Isolation, characterization, and expression of the Corynebacterium glutamicum betP gene, encoding the transport system for the compatible solute glycine betaine

CycA-Dependent Glycine Assimilation Is Connected to Novobiocin Susceptibility in Escherichia coli

Escherichia coli serine hydroxymethyltransferase (GlyA) converts serine to glycine, and glyA mutants are auxotrophic for glycine. CycA is a transporter that mediates glycine uptake. Deleting glyA in E. coli strain W3110 led to activation of CysB, which was related to novobiocin (NOV) susceptibility. Moreover, deleting glyA resulted in increased sensitivity to NOV, and this could be reversed by high concentrations of glycine. Reverse mutants of ΔglyA were selected and one of them had a mutation in yrdC, the gene encoding threonylcarbamoyl-AMP synthase. Subsequent proteome analysis showed that deleting glyA led to increased expression of TcyP and TdcB, making this bacterium dependent on CycA for glycine assimilation. Furthermore, deleting cycA in a ΔglyA background caused a severe growth defect on Luria-Bertani medium, which could be complemented by high concentrations of exogenous glycine. Mutation of yrdC led to decreased expression of TdcB but increased expression of ThrA/B/C and LtaE, which favored the conversion of threonine to glycine and thus avoided the dependence on CycA. Correspondingly, deleting of tcyP, tdcB, or gshA could reverse the NOV-sensitive phenotype of ΔglyA mutants. Overexpression of cycA resulted in increased sensitivity to NOV, whereas deleting this gene caused NOV resistance. Moreover, overexpression of cycA led to increased accumulation of NOV upon drug treatment. Therefore, inactivation of glyA in E. coli led to CycA-dependent glycine assimilation, which enhanced the accumulation of NOV and then made the bacterium more sensitive to this drug. These findings broaden our understanding of glycine metabolism and mechanisms of NOV susceptibility. IMPORTANCE Novobiocin (NOV) has been used in clinical practice as an ATPase inhibitor for decades. However, because it has been withdrawn from the market, pharmaceutical companies are searching for other ATPase inhibitors. Thus, probing the mechanisms of susceptibility to NOV will be beneficial to those efforts. In this study, we showed that inactivation of glyA in E. coli led to CycA-dependent glycine assimilation, which accompanied the accumulation of NOV and thereby increased the sensitivity to this drug. To date, this is the first report demonstrating the linkage between glycine assimilation and NOV susceptibility, and it is also the first report showing that YrdC is able to modulate the metabolic flux of threonine.

Hyperosmotic stress allosterically reconfigures betaine binding pocket in BetP

The transporter BetP in C. glutamicum is essential in maintaining bacterial cell viability during hyperosmotic stress and functions by co-transporting betaine and Na⁺ into bacterial cells. Hyperosmotic stress leads to increased intracellular K⁺ concentrations which in turn promotes betaine binding. While structural details of multiple end state conformations of BetP have provided high resolution snapshots, how K⁺ sensing by the C-terminal domain is allosterically relayed to the betaine binding site is not well understood. In this study, we describe conformational dynamics in solution of BetP using amide hydrogen/deuterium exchange mass spectrometry (HDXMS). These reveal how K⁺ alters conformation of the disordered C- and N-terminal domains to allosterically reconfigure transmembrane helices 3,8 and 10 (TM 3, 8, 10) to enhance betaine interactions. A map of the betaine binding site, at near single amino acid resolution, reveals a critical extrahelical H-bond mediated by TM3 with betaine.

The organosulfur compound dimethylsulfoniopropionate (DMSP) is utilized as an osmoprotectant by Vibrio species

Dimethylsulfoniopropionate (DMSP), a key component of the global geochemical sulfur cycle, is a secondary metabolite produced in large quantities by marine phytoplankton and utilized as an osmoprotectant, thermoprotectant and antioxidant. Marine bacteria can use two pathways to degrade and catabolize DMSP, a demethylation pathway and a cleavage pathway that produces the climate active gas dimethylsulfide (DMS). Whether marine bacteria can also accumulate DMSP as an osmoprotectant to maintain the turgor pressure of the cell in response to changes in external osmolarity has received little attention. The marine halophile Vibrio parahaemolyticus, contains at least six osmolyte transporters, four betaine carnitine choline transport (BCCT) carriers BccT1-BccT4 and two ABC-family ProU transporters. In this study, we showed that DMSP is used as an osmoprotectant by V. parahaemolyticus and several other Vibrio species including V. cholerae and V. vulnificus Using a V. parahaemolyticus proU double mutant, we demonstrated that these ABC transporters are not required for DMSP uptake. However, a bccT null mutant lacking all four BCCTs had a growth defect compared to wild type in high salinity media supplemented with DMSP. Using mutants possessing only one functional BCCT in growth pattern assays, we identified two BCCT-family transporters, BccT1 and BccT2, which are carriers of DMSP. The only V. parahaemolyticus BccT homolog that V. cholerae and V. vulnificus possess is BccT3 and functional complementation in Escherichia coli MKH13 showed V. cholerae VcBccT3 could transport DMSP. In V. vulnificus strains, we identified and characterized an additional BCCT family transporter, which we named BccT5 that was also a carrier for DMSP.Importance DMSP is present in the marine environment, produced in large quantities by marine phytoplankton as an osmoprotectant, and is an important component of the global geochemical sulfur cycle. This algal osmolyte has not been previously investigated for its role in marine heterotrophic bacterial osmotic stress response. Vibrionaceae are marine species, many of which are halophiles exemplified by V. parahaemolyticus, a species that possesses at least six transporters for the uptake of osmolytes. Here, we demonstrated that V. parahaemolyticus and other Vibrio species can accumulate DMSP as an osmoprotectant and show that several BCCT family transporters uptake DMSP. These studies suggest that DMSP is a significant bacterial osmoprotectant, which may be important for understanding the fate of DMSP in the environment. DMSP is produced and present in coral mucus and Vibrio species form part of the microbial communities associated with them. The function of DMSP in these interactions is unclear, but could be an important driver for these associations allowing Vibrio proliferation. This work suggests that DMSP likely has an important role in heterotrophic bacteria ecology than previously appreciated.

Stressed out: Bacterial response to high salinity using compatible solute biosynthesis and uptake systems, lessons from Vibrionaceae

Bacteria have evolved mechanisms that allow them to adapt to changes in osmolarity and some species have adapted to live optimally in high salinity environments such as in the marine ecosystem. Most bacteria that live in high salinity do so by the biosynthesis and/or uptake of compatible solutes, small organic molecules that maintain the turgor pressure of the cell. Osmotic stress response mechanisms and their regulation among marine heterotrophic bacteria are poorly understood. In this review, we discuss what is known about compatible solute metabolism and transport and new insights gained from studying marine bacteria belonging to the family Vibrionaceae.

