A yeast protein similar to bacterial two-component regulators

Histidine kinases and two-component signal transduction systems

The phosphorylation of histidine is the first step in many signal transduction cascades in bacteria, yeast and higher plants. The transfer of a very reactive phosphoryl group from phosphorylated histidine kinase to an acceptor is an essential step in many cellular signaling responses.

Role of the mitogen-activated protein kinase Hog1p in morphogenesis and virulence of Candida albicans

The relevance of the mitogen-activated protein (MAP) kinase Hog1p in Candida albicans was addressed through the characterization of C. albicans strains without a functional HOG1 gene. Analysis of the phenotype of hog1 mutants under osmostressing conditions revealed that this mutant displays a set of morphological alterations as the result of a failure to complete the final stages of cytokinesis, with parallel defects in the budding pattern. Even under permissive conditions, hog1 mutants displayed a different susceptibility to some compounds such as nikkomycin Z or Congo red, which interfere with cell wall functionality. In addition, the hog1 mutant displayed a colony morphology different from that of the wild-type strain on some media which promote morphological transitions in C. albicans. We show that C. albicans hog1 mutants are derepressed in the serum-induced hyphal formation and, consistently with this behavior, that HOG1 overexpression in Saccharomyces cerevisiae represses the pseudodimorphic transition. Most interestingly, deletion of HOG1 resulted in a drastic increase in the mean survival time of systemically infected mice, supporting a role for this MAP kinase pathway in virulence of pathogenic fungi. This finding has potential implications in antifungal therapy.

Caffeine-mediated override of checkpoint controls. A requirement for rhp6 (Schizosaccharomyces pombe)

Cells exposed to inhibitors of DNA synthesis or suffering DNA damage are arrested or delayed in interphase through the action of checkpoint controls. If the arrested cell is exposed to caffeine, relatively normal cell cycle progression is resumed and, as observed in checkpoint control mutants, loss of checkpoint control activity is associated with a reduction in cell viability. To address the mechanism of caffeine's action on cell progression, fission yeast mutants that take up caffeine but are not sensitized to hydroxyurea (HU) by caffeine were selected. Mutants 788 and 1176 are point mutants of rhp6, the fission yeast homolog of the budding yeast RAD6 gene. Mutant rhp6-788 is slightly HU sensitive, radiosensitive, and exhibits normal checkpoint responses to HU, radiation, or inactivation of DNA ligase. However, the addition of caffeine does not override the associated cell cycle blocks. Both point and deletion mutations show synthetic lethality at room temperature with temperature-sensitive mutations in cyclin B (cdc13-117) or the phosphatase cdc25 (cdc25-22). These observations suggest that the rhp6 gene product, a ubiquitin-conjugating enzyme required for DNA damage repair, promotes entry to mitosis in response to caffeine treatment.

Cell cycle-dependent polar localization of an essential bacterial histidine kinase that controls DNA replication and cell division

The master CtrA response regulator functions in Caulobacter to repress replication initiation in different phases of the cell cycle. Here, we identify an essential histidine kinase, CckA, that is responsible for CtrA activation by phosphorylation. Although CckA is present throughout the cell cycle, it moves to a cell pole in S phase, and upon cell division it disperses. Removal of the membrane-spanning region of CckA results in loss of polar localization and cell death. We propose that polar CckA functions to activate CtrA just after the initiation of DNA replication, thereby preventing premature reinitiations of chromosome replication. Thus, dynamic changes in cellular location of critical signal proteins provide a novel mechanism for the control of the prokaryote cell cycle.

The extracellular domain of the Saccharomyces cerevisiae Sln1p membrane osmolarity sensor is necessary for kinase activity

The function of the extracellular domain (ECD) of Sln1p, a plasma membrane two-transmembrane domain (TMD) sensor of the high-osmolarity glycerol (HOG) response pathway, has been studied in the yeast Saccharomyces cerevisiae. Truncations of SLN1 that retain an intact kinase domain are capable of complementing the lethality of an sln1Delta strain. By observing levels of Hog1p phosphorylation as well as the phosphorylation state of Sln1p, the kinase activities of various SLN1 constructions were determined. In derivatives that do not contain the first TMD, Sln1p activity was no longer dependent on medium osmolarity but appeared to be constitutively active even under conditions of high osmolarity. Removal of the first TMD (DeltaTMD1 construct) gave a protein that was strongly phosphorylated whereas Hog1p was largely dephosphorylated, as expected if the active form of Sln1p is phosphorylated. When both TMDs as well as the ECD were deleted, so that the kinase domain is cytosolic, Sln1p was not phosphorylated whereas Hog1p became constitutively hyperphosphorylated. Surprisingly, this hyperactivity of the HOG mitogen-activated protein kinase signaling pathway was not sufficient to result in cell lethality. When the ECD of the DeltaTMD1 construct was replaced with a leucine zipper motif, Sln1p was hyperactive, so that Hog1p became mostly unphosphorylated. In contrast, when the Sln1p/leucine zipper construct was crippled by a mutation of one of the internal leucines, the Sln1 kinase was inactive. These experiments are consistent with the hypothesis that the ECD of Sln1p functions as a dimerization and activation domain but that osmotic regulation of activity requires the presence of the first TMD.

An Arabidopsis GSK3/shaggy-like gene that complements yeast salt stress-sensitive mutants is induced by NaCl and abscisic acid

GSK3/shaggy-like genes encode kinases that are involved in a variety of biological processes. By functional complementation of the yeast calcineurin mutant strain DHT22-1a with a NaCl stress-sensitive phenotype, we isolated the Arabidopsis cDNA AtGSK1, which encodes a GSK3/shaggy-like protein kinase. AtGSK1 rescued the yeast calcineurin mutant cells from the effects of high NaCl. Also, the AtGSK1 gene turned on the transcription of the NaCl stress-inducible PMR2A gene in the calcineurin mutant cells under NaCl stress. To further define the role of AtGSK1 in the yeast cells we introduced a deletion mutation at the MCK1 gene, a yeast homolog of GSK3, and examined the phenotype of the mutant. The mck1 mutant exhibited a NaCl stress-sensitive phenotype that was rescued by AtGSK1. Also, constitutive expression of MCK1 complemented the NaCl-sensitive phenotype of the calcineurin mutants. Therefore, these results suggest that Mck1p is involved in the NaCl stress signaling in yeast and that AtGSK1 may functionally replace Mck1p in the NaCl stress response in the calcineurin mutant. To investigate the biological function of AtGSK1 in Arabidopsis we examined the expression of AtGSK1. Northern-blot analysis revealed that the expression is differentially regulated in various tissues with a high level expression in flower tissues. In addition, the AtGSK1 expression was induced by NaCl and exogenously applied ABA but not by KCl. Taken together, these results suggest that AtGSK1 is involved in the osmotic stress response in Arabidopsis.

Serine/threonine protein kinases and phosphatases in filamentious fungi

Protein phosphorylation and dephosphorylation are one of the central currencies by which living cells perceive and respond to environmental cues. A number of fundamental processes in fungi such as the cell cycle, transcription, and mating have been shown to require protein phosphorylation. The analysis of protein kinases and phosphatases in filamentous fungi is in its infancy; however, it has already become clear that kinases and phosphatases are likely to be important mediators of fungal proliferation and development as well as signal transduction and infection-related morphogenesis. In this review, we describe, summarize, and consider the rapidly expanding field of protein phosphorylation/dephosphorylation in various aspects of filamentous fungal growth and development.

