Application of high-throughput technologies to a structural proteomics-type analysis of Bacillus anthracis.

A collaborative project between two Structural Proteomics In Europe (SPINE) partner laboratories, York and Oxford, aimed at high-throughput (HTP) structure determination of proteins from Bacillus anthracis, the aetiological agent of anthrax and a biomedically important target, is described. Based upon a target-selection strategy combining ;low-hanging fruit' and more challenging targets, this work has contributed to the body of knowledge of B. anthracis, established and developed HTP cloning and expression technologies and tested HTP pipelines. Both centres developed ligation-independent cloning (LIC) and expression systems, employing custom LIC-PCR, Gateway and In-Fusion technologies, used in combination with parallel protein purification and robotic nanolitre crystallization screening. Overall, 42 structures have been solved by X-ray crystallography, plus two by NMR through collaboration between York and the SPINE partner in Utrecht. Three biologically important protein structures, BA4899, BA1655 and BA3998, involved in tRNA modification, sporulation control and carbohydrate metabolism, respectively, are highlighted. Target analysis by biophysical clustering based on pI and hydropathy has provided useful information for future target-selection strategies. The technological developments and lessons learned from this project are discussed. The success rate of protein expression and structure solution is at least in keeping with that achieved in structural genomics programs.

SPINE high-throughput crystallization, crystal imaging and recognition techniques: current state, performance analysis, new technologies and future aspects.

This paper reviews the developments in high-throughput and nanolitre-scale protein crystallography technologies within the remit of workpackage 4 of the Structural Proteomics In Europe (SPINE) project since the project's inception in October 2002. By surveying the uptake, use and experience of new technologies by SPINE partners across Europe, a picture emerges of highly successful adoption of novel working methods revolutionizing this area of structural biology. Finally, a forward view is taken of how crystallization methodologies may develop in the future.

Structure of proline 3-hydroxylase. Evolution of the family of 2-oxoglutarate dependent oxygenases.

Iron (II)/2-oxoglutarate (2-OG)-dependent oxygenases catalyse oxidative reactions in a range of metabolic processes including the hydroxylation of proline and lysine residues during the post-translational modification of collagen. 2-OG oxygenases commonly require ascorbate for full activity. In the vitamin C deficient disease, scurvy, reduced activity of 2-OG oxygenases results in impaired formation of collagen. Here we report the crystal structure of bacterial proline 3-hydroxylase from Streptomyces sp., an enzyme which hydroxylates proline at position 3, the first of a 2-OG oxygenase catalysing oxidation of a free alpha-amino acid. Structures were obtained for the enzyme in the absence of iron (to 2.3A resolution, R=20.2%, Rfree=25.3%) and that complexed to iron (II) (to 2.4A resolution, R=19.8%, Rfree=22.6%). The structure contains conserved motifs present in other 2-OG oxygenases including a 'jelly roll' beta strand core and residues binding iron and 2-oxoglutarate, consistent with divergent evolution within the extended family. The structure differs significantly from many other 2-OG oxygenases in possessing a discrete C-terminal helical domain. Analysis of the structure suggests a model for proline binding and a mechanism for uncoupling of proline and 2-OG turnover.

Kinetic and crystallographic studies on deacetoxycephalosporin C synthase (DAOCS).

Deacetoxycephalosporin C synthase (DAOCS) is an iron(II) and 2-oxoglutarate-dependent oxygenase that catalyzes the conversion of penicillin N to deacetoxycephalosporin C, the committed step in the biosynthesis of cephalosporin antibiotics. The crystal structure of DAOCS revealed that the C terminus of one molecule is inserted into the active site of its neighbor in a cyclical fashion within a trimeric unit. This arrangement has hindered the generation of crystalline enzyme-substrate complexes. Therefore, we constructed a series of DAOCS mutants with modified C termini. Oxidation of 2-oxoglutarate was significantly uncoupled from oxidation of the penicillin substrate in certain truncated mutants. The extent of uncoupling varied with the number of residues deleted and the penicillin substrate used. Crystal structures were determined for the DeltaR306 mutant complexed with iron(II) and 2-oxoglutarate (to 2.10 A) and the DeltaR306A mutant complexed with iron(II), succinate and unhydrated carbon dioxide (to 1.96 A). The latter may mimic a product complex, and supports proposals for a metal-bound CO(2) intermediate during catalysis.

