Biosynthesis and characterization of lipid-linked sugars and glycoproteins in aorta

Synthesis, Processing, and Function of N-Glycans in N-Glycoproteins

Many membrane-resident and secreted proteins, including growth factors and their receptors are N-glycosylated. The initial N-glycan structure is synthesized in the endoplasmic reticulum (ER) as a branched structure on a lipid anchor (dolicholpyrophosphate) and then co-translationally, "en bloc" transferred and linked via N-acetylglucosamine to asparagine within a specific N-glycosylation acceptor sequence of the nascent recipient protein. In the ER and then the Golgi apparatus, the N-linked glycan structure is modified by hydrolytic removal of sugar residues ("trimming") followed by re-glycosylation with additional sugar residues ("processing") such as galactose, fucose or sialic acid to form complex N-glycoproteins. While the sequence of the reactions leading to biosynthesis, "en bloc" transfer and processing of N-glycans is well investigated, it is still not completely understood how N-glycans affect the biological fate and function of N-glycoproteins. This review will discuss the biology of N-glycoprotein synthesis, processing and function with specific reference to the physiology and pathophysiology of the immune and nervous system, as well as infectious diseases such as Covid-19.

Synthesis, Processing, and Function of N-glycans in N-glycoproteins

Many membrane-resident and secrected proteins, including growth factors and their receptors, are N-glycosylated. The initial N-glycan structure is synthesized in the endoplasmic reticulum (ER) as a branched structure on a lipid anchor (dolichol pyrophosphate) and then co-translationally, "en bloc" transferred and linked via N-acetylglucosamine to asparagine within a specific N-glycosylation acceptor sequence of the nascent recipient protein. In the ER and then the Golgi apparatus, the N-linked glycan structure is modified by hydrolytic removal of sugar residues ("trimming") followed by re-glycosylation with additional sugar residues ("processing") such as galactose, fucose, or sialic acid to form complex N-glycoproteins. While the sequence of the reactions leading to biosynthesis, "en bloc" transfer and processing of N-glycans is well investigated, it is still not completely understood how N-glycans affect the biological fate and function of N-glycoproteins. This review discusses the biology of N-glycoprotein synthesis, processing, and function with specific reference to the physiology and pathophysiology of the nervous system.

Protein glycosylation in rat fibroblast cells expressing deletion variants of the human hsp70 gene

Elevated heat resistance is often associated with high cellular levels of hsp70. In the rat cell line, M21, increased heat resistance is associated with both the expression of an intact exogenous human hsp70 gene, and of an exogenous stress glycoprotein, GP62. In this study, we examined heat-stress induced alterations in protein glycosylation in two variant lines of M21 that contain specific deletions in the exogenous human hsp70 gene. The deletion mutant, MVH delta Sm, lacks the nucleolar localization sequence for human hsp70 gene, whereas the other mutant, MVH delta Bg, is characterized by deletion of the ATP binding domain in the human hsp70 gene. After heat induction, the MVH delta Sm mutants exhibited constitutive and heat-induced endogenous hsp70 mRNA levels, protein glycosylation, as well as hsp70 and GP62 protein levels comparable to those in the parent cell line, Rat-1. Cellular heat sensitivity of MVH delta Sm mutants was also similar to that of Rat-1 cells, although the mutant cells had a reduced capacity for thermotolerance development. On the other hand, MVH delta Bg mutants before thermotolerance induction, exhibited constitutive endogenous and human exogenous hsp70 mRNA levels similar to those in MVH delta Sm mutants. However, after thermotolerance induction, MVH delta Bg mutants showed lower levels of heat-induced endogenous hsp70 mRNA than MVH delta Sm mutants, an overall reduction in protein glycosylation, low hsp70 and GP62 levels, and increased heat sensitivity when compared to the parent Rat-1 cell line. Incorporation of D-[2-3H]mannose into oligosaccharide precursor pools and glycoproteins was consistent with protein glycosylation pattern for each cell line. Acute heat stress resulted in the selective glycosylation of the 'prompt' glycoprotein, P-SG64, in the two deletion mutants, similar to that in M21 cells expressing the intact human hsp70 gene. The data indicate that both protein glycosylation and hsp70 expression generally correlate with cellular heat resistance and thermotolerance expression. The presence of full or partially deleted copies of human hsp70 modulates thermotolerance and protein glycosylation in a complex manner that is not yet fully understood.

