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

Bacterial One- and Two-Hybrid Assays to Monitor Transmembrane Helix Interactions

In transenvelope multiprotein machines such as bacterial secretion systems, protein-protein interactions not only occur between soluble domains but might also be mediated by helix-helix contacts in the inner membrane. Several assays have been therefore developed to test homotypic and heterotypic interactions between transmembrane α-helices in their native membrane environment. Here, we provide detailed protocols for two genetic assays, TOXCAT and GALLEX, which are based on the reconstitution of dimeric regulators allowing the control of expression of reporter genes.

The periplasmic coiled coil formed by the assembly platform proteins PulL and PulM is critical for function of the Klebsiella type II secretion system

Bacteria use type II secretion systems (T2SS) to secrete to their surface folded proteins that confer diverse functions, from nutrient acquisition to virulence. In the Klebsiella species, T2SS-mediated secretion of pullulanase (PulA) requires assembly of a dynamic filament called the endopilus. The inner membrane assembly platform (AP) subcomplex is essential for endopilus assembly and PulA secretion. AP components PulL and PulM interact with each other through their C-terminal globular domains and transmembrane segments. Here, we investigated the roles of their periplasmic helices, predicted to form a coiled coil, in assembly and function of the PulL-PulM complex. PulL and PulM variants lacking these periplasmic helices were defective for interaction in the bacterial two-hybrid (BACTH) assay. Their functions in PulA secretion and assembly of PulG subunits into endopilus filaments were strongly reduced. Interestingly, deleting the cytoplasmic peptide of PulM nearly abolished the function of variant PulMΔN and its interaction with PulG, but not with PulL, in the BACTH assay. Nevertheless, PulL was specifically proteolyzed in the presence of the PulMΔN variant, suggesting that PulM N-terminal peptide stabilizes PulL in the cytoplasm. We discuss the implications of these results for the T2S endopilus and type IV pilus assembly mechanisms.

Structure and dynamic association of an assembly platform subcomplex of the bacterial type II secretion system

Type II secretion systems (T2SSs) allow diderm bacteria to secrete hydrolytic enzymes, adhesins, or toxins important for growth and virulence. To promote secretion of folded proteins, T2SSs assemble periplasmic filaments called pseudopili or endopili at an inner membrane subcomplex, the assembly platform (AP). Here, we combined biophysical approaches, nuclear magnetic resonance (NMR) and X-ray crystallography, to study the Klebsiella AP components PulL and PulM. We determined the structure and associations of their periplasmic domains and describe the structure of the heterodimer formed by their ferredoxin-like domains. We show how structural complementarity and plasticity favor their association during the secretion process. Cysteine scanning and crosslinking data provided additional constraints to build a structural model of the PulL-PulM assembly in the cellular context. Our structural and functional insights, together with the relative cellular abundance of its components, support the role of AP as a dynamic hub that orchestrates pilus polymerization.

Pathogenicity Islands in Uropathogenic Escherichia coli Clinical Isolate of the Globally Disseminated O25:H4-ST131 Pandemic Clonal Lineage: First Report from Egypt

Uropathogenic Escherichia coli (UPEC) is the main etiological agent of urinary tract infections (UTIs). The pathogenesis of UTIs relies upon UPEC's acquisition of virulence determinants that are commonly inserted into large chromosomal blocks which are termed 'pathogenicity islands' (PAIs). In this study, we investigated the virulence-associated genes embedded in the chromosome of a UPEC Egyptian strain, EC14142. Additionally, we present a detailed characterization of the PAIs in the EGY_EC14142 chromosome. The isolate displayed a multidrug-resistant phenotype, and whole genome sequencing indicated that it belonged to the globally disseminated O25:H4-ST131 pandemic lineage and the H30-Rx clade. EGY_EC14142 carried genes that are responsible for resistance to aminoglycosides, fluoroquinolones, extended-spectrum β-lactams, macrolides, folate pathway antagonists, and tetracyclines. It encoded five PAIs with a high similarity to PAI II536, PAI IV536, PAI V536, PAI-536-icd, and PAIusp. The genome analysis of EGY_EC14142 with other closely related UPEC strains revealed that they have a high nucleotide sequence identity. The constructed maximum-likelihood phylogenetic tree showed the close clonality of EGY_EC14142 with the previously published ST131 UPEC international isolates, thus endorsing the broad geographical distribution of this clone. This is the first report characterizing PAIs in a UPEC Egyptian strain belonging to the globally disseminated pandemic clone O25:H4-ST131.

