Expression, purification, crystallization and preliminary X-ray analysis of YaeQ (XAC2396) from Xanthomonas axonopodis pv. citri

Xanthomonas axonopodis pv. citri YaeQ (XAC2396) is a member of a family of bacterial proteins conserved in several Gram-negative pathogens. Here, the cloning, expression, purification and crystallization of the 182-residue (20.6 kDa) YaeQ protein are described. Recombinant YaeQ containing selenomethionine was crystallized in space group P2(1) and crystals diffracted to 1.9 A resolution at a synchrotron source. The unit-cell parameters are a = 39.75, b = 91.88, c = 48.03 A, beta = 108.37 degrees. The calculated Matthews coefficient suggests the presence of two YaeQ molecules in the asymmetric unit. Initial experimental phases were calculated by the multiple-wavelength anomalous dispersion technique and an interpretable electron-density map was obtained.

Structure of CDP-D-glucose 4,6-dehydratase from Salmonella typhi complexed with CDP-D-xylose

Tyvelose is a unique 3,6-dideoxyhexose found in the O antigens of some pathogenic species of Yersinia and Salmonella. It is produced via a complex biochemical pathway that employs CDP-D-glucose as the starting ligand. CDP-D-glucose 4,6-dehydratase catalyzes the first irreversible step in the synthesis of this 3,6-dideoxysugar by converting CDP-D-glucose to CDP-4-keto-6-deoxyglucose via an NAD+ -dependent intramolecular oxidation-reduction reaction. Here, the cloning, protein purification and X-ray crystallographic analysis of CDP-D-glucose 4,6-dehydratase from Salmonella typhi complexed with the substrate analog CDP-D-xylose are described. Each subunit of the tetrameric enzyme folds into two domains. The N-terminal region contains a Rossmann fold and provides the platform for NAD(H) binding. The C-terminal motif is primarily composed of alpha-helices and houses the binding pocket for the CDP portion of the CDP-D-xylose ligand. The xylose moiety extends into the active-site cleft that is located between the two domains. Key residues involved in anchoring the sugar group to the protein include Ser134, Tyr159, Asn197 and Arg208. Strikingly, Ser134 O gamma and Tyr159 O eta sit within 2.9 A of the 4'-hydroxyl group of xylose. Additionally, the side chains of Asp135 and Lys136 are located at 3.5 and 3.2 A, respectively, from C-5 of xylose. In the structurally related dTDP-D-glucose 4,6-dehydratase, the Asp/Lys pair is replaced with an Asp/Glu couple. On the basis of this investigation, it can be speculated that Tyr159 serves as the catalytic base to abstract the 4'-hydroxyl proton from the sugar and that Asp135 and Lys136 play critical roles in the subsequent dehydration step that leads to the final product.

Effects of dietary supplements on space radiation-induced oxidative stress in Sprague-Dawley rats

Of particular concern for the health of astronauts during space travel is radiation from protons and high-mass, high-atomic-number (Z), and high-energy particles (HZE particles). Space radiation is known to induce oxidative stress in astronauts after extended space flight. In the present study, the total antioxidant status was used as a biomarker to evaluate oxidative stress induced by gamma rays, protons and HZE-particle radiation. The results demonstrate that the plasma level of total antioxidants in Sprague-Dawley rats was significantly decreased (P < 0.01) in a dose-dependent manner within 4 h after exposure to gamma rays. Exposure to protons and HZE-particle radiation also significantly decreased the serum or plasma level of total antioxidants in the irradiated animals. Diet supplementation with L-selenomethionine alone or a combination of selected antioxidant agents was shown to partially or completely prevent the decrease in the serum or plasma levels of total antioxidants in animals exposed to gamma rays, protons or HZE particles. These findings suggest that exposure to space radiation may compromise the capacity of the host antioxidant defense and that this adverse biological effect can be prevented at least partially by dietary supplementation with L-selenomethionine and antioxidants.

Identification of selenomethionine in selenized yeast using two-dimensional liquid chromatography-mass spectrometry based proteomic analysis

Selenium-enriched yeast has been commonly used as a nutritional supplement. Here we describe a protocol used to investigate the metabolic fate of inorganic selenium in yeast. We provide definitive, mass spectrometry based evidence for the non-specific incorporation of selenomethionine in the yeast proteome involving the replacement of about 30% of all methionine with selenomethionine.