Protein glycosylation in a heat-resistant rat fibroblast cell model expressing human HSP70

Thermotolerance and heat resistance are frequently associated with elevated levels of heat shock proteins (HSPs). Elevated heat resistance is also found to be associated with the overexpression of high levels of HSP70, as seen in M21 cells, derived from the Rat-1 line. In the present study, we report that M21 cells also feature an increase in general protein glycosylation and specific expression of the stress glycoprotein, GP62, both of which correlate with cellular heat resistance. The expression of GP50, a major stress glycoprotein in cell lines such as CHO, however, did not correlate with cellular heat resistance in M21 cells. Protein glycosylation that occurs during acute heat stress ("prompt" glycosylation) was associated with the glycosylation of a major "prompt" stress glycoprotein, P-SG64 (M(r) of 64,000), that was identified by immunoblotting as a glycosylated form of calreticulin. The higher level of protein glycosylation in M21 cells correlated well with increased D-[2-3H]mannose incorporation into precursor pools of dolichyl phosphomannose and dolichyl pyrophosphoryl oligosaccharides and into glycoproteins. Thus, heat resistance in M21 cells is associated not only with expression of high levels of HSP70, but also with a concomitant increase in protein glycosylation. These data support the hypothesis that stress-induced protein glycosylation is a component of cellular stress response, either in association with HSPs or as an independent mechanism.

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.

Control of N-linked oligosaccharide synthesis: cellular levels of dolichyl phosphate are not the only regulatory factor

When MDCK cells were incubated in the presence of the protein synthesis inhibitor puromycin or cycloheximide, there was a rapid and concentration-dependent inhibition in the incorporation of [2-3H]mannose into lipid-linked oligosaccharide and into protein. However, mannose incorporation into dolichyl-P-mannose was not affected. Interestingly, these inhibitors did block [6-3H]glucosamine incorporation into dolichyl-PP-GlcNAc as well as into lipid-linked oligosaccharides. Similar results were obtained when other cell lines were used and also when inhibitors of protein glycosylation such as beta-hydroxynorvaline and beta-fluoroasparagine were used. Cells incubated in puromycin did not show any changes in the levels of sugar nucleotides, GDP-mannose or UDP-GlcNAc, or in the in vitro activities of the glycosyltransferases that add mannose to the lipid-linked oligosaccharides. The inhibition of mannose incorporation into lipid-linked oligosaccharides could not be overcome by addition of dolichyl-P to the inhibited cells, even though the addition of dolichyl-P to control cells stimulated mannose incorporation into dolichyl-P-mannose, lipid-linked oligosaccharides, and protein from 3- to 5-fold. Thus, limitations in the levels of dolichyl-P do not appear to be a major factor in this inhibition. On the other hand, addition of the tripeptide acceptor N-acyl-Asn-Try-Thr did overcome the puromycin inhibition to some extent, suggesting that accumulation of some intermediate such as lipid-linked oligosaccharides might be involved in the inhibition.

The function of galactosyl phosphorylpolyprenol in biosynthesis of lipoteichoic acid in Bacillus coagulans

