Structures of synthetic O-antigen fragments from serotype 2a Shigella flexneri in complex with a protective monoclonal antibody.

The anti-LPS IgG mAb F22-4, raised against Shigella flexneri serotype 2a bacteria, protects against homologous, but not heterologous, challenge in an experimental animal model. We report the crystal structures of complexes formed between Fab F22-4 and two synthetic oligosaccharides, a decasaccharide and a pentadecasaccharide that were previously shown to be both immunogenic and antigenic mimics of the S. flexneri serotype 2a O-antigen. F22-4 binds to an epitope contained within two consecutive 2a serotype pentasaccharide repeat units (RU). Six sugar residues from a contiguous nine-residue segment make direct contacts with the antibody, including the nonreducing rhamnose and both branching glucosyl residues from the two RUs. The glucosyl residue, whose position of attachment to the tetrasaccharide backbone of the RU defines the serotype 2a O-antigen, is critical for recognition by F22-4. Although the complete decasaccharide is visible in the electron density maps, the last four pentadecasaccharide residues from the reducing end, which do not contact the antibody, could not be traced. Although considerable mobility in the free oligosaccharides can thus be expected, the conformational similarity between the individual RUs, both within and between the two complexes, suggests that short-range transient ordering to a helical conformation might occur in solution. Although the observed epitope includes the terminal nonreducing residue, binding to internal epitopes within the polysaccharide chain is not precluded. Our results have implications for vaccine development because they suggest that a minimum of two RUs of synthetic serotype 2a oligosaccharide is required for optimal mimicry of O-Ag epitopes.

Crystal structure of the beta chain of a T cell antigen receptor.

The crystal structure of the extracellular portion of the beta chain of a murine T cell antigen receptor (TCR), determined at a resolution of 1.7 angstroms, shows structural homology to immunoglobulins. The structure of the first and second hypervariable loops suggested that, in general, they adopt more restricted sets of conformations in TCR beta chains than those found in immunoglobulins; the third hypervariable loop had certain structural characteristics in common with those of immunoglobulin heavy chain variable domains. The variable and constant domains were in close contact, presumably restricting the flexibility of the beta chain. This may facilitate signal transduction from the TCR to the associated CD3 molecules in the TCR-CD3 complex.

Structural basis of HIV-1 and HIV-2 protease inhibition by a monoclonal antibody.

BACKGROUND: Since the demonstration that the protease of the human immunodeficiency virus (HIV Pr) is essential in the viral life cycle, this enzyme has become one of the primary targets for antiviral drug design. The murine monoclonal antibody 1696 (mAb1696), produced by immunization with the HIV-1 protease, inhibits the catalytic activity of the enzyme of both the HIV-1 and HIV-2 isolates with inhibition constants in the low nanomolar range. The antibody cross-reacts with peptides that include the N terminus of the enzyme, a region that is highly conserved in sequence among different viral strains and that, furthermore, is crucial for homodimerization to the active enzymatic form. RESULTS: We report here the crystal structure at 2.7 A resolution of a recombinant single-chain Fv fragment of mAb1696 as a complex with a cross-reactive peptide of the HIV-1 protease. The antibody-antigen interactions observed in this complex provide a structural basis for understanding the origin of the broad reactivity of mAb-1696 for the HIV-1 and HIV-2 proteases and their respective N-terminal peptides. CONCLUSION: A possible mechanism of HIV-protease inhibition by mAb1696 is proposed that could help the design of inhibitors aimed at binding inactive monomeric species.

Crystal structure of Urtica dioica agglutinin, a superantigen presented by MHC molecules of class I and class II.

BACKGROUND: Urtica dioica agglutinin (UDA), a monomeric lectin extracted from stinging nettle rhizomes, is specific for saccharides containing N-acetylglucosamine (GlcNAc). The lectin behaves as a superantigen for murine T cells, inducing the exclusive proliferation of Vbeta8.3(+) lymphocytes. UDA is unique among known T cell superantigens because it can be presented by major histocompatibility complex (MHC) molecules of both class I and II. RESULTS: The crystal structure of UDA has been determined in the ligand-free state, and in complex with tri-acetylchitotriose and tetra-acetylchitotetraose at 1.66 A, 1.90 A and 1.40 A resolution, respectively. UDA comprises two hevein-like domains, each with a saccharide-binding site. A serine and three aromatic residues at each site form the principal contacts with the ligand. The N-terminal domain binding site can centre on any residue of a chito-oligosaccharide, whereas that of the C-terminal domain is specific for residues at the nonreducing terminus of the ligand. We have shown previously that oligomers of GlcNAc inhibit the superantigenic activity of UDA and that the lectin binds to glycans on the MHC molecule. We show that UDA also binds to glycans on the T cell receptor (TCR). CONCLUSIONS: The presence of two saccharide-binding sites observed in the structure of UDA suggests that its superantigenic properties arise from the simultaneous fixation of glycans on the TCR and MHC molecules of the T cell and antigen-presenting cell, respectively. The well defined spacing between the two binding sites of UDA is probably a key factor in determining the specificity for Vbeta8.3(+) lymphocytes.

