Gene expression shapes the patterns of parallel evolution of herbicide resistance in the agricultural weed Monochoria vaginalis.

The evolution of herbicide resistance in weeds is an example of parallel evolution, through which genes encoding herbicide target proteins are repeatedly represented as evolutionary targets. The number of herbicide target-site genes differs among species, and little is known regarding the effects of duplicate gene copies on the evolution of herbicide resistance. We investigated the evolution of herbicide resistance in Monochoria vaginalis, which carries five copies of sulfonylurea target-site acetolactate synthase (ALS) genes. Suspected resistant populations collected across Japan were investigated for herbicide sensitivity and ALS gene sequences, followed by functional characterization and ALS gene expression analysis. We identified over 60 resistant populations, all of which carried resistance-conferring amino acid substitutions exclusively in MvALS1 or MvALS3. All MvALS4 alleles carried a loss-of-function mutation. Although the enzymatic properties of ALS encoded by these genes were not markedly different, the expression of MvALS1 and MvALS3 was prominently higher among all ALS genes. The higher expression of MvALS1 and MvALS3 is the driving force of the biased representation of genes during the evolution of herbicide resistance in M. vaginalis. Our findings highlight that gene expression is a key factor in creating evolutionary hotspots.

Regulation of plant ER oxidoreductin 1 (ERO1) activity for efficient oxidative protein folding.

In the endoplasmic reticulum (ER), ER oxidoreductin 1 (ERO1) catalyzes intramolecular disulfide-bond formation within its substrates in coordination with protein-disulfide isomerase (PDI) and related enzymes. However, the molecular mechanisms that regulate the ERO1-PDI system in plants are unknown. Reduction of the regulatory disulfide bonds of the ERO1 from soybean, GmERO1a, is catalyzed by enzymes in five classes of PDI family proteins. Here, using recombinant proteins, vacuum-ultraviolet circular dichroism spectroscopy, biochemical and protein refolding assays, and quantitative immunoblotting, we found that GmERO1a activity is regulated by reduction of intramolecular disulfide bonds involving Cys-121 and Cys-146, which are located in a disordered region, similarly to their locations in human ERO1. Moreover, a GmERO1a variant in which Cys-121 and Cys-146 were replaced with Ala residues exhibited hyperactive oxidation. Soybean PDI family proteins differed in their ability to regulate GmERO1a. Unlike yeast and human ERO1s, for which PDI is the preferred substrate, GmERO1a directly transferred disulfide bonds to the specific active center of members of five classes of PDI family proteins. Of these proteins, GmPDIS-1, GmPDIS-2, GmPDIM, and GmPDIL7 (which are group II PDI family proteins) failed to catalyze effective oxidative folding of substrate RNase A when there was an unregulated supply of disulfide bonds from the C121A/C146A hyperactive mutant GmERO1a, because of its low disulfide-bond isomerization activity. We conclude that regulation of plant ERO1 activity is particularly important for effective oxidative protein folding by group II PDI family proteins.

The high-resolution crystal structure of lobster hemocyanin shows its enzymatic capability as a phenoloxidase.

Hemocyanin (Hc) and phenoloxidase (PO) are members of the type 3 copper protein family. Although arthropod Hc and PO exhibit similar three-dimensional structures of the copper-containing active site, Hc functions as an oxygen transport protein, showing minimal or no phenoloxidase activity. Here, we present the crystal structure of the oxy form of Hc from Panulirus japonicus (PjHc) at 1.58 Å resolution. The structure of the di-copper active site of PjHc was found to be almost identical to that of PO. Although conserved amino acids and the water molecule crucial for the enzymatic activity were observed in PjHc at almost the same positions as those in PO, PjHc showed no enzymatic activity under our experimental conditions. One striking difference between PjHc and arthropod PO was the presence of a "blocker residue" near the binuclear copper site of PjHc. This blocker residue comprised a phenylalanine residue tightly stacked with an imidazole ring of a CuA coordinated histidine and hindered substrates from accessing the active site. Our results suggest that the blocker residue is also a determining factor of the catalytic activity of type 3 copper proteins.

Cooperative Protein Folding by Two Protein Thiol Disulfide Oxidoreductases and 1 in Soybean.

