Akira Nakamura

<title>Abstract</title>The maltose-binding protein (MBP) fusion tag is one of the most commonly utilized crystallization chaperones for proteins of interest. Recently, this MBP-mediated crystallization technique was adapted to <italic>Arabidopsis thaliana</italic> (At) BRZ-INSENSITIVE-LONG (BIL1)/BRASSINAZOLE-RESISTANT (BZR1), a member of the plant-specific BZR TFs, and revealed the first structure of AtBIL1/BZR1 in complex with target DNA. However, it is unclear how the fused MBP affects the structural features of the AtBIL1/BZR1-DNA complex. In the present study, we highlight the potential utility of the MBP crystallization chaperone by comparing it with the crystallization of unfused AtBIL1/BZR1 in complex with DNA. Furthermore, we assessed the validity of the MBP-fused AtBIL1/BZR1-DNA structure by performing detailed dissection of crystal packings and molecular dynamics (MD) simulations with the removal of the MBP chaperone. Our MD simulations define the structural basis underlying the AtBIL1/BZR1-DNA assembly and DNA binding specificity by AtBIL1/BZR1. The methodology employed in this study, the combination of MBP-mediated crystallization and MD simulation, demonstrates promising capabilities in deciphering the protein-DNA recognition code.

Nuclear factor of activated T cells (NFAT) leads to the transcription of diverse inducible genes involved in many biological processes; therefore, aberrant NFAT expression is responsible for the development and exacerbation of various disorders. Since five isoforms of NFAT (NFATc1-c4, NFAT5) exhibit distinct and overlapping functions, selective control of a part, but not all, of NFAT family members is desirable. By comparing the binding activity of each NFATc1-c4 with its regulatory enzyme, calcineurin (CN), using a quantitative immunoprecipitation assay, we found a new CN-binding region (CNBR) selectively functioning in NFATc1 and NFATc4. This region, termed CNBR3, is located between two preexisting CNBR1 and CNBR2, within the Ca2+ regulatory domain. The nuclear translocation of NFATc1 but not NFATc2 in T cells was suppressed by ectopic expression of CNBR3 and, accordingly, NFATc1-dependent cytokine expression was downregulated. Through competition assays using NFATc1-derived partial peptides and mass spectrometry with photoaffinity technology, we identified 18 amino acids in NFATc1 (Arg258 to Pro275 ) and 13 amino acids in CN catalytic subunit (CNA) (Asn77 to Gly89 ) responsible for CNA/CNBR3 binding in which Cys263 and Asp82 , respectively, played crucial roles. The possible selective regulation of NFAT-mediated biological processes by targeting this new CN/NFAT-binding region is suggested.

Alzheimer's disease pathology is characterized by extracellular deposits of amyloid-β (Aβ) and intracellular inclusions of hyperphosphorylated tau. Although genetic studies of familial Alzheimer's disease suggest a causal link between Aβ and disease symptoms, the failure of various Aβ-targeted strategies to slow or halt disease progression has led to consideration of the idea that inhibition of tau aggregation might be a more promising therapeutic approach. Methylene blue (MB), which inhibits tau aggregation and rescue memory deficits in a mouse model of tauopathy, however, lacked efficacy in a recent Phase III clinical trial. In order to gain insight into this failure, the present study was designed to examine the mechanism through which MB inhibits tau aggregation. We found that MB inhibits heparin-induced tau aggregation in vitro, as measured by thioflavin T fluorescence. Further, MB reduced the amount of tau in precipitants recovered after ultracentrifugation of the aggregation mixture. Atomic force microscopy revealed that MB reduces the number of tau fibrils but increases the number of granular tau oligomers. The latter result was confirmed by sucrose gradient centrifugation: MB treatment was associated with higher levels of granular tau oligomers (fraction 3) and lower levels of tau fibrils (fractions 5 and 6). We previously demonstrated that the formation of granular tau oligomers, rather than tau fibrils, is essential for neuronal death. Thus, the fact that MB actions are limited to inhibition of tau fibril formation provides a mechanistic explanation for the poor performance of MB in the recent Phase III clinical trial.