Crystallization and preliminary X-ray diffraction analysis of the beta-chain of a T-cell antigen receptor.

A secreted form of the beta-chain of a T-cell receptor specific for a hemagglutinin peptide of influenza virus in the context of the major histocompatibility complex class II I-Ed molecule has been crystallized in a form suitable for X-ray diffraction analysis. The crystals are tetragonal, space group P4(1)2(1)2 (or P4(3)2(1)2), with cell dimensions a = b = 71.4 A, c = 312.9 A, and diffract to beyond 3.5 A resolution. The beta-chain appears to behave as a stable homodimer in solution.

Structure of RNA in satellite tobacco necrosis virus. A low resolution neutron diffraction study using 1H2O/2H2O solvent contrast variation.

The crystal structure of satellite tobacco necrosis virus has been studied by neutron diffraction at 16 A resolution using the technique of 1H2O/2H2O solvent contrast variation to distinguish between the regions of protein and nucleic acid. The RNA density is essentially localized in a region just inside the protein coat, leading to a significant interaction between the two components. From the appearance of the RNA density we conclude that the protein coat imposes partial icosahedral symmetry on a significant proportion of the nucleic acid. The shape and dimensions of the major part of this density suggests that about 72% of the total RNA could be double-helical in structure. The most important interaction between the two components of the virus occurs between the N-terminal triple-helical arms of the protein subunits and those regions of the RNA density that could have a double-helical secondary structure.

Exchange of individual hydrogens for a protein in a crystal and in solution.

A preliminary comparison of the solvent exchange of individual hydrogens of a protein in solution and in a crystal has been possible by using data for lysozyme from 1H nuclear magnetic resonance and neutron diffraction studies. It is suggested that this approach enables a direct comparison of local dynamical behaviour in the two states. The results indicate markedly similar behaviour for many residues, but significant differences are indicated in several regions of the protein.

Crystal structure of the nucleosome core particle at 16 A resolution.

The crystal structure of the nucleosome core particle has been studied by neutron diffraction to a resolution of 16 A. By using H2O/D2O solvent contrast variation, the structures of the DNA and histone core were analysed separately. The DNA, as seen at this resolution, forms a super-helix of pitch 25.8 A, radius 42.1 A and 1.8 turns in length. The histone core itself is approximately helical and follows the DNA along the inside of the super-helix, giving the nucleosome core particle an overall 2-fold axis of symmetry. Four regions can be distinguished in the protein density, which we interpret as dimers of histones within the octameric core. The dimers have been assigned on the basis of other evidence as being of two kinds, (H2A-H2B) and (H3-H4). Because solvent contrast variation can distinguish between hydrophobic and hydrophilic regions in the protein density, our results suggest that the interface between the monomers of each dimer is probably quite hydrophobic in character, while the interaction between dimers is weaker and/or more hydrophilic. The protein is in contact with most of the DNA and there are some regions where it may penetrate between the turns of the super-helix. In particular, the tetramer (H4-H3)-(H3-H4) is in close contact with the central part of the DNA, but significant contacts are seen also between the histones H3 and the extremities of the super-helix, thus explaining the stability of a nucleosome-like particle depleted of H2A and H2B. Significant departures from the molecular 2-fold axis of symmetry occur in the relative arrangements of the two (H2A-H2B) dimers.

Crystal structure of a dominant B-cell epitope from the preS2 region of hepatitis B virus in the form of an inserted peptide segment in maltodextrin-binding protein.

