The turgor sensor KdpD of Escherichia coli is a homodimer

Structural basis of KdpD histidine kinase binding to the second messenger c-di-AMP

The KdpDE two-component system regulates potassium homeostasis and virulence in various bacterial species. The KdpD histidine kinases (HK) of this system contain a universal stress protein (USP) domain which binds to the second messenger cyclic-di-adenosine monophosphate (c-di-AMP) for regulating transcriptional output from this two-component system in Firmicutes such as Staphylococcus aureus. However, the structural basis of c-di-AMP specificity within the KdpD-USP domain is not well understood. Here, we resolved a 2.3 Å crystal structure of the S. aureus KdpD-USP domain (USPSa) complexed with c-di-AMP. Binding affinity analyses of USPSa mutants targeting the observed USPSa:c-di-AMP structural interface enabled the identification of the sequence residues that are required for c-di-AMP specificity. Based on the conservation of these residues in other Firmicutes, we identified the binding motif, (A/G/C)XSXSX2N(Y/F), which allowed us to predict c-di-AMP binding in other KdpD HKs. Furthermore, we found that the USPSa domain contains structural features distinct from the canonical standalone USPs that bind ATP as a preferred ligand. These features include inward-facing conformations of its β1-α1 and β4-α4 loops, a short α2 helix, the absence of a triphosphate-binding Walker A motif, and a unique dual phospho-ligand binding mode. It is therefore likely that USPSa-like domains in KdpD HKs represent a novel subfamily of the USPs.

Structure and function of the juxtamembrane GAF domain of potassium biosensor KdpD

KdpD/KdpE two-component signaling system regulates expression of a high affinity potassium transporter responsible for potassium homeostasis. The C-terminal module of KdpD consists of a GAF domain linked to a histidine kinase domain. Whereas certain GAF domains act as regulators by binding cyclic nucleotides, the role of the juxtamembrane GAF domain in KdpD is unknown. We report the high-resolution crystal structure of KdpD GAF domain (KdpD^G ) consisting of five α-helices, four β-sheets and two large loops. KdpD^G forms a symmetry-related dimer, wherein parallelly arranged monomers contribute to a four-helix bundle at the dimer-interface, SAXS analysis of KdpD C-terminal module reveals an elongated structure that is a dimer in solution. Substitution of conserved residues with various residues that disrupt the dimer interface produce a range of effects on gene expression demonstrating the importance of the interface in inactive to active transitions during signaling. Comparison of ligand binding site of the classic cyclic nucleotide-binding GAF domains to KdpD^G reveals structural differences arising from naturally occurring substitutions in primary sequence of KdpD^G that modifies the canonical NKFDE sequence motif required for cyclic nucleotide binding. Together these results suggest a structural role for KdpD^G in dimerization and transmission of signal to the kinase domain.

Structural insights into the signal transduction mechanism of the K+-sensing two-component system KdpDE

Two-component systems (TCSs), which consist of a histidine kinase (HK) sensor and a response regulator (RR), are important for bacteria to quickly sense and respond to various environmental signals. HKs and RRs typically function as a cognate pair, interacting only with one another to transduce signaling. Precise signal transduction in a TCS depends on the specific interactions between the receiver domain (RD) of the RR and the dimerization and histidine phosphorylation domain (DHp) of the HK. Here, we determined the complex structure of KdpDE, a TCS consisting of the HK KdpD and the RR KdpE, which is responsible for K⁺ homeostasis. Both the RD and the DNA binding domain (DBD) of KdpE interacted with KdpD. Although the RD of KdpE and the DHp of KdpD contributed to binding specificity, the DBD mediated a distinct interaction with the catalytic ATP-binding (CA) domain of KdpD that was indispensable for KdpDE-mediated signal transduction. Moreover, the DBD-CA interface largely overlapped with that of the DBD-DNA complex, leading to competition between KdpD and its target promoter in a KdpE phosphorylation-dependent manner. In addition, the extended C-terminal tail of the CA domain was critical for stabilizing the interaction with KdpDE and for signal transduction. Together, these data provide a molecular basis for specific KdpD and KdpE interactions that play key roles in efficient signal transduction and transcriptional regulation by this TCS.