Correction to: Scientific Reports https://doi.org/10.1038/s41598-017-11542-0, published online 12 September 2017 This Article contains a typographical error in Table 1. In the 'Data Collection' column, "R meas" is incorrectly listed as "R sym".

Sperm chemotaxis toward a chemoattractant is very important for the success of fertilization. Calaxin, a member of the neuronal calcium sensor protein family, directly acts on outer-arm dynein and regulates specific flagellar movement during sperm chemotaxis of ascidian, Ciona intestinalis. Here, we present the crystal structures of calaxin both in the open and closed states upon Ca2+ and Mg2+ binding. The crystal structures revealed that three of the four EF-hands of a calaxin molecule bound Ca2+ ions and that EF2 and EF3 played a critical role in the conformational transition between the open and closed states. The rotation of α7 and α8 helices induces a significant conformational change of a part of the α10 helix into the loop. The structural differences between the Ca2+-and Mg2+-bound forms indicates that EF3 in the closed state has a lower affinity for Mg2+, suggesting that calaxin tends to adopt the open state in Mg2+-bound form. SAXS data supports that Ca2+-binding causes the structural transition toward the closed state. The changes in the structural transition of the C-terminal domain may be required to bind outer-arm dynein. These results provide a novel mechanism for recognizing a target protein using a calcium sensor protein.

BRZ-INSENSITIVE-LONG HYPOCOTYL 1 (BIL1)/BRASSINAZOLE-RESISTANT 1 (BZR1) is a master transcription factor of brassinosteroid (BR) signalling. The varieties of nucleobase recognition of the NN-BRRE-core motif (NNCGTG), one of variant G-box motifs, distinguish BIL1/BZR1 from basic helix-loop-helix transcription factors, underlying the specific regulation of BR-responsive genes. Here, we show the non-canonical bHLH dimer formation of BIL1/BZR1 to optimize the interaction network with DNA and the orientation of a key residue for NN-BRRE-core motif recognition.

HYPOSENSITIVE TO LIGHT (HTL) and DWARF14 (D14) mediate the perception of karrikin and strigolactone, which stimulates germination of the parasitic weed Striga. However, their role in parasitic seeds is poorly understood, and the basis for their differing responsiveness remains unclear. Here, we show that Striga hermonthica HTL proteins (ShHTLs) in 'conserved' and 'intermediate' clades are able to bind karrikin. The 'divergent' clade is able to hydrolyze strigolactone. Unexpectedly, we find that ShD14 is also capable of hydrolyzing strigolactone. Through comparative analysis of ShHTLs and ShD14 crystal structures, we provide insights into the basis for their selectivity. Moreover, we show that both ShD14 and divergent clade ShHTLs, but not conserved and intermediate clade ShHTLs, can interact with the putative downstream signaling component ShMAX2 in the presence of the synthetic strigolactone, rac-GR24. These findings provide insight into how strigolactone is perceived and how ligand specificity is determined.

<p>Alginate-recognition subsites of alginate lyase FlAlyA were characterized as potential targets for engineering alginate oligosaccharides that are useful biomaterials.</p>

Fenugreek is a dietary supplement for anti-aging and human health. (2S, 3R, 4S)-4-hydroxyisoleucine (4-HIL), which is extracted from fenugreek seeds, is expected to be a promising orally active drug for diabetes and diabetic nephropathy because of its insulinotropic effect. Although several chemical synthesis methods of 4-HIL have been proposed, these methods require multistep reactions to control the stereochemistry of 4-HIL. In this study, we modified the key enzyme 4-HIL dehydrogenase (HILDH) to overcome the biggest limitation in commercial-scale production of 4-HIL. As a result, an effective one-step carbonyl reduction to produce (2S, 3R, 4S)-4-HIL was successfully accomplished with strict stereoselectivity (&gt;99% de). Mass production of (2S, 3R, 4S)-4-HIL by our synthetic method could have a significant contribution to the prevention of diabetes, dyslipidemia, and Alzheimer's disease. (120 words/200 words)