Investigations of Dimethylglycine, Glycine Betaine, and Ectoine Uptake by a Betaine-Carnitine-Choline Transporter Family Transporter with Diverse Substrate Specificity in Vibrio Species

Fluctuations in osmolarity are one of the most prevalent stresses to which bacteria must adapt, both hypo- and hyperosmotic conditions. Most bacteria cope with high osmolarity by accumulating compatible solutes (osmolytes) in the cytoplasm to maintain the turgor pressure of the cell. Vibrio parahaemolyticus, a halophile, utilizes at least six compatible solute transporters for the uptake of osmolytes: two ABC family ProU transporters and four betaine-carnitine-choline transporter (BCCT) family transporters. The full range of compatible solutes transported by this species has yet to be determined. Using an osmolyte phenotypic microarray plate for growth analyses, we expanded the known osmolytes used by V. parahaemolyticus to include N,N-dimethylglycine (DMG), among others. Growth pattern analysis of four triple-bccT mutants, possessing only one functional BCCT, indicated that BccT1 (VP1456), BccT2 (VP1723), and BccT3 (VP1905) transported DMG. BccT1 was unusual in that it could take up both compounds with methylated head groups (glycine betaine [GB], choline, and DMG) and cyclic compounds (ectoine and proline). Bioinformatics analysis identified the four coordinating amino acid residues for GB in the BccT1 protein. In silico modeling analysis demonstrated that GB, DMG, and ectoine docked in the same binding pocket in BccT1. Using site-directed mutagenesis, we showed that a strain with all four residues mutated resulted in the loss of uptake of GB, DMG, and ectoine. We showed that three of the four residues were essential for ectoine uptake, whereas only one of the residues was important for GB uptake. Overall, we have demonstrated that DMG is a highly effective compatible solute for Vibrio species and have elucidated the amino acid residues in BccT1 that are important for the coordination of GB, DMG, and ectoine transport.IMPORTANCE Vibrio parahaemolyticus possesses at least six osmolyte transporters, which allow the bacterium to adapt to high-salinity conditions. In this study, we identified several additional osmolytes that were utilized by V. parahaemolyticus We demonstrated that the compound DMG, which is present in the marine environment, was a highly effective osmolyte for Vibrio species. We determined that DMG is transported via BCCT family carriers, which have not been shown previously to take up this compound. BccT1 was a carrier for GB, DMG, and ectoine, and we identified the amino acid residues essential for the coordination of these compounds. The data suggest that for BccT1, GB is more easily accommodated than ectoine in the transporter binding pocket.

Traversing the "Omic" landscape of microbial halotolerance for key molecular processes and new insights

Post-2005, the biology of the salt afflicted habitats is predominantly studied employing high throughput "Omic" approaches comprising metagenomics, transcriptomics, metatranscriptomics, metabolomics, and proteomics. Such "Omic-based" studies have deciphered the unfamiliar details about microbial salt-stress biology. The MAGs (Metagenome-assembled genomes) of uncultured halophilic microbial lineages such as Nanohaloarchaea and haloalkaliphilic members within CPR (Candidate Phyla Radiation) have been reconstructed from diverse hypersaline habitats. The study of MAGs of such uncultured halophilic microbial lineages has unveiled the genomic basis of salt stress tolerance in "yet to culture" microbial lineages. Furthermore, functional metagenomic approaches have been used to decipher the novel genes from uncultured microbes and their possible role in microbial salt-stress tolerance. The present review focuses on the new insights into microbial salt-stress biology gained through different "Omic" approaches. This review also summarizes the key molecular processes that underlie microbial salt-stress response, and their role in microbial salt-stress tolerance has been confirmed at more than one "Omic" levels.

Genomic Blueprint of Glycine Betaine Metabolism in Coral Metaorganisms and Their Contribution to Reef Nitrogen Budgets

The osmolyte glycine betaine (GB) ranks among the few widespread biomolecules in all three domains of life. In corals, tissue concentrations of GB are substantially higher than in the ambient seawater. However, the synthetic routes remain unresolved, questioning whether intracellular GB originates from de novo synthesis or heterotrophic input. Here we show that the genomic blueprint of coral metaorganisms encode the biosynthetic and degradation machinery for GB. Member organisms also adopted the prokaryotic high-affinity carrier-mediated uptake of exogenous GB, rendering coral reefs potential sinks of marine dissolved GB. The machinery metabolizing GB is highly expressed in the coral model Aiptasia and its microalgal symbionts, signifying GB's role in the cnidarian-dinoflagellate symbiosis. We estimate that corals store between 10⁶–10⁹ grams of GB globally, representing about 16% of their nitrogen biomass. Our findings provide a framework for further mechanistic studies addressing GB's role in coral biology and reef ecosystem nitrogen cycling.

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Coral tissues contain high concentrations of the osmolyte glycine betaine

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Corals and their microbial symbionts can produce and degrade glycine betaine

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High gene expression patterns signifies role in coral-microbial symbiosis

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Glycine betaine is estimated to encompass 16% of the coral's nitrogen biomass

Comparison of the functional properties of trimeric and monomeric CaiT of Escherichia coli

Secondary transporters exist as monomers, dimers or higher state oligomers. The significance of the oligomeric state is only partially understood. Here, the significance of the trimeric state of the L-carnitine/γ-butyrobetaine antiporter CaiT of Escherichia coli was investigated. Amino acids important for trimer stability were identified and experimentally verified. Among others, CaiT-D288A and -D288R proved to be mostly monomeric in detergent solution and after reconstitution into proteoliposomes, as shown by blue native gel electrophoresis, gel filtration, and determination of intermolecular distances. CaiT-D288A was fully functional with kinetic parameters similar to the trimeric wild-type. Significant differences in amount and stability in the cell membrane between monomeric and trimeric CaiT were not observed. Contrary to trimeric CaiT, addition of substrate had no or only a minor effect on the tryptophan fluorescence of monomeric CaiT. The results suggest that physical contacts between protomers are important for the substrate-induced changes in protein fluorescence and the underlying conformational alterations.

Guardians in a stressful world: the Opu family of compatible solute transporters from Bacillus subtilis

The development of a semi-permeable cytoplasmic membrane was a key event in the evolution of microbial proto-cells. As a result, changes in the external osmolarity will inevitably trigger water fluxes along the osmotic gradient. The ensuing osmotic stress has consequences for the magnitude of turgor and will negatively impact cell growth and integrity. No microorganism can actively pump water across the cytoplasmic membrane; hence, microorganisms have to actively adjust the osmotic potential of their cytoplasm to scale and direct water fluxes in order to prevent dehydration or rupture. They will accumulate ions and physiologically compliant organic osmolytes, the compatible solutes, when they face hyperosmotic conditions to retain cell water, and they rapidly expel these compounds through the transient opening of mechanosensitive channels to curb water efflux when exposed to hypo-osmotic circumstances. Here, we provide an overview on the salient features of the osmostress response systems of the ubiquitously distributed bacterium Bacillus subtilis with a special emphasis on the transport systems and channels mediating regulation of cellular hydration and turgor under fluctuating osmotic conditions. The uptake of osmostress protectants via the Opu family of transporters, systems of central importance for the management of osmotic stress by B. subtilis, will be particularly highlighted.

