Structural and functional insights into the pilotin-secretin complex of the type II secretion system

Contribution of the type IV pili secretin tapQ to motility, growth, adhesion, stress tolerance and virulence of Aeromonas hydrophila

Aeromonas hydrophila (A. hydrophila) is a widely distributed opportunistic aquatic pathogen. Type IV pili (T4P) are regarded as key virulence factors of A. hydrophila that play a pivotal role in virulence, and tapQ is a component of T4P. However, the role and mechanism of tapQ in the pathogenicity of A. hydrophila are not clear. In this study, a stable genetically tapQ mutant strain of A. hydrophila (ΔtapQ-AH) was constructed. The median lethal dose value of ΔtapQ-AH in Carassius auratus was 0.82-fold that of wild-type strain of A. hydrophila (WT-AH), indicating that ΔtapQ-AH packs a stronger dose of virulence. The ΔtapQ-AH exhibited a significant increase in swimming, biofilm formation, adhesion, osmotic and oxidative stress resistance abilities compared with WT-AH. Twitching and growth abilities showed significant reductions. Exploring the molecular mechanism of the effect of tapQ gene deletion on virulence and biological properties, we found that tapQ gene mutation would affect gene expression of the phage shock protein (Psp) system, type II secretion system, flagellum, type IV pili, type VI secretion system and outer membrane protein. Taken together, this study initially reveals the role of the tapQ gene and offers novel insights into the mechanisms of virulence regulation in A. hydrophila.

Pilotins are mobile T3SS components involved in assembly and substrate specificity of the bacterial type III secretion system

In animal pathogens, assembly of the type III secretion system injectisome requires the presence of so-called pilotins, small lipoproteins that assist the formation of the secretin ring in the outer membrane. Using a combination of functional assays, interaction studies, proteomics, and live-cell microscopy, we determined the contribution of the pilotin to the assembly, function, and substrate selectivity of the T3SS and identified potential new downstream roles of pilotin proteins. In absence of its pilotin SctG, Yersinia enterocolitica forms few, largely polar injectisome sorting platforms and needles. Accordingly, most export apparatus subcomplexes are mobile in these strains, suggesting the absence of fully assembled injectisomes. Remarkably, while absence of the pilotin all but prevents export of early T3SS substrates, such as the needle subunits, it has little effect on secretion of late T3SS substrates, including the virulence effectors. We found that although pilotins interact with other injectisome components such as the secretin in the outer membrane, they mostly localize in transient mobile clusters in the bacterial membrane. Together, these findings provide a new view on the role of pilotins in the assembly and function of type III secretion injectisomes.

Insights into dynamics and gating properties of T2SS secretins

Secretins are outer membrane (OM) channels found in various bacterial nanomachines that secrete or assemble large extracellular structures. High-resolution 3D structures of type 2 secretion system (T2SS) secretins revealed bimodular channels with a C-module, holding a conserved central gate and an optional top gate, followed by an N-module for which multiple structural organizations have been proposed. Here, we perform a structure-driven in vivo study of the XcpD secretin, which validates one of the organizations of the N-module whose flexibility enables alternative conformations. We also show the existence of the central gate in vivo and its required flexibility, which is key for substrate passage and watertightness control. Last, functional, genomic, and phylogenetic analyses indicate that the optional top gate provides a gain of watertightness. Our data illustrate how the gating properties of T2SS secretins allow these large channels to overcome the duality between the necessity of preserving the OM impermeability while simultaneously promoting the secretion of large, folded effectors.

Scaffolding Protein GspB/OutB Facilitates Assembly of the Dickeya dadantii Type 2 Secretion System by Anchoring the Outer Membrane Secretin Pore to the Inner Membrane and to the Peptidoglycan Cell Wall

The phytopathogenic proteobacterium Dickeya dadantii secretes an array of plant cell wall-degrading enzymes and other virulence factors via the type 2 secretion system (T2SS). T2SSs are widespread among important plant, animal, and human bacterial pathogens. This multiprotein complex spans the double membrane cell envelope and secretes fully folded proteins through a large outer membrane pore formed by 15 subunits of the secretin GspD. Secretins are also found in the type 3 secretion system and the type 4 pili. Usually, specialized lipoproteins termed pilotins assist the targeting and assembly of secretins into the outer membrane. Here, we show that in D. dadantii, the pilotin acts in concert with the scaffolding protein GspB. Deletion of gspB profoundly impacts secretin assembly, pectinase secretion, and virulence. Structural studies reveal that GspB possesses a conserved periplasmic homology region domain that interacts directly with the N-terminal secretin domain. Site-specific photo-cross-linking unravels molecular details of the GspB-GspD complex in vivo. We show that GspB facilitates outer membrane targeting and assembly of the secretin pores and anchors them to the inner membrane while the C-terminal extension of GspB provides a scaffold for the secretin channel in the peptidoglycan cell wall. Phylogenetic analysis shows that in other bacteria, GspB homologs vary in length and domain composition and act in concert with either a cognate ATPase GspA or the pilotin GspS.