In vivo incorporation of selenomethionine in proteins using Pseudomonas aeruginosa as expression host: case study—the outer membrane receptor FpvA

The number of protein structures solved using multiwavelength anomalous diffraction methods coupled with selenomethionine substitution has grown dramatically over the last years. We show using the outer membrane pyoverdin receptor FpvA that Pseudomonas aeruginosa can be used for producing proteins with a high level of selenomethionine incorporation. To circumvent problems encountered with mass spectroscopy analysis of purified membrane proteins, in-gel trypsin digestion of FpvA coupled with MALDI mass spectrometry analysis of the resulting peptides was used to determine the extent of selenomethionine incorporation. Selenomethionine incorporation greater than 95% was achieved using P. aeruginosa as an overexpression system.

The Structural Basis for Substrate Promiscuity in 2-Keto-3-deoxygluconate Aldolase from the Entner-Doudoroff Pathway in Sulfolobus solfataricus

The hyperthermophilic Archaea Sulfolobus solfataricus grows optimally above 80 degrees C and metabolizes glucose by a non-phosphorylative variant of the Entner-Doudoroff pathway. In this pathway glucose dehydrogenase and gluconate dehydratase catalyze the oxidation of glucose to gluconate and the subsequent dehydration of gluconate to D-2-keto-3-deoxygluconate (KDG). KDG aldolase (KDGA) then catalyzes the cleavage of KDG to D-glyceraldehyde and pyruvate. It has recently been shown that all the enzymes of this pathway exhibit a catalytic promiscuity that also enables them to be used for the metabolism of galactose. This phenomenon, known as metabolic pathway promiscuity, depends crucially on the ability of KDGA to cleave KDG and D-2-keto-3-deoxygalactonate (KDGal), in both cases producing pyruvate and D-glyceraldehyde. In turn, the aldolase exhibits a remarkable lack of stereoselectivity in the condensation reaction of pyruvate and D-glyceraldehyde, forming a mixture of KDG and KDGal. We now report the structure of KDGA, determined by multiwavelength anomalous diffraction phasing, and confirm that it is a member of the tetrameric N-acetylneuraminate lyase superfamily of Schiff base-forming aldolases. Furthermore, by soaking crystals of the aldolase at more than 80 degrees C below its temperature activity optimum, we have been able to trap Schiff base complexes of the natural substrates pyruvate, KDG, KDGal, and pyruvate plus D-glyceraldehyde, which have allowed rationalization of the structural basis of promiscuous substrate recognition and catalysis. It is proposed that the active site of the enzyme is rigid to keep its thermostability but incorporates extra functionality to be promiscuous.

Insight into the Polar Reactivity of the Onium Chalcogen Analogues ofS-Adenosyl-l-methionine†

S-Adenosyl-L-methionine (AdoMet) is one of Nature's most diverse metabolites, used not only in a large number of biological reactions but amenable to several different modes of reactivity. The types of transformations in which it is involved include decarboxylation, electrophilic addition to any of the three carbons bonded to the central sulfur atom, proton removal at carbons adjacent to the sulfonium, and reductive cleavage to generate 5'-deoxyadenosyl 5'-radical intermediates. At physiological pH and temperature, AdoMet is subject to three spontaneous degradation pathways, the first of which is racemization of the chiral sulfonium group, which takes place in a pH-independent manner. The two remaining pathways are pH-dependent and include (1) intramolecular attack of the alpha-carboxylate group onto the gamma-carbon, affording L-homoserine lactone (HSL) and 5'-methylthioadenosine (MTA), and (2) deprotonation at C-5', initiating a cascade that results in formation of adenine and S-ribosylmethionine. Herein, we describe pH-dependent stability studies of AdoMet and its selenium and tellurium analogues, Se-adenosyl-L-selenomethionine and Te-adenosyl-L-telluromethionine (SeAdoMet and TeAdoMet, respectively), at 37 degrees C and constant ionic strength, which we use as a probe of their relative intrinsic reactivities. We find that with AdoMet intramolecular nucleophilic attack to afford HSL and MTA exhibits a pH-rate profile having two titratable groups with apparent pK(a) values of 1.2 +/- 0.4 and 8.2 +/- 0.05 and displaying first-order rate constants of <0.7 x 10(-6) s(-1) at pH values less than 0.5, approximately 3 x 10(-6) s(-1) at pH values between 2 and 7, and approximately 15 x 10(-6) s(-1) at pH values greater than 9. Degradation via deprotonation at C-5' follows a pH-rate profile having one titratable group with an apparent pK(a) value of approximately 11.5. The selenium analogue decays significantly faster via intramolecular nucleophilic attack, also exhibiting a pH-rate profile with two titratable groups with pK(a) values of approximately 0.86 and 8.0 +/- 0.1 with first-order rate constants of <7 x 10(-6) s(-1) at pH values less than 0.9, approximately 32 x 10(-6) s(-1) at pH values between 2 and 7, and approximately 170 x 10(-6) s(-1) at pH values greater than 9. Degradation via deprotonation at C-5' proceeds with one titratable group displaying an apparent pK(a) value of approximately 14.1. Unexpectedly, TeAdoMet did not decay at an observable rate via either of these two pathways. Last, enzymatically synthesized AdoMet was found to racemize at rates that were consistent with earlier studies (Hoffman, J. L. (1986) Biochemistry 25, 4444-4449); however, SeAdoMet and TeAdoMet did not racemize at detectable rates. In the accompanying paper, we use the information obtained in these model studies to probe the mechanism of cyclopropane fatty acid synthase via use of the onium chalcogens of AdoMet as methyl donors.

