The biosynthesis of the major lipid-linked oligosaccharide of Chinese hamster ovary cells occurs by the ordered addition of mannose residues

Structural Insight into the Mechanism of N-Linked Glycosylation by Oligosaccharyltransferase

Asparagine-linked glycosylation, also known as N-linked glycosylation is an essential and highly conserved post-translational protein modification that occurs in all three domains of life. This modification is essential for specific molecular recognition, protein folding, sorting in the endoplasmic reticulum, cell-cell communication, and stability. Defects in N-linked glycosylation results in a class of inherited diseases known as congenital disorders of glycosylation (CDG). N-linked glycosylation occurs in the endoplasmic reticulum (ER) lumen by a membrane associated enzyme complex called the oligosaccharyltransferase (OST). In the central step of this reaction, an oligosaccharide group is transferred from a lipid-linked dolichol pyrophosphate donor to the acceptor substrate, the side chain of a specific asparagine residue of a newly synthesized protein. The prokaryotic OST enzyme consists of a single polypeptide chain, also known as single subunit OST or ssOST. In contrast, the eukaryotic OST is a complex of multiple non-identical subunits. In this review, we will discuss the biochemical and structural characterization of the prokaryotic, yeast, and mammalian OST enzymes. This review explains the most recent high-resolution structures of OST determined thus far and the mechanistic implication of N-linked glycosylation throughout all domains of life. It has been shown that the ssOST enzyme, AglB protein of the archaeon Archaeoglobus fulgidus, and the PglB protein of the bacterium Campylobactor lari are structurally and functionally similar to the catalytic Stt3 subunit of the eukaryotic OST enzyme complex. Yeast OST enzyme complex contains a single Stt3 subunit, whereas the human OST complex is formed with either STT3A or STT3B, two paralogues of Stt3. Both human OST complexes, OST-A (with STT3A) and OST-B (containing STT3B), are involved in the N-linked glycosylation of proteins in the ER. The cryo-EM structures of both human OST-A and OST-B complexes were reported recently. An acceptor peptide and a donor substrate (dolichylphosphate) were observed to be bound to the OST-B complex whereas only dolichylphosphate was bound to the OST-A complex suggesting disparate affinities of two OST complexes for the acceptor substrates. However, we still lack an understanding of the independent role of each eukaryotic OST subunit in N-linked glycosylation or in the stabilization of the enzyme complex. Discerning the role of each subunit through structure and function studies will potentially reveal the mechanistic details of N-linked glycosylation in higher organisms. Thus, getting an insight into the requirement of multiple non-identical subunits in the N-linked glycosylation process in eukaryotes poses an important future goal.

User-friendly extraction and multistage tandem mass spectrometry based analysis of lipid-linked oligosaccharides in microalgae

BACKGROUND

Protein N-glycosylation is initiated within the endoplasmic reticulum through the synthesis of a lipid-linked oligosaccharides (LLO) precursor. This precursor is then transferred en bloc on neo-synthesized proteins through the action of the oligosaccharyltransferase giving birth to glycoproteins. The N-linked glycans bore by the glycoproteins are then processed into oligomannosides prior to the exit of the glycoproteins from the endoplasmic reticulum and its entrance into the Golgi apparatus. In this compartment, the N-linked glycans are further maturated in complex type N-glycans. This process has been well studied in a lot of eukaryotes including higher plants. In contrast, little information regarding the LLO precursor and synthesis of N-linked glycans is available in microalgae.

METHODS

In this report, a user-friendly extraction method combining microsomal enrichment and solvent extractions followed by purification steps is described. This strategy is aiming to extract LLO precursor from microalgae. Then, the oligosaccharide moiety released from the extracted LLO were analyzed by multistage tandem mass spectrometry in two models of microalgae namely the green microalgae, Chlamydomonas reinhardtii and the diatom, Phaeodactylum tricornutum.

RESULTS

The validity of the developed method was confirmed by the analysis of the oligosaccharide structures released from the LLO of two xylosyltransferase mutants of C. reinhardtii confirming that this green microalga synthesizes a linear Glc3Man5GlcNAc2 identical to the one of the wild-type cells. In contrast, the analysis of the oligosaccharide released from the LLO of the diatom P. tricornutum demonstrated for the first time a Glc2Man9GlcNAc2 structure.

CONCLUSION

The method described in this article allows the fast, non-radioactive and reliable multistage tandem mass spectrometry characterization of oligosaccharides released from LLO of microalgae including the ones belonging to the Phaeodactylaceae and Chlorophyceae classes, respectively. The method is fully adaptable for extracting and characterizing the LLO oligosaccharide moiety from microalgae belonging to other phyla.

Biochemical characterization and membrane topology of Alg2 from Saccharomyces cerevisiae as a bifunctional alpha1,3- and 1,6-mannosyltransferase involved in lipid-linked oligosaccharide biosynthesis

N-Linked glycosylation involves the ordered, stepwise synthesis of the unique lipid-linked oligosaccharide precursor Glc(3)Man(9) GlcNAc(2)-PP-Dol on the endoplasmic reticulum (ER), catalyzed by a series of glycosyltransferases. Here we characterize Alg2 as a bifunctional enzyme that is required for both the transfer of the alpha1,3- and the alpha1,6-mannose-linked residue from GDP-mannose to Man(1)GlcNAc(2)-PP-Dol forming the Man(3)GlcNAc(2)-PP-Dol intermediate on the cytosolic side of the ER. Alg2 has a calculated mass of 58 kDa and is predicted to contain four transmembrane-spanning helices, two at the N terminus and two at the C terminus. Contradictory to topology predictions, we prove that only the two N-terminal domains fulfill this criterion, whereas the C-terminal hydrophobic sequences contribute to ER localization in a nontransmembrane manner. Surprisingly, none of the four domains is essential for transferase activity because truncated Alg2 variants can exert their function as long as Alg2 is associated with the ER by either its N- or C-terminal hydrophobic regions. By site-directed mutagenesis we demonstrate that an EX(7)E motif, conserved in a variety of glycosyltransferases, is not important for Alg2 function in vivo and in vitro. Instead, we identify a conserved lysine residue, Lys(230), as being essential for activity, which could be involved in the binding of the phosphate of the glycosyl donor.

