Omp85 from the thermophilic cyanobacterium Thermosynechococcus elongatus differs from proteobacterial Omp85 in structure and domain composition

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.

Cell envelope diversity and evolution across the bacterial tree of life

The bacterial cell envelope is a complex multilayered structure conserved across all bacterial phyla. It is categorized into two main types based on the number of membranes surrounding the cell. Monoderm bacteria are enclosed by a single membrane, whereas diderm cells are distinguished by the presence of a second, outer membrane (OM). An ancient divide in the bacterial domain has resulted in two major clades: the Gracilicutes, consisting strictly of diderm phyla; and the Terrabacteria, encompassing monoderm and diderm species with diverse cell envelope architectures. Recent structural and phylogenetic advancements have improved our understanding of the diversity and evolution of the OM across the bacterial tree of life. Here we discuss cell envelope variability within major bacterial phyla and focus on conserved features found in diderm lineages. Characterizing the mechanisms of OM biogenesis and the evolutionary gains and losses of the OM provides insights into the primordial cell and the last universal common ancestor from which all living organisms subsequently evolved.

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.

A minimum functional form of the Escherichia coli BAM complex constituted by BamADE assembles outer membrane proteins in vitro

The biogenesis of outer membrane proteins is mediated by the β-barrel assembly machinery (BAM), which is a heteropentomeric complex composed of five proteins named BamA-E in Escherichia coli. Despite great progress in the BAM structural analysis, the molecular details of BAM-mediated processes as well as the exact function of each BAM component during OMP assembly are still not fully understood. To enable a distinguishment of the function of each BAM component, it is the aim of the present work to examine and identify the effective minimum form of the E. coli BAM complex by use of a well-defined reconstitution strategy based on a previously developed versatile assay. Our data demonstrate that BamADE is the core BAM component and constitutes a minimum functional form for OMP assembly in E. coli, which can be stimulated by BamB and BamC. While BamB and BamC have a redundant function based on the minimum form, both together seem to cooperate with each other to substitute for the function of the missing BamD or BamE. Moreover, the BamAE470K mutant also requires the function of BamD and BamE to assemble OMPs in vitro, which vice verse suggests that BamADE are the effective minimum functional form of the E. coli BAM complex.

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.

TtOmp85, a single Omp85 member protein functions as a β-barrel protein insertase and an autotransporter translocase without any accessory proteins

The biogenesis of outer membrane proteins requires the function of β-barrel assembly machinery (BAM), whose function is highly conserved while its composition is variable. The Escherichia coli BAM is composed of five subunits, while Thermus thermophilus seems to contain a single BAM protein, named TtOmp85. To search for the primitive form of a functional BAM, we investigated and compared the function of TtOmp85 and E. coli BAM by use of a reconstitution assay that examines the integration of OmpA and BamA from E. coli and TtoA from T. thermophilus, as well as the translocation of the E. coli Ag43. Our results show that a single TtOmp85 protein can substitute for the collective function of the five subunits constituting E. coli BAM.

Building Better Barrels - β-barrel Biogenesis and Insertion in Bacteria and Mitochondria

β-barrel proteins are folded and inserted into outer membranes by multi-subunit protein complexes that are conserved across different types of outer membranes. In Gram-negative bacteria this complex is the barrel-assembly machinery (BAM), in mitochondria it is the sorting and assembly machinery (SAM) complex, and in chloroplasts it is the outer envelope protein Oep80. Mitochondrial β-barrel precursor proteins are translocated from the cytoplasm to the intermembrane space by the translocase of the outer membrane (TOM) complex, and stabilized by molecular chaperones before interaction with the assembly machinery. Outer membrane bacterial BamA interacts with four periplasmic accessory proteins, whereas mitochondrial Sam50 interacts with two cytoplasmic accessory proteins. Despite these major architectural differences between BAM and SAM complexes, their core proteins, BamA and Sam50, seem to function the same way. Based on the new SAM complex structures, we propose that the mitochondrial β-barrel folding mechanism follows the budding model with barrel-switching aiding in the release of new barrels. We also built a new molecular model for Tom22 interacting with Sam37 to identify regions that could mediate TOM-SAM supercomplex formation.

Substrate-dependent arrangements of the subunits of the BAM complex determined by neutron reflectometry

In Gram-negative bacteria, the β-barrel assembly machinery (BAM) complex catalyses the assembly of β-barrel proteins into the outer membrane, and is composed of five subunits: BamA, BamB, BamC, BamD and BamE. Once assembled, - β-barrel proteins can be involved in various functions including uptake of nutrients, export of toxins and mediating host-pathogen interactions, but the precise mechanism by which these ubiquitous and often essential β-barrel proteins are assembled is yet to be established. In order to determine the relative positions of BAM subunits in the membrane environment we reconstituted each subunit into a biomimetic membrane, characterizing their interaction and structural changes by Quartz Crystal Microbalance with Dissipation monitoring (QCM-D) and neutron reflectometry. Our results suggested that the binding of BamE, or a BamDE dimer, to BamA induced conformational changes in the polypeptide transported-associated (POTRA) domains of BamA, but that BamB or BamD alone did not promote any such changes. As monitored by neutron reflectometry, addition of an unfolded substrate protein extended the length of POTRA domains further away from the membrane interface as part of the mechanism whereby the substrate protein was folded into the membrane.

Genomic insights into cyanobacterial protein translocation systems

Cyanobacteria are ubiquitous oxygenic photosynthetic bacteria with a versatile metabolism that is highly dependent on effective protein targeting. Protein sorting in diderm bacteria is not trivial and, in cyanobacteria, even less so due to the presence of a complex membrane system: the outer membrane, the plasma membrane and the thylakoid membrane. In cyanobacteria, protein import into the thylakoids is essential for photosynthesis, export to the periplasm fulfills a multifunctional role in maintaining cell homeostasis, and secretion mediates motility, DNA uptake and environmental interactions. Intriguingly, only one set of genes for the general secretory and the twin-arginine translocation pathways seem to be present. However, these systems have to operate in both plasma and thylakoid membranes. This raises the question of how substrates are recognized and targeted to their correct, final destination. Additional complexities arise when a protein has to be secreted across the outer membrane, where very little is known regarding the mechanisms involved. Given their ecological importance and biotechnological interest, a better understanding of protein targeting in cyanobacteria is of great value. This review will provide insights into the known knowns of protein targeting, propose hypotheses based on available genomic sequences and discuss future directions.

