Conserved features in TamA enable interaction with TamB to drive the activity of the translocation and assembly module

A conserved C-terminal domain of TamB interacts with multiple BamA POTRA domains in Borreliella burgdorferi

Lyme disease is the leading tick-borne infection in the United States, caused by the pathogenic spirochete Borreliella burgdorferi, formerly known as Borrelia burgdorferi. Diderms, or bacteria with dual-membrane ultrastructure, such as B. burgdorferi, have multiple methods of transporting and integrating outer membrane proteins (OMPs). Most integral OMPs are transported through the β-barrel assembly machine (BAM) complex. This complex consists of the channel-forming OMP BamA and accessory lipoproteins that interact with the five periplasmic, polypeptide transport-associated (POTRA) domains of BamA. Another system, the translocation and assembly module (TAM) system, has also been implicated in OMP assembly and export. The TAM system consists of two proteins, the BamA paralog TamA which has three POTRA domains and the inner membrane protein TamB. TamB is characterized by a C-terminal DUF490 domain that interacts with the POTRA domains of TamA. Interestingly, while TamB is found in almost all diderms, including B. burgdorferi, TamA is found almost exclusively in Proteobacteria. This strongly suggests a TamA-independent role of TamB in most diderms. We previously demonstrated that BamA interacts with TamB in B. burgdorferi and hypothesized that this is facilitated by the BamA POTRA domains interacting with the TamB DUF490 domain. In this study, we utilized protein-protein co-purification assays to empirically demonstrate that the B. burgdorferi TamB DUF490 domain interacts with BamA POTRA2 and POTRA3. We also observed that the DUF490 domain of TamB interacts with the accessory lipoprotein BamB. To examine if the BamA-TamB interaction is more ubiquitous among diderms, we examined BamA-TamB interactions in Salmonella enterica serovar Typhimurium (St). Interestingly, even though St encodes a TamA protein that interacts with TamB, we observed that the TamB DUF490 of St interacts with BamA in this organism. Our combined findings strongly suggest that the TamB-BamA interaction occurs independent of the TamA component of the TAM protein export system.

Terrabacteria: redefining bacterial envelope diversity, biogenesis and evolution

The bacterial envelope is one of the oldest and most essential cellular components and has been traditionally divided into Gram-positive (monoderm) and Gram-negative (diderm). Recent landmark studies have challenged a major paradigm in microbiology by inferring that the last bacterial common ancestor had a diderm envelope and that the outer membrane (OM) was lost repeatedly in evolution to give rise to monoderms. Intriguingly, OM losses appear to have occurred exclusively in the Terrabacteria, one of the two major clades of bacteria. In this Review, we present current knowledge about the Terrabacteria. We describe their diversity and phylogeny and then highlight the vast phenotypic diversity of the Terrabacteria cell envelopes, which display large deviations from the textbook examples of diderms and monoderms, challenging the classical Gram-positive-Gram-negative divide. We highlight the striking differences in the systems involved in OM biogenesis in Terrabacteria with respect to the classical diderm experimental models and how they provide novel insights into the diversity and biogenesis of the bacterial cell envelope. We also discuss the potential evolutionary steps that might have led to the multiple losses of the OM and speculate on how the very first OM might have emerged before the last bacterial common ancestor.

The translocation assembly module (TAM) catalyzes the assembly of bacterial outer membrane proteins in vitro

The translocation and assembly module (TAM) has been proposed to play a crucial role in the assembly of a small subset of outer membrane proteins (OMPs) in Proteobacteria based on experiments conducted in vivo using tamA and tamB mutant strains and in vitro using biophysical methods. TAM consists of an OMP (TamA) and a periplasmic protein that is anchored to the inner membrane by a single α helix (TamB). Here we examine the function of the purified E. coli complex in vitro after reconstituting it into proteoliposomes. We find that TAM catalyzes the assembly of four model OMPs nearly as well as the β-barrel assembly machine (BAM), a universal heterooligomer that contains a TamA homolog (BamA) and that catalyzes the assembly of almost all E. coli OMPs. Consistent with previous results, both TamA and TamB are required for significant TAM activity. Our study provides direct evidence that TAM can function as an independent OMP insertase and describes a new method to gain insights into TAM function.

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 translocation assembly module (TAM) catalyzes the assembly of bacterial outer membrane proteins in vitro

The bacterial translocation a ssembly m odule (TAM) contains an outer membrane protein (OMP) (TamA) and an elongated periplasmic protein that is anchored to the inner membrane by a single α helix (TamB). TAM has been proposed to play a critical role in the assembly of a small subset of OMPs produced by Proteobacteria based on experiments conducted in vivo using tamA and/or tamB deletion or mutant strains and in vitro using biophysical methods. Recent genetic experiments, however, have strongly suggested that TAM promotes phospholipid homeostasis. To test the idea that TAM catalyzes OMP assembly directly, we examined the function of the purified E. coli complex in vitro after reconstituting it into proteoliposomes. Remarkably, we find that TAM catalyzes the assembly of four model OMPs nearly as well as the β- b arrel a ssembly m achinery (BAM), a universal heterooligomer that contains a TamA homolog (BamA) and that catalyzes the assembly of almost all E. coli OMPs. Consistent with previous results, both TamA and TamB are required for significant TAM activity. Our results provide strong evidence that although their peripheral subunits are unrelated, both BAM and TAM function as independent OMP insertases. Furthermore, our study describes a new method to gain insights into TAM function.

