Biology of animal lectins

The structure of Silurus asotus (catfish) roe lectin (SAL): identification of a noncovalent trimer by mass spectrometry and analytical ultracentrifugation

We identified a noncovalent trimer of Sirurus asotus roe lectin (SAL) at Mr 95,362 along with its monomer at M(r) 31,750 by electrospray ionization mass spectrometry when SAL was dissolved in 0.5% acetic acid, sprayed into the ion source with methanol as a sheath liquid, and desolvated at 75 degrees C in a heated capillary column. The molecular weight of SAL, determined by the sedimentation equilibrium method, was 95,200 and the sedimentation coefficient (S20,w) of SAL in water was 5.58. SAL existed as a noncovalent trimer in solution and showed the ability to agglutinate rabbit erythrocytes. SAL showed three peaks (sal 1, sal 2, and sal 3) by C8 reverse-phase HPLC, and these appeared to be a monomer, a dimer, and a trimer, respectively, by matrix-assisted laser desorption ionization-time of flight mass spectrometry, sal 1 and sal 2 were shown to have a structure interchangeable with that of sal 3 in water.

Making a fitting choice: common aspects of sugar-binding sites in plant and animal lectins

Comparing sequences of plant and animal lectins reveals that the ability to bind any one type of sugar has evolved several times independently in diverse protein frameworks. Conversely, families of lectins that share common structural features often contain members that recognize different groups of sugars. In the context of this combination of convergent and divergent evolution, our knowledge of the structures of lectin-sugar complexes provides valuable insights into the principles that underlie specific sugar binding.

Brain contains HNK-1 immunoreactive O-glycans of the sulfoglucuronyl lactosamine series that terminate in 2-linked or 2,6-linked hexose (mannose)

The monoclonal antibody HNK-1 originally raised to an antigenic marker of natural killer cells also binds to selected regions in nervous tissue. The antigen is a carbohydrate that has attracted much interest as its expression is developmentally regulated in nervous tissue, and it is found, and proposed to be a ligand, on several of the adhesive glycoproteins of the nervous system. It is also expressed on glycolipids and proteoglycans, and is the target of monoclonal auto-antibodies that give rise to a demyelinating disease. The epitope, as characterized on glycolipids isolated from the nervous system, is expressed on 3-sulfated glucuronic acid joined by beta1-3-linkage to a neolacto backbone. Here we exploit the neoglycolipid technology, in conjunction with immunodetection and in situ liquid secondary ion mass spectrometry, to characterize HNK-1-positive oligosaccharide chains derived by reductive alkaline release from total brain glycopeptides. The immunoreactive oligosaccharides detected are tetra- to octasaccharides that are very minor components among a heterogeneous population, each representing less than 0.1% of the starting material. Their peripheral and backbone sequences resemble those of the HNK-1-positive glycolipids. An unexpected finding is that they terminate not with N-acetylgalactosaminitol but with hexitol (2-substituted and 2,6-disubstituted). In a tetrasaccharide investigated in the greatest detail, the hexitol is identified as 2-substituted mannitol.

Optimizing lectin-carbohydrate interactions: improved binding of divalent alpha-mannosylated ligands towards Concanavalin A

The synthesis and binding properties to Jack bean phytohaemagglutinin in (Concanavalin A, Con A) of a new family of divalent alpha-D-mannopyranoside ligands are described. The synthesis of these ligands is based on the coupling of commercially available diamines to p-isothiocyanatophenyl 2,3,4,6 tetra-O-acetyl-alpha-D-mannopyranoside (4). The resulting dimers 6, 15 to 22 and 30 were tested for their relative inhibitory potency by solid-phase enzyme-linked lectin assays (ELLA) using methyl alpha-D-mannopyranoside as standard. Divalent mannosylated ligand 35 bearing a non-aromatic aglycon was also tested for comparison purposes. Concentrations necessary for 50% inhibition (IC50s) of binding of yeast mannan to Jack bean phytohaemagglutinin (Con A) were determined. The inhibitions showed dimers to be approximately 10- to 90-fold more potent than methyl alpha-D-mannopyranoside. Variations in the intra-mannosyl distance proved to be an important factor for optimum binding.

Phagocyte-bacteria interactions

Recognition and phagocytosis of micro-organisms in a serum-poor environment represent innate immunity against many extracellular pathogens. As a paradigm for such processes, we discuss the recognition of Klebsiella pneumoniae by alveolar macrophages and monocyte-derived macrophages in the absence of serum. Macrophages recognize and subsequently kill Klebsiella expressing Man-alpha 2/3-Man or Rha-alpha 2/3-Rha sequences in their capsular polysaccharides by two mechanisms: (a) recognition of the capsular structures by macrophage mannose receptors, and (b) opsonization by the lung surfactant protein A (SP-A), which binds to the capsular polysaccharides of Klebsiella and to SP-A receptors on the macrophages. Sp-A may also enhance phagocytosis by increasing the activity of macrophage mannose receptors. We conclude that a specific microbial surface structure may be a target for recognition by macrophages via several mechanisms, as exemplified in the case of Klebsiella capsular polysaccharides. Multiple recognition mechanisms of pathogens by macrophages may be essential to provide innate immunity to reduce the frequency of infections caused by a relatively less virulent bacterium in the immuno-compromised host.

Difference in the binding mode of two mannose-binding proteins: demonstration of a selective minicluster effect

Serum-type and liver-type mannose-binding proteins (MBP) are both present in higher animals and both are composed of a carbohydrate-recognition domain (CRD) and a collagenous domain. Although known as mannose-binding proteins, these proteins bind N-acetylglucosamine and other related sugars quite well. An earlier specificity study using cloned CRD portions of both types of MBP from rat [Childs, R. A., Feizi, T., Yuen, C.-T., Drickamer, K., & Quesenberry, M. (1990) J. Biol. Chem. 265, 20770-20777] revealed that the liver MBP CRD binds the trimannosyl core structure of N-glycosides, whereas the serum MBP CRD does not. We studied the substrate preferences of these CRDs using both solid and solution phase assays, testing monosaccharides, glycoproteins, and synthetic cluster ligands. While there was no significant difference in the monosaccharide binding specificities of the two CRDs, they displayed very different affinities for natural glycoproteins and mannose-containing cluster glycosides. Most interestingly, synthetic cluster ligands with two terminal GlcNAc moieties have affinity equal to monovalent GlcNAc ligands toward both CRDs, whereas a series of structurally similar Man-terminated divalent ligand displays about 20-fold enhanced affinity toward liver CRD only. A plausible explanation is that the liver MBP CRD has two sugar binding sites per subunit, one of which binds only mannose, and the other, both mannose and N-acetylglucosamine. In contrast, the serum MBP CRD has only one site of the latter type. Results of isothermal titration calorimetry support this hypothesis.