Laminarinase from Flavobacterium sp. strain UMI-01, a new member of the glycosyl hydrolase 16 family of a marine bacterium associated with seaweeds, mainly degrades beta-1,3-glucosyl linkages of beta-glucan (such as laminarin) through the hydrolysis of glycosidic bonds. We determined the crystal structure of ULam111 at 1.60-angstrom resolution to understand the structural basis for its thermostability and substrate specificity. A calcium-binding motif located on the opposite side of the beta-sheet from catalytic cleft increased its degrading activity and thermostability. The disulfide bridge Cys31-Cys34, located on the beta 2-beta 3 loop near the substrate-binding site, is responsible for the thermostability of ULam111. The substrates of beta-1,3-linked laminarin and beta-1,3-1,4-linked glucan bound to the catalytic cleft in a completely different mode at subsite -3. Asn33 and Trp113, together with Phe212, formed hydrogen bonds with preferred substrates to degrade beta-1,3-linked laminarin based on the structural comparisons. Our structural information provides new insights concerning thermostability and substrate recognition that will enable the design of industrial biocatalysts.

Mitochondrial isocitrate dehydrogenase 2 (IDH2) converts NADP(+) to NADPH and promotes regeneration of reduced glutathione (GSH) by supplying NADPH to glutathione reductase or thioredoxin reductase. We have previously shown that under calorie restriction, mitochondrial deacetylase Sirt3 deacetylates and activates IDH2, thereby regulating the mitochondrial glutathione antioxidant defense system in mice. To investigate the regulatory mechanism of mIDH2 (mouse mitochondrial IDH2), we used lysine-to-glutamine (KQ) mutants to mimic acetylated lysines and screened 15 KQ mutants. Among these mutants, the activities of the K256Q and K413Q proteins were less than 50% of the wild-type value. We then solved the crystal structures of the wild-type mIDH2 and the K256Q mutant proteins, revealing conformational changes in the substrate-binding pocket. Structural data suggested that positively charged Lys256 was important in stabilizing the pocket because it repelled a lysine cluster on the other side. Glutamine (or acetylated lysine) was neutral and thus caused the pocket size to decrease, which might be the main reason for the lower activity of the K256Q mutant. Together, our data provide the first structure of an acetylation mimic of mIDH2 and new insights into the regulatory mechanism of acetylation of mIDH2.

Alginate is an abundant algal polysaccharide, composed of beta-D-mannuronate and its C5 epimer alpha-L-guluronate, that is a useful biomaterial in cell biology and tissue engineering, with applications in cancer and aging research. The alginate lyase (EC 4.2.2.3) from Aplysia kurodai, AkAly30, is a eukaryotic member of the polysaccharide lyase 14 (PL-14) family and degrades alginate by cleaving the glycosidic bond through a beta-elimination reaction. Here, we present the structural basis for the substrate specificity, with a preference for polymannuronate, of AkAly30. The crystal structure of AkAly30 at a 1.77 angstrom resolution and the putative substrate-binding model show that the enzyme adopts a beta-jelly roll fold at the core of the structure and that Lys-99, Tyr-140, and Tyr-142 form catalytic residues in the active site. Their arrangements allow the carboxyl group of mannuronate residues at subsite +1 to form ionic bonds with Lys-99. The coupled tyrosine forms a hydrogen bond network with the glycosidic bond, and the hydroxy group of Tyr-140 is located near the C5 atom of the mannuronate residue. These interactions could promote the beta-elimination of the mannuronate residue at subsite +1. More interestingly, Gly-118 and the disulfide bond formed by Cys-115 and Cys-124 control the conformation of an active-site loop, which makes the space suitable for substrate entry into subsite -1. The cleavage efficiency of AkAly30 is enhanced relative to that of mutants lacking either Gly-118 or the Cys-115-Cys-124 disulfide bond. The putative binding model and mutagenesis studies provide a novel substrate recognition mode explaining the polymannuronate specificity of PL-14 alginate lyases.