A new type of congenital disorders of glycosylation (CDG-Ii) provides new insights into the early steps of dolichol-linked oligosaccharide biosynthesis

Deficiency of GDP-Man:Man1GlcNAc2-PP-dolichol mannosyltransferase (hALG2), is the cause of a new type of congenital disorders of glycosylation (CDG) designated CDG-Ii. The patient presented normal at birth but developed in the 1st year of life a multisystemic disorder with mental retardation, seizures, coloboma of the iris, hypomyelination, hepatomegaly, and coagulation abnormalities. An accumulation of Man1GlcNAc2-PP-dolichol and Man2GlcNAc2-PP-dolichol was observed in skin fibroblasts of the patient. Incubation of patient fibroblast extracts with Man1GlcNAc2-PP-dolichol and GDP-mannose revealed a severely reduced activity of the mannosyltransferase elongating Man1GlcNAc2-PP dolichol. Because the Saccharomyces cerevisiae mutant alg2-1 was known to accumulate the same shortened dolichol-linked oligosaccharides as the patient, the yeast ALG2 sequence was used to identify the human ortholog. Genetic analysis revealed that the patient was heterozygous for a single nucleotide deletion and a single nucleotide substitution in the human ortholog of yeast ALG2. Expression of wild type but not of mutant hALG2 cDNA restored the mannosyltransferase activity and the biosynthesis of dolichol-linked oligosaccharides both in patient fibroblasts and in the alg2-1 yeast cells. hALG2 was shown to act as an alpha1,3-mannosyltransferase. The resulting Manalpha1,3-ManGlcNAc2-PP dolichol is further elongated by a yet unknown alpha1,6-mannosyltransferase.

Large-scale isolation of dolichol-linked oligosaccharides with homogeneous oligosaccharide structures: determination of steady-state dolichol-linked oligosaccharide compositions

The dolichol-linked oligosaccharide donor (Glc(3)Man(9)GlcNAc(2)-PP-Dol) for N-linked glycosylation of proteins is assembled in a series of reactions that initiate on the cytoplasmic face of the rough endoplasmic reticulum and terminate within the lumen. The biochemical analysis of the oligosaccharyltransferase and the glycosyltransferases that mediate assembly of dolichol-linked oligosaccharides (OS-PP-Dol) has been hindered by the lack of structurally homogeneous substrate preparations. We have developed an improved method for the preparative-scale isolation of dolichol-linked oligosaccharides from vertebrate tissues and yeast cells. Preparations that were highly enriched in either Glc(3)Man(9)GlcNAc(2)-PP-Dol or Man(9)GlcNAc(2)-PP-Dol were obtained from porcine pancreas and a Man(5)GlcNAc(2)-PP-Dol preparation was obtained from an alg3 yeast culture. Chromatography of the OS-PP-Dol preparations on an aminopropyl silica column was used to obtain dolichol-linked oligosaccharides with defined structures. A single chromatography step could achieve near-baseline resolution of dolichol-linked oligosaccharides that differed by one sugar residue. A sensitive oligosaccharyltransferase endpoint assay was used to determine the concentration and composition of the OS-PP-Dol preparations. Typical yields of Glc(3)Man(9)GlcNAc(2)-PP-Dol, Man(9)GlcNAc(2)-PP-Dol, and Man(5)GlcNAc(2)-PP-Dol ranged between 5 and 15 nmol per chromatographic run. The homogeneity of these preparations ranged between 85 and 98% with respect to oligosaccharide composition. Purification of dolichol-linked oligosaccharides from cultures of alg mutant yeast strains provides a general method to obtain authentic OS-PP-Dol assembly intermediates of high purity. The analytical methods described here can be used to accurately evaluate the steady-state dolichol-linked oligosaccharide compositions of wild-type and mutant cell lines.

Sequential assembly and polymerization of the polyprenol-linked pentasaccharide repeating unit of the xanthan polysaccharide in Xanthomonas campestris

Lipid-linked intermediates are involved in the synthesis of the exopolysaccharide xanthan produced by the bacterium Xanthomonas campestris (L. Ielpi, R. O. Couso, and M. A. Dankert, FEBS Lett. 130:253-256, 1981). In this study, the stepwise assembly of the repeating pentasaccharide unit of xanthan is described. EDTA-treated X. campestris cells were used as both enzyme preparation and lipid-P acceptor, and UDP-Glc, GDP-Man, and UDP-glucuronic acid were used as sugar donors. A linear pentasaccharide unit is assembled on a polyprenol-P lipid carrier by the sequential addition of glucose-1-P, glucose, mannose, glucuronic acid, and mannose. The in vitro synthesis of pentasaccharide-P-P-polyprenol was also accompanied by the incorporation of radioactivity into a polymeric product, which was characterized as xanthan, on the basis of gel filtration and permethylation studies. Results from two-stage reactions showed that essentially pentasaccharide-P-P-polyprenol is polymerized. In addition, the direction of chain elongation has been studied by in vivo experiments. The polymerization of lipid-linked repeat units occurs by the successive transfer of the growing chain to a new pentasaccharide-P-P-polyprenol. The reaction involves C-1 of glucose at the reducing end of the polyprenol-linked growing chain and C-4 of glucose at the nonreducing position of the newly formed polyprenol-linked pentasaccharide, generating a branched polymer with a trisaccharide side chain.

