Evidence that the 14 kDa soluble beta-galactoside-binding lectin in man is encoded by a single gene

Structural Basis Underlying the Binding Preference of Human Galectins-1, -3 and -7 for Galβ1-3/4GlcNAc

Galectins represent β-galactoside-binding proteins and are known to bind Galβ1-3/4GlcNAc disaccharides (abbreviated as LN1 and LN2, respectively). Despite high sequence and structural homology shared by the carbohydrate recognition domain (CRD) of all galectin members, how each galectin displays different sugar-binding specificity still remains ambiguous. Herein we provided the first structural evidence of human galectins-1, 3-CRD and 7 in complex with LN1. Galectins-1 and 3 were shown to have higher affinity for LN2 than for LN1, while galectin-7 displayed the reversed specificity. In comparison with the previous LN2-complexed structures, the results indicated that the average glycosidic torsion angle of galectin-bound LN1 (ψ^LN1 ≈ 135°) was significantly differed from that of galectin-bound LN2 (ψ^LN2 ≈ -108°), i.e. the GlcNAc moiety adopted a different orientation to maintain essential interactions. Furthermore, we also identified an Arg-Asp/Glu-Glu-Arg salt-bridge network and the corresponding loop (to position the second Asp/Glu residue) critical for the LN1/2-binding preference.

Copy-number variation of functional galectin genes: studying animal galectin-7 (p53-induced gene 1 in man) and tandem-repeat-type galectins-4 and -9

Galectins are potent adhesion/growth-regulatory effectors with characteristic expression profiles. Understanding the molecular basis of gene regulation in each case requires detailed information on copy number of genes and sequence(s) of their promoter(s). Our report reveals plasticity in this respect between galectins and species. We here describe occurrence of a two-gene constellation for human galectin (Gal)-7 and define current extent of promoter-sequence divergence. Interestingly, cross-species genome analyses also detected single-copy display. Because the regulatory potential will then be different, extrapolations of expression profiles are precluded between respective species pairs. Gal-4 coding in chromosomal vicinity was found to be confined to one gene, whereas copy-number variation also applied to Gal-9. The example of rat Gal-9 teaches the lesson that the presence of multiple bands in Southern blotting despite a single-copy gene constellation is attributable to two pseudogenes. The documented copy-number variability should thus be taken into consideration when studying regulation of galectin genes, in a species and in comparison between species.

Detection of ligand- and solvent-induced shape alterations of cell-growth-regulatory human lectin galectin-1 in solution by small angle neutron and x-ray scattering

The bioactivity of galectin-1 in cell growth regulation and adhesion prompted us to answer the questions whether ligand presence and a shift to an aprotic solvent typical for bioaffinity chromatography might alter the shape of the homodimeric human lectin in solution. We used small angle neutron and synchrotron x-ray scattering studies for this purpose. Upon ligand accommodation, the radius of gyration of human galectin-1 decreased from 19.1 +/- 0.1 A in the absence of ligand to 18.2 +/- 0.1 A. In the aprotic solvent dimethyl sulfoxide, which did not impair binding capacity, galectin-1 formed dimers of a dimer, yielding tetramers with a cylindrical shape. Intriguingly, no dissociation into subunits occurred. In parallel, NMR monitoring was performed. The spectral resolution was in accord with these data. In contrast to the properties of the human protein, a nonhomologous agglutinin from mistletoe sharing galactose specificity was subject to a reduction in the radius of gyration from approximately 62 A in water to 48.7 A in dimethyl sulfoxide. Evidently, the solvent caused opposite responses in the two tested galactoside-binding lectins with different folding patterns. We have hereby proven that ligand presence and an aprotic solvent significantly affect the shape of galectin-1 in solution.

Partial identification by site-directed mutagenesis of a cell growth inhibitory site on the human galectin-1 molecule

BACKGROUND

Previous work, by us and others, has shown that mammalian galectins-1 have a growth-inhibitory activity for mammalian cells which is apparently independent of their beta-galactoside binding site.

RESULTS

We have made recombinant human galectin-1 as a bacterial fusion protein with an N-terminal hexahistidine tag. This protein displays both haemagglutination and growth-inhibitory activities, even in the presence of the hexahistidine tag. Site-directed mutagenesis of this protein has confirmed the independent nature of the protein sites responsible for the two biological activities. Mutant proteins were created, which displayed each activity in the absence of the other.

CONCLUSIONS

Human galectin-1 possesses a growth-inhibitory site, which is not part of the beta-galactoside binding site. A surface loop, comprising amino acid residues 25-30, and joining two internal beta-strands, forms part of the growth-inhibitory site. This region is relatively close to the N-terminus of the protein, and N-terminal substitutions or extensions also affect growth-inhibitory activity. Further experiments will be necessary to fully define this site.

