The structure of the conserved type six secretion protein TssL (DotU) from Francisella novicida

Pathogenicity and virulence of Francisella tularensis

Tularaemia is a zoonotic disease caused by the Gram-negative bacterium, Francisella tularensis. Depending on its entry route into the organism, F. tularensis causes different diseases, ranging from life-threatening pneumonia to less severe ulceroglandular tularaemia. Various strains with different geographical distributions exhibit different levels of virulence. F. tularensis is an intracellular bacterium that replicates primarily in the cytosol of the phagocytes. The main virulence attribute of F. tularensis is the type 6 secretion system (T6SS) and its effectors that promote escape from the phagosome. In addition, F. tularensis has evolved a peculiar envelope that allows it to escape detection by the immune system. In this review, we cover tularaemia, different Francisella strains, and their pathogenicity. We particularly emphasize the intracellular life cycle, associated virulence factors, and metabolic adaptations. Finally, we present how F. tularensis largely escapes immune detection to be one of the most infectious and lethal bacterial pathogens.

Structure and Function of the Dot/Icm T4SS

The Legionella pneumophila Dot/Icm type IV secretion system (T4SS) delivers effector proteins into host cells during infection. Despite its significance as a potential drug target, our current understanding of its atomic structure is limited to isolated subcomplexes. In this study, we used subtomogram averaging and integrative modeling to construct a nearly-complete model of the Dot/Icm T4SS accounting for seventeen protein components. We locate and provide insights into the structure and function of six new components including DotI, DotJ, DotU, IcmF, IcmT, and IcmX. We find that the cytosolic N-terminal domain of IcmF, a key protein forming a central hollow cylinder, interacts with DotU, providing insight into previously uncharacterized density. Furthermore, our model, in combination with analyses of compositional heterogeneity, explains how the cytoplasmic ATPase DotO is connected to the periplasmic complex via interactions with membrane-bound DotI/DotJ proteins. Coupled with in situ infection data, our model offers new insights into the T4SS-mediated secretion mechanism.

Structural Characterization of TssL from Acinetobacter baumannii: a Key Component of the Type VI Secretion System

The type VI secretion system (T6SS) is a complex molecular nanomachine used by Gram-negative bacteria to deliver diverse effectors into adjacent cells. A membrane complex (MC) anchors this transport system to the bacterial cell wall. One of the proteins forming the MC is TssL, a cytoplasmic protein bound to the inner membrane through a single transmembrane helix. Here, we report the structure of the cytoplasmic N-terminal region of TssL from Acinetobacter baumannii, a bacterium encoding in a single locus a secretion system that is a special case among other T6SSs. The protein structure, consisting of two antiparallel alpha-helical bundles connected by a short loop, reveals several interesting particularities compared with homologous proteins from other organisms. In addition, we demonstrate the structural significance of residues Asp98 and Glu99, which are strongly conserved among T6SS-encoding Gram-negative bacteria. Mutations in these two residues strongly impact protein dynamics, expression, and functionality. Our results improve our understanding of the T6SS of A. baumannii, which remains largely understudied compared with that of other pathogens.IMPORTANCE Several Acinetobacter species carry one functional type VI secretion system (T6SS). The T6SS is encoded in a single locus containing 16 conserved genes, most of which code for proteins essential to T6SS activity. One of these key components is TssL, a cytoplasmic protein bound to the inner membrane. Despite its importance and its particular characteristics, the structure of T6SS in A. baumannii remains understudied. Here, we present structural, in silico, and in vivo studies of TssL, highlighting the importance of two well-conserved residues and improving our understanding of this secretion system in this bacterium.

