Behaviour of topological marker proteins targeted to the Tat protein transport pathway

Identification of novel tail-anchored membrane proteins integrated by the bacterial twin-arginine translocase

The twin-arginine protein transport (Tat) system exports folded proteins across the cytoplasmic membranes of prokaryotes and the energy transducing-membranes of plant thylakoids and mitochondria. Proteins are targeted to the Tat machinery by N-terminal signal peptides with a conserved twin-arginine motif, and some substrates are exported as heterodimers where the signal peptide is present on one of the partner proteins. A subset of Tat substrates is found in the membrane. Tat-dependent membrane proteins usually have large globular domains and a single transmembrane helix present at the N- or C-terminus. Five Tat substrates that have C-terminal transmembrane helices have previously been characterized in the model bacterium Escherichia coli. Each of these is an iron-sulfur cluster-containing protein involved in electron transfer from hydrogen or formate. Here we have undertaken a bioinformatic search to identify further tail-anchored Tat substrates encoded in bacterial genomes. Our analysis has revealed additional tail-anchored iron-sulfur proteins associated in modules with either a b-type cytochrome or a quinol oxidase. We also identified further candidate tail-anchored Tat substrates, particularly among members of the actinobacterial phylum, that are not predicted to contain cofactors. Using reporter assays, we show experimentally that six of these have both N-terminal Tat signal peptides and C-terminal transmembrane helices. The newly identified proteins include a carboxypeptidase and a predicted protease, and four sortase substrates for which membrane integration is a prerequisite for covalent attachment to the cell wall.

History of Maturation of Prokaryotic Molybdoenzymes-A Personal View

In prokaryotes, the role of Mo/W enzymes in physiology and bioenergetics is widely recognized. It is worth noting that the most diverse family of Mo/W enzymes is exclusive to prokaryotes, with the probable existence of several of them from the earliest forms of life on Earth. The structural organization of these enzymes, which often include additional redox centers, is as diverse as ever, as is their cellular localization. The most notable observation is the involvement of dedicated chaperones assisting with the assembly and acquisition of the metal centers, including Mo/W-bisPGD, one of the largest organic cofactors in nature. This review seeks to provide a new understanding and a unified model of Mo/W enzyme maturation.

Isolation of full-length IgG antibodies from combinatorial libraries expressed in the cytoplasm of Escherichia coli

Here we describe a facile and robust genetic selection for isolating full-length IgG antibodies from combinatorial libraries expressed in the cytoplasm of redox-engineered Escherichia coli cells. The method is based on the transport of a bifunctional substrate comprised of an antigen fused to chloramphenicol acetyltransferase, which allows positive selection of bacterial cells co-expressing cytoplasmic IgGs called cyclonals that specifically capture the chimeric antigen and sequester the antibiotic resistance marker in the cytoplasm. The utility of this approach is first demonstrated by isolating affinity-matured cyclonal variants that specifically bind their cognate antigen, the leucine zipper domain of a yeast transcriptional activator, with subnanomolar affinities, which represent a ~20-fold improvement over the parental IgG. We then use the genetic assay to discover antigen-specific cyclonals from a naïve human antibody repertoire, leading to the identification of lead IgG candidates with affinity and specificity for an influenza hemagglutinin-derived peptide antigen.

Enhancing acetone production from H2 and CO2 using supplemental electron acceptors in an engineered Moorella thermoacetica

Acetogens grow autotrophically and use hydrogen (H₂) as the energy source to fix carbon dioxide (CO₂). This feature can be applied to gas fermentation, contributing to a circular economy. A challenge is the gain of cellular energy from H₂ oxidation, which is substantially low, especially when acetate formation coupled with ATP production is diverted to other chemicals in engineered strains. Indeed, an engineered strain of the thermophilic acetogen Moorella thermoacetica that produces acetone lost autotrophic growth on H₂ and CO₂. We aimed to recover autotrophic growth and enhance acetone production, in which ATP production was assumed to be a limiting factor, by supplementing with electron acceptors. Among the four selected electron acceptors, thiosulfate and dimethyl sulfoxide (DMSO) enhanced both bacterial growth and acetone titers. DMSO was the most effective and was further analyzed. We showed that DMSO supplementation enhanced intracellular ATP levels, leading to increased acetone production. Although DMSO is an organic compound, it functions as an electron acceptor, not a carbon source. Thus, supplying electron acceptors is a potential strategy to complement the low ATP production caused by metabolic engineering and to improve chemical production from H₂ and CO₂.

Formate hydrogenlyase, formic acid translocation and hydrogen production: dynamic membrane biology during fermentation

Formate hydrogenlyase-1 (FHL-1) is a complex-I-like enzyme that is commonly found in gram-negative bacteria. The enzyme comprises a peripheral arm and a membrane arm but is not involved in quinone reduction. Instead, FHL-1 couples formate oxidation to the reduction of protons to molecular hydrogen (H₂). Escherichia coli produces FHL-1 under fermentative conditions where it serves to detoxify formic acid in the environment. The membrane biology and bioenergetics surrounding E. coli FHL-1 have long held fascination. Here, we review recent work on understanding the molecular basis of formic acid efflux and influx. We also consider the structure and function of E. coli FHL-1, its relationship with formate transport, and pay particular attention to the molecular interface between the peripheral arm and the membrane arm. Finally, we highlight the interesting phenotype of genetic mutation of the ND1 Loop, which is located at that interface.

