Metabolism of lipid-linked N-acetylglucosamine intermediates

Role of Flippases in Protein Glycosylation in the Endoplasmic Reticulum

Glycosylation is essential to the synthesis, folding, and function of glycoproteins in eukaryotes. Proteins are co- and posttranslationally modified by a variety of glycans in the endoplasmic reticulum (ER); modifications include C- and O-mannosylation, N-glycosylation, and the addition of glycosylphosphatidylinositol membrane anchors. Protein glycosylation in the ER of eukaryotes involves enzymatic steps on both the cytosolic and lumenal surfaces of the ER membrane. The glycans are first assembled as precursor glycolipids, on the cytosolic surface of the ER, which are tethered to the membrane by attachment to a long-chain polyisoprenyl phosphate (dolichol) containing a reduced α-isoprene. The lipid-anchored building blocks then migrate transversely (flip) across the ER membrane to the lumenal surface, where final assembly of the glycan is completed. This strategy allows the cell to export high-energy biosynthetic intermediates as lipid-bound glycans, while constraining the glycosyl donors to the site of assembly on the membrane surface. This review focuses on the flippases that participate in protein glycosylation in the ER.

Suppression of Rft1 expression does not impair the transbilayer movement of Man5GlcNAc2-P-P-dolichol in sealed microsomes from yeast

To further evaluate the role of Rft1 in the transbilayer movement of Man(5)GlcNAc(2)-P-P-dolichol (M5-DLO), a series of experiments was conducted with intact cells and sealed microsomal vesicles. First, an unexpectedly large accumulation (37-fold) of M5-DLO was observed in Rft1-depleted cells (YG1137) relative to Glc(3)Man(9)GlcNAc(2)-P-P-Dol in wild type (SS328) cells when glycolipid levels were compared by fluorophore-assisted carbohydrate electrophoresis analysis. When sealed microsomes from wild type cells and cells depleted of Rft1 were incubated with GDP-[(3)H]mannose or UDP-[(3)H]GlcNAc in the presence of unlabeled GDP-Man, no difference was observed in the rate of synthesis of [(3)H]Man(9)GlcNAc(2)-P-P-dolichol or Man(9)[(3)H]GlcNAc(2)-P-P-dolichol, respectively. In addition, no difference was seen in the level of M5-DLO flippase activity in sealed wild type and Rft1-depleted microsomal vesicles when the activity was assessed by the transport of GlcNAc(2)-P-P-Dol(15), a water-soluble analogue. The entry of the analogue into the lumenal compartment was confirmed by demonstrating that [(3)H]chitobiosyl units were transferred to endogenous peptide acceptors via the yeast oligosaccharyltransferase when sealed vesicles were incubated with [(3)H]GlcNAc(2)-P-P-Dol(15) in the presence of an exogenously supplied acceptor peptide. In addition, several enzymes involved in Dol-P and lipid intermediate biosynthesis were found to be up-regulated in Rft1-depleted cells. All of these results indicate that although Rft1 may play a critical role in vivo, depletion of this protein does not impair the transbilayer movement of M5-DLO in sealed microsomal fractions prepared from disrupted cells.

Biosynthesis of dolichyl diphosphate N-acetylglucosamine intermediates in Ascaridia galli microsomes

1. N-acetylglucosamine-1-phosphate transferase was demonstrated in the microsomal fraction of Ascaridia galli. 2. The transferase reaction depends on exogenous dolichyl phosphate as lipid acceptor and was found to be inhibited by tunicamycin. 3. The enzyme activity was optimal in the presence of sodium deoxycholate as detergent and Mg cations after 10 min of incubation. 4. The product of the transferase reaction--dolichyl diphosphate N-acetylglucosamine was converted into lipid-disaccharide-dolichyl diphosphate N,N'-diacetylchitobiose. 5. The maximum level of the conversion was achieved at 5 mM concentration of unlabelled UDP-N-acetylglucosamine, while this conversion was negligible at lower UDP-N-acetylglucosamine concentrations (0.1 and 0.5 mM).

