Crystal structure of the transport unit of the autotransporter adhesin involved in diffuse adherence from Escherichia coli

The Love and Hate Relationship between T5SS and Other Secretion Systems in Bacteria

Bacteria have existed on Earth for billions of years, exhibiting ubiquity and involvement in various biological activities. To ensure survival, bacteria usually release and secrete effector proteins to acquire nutrients and compete with other microorganisms for living space during long-term evolution. Consequently, bacteria have developed a range of secretion systems, which are complex macromolecular transport machines responsible for transporting proteins across the bacterial cell membranes. Among them, one particular secretion system that stands out from the rest is the type V secretion system (T5SS), known as the "autotransporter". Bacterial activities mediated by T5SS include adherence to host cells or the extracellular matrix, invasion of host cells, immune evasion and serum resistance, contact-dependent growth inhibition, cytotoxicity, intracellular flow, protease activity, autoaggregation, and biofilm formation. In a bacterial body, it is not enough to rely on T5SS alone; in most cases, T5SS cooperates with other secretion systems to carry out bacterial life activities, but regardless of how good the relationship is, there is friction between the secretion systems. T5SS and T1SS/T2SS/T3SS/T6SS all play a synergistic role in the pathogenic processes of bacteria, such as nutrient acquisition, pathogenicity enhancement, and immune modulation, but T5SS indirectly inhibits the function of T4SS. This could be considered a love-hate relationship between secretion systems. This paper uses the systematic literature review methodology to review 117 journal articles published within the period from 1995 to 2024, which are all available from the PubMed, Web of Science, and Scopus databases and aim to elucidate the link between T5SS and other secretion systems, providing clues for future prevention and control of bacterial diseases.

Genome-Wide Analysis of Antigen 43 (Ag43) Variants: New Insights in Their Diversity, Distribution and Prevalence in Bacteria

Antigen 43 (Ag43) expression induces aggregation and biofilm formation that has consequences for bacterial colonisation and infection. Ag43 is secreted through the Type 5 subtype "a" secretion system (T5aSS) and is a prototypical member of the family of self-associating autotransporters (SAATs). As a T5aSS protein, Ag43 has a modular architecture comprised of (i) a signal peptide, (ii) a passenger domain that can be subdivided into three subdomains (SL, EJ, and BL), (iii) an autochaperone (AC) domain, and (iv) an outer membrane translocator. The cell-surface SL subdomain is directly involved in the "Velcro-handshake" mechanism resulting in bacterial autoaggregation. Ag43 is considered to have a ubiquitous distribution in E. coli genomes and many strains harbour multiple agn43 genes. However, recent phylogenetic analyses indicated the existence of four distinct Ag43 classes exhibiting different propensities for autoaggregation and interactions. Given the knowledge of the diversity and distribution of Ag43 in E. coli genomes is incomplete, we have performed a thorough in silico investigation across bacterial genomes. Our comprehensive analyses indicate that Ag43 passenger domains cluster in six phylogenetic classes associated with different SL subdomains. The diversity of Ag43 passenger domains is a result of the association of the SL subtypes with two different EJ-BL-AC modules. We reveal that agn43 is almost exclusively present among bacterial species of the Enterobacteriaceae family and essentially in the Escherichia genus (99.6%) but that it is not ubiquitous in E. coli. The gene is typically present as a single copy but up to five copies of agn43 with different combinations of classes can be observed. The presence of agn43 as well as its different classes appeared to differ between Escherichia phylogroups. Strikingly, agn43 is present in 90% of E. coli from E phylogroup. Our results shed light on Ag43 diversity and provide a rational framework for investigating its role in E. coli ecophysiology and physiopathology.