A Complex of LaoA and LaoB Acts as a Tat-Dependent Dehydrogenase for Long-Chain Alcohols in Pseudomonas aeruginosa

The opportunistic pathogen Pseudomonas aeruginosa can utilize unusual carbon sources, like sodium dodecyl sulfate (SDS) and alkanes. Whereas the initiating enzymatic steps of the corresponding degradation pathways have been characterized in detail, the oxidation of the emerging long-chain alcohols has received little attention. Recently, the genes for the Lao (long-chain-alcohol/aldehyde oxidation) system were discovered to be involved in the oxidation of long-chain alcohols derived from SDS and alkane degradation. In the Lao system, LaoA is predicted to be an alcohol dehydrogenase/oxidase; however, according to genetic studies, efficient long-chain-alcohol oxidation additionally required the Tat-dependent protein LaoB. In the present study, the Lao system was further characterized. In vivo analysis revealed that the Lao system complements the substrate spectrum of the well-described Exa system, which is required for growth with ethanol and other short-chain alcohols. Mutational analysis revealed that the Tat site of LaoB was required for long-chain-alcohol oxidation activity, strongly suggesting a periplasmic localization of the complex. Purified LaoA was fully active only when copurified with LaoB. Interestingly, in vitro activity of the purified LaoAB complex also depended on the presence of the Tat site. The copurified LaoAB complex contained a flavin cofactor and preferentially oxidized a range of saturated, unbranched primary alcohols. Furthermore, the LaoAB complex could reduce cytochrome c₅₅₀-type redox carriers like ExaB, a subunit of the Exa alcohol dehydrogenase system. LaoAB complex activity was stimulated by rhamnolipids in vitro. In summary, LaoAB constitutes an unprecedented protein complex with specific properties apparently required for oxidizing long-chain alcohols. IMPORTANCE Pseudomonas aeruginosa is a major threat to public health. Its ability to thrive in clinical settings, water distribution systems, or even jet fuel tanks is linked to detoxification and degradation of diverse hydrophobic substrates that are metabolized via alcohol intermediates. Our study illustrates a novel flavoprotein long-chain-alcohol dehydrogenase consisting of a facultative two-subunit complex, which is unique among related enzymes, while the homologs of the corresponding genes are found in numerous bacterial genomes. Understanding the catalytic and compartmentalization processes involved is of great interest for biotechnological and hygiene research, as it may be a potential starting point for rationally designing novel antibacterial substances with high specificity against this opportunistic pathogen.

Mechanisms of Energy Transduction by Charge Translocating Membrane Proteins

Life relies on the constant exchange of different forms of energy, i.e., on energy transduction. Therefore, organisms have evolved in a way to be able to harvest the energy made available by external sources (such as light or chemical compounds) and convert these into biological useable energy forms, such as the transmembrane difference of electrochemical potential (Δμ̃). Membrane proteins contribute to the establishment of Δμ̃ by coupling exergonic catalytic reactions to the translocation of charges (electrons/ions) across the membrane. Irrespectively of the energy source and consequent type of reaction, all charge-translocating proteins follow two molecular coupling mechanisms: direct- or indirect-coupling, depending on whether the translocated charge is involved in the driving reaction. In this review, we explore these two coupling mechanisms by thoroughly examining the different types of charge-translocating membrane proteins. For each protein, we analyze the respective reaction thermodynamics, electron transfer/catalytic processes, charge-translocating pathways, and ion/substrate stoichiometries.

Identification of a Formate-Dependent Uric Acid Degradation Pathway in Escherichia coli

Purine is a nitrogen-containing compound that is abundant in nature. In organisms that utilize purine as a nitrogen source, purine is converted to uric acid, which is then converted to allantoin. Allantoin is then converted to ammonia. In Escherichia coli, neither urate-degrading activity nor a gene encoding an enzyme homologous to the known urate-degrading enzymes had previously been found. Here, we demonstrate urate-degrading activity in E. coli We first identified aegA as an E. coli gene involved in oxidative stress tolerance. An examination of gene expression revealed that both aegA and its paralog ygfT are expressed under both microaerobic and anaerobic conditions. The ygfT gene is localized within a chromosomal gene cluster presumably involved in purine catabolism. Accordingly, the expression of ygfT increased in the presence of exogenous uric acid, suggesting that ygfT is involved in urate degradation. Examination of the change of uric acid levels in the growth medium with time revealed urate-degrading activity under microaerobic and anaerobic conditions in the wild-type strain but not in the aegA ygfT double-deletion mutant. Furthermore, AegA- and YgfT-dependent urate-degrading activity was detected only in the presence of formate and formate dehydrogenase H. Collectively, these observations indicate the presence of urate-degrading activity in E. coli that is operational under microaerobic and anaerobic conditions. The activity requires formate, formate dehydrogenase H, and either aegA or ygfT We also identified other putative genes which are involved not only in formate-dependent but also in formate-independent urate degradation and may function in the regulation or cofactor synthesis in purine catabolism.IMPORTANCE The metabolic pathway of uric acid degradation to date has been elucidated only in aerobic environments and is not understood in anaerobic and microaerobic environments. In the current study, we showed that Escherichia coli, a facultative anaerobic organism, uses uric acid as a sole source of nitrogen under anaerobic and microaerobic conditions. We also showed that formate, formate dehydrogenase H, and either AegA or YgfT are involved in uric acid degradation. We propose that formate may act as an electron donor for a uric acid-degrading enzyme in this bacterium.

The function, biogenesis and regulation of the electron transport chains in Campylobacter jejuni: New insights into the bioenergetics of a major food-borne pathogen

Campylobacter jejuni is a zoonotic Epsilonproteobacterium that grows in the gastrointestinal tract of birds and mammals, and is the most frequent cause of food-borne bacterial gastroenteritis worldwide. As an oxygen-sensitive microaerophile, C. jejuni has to survive high environmental oxygen tensions, adapt to oxygen limitation in the host intestine and resist host oxidative attack. Despite its small genome size, C. jejuni is a versatile and metabolically active pathogen, with a complex and highly branched set of respiratory chains allowing the use of a wide range of electron donors and alternative electron acceptors in addition to oxygen, including fumarate, nitrate, nitrite, tetrathionate and N- or S-oxides. Several novel enzymes participate in these electron transport chains, including a tungsten containing formate dehydrogenase, a Complex I that uses flavodoxin and not NADH, a periplasmic facing fumarate reductase and a cytochrome c tetrathionate reductase. This review presents an updated description of the composition and bioenergetics of these various respiratory chains as they are currently understood, including recent work that gives new insights into energy conservation during electron transport to various alternative electron acceptors. The regulation of synthesis and assembly of the electron transport chains is also discussed. A deeper appreciation of the unique features of the respiratory systems of C. jejuni may be helpful in informing strategies to control this important pathogen.

