Influence of tat mutations on the ribose-binding protein translocation in Escherichia coli

Large-scale evolutionary analyses on SecB subunits of bacterial sec system

Protein secretion systems are extremely important in bacteria because they are involved in many fundamental cellular processes. Of the various secretion systems, the Sec system is composed of seven different subunits in bacteria, and subunit SecB brings secreted preproteins to subunit SecA, which with SecYEG and SecDF forms a complex for the translocation of secreted preproteins through the inner membrane. Because of the wide existence of Sec system across bacteria, eukaryota, and archaea, each subunit of the Sec system has a complicated evolutionary relationship. Until very recently, 5,162 SecB sequences have been documented in UniProtKB, however no phylogenetic study has been conducted on a large sampling of SecBs from bacterial Sec secretion system, and no statistical study has been conducted on such size of SecBs in order to exhaustively investigate their variances of pairwise p-distance along taxonomic lineage from kingdom to phylum, to class, to order, to family, to genus and to organism. To fill in these knowledge gaps, 3,813 bacterial SecB sequences with full taxonomic lineage from kingdom to organism covering 4 phyla, 11 classes, 41 orders, 82 families, 269 genera, and 3,744 organisms were studied. Phylogenetic analysis revealed how the SecBs evolved without compromising their function with examples of 3-D structure comparison of two SecBs from Proteobacteria, and possible factors that affected the SecB evolution were considered. The average pairwise p-distances showed that the variance varied greatly in each taxonomic group. Finally, the variance was further partitioned into inter- and intra-clan variances, which could correspond to vertical and horizontal gene transfers, with relevance for Achromobacter, Brevundimonas, Ochrobactrum, and Pseudoxanthomonas.

Bacterial lipid modification of proteins requires appropriate secretory signals even for expression - implications for biogenesis and protein engineering

Sec- and Tat-mediated bacterial lipid modification of proteins are important posttranslational processes owing to their vital roles in cellular functions, membrane targeting and biotechnological applications like ELISA, biosensor, adjuvant-free vaccines, liposomal drug delivery etc. However a better understanding of the tight coupling of secretory and lipid modification machineries and the processes associated will help unravel this essential biological event and utilize it for engineering applications. Further, there is a need for a systematic and convincing investigation into membrane targeting, solubilization and ease-of-purification of engineered lipoproteins to facilitate scientists in readily applying this new protein engineering tool. Therefore, in this study, we have investigated systematically recombinant expression, translocation, solubilization and purification of three White Spot Syndrome Viral (WSSV) proteins, ICP11, VP28 and VP281. Our study shows that the lipid modification and secretion processes are tightly coupled to the extent that mismatch between folding kinetics and signal sequence of target proteins could lead to transcriptional-translational uncoupling or aborted translation. The proteins expressed as lipoproteins through Tat-pathway were targeted to the inner membrane achieving considerable enrichment. These His-tagged proteins were then purified to apparent homogeneity in detergent-free form using single-step Immobilized Metal Affinity Chromatography. This study has interesting findings in lipoprotein biogenesis enhancing the scope of this unique post-translational protein engineering tool for obtaining pure detergent-free, membrane or hydrophobic surface-associating diagnostic targets and vaccine candidates for WSSV.

Novel GFP expression using a short N-terminal polypeptide through the defined twin-arginine translocation (Tat) pathway

Escherichia coli is frequently used as a convenient host organism for soluble recombinant protein expression. However, additional strategies are needed for proteins with complex folding characteristics. Here, we suggested that the acidic, neutral, and alkaline isoelectric point (pI) range curves correspond to the channels of the E. coli type-II cytoplasmic membrane translocation (periplasmic translocation) pathways of twin-arginine translocation (Tat), Yid, and general secretory pathway (Sec), respectively, for unfolded and folded target proteins by examining the characteristic pI values of the N-termini of the signal sequences or the leader sequences, matching with the known diameter of the translocation channels, and analyzing the N-terminal pI value of the signal sequences of the Tat substrates. To confirm these proposed translocation pathways, we investigated the soluble expression of the folded green fluorescent protein (GFP) with short N-terminal polypeptides exhibiting pI and hydrophilicity separately or collectively. This, in turn, revealed the existence of an anchor function with a specific directionality based on the N-terminal pI value (termed as N-terminal pI-specific directionality) and distinguished the presence of the E. coli type-II cytoplasmic membrane translocation pathways of Tat, Yid, and Sec for the unfolded and folded target proteins. We concluded that the pI value and hydrophilicity of the short N-terminal polypeptide, and the total translational efficiency of the target proteins based on the ΔGRNA value of the N-terminal coding regions are important factors for promoting more efficient translocation (secretion) through the largest diameter of the Tat channel. These results show that the short N-terminal polypeptide could substitute for the Tat signal sequence with improved efficiency.