Oligomeric states in sodium ion-dependent regulation of cyanobacterial histidine kinase-2

Two-component signal transduction systems (TCSs) consist of sensor histidine kinases and response regulators. TCSs mediate adaptation to environmental changes in bacteria, plants, fungi and protists. Histidine kinase 2 (Hik2) is a sensor histidine kinase found in all known cyanobacteria and as chloroplast sensor kinase in eukaryotic algae and plants. Sodium ions have been shown to inhibit the autophosphorylation activity of Hik2 that precedes phosphoryl transfer to response regulators, but the mechanism of inhibition has not been determined. We report on the mechanism of Hik2 activation and inactivation probed by chemical cross-linking and size exclusion chromatography together with direct visualisation of the kinase using negative-stain transmission electron microscopy of single particles. We show that the functional form of Hik2 is a higher-order oligomer such as a hexamer or octamer. Increased NaCl concentration converts the active hexamer into an inactive tetramer. The action of NaCl appears to be confined to the Hik2 kinase domain.

Non-canonical activation of histidine kinase KdpD by phosphotransferase protein PtsN through interaction with the transmitter domain

The two-component system KdpD/KdpE governs K⁺ homeostasis by controlling synthesis of the high affinity K⁺ transporter KdpFABC. When sensing low environmental K⁺ concentrations, the dimeric kinase KdpD autophosphorylates in trans and transfers the phosphoryl-group to the response regulator KdpE, which subsequently activates kdpFABC transcription. In Escherichia coli, KdpD can also be activated by interaction with the non-phosphorylated form of the accessory protein PtsN. PtsN stimulates KdpD kinase activity thereby increasing phospho-KdpE levels. Here, we analyzed the interplay between KdpD/KdpE and PtsN. PtsN binds specifically to the catalytic DHp domain of KdpD, which is also contacted by KdpE. Accordingly, PtsN and KdpE compete for binding, providing a paradox. Low levels of non-phosphorylated PtsN stimulate, whereas high amounts reduce kdpFABC expression by blocking access of KdpE to KdpD. Ligand fishing experiments provided insight as they revealed ternary complex formation of PtsN/KdpD₂ /KdpE in vivo demonstrating that PtsN and KdpE bind different protomers in the KdpD dimer. PtsN may bind one protomer to stimulate phosphorylation of the second KdpD protomer, which then phosphorylates bound KdpE. Phosphorylation of PtsN prevents its incorporation in ternary complexes. Interaction with the conserved DHp domain enables PtsN to regulate additional kinases such as PhoR.

Identification of ligand specificity determinants in lantibiotic bovicin HJ50 and the receptor BovK, a multitransmembrane histidine kinase

Lantibiotic bovicin HJ50 is produced by Streptococcus bovis HJ50 and acts as the extracellular signal to autoregulate its own biosynthesis through BovK/R two-component system. Bovicin HJ50 shows a linear N-terminal and glubolar C-terminal structure, and the sensor histidine kinase BovK contains eight transmembrane segments lacking any extensive surface-exposed sensory domain. The signal recognition mechanism between bovicin HJ50 and BovK is still unknown. We performed saturated alanine scanning mutagenesis and other amino acid substitutions on bovicin HJ50 using a semi-in vitro biosynthesis. Results of the mutants inducing activities indicated that several charged and hydrophobic amino acids in ring B of bovicin HJ50, as well as two glycines were key residues to recognize BovK. Circular dichroism analyses indicated that both glycines contributed to bovicin HJ50 structural changes in the membrane. Biotin-labeled bovicin HJ50 could interact with the N-terminal sensor of BovK, and several charged residues and a conserved hydrophobic region in the N-terminal portion of BovK sensor domain were important for interacting with the signal bovicin HJ50. By combining the results, we suggested a mechanism of bovicin HJ50 recognizing and activating BovK mainly through electrostatic and hydrophobic interactions.

