BamA forms a translocation channel for polypeptide export across the bacterial outer membrane

The Assembly of the Inverse Autotransporter Protein YeeJ is Driven by its C-terminal β-strand

Autotransporter proteins are bacterial outer membrane proteins that display passenger domains with various functions through a β-barrel shaped translocation domain. YeeJ is an autotransporter protein from E. coli MG1655. In contrast to most other autotransporter proteins, its passenger domain is located at the C-terminus of the translocation domain. Due to this inverted domain organization, YeeJ belongs to autotransporter proteins of type Ve. To investigate the assembly of YeeJ, the fluorescence of a heterologous mCherry passenger domain was measured to quantify its assembly. Based on AlphaFold2 models of 119 sequences similar to YeeJ, a sequence conservation logo for the β1- and the β12-strand of type Ve autotransporter proteins was generated. Then, the effect of mutations in these strands on the assembly of YeeJ were analyzed. Mutations of the N-terminal aromatic amino acid of the β1-strand did not affect the assembly of the translocation domain and the display of the passenger domain. Likewise, exchange of the β1-strand with the β3-strand did not impair the assembly of the autotransporter fusion protein. Mutation of the C-terminal aromatic amino acid of the β12-strand strongly impaired surface display of the mCherry passenger domain. This amino acid has been shown before as an essential feature of the β-signals of classical autotransporter proteins and outer membrane β-barrel proteins in general. We therefore propose that the β12-strand of YeeJ acts as its β-signal and that the assembly of the YeeJ β-barrel is driven by its C-terminal β-strand, like in most other autotransporter proteins, despite its inverted domain organization.

Modulation with anti-Oma87 antibodies of cytotoxicity, adherence, and internalization of Acinetobacter baumannii in human cervical carcinoma epithelial cells

BamA, an Omp85 superfamily member, is universally conserved and essential for cell viability. Using anti-Oma87 antibodies, we focus on understanding the effect of Oma87 of Acinetobacter baumannii on pathogenicity. Oma87 was expressed, purified, and used to induce anti-Oma87 antibodies in BALB/c mice. Acute toxicity of the protein was evaluated in mice. HeLa cells were infected with both live and killed A. baumannii 19606 and a clinical isolate. The effects of anti-Oma87 sera on A. baumannii adherence, internalization, and proliferation in HeLa cells were studied. The roles of microfilaments and microtubules in A. baumannii invasion were demonstrated by Actin disruption. Reduced bacterial population and biofilm formation were noted. The ability of A. baumannii to provoke autophagy through Oma87 induction leads to incomplete autophagy and potentially facilitates bacterial replication. Actin-mediated uptake, attachment, and invasion demonstrated A. baumannii survival and multiplication within vacuoles in the host cell. The findings underscore the potential of Oma87 as a therapeutic intervention target in infections caused by A. baumannii. This comprehensive analysis contributes valuable information for understanding the virulence mechanisms of A. baumannii, potentially guiding future strategies to combat infections caused by this pathogen.

β-barrel membrane proteins fold via hybrid-barrel intermediate states

Gram-negative bacteria and eukaryotic organelles of bacterial origin contain outer membrane proteins that possess a transmembrane "β-barrel" domain. The conserved β-barrel assembly machine (BAM) and the sorting and assembly machine (SAM) are required for the folding and membrane insertion of β-barrels in Gram-negative bacteria and mitochondria, respectively. Although the mechanisms by which β-barrels are folded are incompletely understood, advances in cryo-electron microscopy (cryo-EM) have recently yielded unprecedented insights into their folding process. Here we highlight recent studies that show that both bacterial and mitochondrial β-barrels fold via the formation of remarkable "hybrid-barrel" intermediate states during their interaction with the folding machinery. We discuss how these results align with a general model of β-barrel folding.

