Improving expression and solubility of rice proteins produced as fusion proteins in Escherichia coli

Use of the selected metal-dependent enzymes for exploring applicability of human annexin A1 as a purification tag

Several fusion tags have been developed for non-chromatographic fusion protein purification. Previously, we identified that human annexin A1 as a novel N-terminal purification tag was used for purifying the fusion proteins produced in Escherichia coli through precipitation in 10 mM Ca2+ buffer, and redissolution of the precipitate in 15 mM EDTA buffer. In this work, we selected four metal-dependent enzymes including E. coli 5-aminolevulinate dehydratase, yeast 3-hydroxyanthranilate 3,4-dioxygenase, maize serine racemase and copper amine oxidase for investigating the annexin A1 tag applicability. Fusion of the His6-tag or the enzyme changed the behavior of precipitation-redissolution. The relatively high recovery yields of three tagged enzymes with the improved purities were obtained through two rounds of purification, whereas low recovery yield of the annexin A1 tagged maize amine oxidase was prepared. The added EDTA displayed different abilities to redissolve the fusion proteins precipitates in two precipitation-redissolution cycles. It inactivated three enzymes and obviously inhibited the activity of the fused maize serine racemase. Based on current findings, we believe that four enzymes could be applied for evaluating applicability of the proteins or peptides as affinity tags for chromatographic purification in a calcium dependent manner.

Expression, purification and crystallization of TGW6, which limits grain weight in rice

Rice is the staple food for over half the world's population. Genes associated with rice yield include THOUSAND GRAIN WEIGHT 6 (TGW6), which negatively regulates the number of endosperm cells as well as grain weight. The 1-bp deletion allele of tgw6 cloned from the Indian landrace rice cultivar Kasalath, which has lost function, enhances both grain size and yield. TGW6 has been utilized as a target for breeding and genome editing to increase the yield of rice. In the present study, we describe an improved heterologous expression system of TGW6 in Escherichia coli to enable purification of the recombinant protein. The best expression was achieved using codon optimized TGW6 with a 30 amino acid truncation at the N-terminus (Δ30TGW6) in the Rosetta-gami 2(DE3) host strain. Furthermore, we found that calcium ions were critical for the purification of stable Δ30TGW6. Crystals of Δ30TGW6 were obtained using the sitting-drop vapor-diffusion method at 283 K, which diffracted X-rays to at least 2.6 Å resolution. Herein, we established an efficient procedure for the production and purification of TGW6 in sufficient quantities for structural and functional studies. Detailed information concerning the molecular mechanism of TGW6 will enable the design of more efficient ways to control the activity of the enzyme.

Structure of the Complete Dimeric Human GDAP1 Core Domain Provides Insights into Ligand Binding and Clustering of Disease Mutations

Charcot-Marie-Tooth disease (CMT) is one of the most common inherited neurological disorders. Despite the common involvement of ganglioside-induced differentiation-associated protein 1 (GDAP1) in CMT, the protein structure and function, as well as the pathogenic mechanisms, remain unclear. We determined the crystal structure of the complete human GDAP1 core domain, which shows a novel mode of dimerization within the glutathione S-transferase (GST) family. The long GDAP1-specific insertion forms an extended helix and a flexible loop. GDAP1 is catalytically inactive toward classical GST substrates. Through metabolite screening, we identified a ligand for GDAP1, the fatty acid hexadecanedioic acid, which is relevant for mitochondrial membrane permeability and Ca²⁺ homeostasis. The fatty acid binds to a pocket next to a CMT-linked residue cluster, increases protein stability, and induces changes in protein conformation and oligomerization. The closest homologue of GDAP1, GDAP1L1, is monomeric in its full-length form. Our results highlight the uniqueness of GDAP1 within the GST family and point toward allosteric mechanisms in regulating GDAP1 oligomeric state and function.

Structural investigation of APRs to improve the solubility of outer membrane protease (PgtE) from Salmonella enterica serotype typhi- A multi-constraint approach

Outer membrane proteins were playing a crucial role on the several functions controlled by cell membranes even though they are not naturally expressed at higher levels. In order to obtain biologically active protein, the denaturation of these inclusion bodies must be optimized using chaotropic agents. Hence, this study focuses on improving the yield of Outer Membrane Protease (PgtE) from Salmonella enterica serotype Typhi (S. Typhi) using chaotropes and additives. Denaturation methods were tried with various pH, detergents, and reducing agents were used to optimize the solubility of PgtE with biologically active form. Due to the aggregation, we failed to achieve the maximum yield of PgtE. Consequently, we predicted 9 Aggregation Prone Regions (APRs) in PgtE, which are mutated by known structural Gatekeepers. We calculated the Aggregation Index (AI) of PgtE with 10 mM of aspartic acid as an additive in optimized buffer. In addition, the mutations at specific positions within the protein structure can act as APRs suppressors without affecting protein stability with CABS flex dynamics. The multiple sequence analysis demonstrate that aspartic acid is appropriate denaturing additive for other Gram-negative pathogens of Omptin family.

Physical and thermodynamic characterization of the rice gibberellin receptor/gibberellin/DELLA protein complex

Gibberellins (GAs) are phytohormones that regulate various developmental processes in plants. The initial GA signalling events involve the binding of a GA to the soluble GA receptor protein GID1, followed by the binding of the complex to the negative transcriptional regulator of GA signaling, the DELLA protein. Although X-ray structures for certain Arabidopsis GID1/GA/DELLA protein complexes have previously been determined, examination of these complexes did not fully clarify how a DELLA protein recognizes and binds to a GID1/GA complex. Herein, we present a study aimed at physically defining, via a combination of gel chromatography, isothermal titration calorimetry (ITC), small-angle X-ray scattering experiments (SAXS), NMR spectroscopy and mutagenesis, how the rice DELLA protein (SLR1) binds to the rice GID1/GA complex. We have identified the shortest SLR1 sequence (M28-A112) that binds the rice GID/GA complex tightly. The binding constant for the ternary complex that includes SLR1(M28-A112) is 2.9 × 10⁷ M⁻¹; the binding is enthalpically driven and does not depend on the chemical nature of the bound GA. Furthermore, the results of SAXS, ITC, and gel filtration experiments indicate that when free in solution, SLR1(M28-A112) is a natively unfolded protein. The NMR experiments expand this observation to show that the unfolded mutant also contains a small amount of marginally stable secondary structure. Conversely, the protein has a highly ordered structure when bound one-to-one to GID1/GA.