Expression level of a chimeric kinase governs entry into sporulation in Bacillus subtilis

Upon starvation, Bacillus subtilis cells switch from growth to sporulation. It is believed that the N-terminal sensor domain of the cytoplasmic histidine kinase KinA is responsible for detection of the sporulation-specific signal(s) that appears to be produced only under starvation conditions. Following the sensing of the signal, KinA triggers autophosphorylation of the catalytic histidine residue in the C-terminal domain to transmit the phosphate moiety, via phosphorelay, to the master regulator for sporulation, Spo0A. However, there is no direct evidence to support the function of the sensor domain, because the specific signal(s) has never been found. To investigate the role of the N-terminal sensor domain, we replaced the endogenous three-PAS repeat in the N-terminal domain of KinA with a two-PAS repeat derived from Escherichia coli and examined the function of the resulting chimeric protein. Despite the introduction of a foreign domain, we found that the resulting chimeric protein, in a concentration-dependent manner, triggered sporulation by activating Spo0A through phosphorelay, irrespective of nutrient availability. Further, by using chemical cross-linking, we showed that the chimeric protein exists predominantly as a tetramer, mediated by the N-terminal domain, as was found for KinA. These results suggest that tetramer formation mediated by the N-terminal domain, regardless of the origin of the protein, is important and sufficient for the kinase activity catalyzed by the C-terminal domain. Taken together with our previous observations, we propose that the primary role of the N-terminal domain of KinA is to form a functional tetramer, but not for sensing an unknown signal.

Oligomeric sensor kinase DcuS in the membrane of Escherichia coli and in proteoliposomes: chemical cross-linking and FRET spectroscopy

DcuS is the membrane-integral sensor histidine kinase of the DcuSR two-component system in Escherichia coli that responds to extracellular C(4)-dicarboxylates. The oligomeric state of full-length DcuS was investigated in vitro and in living cells by chemical cross-linking and by fluorescence resonance energy transfer (FRET) spectroscopy. The FRET results were quantified by an improved method using background-free spectra of living cells for determining FRET efficiency (E) and donor fraction {f(D) = (donor)/[(donor) + (acceptor)]}. Functional fusions of cyan fluorescent protein (CFP) and yellow fluorescent protein (YFP) variants of green fluorescent protein to DcuS were used for in vivo FRET measurements. Based on noninteracting membrane proteins and perfectly interacting proteins (a CFP-YFP fusion), the results of FRET of cells coexpressing DcuS-CFP and DcuS-YFP were quantitatively evaluated. In living cells and after reconstitution of purified recombinant DcuS in proteoliposomes, DcuS was found as a dimer or higher oligomer, independent of the presence of an effector. Chemical cross-linking with disuccinimidyl suberate showed tetrameric, in addition to dimeric, DcuS in proteoliposomes and in membranes of bacteria, whereas purified DcuS in nondenaturing detergent was mainly monomeric. The presence and amount of tetrameric DcuS in vivo and in proteoliposomes was not dependent on the concentration of DcuS. Only membrane-embedded DcuS (present in the oligomeric state) is active in (auto)phosphorylation. Overall, the FRET and cross-linking data demonstrate the presence in living cells, in bacterial membranes, and in proteoliposomes of full-length DcuS protein in an oligomeric state, including a tetramer.

The complexity of the 'simple' two-component system KdpD/KdpE in Escherichia coli

The KdpD/KdpE two-component system of Escherichia coli activates the expression of the kdpFABC operon encoding the high-affinity K(+) uptake system KdpFABC in response to K(+) limitation or salt stress. Earlier, it was proposed that the histidine kinase KdpD is a turgor sensor; recent studies suggest that KdpD integrates three chemical stimuli from the cytoplasm. The histidine kinase KdpD contains several structural features and subdomains that are important for stimulus perception, modulation of the kinase to phosphatase ratio, and signaling. The response regulator KdpE receives the phosphoryl group from KdpD and induces kdpFABC transcription. The three-dimensional structure of the receiver domain was resolved, providing insights into the activation mechanism of this transcriptional regulator. Two accessory components, the universal stress protein UspC and the phosphotransferase system component IIA(Ntr), are known to interact with KdpD, allowing the modulation of kdpFABC expression under certain physiological conditions. Here, we will discuss the complexity of a 'simple' two-component system and its interconnectivity with metabolism and the general stress response.