Biosynthesis of prolyl hydroxylase: evidence for two separate dolichol-media pathways of glycosylation

Prolyl hydroxylase is a glycoprotein containing two nonidentical subunits, alpha and beta. The alpha subunit of prolyl hydroxylase isolated from 13-day-old chick embryos contains a single high mannose oligosaccharide having seven mannosyl residues. Two forms of alpha subunit have been shown to exist in enzyme purified from tendon cells of 17-day-old chick embryos, one of which (alpha) appears to be identical in molecular weight and carbohydrate content with the single alpha of enzyme from 13-day-old chick embryos, as well as another form (alpha') that contains two oligosaccharides, each containing eight mannosyl units [see Kedersha, N. L., Tkacz, J. S., & Berg, R. A. (1985) Biochemistry (preceding paper in this issue)]. Biosynthetic labeling studies were performed with chick tendon cells using [2-3H]mannose, [6-3H]glucosamine, [14C(U)]mannose, and [14C(U)]glucose. Analysis of the labeled products using polyacrylamide gel electrophoresis in sodium dodecyl sulfate showed that only the oligosaccharides on alpha' incorporated measurable mannose or glucosamine isotopes; however, both alpha subunits incorporated 14C amino acid mix and [14C(U)]glucose [metabolically converted to [14C(U)]mannose] under similar conditions. Pulse-chase labeling studies using 14C amino acid mix demonstrated that both glycosylated polypeptide chains alpha and alpha' were synthesized simultaneously and that no precursor product relationship between alpha and alpha' was apparent. In the presence of tunicamycin, neither alpha nor alpha' was detected; a single polypeptide of greater mobility appeared instead. Incubation of the cells with inhibitory concentrations of glucosamine partially depressed the glycosylation of alpha' but allowed the glycosylation of alpha.(ABSTRACT TRUNCATED AT 250 WORDS)

Biosynthesis of dolichyl pyrophosphate N-acetylglucosaminyl intermediates in liver microsomes from hibernating ground squirrels (Citellus citellus L.)

Dolichyl pyrophosphate N-acetyl[14C]glucosamine was synthesized after incubation of liver microsomes from hibernating ground squirrels with UDP-N-acetyl[14C )glucosamine. The radioactivity of glycolipid formed by liver microsomes from hibernating ground squirrels was about 2-fold greater than by liver microsomes from active animals. Addition of exogenous dolichyl phosphate to the incubation mixture increased the formation of dolichyl pyrophosphate N-acetyl[14C]glucosamine by microsomes from both active and hibernating ground squirrels about 6 times. Liver microsomes from hibernating ground squirrels converted dolichyl pyrophosphate N-acetyl[14C]glucosamine into dolichyl pyrophosphate N,N'-diacetyl[14C]chitobiose in the presence of unlabelled UDP-N-acetylglucosamine. This conversion was maximal at 1.0 M concentration of unlabelled UDP-N-acetylglucosamine. The level of dolichyl phosphate assessed by the level of dolichyl pyrophosphate N-acetylglucosamine formation was nearly 2 times greater in liver microsomes from hibernating ground squirrels than from active animals.

Influence of metal ions on the biosynthesis of N-acetylglucosaminyl polyprenols by the retina

The following enzymatic process was investigated, catalyzed by an enzyme preparation from the retina of the embryonic chick: UDP-GlcNAc + dolichol phosphate GDPmannose leads to metal ions GlcNAc-P-P-polyprenol + (GlcNAc)2-P-P-polyprenol + Man-(GlcNAc)2-P-P-polyprenol. These reactions were carried out in the presence of a dolichol phosphate mannose-synthesizing system, shown previously to be an activator of GlcNAc-lipid synthesis. The process was also strongly influenced by the choice of the divalent cation used during the reactions. In the presence of Mg2+, not only was the extent of incorporation of radioactivity from UDP-[3H]GlcNAc increased 4-fold into the GlcNAc lipids, as compared to Mn2+, but the relative distribution of the products was affected as well. In the presence of Mg2+ the reaction was driven mainly in the direction of the formation of the first intermediate of the dolichol pathway, GlcNAc-P-P-polyprenol. Many of the other characteristics of the GlcNAc-transferases, such as pH optimum, requirement for dolichol phosphate and specificity for stimulation by sugar nucleotides, were similar for either the Mn2+ or Mg2+ systems. Retinol phosphate could not replace the requirement for dolichol phosphate. The influence of metal ions, in addition to the stimulation by dolichol phosphate mannose, on GlcNAc-lipid synthesis may be aspects of metabolic regulation of the dolichol pathway.