High-affinity heterotetramer formation between the large myelin-associated glycoprotein and the dynein light chain DYNLL1

The close association of myelinated axons and their myelin sheaths involves numerous intercellular molecular interactions. For example, myelin-associated glycoprotein (MAG) mediates myelin-to-axon adhesion and signalling via molecules on the axonal surface. However, knowledge about intracellular binding partners of myelin proteins, including MAG, has remained limited. The two splice isoforms of MAG, S- and L-MAG, display distinct cytoplasmic domains and spatiotemporal expression profiles. We used yeast two-hybrid screening to identify interaction partners of L-MAG and found the dynein light chain DYNLL1 (also termed dynein light chain 8). DYNLL1 homodimers are known to facilitate dimerization of target proteins. L-MAG and DYNLL1 associate with high affinity, as confirmed with recombinant proteins in vitro. Structural analyses of the purified complex indicate that the DYNLL1-binding segment is localized close to the L-MAG C terminus, next to the Fyn kinase Tyr phosphorylation site. The crystal structure of the complex between DYNLL1 and its binding segment on L-MAG shows 2 : 2 binding in a parallel arrangement, indicating a heterotetrameric complex. The homology between L-MAG and previously characterized DYNLL1-ligands is limited, and some details of binding site interactions are unique for L-MAG. The structure of the complex between the entire L-MAG cytoplasmic domain and DYNLL1, as well as that of the extracellular domain of MAG, were modelled based on small-angle X-ray scattering data, allowing structural insights into L-MAG interactions on both membrane surfaces. Our data imply that DYNLL1 dimerizes L-MAG, but not S-MAG, through the formation of a specific 2 : 2 heterotetramer. This arrangement is likely to affect, in an isoform-specific manner, the functions of MAG in adhesion and myelin-to-axon signalling. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/. Read the Editorial Highlight for this article on page 712.

Plant Stature Related receptor-like Kinanse2 (PSRK2) acts as a factor that determines stem elongation toward gibberellins response in rice

Gibberellins (GAs) are a family of plant hormones that are important to multiple aspects of plant growth and development, especially stem elongation. A PSRK2 was obtained through screening and identifying RLK dominant negative mutants. Phenotype of the loss-of-function mutants, psrk2-DN and psrk2-RNAi, showed that PSRK2 could influence the length of the uppermost and fourth internodes, indicating that PSRK2 might regulate cell division in the intercalary meristems and/or cell elongation in the internodes. Moreover, the expression pattern showed that PSRK2 was strongly expressed in the joined-nodes after the start-up of reproductive growth, but undetectable in leaves. PSRK2 expression was also found to be induced by GA₃, and PSRK2 was involved in GA signaling in cereal aleurone cells, and PSRK2 influence the relative length of the second leaf sheaths in seedling stage. These results indicate PSRK2 is a component of GA signaling pathway that controls stem elongation by negatively regulating GA responses. Abbreviations: Os: Oryza sativa; At: Arabidopsis thaliana; RNAi: RNA interfere; DN: Dominate Negative; SMART: Simple Modular Architecture Research Tool; Uni : Uniconazol; PSRK2: Plant Stature Related receptor-like Kinase 2; RLK: Receptor-like Kinase; GA: Gibberellin; IAA: indole-3-acetic acid; BL: Brassinosteroid.

Membrane-localized ubiquitin ligase ATL15 functions in sugar-responsive growth regulation in Arabidopsis

Ubiquitin ligases play important roles in regulating various cellular processes by modulating the protein function of specific ubiquitination targets. The Arabidopsis Tóxicos en Levadura (ATL) family is a group of plant-specific RING-type ubiquitin ligases that localize to membranes via their N-terminal transmembrane-like domains. To date, 91 ATL isoforms have been identified in the Arabidopsis genome, with several ATLs reported to be involved in regulating plant responses to environmental stresses. However, the functions of most ATLs remain unknown. This study, involving transcriptome database analysis, identifies ATL15 as a sugar responsive ATL gene in Arabidopsis. ATL15 expression was rapidly down-regulated in the presence of sugar. The ATL15 protein showed ubiquitin ligase activity in vitro and localized to plasma membrane and endomembrane compartments. Further genetic analyses demonstrated that the atl15 knockout mutants are insensitive to high glucose concentrations, whereas ATL15 overexpression depresses plant growth. In addition, endogenous glucose and starch amounts were reciprocally affected in the atl15 knockout mutants and the ATL15 overexpressors. These results suggest that ATL15 protein plays a significant role as a membrane-localized ubiquitin ligase that regulates sugar-responsive plant growth in Arabidopsis.

Arabidopsis CBL-Interacting Protein Kinases Regulate Carbon/Nitrogen-Nutrient Response by Phosphorylating Ubiquitin Ligase ATL31

In response to the ratio of available carbon (C) and nitrogen (N) nutrients, plants regulate their metabolism, growth, and development, a process called the C/N-nutrient response. However, the molecular basis of C/N-nutrient signaling remains largely unclear. In this study, we identified three CALCINEURIN B-LIKE (CBL)-INTERACTING PROTEIN KINASES (CIPKs), CIPK7, CIPK12, and CIPK14, as key regulators of the C/N-nutrient response during the post-germination growth in Arabidopsis. Single-knockout mutants of CIPK7, CIPK12, and CIPK14 showed hypersensitivity to high C/low N conditions, which was enhanced in their triple-knockout mutant, indicating that they play a negative role and at least partly function redundantly in the C/N-nutrient response. Moreover, these CIPKs were found to regulate the function of ATL31, a ubiquitin ligase involved in the C/N-nutrient response via the phosphorylation-dependent ubiquitination and proteasomal degradation of 14-3-3 proteins. CIPK7, CIPK12, and CIPK14 physically interacted with ATL31, and CIPK14, acting with CBL8, directly phosphorylated ATL31 in a Ca²⁺-dependent manner. Further analyses showed that these CIPKs are required for ATL31 phosphorylation and stabilization, which mediates the degradation of 14-3-3 proteins in response to C/N-nutrient conditions. These findings provide new insights into C/N-nutrient signaling mediated by protein phosphorylation.