Phylogenetic Classification and Functional Review of Autotransporters

Autotransporters are the core component of a molecular nano-machine that delivers cargo proteins across the outer membrane of Gram-negative bacteria. Part of the type V secretion system, this large family of proteins play a central role in controlling bacterial interactions with their environment by promoting adhesion to surfaces, biofilm formation, host colonization and invasion as well as cytotoxicity and immunomodulation. As such, autotransporters are key facilitators of fitness and pathogenesis and enable co-operation or competition with other bacteria. Recent years have witnessed a dramatic increase in the number of autotransporter sequences reported and a steady rise in functional studies, which further link these proteins to multiple virulence phenotypes. In this review we provide an overview of our current knowledge on classical autotransporter proteins, the archetype of this protein superfamily. We also carry out a phylogenetic analysis of their functional domains and present a new classification system for this exquisitely diverse group of bacterial proteins. The sixteen phylogenetic divisions identified establish sensible relationships between well characterized autotransporters and inform structural and functional predictions of uncharacterized proteins, which may guide future research aimed at addressing multiple unanswered aspects in this group of therapeutically important bacterial factors.

Bioelectronic control of a microbial community using surface-assembled electrogenetic cells to route signals

We developed a bioelectronic communication system that is enabled by a redox signal transduction modality to exchange information between a living cell-embedded bioelectronics interface and an engineered microbial network. A naturally communicating three-member microbial network is 'plugged into' an external electronic system that interrogates and controls biological function in real time. First, electrode-generated redox molecules are programmed to activate gene expression in an engineered population of electrode-attached bacterial cells, effectively creating a living transducer electrode. These cells interpret and translate electronic signals and then transmit this information biologically by producing quorum sensing molecules that are, in turn, interpreted by a planktonic coculture. The propagated molecular communication drives expression and secretion of a therapeutic peptide from one strain and simultaneously enables direct electronic feedback from the second strain, thus enabling real-time electronic verification of biological signal propagation. Overall, we show how this multifunctional bioelectronic platform, termed a BioLAN, reliably facilitates on-demand bioelectronic communication and concurrently performs programmed tasks.

Autodisplay of an endo-1,4-β-xylanase from Clostridium cellulovorans in Escherichia coli for xylans degradation

The goal of this work was the autodisplay of the endo β-1,4-xylanase (XynA) from Clostridium cellulovorans in Escherichia coli using the AIDA system to carry out whole-cell biocatalysis and hydrolysate xylans. For this, pAIDA-xynA vector containing a synthetic xynA gene was fused to the signal peptide of the toxin subunit B Vibro cholere (ctxB) and the auto-transporter of the synthetic aida gene, which encodes for the connector peptide and β-barrel of the auto-transporter (AT-AIDA). E. coli TOP10 cells were transformed and the biocatalyst was characterized using beechwood xylans as substrate. Optimal operational conditions were temperature of 55 °C and pH 6.5, and the Michaelis-Menten catalytic constants V_max and K_m were 149 U/g_DCW and 6.01 mg/mL, respectively. Xylanase activity was inhibited by Cu²⁺, Zn²⁺ and Hg²⁺ as well as EDTA, detergents, and organic acids, and improved by Ca²⁺, Co²⁺ and Mn²⁺ ions. Ca²⁺ ion strongly enhanced the xylanolytic activity up to 2.4-fold when 5 mM CaCl₂ were added. Also, Ca²⁺ improved enzyme stability at 60 and 70 °C. Results suggest that pAIDA-xynA vector has the ability to express functional xylanase to perform whole-cell biocatalysis in order to hydrolysate xylans from hemicellulose feedstock.

BamA is required for autotransporter secretion

BACKGROUND

In Gram-negative bacteria, type Va and Vc autotransporters are proteins that contain both a secreted virulence factor (the "passenger" domain) and a β-barrel that aids its export. While it is known that the folding and insertion of the β-barrel domain utilize the β-barrel assembly machinery (BAM) complex, how the passenger domain is secreted and folded across the membrane remains to be determined. The hairpin model states that passenger domain secretion occurs independently through the fully-formed and membrane-inserted β-barrel domain via a hairpin folding intermediate. In contrast, the BamA-assisted model states that the passenger domain is secreted through a hybrid of BamA, the essential subunit of the BAM complex, and the β-barrel domain of the autotransporter.

METHODS

To ascertain the models' plausibility, we have used molecular dynamics to simulate passenger domain secretion for two autotransporters, EspP and YadA.