POTRA domains of the TamA insertase interact with the outer membrane and modulate membrane properties

The outer membrane (OM) of gram-negative bacteria serves as a vital organelle that is densely populated with OM proteins (OMPs) and plays pivotal roles in cellular functions and virulence. The assembly and insertion of these OMPs into the OM represent a fundamental process requiring specialized molecular chaperones. One example is the translocation and assembly module (TAM), which functions as a transenvelope chaperone promoting the folding of specific autotransporters, adhesins, and secretion systems. The catalytic unit of TAM, TamA, comprises a catalytic β-barrel domain anchored within the OM and three periplasmic polypeptide-transport-associated (POTRA) domains that recruit the TamB subunit. The latter acts as a periplasmic ladder that facilitates the transport of unfolded OMPs across the periplasm. In addition to their role in recruiting the auxiliary protein TamB, our data demonstrate that the POTRA domains mediate interactions with the inner surface of the OM, ultimately modulating the membrane properties. Through the integration of X-ray crystallography, molecular dynamic simulations, and biomolecular interaction methodologies, we located the membrane-binding site on the first and second POTRA domains. Our data highlight a binding preference for phosphatidylglycerol, a minor lipid constituent present in the OM, which has been previously reported to facilitate OMP assembly. In the context of the densely OMP-populated membrane, this association may serve as a mechanism to secure lipid accessibility for nascent OMPs through steric interactions with existing OMPs, in addition to creating favorable conditions for OMP biogenesis.

The patatin-like protein PlpD forms structurally dynamic homodimers in the Pseudomonas aeruginosa outer membrane

Members of the Omp85 superfamily of outer membrane proteins (OMPs) found in Gram-negative bacteria, mitochondria and chloroplasts are characterized by a distinctive 16-stranded β-barrel transmembrane domain and at least one periplasmic POTRA domain. All previously studied Omp85 proteins promote critical OMP assembly and/or protein translocation reactions. Pseudomonas aeruginosa PlpD is the prototype of an Omp85 protein family that contains an N-terminal patatin-like (PL) domain that is thought to be translocated across the OM by a C-terminal β-barrel domain. Challenging the current dogma, we find that the PlpD PL-domain resides exclusively in the periplasm and, unlike previously studied Omp85 proteins, PlpD forms a homodimer. Remarkably, the PL-domain contains a segment that exhibits unprecedented dynamicity by undergoing transient strand-swapping with the neighboring β-barrel domain. Our results show that the Omp85 superfamily is more structurally diverse than currently believed and suggest that the Omp85 scaffold was utilized during evolution to generate novel functions.

ATP-independent assembly machinery of bacterial outer membranes: BAM complex structure and function set the stage for next-generation therapeutics

Diderm bacteria employ β-barrel outer membrane proteins (OMPs) as their first line of communication with their environment. These OMPs are assembled efficiently in the asymmetric outer membrane by the β-Barrel Assembly Machinery (BAM). The multi-subunit BAM complex comprises the transmembrane OMP BamA as its functional subunit, with associated lipoproteins (e.g., BamB/C/D/E/F, RmpM) varying across phyla and performing different regulatory roles. The ability of BAM complex to recognize and fold OM β-barrels of diverse sizes, and reproducibly execute their membrane insertion, is independent of electrochemical energy. Recent atomic structures, which captured BAM-substrate complexes, show the assembly function of BamA can be tailored, with different substrate types exhibiting different folding mechanisms. Here, we highlight common and unique features of its interactome. We discuss how this conserved protein complex has evolved the ability to effectively achieve the directed assembly of diverse OMPs of wide-ranging sizes (8-36 β-stranded monomers). Additionally, we discuss how darobactin-the first natural membrane protein inhibitor of Gram-negative bacteria identified in over five decades-selectively targets and specifically inhibits BamA. We conclude by deliberating how a detailed deduction of BAM complex-associated regulation of OMP biogenesis and OM remodeling will open avenues for the identification and development of effective next-generation therapeutics against Gram-negative pathogens.

Bacterial surface display of human lectins in Escherichia coli

Lectin-glycan interactions sustain fundamental biological processes involved in development and disease. Owing to their unique sugar-binding properties, lectins have great potential in glycobiology and biomedicine. However, their relatively low affinities and broad specificities pose a significant challenge when used as analytical reagents. New approaches for expression and engineering of lectins are in demand to overcome current limitations. Herein, we report the application of bacterial display for the expression of human galectin-3 and mannose-binding lectin in Escherichia coli. The analysis of the cell surface expression and binding activity of the surface-displayed lectins, including point and deletion mutants, in combination with molecular dynamics simulation, demonstrate the robustness and suitability of this approach. Furthermore, the display of functional mannose-binding lectin in the bacterial surface proved the feasibility of this method for disulfide bond-containing lectins. This work establishes for the first time bacterial display as an efficient means for the expression and engineering of human lectins, thereby increasing the available toolbox for glycobiology research.