Helical jackknives control the gates of the double-pore K+ uptake system KtrAB

Ion channel gating is essential for cellular homeostasis and is tightly controlled. In some eukaryotic and most bacterial ligand-gated K⁺ channels, RCK domains regulate ion fluxes. Until now, a single regulatory mechanism has been proposed for all RCK-regulated channels, involving signal transduction from the RCK domain to the gating area. Here, we present an inactive ADP-bound structure of KtrAB from Vibrio alginolyticus, determined by cryo-electron microscopy, which, combined with EPR spectroscopy and molecular dynamics simulations, uncovers a novel regulatory mechanism for ligand-induced action at a distance. Exchange of activating ATP to inactivating ADP triggers short helical segments in the K⁺-translocating KtrB dimer to organize into two long helices that penetrate deeply into the regulatory RCK domains, thus connecting nucleotide-binding sites and ion gates. As KtrAB and its homolog TrkAH have been implicated as bacterial pathogenicity factors, the discovery of this functionally relevant inactive conformation may advance structure-guided drug development.

DOI:http://dx.doi.org/10.7554/eLife.24303.001

Lipid-Protein Interactions in the Regulated Betaine Symporter BetP Probed by Infrared Spectroscopy

The Na(+)-coupled betaine symporter BetP senses changes in the membrane state and increasing levels of cytoplasmic K(+) during hyperosmotic stress latter via its C-terminal domain and regulates transport activity according to both stimuli. This intriguing sensing and regulation behavior of BetP was intensively studied in the past. It was shown by several biochemical studies that activation and regulation depends crucially on the lipid composition of the surrounding membrane. In fact, BetP is active and regulated only when negatively charged lipids are present. Recent structural studies have revealed binding of phosphatidylglycerol lipids to functional important parts of BetP, suggesting a functional role of lipid interactions. However, a regulatory role of lipid interactions could only be speculated from the snapshot provided by the crystal structure. Here, we investigate the nature of lipid-protein interactions of BetP reconstituted in closely packed two-dimensional crystals of negatively charged lipids and probed at the molecular level with Fourier transform infrared (FTIR) spectroscopy. The FTIR data indicate that K(+) binding weakens the interaction of BetP especially with the anionic lipid head groups. We suggest a regulation mechanism in which lipid-protein interactions, especially with the C-terminal domain and the functional important gating helices transmembrane helice 3 (TMH3) and TMH12, confine BetP to its down-regulated transport state. As BetP is also activated by changes in the physical state of the membrane, our results point toward a more general mechanism of how active transport can be modified by dynamic lipid-protein interactions.

Anaerobic growth of Corynebacterium glutamicum via mixed-acid fermentation

Corynebacterium glutamicum, a model organism in microbial biotechnology, is known to metabolize glucose under oxygen-deprived conditions to l-lactate, succinate, and acetate without significant growth. This property is exploited for efficient production of lactate and succinate. Our detailed analysis revealed that marginal growth takes place under anaerobic conditions with glucose, fructose, sucrose, or ribose as a carbon and energy source but not with gluconate, pyruvate, lactate, propionate, or acetate. Supplementation of glucose minimal medium with tryptone strongly enhanced growth up to a final optical density at 600 nm (OD600) of 12, whereas tryptone alone did not allow growth. Amino acids with a high ATP demand for biosynthesis and amino acids of the glutamate family were particularly important for growth stimulation, indicating ATP limitation and a restricted carbon flux into the oxidative tricarboxylic acid cycle toward 2-oxoglutarate. Anaerobic cultivation in a bioreactor with constant nitrogen flushing disclosed that CO2 is required to achieve maximal growth and that the pH tolerance is reduced compared to that under aerobic conditions, reflecting a decreased capability for pH homeostasis. Continued growth under anaerobic conditions indicated the absence of an oxygen-requiring reaction that is essential for biomass formation. The results provide an improved understanding of the physiology of C. glutamicum under anaerobic conditions.

The chemical biology of dimethylsulfoniopropionate

This review addresses synthesis, biosynthesis, transport and degradation of dimethylsulfoniopropionate and its derivatives.

Deciphering the role of multiple betaine-carnitine-choline transporters in the Halophile Vibrio parahaemolyticus

Vibrio parahaemolyticus is a halophile that is the predominant cause of bacterial seafood-related gastroenteritis worldwide. To survive in the marine environment, V. parahaemolyticus must have adaptive strategies to cope with salinity changes. Six putative compatible solute (CS) transport systems were previously predicted from the genome sequence of V. parahaemolyticus RIMD2210633. In this study, we determined the role of the four putative betaine-carnitine-choline transporter (BCCT) homologues VP1456, VP1723, VP1905, and VPA0356 in the NaCl stress response. Expression analysis of the four BCCTs subjected to NaCl upshock showed that VP1456, VP1905, and VPA0356, but not VP1723, were induced. We constructed in-frame single-deletion mutant strains for all four BCCTs, all of which behaved similarly to the wild-type strain, demonstrating a redundancy of the systems. Growth analysis of a quadruple mutant and four BCCT triple mutants demonstrated the requirement for at least one BCCT for efficient CS uptake. We complemented Escherichia coli MHK13, a CS synthesis- and transporter-negative strain, with each BCCT and examined CS uptake by growth analysis and (1)H nuclear magnetic resonance (NMR) spectroscopy analyses. These data demonstrated that VP1456 had the most diverse substrate transport ability, taking up glycine betaine (GB), proline, choline, and ectoine. VP1456 was the sole ectoine transporter. In addition, the data demonstrated that VP1723 can transport GB, proline, and choline, whereas VP1905 and VPA0356 transported only GB. Overall, the data showed that the BCCTs are functional and that there is redundancy among them.

Analysis of strains lacking known osmolyte accumulation mechanisms reveals contributions of osmolytes and transporters to protection against abiotic stress

Osmolyte accumulation and release can protect cells from abiotic stresses. In Escherichia coli, known mechanisms mediate osmotic stress-induced accumulation of K(+) glutamate, trehalose, or zwitterions like glycine betaine. Previous observations suggested that additional osmolyte accumulation mechanisms (OAMs) exist and their impacts may be abiotic stress specific. Derivatives of the uropathogenic strain CFT073 and the laboratory strain MG1655 lacking known OAMs were created. CFT073 grew without osmoprotectants in minimal medium with up to 0.9 M NaCl. CFT073 and its OAM-deficient derivative grew equally well in high- and low-osmolality urine pools. Urine-grown bacteria did not accumulate large amounts of known or novel osmolytes. Thus, CFT073 showed unusual osmotolerance and did not require osmolyte accumulation to grow in urine. Yeast extract and brain heart infusion stimulated growth of the OAM-deficient MG1655 derivative at high salinity. Neither known nor putative osmoprotectants did so. Glutamate and glutamine accumulated after growth with either organic mixture, and no novel osmolytes were detected. MG1655 derivatives retaining individual OAMs were created. Their abilities to mediate osmoprotection were compared at 15°C, 37°C without or with urea, and 42°C. Stress protection was not OAM specific, and variations in osmoprotectant effectiveness were similar under all conditions. Glycine betaine and dimethylsulfoniopropionate (DMSP) were the most effective. Trimethylamine-N-oxide (TMAO) was a weak osmoprotectant and a particularly effective urea protectant. The effectiveness of glycine betaine, TMAO, and proline as osmoprotectants correlated with their preferential exclusion from protein surfaces, not with their propensity to prevent protein denaturation. Thus, their effectiveness as stress protectants correlated with their ability to rehydrate the cytoplasm.