We report here the crystal structure of MalE-B363, a recombinant construction of maltodextrin-binding protein bearing a dominant B-cell epitope sequence from the preS2 region of the hepatitis B surface antigen. The inserted peptide sequence, which replaces the seven carboxy-terminal residues of maltodextrin-binding protein, carries the 14 amino acid residue epitope contained between residues 132 and 145 from the preS2 region. The epitope sequence is flanked on either side by additional residues that result from the genetically engineered insertion, bringing the total length of the foreign peptide to 26 amino acid residues. The hybrid protein has been previously shown to be recognised by monoclonal antibodies elicited by the native viral antigen. Three independent molecules of MalE-B363 are present in the asymmetric unit of the crystal. All 14 epitope residues could be traced for one molecule, ten epitope residues had significant electron density for the second, but no density was visible for the epitope of the third. The conformation of the amino-terminal segment of the epitope from Gln132(e) to Gly138(e) is similar in the two molecules of MalE-B363 for which the foreign peptide could be traced. Moreover, the conformation of a smaller segment, comprising residues Asp133(e) to Arg137(e), is similar to that present in the previously determined crystal structure of MalE-B133, another insertion/deletion mutant of maltodextrin-binding protein bearing the preS2 epitope. The presence of a common structural motif for the same sequence in disparate molecular environments suggests that this conformation might be present also in the native viral antigen. This could provide a structural basis to explain the cross-reactivity of anti-preS2 monoclonal antibodies with these hybrid proteins.

Role of B13 Glu in insulin assembly. The hexamer structure of recombinant mutant (B13 Glu-->Gln) insulin.

The assembly of the insulin hexamer brings the six B13 glutamate side-chains at the centre into close proximity. Their mutual repulsion is unfavourable and zinc co-ordination to B10 histidine is necessary to stabilize the well known zinc-containing hexamers. Since B13 is always a carboxylic acid in all known sequences of hexamer forming insulins, it is likely to be important in the hormone's biology. The mutation of B13 Glu-->Gln leads to a stable zinc-free hexamer with somewhat reduced potency. The structures of the zinc-free B13 Gln hexamer and the 2Zn B13 insulin hexamer have been determined by X-ray analysis and refined with 2.5 A and 2.0 A diffraction data, respectively. Comparisons show that in 2Zn B13 Gln insulin, the hexamer structure (T6) is very like that of the native hormone. On the other hand, the zinc-free hexamer assumes a quaternary structure (T3/R3) seen in the native 4Zn insulin hexamer, and normally associated only with high chloride ion concentrations in the medium. The crystal structures show the B13 Gln side-chains only contact water in contrast to the B13 glutamate in 2Zn insulin. The solvation of the B13 Gln may be associated with this residue favouring helix at B1 to B8. The low potency of the B13 Gln insulin also suggests the residue influences the hormone's conformation.

Preliminary crystallographic study of a complex between the Fab fragment of a monoclonal anti-lysozyme antibody (D1.3) and the Fab fragment from an anti-idiotopic antibody against D1.3.

An anti-lysozyme antibody, D1.3, was used as immunogen to obtain syngeneic (Balb/c) monoclonal anti-idiotopic antibodies. The complex between Fab D1.3 and the Fab fragment from the anti-idiotopic antibody E225 has been crystallized. The crystals are monoclinic, space group P2(1), with a = 75.7 A, b = 77.4 A, c = 97.2 A, beta = 111.90 degrees and one molecule of the complex in the asymmetric unit. X-ray photographs show reflections extending to a resolution of about 3 A. Although twinning occurs frequently in the large crystals obtained, this material is suitable for high-resolution X-ray analysis.

Crystallization and preliminary X-ray diffraction study of the bacterially expressed Fv from the monoclonal anti-lysozyme antibody D1.3 and of its complex with the antigen, lysozyme.

The associated heavy (VH) and light (VL) chain variable domains (Fv) of the monoclonal anti-lysozyme antibody D1.3, secreted from Escherichia coli, have been crystallized in their antigen-bound and free forms. FvD1.3 gives tetragonal crystals, space group P4(1)2(1)2 (or P4(3)2(1)2), with a = 90.6 A, c = 56.4 A. The FvD1.3-lysozyme complex crystallizes in space group C2, with a = 129.2 A, b = 60.8 A, c = 56.9 A and beta = 119.3 degrees. The crystals contain one molecule of Fv or of the Fv-lysozyme complex in their asymmetric units and diffract X-rays to high resolution, making them suitable for X-ray crystallographic studies.

Three-dimensional structure of an Fab-peptide complex: structural basis of HIV-1 protease inhibition by a monoclonal antibody.