How are signals transduced across the cytoplasmic membrane? Transport proteins as transmitter of information

In order to adapt to ever changing environmental conditions, bacteria sense environmental stimuli, and convert them into signals that are transduced intracellularly. Several mechanisms have evolved by which receptors transmit signals across the cytoplasmic membrane. Stimulus perception may trigger receptor dimerization and/or conformational changes. Another mechanism involves the proteolytic procession of a receptor whereby a diffusible cytoplasmic protein is generated. Finally, there is increasing evidence that transport proteins play an important role in transducing signals across the membrane. Transport proteins either directly translocate signaling molecules into the cytoplasm, or transmit information via conformational changes to their interacting partners such as membrane-integrated or soluble components of signal transduction cascades. Employing transport proteins as sensors and regulators of signal transduction represents a sophisticated way of interconnecting metabolic flux and transcriptional regulation in cells.

Reduction of turgor is not the stimulus for the sensor kinase KdpD of Escherichia coli

Stimulus perception by the KdpD/KdpE two-component system of Escherichia coli is still controversial with respect to the nature of the stimulus that is perceived by the sensor kinase KdpD. Limiting potassium concentrations in the medium or high osmolality leads to KdpD/KdpE signal transduction, resulting in kdpFABC expression. It has been hypothesized that changes in turgor are sensed by KdpD through alterations in the physical state of the cytoplasmic membrane. However, in this study the quantitative determination of expression levels of the kdpFABC operon revealed that the system responds very effectively to K(+)-limiting conditions in the medium but barely and to various degrees to salt and sugar stress. Since the current view of stimulus perception calls for mainly intracellular parameters, which might be sensed by KdpD, we set out to test the cytoplasmic concentrations of ATP, K(+), Na(+), glutamate, proline, glycine, trehalose, putrescine, and spermidine under K(+)-limiting conditions. As a first result, the determination of the cytoplasmic volume, which is a prerequisite for such measurements, revealed that a transient shrinkage of the cytoplasmic volume, which is indicative of a reduction in turgor, occurred only under osmotic upshift but not under K(+)-limiting conditions. Furthermore, the intracellular ATP concentration significantly increased under osmotic upshift, whereas only a slight increase occurred after a potassium downshift. Finally, the cytoplasmic K(+) concentration rose severalfold only after an osmotic upshock. For the first time, these data indicate that stimulus perception by KdpD correlates neither with changes in the cytoplasmic volume nor with changes in the intracellular ATP or K(+) concentration or those of the other solutes tested. In conclusion, we propose that a reduction in turgor cannot be the stimulus for KdpD.

Ligand-induced asymmetry in histidine sensor kinase complex regulates quorum sensing

Bacteria sense their environment using receptors of the histidine sensor kinase family, but how kinase activity is regulated by ligand binding is not well understood. Autoinducer-2 (AI-2), a secreted signaling molecule originally identified in studies of the marine bacterium Vibrio harveyi, regulates quorum-sensing responses and allows communication between different bacterial species. AI-2 signal transduction in V. harveyi requires the integral membrane receptor LuxPQ, comprised of periplasmic binding protein (LuxP) and histidine sensor kinase (LuxQ) subunits. Combined X-ray crystallographic and functional studies show that AI-2 binding causes a major conformational change within LuxP, which in turn stabilizes a quaternary arrangement in which two LuxPQ monomers are asymmetrically associated. We propose that formation of this asymmetric quaternary structure is responsible for repressing the kinase activity of both LuxQ subunits and triggering the transition of V. harveyi into quorum-sensing mode.