Crystallization and functional analysis of a soluble deglycosylated form of the human costimulatory molecule B7-1.

The interactions of B7-1 with CD28 and CTLA-4 modulate the course of human immune responses, making B7-1 an important target for developing structure-based therapeutics. B7-1 is, however, one of the most heavily glycosylated proteins found at the leukocyte cell surface, complicating the structural analysis of this molecule. Methods for the production, crystallization and selenomethionine labelling of a soluble deglycosylated form of this molecule are described. The protein readily forms both tetragonal plate and bipyramidal crystals belonging to space groups I4(1)22, with unit-cell parameters a = b = 56.9, c = 298.7 A, and P4(1)22 (or P4(3)22), with unit-cell parameters a = b = 89.0, c = 261.9 A, respectively. The I4(1)22 and primitive crystal forms diffract to 2.7 and 3.5 A, respectively. Surface plasmon resonance-based assays indicate that the ligand-binding properties of sB7-1 are unaffected by deglycosylation. Since none of the methods relied on any special structural properties of sB7-1, it is proposed that this novel combination of procedures could in principle be adapted to the systematic analysis of many other glycoproteins of structural or functional interest.

Alteration of the co-substrate selectivity of deacetoxycephalosporin C synthase. The role of arginine 258.

Deacetoxycephalosporin C synthase is an iron(II) 2-oxoglutaratedependent oxygenase that catalyzes the oxidative ring-expansion of penicillin N to deacetoxycephalosporin C. The wild-type enzyme is only able to efficiently utilize 2-oxoglutarate and 2-oxoadipate as a 2-oxoacid co-substrate. Mutation of arginine 258, the side chain of which forms an electrostatic interaction with the 5-carboxylate of the 2-oxoglutarate co-substrate, to a glutamine residue reduced activity to about 5% of the wild-type enzyme with 2-oxoglutarate. However, other aliphatic 2-oxoacids, which were not co-substrates for the wild-type enzyme, were utilized by the R258Q mutant. These 2-oxoacids "rescued" catalytic activity to the level observed for the wild-type enzyme as judged by penicillin N and G conversion. These co-substrates underwent oxidative decarboxylation as observed for 2-oxoglutarate in the normal reaction with the wild-type enzyme. Crystal structures of the iron(II)- 2-oxo-3-methylbutanoate (1.5 A), and iron(II)-2-oxo-4-methylpentanoate (1.6 A) enzyme complexes were obtained, which reveal the molecular basis for this "chemical co-substrate rescue" and help to rationalize the co-substrate selectivity of 2-oxoglutaratedependent oxygenases.

Structural origins of the selectivity of the trifunctional oxygenase clavaminic acid synthase.

Clavaminate synthase (CAS), a remarkable Fe(II)/2-oxoglutarate oxygenase, catalyzes three separate oxidative reactions in the biosynthesis of clavulanic acid, a clinically used inhibitor of serine beta-lactamases. The first CAS-catalyzed step (hydroxylation) is separated from the latter two (oxidative cyclization/desaturation) by the action of an amidinohydrolase. Here, we describe crystal structures of CAS in complex with Fe(II), 2-oxoglutarate (2OG) and substrates (N-alpha-acetyl-L-arginine and proclavaminic acid). They reveal how CAS catalyzes formation of the clavam nucleus, via a process unprecedented in synthetic organic chemistry, and suggest how it discriminates between substrates and controls reaction of its highly reactive ferryl intermediate. The presence of an unpredicted jelly roll beta-barrel core in CAS implies divergent evolution within the family of 2OG and related oxygenases. Comparison with other non-heme oxidases/oxygenases reveals flexibility in the position which dioxygen ligates to the iron, in contrast to the analogous heme-using enzymes.