F11.2.32, a monoclonal antibody raised against HIV-1 protease (Kd = 5 nM), which inhibits proteolytic activity of the enzyme (K(inh) = 35(+/-3)nM), has been studied by crystallographic methods. The three-dimensional structure of the complex between the Fab fragment and a synthetic peptide, spanning residues 36 to 46 of the protease, has been determined at 2.2 A resolution, and that of the Fab in the free state has been determined at 2.6 A resolution. The refined model of the complex reveals ten well-ordered residues of the peptide (P36 to P45) bound in a hydrophobic cavity at the centre of the antigen-binding site. The peptide adopts a beta hairpin-like structure in which residues P38 to P42 form a type II beta-turn conformation. An intermolecular antiparallel beta-sheet is formed between the peptide and the CDR3-H loop of the antibody; additional polar interactions occur between main-chain atoms of the peptide and hydroxyl groups from tyrosine residues protruding from CDR1-L and CDR3-H. Three water molecules, located at the antigen-antibody interface, mediate polar interactions between the peptide and the most buried hypervariable loops, CDR3-L and CDR1-H. A comparison between the free and complexed Fab fragments shows that significant conformational changes occur in the long hypervariable regions, CDR1-L and CDR3-H, upon binding the peptide. The conformation of the bound peptide, which shows no overall structural similarity to the corresponding segment in HIV-1 protease, suggests that F11.2.32 might inhibit proteolysis by distorting the native structure of the enzyme.

Three-dimensional structures of the free and the antigen-complexed Fab from monoclonal anti-lysozyme antibody D44.1.

The three-dimensional structures of the free and antigen-complexed Fabs from the mouse monoclonal anti-hen egg white lysozyme antibody D44.1 have been solved and refined by X-ray crystallographic techniques. The crystals of the free and lysozyme-bound Fabs were grown under identical conditions and their X-ray diffraction data were collected to 2.1 and 2.5 A, respectively. Two molecules of the Fab-lysozyme complex in the asymmetric unit of the crystals show nearly identical conformations and thus confirm the essential structural features of the antigen-antibody interface. Three buried water molecules enhance the surface complementarity at the interface and provide hydrogen bonds to stabilize the complex. Two hydrophobic buried holes are present at the interface which, although large enough to accommodate solvent molecules, are void. The combining site residues of the complexed FabD44.1 exhibit reduced temperature factors compared with those of the free Fab. Furthermore, small perturbations in atomic positions and rearrangements of side-chains at the combining site, and a relative rearrangement of the variable domains of the light (VL) and the heavy (VH) chains, detail a Fab accommodation of the bound lysozyme. The amino acid sequence of the VH domain, as well as the epitope of lysozyme recognized by D44.1 are very close to those previously reported for the monoclonal antibody HyHEL-5. A feature central to the FabD44.1 and FabHyHEL-5 complexes with lysozyme are three salt bridges between VH glutamate residues 35 and 50 and lysozyme arginine residues 45 and 68. The presence of the three salt bridges in the D44.1-lysozyme interface indicates that these bonds are not responsible for the 1000-fold increase in affinity for lysozyme that HyHEL-5 exhibits relative to D44.1.

Structural and functional studies of FkpA from Escherichia coli, a cis/trans peptidyl-prolyl isomerase with chaperone activity.

The protein FkpA from the periplasm of Escherichia coli exhibits both cis/trans peptidyl-prolyl isomerase (PPIase) and chaperone activities. The crystal structure of the protein has been determined in three different forms: as the full-length native molecule, as a truncated form lacking the last 21 residues, and as the same truncated form in complex with the immunosuppressant ligand, FK506. FkpA is a dimeric molecule in which the 245-residue subunit is divided into two domains. The N-terminal domain includes three helices that are interlaced with those of the other subunit to provide all inter-subunit contacts maintaining the dimeric species. The C-terminal domain, which belongs to the FK506-binding protein (FKBP) family, binds the FK506 ligand. The overall form of the dimer is V-shaped, and the different crystal structures reveal a flexibility in the relative orientation of the two C-terminal domains located at the extremities of the V. The deletion mutant FkpNL, comprising the N-terminal domain only, exists in solution as a mixture of monomeric and dimeric species, and exhibits chaperone activity. By contrast, a deletion mutant comprising the C-terminal domain only is monomeric, and although it shows PPIase activity, it is devoid of chaperone function. These results suggest that the chaperone and catalytic activities reside in the N and C-terminal domains, respectively. Accordingly, the observed mobility of the C-terminal domains of the dimeric molecule could effectively adapt these two independent folding functions of FkpA to polypeptide substrates.