The transmembrane supercomplex mediating the biogenesis of OMPs in Gram-negative bacteria assumes a circular conformational change upon activation

The cell envelope of Gram-negative bacteria is composed of the inner (plasma) and the outer membrane. In the outer membrane, the outer membrane β-barrel proteins (OMPs) serve multiple functions. They are synthesized in the cytoplasm and finally inserted into the outer membrane through a critical and complex pathway facilitated by many protein factors. Recently, a new model for the biogenesis of OMPs in Gram-negative bacteria was proposed, in which a supercomplex containing multiple proteins spans the inner and outer membrane, to mediate the biogenesis of OMPs. The core part of the transmembrane supercomplex is the inner membrane protein translocon and the outer membrane β-barrel assembly machinery (BAM) complex. Some components of the supercomplex, such as the BamA subunit of the BAM complex, are essential and conserved across species. The other components, for example, the BamB subunit and the primary periplasmic chaperone SurA, are also required for the supercomplex to gain complete function and full efficiency. How BamB and SurA behave in the supercomplex, however, is less well understood. Therefore, the crosstalk between BamA, BamB and SurA was investigated mainly through in vivo protein photo-cross-linking experiments and protein modeling. Moreover, theoretical structures for part of the supercomplex consisting of SurA and the BAM complex were constructed. The modeling data are consistent with the experimental results. The theoretical structures computed in this work provide a more comprehensive view of the mechanism of the supercomplex, demonstrating a circular conformational change of the supercomplex when it is active.

Identification of a Compound That Inhibits the Growth of Gram-Negative Bacteria by Blocking BamA-BamD Interaction

The demand for novel antibiotics is imperative for drug-resistant Gram-negative bacteria which causes diverse intractable infection disease in clinic. Here, a comprehensive screening was implemented to identify potential agents that disrupt the assembly of β-barrel outer-membrane proteins (OMPs) in the outer membrane (OM) of Gram-negative bacteria. The assembly of OMPs requires ubiquitous β-barrel assembly machinery (BAM). Among the five protein subunits in BAM, the interaction between BamA and BamD is essential for the function of this complex. We first established a yeast two-hybrid (Y2H) system to confirm the interaction between BamA and BamD, and then screened agents that specifically disrupt this interaction. From this screen, we identified a compound IMB-H4 that specially blocks BamA–BamD interaction and selectively inhibits the growth of Escherichia coli and other Gram-negative bacteria. Moreover, our results suggest that IMB-H4 disrupts BamA–BamD interaction by binding to BamA. Strikingly, E. coli cells having been treated with IMB-H4 showed impaired OM integrity and decreased the abundance of OMPs. Therefore, an antibacterial agent was identified successfully using Y2H system, and this compound likely blocks the assembly of OMPs by targeting BamA–BamD interaction in Gram-negative bacteria.

Structural insights into the mechanism of a novel protein targeting pathway in Gram-negative bacteria

Many nascent polypeptides synthesized in the cytoplasm are translocated across membranes via a specific ‘translocon’ composed of protein complexes. Recently, a novel targeting pathway for the outer membrane β‐barrel proteins (OMPs) in Gram‐negative bacteria was discovered. The cell envelope of Gram‐negative bacteria is composed of the inner (plasma) membrane (IM) and the outer membrane (OM). In this new pathway, a SecA^N protein, which is mainly present in the IM as a homo‐oligomer, translocates nascent OMPs across the IM; at the same time, SecA^N directly interacts with the β‐barrel assembly machinery (BAM) complex embedded within the OM. A supercomplex (containing SecA^N, the BAM complex and many other proteins) spans the IM and OM, and is involved in the biogenesis of OMPs. Investigation of the function of SecA^N and the supercomplex, as well as the translocation mechanism, will require elucidation of their structures. However, no such structures are available. Therefore, here, I describe the use of protein modeling to build homology models for SecA^N and theoretical structures for the core‐complex composed of SecA^N and the BAM complex, which is a key part of the supercomplex. The modeling data are consistent with previous experimental observations and demonstrated a conformational change of the core‐complex. I conclude by proposing mechanisms for how SecA^N and the supercomplex function in the biogenesis of OMPs.

Flavin-Based Fluorescent Protein EcFbFP Auto-Guided Surface Display of Methyl Parathion Hydrolase in Escherichia coli

Methyl parathion hydrolase (MPH) plays an important role in degrading a range of organophosphorus compounds. In order to display MPH on the cell surface of Escherichia coli strain RosettaBlue™, the Flavin-based fluorescent protein EcFbFP was severed as an auto-anchoring matrix. With net negative charges of EcFbFP supplying the driving forces, fusion protein MPH-EcFbFP through a two-step auto-surface display process was finally verified by (a) inner membrane translocation and (b) anchoring at outer membrane. Cells with surface-displayed MPH obtained activity of 0.12 U/OD600 against substrate methyl parathion. MPH when fused with engineered EcFbFP containing 20 net negative charges exhibited fivefold higher anchoring efficiency and tenfold higher enzymatic catalytic activity of 1.10 U/OD600. The above result showed that MPH was successfully displayed on cell surface and can be used for biodegradation of methyl parathion.

How BamA recruits OMP substrates via poly-POTRAs domain

Almost all the outer membrane proteins (OMPs) fold into an invariant β-barrel fold via the polypeptide-transport-associated (POTRA) motif and β-barrel assembly machinery (BAM). However, whether and how poly-POTRAs interact with OMPs remain largely unknown. Here, we have characterized the structures of Haemophilus influenzae poly-POTRAs via X-ray crystallography, small angle X-ray scattering, and molecular dynamics simulation. Unexpectedly, crystal packing reveals a putative OMP travel pathway spiraled by the conserved α2-β2 edges in poly-POTRAs. Supportively, the structure-based mutations targeting the OMP binding sites significantly disrupt OMP biogenesis, resulting in severe cell growth defects. Another notable feature in H. influenzae POTRA structures is flexibility. As characterized by ELISA assays, poly-POTRAs could recruit OMP substrates in a step-wise manner. More importantly, the restriction of POTRA-POTRA linkage and flexibility significantly impairs the BamA function and causes cell growth defect. Altogether, these results suggest that the β-strand augmentations and intrinsic flexibility are important factors for BamA-OMP recruitment.-Ma, X., Wang, Q., Li, Y., Tan, P., Wu, H., Wang, P., Dong, X., Hong, L., Meng, G. How BamA recruits OMP substrates via poly-POTRAs domain.