YdbH and YnbE form an intermembrane bridge to maintain lipid homeostasis in the outer membrane of Escherichia coli

The outer membrane (OM) of didermic gram-negative bacteria is essential for growth, maintenance of cellular integrity, and innate resistance to many antimicrobials. Its asymmetric lipid distribution, with phospholipids in the inner leaflet and lipopolysaccharides (LPS) in the outer leaflet, is required for these functions. Lpt proteins form a transenvelope bridge that transports newly synthesized LPS from the inner membrane (IM) to OM, but how the bulk of phospholipids are transported between these membranes is poorly understood. Recently, three members of the AsmA-like protein family, TamB, YhdP, and YdbH, were shown to be functionally redundant and were proposed to transport phospholipids between IM and OM in Escherichia coli. These proteins belong to the repeating β-groove superfamily, which includes eukaryotic lipid-transfer proteins that mediate phospholipid transport between organelles at contact sites. Here, we show that the IM-anchored YdbH protein interacts with the OM lipoprotein YnbE to form a functional protein bridge between the IM and OM in E. coli. Based on AlphaFold-Multimer predictions, genetic data, and in vivo site-directed cross-linking, we propose that YnbE interacts with YdbH through β-strand augmentation to extend the continuous hydrophobic β-groove of YdbH that is thought to shield acyl chains of phospholipids as they travel through the aqueous intermembrane periplasmic compartment. Our data also suggest that the periplasmic protein YdbL prevents extensive amyloid-like multimerization of YnbE in cells. We, therefore, propose that YdbL has a chaperone-like function that prevents uncontrolled runaway multimerization of YnbE to ensure the proper formation of the YdbH-YnbE intermembrane bridge.

The TAM, a Translocation and Assembly Module for protein assembly and potential conduit for phospholipid transfer

The assembly of β-barrel proteins into the bacterial outer membrane is an essential process enabling the colonization of new environmental niches. The TAM was discovered as a module of the β-barrel protein assembly machinery; it is a heterodimeric complex composed of an outer membrane protein (TamA) bound to an inner membrane protein (TamB). The TAM spans the periplasm, providing a scaffold through the peptidoglycan layer and catalyzing the translocation and assembly of β-barrel proteins into the outer membrane. Recently, studies on another membrane protein (YhdP) have suggested that TamB might play a role in phospholipid transport to the outer membrane. Here we review and re-evaluate the literature covering the experimental studies on the TAM over the past decade, to reconcile what appear to be conflicting claims on the function of the TAM.

Redundant essentiality of AsmA-like proteins in Pseudomonas aeruginosa

The outer membrane (OM) is an essential structure of Gram-negative bacteria that provides mechanical strength and protection from large and/or hydrophobic toxic molecules, including many antibiotics. The OM is composed of glycerophospholipids (GPLs) and lipopolysaccharide (LPS) in the inner and outer leaflets, respectively, and hosts integral β-barrel proteins and lipoproteins. While the systems responsible for translocation and insertion of LPS and OM proteins have been elucidated, the mechanism(s) mediating transport of GPLs from the inner membrane to the OM has remained elusive for decades. Very recently, studies performed in Escherichia coli proposed a role in this process for AsmA-like proteins that are predicted to share structural features with eukaryotic lipid transporters. In this study, we provide the first systematic investigation of AsmA-like proteins in a bacterium other than E. coli, the opportunistic human pathogen Pseudomonas aeruginosa. Bioinformatic analyses revealed that P. aeruginosa possesses seven AsmA-like proteins. Deletion of asmA-like genes in many different combinations, coupled with conditional mutagenesis, revealed that four AsmA-like proteins are redundantly essential for growth and OM integrity in P. aeruginosa, including a novel AsmA-like protein (PA4735) that is not present in E. coli. Cells depleted of AsmA-like proteins showed severe defects in the OM permeability barrier that were partially rescued by lowering the synthesis or transport of LPS. Since fine balancing of GPL and LPS levels is crucial for OM integrity, this evidence supports the role of AsmA-like proteins in GPL transport toward the OM.

IMPORTANCE

Given the importance of the outer membrane (OM) for viability and antibiotic resistance in Gram-negative bacteria, in the last decades, several studies have focused on the characterization of the systems involved in OM biogenesis, which have also been explored as targets for antibacterial drug development. However, the mechanism mediating translocation of glycerophospholipids (GPLs) to the OM remained unknown until recent studies provided evidence that AsmA-like proteins could be responsible for this process. Here, we demonstrate for the first time that AsmA-like proteins are essential and redundant for growth and OM integrity in a Gram-negative bacterium other than the model organism Escherichia coli and demonstrate that the human pathogen Pseudomonas aeruginosa has an additional essential AsmA-like protein that is not present in E. coli, thus expanding the range of AsmA-like proteins that play key functions in Gram-negative bacteria.

