Intra- and interprotein phosphorylation between two-hybrid histidine kinases controls Myxococcus xanthus developmental progression

Histidine-aspartate phosphorelay signaling systems are used to couple stimuli to cellular responses. A hallmark feature is the highly modular signal transmission modules that can form both simple "two-component" systems and sophisticated multicomponent systems that integrate stimuli over time and space to generate coordinated and fine-tuned responses. The deltaproteobacterium Myxococcus xanthus contains a large repertoire of signaling proteins, many of which regulate its multicellular developmental program. Here, we assign an orphan hybrid histidine protein kinase, EspC, to the Esp signaling system that negatively regulates progression through the M. xanthus developmental program. The Esp signal system consists of the hybrid histidine protein kinase, EspA, two serine/threonine protein kinases, and a putative transport protein. We demonstrate that EspC is an essential component of this system because ΔespA, ΔespC, and ΔespA ΔespC double mutants share an identical developmental phenotype. Neither substitution of the phosphoaccepting histidine residue nor deletion of the entire catalytic ATPase domain in EspC produces an in vivo mutant developmental phenotype. In contrast, substitution of the receiver phosphoaccepting residue yields the null phenotype. Although the EspC histidine kinase can efficiently autophosphorylate in vitro, it does not act as a phosphodonor to its own receiver domain. Our in vitro and in vivo analyses suggest the phosphodonor is instead the EspA histidine kinase. We propose EspA and EspC participate in a novel hybrid histidine protein kinase signaling mechanism involving both inter- and intraprotein phosphotransfer. The output of this signaling system appears to be the combined phosphorylated state of the EspA and EspC receiver modules. This system regulates the proteolytic turnover of MrpC, an important regulator of the developmental program.     

Four unusual two-component signal transduction homologs, RedC to RedF, are necessary for timely development in Myxococcus xanthus

We identified a cluster of four two-component signal transduction genes that are necessary for proper progression of Myxococcus xanthus through development. redC to redF mutants developed and sporulated early, resulting in small, numerous, and disorganized fruiting bodies. Yeast two-hybrid analyses suggest that RedCDEF act in a single signaling pathway. The previously identified espA gene displays a phenotype similar to that of redCDEF. However, combined mutants defective in espA redCDEF exhibited a striking additive developmental phenotype, suggesting that EspA and RedC to RedF play independent roles in controlling developmental progression.     

Sporulation timing in Myxococcus xanthus is controlled by the espAB locus

The fruiting body development of Myxococcus xanthus consists of two separate but interacting pathways: one for aggregation of many cells to form raised mounds and the other for sporulation of individual cells into myxospores. Sporulation of individual cells normally occurs after mound formation, and is delayed at least 30 h after starvation under our laboratory conditions. This suggests that M. xanthus has a mechanism that monitors progress towards aggregation prior to triggering sporulation. A null mutation in a newly identified gene, espA (early sporulation), causes sporulation to occur much earlier compared with the wild type (16 h earlier). In contrast, a null mutation in an adjacent gene, espB, delays sporulation by about 16 h compared with the wild type. Interestingly, it appears that the espA mutant does not require raised mounds for sporulation. Many mutant cells sporulate outside the fruiting bodies. In addition, the mutant can sporulate, without aggregation into raised mounds, under some conditions in which cells normally do not form fruiting bodies. Based on these observations, it is hypothesized that EspA functions as an inhibitor of sporulation during early fruiting body development while cells are aggregating into raised mounds. The aggregation-independent sporulation of the espA mutant still requires starvation and high cell density. The espA and espB genes are expressed as an operon and their translations appear to be coupled. Expression occurs only under developmental conditions and does not occur during vegetative growth or during glycerol-induced sporulation. Sequence analysis of EspA indicates that it is a histidine protein kinase with a fork head-associated (FHA) domain at the N-terminus and a receiver domain at the C-terminus. This suggests that EspA is part of a two-component signal transduction system that regulates the timing of sporulation initiation.     

Two Ser/Thr protein kinases essential for efficient aggregation and spore morphogenesis in Myxococcus xanthus

Myxococcus xanthus has a complex life cycle that involves vegetative growth and development. Previously, we described the espAB locus that is involved in timing events during the initial stages of fruiting body formation. Deletion of espA caused early aggregation and sporulation, whereas deletion of espB caused delayed aggregation and sporulation resulting in reduced spore yields. In this study, we describe two genes, pktA5 and pktB8, that flank the espAB locus and encode Ser/Thr protein kinase (STPK) homologues. Cells deficient in pktA5 or pktB8 formed translucent mounds and produced low spore yields, similar in many respects to espB mutants. Double mutant analysis revealed that espA was epistatic to pktA5 and pktB8 with respect to aggregation and fruiting body morphology, but that pktA5 and pktB8 were epistatic to espA with respect to sporulation efficiency. Expression profiles of pktA5-lacZ and pktB8-lacZ fusions and Western blot analysis showed that the STPKs are expressed under vegetative and developmental conditions. In vitro kinase assays demonstrated that the RD kinase, PktA5, autophosphorylated on threonine residue(s) and phosphorylated the artificial substrate, myelin basic protein. In contrast, autophosphorylation of the non-RD kinase, PktB8, was not observed in vitro; however, the phenotype of a pktB8 kinase-dead point mutant resembled the pktB8 deletion mutant, indicating that this residue was important for function and that it likely functions as a kinase in vivo. Immunoprecipitation of Tap-tagged PktA5 and PktB8 revealed an interaction with EspA during development in M. xanthus. These results, taken together, suggest that PktA5 and PktB8 are STPKs that function during development by interacting with EspA and EspB to regulate M. xanthus development.     