Modular organization of carbohydrate recognition domains in animal lectins

In spite of the great diversity of animal lectins, a common characteristic is their ability to bind sugars by means of discrete, modular carbohydrate recognition domains, CRDs. Three different groups of animal lectins-galectins, P-type and C-type lectins- have different types of CRDs which they arrange in a number of combinations, in three dimensions, in order to increase the affinity for oligosaccharides associated with glycoconjugates. The necessity of combining multiple CRDs in a native lectin molecule in order to increase the affinity for multiple ligands is of great importance physiologically, since many of the carbohydrate structures associated with proteins exist in a variety of different conformations. Recent work has clarified the structural basis for carbohydrate recognition by some of these lectins.

Mechanism of Ca2+ and monosaccharide binding to a C-type carbohydrate-recognition domain of the macrophage mannose receptor

Site-directed mutagenesis has been used to identify residues that ligate Ca2+ and sugar to the fourth C-type carbohydrate-recognition domain (CRD) of the macrophage mannose receptor. CRD-4 is the only one of the eight CRDs of the mannose receptor to exhibit detectable monosaccharide binding when expressed in isolation, and it is central to ligand binding by the receptor. CRD-4 requires two Ca2+ for sugar binding, like the CRD of rat serum mannose-binding protein (MBP-A). Sequence comparisons between the two CRDs suggest that the binding site for one Ca2+, which ligates directly to the bound sugar in MBP-A, is conserved in CRD-4 but that the auxiliary Ca2+ binding site is not. Mutation of the four residues at positions in CRD-4 equivalent to the auxiliary Ca2+ binding site in MBP-A indicates that only one, Asn728, is involved in ligation of Ca2+. Alanine-scanning mutagenesis was used to identify two other asparagine residues and one glutamic acid residue that are probably involved in ligation of the auxiliary Ca2+ to CRD-4. Sequence comparisons with other C-type CRDs suggest that the proposed binding site for the auxiliary Ca2+ in CRD-4 of the mannose receptor is unique. Evidence that the conserved Ca2+ in CRD-4 bridges between the protein and bound sugar in a manner analogous to MBP-A was obtained by mutation of one of the amino acid side chains at this site. Ring current shifts seen in the 1H NMR spectra of methyl glycosides of mannose, GlcNAc, and fucose in the presence of CRD-4 and site-directed mutagenesis indicate that a stacking interaction with Tyr729 is also involved in binding of sugars to CRD-4. This interaction contributes about 25% of the total free energy of binding to mannose. C-5 and C-6 of mannose interact with Tyr729, whereas C-2 of GlcNAc is closest to this residue, indicating that these two sugars bind to CRD-4 in opposite orientations. Sequence comparisons with other mannose/GlcNAc-specific C-type CRDs suggest that use of a stacking interaction in the binding of these sugars is probably unique to CRD-4 of the mannose receptor.

Innate immunity: impact on the adaptive immune response

For many years, innate immunity has been considered as a separate entity from the adaptive immune response and has been regarded to be of secondary importance in the hierarchy of immune functions. For the past few years, however, interest in innate immunity has grown enormously, so that now it is studied intensively in many laboratories that seek to integrate these two distinct types of immune function. Our intent in this review is to point out the similarities and differences in these two types of host response to infection, and to indicate our present level of understanding of how these can be integrated into a more complete description of the immune response.

The oligomerization domain of the asialoglycoprotein receptor preferentially forms 2:2 heterotetramers in vitro

The human hepatic asialoglycoprotein receptor is a noncovalent hetero-oligomer composed of two homologous subunits, H1 and H2, with an as yet unknown stoichiometry. Ligand specificity and binding affinity depend on the arrangement of the subunits in the complex. An 80-amino acid segment connecting the transmembrane and the carbohydrate binding domains contains heptad repeats characteristic of alpha-helical coiled coil structure. We expressed and purified corresponding peptides, H1S and H2S, and confirmed by circular dichroism spectroscopy that they can assume alpha-helical conformation. Oxidative cross-linking of amino-terminal cysteines generated specific covalent oligomers, indicating that separately H1S forms trimers and H2S tetramers. Upon mixing, covalent heterotetramers were formed with a preferred stoichiometry of 2 H1S and 2 H2S peptides. These results suggest that the stalk segments of the receptor subunits oligomerize to constitute an alpha-helical coiled coil stalk on top of which the carbohydrate binding domains are exposed for ligand binding. We propose that the functional asialoglycoprotein receptor is a 2:2 heterotetramer.

Synthesis and application of alpha-D-mannosyl clusters as photoaffinity ligands for mannose-binding proteins: concanavalin A as a model receptor

Mono-, di-, and tri-antennary alpha-D-mannopyranosyl derivatives were synthesized as oligosaccharide mimics. The compounds vary in the length of the acyclic aglyconic spacers linking, in the case of the di- and tri-antennary derivatives, the glycosyl endgroups. By haemagglutination assay (Coombs test) the affinity of the alpha-D-mannopyranosyl ligands against Con A was determined in comparison with methyl alpha-D-mannopyranoside. Affinity enhancement and strong cross-linking capacity were found with the di- and tri-antennary compounds where alpha-D-mannopyranosyl endgroups are separated by spacers of 5-37 atoms in length. The optimal ligand had Ki of 5.1 microM in comparison with 3.12 microM for methyl alpha-D-mannopyranoside. The monoantennary compound and, to certain extents, one di- and short tri-antennary ligands with short spacer lengths did not differ significantly in affinity from methyl alpha-D-mannopyranoside. A triantennary, radiolabelled ligand equipped with a photolabile diazirino group was used to covalently modify Con A by photoaffinity labelling. A significant degree of covalent cross-linking of Con A monomers was observed.

The 2.0 A structure of a cross-linked complex between snowdrop lectin and a branched mannopentaose: evidence for two unique binding modes

BACKGROUND

Galanthus nivalis agglutinin (GNA), a mannose-specific lectin from snowdrop bulbs, is a tetrameric member of the family of Amaryllidaceae lectins that exhibit antiviral activity towards HIV. Its subunits are composed of three pseudo-symmetrically related beta sheet domains, each with a conserved mannose-binding site. Crystal structures of monosaccharide and disaccharide complexes of GNA have revealed that all 12 binding sites of the tetramer are functional, and that the degree of occupancy is dependent on the availability of subsidiary interactions from neighboring subunits. The complex of GNA with a branched mannopentaose ((Manalpha1,6-(alpha1, 3-Man)Man-alpha1,6-(alpha1,3-Man)Man) described here simulates a more biologically relevant complex.