Genomic Characterization of International High-Risk Clone ST410 Escherichia coli Co-Harboring ESBL-Encoding Genes and blaNDM-5 on IncFIA/IncFIB/IncFII/IncQ1 Multireplicon Plasmid and Carrying a Chromosome-Borne blaCMY-2 from Egypt

The accelerated dispersion of multidrug-resistant (MDR) Escherichia coli due to the production of extended-spectrum β-lactamases (ESBLs) or AmpC enzymes has been noted in Egypt, presenting a serious treatment challenge. In this study, we investigate the prevalence of ESBLs and AmpC enzymes among 48 E. coli isolates collected from patients with urinary tract infections admitted to a teaching hospital in Alexandria. Phenotypic and genotypic methods of detection are conducted. Isolates producing both enzymes are tested for the mobilization of their genes by a broth mating experiment. Whole genome sequencing (WGS) is performed for isolate EC13655. The results indicate that 80% of the isolates are MDR, among which 52% and 13% were ESBL and AmpC producers, respectively. Conjugation experiments fail to show the mobilization of blaCMY-2 in EC13655, which was chosen for WGS. In silico analysis reveals that the isolate belongs to a ST410-H24Rx high-risk clone. It coharbors the ESBL-encoding genes blaCTX-M-15, blaTEM-1, and blaOXA-1 on an IncFIA/IncFIB/IncFII/IncQ1 multireplicon plasmid. The chromosomal location of blaCMY-2 is detected with a flanking upstream copy of ISEcp1. This chromosomal integration of blaCMY-2 establishes the stable maintenance of the gene and thus, necessitates an imperative local surveillance to reduce further spread of such strains in different clinical settings.

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.

1H, 15 N and 13C resonance assignments of the C-terminal domain of PulL, a component of the Klebsiella oxytoca type II secretion system

Type II secretion systems (T2SS) allow Gram-negative bacteria to transport toxins and enzymes from the periplasm to the external milieu, and are thus important for the pathogenicity of bacteria. To drive secretion, T2SS assemble filaments called pseudopili closely related to bacterial type IV pili. These filaments are non-covalent polymers of proteins that are assembled by an inner membrane complex called the assembly platform connected to a cytoplasmic ATPase motor. In the Klebsiella oxytoca T2SS, the PulL protein from the assembly platform is essential for pseudopilus assembly and protein secretion. However, its role in these processes is not well understood. To decipher the molecular basis of PulL function, we used solution NMR to study its structure and interactions with other components of the machinery. Here as a first step, we report the 1H, 15 N and 13C backbone and side-chain chemical shift assignments of the C-terminal periplasmic domain of PulL and its secondary structure based on NMR data.

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.

Core architecture of a bacterial type II secretion system

Bacterial type II secretion systems (T2SSs) translocate virulence factors, toxins and enzymes across the cell outer membrane. Here we use negative stain and cryo-electron microscopy to reveal the core architecture of an assembled T2SS from the pathogen Klebsiella pneumoniae. We show that 7 proteins form a ~2.4 MDa complex that spans the cell envelope. The outer membrane complex includes the secretin PulD, with all domains modelled, and the pilotin PulS. The inner membrane assembly platform components PulC, PulE, PulL, PulM and PulN have a relative stoichiometric ratio of 2:1:1:1:1. The PulE ATPase, PulL and PulM combine to form a flexible hexameric hub. Symmetry mismatch between the outer membrane complex and assembly platform is overcome by PulC linkers spanning the periplasm, with PulC HR domains binding independently at the secretin base. Our results show that the T2SS has a highly dynamic modular architecture, with implication for pseudo-pilus assembly and substrate loading.

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.

Structure and oligomerization of the periplasmic domain of GspL from the type II secretion system of Pseudomonas aeruginosa

The ability of bacteria to infect a host relies in part on the secretion of molecular virulence factors across the cell envelope. Pseudomonas aeruginosa, a ubiquitous environmental bacterium causing opportunistic infections in humans, employs the type II secretion system (T2SS) to transport effector proteins across its cellular envelope as part of a diverse array of virulence strategies. General secretory pathway protein L (GspL) is an essential inner-membrane component of the T2SS apparatus, and is thought to facilitate transduction of the energy from ATP hydrolysis in the cytoplasm to the periplasmic components of the system. However, our incomplete understanding of the assembly principles of the T2SS machinery prevents the mechanistic deconvolution of T2SS-mediated protein secretion. Here we show via two crystal structures that the periplasmic ferredoxin-like domain of GspL (GspLfld) is a dimer stabilized by hydrophobic interactions, and that this interface may allow significant interdomain plasticity. The general dimerization mode of GspLfld is shared with GspL from Vibrio parahaemolyticus suggesting a conserved oligomerization mode across the GspL family. Furthermore, we identified a tetrameric form of the complete periplasmic segment of GspL (GspLperi) which indicates that GspL may be able to adopt multiple oligomeric states as part of its dynamic role in the T2SS apparatus.