Abrogation of the twin arginine transport system in Salmonella enterica serovar Typhimurium leads to colonization defects during infection

TatC (STM3975) is a highly conserved component of the Twin Arginine Transport (Tat) systems that is required for transport of folded proteins across the inner membrane in gram-negative bacteria. We previously identified a ΔtatC mutant as defective in competitive infections with wild type ATCC14028 during systemic infection of Salmonella-susceptible BALB/c mice. Here we confirm these results and show that the ΔtatC mutant is internalized poorly by cultured J774-A.1 mouse macrophages a phenotype that may be related to the systemic infection defect. This mutant is also defective for short-term intestinal and systemic colonization after oral infection of BALB/c mice and is shed in reduced numbers in feces from orally infected Salmonella-resistant (CBA/J) mice. We show that the ΔtatC mutant is highly sensitive to bile acids perhaps resulting in the defect in intestinal infection that we observe. Finally, the ΔtatC mutant has an unusual combination of motility phenotypes in Salmonella; it is severely defective for swimming motility but is able to swarm well. The ΔtatC mutant has a lower amount of flagellin on the bacterial surface during swimming motility but normal levels under swarming conditions.

Overflow of a hyper-produced secretory protein from the Bacillus Sec pathway into the Tat pathway for protein secretion as revealed by proteogenomics

Bacteria secrete numerous proteins into their environment for growth and survival under complex and ever-changing conditions. The highly different characteristics of secreted proteins pose major challenges to the cellular protein export machinery and, accordingly, different pathways have evolved. While the main secretion (Sec) pathway transports proteins in an unfolded state, the twin-arginine translocation (Tat) pathway transports folded proteins. To date, these pathways were believed to act in strictly independent ways. Here, we have employed proteogenomics to investigate the secretion mechanism of the esterase LipA of Bacillus subtilis, using a serendipitously obtained hyper-producing strain. While LipA is secreted Sec-dependently under standard conditions, hyper-produced LipA is secreted predominantly Tat-dependently via an unprecedented overflow mechanism. Two previously identified B. subtilis Tat substrates, PhoD and YwbN, require each a distinct Tat translocase for secretion. In contrast, hyper-produced LipA is transported by both Tat translocases of B. subtilis, showing that they have distinct but overlapping specificities. The identified overflow secretion mechanism for LipA focuses interest on the possibility that secretion pathway choice can be determined by environmental and intracellular conditions. This may provide an explanation for the previous observation that many Sec-dependently transported proteins have potential twin-arginine signal peptides for export via the Tat pathway.

Sec- and Tat-dependent translocation of beta-lactamases across the Escherichia coli inner membrane

beta-Lactamases represent the major resistance mechanism of gram-negative bacteria against beta-lactam antibiotics. The amino acid sequences of these proteins vary widely, but all are located in the periplasm of bacteria. In this study, we investigated the translocation mechanism of representative beta-lactamases in an Escherichia coli model. N-terminal signal sequence analyses, antibiotic activity assay, and direct measurement of translocation of a green fluorescent protein (GFP) reporter fused to beta-lactamases revealed that most were exported via the Sec pathway. However, the Stenotrophomonas maltophilia L2 beta-lactamase was exported via the E. coli Tat translocase, while the S. maltophilia L1 beta-lactamase was Sec dependent. These results show the possible Tat-dependent translocation of beta-lactamases in the E. coli model system. In addition, the mutation of the cytoskeleton-encoding gene mreB, which may be involved in the spatial organization of penicillin-binding proteins, decreased the MIC of beta-lactams for beta-lactamase-producing E. coli. These findings provide new knowledge about beta-lactamase translocation, a putative new target for addressing beta-lactamase-mediated resistance.