Functional dissection of structural regions of the Thermus thermophilus competence protein PilW: Implication in secretin complex stability, natural transformation and pilus functions

Uptake of DNA from the environment into the bacterial cytoplasm is mediated by a macromolecular transport machinery that is similar in structure and function to type IV pili (T4P) and, indeed, DNA translocator and T4P share common components. One is the secretin PilQ which is assembled into homopolymeric complexes forming highly dynamic outer membrane (OM) channels mediating pilus extrusion and DNA uptake. How PilQ interacts with the motor is still enigmatic. Here, we have used biochemical and genetic techniques to study the interaction of PilQ with PilW, a unique protein which is essential for natural transformation and T4P extrusion of T. thermophilus. PilQ and PilW form high molecular mass complexes in the OM of T. thermophilus. When pilW was deleted, PilQ complexes were no longer observed but only PilQ monomers, accompanied by a loss of DNA uptake as well as a loss of T4P and twitching motility. Piliation of a ΔpilT2/ΔpilW double mutant suggests that PilW is important for stable assembly of PilQ complexes. To analyze the role of different regions of PilW, partial deletions (pilW_∆2-40, pilW_∆50-150, pilW_∆163-216 and pilW_∆216-292) were generated and the effect on DNA uptake, PilQ complex formation and T4P functions such as twitching motility, biofilm formation and cell-cell interaction was studied. These studies revealed that a central disordered region in PilW is required for pilus dynamics. We propose that PilW is part of a protein network that connects the transport ATPase to drive different functions of the DNA translocator and T4P.

The structure and mechanism of the bacterial type II secretion system

The type II secretion system (T2SS) is a multi-protein complex used by many bacteria to move substrates across their cell membrane. Substrates released into the environment serve as local and long-range effectors that promote nutrient acquisition, biofilm formation, and pathogenicity. In both animals and plants, the T2SS is increasingly recognized as a key driver of virulence. The T2SS spans the bacterial cell envelope and extrudes substrates through an outer membrane secretin channel using a pseudopilus. An inner membrane assembly platform and a cytoplasmic motor controls pseudopilus assembly. This microreview focuses on the structure and mechanism of the T2SS. Advances in cryo-electron microscopy are enabling increasingly elaborate sub-complexes to be resolved. However, key questions remain regarding the mechanism of pseudopilus extension and retraction, and how this is coupled with the choreography of the substrate moving through the secretion system. The T2SS is part of an ancient type IV filament superfamily that may have been present within the last universal common ancestor (LUCA). Overall, mechanistic principles that underlie T2SS function have implication for other closely related systems such as the type IV and tight adherence pilus systems.

Characterization of the Pilotin-Secretin Complex from the Salmonella enterica Type III Secretion System Using Hybrid Structural Methods

The type III secretion system (T3SS) is a multi-membrane-spanning protein channel used by Gram-negative pathogenic bacteria to secrete effectors directly into the host cell cytoplasm. In the many species reliant on the T3SS for pathogenicity, proper assembly of the outer membrane secretin pore depends on a diverse family of lipoproteins called pilotins. We present structural and biochemical data on the Salmonella enterica pilotin InvH and the S domain of its cognate secretin InvG. Characterization of InvH by X-ray crystallography revealed a dimerized, α-helical pilotin. Size-exclusion-coupled multi-angle light scattering and small-angle X-ray scattering provide supporting evidence for the formation of an InvH homodimer in solution. Structures of the InvH-InvG heterodimeric complex determined by X-ray crystallography and NMR spectroscopy indicate a predominantly hydrophobic interface. Knowledge of the interaction between InvH and InvG brings us closer to understanding the mechanisms by which pilotins assemble the secretin pore.