Agarose isoelectric focusing can improve resolution of membrane proteins in the two-dimensional electrophoresis of bacterial proteins

2-D separation of bacterial membrane proteins is still difficult despite using high-resolution IPG-IEF/SDS-PAGE. We were searching for alternative methods to avoid typical problems such as precipitation, low solubility, and aggregation of membrane proteins in the 1-D separation with IPG-IEF. Blue native electrophoresis (BNE) and agarose IEF (A-IEF) were tested for their separation capacity and their capability of replacing IPG-IEF in the first dimension. SDS-PAGE was chosen for the second dimension on account of its outstanding resolution. We could confirm that only A-IEF was a useful replacement for the IPG-IEF in the first dimension resulting in 2-D protein distributions with additional membrane protein spots not being found after IPG-IEF/SDS-PAGE. A second interesting result was that the agarose IEF mediates the possibility of separation of membrane proteins in a partially native state in the first dimension. This native A-IEF resulted in drastically changed spot patterns with an acidic shift of nearly all spots and divergent distribution of proteins compared to non-native A-IEF and IPG-IEF. We found out that native and non-native A-IEF are powerful tools to supplement IPG-IEF/SDS-PAGE.

New type of kdp region with a split sensor-kinase kdpD gene located within two divergent kdp operons from the thermoacidophilic bacterium Alicyclobacillus acidocaldarius

The kdp region from the thermoacidophilic bacterium Alicyclobacillus acidocaldarius consists of two divergent operons: kdpZFABCN, which is tenfold induced at low K+ concentrations and encodes the K+-translocating P-type ATPase KdpZFABC as well as KdpN, a novel covalent homo-dimer of the cytoplasmic N-terminal part from sensor kinase KdpD; and secondly, the constitutively expressed kdpHE operon, encoding the remainder of KdpD and the response regulator KdpE.

Detection and analysis of protein–protein interactions in organellar and prokaryotic proteomes by native gel electrophoresis: (Membrane) protein complexes and supercomplexes

It is an essential and challenging task to unravel protein-protein interactions in their actual in vivo context. Native gel systems provide a separation platform allowing the analysis of protein complexes on a rather proteome-wide scale in a single experiment. This review focus on blue-native (BN)-PAGE as the most versatile and successful gel-based approach to separate soluble and membrane protein complexes of intricate protein mixtures derived from all biological sources. BN-PAGE is a charge-shift method with a running pH of 7.5 relying on the gentle binding of anionic CBB dye to all membrane and many soluble protein complexes, leading to separation of protein species essentially according to their size and superior resolution than other fractionation techniques can offer. The closely related colorless-native (CN)-PAGE, whose applicability is restricted to protein species with intrinsic negative net charge, proved to provide an especially mild separation capable of preserving weak protein-protein interactions better than BN-PAGE. The essential conditions determining the success of detecting protein-protein interactions are the sample preparations, e.g. the efficiency/mildness of the detergent solubilization of membrane protein complexes. A broad overview about the achievements of BN- and CN-PAGE studies to elucidate protein-protein interactions in organelles and prokaryotes is presented, e.g. the mitochondrial protein import machinery and oxidative phosphorylation supercomplexes. In many cases, solubilization with digitonin was demonstrated to facilitate an efficient and particularly gentle extraction of membrane protein complexes prone to dissociation by treatment with other detergents. In general, analyses of protein interactomes should be carried out by both BN- and CN-PAGE.