Direct interactions between the secreted effector and the T2SS components GspL and GspM reveal a new effector-sensing step during type 2 secretion

In many Gram-negative bacteria, the type 2 secretion system (T2SS) plays an important role in virulence because of its capacity to deliver a large amount of fully folded protein effectors to the extracellular milieu. Despite our knowledge of most T2SS components, the mechanisms underlying effector recruitment and secretion by the T2SS remain enigmatic. Using complementary biophysical and biochemical approaches, we identified here two direct interactions between the secreted effector CbpD and two components, XcpY_L and XcpZ_M, of the T2SS assembly platform (AP) in the opportunistic pathogen Pseudomonas aeruginosa Competition experiments indicated that CbpD binding to XcpY_L is XcpZ_M-dependent, suggesting sequential recruitment of the effector by the periplasmic domains of these AP components. Using a bacterial two-hybrid system, we then tested the influence of the effector on the AP protein-protein interaction network. Our findings revealed that the presence of the effector modifies the AP interactome and, in particular, induces XcpZ_M homodimerization and increases the affinity between XcpY_L and XcpZ_M The observed direct relationship between effector binding and T2SS dynamics suggests an additional synchronizing step during the type 2 secretion process, where the activation of the AP of the T2SS nanomachine is triggered by effector binding.

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.

Polar N-terminal Residues Conserved in Type 2 Secretion Pseudopilins Determine Subunit Targeting and Membrane Extraction Steps during Fibre Assembly

Bacterial type 2 secretion systems (T2SS), type 4 pili, and archaeal flagella assemble fibres from initially membrane-embedded pseudopilin and pilin subunits. Fibre subunits are made as precursors with positively charged N-terminal anchors, whose cleavage via the prepilin peptidase, essential for pilin membrane extraction and assembly, is followed by N-methylation of the mature (pseudo)pilin N terminus. The conserved Glu residue at position 5 (E5) of mature (pseudo)pilins is essential for assembly. Unlike T4 pilins, where E5 residue substitutions also abolish N-methylation, the E5A variant of T2SS pseudopilin PulG remains N-methylated but is affected in interaction with the T2SS component PulM. Here, biochemical and functional analyses showed that the PulM interaction defect only partly accounts for the PulG^E5A assembly defect. First, PulG^T2A variant, equally defective in PulM interaction, remained partially functional. Furthermore, pseudopilus assembly defect of pulG(E5A) mutant was stronger than that of the pulM deletion mutant. To understand the dominant effect of E5A mutation, we used molecular dynamics simulations of PulG^E5A, methylated PulG^WT (MePulG^WT), and MePulG^E5A variant in a model membrane. These simulations pointed to a key role for an intramolecular interaction between the pseudopilin N-terminal amine and E5 to limit polar interactions with membrane phospholipids. N-methylation of the N-terminal amine further limited its interactions with phospholipid head-groups to facilitate pseudopilin membrane escape. By binding to polar residues in the conserved N-terminal region of PulG, we propose that PulM acts as chaperone to promote pseudopilin recruitment and coordinate its membrane extraction with subsequent steps of the fibre assembly process.

Fusion Reporter Approaches to Monitoring Transmembrane Helix Interactions in Bacterial Membranes

In transenvelope multiprotein machines such as bacterial secretion systems, protein-protein interactions not only occur between soluble domains but might also be mediated by helix-helix contacts in the inner membrane. Here we describe genetic assays commonly used to test interactions between transmembrane α-helices in their native membrane environment. These assays are based on the reconstitution of dimeric regulators allowing the control of expression of reporter genes. We provide detailed protocols for the TOXCAT and GALLEX assays used to monitor homotypic and heterotypic transmembrane helix-helix interactions.

