Biosynthesis of N- and O-linked oligosaccharides of the low density lipoprotein receptor

In human fibroblasts, the receptor for low density lipoprotein (LDL) is synthesized as a precursor of apparent Mr = 120,000 which is converted to a mature form of apparent Mr = 160,000, as determined by migration in sodium dodecyl sulfate (SDS)-polyacrylamide gels (Tolleshaug, H., Goldstein, J. L., Schneider, W. J., and Brown, M. S. (1982) Cell 30, 715-724). The current paper describes the relationship of N- and O-glycosylation to this post-translational modification. Oligosaccharides were analyzed from precursor and mature forms of LDL receptors that had been immunoprecipitated from cells grown in media containing radioactive sugars. In human epidermoid carcinoma A-431 cells, the receptor precursor appears to contain one N-linked high mannose oligosaccharide and approximately 6-9 N-acetylgalactosamine residues linked O-glycosidically to Ser/Thr residues. In the mature receptor, the O-linked oligosaccharides are mono- and disialylated species having the core structure of galactose leads to N-acetylgalactosamine leads to Ser/Thr. The single N-linked oligosaccharide of the mature receptor can either be a tri- or tetraantennary complex-type species. Similar results were obtained with normal human fibroblast receptor except that the O-linked oligosaccharides on the precursor are neutral disaccharides, of which one component is GalNAc and the N-linked complex type unit on the mature receptor is less branched. Since the addition of GalNAc residues to Ser/Thr residues precedes the conversion of N-linked high mannose-type oligosaccharides to complex-type structures, the transfer of N-acetylgalactosamine must occur prior to the entry of glycoproteins into the region of the Golgi containing the processing enzyme alpha-mannosidase I. We also studied the receptor from tunicamycin-treated cells and after treatment with neuraminidase. In addition, we analyzed the receptor synthesized by a lectin-resistant clone of Chinese hamster ovary cells that is deficient in adding galactose residues to both N- and O-linked oligosaccharides. These studies suggest that the apparent differences in molecular weight between the precursor and mature forms of the LDL receptor are largely, if not entirely, due to the addition of sialic acid and galactose residues to the O-linked GalNAc residues.(ABSTRACT TRUNCATED AT 400 WORDS)     

Cytochemical characterization of glycoproteins in the developing acrosome of rats. An ultrastructural study using lectin histochemistry, enzymes and chemical deglycosylation.

The composition and distribution of rat acrosomal glycoproteins during spermiogenesis have been investigated at light and electron microscopic level by means of a variety of morphological techniques including the application of lectins conjugated to peroxidase, digoxigenin and colloidal gold, enzyme and chemical deglycosylation procedures and conventional histochemistry. Results obtained with lectin histochemistry in combination with beta-elimination reaction and endoglucosaminidase F/peptide N-glycosidase F digestion suggest that glycoproteins of mature acrosomes contain both N- and O-linked oligosaccharides. N-linked chains of acrosomal glycoproteins contain mannose and external residues of N-acetylglucosamine and galactose. They also have fucose residues linked to the core region of the oligosaccharide side chains. O-linked oligosaccharide chains contain external residues of both galactose and N-acetylgalactosamine. Mannose, fucose, galactose and N-acetylglucosamine residues were detected in acrosomes at all steps of spermiogenesis. N-acetylgalactosamine residues were only observed in the late steps of the spermiogenesis. N-acetylneuraminic acid residues were not detected throughout the acrosomal development. At initial stages of acrosome formation, glycoproteins were preferentially distributed over the acrosomic granules. In cap phase spermatids, lectin binding sites were homogeneously distributed throughout the acrosomes; however, in mature spermatozoa, glycoproteins were predominantly located over the outer acrosomal membrane.     