RESULTS

Two unique mannopentaose binding modes co-exist in the tetragonal structure (1 subunit/asymmetric unit) of the complex. In one, the conserved monosaccharide-binding pocket in domain 1 (CRD 1) is utilized for cross-linkage of twofold related GNA dimers by the outer 3,6 tri-Man arm, which alternates between two orientations consistent with crystal symmetry. Inter-linked dimers assemble helically along the 41 crystal axis forming a pore-like structure. In the second binding mode, the complete 3,6 tri-Man arm binds to an extended binding region in domain 3 (CRD 3) with subsites for each terminal Man and the internal Man positioned in the conserved monosaccharide pocket. The two remaining mannose residues are not visible in either binding mode.

CONCLUSIONS

This structure provides insights into possible mechanisms of the cross-linkage that is known to occur when lectins interact with specific multivalent cell surface receptors during events such as agglutination and mitogenic stimulation. By virtue of the large number of sites available for mannose binding, GNA has multiple possibilities of forming unique lattice structures. The two distinctly different binding modes observed in this study confirm that high affinity mannose binding occurs only at the two domain sites located near dimer interfaces.

Macromolecular recognition: effect of multivalency in the inhibition of binding of yeast mannan to concanavalin A and pea lectins by mannosylated dendrimers

The synthesis and binding properties of a new family of high affinity alpha-D-mannopyranoside ligands are described. The synthesis of the new multivalent ligands is based on the scaffolding of multiantennary branches of L-lysine residues having electrophilic N-chloroacetylated end groups as core structures. An alpha-D-mannopyranoside with p-substituted aryl aglycon ending with a thiol group was prepared and covalently attached to each of the branches of the dendritic structures. The resulting glycodendrimers with 2 (12), 4 (14), 8 (16), and 16 (18) mannoside residues were tested for their relative inhibitory potency by solid-phase enzyme-linked lectin assays (ELLA) using methyl and p-nitrophenyl alpha-D-mannopyranosides as standards. Concentrations necessary for 50% inhibition (IC50s) of binding of yeast mannan to Jack bean phytohemagglutinin (Canavalia ensiformis, concanavalin A) and to pea lectin (Pisum sativum) were determined. Analogous mannosylated copolyacrylamides were also prepared for comparison. The IC50 values were also plotted as a function of dendrimer valencies. The inhibitions showed 16-mer 18 to be approximately 600- and 2000-fold more potent than methyl alpha-D-mannopyranoside, and 66- and 1383-fold more potent than p-nitrophenyl alpha-D-mannopyranosides with Con A and pea lectins, respectively. Even when these numbers are expressed relative to single mannopyranoside residues per dendrimers, the relative potencies against the aromatic mannoside are still 4- and 86-fold better against Con A and pea lectins. These results unequivocally indicate that the optimum inhibitory binding properties of the new mannosylated dendrimers vary with both dendrimers and lectin valencies.

Analysis of the binding specificities of oligomannoside-binding proteins using methylated monosaccharides

The binding specificities of the closely related lectins from Canavalia ensiformis and Dioclea grandiflora were examined using specifically O-alkylated mono- and disaccharides. Both lectins accept any substitution at the monosaccharide C2 hydroxyl group. The binding energy of C2-alkylated ligands-concanavalin A complexes increases by 1 kcal mol-1 for the C2-O-ethyl ligand, while the binding energies of the corresponding complexes with the Dioclea lectin are identical. Both lectins accept methyl, but not ethyl, substitution of the C3 hydroxyl, in contrast to earlier reports. The results are interpreted in terms of existing models of the concanavalin A binding site. While the results are consistent with a model of the concanavalin A extended binding site that places the non-reducing terminus of all disaccharides in the monosaccharide binding site, they point to the dangers of interpreting the binding behavior of unnatural saccharide ligands on the basis of crystallographic data obtained with native ligands.

Improvements on the purification of mannan-binding lectin and demonstration of its Ca(2+)-independent association with a C1s-like serine protease

Mannan-binding lectin (MBL), previously called 'mannan-binding protein' or MBP, is a plasma C-type lectin which, upon binding to carbohydrate structures on micro-organisms, activates the classical pathway of complement. Purification of MBL relies on its Ca(2+)-dependent affinity for carbohydrate, but existing methods are susceptible to contamination by anti-carbohydrate antibodies. In the present study a sequential-sugar-elution method has been developed which can achieve a preparation of virtually pure MBL and its associated serine protease (MBL-associated serine protease, MASP) by two steps of affinity chromatography. In further separation of MASP from MBL, it was found that activated MASP was associated with MBL independent of Ca2+. Since MBL was found to bind to underivatized Sepharose 4B, the MBL-MASP complex was purified using Sepharose 4B and protease inhibitors were included to purify the complex with MASP in its proenzyme form. Analysis of thus-purified MBL-MASP complex by gel filtration on a Sephacryl S-300 column at pH 7.8 showed that the proenzyme MASP was also associated with MBL independently of Ca2+, but that the complex could be disrupted at a low pH (5.0). Therefore the mechanism of MBL-MASP-mediated complement activation appears to be significantly different from the C1-mediated classical pathway.

Syntheses and some applications of chemically defined multivalent glycoconjugates

Classical multivalent neoglycoproteins have been widely used to study a great number of carbohydrate-protein interactions. The synthesis of other neoglycoconjugates with various shapes, valencies, and conformations has reached considerable levels of sophistication and holds promise as a new tool for glycobiology and biomedical applications. Within the last few years, advances have been made towards both the syntheses and understanding of the antigenic properties of water-soluble glycopolymers. Some of these glycopolymers are finding applications as inhibitors of microbial adhesins and as carriers for drug delivery to specific cells. Novel dendritic carbohydrate structures are emerging as potent ligands for carbohydrate-binding proteins.

