[The biosynthesis of cerebral glycoproteins: studies on mitochondrial mannosyl transferase (author's transl)]

Study of O-glycan sialylation in C6 cultured glioma cells: evidence for post-translational regulation of a beta-galactoside alpha 2,3 sialyltransferase activity by N-glycosylation

We have studied the Gal beta 1-3GalNAc-R alpha 2,3 sialyltransferase from C6 glioma cells transferring Neu5Ac from CMP-Neu5Ac onto O-glycans of glycoproteins. Using synchronized C6 glioma cells, we showed that the alpha 2,3 sialyltransferase activity was inhibited by tunicamycin to a greater extend than DNA and protein biosynthesis suggesting inhibition of N-glycosylation of this enzyme. Additional demonstration of N-glycosylation of the alpha 2,3 sialytransferase was provided through ConA-Sepharose binding. Treatment of partially purified alpha 2,3 sialytransferase by peptide-N-glycosidase F showed a significative inhibition demonstrating that N-glycan moiety is required for complete activity of the C6 glioma cell alpha 2,3 sialyltransferase.

Effect of retinoic acid on two glycosyltransferase activities in C6 cultured glioma cells

1. Activity of two glycosyltransferases was studied in retinoic acid-treated C6 cultured glioma cells. 2. The beta-galactoside alpha 2,3-sialyltransferase transferring N-acetylneuramin onto the O-glycans residues of glycoproteins was activated up to twice after chronic treatment (from 24 to 96 hr) with all-trans retinoic acid. 3. No effect was observed for shorter treatments. 4. On the opposite, the N-glycan galactosyltransferase activity remained unchanged whatever the length of retinoic acid treatment was. 5. The activatory effect was not dependent on isomery, as all-trans and 13-cis retinoic acid isomers were both activators of the C6 glioma cell sialyltransferase. 6. Measurement of adhesion of retinoic acid-treated cells using labelled plasma membranes showed an enhancement of adhesion in correlation with enhancement of sialyltransferase activity.

Evidence for an O-glycan sialylation system in brain. Characterization of a beta-galactoside alpha 2,3-sialyltransferase from rat brain regulating the expression of an alpha-N-acetylgalactosaminide alpha 2,6-sialyltransferase activity

We present evidence for the existence in rat brain of several sialyltransferases able to sialylate sequentially asialofetuin. [14C]Sialylated glycans of asialofetuin were analyzed by gel filtration. Three types of [14C]sialylated glycans were synthesized: N-glycans and monosialylated and disialylated O-glycans. The varying effects of N-ethylmaleimide, lysophosphatidylcholine (lysoPtdCho) and trypsin, were helpful in the identification of these different sialyltransferases. One of them, selectively inhibited by N-ethylmaleimide, was identified as the Neu5Ac alpha 2----3Gal beta 1----3GalNAc-R:alpha 2----6 sialyltransferase previously described [Baubichon-Cortay, H., Serres-Guillaumond, M., Louisot, P. and Broquet, P. (1986) Carbohydr. Res. 149, 209-223]. This enzyme was responsible for the synthesis of disialylated O-glycans. LysoPtdCho and trypsin selectively inhibited the enzyme responsible for the synthesis of monosialylated O-glycan. N-ethylmaleimide, lysoPtdCho and trypsin did not inhibit Neu5Ac transfer onto N-glycans, giving evidence for three different molecular species. To identify the enzyme responsible for monosialylated O-glycan synthesis, we used another substrate: Gal beta 1----3GalNAc--protein obtained after galactosylation of desialylated ovine mucin by a GalNAc-R:beta 1----3 galactosyltransferase from porcine submaxillary gland. This acceptor was devoid of N-glycans and of NeuAc in alpha 2----3 linkages on the galactose residue. When using N-ethylmaleimide we obtained the synthesis of only one product, a monosialylated structure. After structural analysis by HPLC on SAX and SiNH2 columns, we identified this product as Neu5Ac alpha 2----3Gal beta 1----3GalNAc. The enzyme leading to synthesis of this monosialylated O-glycan was identified as a Gal beta 1----3GalNAc-R:alpha 2----3 sialyltransferase. When using lysoPtdCho and trypsin, sialylation was completely abolished, although the Neu5Ac alpha 2----3Gal beta 1----3GalNAc-R:alpha 2----6 sialyltransferase was not inhibited. We provided thus evidence for the interpendence between the two enzymes, the alpha 2----3 sialyltransferase regulates the alpha 2----6 sialyltransferase activity since it synthesizes the alpha 2----6 sialyltransferase substrate.

