T3S protein transport assay system using an inverted cytoplasmic membrane vesicle (IMV) for accurate and controlled measurements of flagellar protein export. We show that the flagellar T3SS in the IMV fully retains export activity. The flagellar hook was constructed inside the lumen of the IMV by adding purified component proteins externally to the IMV solution. We reproduced the hook length control and export specificity switch in the IMV consistent with that seen in the native cell. Previous in vivo analyses showed that flagellar protein export is driven by proton motive force (PMF) and facilitated by ATP hydrolysis by FliI, a T3SS-specific ATPase. Our in vitro assay recapitulated these previous in vivo observations but furthermore clearly demonstrated that even ATP hydrolysis by FliI alone can drive flagellar protein export. Moreover, this assay showed that addition of the FliH 2 /FliI complex to the assay solution at a concentration similar to that in the cell dramatically enhanced protein export, confirming that the FliH 2 /FliI complex in the cytoplasm is important for effective protein transport.

IMPORTANCE The type III secretion system (T3SS) is the functional core of the injectisome, a bacterial protein transporter used to deliver virulence proteins into host cells, and bacterial flagella, critical for many pathogens. The molecular mechanism of protein transport is still unclear due to difficulties in accurate measurements of protein transport under well-controlled conditions in vivo . We succeeded in developing an in vitro transport assay system of the flagellar T3SS using inverted membrane vesicles (IMVs). Flagellar hook formation was reproduced in the IMV, suggesting that the export apparatus in the IMV retains a protein transport activity similar to that in the cell. Using this system, we revealed that ATP hydrolysis by the T3SS ATPase can drive protein export without PMF.

Structural Insight Into Conformational Changes Induced by ATP Binding in a Type III Secretion-Associated ATPase From Shigella flexneri

Gram-negative bacteria utilize the type III secretion system (T3SS) to inject effector proteins into the host cell cytoplasm, where they subvert cellular functions and assist pathogen invasion. The conserved type III-associated ATPase is critical for the separation of chaperones from effector proteins, the unfolding of effector proteins and translocating them through the narrow channel of the secretion apparatus. However, how ATP hydrolysis is coupled to the mechanical work of the enzyme remains elusive. Herein, we present a complete description of nucleoside triphosphate binding by surface presentation antigens 47 (Spa47) from Shigella flexneri, based on crystal structures containing ATPγS, a catalytic magnesium ion and an ordered water molecule. Combining the crystal structures of Spa47-ATPγS and unliganded Spa47, we propose conformational changes in Spa47 associated with ATP binding, the binding of ATP induces a conformational change of a highly conserved luminal loop, facilitating ATP hydrolysis by the Spa47 ATPase. Additionally, we identified a specific hydrogen bond critical for ATP recognition and demonstrated that, while ATPγS is an ideal analog for probing ATP binding, AMPPNP is a poor ATP mimic. Our findings provide structural insight pertinent for inhibitor design.

Frequent pauses in Escherichia coli flagella elongation revealed by single cell real-time fluorescence imaging

The bacterial flagellum is a large extracellular protein organelle that extrudes from the cell surface. The flagellar filament is assembled from tens of thousands of flagellin subunits that are exported through the flagellar type III secretion system. Here, we measure the growth of Escherichia coli flagella in real time and find that, although the growth rate displays large variations at similar lengths, it decays on average as flagella lengthen. By tracking single flagella, we show that the large variations in growth rate occur as a result of frequent pauses. Furthermore, different flagella on the same cell show variable growth rates with correlation. Our observations are consistent with an injection-diffusion model, and we propose that an insufficient cytoplasmic flagellin supply is responsible for the pauses in flagellar growth in E. coli.

The bacterial flagellar filament is assembled from tens of thousands of flagellin subunits that are exported by a dedicated secretion system. Here, the authors show that, on average, the growth rate of flagella in E. coli decays as flagella lengthen, with large variations due to frequent pauses.

