Multiple-wavelength Anomalous Diffraction Method Research Articles

Crystal structure analysis of c4763, a uropathogenic Escherichia coli-specific protein.

Urinary-tract infections (UTIs), which are some of the most common infectious diseases in humans, can cause sepsis and death without proper treatment. Therefore, it is necessary to understand their pathogenicity for proper diagnosis and therapeutics. Uropathogenic Escherichia coli, the major causative agents of UTIs, contain several genes that are absent in nonpathogenic strains and are therefore considered to be relevant to UTI pathogenicity. c4763 is one of the uropathogenic E. coli-specific proteins, but its function is unknown. To investigate the function of c4763 and its possible role in UTI pathogenicity, its crystal structure was determined at a resolution of 1.45 Å by a multiple-wavelength anomalous diffraction method. c4763 is a homodimer with 129 residues in one subunit that contains a GGCT-like domain with five α-helices and seven β-strands. c4763 shows structural similarity to the C-terminal domain of allophanate hydrolase from Kluyveromyces lactis, which is involved in the degradation of urea. These results suggest that c4763 might be involved in the utilization of urea, which is necessary for bacterial survival in the urinary tract. Further biochemical and physiological investigation will elucidate its functional relevance in UTIs.

Structure of mannosyl-3-phosphoglycerate phosphatase fromPyrococcus horikoshii

Mannosyl-3-phosphoglycerate phosphatase (MPGP) catalyzes the dephosphorylation of alpha-mannosyl-3-phosphoglycerate (MPG) to produce alpha-mannosylglycerate (MG). In the hyperthermophile Pyrococcus horikoshii, MPGP plays a role in a series of enzyme reactions that are involved in the MG-biosynthesis pathway, which is important for maintaining life under conditions of high salt concentration. Crystal structures of P. horikoshii MPGP (PhoMPGP) in the holo form and in the apo form lacking the magnesium ion were determined by the multiple-wavelength anomalous diffraction method using SeMet-substituted PhoMPGP. PhoMPGP consists of two domains: a core domain that is conserved in the haloacid dehalogenase superfamily and a cap domain that is specific to the C2B cap subclass of the superfamily. Apo-form crystals contain two PhoMPGP molecules: one in the open conformation and the other in the closed conformation. In holo-form crystals both of the two molecules are in the closed conformation with phosphate and magnesium ions. PhoMPGP has a specific hairpin loop that is bent towards the active site in the closed conformation of both the apo and holo forms. PhoMPGP has a cavity between the two domains which is considered to be the substrate-binding site as a phosphate ion is located in the cavity, mimicking the binding manner of the phosphate group of MPG. The cavity is sequestered in the closed conformation such that a conformational change is indispensable for the release of products. A salt bridge from the general acid/base Asp10 to Arg170 is observed in the holo-form PhoMPGP which is not present in the open form. The importance of the conformational change in the activity of PhoMPGP is discussed.

Structural and functional characterization of a novel phosphatase from the Arabidopsis thaliana gene locus At1g05000

The crystal structure of the protein product of the gene locus At1g05000, a hypothetical protein from A. thaliana, was determined by the multiple-wavelength anomalous diffraction method and was refined to an R factor of 20.4% (R(free) = 24.9%) at 3.3 A. The protein adopts the alpha/beta fold found in cysteine phosphatases, a superfamily of phosphatases that possess a catalytic cysteine and form a covalent thiol-phosphate intermediate during the catalytic cycle. In At1g05000, the analogous cysteine (Cys(150)) is located at the bottom of a positively-charged pocket formed by residues that include the conserved arginine (Arg(156)) of the signature active site motif, HCxxGxxRT. Of 74 model phosphatase substrates tested, purified recombinant At1g05000 showed highest activity toward polyphosphate (poly-P(12-13)) and deoxyribo- and ribonucleoside triphosphates, and less activity toward phosphoenolpyruvate, phosphotyrosine, phosphotyrosine-containing peptides, and phosphatidyl inositols. Divalent metal cations were not required for activity and had little effect on the reaction.