The protein secretion systems in Listeria: inside out bacterial virulence

Listeria monocytogenes, the etiologic agent of listeriosis, remains a serious public health concern with its frequent occurrence in food coupled with a high mortality rate. The capacity of a bacterium to secrete proteins to or beyond the bacterial cell surface is of crucial importance in the understanding of biofilm formation and bacterial pathogenesis to further develop defensive strategies. Recent findings in protein secretion in Listeria together with the availability of complete genome sequences of several pathogenic L. monocytogenes strains, as well as nonpathogenic Listeria innocua Clip11262, prompted us to summarize the listerial protein secretion systems. Protein secretion would rely essentially on the Sec (Secretion) pathway. The twin-arginine translocation pathway seems encoded in all but one sequenced Listeria. In addition, a functional flagella export apparatus, a fimbrilin-protein exporter, some holins and a WXG100 secretion system are encoded in listerial genomes. This critical review brings new insights into the physiology and virulence of Listeria species.

The Tat pathway in bacteria and chloroplasts (review)

Both in prokaryotic organisms and in chloroplasts, a specialized protein transport pathway exists which is capable of translocating proteins in a fully folded conformation. Transport is mediated in both instances by signal peptides harbouring a twin-arginine consensus motif (twin-arginine translocation (Tat) pathway). The Tat translocase comprises the three functionally different membrane proteins TatA, TatB, and TatC. While TatB and TatC are involved in the specific recognition of the substrate, TatA might be the major pore-forming component. Current evidence suggests that a functional Tat translocase is assembled from separate TatBC and TatA assemblies only on demand, i.e., in the presence of transport substrate and a transmembrane H+-motive force.

YidC-dependent translocation of green fluorescence protein fused to the FliP cleavable signal peptide

Escherichia coli FliP is a rare bacterial polytopic membrane protein synthesized with a cleavable, highly hydrophobic signal peptide. More hydrophilic Tat-dependent or Sec-dependent signal peptide is functionally capable of substituting for the FliP signal peptide, but a signal anchor of inner membrane protein fails to do so. To assess the intrinsic characteristics of the FliP signal peptide in mediating protein translocation, we fused it to green fluorescence protein and observed that the translocation of the chimera (FliPss-GFP) was dependent of Ffh, SecA, SecY and SecD. In addition, we showed for the first time the involvement of YidC in protein translocation across the inner membrane.

Recombinant protein secretion in Escherichia coli

The secretory production of recombinant proteins by the Gram-negative bacterium Escherichia coli has several advantages over intracellular production as inclusion bodies. In most cases, targeting protein to the periplasmic space or to the culture medium facilitates downstream processing, folding, and in vivo stability, enabling the production of soluble and biologically active proteins at a reduced process cost. This review presents several strategies that can be used for recombinant protein secretion in E. coli and discusses their advantages and limitations depending on the characteristics of the target protein to be produced.

Bactericidal activity of colicin V is mediated by an inner membrane protein, SdaC, of Escherichia coli

Colicin V (ColV) is a peptide antibiotic that kills sensitive cells by disrupting their membrane potential once it gains access to the inner membrane from the periplasmic face. Recently, we constructed a translocation suicide probe, RR-ColV, that is translocated into the periplasm via the TAT pathway and thus kills the host cells. In this study, we obtained an RR-ColV-resistant mutant by using random Tn10 transposition mutagenesis. Sequencing analysis revealed that the mutant carried a Tn10 insertion in the sdaC (also called dcrA) gene, which is involved in serine uptake and is required for C1 phage adsorption. ColV activity was detected both in the cytoplasm and in the periplasm of this mutant, indicating that RR-ColV was translocated into the periplasm but failed to interact with the inner membrane. The sdaC::Tn10 mutant was resistant only to ColV and remained sensitive to colicins Ia, E3, and A. Most importantly, the sdaC::Tn10 mutant was killed when ColV was anchored to the periplasmic face of the inner membrane by fusion to EtpM, a type II integral membrane protein. Taken together, these results suggest that the SdaC/DcrA protein serves as a specific inner membrane receptor for ColV.