Atomic Structure of the Francisella T6SS Central Spike Reveals a Unique α-Helical Lid and a Putative Cargo

Francisella bacteria rely on a phylogenetically distinct type VI secretion system (T6SS) to escape host phagosomes and cause the fatal disease tularemia, but the structural and molecular mechanisms involved are unknown. Here we report the atomic structure of the Francisella T6SS central spike complex, obtained by cryo-electron microscopy. Our structural and functional studies demonstrate that, unlike the single-protein spike composition of other T6SS subtypes, Francisella T6SS's central spike is formed by two proteins, PdpA and VgrG, akin to T4-bacteriophage gp27 and gp5, respectively, and that PdpA has unique characteristics, including a putative cargo within its cavity and an N-terminal helical lid. Structure-guided mutagenesis demonstrates that the PdpA N-terminal lid and C-terminal spike are essential to Francisella T6SS function. PdpA is thus both an adaptor, connecting VgrG to the tube, and a likely carrier of secreted cargo. These findings are important to understanding Francisella pathogenicity and designing therapeutics to combat tularemia.

Structure and Activity of the Type VI Secretion System

The type VI secretion system (T6SS) is a multiprotein machine that uses a spring-like mechanism to inject effectors into target cells. The injection apparatus is composed of a baseplate on which is built a contractile tail tube/sheath complex. The inner tube, topped by the spike complex, is propelled outside of the cell by the contraction of the sheath. The injection system is anchored to the cell envelope and oriented towards the cell exterior by a trans-envelope complex. Effectors delivered by the T6SS are loaded within the inner tube or on the spike complex and can target prokaryotic and/or eukaryotic cells. Here we summarize the structure, assembly, and mechanism of action of the T6SS. We also review the function of effectors and their mode of recruitment and delivery.

In situ and high-resolution cryo-EM structure of a bacterial type VI secretion system membrane complex

Bacteria have evolved macromolecular machineries that secrete effectors and toxins to survive and thrive in diverse environments. The type VI secretion system (T6SS) is a contractile machine that is related to Myoviridae phages. It is composed of a phage tail-like structure inserted in the bacterial cell envelope by a membrane complex (MC) comprising the TssJ, TssL and TssM proteins. We previously reported the low-resolution negative-stain electron microscopy structure of the enteroaggregative Escherichia coli MC and proposed a rotational 5-fold symmetry with a TssJ:TssL:TssM stoichiometry of 2:2:2. Here, cryo-electron tomography analyses of the T6SS MC confirm the 5-fold symmetry in situ and identify the regions of the structure that insert into the bacterial membranes. A high-resolution model obtained by single-particle cryo-electron microscopy highlights new features: five additional copies of TssJ, yielding a TssJ:TssL:TssM stoichiometry of 3:2:2, an 11-residue loop in TssM, protruding inside the lumen of the MC and constituting a functionally important periplasmic gate, and hinge regions. Based on these data, we propose an updated model on MC structure and dynamics during T6SS assembly and function.

Crystal structure of the periplasmic domain of TssL, a key membrane component of Type VI secretion system

Type VI secretion system (T6SS), as a macromolecular system, is commonly found in Gram-negative bacteria and responsible for exporting effectors. T6SS consists of 13 core proteins. TssL is a component of the membrane complex and plays a pivotal role in T6SS. Here, we report the crystal structure of the C-terminal periplasmic domain of TssL (TssL_Cter) from Serratia marcescens FS14. The TssL_Cter (310-503) contain a five-stranded anti-parallel β-sheet flanked by five α-helices and a short N-terminal helix. Structural comparisons revealed that it belongs to the OmpA-like family with a remarked difference in the conformation of the loop3-5. In OmpA-like family, the corresponding loop is located close to loop2-3, forming a cavity with a small opening together with the longest α5, whereas in TssL_Cter, loop3-5 flipped away from this cavity region. In addition, significant differences in the peptidoglycan (PG) binding site suggest that big conformational change must take place to accomplish the PG binding for TssL_Cter. Furthermore, a long flexible loop between helices α1 and α2 is unique in TssL. TssL would have a big conformational change during the delivery of the Hcp needle and effectors. So we speculate that the long flexible endows TssL the adaptation of its evolutionary new function.