The small molecule nitazoxanide selectively disrupts BAM-mediated folding of the outer membrane usher protein

Bacterial pathogens assemble adhesive surface structures termed pili or fimbriae to initiate and sustain infection of host tissues. Uropathogenic Escherichia coli, the primary causative agent of urinary tract infections, expresses type 1 and P pili required for colonization of the bladder and kidney, respectively. These pili are assembled by the conserved chaperone-usher (CU) pathway, in which a periplasmic chaperone works together with an outer membrane (OM) usher protein to build and secrete the pilus fiber. Previously, we found that the small molecule and antiparasitic drug nitazoxanide (NTZ) inhibits CU pathway-mediated pilus biogenesis in E. coli by specifically interfering with proper maturation of the usher protein in the OM. The usher is folded and inserted into the OM by the β-barrel assembly machine (BAM) complex, which in E. coli comprises five proteins, BamA-E. Here, we show that sensitivity of the usher to NTZ is modulated by BAM expression levels and requires the BamB and BamE lipoproteins. Furthermore, a genetic screen for NTZ-resistant bacterial mutants isolated a mutation in the essential BamD lipoprotein. These findings suggest that NTZ selectively interferes with an usher-specific arm of the BAM complex, revealing new details of the usher folding pathway and BAM complex function. Evaluation of a set of NTZ derivatives identified compounds with increased potency and disclosed that NTZ's nitrothiazole ring is critical for usher inhibition. In summary, our findings indicate highly specific effects of NTZ on the usher folding pathway and have uncovered NTZ analogs that specifically decrease usher levels in the OM.

Type V Secretion in Gram-Negative Bacteria

Type V, or "autotransporter," secretion is a term used to refer to several simple protein export pathways that are found in a wide range of Gram-negative bacteria. Autotransporters are generally single polypeptides that consist of an extracellular ("passenger") domain and a β barrel domain that anchors the protein to the outer membrane (OM). Although it was originally proposed that the passenger domain is secreted through a channel formed solely by the covalently linked β barrel domain, experiments performed primarily on the type Va, or "classical," autotransporter pathway have challenged this hypothesis. Several lines of evidence strongly suggest that both the secretion of the passenger domain and the membrane integration of the β barrel domain are catalyzed by the barrel assembly machinery (Bam) complex, a conserved hetero-oligomer that plays an essential role in the assembly of most integral OM proteins. The secretion reaction appears to be driven at least in part by the folding of the passenger domain in the extracellular space. Although many aspects of autotransporter biogenesis remain to be elucidated, it will be especially interesting to determine whether the different classes of proteins that fall under the type V rubric-most of which have not been examined in detail-are assembled by the same basic mechanism as classical autotransporters.

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.

A unified model for BAM function that takes into account type Vc secretion and species differences in BAM composition

Transmembrane proteins in the outer membrane of Gram-negative bacteria are almost exclusively β-barrels. They are inserted into the outer membrane by a conserved and essential protein complex called the BAM (for β-barrel assembly machinery). In this commentary, we summarize current research into the mechanism of this protein complex and how it relates to type V secretion. Type V secretion systems are autotransporters that all contain a β-barrel transmembrane domain inserted by BAM. In type Vc systems, this domain is a homotrimer. We argue that none of the current models are sufficient to explain BAM function particularly regarding type Vc secretion. We also find that current models based on the well-studied model system Escherichia coli mostly ignore the pronounced differences in BAM composition between different bacterial species. We propose a more holistic view on how all OMPs, including autotransporters, are incorporated into the lipid bilayer.

Structural components involved in plastid protein import

Import of preproteins into chloroplasts is an essential process, requiring two major multisubunit protein complexes that are embedded in the outer and inner chloroplast envelope membrane. Both the translocon of the outer chloroplast membrane (Toc), as well as the translocon of the inner chloroplast membrane (Tic) have been studied intensively with respect to their individual subunit compositions, functions and regulations. Recent advances in crystallography have increased our understanding of the operation of these proteins in terms of their interactions and regulation by conformational switching. Several subdomains of components of the Toc translocon have been studied at the structural level, among them the polypeptide transport-associated (POTRA) domain of the channel protein Toc75 and the GTPase domain of Toc34. In this review, we summarize and discuss the insight that has been gained from these structural analyses. In addition, we present the crystal structure of the Toc64 tetratrico-peptide repeat (TPR) domain in complex with the C-terminal domains of the heat-shock proteins (Hsp) Hsp90 and Hsp70.

BamA β16C strand and periplasmic turns are critical for outer membrane protein insertion and assembly

Outer membrane (OM) β-barrel proteins play important roles in importing nutrients, exporting wastes and conducting signals in Gram-negative bacteria, mitochondria and chloroplasts. The outer membrane proteins (OMPs) are inserted and assembled into the OM by OMP85 family proteins. In Escherichia coli, the β-barrel assembly machinery (BAM) contains four lipoproteins such as BamB, BamC, BamD and BamE, and one OMP BamA, forming a 'top hat'-like structure. Structural and functional studies of the E. coli BAM machinery have revealed that the rotation of periplasmic ring may trigger the barrel β1C-β6C scissor-like movement that promote the unfolded OMP insertion without using ATP. Here, we report the BamA C-terminal barrel structure of Salmonella enterica Typhimurium str. LT2 and functional assays, which reveal that the BamA's C-terminal residue Trp, the β16C strand of the barrel and the periplasmic turns are critical for the functionality of BamA. These findings indicate that the unique β16C strand and the periplasmic turns of BamA are important for the outer membrane insertion and assembly. The periplasmic turns might mediate the rotation of the periplasmic ring to the scissor-like movement of BamA β1C-β6C, triggering the OMP insertion. These results are important for understanding the OMP insertion in Gram-negative bacteria, as well as in mitochondria and chloroplasts.