Biosynthesis of lipid-linked oligosaccharides in embryonic liver. Formation of dolichyl pyrophosphate N-acetylglucosaminyl intermediates

Liver microsomes from pig embryos synthesized dolichyl pyrophosphate N-acetylglucosamine and converted it to dolichyl pyrophosphate N,N'-diacetylchitobiose. N-acetylglucosaminyl transferase activity towards dolichol was about 2-fold greater in microsomes from embryonic liver than in microsomes from adult liver. A maximum level of conversion of dolichyl pyrophosphate N-acetylglucosamine to dolichyl pyrophosphate N,N'-diacetylchitobiose was achieved at 5 mM concentration of unlabelled UDP-N-acetylglucosamine, while this conversion was negligible at lower UDP-N-acetylglucosamine concentrations (0.1 and 0.5 mM). The level of dolichyl phosphate, assessed by the level of dolichyl pyrophosphate N-acetylglucosamine synthesis was 2-fold higher in microsomes from embryonic liver than that in microsomes from adult liver. Tunicamycin (1 microgram/ml) inhibited completely the formation of dolichyl pyrophosphate N-acetyl-glucosamine in embryonic liver microsomes, while the inhibitory effect of UMP (1 mM) was about 70%.

Dolichol-dependent synthesis of chitobiosyl proteins and their further mannosylation. A second route for glycosylation of proteins in rat-spleen lymphocytes?

Incubation of whole lymphocytes with UDP-N-acetyl [3H]glucosamine used as the only precursor leads to the formation of dolichyl diphosphate [3H]chitobiose, DolPP-(GlcNAc)2, and dolichyl diphosphate N-acetyl-[3H]glucosamine, DolPP-GlcNAc. Although very few dolichyl diphosphate oligosaccharides are formed, a high level of radioactivity is recovered with proteins and has been characterized, using hydrazinolysis procedure, as [3H]chitobiosyl and N-acetyl[3H]glucosaminyl units. Addition of tunicamycin inhibits, to the same extent, both the synthesis of DolPP-(GlcNAc)1-2 and the incorporation of the N-acetyl[3H] glucosaminyl residues onto proteins, indicating that these carbohydrate units are transferred onto proteins acceptors from their dolichol derivatives. Chase experiments have indicated that, in fact, the DolPP-(GlcNAc)1-2 were utilized in two ways: either their transfer onto proteins or their degradation into water-soluble saccharidic material. Moreover, the transfer reaction appears to be a slow process compared to the degradation since the radioactivity chased from the DolPP-(GlcNAc)1-2 is not recovered on proteins. This fact allows to show that part of the [3H]chitobiose previously bound to proteins is further converted into oligomannosidic glycans in the presence of GDP-mannose. This direct mannosylation of chitobiosyl-proteins may represent a second route for the N-glycosylation of proteins.

The dolichol pathway of protein glycosylation in rat liver. Stimulation by GTP of the incorporation of N-acetylglucosamine in endogenous lipids and proteins of rough microsomes treated with pyrophosphate