Structural insights into functional lipid-protein interactions in secondary transporters

BACKGROUND

Structural evidences with functional corroborations have revealed distinct features of lipid-protein interactions especially in channels and receptors. Many membrane embedded transporters are also known to require specific lipids for their functions and for some of them cellular and biochemical data suggest tight regulation by the lipid bilayer. However, molecular details on lipid-protein interactions in transporters are sparse since lipids are either depleted from the detergent solubilized transporters in three-dimensional crystals or not readily resolved in crystal structures. Nevertheless the steady increase in the progress of transporter structure determination contributed more examples of structures with resolved lipids.

SCOPE OF REVIEW

This review gives an overview on transporter structures in complex with lipids reported to date and discusses commonly encountered difficulties in the identification of functionally significant lipid-protein interactions based on those structures and functional in vitro data. Recent structures provided molecular details into regulation mechanism of transporters by specific lipids. The review highlights common findings and conserved patterns for distantly related transporter families to draw a more general picture on the regulatory role of lipid-protein interactions.

MAJOR CONCLUSIONS

Several common themes of the manner in which lipids directly influence membrane-mediated folding, oligomerization and structure stability can be found. Especially for LeuT-like fold transporters similarities in structurally resolved lipid-protein interactions suggest a common way in which transporter conformations are affected by lipids even in evolutionarily distinct transporters. Lipids appear to play an additional role as joints mechanically reinforcing the inverted repeat topology, which is a major determinant in the alternating access mechanism of secondary transporters.

GENERAL SIGNIFICANCE

This review brings together and adds to the repertoire of knowledge on lipid-protein interactions of functional significance presented in structures of membrane transporters. Knowledge of specific lipid-binding sites and modes of lipid influence on these proteins not only accomplishes the molecular description of transport cycle further, but also sheds light into localization dependent differences of transporter function. This article is part of a Special Issue entitled Structural biochemistry and biophysics of membrane proteins.

Analysis of putative protomer crosstalk in the trimeric transporter BetP: The heterotrimer approach

The homotrimeric, secondary active betaine carrier BetP from Corynebacterium glutamicum is a model system for stress-regulated transport in bacteria. Its activity responds to hyperosmotic stress and it harbors two different functions, transport catalysis (betaine uptake) and stimulus sensing, resp. activity regulation. Structural information from 2D and 3D crystals as well as functional analysis of monomerized BetP suggested the presence of conformational crosstalk between the individual protomers. To study whether the oligomeric state is functionally significant on a mechanistic level we generated heterooligomeric complexes of BetP in which single protomers within the trimer can be addressed. By testing dominant negative effects in a trimer of one active protomer combined with two protomers in which transport and regulation were abolished, we provide experimental evidence for the absence of functionally significant conformational crosstalk between the protomers on the level of both transport and regulation. This is supported by experiments using mutant forms of putative interacting signal donor and acceptor domains of individual BetP protomers. This result has important consequences for oligomeric transport proteins in general and BetP in particular.

Structural evidence for functional lipid interactions in the betaine transporter BetP

Bilayer lipids contribute to the stability of membrane transporters and are crucially involved in their proper functioning. However, the molecular knowledge of how surrounding lipids affect membrane transport is surprisingly limited and despite its general importance is rarely considered in the molecular description of a transport mechanism. One reason is that only few atomic resolution structures of channels or transporters reveal a functional interaction with lipids, which are difficult to detect in X-ray structures per se. Overcoming these difficulties, we report here on a new structure of the osmotic stress-regulated betaine transporter BetP in complex with anionic lipids. This lipid-associated BetP structure is important in the molecular understanding of osmoregulation due to the strong dependence of activity regulation in BetP on the presence of negatively charged lipids. We detected eight resolved palmitoyl-oleoyl phosphatidyl glycerol (PG) lipids mimicking parts of the membrane leaflets and interacting with key residues in transport and regulation. The lipid-protein interactions observed here in structural detail in BetP provide molecular insights into the role of lipids in osmoregulated secondary transport.

Simulation studies of the mechanism of membrane transporters

Membrane transporters facilitate active transport of their specific substrates, often against their electrochemical gradients across the membrane, through coupling the process to various sources of cellular energy, for example, ATP binding and hydrolysis in primary transporters, and pre-established electrochemical gradient of molecular species other than the substrate in the case of secondary transporters. In order to provide efficient energy-coupling mechanisms, membrane transporters have evolved into molecular machines in which stepwise binding, translocation, and transformation of various molecular species are closely coupled to protein conformational changes that take the transporter from one functional state to another during the transport cycle. Furthermore, in order to prevent the formation of leaky states and to be able to pump the substrate against its electrochemical gradient, all membrane transporters use the widely-accepted "alternating access mechanism," which ensures that the substrate is only accessible from one side of the membrane at a given time, but relies on complex and usually global protein conformational changes that differ for each family of membrane transporters. Describing the protein conformational changes of different natures and magnitudes is therefore at the heart of mechanistic studies of membrane transporters. Here, using a number of membrane transporters from diverse families, we present common protocols used in setting up and performing molecular dynamics simulations of membrane transporters and in analyzing the results, in order to characterize relevant motions of the system. The emphasis will be on highlighting how optimal design of molecular dynamics simulations combined with mechanistically oriented analysis can shed light onto key functionally relevant protein conformational changes in this family of membrane proteins.

Conformational changes of the betaine transporter BetP from Corynebacterium glutamicum studied by pulse EPR spectroscopy

The betaine transporter BetP from Corynebacterium glutamicum is activated by hyperosmotic stress critically depending on the presence and integrity of its sensory C-terminal domain. The conformational properties of the trimeric BetP reconstituted in liposomes in the inactive state and during osmotic activation were investigated by site-directed spin labeling and electron paramagnetic resonance (EPR) spectroscopy. Comparison of intra- and intermolecular inter spin distance distributions obtained by double electron-electron resonance (DEER) EPR with the crystal structure of BetP by means of a rotamer library analysis suggest a rotation of BetP protomers within the trimer by about 15° as compared to the X-ray structure. Furthermore, we observed conformational changes upon activation of BetP, which are reflected in changes of the distances between positions 545 and 589 of different protomers in the trimer. Introduction of proline at positions 550 and 572, both leading to BetP variants with a permanent (low level) transport activity, caused changes of the DEER data similar to those observed for the activated and inactivated state, respectively. This indicates that not only displacements of the C-terminal domain in general but also concomitant interactions of its primary structure with surrounding protein domains and/or lipids are crucial for the activity regulation of BetP.

