A monomeric histidine kinase derived from EnvZ, an Escherichia coli osmosensor

Diversity of function and higher-order structure within HWE sensor histidine kinases

Integral to the protein structure/function paradigm, oligomeric state is typically conserved along with function across evolution. However, notable exceptions such as the hemoglobins show how evolution can alter oligomerization to enable new regulatory mechanisms. Here, we examine this linkage in histidine kinases (HKs), a large class of widely distributed prokaryotic environmental sensors. While the majority of HKs are transmembrane homodimers, members of the HWE/HisKA2 family can deviate from this architecture as exemplified by our finding of a monomeric soluble HWE/HisKA2 HK (EL346, a photosensing light-oxygen-voltage [LOV]-HK). To further explore the diversity of oligomerization states and regulation within this family, we biophysically and biochemically characterized multiple EL346 homologs and found a range of HK oligomeric states and functions. Three LOV-HK homologs are primarily dimeric with differing structural and functional responses to light, while two Per-ARNT-Sim-HKs interconvert between differentially active monomers and dimers, suggesting dimerization might control enzymatic activity for these proteins. Finally, we examined putative interfaces in a dimeric LOV-HK, finding that multiple regions contribute to dimerization. Our findings suggest the potential for novel regulatory modes and oligomeric states beyond those traditionally defined for this important family of environmental sensors.

Robust and tunable signal processing in mammalian cells via engineered covalent modification cycles

Engineered signaling networks can impart cells with new functionalities useful for directing differentiation and actuating cellular therapies. For such applications, the engineered networks must be tunable, precisely regulate target gene expression, and be robust to perturbations within the complex context of mammalian cells. Here, we use bacterial two-component signaling proteins to develop synthetic phosphoregulation devices that exhibit these properties in mammalian cells. First, we engineer a synthetic covalent modification cycle based on kinase and phosphatase proteins derived from the bifunctional histidine kinase EnvZ, enabling analog tuning of gene expression via its response regulator OmpR. By regulating phosphatase expression with endogenous miRNAs, we demonstrate cell-type specific signaling responses and a new strategy for accurate cell type classification. Finally, we implement a tunable negative feedback controller via a small molecule-stabilized phosphatase, reducing output expression variance and mitigating the context-dependent effects of off-target regulation and resource competition. Our work lays the foundation for establishing tunable, precise, and robust control over cell behavior with synthetic signaling networks.

Microcin PDI regulation and proteolytic cleavage are unique among known microcins

Microcin PDI inhibits a diversity of pathogenic Escherichia coli through the action of an effector protein, McpM. In this study we demonstrated that expression of the inhibitory phenotype is induced under low osmolarity conditions and expression is primarily controlled by the EnvZ/OmpR two-component regulatory system. Functional, mutagenesis and complementation experiments were used to empirically demonstrate that EnvZ is required for the inhibitory phenotype and that regulation of mcpM is dependent on binding of the phosphorylated OmpR to the mcpM promoter region. The phosphorylated OmpR may recognize three different binding sites within this promoter region. Site-directed mutagenesis revealed that the McpM precursor peptide includes two leader peptides that undergo sequential cleavage at positions G17/G18 and G35/A36 during export through the type I secretion system. Competition assays showed that both cleaved products are required for the PDI phenotype although we could not distinguish loss of function from loss of secretion in these assays. McpM has four cysteines within the mature peptide and site-directed mutagenesis experiments demonstrated that the first two cysteines are necessary for McpM to inhibit susceptible cells. Together these data combined with previous work indicate that MccPDI is unique amongst the microcins that have been described to date.

Information processing in the adaptation of Saccharomyces cerevisiae to osmotic stress: an analysis of the phosphorelay system

Cellular signaling is key for organisms to survive immediate stresses from fluctuating environments as well as relaying important information about external stimuli. Effective mechanisms have evolved to ensure appropriate responses for an optimal adaptation process. For them to be functional despite the noise that occurs in biochemical transmission, the cell needs to be able to infer reliably what was sensed in the first place. For example Saccharomyces cerevisiae are able to adjust their response to osmotic shock depending on the severity of the shock and initiate responses that lead to near perfect adaptation of the cell. We investigate the Sln1-Ypd1-Ssk1-phosphorelay as a module in the high-osmolarity glycerol pathway by incorporating a stochastic model. Within this framework, we can imitate the noisy perception of the cell and interpret the phosphorelay as an information transmitting channel in the sense of C.E. Shannon's "Information Theory". We refer to the channel capacity as a measure to quantify and investigate the transmission properties of this system, enabling us to draw conclusions on viable parameter sets for modeling the system.

