FliK, the protein responsible for flagellar hook length control in Salmonella, is exported during hook assembly

FliH and FliI help FlhA bring strict order to flagellar protein export in Salmonella

The flagellar type III secretion system (fT3SS) switches substrate specificity from rod-hook-type to filament-type upon hook completion, terminating hook assembly and initiating filament assembly. The C-terminal cytoplasmic domain of FlhA (FlhAC) forms a homo-nonameric ring and is directly involved in substrate recognition, allowing the fT3SS to coordinate flagellar protein export with assembly. The highly conserved GYXLI motif (residues 368-372) of FlhAC induces dynamic domain motions of FlhAC required for efficient and robust flagellar protein export by the fT3SS, but it remains unknown whether this motif is also important for ordered protein export by the fT3SS. Here we analyzed two GYXLI mutants, flhA(GAAAA) and flhA(GGGGG), and provide evidence suggesting that the GYXLI motif in FlhAC requires the flagellar ATPase complex not only to efficiently remodel the FlhAC ring structure for the substrate specificity switching but also to correct substrate recognition errors that occur during flagellar assembly.

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.

Functional Implications of Dynamic Structures of Intrinsically Disordered Proteins Revealed by High-Speed AFM Imaging

The unique functions of intrinsically disordered proteins (IDPs) depend on their dynamic protean structure that often eludes analysis. High-speed atomic force microscopy (HS-AFM) can conduct this difficult analysis by directly visualizing individual IDP molecules in dynamic motion at sub-molecular resolution. After brief descriptions of the microscopy technique, this review first shows that the intermittent tip-sample contact does not alter the dynamic structure of IDPs and then describes how the number of amino acids contained in a fully disordered region can be estimated from its HS-AFM images. Next, the functional relevance of a dumbbell-like structure that has often been observed on IDPs is discussed. Finally, the dynamic structural information of two measles virus IDPs acquired from their HS-AFM and NMR analyses is described together with its functional implications.

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).

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.

Recognition of discrete export signals in early flagellar subunits during bacterial Type III secretion

Type III Secretion Systems (T3SS) deliver subunits from the bacterial cytosol to nascent cell surface flagella. Early flagellar subunits that form the rod and hook substructures are unchaperoned and contain their own export signals. A gate recognition motif (GRM) docks them at the FlhBc component of the FlhAB-FliPQR export gate, but the gate must then be opened and subunits must be unfolded to pass through the flagellar channel. This induced us to seek further signals on the subunits. Here, we identify a second signal at the extreme N-terminus of flagellar rod and hook subunits and determine that key to the signal is its hydrophobicity. We show that the two export signal elements are recognised separately and sequentially, as the N-terminal signal is recognised by the flagellar export machinery only after subunits have docked at FlhB_C via the GRM. The position of the N-terminal hydrophobic signal in the subunit sequence relative to the GRM appeared to be important, as a FlgD deletion variant (FlgD_short), in which the distance between the N-terminal signal and the GRM was shortened, 'stalled' at the export machinery and was not exported. The attenuation of motility caused by FlgD_short was suppressed by mutations that destabilised the closed conformation of the FlhAB-FliPQR export gate, suggesting that the hydrophobic N-terminal signal might trigger opening of the flagellar export gate.

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.

The Lon protease temporally restricts polar cell differentiation events during the Caulobacter cell cycle

The highly conserved protease Lon has important regulatory and protein quality control functions in cells from the three domains of life. Despite many years of research on Lon, only a few specific protein substrates are known in most organisms. Here, we used a quantitative proteomics approach to identify novel substrates of Lon in the dimorphic bacterium Caulobacter crescentus. We focused our study on proteins involved in polar cell differentiation and investigated the developmental regulator StaR and the flagella hook length regulator FliK as specific Lon substrates in detail. We show that Lon recognizes these proteins at their C-termini, and that Lon-dependent degradation ensures their temporally restricted accumulation in the cell cycle phase when their function is needed. Disruption of this precise temporal regulation of StaR and FliK levels in a Δlon mutant contributes to defects in stalk biogenesis and motility, respectively, revealing a critical role of Lon in coordinating developmental processes with cell cycle progression. Our work underscores the importance of Lon in the regulation of complex temporally controlled processes by adjusting the concentrations of critical regulatory proteins. Furthermore, this study includes the first characterization of FliK in C. crescentus and uncovers a dual role of the C-terminal amino acids of FliK in protein function and degradation.

The FlhA linker mediates flagellar protein export switching during flagellar assembly

The flagellar protein export apparatus switches substrate specificity from hook-type to filament-type upon hook assembly completion, thereby initiating filament assembly at the hook tip. The C-terminal cytoplasmic domain of FlhA (FlhA_C) serves as a docking platform for flagellar chaperones in complex with their cognate filament-type substrates. Interactions of the flexible linker of FlhA (FlhA_L) with its nearest FlhA_C subunit in the FlhA_C ring is required for the substrate specificity switching. To address how FlhA_L brings the order to flagellar assembly, we analyzed the flhA(E351A/W354A/D356A) ΔflgM mutant and found that this triple mutation in FlhA_L increased the secretion level of hook protein by 5-fold, thereby increasing hook length. The crystal structure of FlhA_C(E351A/D356A) showed that FlhA_L bound to the chaperone-binding site of its neighboring subunit. We propose that the interaction of FlhA_L with the chaperon-binding site of FlhA_C suppresses filament-type protein export and facilitates hook-type protein export during hook assembly.