Structure of Chlorobium tepidum Sepiapterin Reductase Complex Reveals the Novel Substrate Binding Mode for Stereospecific Production of l-threo-Tetrahydrobiopterin

Sepiapterin reductase (SR) is involved in the last step of tetrahydrobiopterin (BH(4)) biosynthesis by reducing the di-keto group of 6-pyruvoyl tetrahydropterin. Chlorobium tepidum SR (cSR) generates a distinct BH(4) product, L-threo-BH(4) (6R-(1'S,2'S)-5,6,7,8-BH(4)), whereas animal enzymes produce L-erythro-BH(4) (6R-(1'R,2'S)-5,6,7,8-BH(4)) although it has high amino acid sequence similarities to the other animal enzymes. To elucidate the structural basis for the different reaction stereospecificities, we have determined the three-dimensional structures of cSR alone and complexed with NADP and sepiapterin at 2.1 and 1.7 A resolution, respectively. The overall folding of the cSR, the binding site for the cofactor NADP(H), and the positions of active site residues were quite similar to the mouse and the human SR. However, significant differences were found in the substrate binding region of the cSR. In comparison to the mouse SR complex, the sepiapterin in the cSR is rotated about 180 degrees around the active site and bound between two aromatic side chains of Trp-196 and Phe-99 so that its pterin ring is shifted to the opposite side, but its side chain position is not changed. The swiveled sepiapterin binding results in the conversion of the side chain configuration, exposing the opposite face for hydride transfer from NADPH. The different sepiapterin binding mode within the conserved catalytic architecture presents a novel strategy of switching the reaction stereospecificities in the same protein fold.

The structure at 1.7 A resolution of the protein product of the At2g17340 gene from Arabidopsis thaliana.

The crystal structure of the At2g17340 protein from A. thaliana was determined by the multiple-wavelength anomalous diffraction method and was refined to an R factor of 16.9% (Rfree = 22.1%) at 1.7 A resolution. At2g17340 is a member of the Pfam01937.11 protein family and its structure provides the first insight into the structural organization of this family. A number of fully and highly conserved residues defined by multiple sequence alignment of members of the Pfam01937.11 family were mapped onto the structure of At2g17340. The fully conserved residues are involved in the coordination of a metal ion and in the stabilization of loops surrounding the metal site. Several additional highly conserved residues also map into the vicinity of the metal-binding site, while others are clearly involved in stabilizing the hydrophobic core of the protein. The structure of At2g17340 represents a new fold in protein conformational space.

Structure of the RNA-processing inhibitor RraA fromThermus thermophilis

The menG gene product, thought to catalyze the final methylation in vitamin K(2) synthesis, has recently been shown to inhibit RNase E in Eschericha coli. The structure of the protein, since renamed RraA, has been solved to 2.3 A using the multiple-wavelength anomalous diffraction method and selenomethionine-substituted protein from Thermus thermophilus. The six molecules in the asymmetric unit are arranged as two similar trimers which have a degree of interaction, suggesting biological significance. The fold does not support the postulated methylation function. Genomic analysis, specifically a lack of an RNase E homologue in cases where homologues to RraA exist, indicates that the function is still obscure.

Reconstruction of magnetization density in two-dimensional samples from soft X-ray speckle patterns using the multiple-wavelength anomalous diffraction method.

A non-destructive technique for imaging magnetic domains in thin films and two-dimensional magnetic structures using coherent soft X-ray scattering and the multiple-wavelength anomalous diffraction method (MAD) is proposed. The method exploits the strong energy dependence in the magnetic scattering amplitude for 3d transition metals near the L(2,3) absorption edges and 4f elements near the M(4,5) absorption edges. The phase information required in the reconstruction algorithm is derived from the interference between the charge and magnetic scattering amplitudes. Magnetic speckle patterns from the magnetic domain distribution in an artificially defined Fe thin film are used to demonstrate this reconstruction algorithm. Circularly and linearly polarized incident light are examined separately to investigate the effect of polarization on the capability of the method.

