Influence of the passenger domain of a model autotransporter on the properties of its translocator domain

Autotransporters are a superfamily of proteins secreted by Gram-negative bacteria including many virulence factors. They are modular proteins composed of an N-terminal signal peptide, a surface-exposed ‘passenger’ domain carrying the activity of the protein, and a C-terminal ‘translocator’ domain composed of an α-helical linker region and a transmembrane β-barrel. The translocator domain plays an essential role for the secretion of the passenger domain across the outer membrane; however, the mechanism of autotransport remains poorly understood. The whooping cough agent Bordetella pertussis produces an autotransporter serine-protease, SphB1, which is involved in the maturation of an adhesin at the bacterial surface. SphB1 also mediates the proteolytic maturation of its own precursor. We used SphB1 as a model autotransporter and performed the first comparisons of the biochemical and biophysical properties of an isolated translocator domain with those of the same domain preceded by the C-terminal moiety of its natural passenger. By using cross-linking and dynamic light scattering, we provide evidence that the passenger domain promotes the auto-association of SphB1, although these interactions appear rather labile. Electrophysiological studies revealed that the passenger domain of the autotransporter appears to maintain the translocator channel in a low-conductance conformation, most likely by stabilizing the α-helix inside the pore. That the passenger may significantly influence AT physicochemical properties is likely to be relevant for the in vivo maturation and stability of AT proteins.     

Structure of the translocator domain of a bacterial autotransporter

Autotransporters are virulence-related proteins of Gram-negative bacteria that are secreted via an outer-membrane-based C-terminal extension, the translocator domain. This domain supposedly is sufficient for the transport of the N-terminal passenger domain across the outer membrane. We present here the crystal structure of the in vitro-folded translocator domain of the autotransporter NalP from Neisseria meningitidis, which reveals a 12-stranded beta-barrel with a hydrophilic pore of 10 x 12.5 A that is filled by an N-terminal alpha-helix. The domain has pore activity in vivo and in vitro. Our data are consistent with the model of passenger-domain transport through the hydrophilic channel within the beta-barrel, and inconsistent with a model for transport through a central channel formed by an oligomer of translocator domains. However, the dimensions of the pore imply translocation of the secreted domain in an unfolded form. An alternative model, possibly covering the transport of folded domains, is that passenger-domain transport involves the Omp85 complex, the machinery required for membrane insertion of outer-membrane proteins, on which autotransporters are dependent.     

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

Abstract

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 β-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.     

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 β-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.     

The periplasmic folding of a cysteineless autotransporter passenger domain interferes with its outer membrane translocation

Autotransporters are single polypeptides consisting of an outer membrane translocation domain mediating the translocation of a passenger domain. The periplasmic folding state of the passenger domain is controversial. By comparisons of passenger domains differing in their folding properties, our results suggest that periplasmic folding of passenger domains interferes with translocation.     

Structure of the outer membrane translocator domain of the Haemophilus influenzae Hia trimeric autotransporter

Autotransporter proteins are defined by the ability to drive their own secretion across the bacterial outer membrane. The Hia autotransporter of Haemophilus influenzae belongs to the trimeric autotransporter subfamily and mediates bacterial adhesion to the respiratory epithelium. In this report, we present the crystal structure of the C-terminal end of Hia, corresponding to the entire Hia translocator domain and part of the passenger domain (residues 992-1098). This domain forms a beta-barrel with 12 transmembrane beta-strands, including four strands from each subunit. The beta-barrel has a central channel of 1.8 nm in diameter that is traversed by three N-terminal alpha-helices, one from each subunit. Mutagenesis studies demonstrate that the transmembrane portion of the three alpha-helices and the loop region between the alpha-helices and the neighboring beta-strands are essential for stability of the trimeric structure of the translocator domain, and that trimerization of the translocator domain is a prerequisite for translocator activity. Overall, this study provides important insights into the mechanism of translocation in trimeric autotransporters.     