The BCCT family of carriers: from physiology to crystal structure

Increases in the environmental osmolarity are key determinants for the growth of microorganisms. To ensure a physiologically acceptable level of cellular hydration and turgor at high osmolarity, many bacteria accumulate compatible solutes. Osmotically controlled uptake systems allow the scavenging of these compounds from scarce environmental sources as effective osmoprotectants. A number of these systems belong to the BCCT family (betaine-choline-carnitine-transporter), sodium- or proton-coupled transporters (e.g. BetP and BetT respectively) that are ubiquitous in microorganisms. The BCCT family also contains CaiT, an L-carnitine/γ-butyrobetaine antiporter that is not involved in osmotic stress responses. The glycine betaine transporter BetP from Corynebacterium glutamicum is a representative for osmoregulated symporters of the BCCT family and functions both as an osmosensor and osmoregulator. The crystal structure of BetP in an occluded conformation in complex with its substrate glycine betaine and two crystal structures of CaiT in an inward-facing open conformation in complex with L-carnitine and γ-butyrobetaine were reported recently. These structures and the wealth of biochemical data on the activity control of BetP in response to osmotic stress enable a correlation between the sensing of osmotic stress by a transporter protein with the ensuing regulation of transport activity. Molecular determinants governing the high-affinity binding of the compatible solutes by BetP and CaiT, the coupling in symporters and antiporters, and the osmoregulatory properties are discussed in detail for BetP and various BCCT carriers.

Bacterial stimulus perception and signal transduction: response to osmotic stress

When exposed to osmotic stress from the environment, bacteria act to maintain cell turgor and hydration by responding both on the level of gene transcription and protein activity. Upon a sudden decrease in external osmolality, internal solutes are released by the action of membrane embedded mechanosensitive channels. In response to an osmotic upshift, the concentration of osmolytes in the cytoplasm is increased both by de novo synthesis and by active uptake. In order to coordinate these processes of osmoregulation, cells are equipped with systems and mechanisms of sensing physical stimuli correlated to changes in the external osmolality (osmosensing), with pathways to transduce these stimuli into useful signals which can be processed in the cell (signal transduction), and mechanisms of regulating proper responses in the cell to recover from the environmental stress and to maintain all necessary physiological functions (osmoregulation). These processes will be described by a number of representative examples, mainly of osmoreactive transport systems with a focus on available data of their molecular mechanism.

Identification of the membrane protein SucE and its role in succinate transport in Corynebacterium glutamicum

Succinic acid is excreted during anaerobiosis by many bacteria, and manifold applications are known making the biotechnological production of succinate attractive. Although the pathways for succinate formation are known, succinate export is not understood in most of the succinate producing bacteria. Here, we present a bioinformatic approach for identification of a putative succinate export system in Corynebacterium glutamicum. The subsequent screening revealed that a mutant in the gene cg2425 is impaired in succinate production or transport under anaerobic conditions. A function of the Cg2425 protein as import system was excluded. In contrast, a role of the Cg2425 protein as succinate export system was indicated by accumulation of increased amounts of internal succinate under anaerobic conditions in a Cg2425-dependent manner and a concomitant impairment of external succinate accumulation. In conclusion, we propose that Cg2425 participates in succinate export in C. glutamicum and suggest the name SucE for the protein.

Ligand binding and crystal structures of the substrate-binding domain of the ABC transporter OpuA

BACKGROUND

The ABC transporter OpuA from Lactococcus lactis transports glycine betaine upon activation by threshold values of ionic strength. In this study, the ligand binding characteristics of purified OpuA in a detergent-solubilized state and of its substrate-binding domain produced as soluble protein (OpuAC) was characterized.

PRINCIPAL FINDINGS

The binding of glycine betaine to purified OpuA and OpuAC (K(D) = 4-6 microM) did not show any salt dependence or cooperative effects, in contrast to the transport activity. OpuAC is highly specific for glycine betaine and the related proline betaine. Other compatible solutes like proline and carnitine bound with affinities that were 3 to 4 orders of magnitude lower. The low affinity substrates were not noticeably transported by membrane-reconstituted OpuA. OpuAC was crystallized in an open (1.9 A) and closed-liganded (2.3 A) conformation. The binding pocket is formed by three tryptophans (Trp-prism) coordinating the quaternary ammonium group of glycine betaine in the closed-liganded structure. Even though the binding site of OpuAC is identical to that of its B. subtilis homolog, the affinity for glycine betaine is 4-fold higher.

CONCLUSIONS

Ionic strength did not affect substrate binding to OpuA, indicating that regulation of transport is not at the level of substrate binding, but rather at the level of translocation. The overlap between the crystal structures of OpuAC from L.lactis and B.subtilis, comprising the classical Trp-prism, show that the differences observed in the binding affinities originate from outside of the ligand binding site.

Regulative interactions of the osmosensing C-terminal domain in the trimeric glycine betaine transporter BetP from Corynebacterium glutamicum

Activation of the osmoregulated trimeric betaine transporter BetP from Corynebacterium glutamicum was shown to depend mainly on the correct folding and integrity of its 55 amino acid long, partly α-helical C-terminal domain. Reorientation of the three C-terminal domains in the BetP trimer indicates different lipid-protein and protein-protein interactions of the C-terminal domain during osmoregulation. A regulation mechanism is suggested where this domain switches the transporter from the inactive to the active state. Interpretation of recently obtained electron and X-ray crystallography data of BetP led to a structure-function based model of C-terminal molecular switching involved in osmoregulation.

Pseudomonas syringae BetT is a low-affinity choline transporter that is responsible for superior osmoprotection by choline over glycine betaine

The plant pathogen Pseudomonas syringae derives better osmoprotection from choline than from glycine betaine, unlike most bacteria that have been characterized. In this report, we identified a betaine/carnitine/choline family transporter (BCCT) in P. syringae pv. tomato strain DC3000 that mediates the transport of choline and acetylcholine. This transporter has a particularly low affinity (K(m) of 876 microM) and high capacity (V(max) of 80 nmol/min/mg of protein) for choline transport relative to other known BCCTs. Although BetT activity increased in response to hyperosmolarity, BetT mediated significant uptake under low-osmolarity conditions, suggesting a role in transport for both osmoprotection and catabolism. Growth studies with mutants deficient in BetT and other choline transporters demonstrated that BetT was responsible for the superior osmoprotection conferred to P. syringae by choline over glycine betaine when these compounds were provided at high concentrations (>100 microM). These results suggest that P. syringae has evolved to survive in relatively choline-rich habitats, a prediction that is supported by the common association of P. syringae with plants and the widespread production of choline, but genus- and species-specific production of glycine betaine, by plants. Among the three putative BCCT family transporters in Pseudomonas aeruginosa and six in Pseudomonas putida, different transporters were predicted to function based on similarity to Escherichia coli BetT than to P. syringae BetT. Functional P. putida and P. aeruginosa transporters were identified, and their possession of a long C-terminal tail suggested an osmoregulatory function for this tail; this function was confirmed for P. syringae BetT using deletion derivatives.