Secretion of recombinant proteins from E. coli

The microorganism Escherichia coli is commonly used for recombinant protein production. Despite several advantageous characteristics like fast growth and high protein yields, its inability to easily secrete recombinant proteins into the extracellular medium remains a drawback for industrial production processes. To overcome this limitation, a multitude of approaches to enhance the extracellular yield and the secretion efficiency of recombinant proteins have been developed in recent years. Here, a comprehensive overview of secretion mechanisms for recombinant proteins from E. coli is given and divided into three main sections. First, the structure of the E. coli cell envelope and the known natural secretion systems are described. Second, the use and optimization of different one- or two-step secretion systems for recombinant protein production, as well as further permeabilization methods are discussed. Finally, the often-overlooked role of cell lysis in secretion studies and its analysis are addressed. So far, effective approaches for increasing the extracellular protein concentration to more than 10 g/L and almost 100% secretion efficiency exist, however, the large range of optimization methods and their combinations suggests that the potential for secretory protein production from E. coli has not yet been fully realized.

Flagellin glycosylation with pseudaminic acid in Campylobacter and Helicobacter: prospects for development of novel therapeutics

Many pathogenic bacteria require flagella-mediated motility to colonise and persist in their hosts. Helicobacter pylori and Campylobacter jejuni are flagellated epsilonproteobacteria associated with several human pathologies, including gastritis, acute diarrhea, gastric carcinoma and neurological disorders. In both species, glycosylation of flagellin with an unusual sugar pseudaminic acid (Pse) plays a crucial role in the biosynthesis of functional flagella, and thereby in bacterial motility and pathogenesis. Pse is found only in pathogenic bacteria. Its biosynthesis via six consecutive enzymatic steps has been extensively studied in H. pylori and C. jejuni. This review highlights the importance of flagella glycosylation and details structural insights into the enzymes in the Pse pathway obtained via a combination of biochemical, crystallographic, and mutagenesis studies of the enzyme-substrate and -inhibitor complexes. It is anticipated that understanding the underlying structural and molecular basis of the catalytic mechanisms of the Pse-synthesising enzymes will pave the way for the development of novel antimicrobials.

Evidence for the hook supercoiling mechanism of the bacterial flagellum

The bacterial flagellar hook is a short, highly curved tubular structure connecting the basal body as a rotary motor and the filament as a helical propeller to function as a universal joint to transmit motor torque to the filament regardless of its orientation. This highly curved form is known to be part of a supercoil as observed in the polyhook structure. The subunit packing interactions in the Salmonella hook structure solved in the straight form gave clear insights into the mechanisms of its bending flexibility and twisting rigidity. Salmonella FlgE consists of four domains, D0, Dc, D1 and D2, arranged from inside to outside of the tube, and an atomic model of the supercoiled hook built to simulate the hook shape observed in the native flagellum suggested that the supercoiled form is stabilized by near-axial interactions of the D2 domains on the inner surface of the supercoil. Here we show that the deletion of domain D2 from FlgE makes the hook straight, providing evidence to support the proposed hook supercoiling mechanism that it is the near-axial interactions between the D2 domains that stabilize the highly curved hook structure.

Novel insights into the mechanism of well-ordered assembly of bacterial flagellar proteins in Salmonella

The FliI ATPase of the flagellar type III protein export apparatus forms the FliH₂FliI complex along with its regulator FliH. The FliH₂FliI complex is postulated to bring export substrates from the cytoplasm to the docking platform made of FlhA and FlhB although not essential for flagellar protein export. Here, to clarify the role of the FliH₂FliI complex in flagellar assembly, we analysed the effect of FliH and FliI deletion on flagellar protein export and assembly. The hook length was not controlled properly in the ∆fliH-fliI flhB(P28T) mutant compared to wild-type cells, whose hook length is controlled to about 55 nm within 10% error. The FlhA(F459A) mutation increased the export level of the hook protein FlgE and the ruler protein FliK by about 10-fold and 3-fold, respectively, and improved the hook length control in the absence of FliH and FliI. However, the ∆fliH-fliI flhB(P28T) flhA(F459A) mutant did not produce flagellar filaments efficiently, and a large amount of flagellin monomers were leaked out into the culture media. Neither the hook length control nor flagellin leakage was affected by the FlhB(P28T) and FlhA(F459A) mutations. We will discuss a hierarchical protein export mechanism of the bacterial flagellum.

