The osmoreactive betaine carrier BetP from Corynebacterium glutamicum is a sensor for cytoplasmic K+

Flipping the switch: dynamic modulation of membrane transporter activity in bacteria

The controlled entry and expulsion of small molecules across the bacterial cytoplasmic membrane is essential for efficient cell growth and cellular homeostasis. While much is known about the transcriptional regulation of genes encoding transporters, less is understood about how transporter activity is modulated once the protein is functional in the membrane, a potentially more rapid and dynamic level of control. In this review, we bring together literature from the bacterial transport community exemplifying the extensive and diverse mechanisms that have evolved to rapidly modulate transporter function, predominantly by switching activity off. This includes small molecule feedback, inhibition by interaction with small peptides, regulation through binding larger signal transduction proteins and, finally, the emerging area of controlled proteolysis. Many of these examples have been discovered in the context of metal transport, which has to finely balance active accumulation of elements that are essential for growth but can also quickly become toxic if intracellular homeostasis is not tightly controlled. Consistent with this, these transporters appear to be regulated at multiple levels. Finally, we find common regulatory themes, most often through the fusion of additional regulatory domains to transporters, which suggest the potential for even more widespread regulation of transporter activity in biology.

Insights into autoregulation of a membrane protein complex by its cytoplasmic domains

Membrane transporters mediate the passage of molecules across membranes and are essential for cellular function. While the transmembrane region of these proteins is responsible for substrate transport, often the cytoplasmic regions are required for modulating their activity. However, it can be difficult to obtain atomic-resolution descriptions of these autoregulatory domains by classical structural biology techniques, especially if they lack a single, defined structure. The betaine permease, BetP, a homotrimer, is a prominent and well-studied example of a membrane protein whose autoregulation depends on cytoplasmic N- and C-terminal segments. These domains sense and transduce changes in K+ concentration and in lipid bilayer properties caused by osmotic stress. However, structural data for these terminal domains is incomplete, which hinders a clear description of the molecular mechanism of autoregulation. Here we used microsecond-scale molecular simulations of the BetP trimer to compare reported conformations of the 45-amino-acid long C-terminal tails. The simulations provide support for the idea that the conformation derived from electron microscopy (EM) data represents a more stable global orientation of the C-terminal segment under downregulating conditions while also providing a detailed molecular description of its dynamics and highlighting specific interactions with lipids, ions, and neighboring transporter subunits. A missing piece of the molecular puzzle is the N-terminal segment, whose dynamic nature has prevented structural characterization. Using Rosetta to generate ensembles of de novo conformations in the context of the EM-derived structure robustly identifies two features of the N-terminal tail, namely 1) short helical elements and 2) an orientation that would confine potential interactions to the protomer in the counterclockwise direction (viewed from the cytoplasm). Since each C-terminal tail only contacts the protomer in the clockwise direction, these results indicate an intricate interplay between the three protomers of BetP in the downregulated protein and a multidirectionality that may facilitate autoregulation of transport.

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.

CosR Is a Global Regulator of the Osmotic Stress Response with Widespread Distribution among Bacteria

Bacteria accumulate small, organic compounds called compatible solutes via uptake from the environment or biosynthesis from available precursors to maintain the turgor pressure of the cell in response to osmotic stress. The halophile Vibrio parahaemolyticus has biosynthesis pathways for the compatible solutes ectoine (encoded by ectABC-asp_ect) and glycine betaine (encoded by betIBA-proXWV), four betaine-carnitine-choline transporters (encoded by bccT1 to bccT4), and a second ProU transporter (encoded by proVWX). All of these systems are osmotically inducible with the exception of bccT2 Previously, it was shown that CosR, a MarR-type regulator, was a direct repressor of ectABC-asp_ect in Vibrio species. In this study, we investigated whether CosR has a broader role in the osmotic stress response. Expression analyses demonstrated that betIBA-proXWV, bccT1, bccT3, bccT4, and proVWX are repressed in low salinity. Examination of an in-frame cosR deletion mutant showed that expression of these systems is derepressed in the mutant at low salinity compared with the wild type. DNA binding assays demonstrated that purified CosR binds directly to the regulatory region of both biosynthesis systems and four transporters. In Escherichia coli green fluorescent protein (GFP) reporter assays, we demonstrated that CosR directly represses transcription of betIBA-proXWV, bccT3, and proVWX Similar to Vibrio harveyi, we showed betIBA-proXWV was directly activated by the quorum-sensing LuxR homolog OpaR, suggesting a conserved mechanism of regulation among Vibrio species. Phylogenetic analysis demonstrated that CosR is ancestral to the Vibrionaceae family, and bioinformatics analysis showed widespread distribution among Gammaproteobacteria in general. Incidentally, in Aliivibrio fischeri, Aliivibrio finisterrensis, Aliivibrio sifiae, and Aliivibrio wodanis, an unrelated MarR-type regulator gene named ectR was clustered with ectABC-asp, which suggests the presence of another novel ectoine biosynthesis regulator. Overall, these data show that CosR is a global regulator of osmotic stress response that is widespread among bacteria.IMPORTANCE Vibrio parahaemolyticus can accumulate compatible solutes via biosynthesis and transport, which allow the cell to survive in high salinity conditions. There is little need for compatible solutes under low salinity conditions, and biosynthesis and transporter systems need to be repressed. However, the mechanism(s) of this repression is not known. In this study, we showed that CosR played a major role in the regulation of multiple compatible solute systems. Phylogenetic analysis showed that CosR is present in all members of the Vibrionaceae family as well as numerous Gammaproteobacteria Collectively, these data establish CosR as a global regulator of the osmotic stress response that is widespread in bacteria, controlling many more systems than previously demonstrated.

Sensing and regulation of cell volume - we know so much and yet understand so little: TRPV4 as a sensor of volume changes but possibly without a volume-regulatory role?