Bacterial secretins: Mechanisms of assembly and membrane targeting

Secretion systems are employed by bacteria to transport macromolecules across membranes without compromising their integrities. Processes including virulence, colonization, and motility are highly dependent on the secretion of effector molecules toward the immediate cellular environment, and in some cases, into the host cytoplasm. In Type II and Type III secretion systems, as well as in Type IV pili, homomultimeric complexes known as secretins form large pores in the outer bacterial membrane, and the localization and assembly of such 1 MDa molecules often relies on pilotins or accessory proteins. Significant progress has been made toward understanding details of interactions between secretins and their partner proteins using approaches ranging from bacterial genetics to cryo electron microscopy. This review provides an overview of the mode of action of pilotins and accessory proteins for T2SS, T3SS, and T4PS secretins, highlighting recent near-atomic resolution cryo-EM secretin complex structures and underlining the importance of these interactions for secretin functionality.

Soluble versions of outer membrane cytochromes function as exporters for heterologously produced cargo proteins

This study reveals that it is possible to secrete truncated versions of outer membrane cytochromes into the culture supernatant and that these proteins can provide a basis for the export of heterologously produced proteins. Different soluble and truncated versions of the outer membrane cytochrome MtrF were analyzed for their suitability to be secreted. A protein version with a very short truncation of the N-terminus to remove the recognition sequence for the addition of a lipid anchor is secreted efficiently to the culture supernatant, and moreover this protein could be further truncated by a deletion of 160 amino acid and still is detectable in the supernatant. By coupling a cellulase to this soluble outer membrane cytochrome, the export efficiency was measured by means of relative cellulase activity. We conclude that outer membrane cytochromes of S. oneidensis can be applied as transporters for the export of target proteins into the medium using the type II secretion pathway.

Yeast can express and assemble bacterial secretins in the mitochondrial outer membrane

Secretins form large multimeric pores in the outer membrane (OM) of Gram-negative bacteria. These pores are part of type II and III secretion systems (T2SS and T3SS, respectively) and are crucial for pathogenicity. Recent structural studies indicate that secretins form a structure rich in β-strands. However, little is known about the mechanism by which secretins assemble into the OM. Based on the conservation of the biogenesis of β-barrel proteins in bacteria and mitochondria, we used yeast cells as a model system to study the assembly process of secretins. To that end, we analyzed the biogenesis of PulD (T2SS), SsaC (T3SS) and InvG (T3SS) in wild type cells or in cells mutated for known mitochondrial import and assembly factors. Our results suggest that secretins can be expressed in yeast cells, where they are enriched in the mitochondrial fraction. Interestingly, deletion of mitochondrial import receptors like Tom20 and Tom70 reduces the mitochondrial association of PulD but does not affect that of InvG. SsaC shows another dependency pattern and its membrane assembly is enhanced by the absence of Tom70 and compromised in cells lacking Tom20 or the topogenesis of outer membrane β-barrel proteins (TOB) complex component, Mas37. Collectively, these findings suggest that various secretins can follow different pathways to assemble into the bacterial OM.

Architecture, Function, and Substrates of the Type II Secretion System

The type II secretion system (T2SS) delivers toxins and a range of hydrolytic enzymes, including proteases, lipases, and carbohydrate-active enzymes, to the cell surface or extracellular space of Gram-negative bacteria. Its contribution to survival of both extracellular and intracellular pathogens as well as environmental species of proteobacteria is evident. This dynamic, multicomponent machinery spans the entire cell envelope and consists of a cytoplasmic ATPase, several inner membrane proteins, a periplasmic pseudopilus, and a secretin pore embedded in the outer membrane. Despite the trans-envelope configuration of the T2S nanomachine, proteins to be secreted engage with the system first once they enter the periplasmic compartment via the Sec or TAT export system. Thus, the T2SS is specifically dedicated to their outer membrane translocation. The many sequence and structural similarities between the T2SS and type IV pili suggest a common origin and argue for a pilus-mediated mechanism of secretion. This minireview describes the structures, functions, and interactions of the individual T2SS components and the general architecture of the assembled T2SS machinery and briefly summarizes the transport and function of a growing list of T2SS exoproteins. Recent advances in cryo-electron microscopy, which have led to an increased understanding of the structure-function relationship of the secretin channel and the pseudopilus, are emphasized.

Assembly and targeting of secretins in the bacterial outer membrane

In Gram-negative bacteria, secretion of toxins ensure the survival of the bacterium. Such toxins are secreted by sophisticated multiprotein systems. The most conserved part in some of these secretion systems are components, called secretins, which form the outer membrane ring in these systems. Recent structural studies shed some light on the oligomeric organization of secretins. However, the mechanisms by which these proteins are targeted to the outer membrane and assemble there into ring structures are still not fully understood. This review discusses the various species-specific targeting and assembly pathways that are taken by secretins in order to form their functional oligomers.

