Overproduction of the secretin OutD suppresses the secretion defect of an Erwinia chrysanthemi outB mutant

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.

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.

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.

Assembly of the type two secretion system in Aeromonas hydrophila involves direct interaction between the periplasmic domains of the assembly factor ExeB and the secretin ExeD

The type two secretion system is a large, trans-envelope apparatus that secretes toxins across the outer membrane of many Gram-negative bacteria. In Aeromonas hydrophila, ExeA interacts with peptidoglycan and forms a heteromultimeric complex with ExeB that is required for assembly of the ExeD secretin of the secretion system in the outer membrane. While the peptidoglycan-ExeAB (PG-AB) complex is required for ExeD assembly, the assembly mechanism remains unresolved. We analyzed protein-protein interactions to address the hypothesis that ExeD assembly in the outer membrane requires direct interaction with the PG-AB complex. Yeast and bacterial two hybrid analyses demonstrated an interaction between the periplasmic domains of ExeB and ExeD. Two-codon insertion mutagenesis of exeD disrupted lipase secretion, and immunoblotting of whole cells demonstrated significantly reduced secretin in mutant cells. Mapping of the two-codon insertions and deletion analysis showed that the ExeB-ExeD interaction involves the N0 and N1 subdomains of ExeD. Rotational anisotropy using the purified periplasmic domains of ExeB and ExeD determined that the apparent dissociation constant of the interaction is 1.19±0.16 µM. These results contribute important support for a putative mechanism by which the PG-AB complex facilitates assembly of ExeD through direct interaction between ExeB and ExeD. Furthermore, our results provide novel insight into the assembly function of ExeB that may contribute to elucidating the role of homologous proteins in secretion of toxins from other Gram negative pathogens.

Assembly of the type II secretion system

The type II secretion system is utilized by many Gram-negative bacteria to export folded proteins to the surface and/or the extracellular environment of the cell. Although the function of the system is to move proteins from the periplasm to the outside of the cell, it is a large trans-envelope structure composed of more than a dozen different proteins present in multiple copies, including peripheral, integral inner membrane and integral outer membrane proteins plus a pseudopilus stretching between them. The establishment of this structure as an integral component of the entire envelope including the peptidoglycan layer between the two membranes requires assembly. Many of the participants and processes involved in this assembly have now been established, while other aspects remain to be discovered or more fully understood.

Type II secretion: the substrates that won't go away

Type II secretion systems (T2SSs) generally release their substrates into the culture medium. A few T2SS substrates remain anchored to or bound at the surface of the bacteria after secretion. Since they handle already folded proteins, T2SSs are the best way for bacteria to target, at their surface, proteins containing a cofactor, proteins that have to be folded in the cytoplasm or in the periplasm, or multimeric proteins. However, how a T2SS deals with membrane-anchored proteins is not yet understood. While this type of protein has until now been overlooked, new proteomic approaches will facilitate its identification.

Commensal effect of pectate lyases secreted from Dickeya dadantii on proliferation of Escherichia coli O157:H7 EDL933 on lettuce leaves

The outbreaks caused by enterohemorrhagic Escherichia coli O157:H7 on leafy greens have raised serious and immediate food safety concerns. It has been suggested that several phytopathogens aid in the persistence and proliferation of the human enteropathogens in the phyllosphere. In this work, we examined the influence of virulence mechanisms of Dickeya dadantii 3937, a broad-host-range phytopathogen, on the proliferation of the human pathogen E. coli O157:H7 EDL933 (EDL933) on postharvest lettuce by coinoculation of EDL933 with D. dadantii 3937 derivatives that have mutations in virulence-related genes. A type II secretion system (T2SS)-deficient mutant of D. dadantii 3937, A1919 (ΔoutC), lost the capability to promote the multiplication of EDL933, whereas Ech159 (ΔrpoS), a stress-responsive σ factor RpoS-deficient mutant, increased EDL933 proliferation on lettuce leaves. A spectrophotometric enzyme activity assay revealed that A1919 (ΔoutC) was completely deficient in the secretion of pectate lyases (Pels), which play a major role in plant tissue maceration. In contrast to A1919 (ΔoutC), Ech159 (ΔrpoS) showed more than 2-fold-greater Pel activity than the wild-type D. dadantii 3937. Increased expression of pelD (encodes an endo-pectate lyase) was observed in Ech159 (ΔrpoS) in planta. These results suggest that the pectinolytic activity of D. dadantii 3937 is the dominant determinant of enhanced EDL933 proliferation on the lettuce leaves. In addition, RpoS, the general stress response σ factor involved in cell survival in suboptimal conditions, plays a role in EDL933 proliferation by controlling the production of pectate lyases in D. dadantii 3937.