Functional dissection of the transmitter module of the histidine kinase NtrB in Escherichia coli

Signal transduction by two-component systems involves phosphorylation and thereby activation of the response regulator by the cognate histidine kinase. Bifunctional histidine kinases have two opposing activities: depending on the environmental stimuli they either promote phosphorylation or stimulate the rapid dephosphorylation of the response regulator. To determine the mechanism of this switch, we analyzed the domain organization of the bifunctional histidine kinase NtrB. Based on sequence alignments with other histidine kinases and a deletion analysis, we defined three separate subdomains of the transmitter module, the H domain (amino acids 123-221), the N domain (amino acids 221-269), and the G domain (amino acids 269-349). The transmitter module, when separately expressed, exhibited a constitutive positive phenotype. In contrast, in the absence of the G domain, the H domain exhibits a constitutive negative phenotype. This negative regulatory activity of the H domain is inhibited by the G domain. The G domain could be physically uncoupled; when coexpressed with the H-N fragment, the constitutive positive phenotype of the transmitter was restored. We demonstrate, in vitro, that the constitutive negative phenotype of the fragments lacking the G domain is caused by stimulation of dephosphorylation of the response regulator NtrC-P. Based on our analysis, we suggest that the function of the sensor domain is to control the interaction of the H and G domains. If these subdomains interact, NtrB acts as a positive regulator; if they cannot interact, NtrB acts as a negative regulator.

Three-dimensional crystal structure of the transcription factor PhoB receiver domain

PhoB is the response regulator of the two-component signal transduction system activated under phosphate starvation conditions. This protein is a transcription factor that activates more than 30 genes of the pho regulon and consists of two domains: a DNA binding domain and a dimerization domain, the latter being homologous to the receiver domain described for two-component response regulators. Activation by phosphorylation induces dimerization of the protein and the consequent binding to the DNA direct repeat pho box, where it promotes the binding of RNA polymerase. In the absence of phosphorylation, the activating dimerization process can be mimicked by deletion of the DNA binding domain. The three-dimensional crystal structure of the receiver domain of PhoB from Escherichia coli has been solved by multiple anomalous diffraction using a gold derivative obtained by co-crystallization, and refined using data to 1.9 A resolution. The crystal structure reveals an alpha/beta doubly wound fold, similar to other known receivers, the most conspicuous difference being the displacement of helix alpha4 towards its N terminus. The active site includes the acidic triad Asp53 (the site of phosphorylation), Asp10 and Glu9. Lys105, from loop beta5alpha5, and Glu88, from helix alpha4, interact with Asp53 via an H-bond and a water bridge, respectively. In the asymmetric unit of the crystal there are two molecules linked by a complementary hydrophobic surface, which involves helix alpha1, loop beta5alpha5 and the N terminus of helix alpha5, and is connected to the active site through the fully conserved residue Lys105 from loop beta5alpha5. The possibility that this surface is the functional surface used for the activating dimerization is discussed.

Alternative splicing results in differential expression, activity, and localization of the two forms of arginyl-tRNA-protein transferase, a component of the N-end rule pathway

The N-end rule relates the in vivo half-life of a protein to the identity of its N-terminal residue. The underlying ubiquitin-dependent proteolytic system, called the N-end rule pathway, is organized hierarchically: N-terminal aspartate and glutamate (and also cysteine in metazoans) are secondary destabilizing residues, in that they function through their conjugation, by arginyl-tRNA-protein transferase (R-transferase), to arginine, a primary destabilizing residue. We isolated cDNA encoding the 516-residue mouse R-transferase, ATE1p, and found two species, termed Ate1-1 and Ate1-2. The Ate1 mRNAs are produced through a most unusual alternative splicing that retains one or the other of the two homologous 129-bp exons, which are adjacent in the mouse Ate1 gene. Human ATE1 also contains the alternative 129-bp exons, whereas the plant (Arabidopsis thaliana) and fly (Drosophila melanogaster) Ate1 genes encode a single form of ATE1p. A fusion of ATE1-1p with green fluorescent protein (GFP) is present in both the nucleus and the cytosol, whereas ATE1-2p-GFP is exclusively cytosolic. Mouse ATE1-1p and ATE1-2p were examined by expressing them in ate1Delta Saccharomyces cerevisiae in the presence of test substrates that included Asp-betagal (beta-galactosidase) and Cys-betagal. Both forms of the mouse R-transferase conferred instability on Asp-betagal (but not on Cys-betagal) through the arginylation of its N-terminal Asp, the ATE1-1p enzyme being more active than ATE1-2p. The ratio of Ate1-1 to Ate1-2 mRNA varies greatly among the mouse tissues; it is approximately 0.1 in the skeletal muscle, approximately 0.25 in the spleen, approximately 3.3 in the liver and brain, and approximately 10 in the testis, suggesting that the two R-transferases are functionally distinct.

Hyperosmotic stress represses the transcription of HXT2 and HXT4 genes in Saccharomyces cerevisiae

Effects of hyperosmotic stress on the transcriptional regulation of the HXT2 and HXT4 genes of Saccharomyces cerevisiae were investigated under glucose-repressed and -depressed growth conditions. Hyperosmotic stress repressed the transcription of these HXT genes up to 81% depending on growth conditions. Preconditioning of yeast cells for the hyperosmotic stress resulted in a much stronger repression of both HXT genes. The negative effect of hyperosmotic stress was much higher for HXT4 than HXT2. These results also show that hyperosmotic stress interferes with the glucose-dependent transcriptional activation or derepression of HXT2 and HXT4 genes transcription in S. cerevisiae.

Intracellular glycerol levels modulate the activity of Sln1p, a Saccharomyces cerevisiae two-component regulator

The HOG mitogen-activated protein kinase pathway mediates the osmotic stress response in Saccharomyces cerevisiae, activating genes like GPD1 (glycerol phosphate dehydrogenase), required for survival under hyperosmotic conditions. Activity of this pathway is regulated by Sln1p, a homolog of the "two-component" histidine kinase family of signal transduction molecules prominent in bacteria. Sln1p also regulates the activity of a Hog1p-independent pathway whose transcriptional output can be monitored using an Mcm1p-dependent lacZ reporter gene. The relationship between the two Sln1p branches is unclear, however, the requirement for unphosphorylated pathway intermediates in Hog1p pathway activation and for phosphorylated intermediates in the activation of the Mcm1p reporter suggests that the two Sln1p branches are reciprocally regulated. To further investigate the signals and molecules involved in modulating Sln1p activity, we have screened for new mutations that elevate the activity of the Mcm1p-dependent lacZ reporter gene. We find that loss of function mutations in FPS1, a gene encoding the major glycerol transporter in yeast activates the reporter in a SLN1-dependent fashion. We propose that elevated intracellular glycerol levels in the fps1 mutant shift Sln1p to the phosphorylated state and trigger the Sln1-dependent activity of the Mcm1 reporter. These observations are consistent with a model in which Sln1p autophosphorylation is triggered by a hypo-osmotic stimulus and indicate that the Sln1p osmosensor is tied generally to osmotic balance, and may not specifically sense an external osmolyte.