The Francisella Type VI Secretion System

Francisella tularensisis subsp. tularensis is an intracellular bacterial pathogen and the causative agent of the life-threatening zoonotic disease tularemia. The Francisella Pathogenicity Island encodes a large secretion apparatus, known as a Type VI Secretion System (T6SS), which is essential for Francisella to escape from its phagosome and multiply within host macrophages and to cause disease in animals. The T6SS, found in one-quarter of Gram-negative bacteria including many highly pathogenic ones, is a recently discovered secretion system that is not yet fully understood. Nevertheless, there have been remarkable advances in our understanding of the structure, composition, and function of T6SSs of several bacteria in the past few years. The system operates like an inside-out headless contractile phage that is anchored to the bacterial membrane via a baseplate and membrane complex. The system injects effector molecules across the inner and outer bacterial membrane and into host prokaryotic or eukaryotic targets to kill, intoxicate, or in the case of Francisella, hijack the target cell. Recent advances include an atomic model of the contractile sheath, insights into the mechanics of sheath contraction, the composition of the baseplate and membrane complex, the process of assembly of the apparatus, and identification of numerous effector molecules and activities. While Francisella T6SS appears to be an outlier among T6SSs, with limited or no sequence homology with other systems, its structure and organization are strikingly similar to other systems. Nevertheless, we have only scratched the surface in uncovering the mysteries of the Francisella T6SS, and there are numerous questions that remain to be answered.

Type VI secretion system: secretion by a contractile nanomachine

The type VI secretion systems (T6SS) are present in about a quarter of all Gram-negative bacteria. Several key components of T6SS are evolutionarily related to components of contractile nanomachines such as phages and R-type pyocins. The T6SS assembly is initiated by formation of a membrane complex that binds a phage-like baseplate with a sharp spike, and this is followed by polymerization of a long rigid inner tube and an outer contractile sheath. Effectors are preloaded onto the spike or into the tube during the assembly by various mechanisms. Contraction of the sheath releases an unprecedented amount of energy, which is used to thrust the spike and tube with the associated effectors out of the effector cell and across membranes of both bacterial and eukaryotic target cells. Subunits of the contracted sheath are recycled by T6SS-specific unfoldase to allow for a new round of assembly. Live-cell imaging has shown that the assembly is highly dynamic and its subcellular localization is in certain bacteria regulated with a remarkable precision. Through the action of effectors, T6SS has mainly been shown to contribute to pathogenicity and competition between bacteria. This review summarizes the knowledge that has contributed to our current understanding of T6SS mode of action.

Production, crystallization and X-ray diffraction analysis of a complex between a fragment of the TssM T6SS protein and a camelid nanobody

The type VI secretion system (T6SS) is a machine evolved by Gram-negative bacteria to deliver toxin effectors into target bacterial or eukaryotic cells. The T6SS is functionally and structurally similar to the contractile tail of the Myoviridae family of bacteriophages and can be viewed as a syringe anchored to the bacterial membrane by a transenvelope complex. The membrane complex is composed of three proteins: the TssM and TssL inner membrane components and the TssJ outer membrane lipoprotein. The TssM protein is central as it interacts with both TssL and TssJ, therefore linking the membranes. Using controlled trypsinolysis, a 32.4 kDa C-terminal fragment of enteroaggregative Escherichia coli TssM (TssM32Ct) was purified. A nanobody obtained from llama immunization, nb25, exhibited subnanomolar affinity for TssM32Ct. Crystals of the TssM32Ct-nb25 complex were obtained and diffracted to 1.9 Å resolution. The crystals belonged to space group P64, with unit-cell parameters a = b = 95.23, c = 172.95 Å. Molecular replacement with a model nanobody indicated the presence of a dimer of TssM32Ct-nb25 in the asymmetric unit.