Crystal structure of the passenger domain of the Escherichia coli autotransporter EspP

Autotransporters represent a large superfamily of known and putative virulence factors produced by Gram-negative bacteria. They consist of an N-terminal “passenger domain” responsible for the specific effector functions of the molecule and a C-terminal “β-domain” responsible for translocation of the passenger across the bacterial outer membrane. Here, we present the 2.5-Å crystal structure of the passenger domain of the extracellular serine protease EspP, produced by the pathogen Escherichia coli O157:H7 and a member of the serine protease autotransporters of Enterobacteriaceae (SPATEs). Like the previously structurally characterized SPATE passenger domains, the EspP passenger domain contains an extended right-handed parallel β-helix preceded by an N-terminal globular domain housing the catalytic function of the protease. Of note, however, is the absence of a second globular domain protruding from this β-helix. We describe the structure of the EspP passenger domain in the context of previous results and provide an alternative hypothesis for the function of the β-helix within SPATEs.     

The Haemophilus influenzae Hia autotransporter contains an unusually short trimeric translocator domain

Gram-negative bacterial autotransporter proteins are a growing group of virulence factors that are characterized by their ability to cross the outer membrane without the help of accessory proteins. A conserved C-terminal beta-domain is critical for targeting of autotransporters to the outer membrane and for translocation of the N-terminal "passenger" domain to the bacterial surface. We have demonstrated previously that the Haemophilus influenzae Hia adhesin belongs to the autotransporter family, with translocator activity residing in the C-terminal 319 residues. To gain further insight into the mechanism of autotransporter protein translocation, we performed a structure-function analysis on Hia. In initial experiments, we generated a series of in-frame deletions and a set of chimeric proteins containing varying regions of the Hia C terminus fused to a heterologous passenger domain and discovered that the final 76 residues of Hia are both necessary and sufficient for translocation. Analysis by flow cytometry revealed that the region N-terminal to this shortened translocator domain is surface localized, further suggesting that this region is not involved in beta-barrel formation or in translocation of the passenger domain. Western analysis demonstrated that the translocation-competent regions of the C terminus migrated at masses consistent with trimers, suggesting that the Hia C terminus oligomerizes. Furthermore, fusion proteins containing a heterologous passenger domain demonstrated that similarly small C-terminal regions of Yersinia sp. YadA and Neisseria meningitidis NhhA are translocation-competent. These data provide experimental support for a unique subclass of autotransporters characterized by a short trimeric translocator domain.     

Hydrophobic residues of the autotransporter EspP linker domain are important for outer membrane translocation of its passenger

The autotransporter family of proteins is an important class of Gram-negative secreted virulence factors. Their secretion mechanism comprises entry to the periplasm via the Sec apparatus, followed by formation of an outer membrane beta barrel, which allows the N-terminal passenger domain to pass to the extracellular space. Several groups have identified a region immediately upstream of the beta domain that is important for outer membrane translocation, the so-called linker region. Here we characterize this region in EspP, a prototype of the serine protease autotransporters of enterobacteriaceae. We hypothesized that the folding of this region would be important in the outer membrane translocation process. We tested this hypothesis using a mutagenesis approach in conjunction with a series of nested deletions and found that in the absence of a complete passenger, mutations to the C-terminal helix, but not the upstream linker, significantly decrease secretion efficiency. However, in the presence of the passenger mutations to the amino-terminal region of the linker decrease secretion efficiency. Moreover, amino acids of hydrophobic character play a crucial role in linker function, suggesting the existence of a hydrophobic core or hydrophobic interaction necessary for outer membrane translocation of autotransporter proteins.     

A conserved region within the Bordetella pertussis autotransporter BrkA is necessary for folding of its passenger domain

Autotransporter secretion represents a unique mechanism that Gram-negative bacteria employ to deliver proteins to their cell surface. BrkA is a Bordetella pertussis autotransporter protein that mediates serum resistance and contributes to adherence of the bacterium to host cells. BrkA is a 103 kDa protein that is cleaved to form a 73 kDa alpha-domain and a 30 kDa beta domain. The alpha domain, also referred to as the passenger domain, is responsible for the effector functions of the protein, whereas the beta domain serves as a transporter. In an effort to characterize BrkA secretion, we have shown that BrkA has a 42 amino acid signal peptide for transit across the cytoplasmic membrane, and a translocation unit made up of a short linker region fused to the beta-domain to ferry the passenger domain to the bacterial surface through a channel formed by the beta-domain. In this report, we provide genetic, biochemical and structural evidence demonstrating that a region within the BrkA passenger (Glu601-Ala692) is necessary for folding the passenger. This region is not required for surface display in the outer membrane protease OmpT-deficient Escherichia coli strain UT5600. However, a BrkA mutant protein bearing a deletion in this region is susceptible to digestion when expressed in E. coli strains expressing OmpT suggesting that the region is required to maintain a stable structure. The instability of the deletion mutant can be rescued by surface expressing Glu601-Ala692in trans suggesting that this region is acting as an intramolecular chaperone to effect folding of the passenger concurrent with or following translocation across the outer membrane.     