A dimerized coiled-coil domain and an adjoining part of geminin interact with two sites on Cdt1 for replication inhibition

Geminin is a cellular protein that associates with Cdt1 and inhibits Mcm2-7 loading during S phase. It prevents multiple cycles of replication per cell cycle and prevents episome replication. It also directly inhibits the HoxA11 transcription factor. Here we report that geminin forms a parallel coiled-coil homodimer with atypical residues in the dimer interface. Point mutations that disrupt the dimerization abolish interaction with Cdt1 and inhibition of replication. An array of glutamic acid residues on the coiled-coil domain surface interacts with positive charges in the middle of Cdt1. An adjoining region interacts independently with the N-terminal 100 residues of Cdt1. Both interactions are essential for replication inhibition. The negative residues on the coiled-coil domain and a different part of geminin are also required for interaction with HoxA11. Therefore a rigid cylinder with negative surface charges is a critical component of a bipartite interaction interface between geminin and its cellular targets.

Selenium and selenomethionine levels in prostate cancer patients

OBJECTIVE

Selenium (Se) and selenomethionine (Se-Met) have been identified as potential chemopreventive agents of prostate cancer. Using an assay for the speciation (separation and identification) and quantification of selenoamino acids, Se-Met was profiled in serum and prostate tissue from prostate cancer patients. Total Se was measured also.

METHODS

Serum and prostate tissue samples were analyzed using high performance liquid chromatography ion channel plasma-mass spectrometry (HPLC ICP-MS). Samples were provided by 25 subjects undergoing radical prostatectomy for the treatment of prostate cancer.

RESULTS

The mean serum Se-Met concentration was 59.4 +/- 19.5 ng/ml. Mean total serum Se levels were 133.8 +/- 29.2 ng/ml. Total Se levels in paired samples from the transitional and peripheral zones of the prostate were also compared. Total mean Se levels in peripheral zone tissue was higher than in transitional zone tissue (0.432 +/- 0.212 microg/g versus 0.293 +/- 0.172 microg/g, P = 0.01). Se-Met was detected in the majority of prostate tissues.

CONCLUSION

This group of prostate cancer patients had total Se levels in serum and in prostate tissue in a range expected for normal Se intake. There was a significantly higher total Se level found in peripheral versus transitional zone tissues. This is the first time that Se-Met has been detected in prostate tissue.

Structural analysis of Salmonella enterica effector protein SopD

Salmonella outer protein D (SopD) is a type III secreted virulence effector protein from Salmonella enterica. Full-length SopD and SopD lacking 16 amino acids at the N-terminus (SopDDeltaN) have been expressed as fusions with GST in Escherichia coli, purified with a typical yield of 20-30 mg per litre of cell culture and crystallized. Biophysical characterization has been carried out mainly on SopDDeltaN. Analytical size exclusion chromatography shows that SopDDeltaN is monomeric and probably globular in aqueous solution. The secondary structure composition, calculated from the CD spectrum, is mixed (38% alpha-helix and 26% beta-strand). Sequence analysis indicates that SopD contains a coiled coil motif, as found in numerous other type III secretion system-associated proteins. This suggests that SopD has the potential for one or more heterotypic protein-protein interactions. Limited trypsin digestion of SopDDeltaN, monitored by both one-dimensional proton NMR spectroscopy and SDS-PAGE, shows that the protein has a large, protease-resistant core domain of 286 amino acid residues. This single-domain architecture suggests that SopD lacks a cognate chaperone. In crystallization trials, SopDDeltaN produced better crystals than either full-length SopD or trypsin-digested SopDDeltaN. Diffraction to 3.0 A resolution has so far been obtained from crystals of SopDDeltaN.