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.

Kinetic characterization of the WalRKSpn (VicRK) two-component system of Streptococcus pneumoniae: dependence of WalKSpn (VicK) phosphatase activity on its PAS domain

The WalRK two-component system plays important roles in maintaining cell wall homeostasis and responding to antibiotic stress in low-GC Gram-positive bacteria. In the major human pathogen, Streptococcus pneumoniae, phosphorylated WalR(Spn) (VicR) response regulator positively controls the transcription of genes encoding the essential PcsB division protein and surface virulence factors. WalR(Spn) is phosphorylated by the WalK(Spn) (VicK) histidine kinase. Little is known about the signals sensed by WalK histidine kinases. To gain information about WalK(Spn) signal transduction, we performed a kinetic characterization of the WalRK(Spn) autophosphorylation, phosphoryltransferase, and phosphatase reactions. We were unable to purify soluble full-length WalK(Spn). Consequently, these analyses were performed using two truncated versions of WalK(Spn) lacking its single transmembrane domain. The longer version (Delta35 amino acids) contained most of the HAMP domain and the PAS, DHp, and CA domains, whereas the shorter version (Delta195 amino acids) contained only the DHp and CA domains. The autophosphorylation kinetic parameters of Delta35 and Delta195 WalK(Spn) were similar [K(m)(ATP) approximately 37 microM; k(cat) approximately 0.10 min(-1)] and typical of those of other histidine kinases. The catalytic efficiency of the two versions of WalK(Spn) approximately P were also similar in the phosphoryltransfer reaction to full-length WalR(Spn). In contrast, absence of the HAMP-PAS domains significantly diminished the phosphatase activity of WalK(Spn) for WalR(Spn) approximately P. Deletion and point mutations confirmed that optimal WalK(Spn) phosphatase activity depended on the PAS domain as well as residues in the DHp domain. In addition, these WalK(Spn) DHp domain and DeltaPAS mutations led to attenuation of virulence in a murine pneumonia model.

How to switch off a histidine kinase: crystal structure of Geobacillus stearothermophilus KinB with the inhibitor Sda

Entry to sporulation in bacilli is governed by a histidine kinase phosphorelay, a variation of the predominant signal transduction mechanism in prokaryotes. Sda directly inhibits sporulation histidine kinases in response to DNA damage and replication defects. We determined a 2.0-A-resolution X-ray crystal structure of the intact cytoplasmic catalytic core [comprising the dimerization and histidine phosphotransfer domain (DHp domain), connected to the ATP binding catalytic domain] of the Geobacillus stearothermophilus sporulation kinase KinB complexed with Sda. Structural and biochemical analyses reveal that Sda binds to the base of the DHp domain and prevents molecular transactions with the DHp domain to which it is bound by acting as a simple molecular barricade. Sda acts to sterically block communication between the catalytic domain and the DHp domain, which is required for autophosphorylation, as well as to sterically block communication between the response regulator Spo0F and the DHp domain, which is required for phosphotransfer and phosphatase activities.

Functional and structural characterization of EnvZ, an osmosensing histidine kinase of E. coli