Bacterial flagellar axial structure and its construction

The bacterial flagellum is a motile organelle composed of thousands of protein subunits. The filamentous part that extends from the cell membrane is called the axial structure and consists of three major parts, the filament, hook, and rod, and other minor components. Each of the three main parts shares a similar self-assembly mechanism and a common basic architecture of subunit arrangement while showing quite distinct mechanical properties to achieve its specific function. Structural and molecular mechanisms to produce these various mechanical properties of the axial structure, such as the filament, the hook, and the rod, have been revealed by the complementary use of X-ray crystallography and cryo-electron microscopy. In addition, the mechanism of growth of the axial structure is beginning to be revealed based on the molecular structure.

A triangular loop of domain D1 of FlgE is essential for hook assembly but not for the mechanical function

The bacterial flagellar hook is a short, curved tubular structure made of FlgE. The hook connects the basal body as a rotary motor and the filament as a helical propeller and functions as a universal joint to smoothly transmit torque produced by the motor to the filament. Salmonella FlgE consists of D0, Dc, D1 and D2 domains. Axial interactions between a triangular loop of domain D1 (D1-loop) and domain D2 are postulated to be responsible for hook supercoiling. In contrast, Bacillus FlgE lacks the D1-loop and domain D2. Here, to clarify the roles of the D1-loop and domain D2 in the mechanical function, we carried out deletion analysis of Salmonella FlgE. A deletion of the D1-loop conferred a loss-of-function phenotype whereas that of domain D2 did not. The D1-loop deletion inhibited hook polymerization. Suppressor mutations of the D1-loop deletion was located within FlgD, which acts as the hook cap to promote hook assembly. This suggests a possible interaction between the D1-loop of FlgE and FlgD. Suppressor mutant cells produced straight hooks, but retained the ability to form a flagellar bundle behind a cell body, suggesting that the loop deletion does not affect the bending flexibility of the Salmonella hook.

Assembly and stoichiometry of the core structure of the bacterial flagellar type III export gate complex

The bacterial flagellar type III export apparatus, which is required for flagellar assembly beyond the cell membranes, consists of a transmembrane export gate complex and a cytoplasmic ATPase complex. FlhA, FlhB, FliP, FliQ, and FliR form the gate complex inside the basal body MS ring, although FliO is required for efficient export gate formation in Salmonella enterica. However, it remains unknown how they form the gate complex. Here we report that FliP forms a homohexameric ring with a diameter of 10 nm. Alanine substitutions of conserved Phe-137, Phe-150, and Glu-178 residues in the periplasmic domain of FliP (FliP_P) inhibited FliP₆ ring formation, suppressing flagellar protein export. FliO formed a 5-nm ring structure with 3 clamp-like structures that bind to the FliP₆ ring. The crystal structure of FliP_P derived from Thermotoga maritia, and structure-based photo-crosslinking experiments revealed that Phe-150 and Ser-156 of FliP_P are involved in the FliP–FliP interactions and that Phe-150, Arg-152, Ser-156, and Pro-158 are responsible for the FliP–FliO interactions. Overexpression of FliP restored motility of a ∆fliO mutant to the wild-type level, suggesting that the FliP₆ ring is a functional unit in the export gate complex and that FliO is not part of the final gate structure. Copurification assays revealed that FlhA, FlhB, FliQ, and FliR are associated with the FliO/FliP complex. We propose that the assembly of the export gate complex begins with FliP₆ ring formation with the help of the FliO scaffold, followed by FliQ, FliR, and FlhB and finally FlhA during MS ring formation.