Methyl selenium metabolites decrease prostate-specific antigen expression by inducing protein degradation and suppressing androgen-stimulated transcription

Prostate-specific antigen (PSA) is widely used clinically for prostate cancer diagnostics and as an indicator of therapeutic efficacy and recurrence. Several human chemoprevention trials are being conducted to validate the prostate cancer prevention efficacy of selenium and PSA is used in these trials as a biomarker of response. A better understanding of the effects of selenium metabolites on the kinetics of PSA turnover and secretion in prostate cancer cells treated with selenium at concentrations which are achievable physiologically will be important for interpreting the results of these trials. This study addresses whether the putative active anticancer selenium metabolite methylselenol or its precursor methylseleninic acid (MSeA) specifically inhibits PSA expression in the androgen-responsive LNCaP prostate cancer cell model. The results show that exposure to sub-apoptotic concentrations of MSeA and methylselenol inhibited PSA protein expression and secretion, whereas sodium selenite and selenomethionine lacked inhibitory effect. The inhibition was detectable at 3 h of exposure and required a threshold level of MSeA to sustain. Turnover experiments showed that MSeA caused rapid PSA degradation, which was partially blocked by lysosomal inhibitors, but not by a proteasomal inhibitor. Furthermore, MSeA treatment reduced PSA mRNA level, down-regulated androgen receptor protein expression, and inhibited androgen-stimulated PSA promoter transcription. In summary, methylselenol or MSeA specifically and rapidly inhibited PSA expression through two mechanisms of action: inducing PSA protein degradation and suppressing androgen-stimulated PSA transcription. These findings may have important mechanistic implications for the prostate specific cancer chemopreventive action of selenium.

Metabolism of selenomethionine by rainbow trout (Oncorhynchus mykiss) embryos can generate oxidative stress

Although selenium is required by vertebrates, toxicity can arise at concentrations only slightly greater than those they require. The toxicity of Se is thought to arise from its ability to substitute for sulfur during the assembly of proteins. However, recent studies also indicate that some forms of selenium are capable of generating oxidative stress in an in vitro test system that includes glutathione. L-Selenomethionine, the predominant form of selenium in the eggs of oviparous vertebrates, does not generate oxidative radicals in this system, but lesions consistent with oxidative stress have been identified in fish and birds with high concentrations of Se. Here we report on the ability of rainbow trout embryos to transform L-Selenomethionine to a form capable of producing a superoxide radical. Oxidative stress appears to be generated by methioninase enzyme activity in the embryos that liberates methylselenol from l-Selenomethionine. Methylselenol redox cycles in the presence of glutathione producing superoxide and likely accounts for oxidative lesions present in fish and birds environmentally exposed to excessive loads of selenomethionine.

Crystallization and preliminary crystallographic analysis of BbCRASP-1, a complement regulator-acquiring surface protein ofBorrelia burgdorferi

Borrelia burgdorferi is the causative agent of Lyme disease. Serum-resistant strains of the pathogen are able to reduce the host's immune response to infection by recruiting fluid-phase complement regulators from the serum. B. burgdorferi complement regulator-acquiring surface protein-1 (BbCRASP-1) binds factor H and factor-H-like protein-1 to the bacterial surface, where they actively down-regulate complement response. Crystals of native and selenomethionine-substituted BbCRASP-1 have been obtained and a native data set to 2.7 A as well as selenomethionine MAD data to 3.2 A resolution have been collected. The selenium substructure has been solved and initial phases have been refined to 3.0 A by density-modification methods. Model building and refinement are under way.