Structure and assembly of pilotin-dependent and -independent secretins of the type II secretion system

The type II secretion system (T2SS) is a cell envelope-spanning macromolecular complex that is prevalent in Gram-negative bacterial species. It serves as the predominant virulence mechanism of many bacteria including those of the emerging human pathogens Vibrio vulnificus and Aeromonas hydrophila. The system is composed of a core set of highly conserved proteins that assemble an inner membrane platform, a periplasmic pseudopilus and an outer membrane complex termed the secretin. Localization and assembly of secretins in the outer membrane requires recognition of secretin monomers by two different partner systems: an inner membrane accessory complex or a highly sequence-diverse outer membrane lipoprotein, termed the pilotin. In this study, we addressed the question of differential secretin assembly mechanisms by using cryo-electron microscopy to determine the structures of the secretins from A. hydrophila (pilotin-independent ExeD) and V. vulnificus (pilotin-dependent EpsD). These structures, at approximately 3.5 Å resolution, reveal pentadecameric stoichiometries and C-terminal regions that carry a signature motif in the case of a pilotin-dependent assembly mechanism. We solved the crystal structure of the V. vulnificus EpsS pilotin and confirmed the importance of the signature motif for pilotin-dependent secretin assembly by performing modelling with the C-terminus of EpsD. We also show that secretin assembly is essential for membrane integrity and toxin secretion in V. vulnificus and establish that EpsD requires the coordinated activity of both the accessory complex EpsAB and the pilotin EpsS for full assembly and T2SS function. In contrast, mutation of the region of the S-domain that is normally the site of pilotin interactions has little effect on assembly or function of the ExeD secretin. Since secretins are essential outer membrane channels present in a variety of secretion systems, these results provide a structural and functional basis for understanding the key assembly steps for different members of this vast pore-forming family of proteins.

Unraveling the Self-Assembly of the Pseudomonas aeruginosa XcpQ Secretin Periplasmic Domain Provides New Molecular Insights into Type II Secretion System Secreton Architecture and Dynamics

The type II secretion system (T2SS) releases large folded exoproteins across the envelope of many Gram-negative pathogens. This secretion process therefore requires specific gating, interacting, and dynamics properties mainly operated by a bipartite outer membrane channel called secretin. We have a good understanding of the structure-function relationship of the pore-forming C-terminal domain of secretins. In contrast, the high flexibility of their periplasmic N-terminal domain has been an obstacle in obtaining the detailed structural information required to uncover its molecular function. In Pseudomonas aeruginosa, the Xcp T2SS plays an important role in bacterial virulence by its capacity to deliver a large panel of toxins and degradative enzymes into the surrounding environment. Here, we revealed that the N-terminal domain of XcpQ secretin spontaneously self-assembled into a hexamer of dimers independently of its C-terminal domain. Furthermore, and by using multidisciplinary approaches, we elucidate the structural organization of the XcpQ N domain and demonstrate that secretin flexibility at interdimer interfaces is mandatory for its function.

Bacterial secretins are large homooligomeric proteins constituting the outer membrane pore-forming element of several envelope-embedded nanomachines essential in bacterial survival and pathogenicity. They comprise a well-defined membrane-embedded C-terminal domain and a modular periplasmic N-terminal domain involved in substrate recruitment and connection with inner membrane components. We are studying the XcpQ secretin of the T2SS present in the pathogenic bacterium Pseudomonas aeruginosa. Our data highlight the ability of the XcpQ N-terminal domain to spontaneously oligomerize into a hexamer of dimers. Further in vivo experiments revealed that this domain adopts different conformations essential for the T2SS secretion process. These findings provide new insights into the functional understanding of bacterial T2SS secretins.

The role of EscD in supporting EscC polymerization in the type III secretion system of enteropathogenic Escherichia coli

The type III secretion system (T3SS) is a multi-protein complex that plays a central role in the virulence of many Gram-negative bacterial pathogens. In enteropathogenic Escherichia coli, a prevalent cause of diarrheal diseases, the needle complex base of the T3SS is formed by multi-rings: two concentric inner-membrane rings made by the two oligomerizing proteins (EscD and EscJ), and an outer ring made of a single oligomerizing protein (EscC). Although the oligomerization activity of these proteins is critical for their function and can, therefore, affect the virulence of the pathogen, the mechanisms underlying the oligomerization of these proteins have yet to be identified. In this study, we report that the proteins forming the inner-membrane T3SS rings, EscJ and EscD proteins, are crucial for the oligomerization of EscC. Moreover, we elucidate the oligomerization process of EscD and determine the contribution of individual regions of the protein to its self-oligomerization activity. We show that the oligomerization motif of EscD is located at its N-terminal portion and that its transmembrane domain can self-oligomerize, thus contributing to the self-oligomerization of the full-length EscD.

