The structural features of concanavalin A governing non-proline peptide isomerization.

The reversible binding of manganese and calcium to concanavalin A determines the carbohydrate binding of the lectin by inducing large conformational changes. These changes are governed by the isomerization of a non-proline peptide bond, Ala-207-Asp-208, positioned in a beta-strand in between the calcium binding site S2 and the carbohydrate specificity-determining loop. The replacement of calcium by manganese allowed us to investigate the structures of the carbohydrate binding, locked state and the inactive, unlocked state of concanavalin A, both with and without metal ions bound. Crystals of unlocked metal-free concanavalin A convert to the locked form with the binding of two Mn(2+) ions. Removal of these ions from the crystals traps metal-free concanavalin A in its locked state, a minority species in solution. The ligation of a metal ion in S2 to unlocked concanavalin A causes bending of the beta-strand foregoing the S2 ligand residues Asp-10 and Tyr-12. This bending disrupts conventional beta-sheet hydrogen bonding and forces the Thr-11 side chain against the Ala-207-Asp-208 peptide bond. The steric strain exerted by Thr-11 is presumed to drive the trans-to-cis isomerization. Upon isomerization, Asp-208 flips into its carbohydrate binding position, and the conformation of the carbohydrate specificity determining loop changes dramatically.

Crystal structure of CcdB, a topoisomerase poison from E. coli.

The crystal structure of CcdB, a protein that poisons Escherichia coli gyrase, was determined in three crystal forms. The protein consists of a five-stranded antiparallel beta-pleated sheet followed by a C-terminal alpha-helix. In one of the loops of the sheet, a second small three-stranded antiparallel beta-sheet is inserted that sticks out of the molecule as a wing. This wing contains the LysC proteolytic cleavage site that is protected by CcdA and, therefore, forms a likely CcdA recognition site. A dimer is formed by sheet extension and by extensive hydrophobic contacts involving three of the five methionine residues and the C terminus of the alpha-helix. The surface of the dimer on the side of the alpha-helix is overall negatively charged, while the opposite side as well as the wing sheet is dominated by positive charges. We propose that the CcdB dimer binds into the central hole of the 59 kDa N-terminal fragment of GyrA, after disruption of the head dimer interface of GyrA.

Quaternary structure of UEA-II, the chitobiose specific lectin from gorse.

The chitobiose specific Ulex europaeus lectin II crystallizes in space group P3221 with unit-cell dimensions a = b = 105.54, c = 176.26 A. The asymmetric unit contains a complete lectin tetramer. The crystals were shown to diffract to 4.5 A on a rotating-anode source and to 2.7 A at the Daresbury synchrotron source. Molecular replacement and subsequent rigid-body refinement using data to 4.5 A yielded a solution corresponding to a tetramer very similar to that of phytohemagglutinin-L and soybean agglutinin. The monomers in the Ulex lectin tetramer are rotated approximately 5 degrees compared with the phytohemagglutinin-L and soybean agglutinin structures.

Crystallization of ccdB.

CcdB is a small dimeric protein that poisons DNA-topoisomerase II complexes. Its crystallization properties in terms of precipitant type, precipitant concentration, pH and protein concentration have been investigated leading to a novel crystal form which, in contrast to previously reported crystals, is suitable for structure determination using the multiple isomorphous replacement (MIR) method. The space group of this new form is C2, with unit-cell parameters a = 74.94, b = 36.24, c = 35.77 A, beta = 115.27 degrees. The asymmetric unit contains a single monomer. Flash-frozen crystals diffract to at least 1.5 A resolution, while room-temperature diffraction can be observed up to 1.6 A. The double mutant S74C/G77Q, which acts as a super-killer, crystallizes in space group I222 (or I212121) with unit-cell dimensions a = 105.58, b = 105.80, c = 91.90 A. These crystals diffract to 2.5 A resolution.

Expression, purification, crystallization and preliminary X-ray analysis of cyclophilin A from the bovine parasite Trypanosoma brucei brucei.

Cyclophilin A from the bovine parasite Trypanosoma brucei brucei has been cloned, expressed in Escherichia coli, purified and crystallized in the presence of cyclosporin A using ammonium sulfate as a precipitant. The crystals belong to the orthorhombic crystal system with unit-cell dimensions of a = 118.61, b = 210.15 and c = 153.21 A. A data set complete to 2.7 A has been collected using rotating-anode radiation, however the crystals diffract to at least 2.1 A resolution using synchrotron radiation.