A 20-residue peptide of the inner membrane protein OutC mediates interaction with two distinct sites of the outer membrane secretin OutD and is essential for the functional type II secretion system in Erwinia chrysanthemi

The type II secretion system (T2SS) is widely exploited by proteobacteria to secrete enzymes and toxins involved in bacterial survival and pathogenesis. The outer membrane pore formed by the secretin OutD and the inner membrane protein OutC are two key components of the secretion complex, involved in secretion specificity. Here, we show that the periplasmic regions of OutC and OutD interact directly and map the interaction site of OutC to a 20-residue peptide named OutCsip (secretin interacting peptide, residues 139-158). This peptide interacts in vitro with two distinct sites of the periplasmic region of OutD, one located on the N0 subdomain and another overlapping the N2-N3' subdomains. The two interaction sites of OutD have different modes of binding to OutCsip. A single substitution, V143S, located within OutCsip prevents its interaction with one of the two binding sites of OutD and fully inactivates the T2SS. We show that the N0 subdomain of OutD interacts also with a second binding site within OutC located in the region proximal to the transmembrane segment. We suggest that successive interactions between these distinct regions of OutC and OutD may have functional importance in switching the secretion machine.

Novel mechanism of outer membrane targeting of proteins in Gram-negative bacteria

SUMMARY

In Gram-negative bacteria, all the proteins destined for the outer membrane are synthesized with a signal sequence that is cleaved, either by the signal peptidase LepB for integral outer membrane proteins or by LspA for lipoproteins, when they cross the cytoplasmic membrane. The Dickeya dadantii protein PnlH does not possess a cleavable signal sequence but is anchored in the outer membrane by an N-terminal targeting signal. Addition of the 41 N-terminal amino acids of PnlH is sufficient for anchoring various hybrid proteins in the outer membrane. This targeting signal presents some of the characteristics of a Tat (twin arginine translocation) signal sequence but without an obvious cleavage site. We found that the Tat translocation pathway is required for the targeting process. This new mechanism of outer membrane protein targeting is probably widespread as PnlH was also addressed to the outer membrane when expressed in Escherichia coli. As PnlH was not detected as a substrate by Tat signal sequence prediction programmes, this would suggest that there may be many other unknown Tat-dependent outer membrane proteins.

Bacterial outer membrane secretin PulD assembles and inserts into the inner membrane in the absence of its pilotin

Dodecamerization and insertion of the outer membrane secretin PulD is entirely determined by the C-terminal half of the polypeptide (PulD-CS). In the absence of its cognate chaperone PulS, PulD-CS and PulD mislocalize to the inner membrane, from which they are extractable with detergents but not urea. Electron microscopy of PulD-CS purified from the inner membrane revealed apparently normal dodecameric complexes. Electron microscopy of PulD-CS and PulD in inner membrane vesicles revealed inserted secretin complexes. Mislocalization of PulD or PulD-CS to this membrane induces the phage shock response, probably as a result of a decreased membrane electrochemical potential. Production of PulD in the absence of the phage shock response protein PspA and PulS caused a substantial drop in membrane potential and was lethal. Thus, PulD-CS and PulD assemble in the inner membrane if they do not associate with PulS. We propose that PulS prevents premature multimerization of PulD and accompanies it through the periplasm to the outer membrane. PulD is the first bacterial outer membrane protein with demonstrated ability to insert efficiently into the inner membrane.

Purification and characterization of the N-terminal domain of ExeA: a novel ATPase involved in the type II secretion pathway of Aeromonas hydrophila

Aeromonas hydrophila secretes a number of degradative enzymes and toxins into the external milieu via the type II secretory pathway or secreton. ExeA is an essential component of this system and is necessary for the localization and/or multimerization of the secretin ExeD. ExeA contains two sequence motifs characteristic of the Walker superfamily of ATPases. Previous examination of substitution derivatives altered in these motifs suggested that ATP binding or hydrolysis is required for ExeAB complex formation and subsequent secretion function. To directly examine ExeA function, the N-terminal cytoplasmic domain of ExeA with the addition of a C-terminal hexahistidine tag (cytExeA) was overproduced in Escherichia coli and purified by metal chelate affinity and anion-exchange chromatographic techniques. Purified preparations of cytExeA exhibited ATPase activity in the presence of several divalent cations, Mg2+ being the preferred cation, with an optimum reaction temperature of approximately 37 to 42 degrees C and an optimum pH of 7 to 8. cytExeA exhibited an apparent K(m) for Mg-ATP of 0.22 mM and a V(max) of 0.72 nmol min(-1) mg(-1) of protein. cytExeA displayed low specificity for nucleoside triphosphate substrates and was significantly inhibited by F-type ATPase inhibitors. Gel filtration analyses of cytExeA, ExeA, and ExeAB indicated that ExeA dimerizes and forms a very large complex with ExeB. These findings support a model whereby ExeAB utilizes energy derived from ATP hydrolysis to facilitate the correct localization and multimerization of the ExeD secretin.