The perception of two plant germination inducers, karrikins and strigolactones, are mediated by the proteins KAI2 and D14. Recently, KAI2-type proteins from parasitic weeds, which are possibly related to seed germination induced by strigolactone, have been classified into three clades characterized by different responses to karrikin/strigolactone. Here we characterized a karrikin-binding protein in Striga (ShKAI2iB) that belongs to intermediate-evolving KAI2 and provided the structural bases for its karrikin-binding specificity. Binding assays showed that ShKAI2iB bound karrikins but not strigolactone, differing from other KAI2 and D14. The crystal structures of ShKAI2iB and ShKAI2i-Bkarrikin complex revealed obvious structural differences in a helix located at the entry of its ligand-binding cavity. This results in a smaller closed pocket, which is also the major cause of ShKAI2iB's specificity of binding karrikin. Our structural study also revealed that a few non-conserved amino acids led to the distinct ligand-binding profile of ShKAI2iB, suggesting that the evolution of KAI2 resulted in its diverse functions.

Precise protein structure determination provides significant information on life science research, although high-quality crystals are not easily obtained. We developed a system for producing high-quality protein crystals with high throughput. Using this system, gravity-controlled crystallization are made possible by a magnetic microgravity environment. In addition, in-situ and real-time observation and time-lapse imaging of crystal growth are feasible for over 200 solution samples independently. In this paper, we also report results of crystallization experiments for two protein samples. Crystals grown in the system exhibited magnetic orientation and showed higher and more homogeneous quality compared with the control crystals. The structural analysis reveals that making use of the magnetic microgravity during the crystallization process helps us to build a well-refined protein structure model, which has no significant structural differences with a control structure. Therefore, the system contributes to improvement in efficiency of structural analysis for "difficult" proteins, such as membrane proteins and supermolecular complexes.

L-threo-3,4-Dihydroxyphenylserine (L-DOPS, Droxidopa) is a psychoactive drug and synthetic amino acid precursor that acts as a prodrug to the neurotransmitters. SadA, a dioxygenase from Burkholderia ambifaria AMMD, is an Fe(II)- and alpha-ketoglutarate (KG)-dependent enzyme that catalyzes N-substituted branched-chain or aromatic L-amino acids. SadA is able to produce N-succinyl-L-threo-3,4-dimethoxyphenylserine (NSDOPS), which is a precursor of L-DOPS, by catalyzing the hydroxylation of N-succiny1-3,4-dimethoxyphenylalanine (NSDOPA). However, the catalytic activity of SadA toward NSDOPS is much lower than that toward N-succinyl branched-chain L-amino acids. Here, we report an improved biocatalytic synthesis of NSDOPS with SadA. Structure-based protein engineering was applied to improve the alpha-KG turnover activity for the synthesis of NSDOPS. The G79A, G79A/F261W or G79A/F261R mutant showed a more than 6-fold increase in activity compared to that of the wild-type enzyme. The results provide a new insight into the substrate specificity toward NSDOPA and will be useful for the rational design of SadA mutants as a target of industrial biocatalysts. (C) 2014 Elsevier Inc. All rights reserved.

AMP phosphorylase (AMPpase) catalyzes the initial reaction in a novel AMP metabolic pathway recently found in archaea, converting AMP and phosphate into adenine and ribose 1,5-bisphosphate. Gel-filtration chromatography revealed that AMPpase from Thermococcus kodakarensis (Tk-AMPpase) forms an exceptionally large macromolecular structure (&gt;40-mers) in solution. To investigate its unique multimerization feature, we determined the first crystal structures of Tk-AMPpase, in the apo-form and in complex with substrates. Structures of two truncated forms of Tk-AMPpase (Tk-AMPpase Delta N84 and Tk-AMPpase Delta C10) clarified that this multimerization is achieved by two dimer interfaces within a single molecule: one by the central domain and the other by the C-terminal domain, which consists of an unexpected domain-swapping interaction. The N-terminal domain, characteristic of archaeal enzymes, is essential for enzymatic activity, participating in nnultimerization as well as domain closure of the active site upon substrate binding. Moreover, biochemical analysis demonstrated that the macromolecular assembly of Tk-AMPpase contributes to its high thermostability, essential for an enzyme from a hyperthermophile. Our findings unveil a unique archaeal nucleotide phosphorylase that is distinct in both function and structure from previously known members of the nucleoside phosphorylase II family. (C) 2013 Elsevier Ltd. All rights reserved.

