Export of active green fluorescent protein to the periplasm by the twin-arginine translocase (Tat) pathway in Escherichia coli

Independent mobility of proteins and lipids in the plasma membrane of Escherichia coli

Fluidity is essential for many biological membrane functions. The basis for understanding membrane structure remains the classic Singer-Nicolson model, in which proteins are embedded within a fluid lipid bilayer and able to diffuse laterally within a sea of lipid. Here we report lipid and protein diffusion in the plasma membrane of live cells of the bacterium Escherichia coli, using Fluorescence Recovery after Photobleaching (FRAP) and Total Internal Reflection Fluorescence (TIRF) microscopy to measure lateral diffusion coefficients. Lipid and protein mobility within the membrane were probed by visualizing an artificial fluorescent lipid and a simple model membrane protein consisting of a single membrane-spanning alpha-helix with a Green Fluorescent Protein (GFP) tag on the cytoplasmic side. The effective viscosity of the lipid bilayer is strongly temperature-dependent, as indicated by changes in the lipid diffusion coefficient. Surprisingly, the mobility of the model protein was unaffected by changes in the effective viscosity of the bulk lipid, and TIRF microscopy indicates that it clusters in segregated, mobile domains. We suggest that this segregation profoundly influences the physical behaviour of the protein in the membrane, with strong implications for bacterial membrane function and bacterial physiology.

Twin-arginine signal peptide of Bacillus subtilis YwbN can direct Tat-dependent secretion of methyl parathion hydrolase

The twin-arginine translocation (Tat) pathway exports folded proteins across the cytoplasmic membranes of bacteria and archaea. Two parallel Tat pathways (TatAdCd and TatAyCy systems) with distinct substrate specificities have previously been discovered in Bacillus subtilis. In this study, to secrete methyl parathion hydrolase (MPH) into the growth medium, the twin-arginine signal peptide of B. subtilis YwbN was used to target MPH to the Tat pathway of B. subtilis. Western blot analysis and MPH assays demonstrated that active MPH was secreted into the culture supernatant of wild-type cells. No MPH secretion occurred in a total-tat2 mutant, indicating that the observed export in wild-type cells was mediated exclusively by the Tat pathway. Export was fully blocked in a tatAyCy mutant. In contrast, the tatAdCd mutant was still capable of secreting MPH. These results indicated that the MPH secretion directed by the YwbN signal peptide was specifically mediated by the TatAyCy system. The N-terminal sequence of secreted MPH was determined as AAPQVR, demonstrating that the YwbN signal peptide had been processed correctly. This is the first report of functional secretion of a heterologous protein via the B. subtilis TatAyCy system. This study highlights the potential of the TatAyCy system to be used for secretion of other heterologous proteins in B. subtilis.

The canonical twin-arginine translocase components are not required for secretion of folded green fluorescent protein from the ancestral strain of Bacillus subtilis

Cellular processes, such as the digestion of macromolecules, phosphate acquisition, and cell motility, require bacterial secretion systems. In Bacillus subtilis, the predominant protein export pathways are Sec (generalized secretory pathway) and Tat (twin-arginine translocase). Unlike Sec, which secretes unfolded proteins, the Tat machinery secretes fully folded proteins across the plasma membrane and into the medium. Proteins are directed for Tat-dependent export by N-terminal signal peptides that contain a conserved twin-arginine motif. Thus, utilizing the Tat secretion system by fusing a Tat signal peptide is an attractive strategy for the production and export of heterologous proteins. As a proof of concept, we expressed green fluorescent protein (GFP) fused to the PhoD Tat signal peptide in the laboratory and ancestral strains of B. subtilis. Secretion of the Tat-GFP construct, as well as secretion of proteins in general, was substantially increased in the ancestral strain. Furthermore, our results show that secreted, fluorescent GFP could be purified directly from the extracellular medium. Nonetheless, export was not dependent on the known Tat secretion components or the signal peptide twin-arginine motif. We propose that the ancestral strain contains additional Tat components and/or secretion regulators that were abrogated following domestication.

Transcriptomic analysis of carboxylic acid challenge in Escherichia coli: beyond membrane damage

Carboxylic acids are an attractive biorenewable chemical. Enormous progress has been made in engineering microbes for production of these compounds though titers remain lower than desired. Here we used transcriptome analysis of Escherichia coli during exogenous challenge with octanoic acid (C8) at pH 7.0 to probe mechanisms of toxicity. This analysis highlights the intracellular acidification and membrane damage caused by C8 challenge. Network component analysis identified transcription factors with altered activity including GadE, the activator of the glutamate-dependent acid resistance system (AR2) and Lrp, the amino acid biosynthesis regulator. The intracellular acidification was quantified during exogenous challenge, but was not observed in a carboxylic acid producing strain, though this may be due to lower titers than those used in our exogenous challenge studies. We developed a framework for predicting the proton motive force during adaptation to strong inorganic acids and carboxylic acids. This model predicts that inorganic acid challenge is mitigated by cation accumulation, but that carboxylic acid challenge inverts the proton motive force and requires anion accumulation. Utilization of native acid resistance systems was not useful in terms of supporting growth or alleviating intracellular acidification. AR2 was found to be non-functional, possibly due to membrane damage. We proposed that interaction of Lrp and C8 resulted in repression of amino acid biosynthesis. However, this hypothesis was not supported by perturbation of lrp expression or amino acid supplementation. E. coli strains were also engineered for altered cyclopropane fatty acid content in the membrane, which had a dramatic effect on membrane properties, though C8 tolerance was not increased. We conclude that achieving higher production titers requires circumventing the membrane damage. As higher titers are achieved, acidification may become problematic.

Disassembly of the divisome in Escherichia coli: evidence that FtsZ dissociates before compartmentalization

In most bacteria cell division is mediated by a protein super-complex called the divisome that co-ordinates the constriction and scission of the cell envelope. FtsZ is the first of the divisome proteins to accumulate at the division site and is widely thought to function as a force generator that constricts the cell envelope. In this study we have used a combination of confocal fluorescence microscopy and fluorescence recovery after photobleaching (FRAP) to determine if divisome proteins are present at the septum at the time of cytoplasmic compartmentalization in Escherichia coli. Our data suggest that many are, but that FtsZ and ZapA disassemble before the cytoplasm is sealed by constriction of the inner membrane. This observation implies that FtsZ cannot be a force generator during the final stage(s) of envelope constriction in E. coli.