Incubation of UDP-[14C]galactose with membranes of Bacillus coagulans led to the formation of a radioactive glycolipid, which was tentatively characterized as beta-galactosyl phosphorylpolyprenol (Gal-P-prenol) on the basis of its chromatographic behavior and data from structural analysis of its sugar 1-phosphate moiety. The sugar moiety of [14C]Gal-P-prenol was shown to be incorporated into a membrane-bound polymer, which coincided with the diacyl form of lipoteichoic acid in its chromatographic behavior on columns of Sephacryl S-300, DEAE-Sephacel and octyl-Sepharose. Hydrogen fluoride hydrolysis of the polymer afforded an alpha-galactoside identical with Gal(alpha 1----2)Gro obtained from lipoteichoic acids. The incorporation of galactose residues from [14C]Gal-P-prenol into the polymer was greatly enhanced by exogenous lipoteichoic acids, especially of the diacyl and monoacyl forms. The optimal pH and metal concentration for the Gal-P-prenol formation, respectively, were found to be 8.4 and 10 mM (MgCl2), whereas those for the transfer of galactose from this lipid intermediate to polymer were 4.5 and 16 mM (CaCl2). The above results lead to the conclusion that Gal-P-prenol serves as the direct galactosyl donor in the synthesis of lipoteichoic acids in B. coagulans.

Characterization of a glucosyltransferase activity in liver plasma membrane: modulation by cations and lipidic effectors

1. Glucosyltransferase activity incorporating [14C]glucose from UDP-[14C]glucose onto endogenous lipidic acceptors was localized primarily in the plasma membrane of liver. 2. Incubation of plasma membrane by phosphatidyl-choline liposomes loaded with dolichyl-phosphate stimulated the enzymatic activity. 3. This enzyme required Mg2+ for maximal catalitic activity. Ca2+ could substitute Mg2+. 4. Mn2+ acted as a partial non-competitive inhibitor of the Mg2+-activated glucosyltransferase. 5. This enzyme can be modulated by neutral and acidic phospholipids; the most efficient were phosphatidyl-serine and phosphatidyl-inositol. 6. The enzymatic activity was not significantly changed by cholesterol alone but it is greatly enhanced by liposomes loaded with dolichyl-phosphate and cholesterol.

Effects of glucose starvation and puromycin treatment on lipid-linked oligosaccharide precursors and biosynthetic enzymes in Chinese hamster ovary cells in vivo and in vitro

Previous studies from several laboratories have demonstrated that glucose-starved Chinese hamster ovary (CHO) cells and other cells in culture switch from synthesis of the normal Glc3Man9GlcNAc2-P-P-Dol to Man5-GlcNAc2-P-P-Dol. In this study we have investigated this phenomenon in CHO cells in vitro and in vivo in order to determine the possible site of this block. Our results demonstrate that enzymatic activities responsible for Man9GlcNA2 synthesis in vitro are normal in glucose-starved cells. In vivo, however, the pool of GDP-[3H]Man is severely depleted, while [3H]mannose incorporation into lipid-linked and protein-bound Man5GlcNAc2 is increased. This result suggests that the available GDP-Man in starved cells is utilized to synthesize Man5GlcNAc2 preferentially, resulting in a reduction of Dol-P-Man and Man6-Man9 GlcNAc2 synthesis in vivo in glucose-starved cells. Conditions which prevent the depletion of GDP-[3H]Man in glucose-starved cells, such as puromycin or cycloheximide treatment, result in normal synthesis of Man9GlcNAc2 by glucose-starved cells. An unexpected finding in the course of this study is that puromycin or cycloheximide treatment of cells, which is known to inhibit lipid-linked oligosaccharide synthesis in glucose-fed cells, has no such inhibitory effect on glucose-starved cells.

Synthesis of sulfated glycoproteins by glial precursor cells

Populations of enriched glial precursor cells and astrocytes isolated from primary cultures of newborn rat brain were used to study the synthesis of sulfated glycoproteins. Both cell types incorporated [3H]glucosamine and [35S]sulfate into carbohydrate side chains of proteoglycans and glycoproteins. The rate of incorporation of [3H]glucosamine into the oligosaccharides and the pattern of distribution of the label into high mannose and complex glycopeptides recovered from the glycoproteins appeared to be similar for the two glial cell types. However, clear differences were noted in the rate of oligosaccharide sulfation activities. Thus the cultures of precursor glia were about four times more active than cultures enriched in astroglia in their ability to incorporate [35S]sulfate into glycoproteins.