Characterization of the N- and O-linked oligosaccharides in glycoproteins synthesized by Schistosoma mansoni schistosomula

This report describes the structural analyses of the O- and N-linked oligosaccharides contained in glycoproteins synthesized by 48-hr-old Schistosoma mansoni schistosomula. Schistosomula were prepared by mechanical transformation of cercariae and were then incubated in media containing either [2-3H] mannose, [6-3H]glucosamine, or [6-3H]galactose to metabolically radiolabel the oligosaccharide moieties of newly synthesized glycoproteins. Analysis by SDS-polyacrylamide gel electrophoresis and fluorography demonstrated that many glycoproteins were metabolically radiolabeled with the radioactive mannose and glucosamine precursors, whereas few glycoproteins were labeled by the radioactive galactose precursor. Glycopeptide were prepared from the radiolabeled glycoproteins by digestion with pronase and fractionated by chromatography on columns of concanavalin A-Sepharose and pea lectin-agarose. The structures of the oligosaccharide chains in the glycopeptides were analyzed by a variety of techniques. The major O-linked sugars were not bound by concanavalin A-Sepharose and consisted of simple O-linked monosaccharides that were terminal O-linked N-acetylgalactosamine, the minor type, and terminal O-linked N-acetylglucosamine, the major type. The N-linked oligosaccharides were found to consist of high mannose- and complex-type chains. The high mannose-type N-linked chains, which were bound with high affinity by concanavalin A-Sepharose, ranged in size from Man6GlcNAc2 to Man9GlcNAc2. The complex-type chains contained mannose, fucose, N-acetylglucosamine, and N-acetylgalactosamine. No sialic acid was present in any metabolically radiolabeled glycoproteins from schistosomula.     

Partial characterization of oligosaccharides expressed on midgut microvillar glycoproteins of the mosquito, Anopheles stephensi Liston

Midguts of the malaria-transmitting mosquito, Anopheles stephensi, were homogenized and microvillar membranes prepared by calcium precipitation and differential centrifugation. Oligosaccharides present on the microvillar glycoproteins were identified by lectin blotting before and after in vitro and in situ treatments with endo- and exo-glycosidases. Twenty-eight glycoproteins expressed a structurally restricted range of terminal sugars and oligosaccharide linkages. Twenty-three glycoproteins expressed oligomannose and/or hybrid N-linked oligosaccharides, some with alpha1-6 linked fucose as a core residue. Complex-type N-linked oligosaccharides on eight glycoproteins all possessed terminal N-acetylglucosamine, and alpha- and beta-linked N-acetylgalactosamine. Eight glycoproteins expressed O-linked oligosaccharides all containing N-acetylgalactosamine with or without further substitutions of fucose and/or galactose. Galactosebeta1-3/4/6N-acetylglucosamine-, sialic acidalpha2-3/6galactose-, fucosealpha1-2galactose- and galactosealpha1-3galactose- were not detected. Terminal alpha-linked N-acetylgalactosamine residues on N-linked oligosaccharides are described for the first time in insects. The nature and function of these midgut glycoproteins have yet to be identified, but the oligosaccharide side chains are candidate receptors for ookinete binding and candidate targets for transmission blocking strategies.     

Cytochemical characterization of glycoproteins in the developing acrosome of rats

Summary The composition and distribution of rat acrosomal glycoproteins during spermiogenesis have been investigated at light and electron microscopic level by means of a variety of morphological techniques including the application of lectins conjugated to peroxidase, digoxigenin and colloidal gold, enzyme and chemical deglycosylation procedures and conventional histochemistry. Results obtained with lectin histochemistry in combination with -elimination reaction and endoglucosaminidase F/peptide N-glycosidase F digestion suggest that glycoproteins of mature acrosomes contain both N- and O-linked oligosaccharides. N-linked chains of acrosomal glycoproteins contain mannose and external residues of N-acetylglucosamine and galactose. They also have fucose residues linked to the core region of the oligosaccharide side chains. O-linked oligosaccharide chains contain external residues of both galactose and N-acetylgalactosamine. Mannose, fucose, galactose and N-acetylglucosamine residues were detected in acrosomes at all steps of spermiogenesis. N-acetylgalactosamine residues were only observed in the late steps of the spermiogenesis. N-acetylneuraminic acid residues were not detected throughout the acrosomal development. At initial stages of acrosome formation, glycoproteins were preferentially distributed over the acrosomic granules. In cap phase spermatids, lectin binding sites were homogeneously distributed throughout the acrosomes; however, in mature spermatozoa, glycoproteins were predominantly located over the outer acrosomal membrane.     