Oxidized galectin-1 promotes axonal regeneration in peripheral nerves but does not possess lectin properties

Galectin-1 has recently been identified as a factor that regulates initial axonal growth in peripheral nerves after axotomy. Although galectin-1 is a well-known beta-galactoside-binding lectin, its potential to promote axonal regeneration as a lectin has not been reported. It is essential that the process of initial repair in peripheral nerves after axotomy is well clarified. We therefore undertook to investigate the relation between the structure and axonal regeneration-promoting activity of galectin-1. Recombinant human galectin-1 secreted into the culture supernatant of transfected COS1 cells (rhGAL-1/COS1) was purified under nonreducing conditions and subjected to structural analysis. Mass spectrometric analysis of peptide fragments from rhGAL-1/COS1 revealed that the secreted protein exists as an oxidized form containing three intramolecular disulfide bonds (Cys2-Cys130, Cys16-Cys88 and Cys42-Cys60). Recombinant human galectin-1 (rhGAL-1) and a galectin-1 mutant in which all six cysteine residues were replaced by serine (CSGAL-1) were expressed in and purified from Escherichia coli for further analysis; the purified rhGAL-1 was subjected to oxidation, which induced the same pattern of disulfide linkages as that observed in rhGAL-1/COS1. Oxidized rhGAL-1 enhanced axonal regeneration from the transected nerve sites of adult rat dorsal root ganglion explants with associated nerve stumps (5.0-5000 pg. mL-1), but it lacked lectin activity. In contrast, CSGAL-1 induced hemagglutination of rabbit erythrocytes but lacked axonal regeneration-promoting activity. These results indicate that galectin-1 promotes axonal regeneration only in the oxidized form containing three intramolecular disulfide bonds, not in the reduced form which exhibits lectin activity.

Sarcolectin (SCL): structure and expression of the recombinant molecule

Interferons (IFNs) are major cytokines, responsible for down-regulating cell growth and for promoting cell differentiation. The sarcolectin (SCL) protein presented here blocks in the cells the established IFN-dependent interphase and stimulates DNA synthesis, probably in co-ordination with more specific growth factors or hormones. The SCL-DNA structure is closely related to that of cytokeratine K2C7 intermediate filaments, but the SCL is a monomer, or sometimes a dimer, which is excreted into the serum, where it is frequently bound to albumin. Its specific biological functions are carried by the beta sheets, and can be found on the two terminal domains of the molecule, the lectinic properties being located mainly on the N-terminus. The recombinant SCL molecule possesses the same biological functions as the native one, since it inhibits the IFN-dependent antiviral state both in human and in mouse cell cultures. On the contrary, antibodies raised against amino acids 41-55 located on the N-terminal domain of SCL inhibit this antagonistic effect. We postulate that the IFN and SCL proteins, because of their opposite biological functions, are in balance and are part of a feedback system operating the regulation of normal growth. In pathological cases, SCL could play a role in the development of tumors, as we have found in juvenile osteosarcomas or in AIDS cases.

Fungal galectins, sequence and specificity of two isolectins from Coprinus cinereus

Galectins are members of a genetically related family of beta-galactoside-binding lectins. At least eight distinct mammalian galectins have been identified. More distantly related, but still conserving amino acid residues critical for carbohydrate-binding, are galectins in chicken, eel, frog, nematode, and sponge. Here we report that galectins are also expressed in a species of fungus, the inky cap mushroom, Coprinus cinereus. Two dimeric galectins are expressed during fruiting body formation which are 83% identical to each other in amino acid sequence and conserve all key residues shared by members of the galectin family. Unlike most galectins, these have no N-terminal post-translational modification and no cysteine residues. We expressed one of these as a recombinant protein and studied its carbohydrate-binding specificity using a novel nonradioactive assay. Binding specificity has been well studied for a number of other galectins, and like many of these, the recombinant C. cinereus galectin shows particular affinity for blood group A structures. These results demonstrate not only that the galectin gene family is evolutionarily much older than previously realized but also that fine specificity for complex saccharide structures has been conserved. Such conservation implies that galectins evolved to perform very basic cellular functions, presumably by interaction with glycoconjugates bearing complex lactoside carbohydrates resembling blood group A.

Biphasic modulation of cell growth by recombinant human galectin-1

Human soluble galactose-binding lectin (galectin-1) has been expressed as an Escherichia coli fusion protein, following the amplification by polymerase chain reaction of cDNA prepared from a human osteosarcoma cell line. The fusion protein is a functional beta-galactoside-binding lectin, as is the recombinant galectin when purified from the cleaved fusion protein. The recombinant galectin has a biphasic effect on cell proliferation. Unlike the fusion protein, it functions as a human cell growth inhibitor, confirming earlier findings with natural human galectin-1, though it is less effective than the natural galectin. This reaction is not significantly inhibited by lactose, and is thus largely independent of the beta-galactoside-binding site. At lower concentrations, recombinant galectin-1 is mitogenic, this activity being susceptible to inhibition by lactose, and thus attributable to the beta-galactoside-binding ability of the protein. Some tumour cells are susceptible to the growth-inhibitory effect, and the galectin-1 gene is expressed in both normal and tumour cells.

