Transfer of N, N'-diacetylchitobiose from dolichyl diphosphate into a gray matter membrane glycoprotein

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.

Specific transbilayer translocation of dolichol-linked oligosaccharides by an endoplasmic reticulum flippase

The oligosaccharide donor for protein N-glycosylation, Glc(3)Man(9)GlcNAc(2)-PP-dolichol, is synthesized via a multistep pathway that starts on the cytoplasmic face of the endoplasmic reticulum (ER) and ends in the lumen where the glycosylation reaction occurs. This necessitates transbilayer translocation or flipping of the lipid intermediate Man(5)GlcNAc(2)-PP-dolichol (M5-DLO) across the ER membrane. The mechanism by which M5-DLO-or any other lipid-is flipped across the ER is unknown, except that specific transport proteins or flippases are required. We recently demonstrated M5-DLO flipping activity in proteoliposomes reconstituted from detergent-solubilized ER membrane proteins and showed that it was ATP-independent and required a trypsin-sensitive protein that sedimented at approximately 4S. By using an activity-enriched fraction devoid of glycerophospholipid flippase activity, we now report that M5-DLO is rapidly flipped in the reconstituted system with a time constant tau <2 min, whereas its triantennary structural isomer is flipped slowly with tau >200 min. DLOs larger than M5-DLO are also poorly translocated, with tau ranging from approximately 10 min to >200 min. We conclude that (i) the number and arrangement of mannoses in the DLO glycan has a profound effect on the ability of the DLO to be translocated by the flippase, (ii) glycan size per se does not dictate whether a DLO will be flipped, and (iii) the flippase is highly specific for M5-DLO. Our results suggest a simple structural model for the interaction between the DLO head group and the flippase.

Regulation of the biosynthesis of N-acetylglucosaminylpyrophosphoryldolichol, feedback and product inhibition

The assembly of the core oligosaccharide region of asparagine-linked glycoproteins proceeds by means of the dolichol pathway. The first step of this pathway, the reaction of dolichol phosphate with UDP-GlcNAc to form N-acetylglucosaminylpyrophosphoryldolichol (GlcNAc-P-P-dolichol), is under investigation as a possible site of metabolic regulation. This report describes feedback inhibition of this reaction by the second intermediate of the pathway, N-acetylglucosaminyl-N-acetylglucosaminylpyrophosphoryldolichol (GlcNAc-GlcNAc-P-P-dolichol), and product inhibition by GlcNAc-P-P-dolichol itself. These influences were revealed when the reactions were carried out in the presence of showdomycin, a nucleoside antibiotic, present at concentrations that block the de novo formation of GlcNAc-GlcNAc-P-P-dolichol but not that of GlcNAc-P-P-dolichol. The apparent K(i) values for GlcNAc-P-P-dolichol and GlcNAc-GlcNAc-P-P-dolichol under basal conditions were 4.4 and 2.8 microM, respectively. Inhibition was also observed under conditions where mannosyl-P-dolichol (Man-P-dol) stimulated the biosynthesis of GlcNAc-P-P-dolichol; the apparent K(i) values for GlcNAc-P-P-dolichol and GlcNAc-GlcNAc-P-P-dolichol were 2.2 and 11 microM, respectively. Kinetic analysis of the types of inhibition indicated competitive inhibition by GlcNAc-P-P-dolichol toward the substrate UDP-GlcNAc and non-competitive inhibition toward dolichol phosphate. Inhibition by GlcNAc-GlcNAc-P-P-dolichol was uncompetitive toward UDP-GlcNAc and competitive toward dolichol phosphate. A model is presented for the kinetic mechanism of the synthesis of GlcNAc-P-P-dolichol. GlcNAc-P-P-dolichol also exerts a stimulatory effect on the biosynthesis of Man-P-dol, i.e. a reciprocal relationship to that previously observed between these two intermediates of the dolichol pathway. This network of inhibitory and stimulatory influences may be aspects of metabolic control of the pathway and thus of glycoprotein biosynthesis in general.

Site of stimulation by mannosyl-P-dolichol of GlcNAc-lipid formation by microsomes of embryonic chick retina

Mannosyl-P-dolichol (man-P-dol) has been shown to stimulate the early reactions of the dolichol pathway, specifically, the biosynthesis of GlcNAc-P-P-dol and GlcNAc-GlcNAc-P-P-dol, and thus may play a regulatory role in glycoprotein biosynthesis. The site of action of man-P-dol has previously been suggested to be the GlcNAc-transferase concerned with the formation of the monoglucosaminyl derivative. Since the concentration of the chitobiosyl compound also increases as a result of the presence of man-P-dol, the immediate site of the activation was reexamined. The effect of man-P-dol on the formation of GlcNAc-GlcNAc-P-P-dol using GlcNAc-P-P-dol synthesized in situ or added exogenously as the substrate was investigated. In addition, the distribution of radioactivity in the glucosaminyl constituents of the products under the stimulatory conditions was determined. The results of these studies supported the conclusion that the stimulation of GlcNAc-lipid synthesis by man-P-dol is due to the enhanced synthesis of GlcNAc-P-P-dol. It is not a result of the activation of the GlcNAc-transferase catalyzing the attachment of the second GlcNAc residue for the biosynthesis of the chitobiosyl derivative.

