Cell-free labeling in thyroid rough microsomes of lipid-linked and protein-linked oligosaccharides: II. Glucosylated units

Pig thyroid rough microsomes catalyzed the transfer of glucose from UDP-[¹⁴C]Glc to glycolipids extractable with chloroform/methanol, glycolipids extractable with a water-saturated chloroform/methanol and to a residual material. Kinetics of labeling were compatible with a precursor-product relationship between the second type of glycolipid and residuals.The [¹⁴C] Glc-glycolipids soluble in CHCl₃/CH₃OH/H₂O, 10 : 10 : 3, behaved on DEAE-cellulose mainly as pyrophospho derivatives, with some less acidic radioactivity, probably $\text{dolichol}\text{-P-[}{}^{14}\text{C}\text{]}\text{Glc}$. Their saccharide moieties released by mild acid appeared polydisperse on paper chromatography, a part of them being estimated larger than a nonasaccharide marker GlcNAc-[Man]₈. The ¹⁴C-labeled glucosylated glycoproteins have represented all the considerable polymeric label remaining after lipid extraction. Their pronase glycopeptides were submitted to a differential reductive alkaline hydrolysis and it was concluded that their [¹⁴C] glucose belongs mainly to N-glycosically linked units. On gel filtration, the released saccharides exhibited an average size of nine monosaccharide units (from six to twelve with a relatively high proportion of material containing more than nine sugars).In a [¹⁴C] Glc-microsomal extract, 29% of the non-lipid radioactivity was found immunoreactive with an antiserum to pig thyroglobulin.     

Biosynthesis of lipid-linked oligosaccharides. I. Preparation of lipid-linked oligosaccharide substrates.

In order to purify the glycosyltransferases involved in the assembly of lipid-linked oligosaccharides and to be able to study the acceptor substrate specificity of these enzymes, methods were developed to prepare and purify a variety of lipid-linked oligosaccharides, differing in the structure of the oligosaccharide moiety. Thus, Man9 (GlcNAc)2-pyrophosphoryl-dolichol was prepared by isolation and enzymatic synthesis using porcine pancreatic microsomes, while Glc3Man9(GlcNAc)2-PP-dolichol was isolated from Madin-Darby canine kidney cells. Treatment of these oligosaccharide lipids with a series of selected glycosidases led to the preparation of Man alpha 1,2Man alpha 1,2Man alpha 1,3[Man alpha 1,6(Man alpha 1,3)Man alpha 1,6]Man beta 1,4GlcNAc beta 1,4GlcNAc-PP-dolichol; Man alpha 1,2Man alpha 1,2Man alpha 1,3[Man alpha 1,6]Man beta 1,4GlcNAc beta 1, 4GlcNac-PP-dolichol; and Man alpha 1,6(Man alpha 1,3)Man alpha 1, 6[Man alpha 1,3]Man beta 1,4GlcNAc-beta 1,4GlcNAc-PP-dolichol. The preparation, isolation, and characterization of each of these lipid-linked oligosaccharide substrates are described.     

Biosynthesis of protein-linked oligosaccharides in trypanosomatid flagellates

In this article, Armando Parodi presents a summary of the knowledge of the structure and biosynthesis of mammalian Asn-linked (N-linked) oligosacchorides and compares this with what is known in trypanosomatids.     

Biosynthesis of lipid-linked oligosaccharides. Isolation and structure of a second lipid-linked oligosaccharide in Chinese hamster ovary cells.

Previous work has shown that vesicular stomatitis virus-infected Chinese hamster ovary cells contain a major high molecular weight lipid-linked oligosaccharide which is transferred en bloc to protein during the formation of the asparagine-linked complex-type oligosaccharides of the vesicular stomatitis virus G protein (Tabas, I., Schlesinger, S., and Kornfeld, S. (1978) J. Biol. Chem. 253, 716-722). We now report the characterization of a second, lower molecular weight lipid-linked oligosaccharide. The oligosaccharide portion of this molecule was isolated and its structure was determined by methylation analysis, digestion with exoglycosidases, acetolysis and Smith periodate degradation to be: (formula: see text). Several lines of evidence are presented which indicate that this lipid-linked oligosaccharide is primarily involved in the assembly of the major lipid-linked oligosaccharide rather than in the direct glycosylation of proteins.     