Temporal aspects of O-glycosylation of glycoprotein C from herpes simplex virus type-1.

Herpes simplex virus type-1 glycoprotein C (gC1) contains several O-linked oligosaccharides clustered near N-linked chains, and Pronase digestion produces glycopeptides carrying both oligosaccharide types. We have taken advantage of this fact to investigate the temporal relationship between the initiation of O-linked chains and the processing of N-linked oligosaccharides. gC1 was isolated from herpes-simplex-virus-infected BHK (baby-hamster kidney) cells after short labelling periods with [3H]glucosamine, and the labelled Pronase-cleaved glycopeptides fractionated on concanavalin A-Sepharose. N-[3H]Acetylgalactosamine, mostly convertible into free N-[3H]acetylgalactosaminitol on mild alkaline-borohydride treatment, was found in glycopeptides with an affinity to concanavalin A-Sepharose corresponding to that of glycopeptides carrying Man8GlcNAc2 or larger N-linked chains. Since there is evidence that the processing of N-linked chains up to Man8GlcNAc2 involves enzymes located in the rough endoplasmic reticulum, current results strongly suggest that gC1 acquires O-linked N-acetylgalactosamine before the glycoprotein routing to the Golgi apparatus. The addition of the second sugar to the nascent O-linked chain appeared to occur after a relatively long lag time.     

The oligosaccharide moieties of the epidermal growth factor receptor in A-431 cells. Presence of complex-type N-linked chains that contain terminal N-acetylgalactosamine residues

The receptor for epidermal growth factor (EGF) in the human epidermoid carcinoma cell line A-431 is a glycoprotein of apparent molecular weight = 170,000. During biosynthesis, the receptor is first detected as a precursor of apparent Mr = 160,000. In this report we describe our studies on the structures of the oligosaccharide moieties of the mature receptor and its precursor. A-431 cells were grown in medium containing radioactive sugars and the radiolabeled receptors were purified by immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Radiolabeled glycopeptides were prepared from the purified receptor by proteolysis, and their structures were examined by a variety of techniques. The mature EGF receptor contains both complex-type and high mannose-type Asn-linked oligosaccharides in the approximate ratio of 2 to 1, while the precursor contains only high mannose-type chains. A number of experimental results demonstrate that the mature receptor does not contain oligosaccharides in O-linkage through N-acetylgalactosamine to either serine or threonine. The high mannose-type oligosaccharides in both precursor and mature receptor can be cleaved by endo-beta-N-acetylglucosaminidase H and occur in the mature receptor as Man9GlcNAc2 (6%), Man8GlcNAc2 (49%), Man7GlcNAc2 (25%), and Man6GlcNAc2 (20%), whereas, in the receptor precursor the high mannose chains occur primarily as Man8GlcNAc2 (70%). The complex-type oligosaccharides in the mature receptor are predominantly tri- or tetraantennary species and are unusual in several respects. (i) Many of the chains do not contain sialic acid, while the remaining chains contain 1-2 sialic acid residues. (ii) Half of the [3H] mannose-derived radioactivity was recovered as [3H] fucose and the remaining half as [3H] mannose, indicating that there may be an average of 3 fucose residues/chain. (iii) About one-third of the [3H] glucosamine-derived radioactivity in these glycopeptides was recovered as N-acetylgalactosamine and these residues are all alpha-linked and occur at the nonreducing termini. These data demonstrate that the complex-type Asn-linked oligosaccharides in the EGF receptor from A-431 cells contain sugar residues related to human blood type A. In light of other recent studies, these results suggest that in A-431 cells blood group determinants in surface glycoproteins are contained in Asn-linked but not O-linked oligosaccharides.     