An octameric PqiC toroid stabilises the outer-membrane interaction of the PqiABC transport system

The E. coli Paraquat Inducible (Pqi) Pathway is a putative Gram-negative phospholipid transport system. The pathway comprises three components: an integral inner membrane protein (PqiA), a periplasmic spanning MCE family protein (PqiB) and an outer membrane lipoprotein (PqiC). Interactions between all complex components, including stoichiometry, remain uncharacterised; nevertheless, once assembled into their quaternary complex, the trio of Pqi proteins are anticipated to provide a continuous channel between the inner and outer membranes of diderms. Here, we present X-ray structures of both the native and a truncated, soluble construct of the PqiC lipoprotein, providing insight into its biological assembly, and utilise neutron reflectometry to characterise the nature of the PqiB-PqiC-membrane interaction. Finally, we employ phenotypic complementation assays to probe specific PqiC residues, which imply the interaction between PqiB and PqiC is less intimate than previously anticipated.

TamAB is regulated by PhoPQ and functions in outer membrane homeostasis during Salmonella pathogenesis

Salmonella survive and replicate in macrophages, which normally kill bacteria by exposing them to a variety of harsh conditions and antimicrobial effectors, many of which target the bacterial cell envelope. The PhoPQ two-component system responds to the phagosome environment and induces factors that protect the outer membrane, allowing adaptation and growth in the macrophage. We show that PhoPQ induces the transcription of the tamAB operon both in vitro and in macrophages. The TamA protein is structurally similar to BamA, an essential protein in the Bam complex that assembles β-barrel proteins in the outer membrane, while TamB is an AsmA-family protein implicated in lipid transport between the inner and outer membranes. We show that the Bam machinery is stressed in vitro under low Mg2+, low pH conditions that mimic the phagosome. Not surprisingly, mutations affecting Bam function confer significant virulence defects. Although loss of TamAB alone confers no virulence defect, a tamAB deletion confers a synthetic phenotype in bam mutant backgrounds in animals and macrophages, and in vitro upon treatment with vancomycin or sodium dodecyl sulfate. Mutations affecting YhdP, which functions in partial redundancy with TamB, also confer synthetic phenotypes with bam mutations in the animal, but this interaction is not evident in vitro. Thus, in the harsh phagocytic environment of the macrophage, the outer membrane Bam machinery is compromised, and the TamAB system, and perhaps other PhoPQ-regulated factors, is induced to compensate. It is most likely that TamAB and other systems assist the Bam complex indirectly by affecting outer membrane properties. IMPORTANCE The TamAB system has been implicated in both outer membrane protein localization and phospholipid transport between the inner and outer membranes. We show that the β-barrel protein assembly complex, Bam, is stressed under conditions thought to mimic the macrophage phagosome. TamAB expression is controlled by the PhoPQ two-component system and induced in macrophages. This system somehow compensates for the Bam complex as evidenced by the fact that mutations affecting the two systems confer synthetic phenotypes in animals, macrophages, and in vitro in the presence of vancomycin or SDS. This study has implications concerning the role of TamAB in outer membrane homeostasis. It also contributes to our understanding of the systems necessary for Salmonella to adapt and reproduce within the macrophage phagosome.

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.

Differential Genetic Strategies of Burkholderia vietnamiensis and Paraburkholderia kururiensis for Root Colonization of Oryza sativa subsp. japonica and O. sativa subsp. indica , as Revealed by Transposon Mutagenesis Sequencing

Burkholderiaceae are frequent and abundant colonizers of the rice rhizosphere and interesting candidates to investigate for growth promotion. Species of Paraburkholderia have repeatedly been described to stimulate plant growth.

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.

Adaptation of the periplasm to maintain spatial constraints essential for cell envelope processes and cell viability

The cell envelope of Gram-negative bacteria consists of two membranes surrounding a periplasm and peptidoglycan layer. Molecular machines spanning the cell envelope depend on spatial constraints and load-bearing forces across the cell envelope and surface. The mechanisms dictating spatial constraints across the cell envelope remain incompletely defined. In Escherichia coli, the coiled-coil lipoprotein Lpp contributes the only covalent linkage between the outer membrane and the underlying peptidoglycan layer. Using proteomics, molecular dynamics, and a synthetic lethal screen, we show that lengthening Lpp to the upper limit does not change the spatial constraint but is accommodated by other factors which thereby become essential for viability. Our findings demonstrate E. coli expressing elongated Lpp does not simply enlarge the periplasm in response, but the bacteria accommodate by a combination of tilting Lpp and reducing the amount of the covalent bridge. By genetic screening, we identified all of the genes in E. coli that become essential in order to enact this adaptation, and by quantitative proteomics discovered that very few proteins need to be up- or down-regulated in steady-state levels in order to accommodate the longer Lpp. We observed increased levels of factors determining cell stiffness, a decrease in membrane integrity, an increased membrane vesiculation and a dependance on otherwise non-essential tethers to maintain lipid transport and peptidoglycan biosynthesis. Further this has implications for understanding how spatial constraint across the envelope controls processes such as flagellum-driven motility, cellular signaling, and protein translocation

A noncanonical chaperone interacts with drug efflux pumps during their assembly into bacterial outer membranes