RESULTS

We observed that each protein's β-barrel is unable to accommodate the secreting passenger domain in a hairpin configuration without major structural distortions. Additionally, the force required for secretion through EspP's β-barrel is more than that through the BamA β-barrel.

CONCLUSIONS

Secretion of autotransporters most likely occurs through an incompletely formed β-barrel domain of the autotransporter in conjunction with BamA.

GENERAL SIGNIFICANCE

Secretion of virulence factors is a process used by practically all pathogenic Gram-negative bacteria. Understanding this process is a necessary step towards limiting their infectious capacity.

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.

Identification of a novel post-insertion step in the assembly of a bacterial outer membrane protein

Although the barrel assembly machinery (Bam) complex has been shown to facilitate the insertion of β barrel proteins into the bacterial outer membrane (OM), the stage at which β barrels fold is unknown. Here, we describe insights into β barrel assembly that emerged from an analysis of a member of the autotransporter family of OM proteins (EspP) in Escherichia coli. EspP contains an extracellular 'passenger' domain that is translocated across the OM and then released from the covalently linked β barrel domain in an intra-barrel cleavage reaction. We found that the mutation of an unusual lipid-exposed lysine residue impairs a previously unidentified late folding step that follows both the membrane insertion of the β barrel domain and the secretion of the passenger domain but that precedes proteolytic maturation. Our results demonstrate that β barrel assembly can be completed at a post-insertion stage and raise the possibility that interactions with membrane lipids can promote folding in vivo. Furthermore, by showing that the passenger domain is secreted before the β barrel domain is fully assembled, our results also provide evidence against the long-standing hypothesis that autotransporters are autonomous protein secretion systems.

Autotransporter-Mediated Display of a Naïve Affibody Library on the Outer Membrane of Escherichia coli

Development of new affinity proteins using combinatorial protein engineering is today established for generation of monoclonal antibodies and also essential for discovery of binders that are based on non-immunoglobulin proteins. Phage display is most frequently used, but yeast display is becoming increasingly popular, partly due to the option of utilizing fluorescence-activated cell sorting (FACS) for isolation of new candidates. Escherichia coli has several valuable properties for library applications and in particular the high transformation efficiency. The use of various autotransporters and intimins for secretion and anchoring on the outer membrane have shown promising results and particularly for directed evolution of different enzymes. Here, the authors report on display of a large naïve affibody library on the outer membrane of E. coli using the autotransporter Adhesin Involved in Diffuse Adherence (AIDA-I). The expression cassette is first engineered by removing non-essential sequences, followed by introduction of an affibody library, comprising more than 10⁹ variants, into the new display vector. The quality of the library and general performance of the method is assessed by FACS against five different targets, which resulted in a panel of binders with down to nanomolar affinities, suggesting that the method has potential as a complement to phage display for generation of affibody molecules.

Molecular basis for the folding of β-helical autotransporter passenger domains

Bacterial autotransporters comprise a C-terminal β-barrel domain, which must be correctly folded and inserted into the outer membrane to facilitate translocation of the N-terminal passenger domain to the cell exterior. Once at the surface, the passenger domains of most autotransporters are folded into an elongated β-helix. In a cellular context, key molecules catalyze the assembly of the autotransporter β-barrel domain. However, how the passenger domain folds into its functional form is poorly understood. Here we use mutational analysis on the autotransporter Pet to show that the β-hairpin structure of the fifth extracellular loop of the β-barrel domain has a crucial role for passenger domain folding into a β-helix. Bioinformatics and structural analyses, and mutagenesis of a homologous autotransporter, suggest that this function is conserved among autotransporter proteins with β-helical passenger domains. We propose that the autotransporter β-barrel domain is a folding vector that nucleates folding of the passenger domain.

Autotransporter passenger domains are presented on or released from the bacterial surface upon translocation through an outer membrane β-barrel anchor. Here the authors study the two E. coli autotransporters Pet and EspP and propose that the β-barrel anchor acts as a vector to nucleate the folding of the passenger domain.

