A single-domain FlgJ contributes to flagellar hook and filament formation in the Lyme disease spirochete Borrelia burgdorferi

Diving into the complexity of the spirochetal endoflagellum

Spirochaetes, a phylum that includes medically important pathogens such as the causative agents of Lyme disease, syphilis, and leptospirosis, are in many ways highly unique bacteria. Their cell morphology, subcellular organization, and metabolism reveal atypical features. Spirochetal motility is also singular, dependent on the presence of periplasmic flagella or endoflagella, inserted subterminally at cell poles and not penetrating the outer membrane and elongating outside the cell as in enterobacteria. In this review we present a comprehensive comparative genomics analysis of endoflagellar systems in spirochetes, highlighting recent findings on the flagellar basal body and filament. Continued progress in understanding the function and architecture of spirochetal flagella is uncovering paradigm-shifting mechanisms of bacterial motility.

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.

BB0259 Encompasses a Peptidoglycan Lytic Enzyme Function for Proper Assembly of Periplasmic Flagella in Borrelia burgdorferi

Assembly of the bacterial flagellar rod, hook, and filament requires penetration through the peptidoglycan (PG) sacculus and outer membrane. In most β- and γ-proteobacteria, the protein FlgJ has two functional domains that enable PG hydrolyzing activity to create pores, facilitating proper assembly of the flagellar rod. However, two distinct proteins performing the same functions as the dual-domain FlgJ are proposed in δ- and ε-proteobacteria as well as spirochetes. The Lyme disease spirochete Borrelia burgdorferi genome possesses a FlgJ and a PG lytic SLT enzyme protein homolog (BB0259). FlgJ in B. burgdorferi is crucial for flagellar hook and filament assembly but not for the proper rod assembly reported in other bacteria. However, BB0259 has never been characterized. Here, we use cryo-electron tomography to visualize periplasmic flagella in different bb0259 mutant strains and provide evidence that the E580 residue of BB0259 is essential for PG-hydrolyzing activity. Without the enzyme activity, the flagellar hook fails to penetrate through the pores in the cell wall to complete assembly of an intact periplasmic flagellum. Given that FlgJ and BB0259 interact with each other, they likely coordinate the penetration through the PG sacculus and assembly of a functional flagellum in B. burgdorferi and other spirochetes. Because of its role, we renamed BB0259 as flagellar-specific lytic transglycosylase or LTaseBb.

The Role of the Flagellar Protein FlgJ in the Virulence of Brucella abortus

Brucella abortus is a facultative intracellular pathogen that causes a zoonosis called brucellosis. This disease leads to abortion and infertility in cattle, and diverse complications in humans. B. abortus is a successful intracellular bacterium that has developed the ability to evade the host's immune system and it replicates in professional and non-professional phagocytic cells, persisting in the different tissues, and organs of its hosts. It has been described that Brucella expresses a polar flagellum under certain conditions, but its function is still unknown. In this study we evaluated the role of the FlgJ, a protein, presumably a peptidoglycan hydrolase involved in flagellum formation and in the virulence of B. abortus strain 2308. B. abortus 2308 ΔflgJ mutant and complemented strains were constructed to study the function of the FlgJ protein in the context of the virulence of this pathogen in in vitro and in vivo assays. The results showed that the elimination of the flgJ gene delays the growth rate of B. abortus in culture, reduces its intracellular survival capacity in professional and non-professional phagocytic cells, rendering it unable to escape from the endocytic route and not reaching the endoplasmic reticulum. It also negatively affects their persistence in BALB/c mice. Functionally, the B. abortus 2308 flgJ gene restored motility to an E. coli flgJ mutant gene. Furthermore, it was discovered that the production of FlgJ protein is associated with the bacterial adherence by B. abortus. Therefore, although the specific function of the polar flagellum for Brucella is unknown, the data indicates that the flagellar flgJ gene and its product are required for full virulence of B. abortus 2308, since its deletion significantly reduces the fitness of this pathogen in vitro and in vivo.