Efficient export of prefolded, disulfide-bonded recombinant proteins to the periplasm by the Tat pathway in Escherichia coli CyDisCo strains

Numerous high-value therapeutic proteins are produced in Escherichia coli and exported to the periplasm, as this approach simplifies downstream processing and enables disulfide bond formation. Most recombinant proteins are exported by the Sec pathway, which transports substrates across the plasma membrane in an unfolded state. The Tat system also exports proteins to the periplasm, but transports them in a folded state. This system has attracted interest because of its tendency to transport correctly folded proteins, but this trait renders it unable to export proteins containing disulfide bonds since these are normally acquired only in the periplasm; reduced substrates tend to be recognized as incorrectly folded and rejected. In this study we have used a series of novel strains (termed CyDisCo) which oxidise disulfide bonds in the cytoplasm, and we show that these cells efficiently export a range of disulfide-containing proteins when a Tat signal peptide is attached. These test proteins include alkaline phosphatase (PhoA), a phytase containing four disulfide bonds (AppA), an antiinterleukin 1β scFv and human growth hormone. No export of PhoA or AppA is observed in wild-type cells lacking the CyDisCo factors. The PhoA, AppA and scFv proteins were exported in an active form by Tat in the CyDisCo strain, and mass spectrometry showed that the vast majority of the scFv protein was disulfide-bonded and correctly processed. The evidence indicates that this combination of Tat + CyDisCo offers a novel means of exporting active, correctly folded disulfide bonded proteins to the periplasm.

Tat pathway-mediated translocation of the Sec pathway substrate OprM, an outer membrane subunit of the resistance nodulation division xenobiotic extrusion pumps, in Pseudomonas Aeruginosa

BACKGROUND

Pseudomonas aeruginosa produces the Sec and Tat protein secretion machineries. The latter appears to be involved in the secretion of virulence factors, including phospholipase C (PlcH), and hence is a potential target of chemotherapeutic agents.

METHODS

The signal sequence of OprM, the outer membrane subunit of the xenobiotic extrusion pumps, was substituted with that of PlcH. The antibiotic susceptibility of oprM-deficient cells expressing the hybrid protein PlcH-OprM was evaluated using the agar dilution method.

RESULTS

The PlcH-OprM-expressing cells showed resistance to various MexAB-OprM substrate antibiotics. To evaluate the translocation route of PlcH-OprM, tatC encoding an indispensable component of the Tat machinery was knocked out in oprM-deficient cells. The tatC-oprM double mutant expressing PlcH-OprM exhibited antibiotic hypersusceptibility like the oprM-deficient cells, indicating that PlcH-OprM was translocated across the inner membrane exclusively through the Tat system.

CONCLUSIONS

This system can be used for the screening of Tat system inhibitors and will be an excellent model for the study of secretion and biogenesis of the β-barrel outer membrane proteins.

Twin-arginine translocation system (tat) mutants of Salmonella are attenuated due to envelope defects, not respiratory defects

The twin-arginine translocation system (Tat) transports folded proteins across the cytoplasmic membrane and is critical to virulence in Salmonella and other pathogens. Experimental and bioinformatic data indicate that 30 proteins are exported via Tat in Salmonella Typhimurium. However, there are no data linking specific Tat substrates with virulence. We inactivated every Tat-exported protein and determined the virulence phenotype of mutant strains. Although a tat mutant is highly attenuated, no single Tat-exported substrate accounts for this virulence phenotype. Rather, the attenuation is due primarily to envelope defects caused by failure to translocate three Tat substrates, the N-acetylmuramoyl-l-alanine amidases, AmiA and AmiC, and the cell division protein, SufI. Strikingly, neither the amiA amiC nor the sufI mutations alone conferred any virulence defect. Although AmiC and SufI have previously been localized to the divisome, the synthetic phenotypes observed are the first to suggest functional overlap. Many Tat substrates are involved in anaerobic respiration, but we show that a mutant completely deficient in anaerobic respiration retains full virulence in both the oral and systemic phases of infection. Similarly, an obligately aerobic mutant is fully virulent. These results suggest that in the classic mouse model of infection, S. Typhimurium is replicating only in aerobic environments.

High-level secretion of a recombinant protein to the culture medium with a Bacillus subtilis twin-arginine translocation system in Escherichia coli

The twin-arginine translocation (Tat) system transports folded proteins across the plasma membrane in bacteria, and heterologous proteins can be exported by this pathway if a Tat-type signal peptide is present at the N-terminus. The system thus has potential for biopharmaceutical production in Escherichia coli, where export to the periplasm is often a favoured approach. Previous studies have shown that E. coli cells can export high levels of protein by the Tat pathway, and the protein product accummulates almost exclusively in the periplasm. In this study, we analysed E. coli cells that express the Bacillus subtilis TatAdCd system in place of the native TatABC system. We show that a heterologous model protein, comprising the TorA signal peptide linked to green fluorescent protein (TorA-GFP), is efficiently exported by the TatAdCd system. However, whereas the GFP is exported initially to the periplasm during batch fermentation, the mature protein is increasingly found in the extracellular culture medium. By the end of a 16-h fermentation, ~ 90% of exported GFP is present in the medium as active mature protein. The total protein profiles of the medium and periplasm are essentially identical, confirming that the outer membrane becomes leaky during the fermentation process. The cells are otherwise intact, and there is no large-scale release of cytoplasmic contents. Export levels are relatively high, with ~ 0.35 g GFP·L⁻¹ culture present in the medium. This system thus offers a means of producing recombinant protein in E. coli and harvesting directly from the medium, with potential advantages in terms of ease of purification and downstream processing.

Broad host range vectors for expression of proteins with (Twin-) Strep-tag, His-tag and engineered, export optimized yellow fluorescent protein

Background

In current protein research, a limitation still is the production of active recombinant proteins or native protein associations to assess their function. Especially the localization and analysis of protein-complexes or the identification of modifications and small molecule interaction partners by co-purification experiments requires a controllable expression of affinity- and/or fluorescence tagged variants of a protein of interest in its native cellular background. Advantages of periplasmic and/or homologous expressions can frequently not be realized due to a lack of suitable tools. Instead, experiments are often limited to the heterologous production in one of the few well established expression strains.

Results

Here, we introduce a series of new RK2 based broad host range expression plasmids for inducible production of affinity- and fluorescence tagged proteins in the cytoplasm and periplasm of a wide range of Gram negative hosts which are designed to match the recently suggested modular Standard European Vector Architecture and database. The vectors are equipped with a yellow fluorescent protein variant which is engineered to fold and brightly fluoresce in the bacterial periplasm following Sec-mediated export, as shown from fractionation and imaging studies. Expression of Strep-tag^®II and Twin-Strep-tag^® fusion proteins in Pseudomonas putida KT2440 is demonstrated for various ORFs.

Conclusion

The broad host range constructs we have produced enable good and controlled expression of affinity tagged protein variants for single-step purification and qualify for complex co-purification experiments. Periplasmic export variants enable production of affinity tagged proteins and generation of fusion proteins with a novel engineered Aequorea-based yellow fluorescent reporter protein variant with activity in the periplasm of the tested Gram-negative model bacteria Pseudomonas putida KT2440 and Escherichia coli K12 for production, localization or co-localization studies. In addition, the new tools facilitate metabolic engineering and yield assessment for cytoplasmic or periplasmic protein production in a number of different expression hosts when yields in one initially selected are insufficient.