The secretome of Acinetobacter baumannii ATCC 17978 type II secretion system reveals a novel plasmid encoded phospholipase that could be implicated in lung colonization

Acinetobacter baumannii infections are compounded with a striking lack of treatment options. In many Gram-negative bacteria, secreted proteins play an important early role in avoiding host defences. Typically, these proteins are targeted to the external environment or into host cells using dedicated transport systems. Despite the fact that medically relevant species of Acinetobacter possess a type II secretion system (T2SS), only recently, its significance as an important pathway for delivering virulence factors has gained attention. Using in silico analysis to characterize the genetic determinants of the T2SS, which are found clustered in other organisms, in Acinetobacter species, they appear to have a unique genetic organization and are distributed throughout the genome. When compared to other T2SS orthologs, individual components of the T2SS apparatus showed the highest similarity to those of Pseudomonas aeruginosa. A mutant of Acinetobacter baumannii strain ATCC 17978 lacking the secretin component of the T2SS (ΔgspD), together with a trans-complemented mutant, were tested in a series of in vitro and in vivo assays to determine the role of T2SS in pathogenicity. The ΔgspD mutant displayed decreased lipolytic activity, associated with attenuated colonization ability in a murine pneumonia model. These phenotypes are linked to LipAN, a novel plasmid-encoded phospholipase, identified through mass spectroscopy as a T2SS substrate. Recombinant LipAN showed specific phospholipase activity in vitro. Proteomics on the T2-dependent secretome of ATCC 17978 strain revealed its potential dedication to the secretion of a number of lipolytic enzymes, among others which could contribute to its virulence. This study highlights the role of T2SS as an active contributor to the virulence of A. baumannii potentially through secretion of a newly identified phospholipase.

Type IV Pilus Alignment Subcomplex Proteins PilN and PilO Form Homo- and Heterodimers in Vivo

Pseudomonas aeruginosa is a leading cause of hospital-acquired infections and is resistant to many antibiotics. Type IV pili (T4P) are among the key virulence factors used by P. aeruginosa for host cell attachment, biofilm formation, and twitching motility, making this system a promising target for novel therapeutics. Point mutations in the conserved PilMNOP alignment subcomplex were previously shown to have distinct effects on assembly and disassembly of T4P, suggesting that it may function in a dynamic manner. We introduced mutations encoding Cys substitutions into pilN and/or pilO on the chromosome to maintain normal stoichiometry and expression levels and captured covalent PilNO heterodimers, as well as PilN and PilO homodimers, in vivo Most covalent PilN or PilO homodimers had minimal functional impact in P. aeruginosa, suggesting that homodimers are a physiologically relevant state. However, certain covalent homo- or heterodimers eliminated twitching motility, suggesting that specific PilNO configurations are essential for T4P function. These data were verified using soluble N-terminal truncated fragments of PilN and PilO Cys mutants, which purified as a mixture of homo- and heterodimers at volumes consistent with a tetramer. Deletion of genes encoding alignment subcomplex components, PilM or PilP, but not other T4P components, including the motor ATPases PilB or PilT, blocked in vivo formation of disulfide-bonded PilNO heterodimers, suggesting that both PilM and PilP influence the heterodimer interface. Combined, our data suggest that T4P function depends on rearrangements at PilN and PilO interfaces.

Architecture of the type IVa pilus machine

Type IVa pili are filamentous cell surface structures observed in many bacteria. They pull cells forward by extending, adhering to surfaces, and then retracting. We used cryo-electron tomography of intact Myxococcus xanthus cells to visualize type IVa pili and the protein machine that assembles and retracts them (the type IVa pilus machine, or T4PM) in situ, in both the piliated and nonpiliated states, at a resolution of 3 to 4 nanometers. We found that T4PM comprises an outer membrane pore, four interconnected ring structures in the periplasm and cytoplasm, a cytoplasmic disc and dome, and a periplasmic stem. By systematically imaging mutants lacking defined T4PM proteins or with individual proteins fused to tags, we mapped the locations of all 10 T4PM core components and the minor pilins, thereby providing insights into pilus assembly, structure, and function.

Biogenesis of Pseudomonas aeruginosa type IV pili and regulation of their function

Type IV pili (T4P) are bacterial virulence factors involved in a wide variety of functions including deoxyribonucleic acid uptake, surface attachment, biofilm formation and twitching motility. While T4P are common surface appendages, the systems that assemble them and the regulation of their function differ between species. Pseudomonas aeruginosa, Neisseria spp. and Myxococcus xanthus are common model systems used to study T4P biology. This review focuses on recent advances in P. aeruginosa T4P structural biology, and the regulatory pathways controlling T4P biogenesis and function.