Bacteria have membrane-spanning efflux pumps to secrete toxic compounds ranging from heavy metal ions to organic chemicals, including antibiotic drugs. The overall architecture of these efflux pumps is highly conserved: with an inner membrane energy-transducing subunit coupled via an adaptor protein to an outer membrane conduit subunit that enables toxic compounds to be expelled into the environment. Here, we map the distribution of efflux pumps across bacterial lineages to show these proteins are more widespread than previously recognised. Complex phylogenetics support the concept that gene cassettes encoding the subunits for these pumps are commonly acquired by horizontal gene transfer. Using TolC as a model protein, we demonstrate that assembly of conduit subunits into the outer membrane uses the chaperone TAM to physically organise the membrane-embedded staves of the conduit subunit of the efflux pump. The characteristics of this assembly pathway have impact for the acquisition of efflux pumps across bacterial species and for the development of new antimicrobial compounds that inhibit efflux pump function.

A crosslinking study reveals novel insights into how the chaperone TAM helps Gram-negative bacteria insert the drug efflux pump subunit TolC into their outer membrane. Bioinformatic analyses show that TolC-like proteins can be found in all LPS-containing bacteria, but also in some monodermic Firmicutes.

Accelerating biological insight for understudied genes

The rapid expansion of genome sequence data is increasing the discovery of protein-coding genes across all domains of life. Annotating these genes with reliable functional information is necessary to understand evolution, to define the full biochemical space accessed by nature, and to identify target genes for biotechnology improvements. The vast majority of proteins are annotated based on sequence conservation with no specific biological, biochemical, genetic, or cellular function identified. Recent technical advances throughout the biological sciences enable experimental research on these understudied protein-coding genes in a broader collection of species. However, scientists have incentives and biases to continue focusing on well documented genes within their preferred model organism. This perspective suggests a research model that seeks to break historic silos of research bias by enabling interdisciplinary teams to accelerate biological functional annotation. We propose an initiative to develop coordinated projects of collaborating evolutionary biologists, cell biologists, geneticists, and biochemists that will focus on subsets of target genes in multiple model organisms. Concurrent analysis in multiple organisms takes advantage of evolutionary divergence and selection, which causes individual species to be better suited as experimental models for specific genes. Most importantly, multisystem approaches would encourage transdisciplinary critical thinking and hypothesis testing that is inherently slow in current biological research.

Membrane thinning and lateral gating are consistent features of BamA across multiple species

In Gram-negative bacteria, the folding and insertion of β-barrel outer membrane proteins (OMPs) to the outer membrane are mediated by the β-barrel assembly machinery (BAM) complex. Two leading models of this process have been put forth: the hybrid barrel model, which claims that a lateral gate in BamA’s β-barrel can serve as a template for incoming OMPs, and the passive model, which claims that a thinned membrane near the lateral gate of BamA accelerates spontaneous OMP insertion. To examine the key elements of these two models, we have carried out 45.5 μs of equilibrium molecular dynamics simulations of BamA with and without POTRA domains from Escherichia coli, Salmonella enterica, Haemophilus ducreyi and Neisseria gonorrhoeae, together with BamA’s homolog, TamA from E. coli, in their native, species-specific outer membranes. In these equilibrium simulations, we consistently observe membrane thinning near the lateral gate for all proteins. We also see occasional spontaneous lateral gate opening and sliding of the β-strands at the gate interface for N. gonorrhoeae, indicating that the gate is dynamic. An additional 14 μs of free-energy calculations shows that the energy necessary to open the lateral gate in BamA/TamA varies by species, but is always lower than the Omp85 homolog, FhaC. Our combined results suggest OMP insertion utilizes aspects of both the hybrid barrel and passive models.

Gram-negative bacteria such as Escherichia coli have a second, outer membrane surrounding them. This outer membrane provides an additional layer of protection, but also presents an additional challenge in its construction, exacerbated by the lack of chemical energy in this region of the bacterial cell. For example, proteins in the outer membrane are inserted via BamA, itself an integral membrane protein. The precise mechanisms by which BamA assists in the insertion process are still unclear. Here, we use extensive simulations in atomistic detail of BamA from multiple species in its native outer membrane environment to shed light on this process. We find that the lateral gate of BamA, a proposed pathway into the membrane, is dynamic, although to a degree varying by species. On the other hand, thinning of the outer membrane near BamA’s lateral gate is observed consistently across all simulations. We conclude that multiple features of BamA contribute to protein insertion into the outer membrane.