A novel dioxygenase from Burkholderia ambifaria AMMD (SadA) stereoselectively catalyzes the C3-hydroxylation of Nsubstituted branched-chain or aromatic L-amino acids, especially N-succinyl-L-leucine, coupled with the conversion of alpha-ketoglutarate to succinate and CO2. To elucidate the structural basis of the substrate specificity and stereoselective hydroxylation, we determined the crystal structures of the SadA. Zn(II) and SadA. Zn(II). alpha-KG complexes at 1.77 angstrom and 1.98 angstrom resolutions, respectively. SadA adopted a double-stranded beta-helix fold at the core of the structure. In addition, an HXD/EXnH motif in the active site coordinated a Zn(II) as a substitute for Fe(II). The alpha-KG molecule also coordinated Zn(II) in a bidentate manner via its 1-carboxylate and 2-oxo groups. Based on the SadA. Zn(II). alpha-KG structure and mutation analyses, we constructed substrate-binding models with N-succinyl-L-leucine and N-succinyl-L-phenylalanine, which provided new insight into the substrate specificity. The results will be useful for the rational design of SadA variants aimed at the recognition of various N-succinyl L-amino acids.

We have developed a protein crystal formation system, which can, on the ground, suppress convections, exerting magnetic forces on the solution including proteins. The protein formation system consists of a superconducting magnet system, protein crystallization cells, and observational equipment the latter two devices are placed in the magnet bore. The superconducting magnet system includes two coil groups, each generating a magnetic field opposing in direction to each other. The product of magnetic flux density and its spatial gradient is proportional to the magnetic force and needs to be approximately 1400 T 2m in order to levitate water. The magnet is operated in persistent current mode. The crystallization cells are made of a transparent plastic on a plate. The experimental space and cells can be controlled between 4°C and 20°C. The observational equipment, which is a kind of periscope, enables in-situ observation of protein solution in high magnetic fields. In order to optimize experimental conditions, we have also carried out a simulation study based on a solution flow model. © 2011 IEEE.

Ferrous ion- and a-ketoglutarate-dependent dioxygenase from Burkholderia ambifaria AMMD (SadA) catalyzes the C3-hydroxylation of N-substituted branched-chain l-amino acids, especially N-succinyl-l-leucine, coupled to the conversion of alpha-ketoglutarate to succinate and CO2. SadA was expressed in Escherichia coli, purified and crystallized using the sitting-drop vapour-diffusion method at 293 K. Crystals of selenomethionine-substituted SadA were obtained using a reservoir solution containing PEG 3000 as the precipitant at pH 9.5 and diffracted X-rays to 2.4 angstrom resolution. The crystal belonged to space group P212121, with unit-cell parameters a = 49.3, b = 70.9, c = 148.2 angstrom. The calculated Matthews coefficient (VM = 2.1 angstrom 3 Da-1, 41% solvent content) suggested that the crystal contains two molecules per asymmetric unit.

Flavin reductase HpaC(St) catalyzes the reduction of free flavins using NADH or NADPH. High hydrostatic pressure was used for the solubilization and refolding of HpaC(St), which was expressed as inclusion bodies in Escherichia coli to achieve high yield in a flavin-free form. The refolded HpaC(St) was purified using Ni-affinity chromatography followed by a heat treatment, which gave a single band on SDS-PAGE. The purified refolded HpaC(St) did not contain FMN, unlike the same enzyme expressed as a soluble protein. After the addition of FMN to the protein solution, the refolded enzyme showed a higher activity than the enzyme expressed as the soluble protein. Crystals of the refolded enzyme were obtained by adding FMN, FAD, or riboflavin to the protein solution and without the addition of Flavin compound. (C) 2012 Elsevier Inc. All rights reserved.