Non-peptide guided auto-secretion of recombinant proteins by super-folder green fluorescent protein in Escherichia coli

Protein secretion in Escherichia coli is usually led by a signal peptide that targets the protein to specific secretory pathways. In this study, we demonstrated that the superfolder green fluorescent protein (sfGFP) could be served as a non-signal peptide to guide protein auto-secretion in E. coli. This auto-secretion was characterized as a three-step process through the sub-cellular localization analysis: inner membrane trans-location followed by anchoring at outer membrane, and then being released into culture media. We further determined that the beta-barrel structure and net negative charges of sfGFP played important roles in its auto-extracellular secretion property. Using sfGFP as a carrier, heterologous proteins ranging from peptide to complex protein, including antibacterial peptide PG4, endo-beta-N-acethylglucosamindase H (Endo H), human arginase-1 (ARG1), and glutamate decarboxylase (GAD) were all successfully expressed and secreted extracellularly when fused to the carboxyl end of sfGFP. Besides facilitating the extracellular secretion, sfGFP fusion proteins can also be correctly folded and formed the active complex protein structure, including the trimetric human ARG1 and homo-hexametric GAD. This is the first report that sfGFP can guide the secretion of recombinant proteins out of the cells from cytoplasm in E. coli without affecting their conformation and function.

The POTRA domains of Toc75 exhibit chaperone-like function to facilitate import into chloroplasts

Protein trafficking across membranes is an essential function in cells; however, the exact mechanism for how this occurs is not well understood. In the endosymbionts, mitochondria and chloroplasts, the vast majority of proteins are synthesized in the cytoplasm as preproteins and then imported into the organelles via specialized machineries. In chloroplasts, protein import is accomplished by the TOC (translocon on the outer chloroplast membrane) and TIC (translocon on the inner chloroplast membrane) machineries in the outer and inner envelope membranes, respectively. TOC mediates initial recognition of preproteins at the outer membrane and includes a core membrane channel, Toc75, and two receptor proteins, Toc33/34 and Toc159, each containing GTPase domains that control preprotein binding and translocation. Toc75 is predicted to have a β-barrel fold consisting of an N-terminal intermembrane space (IMS) domain and a C-terminal 16-stranded β-barrel domain. Here we report the crystal structure of the N-terminal IMS domain of Toc75 from Arabidopsis thaliana, revealing three tandem polypeptide transport-associated (POTRA) domains, with POTRA2 containing an additional elongated helix not observed previously in other POTRA domains. Functional studies show an interaction with the preprotein, preSSU, which is mediated through POTRA2-3. POTRA2-3 also was found to have chaperone-like activity in an insulin aggregation assay, which we propose facilitates preprotein import. Our data suggest a model in which the POTRA domains serve as a binding site for the preprotein as it emerges from the Toc75 channel and provide a chaperone-like activity to prevent misfolding or aggregation as the preprotein traverses the intermembrane space.

Two-Partner Secretion: Combining Efficiency and Simplicity in the Secretion of Large Proteins for Bacteria-Host and Bacteria-Bacteria Interactions

Initially identified in pathogenic Gram-negative bacteria, the two-partner secretion (TPS) pathway, also known as Type Vb secretion, mediates the translocation across the outer membrane of large effector proteins involved in interactions between these pathogens and their hosts. More recently, distinct TPS systems have been shown to secrete toxic effector domains that participate in inter-bacterial competition or cooperation. The effects of these systems are based on kin vs. non-kin molecular recognition mediated by specific immunity proteins. With these new toxin-antitoxin systems, the range of TPS effector functions has thus been extended from cytolysis, adhesion, and iron acquisition, to genome maintenance, inter-bacterial killing and inter-bacterial signaling. Basically, a TPS system is made up of two proteins, the secreted TpsA effector protein and its TpsB partner transporter, with possible additional factors such as immunity proteins for protection against cognate toxic effectors. Structural studies have indicated that TpsA proteins mainly form elongated β helices that may be followed by specific functional domains. TpsB proteins belong to the Omp85 superfamily. Open questions remain on the mechanism of protein secretion in the absence of ATP or an electrochemical gradient across the outer membrane. The remarkable dynamics of the TpsB transporters and the progressive folding of their TpsA partners at the bacterial surface in the course of translocation are thought to be key elements driving the secretion process.

Mitochondrial-bacterial hybrids of BamA/Tob55 suggest variable requirements for the membrane integration of β-barrel proteins

β-Barrel proteins are found in the outer membrane (OM) of Gram-negative bacteria, chloroplasts and mitochondria. The assembly of these proteins into the corresponding OM is facilitated by a dedicated protein complex that contains a central conserved β-barrel protein termed BamA in bacteria and Tob55/Sam50 in mitochondria. BamA and Tob55 consist of a membrane-integral C-terminal domain that forms a β-barrel pore and a soluble N-terminal portion comprised of one (in Tob55) or five (in BamA) polypeptide transport-associated (POTRA) domains. Currently the functional significance of this difference and whether the homology between BamA and Tob55 can allow them to replace each other are unclear. To address these issues we constructed hybrid Tob55/BamA proteins with differently configured N-terminal POTRA domains. We observed that constructs harboring a heterologous C-terminal domain could not functionally replace the bacterial BamA or the mitochondrial Tob55 demonstrating species-specific requirements. Interestingly, the various hybrid proteins in combination with the bacterial chaperones Skp or SurA supported to a variable extent the assembly of bacterial β-barrel proteins into the mitochondrial OM. Collectively, our findings suggest that the membrane assembly of various β-barrel proteins depends to a different extent on POTRA domains and periplasmic chaperones.

Type V Secretion Systems in Bacteria

Type V secretion denotes a variety of secretion systems that cross the outer membrane in Gram-negative bacteria but that depend on the Sec machinery for transport through the inner membrane. They are possibly the simplest bacterial secretion systems, because they consist only of a single polypeptide chain (or two chains in the case of two-partner secretion). Their seemingly autonomous transport through the outer membrane has led to the term "autotransporters" for various subclasses of type V secretion. In this chapter, we review the structure and function of these transporters and review recent findings on additional factors involved in the secretion process, which have put the term "autotransporter" to debate.