Gluconoylated and glycosylated polylysines as vectors for gene transfer into cystic fibrosis airway epithelial cells

To provide an alternative to viral vectors for the transfer of genes into airway epithelial cells in cystic fibrosis (CF), a novel set of substituted polylysines were employed. Polylysine was partially neutralized by blocking a number of positively charged residues with gluconoyl groups. In addition, polylysine was substituted with sugar residues on a specified number of amino groups. Using the gluconoylated polylysine as vector, the pCMVLuc plasmid gave high expression of the reporter gene luciferase in immortalized CF/T43 cells. The luciferase activity was 75-fold greater in the presence of 100 microM chloroquine. Luciferase gene expression persisted at high levels for up to at least 120 hr following transfection. Glycosylated polylysines/pCMVLuc complexes were compared to the gluconoylated polylysine/pCMVLuc complex and beta-Gal-, alpha-Glc-, and Lac-substituted polylysines gave 320%, 300%, and 290%, respectively, higher expression of the reporter gene luciferase. Luciferase expression ranged from 35 to 2 ng of luciferase per milligram of cell protein in the order: beta-Gal = alpha-Glc = Lac > alpha-Gal = Rha = Man > beta-GalNAc > alpha-GalNAc = alpha-Fuc, suggesting that the transfection efficiency is sugar dependent. Most importantly, in primary cultures of both CF and non-CF airway epithelial cells grown from tracheal tissue explants, lactosylated polylysine gave uniformly high expression of luciferase. The glycosylated polylysines provide an attractive nonviral approach for the transfer of genes into airway epithelial cells.

Characterizations of diverse residue clusters in protein three-dimensional structures

We present new methods for identifying and analyzing statistically significant residue clusters that occur in three-dimensional (3D) protein structures. Residue clusters of different kinds occur in many contexts. They often feature the active site (e.g., in substrate binding), the interface between polypeptide units of protein complexes, regions of protein-protein and protein-nucleic acid interactions, or regions of metal ion coordination. The methods are illustrated with 3D clusters centering on four themes. (i) Acidic or histidine-acidic clusters associated with metal ions. (ii) Cysteine clusters including coordination of metals such as zinc or iron-sulfur structures, cysteine knots prominent in growth factors, multiple sets of buried disulfide pairings that putatively nucleate the hydrophobic core, or cysteine clusters of mostly exposed disulfide bridges. (iii) Iron-sulfur proteins and charge clusters. (iv) 3D environments of multiple histidine residues. Study of diverse 3D residue clusters offers a new perspective on protein structure and function. The algorithms can aid in rapid identification of distinctive sites, suggest correlations among protein structures, and serve as a tool in the analysis of new structures.

Structural and electron microscopic analysis of neurocan and recombinant neurocan fragments

Neurocan, a nervous tissue-specific chondroitin sulfate proteoglycan of the aggrecan family which has been shown to interact with neural cell adhesion molecules and tenascin, could be visualized by rotary shadowing electron microscopy as two globular domains interconnected by an extended flexible filament of 60-90 nm. Several recombinant neurocan fragments generated in the human embryonic kidney cell line 293 represent as observed by electron microscopy the expected parts of this structure, which indicates a correct folding of these molecules. Biological activity of the recombinant N-terminal globular domain could be demonstrated by its coelution with hyaluronan in gel permeation chromatography. In addition, the modification of the recombinant fragments with certain carbohydrate structures was analyzed. High mannose oligosaccharides could be mapped to the N-terminal globular domain of the brain-derived molecule. Only recombinant fragments containing parts of the central region of the molecule were modified with chondroitin sulfate chains and with the HNK-1 epitope, and could be considerably altered in their migratory behavior on SDS-polyacrylamide gel electrophoresis by neuraminidase treatment. These findings and the electron microscopic shape indicate a mucin-like character for the central neurocan region.

Ca2+-dependent antifreeze proteins. Modulation of conformation and activity by divalent metal ions

The antifreeze proteins (AFPs) are structurally diverse molecules that share an ability to bind to ice crystals and inhibit their growth. The type II fish AFPs of Atlantic herring and smelt are unique among known AFPs in their requirement of a cofactor for antifreeze activity. These AFPs are homologous with the carbohydrate-recognition domains of Ca2+-dependent (C-type) lectins and require Ca2+ for their activity. To investigate the role of metal ions in the structure and function of type II AFPs, the binding of Ca2+ and other divalent cations to herring AFP was investigated. Binding studies using 45Ca2+ demonstrated that the AFP has a single Ca2+-binding site with a Kd of 9 microM. Proteolysis protection studies and measurement of antifreeze activity revealed a conformational change from a protease-sensitive and inactive apoAFP to a protease-resistant active AFP upon Ca2+ binding. Other divalent metal ions including Mn2+, Ba2+, and Zn2+ bind at the Ca2+-binding site and induce a similar change. A saturatable increase in tryptophan emission intensity at 340 nm also occurred upon Ca2+ addition. Whereas antifreeze activity appeared normal when Ca2+ or Mn2+ were bound, it was much lower in the presence of other metal ions. When Ba2+ was bound to the AFP, ice crystals showed a distinct difference in morphology. These studies demonstrate that herring AFP specifically binds Ca2+ and, consequently, adopts a conformation that is essential for its ice-binding activity.

Surface-epitope masking and expression cloning identifies the human prostate carcinoma tumor antigen gene PCTA-1 a member of the galectin gene family

The selective production of monoclonal antibodies (mAbs) reacting with defined cell surface-expressed molecules is now readily accomplished with an immunological subtraction approach, surface-epitope masking (SEM). Using SEM, prostate carcinoma (Pro 1.5) mAbs have been developed that react with tumor-associated antigens expressed on human prostate cancer cell lines and patient-derived carcinomas. Screening a human LNCaP prostate cancer cDNA expression library with the Pro 1.5 mAb identifies a gene, prostate carcinoma tumor antigen-1 (PCTA-1). PCTA-1 encodes a secreted protein of approximately 35 kDa that shares approximately 40% sequence homology with the N-amino terminal region of members of the S-type galactose-binding lectin (galectin) gene family. Specific galectins are found on the surface of human and marine neoplastic cells and have been implicated in tumorigenesis and metastasis. Primer pairs within the 3' untranslated region of PCTA-1 and reverse transcription-PCR demonstrate selective expression of PCTA-1 by prostate carcinomas versus normal prostate and benign prostatic hypertrophy. These findings document the use of the SEM procedure for generating mAbs reacting with tumor-associated antigens expressed on human prostate cancers. The SEM-derived mAbs have been used for expression cloning the gene encoding this human tumor antigen. The approaches described in this paper, SEM combined with expression cloning, should prove of wide utility for developing immunological reagents specific for and identifying genes relevant to human cancer.