Most proteins produced in the endoplasmic reticulum (ER) of eukaryotic cells fold via disulfide formation (oxidative folding). Oxidative folding is catalyzed by protein disulfide isomerase (PDI) and PDI-related ER protein thiol disulfide oxidoreductases (ER oxidoreductases). In yeast and mammals, ER oxidoreductin-1s (Ero1s) supply oxidizing equivalent to the active centers of PDI. In this study, we expressed recombinant soybean Ero1 (GmERO1a) and found that GmERO1a oxidized multiple soybean ER oxidoreductases, in contrast to mammalian Ero1s having a high specificity for PDI. One of these ER oxidoreductases, GmPDIM, associated in vivo and in vitro with GmPDIL-2, was unable to be oxidized by GmERO1a. We therefore pursued the possible cooperative oxidative folding by GmPDIM, GmERO1a, and GmPDIL-2 in vitro and found that GmPDIL-2 synergistically accelerated oxidative refolding. In this process, GmERO1a preferentially oxidized the active center in the A': domain among the A: , A': , and B: domains of GmPDIM. A disulfide bond introduced into the active center of the A': domain of GmPDIM was shown to be transferred to the active center of the A: domain of GmPDIM and the A: domain of GmPDIM directly oxidized the active centers of both the A: or A': domain of GmPDIL-2. Therefore, we propose that the relay of an oxidizing equivalent from one ER oxidoreductase to another may play an essential role in cooperative oxidative folding by multiple ER oxidoreductases in plants.

Roles of transferrin receptors in erythropoiesis.

Erythropoiesis requires large amounts of iron for hemoglobin synthesis, which is mainly provided by macrophages and the intestines in a transferrin (Tf)-bound form. Bone marrow erythroblasts incorporate Tf through endocytosis, which is mediated by transferrin receptor 1 (TFR1). Recently, human TFR1, aside from its role as a Tf receptor, was also found to be a receptor for the H-subunit of ferritin (FTH). In humans, hematopoietic erythroid precursor cells express high levels of TFR1 and specifically take up the FTH homopolymer (H-ferritin). H-ferritin inhibits the formation of burst forming unit-erythroid colonies in vitro. TFR2, which is also a Tf receptor, is predominantly expressed in hepatocytes and erythroid precursor cells. In the liver, TFR2 forms a complex with HFE, a hereditary hemochromatosis-associated protein, and acts as an iron sensor. In mice, hepatocyte-specific knockout of the TFR2 gene has been shown to cause systemic iron-overload with decreased expression of hepcidin, the central regulator of iron homeostasis. In erythroid cells, TFR2 forms a complex with the erythropoietin receptor and facilitates its trafficking to the cell membrane. Moreover, hematopoietic cell-specific knockout of the TFR2 gene causes microcytic erythrocytosis in mice. This review focuses on the molecular evolution and functions of these TFRs and their ligands.

Expression and characterization of protein disulfide isomerase family proteins in bread wheat.

BACKGROUND: The major wheat seed proteins are storage proteins that are synthesized in the rough endoplasmic reticulum (ER) of starchy endosperm cells. Many of these proteins have intra- and intermolecular disulfide bonds. In eukaryotes, the formation of most intramolecular disulfide bonds in the ER is thought to be catalyzed by protein disulfide isomerase (PDI) family proteins. The cDNAs that encode eight groups of bread wheat (Triticum aestivum L.) PDI family proteins have been cloned, and their expression levels in developing wheat grains have been determined. The purpose of the present study was to characterize the enzymatic properties of the wheat PDI family proteins and clarify their expression patterns in wheat caryopses. RESULTS: PDI family cDNAs, which are categorized into group I (TaPDIL1Aα, TaPDIL1Aß, TaPDIL1Aγ, TaPDIL1Aδ, and TaPDIL1B), group II (TaPDIL2), group III (TaPDIL3A), group IV (TaPDIL4D), and group V (TaPDIL5A), were cloned. The expression levels of recombinant TaPDIL1Aα, TaPDIL1B, TaPDIL2, TaPDIL3A, TaPDIL4D, and TaPDIL5A in Escherichia coli were established from the cloned cDNAs. All recombinant proteins were expressed in soluble forms and purified. Aside from TaPDIL3A, the recombinant proteins exhibited oxidative refolding activity on reduced and denatured ribonuclease A. Five groups of PDI family proteins were distributed throughout wheat caryopses, and expression levels of these proteins were higher during grain filling than in the late stage of maturing. Localization of these proteins in the ER was confirmed by fluorescent immunostaining of the immature caryopses. In mature grains, the five groups of PDI family proteins remained in the aleurone cells and the protein matrix of the starchy endosperm. CONCLUSIONS: High expression of PDI family proteins during grain filling in the starchy endosperm suggest that these proteins play an important role in forming intramolecular disulfide bonds in seed storage proteins. In addition, these PDI family proteins that remain in the aleurone layers of mature grains likely assist in folding newly synthesized hydrolytic enzymes during germination.