The fliK Gene Is Required for the Resistance of Bacillus thuringiensis to Antimicrobial Peptides and Virulence in Drosophila melanogaster

Antimicrobial peptides (AMPs) are essential effectors of the host innate immune system and they represent promising molecules for the treatment of multidrug resistant microbes. A better understanding of microbial resistance to these defense peptides is thus prerequisite for the control of infectious diseases. Here, using a random mutagenesis approach, we identify the fliK gene, encoding an internal molecular ruler that controls flagella hook length, as an essential element for Bacillus thuringiensis resistance to AMPs in Drosophila. Unlike its parental strain, that is highly virulent to both wild-type and AMPs deficient mutant flies, the fliK deletion mutant is only lethal to the latter's. In agreement with its conserved function, the fliK mutant is non-flagellated and exhibits highly compromised motility. However, comparative analysis of the fliK mutant phenotype to that of a fla mutant, in which the genes encoding flagella proteins are interrupted, indicate that B. thuringiensis FliK-dependent resistance to AMPs is independent of flagella assembly. As a whole, our results identify FliK as an essential determinant for B. thuringiensis virulence in Drosophila and provide new insights on the mechanisms underlying bacteria resistance to AMPs.

A Proline-Rich Element in the Type III Secretion Protein FlhB Contributes to Flagellar Biogenesis in the Beta- and Gamma-Proteobacteria

Flagella are bacterial organelles of locomotion. Their biogenesis is highly coordinated in time and space and relies on a specialized flagellar type III secretion system (fT3SS) required for the assembly of the extracellular hook, rod, and filament parts of this complex motor device. The fT3SS protein FlhB switches secretion substrate specificity once the growing hook reaches its determined length. Here we present the crystal structure of the cytoplasmic domain of the transmembrane protein FlhB. The structure visualizes a so-far unseen proline-rich region (PRR) at the very C-terminus of the protein. Strains lacking the PRR show a decrease in flagellation as determined by hook- and filament staining, indicating a role of the PRR during assembly of the hook and filament structures. Phylogenetic analysis shows that the PRR is a primary feature of FlhB proteins of flagellated beta- and gamma-proteobacteria. Taken together, our study adds another layer of complexity and organismic diversity to the process of flagella biogenesis.

The PopN Gate-keeper Complex Acts on the ATPase PscN to Regulate the T3SS Secretion Switch from Early to Middle Substrates in Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic bacterium of which the main virulence factor is the Type III Secretion System. The ATPase of this machinery, PscN (SctN), is thought to be localized at the base of the secretion apparatus and to participate in the recognition, chaperone dissociation and unfolding of exported T3SS proteins. In this work, a protein-protein interaction ELISA revealed the interaction of PscN with a wide range of exported T3SS proteins including the needle, translocator, gate-keeper and effector. These interactions were further confirmed by Microscale Thermophoresis that also indicated a preferential interaction of PscN with secreted proteins or protein-chaperone complex rather than with chaperones alone, in line with the release of the chaperones in the bacterial cytoplasm after the dissociation from their exported proteins. Moreover, we suggest a new role of the gate-keeper complex and the ATPase in the regulation of early substrates recognition by the T3SS. This finding sheds a new light on the mechanism of secretion switching from early to middle substrates in P. aeruginosa.

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.

In Vitro Autonomous Construction of the Flagellar Axial Structure in Inverted Membrane Vesicles

The bacterial flagellum is a filamentous organelle extending from the cell surface. The axial structure of the flagellum consists of the rod, hook, junction, filament, and cap. The axial structure is formed by axial component proteins exported via a specific protein export apparatus in a well-regulated manner. Although previous studies have revealed the outline of the flagellar construction process, the mechanism of axial structure formation, including axial protein export, is still obscure due to difficulties in direct observation of protein export and assembly in vivo. We recently developed an in vitro flagellar protein transport assay system using inverted membrane vesicles (IMVs) and succeeded in reproducing the early stage of flagellar assembly. However, the late stage of the flagellar formation process remained to be examined in the IMVs. In this study, we showed that the filament-type proteins are transported into the IMVs to produce the filament on the hook inside the IMVs. Furthermore, we provide direct evidence that coordinated flagellar protein export and assembly can occur at the post-translational level. These results indicate that the ordered construction of the entire flagellar structure can be regulated by only the interactions between the protein export apparatus, the export substrate proteins, and their cognate chaperones.

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 .

Structural Insights into the Substrate Specificity Switch Mechanism of the Type III Protein Export Apparatus

Bacteria use a type III protein export apparatus for construction of the flagellum, which consists of the basal body, the hook, and the filament. FlhA forms a homo-nonamer through its C-terminal cytoplasmic domains (FlhA_C) and ensures the strict order of flagellar assembly. FlhA_C goes through dynamic domain motions during protein export, but it remains unknown how it occurs. Here, we report that the FlhA(G368C) mutation biases FlhA_C toward a closed form, thereby reducing the binding affinity of FlhA_C for flagellar export chaperones in complex with their cognate filament-type substrates. The G368C mutations also restrict the conformational flexibility of a linker region of FlhA (FlhA_L), suppressing FlhA_C ring formation. We propose that interactions of FlhA_L with its neighboring subunit converts FlhA_C in the ring from a closed conformation to an open one, allowing the chaperon/substrate complexes to bind to the FlhA_C ring to form the filament at the hook tip.

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.

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 .

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.