Cellular volume changes lead to initiation of cell volume regulatory events, the molecular identity of which remains unresolved. We here discuss experimental challenges associated with investigation of volume regulation during application of large, non-physiological osmotic gradients. The TRPV4 ion channel responds to volume increase irrespectively of the molecular mechanism underlying cell swelling, and is thus considered a sensor of volume changes. Evidence pointing towards the involvement of TRPV4 in subsequent volume regulatory mechanisms is intriguing, yet far from conclusive. We here present an experimental setting with astrocytic cell swelling in the absence of externally applied osmotic gradients, and the lack of evidence for involvement of TRPV4 in this regulatory volume response. Our aim with these new data and the preceding discussion is to stimulate further experimental effort in this area of research to clarify the role of TRPV4 and other channels and transporters in regulatory volume responses.

Responses of Microorganisms to Osmotic Stress

The cytoplasm of bacterial cells is a highly crowded cellular compartment that possesses considerable osmotic potential. As a result, and owing to the semipermeable nature of the cytoplasmic membrane and the semielastic properties of the cell wall, osmotically driven water influx will generate turgor, a hydrostatic pressure considered critical for growth and viability. Both increases and decreases in the external osmolarity inevitably trigger water fluxes across the cytoplasmic membrane, thus impinging on the degree of cellular hydration, molecular crowding, magnitude of turgor, and cellular integrity. Here, we assess mechanisms that permit the perception of osmotic stress by bacterial cells and provide an overview of the systems that allow them to genetically and physiologically cope with this ubiquitous environmental cue. We highlight recent developments implicating the secondary messenger c-di-AMP in cellular adjustment to osmotic stress and the role of osmotic forces in the life of bacteria-assembled in biofilms.

Interpretation of spectroscopic data using molecular simulations for the secondary active transporter BetP

Mechanistic understanding of dynamic membrane proteins such as transporters, receptors, and channels requires accurate depictions of conformational ensembles, and the manner in which they interchange as a function of environmental factors including substrates, lipids, and inhibitors. Spectroscopic techniques such as electron spin resonance (ESR) pulsed electron-electron double resonance (PELDOR), also known as double electron-electron resonance (DEER), provide a complement to atomistic structures obtained from x-ray crystallography or cryo-EM, since spectroscopic data reflect an ensemble and can be measured in more native solvents, unperturbed by a crystal lattice. However, attempts to interpret DEER data are frequently stymied by discrepancies with the structural data, which may arise due to differences in conditions, the dynamics of the protein, or the flexibility of the attached paramagnetic spin labels. Recently, molecular simulation techniques such as EBMetaD have been developed that create a conformational ensemble matching an experimental distance distribution while applying the minimal possible bias. Moreover, it has been proposed that the work required during an EBMetaD simulation to match an experimentally determined distribution could be used as a metric with which to assign conformational states to a given measurement. Here, we demonstrate the application of this concept for a sodium-coupled transport protein, BetP. Because the probe, protein, and lipid bilayer are all represented in atomic detail, the different contributions to the work, such as the extent of protein backbone movements, can be separated. This work therefore illustrates how ranking simulations based on EBMetaD can help to bridge the gap between structural and biophysical data and thereby enhance our understanding of membrane protein conformational mechanisms.

Caging and Photoactivation in Single-Molecule Förster Resonance Energy Transfer Experiments

Caged organic fluorophores are established tools for localization-based super-resolution imaging. Their use relies on reversible deactivation of standard organic fluorophores by chemical reduction or commercially available caged dyes with ON switching of the fluorescent signal by ultraviolet (UV) light. Here, we establish caging of cyanine fluorophores and caged rhodamine dyes, i.e., chemical deactivation of fluorescence, for single-molecule Förster resonance energy transfer (smFRET) experiments with freely diffusing molecules. They allow temporal separation and sorting of multiple intramolecular donor–acceptor pairs during solution-based smFRET. We use this “caged FRET” methodology for the study of complex biochemical species such as multisubunit proteins or nucleic acids containing more than two fluorescent labels. Proof-of-principle experiments and a characterization of the uncaging process in the confocal volume are presented. These reveal that chemical caging and UV reactivation allow temporal uncoupling of convoluted fluorescence signals from, e.g., multiple spectrally similar donor or acceptor molecules on nucleic acids. We also use caging without UV reactivation to remove unwanted overlabeled species in experiments with the homotrimeric membrane transporter BetP. We finally outline further possible applications of the caged FRET methodology, such as the study of weak biochemical interactions, which are otherwise impossible with diffusion-based smFRET techniques because of the required low concentrations of fluorescently labeled biomolecules.

A general method for determining secondary active transporter substrate stoichiometry

The number of ions required to drive substrate transport through a secondary active transporter determines the protein’s ability to create a substrate gradient, a feature essential to its physiological function, and places fundamental constraints on the transporter’s mechanism. Stoichiometry is known for a wide array of mammalian transporters, but, due to a lack of readily available tools, not for most of the prokaryotic transporters for which high-resolution structures are available. Here, we describe a general method for using radiolabeled substrate flux assays to determine coupling stoichiometries of electrogenic secondary active transporters reconstituted in proteoliposomes by measuring transporter equilibrium potentials. We demonstrate the utility of this method by determining the coupling stoichiometry of VcINDY, a bacterial Na⁺-coupled succinate transporter, and further validate it by confirming the coupling stoichiometry of vSGLT, a bacterial sugar transporter. This robust thermodynamic method should be especially useful in probing the mechanisms of transporters with available structures.