Differential stabilities of phosphorylated response regulator domains reflect functional roles of the yeast osmoregulatory SLN1 and SSK1 proteins

Osmoregulation in Saccharomyces cerevisiae involves a multistep phosphorelay system requiring three proteins, SLN1, YPD1, and SSK1, that are related to bacterial two-component signaling proteins, in particular, those involved in regulating sporulation in Bacillus subtilis and anaerobic respiration in Escherichia coli. The SLN1-YPD1-SSK1 phosphorelay regulates a downstream mitogen-activated protein kinase cascade which ultimately controls the concentration of glycerol within the cell under hyperosmotic stress conditions. The C-terminal response regulator domains of SLN1 and SSK1 and full-length YPD1 have been overexpressed and purified from E. coli. A heterologous system consisting of acetyl phosphate, the bacterial chemotaxis response regulator CheY, and YPD1 has been developed as an efficient means of phosphorylating SLN1 and SSK1 in vitro. The homologous regulatory domains of SLN1 and SSK1 exhibit remarkably different phosphorylated half-lives, a finding that provides insight into the distinct roles that these phosphorylation-dependent regulatory domains play in the yeast osmosensory signal transduction pathway.

Signal decay through a reverse phosphorelay in the Arc two-component signal transduction system

Escherichia coli senses and signals anoxic or low redox conditions in its growth environment by the Arc two-component system. Under those conditions, the tripartite sensor kinase ArcB undergoes autophosphorylation at the expense of ATP and subsequently transphosphorylates its cognate response regulator ArcA through a His --> Asp --> His --> Asp phosphorelay pathway. In this study we used various combinations of wild-type and mutant ArcB domains to analyze in vitro the pathway for signal decay. The results indicate that ArcA-P dephosphorylation does not occur by direct hydrolysis but by transfer of the phosphoryl group to the secondary transmitter and subsequently to the receiver domain of ArcB. This reverse phosphorelay involves both the conserved His-717 of the secondary transmitter domain and the conserved Asp-576 of the receiver domain of ArcB but not the conserved His-292 of its primary transmitter domain. This novel pathway for signal decay may generally apply to signal transduction systems with tripartite sensor kinases.

Evidence for common sites of contact between the antisigma factor SpoIIAB and its partners SpoIIAA and the developmental transcription factor sigmaF in Bacillus subtilis

The activity of the developmental transcription factor sigmaF in Bacillus subtilis is governed by a switch involving the dual function protein SpoIIAB. SpoIIAB is an antisigma factor that forms complexes with sigmaF and with an alternative partner protein SpoIIAA. SpoIIAB is also a protein kinase that can inactivate SpoIIAA by phosphorylating it on a serine residue. We sought to identify amino acids in SpoIIAB that are involved in the formation of the SpoIIAB-SpoIIAA complex by screening for mutants that were defective in the activation of sigmaF. This genetic screen, in combination with biochemical analysis and the construction of loss-of-side-chain (alanine substitution) mutants, led to the identification of amino acid side-chains in the N-terminal region of SpoIIAB that could contact SpoIIAA. Unexpectedly, the same amino acid side-chains (R20 and N50) that appear to touch SpoIIAA are required for binding to, and may represent sites of contact with, sigmaF. We propose that the N-terminal region of SpoIIAB forms a binding surface that is responsible for the formation of both the SpoIIAB-SpoIIAA and the SpoIIAB-sigmaF complexes, and that in some cases the same amino acid side-chains contact both partner proteins. N50 is also the defining residue of a region of amino acid sequence homology known as the N-box that is shared by SpoIIAB and related serine protein kinases, as well as by members of a mechanistically dissimilar family of protein kinases that undergo autophosphorylation at a histidine residue. We discuss the implications of this finding for the mechanism of histidine autophosphorylation.

The yeast histidine protein kinase, Sln1p, mediates phosphotransfer to two response regulators, Ssk1p and Skn7p

The Saccharomyces cerevisiae Sln1 protein is a 'two-component' regulator involved in osmotolerance. Two-component regulators are a family of signal-transduction molecules with histidine kinase activity common in prokaryotes and recently identified in eukaryotes. Phosphorylation of Sln1p inhibits the HOG1 MAP kinase osmosensing pathway via a phosphorelay mechanism including Ypd1p and the response regulator, Ssk1p. SLN1 also activates an MCM1-dependent reporter gene, P-lacZ, but this function is independent of Ssk1p. We present genetic and biochemical evidence that Skn7p is the response regulator for this alternative Sln1p signaling pathway. Thus, the yeast Sln1 phosphorelay is actually more complex than appreciated previously; the Sln1 kinase and Ypd1 phosphorelay intermediate regulate the activity of two distinct response regulators, Ssk1p and Skn7p. The established role of Skn7p in oxidative stress is independent of the conserved receiver domain aspartate, D427. In contrast, we show that Sln1p activation of Skn7p requires phosphorylation of D427. The expression of TRX2, previously shown to exhibit Skn7p-dependent oxidative-stress activation, is also regulated by the SLN1 phosphorelay functions of Skn7p. The identification of genes responsive to both classes of Skn7p function suggests a central role for Skn7p and the SLN1-SKN7 pathway in integrating and coordinating cellular response to various types of environmental stress.

MAP kinase pathways in the yeast Saccharomyces cerevisiae

A cascade of three protein kinases known as a mitogen-activated protein kinase (MAPK) cascade is commonly found as part of the signaling pathways in eukaryotic cells. Almost two decades of genetic and biochemical experimentation plus the recently completed DNA sequence of the Saccharomyces cerevisiae genome have revealed just five functionally distinct MAPK cascades in this yeast. Sexual conjugation, cell growth, and adaptation to stress, for example, all require MAPK-mediated cellular responses. A primary function of these cascades appears to be the regulation of gene expression in response to extracellular signals or as part of specific developmental processes. In addition, the MAPK cascades often appear to regulate the cell cycle and vice versa. Despite the success of the gene hunter era in revealing these pathways, there are still many significant gaps in our knowledge of the molecular mechanisms for activation of these cascades and how the cascades regulate cell function. For example, comparison of different yeast signaling pathways reveals a surprising variety of different types of upstream signaling proteins that function to activate a MAPK cascade, yet how the upstream proteins actually activate the cascade remains unclear. We also know that the yeast MAPK pathways regulate each other and interact with other signaling pathways to produce a coordinated pattern of gene expression, but the molecular mechanisms of this cross talk are poorly understood. This review is therefore an attempt to present the current knowledge of MAPK pathways in yeast and some directions for future research in this area.

