Channel properties of the translocator domain of the autotransporter Hbp of Escherichia coli.

Autotransporters produced by Gram-negative bacteria consist of an N-terminal signal sequence, a C-terminal translocator domain (TD), and a passenger domain in between. The TD facilitates the secretion of the passenger across the outer membrane. It generally consists of a channel-forming ß-barrel that can be plugged by an α-helix that is formed by a polypeptide fragment immediately N-terminal to the barrel domain in the sequence. In this work, we characterized the TD of the hemoglobin protease Hbp of Escherichia coli by comparing its properties with the TDs of NalP of Neisseria meningitidis and IgA protease of Neisseria gonorrhoeae. All TDs were produced in inclusion bodies and folded in vitro. In the case of the TD of Hbp, this procedure resulted in autocatalytic intramolecular processing, which mimicked the in vivo processing. Liposome-swelling assays and planar lipid bilayer experiments revealed that the pore of the Hbp TD was largely obstructed. In contrast, an Hbp TD variant that lacked only one amino-acid residue from the N terminus showed the opening and closing of a channel comparable to what was reported for the TD of NalP. Additionally, the naturally processed helix contributed to the stability of the TD, as shown by chemical denaturation monitored by tryptophan fluorescence. Overall these results show that Hbp is processed by an autocatalytic intramolecular mechanism resulting in the stable docking of the α-helix in the barrel. In addition, we could show that the α-helix contributes to the stability of TDs.

Pbp, a cell-surface exposed plasminogen binding protein of Bacteroides fragilis.

The Gram-negative anaerobic bacterium B. fragilis is a member of the commensal flora of the human intestine, but is also frequently found in severe intra-abdominal infections. Several B. fragilis virulence factors have been implicated in the development of these infections. A B. fragilis protein of circa 60-kDa was identified as a putative plasminogen binding protein (Pbp). The corresponding gene was located, cloned, sequenced and the subcellular localization of the protein was investigated. Pbp was both determined in the outer membrane of B. fragilis and of E. coli that expressed the cloned protein. Protease accessibility studies showed that the protein is expressed at the cell surface. Importantly, we demonstrated that Pbp is sufficient and required for plasminogen binding to whole cells in both E. coli and B. fragilis. Pbp-like proteins were also detected in some other Bacteroides subspecies. The role of this potential B. fragilis virulence factor in pathogenicity is discussed.

Limited tolerance towards folded elements during secretion of the autotransporter Hbp.

Many virulence factors secreted by pathogenic Gram-negative bacteria belong to the autotransporter (AT) family. ATs consist of a passenger domain, which is the actual secreted moiety, and a beta-domain that facilitates the transfer of the passenger domain across the outer membrane. Here, we analysed folding and translocation of the AT passenger, using Escherichia coli haemoglobin protease (Hbp) as a model protein. Dual cysteine mutagenesis, instigated by the unique crystal structure of the Hbp passenger, resulted in intramolecular disulphide bond formation dependent on the periplasmic enzyme DsbA. A small loop tied off by a disulphide bond did not interfere with secretion of Hbp. In contrast, a bond between different domains of the Hbp passenger completely blocked secretion resulting in degradation by the periplasmic protease DegP. In the absence of DegP, a translocation intermediate accumulated in the outer membrane. A similar jammed intermediate was formed upon insertion of a calmodulin folding moiety into Hbp. The data suggest that Hbp can fold in the periplasm but must retain a certain degree of flexibility and/or modest width to allow translocation across the outer membrane.

A novel protein, TtpC, is a required component of the TonB2 complex for specific iron transport in the pathogens Vibrio anguillarum and Vibrio cholerae.

Active transport across the outer membrane in gram-negative bacteria requires the energy that is generated by the proton motive force in the inner membrane. This energy is transduced to the outer membrane by the TonB protein in complex with the proteins ExbB and ExbD. In the pathogen Vibrio anguillarum we have identified two TonB systems, TonB1 and TonB2, the latter is used for ferric-anguibactin transport and is transcribed as part of an operon that consists of orf2, exbB2, exbD2, and tonB2. This cluster was identified by a polar transposon insertion in orf2 that resulted in a strain deficient for ferric-anguibactin transport. Only the entire cluster (orf2, exbB2, exbD2 and tonB2) could complement for ferric-anguibactin transport, while just the exbB2, exbD2, and tonB2 genes were unable to restore transport. This suggests an essential role for this Orf2, designated TtpC, in TonB2-mediated transport in V. anguillarum. A similar gene cluster exists in V. cholerae, i.e., with the homologues of ttpC-exbB2-exbD2-tonB2, and we demonstrate that TtpC from V. cholerae also plays a role in the TonB2-mediated transport of enterobactin in this human pathogen. Furthermore, we also show that in V. anguillarum the TtpC protein is found as part of a complex that might also contain the TonB2, ExbB2, and ExbD2 proteins. This novel component of the TonB2 system found in V. anguillarum and V. cholerae is perhaps a general feature in bacteria harboring the Vibrio-like TonB2 system.

Crystal structure of hemoglobin protease, a heme binding autotransporter protein from pathogenic Escherichia coli.

The acquisition of iron is essential for the survival of pathogenic bacteria, which have consequently evolved a wide variety of uptake systems to extract iron and heme from host proteins such as hemoglobin. Hemoglobin protease (Hbp) was discovered as a factor involved in the symbiosis of pathogenic Escherichia coli and Bacteroides fragilis, which cause intra-abdominal abscesses. Released from E. coli, this serine protease autotransporter degrades hemoglobin and delivers heme to both bacterial species. The crystal structure of the complete passenger domain of Hbp (110 kDa) is presented, which is the first structure from this class of serine proteases and the largest parallel beta-helical structure yet solved.