Dual recognition of multiple signals in bacterial outer membrane proteins enhances assembly and maintains membrane integrity

Outer membrane proteins (OMPs) are essential components of the outer membrane of Gram-negative bacteria. In terms of protein targeting and assembly, the current dogma holds that a 'β-signal' imprinted in the final β-strand of the OMP engages the β-barrel assembly machinery (BAM) complex to initiate membrane insertion and assembly of the OMP into the outer membrane. Here, we revealed an additional rule that signals equivalent to the β-signal are repeated in other, internal β-strands within bacterial OMPs, by peptidomimetic and mutational analysis. The internal signal is needed to promote the efficiency of the assembly reaction of these OMPs. BamD, an essential subunit of the BAM complex, recognizes the internal signal and the β-signal, arranging several β-strands and partial folding for rapid OMP assembly. The internal signal-BamD ordering system is not essential for bacterial viability but is necessary to retain the integrity of the outer membrane against antibiotics and other environmental insults.

Membrane insertases at a glance

Protein translocases, such as the bacterial SecY complex, the Sec61 complex of the endoplasmic reticulum (ER) and the mitochondrial translocases, facilitate the transport of proteins across membranes. In addition, they catalyze the insertion of integral membrane proteins into the lipid bilayer. Several membrane insertases cooperate with these translocases, thereby promoting the topogenesis, folding and assembly of membrane proteins. Oxa1 and BamA family members serve as core components in the two major classes of membrane insertases. They facilitate the integration of proteins with α-helical transmembrane domains and of β-barrel proteins into lipid bilayers, respectively. Members of the Oxa1 family were initially found in the internal membranes of bacteria, mitochondria and chloroplasts. Recent studies, however, also identified several Oxa1-type insertases in the ER, where they serve as catalytically active core subunits in the ER membrane protein complex (EMC), the guided entry of tail-anchored (GET) and the GET- and EMC-like (GEL) complex. The outer membrane of bacteria, mitochondria and chloroplasts contain β-barrel proteins, which are inserted by members of the BamA family. In this Cell Science at a Glance article and the accompanying poster, we provide an overview of these different types of membrane insertases and discuss their function.

Structural basis of BAM-mediated outer membrane β-barrel protein assembly

The outer membrane structure is common in Gram-negative bacteria, mitochondria and chloroplasts, and contains outer membrane β-barrel proteins (OMPs) that are essential interchange portals of materials1-3. All known OMPs share the antiparallel β-strand topology4, implicating a common evolutionary origin and conserved folding mechanism. Models have been proposed for bacterial β-barrel assembly machinery (BAM) to initiate OMP folding5,6; however, mechanisms by which BAM proceeds to complete OMP assembly remain unclear. Here we report intermediate structures of BAM assembling an OMP substrate, EspP, demonstrating sequential conformational dynamics of BAM during the late stages of OMP assembly, which is further supported by molecular dynamics simulations. Mutagenic in vitro and in vivo assembly assays reveal functional residues of BamA and EspP for barrel hybridization, closure and release. Our work provides novel insights into the common mechanism of OMP assembly.

The patatin-like protein PlpD forms novel structurally dynamic homodimers in the Pseudomonas aeruginosa outer membrane

Members of the Omp85 superfamily of outer membrane proteins (OMPs) found in Gram-negative bacteria, mitochondria and chloroplasts are characterized by a distinctive 16-stranded β-barrel transmembrane domain and at least one periplasmic POTRA domain. All previously studied Omp85 proteins promote critical OMP assembly and/or protein translocation reactions. Pseudomonas aeruginosa PlpD is the prototype of an Omp85 protein family that contains an N-terminal patatin-like (PL) domain that is thought to be translocated across the OM by a C-terminal β-barrel domain. Challenging the current dogma, we found that the PlpD PL-domain resides exclusively in the periplasm and, unlike previously studied Omp85 proteins, PlpD forms a homodimer. Remarkably, the PL-domain contains a segment that exhibits unprecedented dynamicity by undergoing transient strand-swapping with the neighboring β-barrel domain. Our results show that the Omp85 superfamily is more structurally diverse than currently believed and suggest that the Omp85 scaffold was utilized during evolution to generate novel functions.