The role of intrinsic disorder and dynamics in the assembly and function of the type II secretion system

Many Gram-negative commensal and pathogenic bacteria use a type II secretion system (T2SS) to transport proteins out of the cell. These exported proteins or substrates play a major role in toxin delivery, maintaining biofilms, replication in the host and subversion of host immune responses to infection. We review the current structural and functional work on this system and argue that intrinsically disordered regions and protein dynamics are central for assembly, exo-protein recognition, and secretion competence of the T2SS. The central role of intrinsic disorder-order transitions in these processes may be a particular feature of type II secretion.

Recognizing asymmetry in pseudo-symmetry; structural insights into the interaction between amphipathic α-helices and X-bundle proteins

An α-helix bundle is a small and compact protein fold always composed of more than 2 α-helices that typically run nearly parallel or antiparallel to each other. The repertoire of arrangements of α-helix bundle is such that these domains bind to a myriad of molecular entities including DNA, RNA, proteins and small molecules. A special instance of α-helical bundle is the X-type in which the arrangement of two α-helices interact at 45° to form an X. Among those, some X-helix bundle proteins bind to the hydrophobic section of an amphipathic α-helix in a seemingly orientation and sequence specific manner. In this review, we will compare the binding mode of amphipathic α-helices to X-helix bundle and α-helical bundle proteins. From these structures, we will highlight potential regulatory paradigms that may control the specific interactions of X-helix bundle proteins to amphipathic α-helices. This article is part of a Special Issue entitled: Biophysics in Canada, edited by Lewis Kay, John Baenziger, Albert Berghuis and Peter Tieleman.

Lipids assist the membrane insertion of a BAM-independent outer membrane protein

Like several other large, multimeric bacterial outer membrane proteins (OMPs), the assembly of the Klebsiella oxytoca OMP PulD does not rely on the universally conserved β-barrel assembly machinery (BAM) that catalyses outer membrane insertion. The only other factor known to interact with PulD prior to or during outer membrane targeting and assembly is the cognate chaperone PulS. Here, in vitro translation-transcription coupled PulD folding demonstrated that PulS does not act during the membrane insertion of PulD, and engineered in vivo site-specific cross-linking between PulD and PulS showed that PulS binding does not prevent membrane insertion. In vitro folding kinetics revealed that PulD is atypical compared to BAM-dependent OMPs by inserting more rapidly into membranes containing E. coli phospholipids than into membranes containing lecithin. PulD folding was fast in diC_14:0-phosphatidylethanolamine liposomes but not diC_14:0-phosphatidylglycerol liposomes, and in diC_18:1-phosphatidylcholine liposomes but not in diC_14:1-phosphatidylcholine liposomes. These results suggest that PulD efficiently exploits the membrane composition to complete final steps in insertion and explain how PulD can assemble independently of any protein-assembly machinery. Lipid-assisted assembly in this manner might apply to other large OMPs whose assembly is BAM-independent.

A novel pathway for outer membrane protein biogenesis in Gram-negative bacteria

The understanding of the biogenesis of the outer membrane of Gram‐negative bacteria is of critical importance due to the emergence of bacteria that are becoming resistant to available antibiotics. A problem that is most serious for Gram‐negative bacteria, with essentially few antibiotics under development or likely to be available for clinical use in the near future. The understanding of the Gram‐negative bacterial outer membrane is therefore critical to developing new antimicrobial agents, as this membrane makes direct contact with the external milieu, and the proteins present within this membrane are the instruments of microbial warfare, playing key roles in microbial pathogenesis, virulence and multidrug resistance. To date, a single outer membrane complex has been identified as essential for the folding and insertion of proteins into the outer membrane, this is the β‐barrel assembly machine (BAM) complex, which in some cases is supplemented by the Translocation and Assembly Module (TAM). In this issue of Molecular Microbiology, Dunstan et al. have identified a novel pathway for the insertion of a subset of integral membrane proteins into the Gram‐negative outer membrane that is independent of the BAM complex and TAM.