The Translocation and Assembly Module (TAM) of Edwardsiella tarda Is Essential for Stress Resistance and Host Infection

Translocation and assembly module (TAM) is a protein channel known to mediate the secretion of virulence factors during pathogen infection. Edwardsiella tarda is a Gram-negative bacterium that is pathogenic to a wide range of farmed fish and other hosts including humans. In this study, we examined the function of the two components of the TAM, TamA and TamB, of E. tarda (named tamA_Et and tamB_Et, respectively). TamA_Et was found to localize on the surface of E. tarda and be recognizable by TamA_Et antibody. Compared to the wild type, the tamA and tamB knockouts, TX01ΔtamA and TX01ΔtamB, respectively, were significantly reduced in motility, flagella formation, invasion into host cells, intracellular replication, dissemination in host tissues, and inducing host mortality. The lost virulence capacities of TX01ΔtamA and TX01ΔtamB were restored by complementation with the tamA_Et and tamB_Et genes, respectively. Furthermore, TX01ΔtamA and TX01ΔtamB were significantly impaired in the ability to survive under low pH and oxidizing conditions, and were unable to maintain their internal pH balance and cellular structures in acidic environments, which led to increased susceptibility to lysozyme destruction. Taken together, these results indicate that TamA_Et and TamB_Et are essential for the virulence of E. tarda and required for E. tarda to survive under stress conditions.

Patterns of inter- and intrasubspecific homologous recombination inform eco-evolutionary dynamics of Xylella fastidiosa

High rates of homologous recombination (HR) in the bacterial plant pathogen Xylella fastidiosa have been previously detected. This study aimed to determine the extent and explore the ecological significance of HR in the genomes of recombinants experimentally generated by natural transformation and wild-type isolates. Both sets of strains displayed widespread HR and similar average size of recombined fragments consisting of random events (2-10 kb) of inter- and intrasubspecific recombination. A significantly higher proportion and greater lengths (>10 kb, maximum 31.5 kb) of recombined fragments were observed in subsp. morus and in strains isolated in Europe from intercepted coffee plants shipped from the Americas. Such highly recombinant strains pose a serious risk of emergence of novel variants, as genetically distinct and formerly geographically isolated genotypes are brought in close proximity by global trade. Recently recombined regions in wild-type strains included genes involved in regulation and signaling, host colonization, nutrient acquisition, and host evasion, all fundamental traits for X. fastidiosa ecology. Identification of four recombinant loci shared between wild-type and experimentally generated recombinants suggests potential hotspots of recombination in this naturally competent pathogen. These findings provide insights into evolutionary forces possibly affecting the adaptive potential to colonize the host environments of X. fastidiosa.

The Treponema pallidum Outer Membrane

The outer membrane (OM) of Treponema pallidum, the uncultivatable agent of venereal syphilis, has long been the subject of misconceptions and controversy. Decades ago, researchers postulated that T. pallidum's poor surface antigenicity is the basis for its ability to cause persistent infection, but they mistakenly attributed this enigmatic property to the presence of a protective outer coat of serum proteins and mucopolysaccharides. Subsequent studies revealed that the OM is the barrier to antibody binding, that it contains a paucity of integral membrane proteins, and that the preponderance of the spirochete's immunogenic lipoproteins is periplasmic. Since the advent of recombinant DNA technology, the fragility of the OM, its low protein content, and the lack of sequence relatedness between T. pallidum and Gram-negative outer membrane proteins (OMPs) have complicated efforts to characterize molecules residing at the host-pathogen interface. We have overcome these hurdles using the genomic sequence in concert with computational tools to identify proteins predicted to form β-barrels, the hallmark conformation of OMPs in double-membrane organisms and evolutionarily related eukaryotic organelles. We also have employed diverse methodologies to confirm that some candidate OMPs do, in fact, form amphiphilic β-barrels and are surface-exposed in T. pallidum. These studies have led to a structural homology model for BamA and established the bipartite topology of the T. pallidum repeat (Tpr) family of proteins. Recent bioinformatics has identified several structural orthologs for well-characterized Gram-negative OMPs, suggesting that the T. pallidum OMP repertoire is more Gram-negative-like than previously supposed. Lipoprotein adhesins and proteases on the spirochete surface also may contribute to disease pathogenesis and protective immunity.

Biogenesis and function of the autotransporter adhesins YadA, intimin and invasin

Bacteria often express numerous virulence factors. These virulence factors make them successful pathogens, by e.g. mediating attachment to host cells and thereby facilitating persistence or invasion, or by contributing to the evasion of the host immune system to allow proliferation and spread within the host and in the environment. The site of first contact of Gram negative bacteria with the host is the bacterial outer membrane (OM). Consisting of an asymmetrical lipid bilayer with phospholipids forming the inner, and lipopolysaccharides forming the outer leaflet, the OM harbors numerous integral membrane proteins that are almost exclusively β-barrel proteins. One distinct family of OM β-barrel proteins strongly linked to bacterial virulence are the autotransporter (AT) proteins. During the last years huge progress has been made to better understand the mechanisms underlying the insertion of AT proteins into the OM and also AT function for interaction with the host. This review shortly summarizes our current knowledge about outer membrane protein (OMP) and more specifically AT biogenesis and function. We focused on the AT proteins that we haved studied in most detail: i.e. the Yersinia adhesin A (YadA) and invasin of Yersinia enterocolitica (Ye) as well as its homolog intimin (Int) expressed by enteropathogenic Escherichia coli. In addition, this review provides a short outlook about how we could possibly use this knowledge to fight infection.