Crystal structure of class III chitinase from pomegranate provides the insight into its metal storage capacity.

Chitinase hydrolyzes the ß-1,4-glycosidic bond in chitin. In higher plants, this enzyme has been regarded as a pathogenesis-related protein. Recently, we identified a class III chitinase, which functions as a calcium storage protein in pomegranate (Punica granatum) seed (PSC, pomegranate seed chitinase). Here, we solved a crystal structure of PSC at 1.6 Å resolution. Although its overall structure, including the structure of catalytic site and non-proline cis-peptides, was closely similar to those of other class III chitinases, PSC had some unique structural characteristics. First, there were some metal-binding sites with coordinated water molecules on the surface of PSC. Second, many unconserved aspartate residues were present in the PSC sequence which rendered the surface of PSC negatively charged. This acidic electrostatic property is in contrast to that of hevamine, well-characterized plant class III chitinase, which has rather a positively charged surface. Thus, the crystal structure provides a clue for metal association property of PSC.

The iron content and ferritin contribution in fresh, dried, and toasted nori, Pyropia yezoensis.

Iron is one of the essential trace elements for humans. In this study, the iron contents in fresh, dried, and toasted nori (Pyropia yezoensis) were analyzed. The mean iron content of fresh, dried, and toasted nori were 19.0, 22.6, and 26.2 mg/100 g (dry weight), respectively. These values were superior to other food of plant origin. Furthermore, most of the iron in nori was maintained during processing, such as washing, drying, and toasting. Then, the form of iron in fresh, dried, and toasted nori was analyzed. As a result, an iron storage protein ferritin contributed to iron storage in raw and dried nori, although the precise rate of its contribution is yet to be determined, while ferritin protein cage was degraded in the toasted nori. It is the first report that verified the ferritin contribution to iron storage in such edible macroalgae with commercial importance.

Development of a novel transgenic rice with hypocholesterolemic activity via high-level accumulation of the α' subunit of soybean ß-conglycinin.

Soybean 7S globulin, known as ß-conglycinin, has been shown to regulate human plasma cholesterol and triglyceride levels. Furthermore, the α' subunit of ß-conglycinin has specifically been shown to possess low-density lipoprotein (LDL)-cholesterol-lowering activity. Therefore, accumulation of the α' subunit of ß-conglycinin in rice seeds could lead to the production of new functional rice that could promote human health. Herein, we used the low-glutelin rice mutant 'Koshihikari' (var. a123) and suppressed its glutelins and prolamins, the major seed storage proteins of rice, by RNA interference. The accumulation levels of the α' subunit in the lines with suppressed glutelin and prolamin levels were >20 mg in 1 g of rice seeds, which is considerably higher than those in previous studies. Oral administration of the transgenic rice containing the α' subunit exhibited a hypocholesterolemic activity in rats; the serum total cholesterol and LDL cholesterol levels were significantly reduced when compared to those of the control rice (var. a123). The cholesterol-lowering action by transgenic rice accumulating the α' subunit induces a significant increase in fecal bile acid excretion and a tendency to increase in fecal cholesterol excretion. This is the first report that transgenic rice exhibits a hypocholesterolemic activity in rats in vivo by using the ß-conglycinin α' subunit.

Evaluation of genetic consequences of stocking on the southern-margin populations of white-spotted charr.