The Ruler Protein EscP of the Enteropathogenic Escherichia coli Type III Secretion System Is Involved in Calcium Sensing and Secretion Hierarchy Regulation by Interacting with the Gatekeeper Protein SepL

The type III secretion system (T3SS) is a multiprotein complex that plays a central role in the virulence of many Gram-negative bacterial pathogens. To ensure that effector proteins are efficiently translocated into the host cell, bacteria must be able to sense their contact with the host cell. In this study, we found that EscP, which was previously shown to function as the ruler protein of the enteropathogenic Escherichia coli T3SS, is also involved in the switch from the secretion of translocator proteins to the secretion of effector proteins. In addition, we demonstrated that EscP can interact with the gatekeeper protein SepL and that the EscP-SepL complex dissociates upon a calcium concentration drop. We suggest a model in which bacterial contact with the host cell is accompanied by a drop in the calcium concentration that causes SepL-EscP complex dissociation and triggers the secretion of effector proteins.

IMPORTANCE

The emergence of multidrug-resistant bacterial strains, especially those of pathogenic bacteria, has serious medical and clinical implications. At the same time, the development and approval of new antibiotics have been limited for years. Recently, antivirulence drugs have received considerable attention as a novel antibiotic strategy that specifically targets bacterial virulence rather than growth, an approach that applies milder evolutionary pressure on the bacteria to develop resistance. A highly attractive target for the development of antivirulence compounds is the type III secretion system, a specialized secretory system possessed by many Gram-negative bacterial pathogens for injecting virulence factors (effectors) into host cells. In this study, we shed light on the molecular mechanism that allows bacteria to sense their contact with the host cell and to respond with the timed secretion of effector proteins. Understanding this critical step for bacterial virulence may provide a new therapeutic strategy.

Coupling of Flagellar Gene Expression with Assembly in Salmonella enterica

There are more than 70 genes in the flagellar and chemosensory regulon of Salmonella enterica. These genes are organized into a transcriptional hierarchy of three promoter classes. At the top of the transcriptional hierarchy is the flhDC operon, also called the flagellar master operon, which is transcribed from the flagellar class 1 promoter region. The protein products of the flhDC operon form a hetero-multimeric complex, FlhD₄C₂, which directs σ⁷⁰ RNA polymerase to transcribe from class 2 flagellar promoters. Products of flagellar class 2 transcription are required for the structure and assembly of the hook-basal body (HBB) complex. One of the class 2 flagellar genes, fliA, encodes an alternative sigma transcription factor, σ²⁸, which directs transcription from flagellar class 3 promoters. The class 3 promoters direct transcription of gene products needed after HBB completion including the motor force generators, the filament, and the chemosensory genes. Flagellar gene transcription is coupled to assembly at the level of hook-basal body completion. Two key proteins, σ²⁸ and FliT, play assembly roles prior to HBB completion and upon HBB completion act as positive and negative regulators, respectively. HBB completion signals a secretion-specificity switch in the flagellar type III secretion system, which results in the secretion of σ²⁸ and FliT antigonists allowing these proteins to perform their roles in transcriptional regulation of flagellar genes. Genetic methods have provided the principle driving forces in our understanding of how flagellar gene expression is controlled and coupled to the assembly process.

Type Three Secretion System in Attaching and Effacing Pathogens

Enteropathogenic Escherichia coli and enterohemorrhagic E. coli are diarrheagenic bacterial human pathogens that cause severe gastroenteritis. These enteric pathotypes, together with the mouse pathogen Citrobacter rodentium, belong to the family of attaching and effacing pathogens that form a distinctive histological lesion in the intestinal epithelium. The virulence of these bacteria depends on a type III secretion system (T3SS), which mediates the translocation of effector proteins from the bacterial cytosol into the infected cells. The core architecture of the T3SS consists of a multi-ring basal body embedded in the bacterial membranes, a periplasmic inner rod, a transmembrane export apparatus in the inner membrane, and cytosolic components including an ATPase complex and the C-ring. In addition, two distinct hollow appendages are assembled on the extracellular face of the basal body creating a channel for protein secretion: an approximately 23 nm needle, and a filament that extends up to 600 nm. This filamentous structure allows these pathogens to get through the host cells mucus barrier. Upon contact with the target cell, a translocation pore is assembled in the host membrane through which the effector proteins are injected. Assembly of the T3SS is strictly regulated to ensure proper timing of substrate secretion. The different type III substrates coexist in the bacterial cytoplasm, and their hierarchical secretion is determined by specialized chaperones in coordination with two molecular switches and the so-called sorting platform. In this review, we present recent advances in the understanding of the T3SS in attaching and effacing pathogens.

Regulation of Flagellar Gene Expression in Bacteria

The flagellum of a bacterium is a supramolecular structure of extreme complexity comprising simultaneously both a unique system of protein transport and a molecular machine that enables the bacterial cell movement. The cascade of expression of genes encoding flagellar components is closely coordinated with the steps of molecular machine assembly, constituting an amazing regulatory system. Data on structure, assembly, and regulation of flagellar gene expression are summarized in this review. The regulatory mechanisms and correlation of the process of regulation of gene expression and flagellum assembly known from the literature are described.

HrcU and HrpP are pathogenicity factors in the fire blight pathogen Erwinia amylovora required for the type III secretion of DspA/E

Background

Many Gram-negative bacterial pathogens mediate host-microbe interactions via utilization of the type III secretion (T3S) system. The T3S system is a complex molecular machine consisting of more than 20 proteins. Collectively, these proteins translocate effectors across extracellular space and into the host cytoplasm. Successful translocation requires timely synthesis and allocation of both structural and secreted T3S proteins. Based on amino acid conservation in animal pathogenic bacteria, HrcU and HrpP were examined for their roles in regulation of T3S hierarchy.