Collapsin response mediator protein 2: high-resolution crystal structure sheds light on small-molecule binding, post-translational modifications, and conformational flexibility

Collapsin response mediator protein 2 (CRMP-2) is a neuronal protein involved in axonal pathfinding. Intense research is focusing on its role in various neurological diseases. Despite a wealth of studies, not much is known about the molecular mechanisms of CRMP-2 function in vivo. The detailed structure-function relationships of CRMP-2 have also largely remained unknown, in part due to the fact that the available crystal structures lack the C-terminal tail, which is known to be a target for many post-translational modifications and protein interactions. Although CRMP-2, and other CRMPs, belong to the dihydropyrimidinase family, they have lost the enzymatic active site. Drug candidates for CRMP-2-related processes have come up during the recent years, but no reports of CRMP-2 complexes with small molecules have emerged. Here, CRMP-2 was studied at 1.25-Å resolution using X-ray crystallography. In addition, ligands were docked into the homotetrameric structure, and the C-terminal tail of CRMP-2 was produced recombinantly and analyzed. We have obtained the human CRMP-2 crystal structure at atomic resolution and could identify small-molecule binding pockets in the protein. Structures obtained in different crystal forms highlight flexible regions near possible ligand-binding pockets. We also used the CRMP-2 structure to analyze known or suggested post-translational modifications at the 3D structural level. The high-resolution CRMP-2 structure was also used for docking experiments with the sulfur amino acid metabolite lanthionine ketimine and its ester. We show that the C-terminal tail is intrinsically disordered, but it has conserved segments that may act as interaction sites. Our data provide the most accurate structural data on CRMPs to date and will be useful in further computational and experimental studies on CRMP-2, its function, and its binding to small-molecule ligands.

Characterization of ubiquitin ligase SlATL31 and proteomic analysis of 14-3-3 targets in tomato fruit tissue (Solanum lycopersicum L.)

The 14-3-3 proteins participate in many aspects of plant physiology by interacting with phosphorylated proteins and thereby regulating target protein functions. In Arabidopsis plant, the ubiquitin ligase ATL31 controls 14-3-3 stability via both direct interaction and ubiquitination, and this consequently regulates post-germinative growth in response to carbon and nitrogen nutrient availability. Since 14-3-3 proteins regulate the activities of many key enzymes related to nutrient metabolism, one would anticipate that they should play an essential role not only in vegetative but also in reproductive tissue. Because fruit yield largely depends on carbon and nitrogen availability and their utilization, the function of 14-3-3 proteins was analyzed in tomato fruit tissue. Here, we isolated and characterized an ubiquitin ligase SlATL31 (Solyc03g112340) from tomato and demonstrated that SlATL31 has ubiquitin ligase activity as well as interaction with tomato 14-3-3 proteins, suggesting the possibility that the SlATL31 functions as an ubiquitin ligase for 14-3-3 similarly to its Arabidopsis ortholog. Furthermore, we performed proteomic analysis of 14-3-3 interacting proteins and identified 106 proteins as putative 14-3-3 targets including key enzymes for carbon metabolism and photosynthesis. This 14-3-3 interactome result and available transcriptome profile suggest a considerable yet complex role of 14-3-3 proteins in tomato fruit tissue.

BIOLOGICAL SIGNIFICANCE

Considerable cumulative evidence exists which implies that 14-3-3 proteins are involved in the regulation of plant primary metabolism. Here we provide the first report of 14-3-3 interactome analysis and identify putative 14-3-3 targets in tomato fruit tissue, which may be highly important given the documented metabolic shifts, which occur during fruit development and ripening. These data open future research avenues by which to understand the regulation of the role of post-translational regulation in tomato fruit development.

Expression platforms for producing eukaryotic proteins: a comparison of E. coli cell-based and wheat germ cell-free synthesis, affinity and solubility tags, and cloning strategies

Vectors designed for protein production in Escherichia coli and by wheat germ cell-free translation were tested using 21 well-characterized eukaryotic proteins chosen to serve as controls within the context of a structural genomics pipeline. The controls were carried through cloning, small-scale expression trials, large-scale growth or synthesis, and purification. Successfully purified proteins were also subjected to either crystallization trials or (1)H-(15)N HSQC NMR analyses. Experiments evaluated: (1) the relative efficacy of restriction/ligation and recombinational cloning systems; (2) the value of maltose-binding protein (MBP) as a solubility enhancement tag; (3) the consequences of in vivo proteolysis of the MBP fusion as an alternative to post-purification proteolysis; (4) the effect of the level of LacI repressor on the yields of protein obtained from E. coli using autoinduction; (5) the consequences of removing the His tag from proteins produced by the cell-free system; and (6) the comparative performance of E. coli cells or wheat germ cell-free translation. Optimal promoter/repressor and fusion tag configurations for each expression system are discussed.

Over-expression, purification and isotopic labeling of a tag-less human glucose-dependent insulinotropic polypeptide (hGIP)

Glucose-dependent insulinotropic polypeptide (GIP), a gut peptide released in response to food intake brings about secretion of insulin in a glucose-dependent manner upon binding to its receptor, GIPR. GIP–GIPR has emerged as a new vista for anti-diabetic drug discovery and their interaction is being probed at the atomic level to aid rational drug design. In order to probe this interaction on cells, the current study attempts towards expressing ¹⁵N-labeled GIP using classical molecular biology tools. We have developed a methodology to obtain GIP devoid of extra amino acid(s); a prerequisite to the intended interaction study. The synthetic GIP cDNA with a Factor Xa protease site at the N-terminus of GIP was inserted in the vector pET32a(+); the fusion protein thus expressed was eventually cleaved to obtain GIP. After successful Factor Xa cleavage, the cleaved GIP was confirmed by western blot. Subsequently, the (¹⁵N)GIP was obtained using the aforementioned procedure and confirmed by MALDI-TOF.

Human Ind1 expression causes over-expression of E. coli beta-lactamase ampicillin resistance protein

Ind1, a mitochondrial P-loop NTPase is essential for assembly of respiratory complex-I. Respiratory complex-I (NADH: ubiquinone oxidoreductase), a large (mitochondrial inner membrane) enzyme, is made of 45 subunits and 8 iron-sulfur clusters. Ind1, an iron-sulfur cluster protein involved in the maturation of respiratory complex and binds an Fe/S cluster via a conserved CXXC motif in a labile way. Ind1 has been proposed as a specialized biogenesis factor involved in delivering the Fe/S clusters to the apo complex-I subunits. The IND1 gene is conserved in eukaryotes and is present in genomes of the species that retain functional respiratory complex-I. Depletion of human Ind1 causes ultra-structural changes in depleted mitochondria, including the loss of cristae membranes, massive remodeling of respiratory super complexes, and increased lactate production. Ind1 sequence bears known nucleotide binding domain motifs and was first classified as Nucleotide Binding Protein-Like (NUBPL). Despite the obvious importance of Ind1, very little is known about this protein; in particular its structure as well as its Fe/S cluster binding properties. In the present work we show that the expression of native huInd1 in Escherichia coli stimulates over-expression of the beta-lactamase TEM-1 from E. coli. The homology modeling of huInd1 shows hallmark of Rossmann fold, where a central beta sheet is covered by helices on either side. In the light of the modeled structure of huInd1, we hypothesize that huInd1 binds to the untranslated region (UTR) of the TEM-1 mRNA at 3' site and thereby reducing the possibility of its endonucleolytic cleavage, resulting in over-expression of TEM-1.