Exclusively membrane-inserted state of an uncleavable Tat precursor protein suggests lateral transfer into the bilayer from the translocon

In bacteria, the export of proteins by the twin-arginine translocase (Tat) pathway is directed by cleavable N-terminal signal peptides. We studied the relationship between transport and maturation using a substrate, YedY, that contains an Ala > Leu substitution at the -1 position of the signal peptide. This blocks maturation and leads to the accumulation of a membrane-bound precursor form with the mature domain exposed to the periplasm. Its accumulation does not block transport of other Tat substrates, indicating that exit from the translocation channel has taken place, and the precursor protein is fir mLy integrated into the membrane bilayer. The membrane-integrated nature of the precursor, and complete absence of precursor protein in the periplasm, strongly suggest that the precursor has undergone lateral transfer into the bilayer during translocation. We propose that subsequent proteolytic processing releases the mature protein into the periplasm. A delay in processing results in an inhibition of cell growth, emphasizing a requirement for efficient maturation of Tat substrates.

Mislocalization of Rieske protein PetA predominantly accounts for the aerobic growth defect of Tat mutants in Shewanella oneidensis

Shewanella oneidensis exhibits a remarkable versatility in respiration, which largely relies on its various respiratory pathways. Most of these pathways are composed of secretory terminal reductases and multiple associated electron transport proteins that contain cofactors such as Fe-S, molybdopterin, and NiFe. The majority of these cofactors are inserted enzymatically in the cytoplasm, and thus are substrates of the twin-arginine translocation (Tat) protein export system, which transports fully folded proteins. Using genomic array footprinting, we discovered that loss of TatA or TatC caused a reduction in the growth rate of S. oneidensis under aerobic conditions. Mutational analysis of the predicted Tat substrates revealed that PetA, the Rieske Fe-S subunit of the ubiquinol-cytochrome c reductase, predominantly dictates the aerobic growth defect of tat mutants in S. oneidensis. In addition, evidence is presented that the signal sequence in PetA appears to be resistant to cleavage after the protein is inserted into the cytoplasmic membrane.

Topological mapping methods for α-helical bacterial membrane proteins--an update and a guide

Integral membrane proteins with α-helical transmembrane segments (TMS) are known to play important and diverse roles in prokaryotic cell physiology. The net hydrophobicity of TMS directly corresponds to the observed difficulties in expressing and purifying these proteins, let alone producing sufficient yields for structural studies using two-/three-dimensional (2D/3D) crystallographic or nuclear magnetic resonance methods. To gain insight into the function of these integral membrane proteins, topological mapping has become an important tool to identify exposed and membrane-embedded protein domains. This approach has led to the discovery of protein tracts of functional importance and to the proposition of novel mechanistic hypotheses. In this review, we synthesize the various methods available for topological mapping of α-helical integral membrane proteins to provide investigators with a comprehensive reference for choosing techniques suited to their particular topological queries and available resources.

Absence of long-range diffusion of OmpA in E. coli is not caused by its peptidoglycan binding domain

Background

It is widely believed that integral outer membrane (OM) proteins in bacteria are able to diffuse laterally in the OM. However, stable, immobile proteins have been identified in the OM of Escherichia coli. In explaining the observations, a hypothesized interaction of the immobilized OM proteins with the underlying peptidoglycan (PG) cell wall played a prominent role.

Results

OmpA is an abundant outer membrane protein in E. coli containing a PG-binding domain. We use FRAP to investigate whether OmpA is able to diffuse laterally over long-range (> ~100 nm) distances in the OM. First, we show that OmpA, containing a PG binding domain, does not exhibit long-range lateral diffusion in the OM. Then, to test whether PG interaction was required for this immobilization, we genetically removed the PG binding domain and repeated the FRAP experiment. To our surprise, this did not increase the mobility of the protein in the OM.

Conclusions

OmpA exhibits an absence of long-range (> ~100 nm) diffusion in the OM that is not caused by its PG binding domain. Therefore, other mechanisms are needed to explain this observation, such as the presence of physical barriers in the OM, or strong interactions with other elements in the cell envelope.

Fluorescence imaging of GFP-fused periplasmic components of Na+-driven flagellar motor using Tat pathway in Vibrio alginolyticus

The twin-arginine translocation (Tat) system works to export folded proteins across the cytoplasmic membrane via specific signal peptides harbouring a twin-arginine motif. In Escherichia coli, a functional GFP is exported to the periplasm through the Tat pathway by fusion of the signal peptide of TorA, which is one of the periplasmic proteins exported by the Tat pathway. In this study, we fused the signal peptide of Vibrio alginolyticus TorA (TorASP) to GFP and demonstrate the export of functional GFP to the periplasm of V. alginolyticus. We also made fusions of TorASP-GFP with MotX, MotY and FlgT, which are periplasmic components of the Na(+)-driven flagellar motor. Those fusion proteins were localized to the flagellar motor independent of the Na(+) concentration in the environment.

self-assembling GFP: a versatile tool for plant (membrane) protein analyses

The investigation of cellular processes on the molecular level is important to understand the functional network within plant cells. self-assembling GFP has evolved to be a versatile tool for (membrane) protein analyses. Based on the autocatalytical reassembling property of the nonfluorescent strands 1-10 and 11, protein distribution and membrane protein topology can be analyzed in vivo. Here, we provide basic protocols to determine membrane protein topology in Arabidopsis thaliana protoplasts.

Engineered fluorescent proteins illuminate the bacterial periplasm

The bacterial periplasm is of special interest whenever cell factories are designed and engineered. Recombinantely produced proteins are targeted to the periplasmic space of Gram negative bacteria to take advantage of the authentic N-termini, disulfide bridge formation and easy accessibility for purification with less contaminating cellular proteins. The oxidizing environment of the periplasm promotes disulfide bridge formation - a prerequisite for proper folding of many proteins into their active conformation. In contrast, the most popular reporter protein in all of cell biology, Green Fluorescent Protein (GFP), remains inactive if translocated to the periplasmic space prior to folding. Here, the self-catalyzed chromophore maturation is blocked by formation of covalent oligomers via interchain disulfide bonds in the oxidizing environment. However, different protein engineering approaches addressing folding and stability of GFP resulted in improved proteins with enhanced folding properties. Recent studies describe GFP variants that are not only active if translocated in their folded form via the twin-arginine translocation (Tat) pathway, but actively fold in the periplasm following general secretory pathway (Sec) and signal recognition particle (SRP) mediated secretion. This mini-review highlights the progress that enables new insights into bacterial export and periplasmic protein organization, as well as new biotechnological applications combining the advantages of the periplasmic production and the Aequorea-based fluorescent reporter proteins.