Further evidence by site-directed mutagenesis that conserved hydrophilic residues form a carbohydrate-binding site of human galectin-1

To identify critical amino acid residues for carbohydrate binding of galectins (soluble beta-galactoside-binding lectins found in the animal kingdom). site-directed mutagenesis was performed on human galectin-1. On the basis of the previous results (Hirabayashi and Kasai (1992) J Biol Chem 266:23648-53), more systematic mutagenesis experiments were performed in order to confirm the concept that conserved hydrophilic residues play a central role. When a homologous substitution was made for highly conserved His44, Arg48 or Asn61, the resultant mutant (H44Q, R48H or N61D, respectively) almost completely lacked carbohydrate-binding ability, as found previously for Asn46, Glu71 and Arg73 mutants. This suggests these six hydrophilic residues are essential. On the other hand, when less conserved Lys63, Arg111 or Asp125 were substituted, the resultant mutant (K63H, R111H or D125E, respectively) retained almost the same affinities to asialofetuin and lactose as the wild-type galectin. Therefore, none of these residues is directly involved in the binding. These results, together with the previous observation that the above six essential residues are all encoded in the largest exon of the gene and are located close to each other in the central, most hydrophilic region of the protein, suggest that the residues form a carbohydrate-binding site of galectin.

The adhesive specificity of the soluble human lectin, IgE-binding protein, toward lipid-linked oligosaccharides. Presence of the blood group A, B, B-like, and H monosaccharides confers a binding activity to tetrasaccharide (lacto-N-tetraose and lacto-N-neotetraose) backbones

The immunoglobulin E-binding protein, epsilon BP (also known as CBP35, Mac-2, L-34, and L-29), is a beta-galactoside-binding protein of approximately 30 kDa and a member of the animal lectin family termed S-type or S-Lac. Multiple biological activities have been attributed to this lectin such as mediation of IgE binding to the surface of Langerhans cells and activation of mast cells through binding to the high affinity IgE receptor. In order to better understand the cell-binding activity and the proposed role for epsilon BP as a biological response modifier, we have studied the specificity of binding of the radioiodinated epsilon BP to a series of lipid-linked, structurally defined oligosaccharide sequences of the lacto/neolacto family. The results show that the minimum lipid-linked oligosaccharides that can support epsilon BP binding are pentasaccharides of the lacto/neolacto series and that the lectin binds more strongly to oligosaccharides of this family that bear the blood group A, B, or B-like determinants than to those bearing blood group H. This preferential binding of epsilon BP is also manifest with whole cells, as erythrocytes of blood groups A and B are more strongly bound by epsilon BP than those of blood group O. Blood group Le(a) and Le(x) sequences are not bound by the lectin.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of a 14-kDa laminin binding protein (HLBP14) in human melanoma cells that is identical to the 14-kDa galactoside binding lectin

The carbohydrate moieties present on laminin play a crucial role in the multiple biological activities of this basement membrane glycoprotein. We report the identification of a human laminin binding protein with an apparent molecular mass of 14 kDa on sodium dodecyl sulfate-polyacrylamide gels that was found, after purification and amino acid microsequencing, to be identical to the previously described 14-kDa galactoside binding soluble L-14 lectin. We have designated this human laminin binding protein as HLBP14. HLBP14 was purified from human melanoma cells in culture by laminin affinity chromatography and gel electroelution. We demonstrate that HLBP14 binds specifically to the poly-N-acetyllactosamine residues of murine laminin and does not bind to other glycoproteins that do not contain such structures, such as fibronectin. HLBP14 was eluted from a murine laminin column by lactose, N-acetyllactosamine, and galactose but not by other control saccharides, including glucose, fucose, mannose, and melibiose. It did not bind to laminin treated with endo-beta-galactosidase. Lactose also eluted HLBP14 off a human laminin affinity column, implying that human laminin also contains poly-N-acetyllactosamine residues. On immunoblots, polyclonal antibodies raised against HLBP14 recognized HLBP14 as well as 31- and 67-kDa molecules that are also laminin binding proteins, indicating that these proteins share common epitopes. L-14, a dimeric lactose binding lectin, is expressed in a wide variety of tissues. Although the expression of this molecule has been linked to a variety of biological events, the elucidation of its specific functions has been elusive. The observation that HLBP14, a human cancer cell laminin binding protein, is identical to L-14 strongly suggests that the functions attributed to this lectin could be mediated, at least in part, through its ability to interact with the poly-N-acetyllactosamine residues of laminin. HLBP14 could potentially play a role during tumor invasion and metastasis by modulating the interactions between cancer cells and laminin.