Dimeric H+-ATP synthase in the chloroplast of Chlamydomonas reinhardtii

H+-ATP synthase is the dominant ATP production site in mitochondria and chloroplasts. So far, dimerization of ATP synthase has been observed only in mitochondria by biochemical and electron microscopic investigations. Although the physiological relevance remains still enigmatic, dimerization was proposed to be a unique feature of the mitochondrion [Biochim. Biophys. Acta 1555 (2002) 154]. It is hard to imagine, however, that closely related protein complexes of mitochondria and chloroplast should show such severe differences in structural organization. We present the first evidences for dimerization of chloroplast ATP synthases within the thylakoid membrane. By investigation of the thylakoid membrane of Chlamydomonas reinhardtii by blue-native polyacrylamide gel electrophoresis, dimerization of the chloroplast ATP synthase was detected. Chloroplast ATP synthase dimer dissociates into monomers upon incubation with vanadate or phosphate but not by incubation with molybdate, while the mitochondrial dimer is not affected by the incubation. This suggests a distinct dimerization mechanism for mitochondrial and chloroplast ATP synthase. Since vanadate and phosphate bind to the active sites, contact sites located on the hydrophilic CF1 part are suggested for the chloroplast ATP synthase dimer. As the degree of dimerization varies with phosphate concentration, dimerization might be a response to low phosphate concentrations.

The N-terminal input domain of the sensor kinase KdpD of Escherichia coli stabilizes the interaction between the cognate response regulator KdpE and the corresponding DNA-binding site

The sensor kinase/response regulator system KdpD/KdpE of Escherichia coli regulates expression of the kdpFABC operon, which encodes the high affinity K+ transport system KdpFABC. The membrane-bound sensor kinase KdpD consists of an N-terminal input domain (comprising a large cytoplasmic domain and four transmembrane domains) and a cytoplasmic C-terminal transmitter domain. Here we show that the cytoplasmic N-terminal domain of KdpD (KdpD/1-395) alone supports semi-constitutive kdpFABC expression, which becomes dependent on the extracellular K+ concentration under K+-limiting growth conditions. However, it should be noted that the non-phosphorylatable derivative KdpD/H673Q or the absence of KdpD abolishes kdpFABC expression completely. KdpD/1-395 mediated kdpFABC expression requires the corresponding response regulator KdpE with an intact phosphorylation site. Experiments with an Escherichia coli mutant unable to synthesize acetyl phosphate as well as transposon mutagenesis suggest that KdpE is phosphorylated in vivo by low molecular weight phosphodonors in the absence of the full-length sensor kinase. Various biochemical approaches provide first evidence that kdpFABC expression mediated by KdpD/1-395 is due to a stabilizing effect of this domain on the binding of KdpE approximately P to its corresponding DNA-binding site. Such a stabilizing effect of a sensor kinase domain on the DNA-protein interaction of the cognate response regulator has never been observed before for any other sensor kinase. It describes a new mechanism in bacterial two-component signal transduction.

The sensor protein KdpD inserts into the Escherichia coli membrane independent of the Sec translocase and YidC

KdpD is a sensor kinase protein in the inner membrane of Escherichia coli containing four transmembrane regions. The periplasmic loops connecting the transmembrane regions are intriguingly short and protease mapping allowed us to only follow the translocation of the second periplasmic loop. The results show that neither the Sec translocase nor the YidC protein are required for membrane insertion of the second loop of KdpD. To study the translocation of the first periplasmic loop a short HA epitope tag was genetically introduced into this region. The results show that also the first loop was translocated independently of YidC and the Sec translocase. We conclude that KdpD resembles a new class of membrane proteins that insert into the membrane without enzymatic assistance by the known translocases. When the second periplasmic loop was extended by an epitope tag to 27 amino acid residues, the membrane insertion of this loop of KdpD depended on SecE and YidC. To test whether the two periplasmic regions are translocated independently of each other, the KdpD protein was split between helix 2 and 3 into two approximately equal-sized fragments. Both constructed fragments, which contained KdpD-N (residues 1-448 of KdpD) and the KdpD-C (residues 444-894 of KdpD), readily inserted into the membrane. Similar to the epitope-tagged KdpD protein, only KdpD-C depended on the presence of the Sec translocase and YidC. This confirms that the four transmembrane helices of KdpD are inserted pairwise, each translocation event involving two transmembrane helices and a periplasmic loop.