Fucosyltransferases: differential plasma and tissue alterations in hepatocellular carcinoma and cirrhosis

To determine whether abnormal metabolism of L-fucose in hepatocellular carcinoma is accompanied by alterations in the activities of fucosyltransferases, the latter were determined in plasma and liver tissue of patients with this disease and in cirrhotic and normal subjects. Activities of alpha-2/alpha-3 and alpha-6-L-fucosyltransferases were all significantly greater in plasma from patients with hepatocellular carcinoma than in plasma from cirrhotic patients or normal subjects (p less than 0.025). The activity of each enzyme was dependent, to a similar extent, on Mn2+, Mg2+ and triton X-10, irrespective of the source, and all displayed pH optimums in the range of 7.5 to 8.0. In contrast, activities of alpha-2/alpha-3 fucosyltransferases were significantly lower (p less than 0.025) in homogenates prepared from tumorous liver tissue than in that prepared from nontumorous tissue from hepatocellular carcinoma and cirrhotic patients, whereas for the alpha-6 enzyme the situation was reversed (typically, tumor tissue levels were 5 pmol/hr/mg; in nontumor tissue they were 2 pmol/hr/mg). Activities of galactosyl and mannosyltransferase in tumor tissue were greater in all cases than in nontumor cirrhotic tissue. Plasma fucosyltransferases are specifically elevated in hepatocellular carcinoma but different mechanisms appear to underlie the changes seen for alpha-2/alpha-3 and alpha-6-L-fucosyltransferases.

Biochemical and genetic analysis of an antigenic determinant found on N-linked oligosaccharides in Dictyostelium

Dictyostelium discoideum synthesizes many highly immunogenic carbohydrates of unknown structure and function. We have used monoclonal antibodies prepared against one of these called CA1 to investigate its structure and the consequences of its loss. CA1 is preferentially expressed on lysosomal enzymes as a specific arrangement of mannose-6-SO4 residues on N-linked oligosaccharides. Mutant strains HL241 and HL243 do not express CA1, and synthesize a truncated lipid-linked oligosaccharide (LLO) precursor that lacks the critical mannose residues needed for expression. The lesion appears to result from the loss of mannosyl transferase activity involved in LLO biosynthesis. The truncated LLO is poorly transferred to an artificial peptide acceptor in a cell-free N-glycosylation assay, and this appears to result from improper topological localization of the LLO or to a lower affinity of the LLO for the oligosaccharyl transferase. Although both mutants share these lesions, they are biochemically and genetically distinct. Only HL243 is lower in N-glycosylation in intact cells, and this is not a result of an altered structure of the LLO. There are other differences between the strains. HL241 can form fruiting bodies at a slower rate than normal while HL243 cannot aggregate. Genetic analysis of defects shows that the CA1 lesion in HL241 is recessive, while the lesion in both CA1 and in development are dominant and co-segregate in HL243 and are, therefore, likely to be in the same gene. Lysosomal enzyme targeting is normal but enzyme processing proceeds at a 2-3 fold slower rate in HL241 and HL243 compared to wild-type. Strain HL244 does not express CA1 since it completely lacks protein sulfation, but lysosomal enzyme targeting and processing proceeds at a normal rate, showing that sulfate is not essential for these processes. Alterations in oligosaccharide structure can have individualized effects on the biosynthesis of lysosomal enzymes. The results presented here illustrate how this approach can be used to study both the structure and function of carbohydrate epitopes.

Incompletely processed LH molecules synthesized by rat gonadotrophs treated with inhibitors of oligosaccharide processing

Inhibitors of N-linked oligosaccharide processing are useful tools for studies on the biological function of the oligosaccharide structures in glycoprotein hormones. We have synthesized molecules of lutropin (LH) containing high-mannose- and hybrid-type oligosaccharides using rat gonadotroph-enriched primary cultures in the presence of castanospermine (a glucosidase I inhibitor) or swainsonine (a mannosidase II inhibitor), in order to compare the actions of these molecules with that of the hormone containing complex-type oligosaccharides in the activation of the receptor-adenylate cyclase system. Treatment of gonadotrophs with the above inhibitors caused an increase in the synthesis of highly basic LH molecules (pI 9.6-10.0), because addition of charged carbohydrate moieties to these molecules was prevented. Characterization of the oligosaccharide structure performed by enzymatic treatment (endoglycosidase H and neuraminidase) and the use of immobilized lectins (wheat germ agglutinin and Ricinus communis agglutinin-120) showed that these inhibitor-synthesized LH molecules contained high-mannose- and hybrid-type (asialo and sialylated) oligosaccharides. Their immunological properties were similar to that of complex-type oligosaccharide LH, but they had significantly higher receptor-binding ability in comparison with a sialylated complex-type oligosaccharide LH (about 12-fold) and an asialo complex-type oligosaccharide LH (about 3-fold). It was noted that the incompletely processed molecules were less potent than complex-type oligosaccharide LH in the activation of adenylate cyclase of Leydig cells, showing about 40-60% of the activity induced by the sialylated complex-type oligosaccharide molecule. The present data indicate that the inhibition of terminal processing of N-linked oligosaccharides by castanospermine and swainsonine impairs the full hormonal function of rat LH.