Importance of the latex-clearing protein (Lcp) for poly(cis-1,4-isoprene) rubber cleavage in Streptomyces sp. K30

Streptomyces sp. strain K30 induces the formation of an extracellular Lcp (latex-clearing protein) during poly(cis-1,4-isoprene) degradation. To investigate the function of this enzyme in Streptomyces sp. strain K30, the lcp gene was disrupted. This was the first time that the screening for a knock out lcp mutant of Streptomyces sp. strain K30 was successful. The resulting mutant Streptomyces sp. K30_lcpΩKm exhibited reduced growth in liquid mineral salts media containing poly(cis-1,4-isoprene) as the sole carbon and energy source. Additionally, there was no detectable Lcp activity on latex overlay agar plates. When Lcp from Streptomyces sp. strain K30 was heterologously expressed in strains TK23 and TK24 of Streptomyces lividans and a strain of S. erythraea with plasmid pIJ6021::lcp, the recombinant strains acquired the ability to cleave synthetic poly(cis-1,4-isoprene), confirming the involvement of Lcp in initial polymer cleavage. Specific anti-LcpK30 IgGs were employed in Western blot analysis to detect the secretion of Lcp in the supernatant. We have conducted an important experiment to demonstrate Lcp activity using the supernatant of these Lcp-expressing strains in vitro. All three strains obviously secreted a functional Lcp, as indicated by the formation of halo. Functional testing of Lcp with different plasmids in Escherichia coli strains and Pseudomonas strains was, however, not successful.

Functional expression of a penicillin acylase from the extreme thermophile Thermus thermophilus HB27 in Escherichia coli

Background

Penicillin acylases (PACs) are enzymes of industrial relevance in the manufacture of β-lactam antibiotics. Development of a PAC with a longer half-life under the reaction conditions used is essential for the improvement of the operational stability of the process. A gene encoding a homologue to Escherichia coli PAC was found in the genome of the thermophilic bacterium Thermus thermophilus (Tth) HB27. Because of the nature of this PAC and its complex maturation that is crucial to reach its functional heterodimeric final conformation, the overexpression of this enzyme in a heterologous mesophilic host was a challenge. Here we describe the purification and characterization of the PAC protein from Tth HB27 overexpressed in Escherichia coli.

Results

Fusions to a superfolder green fluorescent protein and differential membrane solubilization assays indicated that the native enzyme remains attached through its amino-terminal end to the outer side of the cytoplasmic membrane of Tth cells. In order to overexpress this PAC in E. coli cells, a variant of the protein devoid of its membrane anchoring segment was constructed. The effect of the co-expression of chaperones and calcium supplementation of the culture medium was investigated. The total production of PAC was enhanced by the presence of DnaK/J and GrpE and even more by trigger factor and GroEL/ES. In addition, 10 mM calcium markedly improved both PAC specific and volumetric activities. Recombinant PAC was affinity-purified and proper maturation of the protein was confirmed by SDS-PAGE and MALDI-TOF analysis of the subunits. The recombinant protein was tested for activity towards several penicillins, cephalosporins and homoserine lactones. Hydrophobic acyl-chain penicillins were preferred over the rest of the substrates. Penicillin K (octanoyl penicillin) was the best substrate, with the highest specificity constant value (16.12 mM^-1.seg^-1). The optimum pH was aprox. 4 and the optimum temperature was 75 °C. The half-life of the enzyme at this temperature was 9.2 h.

Conclusions

This is the first report concerning the heterologous expression of a pac gene from a thermophilic microorganism in the mesophilic host E. coli. The recombinant protein was identified as a penicillin K-deacylating thermozyme.

Role of vesicle-inducing protein in plastids 1 in cpTat transport at the thylakoid

VIPP1 has been shown to be required for the proper formation of thylakoid membranes. However, studies on VIPP1 itself, as well as on PspA, its bacterial homolog, suggests that this protein may be involved in a number of additional functions, including protein translocation. The role of VIPP1 in protein translocation in the chloroplast has not been investigated. To this end, we conducted in vitro thylakoid protein transport assays to look at the effect of VIPP1 on the cpTat pathway, which is one of three translocation pathways found in both the chloroplast and its bacterial progenitor. We found that VIPP1 does indeed enhance protein transport through the cpTat pathway by up to 100%. The VIPP1 effect on cpTat activity occurs without interacting with the substrates or components of the translocon, and does not alter the energy potentials driving this translocation pathway. Instead, VIPP1 greatly enhances the amount of substrate bound productively to the thylakoids. Moreover, the presence of increasing VIPP1 concentrations in the reactions resulted in greater interactions between thylakoid membranes. Taken together, these results demonstrate a stimulatory role for VIPP1 in cpTat transport by enhancement of substrate binding, probably to the membrane lipid regions of the thylakoid. We propose a model in which VIPP1 facilitates reorganization of the thylakoid structure to increase substrate access to productive binding regions of the membrane as an early step in the cpTat pathway.

Twin-arginine translocase mutations that suppress folding quality control and permit export of misfolded substrate proteins

The bacterial twin-arginine translocation (Tat) pathway facilitates the transport of correctly folded proteins across the tightly sealed cytoplasmic membrane. Here, we report the isolation and characterization of suppressor mutations in the Tat translocase that allow export of misfolded proteins, which form structures that are not normally tolerated by the wild-type translocase. Selection of suppressors was enabled by a genetic assay that effectively linked in vivo folding and stability of a test protein with Tat export efficiency of a selectable marker protein, namely TEM-1 β-lactamase. By using a test protein named α(3)B-a designed three-helix-bundle protein that forms collapsed, stable molten globules but lacks a uniquely folded structure-translocase mutants that rescued export of this protein were readily identified. Each mutant translocase still efficiently exported folded substrate proteins, indicating that the substrate specificity of suppressors was relaxed but not strictly altered. A subset of the suppressors could also export other misfolded proteins, such as the aggregation-prone α(3)A protein and reduced alkaline phosphatase. Importantly, the isolation of genetic suppressors that inactivate the Tat quality-control mechanism provides direct evidence for the participation of the Tat translocase in structural proofreading of substrate proteins and reveals epitopes in the translocase that are important for this process.