Regulation of Cdc28 cyclin-dependent protein kinase activity during the cell cycle of the yeast Saccharomyces cerevisiae

The cyclin-dependent protein kinase (CDK) encoded by CDC28 is the master regulator of cell division in the budding yeast Saccharomyces cerevisiae. By mechanisms that, for the most part, remain to be delineated, Cdc28 activity controls the timing of mitotic commitment, bud initiation, DNA replication, spindle formation, and chromosome separation. Environmental stimuli and progress through the cell cycle are monitored through checkpoint mechanisms that influence Cdc28 activity at key cell cycle stages. A vast body of information concerning how Cdc28 activity is timed and coordinated with various mitotic events has accrued. This article reviews that literature. Following an introduction to the properties of CDKs common to many eukaryotic species, the key influences on Cdc28 activity-cyclin-CKI binding and phosphorylation-dephosphorylation events-are examined. The processes controlling the abundance and activity of key Cdc28 regulators, especially transcriptional and proteolytic mechanisms, are then discussed in detail. Finally, the mechanisms by which environmental stimuli influence Cdc28 activity are summarized.

NMR structure of the histidine kinase domain of the E. coli osmosensor EnvZ

Bacteria live in capricious environments, in which they must continuously sense external conditions in order to adjust their shape, motility and physiology. The histidine-aspartate phosphorelay signal-transduction system (also known as the two-component system) is important in cellular adaptation to environmental changes in both prokaryotes and lower eukaryotes. In this system, protein histidine kinases function as sensors and signal transducers. The Escherichia coli osmosensor, EnvZ, is a transmembrane protein with histidine kinase activity in its cytoplasmic region. The cytoplasmic region contains two functional domains: domain A (residues 223-289) contains the conserved histidine residue (H243), a site of autophosphorylation as well as transphosphorylation to the conserved D55 residue of response regulator OmpR, whereas domain B (residues 290-450) encloses several highly conserved regions (G1, G2, F and N boxes) and is able to phosphorylate H243. Here we present the solution structure of domain B, the catalytic core of EnvZ. This core has a novel protein kinase structure, distinct from the serine/threonine/tyrosine kinase fold, with unanticipated similarities to both heatshock protein 90 and DNA gyrase B.

Characterization of genes for two-component phosphorelay mediators with a single HPt domain in Arabidopsis thaliana

Three cDNAs that encode two-component phosphorelay-mediator-like proteins were cloned from Arabidopsis thaliana. Putative proteins (ATHP1-3) contain an HPt (Histidine-containing Phospho transfer)-like domain with a conserved histidine and some invariant residues that are involved in phosphorelay. Growth retardation of YPD1-disrupted yeast cells was reversed with ATHPs, which indicates that ATHPs function as phosphorelay mediators in yeast cells. The ATHP genes are expressed more in roots than in other tissues, similar to the expression of genes for a sensor histidine kinase, ATHK1, and response regulators ATRR1-4. These results suggest that ATHPs function as two-component phosphorelay mediators between sensor histidine kinase and response regulators in Arabidopsis.

The serine, threonine, and/or tyrosine-specific protein kinases and protein phosphatases of prokaryotic organisms: a family portrait

Inspection of the genomes for the bacteria Bacillus subtilis 168, Borrelia burgdorferi B31, Escherichia coli K-12, Haemophilus influenzae KW20, Helicobacter pylori 26695, Mycoplasma genitalium G-37, and Synechocystis sp PCC 6803 and for the archaeons Archaeoglobus fulgidus VC-16 DSM4304, Methanobacterium thermoautotrophicum delta H, and Methanococcus jannaschii DSM2661 revealed that each contains at least one ORF whose predicted product displays sequence features characteristic of eukaryote-like protein-serine/threonine/tyrosine kinases and protein-serine/threonine/tyrosine phosphatases. Orthologs for all four major protein phosphatase families (PPP, PPM, conventional PTP, and low molecular weight PTP) were present in the bacteria surveyed, but not all strains contained all types. The three archaeons surveyed lacked recognizable homologs of the PPM family of eukaryotic protein-serine/threonine phosphatases; and only two prokaryotes were found to contain ORFs for potential phosphatases from all four major families. Intriguingly, our searches revealed a potential ancestral link between the catalytic subunits of microbial arsenate reductases and the protein-tyrosine phosphatases; they share similar ligands (arsenate versus phosphate) and features of their catalytic mechanism (formation of arseno-versus phospho-cysteinyl intermediates). It appears that all prokaryotic organisms, at one time, contained the genetic information necessary to construct protein phosphorylation-dephosphorylation networks that target serine, threonine, and/or tyrosine residues on proteins. However, the potential for functional redundancy among the four protein phosphatase families has led many prokaryotic organisms to discard one, two, or three of the four.

Requirement of STE50 for osmostress-induced activation of the STE11 mitogen-activated protein kinase kinase kinase in the high-osmolarity glycerol response pathway

Exposure of yeast cells to increases in extracellular osmolarity activates the HOG1 mitogen-activated protein (MAP) kinase cascade, which is composed of three tiers of protein kinases: (i) the SSK2, SSK22, and STE11 MAP kinase kinase kinases (MAPKKKs), (ii) the PBS2 MAPKK, and (iii) the HOG1 MAP kinase. Activation of the MAP kinase cascade is mediated by two upstream mechanisms. The SLN1-YPD1-SSK1 two-component osmosensor activates the SSK2 and SSK22 MAPKKKs by direct interaction of the SSK1 response regulator with these MAPKKKs. The second mechanism of HOG1 MAP kinase activation is independent of the two-component osmosensor and involves the SHO1 transmembrane protein and the STE11 MAPKKK. Only PBS2 and HOG1 are common to the two mechanisms. We conducted an exhaustive mutant screening to identify additional elements required for activation of STE11 by osmotic stress. We found that strains with mutations in the STE50 gene, in combination with ssk2Delta ssk22Delta mutations, were unable to induce HOG1 phosphorylation after osmotic stress. Both two-hybrid analyses and coprecipitation assays demonstrated that the N-terminal domain of STE50 binds strongly to the N-terminal domain of STE11. The binding of STE50 to STE11 is constitutive and is not affected by osmotic stress. Furthermore, the two proteins relocalize similarly after osmotic shock. It was concluded that STE50 fulfills an essential role in the activation of the high-osmolarity glycerol response pathway by acting as an integral subunit of the STE11 MAPKKK.

Ethylene gas: perception, signaling and response

During the last decade a genetic approach based on the Arabidopsis 'triple response' to the hormone ethylene has allowed the identification of numerous components of the signal transduction pathway. Cloning of the genes and biochemical analysis of the proteins that they encode are uncovering the molecular mechanisms that allow a plant cell to perceive and respond to this gaseous regulator of plant growth/stress responses.

The two-component hybrid kinase regulator CaNIK1 of Candida albicans

Using degenerate primers of highly conserved regions of two-component response regulators for PCR amplification, a two-component response regulator was cloned from Candida albicans that is homologous to nik-1+ of Neurospora crassa. This two-component hybrid kinase, CaNIK1, also shows features of bacterial two-component response regulators, including a putative unorthodox second histidine kinase motif at the carboxy-terminal end. CaNIK1 was expressed at low levels in both the white and opaque switch phenotypes and in the bud and hyphal growth forms of C. albicans strain WO-1, but in both developmental programmes, the level of transcript was modulated (levels were higher in opaque cells and in hyphae). Partial deletion of both CaNIK1 alleles, by which the histidine autokinase- and ATP-binding domains were removed, did not inhibit either high-frequency phenotypic switching or the bud-hypha transition in high salt concentrations, but in both cases the efficiency of the developmental process was reduced.