Crystallization and preliminary analysis of xenobiotic reductase A and ligand complexes fromPseudomonas putidaII-B

Diffraction-quality crystals have been obtained of the xenobiotic reductase A (XenA) from Pseudomonas II-B, which was originally cultured from the contaminated soil of a World War II era munitions-manufacturing plant. Several complete X-ray diffraction data sets have been collected and analyzed. The native XenA data set includes reflections between 35 and 1.65 A. Four-wavelength MAD data sets from selenomethionine-enriched XenA and from three different ligand complexes are also reported. The XenA crystals belong to space group P2(1)2(1)2, with unit-cell parameters a = 84, b = 158, c = 57 A. Experimental phasing from analysis of the MAD data from selenomethionine-enriched XenA reveals the presence of two molecules in the asymmetric unit. They are related by a non-crystallographic 2(1) screw axis nearly parallel to the c axis, but offset by a quarter unit-cell translation. Thus, the local symmetry produces approximate systematic absences along the (00l) principal axis and complicates the space-group determination.

Sulfur and selenium: the role of oxidation state in protein structure and function

Sulfur and selenium occur in proteins as constituents of the amino acids cysteine, methionine, selenocysteine, and selenomethionine. Recent research underscores that these amino acids are truly exceptional. Their redox activity under physiological conditions allows an amazing variety of posttranslational protein modifications, metal free redox pathways, and unusual chalcogen redox states that increasingly attract the attention of biological chemists. Unlike any other amino acid, the "redox chameleon" cysteine can participate in several distinct redox pathways, including exchange and radical reactions, as well as atom-, electron-, and hydride-transfer reactions. It occurs in various oxidation states in the human body, each of which exhibits distinctive chemical properties (e.g. redox activity, metal binding) and biological activity. The position of selenium in the periodic table between the metals and the nonmetals makes selenoproteins ideal catalysts for many biological redox transformations. It is therefore apparent that the chalcogen amino acids cysteine, methionine, selenocysteine, and selenomethionine exhibit a unique biological chemistry that is the source of exciting research opportunities.

Methioninase and selenomethionine but not Se-methylselenocysteine generate methylselenol and superoxide in an in vitro chemiluminescent assay: implications for the nutritional carcinostatic activity of selenoamino acids

Methylselenol from selenium metabolism is postulated to be and most experimental evidence now indicates that it is the selenium metabolite responsible for the dietary chemoprevention of cancers. Using the recombinant enzyme methioninase, methylselenol-generating chemiluminesence by superoxide (O2*-) is shown to be catalytically produced from L-selenomethionine and D,L-selenoethionine, but not from methionine or L-Se-methylselenocysteine (SeMC). Methylselenol enzymaticaly generated by methioninase activity from the substrate selenomethionine arises from an initial putative selenium radical as measured by chemiluminesence in the absence of glutathione (GSH). In the presence of GSH, superoxide was generated as measured by chemiluminesence and superoxide dismutase inhibition of chemiluminescence. Ascorbic acid also quenched the chemiluminesence from the activity of methioninase with selenomethionine. Methylselenol and other redox cycling selenium compounds are almost assuredly accountable for inducing cell-cycle arrest and apoptosis in cancer cells in vitro and in vivo. Methylselenol generated from selenomethionine by methioninase is catalytic alone in oxidizing thiols, i.e. GSH, generating superoxide and inducing oxidative stress in direct proportion to its concentration. Se-methylselenocysteine in vivo is very likely carcinostatic in like manner to selenomethionine by generating methylselenol from other enzymatic activity, i.e. beta-lyase or amino acid oxidases.

Substrate recognition and selectivity of plant glycerol-3-phosphate acyltransferases (GPATs) from Cucurbita moscata and Spinacea oleracea

Stromal glycerol-3-phosphate acyltransferases (GPAT) are responsible for the selective incorporation of saturated and unsaturated fatty-acyl chains into chloroplast membranes, which is an important determinant of a plant's ability to tolerate chilling temperatures. The molecular mechanisms of plant chilling tolerance were elucidated by creating chimeric GPATs between squash (Cucurbita moscata, chilling-sensitive) and spinach (Spinacea oleracea, chilling-tolerant) and the results were interpreted using structural information on squash GPAT determined by X-ray crystallography at 1.55 A resolution. Enzymatic analysis of the chimeric GPATs showed that the chimeric GPATs containing the spinach region from residues 128 to 187 prefer the 18:1 unsaturated fatty acid rather than 16:0 saturated fatty acid. Structure analysis suggests that the size and character of the cavity that is formed from this region determines the specific recognition of acyl chains.