High-yield export of a native heterologous protein to the periplasm by the tat translocation pathway in Escherichia coli

Numerous high-value recombinant proteins that are produced in bacteria are exported to the periplasm as this approach offers relatively easy downstream processing and purification. Most recombinant proteins are exported by the Sec pathway, which transports them across the plasma membrane in an unfolded state. The twin-arginine translocation (Tat) system operates in parallel with the Sec pathway but transports substrate proteins in a folded state; it therefore has potential to export proteins that are difficult to produce using the Sec pathway. In this study, we have produced a heterologous protein (green fluorescent protein; GFP) in Escherichia coli and have used batch and fed-batch fermentation systems to test the ability of the newly engineered Tat system to export this protein into the periplasm under industrial-type production conditions. GFP cannot be exported by the Sec pathway in an active form. We first tested the ability of five different Tat signal peptides to export GFP, and showed that the TorA signal peptide directed most efficient export. Under batch fermentation conditions, it was found that TorA-GFP was exported efficiently in wild type cells, but a twofold increase in periplasmic GFP was obtained when the TatABC components were co-expressed. In both cases, periplasmic GFP peaked at about the 12 h point during fermentation but decreased thereafter, suggesting that proteolysis was occurring. Typical yields were 60 mg periplasmic GFP per liter culture. The cells over-expressed the tat operon throughout the fermentation process and the Tat system was shown to be highly active over a 48 h induction period. Fed-batch fermentation generated much greater yields: using glycerol feed rates of 0.4, 0.8, and 1.2 mL h(-1), the cultures reached OD(600) values of 180 and periplasmic GFP levels of 0.4, 0.85, and 1.1 g L(-1) culture, respectively. Most or all of the periplasmic GFP was shown to be active. These export values are in line with those obtained in industrial production processes using Sec-dependent export approaches.

Kinetics of precursor interactions with the bacterial Tat translocase detected by real-time FRET

The Escherichia coli twin-arginine translocation (Tat) system transports fully folded and assembled proteins across the inner membrane into the periplasmic space. Traditionally, in vitro protein translocation studies have been performed using gel-based transport assays. This technique suffers from low time resolution, and often, an inability to distinguish between different steps in a continuously occurring translocation process. To address these limitations, we have developed an in vitro FRET-based assay that reports on an early step in the Tat translocation process in real-time. The natural Tat substrate pre-SufI was labeled with Alexa532 (donor), and the fluorescent protein mCherry (acceptor) was fused to the C terminus of TatB or TatC. The colored Tat proteins were easily visible during purification, enabling identification of a highly active inverted membrane vesicle (IMV) fraction yielding transport rates with NADH almost an order of magnitude faster than previously reported. When pre-SufI was bound to the translocon, FRET was observed for both Tat proteins. FRET was diminished upon addition of nonfluorescent pre-SufI, indicating that the initial binding step is reversible. When the membranes were energized with NADH, the FRET signal was lost after a short delay. These data suggest a model in which a Tat cargo initially associates with the TatBC complex, and an electric field gradient is required for the cargo to proceed to the next stage of transport. This cargo migration away from the TatBC complex requires a significant fraction of the total transport time.

Identifying and optimizing intracellular protein-protein interactions using bacterial genetic selection

Protein-protein interactions are crucial for the vast majority of biological processes. To fully understand these processes therefore requires methods for identifying protein interactions within the complex cellular environment. To isolate interacting proteins, we have developed a simple and reliable genetic selection by exploiting the inbuilt "hitchhiker" mechanism of the Escherichia coli twin-arginine translocation (Tat) pathway. This method is based on the unique ability of the Tat system to efficiently co-localize noncovalent complexes of two folded polypeptides to the periplasmic space of E. coli. The genetic selection is comprised of two engineered fusion proteins: an N-terminal Tat signal peptide fused to the protein of interest, and the known or putative partner protein fused to mature TEM-1 β-lactamase. The efficiency with which co-localized β-lactamase chimeras are exported in the periplasm, and thus confer ampicillin resistance to cells, is directly linked to the relative binding affinity of the protein-ligand system. Thus, ampicillin resistance can be used as a convenient readout for identifying and optimizing protein interactions in E. coli. Furthermore, because Tat substrates must be correctly folded for export, our method favors the identification of soluble, non-aggregating, protease-resistant protein pairs. Overall, we anticipate that this new selection tool will be useful for discovering and engineering protein drugs, protein complexes for structural biology, factors that inhibit PPIs, and components for synthetic biology.

Development of Dengue type-2 virus replicons expressing GFP reporter gene in study of viral RNA replication

Insertion of green fluorescent protein (GFP) encoding-gene into virus genes has provided a valuable tool for flavivirus research. This study aimed to develop dengue virus (DENV) replicons expressing GFP reporter that would provide a fast in vitro system to analyze functional roles of specific DENV sequences in viral replication. Two classes of recombinant replicon constructs were generated; one was a RNA-launched replicon with a GFP gene directly inserted into a full-length DENV genome (FL-DENV/GFP), and the other consisted of 4 types of DNA-launched DENV subgenomic replicons with GFP replacement at various structural genes (Δ-DENV/GFP). The FL-DENV/GFP resulted in GFP expression in transfected cells with no viable DENV being recovered from the transfection. The Δ-DENV/GFP constructs with partial structural gene deletion (ΔC-, ΔCprM/M-, ΔprM/M-, or ΔE-) expressed bright and long lasting GFP. The GFP expression intensity in living cells correlated well with the level of RNA replication. Various mutations in the 5'noncoding region of DENV-2 previously shown to be important genetic determinants for virus replication and mouse virulence were incorporated into the 5 different replicon constructs. Characterizations of 29 mutants demonstrated that these replicons can provide a useful platform for a quick and powerful in vitro system to analyze genetic determinants of DENV replication. These constructs can also be useful for development of vectors expressing foreign genes for various researches.

Investigation of the impact of Tat export pathway enhancement on E. coli culture, protein production and early stage recovery

The twin arginine translocation (Tat) pathway occurs naturally in E. coli and has the distinct ability to translocate folded proteins across the inner membrane of the cell. It has the potential to export commercially useful proteins that cannot be exported by the ubiquitous Sec pathway. To better understand the bioprocess potential of the Tat pathway, this article addresses the fermentation and downstream processing performances of E. coli strains with a wild-type Tat system exporting the over-expressed substrate protein FhuD. These were compared to strains cell-engineered to over-express the Tat pathway, since the native export capacity of the Tat pathway is low. This low capacity makes the pathway susceptible to saturation by over-expressed substrate proteins, and can result in compromised cell integrity. However, there is concern in the literature that over-expression of membrane proteins, like those of the Tat pathway, can impact negatively upon membrane integrity itself. Under controlled fermentation conditions E. coli cells with a wild-type Tat pathway showed poor protein accumulation, reaching a periplasmic maximum of only 0.5 mg L⁻¹ of growth medium. Cells over-expressing the Tat pathway showed a 25% improvement in growth rate, avoided pathway saturation, and showed 40-fold higher periplasmic accumulation of FhuD. Moreover, this was achieved whilst conserving the integrity of cells for downstream processing: experimentation comparing the robustness of cells to increasing levels of shear showed no detrimental effect from pathway over-expression. Further experimentation on spheroplasts generated by the lysozyme/osmotic shock method--a scaleable way to release periplasmic protein--showed similar robustness between strains. A scale-down mimic of continuous disk-stack centrifugation predicted clarifications in excess of 90% for both intact cells and spheroplasts. Cells over-expressing the Tat pathway performed comparably to cells with the wild-type system. Overall, engineering E. coli cells to over-express the Tat pathway allowed for greater periplasmic yields of FhuD at the fermentation scale without compromising downstream processing performance.