BamA is a pivotal protein in cell envelope synthesis and cell division in Deinococcus radiodurans

The beta-barrel assembly machinery (BAM) is an indispensable complex for protein transportation located at the outer membrane of bacteria. BAM is composed of five subunits (BamA-E) in the model bacterium Escherichia coli. DR_0379 is a BamA homolog in Deinococcus radiodurans, but the other subunits have not been detected in this species. In the present study, deletion of bamA resulted in decreased growth rate and altered morphology of D. radiodurans. ΔbamA cells underwent abnormal cell division, leading to aggregated bacteria of diverse size and shape, and the cell envelope was detached from the cell surface, resulting in reduced resistance to high ionic strength. Oxidative stress resistance was significantly enhanced in the mutant, which may be attributed to increased manganese ion concentration and Mn/Fe ratio. Numerous proteins were released into the medium from ΔbamA cells, including surface layer (S-layer) proteins and various transporters located in the periplasm and outer membrane. These results indicate that BamA affects the synthesis and assembly of the outer membrane and S-layer, and thereby influences material transport and cell division. The findings highlight the special functions of BamA in D. radiodurans, and promote our understanding of the multi-layer structure of the D. radiodurans cell envelope.

MapB, the Brucella suis TamB homologue, is involved in cell envelope biogenesis, cell division and virulence

Brucella species are Gram-negative, facultative intracellular pathogens responsible for a worldwide zoonosis. The envelope of Brucella exhibits unique characteristics that make these bacteria furtive pathogens and resistant to several host defence compounds. We have identified a Brucella suis gene (mapB) that appeared to be crucial for cell envelope integrity. Indeed, the typical resistance of Brucella to both lysozyme and the cationic lipopeptide polymyxin B was markedly reduced in a ∆mapB mutant. MapB turned out to represent a TamB orthologue. This last protein, together with TamA, a protein belonging to the Omp85 family, form a complex that has been proposed to participate in the translocation of autotransporter proteins across the outer membrane (OM). Accordingly, we observed that MapB is required for proper assembly of an autotransporter adhesin in the OM, as most of the autotransporter accumulated in the mutant cell periplasm. Both assessment of the relative amounts of other specific outer membrane proteins (OMPs) and a proteome approach indicated that the absence of MapB did not lead to an extensive alteration in OMP abundance, but to a reduction in the relative amounts of a protein subset, including proteins from the Omp25/31 family. Electron microscopy revealed that ∆mapB cells exhibit multiple anomalies in cell morphology, indicating that the absence of the TamB homologue in B. suis severely affects cell division. Finally, ∆mapB cells were impaired in macrophage infection and showed an attenuated virulence phenotype in the mouse model. Collectively, our results indicate that the role of B. suis TamB homologue is not restricted to participating in the translocation of autotransporters across the OM but that it is essential for OM stability and protein composition and that it is involved in cell envelope biogenesis, a process that is inherently coordinated with cell division.

The TAM: A Translocation and Assembly Module of the β-Barrel Assembly Machinery in Bacterial Outer Membranes

Assembly of proteins into the outer membrane is an essential process in the cell biology of bacteria. The integration of β-barrel proteins into the outer membrane is mediated by a system referred to as the β-barrel assembly machinery (BAM) that includes two related proteins: BamA in the BAM complex and TamA in the TAM (translocation and assembly module). Here we review what is known about the TAM in terms of its function and the structural architecture of its two subunits, TamA and TamB. By linking the energy transduction possibilities in the inner membrane to TamA in the outer membrane, the TAM provides additional capability to the β-barrel assembly machinery. Conservation of the TAM across evolutionary boundaries, and the presence of hybrid BAM/TAM complexes in some bacterial lineages, adds insight to our growing understanding of how bacterial outer membranes are built.

Constraints on lateral gene transfer in promoting fimbrial usher protein diversity and function

Fimbriae are long, adhesive structures widespread throughout members of the family Enterobacteriaceae. They are multimeric extrusions, which are moved out of the bacterial cell through an integral outer membrane protein called usher. The complex folding mechanics of the usher protein were recently revealed to be catalysed by the membrane-embedded translocation and assembly module (TAM). Here, we examine the diversity of usher proteins across a wide range of extraintestinal (ExPEC) and enteropathogenic (EPEC) Escherichia coli, and further focus on a so far undescribed chaperone–usher system, with this usher referred to as UshC. The fimbrial system containing UshC is distributed across a discrete set of EPEC types, including model strains like E2348/67, as well as ExPEC ST131, currently the most prominent multi-drug-resistant uropathogenic E. coli strain worldwide. Deletion of the TAM from a naive strain of E. coli results in a drastic time delay in folding of UshC, which can be observed for a protein from EPEC as well as for two introduced proteins from related organisms, Yersinia and Enterobacter. We suggest that this models why the TAM machinery is essential for efficient folding of proteins acquired via lateral gene transfer.

An investigation into the Omp85 protein BamK in hypervirulent Klebsiella pneumoniae, and its role in outer membrane biogenesis

Members of the Omp85 protein superfamily have important roles in Gram-negative bacteria, with the archetypal protein BamA being ubiquitous given its essential function in the assembly of outer membrane proteins. In some bacterial lineages, additional members of the family exist and, in most of these cases, the function of the protein is unknown. We detected one of these Omp85 proteins in the pathogen Klebsiella pneumoniae B5055, and refer to the protein as BamK. Here, we show that bamK is a conserved element in the core genome of Klebsiella, and its expression rescues a loss-of-function ∆bamA mutant. We developed an E. coli model system to measure and compare the specific activity of BamA and BamK in the assembly reaction for the critical substrate LptD, and find that BamK is as efficient as BamA in assembling the native LptDE complex. Comparative structural analysis revealed that the major distinction between BamK and BamA is in the external facing surface of the protein, and we discuss how such changes may contribute to a mechanism for resistance against infection by bacteriophage.