Tat-dependent protein targeting in prokaryotes and chloroplasts

The twin-arginine translocation (Tat) system operates in the chloroplast thylakoid and the plasma membranes of a wide range of bacteria. It recognizes substrates bearing cleavable signal peptides in which a twin-arginine motif almost invariably plays a key role in recognition by the translocation machinery. These signal peptides are surprisingly similar to those used to specify transport by Sec-type systems, but the Tat pathway differs in fundamental respects from Sec-type and other protein translocases. Its key attribute is its ability to translocate large, fully folded (even oligomeric) proteins across tightly sealed membranes. To date, three key tat genes have been characterised and the first details of the Tat system are beginning to emerge. In this article we review the salient features of Tat systems, with an emphasis on the targeting signals involved, the substrate specificities of Tat systems, our current knowledge of Tat complex structures and the known mechanistic features. Although the article is focused primarily on bacterial systems, we incorporate relevant aspects of plant thylakoid Tat work and we discuss how the plant and bacterial systems may differ in some respects.

Dissecting the twin-arginine translocation pathway using genome-wide analysis

A recently discovered route for protein export, known as the twin-arginine translocation (Tat) pathway, has received much attention owing to several atypical characteristics that distinguish it from other transport mechanisms. For instance, recent evidence has clearly established that this pathway only transports folded polypeptides. Moreover, several studies have demonstrated a vital role for the Tat pathway in bacterial pathogenesis. In this review, we discuss genomic approaches that have been employed to determine the prevalence and capacity of the Tat system and how the information generated from these approaches is helping to connect Tat transport to bacterial physiology and virulence.

Twin-arginine-specific protein export in Escherichia coli

In many prokaryotic organisms, secretory proteins harboring a twin-arginine consensus motif are exported in a fully folded conformation via the twin-arginine translocation (Tat) pathway. In Escherichia coli, Tat involves the three structurally and functionally different membrane proteins TatA, TatB, and TatC. Whereas TatC proteins function in the specific recognition of substrate, TatA might be the major pore-forming subunit.

A putative twin-arginine translocation pathway in Legionella pneumophila

Legionella pneumophila is a facultative intracellular human pathogen causing Legionnaires' disease, a severe form of pneumonia. Because of the importance of secretion pathways in virulence, we were interested in the possible presence of the twin-arginine translocation (Tat) pathway in L. pneumophila. This secretion pathway is used to transport folded proteins, characterized by two arginines in their signal peptide, across the cytoplasmic membrane. We describe here the presence of a putative Tat pathway in L. pneumophila. Three genes encoding Escherichia coli TatA, TatB, and TatC homologues were identified. The tatA and tatB genes were shown to constitute an operon while tatC is monocistronic. RT-PCR analysis revealed expression of the tat genes during both exponential and stationary growth as well as during intracellular growth in Acanthamoeba castellanii. A search for the conserved twin-arginine motif in predicted signal peptides resulted in a list of putative Tat substrates.

Contribution of the twin arginine translocation system to the virulence of enterohemorrhagic Escherichia coli O157:H7

Shiga toxin-producing Escherichia coli O157:H7 is a major food-borne infectious pathogen. In order to analyze the contribution of the twin arginine translocation (TAT) system to the virulence of E. coli O157:H7, we deleted the tatABC genes of the O157:H7 EDL933 reference strain. The mutant displayed attenuated toxicity on Vero cells and completely lost motility on soft agar plates. Further analyses revealed that the Delta tatABC mutation impaired the secretion of the Shiga toxin 1 (Stx1) and abolished the synthesis of H7 flagellin, which are two major known virulence factors of enterohemorrhagic E. coli O157:H7. Expression of the EDL933 stxAB(1) genes in E. coli K-12 conferred verotoxicity on this nonpathogenic strain. Remarkably, cytotoxicity assay and immunoblot analysis showed, for the first time, an accumulation of the holotoxin complex in the periplasm of the wild-type strain and that a much smaller amount of StxA(1) and reduced verotoxicity were detected in the Delta tatC mutant cells. Together, these results establish that the TAT system of E. coli O157:H7 is an important virulence determinant of this enterohemorrhagic pathogen.