EnvZ is an osmosensing histidine kinase located in the inner membrane, and one of the most extensively studied Escherichia coli histidine kinases. Because of its structural complexity, functional and structural studies have been quite challenging. It is a multidomain transmembrane protein consisting of 450 amino acid residues. In addition, it must form a dimer to function as a histidine kinase like all the other histidine kinases. EnvZ consists of the 115-residue periplasmic domain, two transmembrane domains (TM1 and TM2), and the cytoplasmic domain consisting of the 43-residue linker (HAMP) domain and the 228-residue kinase domain. It has been shown that the kinase domain of EnvZ, responsible for its enzymatic activities, contains all of the conserved regions of histidine kinases such as H, F, N, G1, G2, and G3 boxes. Therefore, the 271-residue cytoplasmic domain of EnvZ (termed EnvZc) has been used as a model system to establish fundamental characteristics of histidine kinases. The DNA fragment encoding EnvZc was cloned in pET vector and EnvZc was expressed and purified. It is highly soluble and retains all the enzymatic activities of EnvZ. We demonstrated that it consists of two functional domains, domain A and domain B. NMR spectroscopic studies of these two domains revealed, for the first time, the structure of a histidine kinase. Domain A is responsible for dimerization of EnvZc forming a four-helical bundle containing two alpha-helical hairpin structures, while domain B is a monomer and has an ATP-binding pocket formed by regions conserved among the histidine kinases. In this chapter, we describe functional and structural studies of EnvZc, which can be applied to characterize other histidine kinases.

The pseudo-receiver domain of CikA regulates the cyanobacterial circadian input pathway

CikA (circadian input kinase) is a component of the cyanobacterial circadian clock that aids in synchronizing the endogenous oscillator with the external environment. cikA mutants of the prokaryotic circadian model organism Synechococcus elongatus PCC 7942 fail to reset the phase of the circadian rhythm of gene expression after an environmental time cue, and also exhibit reduced amplitude and shortened period of circadian oscillation. CikA has histidine protein kinase (HPK) activity that is modulated in vitro by GAF and pseudo-receiver (PsR) domains. Here we show that the PsR domain negatively regulates HPK activity in vivo and also serves as an interaction module to dock CikA at a specific subcellular location. Phenotypes conferred by alleles that encode CikA variants showed that all domains except the featureless N-terminus are required for CikA function. Overexpression of all alleles that encode the PsR domain, whether or not the HPK is functional, caused a dominant arrhythmic phenotype, whereas overexpressed variants that lack PsR did not. Subcellular localization of intact CikA identified a polar focus whereas a variant without PsR showed uniform distribution in the cell, consistent with a model in which PsR mediates interaction with other input pathway components.

Metal bridges between the PhoQ sensor domain and the membrane regulate transmembrane signaling

Bacterial histidine kinases respond to environmental stimuli by transducing a signal from an extracytosolic sensor domain to a cytosolic catalytic domain. Among them, PhoQ promotes bacterial virulence and is tightly repressed by the divalent cations such as calcium and magnesium. We have determined the crystal structure of the PhoQ sensor domain from Salmonella typhimurium in the Ca2+-bound state, which reveals a highly negatively charged surface that is in close proximity to the inner membrane. This acidic surface binds at least three Ca2+, which mediate the PhoQ-membrane interaction. Mutagenesis analysis indicates that structural integrity at the membrane proximal region of the PhoQ sensor domain promotes metal-mediated repression. We propose that depletion or displacement of divalent cations leads to charge repulsion between PhoQ and the membrane, which initiates transmembrane signaling through a change in orientation between the PhoQ sensor domain and membrane. Therefore, both PhoQ and the membrane are required for extracytosolic sensing and transmembrane signaling.

Structure of the entire cytoplasmic portion of a sensor histidine-kinase protein

The large majority of histidine kinases (HKs) are multifunctional enzymes having autokinase, phosphotransfer and phosphatase activities, and most of these are transmembrane sensor proteins. Sensor HKs possess conserved cytoplasmic phosphorylation and ATP-binding kinase domains. The different enzymatic activities require participation by one or both of these domains, implying the need for different conformational states. The catalytic domains are linked to the membrane through a coiled-coil segment that sometimes includes other domains. We describe here the first crystal structure of the complete cytoplasmic region of a sensor HK, one from the thermophile Thermotoga maritima in complex with ADPbetaN at 1.9 A resolution. The structure reveals previously unidentified functions for several conserved residues and reveals the relative disposition of domains in a state seemingly poised for phosphotransfer. The structure thereby inspires hypotheses for the mechanisms of autophosphorylation, phosphotransfer and response-regulator dephosphorylation, and for signal transduction through the coiled-coil segment. Mutational tests support the functional relevance of interdomain contacts.