Synthesis of lipid-linked oligosaccharides in Saccharomyces cerevisiae: Man2GlcNAc2 and Man1GlcNAc2 are transferred from dolichol to protein in vivo

Transfer of truncated oligosaccharides to protein in vivo and the structure of Man2GlcNAc2 synthesized by intact yeast (Saccharomyces cerevisiae) were investigated in the alg2 mutant. At the nonpermissive temperature the alg2 mutant accumulates lipid-linked oligosaccharides that migrate on Bio-Gel P4 in the range expected for Man2GlcNAc2 and Man1GlcNAc2 (T.C. Huffaker and P.W. Robbins (1983) Proc. Natl. Acad. Sci. USA 80, 7466-7470). We characterized the oligosaccharides, derived from protein and lipid, by comigration with standards on HPLC and by Smith degradation followed by HPLC. Man2GlcNAc2 and Man1GlcNAc2 are found on protein in alg2, since their release from a protein-containing precipitate of alg2 cells is N-glycanase (peptide-N4[N-acetyl-beta-glucosaminyl]asparagine amidase) dependent. Transfer also occurred in alg2/pAC3 cells, which carry ALG2 on a multicopy plasmid that confers partial correction of the oligosaccharide phenotype. The alg2/pAC3 cells are viable at 36 degrees C. Two isomers of Man2GlcNAc2, Man1----3ManGlcNAc2 and Man1----6ManGlcNAc2, were present on lipid and protein. The transfer of Man2GlcNAc2 and Man1GlcNAc2 to protein by intact cells supports topological models that postulate access by early intermediates to the lumen of the endoplasmic reticulum.

Study of the N-glycoprotein biosynthesis through dolichol intermediates in the mitochondrial membranes

1. In the mitochondria, the biosynthesis of N-glycoprotein products, through the dolichol intermediates pathway, appears in the outer and in the inner membranes. 2. The biosynthesis of dolichol-pyrophosphoryl-N-acetyl-glucosamine, dolichol-pyrophosphoryl-di-N-acetylchitobiose, dolichol-phosphoryl-glucose and dolichol-phosphoryl-mannose is effective in both membranes. 3. The lipid-linked oligosaccharides biosynthesized in both membranes contain high mannose-type oligosaccharides ranging in size from Man9-GlcNac2 to Man4-GlcNac2. 4. The assembly of the dolichol-pyrophosphoryl-oligosaccharides on the trimannosidic core begins by the elongation of the alpha-1,3 mannose branch in the outer membrane and of the alpha-1,6 mannose branch in the inner membrane.

Lutropin molecules synthesized by rat gonadotroph primary cultures in the presence of swainsonine: their oligosaccharides and biological potencies

The biosynthesis of lutropin (luteinizing hormone, LH) containing abnormally processed oligosaccharides has been studied in rat gonadotroph-enriched primary culture in the presence of swainsonine, an inhibitor of the N-linked oligosaccharide processing in the Golgi complex. The inhibitor led to time- and dose-dependent accumulation of highly alkaline LH molecules (pI 9.6, 10.0) with high binding affinity for concanavalin A (ConA), and concomitant decreases of less alkaline components (pI 8.0, 8.5, 8.8) with less affinity for the lectin. The accumulation of LH molecules was much amplified after gonadotrophin-releasing hormone (GnRH) stimulation. The characterization of the oligosaccharide structure performed by means of enzyme treatment and serial chromatography on immobilized ConA, Ricinin communis agglutinin-120 (RCA-120), and wheat germ agglutinin (WGA) showed that swainsonine-induced LH contained principally the hybrid-type oligosaccharide, which was processed by the addition of terminal sialic acid residues on one branch of each N-linked oligosaccharide in both subunits. Small amounts of LH molecules containing asialo or partially sialylated hybrid oligosaccharides were also synthesized. The data for in vitro steroidogenic activities of swainsonine-induced LH indicated that terminal sialic acid on one branch of the hybrid-type oligosaccharides decreased the full agonist activity by approximately 50%.

Biosynthesis, in calf pancreas microsomes, of three lipid-linked oligosaccharide diphosphates from a synthetic dolichyl diphosphate tetrasaccharide

Incubation of calf pancreas microsomes with synthetic alpha-D-Manp-(1----6)-beta-D-Manp-(1----4)-beta-D-GlcpNAc-(1 ----4)-alpha-D- GlcpNAc-PP-Dol and GDP-D-[14C]-mannose gave three major lipid-linked oligosaccharide diphosphates. After release of the phospholipid residue by mild acid hydrolysis, the corresponding [14C]oligosaccharides were analyzed by gel-filtration, liquid chromatography, degradation by endo-N-acetyl-beta-D-glucosaminidases D and H, by jack bean alpha-D-mannosidase and Aspergillus oryzae (1----2)-alpha-D-mannosidase, acetolysis, and binding to concanavalin A-Sepharose. From the results it could be inferred that the following reaction took place in calf pancreas microsomes: alpha-D-Manp-(1----6)-beta-D-Manp-(1----4)-beta-D-GlcpNAc-(1 ----4)-alpha-D- GlcpNAc-PP-Dol + GDP-D-Man gave GDP + alpha-D-Manp-(1----3)-[alpha-D-Manp-(1----6)]-beta-D-Manp -(1----4)- beta-D-GlcpNAc-(1----4)-alpha-D-GlcpNAc-PP-Dol. The next products to be formed were alpha-D-Manp-(1----2)-alpha-D-Manp-(1----3)-[alpha-D-Manp -(1----6)]-beta-D- Manp-(1----4)-beta-D-GlcpNAc-(1----4)-alpha-D-GlcpNAc-PP-Dol, followed by alpha-D-Manp-(1----2)-alpha-D-Manp-(1----2)-alpha-D-Manp+ ++-(1----3)- [alpha-D-Manp-(1----6)]-beta-D-Manp-(1----4)-beta-D-GlcpNAc- (1----4)-alpha- D-GlcpNAc-PP-Dol. The mannose incorporation was enhanced by Triton X-100 and inhibited by Mn2+, and it occurred in the presence of either Mg2+ or EDTA. It is likely that the mannose donor was GDP-mannose since, under the conditions used, the formation of dolichyl mannosyl phosphate was negligible and the dolichyl heptasaccharide diphosphate accumulated.

