Elongation of N-acetyllactosamine repeats in diantennary oligosaccharides

Glycosylation profile of integrin α3β1 changes with melanoma progression

Glycosylation of integrins has been implicated in the modulation of their function. Characterisation of carbohydrate moieties of alpha(3) and beta(1) subunits from non-metastatic (WM35) and metastatic (A375) human melanoma cell lines was carried out on peptide-N-glycosidase F-released glycans using matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). beta(1) integrin subunit from both cell lines displayed tri- and tetraantennary oligosaccharides complex type glycans, but only in A375 cell line was the sialylated tetraantennary complex type glycan (Hex(7)HexNAc(6)FucSia(4)) present. In contrast, only alpha(3) subunit from metastatic cells possessed beta1-6 branched structures. Our data indicate that the beta(1) and alpha(3) subunits expressed by the metastatic A375 cell line carry beta1-6 branched structures, suggesting that these cancer-associated glycan chains may modulate tumor cell adhesion by affecting the ligand binding properties of alpha(3)beta(1) integrin. In direct ligand binding assays, alpha(3)beta(1) integrin from both cell lines binds strongly to fibronectin and to much lesser degree to placental laminin. No binding to collagen IV was observed. Enzymatic removal of sialic acid residues from purified alpha(3)beta(1) integrin stimulates its adhesion to all examined ECM proteins. Our data suggest that the glycosylation profile of alpha(3)beta(1) integrin in human melanoma cells correlates with the acquisition of invasive capacity during melanoma progression.

Synthesis of glyceroyl beta-N-acetyllactosaminide and its derivatives through a condensation reaction by cellulase

A condensation reaction between N-acetyllactosamine and glycerol was directly catalyzed by using a commercially available cellulase preparation from Trichoderma reesei. 1-O-beta-N-Acetyllactosaminyl-(R, S)-glycerols (1) were readily synthesized in a 5% yield based on the N-acetyllactosamine added and conveniently isolated by two-step column chromatographies. The use of a partially purified enzyme increased 2.3-fold the yield of 1, compared to that of the crude enzyme containing beta-D-galactosidase activity. When various alkanols (n:2-4) were used in the condensation reaction, the corresponding alkyl beta-N-acetyllactosaminides were obtained in yields of 0.3-1.1% of the desired compounds.

The components of the Saccharomyces cerevisiae mannosyltransferase complex M-Pol I have distinct functions in mannan synthesis

The yeast Saccharomyces cerevisiae processes N-linked glycans in the Golgi apparatus in two different ways. Whereas most of the proteins of internal membranes receive a simple core-type structure, a long branched polymer termed mannan is attached to the glycans of many of the proteins destined for the cell wall. The first step in mannan synthesis is the initiation and extension of an alpha-1,6-linked polymannose backbone. This requires the sequential action of two enzyme complexes, mannan polymerases (M-Pol) I and II. M-Pol I contains the proteins Mnn9p and Van1p, although the stoichiometry and individual contributions to enzyme action are unclear. We report here that the two proteins are each present as a single copy in the complex. Both proteins contain a DXD motif found in the active site of many glycosyltransferases, and mutations in this motif in Mnn9p or Van1p reveal that both proteins contribute to mannose polymerization. However, the effects of these mutations on both the in vivo and in vitro activity are distinct, suggesting that the two proteins may have different roles in the complex. Finally, we show that a simple glycoprotein based on hen egg lysozyme can be used as a substrate for modification by purified M-Pol I in vitro.

Plasmid vectors with a 5'-hybrid intron facilitate high-level glycoprotein expression in CHO-cells

For biosynthesis of recombinant glycoproteins with specified carbohydrate structures various Chinese hamster ovary (CHO) cell lines are available that express different sets of glycosyl transferases. To examine various forms of glycosylated lysozyme we prepared a vector that directs the synthesis of the recombinant glycoprotein at a high rate. We compared vectors with varied promoter and 5'-untranslated regions. The expression of cDNA of a glycosylated mutant lysozyme was examined under a control of the SV40 early and cytomegalovirus (CMV) promoters alone and in combination with a tripartite leader and a hybrid intervening sequence. We show that in this system a vector with the CMV promoter, the tripartite leader sequence and the intron, referred to as pMCI, is the best of the examined combinations. Using conventional tissue culturing of CHO cells stably transfected with this vector, we were able to isolate glycosylated lysozyme with a yield of 4.5 mg per liter of spent medium.