Membrane fluidity and its roles in the perception of environmental signals

Poikilothermic organisms are exposed to frequent changes in environmental conditions and their survival depends on their ability to acclimate to such changes. Changes in ambient temperature and osmolarity cause fluctuations in the fluidity of cell membranes. Such fluctuations are considered to be critical to the initiation of the regulatory reactions that ultimately lead to acclimation. The mechanisms responsible for the perception of changes in membrane fluidity have not been fully characterized. However, the analysis of genome-wide gene expression using DNA microarrays has provided a powerful new approach to studies of the contribution of membrane fluidity to gene expression and to the identification of environmental sensors. In this review, we focus on the mechanisms that regulate membrane fluidity, on putative sensors that perceive changes in membrane fluidity, and on the subsequent expression of genes that ensures acclimation to a new set of environmental conditions.

Structure and Mechanism of Action of Sda, an Inhibitor of the Histidine Kinases that Regulate Initiation of Sporulation in Bacillus subtilis

Histidine kinases are used extensively in prokaryotes to monitor and respond to changes in cellular and environmental conditions. In Bacillus subtilis, sporulation-specific gene expression is controlled by a histidine kinase phosphorelay that culminates in phosphorylation of the Spo0A transcription factor. Sda provides a developmental checkpoint by inhibiting this phosphorelay in response to DNA damage and replication defects. We show that Sda acts at the first step in the relay by inhibiting autophosphorylation of the histidine kinase KinA. The structure of Sda, which we determined using NMR, comprises a helical hairpin. A cluster of conserved residues on one face of the hairpin mediates an interaction between Sda and the KinA dimerization/phosphotransfer domain. This interaction stabilizes the KinA dimer, and the two proteins form a stable heterotetramer. The data indicate that Sda forms a molecular barricade that inhibits productive interaction between the catalytic and phosphotransfer domains of KinA.

Spontaneous subunit exchange and biochemical evidence for trans-autophosphorylation in a dimer of Escherichia coli histidine kinase (EnvZ)

The EnvZ/OmpR histidyl-aspartyl phosphorelay (HAP) system in Escherichia coli regulates the expression of ompF and ompC, the major outer membrane porin genes, in response to environmental osmolarity changes. Here, we report that dimers of EnvZc, the cytoplasmic domain of EnvZ, undergo spontaneous subunit exchange in solution. By introducing a cysteine substitution (S260C) in the dimerization domain of EnvZc, we were able to crosslink the two subunits in a dimer and trap the heterodimer formed between two different mutant EnvZc. By using a complementing system with two autophosphorylation-defective EnvZc mutants, one containing the H243V mutation at the autophosphorylation site and the other containing the G405A mutation in the ATP-binding domain, we demonstrated that an EnvZc(G405A) subunit can be phosphorylated by an EnvZc(H243V) subunit only when a heterodimer is formed. The rate of subunit exchange is concentration-dependent, with higher rates at higher concentrations of protein. The disulfide-crosslinked EnvZc(G405A) homodimer could not be phosphorylated by EnvZc(H243V), since the heterodimer formation between the two mutant proteins was blocked, indicating that autophosphorylation cannot occur by dimer-dimer interaction. By using MBP-deltaL-EnvZc(S260C) fusion protein (deltaL: the linker region, spanning residues 180-222, was deleted), it was found that in the disulfide-crosslinked MBP-deltaL-EnvZc(S260C)/deltaL-EnvZc(S260C/G405A) heterodimer, only the deltaL-EnvZc(S260C/G405A) subunit was phosphorylated but not the MBP-deltaL-EnvZc(S260C) subunit. Together, the present results provide biochemical evidence that EnvZ autophosphorylation occurs in trans and only within an EnvZ dimer.