The Structure of a Conserved Domain of TamB Reveals a Hydrophobic β Taco Fold

The translocation and assembly module (TAM) plays a role in the transport and insertion of proteins into the bacterial outer membrane. TamB, a component of this system spans the periplasmic space to engage with its partner protein TamA. Despite efforts to characterize the TAM, the structure and mechanism of action of TamB remained enigmatic. Here we present the crystal structure of TamB amino acids 963–1,138. This region represents half of the conserved DUF490 domain, the defining feature of TamB. TamB_963-1138 consists of a concave, taco-shaped β sheet with a hydrophobic interior. This β taco structure is of dimensions capable of accommodating and shielding the hydrophobic side of an amphipathic β strand, potentially allowing TamB to chaperone nascent membrane proteins from the aqueous environment. In addition, sequence analysis suggests that the structure of TamB_963-1138 is shared by a large portion of TamB. This architecture could allow TamB to act as a conduit for membrane proteins.

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The structure of TamB_963-1138 reveals a β taco fold with a hydrophobic interior

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The hydrophobic interior of TamB_963-1138 could chaperone hydrophobic β strands

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Structural analysis suggests the majority of TamB forms a hydrophobic groove

Job contenders: roles of the β-barrel assembly machinery and the translocation and assembly module in autotransporter secretion

In Gram-negative bacteria, autotransporters secrete effector protein domains that are linked to virulence. Although they were once thought to be simple and autonomous secretion machines, mounting evidence reveals that multiple factors of the bacterial envelope are necessary for autotransporter assembly. Secretion across the outer membrane of their soluble effector "passenger domain" is promoted by the assembly of an outer membrane-spanning "β-barrel domain". Both reactions require BamA, an essential component of the β-barrel assembly machinery (BAM complex) that catalyzes the final reaction step by which outer membrane proteins are integrated into the lipid bilayer. A large amount of data generated in the last decade has shed key insights onto the mechanistic coordination of autotransporter β-barrel domain assembly and passenger domain secretion. These results, together with the recently solved structures of the BAM complex, offer an unprecedented opportunity to discuss a detailed model of autotransporter assembly. Importantly, some autotransporters benefit from the presence of an additional machinery, the translocation and assembly module (TAM), a two-membrane spanning complex, which contains a BamA-homologous subunit. Although it remains unclear how the BAM complex and the TAM cooperate, it is evident that multiple preparatory steps are necessary for efficient autotransporter biogenesis.

Structural basis for substrate selection by the translocation and assembly module of the β-barrel assembly machinery

The assembly of proteins into bacterial outer membranes is a key cellular process that we are only beginning to understand, mediated by the β-barrel assembly machinery (BAM). Two crucial elements of that machinery are the core BAM complex and the translocation and assembly module (TAM), with each containing a member of the Omp85 superfamily of proteins: BamA in the BAM complex, TamA in the TAM. Here, we used the substrate protein FimD as a model to assess the selectivity of substrate interactions for the TAM relative to those of the BAM complex. A peptide scan revealed that TamA and BamA bind the β-strands of FimD, and do so selectively. Chemical cross-linking and molecular dynamics are consistent with this interaction taking place between the first and last strand of the TamA barrel domain, providing the first experimental evidence of a lateral gate in TamA: a structural element implicated in membrane protein assembly. We suggest that the lateral gates in TamA and BamA provide different environments for substrates to engage, with the differences observed here beginning to address how the TAM can be more effective than the BAM complex in the folding of some substrate proteins.

The modular nature of the β-barrel assembly machinery, illustrated in Borrelia burgdorferi

Diderm bacteria have an outer membrane that provides defense against environmental factors including antibiotics. Understanding the process of outer membrane biogenesis is, therefore, of critical importance in order to envisage new treatments of these bacterial pathogens. Borrelia burgdorferi is the pathogen responsible for Lyme disease. Its outer membrane contains integral, β-barrel proteins as well as swathes of externally exposed lipoproteins. Previous work has demonstrated that the β-barrel assembly machine (BAM complex) in B. burgdorferi and other Spirochetes shares several similarities with the BAM complex in other bacterial lineages, such as the Proteobacteria that includes Escherichia coli. However, Iqbal et al. () have identified the inner membrane protein TamB as a subunit of the BAM complex in Spirochetes. This latest study highlights the modular nature of the BAM complex, and suggests that in some bacterial lineages the BAM complex and translocation and assembly module (the TAM) function as a single unit.