Different reactivity of two brain sialyltransferases towards sulfhydryl reagents. Evidence for a thiol group involved in the nucleotide-sugar binding site of the NeuAc alpha 2-3Gal beta 1-3GalNAc alpha(2-6)sialyltransferase

We have studied the amino-acid residues involved in the catalytic activity of two distinct brain sialyltransferases acting on fetuin and asialofetuin. These two enzymes were strongly inhibited by N-bromosuccinimide, a specific blocking reagent for tryptophan residues. This result suggests the involvement of such residues in the catalytic process of the two sialyltransferases. Furthermore, chemical modifications by various sulfhydryl reagents led to a strong inhibition of the fetuin sialyltransferase while the asialofetuin sialyltransferase was only slightly inhibited. For a more thorough understanding of the thiol inactivation mechanism of the fetuin sialyltransferase, we studied in more detail the reactivity of this enzyme with NEM (N-ethylmaleimide), an irreversible reagent. The time-dependent inactivation followed first-order kinetics and these kinetic data afforded presumptive evidence for the binding of 1 mol NEM per mol of enzyme. Only CMP-NeuAc protected the enzyme against NEM inactivation effectively. MnCl2 did not enhance the protective effect of CMP-NeuAc. The modifications of the fetuin sialyltransferase kinetic parameters by NEM showed a competitive mechanism between NEM and CMP-NeuAc. The results suggest the involvement of a sulfhydryl residue in or near the nucleotide-sugar binding site of the fetuin sialyltransferase (but we could not excluded that CMP-NeuAc binding may induce a change in conformation of the protein, leading to a decreased accessibility of this thiol group located near the nucleotide-sugar binding site). This SH group is essential to the enzyme activity, which is not the case for the asialofetuin sialyltransferase.

Different reactivity to lysophosphatidylcholine, DIDS and trypsin of two brain sialyltransferases specific for O-glycans: a consequence of their topography in the endoplasmic membranes

Some properties of two distinct rat brain sialyltransferases, acting on fetuin and asialofetuin, respectively, were investigated. These two membrane-bound enzymes were both strongly inhibited by charged phospholipids. Neutral phospholipids were without effect except lysophosphatidylcholine (lysoPC) which modulated these two enzymes in a different way. At 5 mM lysoPC, the fetuin sialyltransferase was solubilized and highly activated while the asialofetuin sialyltransferase was inhibited. Preincubation of brain microsomes with 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), known as a specific anion inhibitor and a non-penetrating probe, led to a moderate inhibition of the asialofetuin sialyltransferase just as in the case of the ovomucoid galactosyltransferase (used here as a marker for the luminal side of the Golgi membrane); under similar conditions, the fetuin sialyltransferase was strongly inhibited. In the presence of Triton X-100, which induced a disruption of membranes, all three enzymes were strongly inhibited by DIDS. Trypsin action on intact membranes showed that asialofetuin sialyltransferase, galactosyltransferase and fetuin sialyltransferase were all slightly inhibited. After membrane disruption by Triton X-100, the first two enzymes were completely inactivated by trypsin while the fetuin sialyltransferase was quite insensitive to trypsin treatment. From these data, we suggest that the fetuin sialyltransferase, accessible to DIDS, is an external enzyme, oriented closely towards the cytoplasmic side of the brain microsomal vesicles (endoplasmic and Golgi membranes), whereas the asialofetuin sialyltransferase is an internal enzyme, oriented in a similar manner to the galactosyltransferase. Moreover, the anion site (nucleotide sugar binding site) of the fetuin sialyltransferase must be different from its active site, as this enzyme, when solubilized, is strongly inhibited by DIDS while no degradation is observed in the presence of trypsin.

A brain sialyltransferase having a narrow specificity for O-glycosyl-linked oligosaccharide chains

The existence of a brain sialyltransferase catalyzing the specific transfer of NeuAc on native fetuin was demonstrated. This enzyme was not able to sialylate either asialofetuin or desialylated and nondesialylated orosomucoid, transferrin, and bovine submaxillary mucin. It required the presence of Mn2+ for optimal activity. Moreover, in fetuin, this activity was closely related to the proportion of NeuAc residues, but in liver tissue sialylation occurred only onto asialofetuin. In native fetuin, sialylation took place on O-glycan chains to give an O-disialyltetrasaccharidic structure. The Gal----GalNAc----protein was not an acceptor, but alpha-NeuAc-(2----3)-Gal----GalNAc----protein was, suggesting a specific transfer alpha-(2----6) to the GalNAc residue.