Isolation and partial purification of lipid-linked oligosaccharides from calf pancreas

Lipid-linked oligosaccharides containing at least five mannose residues (0.7 nmol/g of tissue) were isolated from calf pancreas by chloroform-methanol-water extraction and purification on DEAE-cellulose and Sephadex LH-20, and affinity chromatography on concanavalin A-Sepharose in the presence of nonionic detergent. The addition, prior to the chromatographic steps, of 14C-labeled lipid-linked oligosaccharides (synthesized in vitro by calf pancreas microsomes in the presence of GDP-D-[14C]mannose and UDP-D-[14C]glucose, respectively) as internal standards, indicated a final yield ranging from 38 to 50%. Analysis of the oligosaccharide residues by liquid chromatography of the lipid-free preparation, monitored by u.v. absorbance and radioactivity measurement of the tritiated compounds, indicated a heterogeneous mixture of oligosaccharides. Its components, ranging from Man5(GlcNAc)2 to Glc3Man9(GlcNAc)2, cochromatographed with the 14C-labeled derivatives from in vitro synthesis. Calf pancreas contains lipid intermediates bearing at least six mannose residues, such as Man9(GlcNAc)2-P-P-lipid, in almost equal or even higher amounts than Glc3Man9(GlcNAc)2-P-P-dolichol.

The dolichol pathway in the retina: oligosaccharide-lipid biosynthesis

The formation of the oligosaccharide-lipid intermediates of the dolichol pathway by the bovine retina was investigated. Intact retinas were incubated in vitro for various periods of time in the presence of a variety of radioactive sugars (2-[3H]mannose, 6-[3H]glucose, 1-[3H]galactose, 1-[14C]glucosamine) using incubation conditions which have been shown previously to support the glycosylation of rhodopsin. The oligosaccharide-lipids were isolated and partially purified by DEAE cellulose chromatography. After mild acid hydrolysis and reduction, the oligosaccharides were analysed by HPLC. Further identification was obtained by chemical means and after digestion of the oligosaccharides with alpha-mannosidase and endohexosaminidase H. The full array of oligosaccharide-lipids which have been observed in other tissues were detected in the bovine retina, although some striking differences were seen in their relative distribution. Although short-term incubations (up to 15 min) indicated that the major species was the fully glucosylated oligosaccharide-lipid (Glc3Man9GlcNAc2), with longer incubation times the non-glucose-containing intermediate, Man9GlcNAc2, became the predominant species. Since glycerol was the carbon source for these incubations, the possibility was investigated that glucose starvation may have been the basis for this phenomenon, as has been reported in other tissues. It was established that this was not the case. Experiments carried out in the presence of castanospermine and bromoconduritol indicated that alpha-glucosidase activity in the retina may have resulted in the accumulation of the unglucosylated oligosaccharide-lipids. The formation of oligosaccharide-lipid intermediates by cells of the retinal pigment epithelium from the embryonic chick, maintained in cell culture, was also examined. In contrast to the bovine retina, the major species present were the glucose-containing intermediates, similar to other tissues.

Characterization of the high mannose asparagine-linked oligosaccharides synthesized by Schistosoma mansoni adult male worms

This report describes the structures of the high-mannose-type N-linked oligosaccharides in glycoproteins synthesized by Schistosoma mansoni adult male worms. Adult male schistosomes were incubated in vitro in media containing either [2-3H]mannose, [6-3H]glucosamine or [6-3H]galactose to allow metabolic radiolabeling of the oligosaccharide moieties of newly synthesized glycoproteins. Glycopeptides were prepared from the radiolabeled glycoproteins by digestion with Pronase and fractionation by chromatography on concanavalin A-Sepharose. Eleven percent of [3H]mannose incorporated into the schistosome glycopeptides was recovered in high mannose-type Asn-linked oligosaccharides which bound to the immobilized lectin. Upon treatment of [3H]mannose-labeled glycopeptide with endo-beta-N-acetylglucosaminidase H, the high mannose-type chains were released and their structures were determined by high performance liquid chromatography, methylation analysis, acetolysis and exoglycosidase digestion. The major species of high mannose-type chains synthesized by S. mansoni adult males have the composition Man7GlcNAc2, Man8GlcNac2 and Man9GlcNA2. Structural analyses indicate that these oligosaccharides are similar to high mannose-type chains synthesized by mammalian cells.

