Structural studies of the Arabidopsis thaliana ethylene signal transduction pathway

Since ethylene was first recognized as a phytohormone, the scope of its profound and multi-faceted impact on plant growth and development has been continuously growing. In Arabidopsis thaliana the response to ethylene is regulated by a group of five receptors (ETR1, ETR2, ERS1, ERS2 and EIN4), which are located in the endoplasmic reticulum membrane. Ethylene is bound by the membrane-embedded N-terminal part, which is followed by a GAF domain. The remaining C-terminal, cytosolic domains resemble the classical bacterial two-component system consisting of a histidine kinase (HK) and in some receptors a receiver domain. Constitutive triple response 1 (CTR1), whose kinase domain bears most resemblance to the RAF family of Ser/Thr protein kinases, directly interacts with the ethylene receptors and thus links signal reception to the intracellular signalling pathway. Therefore, this signalling pathway presents the interesting case, wherein a two-component signalling system manipulates a MAPKKK and possibly a MAPKKK signalling cascade. Still, the question of receptor deactivation by ethylene and the ensuing signal transduction to CTR1 remain unanswered. A number of constructs comprising domains of the receptors as well as the kinase domain of CTR1 were expressed and purified. The dimerization domain (DHp) of ERS1 could be crystallized and solved by MAD to 1.9 Å resolution. The domain is structurally similar to other domains of this family, normally found in bacteria, consisting of a homodimer forming a coiled-coil and a four-helix bundle. Different to the previously available structures a larger portion of the N-terminal coiled-coil could be determined. When the DHp domain structure is compared with other HKs it is most similar to the phosphatase-competent state, one of the three activities attributed to HKs. A trans-phosphorylation mechanism in the dimer is predicted by topological arrangement of the structure. In addition SAXS data of the cytoplasmic part of ETR1 was collected and a model of the domain architecture is presented, showing their relative location with respect to each other. A different location of the receiver domain compared to a recent bacterial structure is suggested. In addition the three-dimensional structures of the active, tri-phosphorylated and the unphosphorylated, inactive kinase domain of CTR1 in complex with staurosporine were determined at 3.0 Å and 2.5 Å resolution, respectively. They illustrate the conformational rearrangements that form the basis of activity regulation. The active kinase domain forms back-to-back dimers in solution, while the unphosphorylated kinase is a monomer. The back-to-back dimer interface is virtually identical to the one found in B-RAF and a number of mutants were identified interfering with the dimerization and also affecting the kinase activity. Furthermore the effects of activation loop phosphorylation on the activity were explored. The results strongly suggest another layer of activity regulation of CTR1 through dimerization in vivo. Steric restraints further indicate regulation of kinase activity across dimers with a 'front-to-front' activation interface, which points to CTR1 mediated ethylene receptor crosstalk generating a continuous head-to-tail oligomer of kinase domains

Transcriptome analysis of human cancer reveals a functional role of Heme Oxygenase-1 in tumor cell adhesion

<p>Abstract</p> <p>Background</p> <p>Heme Oxygenase-1 (HO-1) is expressed in many cancers and promotes growth and survival of neoplastic cells. Recently, HO-1 has been implicated in tumor cell invasion and metastasis. However, the molecular mechanisms underlying these biologic effects of HO-1 remain largely unknown. To identify a common mechanism of action of HO-1 in cancer, we determined the global effect of HO-1 on the transcriptome of multiple tumor entities and identified a universal HO-1-associated gene expression signature.</p> <p>Results</p> <p>Genome-wide expression profiling of Heme Oxygenase-1 expressing versus HO-1 silenced BeWo choriocarcinoma cells as well as a comparative meta-profiling of the preexisting expression database of 190 human tumors of 14 independent cancer types led to the identification of 14 genes, the expression of which correlated strongly and universally with that of HO-1 (P = 0.00002). These genes included regulators of cell plasticity and extracellular matrix (ECM) remodeling (MMP2, ADAM8, TGFB1, BGN, COL21A1, PXDN), signaling (CRIP2, MICB), amino acid transport and glycosylation (SLC7A1 and ST3GAL2), estrogen and phospholipid biosynthesis (AGPAT2 and HSD17B1), protein stabilization (IFI30), and phosphorylation (ALPPL2). We selected PXDN, an adhesion molecule involved in ECM formation, for further analysis and functional characterization. Immunofluorescence and Western blotting confirmed the positive correlation of expression of PXDN and HO-1 in BeWo cancer cells as well as co-localization of these two proteins in invasive extravillous trophoblast cells. Modulation of HO-1 expression in both loss-of and gain-of function cell models (BeWo and 607B melanoma cells, respectively) demonstrated a direct relationship of HO-1 expression with cell adhesion to Fibronectin and Laminin coated wells. The adhesion-promoting effects of HO-1 were dependent on PXDN expression, as loss of PXDN in HO-1 expressing BeWo and 607B cells led to reduced cell attachment to Laminin and Fibronectin coated wells.</p> <p>Conclusions</p> <p>Collectively, our results show that HO-1 expression determines a distinct 'molecular signature' in cancer cells, which is enriched in genes associated with tumorigenesis. The protein network downstream of HO-1 modulates adhesion, signaling, transport, and other critical cellular functions of neoplastic cells and thus promotes tumor cell growth and dissemination.</p