The Tat-dependent protein translocation pathway

The twin-arginine translocation (Tat) pathway is found in bacteria, archaea, and plant chloroplasts, where it is dedicated to the transmembrane transport of fully folded proteins. These proteins contain N-terminal signal peptides with a specific Tat-system binding motif that is recognized by the transport machinery. In contrast to other protein transport systems, the Tat system consists of multiple copies of only two or three usually small (∼8–30 kDa) membrane proteins that oligomerize to two large complexes that transiently interact during translocation. Only one of these complexes includes a polytopic membrane protein, TatC. The other complex consists of TatA. Tat systems of plants, proteobacteria, and several other phyla contain a third component, TatB. TatB is evolutionarily and structurally related to TatA and usually forms tight complexes with TatC. Minimal two-component Tat systems lacking TatB are found in many bacterial and archaeal phyla. They consist of a ‘bifunctional’ TatA that also covers TatB functionalities, and a TatC. Recent insights into the structure and interactions of the Tat proteins have various important implications.

Sequential closure of the cytoplasm and then the periplasm during cell division in Escherichia coli

To visualize the latter stages of cell division in live Escherichia coli, we have carried out fluorescence recovery after photobleaching (FRAP) on 121 cells expressing cytoplasmic green fluorescent protein and periplasmic mCherry. Our data show conclusively that the cytoplasm is sealed prior to the periplasm during the division event.

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.

Translocation of green fluorescent protein by comparative analysis with multiple signal peptides

Type I and II secretory pathways are used for the translocation of recombinant proteins from the cytoplasm of Escherichia coli. The purpose of this study was to evaluate four signal peptides (HlyA, TorA, GeneIII, and PelB), representing the most common secretion pathways in E. coli, for their ability to target green fluorescent protein (GFP) for membrane translocation. Signal peptide-GFP genetic fusions were designed in accordance with BioFusion standards (BBF RFC 10, BBF RFC 23). The HlyA signal peptide targeted GFP for secretion to the extracellular media via the type I secretory pathway, whereas TAT-dependent signal peptide TorA and Sec-dependent signal peptide GeneIII exported GFP to the periplasm. The PelB signal peptide was inefficient in translocating GFP. The use of biological technical standards simplified the design and construction of functional signal peptide-recombinant protein genetic devices for type I and II secretion in E. coli. The utility of the standardized parts model is further illustrated as constructed biological parts are available for direct application to other studies on recombinant protein translocation.

Using superfolder green fluorescent protein for periplasmic protein localization studies

Studies investigating the subcellular localization of periplasmic proteins have been hampered by problems with the export of green fluorescent protein (GFP). Here we show that a superfolding variant of GFP (sfGFP) is fluorescent following Sec-mediated transport and works best when the cotranslational branch of the pathway is employed.

Exploration of twin-arginine translocation for expression and purification of correctly folded proteins in Escherichia coli

Historically, the general secretory (Sec) pathway of Gram‐negative bacteria has served as the primary route by which heterologous proteins are delivered to the periplasm in numerous expression and engineering applications. Here we have systematically examined the twin‐arginine translocation (Tat) pathway as an alternative, and possibly advantageous, secretion pathway for heterologous proteins. Overall, we found that: (i) export efficiency and periplasmic yield of a model substrate were affected by the composition of the Tat signal peptide, (ii) Tat substrates were correctly processed at their N‐termini upon reaching the periplasm and (iii) proteins fused to maltose‐binding protein (MBP) were reliably exported by the Tat system, but only when correctly folded; aberrantly folded MBP fusions were excluded by the Tat pathway's folding quality control feature. We also observed that Tat export yield was comparable to Sec for relatively small, well‐folded proteins, higher relative to Sec for proteins that required cytoplasmic folding, and lower relative to Sec for larger, soluble fusion proteins. Interestingly, the specific activity of material purified from the periplasm was higher for certain Tat substrates relative to their Sec counterparts, suggesting that Tat expression can give rise to relatively pure and highly active proteins in one step.

High yield secretion of heterologous proteins in Corynebacterium glutamicum using its own Tat-type signal sequence

Efficient protein secretion, the basis of large-scale production of many compounds central to the biotechnology industry, is achieved by signal peptide and propeptide optimization in addition to optimizing host factors affecting heterologous protein production. Here, we fused green fluorescent protein (GFP) to the recently identified Tat-type secretory signal peptide of CgR0949 to demonstrate a high-yield protein secretion system of Corynebacterium glutamicum. The resultant secretion vector facilitated effective secretion of active-form GFP (20 mg l(-1)) into C. glutamicum culture medium. The expression of GFP was enhanced 2.9-fold using the Shine-Dalgarno sequence of triosephosphate isomerase in the secretion vector. Moreover, GFP drastically accumulated in the culture supernatant upon addition of calcium chloride even though Ca(2+) addition did neither enhanced the transcription of gfp nor resulted in the accumulation of cytosolic GFP. Active-form GFP concentration reached 1.8 g l(-1) after 48-h incubation in a jar fermentor. Likewise, α-amylase accumulation in C. glutamicum cultures was also enhanced by Ca(2+) addition, suggesting that Ca(2+) may affect general protein secretion in C. glutamicum.

Protein diffusion in the periplasm of E. coli under osmotic stress

The physical and mechanical properties of the cell envelope of Escherichia coli are poorly understood. We use fluorescence recovery after photobleaching to measure diffusion of periplasmic green fluorescent protein and probe the fluidity of the periplasm as a function of external osmotic conditions. For cells adapted to growth in complete medium at 0.14-1.02 Osm, the mean diffusion coefficient <D(peri)> increases from 3.4 μm² s⁻¹ to 6.6 μm² s⁻¹ and the distribution of D(peri) broadens as growth osmolality increases. This is consistent with a net gain of water by the periplasm, decreasing its biopolymer volume fraction. This supports a model in which the turgor pressure drops primarily across the thin peptidoglycan layer while the cell actively maintains osmotic balance between periplasm and cytoplasm, thus avoiding a substantial pressure differential across the cytoplasmic membrane. After sudden hyperosmotic shock (plasmolysis), the cytoplasm loses water as the periplasm gains water. Accordingly, <D(peri)> increases threefold. The fluorescence recovery after photobleaching is complete and homogeneous in all cases, but in minimal medium, the periplasm is evidently thicker at the cell tips. For the relevant geometries, Brownian dynamics simulations in model cytoplasmic and periplasmic volumes provide analytical formulae for extraction of accurate diffusion coefficients from readily measurable quantities.