ExsB is required for correct assembly of the Pseudomonas aeruginosa type III secretion apparatus in the bacterial membrane and full virulence in vivo

Pseudomonas aeruginosa is responsible for high-morbidity infections of cystic fibrosis patients and is a major agent of nosocomial infections. One of its most potent virulence factors is a type III secretion system (T3SS) that injects toxins directly into the host cell cytoplasm. ExsB, a lipoprotein localized in the bacterial outer membrane, is one of the components of this machinery, of which the function remained elusive until now. The localization of the exsB gene within the exsCEBA regulatory gene operon suggested an implication in the T3SS regulation, while its similarity with yscW from Yersinia spp. argued in favor of a role in machinery assembly. The present work shows that ExsB is necessary for full in vivo virulence of P. aeruginosa. Furthermore, the requirement of ExsB for optimal T3SS assembly and activity is demonstrated using eukaryotic cell infection and in vitro assays. In particular, ExsB promotes the assembly of the T3SS secretin in the bacterial outer membrane, highlighting the molecular role of ExsB as a pilotin. This involvement in the regulation of the T3S apparatus assembly may explain the localization of the ExsB-encoding gene within the regulatory gene operon.

New features of the cell wall of the radio-resistant bacterium Deinococcus radiodurans

We have analyzed the cell wall of the radio-resistant bacterium Deinococcus radiodurans. Unexpectedly, the bacterial envelope appears to be organized in different complexes of high molecular weight. Each complex is composed of several proteins, most of which are coded by genes of unknown function and the majority are constituents of the inner/outer membrane system. One of the most abundant complexes is constituted by the gene DR_0774. This protein is a type of secretin which is a known subunit of the homo-oligomeric channel that represents the main bulk of the type IV piliation family. Finally, a minor component of the pink envelope consists of several inner-membrane proteins. The implications of these findings are discussed.

Peptidoglycan-binding protein TsaP functions in surface assembly of type IV pili

Type IV pili (T4P) are ubiquitous and versatile bacterial cell surface structures involved in adhesion to host cells, biofilm formation, motility, and DNA uptake. In Gram-negative bacteria, T4P pass the outer membrane (OM) through the large, oligomeric, ring-shaped secretin complex. In the β-proteobacterium Neisseria gonorrhoeae, the native PilQ secretin ring embedded in OM sheets is surrounded by an additional peripheral structure, consisting of a peripheral ring and seven extending spikes. To unravel proteins important for formation of this additional structure, we identified proteins that are present with PilQ in the OM. One such protein, which we name T4P secretin-associated protein (TsaP), was identified as a phylogenetically widely conserved component of the secretin complex that co-occurs with genes for T4P in Gram-negative bacteria. TsaP contains an N-terminal carbohydrate-binding lysin motif (LysM) domain and a C-terminal domain of unknown function. In N. gonorrhoeae, lack of TsaP results in the formation of membrane protrusions containing multiple T4P, concomitant with reduced formation of surface-exposed T4P. Lack of TsaP did not affect the oligomeric state of PilQ, but resulted in loss of the peripheral structure around the PilQ secretin. TsaP binds peptidoglycan and associates strongly with the OM in a PilQ-dependent manner. In the δ-proteobacterium Myxococcus xanthus, TsaP is also important for surface assembly of T4P, and it accumulates and localizes in a PilQ-dependent manner to the cell poles. Our results show that TsaP is a novel protein associated with T4P function and suggest that TsaP functions to anchor the secretin complex to the peptidoglycan.

Dynamic interplay between the periplasmic and transmembrane domains of GspL and GspM in the type II secretion system

The type II secretion system (T2SS) is a multiprotein nanomachine that transports folded proteins across the outer membrane of gram-negative bacteria. The molecular mechanisms that govern the secretion process remain poorly understood. The inner membrane components GspC, GspL and GspM possess a single transmembrane segment (TMS) and a large periplasmic region and they are thought to form a platform of unknown function. Here, using two-hybrid and pull-down assays we performed a systematic mapping of the GspC/GspL/GspM interaction regions in the plant pathogen Dickeya dadantii. We found that the TMS of these components interact with each other, implying a complex interaction network within the inner membrane. We also showed that the periplasmic, ferredoxin-like, domains of GspL and GspM drive homo- and heterodimerizations of these proteins. Disulfide bonding analyses revealed that the respective domain interfaces include the equivalent secondary-structure elements, suggesting alternating interactions of the periplasmic domains, L/L and M/M versus L/M. Finally, we found that displacements of the periplasmic GspM domain mediate coordinated shifts or rotations of the cognate TMS. These data suggest a plausible mechanism for signal transmission between the periplasmic and the cytoplasmic portions of the T2SS machine.

