Isolation and characterization of endogenous ligands for liver mannan-binding protein

What contributes to an effective mannose recognition domain?

In general, carbohydrate-lectin interactions are characterized by high specificity but also low affinity. The main reason for the low affinities are desolvation costs, due to the numerous hydroxy groups present on the ligand, together with the typically polar surface of the binding sites. Nonetheless, nature has evolved strategies to overcome this hurdle, most prominently in relation to carbohydrate-lectin interactions of the innate immune system but also in bacterial adhesion, a process key for the bacterium's survival. In an effort to better understand the particular characteristics, which contribute to a successful carbohydrate recognition domain, the mannose-binding sites of six C-type lectins and of three bacterial adhesins were analyzed. One important finding is that the high enthalpic penalties caused by desolvation can only be compensated for by the number and quality of hydrogen bonds formed by each of the polar hydroxy groups engaged in the binding process. In addition, since mammalian mannose-binding sites are in general flat and solvent exposed, the half-lives of carbohydrate-lectin complexes are rather short since water molecules can easily access and displace the ligand from the binding site. In contrast, the bacterial lectin FimH benefits from a deep mannose-binding site, leading to a substantial improvement in the off-rate. Together with both a catch-bond mechanism (i.e., improvement of affinity under shear stress) and multivalency, two methods commonly utilized by pathogens, the affinity of the carbohydrate-FimH interaction can be further improved. Including those just described, the various approaches explored by nature to optimize selectivity and affinity of carbohydrate-lectin interactions offer interesting therapeutic perspectives for the development of carbohydrate-based drugs.

Target Selectivity of FimH Antagonists

Mannose-based FimH antagonists are considered new therapeutics for the treatment of urinary tract infections (UTIs). They prevent the adhesion of uropathogenic Escherichia coli (UPEC) to urothelial cell surfaces triggered by the lectin FimH, which is located at the tip of bacterial type 1 pili. Because all reported FimH antagonists are α-d-mannosides, they are also potential ligands of mannose receptors of the human host system. We therefore investigated the selectivity range of five FimH antagonists belonging to different compound families by comparing their affinities for FimH and eight human mannose receptors. On the basis of the detected selectivity range of approximately 5 orders of magnitude, no adverse side effects resulting from nonselective binding to the human receptors have to be expected. FimH antagonists can therefore be further considered as potential therapeutics for the treatment of UTI.

Subcellular localization and physiological significance of intracellular mannan-binding protein

Mannan-binding protein (MBP) is a C-type mammalian lectin specific for mannose and N-acetylglucosamine. MBP is mainly synthesized in the liver and occurs naturally in two forms, serum MBP (S-MBP) and intracellular MBP (I-MBP). S-MBP activates complement in association with MBP-associated serine proteases via the lectin pathway. Despite our previous study (Mori, K., Kawasaki, T., and Yamashina, I. (1984) Arch. Biochem. Biophys. 232, 223-233), the subcellular localization of I-MBP and its functional implication have not been clarified yet. Here, as an extension of our previous studies, we have demonstrated that the expression of human MBP cDNA reproduces native MBP differentiation of S-MBP and I-MBP in human hepatoma cells. I-MBP shows distinct accumulation in cytoplasmic granules, and is predominantly localized in the endoplasmic reticulum (ER) and involved in COPII vesicle-mediated ER-to-Golgi transport. However, the subcellular localization of either a mutant (C236S/C244S) I-MBP, which lacks carbohydrate-binding activity, or the wild-type I-MBP in tunicamycin-treated cells shows an equally diffuse cytoplasmic distribution, suggesting that the unique accumulation of I-MBP in the ER and COPII vesicles is mediated by an N-glycan-lectin interaction. Furthermore, the binding of I-MBP with glycoprotein intermediates occurs in the ER, which is carbohydrate- and pH-dependent, and is affected by glucose-trimmed high-mannose-type oligosaccharides. These results strongly indicate that I-MBP may function as a cargo transport lectin facilitating ER-to-Golgi traffic in glycoprotein quality control.