A conserved stable core structure in the passenger domain beta-helix of autotransporter virulence proteins

In Gram-negative bacteria, a wide variety of virulence factors are secreted via the autotransporter (AT) pathway. Intriguingly, there is no significant concentration of ATP in the periplasm, nor a proton gradient across the OM, so the energetic origin of efficient secretion of AT proteins is unknown. More than 97% of AT proteins are predicted to contain right-handed parallel beta-helical structure, and the three crystal structures available for AT passenger domains each contain a long right-handed parallel beta-helix. Previous studies have shown that pertactin, an AT from Bordetella pertussis, exhibits three-state folding and has a C-terminal stable core structure. Here, we show that Pet, an unrelated AT from Escherichia coli, also exhibits three-state unfolding and also has a stable core structure. Deletion mutants, mass spectrometry, and N-terminal sequencing demonstrate that the Pet stable core is also located near the C-terminus of the passenger domain. Moreover, sequence analysis suggests that three-state folding and a C-terminal stable core structure could be important general features of the biogenesis of AT proteins in vivo.     

The Haemophilus influenzae Hia autotransporter harbours two adhesive pockets that reside in the passenger domain and recognize the same host cell receptor

Haemophilus influenzae is a human-specific pathogen and a major source of morbidity worldwide. Infection with this organism begins with colonization of the nasopharynx, a process that probably depends on adherence to respiratory epithelium. The Hia autotransporter protein is the major adhesin ex-pressed by a subset of non-typeable H. influenzae strains and promotes high-level adherence to a variety of human epithelial cell lines. In the current study, we discovered that the Hia passenger domain contains two distinct binding pockets, including one at the C-terminal end and a second at the N-terminal end. Competition assays revealed that the two binding pockets interact with the same host cell receptor structure, although with differing affinities. Additional experiments demonstrated that both binding domains are required for full-level bacterial adherence. These observations are reminiscent of eukaryotic cell adhesion molecules and highlight the first example of a bacterial adhesin with two domains that participate in a bivalent interaction with identical host cell receptors. Such an interaction increases avidity, thus stabilizing bacterial adherence to the epithelial surface, despite physical forces such as coughing, sneezing and mucociliary clearance.     

Efficient secretion of a folded protein domain by a monomeric bacterial autotransporter

Bacterial autotransporters are proteins that contain a small C-terminal 'beta domain' that facilitates translocation of a large N-terminal 'passenger domain' across the outer membrane (OM) by an unknown mechanism. Here we used EspP, an autotransporter produced by Escherichia coli 0157:H7, as a model protein to gain insight into the transport reaction. Initially we found that the passenger domain of a truncated version of EspP (EspPDelta1-851) was translocated efficiently across the OM. Blue Native polyacrylamide gel electrophoresis, analytical ultracentrifugation and other biochemical methods showed that EspPDelta1-851 behaves as a compact monomer and strongly suggest that the channel formed by the beta domain is too narrow to accommodate folded polypeptides. Surprisingly, we found that a folded protein domain fused to the N-terminus of EspPDelta1-851 was efficiently translocated across the OM. Further analysis revealed that the passenger domain of wild-type EspP also folds at least partially in the periplasm. To reconcile these data, we propose that the EspP beta domain functions primarily to target and anchor the protein and that an external factor transports the passenger domain across the OM.     