Temporal aspects of the N- and O-glycosylation of human chorionic gonadotropin

The glycoprotein hormone, human chorionic gonadotropin (hCG), contains both N- and O-linked oligosaccharide chains linked to its beta-subunit. Using the human choriocarcinoma cell line, BeWo, we have examined the temporal relationship between N- and O-glycosylation of hCG and the subsequent processing of both types of oligosaccharide chains. The results indicate that, as observed in related cell lines, mature, completely glycosylated forms of the subunits of hCG cannot be detected intracellularly in BeWo cells during pulse-chase experiments with [35S]methionine. To more directly study the temporal relationship between N- and O-glycosylation of hCG in BeWo cells, 14C-amino acids and [3H]glucosamine (which also serves as a precursor to N-acetylgalactosamine) were used to label hCG. The results of these studies are consistent with a model for the N- and O-glycosylation of hCG in which 1) N-glycosylation of hCG occurs co-translationally or very shortly after translation, and 2) the addition of O-linked GalNAc residues to the polypeptide and the addition of peripheral GlcNAc residues to the N-linked oligosaccharide chains occur just prior to secretion, presumably in the Golgi complex.     

A murine cytotoxic T lymphocyte cell line resistant to Vicia villosa lectin is deficient in UDP-GalNAc:beta-galactose beta 1,4-N-acetylgalactosaminyltransferase

We have reported the presence of N-acetylgalactosamine linked beta 1,4 to galactose on O-linked oligosaccharides of a cloned murine cytotoxic T cell line and the absence of these residues from the O-linked structures of a Vicia villosa lectin-resistant mutant line, VV6, derived from parental B6.1.SF.1 cells (Conzelmann, A., and Kornfeld, S. (1984) J. Biol. Chem. 259, 12528-12535). This study shows that B6.1.SF.1 cells contain an enzyme which transfers N-acetylgalactosamine from UDP-GalNAc onto the O-linked tetrasaccharides of human glycophorin A, giving rise to pentasaccharides which contain beta-glycosidically linked N-acetylgalactosamine. Desialylated glycophorin was inactive as an acceptor. The enzyme also transfers N-acetylgalactosamine to the N-linked oligosaccharides of the Tamm-Horsfall glycoprotein. This glycoprotein is known to contain N-linked oligosaccharides with beta-linked N-acetylgalactosamine residues which constitute the Sda blood group determinant. This N-acetylgalactosaminyltransferase could not be detected in VV6 cells which can account for the lack of beta-linked N-acetylgalactosamine residues on its O-linked oligosaccharides. The two cell lines have comparable levels of UDP-GalNAc:apomucin N-acetylgalactosaminyltransferase, demonstrating that the enzyme deficiency in VV6 cells is selective. Both cell lines have a similar glycolipid content, with the major component being asialo-GM1. Since this glycolipid contains N-acetylgalactosamine linked beta 1,4 to galactose, it would appear that the N-acetylgalactosyltransferase involved in the biosynthesis of glycolipids is different from the UDP-GalNAc:glycoprotein N-acetylgalactosaminyltransferase. An independently derived murine CTL line also contains the UDP-GalNAc:glycoprotein N-acetylgalactosaminyltransferase, suggesting that the expression of this enzyme is a common characteristic of this type of cell line.     

Biosynthesis and glycosylation of the human complement regulatory protein decay-accelerating factor