Coupled selection of protein solubility in E. coli using uroporphyrinogen III methyltransferase as red fluorescent reporter

Uroporphyrinogen III methyltransferase (UMT) is a novel reporter owing to the catalytic products accumulated in cells emitting red florescence. Overexpression of UMT confers resistance of the Escherichia coli cells to potassium tellurite that inhibits cell growth. In this study, we applied UMT reporter for monitoring protein solubility of MBP or TEV protease variants under different expression conditions, as well as 12 maize proteins with either the designed linker or N-terminal SUMO tag. Effects of five enzymes involved in heme and siroheme biosynthesis on the reporter were also investigated. With increasing concentrations of potassium tellurite, colony numbers of the mixed cells expressing the selected five proteins with different solubility were decreased, but colonies displaying red fluorescence was identified to be produced the protein with relatively high solubility. The developed UMT reporter system is sensitive for monitoring protein solubility based on coupled fluorescence and chemical selection.

Phosphorylation of Arabidopsis ubiquitin ligase ATL31 is critical for plant carbon/nitrogen nutrient balance response and controls the stability of 14-3-3 proteins

Ubiquitin ligase plays a fundamental role in regulating multiple cellular events in eukaryotes by fine-tuning the stability and activity of specific target proteins. We have previously shown that ubiquitin ligase ATL31 regulates plant growth in response to nutrient balance between carbon and nitrogen (C/N) in Arabidopsis. Subsequent study demonstrated that ATL31 targets 14-3-3 proteins for ubiquitination and modulates the protein abundance in response to C/N-nutrient status. However, the underlying mechanism for the targeting of ATL31 to 14-3-3 proteins remains unclear. Here, we show that ATL31 interacts with 14-3-3 proteins in a phosphorylation-dependent manner. We identified Thr(209), Ser(247), Ser(270), and Ser(303) as putative 14-3-3 binding sites on ATL31 by motif analysis. Mutation of these Ser/Thr residues to Ala in ATL31 inhibited the interaction with 14-3-3 proteins, as demonstrated by yeast two-hybrid and co-immunoprecipitation analyses. Additionally, we identified in vivo phosphorylation of Thr(209) and Ser(247) on ATL31 by MS analysis. A peptide competition assay showed that the application of synthetic phospho-Thr(209) peptide, but not the corresponding unphosphorylated peptide, suppresses the interaction between ATL31 and 14-3-3 proteins. Moreover, Arabidopsis plants overexpressing mutated ATL31, which could not bind to 14-3-3 proteins, showed accumulation of 14-3-3 proteins and growth arrest in disrupted C/N-nutrient conditions similar to wild-type plants, although overexpression of intact ATL31 resulted in repression of 14-3-3 accumulation and tolerance to the conditions. Together, these results demonstrate that the physiological role of phosphorylation at 14-3-3 binding sites on ATL31 is to modulate the binding ability and stability of 14-3-3 proteins to control plant C/N-nutrient response.

Expression of a novel recombinant fusion protein BVN-Tβ4 and its effects on diabetic wound healing

A recombinant fusion protein BVN-Tβ4 is successfully expressed in Escherichia coli, purified in the laboratory and applied onto the wounds of diabetic mice to investigate its effects on diabetic wound healing. Our results show that the recombinant protein BVN-Tβ4 can promote diabetic wound healing in the murine model.

The carbon/nitrogen regulator ARABIDOPSIS TOXICOS EN LEVADURA31 controls papilla formation in response to powdery mildew fungi penetration by interacting with SYNTAXIN OF PLANTS121 in Arabidopsis

The carbon/nitrogen (C/N) balance of plants is not only required for growth and development but also plays an important role in basal immunity. However, the mechanisms that link C/N regulation and basal immunity are poorly understood. We previously demonstrated that the Arabidopsis (Arabidopsis thaliana) Arabidopsis Tóxicos en Levadura31 (ATL31) ubiquitin ligase, a regulator of the C/N response, positively regulates the defense response against bacterial pathogens. In this study, we identified the plasma membrane-localized soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor SYNTAXIN OF PLANTS121 (SYP121) as a novel ATL31 interactor. The syp121-1 loss-of-function mutant showed similar hypersensitivity to C/N stress conditions as the atl31 atl6 double mutant. SYP121 is essential for resistance to penetration by powdery mildew fungus and positively regulates the formation of cell wall appositions (papillae) at fungal entry sites. Microscopic analysis demonstrated that ATL31 was specifically localized around papillae. In addition, ATL31 overexpressors showed accelerated papilla formation, enhancing their resistance to penetration by powdery mildew fungus. Together, these data indicate that ATL31 plays an important role in connecting the C/N response with basal immunity by promoting papilla formation through its association with SYP121.

Proteomics analysis reveals a highly heterogeneous proteasome composition and the post-translational regulation of peptidase activity under pathogen signaling in plants

The proteasome is a large multisubunit complex that plays a crucial role in the removal of damaged or selective ubiquitinated proteins, thereby allowing quality control of cellular proteins and restricted regulation of diverse cellular signaling in eukaryotic cells. Proteasome-dependent protein degradation is involved in almost all aspects of plant growth and responses to environmental stresses including pathogen resistance. Although the molecular mechanism for specifying targets by ubiquitin ligases is well understood, the detailed characterization of the plant proteasome complex remains unclear. One of the most important features of the plant proteasome is that most subunits are encoded by duplicate genes, suggesting the highly heterogeneous composition of this proteasome. Here, we performed affinity purification and a combination of 2-dimensional electrophoresis and mass spectrometry, which identified the detailed composition of paralogous and modified proteins. Moreover, these proteomics approaches revealed that specific subunit composition and proteasome peptidase activity were affected by pathogen-derived MAMPs, flg22 treatment. Interestingly, flg22 treatment did not alter mRNA expression levels of the peptidase genes PBA, PBB1/2, PBE1/2, and total proteasome levels remained unchanged by flg22 as well. These results demonstrate the finely tuned mechanism that regulates proteasome function via putative post-translational modifications in response to environmental stress in plants.