Characterization of compatible solute transporter multiplicity in Corynebacterium glutamicum

The soil bacterium Corynebacterium glutamicum is efficiently protected against hyperosmotic stress by a high redundancy of uptake systems and biosynthesis pathways for compatible solutes. We have previously identified and analyzed four osmoregulated uptake systems for betaine, ectoine, and proline. Because of overlapping substrate specificities, it is not possible to quantify their individual contribution to the stress response in wild-type cells. Using a set of strains in which only one uptake system for compatible solutes is present, we investigated the expression regulation at their transcriptional and translational level. The carrier ectP was found to be regulated at the level of transcription, but the already high maximal uptake capacity of approx. 30 nmol/(min mg cell dry mass, cdm) was not further elevated if the medium osmolality was severely increased, indicating that the amount of EctP is not changed. Thus, EctP may represent the rescue system for C. glutamicum. The betP, lcoP, and proP genes were induced upon hyperosmotic conditions, resulting in a 3-10-fold increase of their transport activity. These systems are thus used to fine-tune the uptake capacity for compatible solutes to the actual demands of the cell. ProP represents the most strongly regulated compatible solute uptake system in C. glutamicum.

Membrane topology and mutational analysis of the osmotically activated BetT choline transporter of Escherichia coli

For osmoprotection, Escherichia coli can synthesize glycine betaine from externally supplied choline by the Bet system (betTIBA products). The major carrier of choline is the high-affinity, proton-driven, secondary transporter BetT, which belongs to the BCCT family of transporters. Fusion proteins consisting of N-terminal fragments of BetT linked to beta-galactosidase (LacZ) or alkaline phosphatase (PhoA) were constructed. By analysis of 51 fusion proteins with 37 unique fusion-points, the predictions that BetT comprised 12 membrane-spanning regions and that its N- and C-terminal extensions of about 12 and 180 amino acid residues, respectively, were situated in the cytoplasm were confirmed. This is believed to represent the first experimental examination of the membrane topology of a BCCT family protein. Osmotic upshock experiments were performed with spectinomycin-treated E. coli cells that had expressed the wild-type or a mutant BetT protein during growth at low osmolality (160 mosmol kg(-1)). The choline transport activity of wild-type BetT increased tenfold when the cells were stressed with 0.4 M NaCl (total osmolality 780 mosmol kg(-1)). The peak activity was recorded 5 min after the upshock and higher or lower concentrations of NaCl reduced the activity. Deletions of 1-12 C-terminal residues of BetT caused a gradual reduction in the degree of osmotic activation from ten- to twofold. Mutant proteins with deletion of 18-101 residues displayed a background transport activity, but they could not be osmotically activated. The data showed that the cytoplasmic C-terminal domain of BetT plays an important role in the regulation of BetT activity and that C-terminal truncations can cause BetT to be permanently locked in a low-transport-activity mode.

Bacterial osmosensing transporters

Cells faced with dehydration because of increasing extracellular osmotic pressure accumulate solutes through synthesis or transport. Water follows, restoring cellular hydration and volume. Prokaryotes and eukaryotes possess arrays of osmoregulatory genes and enzymes that are responsible for solute accumulation under osmotic stress. In bacteria, osmosensing transporters can detect increasing extracellular osmotic pressure and respond by mediating the uptake of organic osmolytes compatible with cellular functions ("compatible solutes"). This chapter reviews concepts and methods critical to the identification and study of osmosensing transporters. Like some experimental media, cytoplasm is a "nonideal" solution so the estimation of key solution properties (osmotic pressure, osmolality, water activity, osmolarity, and macromolecular crowding) is essential for studies of osmosensing and osmoregulation. Because bacteria vary widely in osmotolerance, techniques for its characterization provide an essential context for the elucidation of osmosensory and osmoregulatory mechanisms. Powerful genetic, molecular biological, and biochemical tools are now available to aid in the identification and characterization of osmosensory transporters, the genes that encode them, and the osmoprotectants that are their substrates. Our current understanding of osmosensory mechanisms is based on measurements of osmosensory transporter activity performed with intact cells, bacterial membrane vesicles, and proteoliposomes reconstituted with purified transporters. In the quest to elucidate the structural mechanisms of osmosensing and osmoregulation, researchers are now applying the full range of available biophysical, biochemical, and molecular biological tools to osmosensory transporter prototypes.

Halotolerant cyanobacterium Aphanothece halophytica contains a betaine transporter active at alkaline pH and high salinity

Aphanothece halophytica is a halotolerant alkaliphilic cyanobacterium which can grow in media of up to 3.0 M NaCl and pH 11. This cyanobacterium can synthesize betaine from glycine by three-step methylation using S-adenosylmethionine as a methyl donor. To unveil the mechanism of betaine uptake and efflux in this alkaliphile, we isolated and characterized a betaine transporter. A gene encoding a protein (BetT(A. halophytica)) that belongs to the betaine-choline-carnitine transporter (BCCT) family was isolated. Although the predicted isoelectric pH of a typical BCCT family transporter, OpuD of Bacillus subtilis, is basic, 9.54, that of BetT(A. halophytica) is acidic, 4.58. BetT(A. halophytica) specifically catalyzed the transport of betaine. Choline, gamma-aminobutyric acid, betaine aldehyde, sarcosine, dimethylglycine, and amino acids such as proline did not compete for the uptake of betaine by BetT(A. halophytica). Sodium markedly enhanced betaine uptake rates, whereas potassium and other cations showed no effect, suggesting that BetT(A. halophytica) is a Na(+)-betaine symporter. Betaine uptake activities of BetT(A. halophytica) were high at alkaline pH values, with the optimum pH around 9.0. Freshwater Synechococcus cells overexpressing BetT(A. halophytica) showed NaCl-activated betaine uptake activities with enhanced salt tolerance, allowing growth in seawater supplemented with betaine. Kinetic properties of betaine uptake in Synechococcus cells overexpressing BetT(A. halophytica) were similar to those in A. halophytica cells. These findings indicate that A. halophytica contains a Na(+)-betaine symporter that contributes to the salt stress tolerance at alkaline pH. BetT(A. halophytica) is the first identified transporter for compatible solutes in cyanobacteria.

Osmosensing by bacteria

Osmosensors are proteins that sense environmental osmotic pressure. They mediate or direct osmoregulatory responses that allow cells to survive osmotic changes and extremes. Bacterial osmosensing transporters sense high external osmotic pressure and respond by mediating organic osmolyte uptake, hence cellular rehydration. Detailed studies of osmosensing transporters OpuA, BetP, and ProP suggest that they sense and respond to different osmotic pressure-dependent cellular properties. These studies also suggest that each protein has a cytoplasmic osmosensory or osmoregulatory domain, but that these domains differ in structure and function. It is not yet clear whether each transporter represents a distinct osmosensory mechanism or whether different research groups are approaching the same mechanism by way of different paths. Principles emerging from this research will apply to other osmosensors, including those that initiate signal transduction cascades in prokaryotes and eukaryotes.