Crystal structure of the bacterial type VI secretion system component TssL from Vibrio cholerae

The type VI secretion system (T6SS), commonly found in gram-negative bacteria, is responsible for exporting effector proteins. The T6SS has been reported to be cytotoxic to host cells. While the components and assembly of the T6SS complex have been largely assessed, structural data on T6SS components from virulent bacteria is remarkably insufficient. Here, we report the crystal structure of Vibrio cholerae TssL (VcTssL), a core component of T6SS. In spite of a relatively low sequence identity, the overall structure of VcTssL is largely similar to those from other bacterial homologs except for several differences found in local structural elements. A unique feature attributed to the C-terminal fragment of VcTssL is a crystallographic artifact. This incidental feature of VcTssL may provide insights into screening of molecular partners for the cytoplasmic domain of TssL. Additionally, our results may help in the design of molecular probes for a detailed understanding of the functional relationship between TssL and other T6SS components.

Purification, crystallization and preliminary X-ray crystallographic analysis of TssL from Vibrio cholerae

The type VI secretion system (T6SS) is a macromolecular complex that is conserved in Gram-negative bacteria. The T6SS secretes effector proteins into recipient cells in a contact-dependent manner in order to accomplish cooperative and competitive interactions with the cells. Although the composition and mechanism of the T6SS have been intensively investigated across many Gram-negative bacteria, to date structural information on T6SS components from the important pathogen Vibrio cholerae has been rare. Here, the cloning, purification, crystallization and preliminary X-ray crystallographic analysis of the cytoplasmic domain of TssL, an inner membrane protein of the T6SS, from V. cholerae are reported. Diffraction data were collected to 1.5 Å resolution using synchrotron radiation. The crystal belonged to the hexagonal space group P61, with unit-cell parameters a = 78.4, b = 78.4, c = 49.5 Å. The successful structural characterization of TssL from V. cholerae will contribute to understanding the role of the membrane-associated subunits of the T6SS in more detail.

Architecture and assembly of the Type VI secretion system

The Type VI secretion system (T6SS) delivers protein effectors to diverse cell types including prokaryotic and eukaryotic cells, therefore it participates in inter-bacterial competition and pathogenesis. The T6SS is constituted of an envelope-spanning complex anchoring a cytoplasmic tubular edifice. This tubular structure is evolutionarily, functionally and structurally related to the tail of contractile phages. It is composed of an inner tube tipped by a spike complex, and engulfed within a sheath-like structure. This structure assembles onto a platform called "baseplate" that is connected to the membrane sub-complex. The T6SS functions as a nano-crossbow: upon contraction of the sheath, the inner tube is propelled towards the target cell, allowing effector delivery. This review focuses on the architecture and biogenesis of this fascinating secretion machine, highlighting recent advances regarding the assembly of the membrane or tail complexes. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Fha interaction with phosphothreonine of TssL activates type VI secretion in Agrobacterium tumefaciens

The type VI secretion system (T6SS) is a widespread protein secretion system found in many Gram-negative bacteria. T6SSs are highly regulated by various regulatory systems at multiple levels, including post-translational regulation via threonine (Thr) phosphorylation. The Ser/Thr protein kinase PpkA is responsible for this Thr phosphorylation regulation, and the forkhead-associated (FHA) domain-containing Fha-family protein is the sole T6SS phosphorylation substrate identified to date. Here we discovered that TssL, the T6SS inner-membrane core component, is phosphorylated and the phosphorylated TssL (p-TssL) activates type VI subassembly and secretion in a plant pathogenic bacterium, Agrobacterium tumefaciens. Combining genetic and biochemical approaches, we demonstrate that TssL is phosphorylated at Thr 14 in a PpkA-dependent manner. Further analysis revealed that the PpkA kinase activity is responsible for the Thr 14 phosphorylation, which is critical for the secretion of the T6SS hallmark protein Hcp and the putative toxin effector Atu4347. TssL phosphorylation is not required for the formation of the TssM-TssL inner-membrane complex but is critical for TssM conformational change and binding to Hcp and Atu4347. Importantly, Fha specifically interacts with phosphothreonine of TssL via its pThr-binding motif in vivo and in vitro and this interaction is crucial for TssL interaction with Hcp and Atu4347 and activation of type VI secretion. In contrast, pThr-binding ability of Fha is dispensable for TssM structural transition. In conclusion, we discover a novel Thr phosphorylation event, in which PpkA phosphorylates TssL to activate type VI secretion via its direct binding to Fha in A. tumefaciens. A model depicting an ordered TssL phosphorylation-induced T6SS assembly pathway is proposed.