Galactosyltransferases: physical, chemical, and biological aspects

Galactosyltransferases (GTs) are one of the members of a family of enzymes called glycosyltransferases involved in the biosynthesis of complex carbohydrates. These enzymes catalyze the transfer of galactose from UDP-galactose to an acceptor (glycoprotein, glycolipid) containing terminal N-acetylglucosamine or N-acetylgalactosamine residue. GTs occur in soluble (milk, serum, effusions, etc.) and insoluble (membrane) forms. The GT activities on the outer surface of the cells have been correlated with a host of cellular interactions, including fertilization, cell migration, embryonic induction, chondrogenesis, contact inhibition of growth, cell adhesion, hemostasis, intestinal cell differentiation, and immune recognition. GTs have been purified to homogeneity using affinity chromatography. Most GTs are found active in the pH range 6 to 8 and at temperatures between 35 to 40 degrees C. Manganese is an essential co-factor for GT activity. Isoenzymes of GT have been recognized, especially in tumor tissues, malignant effusions, and sera of cancer patients using polyacrylamide gel electrophoresis in the presence and absence of SDS. Depending on the source of the enzyme, the molecular weights of GTs range between 40,000 to 80,000 daltons. Carcinoma-associated GT isoenzyme has been reported to have a higher molecular weight than the normal GT isoenzyme. Development of monoclonal antibody against the cancer-specific GT isoenzyme will provide help in the development of an immunoassay for the measurement of this isoenzyme in the sera and an aid in the radioimmunolocalization of the tumors in cancer patients.

Subcellular localisation of cerebral fucosyltransferase

GDP-fucose: asialofetuin fucosyltransferase from sheep brain was fractionated on a sucrose gradient into two activity peaks. Using purification on Ficoll adapted from the proposed method [(1980) J. Neurochem. 35, 281-296], double localisation of cerebral fucosyltransferase was confirmed and the subcellular active fractions identified as light microsomes and mitochondria.

Involvement of some amino acid residues in the enzymatic activity of solubilized cerebral fucosyltransferase

We have checked the effect of some chemical reagents specific for amino acid residues on the activity of a solubilized cerebral glycoprotein:fucosyltransferase. Diethylpyrocarbonate, 2,3-butanedione and tetranitromethane specific for histidyl, arginyl, and tyrosyl residues respectively, were strong inhibitors of the enzymatic activity This led us to conclude that these amino acid residues are "essential residues" in the cerebral fucosyltransferase activity.

Substrate specificity of cerebral GDP-fucose: glycoprotein fucosyltransferase

Solubilized sheep brain fucosyltransferase was shown to transfer fucose from GDP-fucose onto glycoprotein and glycopeptide acceptors, such as asialofetuin, asialotransferrin, their glycopeptides and glycopeptides from ovalbumin, but not on to monosaccharides and disaccharides such as galactose, N-acetylglucosamine and lactose. Competition studies between asialofetuin and glycopeptide V from ovalbumin provided evidence that both substrates compete for a common enzyme active site. The position of the fucosyl linkage was then investigated. Endo-beta-N-glucosaminidase D digestion of fucosylated and acetylated glycopeptide V showed that fucose is not linked to asparagine-linked N-acetylglucosamine. Hydrazinolysis and nitrous acid deamination performed on asialofetuin and glycopeptide V proved that fucose is not linked to external galactose or N-acetylglucosamine either. Thus we assume that fucose is linked to the oligomannochitobiosyl core of the glycan, and probably to the second N-acetylglucosamine.

[Mitochondrial N-acetylglucosaminyl transferase connected with viral infection]

A higher level of N-acetylglucosamine incorporation by proteinic and polyprenic endogenous acceptors is observed after infection by Myxovirus. This phenomenon occurs in whole mitochondria as in outer mitochondrial membranes, where it is particularly obvious with proteinic endogenous acceptors. Under viral infection, no new N-acetylglucosaminylated polyprenol is detected. In the case of infected animals as in the case of control animals, compounds P1 (extracted by chloroform/methanol 2:1) are identified by thin layer chromatography as a N-acetylglucosamyl-pyrophosphoryl-dolichol and a N, N'-diacetylchitobiosyl-pyrophosphoryl-dolichol. In the case of infected animals, biosynthesis of proteinic acceptors and dolichol is not modified; therefore the increase of N-acetylglucosamine incorporation is not due to a modification of the endogenous acceptors level. By the use of exogenous dolichol-monophosphate we demonstrate that the increased transfer of [14C] N-acetylglucosamine into polyprenic acceptors is the result of a higher activity of the mitochondrial N-acetylglucosaminyltransferase after viral infection.