Structure and dimerization of a soluble form of B7-1.

B7-1 (CD80) and B7-2 (CD86) are glycoproteins expressed on antigen-presenting cells. The binding of these molecules to the T cell homodimers CD28 and CTLA-4 (CD152) generates costimulatory and inhibitory signals in T cells, respectively. The crystal structure of the extracellular region of B7-1 (sB7-1), solved to 3 A resolution, consists of a novel combination of two Ig-like domains, one characteristic of adhesion molecules and the other previously seen only in antigen receptors. In the crystal lattice, sB7-1 unexpectedly forms parallel, 2-fold rotationally symmetric homodimers. Analytical ultracentrifugation reveals that sB7-1 also dimerizes in solution. The structural data suggest a mechanism whereby the avidity-enhanced binding of B7-1 and CTLA-4 homodimers, along with the relatively high affinity of these interactions, favors the formation of very stable inhibitory signaling complexes.

The effect of cysteine mutations on recombinant deacetoxycephalosporin C synthase from S. clavuligerus.

Cysteines 100, 155, and 197 of recombinant deacetoxycephalosporin C synthase were mutated to alanine residues. The C100A mutant had properties similar to those of the wild-type enzyme, but mutation of Cys-155 and Cys-197 reduced enzyme activity with penicillin N and penicillin G to different extents.

Crystal structure of active site-inhibited human coagulation factor VIIa (des-Gla).

Factor VIIa (FVIIa) is a crucial haemostatic protease consisting of four distinct domains termed the Gla, epidermal growth factor-1 (EGF-1), EGF-2, and protease domains (from N- to C-terminus). The crystal structure of human FVIIa inhibited at the active site with 1, 5-dansyl-Glu-Gly-Arg-chloromethyl ketone and lacking the Gla domain has been solved to a resolution of 2.28 A. The EGF-2 and protease domains were well resolved, whereas no electron density for the EGF-1 domain was observed, suggesting a flexible arrangement or disorder within the crystal. Superposition of the protease domain of the present structure with that previously resolved in the tissue factor (TF)/FVIIai complex revealed that although overall the domain structures are similar, the EGF-2 domain is rotated by 7.5 degrees relative to the protease domain on binding TF. A single cleavage in the protease domain was found, between Arg315 and Lys316 (chymotrypsin numbering 170C-170D) in a FVII-specific insertion loop: this cleavage appeared to be essential for crystallisation. Insertion of the heavy chain N-terminal Ile153 is essentially identical in the two structures, as is the geometry of the active site residues and the inhibitor C-terminal arginine residue. Some differences are seen in the cleaved loop, but changes in TF-contact residues are generally minor. This structure supports the hypothesis that TF binding enables spatial domain arrangements in the flexible FVIIa molecule necessary for procoagulant function and furthermore that active site occupancy induces FVIIa active conformation via N-terminal insertion.

Effects of N-butyldeoxynojirimycin and the Lec3.2.8.1 mutant phenotype on N-glycan processing in Chinese hamster ovary cells: application to glycoprotein crystallization.

Heterologous gene expression in either (1) the glycosylation-defective, mutant Chinese hamster ovary cell line, Lec3.2.8.1, or (2) the presence of the alpha-glucosidase inhibitor, N-butyldeoxynojirimycin facilitates the trimming of N-linked glycans of glycoproteins to single N-acetylglucosamine (GlcNAc) residues with endoglycosidase H (endo H). Both approaches are somewhat inefficient, however, with as little as 12% of the total protein being rendered fully endo H-sensitive under these conditions. It is shown here that the combined effects of these approaches on the restriction of oligosaccharide processing are essentially additive, thereby allowing the production of glycoproteins that are essentially completely endo H-sensitive. The preparation of a soluble chimeric form of CD58, the ligand of the human T-cell surface recognition molecule CD2, illustrates the usefulness of the combined approach when expression levels are low or the deglycosylated protein is unstable at low pH. The endo H-treated chimera produced crystals of space group P3(1)21 or P3(2)21, and unit cell dimensions a = b = 116.4 A, c = 51.4 A alpha = beta = 90 degrees , gamma = 120 degrees , that diffract to a maximum resolution of 1.8 A.