Glycoprotein biosynthesis in plants. Demonstration of lipid-linked oligosaccharides of mannose and N-acetylglucosamine.

Previous studies from this laboratory have shown that particulate preparations from maturing cotton fibers catalyze the transfer of mannose from GDP-[14C]mannose into mannosylphosphorylpolyisoprenol (Forsee, W. T., and Elbein,A. D. (1973) J. Biol. Chem. 248, 2858-2867). In this report, we show that these particulate preparations also catalyze the inocoporation of mannose from GDP-[14C]mannose into lipid-linked oligosaccharides and into glycoprotein. The oligosaccharide-lipids were treated with dilute acid to liberate the water-soluble oligosaccharides and these oligosaccharides could then be separated into seven or eight distinct radioactive peaks by paper chromatography in isobutyric acid/NH4OH/H2betaO (57/4/39). The smallest of the oligosaccharides appears to be a trisaccharide with the structure Man leads to GlcNAc-GlcNAc. Thus the oligosaccharides attached to the lipids apparently range in size from those having 3 glycose units to those having approximately 8 to 10 glycose units. The radioactivity in the smaller-sized oligosaccharide-lipids could be chased into the larger oligosaccharide-lipids by a second incubation in the presence of unlabeled GDP-mannose. The sugar at the reducing ends of the oligosaccharides was identified as GlcNAc while some mannose (20 to 30%) was present in alpha linkages at the nonreducing ends...     

Inhibition of protein synthesis also inhibits synthesis of lipid-linked oligosaccharides.

Studies were initiated to determine whether the formation of lipid-linked oligosaccharides was coupled to the synthesis of protein. Canine kidney cells were grown with [2-3H]mannose or [3H]leucine in the presence of cycloheximide or puromycin and the effect of these inhibitors on the synthesis of proteins and lipid-linked oligosaccharides was measured. In all cases, the inhibition of protein synthesis resulted in a substantial inhibition in the incorporation of mannose into the lipid-linked oligosaccharides, although the synthesis of mannosyl-phosphoryl-dolichol was only slightly inhibited. Cycloheximide had no effect on the in vitro incorporation of mannose into lipid-linked oligosaccharides when GDP-[14C]mannose was incubated with aorta microsomal preparations. The inhibition of lipid-linked oligosaccharides was apparently not due to a decrease in the amount of glycosyltransferases as a result of protein degradation in the absence of protein synthesis, nor was it the result of a more rapid degradation of lipid-linked oligosaccharides. The inhibition also did not appear to be due to limitations in the available dolichyl-phosphate. The results suggest that the formation of lipid-linked oligosaccharides may be regulated by end product inhibition.     

The assembly of lipid-linked oligosaccharides in plant and animal membranes

Membrane preparations from growing regions of pea stems and activelydividing mouse L-cells form lipid-linked saccharides from GDP-mannose and UDP-N-acetylglucosamine. These lipids have properties which are consistent with those of mono-and di-phosphoryl polyisoprenyl derivatives. In experiments using plant membranes, the monophosphoryl derivative labeled with GDP-(¹⁴C) mannose contains mannose only, while the diphosphoryl derivative labeled with the same nucleotide sugar is heterogeneous, containing oligosaccharides corresponding to mannosaccharides of 5, 7, and 9-12 residues. Only the diphosphoryl polyisoprenyl derivatives are labeled with UDP-(¹⁴C)glucosamine and these contain predominantly chitobiose and N-acetylglucosamine itself. Unlabeled GDP-mannose added after UDP-N-acetyl (¹⁴C)glucosamine results in the formation of higher lipid-linked oligosaccharides which are apparently the same as those which are labeled with GDP-(¹⁴C)mannose alone. Incubation of the membranes with GDP-(¹⁴C)mannose in the presence of Mn²⁺, unlabeled UDP-glucose or unlabeled UDP-N-acetylglucosamine results in marked changes in the accumulation of both the polyisoprenyl monophosphoryl mannose and polyisoprenyl diphosphoryl oligosaccharides. Animal cell membranes synthesise lipid-linked oligosaccharides when incubated with UDP-N-acetylglucosamine and GDP-mannose. These oligosaccharides are similar in size to those synthesised by the plant membranes but their formation is more efficient. The potential roles of these compounds in glycoprotein biosynthesis in both plant and animal tissues is discussed.     

