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.

A Crystallin Fold in the Interleukin-4-inducing Principle of Schistosoma mansoni Eggs (IPSE/α-1) Mediates IgE Binding for Antigen-independent Basophil Activation.

The IL-4-inducing principle from Schistosoma mansoni eggs (IPSE/α-1), the major secretory product of eggs from the parasitic worm S. mansoni, efficiently triggers basophils to release the immunomodulatory key cytokine interleukin-4. Activation by IPSE/α-1 requires the presence of IgE on the basophils, but the detailed molecular mechanism underlying activation is unknown. NMR and crystallographic analysis of IPSEΔNLS, a monomeric IPSE/α-1 mutant, revealed that IPSE/α-1 is a new member of the ßγ-crystallin superfamily. We demonstrate that this molecule is a general immunoglobulin-binding factor with highest affinity for IgE. NMR binding studies of IPSEΔNLS with the 180-kDa molecule IgE identified a large positively charged binding surface that includes a flexible loop, which is unique to the IPSE/α-1 crystallin fold. Mutational analysis of amino acids in the binding interface showed that residues contributing to IgE binding are important for IgE-dependent activation of basophils. As IPSE/α-1 is unable to cross-link IgE, we propose that this molecule, by taking advantage of its unique IgE-binding crystallin fold, activates basophils by a novel, cross-linking-independent mechanism.

Cloning, expression, purification and preliminary X-ray analysis of the dimerization domain of ethylene response sensor 1 (ERS1) from Arabidopsis thaliana.

Ethylene signalling is initiated by a group of membrane-bound receptors with similarity to two-component systems. ERS1 belongs, together with ETR1, to subfamily 1, which plays a predominant role in ethylene signalling. The dimerization domain of ERS1 was crystallized in space groups C222(1) and P2(1)2(1)2, with two and four molecules per asymmetric unit, respectively. The crystals diffracted X-ray radiation to 1.9 Šresolution.

Optimization of protein buffer cocktails using Thermofluor.

The stability and homogeneity of a protein sample is strongly influenced by the composition of the buffer that the protein is in. A quick and easy approach to identify a buffer composition which increases the stability and possibly the conformational homogeneity of a protein sample is the fluorescence-based thermal-shift assay (Thermofluor). Here, a novel 96-condition screen for Thermofluor experiments is presented which consists of buffer and additive parts. The buffer screen comprises 23 different buffers and the additive screen includes small-molecule additives such as salts and nucleotide analogues. The utilization of small-molecule components which increase the thermal stability of a protein sample frequently results in a protein preparation of higher quality and quantity and ultimately also increases the chances of the protein crystallizing.

Single isomorphous replacement phasing of selenomethionine-containing proteins using UV-induced radiation damage.

The most commonly used heavy-atom derivative, selenium, requires the use of a tunable beamline to access the Se K edge for experimental phasing using anomalous diffraction methods, whereas X-ray diffraction experiments for selenium-specific ultraviolet radiation-damage-induced phasing can be performed on fixed-wavelength beamlines or even using in-house X-ray sources. Several nonredundant X-ray diffraction data sets were collected from three different selenomethionine (Mse) derivatized protein crystals at energies far below the absorption edge before and after exposing the crystal to ultraviolet (UV) radiation using 266 nm lasers of flux density 1.7 × 10¹5 photons s⁻¹â€…mm⁻² for 10-50 min. A detailed analysis revealed that significant changes in diffracted intensities were induced by ultraviolet irradiation whilst retaining crystal isomorphism. These intensity changes allowed the crystal structures to be solved by the radiation-damage-induced phasing (RIP) technique. Inspection of the crystal structures and electron-density maps demonstrated that covalent bonds between selenium and carbon at all sites located in the core of the proteins or in a hydrophobic environment were much more susceptible to UV radiation-induced cleavage than other bonds typically present in Mse proteins. The rapid UV radiation-induced bond cleavage opens a reliable new paradigm for phasing when no tunable X-ray source is available. The behaviour of Mse derivatives of various proteins provides novel insights and an initial basis for understanding the mechanism of selenium-specific UV radiation damage.