Incorporation of N-acetylglucosamine into endogenous lipid and protein acceptors was investigated on heavy microsomes from rat liver, incubated with UDP-N-acetyl[14C]glucosamine and GDP-mannose in the absence of detergent. This subcellular preparation derived for 95% or more from the rough endoplasmic reticulum and was devoid of Golgi components which contain the enzyme that adds the peripheral N-acetylglucosamine units to glycoproteins. The label was found almost exclusively in dolichyl diphosphate N-acetylglucosamine, except when the subcellular preparation was treated with pyrophosphate and subsequently incubated with the nucleotide sugars in the presence of GTP. Then, the incorporation of N-acetylglucosamine was considerably enhanced, and the additional label was associated with dolichyl diphosphate N,N'-diacetylchitobiose, with dolichyl diphosphate oligosaccharides and with proteins. The time-course of N-acetylglucosamine incorporation in these products was compatible with the pathway of dolichyl diphosphate glycoconjugates for the biosynthesis of the core portion of saccharide chains linked to asparagine residues of glycoproteins. The addition of GDP-mannose to the incubation medium was required to produce labeled dolichyl diphosphate oligosaccharides, but not to incorporate N-acetylglucosamine in protein. It is concluded that rough microsomes are capable of assembling dolichol-linked oligosaccharides from exogenous nucleotide precursors and of transferring N,N'-diacetylchitobiose, or its mannosylated derivatives, from the lipid intermediate to endogenous proteins. However, these metabolic activities are hindered in the original subcellular preparation, and in the absence of GTP. Although the earliest perceptible effect produced jointly by the treatment with pyrophosphate and by GTP was the synthesis of dolichyl diphosphate N,N'-diacetylchitobiose, the primary action of these factors remains uncertain. They may stimulate directly the reaction forming dolichyl diphosphate N,N'-diacetylchitobiose from dolichyl diphosphate N-acetylglucosamine, or activate the synthesis of this latter intermediate from a particular pool of dolichyl monophosphate which is readily converted afterwards into disaccharide and oligosaccharide derivatives and glycosylates protein. The requirement for GTP might have a functional meaning, for GTP acted maximally at a concentration distinctly lower than its actual concentration in liver. The detachment of ribosomes from rough vesicles was the major alteration induced by treatment with pyrophosphate. It is suggested that the removal of ribosomes unmasks the membrane sites where GTP acts.

Tunicamycin-mediated depletion of insulin receptors in 3T3-L1 adipocytes

Tunicamycin, an antibiotic that inhibits protein glycosylation, elicited a rapid depletion of insulin binding activity at the surface of 3T3-L1 adipocytes. Disappearance of insulin receptors occurred more rapidly in the presence of tunicamycin than when protein synthesis was inhibited by cycloheximide and was accompanied by a diminution in sensitivity of the adipocytes to the acute effects of insulin and anti-insulin receptor antibody on hexose uptake and metabolism.

Formation of 2-deoxyglucose-containing lipid-linked oligosaccharides. Interference with glycosylation of glycoproteins

Crude membrane preparations from chick embryo cells catalyse the formation of dolichyl-di-N-acetylchitobiosyl diphosphate [Dol-PP-(GlcNAc)2] from uridine diphosphate N-acetylglucosamine (UDP-GlcNAc). The formation of this glycolipid was stimulated by exogenous dolichyl phosphate and inhibited by tunicamycin. Adding GDP-mannose to the cell-free system containing Dol-PP-(GlcNAc)2 by preincubation led to the formation of a lipid-linked oligosaccharide, containing 8--9 sugar residues. The formation of lipid-linked oligosaccharides was inhibited by GDP-2-deoxy-D-glucose (GDP-dGlc): in this case Dol-PP-(Glc-NAc)2-dGlc accumulated. Subsequent additions of mannosyl residues to this trisaccharide-lipid to form lipid-linked oligosaccharides were not possible. Concomitantly the glycosylation of proteins was blocked. Partially inhibitory conditions were obtained by adding both GDP-dGlc and GDP-Man with an excess of GDP-dGlc. Glycosylation of proteins was observed but the glycopeptides did not contain 2-deoxyglucosyl residues. Also in these cases 2-deoxyglucose-containing glycolipids accumulated. The main glycolipid formed under these conditions was Dol-PP-(GlcNAc)2-Man-dGlc. Lipid-linked oligosaccharides containing 2-deoxyglucose were formed under these conditions, although in small amounts, but were not transferred to protein. So the molecular basis of the inhibitory action of 2-deoxyglucose on glycosylation of protein is the incorporation of 2-deoxyglucosyl residues during early phases of the biosynthesis of the lipid-linked oligosaccharides.