Real-time attack on single Escherichia coli cells by the human antimicrobial peptide LL-37

Natural antimicrobial peptides (AMPs) provide prototypes for the design of unconventional antimicrobial agents. Existing bulk assays measure AMP activity but do not provide details of the growth-halting mechanism. We use fluorescence microscopy to directly observe the attack of the human antimicrobial peptide LL-37 on single Escherichia coli cells in real time. Our findings strongly suggest that disruption of the cytoplasmic membrane is not the growth-halting mechanism. At 8 μM, LL-37 binding saturates the outer membrane (OM) within 1 min. Translocation across the OM and access to the periplasmic space (5-25 min later) correlates in time with the halting of growth. Septating cells are attacked more readily than nonseptating cells. The halting of growth may occur because of LL-37 interference with cell wall biogenesis. Only well after growth halts does the peptide permeabilize the cytoplasmic membrane to GFP and the small dye Sytox Green. The assay enables dissection of antimicrobial design criteria into two parts: translocation across the OM and the subsequent halting of growth.

Superfolder GFP is fluorescent in oxidizing environments when targeted via the Sec translocon

The ability to study proteins in live cells using genetically encoded fluorescent proteins (FPs) has revolutionized cell biology (1-3). Researchers have created numerous FP biosensors and optimized FPs for specific organisms and subcellular environments in a rainbow of colors (4,5). However, expressing FPs in oxidizing environments such as the eukaryotic endoplasmic reticulum (ER) or the bacterial periplasm can impair folding, thereby preventing fluorescence (6,7). A substantial fraction of enhanced green fluorescent protein (EGFP) oligomerizes to form non-fluorescent mixed disulfides in the ER (6) and EGFP does not fluoresce in the periplasm when targeted via the SecYEG translocon (7). To overcome these obstacles, we exploited the highly efficient folding capability of superfolder GFP (sfGFP) (8). Here, we report sfGFP does not form disulfide-linked oligomers in the ER and maltose-binding protein (MBP) signal sequence (peri)-sfGFP (9) is brightly fluorescent in the periplasm of Escherichia coli. Thus, sfGFP represents an important research tool for studying resident proteins of oxidizing environments.

Efficient phage display of intracellularly folded proteins mediated by the TAT pathway

Phage display with filamentous phages is widely applied and well developed, yet proteins requiring a cytoplasmic environment for correct folding still defy attempts at functional display. To extend applicability of phage display, we employed the twin-arginine translocation (TAT) pathway to incorporate proteins fused to the C-terminal domain of the geneIII protein into phage particles. We investigated functionality and display level of fluorescent proteins depending on the translocation pathway, which was the TAT, general secretory (SEC) or signal recognition particle (SRP) pathway mediated by the TorA, PelB or DsbA signal sequences, respectively. Importantly, for green fluorescent protein, yellow fluorescent protein and cyan fluorescent protein, only TAT, but not SEC or SRP, translocation led to fluorescence of purified phage particles, although all three proteins could be displayed regardless of the translocation pathway. In contrast, the monomeric red fluorescent protein mCherry was functionally displayed regardless of the translocation pathway. Hence, correct folding and fluorophor formation of mCherry is not limited to the cytosol. Furthermore, we successfully displayed firefly luciferase as well as an 83 kDa argonaute protein, both containing free cysteines. This demonstrates broad applicability of the TAT-mediated phagemid system for the display of proteins requiring cytoplasmic factors for correct folding and should prove useful for the display of proteins requiring incorporation of co-factors or oligomerization to gain function.

Functional analysis of the twin-arginine translocation pathway in Sodalis glossinidius, a bacterial symbiont of the tsetse fly

This study demonstrates a functional twin-arginine (Tat) translocation pathway present in the tsetse fly symbiont Sodalis glossinidius and its potential to export active heterologous proteins to the periplasm. Functionality was demonstrated using green fluorescent protein (GFP) fused to the Tat signal peptide of Escherichia coli trimethylamine N-oxide reductase (TorA).

Production of secretory and extracellular N-linked glycoproteins in Escherichia coli

The Campylobacter jejuni pgl gene cluster encodes a complete N-linked protein glycosylation pathway that can be functionally transferred into Escherichia coli. In this system, we analyzed the interplay between N-linked glycosylation, membrane translocation and folding of acceptor proteins in bacteria. We developed a recombinant N-glycan acceptor peptide tag that permits N-linked glycosylation of diverse recombinant proteins expressed in the periplasm of glycosylation-competent E. coli cells. With this "glycosylation tag," a clear difference was observed in the glycosylation patterns found on periplasmic proteins depending on their mode of inner membrane translocation (i.e., Sec, signal recognition particle [SRP], or twin-arginine translocation [Tat] export), indicating that the mode of protein export can influence N-glycosylation efficiency. We also established that engineered substrate proteins targeted to environments beyond the periplasm, such as the outer membrane, the membrane vesicles, and the extracellular medium, could serve as substrates for N-linked glycosylation. Taken together, our results demonstrate that the C. jejuni N-glycosylation machinery is compatible with distinct secretory mechanisms in E. coli, effectively expanding the N-linked glycome of recombinant E. coli. Moreover, this simple glycosylation tag strategy expands the glycoengineering toolbox and opens the door to bacterial synthesis of a wide array of recombinant glycoprotein conjugates.

The flagellar protein FliL is essential for swimming in Rhodobacter sphaeroides

In this work we characterize the function of the flagellar protein FliL in Rhodobacter sphaeroides. Our results show that FliL is essential for motility in this bacterium and that in its absence flagellar rotation is highly impaired. A green fluorescent protein (GFP)-FliL fusion forms polar and lateral fluorescent foci that show different spatial dynamics. The presence of these foci is dependent on the expression of the flagellar genes controlled by the master regulator FleQ, suggesting that additional components of the flagellar regulon are required for the proper localization of GFP-FliL. Eight independent pseudorevertants were isolated from the fliL mutant strain. In each of these strains a single nucleotide change in motB was identified. The eight mutations affected only three residues located on the periplasmic side of MotB. Swimming of the suppressor mutants was not affected by the presence of the wild-type fliL allele. Pulldown and yeast two-hybrid assays showed that that the periplasmic domain of FliL is able to interact with itself but not with the periplasmic domain of MotB. From these results we propose that FliL could participate in the coupling of MotB with the flagellar rotor in an indirect fashion.