DOI:http://dx.doi.org/10.7554/eLife.21016.001

Regulatory role of charged clusters in the N-terminal domain of BetP from Corynebacterium glutamicum

The trimeric transporter BetP counteracts hyperosmotic stress by a fast increase in transport rate in order to accumulate the compatible solute betaine. The positively charged α-helical C-terminal domain acts as an osmosensor perceiving the increase in the internal potassium (K+) concentration. A second, still unidentified stimulus originates from stress-induced changes in the physical state of the membrane and depends on the amount of negatively charged lipids. BetP possesses a 60-amino acid (aa)-long negatively charged N-terminal domain, which is predicted to adopt a partly helical fold affecting osmoregulation by an unknown mechanism. It is assumed that the C-terminal domain, the N-terminal domain, and negatively charged lipids interact during stress sensing and regulation. Here, we have investigated the regulatory role of negatively charged clusters in the N-terminal domain. We identified one cluster, Glu24Glu25, to be crucial for osmoregulation. Cross-linking studies revealed an interaction between the C- and N-terminal domains of adjacent protomers modulating transport activation. A regulatory partner-switching mechanism emerges in which the C-terminal domain changes its interaction with the N-terminal domain of its own promoter and negatively charged lipids to an interaction with the N-terminal domain of an adjacent protomer and lipids bound to the central cavity of the BetP trimer.

Contributions of Coulombic and Hofmeister Effects to the Osmotic Activation of Escherichia coli Transporter ProP

Osmosensing transporters mediate osmolyte accumulation to forestall cellular dehydration as the extracellular osmolality increases. ProP is a bacterial osmolyte-H(+) symporter, a major facilitator superfamily member, and a paradigm for osmosensing. ProP activity is a sigmoid function of the osmolality. It is determined by the osmolality, not the magnitude or direction of the osmotic shift, in cells and salt-loaded proteoliposomes. The activation threshold varies directly with the proportion of anionic phospholipid in cells and proteoliposomes. The osmosensory mechanism was probed by varying the salt composition and concentration outside and inside proteoliposomes. Data analysis was based on the hypothesis that the fraction of maximal transporter activity at a particular luminal salt concentration reflects the proportion of ProP molecules in an active conformation. ProP attained the same activity at the same osmolality when diverse, membrane-impermeant salts were added to the external medium. Contributions of Coulombic and/or Hofmeister salt effects to ProP activation were examined by varying the luminal salt cation (K(+) and Na(+)) and anion (chloride, phosphate, and sulfate) composition and then systematically increasing the luminal salt concentration by increasing the external osmolality. ProP activity increased with the sixth power of the univalent cation concentration, independent of the type of anion. This indicates that salt activation of ProP is a Coulombic, cation effect resulting from salt cation accumulation and not site-specific cation binding. Possible origins of this Coulombic effect include folding or assembly of anionic cytoplasmic ProP domains, an increase in local membrane surface charge density, and/or the juxtaposition of anionic protein and membrane surfaces during activation.

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.

Osmotic Stress

Escherichia coli and Salmonella encounter osmotic pressure variations in natural environments that include host tissues, food, soil, and water. Osmotic stress causes water to flow into or out of cells, changing their structure, physics, and chemistry in ways that perturb cell functions. E. coli and Salmonella limit osmotically induced water fluxes by accumulating and releasing electrolytes and small organic solutes, some denoted compatible solutes because they accumulate to high levels without disturbing cell functions. Osmotic upshifts inhibit membrane-based energy transduction and macromolecule synthesis while activating existing osmoregulatory systems and specifically inducing osmoregulatory genes. The osmoregulatory response depends on the availability of osmoprotectants (exogenous organic compounds that can be taken up to become compatible solutes). Without osmoprotectants, K+ accumulates with counterion glutamate, and compatible solute trehalose is synthesized. Available osmoprotectants are taken up via transporters ProP, ProU, BetT, and BetU. The resulting compatible solute accumulation attenuates the K+ glutamate response and more effectively restores cell hydration and growth. Osmotic downshifts abruptly increase turgor pressure and strain the cytoplasmic membrane. Mechanosensitive channels like MscS and MscL open to allow nonspecific solute efflux and forestall cell lysis. Research frontiers include (i) the osmoadaptive remodeling of cell structure, (ii) the mechanisms by which osmotic stress alters gene expression, (iii) the mechanisms by which transporters and channels detect and respond to osmotic pressure changes, (iv) the coordination of osmoregulatory programs and selection of available osmoprotectants, and (v) the roles played by osmoregulatory mechanisms as E. coli and Salmonella survive or thrive in their natural environments.

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.

Dynamics of an interactive network composed of a bacterial two-component system, a transporter and K+ as mediator

KdpD and KdpE form a histidine kinase/response regulator system that senses K⁺ limitation and induces the kdpFABC operon, which encodes a high-affinity K⁺ uptake complex. To define the primary stimulus perceived by KdpD we focused in this study on the dynamics of the Kdp response. Escherichia coli cells were subjected to severe K⁺ limitation, and all relevant parameters of the Kdp response, i.e., levels of kdpFABC transcripts and KdpFABC proteins, as well as extra- and intracellular K⁺ concentrations, were quantitatively analysed over time (0 to 180 min). Unexpectedly, induction of kdpFABC was found to follow a non-monotonic time-course. To interpret this unusual behaviour, a mathematical model that adequately captures the dynamics of the Kdp system was established and used for simulations. We found a strong correlation between KdpD/KdpE activation and the intracellular K⁺ concentration, which is influenced by the uptake of K⁺ via the KdpFABC complex. Based on these results a model is proposed in which KdpD/KdpE phosphorylation is inversely correlated with the intracellular K⁺ concentration. To corroborate this hypothesis an isogenic mutant that produces a defective KdpFABC complex, and the trans-complemented mutant that expresses the KtrAB high-affinity K⁺ uptake system of Vibrio alginolyticus were quantitatively analysed. Experimental data and simulations for the mutants consistently support the tight correlation between KdpD/KdpE activation and the intracellular K⁺ concentration. This study presents a striking example of the non-intuitive dynamics of a functional unit comprising signalling proteins and a transporter with K⁺ as mediator.