The major surface glycoprotein of Trypanosoma cruzi amastigotes are ligands of the human serum mannose-binding protein

Trypanosoma cruzi, an obligate intracellular protozoan parasite, chronically infects mammals and causes Chagas' disease in humans. T. cruzi evasion of the mammalian immune response and establishment of chronic infection are poorly understood. During T. cruzi infection, amastigotes and trypomastigotes disseminate in the mammalian host and invade multiple cell types. Parasite surface carbohydrates and mammalian lectins have been implicated in the invasion of mammalian cells. A recent study has demonstrated that the human mannose-binding protein and the macrophage mannose receptor, two mammalian C-type lectins, bind to T. cruzi (S. J. Kahn, M. Wleklinski, A. Aruffo, A. Farr, D. Coder, and M. Kahn, J. Exp. Med. 182:1243-1258,1995). In this report we identify the major surface glycoproteins, including the SA85-1 glycoproteins, as T. cruzi ligands of the mannose-binding protein. Further characterization of the interaction between the mannose-binding protein and T. cruzi demonstrates that (i) the SA85-1 glycoproteins are expressed by amastigotes and trypomastigotes but only amastigotes express the mannose-binding protein ligand, (ii) treatment of amastigotes with alpha-mannosidase inhibits the binding of mannose-binding protein, and (iii) amastigote binding of mannose-binding protein is stable despite the spontaneous shedding of some glycoproteins from its surface. Together, the data indicate that developmentally regulated glycosylation of surface glycoproteins controls the expression of ligands that affect the interactions between T. cruzi and mannose-binding protein. It has been established that the binding of mannose-binding protein to microorganisms facilitates their uptake into phagocytic cells. Preferential opsonization of amastigotes with mannose-binding proteins may account for their clearance from the circulation and may contribute to the parasite's ability to invade different cell types.

A cytolytic function for a sialic acid-binding lectin that is a member of the pentraxin family of proteins

A variety of invertebrates possess plasma lectins with sialic acid recognition capabilities. One of the best studied of these lectins is limulin, which is a member of the pentraxin family of proteins and is found in the plasma of the American horseshoe crab, Limulus polyphemus. We find that limulin is one of several sialic acid-binding lectins of Limulus plasma and is present at a much lower abundance than Limulus C-reactive protein, the other plasma pentraxin. Limulin was purified by sequential affinity chromatography on phosphorylethanolamine-agarose, which isolates the pentraxins and separates limulin from the other sialic acid-binding lectins of the plasma, followed by fetuin-Sepharose, which binds limulin and separates it from Limulus C-reactive protein, the most abundant pentraxin of the plasma. We show here that limulin is the mediator of the Ca+2-dependent hemolytic activity found in the plasma of Limulus. Plasma that was depleted in the pentraxins by passage over phosphorylethanolamine-agarose or was depleted in the sialic acid-binding lectins by passage over fetuin-Sepharose lacked hemolytic activity. Purified limulin was hemolytic at concentrations of 3-5 nM. The other sialic acid-binding lectins of Limulus plasma and Limulus C-reactive protein were nonhemolytic. Foreign cell cytolysis by limulin represents a novel function for a plasma lectin and is the first documented function for limulin.

Purification and some properties of hemagglutinin from the Myxomycete, Physarum polycephalum

A new hemagglutinin was isolated from the plasmodium of Physarum polycephalum by salting out with ammonium sulphate followed by chromatography on DE-32, DEAE-Toyopearl and hydroxyapatite. This hemagglutinin, named physarumin, was purified 1000-fold over crude extracts. The molecular weight of physarumin was determined to be 22,000 by size exclusion chromatography on Bio-Gel P-60 and 8,700 by SDS-polyacrylamide gel electrophoresis. Physarumin agglutinated rabbit, guinea pig, horse and human erythrocytes. Physarumin-induced hemagglutination was inhibited by fetuin and alpha 1-acid glycoprotein, but not by commercially available mono- and disaccharides. Hemagglutinating activity was blocked by EDTA, and was restored by adding Ca2+ but not by Mg2+.

Prostate cancer--biology of metastasis and its clinical implications

Prostate cancer is one of the most commonly diagnosed cancers and is a major cause of cancer death in men. Although the majority of the diagnosed prostate cancers will remain localized and never produce clinical symptoms during the lifetime of the host, a subset of these cancers will progress to a more malignant state requiring therapeutic intervention. Acquisition of metastatic ability by prostatic cancer cells is the most lethal aspect of prostatic cancer progression. Once this has occurred, definitive therapy is required before the initially localized metastatic cells escape from the prostate. At present, metastatic prostate cancer is incurable. Therefore, there is an urgent need to develop molecular markers that can be used to predict the metastatic potential of prostate cancers. Using somatic cell hybridization, we have demonstrated that acquisition of metastatic ability requires both the loss of metastasis-suppressor function(s) and the activation of oncogenes. In further studies using micro-cell-mediated chromosomal transfer, we located genes on human chromosome, 8, 10cen-q23, 11p11.2-13, and 17pter-q23, which, when introduced into rat prostatic cancer cells, are capable of suppressing their metastatic ability without affecting their tumorigenicity or growth rate in vivo. Initially we focused upon the human chromosome 11p11.2-13 region to clone metastasis-suppressor gene(s) positionally. One such gene, termed KAI-1, encodes a membrane glycoprotein. KAI-1 has been mapped to the p11.2 region of human chromosome 11 by fluorescence in-situ hybridization analysis. Expression of KAI-1 has been detected in all normal human tissues thus far tested, including prostate tissue. When introduced into rat metastatic prostatic cancer cells, KAI-1 significantly suppressed the metastasis without affecting the tumor growth rate. KAI-1 expression is high in human normal prostate and benign prostatic hyperplasia but is dramatically lower in cancer cell lines derived from metastatic prostate tumors.

Croquemort, a novel Drosophila hemocyte/macrophage receptor that recognizes apoptotic cells

Programmed cell death is first observed at stage 11 of embryogenesis in Drosophila. The systematic removal of apoptotic cells is mediated by cells that are derived from the procephalic mesoderm and differentiate into macrophages. We describe a macrophage receptor for apoptotic cells. This receptor, croquemort (catcher of death), is a member of the CD36 superfamily. Croquemort-mediated phagocytosis represents the concept that phagocytosis evolved primarily as a cellular process for the removal of effete cells. Our findings support the idea that the primordial function of macrophages may have been in tissue modeling and that their adapted role is in host defense.