The transmembrane domains of the sensor kinase KdpD of Escherichia coli are not essential for sensing K+ limitation

The sensor kinase/response regulator system KdpD/KdpE of Escherichia coli regulates the expression of the kdpFABC operon, which encodes the high affinity K+ transport system KdpFABC. The membrane-bound sensor kinase KdpD consists of four transmembrane domains, a large cytoplasmic N-terminal domain and a cytoplasmic C-terminal transmitter domain. To elucidate the role of the four transmembrane domains, various deletions were introduced in kdpD and the activities of the resulting truncated derivatives of KdpD were determined. A KdpD protein lacking all four transmembrane domains was able to sense low K+ concentrations, whereas at higher K+ concentrations kdpFABC expression was constitutive. These and further results with various truncated KdpD proteins lacking distinct parts of the transmembrane domains or derivatives in which a linker peptide or two transmembrane domains of PutP, the Na+/proline transporter of Escherichia coli, replaced the missing part indicated that the transmembrane domains are not essential for sensing of K+ limitation, but may be important for the correct positioning of the large N- and C-terminal cytoplasmic domains to each other.

The roles and regulation of potassium in bacteria

Potassium is the major intracellular cation in bacteria as well as in eucaryotic cells. Bacteria accumulate K+ by a number of different transport systems that vary in kinetics, energy coupling, and regulation. The Trk and Kdp systems of enteric organisms have been well studied and are found in many distantly related species. The Ktr system, resembling Trk in many ways, is also found in many bacteria. In most species two or more independent saturable K(+)-transport systems are present. The KefB and KefC type of system that is activated by treatment of cells with toxic electrophiles is the only specific K(+)-efflux system that has been well characterized. Pressure-activated channels of at least three types are found in bacteria; these represent nonspecific paths of efflux when turgor pressure is dangerously high. A close homolog of eucaryotic K+ channels is found in many bacteria, but its role remains obscure. K+ transporters are regulated both by ion concentrations and turgor. A very general property is activation of K+ uptake by an increase in medium osmolarity. This response is modulated by both internal and external concentrations of K+. Kdp is the only K(+)-transport system whose expression is regulated by environmental conditions. Decrease in turgor pressure and/or reduction in external K+ rapidly increase expression of Kdp. The signal created by these changes, inferred to be reduced turgor, is transmitted by the KdpD sensor kinase to the KdpE-response regulator that in turn stimulates transcription of the kdp genes. K+ acts as a cytoplasmic-signaling molecule, activating and/or inducing enzymes and transport systems that allow the cell to adapt to elevated osmolarity. The signal could be ionic strength or specifically K+. This signaling response is probably mediated by a direct sensing of internal ionic strength by each particular system and not by a component or system that coordinates this response by different systems to elevated K+.

Cooperativity in signal transfer through the Uhp system of Escherichia coli

The UhpABC regulatory system in enterobacteria controls the expression of the hexose phosphate transporter UhpT. Signaling is initiated through sensing of extracellular glucose 6-phosphate by membrane-bound UhpC, which in turn modulates the histidine-protein kinase UhpB. Together with the cytoplasmic response regulator UhpA, they constitute a typical two-component regulatory system based on His-to-Asp phosphoryl transfer. Activated (i.e., phosphorylated) UhpA binds to the promoter region of uhpT, resulting in initiation of transcription. We have investigated the contribution of transmembrane signaling (through UhpBC) and intracellular activation (through UhpA) to the overall Uhp response (UhpT expression) in vivo. UhpA activation could be made independent of transmembrane signaling when (Delta)uhpBC cells were grown on pyruvate. Inorganic phosphate interfered with glucose 6-phosphate-dependent, UhpBC-mediated, as well as pyruvate-mediated activation of UhpA. The relationship between the concentration of inducer (glucose 6-phosphate) and the Uhp induction rate was nonhyperbolic, indicating positive cooperativity. The degree of cooperativity was affected by the carbon or energy source available to the cells for growth. As pyruvate-mediated activation of UhpA in (Delta)uhpBC cells could result in considerably stronger UhpT expression than glucose 6-phosphate-dependent activation through UhpBC, the observed positive cooperativity for the overall pathway in wild-type cells may reflect the previously described cooperative binding of UhpA to the uhpT promoter (J. L. Dahl et al., J. Biol. Chem. 272:1910-1919, 1997).