The ethylene-receptor family from Arabidopsis: structure and function

The gaseous hormone ethylene regulates many aspects of plant growth and development. Ethylene is perceived by a family of high-affinity receptors typified by the ETR1 protein from Arabidopsis. The ETR1 gene codes for a protein which contains a hydrophobic N-terminal domain that binds ethylene and a C-terminal domain that is related in sequence to histidine kinase-response regulator two-component signal transducers found in bacteria. A structural model for the ethylene-binding domain is presented in which a Cu(I) ion is coordinated within membrane-spanning alpha-helices of the hydrophobic domain. It is proposed that binding of ethylene to the transition metal would induce a conformational change in the sensor domain that would be propagated to the cytoplasmic transmitter domain of the protein. A total of four additional genes that are related in sequence to ETR1 have been identified in Arabidopsis. Specific missense mutations in any one of the five genes leads to ethylene insensitivity in planta. Models for signal transduction that can account for the genetic dominance of these mutations are discussed.

Hypertonicity regulates the function of human neutrophils by modulating chemoattractant receptor signaling and activating mitogen-activated protein kinase p38

Excessive neutrophil activation causes posttraumatic complications, which may be reduced with hypertonic saline (HS) resuscitation. We tested if this is because of modulated neutrophil function by HS. Clinically relevant hypertonicity (10-25 mM) suppressed degranulation and superoxide formation in response to fMLP and blocked the activation of the mitogen activated protein kinases (MAPK) ERK1/2 and p38, but did not affect Ca2+ mobilization. HS did not suppress oxidative burst in response to phorbol myristate acetate (PMA). This indicates that HS suppresses neutrophil function by intercepting signal pathways upstream of or apart from PKC. HS activated p38 by itself and enhanced degranulation in response to PKC activation. This enhancement was reduced by inhibition of p38 with SB203580, suggesting that p38 up-regulation participates in HS-induced enhancements of degranulation. HS had similar effects on the degranulation of cells that were previously stimulated with fMLP, but had no effect on its own, suggesting that HS enhancement of degranulation requires another signal. We conclude that depending on other stimuli, HS can suppress neutrophil activation by intercepting multiple receptor signals or augment degranulation by enhancing p38 signaling. In patients HS resuscitation may reduce posttraumatic complications by preventing neutrophil activation via chemotactic factors released during reperfusion.

Heat stress activates fission yeast Spc1/StyI MAPK by a MEKK-independent mechanism

Fission yeast Spc1/StyI MAPK is activated by many environmental insults including high osmolarity, oxidative stress, and heat shock. Spc1/StyI is activated by Wis1, a MAPK kinase (MEK), which is itself activated by Wik1/Wak1/Wis4, a MEK kinase (MEKK). Spc1/StyI is inactivated by the tyrosine phosphatases Pyp1 and Pyp2. Inhibition of Pyp1 was recently reported to play a crucial role in the oxidative stress and heat shock responses. These conclusions were based on three findings: 1) osmotic, oxidative, and heat stresses activate Spc1/StyI in wis4 cells; 2) oxidative stress and heat shock activate Spc1/StyI in cells that express Wis1AA, in which MEKK consensus phosphorylation sites were replaced with alanine; and 3) Spc1/StyI is maximally activated in Deltapyp1 cells. Contrary to these findings, we report: 1) Spc1/StyI activation by osmotic stress is greatly reduced in wis4 cells; 2) wis1-AA and Deltawis1 cells have identical phenotypes; and 3) all forms of stress activate Spc1/StyI in Deltapyp1 cells. We also report that heat shock, but not osmotic or oxidative stress, activate Spc1 in wis1-DD cells, which express Wis1 protein that has the MEKK consensus phosphorylation sites replaced with aspartic acid. Thus osmotic and oxidative stress activate Spc1/StyI by a MEKK-dependent process, whereas heat shock activates Spc1/StyI by a novel mechanism that does not require MEKK activation or Pyp1 inhibition.

Stress-responsive expression of genes for two-component response regulator-like proteins in Arabidopsis thaliana

Four cDNAs that encode two-component response regulator-like proteins were cloned from Arabidopsis thaliana. Putative proteins (ATRR1-4) contain a receiver domain with a conserved aspartate residue - a possible phosphorylation site - at the N-terminal half. ATRR2 lacks the C-terminal half; the others contain a C-terminal domain abundant in acidic amino acids or proline residues. ATRR1 and ATRR2 are expressed more in roots than in other tissues and are induced by low temperature, dehydration and high salinity. Levels of ATRR3 and ATRR4 were not affected by stress treatments. These results suggest that ATRRs play distinct physiological roles in Arabidopsis, and that some are involved in stress responses.

Identification of a putative histidine kinase two-component phosphorelay gene (CaHK1) in Candida albicans

We have cloned and analysed the sequence of a putative histidine kinase, two-component gene (CaHK1) from Candida albicans. This gene encodes a 2471 amino acid protein (Cahk1p) with an estimated molecular mass of 281.8 kDa. A homology search of Cahk1p with other proteins in the databases showed that Cahk1p exhibits the greatest homology at its C-terminus with both the sensor and regulator components of prokaryotic and eukaryotic two-component histidine kinases. A further analysis of this homology showed that the Cahk1p possessed both sensor and regulator domains in the same polypeptide. Also, Cahk1p is likely to be a soluble protein. The sensor kinase domain of Cahk1p contains conserved motifs that are characteristic of all histidine kinase proteins, including the putative histidine which is believed to be autophosphorylated during activation, ATP binding motifs and others (F- and N-motifs), with unknown function. The Cahk1p regulator domain also contains conserved aspartate and lysine residues and the putative aspartate, which is secondarily phosphorylated by the autophosphorylated histidine. Finally, according to the codon usage frequency of the CaHK1 gene in comparison with other genes from C. albicans, there would appear to be a low level of expression of the gene.

Antibacterial agents that inhibit two-component signal transduction systems

A class of antibacterials has been discovered that inhibits the growth of Gram-positive pathogenic bacteria. RWJ-49815, a representative of a family of hydrophobic tyramines, in addition to being a potent bactericidal Gram-positive antibacterial, inhibits the autophosphorylation of kinase A of the KinA::Spo0F two-component signal transduction system in vitro. Analogs of RWJ-49815 vary greatly in their ability to inhibit growth of bacteria and this ability correlates directly with their activity as kinase A inhibitors. Compared with the potent quinolone, ciprofloxacin, RWJ-49815 exhibits reduced resistance emergence in a laboratory passage experiment. Inhibition of the histidine protein kinase::response regulator two-component signal transduction pathways may present an opportunity to depress chromosomal resistance emergence by targeting multiple proteins with a single inhibitor in a single bacterium. Such inhibitors may represent a class of antibacterials that potentially may represent a breakthrough in antibacterial therapy.