Characterization of the sialic acid-binding site in sialoadhesin by site-directed mutagenesis

The sialoadhesins are a distinct subgroup of the immunoglobulin superfamily, comprising sialoadhesin, CD22, the myelin-associated glycoprotein, and CD33. They can all mediate sialic acid-dependent binding to cells with distinct specificities. Sialoadhesin is a murine macrophage-restricted cell-surface molecule with 17 extracellular immunoglobulin-like domains that recognizes NeuAc alpha 2-3Gal in N- and O-glycans and interacts preferentially with cells of the granulocytic lineage. Its sialic acid-binding site is located within the NH2-terminal (membrane-distal) V-set domain. Here we have carried out site-directed mutagenesis in an attempt to identify the binding site of sialoadhesin. A subset of nonconservative mutations disrupted sialic acid-dependent binding without affecting binding of three monoclonal antibodies directed to two distinct epitopes of sialoadhesin. A CD8 alpha-based molecular model predicts that these residues form a contiguous binding site on the GFCC'C" beta-sheet of the V-set domain centered around an arginine in the F strand. A conservative mutation of this arginine to lysine also abolished binding. This amino acid is conserved among all members of the sialoadhesin family and is therefore likely to be a key residue in mediating sialic acid-dependent binding of sialoadhesins to cells.

Characterization of the proteins comprising the integral matrix of Strongylocentrotus purpuratus embryonic spicules

In the present study, we enumerate and characterize the proteins that comprise the integral spicule matrix of the Strongylocentrotus purpuratus embryo. Two-dimensional gel electrophoresis of [35S]methionine radiolabeled spicule matrix proteins reveals that there are 12 strongly radiolabeled spicule matrix proteins and approximately three dozen less strongly radiolabeled spicule matrix proteins. The majority of the proteins have acidic isoelectric points; however, there are several spicule matrix proteins that have more alkaline isoelectric points. Western blotting analysis indicates that SM50 is the spicule matrix protein with the most alkaline isoelectric point. In addition, two distinct SM30 proteins are identified in embryonic spicules, and they have apparent molecular masses of approximately 43 and 46 kDa. Comparisons between embryonic spicule matrix proteins and adult spine integral matrix proteins suggest that the embryonic 43-kDa SM30 protein is an embryonic isoform of SM30. An adult 49-kDa spine matrix protein is also identified as a possible adult isoform of SM30. Analysis of the SM30 amino acid sequences indicates that a portion of SM30 proteins is very similar to the carbohydrate recognition domain of C-type lectin proteins.

Introduction of selectin-like binding specificity into a homologous mannose-binding protein

The structures of the ligand-binding C-type carbohydrate-recognition domains of selectin cell adhesion molecules and of mannose-binding proteins (MBPs) are similar to each other even though these proteins bind very different carbohydrate ligands. Our current understanding of ligand binding by E-selectin is based on structural studies of unliganded E-selectin and of MBP-carbohydrate complexes, combined with results from mutagenesis of E-selectin. Five regions of E-selectin that differ in sequence from the corresponding regions of MBP have been introduced into the carbohydrate-recognition domain of MBP. Four of the changes have little effect on ligand binding. Insertion of one stretch of positively charged amino acids alters the sugar binding selectivity of the domain so that it now binds HL-60 cells and serum albumin derivatized with sialyl-Lewis X tetrasaccharide, thus mimicking the properties of E-selectin.

Characterization of carbohydrate-binding protein p33/41: relation with annexin IV, molecular basis of the doublet forms (p33 and p41), and modulation of the carbohydrate binding activity by phospholipids

A protein, p33/41, expressed in bovine kidney and many other tissues was identified as a lectin which binds to sialoglycoproteins and glycosaminoglycans in a calcium-dependent manner. Partial amino acid sequences of p33/41 are highly homologous to those of calcium/phospholipid-binding annexin protein, annexin IV (endonexin), p33/41 exhibited similar calcium/phospholipid-binding activity (Kojima, K., Ogawa, H., Seno, N., Yamamoto, K., Irimura, T., Osawa, T., and Matsumoto, I. (1993) J. Biol. Chem. 267, 20536-20539). To further characterize p33/41, we cloned the p33/41 cDNA and characterized the recombinant protein encoded by this cDNA. Oligonucleotide probes were synthesized based on partial amino acid sequences of p33/41 and used for screening. A p33/41 cDNA clone was isolated encoding a protein of 319 amino acids with a calculated molecular mass of 35,769 Da. The deduced amino acid sequence was identical to that of bovine annexin IV except for one amino acid substitution. The recombinant protein gave two 33-kDa (p33) and 41-kDa (p41) bands on SDS-polyacrylamide gel electrophoresis under non-reducing conditions, and only one 33-kDa band under reducing conditions, as did the native protein. Mass spectrometric analysis combined with site-directed mutagenesis of each of the four cysteine residues of the recombinant protein revealed that p41 is a dimer of p33 cross-linked at Cys-198 via a disulfide bond. The recombinant protein bound to columns of heparin and fetuin glycopeptides in a calcium dependent manner and to phospholipid vesicles composed of phosphatidylserine (PS)/phosphatidylcholine (PC), phosphatidylethanolamine (PE)/PC or phosphatidylinositol (PI)/PC. Furthermore, concurrent binding assays showed that the binding of the recombinant protein to phospholipid vesicles was not affected by heparin, whereas that to heparin was influenced by the phospholipid composition of the vesicles; the highest binding was observed with vesicles composed of PE/PC. These results suggest that p33/41 binds two types of ligands via different sites and that phospholipids modulate the carbohydrate binding activity of p33/41.

Specificity of C-glycoside complexation by mannose/glucose specific lectins

The binding of the mannose/glucose specific lectins from Canavalia ensiformis (concanavalin A) and Dioclea grandiflora to a series of C-glucosides were studied by titration microcalorimetry and fluorescence anisotropy titration. These closely related lectins share a specificity for the trimannoside methyl 3,6-di-O-(alpha-D-mannopyranosyl)-alpha-D-mannopyranoside, and are a useful model system for addressing the feasibility of differentiating between lectins with overlapping carbohydrate specificities. The ligands were designed to address two issues: (1) how the recognition properties of non-hydrolyzable C-glycoside analogues compare with those of the corresponding O-glycosides and (2) the effect of presentation of more than one saccharide recognition epitope on both affinity and specificity. Both lectins bind the C-glycosides with affinities comparable to those of the O-glycoside analogues; however, the ability of both lectins to differentiate between gluco and manno diastereomers was diminished in the C-glycoside series. Bivalent norbornyl C-glycoside esters were bound by the lectin from Canavalia but only weakly by the lectin from Dioclea. In addition to binding the bivalent ligands, concanavalin A discriminated between C-2 epimers, with the manno configuration binding more tightly than the gluco. The stoichiometry of binding of the bivalent ligands to both di- and tetrameric lectin was two binding sites per ligand, rather than the expected 1:1 stoichiometry. Together, these results suggest that concanavalin A may possess more than one class of carbohydrate binding sites and that these additional sites show stereochemical discrimination similar to that of the previously identified monosaccharide binding site. The implications of these findings for possible in vivo roles of plant lectins and for the use of concanavalin A as a research tool are discussed.