Results

Both HrcU and HrpP were shown to be required for disease development in an immature pear infection model and respective mutants were unable to induce a hypersensitive response in tobacco. Using in vitro western blot analyses, both proteins were also shown to be required for the secretion of DspA/E, a type 3 effector and an important pathogenicity factor. Via yeast-two hybridization (Y2H), HrpP and HrcU were revealed to exhibit protein-protein binding. Finally, all HrcU and HrpP phenotypes identified were shown to be dependent on a conserved amino acid motif in the cytoplasmic tail of HrcU.

Conclusions

Collectively, these data demonstrate roles for HrcU and HrpP in regulating T3S and represent the first attempt in understanding T3S heirarchy in E. amylovora.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-016-0702-y) contains supplementary material, which is available to authorized users.

Function of the conserved FHIPEP domain of the flagellar type III export apparatus, protein FlhA

The Type III flagellar protein export apparatus of bacteria consists of five or six membrane proteins, notably FlhA, which controls the export of other proteins and is homologous to the large family of FHIPEP export proteins. FHIPEP proteins contain a highly-conserved cytoplasmic domain. We mutagenized the cloned Salmonella flhA gene for the 692 amino acid FlhA, changing a single, conserved amino acid in the 68-amino acid FHIPEP region. Fifty-two mutations at 30 positions mostly led to loss of motility and total disappearance of microscopically visible flagella, also Western blot protein/protein hybridization showed no detectable export of hook protein and flagellin. There were two exceptions: a D199A mutant strain, which produced short-stubby flagella; and a V151L mutant strain, which did not produce flagella and excreted mainly un-polymerized hook protein. The V151L mutant strain also exported a reduced amount of hook-cap protein FlgD, but when grown with exogenous FlgD it produced polyhooks and polyhook-filaments. A suppressor mutant in the cytoplasmic domain of the export apparatus membrane protein FlhB rescued export of hook-length control protein FliK and facilitated growth of full-length flagella. These results suggested that the FHIPEP region is part of the gate regulating substrate entry into the export apparatus pore.

The Structure of a Type 3 Secretion System (T3SS) Ruler Protein Suggests a Molecular Mechanism for Needle Length Sensing

The type 3 secretion system (T3SS) and the bacterial flagellum are related pathogenicity-associated appendages found at the surface of many disease-causing bacteria. These appendages consist of long tubular structures that protrude away from the bacterial surface to interact with the host cell and/or promote motility. A proposed "ruler" protein tightly regulates the length of both the T3SS and the flagellum, but the molecular basis for this length control has remained poorly characterized and controversial. Using the Pseudomonas aeruginosa T3SS as a model system, we report the first structure of a T3SS ruler protein, revealing a "ball-and-chain" architecture, with a globular C-terminal domain (the ball) preceded by a long intrinsically disordered N-terminal polypeptide chain. The dimensions and stability of the globular domain do not support its potential passage through the inner lumen of the T3SS needle. We further demonstrate that a conserved motif at the N terminus of the ruler protein interacts with the T3SS autoprotease in the cytosolic side. Collectively, these data suggest a potential mechanism for needle length sensing by ruler proteins, whereby upon T3SS needle assembly, the ruler protein's N-terminal end is anchored on the cytosolic side, with the globular domain located on the extracellular end of the growing needle. Sequence analysis of T3SS and flagellar ruler proteins shows that this mechanism is probably conserved across systems.

[Structure and function of the bacterial flagellar type III protein export system in Salmonella ]

The bacterial flagellum is a filamentous organelle that propels the bacterial cell body in liquid media. For construction of the bacterial flagellum beyond the cytoplasmic membrane, flagellar component proteins are transported by its specific protein export apparatus from the cytoplasm to the distal end of the growing flagellar structure. The flagellar export apparatus consists of a transmembrane export gate complex and a cytoplasmic ATPase ring complex. Flagellar substrate-specific chaperones bind to their cognate substrates in the cytoplasm and escort the substrates to the docking platform of the export gate. The export apparatus utilizes ATP and proton motive force across the cytoplasmic membrane as the energy sources to drive protein export and coordinates protein export with assembly by ordered export of substrates to parallel with their order of assembly. In this review, we summarize our current understanding of the structure and function of the flagellar protein export system in Salmonella enterica serovar Typhimurium.

Weak Interactions between Salmonella enterica FlhB and Other Flagellar Export Apparatus Proteins Govern Type III Secretion Dynamics

The bacterial flagellum contains its own type III secretion apparatus that coordinates protein export with assembly at the distal end. While many interactions among export apparatus proteins have been reported, few have been examined with respect to the differential affinities and dynamic relationships that must govern the mechanism of export. FlhB, an integral membrane protein, plays critical roles in both export and the substrate specificity switching that occurs upon hook completion. Reported herein is the quantitative characterization of interactions between the cytoplasmic domain of FlhB (FlhB_C) and other export apparatus proteins including FliK, FlhA_C and FliI. FliK and FlhA_C bound with micromolar affinity. K_D for FliI binding in the absence of ATP was 84 nM. ATP-induced oligomerization of FliI induced kinetic changes, stimulating fast-on, fast-off binding and lowering affinity. Full length FlhB purified under solubilizing, nondenaturing conditions formed a stable dimer via its transmembrane domain and stably bound FliH. Together, the present results support the previously hypothesized central role of FlhB and elucidate the dynamics of protein-protein interactions in type III secretion.