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 the tetrasaccharide lipid intermediate P1-dolichyl P2-[O-alpha-D-mannopyranosyl-(1----6)-O-beta-D-mannopyranosyl-(1----4) -O-(2- acetamido-2-deoxy-beta-D-glucopyranosyl)-(1----4)-2-acetamido-2- deoxy-alpha-D-glucopyranosyl] diphosphate

O-alpha-D-Mannopyranosyl-(1----6)-O-beta-D-mannopyranosyl-(1----4)-O-(2- acetamido-2-deoxy-beta-D-glucopyranosyl)-(1----4)-2-acetamido-2-deoxy- D-glucopyranose was isolated from bovine or ovine mannosidosis urine. After peracetylation, treatment with trimethylsilyl trifluoromethanesulfonate gave a high yield of a peracetyl oxazoline, which was phosphorylated with dibenzyl phosphate to give a dibenzyl glycosyl phosphate that was converted into a peracetyl tetrasaccharide phosphate by catalytic hydrogenolysis. A coupling reaction with P1-dolichyl P2-diphenyl diphosphate, prepared in two stages from pig-liver dolichol, yielded a peracetyl diphosphoric diester, which on O-deacetylation gave P1-dolichyl P2-[O-alpha-D-mannopyranosyl-(1----6)-O-beta-D-mannopyranosyl-(1----4)-O -(2- acetamido-2-deoxy-beta-D-glucopyranosyl)-(1----4)-2-acetamido-2-deoxy- alpha-D- glucopyranosyl] diphosphate. This tetrasaccharide lipid intermediate was active as an acceptor of D-mannose residues from GDP-D-mannose in the presence of calf pancreas microsomes.

Purification and properties of beta-mannosyltransferase that synthesizes Man-beta-GlcNAc-GlcNAc-pyrophosphoryl-dolichol

The beta-mannosyltransferase that adds mannose, from GDP-mannose, to GlcNAc-GlcNAc-pyrophosphoryl-dolichol, to form Man-beta-GlcNAc-GlcNAc-pyrophosphoryl-dolichol was solubilized from pig aorta microsomal preparations, using 0.5% NP-40, and was purified about 116-fold using conventional methods. The purified enzyme was mostly free of alpha 1,3- or alpha 1,6-mannosyltransferase activities, since Man beta-GlcNAc-GlcNAc-PP-dolichol (PP = pyrophosphoryl) accounted for more than 95% of the product when enzyme was incubated with GDP-[14C]mannose and GlcNAc-GlcNAc-PP-dolichol. Very little Man-beta-GlcNAc-GlcNAc-PP-dolichol was formed when GDP-[14C]mannose was replaced by dolichol-phosphoryl-[14C]mannose, indicating that GDP-mannose was the mannosyl donor. The oligosaccharide portion of this lipid was released by mild acid hydrolysis and was characterized by gel filtration as well as by susceptibility to beta-mannosidase and resistance to alpha-mannosidase. The partially purified enzyme could be stabilized by the addition of 20% glycerol and 0.5 mM dithiothreitol to the buffer, and could be kept in this solution for 5 or 6 days in ice. The enzyme was greatly stimulated by the addition of detergent (NP-40) with optimum activity being observed at 0.1%. However, no stimulation was seen with any phospholipid. The partially purified enzyme had a pH optimum of about 7.0, and showed an almost absolute requirement for Mg2+ with optimal activity occurring at about 5 mM Mg2+. Mn2+ and Ca2+ were only slightly active. The Km for GDP-mannose was about 5 X 10(-7) M and that for GlcNAc-GlcNAc-PP-dolichol about 1 X 10(-6) M. Beta-Mannosyltransferase activity was inhibited competitively by a variety of guanosine nucleotides with GDP and GDP-glucose being most active, but GTP, GMP, guanosine, and periodate-oxidized guanosine were also effective. The enzyme was strongly inhibited by p-chloromercuribenzenesulfonic acid and this inhibition was partially prevented by the addition of dithiothreitol.

Characterization of dolichol monophosphate- and dolichol diphosphate-linked saccharides in trypanosomatid flagellates

Dolichol-P- and dolichol-P-P-linked saccharides were isolated from several trypanosomatid flagellates incubated with [U-14C]glucose. Formation of Glc-P-dolichol and Man-P-dolichol was observed in Herpetomonas muscarum and Leishmania adleri, whereas only the latter derivative was synthesized in Trypanosoma dionisii and Leptomonas samueli. The main and largest dolichol-P-P-linked oligosaccharide formed in Trypanosoma conorhini, T. dionisii, L. samueli, Herpetomonas samuelpessoai and H. muscarum appeared to be Man9GlcNAc2, whereas in Blastocrithidia culicis it was Man6GlcNAc2. In L. adleri there were two main oligosaccharides linked to dolichol-P-P, Man6GlcNAc2 and Man5GlcNAc2. The The structures of the oligosaccharides were identical with those of the intermediates in the formation of Glc3Man9GlcNAc2-P-P-dolichol in higher eucaryotes. It was concluded that similarly to Trypanosoma cruzi, Crithidia fasciculata and Leishmania mexicana, no glucosylated derivatives of dolichol-P-P were formed in the additional seven trypanosomatids studied here. The results obtained suggest that the defective step could be in some cases the formation of Glc-P-dolichol and in others, the transfer of glucose residues from the latter compound to dolichol-P-P-linked oligosaccharides.