ERGIC-53 is a functional mannose-selective and calcium-dependent human homologue of leguminous lectins

Based on sequence homologies with leguminous lectins, the intermediate compartment marker ERGIC-53 was proposed to be a member of a putative new class of animal lectins associated with the secretory pathway. Independent, a promyelocytic protein, MR60, was purified by mannose-column chromatography, and a cDNA was isolated that matched MR60 peptide sequences. This cDNA was identical to that of ERGIC-53 and homologies with the animal lectin family of the galectins were noticed. Not all peptide sequences of MR60, however, were found in ERGIC-53, raising the possibility that another protein associated with ERGIC-53 may possess the lectin activity. Here, we provide the first direct evidence for a lectin function of ERGIC-53. Overexpressed ERGIC-53 binds to a mannose column in a calcium-dependent manner and also co-stains with mannosylated neoglycoprotein in a morphological binding assay. By using a sequential elution protocol we show that ERGIC-53 has selectivity for mannose and low affinity for glucose and GlcNAc, but no affinity for galactose. To experimentally address the putative homology of ERGIC-53 to leguminous lectins, a highly conserved protein family with an invariant asparagine essential for carbohydrate binding, we substituted the corresponding asparagine in ERGIC-53. This mutation, as well as a mutation affecting a second site in the putative carbohydrate recognition domain, abolished mannose-column binding and co-staining with mannosylated neoglycoprotein. These findings establish ERGIC-53 as a lectin and provide functional evidence for its relationship to leguminous lectins. Based on its monosaccharide specificity, domain organization, and recycling properties, we propose ERGIC-53 to function as a sorting receptor for glyco-proteins in the early secretory pathway.

Isolation and characterization of carinactivase, a novel prothrombin activator in Echis carinatus venom with a unique catalytic mechanism

The venom of the viper Echis carinatus contains a metalloprotease, ecarin, that is a potent prothrombin activator. We here show that the venom is also rich in another prothrombin activator, which does not belong to any known category of prothrombin activators. The novel enzyme, designated carinactivase-1 (CA-1), consists of two subunits held together non-covalently but very tightly. One subunit is a 62-kDa polypeptide that has metalloprotease activity and is homologous to the single-chain enzyme ecarin; the other subunit of 25 kDa consists of two disulfide-linked polypeptides of 17 and 14 kDa, and this subunit resembles the anticoagulant in the habu snake venom, IX/X-bp, that specifically binds the Gla domains of coagulation factors IX and X in a Ca2+-dependent fashion. The activation of prothrombin by CA-1 requires Ca2+ ions at millimolar concentrations and in the absence of Ca2+ ions this enzyme is virtually inactive. By contrast, activation by ecarin is completely independent of Ca2+ ions. CA-1, unlike ecarin, does not activate prothrombin derivatives, in which binding of Ca2+ ions has been perturbed, namely prethrombin-1 and acarboxyprothrombin. Furthermore, the isolated catalytic subunit, although its activity is greatly reduced as compared to that of the holoenzyme, no longer requires Ca2+ ions for the activation of prothrombin. Reconstitution with the non-catalytic 25-kDa subunit restores high level activity and the dependence on Ca2+ ions. Finally, prothrombin activation by CA-1 is inhibited by prothrombin fragment 1, and the isolated non-catalytic subunit is capable of binding fragment 1 in the presence of Ca2+ ions. From these observations, we postulate the following unique mechanism for the activation of prothrombin by CA-1. The enzyme primarily recognizes the Ca2+-bound conformation of the Gla domain in prothrombin via the 25-kDa regulatory subunit, and the subsequent conversion of prothrombin to active thrombin is catalyzed by the 62-kDa catalytic subunit.

Identification of different galectins by immunoblotting after two-dimensional polyacrylamide get electrophoresis with immobilized pH gradients

Vertebrate soluble beta-galactoside-binding lectins form a growing protein family that recently have been named galectins. Seven different galectins have been sequenced and characterized in mammals, and there is compelling evidence for the existence of other members of this lectin family. Three among six galectins are homodimers with (i) an identical subunit of a relative molecular mass of about 14500, and (ii) amino acid sequence homologies giving rise to possible immunochemical cross-reactivities. They are indistinguishable from each other by conventional sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), even when followed by immunoblotting. However, their different isoelectric points allow their identification using isoelectric focusing and two-dimensional (2-D) polyacrylamide gel electrophoresis. A strategy was developed to identify these galectins in crude extracts from cells and tissues, based on the two-dimensional electrophoresis with immobilized pH gradient (IPG-Dalt) analysis of the specific spots of purified galectins and of the spots of crude extracts, after silver staining. In addition, 2-D immunoblotting using anti-galectin 1 (Gal-1) and anti carbohydrate-binding protein 15 (CPB15) antibodies were performed on brain and leukemia cells (HL60) allowing an identification of related polypeptides. Our results indicate that the use of IPG-Dalt provides a suitable reproducibility and allows the detection of galectins or other galactoside-binding proteins even at basic pIs.

Cellular mechanisms of adaptation of grafts to antibody

New, more effective, strategies of immunosuppression, including those recently designed to induce durable T cell tolerance (by grafting allogeneic or xenogeneic haematopoietic cells into T lymphocyte-depleted recipients), leave humoral rejection as the main barrier to transplantation of vascularized organs between different species. Recent experimental work indicates that hyperacute rejection can be prevented by manipulations of antibodies and complement. In this paper, we review the mechanisms governing the interaction of antibodies with cell surface antigens in vitro and in vivo, and their cellular consequences. Evidence is presented that, in appropriate conditions, antibodies can protect by effecting modification of graft antigenicity (adaptation or accommodation).