The synthesis of complex-type oligosaccharides. I. Structure of the lipid-linked oligosaccharide precursor of the complex-type oligosaccharides of the vesicular stomatitis virus G protein.

The synthesis of the complex-type oligosaccharide unit of the vesicular stomatitis virus G protein is initiated by the en bloc transfer of a high molecular weight oligosaccharide from a lipid carrier to the nascent polypeptide. Following transfer the oligosaccharide is "processed" by removal of glucose and mannose residues and the sugars that constitute the outer branches of the complex-type oligosaccharide are added. The structure of the oligosaccharide moiety of the lipid-linked precursor has been elucidated in order to further define the steps involved in processing. Since it was not feasible to obtain adequate amounts of material for standard structural studies, most of the structural studies were performed on radiolabeled material, with radioactivity incorporated differentially into glucose, mannose, and N-acetylglucosamine. Based on endo-beta-N-acetylglucosaminidase CII digestion, alpha-mannosidase digestion, acetolysis, Smith periodate degradation, methylation analysis, and periodate oxidation, we propose the following structure for the oligosaccharide moiety of the lipid-linked oligosaccharide.     

Abnormal lipid-linked oligosaccharides in class E Thy-1-negative mutant lymphomas.

The glycosylation defect of Thy-1-mutant lymphomas of the class E complementation group has been identified as a block in the synthesis of the lipid-linked oligosaccharide precursor of the asparagine-linked oligosaccharides of glycoproteins. Two major lipid-linked oligosaccharides were isolated from the mutant cells. Both oligosaccharides were smaller than the lipid-linkid oligosaccharides of wild-type lymphomas and, in contrast to the lipid-linked oligosaccharides isolated from wild-type cells, both were resistant to digestion with endoglycosidase H. The oligosaccharides of newly synthesized polypeptides in class E Thy-1-cells were also resistant to endoglycosidase H digestion, providing strong evidence that they are derived from the abnormal lipid-linked oligosaccharides.     

Arrangement of glucose residues in the lipid-linked oligosaccharide precursor of asparaginyl oligosaccharides.

The lipid-linked oligosaccharide synthesized in vitro, in the presence of 1.0 microM UDP-[3H]Glc, GDP-[14C]Man, and UDP-GlcNAc has been isolated and the structure of the oligosaccharide has been analyzed. The oligosaccharide contains 2 N-acetylglucosamine, 9 mannose, and 3 glucose residues. The N-acetylglucosamine residues are located at the reducing terminus. The 3 glucose residues are arranged in a linear order at one of the nonreducing termini in the sequence Glc 1,2--Glc 1,3--Glc--(Man)9 (GlcNAc)2. The structural analysis was made possible largely by the availability of glucosidase preparations of fungal anad microsomal origin which remove glucose residues from the oligosaccharide without releasing mannose residues.     

Biosynthesis of lipid-linked oligosaccharides in embryonic liver. Formation of mannose containing derivatives

In the presence of exogenous dolichyl phosphate mannosyl transferase activity towards dolichyl phosphate was nearly 3-fold higher in microsomes from pig embryonic liver compared to that from adult liver. After incubation of microsomes from embryonic liver with UDP-N-acetylglucosamine and GDP-[14C]mannose lipid-linked tri- to undecasaccharides were discovered in CHCl3-CH3OH (2:1, v/v) and CHCl3-CH3OH-H2O (1:1:0.3, by vol) extracts. The main proportion of the radioactivity was incorporated into penta-, sexta and undecasaccharides. Amphomycin at concentration 500 micrograms/ml inhibited almost completely dolichyl phosphate mannose synthesis in embryonic liver microsomes without inhibition the formation of lipid-linked penta- and sextasaccharides. It was suggested that mannose transferred to lipid-linked tetra- to heptasaccharides comes from GDP-mannose but not from dolichyl phosphate mannose.     