Genetic engineering of CHO cells producing human interferon-gamma by transfection of sialyltransferases

Natural human interferon-gamma (hIFN-gamma) contains mainly biantennary complex-type sugar chains. We previously remodeled the branch structures of N-glycans on hIFN-gamma in Chinese hamster ovary (CHO) cells by overexpressing UDP-N-acetylglucosamine: alpha1,6-D-mannoside beta1,6-N-acetylglucosaminyltransferase (GnT-V). Normal CHO cells primarily produced hIFN-gamma having biantennary sugar chains, whereas a CHO clone, designated IM4/Vh, transfected with GnT-V, primarily produced hIFN-gamma having GlcNAcbeta1-6 branched triantennary sugar chains when sialylation was incomplete and an increase in poly-N-acetyllactosamine (Galbeta1-4GlcNAcbeta1-3)n was observed. In the present study, we introduced mouse Galbeta1-3/4GlcNAc-R alpha2,3-sialyltransferase (ST3Gal IV) and/or rat Galbeta1-4GlcNAc-R alpha2,6-sialyltransferase (ST6Gal I) cDNAs into the IM4/Vh cells to increase the extent of sialylation and to examine the effect of sialyltransferase (ST) type on the linkage of sialic acid. Furthermore, we speculated that sialylation extent might affect the level of poly-N-acetyllactosamine. We isolated four clones expressing different levels of alpha2,3-ST and/or alpha2,6-ST. The extent of sialylation of hIFN-gamma from the IM4/Vh clone was 61.2%, which increased to about 80% in every ST transfectant. The increase occurred regardless of the type of overexpressed ST, and the proportion of alpha2,3- and alpha2,6-sialic acid corresponded to the activity ratio of alpha2,3-ST to alpha2,6-ST. Furthermore, the proportion of N-glycans containing poly-N-acetyllactosamine was significantly reduced (less than 10%) in the ST transfectants compared with the parental IM4/Vh clone (22.9%). These results indicated that genetic engineering of STs is highly effective for regulating the terminal structures of sugar chains on recombinant proteins in CHO cells.

Structural characterization of oligosaccharides in recombinant soluble human interferon receptor 2 using fluorophore-assisted carbohydrate electrophoresis

The N-linked oligosaccharide profiles (banding patterns in gels) and structures of recombinant soluble human interferon receptor 2 (r-shIFNAR2) were determined using fluorophore-assisted carbohydrate electrophoresis (FACE, Glyko, Novato, CA). The method involves releasing N-linked oligosaccharide moieties from a glycoprotein by digestion with peptide-N glycanase (PNGase F), labeling the released oligosaccharides with the fluorescent dye 8-aminonaphthalene-1,3,6-trisulfonate (ANTS), and separating the labeled oligosaccharides by gel electrophoresis. The isolated oligosaccharides in the bands from the profiling gels can then be sequenced using exoglycosidases to reveal the oligosaccharide structures. The oligosaccharide profile of r-shIFNAR2 consists of at least nine oligosaccharide bands. The relative amount of oligosaccharide in each band can vary, depending on the culture conditions of the source cells. FACE structural analysis shows that r-shIFNAR2 contains only core-fucosylated N-linked oligosaccharides, most of which are fully sialylated (approximately 92%). The major types and relative amounts of the oligosaccharides from a representative sample are: disialylated, galactosylated, biantennary (15%); trisialylated, galactosylated, triantennary (19%), tetrasialylated, galactosylated, tetraantennary (30%), and N-acetyllactosamine-containing higher-order oligosaccharides including tri-, tetra-, and pentaantennary (28%). The remaining oligosaccharides are not fully sialylated and/or not fully galactosylated di-, tri-, and tetraantennary structures (approximately 5%) and unidentified structures (approximately 3%). A method for determining the types and structures of the N-acetyllactosamine containing oligosaccharides is also reported in this study.