Signal transduction by MAP kinase cascades in budding yeast

Budding yeast contain at least four distinct MAPK (mitogen activated protein kinase) cascades that transduce a variety of intracellular signals: mating-pheromone response, pseudohyphal/invasive growth, cell wall integrity, and high osmolarity adaptation. Although each MAPK cascade contains a conserved set of three protein kinases, the upstream activation mechanisms for these cascades are diverse, including a trimeric G protein, monomeric small G proteins, and a prokaryotic-like two-component system. Recently, it became apparent that there is extensive sharing of signaling elements among the MAPK pathways; however, little undesirable cross-talk occurs between various cascades. The formation of multi-protein signaling complexes is probably centrally important for this insulation of individual MAPK cascades.

Response regulators implicated in His-to-Asp phosphotransfer signaling in Arabidopsis

The His to Asp phosphotransfer signal transduction mechanism involves three common signaling domains: the transmitter (or His-kinase), the receiver, and the histidine-containing phototransfer (HPt) domain. Typically, a sensor kinase has a His-kinase domain and a response regulator has a receiver domain containing a phosphoaccepting aspartate, whereas a histidine-containing phototransfer domain serves as a mediator of the histidine-to-aspartate phosphotransfer. This signal transduction mechanism was thought to be restricted to prokaryotes. However, many examples have been discovered in diverse eukaryotic species including higher plants. In Arabidopsis, three sensor kinases have been characterized, namely, ETR1, ERS, and CKI1, which were suggested to be involved in ethylene- and cytokinin-dependent signal transduction pathways, respectively. To date, no response regulator has been discovered in higher plants. We identify five distinct Arabidopsis response regulator genes, each encoding a protein containing a receiver-like domain. In vivo and in vitro evidence that ARRs can function as phosphoaccepting response regulators was obtained by employing the Escherichia coli His-Asp phosphotransfer signaling system.

Activation of the yeast SSK2 MAP kinase kinase kinase by the SSK1 two-component response regulator

Exposure of yeast cells to increased extracellular osmolarity induces the HOG1 mitogen-activated protein kinase (MAPK) cascade, which is composed of SSK2, SSK22 and STE11 MAPKKKs, PBS2 MAPKK and HOG1 MAPK. The SSK2/SSK22 MAPKKKs are activated by a 'two-component' osmosensor composed of SLN1, YPD1 and SSK1. The SSK1 C-terminal receiver domain interacts with an N-terminal segment of SSK2. Upon hyperosmotic treatment, SSK2 is autophosphorylated rapidly, and this reaction requires the interaction of SSK1 with SSK2. Autophosphorylation of SSK2 is an intramolecular reaction, suggesting similarity to the mammalian MEKK1 kinase. Dephosphorylation of SSK2 renders the kinase inactive, but it can be re-activated by addition of SSK1 in vitro. A conserved threonine residue (Thr1460) in the activation loop of SSK2 is important for kinase activity. Based on these observations, we propose the following two-step activation mechanism of SSK2 MAPKKK. In the first step, the binding of SSK1 to the SSK1-binding site in the N-terminal domain of SSK2 causes a conformational change in SSK2 and induces its latent kinase activity. In the second step, autophosphorylation of SSK2 renders its activity independent of the presence of SSK1. A similar mechanism might be applicable to other MAPKKKs from both yeast and higher eukaryotes.

An Escherichia coli protein that exhibits phosphohistidine phosphatase activity towards the HPt domain of the ArcB sensor involved in the multistep His-Asp phosphorelay

The Escherichia coli sensory kinase, ArcB, possesses a histidine-containing phosphotransfer (HPt) domain, which is implicated in the His-Asp multistep phosphorelay. We searched for a presumed phosphohistidine phosphatase, if present, which affects the function of the HPt domain through its dephosphorylation activity. Using in vivo screening, we first identified a gene that appeared to interfere with the His-Asp phosphorelay between the HPt domain and the receiver domain of OmpR, provided that the gene product was expressed through a multicopy plasmid. The cloned gene (named sixA) was found to encode a protein consisting of 161 amino acids, which has a noticeable sequence motif, an arginine-histidine-glycine (RHG) signature, at its N-terminus. Such an RHG signature, which presumably functions as a nucleophilic phosphoacceptor, was previously found in a set of divergent enzymes, including eukaryotic fructose-2,6-bisphosphatase, E. coli periplasmic phosphatase and E. coli glucose-1-phosphate phosphatase, and ubiquitous phosphoglycerate mutase. Otherwise, the entire amino acid sequences of none of these enzymes resembles that of SixA. It was demonstrated in vitro that the purified SixA protein exhibited the ability to release the phosphoryl group from the HPt domain of ArcB, but the mutant protein lacking the crucial histidine residue in the RHG signature did not. Evidence was also provided that a deletion mutation of sixA on the chromosome affected the in vivo phosphotransfer signalling. These results support the view that SixA is capable of functioning as a phosphohistidine phosphatase that may be implicated in the His-Asp phosphorelay through regulating the phosphorylation state of the HPt domain.

SacY, a transcriptional antiterminator from Bacillus subtilis, is regulated by phosphorylation in vivo

SacY antiterminates transcription of the sacB gene in Bacillus subtilis in response to the presence of sucrose in the growth medium. We have found that it can substitute for BglG, a homologous protein, in antiterminating transcription of the bgl operon in Escherichia coli. We therefore sought to determine whether, similarly to BglG, SacY is regulated by reversible phosphorylation in response to the availability of the inducing sugar. We show here that two forms of SacY, phosphorylated and nonphosphorylated, exist in B. subtilis cells and that the ratio between them depends on the external level of sucrose. Addition of sucrose to the growth medium after SacY phosphorylation in the cell resulted in its rapid dephosphorylation. The extent of SacY phosphorylation was found to be proportional to the cellular levels of SacX, a putative sucrose permease which was previously shown to have a negative effect on SacY activity. Thus, the mechanism by which the sac sensory system modulates sacB expression in response to sucrose involves reversible phosphorylation of the regulator SacY, and this process appears to depend on the SacX sucrose sensor. The sac system is therefore a member of the novel family of sensory systems represented by bgl.

The Win1 mitotic regulator is a component of the fission yeast stress-activated Sty1 MAPK pathway

The fission yeast Sty1 mitogen-activated protein (MAP) kinase (MAPK) and its activator the Wis1 MAP kinase kinase (MAPKK) are required for cell cycle control, initiation of sexual differentiation, and protection against cellular stress. Like the mammalian JNK/SAPK and p38/CSBP1 MAPKs, Sty1 is activated by a range of environmental insults including osmotic stress, hydrogen peroxide, UV light, menadione, heat shock, and the protein synthesis inhibitor anisomycin. We have recently identified two upstream regulators of the Wis1 MAPKK, namely the Wak1 MAPKKK and the Mcs4 response regulator. Cells lacking Mcs4 or Wak1, however, are able to proliferate under stressful conditions and undergo sexual differentiation, suggesting that additional pathway(s) control the Wis1 MAPKK. We now show that this additional signal information is provided, at least in part, by the Win1 mitotic regulator. We show that Wak1 and Win1 coordinately control activation of Sty1 in response to multiple environmental stresses, but that Wak1 and Win1 perform distinct roles in the control of Sty1 under poor nutritional conditions. Our results suggest that the stress-activated Sty1 MAPK integrates information from multiple signaling pathways.