Coldwater-adapted freshwater fishes, especially their populations along warm-range margins, are most vulnerable to the climate oscillations associated with global warming. Stocking is a major strategy for avoiding the extinction of these species. However, while stocking can reverse the decline of isolated populations, it may also result in a loss of genetic diversity in the native local population due to the introgressive replacement of hatchery genes. To plan an adequate strategy for conserving locally adapted populations, the genetic impacts of stocking on native lineages should be evaluated from small river branches to wide-ranging drainage areas. We investigated the population genetic structure of white-spotted charr (Salvelinus leucomaenis) within its southern range (Lake Biwa basin, Japan). By applying genome-wide SNP analysis to the population's genetic structure, we assessed the extent of genetic introgression resulting from stocking. White-spotted charr in the Lake Biwa watershed constitutes a distinctive genetic group, within which apparent genetic differentiation was observed. The hatchery-reared fish line commonly used for supplementation stocking in the catchment was discernable from the native population, enabling us to analyze genetic introgression across the entire drainage area. Admixed individuals resulting from hatchery introgression were observed in most of the stocked sites that showed relatively high heterozygosity and nucleotide diversity. However, their genetic differentiation was much lower than that of native populations. The supplementation history as well as the road availability contributed substantially to the introgression of hatchery genes. Populations with the native genetic structure remained in the upstream regions of the tested rivers. However, their heterozygosity and nucleotide diversity were low when compared with that of the populations with hatchery supplementation. Our results shed light on the genetic impacts of stocking on isolated native populations and suggest that conventional supplementation methods cannot preserve a unique biodiversity in the distribution margin.

Mass spectrometry based N- and C-terminal sequence determination of a hepatopancreas-type prophenoloxidase from the kuruma prawn, Marsupenaeus japonicus.

We previously identified and characterized a novel hepatopancreas-type prophenoloxidase from kuruma prawn, Marsupenaeus japonicus. In the characterization, this enzyme was indicated to have a feature of a signal peptide at its N-terminus. The putative primary structure was then proposed but its N- and C-terminal sequences remained undetermined. In the present study, the N- and C-terminal amino acid sequences of this prophenoloxidase were determined by de novo sequencing methods using matrix-assisted laser desorption ionization mass spectrometry. The sequence analyses revealed that the N-terminus of the prophenoloxidase was processed, whereas the C-terminus was not. This finding suggests that this enzyme has a signal peptide, and that it is synthesized at the endoplasmic reticulum in hepatopancreas cells and secreted to hemolymph plasma, similar to the case of hemocyanin, another member of the class III copper proteins.

Chitinase III in pomegranate seeds (Punica granatum Linn.): a high-capacity calcium-binding protein in amyloplasts.

Chitinases are a class of ubiquitous proteins that are widely distributed in plants. Defense is the major natural role for chitinases, primarily against fungal pathogens. Little is known regarding their non-defensive roles in seeds. In this study, a new class III chitinase from pomegranate seeds (pomegranate seed chitinase, PSC) was isolated and purified to homogeneity. The native state of PSC is a monomer with a molecular weight of approximately 30 kDa. This chitinase naturally binds calcium ions with high capacity and low affinity, suggesting that PSC is a calcium storage protein. Consistent with this idea, its amino acid sequence (inferred from cDNA) is rich in acidic amino acid residues, especially Asp, similar to reported calcium storage proteins. The presence of calcium considerably improves the stability of the protein but has little effect on its enzymatic activity. Transmission electron microscopy analyses indicate that, similar to phytoferritin, this enzyme is widely distributed in the stroma of amyloplasts of the embryonic cells, suggesting that amyloplasts in seeds could serve as an alternative plastid for calcium storage. Indeed, the transmission electron microscopy results showed that, within the embryonic cells, calcium ions are mainly distributed in the stroma of the amyloplasts, consistent with a role for PSC in calcium storage. Thus, the plant appears to have evolved a new plastid for calcium storage in seeds. During seed germination, the content of this enzyme decreases with time, suggesting that it is involved in the germination process.

A novel type of prophenoloxidase from the kuruma prawn Marsupenaeus japonicus contributes to the melanization of plasma in crustaceans.

Melanization is one of the major immune responses in arthropods. Prophenoloxidases (proPOs) catalyze the oxidation of mono- or o-diphenols, a reaction that is the key initial step of melanin formation. Well-characterized proPOs from crustaceans are synthesized in haemocytes and are released into plasma in response to microbial attack. However, PO activity does exist in the plasma of haemolymph without pathogenic infections. Here, we demonstrate that a novel type of proPO contributes to such PO activity in the plasma fraction of haemolymph of crustaceans. The novel enzyme, which was purified from the plasma of the kuruma prawn (Marsupenaeus japonicus), possessed strong and specific monophenol and o-diphenol oxidation activity compared with that of known haemocyte-type proPO. Amino acid sequence analyses indicated that this enzyme was distinct from the known proPO. The cDNA sequence and deduced amino acid sequence of this enzyme has a putative binuclear copper center, and showed approximately 30% and 20% identity with the primary structures of reported proPO and haemocyanin sequences of the kuruma prawn, respectively. Reverse transcription PCR analysis showed that this enzyme was synthesized in the hepatopancreas rather than in haemocytes. Although the primary structure and enzymatic properties of this novel enzyme suggested that it is a phenoloxidase, its biogenesis, tissue distribution, and oligomeric state resemble those of haemocyanin, which belongs to the same protein family (type III copper protein). This novel proPO enzyme may share a role with the already characterized version, itself a major component of the innate immune system in crustaceans.