Development of an enzyme-linked immunosorbent assay for detection of chicken osteocalcin and its use in evaluation of perch effects on bone remodeling in caged White Leghorns

Osteocalcin (OC) is a sensitive biochemical marker for evaluating bone turnover in mammals. The role of avian OC is less clear because of the need for a chicken assay. Our objectives were to develop an assay using indirect competitive ELISA for detecting chicken serum OC and use the assay to examine the effects of perches on bone remodeling in caged hens. Anti-chicken OC polyclonal antibody was produced by immunization of rabbits with a recombinant OC from Escherichia coli. Chicken OC extracted from bone was used as a coated protein, and purified chicken OC was used for calibration. The limit of detection of the developed OC ELISA was 0.13 ng/mL. The intra- and interassay CV were <7 and <12%, respectively. The sensitivity of the developed OC ELISA was compared with a commercial Rat-Mid OC ELISA in laying hens housed in conventional cages with or without perches. Serum samples were collected from 71-wk-old White Leghorn hens subjected to 4 treatments. Treatment 1 was control chickens that never had access to perches during their life cycle. Treatment 2 chickens had perches during the pullet phase (0 to 16.9 wk of age), whereas treatment 3 chickens had perches only during the egg-laying phase of the life cycle (17 to 71 wk of age). Treatment 4 chickens always had access to perches (0 to 71 wk of age). Correlation between the 2 assays was 0.62 (P < 0.0001). Levels of serum OC using the developed chicken ELISA were higher than that detected using the Rat-Mid ELISA (P < 0.0001). Results from the chicken ELISA assay showed that hens with perch access had higher concentrations of serum OC than hens without perches during egg laying (P = 0.04). Pullet access to perches did not affect serum OC levels in 71-wk-old hens (P = 0.15). In conclusion, a chicken OC ELISA has been validated that is sensitive and accurate with adequate discriminatory power for measuring bone remodeling in chickens.

Efficient soluble expression of secreted matrix metalloproteinase 26 in Brevibacillus choshinensis

Matrix metalloproteinase 26 (MMP-26) is a novel member of the matrix metalloproteinase family with minimal domain constitution and unknown physiological function. The three-dimensional (3D) structure of the enzyme also remains to be deciphered. Previous studies show that MMP-26 may be expressed in Escherichia coli (E. coli) as inclusion bodies and re-natured with catalytic activity. However, the low re-naturation rate of this method limits its usage in structural studies. In this paper, we tried to clone, express and purify the pro form and catalytic form of MMP-26 (ProMMP-26 and CatMMP-26) in several widely used expression vectors and express the recombinant MMP-26 proteins in E. coli cells. These constructs resulted in insoluble expressions or soluble expressions of MMP-26 with little catalytic activity. We then used Brevibacillus choshinensis (B. choshinensis) as the host system for the soluble and active expression of MMP-26. The enzyme was secreted in soluble form in the supernatant of cell culture medium and purified via a two-step purification process that included Ni(2+) affinity chromatography followed by gel filtration. The yields of purified ProMMP-26 and CatMMP-26 were 12 and 18mg/L, respectively, with high purity and homogeneity. Both ProMMP-26 and CatMMP-26 showed gelatin zymography activity and the purified CatMMP-26 had high enzymatic activity against DQ-gelatin substrate. The large-scale soluble and active protein production for future structural studies of MMP-26 is thus feasible using the B. choshinensis host system.

How to achieve high-level expression of microbial enzymes: strategies and perspectives

Microbial enzymes have been used in a large number of fields, such as chemical, agricultural and biopharmaceutical industries. The enzyme production rate and yield are the main factors to consider when choosing the appropriate expression system for the production of recombinant proteins. Recombinant enzymes have been expressed in bacteria (e.g., Escherichia coli, Bacillus and lactic acid bacteria), filamentous fungi (e.g., Aspergillus) and yeasts (e.g., Pichia pastoris). The favorable and very advantageous characteristics of these species have resulted in an increasing number of biotechnological applications. Bacterial hosts (e.g., E. coli) can be used to quickly and easily overexpress recombinant enzymes; however, bacterial systems cannot express very large proteins and proteins that require post-translational modifications. The main bacterial expression hosts, with the exception of lactic acid bacteria and filamentous fungi, can produce several toxins which are not compatible with the expression of recombinant enzymes in food and drugs. However, due to the multiplicity of the physiological impacts arising from high-level expression of genes encoding the enzymes and expression hosts, the goal of overproduction can hardly be achieved, and therefore, the yield of recombinant enzymes is limited. In this review, the recent strategies used for the high-level expression of microbial enzymes in the hosts mentioned above are summarized and the prospects are also discussed. We hope this review will contribute to the development of the enzyme-related research field.

Loss of function of the IAA-glucose hydrolase gene TGW6 enhances rice grain weight and increases yield

Increases in the yield of rice, a staple crop for more than half of the global population, are imperative to support rapid population growth. Grain weight is a major determining factor of yield. Here, we report the cloning and functional analysis of THOUSAND-GRAIN WEIGHT 6 (TGW6), a gene from the Indian landrace rice Kasalath. TGW6 encodes a novel protein with indole-3-acetic acid (IAA)-glucose hydrolase activity. In sink organs, the Nipponbare tgw6 allele affects the timing of the transition from the syncytial to the cellular phase by controlling IAA supply and limiting cell number and grain length. Most notably, loss of function of the Kasalath allele enhances grain weight through pleiotropic effects on source organs and leads to significant yield increases. Our findings suggest that TGW6 may be useful for further improvements in yield characteristics in most cultivars.