Isolation of CaSLN1 and CaNIK1, the genes for osmosensing histidine kinase homologues, from the pathogenic fungus Candida albicans

Recent studies have revealed that fungi possess a mechanism similar to bacterial two-component systems to respond to extracellular changes in osmolarity. In Saccharomyces cerevisiae, Sln1p contains both histidine kinase and receiver (response regulator) domains and acts as an osmosensor protein that regulates the downstream HOG1 MAP kinase cascade. SLN1 of Candida albicans was functionally cloned using an S. cerevisiae strain in which SLN1 expression was conditionally suppressed. Deletion analysis of the cloned gene demonstrated that the receiver domain of C. albicans Sln1p was not necessary to rescue SLN1-deficient S. cerevisiae strains. Unlike S. cerevisiae, a null mutation of C. albicans SLN1 was viable under regular and high osmotic conditions, but it caused a slight growth retardation at high osmolarity. Southern blotting with C. albicans SLN1 revealed the presence of related genes, one of which is highly homologous to the NIK1 gene of Neurospora crassa. Thus, C. albicans harbours both SLN1- and NIK1-type histidine kinases.

Genetic analysis of protein tyrosine phosphatases

Genetic analysis has enhanced our understanding of the biological roles of many protein tyrosine kinases (PTKs). More recently, studies utilizing both spontaneous mutants and mutants induced by homologous recombination techniques have begun to yield key insights into the role of specific protein tyrosine phosphatases (PTPs) and to suggest how PTKs and PTPs interact. Specific PTPs in Saccharomyces cerevesiae and Schizomyces pombe regulate MAP kinase pathways. Several Drosophila receptor PTPs control axonal targeting pathways, whereas the non-receptor PTP Corkscrew (Csw), plays an essential positive signaling role in multiple developmental pathways directed by receptor PTKs. The vertebrate homolog of Csw, SHP-2, also is required for growth factor signaling and normal development. Finally, very recent studies of other mammalian PTPs suggest that they have critical roles in processes as diverse as hematopoiesis and liver and pituitary development.

Expression of the glyoxalase I gene of Saccharomyces cerevisiae is regulated by high osmolarity glycerol mitogen-activated protein kinase pathway in osmotic stress response

Methylglyoxal is a cytotoxic metabolite derived from dihydroxyacetone phosphate, an intermediate of glycolysis. Detoxification of methylglyoxal is performed by glyoxalase I. Expression of the structural gene of glyoxalase I (GLO1) of Saccharomyces cerevisiae under several stress conditions was investigated using the GLO1-lacZ fusion gene, and expression of the GLO1 gene was found to be specifically induced by osmotic stress. The Hog1p is one of the mitogen-activated protein kinases (MAPKs) in S. cerevisiae, and both Msn2p and Msn4p are the transcriptional regulators that are thought to be under the control of Hog1p-MAPK. Expression of the GLO1 gene under osmotic stress was completely repressed in hog1Delta disruptant and was repressed approximately 80 and 50% in msn2Delta and msn4Delta disruptants, respectively. A double mutant of the MSN2 and MSN4 gene was unable to induce expression of the GLO1 gene under highly osmotic conditions. Glucose consumption increased approximately 30% during the adaptive period in osmotic stress in the wild type strain. On the contrary, it was reduced by 15% in the hog1Delta mutant. When the yeast cell is exposed to highly osmotic conditions, glycerol is synthesized as a compatible solute. Glycerol is synthesized from glucose, and a rate-limiting enzyme in glycerol biosynthesis is glycerol-3-phosphate dehydrogenase (GPD1 gene product), which catalyzes reduction of dihydroxyacetone phosphate to glycerol 3-phosphate. Expression of the GPD1 gene is also under the control of Hog1p-MAPK. Methylglyoxal is also synthesized from dihydroxyacetone phosphate; therefore, induction of the GLO1 gene expression by osmotic stress was thought to scavenge methylglyoxal, which increased during glycerol production for adaptation to osmotic stress.

3-phosphohistidine and 6-phospholysine are substrates of a 56-kDa inorganic pyrophosphatase from bovine liver

A 56-kDa inorganic pyrophosphatase isolated from bovine liver hydrolyzed PPi, imidodiphosphate, 3-phosphohistidine, and 6-phospholysine at rates of 0.11, 0.44, 1.09, and 1.22 mumol/min/mg protein, respectively, in a reaction mixture containing 1 mM MgCl2 at pH 8.2. The hydrolysis of imidodiphosphate was influenced by various treatments in a different manner from that of N-phosphorylated amino acids, indicating that the pyrophosphatase has two different catalytic sites for imidodiphosphate and N-phosphorylated amino acids, respectively. Evidence for separate catalytic sites consists of the following findings: the activity on hydrolysis of imidodiphosphate gave a bell-shaped pH curve with a peak at pH 6.5, while the activity on hydrolysis of N-phosphorylated amino acids maintained a high level in the pH range between 6.0 and 9.5. One hundred micromolar p-chloromercuriphenyl sulfonate inhibited the hydrolysis of imidodiphosphate by 35% and did not inhibit that of N-phosphorylated amino acids. Two millimolar magnesium chloride repressed the hydrolysis of imidodiphosphate and had no inhibitory effect on the hydrolysis of N-phosphorylated amino acids. Moreover, methylenediphosphonic acid, an analog of imidodiphosphate, stimulated the hydrolysis of imidodiphosphate in the presence of MgCl2, while it potentiated the substrate inhibition on hydrolysis of N-phosphorylated amino acids.

Post-translational modification of proteins by reversible phosphorylation in prokaryotes

Microorganisms have developed three different systems for catalyzing protein phosphorylation and using this reversible modificaiton to regulate their cellular activities. The first 'classical' system utilizes nucleoside-triphosphates as phosphoryl donors and leads to the modification of protein substrates at serine/threonine or tyrosine residues. The second system, called 'two-component system', requires first a sensor kinase which autophosphorylates at a histidine residue at the expense of adenosine-triphosphate, then a response regulator which is modified in turn at an aspartate residue and thereafter induces a metabolic change within the cell. The third system, called 'PTS system', makes use of phosphoenol pyruvate to generate a phosphoryl group which is passed down a chain of several proteins and finally transferred to a sugar. There is increasing evidence that, contrary to an early concept, these systems and the corresponding enzymes (protein kinases and phosphoprotein phosphatases) share a number of structural and functional similarities with the phosphorylation-dephosphorylation machineries found in eukaryotes. Therefore one can expect that microorganisms will serve, once again, as a basic model for exploring and understanding a key regulatory mechanism, reversible protein phosphorylation, which concerns all organisms.