FlhF regulates the number and configuration of periplasmic flagella in Borrelia burgdorferi

The Lyme disease bacterium Borrelia burgdorferi has 7-11 periplasmic flagella (PF) that arise from the cell poles and extend toward the midcell as a flat-ribbon, which is distinct from other bacteria. FlhF, a signal recognition particle (SRP)-like GTPase, has been found to regulate the flagellar number and polarity; however, its role in B. burgdorferi remains unknown. B. burgdorferi has an FlhF homolog (BB0270). Structural and biochemical analyses show that BB0270 has a similar structure and enzymatic activity as its counterparts from other bacteria. Genetics and cryo-electron tomography studies reveal that deletion of BB0270 leads to mutant cells that have less PF (4 ± 2 PF per cell tip) and fail to form a flat-ribbon, indicative of a role of BB0270 in the control of PF number and configuration. Mechanistically, we demonstrate that BB0270 localizes at the cell poles and controls the number and position of PF via regulating the flagellar protein stability and the polar localization of the MS-ring protein FliF. Our study not only provides the detailed characterizations of BB0270 and its profound impacts on flagellar assembly, morphology and motility in B. burgdorferi, but also unveils mechanistic insights into how spirochetes control their unique flagellar patterns.

The flagella of 'Candidatus Liberibacter asiaticus' and its movement in planta

Citrus huanglongbing (HLB) is the most devastating citrus disease worldwide. 'Candidatus Liberibacter asiaticus' (Las) is the most prevalent HLB causal agent that is yet to be cultured. Here, we analysed the flagellar genes of Las and Rhizobiaceae and observed two characteristics unique to the flagellar proteins of Las: (i) a shorter primary structure of the rod capping protein FlgJ than other Rhizobiaceae bacteria and (ii) Las contains only one flagellin-encoding gene flaA (CLIBASIA_02090), whereas other Rhizobiaceae species carry at least three flagellin-encoding genes. Only flgJ_Atu but not flgJ_Las restored the swimming motility of Agrobacterium tumefaciens flgJ mutant. Pull-down assays demonstrated that FlgJ_Las interacts with FlgB but not with FliE. Ectopic expression of flaA_Las in A. tumefaciens mutants restored the swimming motility of ∆flaA mutant and ∆flaAD mutant, but not that of the null mutant ∆flaABCD. No flagellum was observed for Las in citrus and dodder. The expression of flagellar genes was higher in psyllids than in planta. In addition, western blotting using flagellin-specific antibody indicates that Las expresses flagellin protein in psyllids, but not in planta. The flagellar features of Las in planta suggest that Las movement in the phloem is not mediated by flagella. We also characterized the movement of Las after psyllid transmission into young flush. Our data support a model that Las remains inside young flush after psyllid transmission and before the flush matures. The delayed movement of Las out of young flush after psyllid transmission provides opportunities for targeted treatment of young flush for HLB control.

Cryo-electron tomography of periplasmic flagella in Borrelia burgdorferi reveals a distinct cytoplasmic ATPase complex

Periplasmic flagella are essential for the distinct morphology and motility of spirochetes. A flagella-specific type III secretion system (fT3SS) composed of a membrane-bound export apparatus and a cytosolic ATPase complex is responsible for the assembly of the periplasmic flagella. Here, we deployed cryo-electron tomography (cryo-ET) to visualize the fT3SS machine in the Lyme disease spirochete Borrelia burgdorferi. We show, for the first time, that the cytosolic ATPase complex is attached to the flagellar C-ring through multiple spokes to form the “spoke and hub” structure in B. burgdorferi. This structure not only strengthens structural rigidity of the round-shaped C-ring but also appears to rotate with the C-ring. Our studies provide structural insights into the unique mechanisms underlying assembly and rotation of the periplasmic flagella and may provide the basis for the development of novel therapeutic strategies against several pathogenic spirochetes.

Cryo-electron tomography of periplasmic flagella in the Lyme disease bacterium Borrelia burgdorferi reveals it to have a distinct cytoplasmic ATPase complex and an atypical interaction with the flagellar C-ring.

Type III secretion systems are widely utilized by gram-negative bacteria to assemble flagella or to transport virulence effectors into eukaryotic cells. The central component is known as a type III secretion machine, which consists of a membrane-bound export apparatus and a cytosolic ATPase complex. Powered by the proton motive force and ATP hydrolysis, the secretion machine is responsible for substrate recognition and export. Here, we use the Lyme disease spirochete B. burgdorferi as a model system to unveil unprecedented structural details of the intact flagellar secretion machine by high-throughput cryo-electron tomography (cryo-ET) and subtomogram averaging. We provide the first structural evidence that the cytosolic ATPase complex is attached to the flagellar C-ring through multiple spokes to form the “spoke and hub” structure in B. burgdorferi. The novel architecture of the ATPase complex not only strengthens the flagellar C-ring but also enables an optimal translocation of substrates through the ATPase complex and the export apparatus.