Molecular ruler determines needle length for the Salmonella Spi-1 injectisome

The type-III secretion (T3S) systems of bacteria are part of self-assembling nanomachines: the bacterial flagellum that enables cells to propel themselves through liquid and across hydrated surfaces, and the injectisome that delivers pathogenic effector proteins into eukaryotic host cells. Although the flagellum and injectisome serve different purposes, they are evolutionarily related and share many structural similarities. Core features to these T3S systems are intrinsic length control mechanisms for external cellular projections: the hook of the flagellum and the injectisome needle. We present evidence that the Spi-1 injectisome, like the Salmonella flagellar hook, uses a secreted molecular ruler, InvJ, to determine needle length. This result supports a universal length control mechanism using molecular rulers for T3S systems.

Two-ball structure of the flagellar hook-length control protein FliK as revealed by high-speed atomic force microscopy

The bacterial flagellar hook is a short and uniquely curved tube that connects the basal body to the filament. Hook length is controlled at 55 nm on average by a soluble protein FliK in Salmonella enterica serovar Typhimurium. The N-terminal segment of FliK responsible for measuring the hook length is considered to be intrinsically disordered. Here, we show by high-speed atomic force microscopy that a FliK molecule in solution takes on a shape of two balls linked by a flexible string; the larger ball corresponds to the N-terminal region and the smaller one corresponds to the C-terminal region. The N-terminal domain is stable but the C-terminal domain fluctuates in shape. Based on these and other features of FliK, we propose that the folding of the N-terminal segment at the tip of the growing hook plays a major role in determining the minimal length of the hook.

HpaP modulates type III effector secretion in Ralstonia solanacearum and harbours a substrate specificity switch domain essential for virulence

Many pathogenic bacteria have evolved a type III secretion system (T3SS) to successfully invade their host. This extracellular apparatus allows the translocation of proteins, called type III effectors (T3Es), directly into the host cells. T3Es are virulence factors that have been shown to interfere with the host's immunity or to provide nutrients from the host to the bacteria. The Gram-negative bacterium Ralstonia solanacearum is a worldwide major crop pest whose virulence strongly relies on the T3SS. In R. solanacearum, transcriptional regulation has been extensively studied. However, very few data are available concerning the role played by type III-associated regulators, such as type III chaperones and T3SS control proteins. Here, we characterized HpaP, a putative type III secretion substrate specificity switch (T3S4) protein of R. solanacearum which is not secreted by the bacterium or translocated in the plant cells. HpaP self-interacts and interacts with the PopP1 T3E. HpaP modulates the secretion of early (HrpY pilin) and late (AvrA and PopP1 T3Es) type III substrates. HpaP is dispensable for the translocation of T3Es into the host cells. Finally, we identified two regions of five amino acids in the T3S4 domain that are essential for efficient PopP1 secretion and for HpaP's role in virulence on tomato and Arabidopsis thaliana, but not required for HpaP-HpaP and HpaP-PopP1 interactions. Taken together, our results indicate that HpaP is a putative R. solanacearum T3S4 protein important for full pathogenicity on several hosts, acting as a helper for PopP1 secretion, and repressing AvrA and HrpY secretion.

Interactions of bacterial flagellar chaperone-substrate complexes with FlhA contribute to co-ordinating assembly of the flagellar filament

Assembly of the bacterial flagellar filament is strictly sequential; the junction proteins, FlgK and FlgL, are assembled at the distal end of the hook prior to the FliD cap, which supports assembly of as many as 30 000 FliC molecules into the filament. Export of these proteins requires assistance of flagellar chaperones: FlgN for FlgK and FlgL, FliT for FliD and FliS for FliC. The C-terminal cytoplasmic domain of FlhA (FlhAC ), a membrane component of the export apparatus, provides a binding-site for these chaperone-substrate complexes but it remains unknown how it co-ordinates flagellar protein export. Here, we report that the highly conserved hydrophobic dimple of FlhAC is involved in the export of FlgK, FlgL, FliD and FliC but not in proteins responsible for the structure and assembly of the hook, and that the binding affinity of FlhAC for the FlgN/FlgK complex is slightly higher than that for the FliT/FliD complex and about 14-fold higher than that for the FliS/FliC complex, leading to the proposal that the different binding affinities of FlhAC for these chaperone/substrate complexes may confer an advantage for the efficient formation of the junction and cap structures at the tip of the hook prior to filament formation.

Protein export through the bacterial flagellar type III export pathway

For construction of the bacterial flagellum, which is responsible for bacterial motility, the flagellar type III export apparatus utilizes both ATP and proton motive force across the cytoplasmic membrane and exports flagellar proteins from the cytoplasm to the distal end of the nascent structure. The export apparatus consists of a membrane-embedded export gate made of FlhA, FlhB, FliO, FliP, FliQ, and FliR and a water-soluble ATPase ring complex consisting of FliH, FliI, and FliJ. FlgN, FliS, and FliT act as substrate-specific chaperones that do not only protect their cognate substrates from degradation and aggregation in the cytoplasm but also efficiently transfer the substrates to the export apparatus. The ATPase ring complex facilitates the initial entry of the substrates into the narrow pore of the export gate. The export gate by itself is a proton-protein antiporter that uses the two components of proton motive force, the electric potential difference and the proton concentration difference, for different steps of the export process. A specific interaction of FlhA with FliJ located in the center of the ATPase ring complex allows the export gate to efficiently use proton motive force to drive protein export. The ATPase ring complex couples ATP binding and hydrolysis to its assembly-disassembly cycle for rapid and efficient protein export cycle. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Identification and characterization of a novel flagellum-dependent Salmonella-infecting bacteriophage, iEPS5