Polypeptide Transport-Associated Domains of the Toc75 Channel Protein Are Located in the Intermembrane Space of Chloroplasts

Toc75 is the channel for protein translocation across the chloroplast outer envelope membrane. Toc75 belongs to the Omp85 protein family and consists of three N-terminal polypeptide transport-associated (POTRA) domains that are essential for the functions of Toc75, followed by a membrane-spanning β-barrel domain. In bacteria, POTRA domains of Omp85 family members are located in the periplasm, where they interact with other partner proteins to accomplish protein secretion and outer membrane protein assembly. However, the orientation and therefore the molecular function of chloroplast Toc75 POTRA domains remain a matter of debate. We investigated the topology of Toc75 using bimolecular fluorescence complementation and immunogold electron microscopy. Bimolecular fluorescence complementation analyses showed that in stably transformed plants, Toc75 N terminus is located on the intermembrane space side, not the cytosolic side, of the outer membrane. Immunogold labeling of endogenous Toc75 POTRA domains in pea (Pisum sativum) and Arabidopsis (Arabidopsis thaliana) confirmed that POTRA domains are located in the intermembrane space of the chloroplast envelope.

Yeast Mitochondria as a Model System to Study the Biogenesis of Bacterial β-Barrel Proteins

Beta-barrel proteins are found in the outer membrane of Gram-negative bacteria, mitochondria, and chloroplasts. The evolutionary conservation in the biogenesis of these proteins allows mitochondria to assemble bacterial β-barrel proteins in their functional form. In this chapter, we describe exemplarily how the capacity of yeast mitochondria to process the trimeric autotransporter YadA can be used to study the role of bacterial periplasmic chaperones in this process.

Dynamic interplay of membrane-proximal POTRA domain and conserved loop L6 in Omp85 transporter FhaC

Omp85 transporters mediate protein insertion into, or translocation across, membranes. They have a conserved architecture, with POTRA domains that interact with substrate proteins, a 16-stranded transmembrane β barrel, and an extracellular loop, L6, folded back in the barrel pore. Here using electrophysiology, in vivo biochemical approaches and electron paramagnetic resonance, we show that the L6 loop of the Omp85 transporter FhaC changes conformation and modulates channel opening. Those conformational changes involve breaking the conserved interaction between the tip of L6 and the inner β-barrel wall. The membrane-proximal POTRA domain also exchanges between several conformations, and the binding of FHA displaces this equilibrium. We further demonstrate a dynamic, physical communication between the POTRA domains and L6, which must take place via the β barrel. Our findings thus link all three essential components of Omp85 transporters and indicate that they operate in a concerted fashion in the transport cycle.

Cross-species chimeras reveal BamA POTRA and β-barrel domains must be fine-tuned for efficient OMP insertion

BAM is a conserved molecular machine, the central component of which is BamA. Orthologues of BamA are found in all Gram-negative bacteria, chloroplasts and mitochondria where it is required for the folding and insertion of β-barrel containing integral outer membrane proteins (OMPs) into the outer membrane. BamA binds unfolded β-barrel precursors via the five polypeptide transport-associated (POTRA) domains at its N-terminus. The C-terminus of BamA folds into a β-barrel domain, which tethers BamA to the outer membrane and is involved in OMP insertion. BamA orthologues are found in all Gram-negative bacteria and appear to function in a species-specific manner. Here we investigate the nature of this species-specificity by examining whether chimeric Escherichia coli BamA fusion proteins, carrying either the β-barrel or POTRA domains from various BamA orthologues, can functionally replace E. coli BamA. We demonstrate that the β-barrel domains of many BamA orthologues are functionally interchangeable. We show that defects in the orthologous POTRA domains can be rescued by compensatory mutations within the β-barrel. These data reveal that the POTRA and barrel domains must be precisely aligned to ensure efficient OMP insertion.

TtOmp85, a β-barrel assembly protein, functions by barrel augmentation

Outer membrane proteins are vital for Gram-negative bacteria and organisms that inherited organelles from them. Proteins from the Omp85/BamA family conduct the insertion of membrane proteins into the outer membrane. We show that an eight-stranded outer membrane β-barrel protein, TtoA, is inserted and folded into liposomes by an Omp85 homologue. Furthermore, we recorded the channel conductance of this Omp85 protein in black lipid membranes, alone and in the presence of peptides comprising the sequence of the two N-terminal and the two C-terminal β-strands of TtoA. Only with the latter could a long-living compound channel that exhibits conductance levels higher than those of the Omp85 protein alone be observed. These data support a model in which unfolded outer membrane protein after docking with its C-terminus penetrates into the transmembrane β-barrel of the Omp85 protein and augments its β-sheet at the first strand. Augmentation with successive β-strands leads to a compound, dilated barrel of both proteins.

Structure and function of POTRA domains of Omp85/TPS superfamily

The Omp85/TPS (outer-membrane protein of 85 kDa/two-partner secretion) superfamily is a ubiquitous and major class of β-barrel proteins. This superfamily is restricted to the outer membranes of gram-negative bacteria, mitochondria, and chloroplasts. The common architecture, with an N-terminus consisting of repeats of soluble polypeptide-transport-associated (POTRA) domains and a C-terminal β-barrel pore is highly conserved. The structures of multiple POTRA domains and one full-length TPS protein have been solved, yet discovering roles of individual POTRA domains has been difficult. This review focuses on similarities and differences between POTRA structures, emphasizing POTRA domains in autotrophic organisms including plants and cyanobacteria. Unique roles, specific for certain POTRA domains, are examined in the context of POTRA location with respect to their attachment to the β-barrel pore, and their degree of biological dispensability. Finally, because many POTRA domains may have the ability to interact with thousands of partner proteins, possible modes of these interactions are also explored.

Biogenesis of beta-barrel proteins in evolutionary context

The vast majority of outer membrane (OM) proteins in Gram-negative bacteria belongs to the class of membrane-embedded β-barrel proteins. Besides Gram-negative bacteria, the presence of β-barrel proteins is restricted to the OM of the eukaryotic organelles mitochondria and chloroplasts that were derived from prokaryotic ancestors. The assembly of these proteins into the corresponding OM is in each case facilitated by a dedicated protein complex that contains a highly conserved central β-barrel protein termed BamA/YaeT/Omp85 in Gram-negative bacteria and Tob55/Sam50 in mitochondria. However, little is known about the exact mechanism by which these complexes mediate the integration of β-barrel precursors into the lipid bilayer. Interestingly, previous studies showed that during evolution, these complexes retained the ability to functionally assemble β-barrel proteins from different origins. In this review we summarize the current knowledge on the biogenesis pathway of β-barrel proteins in Gram-negative bacteria, mitochondria and chloroplasts and focus on the commonalities and divergences that evolved between the different β-barrel assembly machineries.