Purification and molecular characterization of a novel 16-kDa galectin from the nematode Caenorhabditis elegans

In our previous study (Hirabayashi, J., Satoh, M., Ohyama, Y., and Kasai, K. (1992) J. Biochem. (Tokyo) 111, 553-555), two beta-galactoside-binding lectins (apparent subunit molecular masses, 16 and 32 kDa, respectively) were identified in the nematode Caenorhabditis elegans. The subsequent study revealed that the 32-kDa lectin is a member of the galectin family. Since the 32-kDa galectin was found to consist of two homologous domains (approximately 16 kDa), 16-kDa lectin was thought to be a degradation product of the 32-kDa galectin. To clarify this, the 16-kDa lectin was purified by an improved procedure employing extraction with a calcium-supplemented buffer. The purified 16-kDa lectin was found to exist as a dimer (approximately 30 kDa) and showed hemagglutinating activity toward trypsinized rabbit erythrocytes, which was inhibited by lactose. Almost the whole sequence of the 16-kDa polypeptide (approximately 95%, 135 amino acids) was determined after digestion with various proteases. Based on the obtained information, a full-length cDNA was cloned with the aid of RNA-polymerase chain reaction. The clone encoded 146 amino acids including initiator methionine (calculated molecular mass, 15,928 Da). Based on these results, it was concluded that the 16-kDa lectin is a novel member of the galectin family, but not a degradation product of the 32-kDa galectin as had previously thought. However, the 16-kDa galectin showed relatively low sequence similarities to both the N-terminal and the C-terminal domains of the 32-kDa galectin (28% and 27% identities, respectively) and to various vertebrate galectins (14-27%). Nonetheless, all of the critical amino acids involved in carbohydrate binding were conserved. These observations suggest that, in spite of phylogenic distance between nematodes and vertebrates, both the 16-kDa and 32-kDa nematode isolectins have conserved essentially the same function(s) as those of vertebrate galectins, probably through recognition of a key disaccharide moiety, "N-acetyllactosamine."

Cell calcium signalling induced by endogenous lectin carbohydrate interaction in the Jurkat T cell line

The effects of the beta-galactoside-binding lectin from human placenta (HPL14) on intracellular calcium concentration ([Ca2+]i) were examined in the human Jurkat T cell line. The lectin induces a concentration dependent increase in [Ca2+]i. This calcium signalling effect is clearly mediated through complementary cell surface galactoglycoconjugates because it can be blocked by beta-galactosides. The observed Ca2+ - response involves both the release of calcium from intracellular stores and a calcium influx from the extracellular space. It is sustained in the presence of 1 mM extracellular calcium whereas it becomes transient when the influx of extracellular calcium was blocked by calcium chelation to EGTA. Voltage-sensitive calcium channel blockers like verapamil and prenylamine were without effect on the action of HPL14. Protection of the sugar binding activity of HPL14 in the absence of a thiol-reducing reagent by carboxamidomethylation (CM-HPL14) or by substitution Cys2 with serine (C2S) results in lectin proteins with considerably decreased calcium signalling efficiency. The recombinant lectin (Rec H) and the mutant protein obtained by substitution of highly conservative Trp68 with tyrosine (W68Y) induce lower levels of [Ca2+]i compared to wild type lectin.

Scavenger receptors in innate immunity

Scavenger receptors are cell-surface proteins expressed by mammalian monocytes and macrophages and by invertebrate hemocytes, among other cell types. They exhibit distinctive ligand-binding properties, recognizing a wide range of ligands that include microbial surface constituents and intact microbes. The ligand-binding properties and expression patterns of these receptors suggest that they may function in one or more host-defense-related processes. Significant advances in scavenger receptor biology have recently been reported, including the identification of several new scavenger receptor genes.

Calcium-dependent lectin activity with novel distribution on thymocyte subsets

In this study we use carbohydrate probes to search for novel cell surface lectins in the immune system. Many carbohydrate binding proteins are members of the C-type lectin superfamily, whose ligand binding is dependent on calcium. To identify potential new members of this superfamily, fluorescein-conjugated carbohydrate polymers were used to probe for calcium-dependent cell surface binding. This approach offers advantages over the use of monoclonal antibody probes since only carbohydrate binding proteins would be identified. We have identified a cell surface lectin, referred to as thy-lec, detected by the probe fucoidan-fuorescein isothiocyanate. This calcium-dependent lectin has a novel distribution on thymocyte subsets. It is present on the surface of immature CD4+8+ and on large, cycling CD4-8- cells and CD8+4- cells, but not on small, mature phenotype CD8+4- or CD4+8- thymocytes. This lectin is not found on mature T cells or other leukocytes in lymph nodes, spleen or bone marrow. It is proposed that this novel cell surface has a function in the maturation of T cells in the thymus.

Remodeling of mouse milk glycoconjugates by transgenic expression of a human glycosyltransferase

The mammary gland is a unique biosynthetic tissue that produces a variety of species-specific glycoconjugates, but the factors regulating the production of specific glycoconjugates are not well understood. To explore the underlying regulation, a fusion gene containing a cDNA encoding the human alpha 1,2-fucosyltransferase (alpha 1,2FT), which generates the H-blood group antigen, flanked by the murine whey acidic protein promoter and a polyadenylation signal, was introduced into mice. Milk samples from transgenic animals contained soluble forms of the alpha 1,2FT, as revealed by Western blots of milk samples using an anti-alpha 1,2FT antiserum and by the demonstration of alpha 1,2FT enzyme activity. Milk from transgenic animals also contained large quantities of 2'-fucosyllactose (Fuc alpha 1-2Gal beta 1-4Glc) and modified glycoproteins containing the H-antigen, whereas milk from control animals lacked these glycoconjugates. Expression levels of 2'-fucosyllactose were high in most animals and represented 1/3 to nearly 1/2 of the total milk oligosaccharides. These results demonstrate that heterologous transgenic expression of a glycosyltransferase can result in the expression of both the transgene and its secondary gene products and that the structures of milk oligosaccharides can be remodeled depending on expression of the appropriate enzyme. Furthermore, these results suggest that the lactating mammary gland may be a unique biosynthetic reactor for the production of biologically active oligosaccharides and glycoconjugates.

The photoreactive carbohydrate probe, a novel method for detection of lectins

One of the main difficulties in the research of lectins is the absence of an adequate technique for their specific and routine detection. Here, we present a photoreactive carbohydrate-probe which could help to overcome this problem. The probe was comopsed by joining four segments: (i) a carbohydrate moiety, (ii) the digoxigenin tag, (iii) the photoreactive cross-linker and (iv) the lysyl-lysine backbone. After incubation with lectins in the dark, the probe can be activated and cross-linked to the lectins by illumination. The result is a lectin with covalently incorporated digoxigenin tag. Such a labelled lectin can be easily identified using anti-digoxigenin antibodies in a Western blot technique. This method is of high specificity and sensitivity and enables direct detection of lectins in complex mixtures, even whole cell homogenates.

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.

Increasing diversity of animal lectin structures

The number of animal proteins known to recognize carbohydrates and the number of their biological roles continue to increase. Comparisons of primary structures show that some of the newly described lectins are akin to previously investigated lectins, whereas others represent new structural groups. Progress has been made in understanding structure-function relationships for several lectins in both the old and the new categories.