Identification and characterization of a ferritin gene and its product from the multicellular green alga Ulva pertusa.

Iron is an essential element for virtually all kingdoms of life, and especially for primary producers in ocean ecosystems. To date, the molecular mechanism of iron utilization by macroalgae remains largely unknown. To elucidate the strategy of iron acquisition and storage in macroalgae, we focused on the function of the iron storage protein ferritin in the sea lettuce, Ulva pertusa, which has abundant iron content. Judging from the primary structure, U. pertusa ferritin (UpFer) can be classified as a land-plant-type ferritin, which is usually found in plastids. The gene of UpFer was expressed in the peripheral, central and rhizoid parts. Western blot analysis showed that UpFER was present and functioned in processed 26- and 22-kDa forms. Furthermore, recombinant UpFER had iron incorporation activity comparable to other ferritins. These results suggest that ferritin also functions as an iron storage protein as in unicellular algae and land plants.

Crystal structure of plant ferritin reveals a novel metal binding site that functions as a transit site for metal transfer in ferritin.

Ferritins are important iron storage and detoxification proteins that are widely distributed in living kingdoms. Because plant ferritin possesses both a ferroxidase site and a ferrihydrite nucleation site, it is a suitable model for studying the mechanism of iron storage in ferritin. This article presents for the first time the crystal structure of a plant ferritin from soybean at 1.8-A resolution. The soybean ferritin 4 (SFER4) had a high structural similarity to vertebrate ferritin, except for the N-terminal extension region, the C-terminal short helix E, and the end of the BC-loop. Similar to the crystal structures of other ferritins, metal binding sites were observed in the iron entry channel, ferroxidase center, and nucleation site of SFER4. In addition to these conventional sites, a novel metal binding site was discovered intermediate between the iron entry channel and the ferroxidase site. This site was coordinated by the acidic side chain of Glu(173) and carbonyl oxygen of Thr(168), which correspond, respectively, to Glu(140) and Thr(135) of human H chain ferritin according to their sequences. A comparison of the ferroxidase activities of the native and the E173A mutant of SFER4 clearly showed a delay in the iron oxidation rate of the mutant. This indicated that the glutamate residue functions as a transit site of iron from the 3-fold entry channel to the ferroxidase site, which may be universal among ferritins.

Role of H-1 and H-2 subunits of soybean seed ferritin in oxidative deposition of iron in protein.

Naturally occurring phytoferritin is a heteropolymer consisting of two different H-type subunits, H-1 and H-2. Prior to this study, however, the function of the two subunits in oxidative deposition of iron in ferritin was unknown. The data show that, upon aerobic addition of 48-200 Fe(2+)/shell to apoferritin, iron oxidation occurs only at the diiron ferroxidase center of recombinant H1 (rH-1). In addition to the diiron ferroxidase mechanism, such oxidation is catalyzed by the extension peptide (a specific domain found in phytoferritin) of rH-2, because the H-1 subunit is able to remove Fe(3+) from the center to the inner cavity better than the H-2 subunit. These findings support the idea that the H-1 and H-2 subunits play different roles in iron mineralization in protein. Interestingly, at medium iron loading (200 irons/shell), wild-type (WT) soybean seed ferritin (SSF) exhibits a stronger activity in catalyzing iron oxidation (1.10 ± 0.13 µm iron/subunit/s) than rH-1 (0.59 ± 0.07 µm iron/subunit/s) and rH-2 (0.48 ± 0.04 µm iron/subunit/s), demonstrating that a synergistic interaction exists between the H-1 and H-2 subunits in SSF during iron mineralization. Such synergistic interaction becomes considerably stronger at high iron loading (400 irons/shell) as indicated by the observation that the iron oxidation activity of WT SSF is ∼10 times larger than those of rH-1 and rH-2. This helps elucidate the widespread occurrence of heteropolymeric ferritins in plants.