Electrophoretic study of conformational changes of a human soluble beta-D-galactoside-binding lectin upon storage

Human brain lectin (HBL), a beta-galactoside specific soluble lectin, was purified by affinity chromatography. An alkylated derivative of this lectin was also prepared. Both native and modified molecules were conserved at -20 degrees C in the presence or absence of beta-mercaptoethanol, a reducing agent which was described to maintain the lectin activity in vitro or in the presence of beta-mercaptoethanol and lactose. The impact of storage conditions, over one year, on the native and derivated lectins, was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, isoelectric focusing and titration curve, using the PhastSystem (Pharmacia). Western-blot analysis using an anti-HBL antibody and size-exclusion high performance liquid chromatography were used to complete the study. The subunit M(r)s were estimated before freezing (T0) and after three and twelve months (T3, T12). They were comparable for all preparations. In all samples tested, isoelectric focusing demonstrated the existence of at least three acidic proteins, with the pI ranging between 4.7-4.9. Titration curves clearly showed pH-dependent conformational changes, resulting in a panel of differently charged molecular species, some of which may be related to different oxidative states of the cysteine residues. We concluded that lectin can be stored at -20 degrees C for at least one year before use as a reagent since the modifications revealed by electrophoretic analysis do not alter the hemagglutination activity and carbohydrate binding properties. The immunoreactivity also remained unchanged.

Localization of endogenous beta-galactoside-binding lectin as a means to distinguish malignant from benign skin tissue

The immunolocalization of a 30-kd endogenous lectin, referred to as galaptin, was studied in various skin tumor types. Basal cell carcinomas expressed little or no galaptin, whereas nonmalignant basaloid cells, squamous cell carcinomas, melanoma, nevi, and stroma showed more prominent galaptin immunostaining. Therefore, the immunodetection of galaptin may aid the dermatopathologist in difficult histologic diagnosis.

Structure and expression of the negative growth factor mouse beta-galactoside binding protein gene

Following the identification of murine beta-galactoside binding protein (mGBP) as an autocrine negative growth factor we have now isolated and characterized the genomic region spanning the mGBP gene and have determined the 5' end of the transcript by primer extension, S1 mapping and mRNA sequence. The gene is found to be contained within 4 kilobases and composed of four exons of 79, 80, 171 and 197 nucleotides separated by three introns of 1200, 1600 and 193 nucleotides. The DNA region upstream of the 5' end of the transcript contains canonical sequences for eukaryotic promoter elements including CAT and TATA boxes and several DNA motifs for potential transcription regulation. The gene is differentially expressed in a variety of normal tissues.

Identification of an autocrine negative growth factor: mouse beta-galactoside-binding protein is a cytostatic factor and cell growth regulator

Murine beta-galactoside-binding protein, a protein classified as a soluble lectin, is shown to be a cell growth-regulatory molecule and a cytostatic factor. The growth-inhibitory effect is not related to lectin properties, and competition assays indicate that the protein binds to specific cell surface receptors with high affinity. It exerts control in G0 and at G2, both as a regulator of cell replication and as a cytostatic factor.

Evidence for export of a muscle lectin from cytosol to extracellular matrix and for a novel secretory mechanism

A soluble lactose-binding lectin with subunit Mr of 14,500 is believed to function by interacting with extracellular glycoconjugates, because it has been detected extracellularly by immunohistochemistry. This localization has been questioned, however, since the lectin lacks a secretion signal sequence, which challenges the contention that it is secreted. We have demonstrated externalization of this lectin from C2 mouse muscle cells by both immunoprecipitation of metabolically labeled protein and immunohistochemical localization. We further show that externalization of the lectin is a developmentally regulated process that accompanies myoblast differentiation and that the lectin codistributes with laminin in myotube extracellular matrix. Immunohistochemical localization during intermediate stages of externalization suggests that the lectin becomes concentrated in evaginations of plasma membrane, which pinch off to form labile lectin-rich extracellular vesicles. This suggests a possible mechanism for lectin export from the cytosol to the extracellular matrix.

Soluble galactoside-binding vertebrate lectins: a protein family with common properties

1. Soluble galactoside-binding lectins could play a key role in vertebrates by specifical binding to complementary glycoconjugates. 2. Their expression and localization are developmentally regulated. 3. They constitute a large family of structurally related proteins which contain a series of conserved aminoacids. 4. Their functional role could vary from an organ to another, and the same lectin may probably mediate several biological activities.