The effect of mannosamine on the formation of lipid-linked oligosaccharides and glycoproteins in canine kidney cells

Madin-Darby canine kidney (MDCK) cells normally form lipid-linked oligosaccharides having mostly the Glc3Man9GlcNAc2 oligosaccharide. However, when MDCK cells are incubated in 1 to 10 mM mannosamine and labeled with [2-3H]mannose, the major oligosaccharides associated with the dolichol were Man5GlcNAc2 and Man6GlcNAc2 structures. Since both of these oligosaccharides were susceptible to digestion by endo-beta-N-acetylglucosaminidase H, the Man5GlcNAc2 must be different in structure than the Man5GlcNAc2 usually found as a biosynthetic intermediate in the lipid-linked oligosaccharides. Methylation analysis also indicated that this Man5GlcNAc2 contained 1----3 linked mannose residues. Since pulse chase studies indicated that the lesion was in biosynthesis, it appears that mannosamine inhibits the in vivo formation of lipid-linked oligosaccharides perhaps by inhibiting the alpha-1,2-mannosyl transferases. Although the lipid-linked oligosaccharides produced in the presence of mannosamine were smaller in size than those of control cells and did not contain glucose, the oligosaccharides were still transferred in vivo to protein. Furthermore, the oligosaccharide portions of the glycoproteins were still processed as shown by the fact that the glycopeptides were of the complex and hybrid types and were labeled with [3H]mannose or [3H]galactose. In contrast, control cells produced complex and high-mannose structures but no hybrid oligosaccharides were detected. The inhibition by mannosamine could be overcome by adding high concentrations of glucose to the medium.

Synthesis and release of sulfated glycoproteins by cultured glial cells

Both primary cultured glial cells and cloned (C-6) glioma cells have been shown to synthesize and release sulfated glycoproteins. It was found that N-linked tri- and tetra-antennary glycopeptides recovered from the glycoproteins contained most of the (35S) sulfate label. C-6 glial cells showed a higher rate of oligosaccharide sulfation than the primary glial cultures. Both cell types exhibited a high rate of release of sulfated glycoproteins into the medium. The ratio of 35S/3H incorporated from (35S) sulfate and (3H) glucosamine in the released material was higher than that of the glycoproteins associated with the cell, indicating an enrichment of sulfated glycoproteins in the secreted materials. Monensin inhibited both the synthesis and the release of sulfated glycoproteins.

Biosynthesis of glycoproteins in human placenta: differential labeling of mannose and heterogeneity of oligosaccharide lipid intermediates

The biosynthesis of lipid-linked oligosaccharides has been studied in first trimester human placentas. Tissue was pulsed with [2-3H]mannose, [1-3H]glucosamine, and [1-3H]galactose (for [3H]glucose incorporation). The lipid-linked oligosaccharides (OSL) were purified on DEAE-cellulose. After cleaving the lipid from OSL the oligosaccharides were purified by paper chromatography and borate high-voltage electrophoresis. Four major oligosaccharides with the composition Glc(0-3)Man9GlcNAc2 thus obtained were characterized enzymatically by digestion with endo-beta-N-acetylglucosaminidase H and alpha-mannosidase, and chemically by methylation, acetolysis, and Smith degradation. While identical Glc(1-3)Man9GlcNAc2 oligosaccharides have been isolated from other in vivo systems, the presence of Man9Glc2 is novel for human placenta. Furthermore, Man9GlcNAc2 is present in appreciable amounts in placenta. Second, one mannose in Man9GlcNAc2 had a significantly higher specific radioactivity than the other mannosyl residues of the Glc(0-3)Man9GlcNAc2 oligosaccharides. Third, the differential labeling in Man9GlcNAc2 and our pulse-chase studies indicate that Man9GlcNAc2 is not a major precursor of Glc3Man9GlcNAc2. The data also suggest that the ninth mannose, which has the highest radioactivity, may be incorporated in a different subcellular compartment and may serve as a regulatory step in the biosynthesis of lipid-linked oligosaccharides. A model is proposed which outlines possible multiple pathways of lipid-linked oligosaccharide biosynthesis in human placenta.