The bacterial flagellar type III export gate complex is a membrane-embedded nanomachine responsible for flagellar protein export and exits in a patch of membrane within the central pore of the basal body MS ring. In this work, we investigate how formation of the export gate complex is initiated. The export gate complex is composed of 5 highly conserved transmembrane proteins: FlhA, FlhB, FliP, FliQ, and FliR. Each subunit protein assembles into the gate during MS ring formation in a well-coordinated manner. The transmembrane protein FliO is required for efficient assembly of the export gate complex in S. enterica but is not essential for flagellar protein export. Here we carry out biochemical and structural analyses of FliP and provide direct evidence suggesting that FliP forms a trimer-of-dimer structure with a diameter of 10 nm. The assembly of the export gate complex begins with FliP₆ ring formation with the help of the FliO scaffold, followed by FliQ, FliR, and FlhB and finally FlhA during MS ring formation. Given the structural and functional similarities between the flagellar and the virulence-factor-delivering injectisome machineries, we propose that the periplasmic domain of FliP homologues of the injectisome could be a good target for novel antibiotics.

The role of the FliD C-terminal domain in pentamer formation and interaction with FliT

Flagellar biogenesis is controlled by a negative feedback loop. When FliD was secreted at the late step of flagellar assembly, the FliD-FliT complex disassembled and free FliT bound to the FlhDC complex, a master regulator of flagellar biogenesis, subsequently inhibiting the overall expression of flagellar proteins. In this study, we analyzed the role of the FliD C-terminal domain in pentamer formation and interaction with FliT. Our study showed that the FliD L443R mutant exists as a monomer in solution, indicating that the Leu443 residue of FliD, which contributes to its interaction with FliT, plays a crucial role in the pentameric oligomerization of FliD. Consistently, the increased levels of free FliT proteins caused by FliD L443R mutation had negative effects on the gene expression of flagellar synthesis and reduced the expression of flagellar proteins. The lengths of flagella in each cell were significantly reduced in L443R mutant strain, suggesting that normal flagellar biogenesis was impeded. These results suggest that the C-terminal domain of FliD plays a crucial role in the pentameric oligmerization of FliD and the binding of FliT to the C-terminal domain of FliD is critical to inhibit the premature assembly of the FliD pentamer in the cytosol.

The role of intrinsically disordered C-terminal region of FliK in substrate specificity switching of the bacterial flagellar type III export apparatus

The bacterial flagellar export switching machinery consists of a ruler protein, FliK, and an export switch protein, FlhB and switches substrate specificity of the flagellar type III export apparatus upon completion of hook assembly. An interaction between the C-terminal domain of FliK (FliK_C ) and the C-terminal cytoplasmic domain of FlhB (FlhB_C ) is postulated to be responsible for this switch. FliK_C has a compactly folded domain termed FliK_T3S4 (residues 268-352) and an intrinsically disordered region composed of the last 53 residues, FliK_CT (residues 353-405). Residues 301-350 of FliK_T3S4 and the last five residues of FliK_CT are critical for the switching function of FliK. FliK_CT is postulated to regulate the interaction of FliK_T3S4 with FlhB_C , but it remains unknown how. Here we report the role of FliK_CT in the export switching mechanism. Systematic deletion analyses of FliK_CT revealed that residues of 351-370 are responsible for efficient switching of substrate specificity of the export apparatus. Suppressor mutant analyses showed that FliK_CT coordinates FliK_T3S4 action with the switching. Site-directed photo-cross-linking experiments showed that Val-302 and Ile-304 in the hydrophobic core of FliK_T3S4 bind to FlhB_C . We propose that FliK_CT may induce conformational rearrangements of FliK_T3S4 to bind to FlhB_C .