The biosynthesis and oligosaccharide structure of the human complement regulatory glycoprotein decay-accelerating factor (DAF) were studied in erythrocytes and cell lines. Initial information relative to carbohydrate moieties of DAF was obtained by enzymatic digestions. The 74,000 Mr erythrocyte DAF was lowered 3000 by endoglycosidase F, whereas endoglycosidase H had no effect, indicating one N-linked complex-type unit. Treatment with endo-alpha-N-acetylgalactosaminidase to remove O-linked oligosaccharides resulted in a 48,000 Mr molecule (67% of the Mr shift being due to sialic acid), which decreased to 45,000 Mr after sequential endoglycosidase F treatment. To additionally define the oligosaccharide structure and identify precursors in biosynthetic pathways, DAF was studied in the HL-60 cell line differentiated by vitamin D toward monocytes. Pulse-chase experiments with [35S]methionine revealed a precursor species of 43,000 Mr that underwent an early post-translational modification to a 46,000 Mr intermediate, and subsequently was chased into a mature species of 80,000 Mr that aligned with 125I surface-labeled DAF from these cells. All three forms of DAF were approximately 3000 lower in Mr in the presence of tunicamycin. The two lower Mr DAF species were sensitive to endoglycosidases F and H but not to neuraminidase or endo-alpha-N-acetylgalactosaminidase. In summary, DAF is synthesized as a 43,000 Mr precursor species containing one N-linked high-mannose unit. Before entering the central region of the Golgi, it is converted to a 46,000 Mr species by an as yet unknown post-translational modification. The 46,000 Mr form is converted to the 74,000 Mr (erythrocyte) or 80,000 Mr (leukocyte) membrane form of DAF by the addition of multiple, sialylated O-linked oligosaccharide chains (responsible for the large electrophoretic mobility shift) and conversion of the single N-linked high-mannose unit to a complex-type structure. The cell-specific Mr variation between red and white blood cells arises during this post-translational modification from the 46,000 Mr biosynthetic intermediate to the mature DAF species expressed on the cell surface.     

Processing of the gp55-116 envelope glycoprotein complex (gB) of human cytomegalovirus.

The processing pathway of the major envelope glycoprotein complex, gp55-116 (gB), of human cytomegalovirus was studied using inhibitors of glycosylation and endoglycosidases. The results of these studies indicated that the mature gp55-116 is synthesized by the addition of both simple and complex N-linked sugars to a nonglycosylated precursor of estimated Mr 105,000. In a rapid processing step, the Mr 105,000 precursor is glycosylated to a protein of Mr 150,000 (gp150) which contains only endoglycosidase H-sensitive sugar linkages. The gp150 is then processed relatively slowly to a Mr 165,000 to 170,000 species (gp165-170), which is then cleaved to yield the mature gp55-116. Monensin prevented the final processing steps of the gp150, including cleavage, suggesting that transport through the Golgi apparatus is required for complete processing. Digestion of the intracellular forms of this complex as well as the virion forms confirmed the above findings and indicated that the mature virion form of gp55 contains 8,000 daltons of N-linked sugars. The virion gp116 contains some 52,000 to 57,000 daltons of N-linked carbohydrates and approximately 5,000 daltons of O-linked sugars.     

Carbohydrate components of lipovitellin of the sand crab Emerita asiatica

Lipovitellin II (Lv II), the major yolk protein of the anomuran crab Emerita asiatica, was purified using heparin-sepharose affinity column chromatography. The purified Lv II was a glycoprotein as it was stainable with periodic acid-Schiff's reagent. Quantitative analysis of sugars showed the presence of fucose, mannose, galactosamine, N-linked oligosaccharides, as well as O-linked oligosaccharides containing N-acetyl hexosamine as the terminal residue. The amount of N-linked oligosaccharides is higher than that of the O-linked oligosaccharides. Biogel P-4 column chromatographic separation of the radiolabeled oligosaccharides of Lv II showed the presence of five different O-linked oligosaccharides and four different N-linked oligosaccharide species. HPTLC separation of the neoglycolipids prepared from the O-linked oligosaccharides also showed the presence of five different O-linked oligosaccharide species. N-linked oligosaccharides contain significant quantities of mannose. Unisil column chromatographic purification in conjunction with HPTLC separation revealed three neutral glycolipid species such as monoglycosylceramide, diglycosylceramide, and triglycosylceramide in the Lv II. The functional significance of these carbohydrate components of the major yolk protein during embryogenesis of the sand crab is discussed.     