Relationship between oligosaccharide-lipid synthesis and protein synthesis in mouse LM cells

Previous studies from several laboratories have reported that inhibition of protein synthesis results in a concomitant reduction in synthesis of oligosaccharide-diphosphoryldolichol. We have investigated this phenomenon in LM cells grown in defined culture medium. The results of this study indicate that incubation of LM cell with cycloheximide at a concentration sufficient to totally arrest polypeptide synthesis, results in a rapid reduction in the synthesis of [3H]mannose-labeled or [3H]glucosamine-labeled oligosaccharide-lipid within 2 min. Cycloheximide treatment had only a slight inhibitory effect on synthesis of GDP-Man. Furthermore, during the first 5 min after the addition of cycloheximide to LM cells, when [3H]mannose incorporation into oligosaccharide-lipid was maximally reduced, the specific activity of the GDP-Man pool was identical to that observed in control cells. Labeling in vivo of the lipid-linked saccharide precursors of oligosaccharide-lipid revealed that cycloheximide had virtually no effect on the synthesis of Man-P-Dol, GlcNAc-PP-Dol, GlcNAc2-PP-Dol, and beta-Man-(GlcNAc)2-PP-Dol (Dol = dolichol). In agreement with this finding, results of experiments in vitro using microsomes prepared from LM cells indicate that cycloheximide did not directly inhibit the enzymes responsible for the synthesis of these lipid-linked saccharide precursors. Supplementation of mouse LM cells by preincubation for 1 h with dolichyl phosphate (5 micrograms/ml) resulted in a 300% stimulation of oligosaccharide-lipid synthesis when compared to non-supplemented cells. However, dolichyl phosphate supplementation of cycloheximide-treated cells failed to restore oligosaccharide-lipid synthesis to the level observed in control cells. In control cells pre-labeled oligosaccharide-lipid turned over rapidly and, as expected, cycloheximide addition to the chase medium significantly retarded this turnover process. However, preincubation of cells with exogenous dolichyl phosphate had little or no effect on the turnover of oligosaccharide-lipid in control or cycloheximide-treated cells. These findings argue against dolichyl phosphate deficiency as the primary cause of reduced oligosaccharide-lipid synthesis when protein synthesis is blocked. The implications of these results, and an alternative hypothesis to explain the effect of inhibition of protein synthesis on oligosaccharide-lipid synthesis based on elevation of intracellular GTP levels, are discussed.

Effect of phospholipids on membrane-bound and solubilized mannosyltransferase activity from aortic wall

Phospholipids interact on the membrane-bound and solubilized mannosyltransferase activity. The biosynthesis of Dol-P-Man is strongly inhibited by phosphatidic acid and lysophosphatidylcholine. The effect of phospholipids is not related to stereospecificity. Chemical properties of phospholipids (ester or ether bond, length of fatty acids and polarity of head groups) are not an essential factor for inhibition. The different parameters involved in enzymatic reaction of glycosylation are not modified by phospholipids, in particular the integrity of GDP-[14C]mannose. The inhibitory effect of lysophosphatidylcholine and phosphatidic acid on mannosyltransferase activities is related to their possible formation of micellar structures which definitely induce a conformation change of this enzyme.

Biological activities of isolated tunicamycin and streptovirudin fractions

The nucleoside antibiotics tunicamycin and streptovirudin were separated by high-performance liquid chromatography into a series of 256-nm-absorbing peaks. Most of the streptovirudin peaks eluted from a Biosil ODS column earlier than those of tunicamycin, indicating that they were less hydrophobic. With the exception of the first peak, 17 other tunicamycin peaks were potent inhibitors of the formation of dolichylpyrophosphoryl-N-acetylglucosamine with 50% inhibition of the solubilized GlcNAc-1-P transferase requiring about 10 ng of antibiotic per mL. These fractions also inhibited the synthesis of dolichylphosphorylglucose, but in these cases about 500 ng/mL was necessary to achieve 50% inhibition. In MDCK cells in culture, the four major tunicamycin peaks inhibited the incorporation of [2-(3)H]mannose into protein by 50% at about 0.2-0.5 microgram/mL, but [3H]leucine incorporation into protein was unaffected, except at high levels of antibiotic (5-10 microgram/mL). Essentially the same results were observed with the streptovirudin fractions except that they were somewhat less active and some inhibition of protein synthesis was observed with several of these peaks.