Fusion with the cold-active esterase facilitates autotransporter-based surface display of the 10th human fibronectin domain in Escherichia coli

Cell surface display is a popular approach for the construction of whole-cell biocatalysts, live vaccines, and screening of combinatorial libraries. To develop a novel surface display system for the popular scaffold protein 10th human fibronectin type III domain (¹⁰Fn3) in Escherichia coli cells, we have used an α-helical linker and a C-terminal translocator domain from previously characterized autotransporter from Psychrobacter cryohalolentis K5^T. The level of ¹⁰Fn3 passenger exposure at the cell surface provided by the hybrid autotransporter Fn877 and its C-terminal variants was low. To improve it, the fusion proteins containing ¹⁰Fn3 and the native autotransporter passenger Est877 or the cold-active esterase EstPc in different orientations were constructed and expressed as passenger domains. Using the whole-cell ELISA and activity assays, we have demonstrated that N-terminal position of EstPc in the passenger significantly improves the efficiency of the surface display of ¹⁰Fn3 in E. coli cells.

Escherichia coli surface display for the selection of nanobodies

Nanobodies (Nbs) are the smallest functional antibody fragments known in nature and have multiple applications in biomedicine or environmental monitoring. Nbs are derived from the variable segment of camelid heavy chain-only antibodies, known as VHH. For selection, libraries of VHH gene segments from naïve, immunized animals or of synthetic origin have been traditionally cloned in E. coli phage display or yeast display systems, and clones binding the target antigen recovered, usually from plastic surfaces with the immobilized antigen (phage display) or using fluorescence-activated cell sorting (FACS; yeast display). This review briefly describes these conventional approaches and focuses on the distinct properties of an E. coli display system developed in our laboratory, which combines the benefits of both phage display and yeast display systems. We demonstrate that E. coli display using an N-terminal domain of intimin is an effective platform for the surface display of VHH libraries enabling selection of high-affinity Nbs by magnetic cell sorting and direct selection on live mammalian cells displaying the target antigen on their surface. Flow cytometry analysis of E. coli bacteria displaying the Nbs on their surface allows monitoring of the selection process, facilitates screening, characterization of antigen-binding clones, specificity, ligand competition and estimation of the equilibrium dissociation constant (KD ).

Autotransporter Adhesins in Escherichia coli Pathogenesis

Most bacteria produce adhesion molecules to facilitate the interaction with host cells and establish successful infections. An important group of bacterial adhesins belong to the autotransporter (AT) superfamily, the largest group of secreted and outer membrane proteins in Gram-negative bacteria. AT adhesins possess diverse functions that facilitate bacterial colonisation, survival and persistence, and as such are often associated with increased bacterial fitness and pathogenic potential. In this review, we will describe AIDA-I type AT adhesins, which comprise the biggest and most diverse group in the AT family. We will focus on Escherichia coli proteins and define general aspects of their biogenesis, distribution, structural properties and key roles in infection.

Orientation and density control of proteins on solid matters by outer membrane coating: Analytical and diagnostic applications

Autodisplay is an expression system for the display of recombinant proteins on the outer membrane (OM) of gram negative bacteria and has been developed for translocation studies, whole cell biocatalysis, bioremediation, inhibitor screening, and enzyme refolding. Recently, affinity proteins such as IgG-binding Z-domains and biotin-binding streptavidin have been autodisplayed on the OM of Escherichia coli for analytical and biomedical applications. The secretion mechanism of the autodisplay system was used and orientation and density control of these affinity proteins were determined. Affinity protein-autodisplaying E. coli cells have been used to coat solid supports in immunoassays. For this purpose, the OM of autodisplayed E. coli cells was separated and isolated by the aid of detergents. The structure of the resulting OM liposomes as well as their physico-chemical parameters, were analyzed. OM liposomes were used subsequently for coating various solid matters including microplates and biosensor transducer surfaces and the formation of OM layers were monitored. OM layer formation on solid matters was shown to increase the sensitivity of immunoassays and biosensors. In this review, analytical and diagnostic applications are described in particular concerning orientation and density control of autodisplayed affinity proteins.