Hydrolysis of a slow cyclic thiophosphate substrate of RNase T1 analyzed by time-resolved crystallography.

Here we present a time-resolved crystallographic analysis of the hydrolysis of exo (Sp) guanosine 2',3'-cyclophosphorothioate by RNase T1. The use of a slow substrate and fast crystallization methods made it possible to perform the study with conventional data-collection techniques. The results support the idea that the hydrolysis reaction proceeds through a mechanism that is the inverse of the transesterification reaction. In addition, the structures provide an explanation for the differential behavior of RNase T1 towards exo- and endo-cyclic thiophosphates.

Crystallization of two related lectins from the legume plant Dolichos biflorus.

The seed lectin DBL and the related stem and leaves lectin DB58 of the tropical legume Dolichos biflorus were crystallized, as well as complexes of DBL with adenine and with GalNAc(alpha1-3)[Fuc(alpha1-2)]Gal. The different crystal forms of DBL diffract to about 2.8 A, while DB58 crystals diffract to 3.3 A.

Crystal structure of arcelin-5, a lectin-like defense protein from Phaseolus vulgaris.

In the seeds of the legume plants, a class of sugar-binding proteins with high structural and sequential identity is found, generally called the legume lectins. The seeds of the common bean (Phaseolus vulgaris) contain, besides two such lectins, a lectin-like defense protein called arcelin, in which one sugar binding loop is absent. Here we report the crystal structure of arcelin-5 (Arc5), one of the electrophoretic variants of arcelin, solved at a resolution of 2.7 A. The R factor of the refined structure is 20.6%, and the free R factor is 27.1%. The main difference between Arc5 and the legume lectins is the absence of the metal binding loop. The bound metals are necessary for the sugar binding capabilities of the legume lectins and stabilize an Ala-Asp cis-peptide bond. Surprisingly, despite the absence of the metal binding site in Arc5, this cis-peptide bond found in all legume lectin structures is still present, although the Asp residue has been replaced by a Tyr residue. Despite the high identity between the different legume lectin sequences, they show a broad range of quaternary structures. The structures of three different dimers and three different tetramers have been solved. Arc5 crystallized as a monomer, bringing the number of known quaternary structures to seven.

Structure of chymopapain at 1.7 A resolution.

The X-ray structure of chymopapain, a cysteine proteinase isolated from the latex of the fruits of Carica papaya L., has been determined by molecular replacement methods and refined to a conventional R factor of 0.19 for all observed reflections in the range from 9.5 to 1.7 A resolution. The crystals used in this study contained a unique molecular species of chymopapain with two moles of thiomethyl attached to the two free cysteines per mole of enzyme. A comparison is made with the other known papaya proteinase X-ray structures: papain, caricain, and glycyl endopeptidase. Their backbone conformations are extremely similar except for two loop regions. Both regions are located at the surface of the protein and far away of the active site cleft. In each X-ray structure the same water network was found at the interface between the two domains of the enzyme. A close examination of the active site groove showed that the specificity restrictions dictated by the S2 subsite did not differ significantly among the four proteinases.

A structure of the complex between concanavalin A and methyl-3,6-di-O-(alpha-D-mannopyranosyl)-alpha-D-mannopyranoside reveals two binding modes.

The structure of concanavalin A in complex with the trimannoside methyl-3,6-di-O-(alpha-D-mannopyranosyl)-alpha-D-mannopyranoside has been determined in a novel space group. In three of the four subunits of the concanavalin A tetramer, the interactions between the protein and the bound saccharide are essentially identical to those reported previously by other authors (Naismith, J. H., and Field, R. A. (1996) J. Biol. Chem. 271, 972-976). In the fourth subunit, however, the alpha1-->3 linkage has a different conformation, resulting in a different part of the alpha1-->3-linked mannose interacting with essentially the same surface of the protein. Furthermore, significant differences are observed in the quaternary associations of the subunits compared with the saccharide-free structures and other carbohydrate complexes, suggesting that the concanavalin A tetramer is a rather flexible entity.