Crystal structure of a Fab complex formed with PfMSP1-19, the C-terminal fragment of merozoite surface protein 1 from Plasmodium falciparum: a malaria vaccine candidate.

Merozoite surface protein 1 (MSP1) is the major protein component on the surface of the merozoite, the erythrocyte-invasive form of the malaria parasite Plasmodium. Present in all species of Plasmodium, it undergoes two distinct proteolytic maturation steps during the course of merozoite development that are essential for invasion of the erythrocyte. Antibodies specific for the C-terminal maturation product, MSP1-19, can inhibit erythrocyte invasion and parasite growth. This polypeptide is therefore considered to be one of the more promising malaria vaccine candidates. We describe here the crystal structure of recombinant MSP1-19 from P.falciparum (PfMSP1-19), the most virulent species of the parasite in humans, as a complex with the Fab fragment of the monoclonal antibody G17.12. This antibody recognises a discontinuous epitope comprising 13 residues on the first epidermal growth factor (EGF)-like domain of PfMSP1-19. Although G17.12 was raised against the recombinant antigen expressed in an insect cell/baculovirus system, it binds uniformly to the surface of merozoites from the late schizont stage, showing that the cognate epitope is exposed on the naturally occurring MSP1 polypeptide complex. Although the epitope includes residues that have been mapped to regions recognised by invasion-inhibiting antibodies studied by other workers, G17.12 does not inhibit erythrocyte invasion or MSP1 processing.

Inhibition of the HIV-1 and HIV-2 proteases by a monoclonal antibody.

The monoclonal antibody 1696, directed against the HIV-1 protease, displays strong inhibitory effects toward the catalytic activity of the enzyme of both the HIV-1 and HIV-2 isolates. This antibody cross-reacts with peptides that include the N-terminus of the enzyme, a region that is well conserved in sequence among different viral strains and which, furthermore, is crucial for homodimerization to the active enzymatic form. This observation, as well as antigen-binding studies in the presence of an active site inhibitor, suggest that 1696 inhibits the HIV protease by destabilizing its active homodimeric form. To characterize further how the antibody 1696 inhibits the HIV-1 and HIV-2 proteases, we have solved the crystal structure of its Fab fragment by molecular replacement and refined it at 3.0 A resolution. The antigen binding site has a deep cavity at its center, which is lined mainly by acidic and hydrophobic residues, and is large enough to accommodate several antigen residues. The structure of the Fab 1696 could form a starting basis for the design of alternative HIV protease-inhibiting molecules of broad specificity.

Preliminary crystallographic studies of an anti-HIV-1 protease antibody that inhibits enzyme activity.

F11.2.32, a monoclonal antibody directed against the HIV-1 protease, displays strong inhibitory effects toward the catalytic activity of the enzyme. The antibody cross-reacts with peptides 36-46 and 36-57 from the protease. Crystals of the Fab have been obtained both in the free state and as complexes formed with the protease peptide fragments, 36-46 and 36-57. Diffraction data have been collected for the free and complexed forms of Fab F11.2.32 and preliminary models for the crystal structures were obtained by molecular replacement.

Structural and functional characterization of a monoclonal antibody specific for the preS1 region of hepatitis B virus.

The monoclonal antibody 5a19, raised against the ay serotype of hepatitis B virus, binds to the segment of the preS1 region comprising residues 37-43, which is implicated in attachment of the virus to hepatocytes. The dissociation constant, derived from kinetic studies using surface plasmon resonance techniques, is in the low nanomolar range. The nucleotide sequence of the variable domains has been determined and the corresponding germ-line genes have been identified. The three-dimensional structure of the Fab fragment has been determined by X-ray crystallography to 2.6 A resolution.

The interaction of metal ions with annexin V: a crystallographic study.

Three closely related rhombohedral crystal structures of human annexin V have been analysed and compared: a low-calcium, a high-calcium and an ytterbium-soaked crystal. The occupancy of the calcium sites increases at higher calcium concentrations, but the calcium is removed rather than replaced during soaking in the ytterbium solution. Instead, other sites are substituted at high calcium concentrations as well as in the presence of ytterbium. Furthermore, a new site is revealed in the ytterbium-soaked crystal which may give a clue to the mechanism of conformational change that takes place in the third domain of annexin V in the presence of very high calcium concentrations and of phospholipids.