A Novel Metagenomic Short-Chain Dehydrogenase/Reductase Attenuates Pseudomonas aeruginosa Biofilm Formation and Virulence on Caenorhabditis elegans

In Pseudomonas aeruginosa, the expression of a number of virulence factors, as well as biofilm formation, are controlled by quorum sensing (QS). N-Acylhomoserine lactones (AHLs) are an important class of signaling molecules involved in bacterial QS and in many pathogenic bacteria infection and host colonization are AHL-dependent. The AHL signaling molecules are subject to inactivation mainly by hydrolases (Enzyme Commission class number EC 3) (i.e. N-acyl-homoserine lactonases and N-acyl-homoserine-lactone acylases). Only little is known on quorum quenching mechanisms of oxidoreductases (EC 1). Here we report on the identification and structural characterization of the first NADP-dependent short-chain dehydrogenase/reductase (SDR) involved in inactivation of N-(3-oxo-dodecanoyl)-L-homoserine lactone (3-oxo-C12-HSL) and derived from a metagenome library. The corresponding gene was isolated from a soil metagenome and designated bpiB09. Heterologous expression and crystallographic studies established BpiB09 as an NADP-dependent reductase. Although AHLs are probably not the native substrate of this metagenome-derived enzyme, its expression in P. aeruginosa PAO1 resulted in significantly reduced pyocyanin production, decreased motility, poor biofilm formation and absent paralysis of Caenorhabditis elegans. Furthermore, a genome-wide transcriptome study suggested that the level of lasI and rhlI transcription together with 36 well known QS regulated genes was significantly (≥10-fold) affected in P. aeruginosa strains expressing the bpiB09 gene in pBBR1MCS-5. Thus AHL oxidoreductases could be considered as potent tools for the development of quorum quenching strategies

Protein kinase domain of CTR1 from Arabidopsis thaliana promotes ethylene receptor cross talk.

Ethylene controls many aspects of plant growth and development. Signaling by the gaseous phytohormone is initiated by disulfide-linked membrane-bound receptors, and the formation of heteromeric receptor clusters contributes to the broad range of ethylene responsiveness. In Arabidopsis thaliana, the TCS-like ethylene receptors interact with the cytosolic serine/threonine kinase constitutive triple response 1 (CTR1), a proposed mitogen-activated protein kinase kinase kinase. In the absence of the hormone, the receptor and therefore CTR1 are active. Hence, ethylene acts as an inverse agonist of its signaling pathway. The three-dimensional structures of the active, triphosphorylated and the unphosphorylated, inactive kinase domain of CTR1 in complex with staurosporine illustrate the conformational rearrangements that form the basis of activity regulation. Additionally, in analytical ultracentrifugation experiments, active kinase domains form back-to-back dimers, while inactive and activation loop variants are monomers. Together with a front-to-front activation interface, the active protein kinase dimers thereby engage in interactions that promote CTR1-mediated cross talk between ethylene receptor clusters. This model provides a structural foundation for the observed high sensitivity of plants to ethylene

Structural Model of the Cytosolic Domain of the Plant Ethylene Receptor 1 (ETR1)