A novel glycan modifies the flagellar filament proteins of the oral bacterium Treponema denticola

While protein glycosylation has been reported in several spirochetes including the syphilis bacterium Treponema pallidum and Lyme disease pathogen Borrelia burgdorferi, the pertinent glycan structures and their roles remain uncharacterized. Herein, a novel glycan with an unusual chemical composition and structure in the oral spirochete Treponema denticola, a keystone pathogen of periodontitis was reported. The identified glycan of mass 450.2 Da is composed of a monoacetylated nonulosonic acid (Non) with a novel extended N7 acyl modification, a 2-methoxy-4,5,6-trihydroxy-hexanoyl residue in which the Non has a pseudaminic acid configuration (L-glycero-L-manno) and is β-linked to serine or threonine residues. This novel glycan modifies the flagellin proteins (FlaBs) of T. denticola by O-linkage at multiple sites near the D1 domain, a highly conserved region of bacterial flagellins that interact with Toll-like receptor 5. Furthermore, mutagenesis studies demonstrate that the glycosylation plays an essential role in the flagellar assembly and motility of T. denticola. To our knowledge, this novel glycan and its unique modification sites have not been reported previously in any bacteria.

A new view into prokaryotic cell biology from electron cryotomography

Electron cryotomography (ECT) enables intact cells to be visualized in 3D in an essentially native state to 'macromolecular' (∼4 nm) resolution, revealing the basic architectures of complete nanomachines and their arrangements in situ. Since its inception, ECT has advanced our understanding of many aspects of prokaryotic cell biology, from morphogenesis to subcellular compartmentalization and from metabolism to complex interspecies interactions. In this Review, we highlight how ECT has provided structural and mechanistic insights into the physiology of bacteria and archaea and discuss prospects for the future.

Hypothetical Protein BB0569 Is Essential for Chemotaxis of the Lyme Disease Spirochete Borrelia burgdorferi

The Lyme disease spirochete Borrelia burgdorferi has five putative methyl-accepting chemotaxis proteins (MCPs). In this report, we provide evidence that a hypothetical protein, BB0569, is essential for the chemotaxis of B. burgdorferi. While BB0569 lacks significant homology to the canonical MCPs, it contains a conserved domain (spanning residues 110 to 170) that is often evident in membrane-bound MCPs such as Tar and Tsr of Escherichia coli. Unlike Tar and Tsr, BB0569 lacks transmembrane regions and recognizable HAMP and methylation domains and is similar to TlpC, a cytoplasmic chemoreceptor of Rhodobacter sphaeroides. An isogenic mutant of BB0569 constantly runs in one direction and fails to respond to attractants, indicating that BB0569 is essential for chemotaxis. Immunofluorescence, green fluorescent protein (GFP) fusion, and cryo-electron tomography analyses demonstrate that BB0569 localizes at the cell poles and is required for chemoreceptor clustering at the cell poles. Protein cross-linking studies reveal that BB0569 forms large protein complexes with MCP3, indicative of its interactions with other MCPs. Interestingly, analysis of B. burgdorferi mcp mutants shows that inactivation of either mcp2 or mcp3 reduces the level of BB0569 substantially and that such a reduction is caused by protein turnover. Collectively, these results demonstrate that the domain composition and function of BB0569 are similar in some respects to those of TlpC but that these proteins are different in their cellular locations, further highlighting that the chemotaxis of B. burgdorferi is unique and different from the Escherichia coli and Salmonella enterica paradigm.

IMPORTANCE

Spirochete chemotaxis differs substantially from the Escherichia coli and Salmonella enterica paradigm, and the basis for controlling the rotation of the bundles of periplasmic flagella at each end of the cell is unknown. In recent years, Borrelia burgdorferi, the causative agent of Lyme disease, has been used as a model organism to understand spirochete chemotaxis and its role in infectious processes of the disease. In this report, BB0569, a hypothetical protein of B. burgdorferi, has been investigated by using an approach of genetic, biochemistry, and cryo-electron tomography analyses. The results indicate that BB0569 has a distinct role in chemotaxis that may be unique to spirochetes and represents a novel paradigm.

The structure and regulation of flagella in Bacillus subtilis

Bacterial flagellar motility is among the most extensively studied physiological systems in biology, but most research has been restricted to using the highly similar Gram-negative species Escherichia coli and Salmonella enterica. Here, we review the recent advances in the study of flagellar structure and regulation of the distantly related and genetically tractable Gram-positive bacterium Bacillus subtilis. B. subtilis has a thicker layer of peptidoglycan and lacks the outer membrane of the Gram-negative bacteria; thus, not only phylogenetic separation but also differences in fundamental cell architecture contribute to deviations in flagellar structure and regulation. We speculate that a large number of flagella and the absence of a periplasm make B. subtilis a premier organism for the study of the earliest events in flagellar morphogenesis and the type III secretion system. Furthermore, B. subtilis has been instrumental in the study of heterogeneous gene transcription in subpopulations and of flagellar regulation at the translational and functional level.