Cell-free protein synthesis of membrane (1,3)-β-d-glucan (curdlan) synthase: co-translational insertion in liposomes and reconstitution in nanodiscs

A membrane-embedded curdlan synthase (CrdS) from Agrobacterium is believed to catalyse a repetitive addition of glucosyl residues from UDP-glucose to produce the (1,3)-β-d-glucan (curdlan) polymer. We report wheat germ cell-free protein synthesis (WG-CFPS) of full-length CrdS containing a 6xHis affinity tag and either Factor Xa or Tobacco Etch Virus proteolytic sites, using a variety of hydrophobic membrane-mimicking environments. Full-length CrdS was synthesised with no variations in primary structure, following analysis of tryptic fragments by MALDI-TOF/TOF Mass Spectrometry. Preparative scale WG-CFPS in dialysis mode with Brij-58 yielded CrdS in mg/ml quantities. Analysis of structural and functional properties of CrdS during protein synthesis showed that CrdS was co-translationally inserted in DMPC liposomes during WG-CFPS, and these liposomes could be purified in a single step by density gradient floatation. Incorporated CrdS exhibited a random orientation topology. Following affinity purification of CrdS, the protein was reconstituted in nanodiscs with Escherichia coli lipids or POPC and a membrane scaffold protein MSP1E3D1. CrdS nanodiscs were characterised by small-angle X-ray scattering using synchrotron radiation and the data obtained were consistent with insertion of CrdS into bilayers. We found CrdS synthesised in the presence of the Ac-AAAAAAD surfactant peptide or co-translationally inserted in liposomes made from E. coli lipids to be catalytically competent. Conversely, CrdS synthesised with only Brij-58 was inactive. Our findings pave the way for future structural studies of this industrially important catalytic membrane protein.

Molecular cloning and expression of ranalexin, a bioactive antimicrobial peptide from Rana catesbeiana in Escherichia coli and assessments of its biological activities

The coding sequence, which corresponds to the mature antimicrobial peptide ranalexin from the frog Rana catesbeiana, was chemically synthesized with preferred codons for expression in Escherichia coli. It was cloned into the vector pET32c (+) to express a thioredoxin-ranalexin fusion protein which was produced in soluble form in E. coli BL21 (DE3) induced under optimized conditions. After two purification steps through affinity chromatography, about 1 mg of the recombinant ranalexin was obtained from 1 L of culture. Mass spectrometrical analysis of the purified recombinant ranalexin demonstrated its identity with ranalexin. The purified recombinant ranalexin is biologically active. It showed antibacterial activities similar to those of the native peptide against Staphylococcus aureus, Streptococcus pyogenes, E. coli, and multidrug-resistant strains of S. aureus with minimum inhibitory concentration values between 8 and 128 μg/ml. The recombinant ranalexin is also cytotoxic in HeLa and COS7 human cancer cells (IC50 = 13-15 μg/ml).

The Arabidopsis ubiquitin ligases ATL31 and ATL6 control the defense response as well as the carbon/nitrogen response

In higher plants, the metabolism of carbon (C) and nitrogen nutrients (N) is mutually regulated and referred to as the C and N balance (C/N). Plants are thus able to optimize their growth depending on their cellular C/N status. Arabidopsis ATL31 and ATL6 encode a RING-type ubiquitin ligases which play a critical role in the C/N status response (Sato et al. in Plant J 60:852-864, 2009). Since many ATL members are involved in the plant defense response, the present study evaluated whether the C/N response regulators ATL31 and ATL6 are involved in defense responses. Our results confirmed that ATL31 and ATL6 expression is up-regulated with the microbe-associated molecular patterns elicitors flg22 and chitin as well as with infections with Pseudomonas syringae pv. tomato DC3000 (Pst. DC3000). Moreover, transgenic plants overexpressing ATL31 and ATL6 displayed increased resistance to Pst. DC3000. In accordance with these data, loss of ATL31 and ATL6 function in an atl31 atl6 double knockout mutant resulted in reduced resistance to Pst. DC3000. In addition, the molecular cross-talk between C/N and the defense response was investigated by mining public databases. The analysis identified the transcription factors MYB51 and WRKY33, which are involved in the defense response, and their transcripts levels correlate closely with ATL31 and ATL6. Further study demonstrated that the expression of ATL31, ATL6 and defense marker genes including MYB51 and WRKY33 were regulated by C/N conditions. Taken together, these results indicate that ATL31 and ATL6 function as key components of both C/N regulation and the defense response in Arabidopsis.

Expression, purification, and functional characterization of a stable helicase domain from a tomato mosaic virus replication protein

Tomato mosaic virus (genus, Tobamovirus) is a member of the alphavirus-like superfamily of positive-strand RNA viruses, which include many plant and animal viruses of agronomical and clinical importance. The RNA of alphavirus-like superfamily members encodes replication-associated proteins that contain a putative superfamily 1 helicase domain. To date, a viral three-dimensional superfamily 1 helicase structure has not been solved. For the study reported herein, we expressed tomato mosaic virus replication proteins that contain the putative helicase domain and additional upstream N-terminal residues in Escherichia coli. We found that an additional 155 residues upstream of the N-terminus of the helicase domain were necessary for stability. We developed an efficient procedure for the expression and purification of this fragment and have examined factors that affect its stability. Finally, we also showed that the stable fragment has nucleoside 5'-triphosphatase activity.

Recombinant expression and functional analysis of proteases from Streptococcus pneumoniae, Bacillus anthracis, and Yersinia pestis

Background

Uncharacterized proteases naturally expressed by bacterial pathogens represents important topic in infectious disease research, because these enzymes may have critical roles in pathogenicity and cell physiology. It has been observed that cloning, expression and purification of proteases often fail due to their catalytic functions which, in turn, cause toxicity in the E. coli heterologous host.

Results

In order to address this problem systematically, a modified pipeline of our high-throughput protein expression and purification platform was developed. This included the use of a specific E. coli strain, BL21(DE3) pLysS to tightly control the expression of recombinant proteins and various expression vectors encoding fusion proteins to enhance recombinant protein solubility. Proteases fused to large fusion protein domains, maltosebinding protein (MBP), SP-MBP which contains signal peptide at the N-terminus of MBP, disulfide oxidoreductase (DsbA) and Glutathione S-transferase (GST) improved expression and solubility of proteases. Overall, 86.1% of selected protease genes including hypothetical proteins were expressed and purified using a combination of five different expression vectors. To detect novel proteolytic activities, zymography and fluorescence-based assays were performed and the protease activities of more than 46% of purified proteases and 40% of hypothetical proteins that were predicted to be proteases were confirmed.

Conclusions

Multiple expression vectors, employing distinct fusion tags in a high throughput pipeline increased overall success rates in expression, solubility and purification of proteases. The combinatorial functional analysis of the purified proteases using fluorescence assays and zymography confirmed their function.

Polydispersity as a parameter for indicating the thermal stability of proteins by dynamic light scattering

A physical parameter for predicting the thermal stability of proteins was provided by a new approach using dynamic light scattering (DLS). The relationship between the melting point measured by differential scanning calorimetry (DSC) and the polydispersity of the hydrodynamic diameter determined by DLS analysis was examined. Calmodulin (CaM) and concanavalin A (ConA) were used as model proteins. The melting point measured by DSC, an indicator for thermal stability, increased and the polydispersity decreased on binding of the proteins to specific ligands, suggesting that the polydispersity could be used an indicator to predict thermal stability. In addition, the increase of thermal stability that resulted from forming a complex could be quantified by polydispersity analysis even when the melting point changed only slightly.