Cysteine-scanning analysis of the dimerization domain of EnvZ, an osmosensing histidine kinase

EnvZ and OmpR are a transmembrane sensor and its cognate response regulator, respectively, regulating the transcription of porin genes in response to medium osmolarity in Escherichia coli. The cytoplasmic domain of EnvZ (EnvZc) possesses both kinase and phosphatase activities and can be dissected into two functional domains, A and B. Here, we performed a cysteine-scanning analysis of domain A, a 67-residue central dimerization and phosphatase domain containing His-243 as the phosphorylation site, and we examined the effects of the cysteine substitution mutations on the enzymatic activities of domain A. The substitution mutations were made at 31 residues, from which 24 mutant domain A proteins were biochemically characterized. From the analysis of the phosphatase activity of purified mutant proteins, it was found that there are two regions in domain A which are important for this activity. Cysteine mutations in these regions dramatically reduce or completely abolish the phosphatase activity of domain A. The mutations that have the most-severe effects on domain A phosphatase activity also significantly reduce the phosphatase activity of EnvZc containing the same mutation. Using an in vitro complementation system with EnvZc(H243V), these cysteine mutants were further characterized for their autophosphorylation activities as well as their phosphotransfer activities. The results indicate that some mutations are specific either for the phosphatase activity or for the kinase activity.

Probing catalytically essential domain orientation in histidine kinase EnvZ by targeted disulfide crosslinking

EnvZ, a dimeric transmembrane histidine kinase, belongs to the family of His-Asp phosphorelay signal transduction systems. The cytoplasmic kinase domain of EnvZ can be dissected into two independently functioning domains, A and B, whose NMR solution structures have been individually determined. Here, we examined the topological arrangement of these two domains in the EnvZ dimer, a structure that is key to understanding the mechanism underlying the autophosphorylation activity of the kinase. A series of cysteine substitution mutants were constructed to test the feasibility of chemical crosslinking between the two domains. These crosslinking data demonstrate that helix I of domain A of one subunit in the EnvZc dimer is in close proximity to domain B of the other subunit in the same dimer, while helix II of domain A of one subunit interacts with domain B of the same subunit in the EnvZc dimer. This is the first demonstration of the topological arrangement between the central dimerization domain containing the active center His residues (domain A) and the ATP-binding catalysis assisting domain (domain B) in a class I histidine kinase.

Formation of the stoichiometric complex of EnvZ, a histidine kinase, with its response regulator, OmpR

EnvZ, a histidine kinase, and its cognate response regulator OmpR of Escherichia coli are responsible for adaptation to external osmotic changes by regulating the levels of the outer membrane porin proteins, OmpF and OmpC. The osmosensor, EnvZ, has dual enzymatic functions with OmpR kinase and OmpR-P phosphatase. Here, we demonstrate that the cytoplasmic kinase domain of EnvZ (EnvZc) and OmpR are able to form a 1:1 complex detected by native PAGE. This indicates that two OmpR molecules can bind to one EnvZc dimer. As this 1:1 EnvZc/OmpR complex is formed even in the presence of a large excess of EnvZc, OmpR binding to EnvZc is co-operative. The complex formation is also observed between EnvZc and phosphorylated OmpR for the phosphatase reaction. OmpR-P bound to EnvZc was readily released upon the addition of OmpR, indicating that OmpR and OmpR-P can compete for the binding to EnvZ. On the basis of these results, a model is discussed to explain how cellular OmpR-P concentrations are regulated in response to medium osmolarity.

Interaction of EnvZ, a sensory histidine kinase, with phosphorylated OmpR, the cognate response regulator

EnvZ is a sensory histidine kinase in Escherichia coli to regulate the phosphorylation of OmpR, its cognate response regulator, required for the expression of genes for outer membrane porin proteins. Here, we re-examined the recent paper Mattison and Kenney, in which the authors reported that phosphorylated OmpR (OmpR-P) is unable to bind to EnvZ, thus casting doubts on the role of the EnvZ phosphatase activity in vivo. Using an identical method, the Kd value for the interaction of the fluorescein-labelled OmpR (Fl-OmpR) with EnvZc was determined to be 1.96 +/- 0.28 micro M. We demonstrated that OmpR-P as well as OmpR inhibited the interaction of Fl-OmpR with EnvZc. Their 50% inhibitory concentrations were 1.09 +/- 0.25 micro M and 0.89 +/- 0.14 micro M, respectively, under the conditions used. The interaction between His-10-OmpR and EnvZc was also inhibited almost equally with OmpR-P and OmpR. Fluorescein labelling of OmpR was highly heterogeneous as detected by mass spectrometry, even though it slightly affected the OmpR phosphorylation (kinase) and the dephosphorylation of OmpR-P (phosphatase), indicating that EnvZc is able to interact with Fl-OmpR or Fl-OmpR-P as well as with OmpR or OmpR-P as a substrate. We demonstrated that OmpR-P is able to interact with EnvZc with a similar affinity to OmpR and serves as an effective substrate for the EnvZ phosphatase. These findings support the hypothesis that osmotic signals regulate the level of the cellular concentration of OmpR-P by modulating the ratio of kinase to phosphatase activity of the bifunctional enzymatic activities of EnvZ.