Twin-arginine translocation of methyl parathion hydrolase in Bacillus subtilis

Secretion of recombinant enzymes to extracellular milieu is important for enhanced degradation of toxic pollutants since the substrates are often inadequately taken up by cells. The twin-arginine translocation (Tat) pathway is a secretion mechanism for the transport of folded proteins across the cytoplasmic membrane of bacteria. Notably, two substrate-specific Tat systems have previously been discovered in Bacillus subtilis. The uptake of organophosphates (OPs) is the rate-limiting factor in whole-cell degradation of OPs. In this study, to secret an OP-hydrolyzing enzyme, methyl parathion hydrolase (MPH), into the growth medium, the twin-arginine (RR-) signal peptide of trimethylamine N-oxide reductase (TorA) from Escherichia coli was used to target MPH to the Tat pathway of B. subtilis. Fractionation studies and MPH assays demonstrated that MPH was secreted into the culture supernatant where it was fully active. Export was fully blocked in a tat mutant, indicating that the observed export in wild-type cells was mediated exclusively by the Tat pathway. The amount of MPH present in the culture medium was estimated to be 6.1 mg/L. N-terminal sequencing of the purified MPH demonstrated that the TorA signal peptide had been processed correctly. The secretion of MPH neither inhibited cell growth nor affected cell viability. The recombinant strain showed the accelerated degradation for OPs and the culture supernatant effectively degraded OPs on vegetables. The recombinant strain may be ideal for large-scale production of MPH at low costs because of simplification of the protein purification step. The Tat pathway of B. subtilis was successfully utilized for extracellular secretion of MPH. This is the first demonstration of Tat-dependent export of an active heterologous protein in B. subtilis using an E. coli Tat signal peptide. This study highlights the potential of the B. subtilis Tat pathway for heterologous protein secretion.

The Tat Protein Export Pathway

Proteins that reside partially or completely outside the bacterial cytoplasm require specialized pathways to facilitate their localization. Globular proteins that function in the periplasm must be translocated across the hydrophobic barrier of the inner membrane. While the Sec pathway transports proteins in a predominantly unfolded conformation, the Tat pathway exports folded protein substrates. Protein transport by the Tat machinery is powered solely by the transmembrane proton gradient, and there is no requirement for nucleotide triphosphate hydrolysis. Proteins are targeted to the Tat machinery by N-terminal signal peptides that contain a consensus twin arginine motif. In Escherichia coli and Salmonella there are approximately thirty proteins with twin arginine signal peptides that are transported by the Tat pathway. The majority of these bind complex redox cofactors such as iron sulfur clusters or the molybdopterin cofactor. Here we describe what is known about Tat substrates in E. coli and Salmonella, the function and mechanism of Tat protein export, and how the cofactor insertion step is coordinated to ensure that only correctly assembled substrates are targeted to the Tat machinery.

Analysis of Tat targeting function and twin-arginine signal peptide activity in Escherichia coli

The Tat system is a protein export system dedicated to the transport of folded proteins across the prokaryotic cytoplasmic membrane and the thylakoid membrane of plant chloroplasts. Proteins are targeted for export by the Tat system via N-terminal signal peptides harbouring an S-R-R-x-F-L-K 'twin-arginine' motif. In this chapter qualitative and quantitative assays for native Tat substrates in the model organism Escherichia coli are described. Genetic screening methods designed to allow the rapid positive selection of Tat signal peptide activity and the first positive selection for mutations that inactivate the Tat pathway are also presented. Finally isothermal titration calorimetry (ITC) methods for measuring the affinity of twin-arginine signal peptide-chaperone interactions are discussed.

Size dependence of protein diffusion in the cytoplasm of Escherichia coli

Diffusion in the bacterial cytoplasm is regarded as the primary method of intracellular protein movement and must play a major role in controlling the rates of cell processes. A number of recent studies have used green fluorescent protein (GFP) tagging and fluorescence microscopy to probe the movement and distribution of proteins in the bacterial cytoplasm. However, the dynamic behavior of indigenous proteins must be controlled by a complex mixture of specific interactions, combined with the basic physical constraints imposed by the viscosity and macromolecular crowding of the cytoplasm. These factors are difficult to unravel in studies with indigenous proteins. To what extent the addition of a GFP tag might affect the movement of a protein through the cytoplasm has also remained unknown. To resolve these problems, we have carried out a systematic study of the size dependence of protein diffusion coefficients in the Escherichia coli cytoplasm, using engineered GFP multimers (from 2 to 6 covalently linked GFP molecules). Diffusion coefficients were measured using confocal fluorescence recovery after photobleaching (FRAP). At least up to 110 kDa (four linked GFP molecules), the diffusion coefficient varies with size roughly as would be predicted from the Einstein-Stokes equation for a classical (Newtonian) fluid. Thus, protein diffusion coefficients are predictable over this range. GFP tagging of proteins has little impact on the diffusion coefficient over this size range and therefore need not significantly perturb protein movement. Two indigenous E. coli proteins were used to show that their specific interactions within the cell are the main controllers of the diffusion rate.

A comprehensive analysis of filamentous phage display vectors for cytoplasmic proteins: an analysis with different fluorescent proteins

Filamentous phage display has been extensively used to select proteins with binding properties of specific interest. Although many different display platforms using filamentous phage have been described, no comprehensive comparison of their abilities to display similar proteins has been conducted. This is particularly important for the display of cytoplasmic proteins, which are often poorly displayed with standard filamentous phage vectors. In this article, we have analyzed the ability of filamentous phage to display a stable form of green fluorescent protein and modified variants in nine different display vectors, a number of which have been previously proposed as being suitable for cytoplasmic protein display. Correct folding and display were assessed by phagemid particle fluorescence, and with anti-GFP antibodies. The poor correlation between phagemid particle fluorescence and recognition of GFP by antibodies, indicates that proteins may fold correctly without being accessible for display. The best vector used a twin arginine transporter leader to transport the displayed protein to the periplasm, and a coil-coil arrangement to link the displayed protein to g3p. This vector was able to display less robust forms of GFP, including ones with inserted epitopes, as well as fluorescent proteins of the Azami green series. It was also functional in mock selection experiments.

Visualizing interactions along the Escherichia coli twin-arginine translocation pathway using protein fragment complementation

The twin-arginine translocation (Tat) pathway is well known for its ability to export fully folded substrate proteins out of the cytoplasm of Gram-negative and Gram-positive bacteria. Studies of this mechanism in Escherichia coli have identified numerous transient protein-protein interactions that guide export-competent proteins through the Tat pathway. To visualize these interactions, we have adapted bimolecular fluorescence complementation (BiFC) to detect protein-protein interactions along the Tat pathway of living cells. Fragments of the yellow fluorescent protein (YFP) were fused to soluble and transmembrane factors that participate in the translocation process including Tat substrates, Tat-specific proofreading chaperones and the integral membrane proteins TatABC that form the translocase. Fluorescence analysis of these YFP chimeras revealed a wide range of interactions such as the one between the Tat substrate dimethyl sulfoxide reductase (DmsA) and its dedicated proofreading chaperone DmsD. In addition, BiFC analysis illuminated homo- and hetero-oligomeric complexes of the TatA, TatB and TatC integral membrane proteins that were consistent with the current model of translocase assembly. In the case of TatBC assemblies, we provide the first evidence that these complexes are co-localized at the cell poles. Finally, we used this BiFC approach to capture interactions between the putative Tat receptor complex formed by TatBC and the DmsA substrate or its dedicated chaperone DmsD. Our results demonstrate that BiFC is a powerful approach for studying cytoplasmic and inner membrane interactions underlying bacterial secretory pathways.