CNI1/ATL31, a RING-type ubiquitin ligase that functions in the carbon/nitrogen response for growth phase transition in Arabidopsis seedlings

Plants are able to sense and respond to changes in the balance between carbon (C) and nitrogen (N) metabolite availability, known as the C/N response. During the transition to photoautotrophic growth following germination, growth of seedlings is arrested if a high external C/N ratio is detected. To clarify the mechanisms for C/N sensing and signaling during this transition period, we screened a large collection of FOX transgenic plants, overexpressing full-length cDNAs, for individuals able to continue post-germinative growth under severe C/N stress. One line, cni1-D (carbon/nitrogen insensitive 1-dominant), was shown to have a suppressed sensitivity to C/N conditions at both the physiological and molecular level. The CNI1 cDNA encoded a predicted RING-type ubiquitin ligase previously annotated as ATL31. Overexpression of ATL31 was confirmed to be responsible for the cni1-D phenotype, and a knock-out of this gene resulted in hypersensitivity to C/N conditions during post-germinative growth. The ATL31 protein was confirmed to contain ubiquitin ligase activity using an in vitro assay system. Moreover, removal of this ubiquitin ligase activity from the overexpressed protein resulted in the loss of the mutant phenotype. Taken together, these data demonstrated that CNI1/ATL31 activity is required for the plant C/N response during seedling growth transition.

Cloning and expression of a novel insulin-releasing peptide, brevinin-2GU from Escherichia coli

Brevinin-2GU, a member of a brevinin-2 family of antimicrobial peptides isolated from an extract of the skin of the Asian frog, Hylarana guntheri, displays significant insulin-releasing activity. In this study, in order to obtain relatively large quantity of functional brevinin-2GU protein, the coding sequence of brevinin-2GU gene was cloned into pET32a (+) vector and expression as a Trx fusion protein in Escherichia coli. The results indicated that the expression level of the fusion protein could reach up to 45% of the total cell proteins. the soluble fusion protein collected from the supernatant of the cell lysate was then separated by Ni(2+)-chelating chromatography and cleaved by Factor Xa protease to release mature brevinin-2GU. Our results showed that the final yield of the brevinin-2GU was 1.7 mg, the purified peptide displayed insulin-releasing activity similar to that of the purified brevinin that was reported earlier. This method allows production of sufficiently large amounts of brevinin-2GU peptide for functional and structural characterizations.

Identification of an Arabidopsis thaliana protein that binds to tomato mosaic virus genomic RNA and inhibits its multiplication

The genomic RNAs of positive-strand RNA viruses carry RNA elements that play positive, or in some cases, negative roles in virus multiplication by interacting with viral and cellular proteins. In this study, we purified Arabidopsis thaliana proteins that specifically bind to 5' or 3' terminal regions of tomato mosaic virus (ToMV) genomic RNA, which contain important regulatory elements for translation and RNA replication, and identified these proteins by mass spectrometry analyses. One of these host proteins, named BTR1, harbored three heterogeneous nuclear ribonucleoprotein K-homology RNA-binding domains and preferentially bound to RNA fragments that contained a sequence around the initiation codon of the 130K and 180K replication protein genes. The knockout and overexpression of BTR1 specifically enhanced and inhibited, respectively, ToMV multiplication in inoculated A. thaliana leaves, while such effect was hardly detectable in protoplasts. These results suggest that BTR1 negatively regulates the local spread of ToMV.

Rapid and easy development of versatile tools to study protein/ligand interactions

The system described here allows the expression of protein fragments into a solvent-exposed loop of a carrier protein, the beta-lactamase BlaP. When using Escherichia coli constitutive expression vectors, a positive selection of antibioresistant bacteria expressing functional hybrid beta-lactamases is achieved in the presence of beta-lactams making further screening of correctly folded and secreted hybrid beta-lactamases easier. Protease-specific recognition sites have been engineered on both sides of the beta-lactamase permissive loop in order to cleave off the exogenous protein fragment from the carrier protein by an original two-step procedure. According to our data, this approach constitutes a suitable alternative for production of difficult to express protein domains. This work demonstrates that the use of BlaP as a carrier protein does not alter the biochemical activity and the native disulphide bridge formation of the inserted chitin binding domain of the human macrophage chitotriosidase. We also report that the beta-lactamase activity of the hybrid protein can be used to monitor interactions between the inserted protein fragments and its ligands and to screen neutralizing molecules.

A set of ligation-independent in vitro translation vectors for eukaryotic protein production

Background

The last decade has brought the renaissance of protein studies and accelerated the development of high-throughput methods in all aspects of proteomics. Presently, most protein synthesis systems exploit the capacity of living cells to translate proteins, but their application is limited by several factors. A more flexible alternative protein production method is the cell-free in vitro protein translation. Currently available in vitro translation systems are suitable for high-throughput robotic protein production, fulfilling the requirements of proteomics studies. Wheat germ extract based in vitro translation system is likely the most promising method, since numerous eukaryotic proteins can be cost-efficiently synthesized in their native folded form. Although currently available vectors for wheat embryo in vitro translation systems ensure high productivity, they do not meet the requirements of state-of-the-art proteomics. Target genes have to be inserted using restriction endonucleases and the plasmids do not encode cleavable affinity purification tags.

Results

We designed four ligation independent cloning (LIC) vectors for wheat germ extract based in vitro protein translation. In these constructs, the RNA transcription is driven by T7 or SP6 phage polymerase and two TEV protease cleavable affinity tags can be added to aid protein purification. To evaluate our improved vectors, a plant mitogen activated protein kinase was cloned in all four constructs. Purification of this eukaryotic protein kinase demonstrated that all constructs functioned as intended: insertion of PCR fragment by LIC worked efficiently, affinity purification of translated proteins by GST-Sepharose or MagneHis particles resulted in high purity kinase, and the affinity tags could efficiently be removed under different reaction conditions. Furthermore, high in vitro kinase activity testified of proper folding of the purified protein.

Conclusion

Four newly designed in vitro translation vectors have been constructed which allow fast and parallel cloning and protein purification, thus representing useful molecular tools for high-throughput production of eukaryotic proteins.