Biogenesis of the somatogenic receptor in rat liver

Certain structural characteristics, in particular the type of oligosaccharide chains associated with the rat liver somatogenic (GH) receptors, were studied in different isolated organelles involved in receptor biosynthesis, maturation, and binding, with the use of ligand-affinity cross-linking, incubation with various oligosaccharide chain-cleaving enzymes, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In an endoplasmic reticulum-enriched fraction, a somatogenic receptor with Mr 33,000, after correction for bound ligand (assuming a 1:1 binding ratio of ligand to receptor) was found to contain N-linked high mannose oligosaccharide chain(s). In an intermediate density fraction, enriched in cis-Golgi, a major receptor of Mr 43,000 was found to contain N-linked complex type of oligosaccharide chains. In a low density membrane fraction, containing trans-Golgi complex membranes and endocytic vesicles, three receptors of Mr 95,000, 55,000, and 43,000 were found. These three receptors contain N-linked complex-type oligosaccharide chains. Neuraminidase treatment resulted in a decrease of the Mr 95,000 and 43,000 receptors to Mr 81,000 and 39,000, respectively. Two specific somatogenic receptors of Mr 95,000 and 43,000 containing N-linked complex type of oligosaccharides were found in an isolated plasma membrane-enriched fraction. When isolated hepatocytes were analyzed, the Mr 95,000 receptor was found to be the major labeled species. Two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis (first dimension nonreducing and the second dimension reducing conditions), showed that the Mr 43,000 receptor is contained within the Mr 95,000 receptor. The data suggest that the Mr 33,000 receptor found in endoplasmic reticulum constitutes a precursor to the Mr 43,000 receptor and that the Mr 43,000 receptor is complexed with an unknown subunit during transport through the Golgi complex to form an Mr 95,000 receptor present on the cell surface.     

Xylosyl transfer to the core protein precursor of the rat chondrosarcoma proteoglycan

Rat chondrosarcoma chondrocytes were labeled with [3H]serine or [3H]mannose as a precursor. Intracellular proteoglycan core protein precursor was purified from cell lysates by immunoprecipitation with polyclonal antibodies against the hyaluronic acid-binding region, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The core precursor was eluted from the gels and treated with alkaline borohydride in order to convert serine residues substituted with xylose or N-acetylgalactosamine to alanine (or with alkaline sulfite to convert them to cysteic acid). After acid hydrolysis, the proportions of labeled serine and alanine (or cysteic acid) were determined by high performance liquid chromatography, and the results were compared with those obtained for the completed proteoglycan molecules isolated from the same cultures. In the completed proteoglycans, about 55% of the serine residues were substituted with xylose or N-acetylgalactosamine, while the corresponding figure for the intracellular precursor molecules was less than 5%. These results indicate, in agreement with our previous kinetic data, that the major part of the xylosyl transfer to the chondrosarcoma proteoglycan core protein precursor must occur late in the processing sequence, i.e. after about 85% of its intracellular lifetime and no more than 7 min before the addition of the rest of the chondroitin sulfate chain. The ratio of [3H]mannose to [3H]fucose in the core precursor was about 19, while that for the complete proteoglycan was about 2. This indicates the presence of high mannose, N-linked oligosaccharides on the core protein precursor which are converted to the complex forms on the completed proteoglycan. These data provide further support that the core precursor resides mainly in the pre-Golgi compartment and that xylosylation occurs mainly in a Golgi compartment.     

Glycosylation of the epidermal growth factor receptor in A-431 cells. The contribution of carbohydrate to receptor function

A-431 cells were treated with inhibitors of either N-linked glycosylation (tunicamycin or glucosamine) or of N-linked oligosaccharide processing (swainsonine or monensin) to examine the glycosylation of epidermal growth factor (EGF) receptors and to determine the effect of glycosylation modification on receptor function. The receptor was found to be an Mr = 130,000 polypeptide to which a relatively large amount of carbohydrate is added co-translationally in the form of N-linked oligosaccharides. Processing of these oligosaccharides accounts for the 10,000-dalton difference in electrophoretic migration between the Mr = 160,000 precursor and Mr = 170,000 mature forms of the receptor. No evidence was found for O-linked oligosaccharides on the receptor. Mr = 160,000 receptors resulting from swainsonine or monensin treatment were present on the cell surface and retained full function, as judged by 125I-EGF binding to intact cells and detergent-solubilized extracts and by in vitro phosphorylation in the absence or presence of EGF. On the other hand, when cells were treated with tunicamycin or glucosamine, ligand binding was reduced by more than 50% in either intact cells or solubilized cell extracts. The Mr = 130,000 receptors synthesized in the presence of these inhibitors were not found on the cell surface. In addition, no Mr = 130,000 phosphoprotein was detected in the in vitro phosphorylation of tunicamycin or glucosamine-treated cells. It appears, therefore, that although terminal processing of N-linked oligosaccharides is not necessary for proper translocation or function of the EGF receptor, the addition of N-linked oligosaccharides is required.     