LaoABCR, a Novel System for Oxidation of Long-Chain Alcohols Derived from SDS and Alkane Degradation in Pseudomonas aeruginosa

The opportunistic pathogen Pseudomonas aeruginosa strain PAO1 is able to use a variety of organic pollutants as growth substrates, including the anionic detergent sodium dodecyl sulfate (SDS) and long-chain alkanes. While the enzymes initiating SDS and alkane degradation are well known, the subsequent enzymatic steps for degradation of the derived primary long-chain alcohols have not yet been identified. By evaluating genes specifically induced during growth with SDS, a gene cluster encoding a putative alcohol dehydrogenase (PA0364/LaoA), a probable inner membrane protein (PA0365/LaoB), and a presumable aldehyde dehydrogenase (PA0366/LaoC) was identified and designated the Lao (long-chain-alcohol/aldehyde-oxidation) system. Growth experiments with deletion mutants with SDS, 1-dodecanol, and alkanes revealed that LaoA and LaoB are involved in the degradation of primary long-chain alcohols. Moreover, detection of 1-dodecanol oxidation in cell extracts by activity staining revealed an interdependency of LaoA and LaoB for efficient 1-dodecanol oxidation. An in silico analysis yielded no well-characterized homologue proteins for LaoA and LaoB. Furthermore, a gene adjacent to the lao gene cluster encodes a putative transcriptional regulator (PA0367/LaoR). A laoR deletion mutant exhibited constitutive expression of LaoA and LaoB, indicating that LaoR is a repressor for the expression of laoABC Taken together, these results showed that the proteins LaoA and LaoB constitute a novel oxidation system for long-chain alcohols derived from pollutants.IMPORTANCE The versatile and highly adaptive bacterium Pseudomonas aeruginosa is able to colonize a variety of habitats, including anthropogenic environments, where it is often challenged with toxic compounds. Its ability to degrade such compounds and to use them as growth substrates can significantly enhance spreading of this opportunistic pathogen in hygienic settings, such as clinics or water distribution systems. Thus, knowledge about the metabolism of P. aeruginosa can contribute to novel approaches for preventing its growth and reducing nosocomial infections. As the Lao system is important for the degradation of two different classes of pollutants, the identification of these novel enzymes can be a useful contribution for developing effective antibacterial strategies.

Deep-Sea Bacterium Shewanella piezotolerans WP3 Has Two Dimethyl Sulfoxide Reductases in Distinct Subcellular Locations

Dimethyl sulfoxide (DMSO) acts as a substantial sink for dimethyl sulfide (DMS) in deep waters and is therefore considered a potential electron acceptor supporting abyssal ecosystems. Shewanella piezotolerans WP3 was isolated from west Pacific deep-sea sediments, and two functional DMSO respiratory subsystems are essential for maximum growth of WP3 under in situ conditions (4°C/20 MPa). However, the relationship between these two subsystems and the electron transport pathway underlying DMSO reduction by WP3 remain unknown. In this study, both DMSO reductases (type I and type VI) in WP3 were found to be functionally independent despite their close evolutionary relationship. Moreover, immunogold labeling of DMSO reductase subunits revealed that the type I DMSO reductase was localized on the outer leaflet of the outer membrane, whereas the type VI DMSO reductase was located within the periplasmic space. CymA, a cytoplasmic membrane-bound tetraheme c-type cytochrome, served as a preferential electron transport protein for the type I and type VI DMSO reductases, in which type VI accepted electrons from CymA in a DmsE- and DmsF-independent manner. Based on these results, we proposed a core electron transport model of DMSO reduction in the deep-sea bacterium S. piezotolerans WP3. These results collectively suggest that the possession of two sets of DMSO reductases with distinct subcellular localizations may be an adaptive strategy for WP3 to achieve maximum DMSO utilization in deep-sea environments.IMPORTANCE As the dominant methylated sulfur compound in deep oceanic water, dimethyl sulfoxide (DMSO) has been suggested to play an important role in the marine biogeochemical cycle of the volatile anti-greenhouse gas dimethyl sulfide (DMS). Two sets of DMSO respiratory systems in the deep-sea bacterium Shewanella piezotolerans WP3 have previously been identified to mediate DMSO reduction under in situ conditions (4°C/20 MPa). Here, we report that the two DMSO reductases (type I and type VI) in WP3 have distinct subcellular localizations, in which type I DMSO reductase is localized to the exterior surface of the outer membrane and type VI DMSO reductase resides in the periplasmic space. A core electron transport model of DMSO reduction in WP3 was constructed based on genetic and physiological data. These results will contribute to a comprehensive understanding of the adaptation mechanisms of anaerobic respiratory systems in benthic microorganisms.

A unifying mechanism for the biogenesis of membrane proteins co-operatively integrated by the Sec and Tat pathways

The majority of multi-spanning membrane proteins are co-translationally inserted into the bilayer by the Sec pathway. An important subset of membrane proteins have globular, cofactor-containing extracytoplasmic domains requiring the dual action of the co-translational Sec and post-translational Tat pathways for integration. Here, we identify further unexplored families of membrane proteins that are dual Sec-Tat-targeted. We establish that a predicted heme-molybdenum cofactor-containing protein, and a complex polyferredoxin, each require the concerted action of two translocases for their assembly. We determine that the mechanism of handover from Sec to Tat pathway requires the relatively low hydrophobicity of the Tat-dependent transmembrane domain. This, coupled with the presence of C-terminal positive charges, results in abortive insertion of this transmembrane domain by the Sec pathway and its subsequent release at the cytoplasmic side of the membrane. Together, our data points to a simple unifying mechanism governing the assembly of dual targeted membrane proteins.