Crystal structure of a camel single-domain VH antibody fragment in complex with lysozyme.

The Camelidae is the only taxonomic family known to possess functional heavy-chain antibodies, lacking light chains. We report here the 2.5 A resolution crystal structure of a camel VH in complex with its antigen, lysozyme. Compared to human and mouse VH domains, there are no major backbone rearrangements in the VH framework. However, the architecture of the region of VH that interacts with a VL in a conventional FV is different from any previously seen. Moreover, the CDR1 region, although in sequence homologous to human CDR1, deviates fundamentally from the canonical structure. Additionally, one half of the CDR3 contacts the VH region which in conventional immunoglobulins interacts with a VL whereas the other half protrudes from the antigen binding site and penetrates deeply into the active site of lysozyme.

The crystallographic structure of phytohemagglutinin-L.

The structure of phytohemagglutinin-L (PHA-L), a leucoagglutinating seed lectin from Phaseolus vulgaris, has been solved with molecular replacement using the coordinates of lentil lectin as model, and refined at a resolution of 2.8 A. The final R-factor of the structure is 20.0%. The quaternary structure of the PHA-L tetramer differs from the structures of the concanavalin A and peanut lectin tetramers, but resembles the structure of the soybean agglutinin tetramer. PHA-L consists of two canonical legume lectin dimers that pack together through the formation of a close contact between two beta-strands. Of the two covalently bound oligosaccharides per monomer, only one GlcNAc residue per monomer is visible in the electron density. In this article we describe the structure of PHA-L, and we discuss the putative position of the high affinity adenine-binding site present in a number of legume lectins. A comparison with transthyretin, a protein that shows a remarkable resemblance to PHA-L, gives further ground to our proposal.

Concanavalin A crystallized in complex with the trisaccharide 3,6-di-O-methyl-(alpha-D-mannopyranosyl)-alpha-D-mannopyranoside.

Concanavalin A was co-crystallized in two crystal forms with 3,6-di-O-methyl- (alpha-D-mannopyranosyl) alpha- D-mannopyranoside, which is primarily responsible for the high-affinity binding of N-linked carbohydrates to concanavalin A. Both crystal forms have space group P2(1) and contain a complete concanavalin A tetramer in the asymmetric unit. Form A was crystallized using polyethylene glycol methyl ether as the precipitant and has unit-cell dimensions a = 59.83, b = 64.84 and c = 125.92 A, beta = 93.87 degrees. Form B was obtained using phosphate as the precipitant and has unit-cell dimensions a = 81.94, b = 66.75 and c = 108.92 A, beta = 97.58 degrees. Form B was stable in the X-ray beam for several days and diffracted to 3.15 A resolution. Form A crystals could not withstand X-ray radiation at room temperature, but produced high-quality data under cryogenic conditions. The latter are suitable for a 2.3 A resolution structure determination by molecular replacement.

Sequential structural changes upon zinc and calcium binding to metal-free concanavalin A.

The lectin concanavalin A (ConA) sequentially binds a transition metal ion in the metal-binding site S1 and a calcium ion in the metal-binding site S2 to form its saccharide-binding site. Metal-free ConA crystals soaked with either Zn2+ (apoZn-ConA) or Co2+ (apoCo-ConA) display partial binding of these ions in the proto-transition metal-binding site, but no further conformational changes are observed. These structures can represent the very first step in going from metal-free ConA toward the holoprotein. In the co-crystals of metal-free ConA with Zn2+ (Zn-ConA), the zinc ion can fully occupy the S1 site. The positions of the carboxylate ligands Asp10 and Asp19 that bridge the S1 and S2 sites are affected. The ligation to Zn2+ orients Asp10 optimally for calcium ligation and stabilizes Asp19 by a hydrogen bond to one of its water ligands. The neutralizing and stabilizing effect of the binding of Zn2+ in S1 is necessary to allow for subsequent Ca2+ binding in the S2 site. However, the S2 site of monometallized ConA is still disrupted. The co-crystals of metal-free ConA with both Zn2+ and Ca2+ contain the active holoprotein (ConA ZnCa). Ca2+ has induced large conformational changes to stabilize its hepta-coordination in the S2 site, which comprise the trans to cis isomerization of the Ala207-Asp208 peptide bond accompanied by the formation of the saccharide-binding site. The Zn2+ ligation in ConA ZnCa is similar to Mn2+, Cd2+, Co2+, or Ni2+ ligation in the S1 site, in disagreement with earlier extended x-ray absorption fine structure results that suggested a lower coordination number for Zn2+.