The SLC6 transporters: perspectives on structure, functions, regulation, and models for transporter dysfunction

The human SLC6 family is composed of approximately 20 structurally related symporters (co-transporters) that use the transmembrane electrochemical gradient to actively import their substrates into cells. Approximately half of the substrates of these transporters are amino acids, with others transporting biogenic amines and/or closely related compounds, such as nutrients and compatible osmolytes. In this short review, five leaders in the field discuss a number of currently important research themes that involve SLC6 transporters, highlighting the integrative role they play across a wide spectrum of different functions. The first essay, by Gary Rudnick, describes the molecular mechanism of their coupled transport which is being progressively better understood based on new crystal structures, functional studies, and modeling. Next, the question of multiple levels of transporter regulation is discussed by Reinhard Krämer, in the context of osmoregulation and stress response by the related bacterial betaine transporter BetP. The role of selected members of the human SLC6 family that function as nutrient amino acid transporters is then reviewed by François Verrey. He discusses how some of these transporters mediate the active uptake of (essential) amino acids into epithelial cells of the gut and the kidney tubule to support systemic amino acid requirements, whereas others are expressed in specific cells to support their specialized metabolism and/or growth. The most extensively studied members of the human SLC6 family are neurotransmitter reuptake transporters, many of which are important drug targets for the treatment of neuropsychiatric disorders. Randy Blakely discusses the role of posttranscriptional modifications of these proteins in regulating transporter subcellular localization and activity state. Finally, Dennis Murphy reviews how natural gene variants and mouse genetic models display consistent behavioral alterations that relate to altered extracellular neurotransmitter levels.

Osmostress-induced cell volume loss delays yeast Hog1 signaling by limiting diffusion processes and by Hog1-specific effects

Signal transmission progresses via a series of transient protein-protein interactions and protein movements, which require diffusion within a cell packed with different molecules. Yeast Hog1, the effector protein kinase of the High Osmolarity Glycerol pathway, translocates transiently from the cytosol to the nucleus during adaptation to high external osmolarity. We followed the dynamics of osmostress-induced cell volume loss and Hog1 nuclear accumulation upon exposure of cells to different NaCl concentrations. While Hog1 nuclear accumulation peaked within five minutes following mild osmotic shock it was delayed up to six-fold under severe stress. The timing of Hog1 nuclear accumulation correlated with the degree of cell volume loss and the cells capacity to recover. Also the nuclear translocation of Msn2, the transcription factor of the general stress response pathway, is delayed upon severe osmotic stress suggesting a general phenomenon. We show by direct measurements that the general diffusion rate of Hog1 in the cytoplasm as well as its rate of nuclear transport are dramatically reduced following severe volume reduction. However, neither Hog1 phosphorylation nor Msn2 nuclear translocation were as much delayed as Hog1 nuclear translocation. Our data provide direct evidence that signaling slows down during cell volume compression, probably as a consequence of molecular crowding. Hence one purpose of osmotic adaptation is to restore optimal diffusion rates for biochemical and cell biological processes. In addition, there may be mechanisms slowing down especially Hog1 nuclear translocation under severe stress in order to prioritize Hog1 cytosolic targets.

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.

Arginine oscillation explains Na+ independence in the substrate/product antiporter CaiT

Most secondary-active transporters transport their substrates using an electrochemical ion gradient. In contrast, the carnitine transporter (CaiT) is an ion-independent, l-carnitine/γ-butyrobetaine antiporter belonging to the betaine/carnitine/choline transporter family of secondary transporters. Recently determined crystal structures of CaiT from Escherichia coli and Proteus mirabilis revealed an inverted five-transmembrane-helix repeat similar to that in the amino acid/Na(+) symporter LeuT. The ion independence of CaiT makes it unique in this family. Here we show that mutations of arginine 262 (R262) make CaiT Na(+)-dependent. The transport activity of R262 mutants increased by 30-40% in the presence of a membrane potential, indicating substrate/Na(+) cotransport. Structural and biochemical characterization revealed that R262 plays a crucial role in substrate binding by stabilizing the partly unwound TM1' helix. Modeling CaiT from P. mirabilis in the outward-open and closed states on the corresponding structures of the related symporter BetP reveals alternating orientations of the buried R262 sidechain, which mimic sodium binding and unbinding in the Na(+)-coupled substrate symporters. We propose that a similar mechanism is operative in other Na(+)/H(+)-independent transporters, in which a positively charged amino acid replaces the cotransported cation. The oscillation of the R262 sidechain in CaiT indicates how a positive charge triggers the change between outward-open and inward-open conformations as a unifying critical step in LeuT-type transporters.

Physicochemical factors controlling the activity and energy coupling of an ionic strength-gated ATP-binding cassette (ABC) transporter

Cells control their volume through the accumulation of compatible solutes. The bacterial ATP-binding cassette transporter OpuA couples compatible solute uptake to ATP hydrolysis. Here, we study the gating mechanism and energy coupling of OpuA reconstituted in lipid nanodiscs. We show that anionic lipids are essential both for the gating and the energy coupling. The tight coupling between substrate binding on extracellular domains and ATP hydrolysis by cytoplasmic nucleotide-binding domains allows the study of transmembrane signaling in nanodiscs. From the tight coupling between processes at opposite sides of the membrane, we infer that the ATPase activity of OpuA in nanodiscs reflects solute translocation. Intriguingly, the substrate-dependent, ionic strength-gated ATPase activity of OpuA in nanodiscs is at least an order of magnitude higher than in lipid vesicles (i.e. with identical membrane lipid composition, ionic strength, and nucleotide and substrate concentrations). Even with the chemical components the same, the lateral pressure (profile) of the nanodiscs will differ from that of the vesicles. We thus propose that membrane tension limits translocation in vesicular systems. Increased macromolecular crowding does not activate OpuA but acts synergistically with ionic strength, presumably by favoring gating interactions of like-charged surfaces via excluded volume effects.

