Human liver lysosomal alpha-glucosidase modified by chemical galactosylation and isolation of specific antibodies

Synthesis of soybean agglutinin in bacterial and mammalian cells

The cDNA of soybean agglutinin (SBA), a glycoprotein lectin, obtained from the mRNA of soybean seeds at mid-maturation, was cloned in a lambda gt 10 phage and subcloned in a pUC-8 plasmid. Probing with a fragment of the lectin gene [Vodkin, L. O., Rhodes, P. R. & Goldberg, R. B. (1983) Cell 34, 1023-1031] afforded a clone of 1012 nucleotides containing the complete coding region of 858 nucleotides for the precursor to soybean agglutinin. The deduced amino acid sequence contains the 253 residues of the mature lectin and an hydrophobic N-terminal signal peptide of 32 amino acids. Expression in Escherichia coli of the cDNA coding for the precursor to the lectin or for the mature lectin led to the accumulation of large quantities of inclusion bodies, from which mature SBA was isolated in small yield (up to 1 mg/l). It was identical with the native lectin in the hemagglutinating activity and carbohydrate specificity, N-terminal sequence and oligomeric structure, but, because it was not glycosylated, its subunit mass was lower by 2 kDa. Our findings show that pre-SBA is processed into the mature form in the bacteria, and that, contrary to what has been suggested [Nagai, K. & Yamaguchi, H. (1993) J. Biochem. (Tokyo) 113, 123-125], glycosylation is not essential for the folding of the lectin, nor for its subunit assembly into a biologically active tetramer. To obtain recombinant SBA in secreted form, the pre-SBA cDNA was subcloned in pTM1 vector and the construct inserted into vaccinia virus. When monkey BS-C-1 cells were infected by the virus, using a double expression protocol, recombinant lectin was secreted into the growth medium, from which it was isolated by immunoaffinity chromatography at a yield similar to that from the bacteria. Except for its lower hemagglutinating activity, the product was indistinguishable from native SBA in all properties tested. It was also susceptible to digestion by endo-beta-N-acetylglucosaminidase H or N-glycanase which caused a decrease of 2 kDa in its subunit mass and gave the same results on lectin blot analysis, indicating that it too is a glycoprotein with a single oligomannose unit.

Mutational studies of the amino acid residues in the combining site of Erythrina corallodendron lectin

High-resolution X-ray crystallography of the complex of the Gal/GalNAc-specific Erythrina corallodendron lectin with lactose identified the amino acid side chains that form contacts with the galactose moiety of the disaccharide. The contribution of these amino acids to the binding of different monosaccharides and oligosaccharides by the lectin was examined by site-directed mutagenesis. Replacement of Phe131, on which the galactose is stacked, by tyrosine, gave a mutant with the same hemagglutinating activity and carbohydrate specificity as the parent lectin, but replacement by alanine or valine resulted in loss of activity. Mutations of Ala88, Asp89, and Asn133 produced mutants that were also inactive whereas those of the other combining site residues, Tyr106, Ala218, and Gln219, were biologically active. None of the active mutants interacted with mannose or glucose. Thus, contrary to an earlier assumption. Ala218 is not responsible for the inability of E. corallodendron lectin to bind these sugars. Our findings also demonstrate that Gln219 is not involved in galactose binding in solution, even though this is implicated by the crystal data. Instead, our data suggest that Gln219 assists in the ligation of N-acetyllactosamine to the lectin, by interacting with the acetamide group of the disaccharide. Comparison with other legume lectins specific for mannose/glucose, galactose, N-acetylgalactosamine, L-fucose or N-acetylglucosamine, shows that only three of the combining site residues of E. corallodendron lectin occupy invariant positions both in their primary and tertiary structures. These residues are an aspartic acid and an asparagine corresponding to positions 89 and 133, respectively, in E. corallodendron lectin, and an aromatic residue, either phenylalanine (as Phe131 in this lectin), tyrosine or tryptophan. We therefore postulate that these three residues are essential for ligand binding by all such lectins, irrespective of their specificity.

Adhesion of human lymphoid cell lines to immobilized carbohydrates and to bone-marrow stromal cell layers by surface sugar receptors

Cell-surface sugar receptors may participate in interactions of lymphoid cells that influence their adhesive properties and proliferation. Their expression on cells of the pre-B line BLIN-I, the B-lymphoblastoid line Croco II, the myeloma line RPMI 8226 and the T-lymphoblastoid line CCRF-CEM was monitored with a panel of 14 types of chemically glycosylated E. coli beta-galactosidase at a non-saturating ligand concentration. Quantitative differences were determined for the capacity of the different cell types to bind constituents of the carbohydrate part of glycoconjugates. They were corroborated by analyses of binding for lactose-, beta-N-acetylgalactosamine-, beta-N-acetylglucosamine- and fucose-exposing neoglycoenzymes up to saturation levels. Values of dissociation constants of the tetrameric enzyme were in the range of 3-300 nM. Several types of sugar receptor led to carbohydrate-inhibitable adhesion of cells to 6 types of nitrocellulose-immobilized neoglycoprotein, their effectiveness being most obvious for the myeloma cells. Analyses of the carbohydrate-ligand-mediated adhesion of the other cell types revealed a comparatively decreased response. Only a few carbohydrates among the 7 types tested were effective in reducing cell adhesion to a far more complex ligand-bearing matrix than immobilized neoglycoproteins, namely bone-marrow stromal cell layers: sialic acid and N-acetylgalactosamine for B-lymphoblastoid cells and rhamnose for pre-B cells. These cellular interactions may encompass sugar receptors on the stromal cells and other types of molecular recognition in addition to the detected activities on the lymphoid cells.