Autodisplay of glucose-6-phosphate dehydrogenase for redox cofactor regeneration at the cell surface

Inherent cofactor regeneration is a pivotal feature of whole cell biocatalysis. For specific biotechnological applications, surface display of enzymes is emerging as a tool to circumvent mass transfer limitations or enzyme stability problems. Even complex reactions can be accomplished applying displayed enzymes. Yet, industrial utilization of the technique is still impeded by lacking cofactor regeneration at the cell surface. Here, we report on the surface display of a glucose-6-phoshate dehydrogenase (G6PDH) via Autodisplay to address this limitation and regenerate NADPH directly at the cell surface. The obtained whole cell biocatalyst demonstrated similar kinetic parameters compared to the purified enzyme, more precisely K_M values of 0.2 mM for NADP⁺ and calculated total turnover numbers of 10⁷ . However, the K_M for the substrate G6P increased by a factor of 7 to yield 1.5 mM. The whole cell biocatalyst was cheaper to produce, easy to separate from the reaction mixture and reusable in consecutive reaction cycles. Furthermore, lyophilization allowed storage at room temperature. The whole cell biocatalyst displaying G6PDH was applicable for NADPH regeneration in combination with soluble as well as surface displayed enzymes and model reactions in combination with bacterial CYP102A1 and human CYP1A2 were realized. Biotechnol. Bioeng. 2017;114: 1658-1669. © 2017 Wiley Periodicals, Inc.

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.

Co-expression of active human cytochrome P450 1A2 and cytochrome P450 reductase on the cell surface of Escherichia coli

Background

Human cytochrome P450 (CYP) enzymes mediate the first step in the breakdown of most drugs and are strongly involved in drug–drug interactions, drug clearance and activation of prodrugs. Their biocatalytic behavior is a key parameter during drug development which requires preparative synthesis of CYP related drug metabolites. However, recombinant expression of CYP enzymes is a challenging bottleneck for drug metabolite biosynthesis. Therefore, we developed a novel approach by displaying human cytochrome P450 1A2 (CYP1A2) and cytochrome P450 reductase (CPR) on the surface of Escherichia coli.

Results

To present human CYP1A2 and CPR on the surface, we employed autodisplay. Both enzymes were displayed on the surface which was demonstrated by protease and antibody accessibility tests. CPR activity was first confirmed with the protein substrate cytochrome c. Cells co-expressing CYP1A2 and CPR were capable of catalyzing the conversion of the known CYP1A2 substrates 7-ethoxyresorufin, phenacetin and the artificial substrate luciferin-MultiCYP, which would not have been possible without interaction of both enzymes. Biocatalytic activity was strongly influenced by the composition of the growth medium. Addition of 5-aminolevulinic acid was necessary to obtain a fully active whole cell biocatalyst and was superior to the addition of heme.

Conclusion

We demonstrated that CYP1A2 and CPR can be co-expressed catalytically active on the cell surface of E. coli. It is a promising step towards pharmaceutical applications such as the synthesis of drug metabolites.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0427-5) contains supplementary material, which is available to authorized users.

A simplified process design for P450 driven hydroxylation based on surface displayed enzymes

New production routes for fine and bulk chemicals are important to establish further sustainable processes in industry. Besides the identification of new biocatalysts and new production routes the optimization of existing processes in regard to an improved utilization of the catalysts are needed. In this paper we describe the successful expression of P450BM3 on the surface of E. coli cells with the Autodisplay system. The successful hydroxylation of palmitic acid by using surface-displayed P450BM3 was shown. Besides optimization of surface protein expression, several cofactor regeneration systems were compared and evaluated. Afterwards, the development of a suitable process for the biocatalytic hydroxylation of fatty acids based on the re-use of the catalysts after a simple centrifugation was investigated. It was shown that the catalyst can be used for several times without any loss in activity. By using surface-displayed P450s in combination with an enzymatic cofactor regeneration system a total turnover number of up to 54,700 could be reached, to the knowledge of the authors the highest value reported for a P450 monooxygenase to date. Further optimizations of the described reaction system can have an enormous impact on the process design for more sustainable bioprocesses. Biotechnol. Bioeng. 2016;113: 1225-1233. © 2015 Wiley Periodicals, Inc.