Heterologous Display of Chlamydia trachomatis PmpD Passenger at the Surface of Salmonella OMVs

Chlamydia trachomatis is the bacterial pathogen that causes most cases of sexually transmitted diseases annually. To combat the global spread of asymptomatic infection, development of effective (mucosal) vaccines that offer both systemic and local immune responses is considered a high priority. In this study, we explored the expression of C. trachomatis full-length (FL) PmpD, as well as truncated PmpD passenger constructs fused to a "display" autotransporter (AT) hemoglobin protease (HbpD) and studied their inclusion into outer membrane vesicles (OMVs) of Escherichia coli and Salmonella Typhimurium. OMVs are considered safe vaccine vectors well-suited for mucosal delivery. By using E. coli AT HbpD-fusions of chimeric constructs we improved surface display and successfully generated Salmonella OMVs decorated with a secreted and immunogenic PmpD passenger fragment (aa68-629) to 13% of the total protein content. Next, we investigated whether a similar chimeric surface display strategy could be applied to other AT antigens, i.e., secreted fragments of Prn (aa35-350) of Bordetella pertussis and VacA (aa65-377) of Helicobacter pylori. The data provided information on the complexity of heterologous expression of AT antigens at the OMV surface and suggested that optimal expression strategies should be developed on an antigen-to-antigen basis.

Surveying membrane landscapes: a new look at the bacterial cell surface

Recent studies applying advanced imaging techniques are changing the way we understand bacterial cell surfaces, bringing new knowledge on everything from single-cell heterogeneity in bacterial populations to their drug sensitivity and mechanisms of antimicrobial resistance. In both Gram-positive and Gram-negative bacteria, the outermost surface of the bacterial cell is being imaged at nanoscale; as a result, topographical maps of bacterial cell surfaces can be constructed, revealing distinct zones and specific features that might uniquely identify each cell in a population. Functionally defined assembly precincts for protein insertion into the membrane have been mapped at nanoscale, and equivalent lipid-assembly precincts are suggested from discrete lipopolysaccharide patches. As we review here, particularly for Gram-negative bacteria, the applications of various modalities of nanoscale imaging are reawakening our curiosity about what is conceptually a 3D cell surface landscape: what it looks like, how it is made and how it provides resilience to respond to environmental impacts.

Small Molecule Antibiotics Inhibit Distinct Stages of Bacterial Outer Membrane Protein Assembly

Several antibacterial compounds have recently been discovered that potentially inhibit the activity of BamA, an essential subunit of a heterooligomer (the barrel assembly machinery or BAM) that assembles outer membrane proteins (OMPs) in Gram-negative bacteria, but their mode of action is unclear. To address this issue, we examined the effect of three inhibitors on the biogenesis of a model E. coli OMP (EspP) in vivo. We found that darobactin potently inhibited the interaction of a conserved C-terminal sequence motif (the “β signal”) with BamA, but had no effect on assembly if added at a postbinding stage. In contrast, Polyphor peptide 7 and MRL-494 inhibited both binding and at least one later step of assembly. Taken together with previous studies that analyzed the binding of darobactin and Polyphor peptide 7 to BamA in vitro, our results strongly suggest that the two compounds inhibit BAM function by distinct competitive and allosteric mechanisms. In addition to providing insights into the properties of the antibacterial compounds, our results also provide direct experimental evidence that supports a model in which the binding of the β signal to BamA initiates the membrane insertion of OMPs.