Formation of unusual mannosamine-containing lipid-linked oligosaccharides in Madin-Darby canine kidney cell cultures.

Glc3Man9(GlcNAc)2-pyrophosphoryl-dolichol is the major lipid-linked oligosaccharide (LLO) produced by Madin-Darby canine kidney cells in culture. However, when these cells are incubated in the presence of millimolar concentrations of mannosamine and labeled with [2-3H]mannose, they accumulate various LLO that have smaller-sized oligosaccharides with unusual structures and the Glc3Man9(GlcNAc)2-pyrophosphoryl-dolichol is not detected. Thus in the presence of 10 mM mannosamine, more than 80% of the oligosaccharides are eluted from concanavalin A-Sepharose with 10 mM alpha-methylglucoside, indicating that they no longer have the tight-binding characteristics of control oligosaccharides. In addition, 20-40% of these oligosaccharides bind to Dowex 50-H+, indicating the presence of mannosamine in these structures. Interestingly enough, these abnormal oligosaccharides are still transferred to protein. The mannosamine-induced oligosaccharides were separated into neutral and basic fractions on a cation exchange resin. The neutral oligosaccharides ranged in size from hexose3(GlcNAc)2 to hexose10(GlcNAc)2 with the major species being Man5(GlcNAc)2 to Man7(GlcNAc)2. These oligosaccharides were almost completely susceptible to digestion by alpha-mannosidase and by endoglucosaminidase H. The basic oligosaccharides showed anomolous behavior on the Bio-Gel P-4 columns and appeared to be of small size on the standard columns, ranging from hexose2 to hexose4. However, most of these oligosaccharides were susceptible to digestion by endoglucosaminidase H as well as by alpha-mannosidase, suggesting that they were of different size and structure than would be predicted from the gel filtration patterns. Significantly, when the basic oligosaccharides were subjected to chemical N-acetylation, or when the gel filtration columns were run at high pH rather than at the usual pH of 3.0, the basic oligosaccharides migrated like much larger oligosaccharides. These data provide strong evidence to indicate that some mannosamine can be incorporated into the LLO, and that these mannosamine-containing oligosaccharides exhibit unusual properties. Preliminary studies indicated that Madin-Darby canine kidney cells do incorporate label from [3H]mannosamine into the LLO.     

Cell-free N-glycosylation in Dictyostelium discoideum: analysis of wild-type and mutants defective in lipid-linked oligosaccharide biosynthesis

N-glycosylation was measured in wild-type cell lysates of Dictyostelium discoideum and in two mutant strains that synthesize a truncated lipid-linked oligosaccharide, Man6GlcNAc2 lacking terminal mannose and glucose residues. Endogenous lipid-linked oligosaccharide (LLO) was transferred to octanoyl-Asn-[125I]Tyr-ThrNH2 by membrane fractions. About 50% of the glycopeptide product remained associated with membranes. Taurocholate and saponin promoted and preserved glycosylation, but NP-40 and Triton X-100 did not. Using this artificial assay, the rate and extent of transfer of the truncated lipid-linked oligosaccharide in extracts of the two mutant strains, HL241 and HL243, was reduced 5-10-fold relative to that of wild-type. The low activity found in the mutant strains appears to result from either reduced affinity of the truncated LLO for the transferase or from its improper topological localization in the membrane. When protein N-glycosylation is measured in living cells it is nearly normal in HL241, but it is 3-4-fold decreased in HL243. Although the results of the in vitro and in vivo assays differ, they are not in conflict. Rather, they suggest that the static in vitro assay may be capable of revealing subtleties in the productive positioning of LLO and the oligosaccharyl transferase. The decrease in glycosylation seen in intact HL243 cells may be a consequence of the pleiotropic effects of the primary mutation rather than a direct result of the altered LLO structure. Genetic analysis showed that the mutation in HL241 is recessive, while the mutation in HL243 is dominant and prevents normal development. Thus, the two mutants share a lesion in lipid-linked oligosaccharide biosynthesis and in cell-free glycosylation, but differ in their in vivo glycosylation. Their primary defects are probably different.     