Protein domain of unknown function 3233 is a translocation domain of autotransporter secretory mechanism in gamma proteobacteria

Vibrio cholerae, the enteropathogenic gram negative bacteria is one of the main causative agents of waterborne diseases like cholera. About 1/3(rd) of the organism's genome is uncharacterised with many protein coding genes lacking structure and functional information. These proteins form significant fraction of the genome and are crucial in understanding the organism's complete functional makeup. In this study we report the general structure and function of a family of hypothetical proteins, Domain of Unknown Function 3233 (DUF3233), which are conserved across gram negative gammaproteobacteria (especially in Vibrio sp. and similar bacteria). Profile and HMM based sequence search methods were used to screen homologues of DUF3233. The I-TASSER fold recognition method was used to build a three dimensional structural model of the domain. The structure resembles the transmembrane beta-barrel with an axial N-terminal helix and twelve antiparallel beta-strands. Using a combination of amphipathy and discrimination analysis we analysed the potential transmembrane beta-barrel forming properties of DUF3233. Sequence, structure and phylogenetic analysis of DUF3233 indicates that this gram negative bacterial hypothetical protein resembles the beta-barrel translocation unit of autotransporter Va secretory mechanism with a gene organisation that differs from the conventional Va system.     

X-ray crystal structure of the passenger domain of plasmid encoded toxin(Pet), an autotransporter enterotoxin from enteroaggregative Escherichia coli (EAEC)

Autotransporters (ATs) represent a superfamily of proteins produced by a variety of pathogenic bacteria, which include the pathogenic groups of Escherichia coli (E. coli) associated with gastrointestinal and urinary tract infections. We present the first X-ray structure of the passenger domain from the Plasmid-encoded toxin (Pet) a 100 kDa protein at 2.3 Å resolution which is a cause of acute diarrhea in both developing and industrialized countries. Pet is a cytoskeleton-altering toxin that induces loss of actin stress fibers. While Pet (pdb code: 4OM9) shows only a sequence identity of 50% compared to the closest related protein sequence, extracellular serine protease plasmid (EspP) the structural features of both proteins are conserved. A closer structural look reveals that Pet contains a β-pleaded sheet at the sequence region of residues 181–190, the corresponding structural domain in EspP consists of a coiled loop. Secondary, the Pet passenger domain features a more pronounced beta sheet between residues 135 and 143 compared to the structure of EspP.     

The Haemophilus influenzae Hap autotransporter mediates microcolony formation and adherence to epithelial cells and extracellular matrix via binding regions in the C-terminal end of the passenger domain

The pathogenesis of non-typable Haemophilus influenzae disease begins with colonization of the nasopharynx and is facilitated by bacterial adherence to respiratory mucosa. The H. influenzae Hap autotransporter is a non-pilus adhesin that promotes adherence to epithelial cells and selected extracellular matrix proteins and mediates bacterial aggregation and microcolony formation. In addition, Hap has serine protease activity. Hap contains a 110 kDa internal passenger domain called HapS and a 45 kDa C-terminal translocator domain called Hapbeta. In the present study, we sought to define the structural basis for Hap adhesive activities. Based on experiments using a panel of monoclonal antibodies against HapS, a deletion derivative lacking most of HapS and a purified fragment of HapS, we established that adherence to epithelial cells is mediated by sequences within the C-terminal 311 residues of HapS. In additional experiments, we discovered that bacterial aggregation is also mediated by sequences within the C-terminal 311 residues of HapS and occurs via HapS-HapS interaction between molecules on neighbouring organisms. Finally, we found that adherence to fibronectin, laminin and collagen IV is mediated in part by sequences within the C-terminal 311 residues of HapS and in full by sequences within the C-terminal 511 residues of HapS. Taken together, these results demonstrate that all Hap adhesive activities reside in the C-terminal portion of HapS. Coupled with earlier observations, the current results establish that HapS adhesive activities and HapS protease activity are contained in separate modules of the protein.     

Influence of the N-terminal domain on the aggregation properties of the prion protein

Prion diseases appear to be caused by the aggregation of the cellular prion protein (PrP(C)) into an infectious form denoted PrP(Sc). The in vitro aggregation of the prion protein has been extensively investigated, yet many of these studies utilize truncated polypeptides. Because the C-terminal portion of PrP(Sc) is protease-resistant and retains infectivity, it is assumed that studies on this fragment are most relevant. The full-length protein can be distinguished from the truncated protein because it contains a largely structured, alpha-helical, C-terminal region in addition to an N terminus that is unstructured in the absence of metal ion binding. Herein, the in vitro aggregation of a truncated portion of the prion protein (PrP 90-231) and a full-length version (PrP 23-231) were compared. In each case, concentration-dependent aggregation was analyzed to discern whether it proceeds by a nucleation-dependent pathway. Both protein constructs appear to aggregate via a nucleated polymerization with a small nucleus size, yet the later steps differ. The full-length protein forms larger aggregates than the truncated protein, indicating that the N terminus may mediate higher-order aggregation processes. In addition, the N terminus has an influence on the assembly state of PrP before aggregation begins, causing the full-length protein to adopt several oligomeric forms in a neutral pH buffer. Our results emphasize the importance of studying the full-length protein in addition to the truncated forms for in vitro aggregation studies in order to make valid hypotheses about the mechanisms of prion aggregation and the distribution of aggregates in vivo.     