The TamB ortholog of Borrelia burgdorferi interacts with the β-barrel assembly machine (BAM) complex protein BamA

Two outer membrane protein (OMP) transport systems in diderm bacteria assist in assembly and export of OMPs. These two systems are the β-barrel assembly machine (BAM) complex and the translocation and assembly module (TAM). The BAM complex consists of the OMP component BamA along with several outer membrane associated proteins. The TAM also consists of an OMP, designated TamA, and a single inner membrane (IM) protein, TamB. Together TamA and TamB aid in the secretion of virulence-associated OMPs. In this study we characterized the hypothetical protein BB0794 in Borrelia burgdorferi. BB0794 contains a conserved DUF490 domain, which is a motif found in all TamB proteins. All spirochetes lack a TamA ortholog, but computational and physicochemical characterization of BB0794 revealed it is a TamB ortholog. Interestingly, BB0794 was observed to interact with BamA and a BB0794 regulatable mutant displayed altered cellular morphology and antibiotic sensitivity. The observation that B. burgdorferi contains a TamB ortholog that interacts with BamA and is required for proper outer membrane biogenesis not only identifies a novel role for TamB-like proteins, but also may explain why most diderms harbor a TamB-like protein while only a select group encodes TamA.

Conserved Features in the Structure, Mechanism, and Biogenesis of the Inverse Autotransporter Protein Family

The bacterial cell surface proteins intimin and invasin are virulence factors that share a common domain structure and bind selectively to host cell receptors in the course of bacterial pathogenesis. The β-barrel domains of intimin and invasin show significant sequence and structural similarities. Conversely, a variety of proteins with sometimes limited sequence similarity have also been annotated as “intimin-like” and “invasin” in genome datasets, while other recent work on apparently unrelated virulence-associated proteins ultimately revealed similarities to intimin and invasin. Here we characterize the sequence and structural relationships across this complex protein family. Surprisingly, intimins and invasins represent a very small minority of the sequence diversity in what has been previously the “intimin/invasin protein family”. Analysis of the assembly pathway for expression of the classic intimin, EaeA, and a characteristic example of the most prevalent members of the group, FdeC, revealed a dependence on the translocation and assembly module as a common feature for both these proteins. While the majority of the sequences in the grouping are most similar to FdeC, a further and widespread group is two-partner secretion systems that use the β-barrel domain as the delivery device for secretion of a variety of virulence factors. This comprehensive analysis supports the adoption of the “inverse autotransporter protein family” as the most accurate nomenclature for the family and, in turn, has important consequences for our overall understanding of the Type V secretion systems of bacterial pathogens.

Genetic analysis of Vibrio parahaemolyticus intestinal colonization

Vibrio parahaemolyticus is the most common cause of seafood-borne gastroenteritis worldwide and a blight on global aquaculture. This organism requires a horizontally acquired type III secretion system (T3SS2) to infect the small intestine, but knowledge of additional factors that underlie V. parahaemolyticus pathogenicity is limited. We used transposon-insertion sequencing to screen for genes that contribute to viability of V. parahaemolyticus in vitro and in the mammalian intestine. Our analysis enumerated and controlled for the host infection bottleneck, enabling robust assessment of genetic contributions to in vivo fitness. We identified genes that contribute to V. parahaemolyticus colonization of the intestine independent of known virulence mechanisms in addition to uncharacterized components of T3SS2. Our study revealed that toxR, an ancestral locus in Vibrio species, is required for V. parahaemolyticus fitness in vivo and for induction of T3SS2 gene expression. The regulatory mechanism by which V. parahaemolyticus ToxR activates expression of T3SS2 resembles Vibrio cholerae ToxR regulation of distinct virulence elements acquired via lateral gene transfer. Thus, disparate horizontally acquired virulence systems have been placed under the control of this ancestral transcription factor across independently evolved human pathogens.

Inner-membrane protein MorC is involved in fimbriae production and biofilm formation in Aggregatibacter actinomycetemcomitans

Fimbrial subunit synthesis, secretion and assembly on the surface of the periodontal pathogen Aggregatibacter actinomycetemcomitans are essential for biofilm formation. A recent quantitative proteomics study employing an afimbriated strain and a developed mutant isogenic for the inner-membrane protein morphogenesis protein C (MorC) revealed that the abundance of the proteins of the fimbrial secretion apparatus in the membrane is dependent on MorC. To investigate further the relationship between MorC and fimbriation, we identified and complemented the defect in fimbriae production in the afimbriated laboratory strain. The transformed strain expressing a plasmid containing genes encoding the WT fimbrial subunit and the prepilin peptidase displayed all of the hallmarks of a fimbriated bacterium including the distinct star-like colony morphology, robust biofilm formation, biofilm architecture composed of discrete microcolonies and the presence of fimbriae. When the identical plasmid was transformed into a morC mutant strain, the bacterium did not display any of the phenotypes of fimbriated strains. Extension of these studies to a naturally fimbriated clinical strain showed that the resulting morC mutant maintained the characteristic colony morphology of fimbriated strains. There was, however, a reduction in the secretion of fimbrial subunits, and fewer fimbriae were observed on the surface of the mutant strain. Furthermore, the morC mutant of the fimbriated strain displayed a significantly altered biofilm microcolony architecture, while maintaining a similar biofilm mass to the parent strain. These results suggest that MorC influences fimbrial secretion and microcolony formation in A. actinomycetemcomitans.