Structure-function of cyanobacterial outer-membrane protein, Slr1270: homolog of Escherichia coli drug export/colicin import protein, TolC

Compared to thylakoid and inner membrane proteins in cyanobacteria, no structure-function information is available presently for integral outer-membrane proteins (OMPs). The Slr1270 protein from the cyanobacterium Synechocystis 6803, over-expressed in Escherichia coli, was refolded, and characterized for molecular size, secondary structure, and ion-channel function. Refolded Slr1270 displays a single band in native-electrophoresis, has an α-helical content of 50-60%, as in E. coli TolC with which it has significant secondary-structure similarity, and an ion-channel function with a single-channel conductance of 80-200pS, and a monovalent ion (K(+):Cl(-)) selectivity of 4.7:1. The pH-dependence of channel conductance implies a role for carboxylate residues in channel gating, analogous to that in TolC.

A comprehensive analysis of the Omp85/TpsB protein superfamily structural diversity, taxonomic occurrence, and evolution

Members of the Omp85/TpsB protein superfamily are ubiquitously distributed in Gram-negative bacteria, and function in protein translocation (e.g., FhaC) or the assembly of outer membrane proteins (e.g., BamA). Several recent findings are suggestive of a further level of variation in the superfamily, including the identification of the novel membrane protein assembly factor TamA and protein translocase PlpD. To investigate the diversity and the causal evolutionary events, we undertook a comprehensive comparative sequence analysis of the Omp85/TpsB proteins. A total of 10 protein subfamilies were apparent, distinguished in their domain structure and sequence signatures. In addition to the proteins FhaC, BamA, and TamA, for which structural and functional information is available, are families of proteins with so far undescribed domain architectures linked to the Omp85 β-barrel domain. This study brings a classification structure to a dynamic protein superfamily of high interest given its essential function for Gram-negative bacteria as well as its diverse domain architecture, and we discuss several scenarios of putative functions of these so far undescribed proteins.

The polypeptide transport-associated (POTRA) domains of TpsB transporters determine the system specificity of two-partner secretion systems

The two-partner secretion (TPS) systems of Gram-negative bacteria secrete large TpsA exoproteins by a dedicated TpsB transporter in the outer membrane. TpsBs contain an N-terminal module located in the periplasm that includes two polypeptide transport-associated (POTRA) domains. These are thought to initiate secretion of a TpsA by binding its N-terminal secretion signal, called the TPS domain. Neisseria meningitidis encodes up to five TpsA proteins that are secreted via only two TpsB transporters: TpsB1 and TpsB2. Of these two, the TpsB2 recognizes the TPS domains of all TpsAs, despite their sequence diversity. By contrast, the TpsB1 shows a limited recognition of a TPS domain that is shared by two TpsAs. The difference in substrate specificity of the TpsBs enabled us to investigate the role of the POTRA domains in the selection of TPS domains. We tested secretion of TPS domains or full-length TpsAs by TpsB mutants with deleted, duplicated, and exchanged POTRA domains. Exchanging the two POTRA domains of a TpsB resulted in a switch in specificity. Furthermore, exchanging a single POTRA domain showed that each of the two domains contributed to the cargo selection. Remarkably, the order of the POTRA domains could be reversed without affecting substrate selection, but this aberrant order did result in an alternatively processed secretion product. Our results suggest that secretion of a TpsA is initiated by engaging both POTRA domains of a TpsB transporter and that these select the cognate TpsAs for secretion.

Heterologous expression of Bartonella adhesin A in Escherichia coli by exchange of trimeric autotransporter adhesin domains results in enhanced adhesion properties and a pathogenic phenotype

Human-pathogenic Bartonella henselae causes cat scratch disease and vasculoproliferative disorders. An important pathogenicity factor of B. henselae is the trimeric autotransporter adhesin (TAA) Bartonella adhesin A (BadA), which is modularly constructed, consisting of a head, a long and repetitive neck-stalk module, and a membrane anchor. BadA is involved in bacterial autoagglutination, binding to extracellular matrix proteins and host cells, and in proangiogenic reprogramming. The slow growth of B. henselae and limited tools for genetic manipulation are obstacles for detailed examination of BadA and its domains. Here, we established a recombinant expression system for BadA mutants in Escherichia coli allowing functional analysis of particular BadA domains. Using a BadA mutant lacking 21 neck-stalk repeats (BadA HN23), the BadA HN23 signal sequence was exchanged with that of E. coli OmpA, and the BadA membrane anchor was additionally replaced with that of Yersinia adhesin A (YadA). Constructs were cloned in E. coli, and hybrid protein expression was detected by immunoblotting, fluorescence microscopy, and flow cytometry. Functional analysis revealed that BadA hybrid proteins mediate autoagglutination and binding to collagen and endothelial cells. In vivo, expression of this BadA construct correlated with higher pathogenicity of E. coli in a Galleria mellonella infection model.

Evolution and targeting of Omp85 homologs in the chloroplast outer envelope membrane

Translocon at the outer-envelope-membrane of chloroplasts 75 (Toc75) is the core component of the chloroplast protein import machinery. It belongs to the Omp85 family whose members exist in various Gram-negative bacteria, mitochondria, and chloroplasts of eukaryotes. Chloroplasts of Viridiplantae contain another Omp85 homolog called outer envelope protein 80 (OEP80), whose exact function is unknown. In addition, the Arabidopsis thaliana genome encodes truncated forms of Toc75 and OEP80. Multiple studies have shown a common origin of the Omp85 homologs of cyanobacteria and chloroplasts but their results about evolutionary relationships among cyanobacterial Omp85 (cyanoOmp85), Toc75, and OEP80 are inconsistent. The bipartite targeting sequence-dependent sorting of Toc75 has been demonstrated but the targeting mechanisms of other chloroplast Omp85 homologs remain largely unexplored. This study was aimed to address these unresolved issues in order to further our understanding of chloroplast evolution. Sequence alignments and recently determined structures of bacterial Omp85 homologs were used to predict structures of chloroplast Omp85 homologs. The results enabled us to identify amino acid residues that may indicate functional divergence of Toc75 from cyanoOmp85 and OEP80. Phylogenetic analyses using Omp85 homologs from various cyanobacteria and chloroplasts provided strong support for the grouping of Toc75 and OEP80 sister to cyanoOmp85. However, this support was diminished when the analysis included Omp85 homologs from other bacteria and mitochondria. Finally, results of import assays using isolated chloroplasts support outer membrane localization of OEP80tr and indicate that OEP80 may carry a cleavable targeting sequence.