The role of the G2 box, a conserved motif in the histidine kinase superfamily, in modulating the function of EnvZ

Histidine kinase EnvZ, a transmembrane osmotic sensor for Escherichia coli, is a bifunctional enzyme having OmpR (its cognate response regulator) kinase and phosphorylated OmpR (OmpR-P) phosphatase activities. Its cytoplasmic domain consists of domain A responsible for dimerization of EnvZ, histidine phosphotransfer and phosphatase activities, and domain B responsible for ATP binding. Here, we have constructed a number of substitution mutations at the G2 box, one of the conserved motifs in domain B, and demonstrated that they influence the phosphatase activity of EnvZ over a wide range. The effects of ADP, a cofactor for the phosphatase activity, were found to be substantially different depending upon the mutations. The effects of these mutations were also examined in vivo using a chimeric Tar-EnvZ construct (Taz1-1), and the results agreed with the in vitro data for the phosphatase and kinase activities for all mutations. Using Taz1-1 carrying the T402A mutation, three independent intragenic suppressor mutations (T235M, S269L and E276K) were isolated, and all were found in domain A. Together, the present results demonstrate for the first time that domain A and domain B are functionally co-ordinated and topologically arranged in a specific manner. The G2 box may modulate the interaction between these two domains in response to extracellular osmolarity.

EnvZ-OmpR interaction and osmoregulation in Escherichia coli

EnvZ, a histidine kinase/phosphatase in Escherichia coli, responds to the osmolarity changes in the medium by regulating the phosphorylation state of the transcription factor OmpR, which controls the expression levels of outer membrane porin proteins OmpF and OmpC. Although both ompR and envZ genes are located on the ompB locus under the control of the ompB promoter and transcribed as a single polycistronic mRNA, the expression of envZ is known to be significantly less than ompR. However, to date no accurate estimation for the amounts of EnvZ and OmpR in the cell has been carried out. Here we examined the levels of EnvZ and OmpR in the wild-type strain MC4100 by quantitative Western blot analysis using anti-OmpR and anti-EnvZc (cytoplasmic domain of EnvZ) antisera. It was observed that during exponential growth in L-broth medium there were approximately 3500 and 100 molecules per cell of OmpR and EnvZ, respectively. The levels of OmpR and EnvZ in MC4100 cells grown in a high osmolarity medium (nutrient broth with 20% sucrose) were about the same as those grown in L-broth, whereas they were 1.7-fold higher than those in a low osmolarity medium (nutrient broth). With His10-OmpR, we also determined that the K(d) value for the EnvZc-OmpR complex formation is 1.20 +/- 0.17 microm. On the basis of these results, the molecular mechanism of osmoregulation of ompF and ompC is discussed.

Mycobacterium tuberculosis WhiB3 interacts with RpoV to affect host survival but is dispensable for in vivo growth

Previous work established that the principal sigma factor (RpoV) of virulent Mycobacterium bovis, a member of the Mycobacterium tuberculosis complex, restores virulence to an attenuated strain containing a point mutation (Arg-515-->His) in the 4.2 domain of RpoV. We used the 4.2 domain of RpoV as bait in a yeast two-hybrid screen of an M. tuberculosis H37Rv library and identified a putative transcription factor, WhiB3, which selectively interacts with the 4.2 domain of RpoV in virulent strains but not with the mutated (Arg-515-->His) allele. Infection of mice and guinea pigs with a M. tuberculosis H37Rv whiB3 deletion mutant strain showed that whiB3 is not necessary for in vivo bacterial replication in either animal model. In contrast, an M. bovis whiB3 deletion mutant was completely attenuated for growth in guinea pigs. However, we found that immunocompetent mice infected with the M. tuberculosis H37Rv whiB3 mutant strain had significantly longer mean survival times as compared with mice challenged with wild-type M. tuberculosis. Remarkably, the bacterial organ burdens of both mutant and wild-type infected mice were identical during the acute and persistent phases of infection. Our results imply that M. tuberculosis replication per se is not a sufficient condition for virulence in vivo. They also indicate a different role for M. bovis and M. tuberculosis whiB3 genes in pathogenesis generated in different animal models. We propose that M. tuberculosis WhiB3 functions as a transcription factor regulating genes that influence the immune response of the host.