Amino acid secondary transporters: toward a common transport mechanism

Solute carriers (SLC) that transport amino acids are key players in health and diseases in humans. Their prokaryotic relatives are often involved in essential physiological processes in microorganisms, e.g. in homeostasis and acidic/osmotic stress response. High-resolution X-ray structures of the sequence-unrelated amino acid transporters unraveled a striking structural similarity between carriers, which were formerly assigned to different families. The highly conserved fold is characterized by two inverted structural repeats of five transmembrane helices each and indicates common mechanistic transport concepts if not an evolutionary link among a large number of amino acid transporters. Therefore, these transporters are classified now into the structural amino acid-polyamine-organocation superfamily (APCS). The APCS includes among others the mammalian SLC6 transporters and the heterodimeric SLC7/SLC3 transporters. However, it has to be noted that the APCS is not limited entirely to amino acid transporters but contains also transporters for, e.g. amino acid derivatives and sugars. For instance, the betaine-choline-carnitine transporter family of bacterial activity-regulated Na(+)- and H(+)-coupled symporters for glycine betaine and choline is also part of this second largest structural superfamily. The APCS fold provides different possibilities to transport the same amino acid. Arginine can be transported by an H(+)-coupled symport or by antiport mechanism in exchange against agmatine for example. The convergence of the mechanistic concept of transport under comparable physiological conditions allows speculating if structurally unexplored amino acid transporters, e.g. the members of the SLC36 and SLC38 family, belong to the APCS, too. In the kidney, which is an organ that depends critically on the regulated amino acid transport, these different SLC transporters have to work together to account for proper function. Here, we will summarize the basic concepts of Na(+)- and H(+)-coupled amino acid symport and amino acid-product antiport in the light of the respective physiological requirements.

Impacts of the osmolality and the lumenal ionic strength on osmosensory transporter ProP in proteoliposomes

H(+) symporter ProP serves as a paradigm for the study of osmosensing. ProP attains the same activity at the same osmolality when the medium outside cells or proteoliposomes is supplemented with diverse, membrane-impermeant solutes. The osmosensory mechanism of ProP has been probed by varying the solvent within membrane vesicles and proteoliposomes. ProP activation was not ion specific, did not require K(+), and could be elicited by large, uncharged solutes polyethylene glycols (PEGS). We hypothesized that ProP is an ionic strength sensor and lumenal macromolecules activate ProP by altering ion activities. The attainable range of lumenal ionic strength was expanded by lowering the phosphate concentration within proteoliposomes. ProP activity at high osmolality, but not the osmolality, yielding half-maximal activity (Π(1/2)/RT), decreased with the lumenal phosphate concentration. This was attributed to acidification of the proteoliposome lumen due to H(+)-proline symport. The ionic strength yielding half-maximal ProP activity was more anion-dependent than Π(1/2)/RT for proteoliposomes loaded with citrate, sulfate, phosphate, chloride, or iodide. The anion effects followed the Hofmeister series. Lumenal bovine serum albumin (BSA) lowered the lumenal ionic strength at which ProP became active. Osmolality measurements documented the non-idealities of solutions including potassium phosphate and other solutes. The impacts of PEGS and BSA on ion activities did not account for their impacts on ProP activity. The effects of the tested solutes on ProP appear to be non-coulombic in nature. They may arise from effects of preferential interactions and macromolecular crowding on the membrane or on ProP.

Fast, multiphase volume adaptation to hyperosmotic shock by Escherichia coli

All living cells employ an array of different mechanisms to help them survive changes in extra cellular osmotic pressure. The difference in the concentration of chemicals in a bacterium's cytoplasm and the external environment generates an osmotic pressure that inflates the cell. It is thought that the bacterium Escherichia coli use a number of interconnected systems to adapt to changes in external pressure, allowing them to maintain turgor and live in surroundings that range more than two-hundred-fold in external osmolality. Here, we use fluorescence imaging to make the first measurements of cell volume changes over time during hyperosmotic shock and subsequent adaptation on a single cell level in vivo with a time resolution on the order of seconds. We directly observe two previously unseen phases of the cytoplasmic water efflux upon hyperosmotic shock. Furthermore, we monitor cell volume changes during the post-shock recovery and observe a two-phase response that depends on the shock magnitude. The initial phase of recovery is fast, on the order of 15–20 min and shows little cell-to-cell variation. For large sucrose shocks, a secondary phase that lasts several hours adds to the recovery. We find that cells are able to recover fully from shocks as high as 1 Osmol/kg using existing systems, but that for larger shocks, protein synthesis is required for full recovery.

Bacterial osmoregulation: a paradigm for the study of cellular homeostasis

To thrive, cells must control their own physical and chemical properties. This process is known as cellular homeostasis. The dilute solutions traditionally favored by experimenters do not simulate the cytoplasm, where macromolecular crowding and preferential interactions among constituents may dominate critical processes. Solutions that do simulate cytoplasmic conditions are now being characterized. Corresponding cytoplasmic properties can be varied systematically by imposing osmotic stress. This osmotic stress approach is revealing how cytoplasmic properties modulate protein folding and protein?nucleic acid interactions. Results suggest that cytoplasmic homeostasis may require adjustments to multiple, interwoven cytoplasmic properties. Osmosensory transporters with diverse structures and bioenergetic mechanisms activate in response to osmotic stress as other proteins inactivate. These transporters are serving as paradigms for the study of in vivo protein-solvent interactions. Experimenters have proposed three different osmosensory mechanisms. Distinct mechanisms may exist, or these proposals may reflect different perceptions of a single, unifying mechanism.

Cystathionine β-synthase (CBS) domains 1 and 2 fulfill different roles in ionic strength sensing of the ATP-binding cassette (ABC) transporter OpuA

The cystathionine β-synthase module of OpuA in conjunction with an anionic membrane surface acts as a sensor of internal ionic strength, which allows the protein to respond to osmotic stress. We now show by chemical modification and cross-linking studies that CBS2-CBS2 interface residues are critical for transport activity and/or ionic regulation of transport, whereas CBS1 serves no functional role. We establish that Cys residues in CBS1, CBS2, and the nucleotide-binding domain are more accessible for cross-linking at high than low ionic strength, indicating that these domains undergo conformational changes when transiting between the active and inactive state. Structural analyses suggest that the cystathionine β-synthase module is largely unstructured. Moreover, we could substitute CBS1 by a linker and preserve ionic regulation of transport. These data suggest that CBS1 serves as a linker and the structured CBS2-CBS2 interface forms a hinge point for ionic strength-dependent rearrangements that are transmitted to the nucleotide-binding domain and thereby affect translocation activity.