Biochemical and structural analysis of the Bacillus subtilis ABC transporter OpuA and its isolated subunits

Adaptation of microorganisms to changing osmotic conditions is a prerequisite for survival and cellular vitality for most microorganisms. In the Gram-positive soil bacterium Bacillus subtilis, five transport systems catalyze the uptake of compatible solutes across the plasma membrane that allow the growth of B. subtilis over a wide range of osmotic conditions. Focus of this review is the osmoprotectant uptake A (OpuA) transporter, a member of the family of substrate-binding protein (SBP)-dependent ATP-binding cassette (ABC) transporters that mediates the uptake of the compatible solutes glycine betaine and proline betaine. OpuA is composed of three subunits: a nucleotide-binding domain (OpuAA) located in the cytosol, a transmembrane domain (OpuAB), and a SBP (OpuAC), which binds glycine betaine and proline betaine with high specificity and targets it to OpuAB for ATP-dependent translocation across the plasma membrane. After a brief introduction in the field of bacterial osmoadaptation, we will summarize our recent findings about the biochemical and structural analysis of the components of the OpuA systems. Our studies covered both the isolated subunits of the OpuA transporter and initial investigations of the whole transporter in vitro.

Heterologous expression of BetL, a betaine uptake system, enhances the stress tolerance of Lactobacillus salivarius UCC118

Given the increasing commercial and clinical relevance of probiotic cultures, improving the technological robustness of what are often process-sensitive cultures is an important biological goal. The nisin-controlled expression system was used to direct the heterologous expression of the listerial betaine uptake system BetL in the probiotic strain Lactobacillus salivarius UCC118. Following nisin induction, strains expressing betL exhibited a significant increase in resistance to several stresses, including elevated osmo-, cryo-, baro-, and chill tolerance, as well as increased resistance to spray- and freeze-drying. The ability to confer additional stress tolerance on a probiotic culture may be an important step in delivering viable cultures for maximal efficacy.

Structure and Function of the Betaine Uptake System BetP of Corynebacterium glutamicum: Strategies to Sense Osmotic and Chill Stress

The soil bacterium Corynebacterium glutamicum has to cope with frequent fluctuations of the external osmolarity and temperature. The consequences of hyperosmotic and chill stress seem to differ, either causing dehydration of the cytoplasm or leading to impairment of cellular functions due to low temperature. Nevertheless, a particular type of regulatory response, namely the accumulation of so-called compatible solutes, is induced under both conditions. Compatible solutes are known to stabilize the native conformation of enzymes, which may be affected by osmotic and chill stress. BetP is a high-affinity uptake carrier for the compatible solute glycine betaine in C. glutamicum. BetP includes, besides its catalytic function, the ability to sense hyperosmotic conditions and chill stress. As a consequence, the carrier is activated in dependence of the extent of these types of stress. The signal input related to these changes of the environmental conditions is based on at least two different mechanisms. In case of hyperosmotic stress, BetP responds to the internal potassium concentration as a measure for hypertonicity, whereas chill stress is detected by an independent signal, most probably changes of the physical state of the membrane.

Influence of membrane composition on osmosensing by the betaine carrier BetP from Corynebacterium glutamicum

The glycine betaine carrier BetP from Corynebacterium glutamicum was recently shown to function as both an osmosensor and osmoregulator in proteoliposomes made from Escherichia coli phospholipids by sensing changes in the internal K+ concentration as a measure of hyperosmotic stress (Rübenhagen, R., Morbach, S., and Krämer, R. (2001) EMBO J. 20, 5412-5420). Furthermore, evidence was provided that a stretch of 25 amino acids of the C-terminal domain of BetP is critically involved in K+ sensing. This K+-sensitive region has been further characterized. Glu572 turned out to be important for osmosensing in E. coli cells and in proteoliposomes made from E. coli phospholipids. BetP mutants E572K, E572P, and E572A/H573A/R574A were unable to detect an increase in the internal K+ concentration in this membrane environment. However, these BetP variants regained their ability to detect osmotic stress in membranes with increased phosphatidylglycerol content, i.e. in intact C. glutamicum cells or in proteoliposomes mimicking the composition of the C. glutamicum membrane. Mutants E572P and Y550P were still insensitive to osmotic stress also in this membrane background. These results led to the following conclusions. (i) The K+ sensor in mutants E572Q, E572D, and E572K is only partially impaired. (ii) Restoration of activity regulation is not possible if the correct conformation or orientation of the C-terminal domain is compromised by a proline residue at position 572 or 550. (iii) Phosphatidylglycerol in the membrane of C. glutamicum seems to stabilize the inactive conformation of BetP C252T and other mutants.

The osmotic activation of transporter ProP is tuned by both its C-terminal coiled-coil and osmotically induced changes in phospholipid composition

Transporter ProP of Escherichia coli (ProPEc) senses extracellular osmolality and mediates osmoprotectant uptake when it is rising or high. A replica of the ProPEc C terminus (Asp468-Arg497) forms an intermolecular alpha-helical coiled-coil. This structure is implicated in the osmoregulation of intact ProPEc, in vivo. Like that from Corynebacterium glutamicum (ProPCg), the ProP orthologue from Agrobacterium tumefaciens (ProPAt) sensed and responded to extracellular osmolality after expression in E. coli. The osmotic activation profiles of all three orthologues depended on the osmolality of the bacterial growth medium, the osmolality required for activation rising as the growth osmolality approached 0.7 mol/kg. Thus, each could undergo osmotic adaptation. The proportion of cardiolipin in a polar lipid extract from E. coli increased with extracellular osmolality so that the osmolality activating ProPEc was a direct function of membrane cardiolipin content. Group A ProP orthologues (ProPEc, ProPAt) share the C-terminal coiled-coil domain and were activated at low osmolalities. Like variant ProPEc-R488I, in which the C-terminal coiled-coil is disrupted, ProPEc derivatives that lack the coiled-coil and Group B orthologue ProPCg required a higher osmolality to activate. The amplitude of ProPEc activation was reduced 10-fold in its deletion derivatives. The coiled-coil structure is not essential for osmotic activation of ProP per se. However, it tunes Group A orthologues to osmoregulate over a low osmolality range. Coiled-coil lesions may impair both coiled-coil formation and interaction of ProPEc with amplifier protein ProQ. Cardiolipin may contribute to ProP adaptation by altering bulk membrane properties or by acting as a ProP ligand.