The Two Sets of DMSO Respiratory Systems of Shewanella piezotolerans WP3 Are Involved in Deep Sea Environmental Adaptation

Dimethyl sulfoxide (DMSO) is an abundant methylated sulfur compound in deep sea ecosystems. However, the mechanism underlying DMSO-induced reduction in benthic microorganisms is unknown. Shewanella piezotolerans WP3, which was isolated from a west Pacific deep sea sediment, can utilize DMSO as the terminal electron acceptor. In this study, two putative dms gene clusters [type I (dmsEFA1B1G1H1) and type II (dmsA2B2G2H2)] were identified in the WP3 genome. Genetic and physiological analyses demonstrated that both dms gene clusters were functional and the transcription of both gene clusters was affected by changes in pressure and temperature. Notably, the type I system is essential for WP3 to thrive under in situ conditions (4°C/20 MPa), whereas the type II system is more important under high pressure or low temperature conditions (20°C/20 MPa, 4°C/0.1 MPa). Additionally, DMSO-dependent growth conferred by the presence of both dms gene clusters was higher than growth conferred by either of the dms gene clusters alone. These data collectively suggest that the possession of two sets of DMSO respiratory systems is an adaptive strategy for WP3 survival in deep sea environments. We propose, for the first time, that deep sea microorganisms might be involved in global DMSO/DMS cycling.

Simultaneous selection and counter-selection for the directed evolution of proteases in E. coli using a cytoplasmic anchoring strategy

With the goal of generating new enzymes that can cleave custom sequences, this article describes a selection strategy for evolving proteases with desirable characteristics. Positive selection and counter-selection are combined to select for and against specified cleavage sequences simultaneously. Cleavage of the positive selection sequence permits E. coli growth, and cleavage of the counter-selection sequence slows growth. Growth occurs when cleavage of the positive selection sequence releases β-lactamase into the periplasm where it can confer antibiotic resistance. The counter-selection traps β-lactamase in the cytoplasm, preventing antibiotic resistance and growth. Thus, proteases with a preference for the positive selection sequence relative to the counter-selection sequence grow more rapidly. This system was used to select a tobacco etch virus (TEV) protease mutant with new substrate compatibility. Biotechnol. Bioeng. 2016;113: 1187-1193. © 2015 Wiley Periodicals, Inc.

The Aerobic and Anaerobic Respiratory Chain of Escherichia coli and Salmonella enterica: Enzymes and Energetics

Escherichia coli contains a versatile respiratory chain that oxidizes 10 different electron donor substrates and transfers the electrons to terminal reductases or oxidases for the reduction of six different electron acceptors. Salmonella is able to use two more electron acceptors. The variation is further increased by the presence of isoenzymes for some substrates. A large number of respiratory pathways can be established by combining different electron donors and acceptors. The respiratory dehydrogenases use quinones as the electron acceptors that are oxidized by the terminal reductase and oxidases. The enzymes vary largely with respect to their composition, architecture, membrane topology, and the mode of energy conservation. Most of the energy-conserving dehydrogenases (FdnGHI, HyaABC, HybCOAB, and others) and the terminal reductases (CydAB, NarGHI, and others) form a proton potential (Δp) by a redox-loop mechanism. Two enzymes (NuoA-N and CyoABCD) couple the redox energy to proton translocation by proton pumping. A large number of dehydrogenases and terminal reductases do not conserve the redox energy in a proton potential. For most of the respiratory enzymes, the mechanism of proton potential generation is known or can be predicted. The H+/2e- ratios for most respiratory chains are in the range from 2 to 6 H+/2e-. The energetics of the individual redox reactions and the respiratory chains is described and related to the H+/2e- ratios.

Biogenesis of Escherichia coli DMSO Reductase: A Network of Participants for Protein Folding and Complex Enzyme Maturation

Protein folding and structure have been of interest since the dawn of protein chemistry. Following translation from the ribosome, a protein must go through various steps to become a functional member of the cellular society. Every protein has a unique function in the cell and is classified on this basis. Proteins that are involved in cellular respiration are the bioenergetic workhorses of the cell. Bacteria are resilient organisms that can survive in diverse environments by fine tuning these workhorses. One class of proteins that allow survival under anoxic conditions are anaerobic respiratory oxidoreductases, which utilize many different compounds other than oxygen as its final electron acceptor. Dimethyl sulfoxide (DMSO) is one such compound. Respiration using DMSO as a final electron acceptor is performed by DMSO reductase, converting it to dimethyl sulfide in the process. Microbial respiration using DMSO is reviewed in detail by McCrindle et al. (Adv Microb Physiol 50:147-198, 2005). In this chapter, we discuss the biogenesis of DMSO reductase as an example of the participant network for complex iron-sulfur molybdoenzyme maturation pathways.

The Aerobic and Anaerobic Respiratory Chain of Escherichia coli and Salmonella enterica: Enzymes and Energetics

Escherichia coli contains a versatile respiratory chain which oxidizes ten different electron donor substrates and transfers the electrons to terminal reductases or oxidases for the reduction of six different electron acceptors. Salmonella is able to use even two more electron acceptors. The variation is further increased by the presence of isoenzymes for some substrates. Various respiratory pathways can be established by combining the oxidation of different electron donors and acceptors which are linked by respiratory quinones. The enzymes vary largely with respect to architecture, membrane topology, and mode of energy conservation. Most of the energy-conserving dehydrogenases (e.g., FdnGHI, HyaABC, and HybCOAB) and of the terminal reductases (CydAB, NarGHI, and others) form a proton potential (Δp) by a redox loop mechanism. Only two enzymes (NuoA-N and CyoABCD) couple the redox energy to proton translocation by proton pumping. A large number of dehydrogenases (e.g., Ndh, SdhABCD, and GlpD) and of terminal reductases (e.g., FrdABCD and DmsABC) do not conserve the redox energy in a proton potential. For most of the respiratory enzymes, the mechanism of proton potential generation is known from structural and biochemical studies or can be predicted from sequence information. The H+/2e- ratios of proton translocation for most respiratory chains are in the range from 2 to 6 H+/2e-. The energetics of the individual redox reactions and of the respiratory chains is described. In contrast to the knowledge on enzyme function are physiological aspects of respiration such as organization and coordination of the electron transport and the use of alternative respiratory enzymes, not well characterized.