Crystallization of glycosylated and nonglycosylated phytohemagglutinin-L.

In the seeds of legume plants a class of sugar-binding proteins can be found, generally called legume lectins. In this paper we present the crystallization of phytohemagglutinin-L (PHA-L), a glycosylated lectin from the seeds of the common bean (Phaseolus vulgaris). Single PHA-L crystals were grown by vapor diffusion, using PEG as precipitant. The protein crystallizes in the monoclinic space group C2, and diffracts to a resolution of 2.7 angstroms. The unit cell parameters are a=106.3 angstroms, 121.2 angstroms, c=90.8 angstroms, and beta=93.7 degrees. The asymmetric unit probably contains one PHA-L tetramer. Crystals of a recombinant nonglycosylated form of PHA-L, grown under identical conditions, and crystals of the native PHA-L, grown in the presence of isopropanol, did not survive the mounting process.

Crystallographic structure of metal-free concanavalin A at 2.5 A resolution.

The three-dimensional structure of demetallized concanavalin A has been determined at 2.5 A resolution and refined to a crystallographic R-factor of 18%. The lectin activity of concanavalin A requires the binding of both a transition metal ion, generally Mn2+, and a Ca2+ ion in two neighboring sites in close proximity to the carbohydrate binding site. Large structural differences between the native and the metal-free lectin are observed in the metal-binding region and consequently for the residues involved in the specific binding of saccharides. The demetallization invokes a series of conformational changes in the protein backbone, apparently initiated mainly by the loss of the calcium ion. Most of the Mn2+ ligands retain their position, but the Ca2+ binding site is destroyed. The Ala207-Asp208 peptide bond, in the beta-strand neighboring the metal-binding sites, undergoes a cis to trans isomerization. The cis conformation for this bond is a highly conserved feature among the leguminous lectins and is critically maintained by the Ca2+ ion in metal-bound concanavalin A. A further and major change adjacent to the isomerized bond is an expansion of the loop containing the monosaccharide ligand residues Leu99 and Tyr100. The dispersion of the ligand residues for the monosaccharide binding site (Asn14, Agr228, Asp208, Leu99, and Tyr100) in metal-free concanavalin A abolishes the lectin's ability to bind saccharides. Since the quaternary structure of legume lectins is essential to their biological role, the tetramer formation was analyzed. In the crystal (pH 5), the metal-free concanavalin A dimers associate into a tetramer that is similar to the native one, but with a drastically reduced number of inter-dimer interactions. This explains the tetramer dissociation into dimers below pH values of 6.5.

NMR, molecular modeling, and crystallographic studies of lentil lectin-sucrose interaction.

The conformational features of sucrose in the combining site of lentil lectin have been characterized through elucidation of a crystalline complex at 1.9-A resolution, transferred nuclear Overhauser effect experiments performed at 600 Mhz, and molecular modeling. In the crystal, the lentil lectin dimer binds one sucrose molecule per monomer. The locations of 229 water molecules have been identified. NMR experiments have provided 11 transferred NOEs. In parallel, the docking study and conformational analysis of sucrose in the combining site of lentil lectin indicate that three different conformations can be accommodated. Of these, the orientation with lowest energy is identical with the one observed in the crystalline complex and provides good agreement with the observed transferred NOEs. These structural investigations indicate that the bound sucrose has a unique conformation for the glycosidic linkage, close to the one observed in crystalline sucrose, whereas the fructofuranose ring remains relatively flexible and does not exhibit any strong interaction with the protein. Major differences in the hydrogen bonding network of sucrose are found. None of the two inter-residue hydrogen bonds in crystalline sucrose are conserved in the complex with the lectin. Instead, a water molecule bridges hydroxyl groups O2-g and O3-f of sucrose.

The structures of RNase A complexed with 3'-CMP and d(CpA): active site conformation and conserved water molecules.