Influence of tertiary structure domain properties on the functionality of apolipoprotein A-I

The tertiary structure of apolipoprotein (apo) A-I and the contributions of structural domains to the properties of the protein molecule are not well defined. We used a series of engineered human and mouse apoA-I molecules in a range of physical-biochemical measurements to address this issue. Circular dichroism measurements of alpha-helix thermal unfolding and fluorescence spectroscopy measurements of 8-anilino-1-napthalenesulfonic acid binding indicate that removal of the C-terminal 54 amino acid residues from human and mouse apoA-I has similar effects; the molecules are only slightly destabilized, and there is a decrease in hydrophobic surface exposure. These results are consistent with both human and mouse apoA-I adopting a two-domain tertiary structure, comprising an N-terminal antiparallel helix bundle domain and a separate less ordered C-terminal domain. Mouse apoA-I is significantly less resistant than human apoA-I to thermal and chemical denaturation; the midpoint of thermal unfolding of mouse apoA-I at 45 degrees C is 15 degrees C lower and the midpoint of guanidine hydrochloride denaturation (D1/2) occurs at 0.5 M as compared to 1.0 M for human apoA-I. These differences reflect the overall greater stability of the helix bundle formed by residues 1-189 in human apoA-I. Measurements of the heats of binding to egg phosphatidylcholine (PC) small unilamellar vesicles and the kinetics of solubilization of dimyristoyl PC multilamellar vesicles indicate that the more stable human helix bundle interacts poorly with lipids as compared to the equivalent mouse N-terminal domain. The C-terminal domain of human apoA-I is much more hydrophobic than that of mouse apoA-I; in the lipid-free state the human C-terminal domain (residues 190-243) is partially alpha-helical and undergoes cooperative unfolding (D1/2 = 0.3 M) whereas the isolated mouse C-terminal domain (residues 187-240) is disordered in dilute solution. The human C-terminal domain binds to lipid surfaces much more avidly than the equivalent mouse domain. Human and mouse apoA-I have very different tertiary structure domain contributions for achieving functionality. It is clear that the stability of the N-terminal helix bundle, and the hydrophobicity and alpha-helix content of the C-terminal domain, are critical factors in determining the overall properties of the apoA-I molecule.     

Influence of domain orientation on the mechanical properties of regenerated cellulose fibers

The determination of the crystal orientation of regenerated cellulose fibers produced under different drawing regimes is presented. Orientation is determined by using wide-angle X-ray diffraction from a synchrotron source and by measuring the azimuthal width of equatorial reflections. The orientation parameter theta is then determined to compare fiber samples. By using a 500 nm beam size, clear differences between the crystal orientations of the skin and the core of the fibers are reported for a range of differently processed fibers for the first time. These results are shown to have implications for the mechanical properties of regenerated cellulose fibers. By applying tensile deformation to fiber bundles it is shown that the most misoriented samples undergo rapid decreases in the orientation parameter, which is an indication of crystal reorientation. However, the more highly oriented fibers undergo little reorientation. An average shear modulus for these fibers is determined by placing the data on a master curve and fitting with a model equation. By using another model for the fibers of low orientation and the shear modulus from the master curve analysis, it is shown that the deformation of less oriented fibers is dominated by shear between crystals, whereas the more oriented filaments are likely to undergo more significant chain deformation. By using a new model for fibers of low orientation, a parameter ksigma is introduced that gives the proportion of the fiber stress that is due to crystal shear. Systematic differences between this parameter for fibers of increasing initial orientation are reported. Moreover it is shown that the fibers of initially lower average orientation are governed by uniform strain, in agreement with the new model, whereas more highly oriented fibers deform under uniform stress. Furthermore, the model that we propose for misoriented domains and the use of a new factor dictating the proportion of shear stress may have general applications in materials engineering.