Insights into the biology of Borrelia burgdorferi gained through the application of molecular genetics

Borrelia burgdorferi, the vector-borne bacterium that causes Lyme disease, was first identified in 1982. It is known that much of the pathology associated with Lyme borreliosis is due to the spirochete's ability to infect, colonize, disseminate, and survive within the vertebrate host. Early studies aimed at defining the biological contributions of individual genes during infection and transmission were hindered by the lack of adequate tools and techniques for molecular genetic analysis of the spirochete. The development of genetic manipulation techniques, paired with elucidation and annotation of the B. burgdorferi genome sequence, has led to major advancements in our understanding of the virulence factors and the molecular events associated with Lyme disease. Since the dawn of this genetic era of Lyme research, genes required for vector or host adaptation have garnered significant attention and highlighted the central role that these components play in the enzootic cycle of this pathogen. This chapter covers the progress made in the Borrelia field since the application of mutagenesis techniques and how they have allowed researchers to begin ascribing roles to individual genes. Understanding the complex process of adaptation and survival as the spirochete cycles between the tick vector and vertebrate host will lead to the development of more effective diagnostic tools as well as identification of novel therapeutic and vaccine targets. In this chapter, the Borrelia genes are presented in the context of their general biological roles in global gene regulation, motility, cell processes, immune evasion, and colonization/dissemination.

Cryoelectron tomography reveals the sequential assembly of bacterial flagella in Borrelia burgdorferi

Periplasmic flagella are essential for the distinctive morphology, motility, and infectious life cycle of the Lyme disease spirochete Borrelia burgdorferi. In this study, we genetically trapped intermediates in flagellar assembly and determined the 3D structures of the intermediates to 4-nm resolution by cryoelectron tomography. We provide structural evidence that secretion of rod substrates triggers remodeling of the central channel in the flagellar secretion apparatus from a closed to an open conformation. This open channel then serves as both a gateway and a template for flagellar rod assembly. The individual proteins assemble sequentially to form a modular rod. The hook cap initiates hook assembly on completion of the rod, and the filament cap facilitates filament assembly after formation of the mature hook. Cryoelectron tomography and mutational analysis thus combine synergistically to provide a unique structural blueprint of the assembly process of this intricate molecular machine in intact cells.

Motility is crucial for the infectious life cycle of Borrelia burgdorferi

The Lyme disease spirochete, Borrelia burgdorferi, exists in a zoonotic cycle involving an arthropod tick and mammalian host. Dissemination of the organism within and between these hosts depends upon the spirochete's ability to traverse through complex tissues. Additionally, the spirochete outruns the host immune cells while migrating through the dermis, suggesting the importance of B. burgdorferi motility in evading host clearance. B. burgdorferi's periplasmic flagellar filaments are composed primarily of a major protein, FlaB, and minor protein, FlaA. By constructing a flaB mutant that is nonmotile, we investigated for the first time the absolute requirement for motility in the mouse-tick life cycle of B. burgdorferi. We found that whereas wild-type cells are motile and have a flat-wave morphology, mutant cells were nonmotile and rod shaped. These mutants were unable to establish infection in C3H/HeN mice via either needle injection or tick bite. In addition, these mutants had decreased viability in fed ticks. Our studies provide substantial evidence that the periplasmic flagella, and consequently motility, are critical not only for optimal survival in ticks but also for infection of the mammalian host by the arthropod tick vector.

The C terminus of the flagellar muramidase SltF modulates the interaction with FlgJ in Rhodobacter sphaeroides

Macromolecular structures such as the bacterial flagellum in Gram-negative bacteria must traverse the cell wall. Lytic transglycosylases are capable of enlarging gaps in the peptidoglycan meshwork to allow the efficient assembly of supramolecular complexes. We have previously shown that in Rhodobacter sphaeroides SltF, the flagellar muramidase, and FlgJ, a flagellar scaffold protein, are separate entities that interact in the periplasm. In this study we show that the export of SltF to the periplasm is dependent on the SecA pathway. A deletion analysis of the C-terminal portion of SltF shows that this region is required for SltF-SltF interaction. These C terminus-truncated mutants lose the capacity to interact with themselves and also bind FlgJ with higher affinity than does the wild-type protein. We propose that this region modulates the interaction with the scaffold protein FlgJ during the assembly process.