Tick histamine-binding proteins: isolation, cloning, and three-dimensional structure.

High-affinity histamine-binding proteins (HBPs) were discovered in the saliva of Rhipicephalus appendiculatus ticks. Their ability to outcompete histamine receptors indicates that they suppress inflammation during blood feeding. The crystal structure of a histamine-bound HBP, determined at 1.25 A resolution, reveals a lipocalin fold novel in containing two binding sites for the same ligand. The sites are orthogonally arranged and highly rigid and form an internal surface of unusual polar character that complements the physicochemical properties of histamine. As soluble receptors of histamine, HBPs offer a new strategy for controlling histamine-based diseases.

Studies on the active site of deacetoxycephalosporin C synthase.

The Fe(II) and 2-oxoglutarate-dependent dioxygenase deacetoxycephalosporin C synthase (DAOCS) from Streptomyces clavuligerus was expressed at ca 25 % of total soluble protein in Escherichia coli and purified by an efficient large-scale procedure. Purified protein catalysed the conversions of penicillins N and G to deacetoxycephems. Gel filtration and light scattering studies showed that in solution monomeric apo-DAOCS is in equilibrium with a trimeric form from which it crystallizes. DAOCS was crystallized +/-Fe(II) and/or 2-oxoglutarate using the hanging drop method. Crystals diffracted to beyond 1.3 A resolution and belonged to the R3 space group (unit cell dimensions: a=b=106.4 A, c=71.2 A; alpha=beta=90 degrees, gamma=120 degrees (in the hexagonal setting)). Despite the structure revealing that Met180 is located close to the reactive oxidizing centre of DAOCS, there was no functional difference between the wild-type and selenomethionine derivatives. X-ray absorption spectroscopic studies in solution generally supported the iron co-ordination chemistry defined by the crystal structures. The Fe K-edge positions of 7121.2 and 7121.4 eV for DAOCS alone and with 2-oxoglutarate were both consistent with the presence of Fe(II). For Fe(II) in DAOCS the best fit to the Extended X-ray Absorption Fine Structure (EXAFS) associated with the Fe K-edge was found with two His imidazolate groups at 1.96 A, three nitrogen or oxygen atoms at 2.11 A and one other light atom at 2.04 A. For the Fe(II) in the DAOCS-2-oxoglutarate complex the EXAFS spectrum was successfully interpreted by backscattering from two His residues (Fe-N at 1.99 A), a bidentate O,O-co-ordinated 2-oxoglutarate with Fe-O distances of 2.08 A, another O atom at 2.08 A and one at 2.03 A. Analysis of the X-ray crystal structural data suggests a binding mode for the penicillin N substrate and possible roles for the C terminus in stabilising the enzyme and ordering the reaction mechanism.

Crystal structure of the CD2-binding domain of CD58 (lymphocyte function-associated antigen 3) at 1.8-A resolution.