Adherence of bacteria to mammalian cells: inhibition by tunicamycin and streptovirudin

Group B streptococci were labeled either by growing the cells in [14C]fructose or by using the surface label 4,4'-[3H]diisothiocyano-1,2-diphenylethane-2,2'-disulfonic acid, which reacts with amino groups. A quantitative assay was developed by using these labeled bacteria to study the adherence of streptococci to canine kidney epithelial cells. The bacteria adhered to kidney cells that had been infected with influenza A virus, but did not adhere to uninfected cells. The binding of 3H-labeled group B streptococci was proportional to the number of bacteria added and showed saturation kinetics. The binding was blocked by the addition of unlabeled group B streptococci but was not affected by addition of streptococci from other groups. It was also blocked by mixing the 3H-labeled streptococci with influenza A virus before adding the bacteria to the kidney cells. When the kidney cells were infected with influenza virus in the presence of either tunicamycin or streptovirudin, these antibiotics inhibited the appearance of viral hemagglutinin in the kidney cells and also prevented the release of mature virus. In these experiments, the adherence of 3h-labeled streptococci was also inhibited. Tunicamycin was shown to block the incorporation of [14C]mannose into lipid-linked oligosaccharides and glycoprotein in both normal and virus-infected kidney cells. These data give strong support to the notion that adherence of streptococci to mammalian cells involves recognition of viral hemagglutinin, a glycoprotein whose synthesis is blocked by certain antibiotics.

Mannosyl transfer by membranes of Aspergillus niger: mannosylation of endogenous acceptors and partial analysis of the products

A smooth membrane fraction of Aspergillus niger catalyzed the transfer of mannose from GDP-mannose to endogenous lipid and protein acceptors. The mannolipid was acidic, as judged by diethylaminoethyl-cellulose chromatography, and had a mobility similar to ficaprenyl phosphate on thin-layer chromatograms. Mannose transfer occurred optimally at pH 6.5 to 7.5 and required Mn(2+) for use of the protein as acceptor, but either Mn(2+) or Mg(2+) with the lipid as acceptor. Glycopeptides of the mannosylated protein ([(14)C]gly) and of an alpha-glucosidase (alpha-glu) secreted by the organism were produced by Pronase digestion and separation of the products on Sephadex G-25. Because ovalbumin has a carbohydrate composition similar to that of alpha-glu and because the carbohydrate structure of ovalbumin is known, ovalbumin glycopeptides (Ov) were similarly obtained and used as standards in determining carbohydrate structures. Oligosaccharide chains of [(14)C]gly, alpha-glu, and Ov were obtained by treatment of the respective glycopeptides with endo-beta-N-acetylglucosaminidase, reduction with NaBT(4), and concanavalin A-Sepharose chromatography. The (3)H-labeled oligosaccharides obtained were subjected to the following treatments: (i) digestion with alpha- and beta-mannosidases, (ii) Smith degradation, and (iii) acetolysis. Subsequently, changes in paper chromatographic mobilities were detected. Also, alpha-glu was permethylated, and the partially methylated alditol acetates were analyzed by gas-liquid chromatography. The resultant proposed structure shows that the oligosaccharide chain of alpha-glu is almost identical to that of an Ov chain, while [(14)C]gly has a structure which is probably the same as that of alpha-glu. It is suggested that the transferase(s) involved in [(14)C]gly synthesis in vitro may be responsible for glycosylation of secreted enzymes.

Study of nuclear mannosyl-transferase: lipids intermediates

Mannosyl-transferase of rat liver nuclei catalyzed the transfer of mannose, from GDP-mannose, to endogeneous lipids and proteins. Two solubility-different glycolipids were characterized: a mannosyl-phosphoryl-dolichol and an oligosaccharide-lipid. The labeling of the two lipids was consistent with a role in mannose transfer to nuclear glycoproteins, but not according to the Lennarz' scheme.