Ribose-1,5-bisphosphate isomerase (R15Pi) is a novel enzyme recently identified as a member of an AMP metabolic pathway in archaea. The enzyme converts D-ribose 1,5-bisphosphate into ribulose 1,5-bisphosphate, providing the substrate for archaeal ribulose-1,5-bisphosphate carboxylase/oxygenases. We here report the crystal structures of R15Pi from Thermococcus kodakarensis KOD1 (Tk-R15Pi) with and without its substrate or product. Tk-R15Pi is a hexameric enzyme formed by the trimerization of dimer units. Biochemical analyses show that Tk-R15Pi only accepts the alpha-anomer of D-ribose 1,5-bisphosphate and that Cys(133) and Asp(202) residues are essential for ribulose 1,5-bisphosphate production. Comparison of the determined structures reveals that the unliganded and product-binding structures are in an open form, whereas the substrate-binding structure adopts a closed form, indicating domain movement upon substrate binding. The conformational change to the closed form optimizes active site configuration and also isolates the active site from the solvent, which may allow deprotonation of Cys(133) and protonation of Asp(202) to occur. The structural features of the substrate-binding form and biochemical evidence lead us to propose that the isomerase reaction proceeds via a cis-phosphoenolate intermediate.

Space-based microgravity environments have been utilized to obtain a highly ordered crystal because of the lack of gravity-induced convection. A superconducting magnet-based quasi-microgravity is also expected to contribute to the enhancement of the quality of protein crystals. We here report a case study on protein crystallization using fifteen kinds of samples in a magnetic field gradient, which was sufficient for magnetic levitation of water droplets. In three cases, rod-type crystals were aligned perpendicular to the crystallization plate, exhibiting magnetic orientation parallel to the direction of the magnetic field. Five proteins showed improvement in crystal quality evaluated by the resolution limit in X-ray diffraction experiments and the overall B-factor of the crystal. Our data support the idea that the reduced-gravity environment produced by a high magnetic field gradient can be used to obtain enhanced-quality protein crystals, aiding in the determination of their precise crystal structures.

A homodimeric GrpE protein functions as a nucleotide exchange factor of the eubacterium DnaK molecular chaperone system. The co-chaperone GrpE accelerates ADP dissociation from, and promotes ATP binding to, DnaK, which cooperatively facilitates the DnaK chaperone cycle with another co-chaperone, DnaJ. GrpE characteristically undergoes two-step conformational changes in response to elevation of the environmental temperature. In the first transition at heat-shock temperatures, a fully reversible and functionally deficient structural alteration takes place in GrpE, and then the higher temperatures lead to the irreversible dissociation of the GrpE dimer into monomers as the second transition. GrpE is also thought to be a thermosensor of the DnaK system, since it is the only member of the DnaK system that changes its structure reversibly and loses its function at heat-shock temperatures of various organisms. We here report the crystal structure of GrpE from Thermus thermophilus HB8 (GrpE(Tth)) at 3.23 angstrom resolution. The resolved structure is compared with that of GrpE from mesophilic Escherichia coli (GrpE(Eco)), revealing structural similarities, particularly in the DnaK interaction regions, and structural characteristics for the thermal stability of GrpE(Tth). In addition, the structure analysis raised the possibility that the polypeptide chain in the reported GrpE(Eco) structure was misinterpreted. Comparison of these two GrpE structures combined with the results of limited proteolysis experiments provides insight into the protein dynamics of GrpE(Tth) correlated with the shift of temperature, and also suggests that the localized and partial unfolding at the plausible DnaK interaction sites of GrpE(Tth) causes functional deficiency of nucleotide exchange factor in response to the heat shock. (C) 2009 Elsevier Ltd. All rights reserved.