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.

Osmotic stress response in C. glutamicum: impact of channel- and transporter-mediated potassium accumulation

Potassium accumulation is an essential aspect of bacterial response to diverse stress situations; consequently its uptake plays a pivotal role. Here, we show that the Gram-positive soil bacterium Corynebacterium glutamicum which is employed for the large-scale industrial production of amino acids requires potassium under conditions of ionic and non-ionic osmotic stress. Besides the accumulation of high concentrations of potassium contributing significantly to the osmotic potential of the cytoplasm, we demonstrate that glutamate is not the counter ion for potassium under these conditions. Interestingly, potassium is required for the activation of osmotic stress-dependent expression of the genes betP and proP. The Kup-type potassium transport system which is present in C. glutamicum in addition to the potassium channel CglK does not contribute to potassium uptake at conditions of hyperosmotic stress. Furthermore, we established a secondary carrier of the KtrAB type from C. jeikeium in C. glutamicum thus providing an experimental comparison of channel- and carrier-mediated potassium uptake under osmotic stress. While at low potassium availability, the presence of the KtrAB transporter improves both potassium accumulation and growth of C. glutamicum upon osmotic stress, at proper potassium supply, the channel CglK is sufficient.

Salt adaptation in Acinetobacter baylyi: identification and characterization of a secondary glycine betaine transporter

Members of the genus Acinetobacter are well known for their metabolic versatility that allows them to adapt to different ecological niches. Here, we have addressed how the model strain Acinetobacter baylyi copes with different salinities and low water activities. A. baylyi tolerates up to 900 mM sodium salts and even higher concentrations of potassium chloride. Growth at high salinities was better in complex than in mineral medium and addition of glycine betaine stimulated growth at high salinities in mineral medium. Cells grown at high salinities took up glycine betaine from the medium. Uptake of glycine betaine was energy dependent and dependent on a salinity gradient across the membrane. Inspection of the genome sequence revealed two potential candidates for glycine betaine transport, both encoding potential secondary transporters, one of the major facilitator superfamily (MFS) class (ACIAD2280) and one of the betaine/choline/carnitine transporter (BCCT) family (ACIAD3460). The latter is essential for glycine betaine transport in A. baylyi. The broad distribution of ACIAD3460 homologues indicates the essential role of secondary transporters in the adaptation of Acinetobacter species to osmotic stress.

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.

Use of liposomes to study cellular osmosensors

When cells are exposed to changes in the osmotic pressure of the external medium, they respond with mechanisms of osmoregulation. An increase of the extracellular osmolality leads to the accumulation of internal solutes by biosynthesis or uptake. Particular bacterial transporters act as osmosensors and respond to increased osmotic pressure by catalyzing uptake of compatible solutes. The functions of osmosensing, osmoregulation , and solute transport of these transporters can be analyzed in molecular detail after solubilization, isolation, and reconstitution into phospholipid vesicles. Using this approach, intrinsic functions of osmosensing transporters are studied in a defined hydrophilic (access to both sides of the membrane) and hydrophobic surrounding (phospholipid membrane), and free of putative interacting cofactors and regulatory proteins.

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.

The role of biomacromolecular crowding, ionic strength, and physicochemical gradients in the complexities of life's emergence

We have developed a general scenario of prebiotic physicochemical evolution during the Earth's Hadean eon and reviewed the relevant literature. We suggest that prebiotic chemical evolution started in microspaces with membranous walls, where external temperature and osmotic gradients were coupled to free-energy gradients of potential chemical reactions. The key feature of this scenario is the onset of an emergent evolutionary transition within the microspaces that is described by the model of complex vectorial chemistry. This transition occurs at average macromolecular crowding of 20 to 30% of the cell volume, when the ranges of action of stabilizing colloidal forces (screened electrostatic forces, hydration, and excluded volume forces) become commensurate. Under these conditions, the macromolecules divide the interior of microspaces into dynamically crowded macromolecular regions and topologically complementary electrolyte pools. Small ions and ionic metabolites are transported vectorially between the electrolyte pools and through the (semiconducting) electrolyte pathways of the crowded macromolecular regions from their high electrochemical potential (where they are biochemically produced) to their lower electrochemical potential (where they are consumed). We suggest a sequence of tentative transitions between major evolutionary periods during the Hadean eon as follows: (i) the early water world, (ii) the appearance of land masses, (iii) the pre-RNA world, (iv) the onset of complex vectorial chemistry, and (v) the RNA world and evolution toward Darwinian thresholds. We stress the importance of high ionic strength of the Hadean ocean (short Debye's lengths) and screened electrostatic interactions that enabled the onset of the vectorial structure of the cytoplasm and the possibility of life's emergence.

Engineering of ion sensing by the cystathionine beta-synthase module of the ABC transporter OpuA

We have previously shown that the C-terminal cystathionine beta-synthase (CBS) domains of the nucleotide-binding domains of the ABC transporter OpuA, in conjunction with an anionic membrane surface function, act as sensor of internal ionic strength (I(in)). Here, we show that a surface-exposed cationic region in the CBS module domain is critical for ion sensing. The consecutive substitution of up to five cationic residues led to a gradual decrease of the ionic strength dependence of transport. In fact, a 5-fold mutant was essentially independent of salt in the range from 0 to 250 mm KCl (or NaCl), supplemented to medium of 30 mm potassium phosphate. Importantly, the threshold temperature for transport was lowered by 5-7 degrees C and the temperature coefficient Q(10) was lowered from 8 to approximately 1.5 in the 5-fold mutant, indicating that large conformational changes are accompanying the CBS-mediated regulation of transport. Furthermore, by replacing the anionic C-terminal tail residues that extend the CBS module with histidines, the transport of OpuA became pH-dependent, presumably by additional charge interactions of the histidine residues with the membrane. The pH dependence was not observed at high ionic strength. Altogether the analyses of the CBS mutants support the notion that the osmotic regulation of OpuA involves a simple biophysical switching mechanism, in which nonspecific electrostatic interactions of a protein module with the membrane are sufficient to lock the transporter in the inactive state.