Microheterogeneity of Serum Glycoproteins in Patients with Chronic Alcohol Abuse Compared with Carbohydrate-deficient Glycoprotein Syndrome Type I

Background: Chronic alcohol abuse alters the normal N-glycosylation of transferrin, producing the carbohydrate-deficient transferrin isoforms. This alteration could be similar to that present in patients with carbohydrate-deficient glycoprotein syndrome type 1 (CDG1). We thus compared the alterations of N-glycans present in patients with alcoholism and patients with CDG1.

Methods: The N-glycans of serum glycoproteins were compared in sera of patients with alcoholism, patients with CDG1, and controls by two-dimensional electrophoresis, neuraminidase, peptide:N-glycosidase F, and endoglycosidase F2 treatments. A specific antibody directed against the amino acid sequence surrounding the N-432 N-glycosylation site of transferrin was prepared (SZ-350 antibody).

Results: In patients with alcoholism, the abnormal transferrin and α1-antitrypsin isoforms were devoid of a variable number of entire N-glycan moieties and were identical with those present in CDG1. In the serum of patients with alcoholism, this finding was less pronounced than in CDG1. In contrast to CDG1, there was no decrease in clusterin or serum amyloid P in patients with alcoholism. The SZ-350 antibody recognized only transferrin isoforms with one or no N-glycan moieties.

Conclusion: Antibodies directed against specific N-glycosylation sites of glycoproteins could be useful for developing more specific immunochemical tests for the diagnosis of chronic alcohol abuse.

Lysosomal segregation of a mannose-rich glycoprotein imparted by the prosequence of myeloperoxidase

The role of the N-terminal sequence of myeloperoxidase in the intracellular targeting was examined by using glycosylated lysozyme as a reporter. A fusion protein was constructed in which the presequence residues-18 through -6 of the lysozyme moiety had been replaced by residues 1-158 of prepromyeloperoxidase. Expression of the fusion protein in Chinese hamster ovary cells demonstrated its partial secretion and partial intracellular retention. The latter was accompanied by trimming the myeloperoxidase prosequence off the lysozyme moiety. The rate of the retention of the lysozyme fusion protein was higher than that of glycosylated lysozyme that had been expressed in cells transfected with cDNA of glycosylated lysozyme. The retention was insensitive to NH4Cl. In the secreted protein, lysozyme contained predominantly complex oligosaccharides as demonstrated by a proteolytic fragmentation in vitro and resistance to endo-beta-N-acetylglucosaminidase H. In contrast, when targeted to lysosomes, the lysozyme moiety of the fusion protein contained predominantly mannose-rich oligosaccharides. In baby hamster kidney cells, the trimming of the oligosaccharides in the lysozyme fragment was less vigorous, and a selective targeting of molecules bearing mannose-rich oligosaccharides to lysosomes was more apparent than in Chinese hamster ovary cells. In the presence of monensin, the formation of complex oligosaccharides in the fusion protein and its secretion were strongly inhibited, whereas the intracellular fragmentation was not. We suggest that the prosequence of myeloperoxidase participates in the intracellular routing of the precursor and that this routing operates on precursors bearing mannose-rich rather than terminally glycosylated oligosaccharides and diverts them from the secretory pathway at a site proximal to the monensin-sensitive compartment of the Golgi apparatus.

Increased elongation of N-acetyllactosamine repeats in doubly glycosylated lysozyme with a particular spacing of the glycosylation sites

Lysozyme is an example of an extensively studied secretory enzyme. Glycosylated mutant human lysozyme has been used as a model in studies on the biosynthesis of N-acetyllactosamine repeats in N-linked oligosaccharides. We examined the biosynthesis of the repeats in two doubly glycosylated mutants and describe here a rapid purification and separation of singly and doubly glycosylated molecules. In one of the mutants, the elongation of the repeats is enhanced if the molecules are doubly glycosylated, but not if the carbohydrate is attached to either site individually. This enhancement is not seen in the other doubly glycosylated mutant. Since lysozyme is not structurally related to glycoproteins bearing carbohydrate with N-acetyllactosamine repeats, we propose that in multivalent substrates the synthesis of the repeats can be promoted by a proper spacing of the elongated carbohydrate antennae in addition to any role of the protein backbone.