Deletion Variants of Autotransporter from Psychrobacter cryohalolentis Increase Efficiency of 10FN3 Exposure on the Surface of Escherichia coli Cells

The autotransporter AT877 from Psychrobacter cryohalolentis belongs to the family of outer membrane proteins containing N-terminal passenger and C-terminal translocator domains that form the basis for the design of display systems on the surface of bacterial cells. It was shown in our previous study that the passenger domain of AT877 can be replaced by the cold-active esterase EstPc or the tenth domain of fibronectin type III (¹⁰Fn3). In order to increase efficiency of the ¹⁰Fn3 surface display in Escherichia coli cells, four deletion variants of the Fn877 hybrid autotransporter were obtained. It was demonstrated that all variants are present in the membrane of bacterial cells and facilitate binding of the antibodies specific against ¹⁰Fn3 on the cell surface. The highest level of binding is provided by the variants Δ239 and Δ310, containing four and seven beta-strands out of twelve that comprise the structure of the translocator domain. Using electrophoresis under semi-native conditions, presence of heat modifiability in the full-size Fn877 and its deletion variants was demonstrated, which indicated preservation of beta structure in their molecules. The obtained results could be used to optimize the bacterial display systems of ¹⁰Fn3, as well as of other heterologous passenger domains.

Rational Metabolic Engineering Combined with Biosensor-Mediated Adaptive Laboratory Evolution for l-Cysteine Overproduction from Glycerol in Escherichia coli

l-Cysteine is an important sulfur-containing amino acid with numerous applications in the pharmaceutical and cosmetic industries. The microbial production of l-cysteine has received substantial attention, and the supply of the precursor l-serine is important in l-cysteine biosynthesis. In this study, to achieve l-cysteine overproduction, we first increased l-serine production by deleting genes involved in the pathway of l-serine degradation to glycine (serine hydroxymethyl transferase, SHMT, encoded by glyA genes) in strain 4W (with l-serine titer of 1.1 g/L), thus resulting in strain 4WG with l-serine titer of 2.01 g/L. Second, the serine-biosensor based on the transcriptional regulator NCgl0581 of C. glutamicum was constructed in E. coli, and the validity and sensitivity of the biosensor were demonstrated in E. coli. Then 4WG was further evolved through adaptive laboratory evolution (ALE) combined with serine-biosensor, thus yielding the strain 4WGX with 4.13 g/L l-serine production. Moreover, the whole genome of the evolved strain 4WGX was sequenced, and ten non-synonymous mutations were found in the genome of strain 4WGX compared with strain 4W. Finally, 4WGX was used as the starting strain, and deletion of the l-cysteine desulfhydrases (encoded by tnaA), overexpression of serine acetyltransferase (encoded by cysE) and the key enzyme of transport pathway (encoded by ydeD) were performed in strain 4WGX. The recombinant strain 4WGX-∆tnaA-cysE-ydeD can produce 313.4 mg/L of l-cysteine using glycerol as the carbon source. This work provides an efficient method for the biosynthesis of value-added commodity products associated with glycerol conversion.

Function of the Omp85 Superfamily of Outer Membrane Protein Assembly Factors and Polypeptide Transporters

The Omp85 protein superfamily is found in the outer membrane (OM) of all gram-negative bacteria and eukaryotic organelles of bacterial origin. Members of the family catalyze both the membrane insertion of β-barrel proteins and the translocation of proteins across the OM. Although the mechanism(s) by which these proteins function is unclear, striking new insights have emerged from recent biochemical and structural studies. In this review we discuss the entire Omp85 superfamily but focus on the function of the best-studied member, BamA, which is an essential and highly conserved component of the bacterial barrel assembly machinery (BAM). Because BamA has multiple functions that overlap with those of other Omp85 proteins, it is likely the prototypical member of the Omp85 superfamily. Furthermore, BamA has become a protein of great interest because of the recent discovery of small-molecule inhibitors that potentially represent an important new class of antibiotics. Expected final online publication date for the Annual Review of Microbiology, Volume 76 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The Escherichia coli outer membrane protein OmpA acquires secondary structure prior to its integration into the membrane

Almost all proteins that reside in the outer membrane (OM) of Gram-negative bacteria contain a membrane-spanning segment that folds into a unique β barrel structure and inserts into the membrane by an unknown mechanism. To obtain further insight into outer membrane protein (OMP) biogenesis, we revisited the surprising observation reported over 20 years ago that the Escherichia coli OmpA β barrel can be assembled into a native structure in vivo when it is expressed as two noncovalently linked fragments. Here, we show that disulfide bonds between β strand 4 in the N-terminal fragment and β strand 5 in the C-terminal fragment can form in the periplasmic space and greatly increase the efficiency of assembly of "split" OmpA, but only if the cysteine residues are engineered in perfect register (i.e., they are aligned in the fully folded β barrel). In contrast, we observed only weak disulfide bonding between β strand 1 in the N-terminal fragment and β strand 8 in the C-terminal fragment that would form a closed or circularly permutated β barrel. Our results not only demonstrate that β barrels begin to fold into a β-sheet-like structure before they are integrated into the OM but also help to discriminate among the different models of OMP biogenesis that have been proposed.