Dynamics and Energy Contributions for Transport of Unfolded Pertactin through a Protein Nanopore

To evaluate the physical parameters governing translocation of an unfolded protein across a lipid bilayer, we studied protein transport through aerolysin, a passive protein channel, at the single-molecule level. The protein model used was the passenger domain of pertactin, an autotransporter virulence protein. Transport of pertactin through the aerolysin nanopore was detected as transient partial current blockades as the unfolded protein partially occluded the aerolysin channel. We compared the dynamics of entry and transport for unfolded pertactin and a covalent end-to-end dimer of the same protein. For both the monomer and the dimer, the event frequency of current blockades increased exponentially with the applied voltage, while the duration of each event decreased exponentially as a function of the electrical potential. The blockade time was twice as long for the dimer as for the monomer. The calculated activation free energy includes a main enthalpic component that we attribute to electrostatic interactions between pertactin and the aerolysin nanopore (despite the low Debye length), plus an entropic component due to confinement of the unfolded chain within the narrow pore. Comparing our experimental results to previous studies and theory suggests that unfolded proteins cross the membrane by passing through the nanopore in a somewhat compact conformation according to the "blob" model of Daoud and de Gennes.

Going beyond E. coli: autotransporter based surface display on alternative host organisms

Autotransporters represent one of the most popular anchoring motifs used to display peptides, proteins or enzymes on the cell surface of a Gram-negative bacterium. Applications range from vaccine delivery to library screenings to biocatalysis and bioremediation. Although the underlying secretion mechanism is supposed to be available in most, if not all, Gram-negative bacteria, autotransporters have to date almost exclusively been used for surface display on Escherichia coli. However, for their utilisation beyond a laboratory scale, in particular for biocatalysis, host bacteria with specific features and industrial applicability are required. A few groups have addressed this issue and demonstrated that bacteria other than E. coli can also be used for autotransporter based surface display. We summarise these studies and discuss opportunities and challenges that arise from surface display of recombinant proteins using the autotransporter pathway in alternative hosts.

The β-Barrel Assembly Machinery Complex

The outer membranes of gram-negative bacteria contain integral membrane proteins, most of which are of β-barrel structure, and critical for bacterial survival. These β-barrel proteins rely on the β-barrel assembly machinery (BAM) complex for their integration into the outer membrane as folded species. The central and essential subunit of the BAM complex, BamA, is a β-barrel protein conserved in all gram-negative bacteria and also found in eukaryotic organelles derived from bacterial endosymbionts. In Escherichia coli, BamA docks with four peripheral lipoproteins, BamB, BamC, BamD and BamE, partner subunits that add to the function of the BAM complex in outer membrane protein biogenesis. By way of introduction to this volume, we provide an overview of the work that has illuminated the mechanism by which the BAM complex drives β-barrel assembly. The protocols and methodologies associated with these studies as well as the challenges encountered and their elegant solutions are discussed in subsequent chapters.

Of linkers and autochaperones: an unambiguous nomenclature to identify common and uncommon themes for autotransporter secretion

Autotransporter (AT) proteins provide a diverse array of important virulence functions to Gram-negative bacterial pathogens, and have also been adapted for protein surface display applications. The 'autotransporter' moniker refers to early models that depicted these proteins facilitating their own translocation across the bacterial outer membrane. Although translocation is less autonomous than originally proposed, AT protein segments upstream of the C-terminal transmembrane β-barrel have nevertheless consistently been found to contribute to efficient translocation and/or folding of the N-terminal virulence region (the 'passenger'). However, defining the precise secretion functions of these AT regions has been complicated by the use of multiple overlapping and ambiguous terms to define AT sequence, structural, and functional features, including 'autochaperone', 'linker' and 'junction'. Moreover, the precise definitions and boundaries of these features vary among ATs and even among research groups, leading to an overall murky picture of the contributions of specific features to translocation. Here we propose a unified, unambiguous nomenclature for AT structural, functional and conserved sequence features, based on explicit criteria. Applied to 16 well-studied AT proteins, this nomenclature reveals new commonalities for translocation but also highlights that the autochaperone function is less closely associated with a conserved sequence element than previously believed.