High-level production of a novel antimicrobial peptide perinerin in Escherichia coli by fusion expression

Perinerin is a small antimicrobial peptide (AMP) isolated from an Asian marine clamworm, Perinereis aibuhitensis Grube. It shows marked activity in vitro against both Gram-negative and Gram-positive bacteria. To obtain it in large amounts, the coding sequence of perinerin was cloned into pET32a(+) vector and expression as a Trx fusion protein in Escherichia coli. The soluble fusion protein collected from the supernatant of the cell lyste was separated by Ni(2+)-chelating chromatography. The purified protein was then cleaved by Factor Xa protease to release mature perinerin. Final purification was achieved by ion-exchange chromatography. Recombinant perinerin exhibited a similar antimicrobial activity to the native perinerin. These works might provide a significant foundation for the following research on the action of mechanism of marine AMPs.

Isolation and characterization of the RAD54 gene from Arabidopsis thaliana

Homologous recombination (HR) is an essential process in maintaining genome integrity and variability. In eukaryotes, the Rad52 epistasis group proteins are involved in meiotic recombination and/or HR repair. One member of this group, Rad54, belongs to the SWI2/SNF2 family of DNA-stimulated ATPases. Recent studies indicate that Rad54 has important functions in HR, both as a chromatin remodelling factor and as a mediator of the Rad51 nucleoprotein filament. Despite the importance of Rad54 in HR, no study of Rad54 from plants has yet been performed. Here, we cloned the full-length AtRAD54 cDNA sequence; an open reading frame of 910 amino acids encodes a protein with a predicted molecular mass of 101.9 kDa. Western blotting analysis showed that the AtRad54 protein was indeed expressed as a protein of approximately 110 kDa in Arabidopsis. The predicted protein sequence of AtRAD54 contains seven helicase domains, which are conserved in all other Rad54s. Yeast two-hybrid analysis revealed an interaction between Arabidopsis Rad51 and Rad54. AtRAD54 transcripts were found in all tissues examined, with the highest levels of expression in flower buds. Expression of AtRAD54 was induced by gamma-irradiation. A T-DNA insertion mutant of AtRAD54 devoid of full-length AtRAD54 expression was viable and fertile; however, it showed increased sensitivity to gamma-irradiation and the cross-linking reagent cisplatin. In addition, the efficiency of somatic HR in the mutant plants was reduced relative to that in wild-type plants. Our findings point to an important role for Rad54 in HR repair in higher plants.

Current strategies for the use of affinity tags and tag removal for the purification of recombinant proteins

Affinity tags are highly efficient tools for protein purification. They allow the purification of virtually any protein without any prior knowledge of its biochemical properties. The use of affinity tags has therefore become widespread in several areas of research e.g., high throughput expression studies aimed at finding a biological function to large numbers of yet uncharacterized proteins. In some cases, the presence of the affinity tag in the recombinant protein is unwanted or may represent a disadvantage for the projected application of the protein, like for clinical use. Therefore, an increasing number of approaches are available at present that are designed for the removal of the affinity tag from the recombinant protein. Most of these methods employ recombinant endoproteases that recognize a specific sequence. These process enzymes can subsequently be removed from the process by affinity purification, since they also include a tag. Here, a survey of the most common affinity tags and the current methods for tag removal is presented, with special emphasis on the removal of N-terminal histidine tags using TAGZyme, a system based on exopeptidase cleavage. In the quest to reduce the significant costs associated with protein purification at large scale, relevant aspects involved in the development of downstream processes for pharmaceutical protein production that incorporate a tag removal step are also discussed. A comparison of the yield of standard vs. affinity purification together with an example of tag removal using TAGZyme is also included.

Improved expression, purification and crystallization of a putative N-acetyl-gamma-glutamyl-phosphate reductase from rice (Oryza sativa)

N-Acetyl-gamma-glutamyl-phosphate reductase (AGPR) catalyzes the third step in an eight-step arginine-biosynthetic pathway that starts with glutamate. This enzyme converts N-acetyl-gamma-glutamyl phosphate to N-acetylglutamate-gamma-semialdehyde by an NADPH-dependent reductive dephosphorylation. AGPR from Oryza sativa (OsAGPR) was expressed in Escherichia coli at 291 K as a soluble fusion protein with an upstream thioredoxin-hexahistidine [Trx-(His)6] extension. OsAGPR(Ala50-Pro366) was purified and crystals were obtained using the sitting-drop vapour-diffusion method at 293 K and diffract X-rays to at least 1.8 A resolution. They belong to the hexagonal space group P6(1), with unit-cell parameters a = 86.11, c = 316.3 A.

Refolding and purification of recombinant OsNifU1A domain II that was expressed by Escherichia coli

OsNifU1A is a NifU-like rice (Oryza sativa) protein, discovered recently. Its amino acid sequence is very homologous to the sequence of cyanobacterial CnfU and to the sequences of NifU C-terminal domains. Based on its sequence, OsNifU1A is probably a modular structure consisting of two CnfU-like domains, with domain I (formed by residues Leu73 to Gly153) and domain II (formed by residues Leu154 to Ser226). Domain I have a conserved Cys-X-X-Cys motif, which may function as an iron-sulfur cluster assembly scaffold. Domain II lacks a Cys-X-X-Cys motif and therefore, cannot function analogously. Other NifU-like proteins, with sequences homologous to OsNifU1A domain II, have been identified during plant genomic projects; however, the biological roles of these domains remain unknown. We successfully constructed an Escherichia coli expression system for OsNifU1A domain II that enabled us to synthesize and purify milligram quantities of protein for use in structural and functional studies. Using the Gateway system, we built DNA sequences corresponding to two OsNifU1A domain II fusion proteins. One construct has a (His)6 sequence upstream of the OsNifU1A domain II sequence; the other has an upstream thioredoxin-(His)6 sequence. Recombinant OsNifU1A domain II fusion proteins were extracted from E. coli inclusion bodies by dissolving them in 6 M guanidine-HCl. About 36% of the total (His)6/OsNifU1A domain II fusion protein initially present remained soluble after guanidine-HCl was completely removed by step-wise dialysis; whereas, recovery of soluble Trx-(His)6 fusion protein was about 60% of the total cell lysate. About 2 mg of 15N-labeled OsNifU1A domain II was purified for NMR spectral studies. Examination of the OsNifU1A domain II 1H-15N HSQC NMR spectrum indicated that the purified protein was monomeric and correctly folded. Therefore, we established an efficient procedure for synthesis and purification of 15N-labeled OsNifU1A domain II in quantities sufficient for heteronuclear NMR solution structure studies.