Cryo-EM structures reveal multiple stages of bacterial outer membrane protein folding

Transmembrane β barrel proteins are folded into the outer membrane (OM) of Gram-negative bacteria by the β barrel assembly machinery (BAM) via a poorly understood process that occurs without known external energy sources. Here, we used single-particle cryo-EM to visualize the folding dynamics of a model β barrel protein (EspP) by BAM. We found that BAM binds the highly conserved "β signal" motif of EspP to correctly orient β strands in the OM during folding. We also found that the folding of EspP proceeds via "hybrid-barrel" intermediates in which membrane integrated β sheets are attached to the essential BAM subunit, BamA. The structures show an unprecedented deflection of the membrane surrounding the EspP intermediates and suggest that β sheets progressively fold toward BamA to form a β barrel. Along with in vivo experiments that tracked β barrel folding while the OM tension was modified, our results support a model in which BAM harnesses OM elasticity to accelerate β barrel folding.

Conversion of the OmpF Porin into a Device to Gather Amyloids on the E. coli Outer Membrane

Protein amyloids are ubiquitous in natural environments. They typically originate from microbial secretions or spillages from mammals infected by prions, currently raising concerns about their infectivity and toxicity in contexts such as gut microbiota or soils. Exploiting the self-assembly potential of amyloids for their scavenging, here, we report the insertion of an amyloidogenic sequence stretch from a bacterial prion-like protein (RepA-WH1) in one of the extracellular loops (L5) of the abundant Escherichia coli outer membrane porin OmpF. The expression of this grafted porin enables bacterial cells to trap on their envelopes the same amyloidogenic sequence when provided as an extracellular free peptide. Conversely, when immobilized on a surface as bait, the full-length prion-like protein including the amyloidogenic peptide can catch bacteria displaying the L5-grafted OmpF. Polyphenolic molecules known to inhibit amyloid assembly interfere with peptide recognition by the engineered OmpF, indicating that this is compatible with the kind of homotypic interactions expected for amyloid assembly. Our study suggests that synthetic porins may provide suitable scaffolds for engineering biosensor and clearance devices to tackle the threat posed by pathogenic amyloids.

Transmembrane β-barrel proteins of bacteria: From structure to function

The outer membrane of Gram-negative bacteria is a specialized organelle conferring protection to the cell against various environmental stresses and resistance to many harmful compounds. The outer membrane has a number of unique features, including an asymmetric lipid bilayer, the presence of lipopolysaccharides and an individual proteome. The vast majority of the integral transmembrane proteins in the outer membrane belongs to the family of β-barrel proteins. These evolutionarily related proteins share a cylindrical, anti-parallel β-sheet core fold spanning the outer membrane. The loops and accessory domains attached to the β-barrel allow for a remarkable versatility in function for these proteins, ranging from diffusion pores and transporters to enzymes and adhesins. We summarize the current knowledge on β-barrel structure and folding and give an overview of their functions, evolution, and potential as drug targets.

Surface-tethered planar membranes containing the β-barrel assembly machinery: a platform for investigating bacterial outer membrane protein folding

The outer membrane of Gram-negative bacteria presents a robust physicochemical barrier protecting the cell from both the natural environment and acting as the first line of defense against antimicrobial materials. The proteins situated within the outer membrane are responsible for a range of biological functions including controlling influx and efflux. These outer membrane proteins (OMPs) are ultimately inserted and folded within the membrane by the β-barrel assembly machine (Bam) complex. The precise mechanism by which the Bam complex folds and inserts OMPs remains unclear. Here, we have developed a platform for investigating Bam-mediated OMP insertion. By derivatizing a gold surface with a copper-chelating self-assembled monolayer, we were able to assemble a planar system containing the complete Bam complex reconstituted within a phospholipid bilayer. Structural characterization of this interfacial protein-tethered bilayer by polarized neutron reflectometry revealed distinct regions consistent with known high-resolution models of the Bam complex. Additionally, by monitoring changes of mass associated with OMP insertion by quartz crystal microbalance with dissipation monitoring, we were able to demonstrate the functionality of this system by inserting two diverse OMPs within the membrane, pertactin, and OmpT. This platform has promising application in investigating the mechanism of Bam-mediated OMP insertion, in addition to OMP function and activity within a phospholipid bilayer environment.