Modulation of KdpD phosphatase implicated in the physiological expression of the kdp ATPase of Escherichia coli

The KdpD sensor kinase and the KdpE response regulator control the expression of the kdpFABC operon, encoding the KdpFABC high-affinity K+ transport system of Escherichia coli. Low turgor pressure has been postulated to be the environmental stimulus to express KdpFABC. KdpD has autokinase, phosphotransferase and, like many sensor kinases, response regulator (phospho-KdpE) specific phosphatase activity. To determine which of these activities are altered in response to the environmental stimulus, we isolated and analysed six kdpD mutants that cause constitutive expression of KdpFABC. In three of the mutants, phosphatase activity was undetectable and, in two, phosphatase was reduced. Kinase activity was unaffected in four of the mutants, but elevated in one. In one mutant, a pseudorevertant of a kdpD null mutation, kinase and phosphatase were both reduced to 20% of the wild-type level. These findings suggest that initiation of signal transduction by KdpD is mediated by the inhibition of the phospho-KdpE-specific phosphatase activity of KdpD, leading to an accumulation of phospho-KdpE, which in turn activates the expression of the KdpFABC system. The data also suggest that levels of activity in vitro may differ from what occurs in vivo, because in vitro conditions cannot replicate those in vivo.

The hydrophilic N-terminal domain complements the membrane-anchored C-terminal domain of the sensor kinase KdpD of Escherichia coli

The putative turgor sensor KdpD is characterized by a large, N-terminal domain of about 400 amino acids, which is not found in any other known sensor kinase. Comparison of 12 KdpD sequences from various microorganisms reveals that this part of the kinase is highly conserved and includes two motifs (Walker A and Walker B) that are very similar to the classical ATP-binding sites of ATP-requiring enzymes. By means of photoaffinity labeling with 8-azido-[alpha-(32)P]ATP, direct evidence was obtained for the existence of an ATP-binding site located in the N-terminal domain of KdpD. The N-terminal domain, KdpD/1-395, was overproduced and purified. Although predicted to be hydrophilic, it was found to be membrane-associated and could be solubilized either by treatment with buffer of low ionic strength or detergent. The membrane-associated form, but not the solubilized one, retained the ability to bind 8-azido-[alpha-(32)P]ATP. Previously, it was shown that the phosphatase activity of a truncated KdpD, KdpD/Delta12-395, is deregulated in vitro (Jung, K., and Altendorf, K. (1998) J. Biol. Chem. 273, 17406-17410). Here, we demonstrated that this effect was reversed in vesicles containing both the truncated KdpD and the N-terminal domain. Furthermore, coexpression of kdpD/Delta12-395 and kdpD/1-395 restored signal transduction in vivo. These results highlight the importance of the N-terminal domain for the function of KdpD and provide evidence for an interaction of this domain and the transmitter domain of the sensor kinase.

Dimerization of signalling modules of the EvgAS and BvgAS phosphorelay systems

Biophysical and biochemical properties of signalling proteins or domains derived from the unorthodox EvgAS and BvgAS two-component phosphorelay systems of Escherichia coli and Bordetella pertussis were investigated. Oligomerization of the effector proteins EvgA and BvgA and of truncated EvgS and BvgS derived signalling proteins containing the receiver and histidine containing phosphotransfer (HPt) domains or comprising only the HPt domains were characterized by native gel electrophoresis, gel permeation experiments and analytical ultracentrifugation. The results obtained by the different methods are consistent with non-phosphorylated EvgA and BvgA proteins being dimers in solution with a dissociation constant significantly below 1 microM. In contrast, all sensor derived domains of EvgS and BvgS were observed to be monomers in vitro. No indications for a phosphorylation induced stimulation of oligomerization of the C-terminal histidine kinase domains could be detected. In agreement with these data, surface plasmon resonance studies revealed a 2:1 stoichiometry in the interaction of EvgA with the immobilized EvgS HPt domain and an affinity constant of 1. 24x10(6) M(-1).