The targeting, docking and anti-proteolysis functions of the secretin chaperone PulS

The Klebsiella oxytoca lipoprotein PulS might function as either or both a pilot and a docking factor in the outer membrane targeting and assembly of the Type II secretion system secretin PulD. In the piloting model, PulS binds to PulD monomers and targets them to the outer membrane via the lipoprotein sorting pathway components LolA and LolB. In this model, PulS also protects the PulD monomers from proteolysis during transit through the periplasm. In the docking model, PulS is targeted alone to the outer membrane, where it acts as a receptor for PulD monomers, allowing them to accumulate and assemble specifically in this membrane. PulS was shown to dissociate from and/or re-associate freely with PulD multimers in zwitterionic detergent, making it difficult to determine whether PulS remains associated with PulD dodecamers in the outer membrane by co-purification. However, PulD protomers in the dodecamer were shown to be stable in the absence of PulS, indicating that PulS is only required to protect the protease-susceptible monomer. DegP was identified as one of the proteases that could contribute to PulD degradation in the absence of PulS. Studies on the in vitro assembly and targeting of PulD into Escherichia coli membrane vesicles demonstrated its strong preference to insert into the inner membrane, as is the case in vivo in the absence of PulS. However, PulD could be targeted to outer membrane fragments in vitro if they were preloaded with PulS, indicating the technical feasibility of the docking model. We conclude that both modes of action might contribute to efficient outer membrane targeting of PulD in vivo, although the piloting function is likely to predominate.

Crystal structure of the pilotin from the enterohemorrhagic Escherichia coli type II secretion system

Bacteria contain several sophisticated macromolecular machineries responsible for translocating proteins across the cell envelope. One prominent example is the type II secretion system (T2SS), which contains a large outer membrane channel, called the secretin. These gated channels require specialized proteins, so-called pilotins, to reach and assemble in the outer membrane. Here we report the crystal structure of the pilotin GspS from the T2SS of enterohemorrhagic Escherichia coli (EHEC), an important pathogen that can cause severe disease in cases of food poisoning. In this four-helix protein, the straight helix α2, the curved helix α3 and the bent helix α4 surround the central N-terminal helix α1. The helices of GspS create a prominent groove, mainly formed by side chains of helices α1, α2 and α3. In the EHEC GspS structure this groove is occupied by extra electron density which is reminiscent of an α-helix and corresponds well with a binding site observed in a homologous pilotin. The residues forming the groove are well conserved among homologs, pointing to a key role of this groove in this class of T2SS pilotins. At the same time, T2SS pilotins in different species can be entirely different in structure, and the pilotins for secretins in non-T2SS machineries have yet again unrelated folds, despite a common function. It is striking that a common complex function, such as targeting and assembling an outer membrane multimeric channel, can be performed by proteins with entirely different folds.

Assembly of the type II secretion system such as found in Vibrio cholerae depends on the novel Pilotin AspS

The Type II Secretion System (T2SS) is a molecular machine that drives the secretion of fully-folded protein substrates across the bacterial outer membrane. A key element in the machinery is the secretin: an integral, multimeric outer membrane protein that forms the secretion pore. We show that three distinct forms of T2SSs can be distinguished based on the sequence characteristics of their secretin pores. Detailed comparative analysis of two of these, the Klebsiella-type and Vibrio-type, showed them to be further distinguished by the pilotin that mediates their transport and assembly into the outer membrane. We have determined the crystal structure of the novel pilotin AspS from Vibrio cholerae, demonstrating convergent evolution wherein AspS is functionally equivalent and yet structurally unrelated to the pilotins found in Klebsiella and other bacteria. AspS binds to a specific targeting sequence in the Vibrio-type secretins, enhances the kinetics of secretin assembly, and homologs of AspS are found in all species of Vibrio as well those few strains of Escherichia and Shigella that have acquired a Vibrio-type T2SS.

The type 2 secretion system (T2SS) is a sophisticated, multi-component molecular machine that drives the secretion of fully-folded protein substrates across the bacterial outer membrane. In Vibrio cholerae, for example, the T2SS mediates the secretion of cholera toxin. We find that there are three distinct forms of T2SS, based on the sequence characteristics of the secretin. A targeting paradigm, developed for the Klebsiella-type secretin PulD, could not previously be applied to the T2SS in Vibrio cholerae and many other bacterial species whose genomes encode no homolog of the crucial targeting factor PulS (also called OutS, EtpO or GspS). Using bioinformatics we find, remarkably, that these bacteria have instead evolved a structurally distinct protein to serve in place of PulS. We crystallized and solved the structure of this distinct factor, AspS, measured its activity in novel assays for T2SS assembly, and show that the protein is essential for the function of the Vibrio-type T2SS. A structural homolog of AspS found here in Pseudomonas suggests widespread use of the pilotin-secretin targeting paradigm for T2SS assembly.