Characterization and quantification of mouse mannan-binding lectins (MBL-A and MBL-C) and study of acute phase responses

Rat monoclonal antibodies (MoAbs) against mouse mannan-binding lectin (MBL)-A and MBL-C were generated and assays for MBL-A and MBL-C were constructed. This allowed for the quantitative analysis of both proteins for the first time. Previously only MBL-A has been quantified using less standardized methods. In a mouse serum pool the concentrations were now determined at 7.5 microg MBL-A and 45 microg MBL-C per ml. On gel permeation chromatography of mouse serum, MBL-A eluted corresponding to a M(r) of 850 kDa whereas the majority of MBL-C eluted corresponding to a Mr of 950 kDa. On sucrose density gradient centrifugation the sedimentation velocities of MBL-A and MBL-C were estimated at 7.3 S and 10.8 S, respectively. The MBL-A and MBL-C levels in 10 laboratory mice strains were compared and found to vary between 4 microg/ml to 12 microg/ml, and 16 microg/ml to 118 microg/ml, respectively. After the induction of acute phase responses by intraperitoneal injection of either casein or lipopolysaccharide (LPS), MBL-A was found to increase approximately two-fold, with a maximum after 32 h, while MBL-C did not increase significantly. In comparison, serum amyloid A component (SAA) peaked at 15 h with an approximate 100-fold increase.

Purification and characterization of two mannan-binding lectins from mouse serum

Mannan-binding lectin (MBL) is a serum protein that activates the complement system after binding to glycoconjugates found on the surface of microorganisms. By molecular cloning two forms of MBL have been identified in the mouse (mMBL-A and mMBL-C), but only mMBL-A has been purified and characterized at the protein level. MBL-C has been termed the liver form of MBL. The present report describes the purification and characterization of mMBL-A and mMBL-C from serum. The two forms of mMBL could be separated both by ion-exchange and carbohydrate-affinity chromatography. The initial identification by immunochemical technique was confirmed by N-terminal amino-acid sequencing. Both proteins give bands corresponding to polypeptide chains of 28 kDa on SDS-PAGE in the reduced state, but mMBL-A migrated more rapidly than mMBL-C in acid/urea-PAGE, in accordance with the calculated pIs. Both forms mediated activation of complement component C4 in mannan-coated microtiter wells. MBL-A showed a higher affinity for d -glucose and alpha-methyl-d -glucose then did MBL-C. Serum concentrations of mMBL-A in laboratory strains and wild mice were found to vary from 5 to 80 microg/ml, with wild mice tending to show higher levels than laboratory strains.

Asymmetry adjacent to the collagen-like domain in rat liver mannose-binding protein

Rat liver mannose-binding protein (MBP-C) is the smallest known member of the collectin family of animal lectins, many of which are involved in defence against microbial pathogens. It consists of an N-terminal collagen-like domain linked to C-terminal carbohydrate-recognition domains. MBP-C, overproduced in Chinese-hamster ovary cells, is post-translationally modified and processed in a manner similar to the native lectin. Analytical ultracentrifugation experiments indicate that MBP-C is trimeric, with a weight-averaged molecular mass of approx. 77 kDa. The rate of sedimentation of MBP-C and its mobility on gel filtration suggest a highly elongated molecule. Anomalous behaviour on gel filtration due to this extended conformation may explain previous suggestions that MBP-C forms a higher oligomer. The polypeptide chains of the MBP-C trimer are linked by disulphide bonds between two cysteine residues at the N-terminal junction of the collagen-like domain. Analysis of an N-terminal tryptic fragment reveals that the disulphide bonding in MBP-C is heterogeneous and asymmetrical. These results indicate that assembly of MBP-C oligomers probably proceeds in a C- to N-terminal direction: trimerization at the C-terminus is followed by assembly of the collagenous domain and finally formation of N-terminal disulphide bonds. The relatively simple organization of MBP-C provides a template for understanding larger, more complex collectins.