Crystallographic analysis of a novel complex of actinomycin D bound to the DNA decamer CGATCGATCG.

The potent anticancer drug actinomycin D (ActD) acts by binding to DNA, thereby interfering with replication and transcription. ActD inhibits RNA polymerase far more specifically than DNA polymerase. Such discrimination is not easily understood by the conventional DNA binding mode of ActD. We have solved and refined at 1.7 A resolution the crystal structure of ActD complexed to CGATCGATCG, which contains no canonical GpC binding sequence. The crystal data are space group P4(3)2(1)2, a = b = 47.01 A, and c = 160.37 A. The structure was solved by the multiple wavelength anomalous diffraction method using a 5-bromo-U DNA. The asymmetric unit of the unit cell contains two independent dimers of a novel slipped duplex complex consisting of two decamer DNA strands bound with two ActD drug molecules. (The DNA in one dimer is numbered C1 to G10 in one strand and C11 to G20 in the complementary strand and in the second dimer, C101 to G110 and C111 to G120, respectively.) The structure reveals a highly unusual ActD binding mode in which the DNA adopts a slipped duplex with the A3-T4/A13-T14 dinucleotides looped out. ActD intercalates between G2-C11* (C11* being from a symmetry-related molecule) and C5-G20 base pairs. Two such slipped duplex-ActD complexes bound to each other by mutually intercalating their T4/T14 bases into the helix cavities (located between C5-G20 and G6-C19 base pairs) of neighboring complexes, forming a dimer of drug-DNA complexes. The binding site mimics the drug binding at the elongation point during transcription. Modeling studies show that the ActD-DNA complex fits snugly in the active site cavity in RNA polymerase but not in DNA polymerase. This may explain the strong preference of ActD inhibition toward transcription.

Crystal Structure of 1-Aminocyclopropane-1-carboxylate Deaminase from Hansenula saturnus

The pyridoxal 5'-phosphate (PLP)-dependent enzyme 1-aminocyclopropane-1-carboxylate deaminase (ACCD) catalyzes a reaction that involves a ring opening of cyclopropanoid amino acid, yielding alpha-ketobutyrate and ammonia. Unlike other PLP-dependent enzymes, this enzyme has no alpha-hydrogen atom in the substrate. Thus, a unique mechanism for the bond cleavage is expected. The crystal structure of ACCD from Hansenula saturnus has been determined at 2.0 A resolution by the multiple wavelength anomalous diffraction method using mercury atoms as anomalous scatterers. The model was built on the electron density map, which was obtained by the density averaging of multiple crystal forms. The final model was refined to an R-factor of 22.5% and an R(free)-factor of 26.8%. The ACCD folds into two domains, each of which has an open twisted alpha/beta structure similar to the beta-subunit of tryptophan synthase. However, in ACCD, unlike in other members of the beta family of PLP-dependent enzymes, PLP is buried deep in the molecule. The structure provides the first view of the catalytic center of the cyclopropane ring opening.

Prospects for X-ray Crystal Structure Analysis of Selenoproteins with SPring-8 Synchrotron Radiation.

The impact of synchrotron radiation as a new X-ray source with its polychromatic nature and associated high intensity and fine collimation has brought important advances in the field of macromolecular crystallography. It has extended structure determinations of proteins to higher resolution, allowed use of smaller crystals with larger unit cells. In particular, selenoprotein is a suitable material for X-ray crystal structure analysis with synchrotron radiation, since its polychromatic nature and anomalous diffraction from the selenium atom(s) in the protein allow the multiple-wavelength anomalous-diffraction (MAD) method to be used for phase determination. RIKEN beam line I (BL45XU), installed in the SPring-8 synchrotron radiation facility, has been designed and developed to optimize MAD data collection based on a ‘trichromatic concept’. This concept facilitates simultaneous data collection by use of a ‘trichromator’, of three intensity data-sets at three different wavelengths from a single protein crystal, and thus results in the minimization of systematic errors in the measurement of anomalous diffraction by the MAD method. The X-ray crystallographic analysis of selenoprotein with SPring-8 synchrotron radiation, therefore, results in very fast data collection and high resolution structural analysis using a single protein crystal, which should lead to elucidation of the structure-function relationship of selenoprotein.