A novel EP-involved pathway for iron release from soya bean seed ferritin.

Iron in phytoferritin from legume seeds is required for seedling germination and early growth. However, the mechanism by which phytoferritin regulates its iron complement to these physiological processes remains unknown. In the present study, protein degradation is found to occur in purified SSF (soya bean seed ferritin) (consisting of H-1 and H-2 subunits) during storage, consistent with previous results that such degradation also occurs during seedling germination. In contrast, no degradation is observed with animal ferritin under identical conditions, suggesting that SSF autodegradation might be due to the EP (extension peptide) on the exterior surface of the protein, a specific domain found only in phytoferritin. Indeed, EP-deleted SSF becomes stable, confirming the above hypothesis. Further support comes from a protease activity assay showing that EP-1 (corresponding to the EP of the H-1 subunit) exhibits significant serine protease-like activity, whereas the activity of EP-2 (corresponding to the EP of the H-2 subunit) is much weaker. Consistent with the observation above, rH-1 (recombinant H-1 ferritin) is prone to degradation, whereas its analogue, rH-2, becomes very stable under identical conditions. This demonstrates that SSF degradation mainly originates from the serine protease-like activity of EP-1. Associated with EP degradation is a considerable increase in the rate of iron release from SSF induced by ascorbate in the amyloplast (pH range, 5.8-6.1). Thus phytoferritin may have facilitated the evolution of the specific domain to control its iron complement in response to cell iron need in the seedling stage.

The basicity of an active-site water molecule discriminates between tyrosinase and catechol oxidase activity.

Tyrosinase (Ty) and catechol oxidase (CO) are members of type-3 copper enzymes. While Ty catalyzes both phenolase and catecholase reactions, CO catalyzes only the latter reaction. In the present study, Ty was found to catalyze the catecholase reaction, but hardly the phenolase reaction in the presence of the metallochaperon called "caddie protein (Cad)". The ability of the substrates to dissociate the motif shielding the active-site pocket seems to contribute critically to the substrate specificity of Ty. In addition, a mutation at the N191 residue, which forms a hydrogen bond with a water molecule near the active center, decreased the inherent ratio of phenolase versus catecholase activity. Unlike the wild-type complex, reaction intermediates were not observed when the catalytic reaction toward the Y98 residue of Cad was progressed in the crystalline state. The increased basicity of the water molecule may be necessary to inhibit the proton transfer from the conjugate acid to a hydroxide ion bridging the two copper ions. The deprotonation of the substrate hydroxyl by the bridging hydroxide seems to be significant for the efficient catalytic cycle of the phenolase reaction.

The universal mechanism for iron translocation to the ferroxidase site in ferritin, which is mediated by the well conserved transit site.

Ferritins are ubiquitous iron storage proteins. Recently, we identified a novel metal-binding site, transit site, in the crystal structure of phytoferritin. To elucidate the function of the transit site in ferritin from other species, we prepared transit-site-deficient mutants of human H ferritin, E140A and E140Q, and their iron oxidation kinetics was analyzed. The initial velocities of iron oxidization were reduced in the variants, especially in E140Q. The crystal structure of E140Q showed that the side chain of the mutated Gln140 was fixed by a hydrogen bond, whereas that of native Glu140 was flexible. These results suggest that the conserved transit site also has a function to assist with the metal ion sequestration to the ferroxidase site in ferritins from vertebrates.

Crystallization and preliminary X-ray analysis of the major peanut allergen Ara h 1 core region.

Peanuts contain some of the most potent food allergens known to date. Ara h 1 is one of the three major peanut allergens. As a first step towards three-dimensional structure elucidation, recombinant Ara h 1 core region was cloned, expressed in Escherichia coli and purified to homogeneity. Crystals were obtained using 0.1 M sodium citrate pH 5.6, 0.1 M NaCl, 15% PEG 400 as precipitant. The crystals diffracted to 2.25 A resolution using synchrotron radiation and belonged to the monoclinic space group C2, with unit-cell parameters a=156.521, b=88.991, c=158.971 A, beta=107.144 degrees. Data were collected at the BL-38B1 station of SPring-8 (Hyogo, Japan).