The binding of the cell surface molecule CD58 (formerly lymphocyte function-associated antigen 3) to its ligand, CD2, significantly increases the sensitivity of antigen recognition by T cells. This was the first heterophilic cell adhesion interaction to be discovered and is now an important paradigm for analyzing the structural basis of cell-cell recognition. The crystal structure of a CD2-binding chimeric form of CD58, solved to 1.8-A resolution, reveals that the ligand binding domain of CD58 has the expected Ig superfamily V-set topology and shares several of the hitherto unique structural features of CD2, consistent with previous speculation that the genes encoding these molecules arose via duplication of a common precursor. Nevertheless, evidence for considerable divergence of CD2 and CD58 is also implicit in the structures. Mutations that disrupt CD2 binding map to the highly acidic surface of the AGFCC'C" beta-sheet of CD58, which, unexpectedly, lacks marked shape complementarity to the equivalent, rather more basic CD58-binding face of human CD2. The specificity of the very weak interactions of proteins mediating cell-cell recognition may often derive largely from electrostatic complementarity, with shape matching at the protein-protein interface being less exact than for interactions that combine specificity with high affinity, such as those involving antibodies.

Crystallization and preliminary X-ray analysis of active site-inhibited human coagulation factor VIIa (des-Gla).

Human coagulation factor VIIai that lacks the Gla domain (residues 1-44) has been prepared, purified, and crystallised. First, recombinant factor VII was activated to form factor VIIa, the active site was then inhibited with 1,5-dansyl-Glu-Gly-Arg-chloromethyl ketone, and finally the Gla domain was removed by chymotryptic digestion, yielding factor VIIai (des-Gla). After further purification single crystals suitable for x-ray analysis were obtained by vapour diffusion. Crystals of factor VIIai (des-Gla) belong to the tetragonal space group P41212 or P43212 with unit cell dimensions a = b = 94.85 A, c = 114.30 A, contain one molecule per asymmetric unit, and diffract to 2.3-A resolution when exposed to synchrotron radiation.

Crystallization and preliminary diffraction studies of the extracellular region of human p58 killer cell inhibitory receptor (KIR2).

Molecules of the human killer cell inhibitory receptor (KIR) family, which belong to the immunoglobulin superfamily (IgSF), are expressed on the surface of natural killer (NK) cells and some subsets of T cells. These receptors function to mediate the inhibition or activation of cytotoxic activity by recognizing HLA class I molecules on the target cell. The extracellular region of a p58 KIR specific for HLA-Cw1,3,7 (KIR2) has been overproduced in Escherichia coli and purified. The recombinant KIR2 has been crystallized in 9-10% poly(ethylene glycol) methyl ether (average Mr = 8000), 50mM HEPES, 8% ethylene glycol, 0.5% octyl-beta-glucoside, pH 7.5, at 294 K using the sitting-drop vapour-diffusion method. Preliminary X-ray diffraction studies reveal the space group to be hexagonal (P6122 or P6522) with lattice constants a = b = 95.3, c = 130.8 A. A native data set (3 A resolution) has been collected at the Photon Factory (lambda = 1.0 A).

Antagonist HIV-1 Gag peptides induce structural changes in HLA B8.

In the cellular immune response, recognition by CTL-TCRs of viral antigens presented as peptides by HLA class I molecules, triggers destruction of the virally infected cell (Townsend, A.R.M., J. Rothbard, F.M. Gotch, G. Bahadur, D. Wraith, and A.J. McMichael. 1986. Cell. 44:959-968). Altered peptide ligands (APLs) which antagonise CTL recognition of infected cells have been reported (Jameson, S.C., F.R. Carbone, and M.J. Bevan. 1993. J. Exp. Med. 177:1541-1550). In one example, lysis of antigen presenting cells by CTLs in response to recognition of an HLA B8-restricted HIV-1 P17 (aa 24-31) epitope can be inhibited by naturally occurring variants of this peptide, which act as TCR antagonists (Klenerman, P., S. Rowland Jones, S. McAdam, J. Edwards, S. Daenke, D. Lalloo, B. Koppe, W. Rosenberg, D. Boyd, A. Edwards, P. Giangrande, R.E. Phillips, and A. McMichael. 1994. Nature (Lond.). 369:403-407). We have characterised two CTL clones and a CTL line whose interactions with these variants of P17 (aa 24-31) exhibit a variety of responses. We have examined the high resolution crystal structures of four of these APLs in complex with HLA B8 to determine alterations in the shape, chemistry, and local flexibility of the TCR binding surface. The variant peptides cause changes in the recognition surface by three mechanisms: changes contributed directly by the peptide, effects transmitted to the exposed peptide surface, and induced effects on the exposed framework of the peptide binding groove. While the first two mechanisms frequently lead to antagonism, the third has more profound effects on TCR recognition.