The bacterial type VI secretion system (T6SS) resembles a contractile phage tail structure and functions to deliver effectors to eukaryotic or prokaryotic target cells for the survival of many pathogenic bacteria. T6SS is highly regulated by various regulatory systems at multiple levels in response to environmental cues. Post-translational regulation via threonine (Thr) phosphorylation is an emerging theme in regulating prokaryotic signaling, including T6SS; the knowledge is mainly contributed by studies of Hcp secretion island 1-encoded T6SS (H1-T6SS) of Pseudomonas aeruginosa. Here, we discover a new phosphorylated target, a T6SS core-component TssL, and demonstrate that this Thr phosphorylation event post-translationally regulates type VI secretion in a plant pathogenic bacterium, Agrobacterium tumefaciens. We provide the first demonstration that the specific binding of Fha, a forkhead-associated domain-containing protein, to the phosphorylated target is required to stimulate type VI secretion. Genetic and biochemical data strongly suggest an ordered TssL-phosphorylation–dependent assembly and secretion pathway.

Structure of the T6SS lipoprotein TssJ1 from Pseudomonas aeruginosa

The type VI secretion system of Pseudomonas aeruginosa has been shown to be responsible for the translocation of bacteriolytic effectors into competing bacteria. A mechanistic understanding of this widely distributed secretion system is developing and structural studies of its components are ongoing. Two representative structures of one highly conserved component, TssJ, from Escherichia coli and Serratia marcescens have been published. Here, the X-ray crystal structure of TssJ1 from P. aeruginosa is presented at 1.4 Å resolution. The overall structure is conserved among the three proteins. This finding suggests that the homologues function in a similar manner and bolsters the understanding of the structure of this family of proteins.

Identification of Francisella novicida mutants that fail to induce prostaglandin E(2) synthesis by infected macrophages

Francisella tularensis is the causative agent of tularemia. We have previously shown that infection with F. tularensis Live Vaccine Strain (LVS) induces macrophages to synthesize prostaglandin E₂ (PGE₂). Synthesis of PGE₂ by F. tularensis infected macrophages results in decreased T cell proliferation in vitro and increased bacterial survival in vivo. Although we understand some of the biological consequences of F. tularensis induced PGE₂ synthesis by macrophages, we do not understand the cellular pathways (neither host nor bacterial) that result in up-regulation of the PGE₂ biosynthetic pathway in F. tularensis infected macrophages. We took a genetic approach to begin to understand the molecular mechanisms of bacterial induction of PGE₂ synthesis from infected macrophages. To identify F. tularensis genes necessary for the induction of PGE₂ in primary macrophages, we infected cells with individual mutants from the closely related strain F. tularensis subspecies novicida U112 (U112) two allele mutant library. Twenty genes were identified that when disrupted resulted in U112 mutant strains unable to induce the synthesis of PGE₂ by infected macrophages. Fourteen of the genes identified are located within the Francisella pathogenicity island (FPI). Genes in the FPI are required for F. tularensis to escape from the phagosome and replicate in the cytosol, which might account for the failure of U112 with transposon insertions within the FPI to induce PGE₂. This implies that U112 mutant strains that do not grow intracellularly would also not induce PGE₂. We found that U112 clpB::Tn grows within macrophages yet fails to induce PGE₂, while U112 pdpA::Tn does not grow yet does induce PGE₂. We also found that U112 iglC::Tn neither grows nor induces PGE₂. These findings indicate that there is dissociation between intracellular growth and the ability of F. tularensis to induce PGE₂ synthesis. These mutants provide a critical entrée into the pathways used in the host for PGE₂ induction.