A cytoplasmic coiled-coil domain is required for histidine kinase activity of the yeast osmosensor, SLN1

The yeast histidine kinase, Sln1p, is a plasma membrane-associated osmosensor that regulates the activity of the osmotic stress MAP kinase pathway. Changes in the osmotic environment of the cell influence the autokinase activity of the cytoplasmic kinase domain of Sln1p. Neither the nature of the stimulus, the mechanism by which the osmotic signal is transduced nor the manner in which the kinase is regulated is currently clear. We have identified several mutations located in the linker region of the Sln1 kinase (just upstream of the kinase domain) that cause hyperactivity of the Sln1 kinase. This region of histidine kinases is largely uncharacterized, but its location between the transmembrane domains and the cytoplasmic kinase domain suggests that it may have a potential role in signal transduction. In this study, we have investigated the Sln1 linker region in order to understand its function in signal transduction and regulation of Sln1 kinase activity. Our results indicate that the linker region forms a coiled-coil structure and suggest a mechanism by which alterations induced by osmotic stress influence kinase activity by altering the alignment of the phospho-accepting histidine with respect to the catalytic domain of the kinase.

K+ stimulates specifically the autokinase activity of purified and reconstituted EnvZ of Escherichia coli

The histidine kinase/response regulator system EnvZ/OmpR of Escherichia coli regulates transcription of the genes ompF and ompC, encoding two porins of the outer membrane. Although the total amount of OmpF and OmpC remains constant, the relative levels of the two proteins fluctuate in a reciprocal manner depending on medium osmolality. The membrane-anchored sensor EnvZ somehow monitors changes in environmental osmolality. To characterize the nature of the stimulus perceived by EnvZ, this protein was overproduced, purified, and reconstituted into proteoliposomes. Autokinase activity of purified and reconstituted EnvZ was stimulated by an increase of the K(+) concentration. Rb(+), Na(+), and NH4(+) also stimulated the activity but to a smaller extent, whereas an osmotic upshift imposed by various sugars or increasing concentrations of glycine betaine, proline, or Tris/MES were without influence. Neither the transfer of the phosphoryl group from EnvZ approximately P to OmpR nor the EnvZ-mediated OmpR approximately P dephosphorylation were affected by one of the tested solutes. Experiments with the reconstructed signal transduction cascade including DNA fragments demonstrated a substantial increase of the amount of phosphorylated OmpR in the presence of K(+) and to a lower extent in the presence of Na(+), Rb(+), and NH4(+). Various K(+) salts were tested indicating that the determined effects were K(+)-specific and not dependent on the anion. In a further in vitro test system, which utilizes right-side-out membrane vesicles, the K(+)-specific activation of EnvZ autokinase from the luminal side was confirmed. These results clearly indicate a regulation of EnvZ autokinase activity by monovalent ions, specifically K(+). Whether K(+) accumulation, which is one of the first responses of E. coli after an osmotic upshift, is related to the stimulation of the EnvZ autokinase activity in vivo is discussed.