Variability in bacterial flagella re-growth patterns after breakage

Many bacteria swim through liquids or crawl on surfaces by rotating long appendages called flagella. Flagellar filaments are assembled from thousands of subunits that are exported through a narrow secretion channel and polymerize beneath a capping scaffold at the tip of the growing filament. The assembly of a flagellum uses a significant proportion of the biosynthetic capacities of the cell with each filament constituting ~1% of the total cell protein. Here, we addressed a significant question whether a flagellar filament can form a new cap and resume growth after breakage. Re-growth of broken filaments was visualized using sequential 3-color fluorescent labeling of filaments after mechanical shearing. Differential electron microscopy revealed the formation of new cap structures on broken filaments that re-grew. Flagellar filaments are therefore able to re-grow if broken by mechanical shearing forces, which are expected to occur frequently in nature. In contrast, no re-growth was observed on filaments that had been broken using ultrashort laser pulses, a technique allowing for very local damage to individual filaments. We thus conclude that assembly of a new cap at the tip of a broken filament depends on how the filament was broken.

Straight and rigid flagellar hook made by insertion of the FlgG specific sequence into FlgE

The bacterial flagellar hook connects the helical flagellar filament to the rotary motor at its base. Bending flexibility of the hook allows the helical filaments to form a bundle behind the cell body to produce thrust for bacterial motility. The hook protein FlgE shows considerable sequence and structural similarities to the distal rod protein FlgG; however, the hook is supercoiled and flexible as a universal joint whereas the rod is straight and rigid as a drive shaft. A short FlgG specific sequence (GSS) has been postulated to confer the rigidity on the FlgG rod, and insertion of GSS at the position between Phe-42 and Ala-43 of FlgE actually made the hook straight. However, it remains unclear whether inserted GSS confers the rigidity as well. Here, we provide evidence that insertion of GSS makes the hook much more rigid. The GSS insertion inhibited flagellar bundle formation behind the cell body, thereby reducing motility. This indicates that the GSS insertion markedly reduced the bending flexibility of the hook. Therefore, we propose that the inserted GSS makes axial packing interactions of FlgE subunits much tighter in the hook to suppress axial compression and extension of the protofilaments required for bending flexibility.

Fuel of the Bacterial Flagellar Type III Protein Export Apparatus

The flagellar type III export apparatus utilizes ATP and proton motive force (PMF) across the cytoplasmic membrane as the energy sources and transports flagellar component proteins from the cytoplasm to the distal growing end of the growing structure to construct the bacterial flagellum beyond the cellular membranes. The flagellar type III export apparatus coordinates flagellar protein export with assembly by ordered export of substrates to parallel with their order of the assembly. The export apparatus is composed of a PMF-driven transmembrane export gate complex and a cytoplasmic ATPase complex. Since the ATPase complex is dispensable for flagellar protein export, PMF is the primary fuel for protein unfolding and translocation. Interestingly, the export gate complex can also use sodium motive force across the cytoplasmic membrane in addition to PMF when the ATPase complex does not work properly. Here, we describe experimental protocols, which have allowed us to identify the export substrate class and the primary fuel of the flagellar type III protein export apparatus in Salmonella enterica serovar Typhimurium.

Design and Preparation of the Fragment Proteins of the Flagellar Components Suitable for X-Ray Crystal Structure Analysis

Terminal disordering in a monomeric state is a common structural property among bacterial flagellar axial proteins. The conformational flexibility of disordered regions of a protein often disturbs its crystallization. Moreover, disordered regions sometimes cause the aggregation problem. Therefore, trimming disordered regions is essential for crystallization of this type of proteins. In this chapter, we describe a simple but powerful method to determine the stable core and metastable fragments of target proteins for crystallization. This method including limited proteolysis in combination with SDS-PAGE and MALDI-TOF mass spectrometry can be applied to almost any proteins containing disordered regions.