S- and N-Oxide Reductases

Escherichia coli is a versatile facultative anaerobe that can respire on a number of terminal electron acceptors, including oxygen, fumarate, nitrate, and S- and N-oxides. Anaerobic respiration using S- and N-oxides is accomplished by enzymatic reduction of these substrates by dimethyl sulfoxide reductase (DmsABC) and trimethylamine N-oxide reductase (TorCA). Both DmsABC and TorCA are membrane-associated redox enzymes that couple the oxidation of menaquinol to the reduction of S- and N-oxides in the periplasm. DmsABC is membrane bound and is composed of a membrane-extrinsic dimer with a 90.4-kDa catalytic subunit (DmsA) and a 23.1-kDa electron transfer subunit (DmsB). These subunits face the periplasm and are held to the membrane by a 30.8-kDa membrane anchor subunit (DmsC). The enzyme provides the scaffold for an electron transfer relay composed of a quinol binding site, five [4Fe-4S] clusters, and a molybdo-bis(molybdopterin guanine dinucleotide) (present nomenclature: Mo-bis-pyranopterin) (Mo-bisMGD) cofactor. TorCA is composed of a soluble periplasmic subunit (TorA, 92.5 kDa) containing a Mo-bis-MGD. TorA is coupled to the quinone pool via a pentaheme c subunit (TorC, 40.4 kDa) in the membrane. Both DmsABC and TorCA require system-specific chaperones (DmsD or TorD) for assembly, cofactor insertion, and/or targeting to the Tat translocon. In this chapter, we discuss the complex regulation of the dmsABC and torCAD operons, the poorly understood paralogues, and what is known about the assembly and translocation to the periplasmic space by the Tat translocon.

Role of the twin arginine protein transport pathway in the assembly of the Streptomyces coelicolor cytochrome bc1 complex

The cytochrome bc1-cytochrome aa3 complexes together comprise one of the major branches of the bacterial aerobic respiratory chain. In actinobacteria, the cytochrome bc1 complex shows a number of unusual features in comparison to other cytochrome bc1 complexes. In particular, the Rieske iron-sulfur protein component of this complex, QcrA, is a polytopic rather than a monotopic membrane protein. Bacterial Rieske proteins are usually integrated into the membrane in a folded conformation by the twin arginine protein transport (Tat) pathway. In this study, we show that the activity of the Streptomyces coelicolor M145 cytochrome bc1 complex is dependent upon an active Tat pathway. However, the polytopic Rieske protein is still integrated into the membrane in a ΔtatC mutant strain, indicating that a second protein translocation machinery also participates in its assembly. Difference spectroscopy indicated that the cytochrome c component of the complex was correctly assembled in the absence of the Tat machinery. We show that the intact cytochrome bc1 complex can be isolated from S. coelicolor M145 membranes by affinity chromatography. Surprisingly, a stable cytochrome bc1 complex containing the Rieske protein can be isolated from membranes even when the Tat system is inactive. These findings strongly suggest that the additional transmembrane segments of the S. coelicolor Rieske protein mediate hydrophobic interactions with one or both of the cytochrome subunits.

A regulatory domain controls the transport activity of a twin-arginine signal peptide

The twin-arginine translocation (Tat) pathway is used by bacteria for the transmembrane transport of folded proteins. Proteins are targeted to the Tat translocase by signal peptides that have common tripartite structures consisting of polar n-regions, hydrophobic h-regions, and polar c-regions. In this work, the signal peptide of [NiFe] hydrogenase-1 from Escherichia coli has been studied. The hydrogenase-1 signal peptide contains an extended n-region that has a conserved primary structure. Genetic and biochemical approaches reveal that the signal peptide n-region is essential for hydrogenase assembly and acts as a regulatory domain controlling transport activity of the signal peptide.

Signal peptide etiquette during assembly of a complex respiratory enzyme

Salmonella enterica serovar Typhimurium is a Gram-negative pathogen capable of respiration with a number of terminal electron acceptors. Tetrathionate reductase is important for the infection process and is encoded by the ttrBCA operon where TtrA and TtrB are metallocofactor-containing proteins targeted to the periplasmic side of the membrane by two different Tat targeting peptides. In this work, the inter-relationship between these two signal peptides has been explored. Molecular genetics and biochemical approaches reveal that the processing of the TtrB Tat signal peptide is dependent on the successful assembly of its partner protein, TtrA. Inactivation of either the TtrA or the TtrB Tat targeting peptides individually was observed to have limited overall effects on assembly of the enzyme or on cellular tetrathionate reductase activity. However, inactivation of both signal peptides simultaneously was found to completely abolish physiological tetrathionate reductase activity. These data suggest both signals are normally active during assembly of the enzyme, and imply a code of conduct exists between the signal peptides where one can compensate for inactivity in the other. Since it appears likely that tetrathionate reductase presents itself for export as a multi-signal complex, these observations also have implications for the mechanism of the bacterial Tat translocase.

Cardiolipin binding in bacterial respiratory complexes: structural and functional implications

The structural and functional integrity of biological membranes is vital to life. The interplay of lipids and membrane proteins is crucial for numerous fundamental processes ranging from respiration, photosynthesis, signal transduction, solute transport to motility. Evidence is accumulating that specific lipids play important roles in membrane proteins, but how specific lipids interact with and enable membrane proteins to achieve their full functionality remains unclear. X-ray structures of membrane proteins have revealed tight and specific binding of lipids. For instance, cardiolipin, an anionic phospholipid, has been found to be associated to a number of eukaryotic and prokaryotic respiratory complexes. Moreover, polar and septal accumulation of cardiolipin in a number of prokaryotes may ensure proper spatial segregation and/or activity of proteins. In this review, we describe current knowledge of the functions associated with cardiolipin binding to respiratory complexes in prokaryotes as a frame to discuss how specific lipid binding may tune their reactivity towards quinone and participate to supercomplex formation of both aerobic and anaerobic respiratory chains. This article is part of a Special Issue entitled: 17th European Bioenergetics Conference (EBEC 2012).