The critical role of DNA in the equilibrium between OmpR and phosphorylated OmpR mediated by EnvZ in Escherichia coli

Escherichia coli modulates its porin expression through a histidine kinase, EnvZ, and its cognate response regulator, OmpR. EnvZ is a bifunctional enzyme that possesses both OmpR kinase and phosphorylated OmpR (OmpR-P) phosphatase activities and thus controls the cellular level of OmpR-P. In an in vitro-assay system, the addition of OmpR to the reaction mixture consisting of the cytoplasmic domain of EnvZ (EnvZc) and ATP produces a barely detectable amount of OmpR-P because of the dual activities of EnvZ. Here we report that DNA fragments containing the upstream promoter regions of the porin genes (ompF and ompC) can shift the equilibrium between OmpR and OmpR-P dramatically toward OmpR-P. Among the four reactions occurring in the mixture, only the EnvZ phosphatase activity was inhibited severely by the specific DNA, in contrast to the previous report by Kenney and her associates that DNA stimulates OmpR phosphorylation by EnvZ [Ames, S. K., Frankema, N. & Kenney, L. J. (1999) Proc. Natl. Acad. Sci. USA 96, 11792-11797]. The autophosphorylation of EnvZc and the phosphotransfer from phosphorylated EnvZc to OmpR were not affected by DNA, whereas the autodephosphorylation of OmpR-P was inhibited slightly. We propose that the apparent inhibitory effect of DNA on the EnvZ phosphatase function is caused by sequestrating OmpR-P from the reaction as a result of OmpR-P binding to DNA.

The critical role of DNA in the equilibrium between OmpR and phosphorylated OmpR mediated by EnvZ in Escherichia coli

Escherichia coli modulates its porin expression through a histidine kinase, EnvZ, and its cognate response regulator, OmpR. EnvZ is a bifunctional enzyme that possesses both OmpR kinase and phosphorylated OmpR (OmpR-P) phosphatase activities and thus controls the cellular level of OmpR-P. In an in vitro-assay system, the addition of OmpR to the reaction mixture consisting of the cytoplasmic domain of EnvZ (EnvZc) and ATP produces a barely detectable amount of OmpR-P because of the dual activities of EnvZ. Here we report that DNA fragments containing the upstream promoter regions of the porin genes (ompF and ompC) can shift the equilibrium between OmpR and OmpR-P dramatically toward OmpR-P. Among the four reactions occurring in the mixture, only the EnvZ phosphatase activity was inhibited severely by the specific DNA, in contrast to the previous report by Kenney and her associates that DNA stimulates OmpR phosphorylation by EnvZ [Ames, S. K., Frankema, N. & Kenney, L. J. (1999) Proc. Natl. Acad. Sci. USA 96, 11792-11797]. The autophosphorylation of EnvZc and the phosphotransfer from phosphorylated EnvZc to OmpR were not affected by DNA, whereas the autodephosphorylation of OmpR-P was inhibited slightly. We propose that the apparent inhibitory effect of DNA on the EnvZ phosphatase function is caused by sequestrating OmpR-P from the reaction as a result of OmpR-P binding to DNA.

Phosphatase activity of histidine kinase EnvZ without kinase catalytic domain

Most histidine kinases are bifunctional enzymes having both kinase and phosphatase activities. The cytoplasmic kinase domain of EnvZ, a transmembrane histidine kinase functioning as an osmosensor in Escherichia coli, consists of two distinct functional subdomains: domain A [EnvZc(223-289)] and domain B [EnvZc(290-450)]. NMR studies demonstrated that domain A consists of a four-helix bundle serving as a dimerization and phosphotransfer domain, and domain B functions as the ATP-binding and catalytic domain. Here we demonstrate that domain A by itself has the phosphatase activity both in vitro and in vivo. This phosphatase activity is Mg(2+) dependent but is not activated by ADP, ATP, or adenosine 5'-[beta, gamma-imido]triphosphate (AMPPNP), each of which may serve as a cofactor for the EnvZ phosphatase activity. Domain B showed a small but distinct effect on the domain A phosphatase activity only in the presence of ADP or AMPPNP. However, when domain B was covalently linked to domain A, dramatic cofactor-dependent enhancement of the phosphatase activity was observed. Extending domain A for another 75 residues at the C terminus or 44 residues at the N terminus did not enhance its phosphatase activity. Substitution mutations at His-243, the autophosphorylation site, demonstrate that the His residue plays an essential role in the phosphatase activity. The so-called X-region mutant L288P that is known to specifically abolish the phosphatase activity in EnvZ had no effect on the domain A phosphatase function. We propose that the EnvZ phosphatase activity is regulated by relative positioning of domains A and B, which is controlled by external signals. We also propose that the His-243 residue participates in both kinase and phosphatase reactions.