Potassium transport in corynebacterium glutamicum is facilitated by the putative channel protein CglK, which is essential for pH homeostasis and growth at acidic pH

We studied the requirement for potassium and for potassium transport activity for the biotechnologically important bacterium Corynebacterium glutamicum, which is used for large-scale production of amino acids. Different from many other bacteria, at alkaline or neutral pH, C. glutamicum is able to grow without the addition of potassium, resulting in very low cytoplasmic potassium concentrations. In contrast, at acidic pH, the ability for growth was found to depend on the presence of K+. For the first time, we provide experimental evidence that a potential potassium channel (CglK) acts as the major potassium uptake system in a bacterium and proved CglK's function directly in its natural membrane environment. A full-length CglK protein and a separate soluble protein harboring the RCK domain can be translated from the cglK gene, and both are essential for full CglK functionality. As a reason for potassium-dependent growth limitation at acidic pH, we identified the impaired capacity for internal pH homeostasis, which depends on the availability and internal accumulation of potassium. Potassium uptake via CglK was found to be relevant for major physiological processes, like the activity of the respiratory chain, and to be crucial for maintenance of the internal pH, as well as for the adjustment of the membrane potential in C. glutamicum.

Regulatory properties and interaction of the C- and N-terminal domains of BetP, an osmoregulated betaine transporter from Corynebacterium glutamicum

The glycine betaine carrier BetP from Corynebacterium glutamicum responds to changes in external osmolality by regulation of its transport activity, and the C-terminal domain was previously identified to be involved in this process. Here we investigate the structural requirements of the C-terminal domain for osmoregulation as well as interacting domains that are relevant for intramolecular signal transduction in response to osmotic stress. For this purpose, we applied a proline scanning approach and amino acid replacements other than proline in selected positions. To analyze the impact of the surrounding membrane, BetP mutants were studied in both C. glutamicum and Escherichia coli, which strongly differ in their phospholipid composition. A region of approximately 25 amino acid residues within the C-terminal domain with a high propensity for alpha-helical structure was found to be essential in terms of its conformational properties for osmodependent regulation. The size of this region was larger in E. coli membranes than in the highly negatively charged C. glutamicum membranes. As a novel aspect of BetP regulation, interaction of the C-terminal domain with one of the cytoplasmic loops as well as with the N-terminal domain was shown to be involved in osmosensing and/or osmoregulation. These results support a functional model of BetP activation that involves the C-terminal domain shifting from interaction with the membrane to interaction with intramolecular domains in response to osmotic stress.

Cardiolipin controls the osmotic stress response and the subcellular location of transporter ProP in Escherichia coli

The phospholipid composition of the membrane and transporter structure control the subcellular location and function of osmosensory transporter ProP in Escherichia coli. Growth in media of increasing osmolality increases, and entry to stationary phase decreases, the proportion of phosphatidate in anionic lipids (phosphatidylglycerol (PG) plus cardiolipin (CL)). Both treatments increase the CL:PG ratio. Transporters ProP and LacY are concentrated with CL (and not PG) near cell poles and septa. The polar concentration of ProP is CL-dependent. Here we show that the polar concentration of LacY is CL-independent. The osmotic activation threshold of ProP was directly proportional to the CL content of wild type bacteria, the PG content of CL-deficient bacteria, and the anionic lipid content of cells and proteoliposomes. CL was effective at a lower concentration in cells than in proteoliposomes, and at a much lower concentration than PG in either system. Thus, in wild type bacteria, osmotic induction of CL synthesis and concentration of ProP with CL at the cell poles adjust the osmotic activation threshold of ProP to match ambient conditions. ProP proteins linked by homodimeric, C-terminal coiled-coils are known to activate at lower osmolalities than those without such structures and coiled-coil disrupting mutations raise the osmotic activation threshold. Here we show that these mutations also prevent polar concentration of ProP. Stabilization of the C-terminal coiled-coil by covalent cross-linking of introduced Cys reverses the impact of increasing CL on the osmotic activation of ProP. Association of ProP C termini with the CL-rich membrane at cell poles may raise the osmotic activation threshold by blocking coiled-coil formation. Mutations that block coiled-coil formation may also block association of the C termini with the CL-rich membrane.

Sodium regulation of GAF domain function

Cyclic nucleotide PDEs (phosphodiesterases) regulate cellular levels of cAMP and cGMP by controlling the rate of degradation. Several mammalian PDE isoforms possess N-terminal GAF (found in cGMP PDEs, Anabaena adenylate cyclases and Escherichia coli FhlA; where FhlA is formate hydrogen lyase transcriptional activator) domains that bind cyclic nucleotides. Similarly, the CyaB1 and CyaB2 ACs (adenylate cyclases) of the cyanobacterium Anabaena PCC 7120 bind cAMP through one (CyaB1) or two (CyaB2) N-terminal GAF domains and mediate autoregulation of the AC domain. Sodium inhibits the activity of CyaB1, CyaB2 and mammalian PDE2A in vitro through modulation of GAF domain function. Furthermore, genetic ablation of cyaB1 and cyaB2 gives rise to Anabaena strains defective in homoeostasis at limiting sodium. Sodium regulation of GAF domain function has therefore been conserved since the eukaryotic/prokaryotic divergence. The GAF domain is the first identified protein domain to directly sense and signal changes in environmental sodium.