Type II protein secretion and its relationship to bacterial type IV pili and archaeal flagella

Homologues of the protein constituents of theKlebsiella pneumoniae(Klebsiella oxytoca) type II secreton (T2S), thePseudomonas aeruginosatype IV pilus/fimbrium biogenesis machinery (T4P) and theMethanococcus voltaeflagellum biogenesis machinery (Fla) have been identified. Known constituents of these systems include (1) a major prepilin (preflagellin), (2) several minor prepilins (preflagellins), (3) a prepilin (preflagellin) peptidase/methylase, (4) an ATPase, (5) a multispanning transmembrane (TM) protein, (6) an outer-membrane secretin (lacking in Fla) and (7) several functionally uncharacterized envelope proteins. Sequence and phylogenetic analyses led to the conclusion that, although many of the protein constituents are probably homologous, extensive sequence divergence during evolution clouds this homology so that a common ancestry can be established for all three types of systems for only two constituents, the ATPase and the TM protein. Sequence divergence of the individual T2S constituents has occurred at characteristic rates, apparently without shuffling of constituents between systems. The same is probably also true for the T4P and Fla systems. The family of ATPases is much larger than the family of TM proteins, and many ATPase homologues function in capacities unrelated to those considered here. Many phylogenetic clusters of the ATPases probably exhibit uniform function. Some of these have a corresponding TM protein homologue although others probably function without one. It is further shown that proteins that compose the different phylogenetic clusters in both the ATPase and the TM protein families exhibit unique structural characteristics that are of probable functional significance. The TM proteins are shown to have arisen by at least two dissimilar intragenic duplication events, one in the bacterial kingdom and one in the archaeal kingdom. The archaeal TM proteins are twice as large as the bacterial TM proteins, suggesting an oligomeric structure for the latter.

Secretins of Pseudomonas aeruginosa: large holes in the outer membrane

Pseudomonas aeruginosa produces a large number of exoproteins, ranging from the ADP-ribosyltransferases exotoxin A and ExoS to degradative enzymes, such as elastase and chitinase. As it is a gram-negative bacterium, P. aeruginosa must be able to transport these exoproteins across both membranes of the cell envelope. In addition, also proteins that are part of cellular appendages, such as type IV pili and flagella, have to cross the cell envelope. Whereas the majority of the proteins transported across the inner membrane are dependent on the Sec channel, the systems for translocation across the outer membrane seem to be more diverse. Gram-negative bacteria have invented a number of different strategies during the course of evolution to achieve this goal. Although these transport machineries seem to be radically different, many of them actually depend on a member of the secretin protein family for their function. Recent results show that secretins form a large complex in the outer membrane, which constitutes the actual translocation channel. Understanding the working mechanism of this protein translocation channel could open up new strategies to target molecular machineries at the heart of many important virulence factors.

Identification of XcpP domains that confer functionality and specificity to the Pseudomonas aeruginosa type II secretion apparatus

Gram-negative bacteria have evolved several types of secretion mechanisms to release proteins into the extracellular medium. One such mechanism, the type II secretory system, is a widely conserved two-step process. The first step is the translocation of signal peptide-bearing exoproteins across the inner membrane. The second step, the translocation across the outer membrane, involves the type II secretory apparatus or secreton. The secretons are made up of 12-15 proteins (Gsp) depending on the organism. Even though the systems are conserved, heterologous secretion is mostly species restricted. Moreover, components of the secreton are not systematically exchangeable, especially with distantly related microorganisms. In closely related species, two components, the GspC and GspD (secretin) family members, confer specificity for substrate recognition and/or secreton assembly. We used Pseudomonas aeruginosa as a model organism to determine which domains of XcpP (GspC member) are involved in specificity. By constructing hybrids between XcpP and OutC, the Erwinia chrysanthemi homologue, we identified a region of 35 residues that was not exchangeable. We showed that this region might influence the stability of the XcpYZ secreton subcomplex. Remarkably, XcpP and OutC have domains, coiled-coil and PDZ, respectively, which exhibit the same function but that are structurally different. Those two domains are exchangeable and we provided evidence that they are involved in the formation of homomultimeric complexes of XcpP.