A novel flagellatropic phage of Salmonella enterica serovar Typhimurium, called iEPS5, was isolated and characterized. iEPS5 has an icosahedral head and a long noncontractile tail with a tail fiber. Genome sequencing revealed a double-stranded DNA of 59,254 bp having 73 open reading frames (ORFs). To identify the receptor for iEPS5, Tn5 transposon insertion mutants of S. Typhimurium SL1344 that were resistant to the phage were isolated. All of the phage-resistant mutants were found to have mutations in genes involved in flagellar formation, suggesting that the flagellum is the adsorption target of this phage. Analysis of phage infection using the ΔmotA mutant, which is flagellated but nonmotile, demonstrated the requirement of flagellar rotation for iEPS5 infection. Further analysis of phage infection using the ΔcheY mutant revealed that iEPS5 could infect host bacteria only when the flagellum is rotating counterclockwise (CCW). These results suggested that the CCW-rotating flagellar filament is essential for phage adsorption and required for successful infection by iEPS5. In contrast to the well-studied flagellatropic phage Chi, iEPS5 cannot infect the ΔfliK mutant that makes a polyhook without a flagellar filament, suggesting that these two flagellatropic phages utilize different infection mechanisms. Here, we present evidence that iEPS5 injects its DNA into the flagellar filament for infection by assessing DNA transfer from SYBR gold-labeled iEPS5 to the host bacteria.

Role of the Dc domain of the bacterial hook protein FlgE in hook assembly and function

The bacterial flagellar hook acts as a universal joint to smoothly transmit torque produced by the motor to the filament. The hook protein FlgE assembles into a 55 nm tubular structure with the help of the hook cap (FlgD). FlgE consists of four domains, D0, Dc, D1 and D2, arranged from the inner to the outer part of the tubular structure of the hook. The Dc domain contributes to the structural stability of the hook, but it is unclear how this Dc domain is responsible for the universal joint mechanism. Here, we carried out a deletion analysis of the FlgE Dc domain. FlgEΔ4/5 with deletion of residues 30 to 49 was not secreted into the culture media. FlgEΔ5 and FlgEΔ6 with deletions of residues 40 to 49 and 50 to 59, respectively, still formed hooks, allowing the export apparatus to export the hook-filament junction proteins FlgK and FlgL and flagellin FliC. However, these deletions inhibited the replacement of the FlgD hook cap by FlgK at the hook tip, thereby abolishing filament formation. Deletion of residues 50 to 59 significantly affected hook morphology. These results suggest that the Dc domain is responsible not only for hook assembly but also for FlgE export, the interaction with FlgK, and the polymorphic supercoiling mechanism of the hook.

Protein export according to schedule: architecture, assembly, and regulation of type III secretion systems from plant- and animal-pathogenic bacteria

Flagellar and translocation-associated type III secretion (T3S) systems are present in most gram-negative plant- and animal-pathogenic bacteria and are often essential for bacterial motility or pathogenicity. The architectures of the complex membrane-spanning secretion apparatuses of both systems are similar, but they are associated with different extracellular appendages, including the flagellar hook and filament or the needle/pilus structures of translocation-associated T3S systems. The needle/pilus is connected to a bacterial translocon that is inserted into the host plasma membrane and mediates the transkingdom transport of bacterial effector proteins into eukaryotic cells. During the last 3 to 5 years, significant progress has been made in the characterization of membrane-associated core components and extracellular structures of T3S systems. Furthermore, transcriptional and posttranscriptional regulators that control T3S gene expression and substrate specificity have been described. Given the architecture of the T3S system, it is assumed that extracellular components of the secretion apparatus are secreted prior to effector proteins, suggesting that there is a hierarchy in T3S. The aim of this review is to summarize our current knowledge of T3S system components and associated control proteins from both plant- and animal-pathogenic bacteria.

Role of EscP (Orf16) in injectisome biogenesis and regulation of type III protein secretion in enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli employs a type III secretion system (T3SS) to translocate virulence effector proteins directly into enterocyte host cells, leading to diarrheal disease. The T3SS is encoded within the chromosomal locus of enterocyte effacement (LEE). The function of some of the LEE-encoded proteins remains unknown. Here we investigated the role of the Orf16 protein in T3SS biogenesis and function. An orf16 deletion mutant showed translocator and effector protein secretion profiles different from those of wild-type cells. The orf16 null strain produced T3S structures with abnormally long needles and filaments that caused weak hemolysis of red blood cells. Furthermore, the number of fully assembled T3SSs was also reduced in the orf16 mutant, indicating that Orf16, though not essential, is required for efficient T3SS assembly. Analysis of protein secretion revealed that Orf16 is a T3SS-secreted substrate and regulates the secretion of the inner rod component EscI. Both pulldown and yeast two-hybrid assays showed that Orf16 interacts with the C-terminal domain of an inner membrane component of the secretion apparatus, EscU; the inner rod protein EscI; the needle protein EscF; and the multieffector chaperone CesT. These results suggest that Orf16 regulates needle length and, along with EscU, participates in a substrate specificity switch from early substrates to translocators. Taken together, our results suggest that Orf16 acts as a molecular measuring device in a way similar to that of members of the Yersinia YscP and flagellar FliK protein family. Therefore, we propose that this protein be renamed EscP.

Molecular characterization of the flagellar hook in Bacillus subtilis

The structure of the Gram-positive flagellum is poorly understood, and Bacillus subtilis encodes three proteins homologous to the flagellar hook protein from Salmonella enterica. Here we generated a modified B. subtilis hook protein that could be fluorescently stained using a cysteine-reactive dye. We used the fluorescently labeled hook to demonstrate that FlgE is the hook structural protein and that FliK regulated hook length. We further demonstrate that two proteins of unknown function, FlhO and FlhP, and the putative hook cap, FlgD, were required for hook assembly, such that when flhO, flhP, or flgD was mutated, hook protein was secreted into the supernatant. All mutants defective in hook completion resulted in homogeneously reduced σ(D)-dependent gene expression due to the action of the anti-sigma factor FlgM.