Animal lectins

Protein and lipid glycosylation is no longer considered as a topic whose appeal is restricted to a limited number of analytical experts perseveringly pursuing the comprehensive cataloguing of structural variants. It is in fact arousing curiosity in various areas of basic and applied bioscience. Well founded by the conspicuous coding potential of the sugar part of cellular glycoconjugates which surpasses the storage capacity of oligonucleotide- or oligopeptide-based code systems, recognition of distinct oligosaccharide ligands by endogenous receptors, i.e. lectins and sugar-binding enzymes or antibodies, is increasingly being discovered to play salient roles in animal physiology. Having inevitably started with a descriptive stage, research on animal lectins has now undubitably reached maturity. Besides listing the current categories for lectin classification and providing presentations of the individual families and their presently delineated physiological significance, this review places special emphasis on tracing common structural and functional themes which appear to reverberate in nominally separated lectin and animal categories as well as lines of research which may come to fruition for medical sciences.

Structural analysis of monosaccharide recognition by rat liver mannose-binding protein

The structural basis of carbohydrate recognition by rat liver mannose-binding protein (MBP-C) has been explored by determining the three-dimensional structure of the C-type carbohydrate-recognition domain (CRD) of MBP-C using x-ray crystallography. The structure was solved by molecular replacement using rat serum mannose-binding protein (MBP-A) as a search model and was refined to maximum Bragg spacings of 1.7 A. Despite their almost identical folds, the dimeric structures formed by the two MBP CRDs differ dramatically. Complexes of MBP-C with methyl glycosides of mannose, N-acetylglucosamine, and fucose were prepared by soaking MBP-C crystals in solutions containing these sugars. Surprisingly, the pyranose ring of mannose is rotated 180 degrees relative to the orientation observed previously in MBP-A, but the local interactions between sugar and protein are preserved. For each of the bound sugars, vicinal, equatorial hydroxyl groups equivalent to the 3- and 4-OH groups of mannose directly coordinate Ca2+ and form hydrogen bonds with residues also serving as Ca2+ ligands. Few interactions are observed between other parts of the sugar and the protein. A complex formed between free galactose and MBP-C reveals a similar mode of binding, with the anomeric hydroxyl group serving as one of the Ca2+ ligands. A second binding site for mannose has also been observed in one of two copies in the asymmetric unit at a sugar concentration of 1.3 M. These structures explain how MBPs recognize a wide range of monosaccharides and suggest how fine specificity differences between MBP-A and MBP-C may be achieved.

Localization and characterization of the carbohydrate-binding site of the porcine lymphocyte mannan-binding protein

Mannan-binding proteins found in the liver and serum of several vertebrate species are supposed to play an important role in the intracellular transport of glycoproteins, as well as in several protective reactions including complement activation and elimination of various pathogens. To study these protective functions at molecular level it is necessary to understand the fine oligosaccharide specificity and mutual relation among various forms of these soluble lectins. We have isolated mannan-binding protein as peripheral membrane proteins of porcine lymphocytes. This lectin was purified to homogeneity and shown to possess many properties in common with the well studied rat liver proteins (mol. mass, subunit composition and general organization of the molecule). Binding studies performed with three series of defined oligosaccharides (high mannose, hybrid type, and complex) on native lectin molecules as well as isolated carbohydrate-binding domains revealed distinctive features of this mannan-binding protein, including its impaired ability to bind the oligosaccharide ligand after reduction and decyclization at core N-acetyl-D-glucosamine 1.

Lectin localization in human nerve by biochemically defined lectin-binding glycoproteins, neoglycoprotein and lectin-specific antibody