Programmed cell death in prokaryotes

Programmed cell death (PCD), also referred to as apoptosis, is a cellular "suicide" mechanism, based on information from its own internal metabolism, environment, developmental history, and genome. This system was described in eukaryotes continuously along evolution, through amoebae, nematodes, insects, and animals. PCD is essential for the proper development or function of a cell system, organ, or survival of the organism as a whole. Research in the last 2 decades has shown that the life cycle of several prokaryotic organisms display developmental programs, similar to metazoan differentiation, that is part of their adaptation to stressful environments. These include warmer cell formation and differentiation in Caulobacter cereus, sporulation in Bacillus and Streptomyces, heterocyst formation in Anabaena, development of bacteroids in Rhizobium, the formation of multicellular fruiting bodies and sporulation in Myxobacteria, and the formation of nonculturable, but viable, cells in various Gram-negative bacteria. Moreover, and more significantly, the photosynthetic bacteria Rhodobacter capsulatus were shown to release nucleoprotein particles designated "gene transfer agent (GTA)" as they enter the stationary phase. GTAs contain DNA of 3.6 x 10(6) molecular weight, representing all parts of the genome, and they may be taken up by other strains of R. capsulatus, and complement mutants. We postulate that these various modes of stress adaptations in bacteria are prokaryotic manifestation, and possibly the phylogenetic precursor, of the eukaryotic phenomenon, programmed cell death, and therefore we propose to designate it "proapoptosis". In addition to their function, apoptosis and proapoptosis share various mechanistic programmed features, including DNA fragmentation and packaging, cell shrinkage, degradation of RNA, proteolysis and synthesis of new proteins, and the involvement of reactive oxygen species.

Xbp1, a stress-induced transcriptional repressor of the Saccharomyces cerevisiae Swi4/Mbp1 family

We have identified Xbp1 (XhoI site-binding protein 1) as a new DNA-binding protein with homology to the DNA-binding domain of the Saccharomyces cerevisiae cell cycle regulating transcription factors Swi4 and Mbp1. The DNA recognition sequence was determined by random oligonucleotide selection and confirmed by gel retardation and footprint analyses. The consensus binding site of Xbp1, GcCTCGA(G/A)G(C/A)g(a/g), is a palindromic sequence, with an XhoI restriction enzyme recognition site at its center. This Xbpl binding site is similar to Swi4/Swi6 and Mbp1/Swi6 binding sites but shows a clear difference from these elements in one of the central core bases. There are binding sites for Xbp1 in the G1 cyclin promoter (CLN1), but they are distinct from the Swi4/Swi6 binding sites in CLN1, and Xbp1 will not bind to Swi4/Swi6 or Mbp1/Swi6 binding sites. The XBP1 promoter contains several stress-regulated elements, and its expression is induced by heat shock, high osmolarity, oxidative stress, DNA damage, and glucose starvation. When fused to the LexA DNA-binding domain, Xbp1 acts as transcriptional repressor, defining it as the first repressor in the Swi4/Mbp1 family and the first potential negative regulator of transcription induced by stress. Overexpression of XBP1 results in a slow-growth phenotype, lengthening of G1, an increase in cell volume, and a repression of G1 cyclin expression. These observations suggest that Xbp1 may contribute to the repression of specific transcripts and cause a transient cell cycle delay under stress conditions.

Structural and functional analyses of activating amino acid substitutions in the receiver domain of NtrC: evidence for an activating surface

The bacterial enhancer-binding protein NtrC activates transcription when phosphorylated on aspartate 54 in its amino (N)-terminal regulatory domain or when altered by constitutively activating amino acid substitutions. The N-terminal domain of NtrC, which acts positively on the remainder of the protein, is homologous to a large family of signal transduction domains called receiver domains. Phosphorylation of an aspartate residue in a receiver domain modulates the function of a downstream target, but the accompanying structural changes are not clear. In the present work we examine structural and functional differences between the wild-type receiver domain of NtrC and mutant forms carrying constitutively activating substitutions. Combinations of such substitutions resulted in both increased structural changes in the N-terminal domain, monitored by NMR chemical shift differences, and increased transcriptional activation by the full-length protein. Structural changes caused by substitutions outside the active site (D86N and A89T) were not only local but extended over a substantial portion of the N-terminal domain including the region from alpha-helix 3 to beta-strand 5 ("3445 face") and propagating to the active site. Interestingly, the activating substitution of glutamate for aspartate at the site of phosphorylation (D54E) also triggered structural changes in the 3445 face. Thus, the active site and the 3445 face appear to interact. Implications with respect to how phosphorylation may affect the structure of receiver domains and how structural changes may be communicated to the remainder of NtrC are discussed.

BglG, the response regulator of the Escherichia coli bgl operon, is phosphorylated on a histidine residue

We have shown previously that the activity of BglG, the response regulator of the bgl system, as a transcriptional antiterminator is modulated by the sensor BglF, which reversibly phosphorylates BglG. We show here that the phosphoryl group on BglG is present as a phosphoramidate, based on the sensitivity of phosphorylated BglG to heat, hydroxylamine, and acidic but not basic conditions. By analyzing the products of base-hydrolyzed phosphorylated BglG by thin-layer chromatography, we show that the phosphorylation occurs on a histidine residue. This result supports the notion that the bgl system is a member of a new family of bacterial sensory systems.

A histidine protein kinase homolog from the endosymbiont of the hydrothermal vent tubeworm Riftia pachyptila

The uncultivated bacterial endosymbionts of the hydrothermal vent tubeworm Riftia pachyptila play a central role in providing their host with fixed carbon. While this intimate association between host and symbiont indicates tight integration and coordination of function via cellular communication mechanisms, no such systems have been identified. To elucidate potential signal transduction pathways in symbionts that may mediate symbiont-host communication, we cloned and characterized a gene encoding a histidine protein kinase homolog isolated from a symbiont fosmid library. The gene, designated rssA (for Riftia symbiont signal kinase), resembles known sensor kinases and encodes a protein capable of phosphorylating response regulators in Escherichia coli. A second open reading frame, rssB (for Riftia symbiont signal regulator), encodes a protein similar to known response regulators. These results suggest that the symbionts utilize a phosphotransfer signal transduction mechanism to communicate external signals that may mediate recognition of or survival within the host. The specific signals eliciting a response by the signal transduction proteins of the symbiont remain to be elucidated.

Two-component kinase-like activity of nm23 correlates with its motility-suppressing activity

Nm23 genes, which encode nucleoside diphosphate kinases, have been implicated in suppressing tumor metastasis. The motility of human breast carcinoma cells can be suppressed by transfection with wild-type nm23-H1, but not by transfections with two nm23-H1 mutants, nm23-H1(S12OG) and nm23-H1(P96S). Here we report that nm23-H1 can transfer a phosphate from its catalytic histidine to aspartate or glutamate residues on 43-kDa membrane proteins. One of the 43-kDa membrane proteins was not phosphorylated by either nm23-H1(P96S) or nm23-H1(S120G), and another was phosphorylated much more slowly by nm23-H1(P96S) and by nm23-H1(S120G) than by wild-type nm23-H1. Nm23-H1 also can transfer phosphate from its catalytic histidine to histidines on ATP-citrate lyase and succinic thiokinase. The rates of phosphorylation of ATP-citrate lyase by nm23-H1(S120G) and nm23-H1(P96S) were similar to that by wild-type nm23-H1. The rate of phosphorylation of succinic thiokinase by nm23-H1(S120) was similar to that by wild-type nm23-H1, and the rate of phosphorylation of succinic thiokinase by nm23-H1(P96S) was about half that by wild-type nm23-H1. Thus, the transfer of phosphate from nm23-H1 to aspartates or glutamates on other proteins appears to correlate better with the suppression of motility than does the transfer to histidines.