A new twist in the assembly of type IV pilus-like fibers

The type II secretion system (T2SS) and the type IV pilus system (T4PS) are structurally related molecular machines that reversibly assemble helical fibers in an ATP-dependent manner. In this issue of Structure, Nivaskumar and colleagues provide support for a "spooling" model of T2SS pseudopilus assembly and suggest that the T2S-and by extension, T4P-system motors may operate in a rotary manner to assemble filaments.

Novel Role for PilNO in Type IV Pilus Retraction Revealed by Alignment Subcomplex Mutations

Type IV pili (T4P) are dynamic protein filaments that mediate bacterial adhesion, biofilm formation, and twitching motility. The highly conserved PilMNOP proteins form an inner membrane alignment subcomplex required for function of the T4P system, though their exact roles are unclear. Three potential interaction interfaces for PilNO were identified: core-core, coiled coils (CC), and the transmembrane segments (TMSs). A high-confidence PilNO heterodimer model was used to select key residues for mutation, and the resulting effects on protein-protein interactions were examined both in a bacterial two-hybrid (BTH) system and in their native Pseudomonas aeruginosa context. Mutations in the oppositely charged CC regions or the TMS disrupted PilNO heterodimer formation in the BTH assay, while up to six combined mutations in the core failed to disrupt the interaction. When the mutations were introduced into the P. aeruginosa chromosome at the pilN or pilO locus, specific changes at each of the three interfaces--including core mutations that failed to disrupt interactions in the BTH system--abrogated surface piliation and/or impaired twitching motility. Unexpectedly, specific CC mutants were hyperpiliated but nonmotile, a hallmark of pilus retraction defects. These data suggest that PilNO participate in both the extension and retraction of T4P. Our findings support a model of multiple, precise interaction interfaces between PilNO; emphasize the importance of studying protein function in a minimally perturbed context and stoichiometry; and highlight potential target sites for development of small-molecule inhibitors of the T4P system.

IMPORTANCE

Pseudomonas aeruginosa is an opportunistic pathogen that uses type IV pili (T4P) for host attachment. The T4P machinery is composed of four cell envelope-spanning subcomplexes. PilN and PilO heterodimers are part of the alignment subcomplex and essential for T4P function. Three potential PilNO interaction interfaces (the core-core, coiled-coil, and transmembrane segment interfaces) were probed using site-directed mutagenesis followed by functional assays in an Escherichia coli two-hybrid system and in P. aeruginosa. Several mutations blocked T4P assembly and/or motility, including two that revealed a novel role for PilNO in pilus retraction, while other mutations affected extension dynamics. These critical PilNO interaction interfaces represent novel targets for small-molecule inhibitors with the potential to disrupt T4P function.

Crystal structure of the full-length ATPase GspE from the Vibrio vulnificus type II secretion system in complex with the cytoplasmic domain of GspL

The type II secretion system (T2SS) is present in many Gram-negative bacteria and is responsible for secreting a large number of folded proteins, including major virulence factors, across the outer membrane. The T2SS consists of 11-15 different proteins most of which are present in multiple copies in the assembled secretion machinery. The ATPase GspE, essential for the functioning of the T2SS, contains three domains (N1E, N2E and CTE) of which the N1E domain is associated with the cytoplasmic domain of the inner membrane protein GspL. Here we describe and analyze the structure of the GspE•cyto-GspL complex from Vibrio vulnificus in the presence of an ATP analog, AMPPNP. There are three such ∼83 kDa complexes per asymmetric unit with essentially the same structure. The N2E and CTE domains of a single V. vulnificus GspE subunit adopt a mutual orientation that has not been seen before in any of the previous GspE structures, neither in structures of related ATPases from other secretion systems. This underlines the tremendous conformational flexibility of the T2SS secretion ATPase. Cyto-GspL interacts not only with the N1E domain, but also with the CTE domain and is even in contact with AMPPNP. Moreover, the cyto-GspL domains engage in two types of mutual interactions, resulting in two essentially identical, but crystallographically independent, "cyto-GspL rods" that run throughout the crystal. Very similar rods are present in previous crystals of cyto-GspL and of the N1E•cyto-GspL complex. This arrangement, now seen four times in three entirely different crystal forms, involves contacts between highly conserved residues suggesting a role in the biogenesis or the secretion mechanism or both of the T2SS.