Phenotypic and Genetic Analysis of det2, a New Mutant That Affects Light-Regulated Seedling Development in Arabidopsis

The greening phenotypes produced by recessive mutations in a gene designated de-etiolated-2 (DET2) are described. Recessive mutations in the DET2 gene uncouple light signals from a number of light-dependent processes. det2 mutations result in dark-grown Arabidopsis thaliana seedlings with many characteristics of light-grown plants, including hypocotyl growth inhibition, cotyledon expansion, primary leaf initiation, anthocyanin accumulation, and derepression of light-regulated gene expression. In contrast to these morphological and gene expression changes, however, the chloroplast development program is not initiated in the dark in det2 mutants, suggesting that light-regulated gene expression precedes the differentiation of etioplasts to chloroplasts. det2 mutations thus reveal at least two classes of downstream light-regulated responses that differ in their timing and control mechanisms. Homozygous det2 mutations also affect photoperiodic responses in light-grown plants, including timing of flowering, dark adaptation of gene expression, and onset of leaf senescence. The phenotype of det1 det2 double mutants is additive, implying that DET1 and DET2 function in distinct pathways that affect downstream light-regulated genes. Furthermore, these pathways are not utilized solely during early seedling development but must also be required to regulate different aspects of the light developmental program during later stages of vegetative growth.     

Initial binding of Shiga toxin-producing Escherichia coli to host cells and subsequent induction of actin rearrangements depend on filamentous EspA-containing surface appendages

Shiga toxin-producing Escherichia coli (STEC) induce so-called attaching and effacing lesions that enable the tight adherence of these pathogens to the gut epithelium. All of the genes necessary for this process are present in the locus of enterocyte effacement, which encodes a type III secretion system, the secreted Esp proteins and the surface protein intimin. In this study we sequenced the espA gene of STEC, generated and characterized a corresponding deletion mutant and raised EspA-specific monoclonal antibodies to analyse the functional role of this protein during infection. EspA was detected in often filament-like structures decorating all bacteria that had attached to HeLa cells. These appendages were especially prominent on bacteria that had not yet induced the formation of actin pedestals, indicating that they mediate the initial contact of STEC to their target cells. Consistently, a deletion of the espA gene completely abolished the capacity of such STEC mutants to bind to HeLa cells and to induce actin rearrangements. Surface appendages similar to those described in this study are also formed by Pseudomonas syringae and may represent a structural element common to many bacterial pathogens that deliver proteins into their target cells via a type III secretion system.     

Molecular modeling and functional analysis of the AtoS-AtoC two-component signal transduction system of Escherichia coli

The AtoS-AtoC two-component signal transduction system positively regulates the expression of the atoDAEB operon in Escherichia coli. Upon acetoacetate induction, AtoS sensor kinase autophosphorylates and subsequently phosphorylates, thereby activating, the response regulator AtoC. In a previous work we have shown that AtoC is phosphorylated at both aspartate 55 and histidine73. In this study, based on known three-dimensional structures of other two component regulatory systems, we modeled the 3D-structure of the receiver domain of AtoC in complex with the putative dimerization/autophosphorylation domain of the AtoS sensor kinase. The produced structural model indicated that aspartate 55, but not histidine 73, of AtoC is in close proximity to the conserved, putative phosphate-donor, histidine (H398) of AtoS suggesting that aspartate 55 may be directly involved in the AtoS-AtoC phosphate transfer. Subsequent biochemical studies with purified recombinant proteins showed that AtoC mutants with alterations of aspartate 55, but not histidine 73, were unable to participate in the AtoS-AtoC phosphate transfer in support of the modeling prediction. In addition, these AtoC mutants displayed reduced DNA-dependent ATPase activity, although their ability to bind their target DNA sequences in a sequence-specific manner was found to be unaltered.     