Characterization of an iron-regulated alpha-enolase of Bacteroides fragilis.

This study describes the identification, cloning and molecular characterization of the alpha-enolase P46 of Bacteroides fragilis. The gram-negative anaerobic bacterium B. fragilis is a member of the commensal flora of the human intestine but is also frequently found in severe intra-abdominal infections. Several virulence factors have been described that may be involved in the development of these infections. Many of these virulence factors are upregulated under conditions of iron- or heme-starvation. We found a major protein of 46 kDa (P46) that is upregulated under iron-depleted conditions. This protein was identified as an alpha-enolase. Alpha-enolases in several gram-positive bacteria and eukaryotic cells are located at the cell surface and function as plasminogen-binding proteins. Localization studies demonstrated that P46 is mainly located in the cytoplasm and partly associated with the inner membrane (IM). Under iron-restricted conditions, however, P46 is localized primarily in the IM fraction. Plasminogen-binding to B. fragilis cells did occur but was not P46 dependent. A 60-kDa protein was identified as a putative plasminogen-binding protein in B. fragilis.

Signal recognition particle (SRP)-mediated targeting and Sec-dependent translocation of an extracellular Escherichia coli protein.

Hemoglobin protease (Hbp) is a hemoglobin-degrading protein that is secreted by a human pathogenic Escherichia coli strain via the autotransporter mechanism. Little is known about the earliest steps in autotransporter secretion, i.e. the targeting to and translocation across the inner membrane. Here, we present evidence that Hbp interacts with the signal recognition particle (SRP) and the Sec-translocon early during biogenesis. Furthermore, Hbp requires a functional SRP targeting pathway and Sec-translocon for optimal translocation across the inner membrane. SecB is not required for targeting of Hbp but can compensate to some extent for the lack of SRP. Hbp is synthesized with an unusually long signal peptide that is remarkably conserved among a subset of autotransporters. We propose that these autotransporters preferentially use the co-translational SRP/Sec route to avoid adverse effects of the exposure of their mature domains in the cytoplasm.

Characterization and crystallization of a novel haemoglobinase from pathogenic Escherichia coli.

A haemoglobin-degrading enzyme from pathogenic Escherichia coli has been cloned, expressed and purified to homogeneity. The pure protein proteolyses haemoglobin and binds haem. In vivo, its role is to remove haem from haemoglobin and pass it to the bacteria, allowing them to overcome the limiting concentration of iron available in the body. The protein has been crystallized using polyethylene glycol to give crystals in a hexagonal space group with unit-cell parameters a = b = 114.6, c = 434.3 A. X-ray data have been collected to 2.5 A resolution. This is the first member of the SPATE (serine protease autotransporters of Enterobacteriaceae) family of autotransporter proteins to be crystallized.

Low resolution solution structure of the Apo form of Escherichia coli haemoglobin protease Hbp.

We have studied the solution properties of the apo form of the haemoglobin protease or "haemoglobinase", Hbp, a principal component of an important iron acquisition system in pathogenic Escherichia coli. Experimental determination of secondary structure content from circular dichroism (CD) spectroscopy, obtained using synchrotron light, showed that the protein contains predominately beta-sheets in agreement with secondary structure prediction from the primary sequence. Next, the size and shape of the protein were probed using analytical ultracentrifugation (AUC) and small angle X-ray scattering (SAXS). These showed that Hbp is a monomer, with an extended conformation. Using ab initio reconstruction methods we have produced a model of Hbp, which shows that the protein adopts an extended crescent-shaped conformation. Analysis of the resulting model gives hydrodynamic parameters in good agreement with those observed experimentally. Thus we are able to construct a hydrodynamically rigorous model of apo-Hbp in solution, not only giving a greater level of confidence to the results of the SAXS reconstruction methods, but providing the first three-dimensional view of this intriguing molecule.

Escherichia coli hemoglobin protease autotransporter contributes to synergistic abscess formation and heme-dependent growth of Bacteroides fragilis.

Intra-abdominal infections (IAI) continue to be a serious clinical problem. Bacterial synergism is an important factor that influences the shift from contamination to IAI, leading to the development of lesions and abscess formation. Escherichia coli and Bacteroides fragilis are particularly abundant in IAI. The underlying molecular mechanisms of this pathogenic synergy are still unclear. The role of the hemoglobin protease (Hbp) autotransporter protein from E. coli in the synergy of IAI was investigated. Hbp is identical to Tsh, a temperature-sensitive hemagglutinin associated with avian pathogenic E. coli. Clinical isolates from miscellaneous extraintestinal infections were phenotypically and genotypically screened for Hbp. The presence of Hbp was significantly associated with E. coli isolated from IAI and other extraintestinal infections. In a murine infection model, Hbp was shown to contribute to the pathogenic synergy of abscess development. Mice immunized with Hbp were protected against mixed infections and did not develop abscess lesions. Furthermore, an E. coli wild-type strain that did not induce abscess formation in the synergy model was transformed with a plasmid encoding the hbp gene, and mixed infections with this strain lead to increased growth of B. fragilis and induction of abscess lesions. Growth-promoting studies showed that purified Hbp is able to deliver heme to B. fragilis strain BE1. In conclusion, results suggest the synergy of abscess formation by E. coli and B. fragilis can be partly explained by the capacity of B. fragilis to intercept Hbp and iron from heme to overcome the iron restrictions imposed by the host.