Large-scale preparation, purification, and crystallization of UDP-N-acetylmuramoyl-L-alanine: D-glutamate ligase from Escherichia coli.

The UDP-N-acetylmuramoyl-L-alanine:D-glutamate ligase from Escherichia coli, an enzyme involved in the biosynthesis of the bacterial peptidoglycan monomer unit, was overproduced and purified to homogeneity on a large scale, yielding 4 mg of protein per liter of bacterial culture. Crystals of the complex with the substrate UDP-MurNAc-L-Ala were grown by the hanging drop method using ammonium sulfate as the precipitant. They are tetragonal with cell dimensions a = b = 65.5 A and c = 134.59 A, space group P4(1) or P4(3), and contain one monomer of 46,842 Da in the asymmetric unit. In order to use the multiple-wavelength anomalous diffraction method for phasing, a selenomethionine derivative of the protein has also been overproduced, purified, and crystallized.

Crystallization and preliminary crystallographic analysis of the Ras binding domain of RalGDS, a guanine nucleotide dissociation stimulator of the Ral protein.

The RalGDS is a guanine nucleotide dissociation stimulator which activates the Ral protein, a Ras-like small GTPase. The C-terminal domain of the RalGDS (C-RalGDS) binds tightly to the effector loop of Ras suggesting that the RalGDS may be a crossing point of two signal tranduction pathways associated with the Ras and Ral proteins. C-RalGDS has been purified and crystallized in space group C2, with unit-cell dimensions a = 108.8, b = 30.7, c = 51.3 A, beta = 91.7 degrees at 277 K and a = 103.8, b = 30.55, c = 51.4 A, beta = 94.9 degrees for data collected at 100 K. The crystals diffract to 1.8 A at a synchrotron radiation source. To use the multiple-wavelength anomalous diffraction method for phasing, a selenomethionine derivative of the protein has also been crystallized.

Effect of the anisotropy of anomalous scattering on the MAD phasing method

The analysis of X-ray diffraction intensities is complicated by the anisotropy of anomalous scattering (AAS) that can occur due to resonance associated with transitions between core electrons and valence molecular orbitals. Substantial AAS has been observed directly in diffraction data near the K edge of selenium in selenolanthionine [Templeton & Templeton (1988). Acta Cryst. A44, 1045-1051] and in pleiochroism of X-ray absorption in selenobiotinyl streptavidin [Hendrickson, Pähler, Smith, Satow, Merritt & Phizackerley (1989). Proc. Natl Acad. Sci. USA, 86, 2190-2194]. The impact of AAS on the multiple-wavelength anomalous diffraction (MAD) method for phase determination is of particular interest in the context of this chemical state of selenium in the light of a general method that has been developed to incorporate selenomethionine into proteins for use in MAD phasing [Hendrickson, Horton & LeMaster (1990). EMBO J. 9, 1665-1672]. The first step of the MAD phasing method necessarily assumes that the anomalous-scattering factors are isotropic and our first aim here is to evaluate the effect of this approximation on initially determined phases. To obtain ultimate phases free from the effects of anisotropy, a least-squares procedure has been written in which global parameters (i.e. pertaining to the whole data set) are refined simultaneously with local parameters (i.e. pertaining to a given node h). The AAS is taken explicityly into account by considering f' and f" as tensors instead of scalars [Templeton & Templeton (1982). Acta Cryst. A38, 62-67], and the components of the f' and f" tensors are among the refinable global parameters. The effectiveness of this procedure is tested with data simulated from the refined atomic model of selenobiotinyl streptavidin. The application of this procedure to actual Photon Factory measurements is also described. The results show that AAS does not cripple the MAD method, and that phases uncorrupted by these effects can be recovered.