Role of a sodium-dependent symporter homologue in the thermosensitivity of beta-lactam antibiotic resistance and cell wall composition in Staphylococcus aureus

Expression of high-level beta-lactam resistance is known to be thermosensitive in many methicillin-resistant Staphylococcus aureus (MRSA) strains, including strain COL, in which the high methicillin MIC for cultures grown at 37 degrees C (800 microg/ml) was reduced to 12 microg/ml at 42 degrees C. COL grew faster at 42 degrees C than at 37 degrees C and at the higher temperature produced cell walls of abnormal composition: there was an over-representation of the monomeric muropeptide without the oligoglycine chain and an increase in the representation of multimers that contained this wall component as the donor molecule. Screening of a Tn551 insertional library for mutants, in which the high and homogenous beta-lactam antibiotic resistance of strain COL is retained at 42 degrees C, identified mutant C245, which expressed high-level methicillin resistance and produced a cell wall of normal composition independent of the temperature. The Tn551 inactivated gene was found, by homology search, to encode for a sodium-dependent symporter, homologues of which are ubiquitous in both prokaryotic and eukaryotic genomes. Inactivation of this putative symporter in several heteroresistant clinical MRSA isolates caused striking increases in the level of their beta-lactam resistance.

Osmosensing Properties of the Histidine Protein Kinase MtrB from

The MtrB-MtrA two component system of Corynebacterium glutamicum was recently shown to be in involved in the osmostress response as well as cell wall metabolism. To address the question of whether the histidine protein kinase MtrB is an osmosensor, the kinase was purified and reconstituted into liposomes in a functionally active form. The activity regulation was investigated by varying systematically physicochemical parameters, which are putative stimuli that could be used by the bacterial cell to detect osmotic conditions. Membrane shrinkage was ruled out as a stimulus for activation of MtrB. Instead, MtrB was shown to be activated upon the addition of various chemical compounds, like sugars, amino acids, and polyethylene glycols. Because of the different chemical nature of the solutes, it seems unlikely that they bind to a specific binding site. Instead, they are proposed to act via a change of the hydration state of the protein shifting MtrB into the active state. For MtrB activation it was essential that these solutes were added at the same side as the cytoplasmic domains of the kinase were located, indicating that hypertonicity is sensed by MtrB via cytoplasmatically located protein domains. This was confirmed by the analysis of two MtrB mutants in which either the large periplasmic loop or the HAMP domain was deleted. These mutants were regulated similar to wild type MtrB. Thus, we postulate that MtrB belongs to a class of histidine protein kinases that sense environmental changes at cytoplasmatic protein domains independently of the periplasmic loop and the cytoplasmic HAMP domain.

Osmolality, temperature, and membrane lipid composition modulate the activity of betaine transporter BetP in Corynebacterium glutamicum

The gram-positive soil bacterium Corynebacterium glutamicum, a major amino acid-producing microorganism in biotechnology, is equipped with several osmoregulated uptake systems for compatible solutes, which is relevant for the physiological response to osmotic stress. The most significant carrier, BetP, is instantly activated in response to an increasing cytoplasmic K(+) concentration. Importantly, it is also activated by chill stress independent of osmotic stress. We show that the activation of BetP by both osmotic stress and chill stress is altered in C. glutamicum cells grown at and adapted to low temperatures. BetP from cold-adapted cells is less sensitive to osmotic stress. In order to become susceptible for chill activation, cold-adapted cells in addition needed a certain amount of osmotic stimulation, indicating that there is cross talk of these two types of stimuli at the level of BetP activity. We further correlated the change in BetP regulation properties in cells grown at different temperatures to changes in the lipid composition of the plasma membrane. For this purpose, the glycerophospholipidome of C. glutamicum grown at different temperatures was analyzed by mass spectrometry using quantitative multiple precursor ion scanning. The molecular composition of glycerophospholipids was strongly affected by the growth temperature. The modulating influence of membrane lipid composition on BetP function was further corroborated by studying the influence of artificial modulation of membrane dynamics by local anesthetics and the lack of a possible influence of internally accumulated betaine on BetP activity.

Structure and Function of Transmembrane Segment XII in Osmosensor and Osmoprotectant Transporter ProP ofEscherichia coli

Escherichia coli transporter ProP acts as both an osmosensor and an osmoregulator. As medium osmolality rises, ProP is activated and mediates H+-coupled uptake of osmolytes like proline. A homology model of ProP with 12-transmembrane (TM) helices and cytoplasmic termini was created, and the protein's topology was substantiated experimentally. Residues 468-497, at the end of the C-terminal domain and linked to TM XII, form an intermolecular, homodimeric alpha-helical coiled-coil that tunes the transporter's response to osmolality. We aim to further define the structure and function of ProP residues Q415-E440, predicted to include TM XII. Each residue was replaced with cysteine (Cys) in a histidine-tagged, Cys-less ProP variant (ProP*). Cys at positions 415-418 and 438-440 were most reactive with Oregon Green Maleimide (OGM), suggesting that residues 419 through 437 are in the membrane. Except for V429-I433, reactivity of those Cys varied with helical periodicity. Cys predicted to face the interior of ProP were more reactive than Cys predicted to face the lipid. The former may be exposed to hydrated polar residues in the protein interior, particularly on the periplasmic side. Intermolecular cross-links formed when ProP* variants with Cys at positions 419, 420, 422, and 439 were treated with DTME. Thus TM XII can participate, along its entire length, in the dimer interface of ProP. Cys substitution E440C rendered ProP* inactive. All other variants retained more than 30% of the proline uptake activity of ProP* at high osmolality. Most variants with Cys substitutions in the periplasmic half of TM XII activated at lower osmolalities than ProP*. Variants with Cys substitutions on one face of the cytoplasmic half of TM XII required a higher osmolality to activate. They included elements of a GXXXG motif that are predicted to form the interface of TM XII with TM VII. These studies define the position of ProP TM XII within the membrane, further support the predicted structure of ProP, reveal the dimerization interface, and show that the structure of TM XII influences the osmolality at which ProP activates.

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.