Expression, purification, crystallization and preliminary crystallographic analysis of a putative GTP-binding protein, YsxC, fromBacillus subtilis

Bacillus subtilis YsxC has been putatively identified as a member of the GTP-binding protein family. Gene-knockout/deletion analysis has suggested that this protein is essential for survival of the microorganism and hence may represent a target for the development of a novel anti-infective agent. The B. subtilis ysxC gene was cloned and the protein was overexpressed in Escherichia coli and subsequently purified. Using hanging-drop vapour-diffusion crystallization techniques, two different crystal forms of YsxC were obtained in the presence and absence of GDP and which have one and two copies of YsxC in the asymmetric unit, respectively. Both crystal forms diffract to beyond 2.0 A resolution and are suitable for structure determination.

The purification, crystallization and preliminary structural characterization of PhzF, a key enzyme in the phenazine-biosynthesis pathway fromPseudomonas fluorescens2-79

Phenazines produced by members of several bacterial genera are biologically active metabolites that function in microbial competitiveness, the suppression of soil-borne plant diseases and virulence in infectious disease. Despite recent progress towards understanding the biochemistry of phenazine synthesis, the key reactions leading to the formation of the heterocyclic scaffold common to all phenazine compounds remain obscure. Pseudomonas fluorescens 2-79 contains seven phenazine (phz) genes that encode components of the pathway for biosynthesis of phenazine-1-carboxylic acid. A central step in this pathway involves the condensation of two identical precursor molecules derived from chorismic acid and is catalysed by the product of the phzF gene. In this study, recombinant PhzF was purified and crystallized from PEG 4000/ammonium sulfate/sodium citrate pH 5.6. The crystals belong to space group P3(1)21 or P3(2)21, with unit-cell parameters a = b = 56.3, c = 156.4 A. They contain one monomer in the asymmetric unit and diffract to better than 1.7 A on synchrotron beamlines. Crystals of seleno-L-methionine-labelled PhzF have been obtained and SAD data are reported.

Thioredoxin h overexpressed in barley seeds enhances selenite resistance and uptake during germination and early seedling development

The uptake, distribution and metabolism of selenite were examined in germinating homozygous barley ( Hordeum vulgare L.) grain with thioredoxin h overexpressed in starchy endosperm. Results were related to the null segregant in which the transgene had segregated out during crossing. Compared with the null segregant, the homozygote showed enhanced germination and root and shoot growth in the presence of 1 and 2 mM sodium selenite. The rate of incorporation of selenite by the homozygote was approximately twice that of the null segregant. Based on X-ray absorption spectroscopy, the major products in both cases were selenomethionine-like species and the red, monoclinic form of elemental selenium, a derivative not previously reported in green plants. Selenite and selenate made up the balance. The distribution of the products formed differed as to the tissue - root, shoot, aleurone, endosperm - but the ratios were similar in the homozygote and null segregant. The results provide evidence that, in addition to the accelerated germination observed previously in water, barley grain overexpressing thioredoxin h are resistant to the inhibitory effects of selenite. These properties raise the possibility that plants overexpressing thioredoxin h could find application in the remediation of polluted environments.

An Extended Winged Helix Domain in General Transcription Factor E/IIEα

Initiation of eukaryotic mRNA transcription requires melting of promoter DNA with the help of the general transcription factors TFIIE and TFIIH. Here we define a conserved and functionally essential N-terminal domain in TFE, the archaeal homolog of the large TFIIE subunit alpha. X-ray crystallography shows that this TFE domain adopts a winged helix-turn-helix (winged helix) fold, extended by specific alpha-helices at the N and C termini. Although the winged helix fold is often found in DNA-binding proteins, we show that TFE is not a typical DNA-binding winged helix protein, because its putative DNA-binding face shows a negatively charged groove and an unusually long wing, and because the domain lacks DNA-binding activity in vitro. The groove and a conserved hydrophobic surface patch on the additional N-terminal alpha-helix may, however, allow for interactions with other general transcription factors and RNA polymerase. Homology modeling shows that the TFE domain is conserved in TFIIE alpha, including the potential functional surfaces.

Selenomethionine incorporation into a protein by cell-free synthesis

Multi-wavelength anomalous diffraction phasing is especially useful for high-throughput structure determinations. Selenomethionine substituted proteins are commonly used for this purpose. However, the cytotoxicity of selenomethionine drastically reduces the efficiency of its incorporation in in vivo expression systems. In the present study, an improved E. coli cell-free protein synthesis system was used to incorporate selenomethionine into a protein, so that highly efficient incorporation could be achieved. A milligram quantity of selenomethionine-containing Ras was obtained using the cell-free system with dialysis. The mass spectrometry analysis showed that more than 95% of the methionine residues were substituted with selenomethionine. The crystal of this protein grew under the same conditions and had the same unit cell constants as those of the native Ras protein. The three-dimensional structure of this protein, determined by multi-wavelength anomalous diffraction phasing, was almost the same as that of the Ras protein prepared by in vivo expression. Therefore, the cell-free synthesis system could become a powerful protein expression method for high-throughput structure determinations by X-ray crystallography.