Structure and assembly of a trans-periplasmic channel for type IV pili in Neisseria meningitidis

Type IV pili are polymeric fibers which protrude from the cell surface and play a critical role in adhesion and invasion by pathogenic bacteria. The secretion of pili across the periplasm and outer membrane is mediated by a specialized secretin protein, PilQ, but the way in which this large channel is formed is unknown. Using NMR, we derived the structures of the periplasmic domains from N. meningitidis PilQ: the N-terminus is shown to consist of two β-domains, which are unique to the type IV pilus-dependent secretins. The structure of the second β-domain revealed an eight-stranded β-sandwich structure which is a novel variant of the HSP20-like fold. The central part of PilQ consists of two α/β fold domains: the structure of the first of these is similar to domains from other secretins, but with an additional α-helix which links it to the second α/β domain. We also determined the structure of the entire PilQ dodecamer by cryoelectron microscopy: it forms a cage-like structure, enclosing a cavity which is approximately 55 Å in internal diameter at its largest extent. Specific regions were identified in the density map which corresponded to the individual PilQ domains: this allowed us to dock them into the cryoelectron microscopy density map, and hence reconstruct the entire PilQ assembly which spans the periplasm. We also show that the C-terminal domain from the lipoprotein PilP, which is essential for pilus assembly, binds specifically to the first α/β domain in PilQ and use NMR chemical shift mapping to generate a model for the PilP:PilQ complex. We conclude that passage of the pilus fiber requires disassembly of both the membrane-spanning and the β-domain regions in PilQ, and that PilP plays an important role in stabilising the PilQ assembly during secretion, through its anchorage in the inner membrane.

A Single Amino Acid Substitution Changes the Self-Assembly Status of a Type IV Piliation Secretin

Secretins form large multimeric complexes in the outer membranes of many Gram-negative bacteria, where they function as dedicated gateways that allow proteins to access the extracellular environment. Despite their overall relatedness, different secretins use different specific and general mechanisms for their targeting, assembly, and membrane insertion. We report that all tested secretins from several type II secretion systems and from the filamentous bacteriophage f1 can spontaneously multimerize and insert into liposomes in an in vitro transcription-translation system. Phylogenetic analyses indicate that these secretins form a group distinct from the secretins of the type IV piliation and type III secretion systems, which do not autoassemble in vitro. A mutation causing a proline-to-leucine substitution allowed PilQ secretins from two different type IV piliation systems to assemble in vitro, albeit with very low efficiency, suggesting that autoassembly is an inherent property of all secretins.

Cysteine scanning mutagenesis and disulfide mapping analysis of arrangement of GspC and GspD protomers within the type 2 secretion system

The type II secretion system (T2SS) secretes enzymes and toxins across the outer membrane of Gram-negative bacteria. The precise assembly of T2SS, which consists of at least 12 core-components called Gsp, remains unclear. The outer membrane secretin, GspD, forms the channels, through which folded proteins are secreted, and interacts with the inner membrane component, GspC. The periplasmic regions of GspC and GspD consist of several structural domains, HR(GspC) and PDZ(GspC), and N0(GspD) to N3(GspD), respectively, and recent structural and functional studies have proposed several interaction sites between these domains. We used cysteine mutagenesis and disulfide bonding analysis to investigate the organization of GspC and GspD protomers and to map their interaction sites within the secretion machinery of the plant pathogen Dickeya dadantii. At least three distinct GspC-GspD interactions were detected, and they involve two sites in HR(GspC), two in N0(GspD), and one in N2(GspD). None of these interactions occurs through static interfaces because the same sites are also involved in self-interactions with equivalent neighboring domains. Disulfide self-bonding of critical interaction sites halts secretion, indicating the transient nature of these interactions. The secretion substrate diminishes certain interactions and provokes an important rearrangement of the HR(GspC) structure. The T2SS components OutE/L/M affect various interaction sites differently, reinforcing some but diminishing the others, suggesting a possible switching mechanism of these interactions during secretion. Disulfide mapping shows that the organization of GspD and GspC subunits within the T2SS could be compatible with a hexamer of dimers arrangement rather than an organization with 12-fold rotational symmetry.