The Campylobacter jejuni CiaC virulence protein is secreted from the flagellum and delivered to the cytosol of host cells

Campylobacter jejuni is a leading cause of bacterial gastroenteritis worldwide. Acute C. jejuni-mediated disease (campylobacteriosis) involves C. jejuni invasion of host epithelial cells using adhesins (e.g., CadF and FlpA) and secreted proteins [e.g., the Campylobacter invasion antigens (Cia)]. The genes encoding the Cia proteins are up-regulated upon co-culture of C. jejuni with epithelial cells. One of the Cia proteins, CiaC, is required for maximal invasion of host cells by C. jejuni. Previous work has also revealed that CiaC is, in part, responsible for host cell cytoskeletal rearrangements that result in membrane ruffling. This study was performed to test the hypothesis that CiaC is delivered to the cytosol of host cells. To detect the delivery of CiaC into cultured epithelial cells, we used the adenylate cyclase domain (ACD) of Bordetella pertussis CyaA as a reporter. In this study, we found that export and delivery of the C. jejuni Cia proteins into human INT 407 epithelial cells required a functional flagellar hook complex composed of FlgE, FlgK, and FlgL. Assays performed with bacterial culture supernatants supported the hypothesis that CiaC delivery requires bacteria-host cell contact. We also found that CiaC was delivered to host cells by cell-associated (bound) bacteria, as judged by experiments performed with inhibitors that specifically target the cell signaling pathways utilized by C. jejuni for cell invasion. Interestingly, the C. jejuni flgL mutant, which is incapable of exporting and delivering the Cia proteins, did not induce INT 407 cell membrane ruffles. Complementation of the flgL mutant with plasmid-encoded flgL restored the motility and membrane ruffling. These data support the hypothesis that the C. jejuni Cia proteins, which are exported from the flagellum, are delivered to the cytosol of host cells.

Functional characterization of the type III secretion substrate specificity switch protein HpaC from Xanthomonas campestris pv. vesicatoria

Pathogenicity of Xanthomonas campestris pv. vesicatoria depends on a type III secretion (T3S) system which translocates effector proteins into eukaryotic cells and is associated with an extracellular pilus and a translocon in the host plasma membrane. T3S substrate specificity is controlled by the cytoplasmic switch protein HpaC, which interacts with the C-terminal domain of the inner membrane protein HrcU (HrcU(C)). HpaC promotes the secretion of translocon and effector proteins but prevents the efficient secretion of the early T3S substrate HrpB2, which is required for pilus assembly. In this study, complementation assays with serial 10-amino-acid HpaC deletion derivatives revealed that the T3S substrate specificity switch depends on N- and C-terminal regions of HpaC, whereas amino acids 42 to 101 appear to be dispensable for the contribution of HpaC to the secretion of late substrates. However, deletions in the central region of HpaC affect the secretion of HrpB2, suggesting that the mechanisms underlying HpaC-dependent control of early and late substrates can be uncoupled. The results of interaction and expression studies with HpaC deletion derivatives showed that amino acids 112 to 212 of HpaC provide the binding site for HrcU(C) and severely reduce T3S when expressed ectopically in the wild-type strain. We identified a conserved phenylalanine residue at position 175 of HpaC that is required for both protein function and the binding of HpaC to HrcU(C). Taking these findings together, we concluded that the interaction between HpaC and HrcU(C) is essential but not sufficient for T3S substrate specificity switching.

An infrequent molecular ruler controls flagellar hook length in Salmonella enterica

The bacterial flagellum consists of a long external filament connected to a membrane-embedded basal body at the cell surface by a short curved structure called the hook. In Salmonella enterica, the hook extends 55 nm from the cell surface. FliK, a secreted molecular ruler, controls hook length. Upon hook completion, FliK induces a secretion-specificity switch to filament-type substrate secretion. Here, we demonstrate that an infrequent ruler mechanism determines hook length. FliK is intermittently secreted during hook polymerization. The probability of the specificity switch is an increasing function of hook length. By uncoupling hook polymerization from FliK expression, we illustrate that FliK secretion immediately triggers the specificity switch in hooks greater than the physiological length. The experimental data display excellent agreement with a mathematical model of the infrequent ruler hypothesis. Merodiploid bacteria expressing simultaneously short and long ruler variants displayed hook-length control by the short ruler, further supporting the infrequent ruler model. Finally, the velocity of FliK secretion determines the probability of a productive FliK interaction with the secretion apparatus to change secretion substrate specificity.

The NMR structure of FliK, the trigger for the switch of substrate specificity in the flagellar type III secretion apparatus

The flagellar cytoplasmic protein FliK controls hook elongation by two successive events: by determining hook length and by stopping the supply of hook protein. These two distinct roles are assigned to different parts of FliK: the N-terminal half (FliK(N)) determines length and the C-terminal half (FliK(C)) switches secretion from the hook protein to the filament protein. The interaction of FliK(C) with FlhB, the switchable secretion gate, triggers the switch. By NMR spectroscopy, we demonstrated that FliK is largely unstructured and determined the structure of a compact domain in FliK(C). The compact domain, denoted the FliK(C) core domain, consists of two α-helices, a β-sheet with two parallel and two antiparallel strands, and several exposed loops. Based on the functional data obtained by a series of deletion mutants of the FliK(C) core domain, we constructed a model of the complex between the FliK(C) core domain and FlhB(C). The model suggested that one of the FliK(C) loops has a high probability of interacting with the C-terminal domain of FlhB (FlhB(C)) as the FliK molecule enters the secretion gate. We suggest that the autocleaved NPTH sequence in FlhB contacts loop 2 of FliK(C) to trigger the switching event. This contact is sterically prevented when NPTH is not cleaved. Thus, the structure of FliK provides insight into the mechanism by which this bifunctional protein triggers a switch in the export of substrates.