Analysis of six tonB gene homologs in Bacteroides fragilis revealed that tonB3 is essential for survival in experimental intestinal colonization and intra-abdominal infection

The opportunistic, anaerobic pathogen and commensal of the human large intestinal tract, Bacteroides fragilis strain 638R, contains six predicted TonB proteins, termed TonB1-6, four ExbBs orthologs, ExbB1-4, and five ExbDs orthologs, ExbD1-5. The inner membrane TonB/ExbB/ExbD complex harvests energy from the proton motive force (Δp) and the TonB C-terminal domain interacts with and transduces energy to outer membrane TonB-dependent transporters (TBDTs). However, TonB's role in activating nearly one hundred TBDTs for nutrient acquisition in B. fragilis during intestinal colonization and extraintestinal infection has not been established. In this study, we show that growth was abolished in the ΔtonB3 mutant when heme, vitamin B12, Fe(III)-ferrichrome, starch, mucin-glycans, or N-linked glycans were used as a substrate for growth in vitro. Genetic complementation of the ΔtonB3 mutant with the tonB3 gene restored growth on these substrates. The ΔtonB1, ΔtonB2, ΔtonB4, ΔtonB5, and ΔtonB6 single mutants did not show a growth defect. This indicates that there was no functional compensation for the lack of TonB3, and it demonstrates that TonB3, alone, drives the TBDTs involved in the transport of essential nutrients. The ΔtonB3 mutant had a severe growth defect in a mouse model of intestinal colonization compared to the parent strain. This intestinal growth defect was enhanced in the ΔtonB3 ΔtonB6 double mutant strain which completely lost its ability to colonize the mouse intestinal tract compared to the parent strain. The ΔtonB1, ΔtonB2, ΔtonB4, and ΔtonB5 mutants did not significantly affect intestinal colonization. Moreover, the survival of the ΔtonB3 mutant strain was completely eradicated in a rat model of intra-abdominal infection. Taken together, these findings show that TonB3 was essential for survival in vivo. The genetic organization of tonB1, tonB2, tonB4, tonB5, and tonB6 gene orthologs indicates that they may interact with periplasmic and nonreceptor outer membrane proteins, but the physiological relevance of this has not been defined. Because anaerobic fermentation metabolism yields a lower Δp than aerobic respiration and B. fragilis has a reduced redox state in its periplasmic space - in contrast to an oxidative environment in aerobes - it remains to be determined if the diverse system of TonB/ExbB/ExbD orthologs encoded by B. fragilis have an increased sensitivity to PMF (relative to aerobic bacteria) to allow for the harvesting of energy under anaerobic conditions.

Bacterial Contact-Dependent Inhibition Protein Binds near the Open Lateral Gate in BamA Prior to Toxin Translocation

Contact-dependent inhibition (CDI) is a mechanism of interbacterial competition in Gram-negative bacteria. The critical component of CDI systems is a large protein named CdiA; it forms a filament on the bacterial cell surface and contains a toxin domain at its C-terminal end. Upon binding to a receptor protein on the surface of a neighboring cell, CdiA delivers the toxin domain through the outer membrane of the neighboring bacterium. The mechanism of that delivery process is poorly understood. We have characterized how CdiA from E. coli EC93 binds to its receptor, BamA, to understand how this binding event might initiate the process of toxin delivery. BamA is an essential protein that assembles β-barrel proteins into the outer membranes of all Gram-negative bacteria; this assembly process depends on BamA's unique ability to open laterally in the lipid bilayer through a gate in its own membrane-embedded β-barrel. Through site-specific photo-cross-linking and mutational analysis, we demonstrate that the BamA-CdiA interaction depends on a small number of non-conserved amino acids on the extracellular surface of BamA, but the protein interface extends over a region near BamA's lateral gate. We further demonstrate that BamA's lateral gate can open without disrupting the interaction with CdiA. CdiA thus appears to initially engage BamA in a manner that could allow it to utilize BamA's lateral gate in subsequent steps in the toxin translocation process.