Developmental regulation of glycosyltransferases involved in biosynthesis of asparagine-linked glycoproteins in mouse mammary gland

Three glycosyltransferases, namely N-acetylglucosaminyl-1-phosphotransferase, mannosyltransferase and glucosyltransferase, involved in the biosynthesis of asparagine-linked glycoproteins in the mouse mammary gland, were identified by characterization of the products formed by these enzymes. The tissue capacities of the glycosyltransferases, measured as pmol product formed g tissue-1 min-1, increase during developmental cycle of the gland until, at late lactational stage, they reach more than 34, 14 and 60 times, respectively, the basal level found in the tissue of virgin animals. Among the three enzymes at late lactational stage the specific activities of glucosyltransferase and N-acetylglucosaminyl-1-phosphotransferase increase 7-fold and 4-fold respectively, whereas mannosyltransferase shows a mere 1.4-fold increase during tissue development. All of the enzymes, both in terms of tissue capacity and specific activity, return to the basal levels of the virgin gland one month after the end of lactation. The activities of the three enzymes in the lactating gland decrease precipitously following treatment of mice with bromocriptine, a drug that interferes with the release of prolactin from the pituitary gland and thereby inhibits milk synthesis and secretion. These results indicate that the three glycosyltransferases are developmentally regulated during the growth and differentiation of the mouse mammary gland.

Modifications of lysosomal enzymes in Dictyostelium discoideum

This paper has two purposes. The first is to review the past studies on the structure, biosynthesis, and immunological properties of a class of glycoproteins, the lysosomal enzymes, in Dictyostelium discoideum. The second purpose is to present new data on the analysis of mutant strains altered in the biosynthesis of the lipid-linked precursor of N-linked oligosaccharides, and on the characterization of new carbohydrate antigenic determinants found on multiple proteins in Dictyostelium. We will also show how a combination of genetic, biochemical and immunochemical approaches have been used to unravel a portion of the glycosylation pathway in Dictyostelium. The long-term goal of these studies is to use Dictyostelium discoideum as a model system to understand the functions of a variety of glycoconjugates in a multicellular organism. The existence of a large number of mutant strains which are altered in a variety of cellular functions, development and the posttranslational modification of multiple proteins, offers a great opportunity to explore this area.

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.

Characterization of the oligosaccharides of prolyl hydroxylase, a microsomal glycoprotein

Prolyl hydroxylase is a tetrameric glycoprotein that catalyzes a vital posttranslational modification in the biosynthesis of collagen. The enzyme purified from whole chick embryos (WCE) possesses two nonidentical subunits, alpha and beta, and has been shown by several techniques to reside in the endoplasmic reticulum of chick embryo fibroblasts. The studies described here demonstrate that the larger of the two subunits (alpha) exists in two forms in chick embryo fibroblasts (CEF); these two forms differ in carbohydrate content. The larger alpha subunit, alpha', contains two N-linked high mannose oligosaccharides, each containing eight mannose units; the smaller subunit, alpha, contains a single seven-mannose N-linked oligosaccharide. Both oligosaccharides could be cleaved by endo-beta-N-acetylglucosaminidase H and completely digested with alpha-mannosidase to yield mannosyl-N-acetylglucosamine.

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.

Characterization of Crithidia fasciculata oligosaccharide-lipid and its elongation by bovine mammary microsomes

It was recently shown that a Man7(GlcNAc)2-lipid species serves as the precursor for the biosynthesis of asparagine-linked glycoproteins in the trypanosomatid Crithidia fasciculata. Preliminary results indicated it to be similar to the intermediate in the major pathway for the biosynthesis of lipid-linked Glc3Man9(GlcNAc)2 in animal systems. To explore the potential of this glycolipid as an acceptor for studying the biosynthesis of mammary glycoproteins, we conducted a detailed structural analysis of the labelled Man7(GlcNAc)2-lipid isolated from exponentially growing cells of C. fasciculata. The results showed its structure to be Man alpha 1----2Man alpha 1----2Man alpha 1----3(Man alpha 1----2Man alpha 1----3Man alpha 1----6)Man beta----GlcNAc beta----GlcNAc, identical to the nonasaccharide synthesized by the animal systems. Incubation of the Man7(GlcNAc)2-lipid with bovine mammary microsomes along with GDP-mannose or mannosyl-phosphodolichol elongates it to give Man9(GlcNAc)2-lipid having the same structure as the corresponding intermediate in animal systems. Inhibition of the elongation reaction by EDTA or amphomycin indicates that the intermediary formation of mannosyl-phosphodolichol is required for the incorporation of mannose residues into the nonasaccharide-lipid. Mannosyl.phosphoretinol failed to serve as a mannosyl donor in this reaction.

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.

The synthesis of a trisaccharide and a tetrasaccharide lipid intermediate. P1-dolichyl P2-[O-beta-D-mannopyranosyl-(1----4)-O-(2-acetamido-2-deoxy- beta-D-glucopyranosyl)-(1----4)-2-acetamido-2-deoxy-alpha-D- glucopyranosyl] diphosphate and P1-dolichyl P2-[O-alpha-D-mannopyranosyl-(1----3)-O-beta-D- mannopyranosyl-(1----4)-O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)- (1----4)-2-acetamido-2-deoxy-alpha-D-glucopyranosyl] diphosphate