Structure-function analysis of the HrpB2-HrcU interaction in the Xanthomonas citri type III secretion system

Bacterial type III secretion systems deliver protein virulence factors to host cells. Here we characterize the interaction between HrpB2, a small protein secreted by the Xanthomonas citri subsp. citri type III secretion system, and the cytosolic domain of the inner membrane protein HrcU, a paralog of the flagellar protein FlhB. We show that a recombinant fragment corresponding to the C-terminal cytosolic domain of HrcU produced in E. coli suffers cleavage within a conserved Asn264-Pro265-Thr266-His267 (NPTH) sequence. A recombinant HrcU cytosolic domain with N264A, P265A, T266A mutations at the cleavage site (HrcU_AAAH) was not cleaved and interacted with HrpB2. Furthermore, a polypeptide corresponding to the sequence following the NPTH cleavage site also interacted with HrpB2 indicating that the site for interaction is located after the NPTH site. Non-polar deletion mutants of the hrcU and hrpB2 genes resulted in a total loss of pathogenicity in susceptible citrus plants and disease symptoms could be recovered by expression of HrpB2 and HrcU from extrachromossomal plasmids. Complementation of the ΔhrcU mutant with HrcU_AAAH produced canker lesions similar to those observed when complemented with wild-type HrcU. HrpB2 secretion however, was significantly reduced in the ΔhrcU mutant complemented with HrcU_AAAH, suggesting that an intact and cleavable NPTH site in HrcU is necessary for total functionally of T3SS in X. citri subsp. citri. Complementation of the ΔhrpB2 X. citri subsp. citri strain with a series of hrpB2 gene mutants revealed that the highly conserved HrpB2 C-terminus is essential for T3SS-dependent development of citrus canker symptoms in planta.

Genetic analysis of the bacterial hook-capping protein FlgD responsible for hook assembly

FlgD of Salmonella enterica is a 232 aa protein that acts as the hook cap to promote assembly of FlgE into the hook structure. The N-terminal 86 residues (FlgD(N)) complement flgD mutants, albeit to a small degree. However, little is known about the role of the C-terminal region of FlgD (FlgD(C)). Here we isolated pseudorevertants from Salmonella flgE mutants. About half of the extragenic mutations lay within FlgD(C) and only one in FlgD(N). These suppressor mutations prevented mutant FlgE subunits from leaking out to some degree. Two weakly motile flgD mutants encoding C-terminally truncated variants, FlgD₁₋₁₉₅ and FlgD((1-138f-s+4aa)), secreted larger amounts of FlgE into the culture medium than wild-type cells. Their hooks were shorter, and their length distributions were broader, with significant tailing towards smaller values. These results suggest that FlgD(C) contributes to efficient hook polymerization. Therefore, we propose that FlgD(N) attaches to the distal end of the hook to promote hook polymerization and that FlgD(C) blocks the exit of newly exported FlgE monomers into the culture medium, allowing FlgE to have more time to assemble into the hook.

The type III secretion injectisome, a complex nanomachine for intracellular 'toxin' delivery

The type III secretion injectisome is a nanomachine that delivers bacterial proteins into the cytosol of eukaryotic target cells. It consists of a cylindrical basal structure spanning the two bacterial membranes and the peptidoglycan, connected to a hollow needle, eventually followed by a filament (animal pathogens) or to a long pilus (plant pathogens). Export employs a type III pathway. During assembly, all the protein subunits of external elements are sequentially exported by the basal structure itself, implying that the export apparatus can switch its substrate specificity over time. The length of the needle is controlled by a protein that it also secreted during assembly and presumably acts as a molecular ruler.

Secretion of early and late substrates of the type III secretion system from Xanthomonas is controlled by HpaC and the C-terminal domain of HrcU

The plant pathogenic bacterium Xanthomonas campestris pv. vesicatoria utilizes a type III secretion (T3S) system to inject effector proteins into eukaryotic cells. T3S substrate specificity is controlled by HpaC, which promotes secretion of translocon and effector proteins but prevents efficient secretion of the early substrate HrpB2. HpaC and HrpB2 interact with the C-terminal domain (HrcU(C) ) of the FlhB/YscU homologue HrcU. Here, we provide experimental evidence that HrcU is proteolytically cleaved at the conserved NPTH motif, which is required for binding of both HpaC and HrpB2 to HrcU(C) . The results of mutant studies showed that cleavage of HrcU contributes to pathogenicity and secretion of late substrates but is dispensable for secretion of HrpB2, which is presumably secreted prior to HrcU cleavage. The introduction of a point mutation (Y318D) into HrcU(C) activated secretion of late substrates in the absence of HpaC and suppressed the hpaC mutant phenotype. However, secretion of HrpB2 was unaffected by HrcU(Y318D) , suggesting that the export of early and late substrates is controlled by independent mechanisms that can be uncoupled. As HrcU(Y318D) did not interact with HrpB2 and HpaC, we propose that the substrate specificity switch leads to the release of HrcU(C) -bound HrpB2 and HpaC.