Molecular modeling and functional analysis of the AtoS–AtoC two-component signal transduction system of Escherichia coli

The AtoS–AtoC two-component signal transduction system positively regulates the expression of the atoDAEB operon in Escherichia coli. Upon acetoacetate induction, AtoS sensor kinase autophosphorylates and subsequently phosphorylates, thereby activating, the response regulator AtoC. In a previous work we have shown that AtoC is phosphorylated at both aspartate 55 and histidine73. In this study, based on known three-dimensional structures of other two component regulatory systems, we modeled the 3D-structure of the receiver domain of AtoC in complex with the putative dimerization/autophosphorylation domain of the AtoS sensor kinase. The produced structural model indicated that aspartate 55, but not histidine 73, of AtoC is in close proximity to the conserved, putative phosphate-donor, histidine (H398) of AtoS suggesting that aspartate 55 may be directly involved in the AtoS–AtoC phosphate transfer. Subsequent biochemical studies with purified recombinant proteins showed that AtoC mutants with alterations of aspartate 55, but not histidine 73, were unable to participate in the AtoS–AtoC phosphate transfer in support of the modeling prediction. In addition, these AtoC mutants displayed reduced DNA-dependent ATPase activity, although their ability to bind their target DNA sequences in a sequence-specific manner was found to be unaltered.     

肠出血性大肠杆菌O157∶H7 EspA和EspB蛋白的重组表达及其免疫效果

目的：通过DNA重组技术表达肠出血性大肠杆菌（EHEC）0157：H7的EspA和EspB蛋白,并分析它们的免疫保护性。方法：采用PCR技术从EHEC0157：H7基因组中扩增espA和espB基因,连接至pET-22b（4-）载体上,转化至宿主细胞大肠杆菌BL21（DE3）,经IPTG诱导表达,用亲和层析纯化目的蛋白,SDS-PAGE测定其相对分子质量,免疫小鼠分析其免疫保护性。结果：重组espA和espB基因片段的测序结果与GenBank中的相应基因序列完全一致,一致性均为100％;得到了纯度为95％以上的重组EspA和EspB蛋白,免疫小鼠所得到的抗体效价均为10^6。结论：重组EspA和EspB蛋白获得了可溶性表达,表达的蛋白具有良好的免疫保护性,为进一步制备疫苗奠定了基础。     

Myxococcus xanthus sasS encodes a sensor histidine kinase required for early developmental gene expression.

Initiation of Myxococcus xanthus multicellular development requires integration of information concerning the cells' nutrient status and density. A gain-of-function mutation, sasB7, that bypasses both the starvation and high cell density requirements for developmental expression of the 4521 reporter gene, maps to the sasS gene. The wild-type sasS gene was cloned and sequenced. This gene is predicted to encode a sensor histidine protein kinase that appears to be a key element in the transduction of starvation and cell density inputs. The sasS null mutants express 4521 at a basal level, form defective fruiting bodies, and exhibit reduced sporulation efficiencies. These data indicate that the wild-type sasS gene product functions as a positive regulator of 4521 expression and participates in M. xanthus development. The N terminus of SasS is predicted to contain two transmembrane domains that would locate the protein to the cytoplasmic membrane. The sasB7 mutation, an E139K missense mutation, maps to the predicted N-terminal periplasmic region. The C terminus of SasS contains all of the conserved residues typical of the sensor histidine protein kinases. SasS is predicted to be the sensor protein in a two-component system that integrates information required for M. xanthus developmental gene expression.     

On the role of conserved histidine 106 in 10-formyltetrahydrofolate dehydrogenase catalysis: connection between hydrolase and dehydrogenase mechanisms

The enzyme, 10-formyltetrahydrofolate dehydrogenase (FDH), converts 10-formyltetrahydrofolate (10-formyl-THF) to tetrahydrofolate in an NADP(+)-dependent dehydrogenase reaction or an NADP(+)-independent hydrolase reaction. The hydrolase reaction occurs in a 310-amino acid long amino-terminal domain of FDH (N(t)-FDH), whereas the dehydrogenase reaction requires the full-length enzyme. The amino-terminal domain of FDH shares some sequence identity with several other enzymes utilizing 10-formyl-THF as a substrate. These enzymes have two strictly conserved residues, aspartate and histidine, in the putative catalytic center. We have shown recently that the conserved aspartate is involved in FDH catalysis. In the present work we studied the role of the conserved histidine, His(106), in FDH function. Site-directed mutagenesis experiments showed that replacement of the histidine with alanine, asparagine, aspartate, glutamate, glutamine, or arginine in N(t)-FDH resulted in expression of insoluble proteins. Replacement of the histidine with another positively charged residue, lysine, produced a soluble mutant with no hydrolase activity. The insoluble mutants refolded from inclusion bodies adopted a conformation inherent to the wild-type N(t)-FDH, but they did not exhibit any hydrolase activity. Substitution of alanine for three non-conserved histidines located close to the conserved one did not reveal any significant changes in the hydrolase activity of N(t)-FDH. Expressed full-length FDH with the substitution of lysine for the His(106) completely lost both the hydrolase and dehydrogenase activities. Thus, our study showed that His(106), besides being an important structural residue, is also directly involved in both the hydrolase and dehydrogenase mechanisms of FDH. Modeling of the putative hydrolase catalytic center/folate-binding site suggested that the catalytic residues, aspartate and histidine, are unlikely to be adjacent to the catalytic cysteine in the aldehyde dehydrogenase catalytic center. We hypothesize that 10-formyl-THF dehydrogenase reaction is not an independent reaction but is a combination of hydrolase and aldehyde dehydrogenase reactions.