Lipid-bound oligosaccharides in insects

Membrane preparations from immature stages of the fruit fly Ceratitis capitata catalyze the transfer of mannose from GDP-[14C]mannose into lipid-linked oligosaccharides. These compounds behave as polyprenyl derivatives and their formation is stimulated by the addition of an acidic glycolipid fraction isolated from insects. The mannose-labeled oligosaccharides are attached to the poly-isoprenol by a pyrophosphoryl linkage and can be released by mild acid hydrolysis. The trisaccharide lipid has been partially characterized. The results indicate that the compound is polyprenyl-pyrophosphate-N,N'-diacetylchitobiose-mannose. Incubation of dolichyl phosphate [14C]mannose or lower 14C-labeled oligosaccharide lipids with unlabeled GDP-mannose and the insect enzyme leads to the labeling of a higher lipid-bound oligosaccharide. When UDP-N-acetyl[14C]glucosamine was incubated with insect membranes a 14C-labeled chitobiosyl lipid was synthesized. If unlabeled GDP-mannose was also present, the 14C label appeared in the trisaccharide and higher oligosaccharide lipids. Preliminary evidence indicates that the insect polyprenyl oligosaccharides described here might participate in glycoprotein biosynthesis.

Cell-free labeling in thyroid rough microsomes of lipid-linked and protein-linked oligosaccharides. I. Mannosylated units

In order to evaluate the possibility in a pig thyroid rough microsomal system of a transfer of pre-assembled sugar cores from sugar-lipids to protein, we have examined after incubation with GDP-[14C]Man the compounds bearing labeled saccharides and have determined some properties of their released saccharide moieties. The [14C]Man material specifically soluble in CHCl3/CH3OH/H2O, 10:10:3, behaved on DEAE-cellulose and when treated with hot alkali and alkaline phosphatase as a lipid pyrophosphate (sometimes accompanied by some dolichol-P-[14C]Man). Its saccharide moiety, released by mild acid, exhibited properties (molecular size, sensitivity to alpha-mannosidase, affinity for concanavalin A and charge modification introduced by a strong reductive alkaline treatment) pointing to a polymannosylated N,N'-diacetylchitobiose containing an average of nine monosaccharide units (from six to twelve). The [14C]mannosylated glycoproteins have represented all the polymeric label remaining after lipid extraction. From the susceptibility of their pronase glycopeptides to a differential reductive alkaline hydrolysis, it was concluded that their label belonged mainly to N-glycosidically linked units. Released saccharides exhibited the same properties as those from lipids, a result substantiating the possibility raised from previous studies of a transfer of pre-assembled moieties.

Biosynthesis of Saccharomyces cerevisiae glycoproteins: nature of some participating glycolipids

A particulate membrane preparation from Saccharomyces cerevisiae catalyzed the incorporation of mannose from GDP-mannose into lipids that were extractable in chloroform-methanol. One lipid has been previously characterized as dolichyl phosphomannose. Another one was purified by chromatography on silicic acid, DEAE-cellulose and Sephadex LH-20 was found to be alkali unstable. The lipid moiety was shown to be dolichol and the glycosydic part contained mannose, glucose and glucosamine. Radioactive mannose was also incorporated at a slower rate into more polar compounds. They were soluble in chloroform-methanol-water and were seen to liberate neutral oligosaccharides after alkaline hydrolysis. Radioactive mannose was also incorporated into substances which behave chemically as glycoproteins since they were insoluble in organic solvents, water and trichloroactic acid. Pronase treatment of the trichloroacetic acid-insoluble material released water-soluble oligosaccharides. When the particulate preparation which had been extracted with chloroform-methanol at-20 C, was incubated with GDP-(U-14C)mannose, radioactivity was incorporated into glycolipids that were soluble in chloroform-methanol-water and into glycoproteins. This result suggests that at least part of the mannose was transferred to endogenous acceptors independent of dolichyl phosphomannose.