Cloning, expression, purification and preliminary X-ray analysis of the protein kinase domain of constitutive triple response 1 (CTR1) from Arabidopsis thaliana.

Ethylene, a gaseous plant hormone, is perceived by a group of membrane-bound receptors. Constitutive triple response 1 (CTR1) from Arabidopsis thaliana directly interacts with ethylene receptors and thus links signal reception to the intracellular signalling pathway. The C-terminal protein kinase domain of CTR1 has been crystallized in its wild-type form and as a kinase-dead mutant. The wild-type crystals diffracted X-ray radiation to 3 Šresolution and the crystals of the kinase-dead mutant diffacted to 2.5 Šresolution. The crystals belonged to space groups P4(1)2(1)2 and P4(2)2(1)2, respectively, with two molecules per asymmetric unit in both cases.

In Vitro Production of Perdeuterated Proteins in H2O for Biomolecular NMR Studies.

The cell-free synthesis is an efficient strategy to produce in large scale protein samples for structural investigations. In vitro synthesis allows for significant reduction of production time, simplification of purification steps and enables production of both soluble and membrane proteins. The cell-free reaction is an open system and can be performed in presence of many additives such as cofactors, inhibitors, redox systems, chaperones, detergents, lipids, nanodisks, and surfactants to allow for the expression of toxic membrane proteins or intrinsically disordered proteins. In this chapter we present protocols to prepare E. coli S30 cellular extracts, T7 RNA polymerase, and their use for in vitro protein expression. Optimizations of the protocol are presented for preparation of protein samples enriched in deuterium, a prerequisite for the study of high-molecular-weight proteins by NMR spectroscopy. An efficient production of perdeuterated proteins is achieved together with a full protonation of all the amide NMR probes, without suffering from residual protonation on aliphatic carbons. Application to the production of the 468 kDa TET2 protein assembly for NMR investigations is presented.

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

The interleukin-4-inducing principle from Schistosoma mansoni eggs (IPSE/alpha-1) triggers the release of large amounts of interleukin-4 from human blood basophils, thus presumably playing an immunomodulatory role during schistosome infection. IPSE/alpha-1 was crystallized and a native X-ray data set was collected to 1.66 A resolution from a single crystal at 100 K using synchrotron radiation. The crystal belonged to space group P6(1) or P6(5), with one molecule per asymmetric unit.

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

Copper 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.

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

Copper (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).

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.

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-C(12)-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.

DNA sequences from Arabidopsis, which encode protein kinases and function as upstream regulators of Snf1 in yeast.

Sucrose nonfermenting-1 (Snf1)-related protein kinase-1 (SnRK1) of plants is a global regulator of carbon metabolism through the modulation of enzyme activity and gene expression. It is structurally and functionally related to the yeast protein kinase, Snf1, and to mammalian AMP-activated protein kinase. Two DNA sequences from Arabidopsis thaliana, previously known only by their data base accession numbers of NM_ 125448.3 (protein ID NP_200863) and NM_114393.3 (protein ID NP_566876) each functionally complemented a Saccharomyces cerevisiae elm1 sak1 tos3 triple mutant. This indicates that the Arabidopsis proteins are able to substitute for one of the missing yeast upstream kinases, which are required for activity of Snf1. Both plant proteins were shown to phosphorylate a peptide with the amino acid sequence of the phosphorylation site in the T-loop of SnRK1 and by inference SnRK1 in Arabidopsis. The proteins encoded by NM_125448.3 and NM_114393.3 have been named AtSnAK1 and AtSnAK2 (Arabidopsis thaliana SnRK1-activating kinase), respectively. We believe this is the first time that upstream activators of SnRK1 have been described in any plant species.