DNA replication initiator protein RepE stringently regulates F plasmid replication by its two distinct molecular association states. A predominant dimer functions as an autogenous repressor, whereas monomers act as replication initiators, and the dimer requires actions of the DnaK molecular chaperone system for monomerization. The structure of the monomeric form is known, whereas the dimeric structure and structural details of the dimer-to-monomer conversion have been unclear. Here we present the crystal structure of the RepE dimer in complex with the repE operator DNA. The dimerization interface is mainly formed by intermolecular P-sheets with several key interactions of charged residues. The conformations of the internal N- and C-terminal domains are conserved between the dimer and monomer, whereas the relative domain orientations are strikingly different, allowing for an efficient oligomeric transition of dual-functional RepE. This domain relocation accompanies secondary structural changes in the linker connecting the two domains, and the linker is included in plausible DnaK/DnaJ-binding regions. These findings suggest an activation mechanism for F plasmid replication by RepE monomerization, which is induced and mediated by the DnaK system.

The replication initiator factor RepE of the F plasmid in Escherichia coli is an essential protein that stringently regulates the F-plasmid copy number. The RepE protein has a dual function: its monomer functions as a replication initiator, while its dimer acts as a transcriptional repressor of the repE gene. The wild-type dimeric RepE protein was expressed as an N-terminal histidine-tagged protein, purified under native conditions with a high salt concentration and crystallized in complex with the repE operator DNA using the sitting-drop vapour-diffusion technique. The crystals diffracted to a resolution of 3.14 angstrom after the application of dehydration and crystal annealing and belong to space group P2(1), with unit-cell parameters a = 60.73, b = 99.32, c = 95.00 angstrom, beta = 108.55 degrees.

UV exposure of DNA molecules induces serious DNA lesions. The cyclobutane pyrimidine dimer (CPD) photolyase repairs CPD-type-lesions by using the energy of visible light. Two chromophores for different roles have been found in this enzyme family; one catalyzes the CPD repair reaction and the other works as an antenna pigment that harvests photon energy. The catalytic cofactor of all known photolyases is FAD, whereas several light-harvesting cofactors are found. Currently, 5,10-methenyltetrahydrofolate (MTHF), 8-hydroxy-5-deaza-riboflavin (8-HDF) and FMN are the known light-harvesting cofactors, and some photolyases lack the chromophore. Three crystal structures of photolyases from Escherichia coli (Ec-photolyase), Anacystis nidulans (An-photolyase), and Thermus thermophilus (Tt-photolyase) have been determined; however, no archaeal photolyase structure is available. A similarity search of archaeal genomic data indicated the presence of a homologous gene, ST0889, on Sulfolobus tokodaii strain7 An enzymatic assay reveals that ST0889 encodes photolyase from S. tokodaii (St-photolyase). We have determined the crystal structure of the St-photolyase protein to confirm its structural features and to investigate the mechanism of the archaeal DNA repair system with light energy. The crystal structure of the St-photolyase is superimposed very well on the three known photolyases including the catalytic cofactor FAD. Surprisingly, another FAD molecule is found at the position of the light-harvesting cofactor. This second FAD molecule is well accommodated in the crystal structure, suggesting that FAD works as a novel light-harvesting cofactor of photolyase. In addition, two of the four CPD recognition residues in the crystal structure of An-photolyase are not found in St-photolyase, which might utilize a different mechanism to recognize the CPD from that of An-photolyase. (c) 2006 Elsevier Ltd. All rights reserved.

The initiator protein RepE of the mini-F plasmid in Escherichia coli plays an essential role in DNA replication, which is regulated by the molecular chaperone-dependent oligomeric state (monomer or dimer). Crosslinking, ultracentrifugation, and gel filtration analyses showed that the solely expressed N-terminal domain (residues 1-144 or 1-152) exists in the dimeric state as in the wild-type RepE protein. This result indicates that the N-terminal domain functions as a dimerization domain of RepE and might be important for the interaction with the molecular chaperones. The N-terminal domain dimer has been crystallized in order to obtain structural insight into the regulation of the monomer/dimer conversion of RepE. (C) 2004 Elsevier Inc. All rights reserved.