Overproducing the BAM complex improves secretion of difficult-to-secrete recombinant autotransporter chimeras

Monomeric autotransporters have been used extensively to transport recombinant proteins or protein domains to the cell surface of Gram-negative bacteria amongst others for antigen display. Genetic fusion of such antigens into autotransporters has yielded chimeras that can be used for vaccination purposes. However, not every fusion construct is transported efficiently across the cell envelope. Problems occur in particular when the fused antigen attains a relatively complex structure in the periplasm, prior to its translocation across the outer membrane. The latter step requires the interaction with periplasmic chaperones and the BAM (β-barrel assembly machinery) complex in the outer membrane. This complex catalyzes insertion and folding of β-barrel outer membrane proteins, including the β-barrel domain of autotransporters. Here, we investigated whether the availability of periplasmic chaperones or the BAM complex is a limiting factor for the surface localization of difficult-to-secrete chimeric autotransporter constructs. Indeed, we found that overproduction of in particular the BAM complex, increases surface display of difficult-to-secrete chimeras. Importantly, this beneficial effect appeared to be generic not only for a number of monomeric autotransporter fusions but also for fusions to trimeric autotransporters. Therefore, overproduction of BAM might be an attractive strategy to improve the production of recombinant autotransporter constructs.

Chlamydia trachomatis Polymorphic Membrane Proteins (Pmps) Form Functional Homomeric and Heteromeric Oligomers

Chlamydiae are Gram-negative, obligate intracellular bacteria, which infect animals and humans. Adhesion to host cells, the first step in the infection process, is mediated by polymorphic membrane proteins (Pmps). Pmps constitute the largest chlamydial protein family, with 9 members (subdivided into six subtypes) in C. trachomatis and 21 in C. pneumoniae, and are characterized by the presence of multiple copies of GGA(I,L,V) and FxxN motifs. Motif-rich fragments of all nine C. trachomatis Pmps act as adhesins and are essential for infection. As autotransporters, most Pmp proteins are secreted through their β-barrel domain and localize on the surface of the chlamydial cell, where most of them are proteolytically processed. Classical autotransporters are monomeric proteins, which can function as toxins, proteases, lipases and monoadhesive adhesins. Here we show that selected recombinant C. trachomatis Pmp fragments form functional adhesion-competent multimers. They assemble into homomeric and heteromeric filaments, as revealed by non-denaturing gel electrophoresis, size-exclusion chromatography and electron microscopy. Heteromeric filaments reach 2 μm in length, significantly longer than homomeric structures. Filament formation was independent of the number of motifs present in the fragment(s) concerned and their relative affinity for host cells. Our functional studies demonstrated that only adhesion-competent oligomers were able to block a subsequent infection. Pre-loading of infectious chlamydial cells with adhesion-competent Pmp oligomers maintained the subsequent infection, while adhesion-incompetent structures reduced infectivity, presumably by blocking the function of endogenous Pmps. The very large number of possible heteromeric and homomeric Pmp complexes represents a novel mechanism to ensure stable adhesion and possibly host cell immune escape.

Folding Control in the Path of Type 5 Secretion

The type 5 secretion system (T5SS) is one of the more widespread secretion systems in Gram-negative bacteria. Proteins secreted by the T5SS are functionally diverse (toxins, adhesins, enzymes) and include numerous virulence factors. Mechanistically, the T5SS has long been considered the simplest of secretion systems, due to the paucity of proteins required for its functioning. Still, despite more than two decades of study, the exact process by which T5SS substrates attain their final destination and correct conformation is not totally deciphered. Moreover, the recent addition of new sub-families to the T5SS raises additional questions about this secretion mechanism. Central to the understanding of type 5 secretion is the question of protein folding, which needs to be carefully controlled in each of the bacterial cell compartments these proteins cross. Here, the biogenesis of proteins secreted by the Type 5 secretion system is discussed, with a focus on the various factors preventing or promoting protein folding during biogenesis.