Temporal association of the N- and O-linked glycosylation events and their implication in the polarized sorting of intestinal brush border sucrase-isomaltase, aminopeptidase N, and dipeptidyl peptidase IV.

The temporal association between O-glycosylation and processing of N-linked glycans in the Golgi apparatus as well as the implication of these events in the polarized sorting of three brush border proteins has been the subject of the current investigation. O-Glycosylation of pro-sucrase-isomaltase (pro-SI), aminopeptidase N (ApN), and dipeptidyl peptidase IV (DPPIV) is drastically reduced when processing of the mannose-rich N-linked glycans is blocked by deoxymannojirimycin, an inhibitor of the Golgi-located mannosidase I. By contrast, O-glycosylation is not affected in the presence of swainsonine, an inhibitor of Golgi mannosidase II. The results indicate that removal of the outermost mannose residues by mannosidase I from the mannose-rich N-linked glycans is required before O-glycosylation can ensue. On the other hand, subsequent mannose residues in the core chain impose no sterical constraints on the progression of O-glycosylation. Reduction or modification of N- and O-glycosylation do not affect the transport of pro-SI, ApN, or DPPIV to the cell surface per se. However, the polarized sorting of two of these proteins, pro-SI and DPPIV, to the apical membrane is substantially altered when O-glycans are not completely processed, while the sorting of ApN is not affected. The processing of N-linked glycans, on the other hand, has no influence on sorting of all three proteins. The results indicate that O-linked carbohydrates are at least a part of the sorting mechanism of pro-SI and DPPIV. The sorting of ApN implicates neither O-linked nor N-linked glycans and is driven most likely by carbohydrate-independent mechanisms.     

Human transferrin receptor contains O-linked oligosaccharides

We have investigated the oligosaccharides in the human transferrin receptor from three different cell lines. During our studies on the structures of the N-linked oligosaccharides of the receptor, we discovered that the receptor contains O-linked oligosaccharides. This report describes the isolation and characterization of these O-linked oligosaccharides. Three different human cell lines--K562, A431, and BeWo--were grown in media containing either [2-3H] mannose or [6-3H]glucosamine. The newly synthesized and radiolabeled transferrin receptors were purified by immunoprecipitation from cell extracts and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The receptor was proteolytically digested or treated directly with mild base/borohydride. The released radiolabeled glycopeptides and oligosaccharides were separated by a variety of chromatographic techniques, and their structures were analyzed. The transferrin receptor from all three cell types contains O-linked oligosaccharides that are released from peptide by mild base/borohydride treatment. The receptor from K562 cells contains at least one O-linked oligosaccharide having two sialic acid residues and a core structure of the disaccharide galactose-N-acetyl-galactosamine. In contrast, the O-linked oligosaccharides in the transferring receptors from both A431 and BeWo cell lines are not as highly sialylated and were identified as both the neutral disaccharide galactose-N-acetylgalactosamine and the neutral monosaccharide N-acetylgalactosamine. In addition, the receptors from all three cell lines contain both complex-type and high mannose-type N-linked oligosaccharides. The complex-type chains in the receptor from A431 cells have properties of blood group A antigens, whereas oligosaccharides in receptors from both BeWo and K562 cells lack these properties. These results are interesting since both A431 and BeWo cells, but not K562 cells, are positive for blood group A antigens. Thus, our results demonstrate that the human transferrin receptor contains O-linked oligosaccharides and that there are differences in the structures of both the O-linked and complex-type N-linked oligosaccharides on the receptors synthesized by different cell types.     