Ethylene initiates important aspects of plant growth and development through disulfide-linked receptor dimers located in the endoplasmic reticulum. The receptors feature a small transmembrane, ethylene binding domain followed by a large cytosolic domain, which serves as a scaffold for the assembly of large molecular weight complexes of different ethylene receptors and other cellular participants of the ethylene signaling pathway. Here we report the crystallographic structures of the ethylene receptor 1 (ETR1) catalytic ATP-binding and the ethylene response sensor 1 dimerization histidine phosphotransfer (DHp) domains and the solution structure of the entire cytosolic domain of ETR1, all from Arabidopsis thaliana. The isolated dimeric ethylene response sensor 1 DHp domain is asymmetric, the result of different helical bending angles close to the conserved His residue. The structures of the catalytic ATP-binding, DHp, and receiver domains of ethylene receptors and of a homologous, but dissimilar, GAF domain were refined against experimental small angle x-ray scattering data, leading to a structural model of the entire cytosolic domain of the ethylene receptor 1. The model illustrates that the cytosolic domain is shaped like a dumbbell and that the receiver domain is flexible and assumes a position different from those observed in prokaryotic histidine kinases. Furthermore the cytosolic domain of ETR1 plays a key role, interacting with all other receptors and several participants of the ethylene signaling pathway. Our model, therefore, provides the first step toward a detailed understanding of the molecular mechanics of this important signal transduction process in plants

Structural Insights into the Nucleotide-Binding Domains of the P1B-type ATPases HMA6 and HMA8 from Arabidopsis thaliana

International audienceCopper is a crucial ion in cells, but needs to be closely controlled due to its toxic potential and ability to catalyse the formation of radicals. In chloroplasts, an important step for the proper functioning of the photosynthetic electron transfer chain is the delivery of copper to plastocyanin in the thylakoid lumen. The main route for copper transport to the thylakoid lumen is driven by two PIB-type ATPases, Heavy Metal ATPase 6 (HMA6) and HMA8, located in the inner membrane of the chloroplast envelope and in the thylakoid membrane, respectively. Here, the crystal structures of the nucleotide binding domain of HMA6 and HMA8 from Arabidopsis thaliana are reported at 1.5Å and 1.75Å resolution, respectively, providing the first structural information on plants Cu+-ATPases. The structures reveal a compact domain, with two short helices on both sides of a twisted beta-sheet. A double mutant, aiding in the crystallization, provides a new crystal contact, but also avoids an internal clash highlighting the benefits of construct modifications. Finally, the histidine in the HP motif of the isolated domains, unable to bind ATP, shows a side chain conformation distinct from nucleotide bound structures

Cloning, expression, purification, crystallization and preliminary X-ray crystallographic analysis of interleukin-4-inducing principle from Schistosoma mansoni eggs (IPSE/alpha-1)

Interleukin-4-inducing principle from S. mansoni eggs (IPSE/alpha-1) was cloned, purified and crystallized using the hanging-drop vapour-diffusion method at 293 K. X-ray diffraction data were collected to 1.66 Å resolution

HMA6 and HMA8 are two chloroplast Cu+-ATPases with different enzymatic properties.

The final version of record is available at http://www.bioscirep.org/content/35/3/e00201International audienceCopper (Cu) plays a key role in the photosynthetic process as cofactor of the plastocyanin (PC), an essential component of the chloroplast photosynthetic electron transfer chain. Encoded by the nuclear genome, PC is translocated in its apo-form into the chloroplast and the lumen of thylakoids where it is processed to its mature form and acquires Cu. In Arabidopsis, Cu delivery into the thylakoids involves two transporters of the PIB-1 ATPases family, heavy metal associated protein 6 (HMA6) located at the chloroplast envelope and HMA8 at the thylakoid membrane. To gain further insight into the way Cu is delivered to PC, we analysed the enzymatic properties of HMA8 and compared them with HMA6 ones using in vitro phosphorylation assays and phenotypic tests in yeast. These experiments reveal that HMA6 and HMA8 display different enzymatic properties: HMA8 has a higher apparent affinity for Cu+ but a slower dephosphorylation kinetics than HMA6. Modelling experiments suggest that these differences could be explained by the electrostatic properties of the Cu+ releasing cavities of the two transporters and/or by the different nature of their cognate Cu+ acceptors (metallochaperone/PC)