The twin-arginine translocation pathway in α-proteobacteria is functionally preserved irrespective of genomic and regulatory divergence

The twin-arginine translocation (Tat) pathway exports fully folded proteins out of the cytoplasm of Gram-negative and Gram-positive bacteria. Although much progress has been made in unraveling the molecular mechanism and biochemical characterization of the Tat system, little is known concerning its functionality and biological role to confer adaptive skills, symbiosis or pathogenesis in the α-proteobacteria class. A comparative genomic analysis in the α-proteobacteria class confirmed the presence of tatA, tatB, and tatC genes in almost all genomes, but significant variations in gene synteny and rearrangements were found in the order Rickettsiales with respect to the typically described operon organization. Transcription of tat genes was confirmed for Anaplasma marginale str. St. Maries and Brucella abortus 2308, two α-proteobacteria with full and partial intracellular lifestyles, respectively. The tat genes of A. marginale are scattered throughout the genome, in contrast to the more generalized operon organization. Particularly, tatA showed an approximately 20-fold increase in mRNA levels relative to tatB and tatC. We showed Tat functionality in B. abortus 2308 for the first time, and confirmed conservation of functionality in A. marginale. We present the first experimental description of the Tat system in the Anaplasmataceae and Brucellaceae families. In particular, in A. marginale Tat functionality is conserved despite operon splitting as a consequence of genome rearrangements. Further studies will be required to understand how the proper stoichiometry of the Tat protein complex and its biological role are achieved. In addition, the predicted substrates might be the evidence of role of the Tat translocation system in the transition process from a free-living to a parasitic lifestyle in these α-proteobacteria.

Conserved signal peptide recognition systems across the prokaryotic domains

The twin-arginine translocation (Tat) pathway is a protein targeting system found in bacteria, archaea, and chloroplasts. Proteins are directed to the Tat translocase by N-terminal signal peptides containing SRRxFLK “twin-arginine” amino acid motifs. The key feature of the Tat system is its ability to transport fully folded proteins across ionically sealed membranes. For this reason the Tat pathway has evolved for the assembly of extracytoplasmic redox enzymes that must bind cofactors, and so fold, prior to export. It is important that only cofactor-loaded, folded precursors are presented for export, and cellular processes have been unearthed that regulate signal peptide activity. One mechanism, termed “Tat proofreading”, involves specific signal peptide binding proteins or chaperones. The archetypal Tat proofreading chaperones belong to the TorD family, which are dedicatedto the assembly of molybdenum-dependent redox enzymes in bacteria. Here, a gene cluster was identified in the archaeon Archaeoglobus fulgidusthat is predicted to encode a putative molybdenum-dependent tetrathionate reductase. The gene cluster also encodes a TorD family chaperone (AF0160 or TtrD) and in this work TtrD is shown to bind specifically to the Tat signal peptide of the TtrA subunit of the tetrathionate reductase. In addition, the 3D crystal structure of TtrD is presented at 1.35 Å resolution and a nine-residue binding epitope for TtrD is identified within the TtrA signal peptide close to the twin-arginine targeting motif. This work suggests that archaea may employ a chaperone-dependent Tat proofreading system that is similar to that utilized by bacteria.

Thiosulfate reduction in Salmonella enterica is driven by the proton motive force

Thiosulfate respiration in Salmonella enterica serovar Typhimurium is catalyzed by the membrane-bound enzyme thiosulfate reductase. Experiments with quinone biosynthesis mutants show that menaquinol is the sole electron donor to thiosulfate reductase. However, the reduction of thiosulfate by menaquinol is highly endergonic under standard conditions (ΔE°' = -328 mV). Thiosulfate reductase activity was found to depend on the proton motive force (PMF) across the cytoplasmic membrane. A structural model for thiosulfate reductase suggests that the PMF drives endergonic electron flow within the enzyme by a reverse loop mechanism. Thiosulfate reductase was able to catalyze the combined oxidation of sulfide and sulfite to thiosulfate in a reverse of the physiological reaction. In contrast to the forward reaction the exergonic thiosulfate-forming reaction was PMF independent. Electron transfer from formate to thiosulfate in whole cells occurs predominantly by intraspecies hydrogen transfer.

A sulfurtransferase is essential for activity of formate dehydrogenases in Escherichia coli

l-Cysteine desulfurases provide sulfur to several metabolic pathways in the form of persulfides on specific cysteine residues of an acceptor protein for the eventual incorporation of sulfur into an end product. IscS is one of the three Escherichia coli l-cysteine desulfurases. It interacts with FdhD, a protein essential for the activity of formate dehydrogenases (FDHs), which are iron/molybdenum/selenium-containing enzymes. Here, we address the role played by this interaction in the activity of FDH-H (FdhF) in E. coli. The interaction of IscS with FdhD results in a sulfur transfer between IscS and FdhD in the form of persulfides. Substitution of the strictly conserved residue Cys-121 of FdhD impairs both sulfur transfer from IscS to FdhD and FdhF activity. Furthermore, inactive FdhF produced in the absence of FdhD contains both metal centers, albeit the molybdenum cofactor is at a reduced level. Finally, FdhF activity is sulfur-dependent, as it shows reversible sensitivity to cyanide treatment. Conclusively, FdhD is a sulfurtransferase between IscS and FdhF and is thereby essential to yield FDH activity.

Correct assembly of iron-sulfur cluster FS0 into Escherichia coli dimethyl sulfoxide reductase (DmsABC) is a prerequisite for molybdenum cofactor insertion

The FS0 [4Fe-4S] cluster of the catalytic subunit (DmsA) of Escherichia coli dimethyl sulfoxide reductase (DmsABC) plays a key role in the electron transfer relay. We have now established an additional role for the cluster in directing molybdenum cofactor assembly during enzyme maturation. EPR spectroscopy indicates that FS0 has a high spin ground state (S = 3/2) in its reduced form, resulting in an EPR spectrum with a peak at g ∼ 5.0. The cluster is predicted to be in close proximity to the molybdo-bis(pyranopterin guanine dinucleotide) (Mo-bisPGD) cofactor, which provides the site of dimethyl sulfoxide reduction. Comparison with nitrate reductase A (NarGHI) indicates that a sequence of residues ((18)CTVNC(22)) plays a role in both FS0 and Mo-bisPGD coordination. A DmsA(ΔN21) mutant prevented Mo-bisPGD binding and resulted in a degenerate [3Fe-4S] cluster form of FS0 being assembled. DmsA belongs to the Type II subclass of Mo-bisPGD-containing catalytic subunits that is distinguished from the Type I subclass by having three rather than two residues between the first two Cys residues coordinating FS0 and a conserved Arg residue rather than a Lys residue following the fourth cluster coordinating Cys. We introduced a Type I Cys group into DmsA in two stages. We changed its sequence from (18)C(A)TVNC(B)GSRC(C)P(27) to (18)C(A)TYC(B)GVGC(C)G(26) (similar to that of formate dehydrogenase (FdnG)) and demonstrated that this eliminated both Mo-bisPGD binding and EPR-detectable FS0. We then combined this change with a DmsA(R61K) mutation and demonstrated that this additional change partially rescued Mo-bisPGD insertion.