Dolichol-linked oligosaccharide and glycoprotein biosyntheses in glial cells in primary culture: development and enzymatic correlates

Primary cultures of cerebral glia derived from neonatal rat brain were utilized to determine 1) the developmental changes of dolichol-linked oligosaccharide and N-linked glycoprotein biosyntheses, 2) the enzymatic correlates of these developmental changes, and 3) the temporal relations between the biosyntheses of dolichol-oligosaccharide and N-linked glycoproteins and the biochemical expression of astrocytic and oligodendroglial differentiation. Marked, parallel developmental increases in the rates of incorporation of [3H]glucosamine into both dolichol-oligosaccharide and glycoprotein were observed, with maximal rates achieved after 9-10 days in culture and little change occurring over the next 10 days in culture. Concerning the enzymatic correlates, dolichol kinase exhibited a moderate developmental increase with a maximum at 5 days in culture, whereas the activities of the three critical enzymes that utilize dolichyl phosphate in the synthesis of the dolichol-linked oligosaccharide, i.e., N-acetylglucosaminylphosphotransferase (GlcNAc-1-P transferase), mannosylphosphoryldolichol (Man-P-Dol) synthase, and glucosylphosphoryldolichol (Glc-P-Dol) synthase, reached maxima after 6-9 days in culture. Both the activity of Man-P-Dol synthase in vitro and the rate of formation of its product, Man-P-Dol, in intact cells were shown to correlate closely with the rates of oligosaccharide and glycoprotein biosyntheses. An important regulatory role for Man-P-Dol synthase and its product, Man-P-Dol, was suggested further by the demonstration of a maturation-dependent activation by Man-P-Dol of GlcNAc-1-P transferase, the first committed step in the pathway. Two enzymatic markers of astrocytic (glutamine synthetase) and oligodendroglial (2',3'-cyclic nucleotide 3'-phosphohydrolase) differentiation exhibited marked developmental increases in activity with onset at the time of attainment of peak rates of dolichol-oligosaccharide and glycoprotein biosyntheses. Importance of the latter processes for glial differentiation is suggested.

Dolichol-linked glycoprotein synthesis in developing mammalian brain: maturational changes of the N-acetylglucosaminylphosphotransferase

The enzyme UDP-N-acetylglucosamine:dolichyl phosphate, N-acetylglucosamine-1-phosphate transferase (GlcNAc-1-P transferase), the first committed step in the dolichol-linked oligosaccharide pathway for glycoprotein biosynthesis, has been studied in developing rat brain. The enzyme was shown to be localized in microsomes, particularly heavy microsomes, and to be activated by Mg2+ and inhibited by tunicamycin. Study of the enzyme with brain development demonstrated two prominent findings. First, the accentuation of enzymatic activity caused by addition of a saturating concentration of dolichyl phosphate was greater in brain of older (3-4 weeks of age and subsequently) animals (25-fold) than in brain of younger (less than two weeks of age) animals (10-fold). This difference suggests that dolichyl phosphate may be limiting for GlcNAc-1-P transferase activity in endoplasmic reticulum of the older animals. Second, a marked (3.5-fold) increase in activity occurred over a discrete time period (3-4 weeks of postnatal life) during brain development. That this increase reflected an increase in enzyme amount rather than in catalytic efficiency was suggested by kinetic studies. Coupled with our previous demonstrations of increases in brain dolichol, dolichol kinase activity, and dolichyl phosphate levels during approximately the same developmental period (Sakakihara, Y. and Volpe, J.J., Dev. Brain Res., 14 (1984) 225-262; Volpe, J.J. et al., Dev. Brain Res., in press), the data suggest a temporally discrete period of activation of the dolichol-linked pathway to glycoproteins. Whether the pathway is regulated coordinately or sequentially is a fertile topic for future study.

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.

[Demonstration by chromatofocalization of isoenzymes of sheep brain fucosyltransferase]

Glycoprotein : fucosyltransferase from sheep brain has previously been solubilized with detergent Triton X 100 and prepurified by hydrophobic chromatography on ethyl-agarose. Additional purification was performed using a new method, namely chromatofocalization. With a small column (30 ml) of P B E 94 Pharmacia eluted by a mixture of polybuffers 76 and 94, we were able to separate quickly four isoenzymes of the cerebral fucosyltransferase in the pH range 5-8. With regard to speed and ease, this technique seems to be very useful in purification of glycosyltransferases.