Molecular recognition can be mediated by protein (lectin)-carbohydrate interaction, explaining the interest in this topic. Plant lectins and, more recently, chemically glycosylated neoglycoproteins principally allow to map the occurrence of components of this putative recognition system. Labelled endogenous lectins and the lectin-binding ligands can add to the panel of glycohistochemical tools. They may be helpful to derive physiologically valid conclusions in this field for mammalian tissues. Consequently, experiments were prompted to employ the abundant beta-galactoside-specific lectin of human nerves in affinity chromatography and in histochemistry to purify and to localize its specific glycoprotein ligands. In comparison to the beta-galactoside-specific plant lectins from Ricinus communis and Erythrina cristagalli, notable similarities were especially detectable in the respective profiles of the mammalian and the Erythrina lectin. They appear to account for rather indistinguishable staining patterns in fixed tissue sections. Inhibitory controls within affinity chromatography, within solid-phase assays for each fraction of lectin-binding glycoproteins and within histochemistry as well as the demonstration of crossreactivity of the three fractions of lectin-binding glycoproteins with the biotinylated Erythrina lectin in blotting ascertained the specificity of the lectin-glycoprotein interaction. In addition to monitoring the accessible cellular ligand part by the endogenous lectin as probe, the comparison of immunohistochemical and glycohistochemical detection of the lectin in serial sections proved these methods for receptor analysis to be rather equally effective. The observation that the biotinylated lectin-binding glycoproteins are also appropriate ligands in glycohistochemical analysis warrants emphasis.(ABSTRACT TRUNCATED AT 250 WORDS)

Structure and evolutionary origin of the gene encoding a human serum mannose-binding protein

The N-terminal sequence of the major human serum mannose-binding protein (MBP1) was shown to be identical at all positions determined with the amino acid sequence predicted from a cDNA clone of a human liver MBP mRNA. An oligonucleotide corresponding to part of the sequence of this cDNA clone was used to isolate a cosmid genomic clone containing a homologous gene. The intron/exon structure of this gene was found to closely resemble that of the gene encoding a rat liver MBP (MBP A). The nucleotide sequence of the exons differed in several places from that of the human cDNA clone published by Ezekowitz, Day & Herman [(1988) J. Exp. Med. 167, 1034-1046]. The MBP molecule comprises a signal peptide, a cysteine-rich domain, a collagen-like domain, a 'neck' region and a carbohydrate-binding domain. Each domain is encoded by a separate exon. This genomic organization lends support to the hypothesis that the gene arose during evolution by a process of exon shuffling. Several consensus sequences that may be involved in controlling the expression of human serum MBP have been identified in the promoter region of the gene. The consensus sequences are consistent with the suggestion that this mammalian serum lectin is regulated as an acute-phase protein synthesized by the liver.

Neoglycolipids as probes of oligosaccharide recognition by recombinant and natural mannose-binding proteins of the rat and man

Oligosaccharide recognition by three mammalian mannose-binding proteins was investigated by using as probes a series of structurally characterized neoglycolipids in t.l.c. binding assays. The neoglycolipids were derived from N-linked oligosaccharides of complex, high-mannose and hybrid types and from human milk oligosaccharides and simple di- and tri-saccharides. The three proteins, namely the recombinant carbohydrate-recognition domain of rat mannose-binding Protein A and the multi-subunit forms of rat and human serum mannose-binding proteins, were shown to have in common reactivity with oligosaccharide probes containing one or more non-reducing terminal N-acetylglucosamine residue(s). Substitution with galactose masks reactivity. The three proteins also bound to non-reducing terminal mannose residues in high-mannose-type oligosaccharides, non-reducing terminal fucose residues in the sequence Fuc alpha 1-4(Gal beta 1-3)GlcNAc and non-reducing terminal glucose residues in dextran oligomers; the recombinant binding domain gave consistently weaker binding. The relative reactivities with the various probes differ for each protein. Overall, the reaction patterns of the three mammalian proteins differ from that of the plant lectin concanavalin A, which showed preferential binding to the high-mannose type, weak binding to biantennary complex type and no binding to the fuco-oligosaccharide and simple oligosaccharide probes. As a group, the three mammalian proteins resemble bovine serum conglutinin and behave as lectins with rather broad sugar specificities directed at certain non-reducing terminal N-acetylglucosamine, mannose, glucose and fucose residues, but with subtle differences in fine specificities. These results illustrate the potential of neoglycolipids in studies of oligosaccharide recognition by natural and recombinant proteins of diverse biological systems.