A putative twin-arginine translocation system in the phytopathogenic bacterium Xylella fastidiosa

The twin-arginine translocation (Tat) pathway of the xylem-limited phytopathogenic bacterium Xylella fastidiosa strain 9a5c, responsible for citrus variegated chlorosis, was explored. The presence of tatA, tatB, and tatC in the X. fastidiosa genome together with a list of proteins harboring 2 consecutive arginines in their signal peptides suggested the presence of a Tat pathway. The functional Tat dependence of X. fastidiosa OpgD was examined. Native or mutated signal peptides were fused to the β-lactamase. Expression of fusion with intact signal peptides mediated high resistance to ampicillin in Escherichia coli tat+ but not in the E. coli tat null mutant. The replacement of the 2 arginines by 2 lysines prevented the export of β-lactamase in E. coli tat+, demonstrating that X. fastidiosa OpgD carries a signal peptide capable of engaging the E. coli Tat machinery. RT-PCR analysis revealed that the tat genes are transcribed as a single operon. tatA, tatB, and tatC genes were cloned. Complementation assays in E. coli devoid of all Tat or TatC components were unsuccessful, whereas X. fastidiosa Tat components led to a functional Tat translocase in E. coli TatB-deficient strain. Additional experiments implicated that X. fastidiosa TatB component could form a functional heterologous complex with the E. coli TatC component.

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.

Biogenesis of membrane bound respiratory complexes in Escherichia coli

Escherichia coli is one of the preferred bacteria for studies on the energetics and regulation of respiration. Respiratory chains consist of primary dehydrogenases and terminal reductases or oxidases linked by quinones. In order to assemble this complex arrangement of protein complexes, synthesis of the subunits occurs in the cytoplasm followed by assembly in the cytoplasm and/or membrane, the incorporation of metal or organic cofactors and the anchoring of the complex to the membrane. In the case of exported metalloproteins, synthesis, assembly and incorporation of metal cofactors must be completed before translocation across the cytoplasmic membrane. Coordination data on these processes is, however, scarce. In this review, we discuss the various processes that respiratory proteins must undergo for correct assembly and functional coupling to the electron transport chain in E. coli. Targeting to and translocation across the membrane together with cofactor synthesis and insertion are discussed in a general manner followed by a review of the coordinated biogenesis of individual respiratory enzyme complexes. Lastly, we address the supramolecular organization of respiratory enzymes into supercomplexes and their localization to specialized domains in the membrane.

Structural analysis of a monomeric form of the twin-arginine leader peptide binding chaperone Escherichia coli DmsD

The redox enzyme maturation proteins play an essential role in the proofreading and membrane targeting of protein substrates to the twin-arginine translocase. Functionally, the most thoroughly characterized redox enzyme maturation protein to date is Escherichia coli DmsD (EcDmsD). Herein, we present the X-ray crystal structure of the monomeric form of the EcDmsD refined to 2.0 A resolution, with clear electron density present for each of its 204 amino acid residues. The structural data presented here complement the biochemical data previously generated regarding the function of these twin-arginine translocase leader peptide binding chaperone proteins. Docking and molecular dynamics simulation experiments were used to provide a proposed model for how this chaperone is able to recognize the leader peptide of its substrate DmsA. The interactions observed in the model are in agreement with previous biochemical data and suggest intimate interactions between the conserved twin-arginine motif of the leader peptide of E. coli DmsA and the most conserved regions on the surface of EcDmsD.

The twin-arginine transport system: moving folded proteins across membranes

The Tat (twin-arginine transport) pathway is a protein-targeting system dedicated to the transmembrane translocation of fully folded proteins. This system is highly prevalent in the cytoplasmic membranes of bacteria and archaea, and is also found in the thylakoid membranes of plant chloroplasts and possibly also in the inner membrane of plant mitochondria. Proteins are targeted to a membrane-embedded Tat translocase by specialized N-terminal twin-arginine signal peptides bearing an SRRXFLK amino acid motif. The genes encoding components of the Tat translocase were discovered approx. 10 years ago, and, since then, research in this area has expanded on a global scale. In this review, the key discoveries in this field are summarized, and recent studies of bacterial twin-arginine signal-peptide-binding proteins are discussed.

Beta-lactamase can function as a reporter of bacterial protein export during Mycobacterium tuberculosis infection of host cells

Mycobacterium tuberculosis is an intracellular pathogen that is able to avoid destruction by host immune defences. Exported proteins of M. tuberculosis, which include proteins localized to the bacterial surface or secreted into the extracellular environment, are ideally situated to interact with host factors. As a result, these proteins are attractive candidates for virulence factors, drug targets and vaccine components. Here we describe a beta-lactamase reporter system capable of identifying exported proteins of M. tuberculosis during growth in host cells. Because beta-lactams target bacterial cell-wall synthesis, beta-lactamases must be exported beyond the cytoplasm to protect against these drugs. When used in protein fusions, beta-lactamase can report on the subcellular location of another protein as measured by protection from beta-lactam antibiotics. Here we demonstrate that a truncated TEM-1 beta-lactamase lacking a signal sequence for export ('BlaTEM-1) can be used in this manner directly in a mutant strain of M. tuberculosis lacking the major beta-lactamase, BlaC. The 'BlaTEM-1 reporter conferred beta-lactam resistance when fused to both Sec and Tat export signal sequences. We further demonstrate that beta-lactamase fusion proteins report on protein export while M. tuberculosis is growing in THP-1 macrophage-like cells. This genetic system should facilitate the study of proteins exclusively exported in the host environment by intracellular M. tuberculosis.