Species-specificity of the BamA component of the bacterial outer membrane protein-assembly machinery

The BamA protein is the key component of the Bam complex, the assembly machinery for outer membrane proteins (OMP) in gram-negative bacteria. We previously demonstrated that BamA recognizes its OMP substrates in a species-specific manner in vitro. In this work, we further studied species specificity in vivo by testing the functioning of BamA homologs of the proteobacteria Neisseria meningitidis, Neisseria gonorrhoeae, Bordetella pertussis, Burkholderia mallei, and Escherichia coli in E. coli and in N. meningitidis. We found that no BamA functioned in another species than the authentic one, except for N. gonorrhoeae BamA, which fully complemented a N. meningitidis bamA mutant. E. coli BamA was not assembled into the N. meningitidis outer membrane. In contrast, the N. meningitidis BamA protein was assembled into the outer membrane of E. coli to a significant extent and also associated with BamD, an essential accessory lipoprotein of the Bam complex.Various chimeras comprising swapped N-terminal periplasmic and C-terminal membrane-embedded domains of N. meningitidis and E. coli BamA proteins were also not functional in either host, although some of them were inserted in the OM suggesting that the two domains of BamA need to be compatible in order to function. Furthermore, conformational analysis of chimeric proteins provided evidence for a 16-stranded β-barrel conformation of the membrane-embedded domain of BamA.

Secretion and activation of the Serratia marcescens hemolysin by structurally defined ShlB mutants

The ShlA hemolysin of Serratia marcescens is secreted across the outer membrane by the ShlB protein; ShlB belongs to the two-partner secretion system (type Vb), a subfamily of the Omp85 outer membrane protein assembly and secretion superfamily. During secretion, ShlA is converted from an inactive non-hemolytic form into an active hemolytic form. The structure of ShlB is predicted to consist of the N-terminal α-helix H1, followed by the two polypeptide-transport-associated domains POTRA P1 and P2, and the β-barrel of 16 β-strands. H1 is inserted into the pore of the β-barrel in the outer membrane; P1 and P2 are located in the periplasm. To obtain insights into the secretion and activation of ShlA by ShlB, we isolated ShlB mutants impaired in secretion and/or activation. The triple H1 P1 P2 mutant did not secrete ShlA. The P1 and P2 deletion derivatives secreted reduced amounts of ShlA, of which P1 showed some hemolysis, whereas P2 was inactive. Deletion of loop 6 (L6), which is conserved among exporters of the Omp85 family, compromised activation but retained low secretion. Secretion-negative mutants generated by random mutagenesis were located in loop 6. The inactive secreted ShlA derivatives were complemented in vitro to active ShlA by an N-terminal ShlA fragment (ShlA242) secreted by ShlB. Deletion of H1 did not impair secretion of hemolytic ShlA. The study defines domains of ShlB which are important for ShlA secretion and activation.

The lipid bilayer-inserted membrane protein BamA of Escherichia coli facilitates insertion and folding of outer membrane protein A from its complex with Skp

Folding of β-barrel membrane proteins, either from a urea-unfolded form or from chaperone-bound aqueous forms, has been characterized for pure lipid bilayers. The impact of preinserted integral proteins from biomembranes has not been examined in biophysical comparisons, but this knowledge is important for the characterization of protein assembly machinery in membranes to distinguish specific effects from unspecific effects. Here, folding was studied for a β-barrel membrane protein, outer membrane protein A (OmpA) from Escherichia coli, in the absence and presence of two other preinserted integral proteins, BamA of the β-barrel assembly machinery complex (BAM) from E. coli and FomA from Fusobacterium nucleatum. Three different preformed lipid membranes of phosphatidylcholine were prepared to compare the folding kinetics of OmpA, namely, proteoliposomes containing either BamA or FomA and pure liposomes. Urea-unfolded OmpA folded faster into phosphatidylcholine bilayers containing FomA than into pure lipid bilayers, but the kinetics of OmpA folding and insertion were fastest for bilayers containing BamA. Incorporation of BamA into lipid bilayers composed of phosphatidylcholine and phosphatidylethanolamine greatly weakened the inhibiting effect of phosphatidylethanolamine on the folding of OmpA. Folding of OmpA from its complex with the periplasmic chaperone Skp into bilayers composed of phosphatidylethanolamine and phosphatidylcholine was inhibited in the absence of BamA but facilitated when BamA was present, indicating an interaction of Skp-OmpA complexes with BamA.

Autotransporter secretion: varying on a theme

Autotransporters are widely distributed among Gram-negative bacteria. They can have a large variety of functions and many of them have a role in virulence. They are synthesized as large precursors with an N-terminal signal sequence that mediates transport across the inner membrane via the Sec machinery and a translocator domain that mediates the transport of the connected passenger domain across the outer membrane to the bacterial cell surface. Like integral outer membrane proteins, the translocator domain folds in a β-barrel structure and requires the Bam machinery for its insertion into the outer membrane. After transport across the outer membrane, the passenger may stay connected via the translocator domain to the bacterial cell surface or it is proteolytically released into the extracellular milieu. Based on the size of the translocator domain and its position relative to the passenger in the precursor, autotransporters are divided into four sub-categories. We review here the current knowledge of the biogenesis, structure and function of various autotransporters.

Two-partner secretion: as simple as it sounds?