Expression of the ExeAB complex of Aeromonas hydrophila is required for the localization and assembly of the ExeD secretion port multimer

Aeromonas hydrophila secretes protein toxins via the type II pathway, involving the products of at least two operons, exeAB (gspAB) and exeC-N (gspC-N). In the studies reported here, aerolysin secretion was restored to C5.84, an exeA::Tn5-751 mutant, by overexpression of exeD alone in trans. Expression studies indicated that these results did not reflect a role of ExeAB in the regulation of the exeC-N operon. Instead, immunoblot analysis showed that ExeD did not multimerize in C5.84, and fractionation of the membranes showed that the monomeric ExeD remained in the inner membrane. Expression of ExeAB, but not either protein alone, from a plasmid in C5.84 resulted in increases in the amount of multimeric ExeD, which correlated with increases in aerolysin secretion. Pulse-chase analysis also suggested that the induction of ExeAB allowed multimerization of previously accumulated monomer ExeD. In C5.84 cells overproducing ExeD, it multimerized even in the absence of ExeAB and, although most remained in the inner membrane, an amount similar to that in wild-type outer membranes fractionated with the outer membrane of the overproducing cells. These results indicate that the secretion defect of exeAB mutants is a result of an inability to assemble the ExeD secretin in the outer membrane. The localization and multimerization of overproduced ExeD in these mutants further suggests that the ExeAB complex plays either a direct or indirect role in the transport of ExeD into the outer membrane.

Biology of type II secretion

The type II secretion pathway or the main terminal branch of the general secretion pathway, as it has also been referred to, is widely distributed among Proteobacteria, in which it is responsible for the extracellular secretion of toxins and hydrolytic enzymes, many of which contribute to pathogenesis in both plants and animals. Secretion through this pathway differs from most other membrane transport systems, in that its substrates consist of folded proteins. The type II secretion apparatus is composed of at least 12 different gene products that are thought to form a multiprotein complex, which spans the periplasmic compartment and is specifically required for translocation of the secreted proteins across the outer membrane. This pathway shares many features with the type IV pilus biogenesis system, including the ability to assemble a pilus-like structure. This review discusses recent findings on the organization of the secretion apparatus and the role of its various components in secretion. Different models for pilus-mediated secretion through the gated pore in the outer membrane are also presented, as are the possible properties that determine whether a protein is recognized and secreted by the type II pathway.

Exchange of Xcp (Gsp) secretion machineries between Pseudomonas aeruginosa and Pseudomonas alcaligenes: species specificity unrelated to substrate recognition

Pseudomonas aeruginosa and Pseudomonas alcaligenes are gram-negative bacteria that secrete proteins using the type II or general secretory pathway, which requires at least 12 xcp gene products (XcpA and XcpP to -Z). Despite strong conservation of this secretion pathway, gram-negative bacteria usually cannot secrete exoproteins from other species. Based on results obtained with Erwinia, it has been proposed that the XcpP and/or XcpQ homologs determine this secretion specificity (M. Linderberg, G. P. Salmond, and A. Collmer, Mol. Microbiol. 20:175-190, 1996). In the present study, we report that XcpP and XcpQ of P. alcaligenes could not substitute for their respective P. aeruginosa counterparts. However, these complementation failures could not be correlated to species-specific recognition of exoproteins, since these bacteria could secrete exoproteins of each other. Moreover, when P. alcaligenes xcpP and xcpQ were expressed simultaneously in a P. aeruginosa xcpPQ deletion mutant, complementation was observed, albeit only on agar plates and not in liquid cultures. After growth in liquid culture the heat-stable P. alcaligenes XcpQ multimers were not detected, whereas monomers were clearly visible. Together, our results indicate that the assembly of a functional Xcp machinery requires species-specific interactions between XcpP and XcpQ and between XcpP or XcpQ and another, as yet uncharacterized component(s).

Expression of the endogenous type II secretion pathway in Escherichia coli leads to chitinase secretion

Escherichia coli K-12, the most widely used laboratory bacterium, does not secrete proteins into the extracellular medium under standard growth conditions, despite possessing chromosomal genes encoding a putative type II secretion machinery (secreton). We show that in wild-type E.coli K-12, divergent transcription of the two operons in the main chromosomal gsp locus, encoding the majority of the secreton components, is silenced by the nucleoid-structuring protein H-NS. In mutants lacking H-NS, the secreton genes cloned on a moderate-copy-number plasmid are expressed and promote efficient secretion of the endogenous, co-regulated endochitinase ChiA. This is the first time that secretion of an endogenous extracellular protein has been demonstrated in E.coli K-12.