Bound water structure and polymorphic amino acids act together to allow the binding of different peptides to MHC class I HLA-B53.

The structure of the human MHC class I molecule HLA-B53 complexed to two nonameric peptide epitopes (from the malaria parasite P. falciparum and the HIV2 gag protein) has been determined by X-ray crystallography at 2.3 angstrom resolution. The structures reveal the architecture of a Pro-specific B pocket common to many HLA-B alleles. Relative to other alleles, the B53 peptide-binding groove is widened by a significant (up to 1.25 angstrom) shift in the position of the alpha 1 helix. Within this groove, bound water molecules, acting in concert with the side chains of polymorphic residues, provide the functional malleability of the MHC, which enables the high affinity/low specificity binding of multiple peptide epitopes.

2.0 A resolution structure of a ternary complex of pig muscle phosphoglycerate kinase containing 3-phospho-D-glycerate and the nucleotide Mn adenylylimidodiphosphate.

The crystal structure of a ternary complex of pig muscle phosphoglycerate kinase (PGK) containing 3-phosphoglycerate (3-PG) and manganese adenylylimidodiphosphate (Mn AMP-PNP) has been determined and refined at 2.0 A resolution. The complex differs from the true substrate ternary complex only in the presence of an imido- rather than an oxylink between beta- and gamma-phosphates of the bound nucleotide. The 3-PG is bound in a similar manner to that observed in binary complexes. The nucleotide is bound in a similar manner to Mg ADP except that the metal ion is coordinated by all three alpha-, beta-, and gamma-phosphates, but not by the protein. The gamma-phosphate, which is transferred in the reaction, is not bound by the protein. One further characteristic of the ternary complex is that Arg-38 moves to a position where its guanidinium group makes a triple interaction with the N-terminal domain, the C-terminal domain, and the 1-carboxyl group of the bound 3-PG. Although a hinge-bending conformation change is seen in the ternary complex, it is no larger than that observed in the 3-PG binary complex. To reduce that distance between two bound substrates to a value consistent with the direct in-line transfer known to occur in PGK, we modeled the closure of a pronounced cleft in the protein structure situated between the bound substrates. This closure suggested a mechanism of catalysis that involves the "capture" of the gamma-phosphate by Arg-38 and the N-terminus of helix-14, which has a conserved Gly-Gly-Gly phosphate binding motif. We propose that nucleophilic attack by the 1-carboxyl group of the 3-PG on the gamma-phosphorus follows the capture of the gamma-phosphate, leading to a pentacoordinate transition state that may be stabilized by hydrogen bonds donated by the NH groups in the N-terminus of helix 14 and the guanidinium group of Arg-38. During the course of the reaction the metal ion is proposed to migrate to a position coordinating the alpha- and beta-phosphates and the carboxyl group of Asp-374. The mechanism is consistent with the structural information from binary and ternary substrate complexes and much solution data, and gives a major catalytic role to Arg-38, as indicated by site-directed mutagenesis.

Production and crystallization of MHC class I B allele single peptide complexes.

Major histocompatibility complex class I B alleles, HLA B8, B53 and B3501 have been cloned, expressed, refolded and crystallized in specific complexes with a number of different 8-mer and 9-mer peptides. For some of these crystallization was initiated by cross-seeding between different B allele complexes. All crystallize in the space group P212121, with similar unit cell dimensions of approximately 52 A X 81 A X 112 A, contain one complex per asymmetric unit and diffract to approximately 2.0 A resolution.