The two-partner secretion (TPS) pathway is a branch of type V secretion. TPS systems are dedicated to the secretion across the outer membrane of long proteins that form extended β-helices. They are composed of a 'TpsA' cargo protein and a 'TpsB' transporter, which belongs to the Omp85 superfamily. This basic design can be supplemented by additional components in some TPS systems. X-ray structures are available for the conserved TPS domain of several TpsA proteins and for one TpsB transporter. However, the molecular mechanisms of two-partner secretion remain to be deciphered, and in particular, the specific role(s) of the TPS domain and the conformational dynamics of the TpsB transporter. Deciphering the TPS pathway may reveal functional features of other transporters of the Omp85 superfamily.

Assembly of the β-Barrel Outer Membrane Proteins in Gram-Negative Bacteria, Mitochondria, and Chloroplasts

In the last decade, there has been an explosion of publications on the assembly of β-barrel outer membrane proteins (OMPs), which carry out diverse cellular functions, including solute transport, protein secretion, and assembly of protein and lipid components of the outer membrane. Of the three outer membrane model systems—Gram-negative bacteria, mitochondria and chloroplasts—research on bacterial and mitochondrial systems has so far led the way in dissecting the β-barrel OMP assembly pathways. Many exciting discoveries have been made, including the identification of β-barrel OMP assembly machineries in bacteria and mitochondria, and potentially the core assembly component in chloroplasts. The atomic structures of all five components of the bacterial β-barrel assembly machinery (BAM) complex, except the β-barrel domain of the core BamA protein, have been solved. Structures reveal that these proteins contain domains/motifs known to facilitate protein-protein interactions, which are at the heart of the assembly pathways. While structural information has been valuable, most of our current understanding of the β-barrel OMP assembly pathways has come from genetic, molecular biology, and biochemical analyses. This paper provides a comparative account of the β-barrel OMP assembly pathways in Gram-negative bacteria, mitochondria, and chloroplasts.

The chloroplast protein import system: from algae to trees

Chloroplasts are essential organelles in the cells of plants and algae. The functions of these specialized plastids are largely dependent on the ~3000 proteins residing in the organelle. Although chloroplasts are capable of a limited amount of semiautonomous protein synthesis - their genomes encode ~100 proteins - they must import more than 95% of their proteins after synthesis in the cytosol. Imported proteins generally possess an N-terminal extension termed a transit peptide. The importing translocons are made up of two complexes in the outer and inner envelope membranes, the so-called Toc and Tic machineries, respectively. The Toc complex contains two precursor receptors, Toc159 and Toc34, a protein channel, Toc75, and a peripheral component, Toc64/OEP64. The Tic complex consists of as many as eight components, namely Tic22, Tic110, Tic40, Tic20, Tic21 Tic62, Tic55 and Tic32. This general Toc/Tic import pathway, worked out largely in pea chloroplasts, appears to operate in chloroplasts in all green plants, albeit with significant modifications. Sub-complexes of the Toc and Tic machineries are proposed to exist to satisfy different substrate-, tissue-, cell- and developmental requirements. In this review, we summarize our understanding of the functions of Toc and Tic components, comparing these components of the import machinery in green algae through trees. We emphasize recent findings that point to growing complexities of chloroplast protein import process, and use the evolutionary relationships between proteins of different species in an attempt to define the essential core translocon components and those more likely to be responsible for regulation. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.

Type V secretion: mechanism(s) of autotransport through the bacterial outer membrane

Autotransport in Gram-negative bacteria denotes the ability of surface-localized proteins to cross the outer membrane (OM) autonomously. Autotransporters perform this task with the help of a β-barrel transmembrane domain localized in the OM. Different classes of autotransporters have been investigated in detail in recent years; classical monomeric but also trimeric autotransporters comprise many important bacterial virulence factors. So do the two-partner secretion systems, which are a special case as the transported protein resides on a different polypeptide chain than the transporter. Despite the great interest in these proteins, the exact mechanism of the transport process remains elusive. Moreover, different periplasmic and OM factors have been identified that play a role in the translocation, making the term ‘autotransport’ debatable. In this review, we compile the wealth of details known on the mechanism of single autotransporters from different classes and organisms, and put them into a bigger perspective. We also discuss recently discovered or rediscovered classes of autotransporters.

The bacterial outer membrane β-barrel assembly machinery

β-Barrel proteins found in the outer membrane of Gram-negative bacteria serve a variety of cellular functions. Proper folding and assembly of these proteins are essential for the viability of bacteria and can also play an important role in virulence. The β-barrel assembly machinery (BAM) complex, which is responsible for the proper assembly of β-barrels into the outer membrane of Gram-negative bacteria, has been the focus of many recent studies. This review summarizes the significant progress that has been made toward understanding the structure and function of the bacterial BAM complex.

The Bam machine: a molecular cooper

The bacterial outer membrane (OM) is an exceptional biological structure with a unique composition that contributes significantly to the resiliency of Gram-negative bacteria. Since all OM components are synthesized in the cytosol, the cell must efficiently transport OM-specific lipids and proteins across the cell envelope and stably integrate them into a growing membrane. In this review, we discuss the challenges associated with these processes and detail the elegant solutions that cells have evolved to address the topological problem of OM biogenesis. Special attention will be paid to the Bam machine, a highly conserved multiprotein complex that facilitates OM β-barrel folding. This article is part of a Special Issue entitled: Protein Folding in Membranes.

X-ray crystallography at the heart of life science

X-ray crystallography is the fundamental research tool that shaped our notion on biological structure & function at the molecular level. It generates the information vital to understand life processes by providing the information required for creating accurate three-dimensional models (namely mapping the position of each and every atom that makes up the studied object). The use of this method begun in the middle of last century following Max von Laue discovery of the phenomenon of diffraction of X-rays by crystals, and the successful application of this discovery for the determination of the electronic distribution within simple inorganic molecules by Sir William Henry Bragg and his son, William Lawrence Bragg. The idea of extension of this method to biological molecules met initially with considerable skepticism. For over two decades many respected scientists doubted whether it could be done. Yet, despite its bottlenecks (some of which are described below), the superiority of X-ray crystallography over all other approaches for shedding light on functional aspects at the molecular level became evident once the first structure was determined. The power of this method inspired continuous efforts and spectacular innovations, which vastly accelerated its incredible expansion. Consequently, over the last six decades biological crystallography has produced a constantly growing number of structures, some of which were considered formidable. This remarkable advance yielded numerous new insights into intricate functional aspects. Owing to space limitation this article focuses on selected studies performed recently and highlights some recent exciting developments.