Interference with glycosylation of glycoproteins. Inhibition of formation of lipid-linked oligosaccharides in vivo.

Influenza-virus-infected cells were labelled with radioactive sugars and extracted to give fractions containing lipid-linked oligosaccharides and glycoproteins. The oligosaccharides linked to lipid were of the 'high-mannose' type and contained glucose. In the glycoprotein fraction, radioactivity was associated with virus proteins and found to occur predominantly in the 'high-mannose' type of glycopeptides. In the presence of the inhibitors 2-deoxy-D-glucose, 2-deoxy-2-amino-D-glucose (glucosamine), 2-deoxy-2-fluoro-D-glucose and 2-deoxy-2-fluoro-D-mannose incorporation of radiolabelled sugars into lipid- and protein-linked oligosaccharides was decreased. Kinetic analysis showed that the inhibitors affected first the assembly of lipid-linked oligosaccharides and then protein glycosylation after a lag period. During inhibition by deoxyglucose and the fluoro sugars lipid-linked oligosaccharides were formed that contained oligosaccharides of decreased molecular weight. No such aberrant forms were found during inhibition by glucosamine. In the case of inhibition by deoxyglucose it was shown that the aberrant oligosaccharides were not transferred to protein. Inhibition of formation of lipid-linked oligosaccharides by deoxyglucose and fluoro sugars was antagonized by mannose, in which case oligosaccharides of normal molecular weight were formed. The inhibition by glucosamine was reversed by its removal from the medium. The reversible effects of these inhibitors exemplify their usefulness as tools in the study of glycosylation processes.     

Proteins of rough microsomal membranes related to ribosome binding. I. Identification of ribophorins I and II, membrane proteins characteristics of rough microsomes

Rat liver rough microsomes (RM) contain two integral membrane proteins which are not found in smooth microsomes (SM) and appear to be related to the presence of ribosome-binding sites. These proteins, of molecular weight 65,000 and 63,000, were designated ribophorins I and II, respectively. They were not released from the microsomal membranes by alkali or acid treatment, or when the ribosomes were detached by incubation with puromycin in a high salt medium. The anionic detergent sodium deoxycholate caused solubilization of the ribophorins, but neutral detergents led to their recovery with the sedimentable ribosomes. Ribosomal aggregates containing both ribophorins, but few other membrane proteins, were obtained from RM treated with the nonionic detergent Kyro EOB (2.5 X10(-2) M) in a low ionic strength medium. Sedimentation patterns produced by these aggregates resembled those of large polysomes but were not affected by RNase treatment. The aggregates, however, were dispersed by mild trypsinization (10 microgram trypsin for 30 min at 0 degrees C), incubation with deoxycholate, or in a medium of high salt concentration. These treatments led to a concomitant degradation or release of the ribophorins. It was estimated, from the staining intensity of protein bands in acrylamide gels, that in the Kyro EOB aggregates there were one to two molecules of each ribophorin per ribosome. Sedimentable complexes without ribosomes containing both ribophorins could also be obtained by dissolving RM previously stripped of ribosomes by puromycin- KCl using cholate, a milder detergent than DOC. Electron microscope examination of the residue obtained from RM treated with Kyro EOB showed that the rapidly sedimenting polysome-like aggregates containing the ribophorins consisted of groups of tightly packed ribosomes which were associated with remnants of the microsomal membranes.     

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%.     

Enzymatic transfer of mannose from mannosyl-phosphoryl-polyprenol to lipid-linked oligosaccharides by pig aorta.