Synthetic benzyl O-(2,3,4,6,-tetra-O-acetyl-beta-D-mannopyranosyl)-(1-4)- (2-acetamido-3,6-di-O-acetyl-2-deoxy-beta-D-glucopyranosyl)-(1-4)- 2-acetamido-3,6-di-O-benzyl-2-deoxy-alpha-D-glucopyranoside was converted, by catalytic hydrogenolysis of the benzyl groups, followed by treatment with acetyl chloride-hydrogen chloride and then "chloride-ion catalysis", into O-(2,3,4,6-tetra-O-acetyl- beta-D-mannopyranosyl)-(1-4)-O-(2-acetamido-3,6-di-O-acetyl-2-deoxy beta- D-glucopyranosyl)- (1-4)-2-methyl-(3,6-di-O-acetyl-1,2-dideoxy-alpha-D- glucopyrano)-[2,1-d]-2-oxazoline. This compound was phosphorylated by treatment with dibenzyl phosphate under strictly anhydrous conditions to give after catalytic hydrogenolysis, a peracetyl trisaccharide phosphate. This was coupled with P1-dolichyl P2-diphenyl diphosphate to give, after O-deacetylation, P1-dolichyl P2-[O- beta-D-mannopyranosyl- (1-4)-O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)- (1-4)-2-acetamido-2-deoxy-alpha-D-glucopyranosyl] diphosphate, a trisaccharide "lipid intermediate". O-(2,3,4,6-Tetra-O-acetyl-alpha-D-mannopyranosyl)-(1-3)-O-(2,4,6-tri-O- acetyl-beta-D-mannopyranosyl)-(1-4)-O-(2-acetamido-3,6-di-O-acetyl-2- deoxy-beta- D-glucopyranosyl)- (1-4)-(2-acetamido-3,6-di-O-acetyl-2-deoxy-alpha-D-glucopyranosyl) phosphate, synthesized from O-alpha-D-mannopyranosyl-(1-3)-O-beta-D- mannopyranosyl-(1-4)-2-acetamido-2-deoxy-D-glucopyranose that had been isolated from mannosidosis urine was coupled with P1-dolichyl P2-diphenyl diphosphate to give, after O-deacetylation, P1-dolichyl P2-[O-alpha-D-mannopyranosyl- (1-3)-O-beta-D-mannopyranosyl-(1-4)-O-(2-acetamido-2-deoxy-beta-D- glucopyranosyl)-(1-4)-2-acetamido-2-deoxy-alpha-D-glucopyranosyl] diphosphate, a tetrasaccharide "lipid intermediate".

Solubilization of mannosyltransferase activities for the biosynthesis of mammary glycoproteins. Elongation of tetrasaccharide-lipid to heptasaccharide-lipid by a solubilized enzyme preparation

The microsomal preparation from the lactating bovine mammary tissue was solubilized by treatment with nonionic detergent, NP-40, at a protein/detergent ratio of 1.5:1 and a detergent concentration of 0.5%. Following centrifugation at 147000 X g for 120 min, the supernatant fraction was incubated with labeled sugar nucleotides, GDP-Man and UDP-GlcNAc. It was found to synthesize a series of lipid-linked saccharides up to (Man)5-(GlcNAc)2. The solubilized glycosyltransferases retained up to about 60% of the activity after two weeks of storage at 4 degrees C. The biosynthesis of glycolipids was stimulated by a mixture of lipids obtained by extracting the mammary microsomes with CHCl3/CH3OH (2:1). A labeled lipid-linked tetrasaccharide of the structure Man alpha 1----3 Man beta----GlcNAc beta----GlcNAc was isolated by labeling baby hamster kidney cells with [2-3H]mannose under conditions of glucose starvation followed by extraction of the cells with CHCl3/CH3OH (2:1) and separation of the lipids by high-performance liquid chromatography. When this lipid-linked tetrasaccharide was incubated with the solubilized bovine mammary microsomes and GDP-Man, it was elongated to a lipid-linked heptasaccharide having the structure Man alpha 1----2Man alpha 1----2Man alpha 1----3(Man alpha 1----6)Man beta----GlcNAc beta----GlcNAc. The kinetics of the elongation reaction also revealed the intermediary formation of smaller amounts of lipid-linked pentasaccharide and hexasaccharide. The elongation reaction did not require any divalent metal ion and had a broad pH optimum between 6.8 and 7.6. The lack of inhibition of the elongation reaction by EDTA or amphomycin support earlier studies that GDP-Man rather than mannosylphosphoryldolichol, is the direct donor of mannosyl residues for the biosynthesis of glycolipids up to (Man)5(GlcNAc)2. Mannosylphosphorylretinol was ineffective as mannosyl donor for the elongation reaction.

Cleavage of dolichyl pyrophosphoryl oligosaccharides by endo-beta-N-acetylglucosaminidase H: comparison of enzymatic and acid hydrolysis techniques for saccharide release

Endo-beta-N-acetylglucosaminidase H (endo H) was found to bring about the complete hydrolysis of dolichyl pyrophosphoryl oligosaccharides. Both glycosylated and unglucosylated polymannose oligosaccharides were released by the enzyme through cleavage of the di-N-acetylchitobiose sequence. The action of the endo H on the oligosaccharide-lipids was facilitated by the inclusion of Triton X-100 (maximal stimulation at concentrations greater than 0.03%) or small amounts of a variety of other detergents; however, sodium dodecyl sulfate (0.1%) was strongly inhibitory. Although incubations were routinely carried out at pH 5.2, the enzyme was noted to be equally effective at pH 6.5 and to retain 75% of its activity toward oligosaccharide-lipid at pH 7.4. While these results broaden the known specificity of the endo H for the aglycon moiety, it was observed that even under optimal conditions the rate of hydrolysis of lipid-linked Glc3Man9GlcNAc2 was substantially slower than that of the same oligosaccharide attached to asparagine in a peptide sequence. The use of endo H, an enzyme which can be obtained free of exoglycosidases, appears to have a number of advantages over mild acid hydrolysis as a tool for cleaving oligosaccharide-lipids. It was found that the latter procedure causes a small but detectable degradation of the sugar chains and, when carried out in the presence of methanol, leads to the release of about 10% of the oligosaccharide as its beta-methyl glycoside. Furthermore, the oligosaccharides released by the endo H can be directly compared to those liberated by this enzyme from glycoproteins; this may prove to be useful in metabolic studies dealing with oligosaccharide-lipid assembly and their involvement in the N-glycosylation of proteins.