Chill activation of compatible solute transporters in Corynebacterium glutamicum at the level of transport activity

The gram-positive soil bacterium Corynebacterium glutamicum harbors four osmoregulated secondary uptake systems for compatible solutes, BetP, EctP, LcoP, and ProP. When reconstituted in proteoliposomes, BetP was shown to sense hyperosmotic conditions via the increase in luminal K(+) and to respond by instant activation. To study further putative ways of stimulus perception and signal transduction, we have investigated the responses of EctP, LcoP, and BetP, all belonging to the betaine-carnitine-choline transporter family, to chill stress at the level of activity. When fully activated by hyperosmotic stress, they showed the expected increase of activity at increasing temperature. In the absence of osmotic stress, EctP was not activated by chill and LcoP to only a very low extent, whereas BetP was significantly stimulated at low temperature. BetP was maximally activated at 10 degrees C, reaching the same transport rate as that observed under hyperosmotic conditions at this temperature. A role of cytoplasmic K(+) in chill-dependent activation of BetP was ruled out, since (i) the cytoplasmic K(+) concentration did not change significantly at lower temperatures and (ii) a mutant BetP lacking the C-terminal 25 amino acids, which was previously shown to have lost the ability to be activated by luminal K(+), was fully competent in chill sensing. When heterologously expressed in Escherichia coli, BetP did not respond to chill stress. This may indicate that the membrane in which BetP is inserted plays an important role in chill activation and thus in signal transduction by BetP, different from the previously established K(+)-mediated process.

DNA microarray analysis of the nitrogen starvation response of Corynebacterium glutamicum

Nitrogen is an essential component of nearly all of the complex macromolecules in a bacterial cell, e.g. proteins, nucleic acids, and cell wall components. Accordingly, most prokaryotes have developed elaborate control mechanisms to provide an optimal supply of nitrogen for cellular metabolism and to cope with situations of nitrogen limitation. In this communication, a global analysis of the Corynebacterium glutamicum nitrogen starvation response by transcriptional profiling using DNA microarrays is presented. Our results show that C. glutamicum reacts to nitrogen starvation with a rearrangement of the cellular transport capacity, changes in metabolic pathways concerning nitrogen assimilation and amino acid biosynthesis, and a decreased capacity for protein synthesis.

OusB, a broad-specificity ABC-type transporter from Erwinia chrysanthemi, mediates uptake of glycine betaine and choline with a high affinity

The ability of Erwinia chrysanthemi to cope with environments of elevated osmolality is due in part to the transport and accumulation of osmoprotectants. In this study we have identified a high-affinity glycine betaine and choline transport system in E. chrysanthemi. By using a pool of Tn5-B21 ousA mutants, we isolated a mutant that could grow in the presence of a toxic analogue of glycine betaine (benzyl-glycine betaine) at high osmolalities. This mutant was impaired in its ability to transport all effective osmoprotectants in E. chrysanthemi. The DNA sequence of the regions flanking the transposon insertion site revealed three chromosomal genes (ousVWX) that encode components of an ABC-type transporter (OusB): OusV (ATPase), OusW (permease), and OusX (periplasmic binding protein). The OusB components showed a significant degree of sequence identity to components of ProU from Salmonella enterica serovar Typhimurium and Escherichia coli. OusB was found to restore the uptake of glycine betaine and choline through functional complementation of an E. coli mutant defective in both ProU and ProP osmoprotectant uptake systems. Competition experiments demonstrated that choline, dimethylsulfoniacetate, dimethylsulfoniopropionate, and ectoine were effective competitors for OusB-mediated betaine transport but that carnitine, pipecolate, and proline were not effective. In addition, the analysis of single and double mutants showed that OusA and OusB were the only osmoprotectant transporters operating in E. chrysanthemi.

Activity regulation of the betaine transporter BetP of Corynebacterium glutamicum in response to osmotic compensation

As a response to hyperosmotic stress bacterial cells accumulate compatible solutes by synthesis or by uptake. Beside the instant activation of uptake systems after an osmotic upshift, transport systems show also a second, equally important type of regulation. In order to adapt the pool size of compatible solutes in the cytoplasm to the actual extent of osmotic stress, cells down-regulate solute uptake when the initial osmotic stress is compensated. Here we describe the role of the betaine transporter BetP, the major uptake carrier for compatible solutes in Corynebacterium glutamicum, in this adaptation process. For this purpose, betP was expressed in cells (C. glutamicum and Escherichia coli), which lack all known uptake systems for compatible solutes. Betaine uptake mediated by BetP as well as by a truncated form of BetP, which is deregulated in its response to hyperosmotic stress, was dissected into the individual substrate fluxes of unidirectional uptake, unidirectional efflux and net uptake. We determined a strong decrease of unidirectional betaine uptake by BetP in the adaptation phase. The observed decrease in net uptake was thus mainly due to a decrease of Vmax of BetP and not a consequence of the presence of separate efflux system(s). These results indicate that adaptation of BetP to osmotic compensation is different from activation by osmotic stress and also different from previously described adaptation mechanisms in other organisms. Cytoplasmic K+, which was shown to be responsible for activation of BetP upon osmotic stress, as well as a number of other factors was ruled out as triggers for the adaptation process. Our results thus indicate the presence of a second type of signal input in the adaptive regulation of osmoregulated carrier proteins.

Bacterial osmosensing: roles of membrane structure and electrostatics in lipid–protein and protein–protein interactions

Bacteria act to maintain their hydration when the osmotic pressure of their environment changes. When the external osmolality decreases (osmotic downshift), mechanosensitive channels are activated to release low molecular weight osmolytes (and hence water) from the cytoplasm. Upon osmotic upshift, osmoregulatory transporters are activated to import osmolytes (and hence water). Osmoregulatory channels and transporters sense and respond to osmotic stress via different mechanisms. Mechanosensitive channel MscL senses the increasing tension in the membrane and appears to gate when the lateral pressure in the acyl chain region of the lipids drops below a threshold value. Transporters OpuA, BetP and ProP are activated when increasing external osmolality causes threshold ionic concentrations in excess of about 0.05 M to be reached in the proteoliposome lumen. The threshold activation concentrations for the OpuA transporter are strongly dependent on the fraction of anionic lipids that surround the cytoplasmic face of the protein. The higher the fraction of anionic lipids, the higher the threshold ionic concentrations. A similar trend is observed for the BetP transporter. The lipid dependence of osmotic activation of OpuA and BetP suggests that osmotic signals are transmitted to the protein via interactions between charged osmosensor domains and the ionic headgroups of the lipids in the membrane. The charged, C-terminal domains of BetP and ProP are important for osmosensing. The C-terminal domain of ProP participates in homodimeric coiled-coil formation and it may interact with the membrane lipids and soluble protein ProQ. The activation of ProP by lumenal, macromolecular solutes at constant ionic strength indicates that its structure and activity may also respond to macromolecular crowding. This excluded volume effect may restrict the range over which the osmosensing domain can electrostatically interact. A simplified version of the dissociative double layer theory is used to explain the activation of the transporters by showing how changes in ion concentration could modulate interactions between charged osmosensor domains and charged lipid or protein surfaces. Importantly, the relatively high ionic concentrations at which osmosensors become activated at different surface charge densities compare well with the predicted dependence of 'critical' ion concentrations on surface charge density. The critical ion concentrations represent transitions in Maxwellian ionic distributions at which the surface potential reaches 25.7 mV for monovalent ions. The osmosensing mechanism is qualitatively described as an "ON/OFF switch" representing thermally relaxed and electrostatically locked protein conformations.