A particulate enzyme preparation prepared from the intimal layer of pig aorta catalyzed the transfer of mannose from mannosyl-phosphoryl-polyprenol (MPP) into a series of oligosaccharides that were linked to lipid. The reaction required detergent with Triton X-100 and NP-40 being best at a concentration of 0.5%. Several other detergents were inactive or only slightly active. The pH optima for this activity was about 7 to 7.5 in Tris buffer and the apparent Km for MPP was about 2 x 10(-7) M. The reaction was not stimulated by the addition of divalent cation and, in fact, was inhibited by the high concentrations of cation. The addition of EDTA did not inhibit the transfer of mannose from MPP and was somewhat stimulatory. The transferase(s) activity was "solubilized" from the particles by treatment with Triton X-100. This solubilized enzyme still formed a series of lipid-linked oligosaccharides from either MPP or GDP-mannose. The oligosaccharides were released from the lipid by mild acid hydrolysis and were separated by paper chromatography. Some five or six radioactive oligosaccharides were formed from either MPP or from GDP-mannose and these oligosaccharides had similar mobilities upon paper chromatography. However, MPP was a better donor for the larger oligosaccharides (i.e. those containing 8, 9, or 10 sugar residues), whereas GDP-mannose was better for formation of the oligosaccharide containing 7 sugar residues. In the presence of EDTA and detergent no MPP was formed from GDP-mannose, but radioactivity was still incorporated into the lipid-linked oligosaccharides. Under these conditions essentially all of the radioactivity was in the oligosaccharide containing 7 sugar residues. Since much of this activity could be released as mannose by acetolysis, GDP-mannose may be the direct mannosyl donor for formation of 1 leads to 6 branches. Oligosaccharides 7, 8, 9, and 10 were isolated and partially characterized in terms of their molecular weights, sugar composition, susceptibility to alpha-mannosidase, and 14C products formed by acetolysis and periodate oxidation. The molecular weights ranged from 1310 for oligosaccharide 7 to 1750 for oligosaccharide 10. Hydrolysis of each oligosaccharide and reduction with NaB3H4 gave the expected ratio of [3H]hexitol to [3H]hexosaminitol based on the molecular weight of the oligosaccharide. However, the hexitol fraction contained [3H]mannitol and [3H]glucitol. Since the amount of radioactivity in glucitol was 2 to 4 times that in mannitol and since only glucosaminitol was found in the amino sugar peak, it seems likely that each 14C-oligosaccharide was contaminated with an unlabeled oligosaccharide of equal molecular weight containing glucose and GlcNAc. Acetolysis of the 14C-oligosaccharides gave rise to 14C peaks of mannose, mannobiose, and mannotriose. In the larger oligosaccharides, most of the radioactivity was in mannobiose whereas in oligosaccharide 7 most of the radioactivity was in mannose...     

Fluoroglucose-inhibition of protein glycosylation in vivo. Inhibition of mannose and glucose incorporation into lipid-linked oligosaccharides

The effects of the glycosylation inhibitor 2-deoxy-2-fluoro-D-glucose on the formation of the lipid-linked oligosaccharides and monosaccharides that are involved in protein glycosylation were investigated. In chick embryo cells treated with fluoroglucose the formation of lipid-linked oligosaccharides cannot go to completion and oligosaccharides with decreased amounts of glucose and mannose can be detected. These oligosaccharides are probably biosynthetic intermediates and serve as acceptors of sugar residues while reversing fluoroglucose-inhibition by the addition of mannose and glucose to the culture medium. In contrast to deoxyglucose, fluoroglucose was not incorporated into lipid-linked oligosaccharides. Fluoroglucose inhibits the formation in vivo of dolichyl phosphate glucose and dolichyl phosphate mannose, but not the transfer of those sugar residues from the lipid monophosphate derivative to the lipid-linked oligosaccharides. The pool size of UDP-glucose, but not of GDP-mannose and UDP-N-acetylglucosamine, was decreased. Also, the formation of lipid-linked N-acetylglucosamine was not affected by fluoroglucose. Fluoroglucose was applied to deplete cellular membranes of endogenous lipid-linked mannose and glucose, and can possibly be used to discern different pathways of glycosylation.