Probing the role of axial methionine in the blue copper center of azurin with unnatural amino acids

Expressed protein ligation was used to replace the axial methionine of the blue copper protein azurin from Pseudomonas aeruginosa with unnatural amino acids. The highly conserved methionine121 residue was replaced with the isostructural amino acids norleucine (Nle) and selenomethionine (SeM). The UV-visible absorption, X- and Q-band EPR, and Cu EXAFS spectra of the variants are slightly perturbed from WT. All variants have a predominant S(Cys) to Cu(II) charge transfer band around 625 nm and narrow EPR hyperfine splittings. The Se EXAFS of the M121SeM variant is also reported. In contrast to the small spectral changes, the reduction potentials of M121SeM, M121Leu, and M121Nle are 25, 135, and 140 mV, respectively, higher than that of WT azurin. The use of unnatural amino acids allowed deconvolution of different factors affecting the reduction potentials of the blue copper center. A careful analysis of the WT azurin and its variants obtained in this work showed the large reduction potential variation was linearly correlated with the hydrophobicity of the axial ligand side chains. Therefore, hydrophobicity is the dominant factor in tuning the reduction potentials of blue copper centers by axial ligands.

Methioninase gene therapy with selenomethionine induces apoptosis in bcl-2-overproducing lung cancer cells

We have previously shown that the toxic pro-oxidant methylselenol is released from selenomethionine (SeMET) by cancer cells transformed with the adenoviral methionine alpha,gamma-lyase (methioninase, MET) gene cloned from Pseudomonas putida. Methylselenol damaged the mitochondria via oxidative stress, and caused cytochrome c release into the cytosol thereby activating caspase enzymes and thereby apoptosis. However, gene therapy strategies are less effective if tumor cells overexpress the antiapoptotic mitochondrial protein bcl-2. In this study, we investigated whether rAdMET/SeMET was effective against bcl-2-overproducing A549 lung cancer cells. We established two clones of the human lung cancer A549 cell line that show moderate and high expression levels of bcl-2, respectively, compared to the parent cell line, which has very low bcl-2 expression. Staurosporine-induced apoptosis was inhibited in the bcl-2-overproducing clones as well as in the parental cell line. In contrast to staurosporine, apoptosis was induced in the bcl-2-overproducing clones as well as the parental cell line by AdMET/SeMET. Apoptosis in the rAdMET-SeMET-treated cells was determined by fragmentation of nuclei, and release of cytochrome c from mitochondria to the cytosol. A strong bystander effect of AdMET/SeMET was observed on A549 cells as well as the bcl-2-overproducing clones. rAdMET/SeMET prodrug gene therapy is therefore a promising novel strategy effective against bcl-2 overexpression, which has blocked other gene therapy strategies.

Crystallization and preliminary X-ray analysis of native and selenomethionine 2-hydroxybiphenyl 3-monooxygenase

2-hydroxybiphenyl 3-monooxygenase (HbpA; EC 1.14.13.44) from Pseudomonas azelaica HBP1 was produced in Escherichia coli both as native and SeMet-labelled protein. The two enzymes were purified to homogeneity and crystallized by the hanging-drop vapour-diffusion method. The crystals belong to the monoclinic space group C2, with unit-cell parameters a = 108.6, b = 196.8, c = 79.3 A, beta = 97.7 degrees for the native protein and a = 108.3, b = 196.8, c = 79.0 A, beta = 97.8 degrees for SeMet HbpA. Crystal-packing considerations led to the assumption of two HbpA subunits per asymmetric unit, which corresponds to a V(M) value of 3.3 A(3) Da(-1) and a solvent content of 62%. The crystals were radiation-sensitive and only had a lifespan of about 120 s when exposed to synchrotron radiation on an undulator beamline. To obtain complete data sets, data were collected from 23 native and 26 derivative crystals. The high-resolution limit was 2.0 A for native and 2.25 A for SeMet HbpA.