The interactions of RNase A with cytidine 3'-monophosphate (3'-CMP) and deoxycytidyl-3',5'-deoxyadenosine (d(CpA)) were analyzed by X-ray crystallography. The 3'-CMP complex and the native structure were determined from trigonal crystals, and the d(CpA) complex from monoclinic crystals. The differences between the overall structures are concentrated in loop regions and are relatively small. The protein-inhibitor contacts are interpreted in terms of the catalytic mechanism. The general base His 12 interacts with the 2' oxygen, as does the electrostatic catalyst Lys 41. The general acid His 119 has 2 conformations (A and B) in the native structure and is found in, respectively, the A and the B conformation in the d(CpA) and the 3'-CMP complex. From the present structures and from a comparison with RNase T1, we propose that His 119 is active in the A conformation. The structure of the d(CpA) complex permits a detailed analysis of the downstream binding site, which includes His 119 and Asn 71. The comparison of the present RNase A structures with an inhibitor complex of RNase T1 shows that there are important similarities in the active sites of these 2 enzymes, despite the absence of any sequence homology. The water molecules were analyzed in order to identify conserved water sites. Seventeen water sites were found to be conserved in RNase A structures from 5 different space groups. It is proposed that 7 of those water molecules play a role in the binding of the N-terminal helix to the rest of the protein and in the stabilization of the active site.

The monosaccharide binding site of lentil lectin: an X-ray and molecular modelling study.

The X-ray crystal structure of lentil lectin in complex with alpha-D-glucopyranose has been determined by molecular replacement and refined to an R-value of 0.20 at 3.0 A resolution. The glucose interacts with the protein in a manner similar to that found in the mannose complexes of concanavalin A, pea lectin and isolectin I from Lathyrus ochrus. The complex is stabilized by a network of hydrogen bonds involving the carbohydrate oxygens O6, O4, O3 and O5. In addition, the alpha-D-glucopyranose residue makes van der Waals contacts with the protein, involving the phenyl ring of Phe123 beta. The overall structure of lentil lectin, at this resolution, does not differ significantly from the highly refined structures of the uncomplexed lectin. Molecular docking studies were performed with mannose and its 2-O and 3-O-m-nitro-benzyl derivatives to explain their high affinity binding. The interactions of the modelled mannose with lentil lectin agree well with those observed experimentally for the protein-carbohydrate complex. The highly flexible Me-2-O-(m-nitro-benzyl)-alpha-D-mannopyranoside and Me-3-O-(m-nitro-benzyl)-alpha-D-mannopyranoside become conformationally restricted upon binding to lentil lectin. For best orientations of the two substrates in the combining site, the loss of entropy is accompanied by the formation of a strong hydrogen bond between the nitro group and one amino acid, Gly97 beta and Asn125 beta, respectively, along with the establishment of van der Waals interactions between the benzyl group and the aromatic amino acids Tyr100 beta and Trp128 beta.

Structural analysis of two crystal forms of lentil lectin at 1.8 A resolution.

The structures of two crystal forms of lentil lectin are determined and refined at high resolution. Orthorhombic lentil lectin is refined at 1.80 A resolution to an R-factor of 0.184 and monoclinic lentil lectin at 1.75 A resolution to an R-factor of 0.175. These two structures are compared to each other and to the other available legume lectin structures. The monosaccharide binding pocket of each lectin monomer contains a tightly bound phosphate ion. This phosphate makes hydrogen bonding contacts with Asp-81 beta, Gly-99 beta, and Asn-125 beta, three residues that are highly conserved in most of the known legume lectin sequences and essential for monosaccharide recognition in all legume lectin crystal structures described thus far. A detailed analysis of the composition and properties of the hydrophobic contact network and hydrophobic nuclei in lentil lectin is presented. Contact map calculations reveal that dense clusters of nonpolar as well as polar side chains play a major role in secondary structure packing. This is illustrated by a large cluster of 24 mainly hydrophobic amino acids that is responsible for the majority of packing interactions between the two beta-sheets. Another series of four smaller and less hydrophobic clusters is found to mediate the packing of a number of loop structures upon the front sheet. A very dense, but not very conserved cluster is found to stabilize the transition metal binding site. The highly conserved and invariant nonpolar residues are distributed asymmetrically over the protein.