Structural diversity in twin-arginine signal peptide-binding proteins

The twin-arginine transport (Tat) system is dedicated to the translocation of folded proteins across the bacterial cytoplasmic membrane. Proteins are targeted to the Tat system by signal peptides containing a twin-arginine motif. In Escherichia coli, many Tat substrates bind redox-active cofactors in the cytoplasm before transport. Coordination of cofactor insertion with protein export involves a "Tat proofreading" process in which chaperones bind twin-arginine signal peptides, thus preventing premature export. The initial Tat signal-binding proteins described belonged to the TorD family, which are required for assembly of N- and S-oxide reductases. Here, we report that E. coli NapD is a Tat signal peptide-binding chaperone involved in biosynthesis of the Tat-dependent nitrate reductase NapA. NapD binds tightly and specifically to the NapA twin-arginine signal peptide and suppresses signal peptide translocation activity such that transport via the Tat pathway is retarded. High-resolution, heteronuclear, multidimensional NMR spectroscopy reveals the 3D solution structure of NapD. The chaperone adopts a ferredoxin-type fold, which is completely distinct from the TorD family. Thus, NapD represents a new family of twin-arginine signal-peptide-binding proteins.

Extracellular respiration

Although it has long been known that microbes can generate energy using diverse strategies, only recently has it become clear that a growing number involve electron transfer to or from extracellular substrates. The best-known example of what we will term 'extracellular respiration' is electron transfer between microbes and minerals, such as iron and manganese (hydr)oxides. This makes sense, given that these minerals are sparingly soluble. What is perhaps surprising, however, is that a number of substrates that might typically be classified as 'soluble' are also respired at the cell surface. There are several reasons why this might be the case: the substrate, in its ecological context, might be associated with a solid surface and thus effectively insoluble; the substrate, while soluble, might simply be too large to transport inside the cell; or the substrate, while benign in one redox state, might become toxic after it is metabolized. In this review, we discuss various examples of extracellular respiration, paying particular attention to what is known about the molecular mechanisms underlying these processes. As will become clear, much remains to be learned about the biochemistry, cell biology and regulation of extracellular respiration, making it a rich field of study for molecular microbiologists.

Protein Secretion and Membrane Insertion Systems in Gram-Negative Bacteria

In contrast to other organisms, gram-negative bacteria have evolved numerous systems for protein export. Eight types are known that mediate export across or insertion into the cytoplasmic membrane, while eight specifically mediate export across or insertion into the outer membrane. Three of the former secretory pathway (SP) systems, type I SP (ISP, ABC), IIISP (Fla/Path) and IVSP (Conj/Vir), can export proteins across both membranes in a single energy-coupled step. A fourth generalized mechanism for exporting proteins across the two-membrane envelope in two distinct steps (which we here refer to as type II secretory pathways [IISP]) utilizes either the general secretory pathway (GSP or Sec) or the twin-arginine targeting translocase for translocation across the inner membrane, and either the main terminal branch or one of several protein-specific export systems for translocation across the outer membrane. We here survey the various well-characterized protein translocation systems found in living organisms and then focus on the systems present in gram-negative bacteria. Comparisons between these systems suggest specific biogenic, mechanistic and evolutionary similarities as well as major differences.

The twin-arginine translocation system and its capability for protein secretion in biotechnological protein production

The biotechnological production of recombinant proteins is challenged by processes that decrease the yield, such as protease action, aggregation, or misfolding. Today, the variation of strains and vector systems or the modulation of inducible promoter activities is commonly used to optimize expression systems. Alternatively, aggregation to inclusion bodies may be a desired starting point for protein isolation and refolding. The discovery of the twin-arginine translocation (Tat) system for folded proteins now opens new perspectives because in most cases, the Tat machinery does not allow the passage of unfolded proteins. This feature of the Tat system can be exploited for biotechnological purposes, as expression systems may be developed that ensure a virtually complete folding of a recombinant protein before purification. This review focuses on the characteristics that make recombinant Tat systems attractive for biotechnology and discusses problems and possible solutions for an efficient translocation of folded proteins.

A scFv antibody mutant isolated in a genetic screen for improved export via the twin arginine transporter pathway exhibits faster folding

Proteins destined for export across the cytoplasmic membrane via the post-translational Sec-dependent route have to be maintained in a largely unfolded state within the cytoplasm. In sharp contrast, only proteins that have folded into a native-like state within the cytoplasm are competent for export via the twin arginine translocation (Tat) pathway. Proteins that contain disulfide bonds, such as scFv antibody fragments, can be translocated via Tat only when expressed in Escherichia coli trxB gor mutant strains having an oxidizing cytoplasm. However, export is poor with the majority of the protein accumulating in the cytoplasm and only a fraction exported to the periplasmic space. Using a high throughput fluorescence screen, we isolated a mutant of the anti-digoxin 26-10 scFv from a large library of random mutants that is exported with a higher yield into the periplasm. In vitro refolding experiments revealed that the mutant scFv exhibits a 250% increase in the rate constant of the critical second phase of folding. This result suggests that Tat export competence is related to the protein folding rate and could be exploited for the isolation of faster folding protein mutants.

Constructing the wonders of the bacterial world: biosynthesis of complex enzymes

The prokaryotic cytoplasmic membrane not only maintains cell integrity and forms a barrier between the cell and its outside environment, but is also the location for essential biochemical processes. Microbial model systems provide excellent bases for the study of fundamental problems in membrane biology including signal transduction, chemotaxis, solute transport and, as will be the topic of this review, energy metabolism. Bacterial respiration requires a diverse array of complex, multi-subunit, cofactor-containing redox enzymes, many of which are embedded within, or located on the extracellular side of, the membrane. The biosynthesis of these enzymes therefore requires carefully controlled expression, assembly, targeting and transport processes. Here, focusing on the molybdenum-containing respiratory enzymes central to anaerobic respiration in Escherichia coli, recent descriptions of a chaperone-mediated 'proofreading' system involved in coordinating assembly and export of complex extracellular enzymes will be discussed. The paradigm proofreading chaperones are members of a large group of proteins known as the TorD family, and recent research in this area highlights common principles that underpin biosynthesis of both exported and non-exported respiratory enzymes.