Characterization of oligosaccharide chains on mouse mammary tumor virus envelope proteins and the implications for the mechanism of their glucocorticoid regulated processing

Glucocorticoid hormone is required for complete posttranslational processing of the glycosylated mouse mammary tumor virus envelope precursor, Pr74env in the murine T-lymphosarcoma cell line, W7MG1. Metabolic labeling studies with [35S]methionine, [3H]galactose, and [3H]mannose, combined with enzymatic digestion analyses with a variety of endoglycosidases, demonstrated that both proteolytic processing and N-linked oligosaccharide maturation depended, either directly or indirectly, on glucocorticoid action. Pr74 is found in both control and hormone-treated cells. In both cases Pr74 molecules carry high mannose and/or hybrid, but not complex, oligosaccharide chains with very little or no sialic acid. When cells are grown with glucocorticoid, Pr74 is converted to gp52 and gp33 with greatly increased efficiency, and these mature glycoproteins carry complex oligosaccharides containing sialic acid. No O-linked carbohydrate was detected on any of these species. According to this evidence, the glucocorticoid-regulated step in this pathway must occur at or before the final mannose trimming step in the Golgi that is required for formation of complex carbohydrate chains.     

Neisseria gonorrhoeae O-linked pilin glycosylation: functional analyses define both the biosynthetic pathway and glycan structure

Neisseria gonorrhoeae expresses an O-linked protein glycosylation pathway that targets PilE, the major pilin subunit protein of the Type IV pilus colonization factor. Efforts to define glycan structure and thus the functions of pilin glycosylation (Pgl) components at the molecular level have been hindered by the lack of sensitive methodologies. Here, we utilized a 'top-down' mass spectrometric approach to characterize glycan status using intact pilin protein from isogenic mutants. These structural data enabled us to directly infer the function of six components required for pilin glycosylation and to define the glycan repertoire of strain N400. Additionally, we found that the N. gonorrhoeae pilin glycan is O-acetylated, and identified an enzyme essential for this unique modification. We also identified the N. gonorrhoeae pilin oligosaccharyltransferase using bioinformatics and confirmed its role in pilin glycosylation by directed mutagenesis. Finally, we examined the effects of expressing the PglA glycosyltransferase from the Campylobacter jejuni N-linked glycosylation system that adds N-acetylgalactosamine onto undecaprenylpyrophosphate-linked bacillosamine. The results indicate that the C. jejuni and N. gonorrhoeae pathways can interact in the synthesis of O-linked di- and trisaccharides, and therefore provide the first experimental evidence that biosynthesis of the N. gonorrhoeae pilin glycan involves a lipid-linked oligosaccharide precursor. Together, these findings underpin more detailed studies of pilin glycosylation biology in both N. gonorrhoeae and N. meningitidis, and demonstrate how components of bacterial O- and N-linked pathways can be combined in novel glycoengineering strategies.     

Localization of alpha 1----3-linked mannoses in the N-linked oligosaccharides of Saccharomyces cerevisiae mnn mutants

Neutral and phosphorylated N-linked oligosaccharides were isolated from Saccharomyces cerevisiae mnn9 and mnn9 gls1 mutant mannoproteins and separated into homologues that differed in the number of terminal alpha 1----3-linked mannoses. In each type of oligosaccharide, the addition of such mannose was shown to occur in an ordered rather than a random fashion. The results confirm and extend an earlier report that dealt with the N-linked oligosaccharides from yeast invertase [Trimble, R.B., & Atkinson, P.H. (1986) J. Biol. Chem. 261, 9815-9824], and they suggest that the postulated processing pathway can be generalized to include phosphorylated and glucose-containing N-linked oligomannosides. We conclude that this processing pathway is identical for the analogous oligosaccharides from the mnn9 and wild-type strains of S. cerevisiae. Analysis of the mnn2 mnn10 mannoprotein revealed that a similar modification occurred at the branched terminus of the outer chain as well as in the core in this mutant.