The use of 1-deoxymannojirimycin to evaluate the role of various alpha-mannosidases in oligosaccharide processing in intact cells

Effect of substrate structure on the activity of Man9-mannosidase from pig liver involved in N-linked oligosaccharide processing

Man9-mannosidase, an alpha 1,2-specific enzyme located in the endoplasmic reticulum and involved in N-linked-oligosaccharide processing, has been isolated from crude pig-liver microsomes and its substrate specificity studied using a variety of free and peptide-bound high-mannose oligosaccharide derivatives. The purified enzyme displays no activity towards synthetic alpha-mannosides, but removes three alpha 1,2-mannose residues from the natural Man9-(GlcNAc)2 substrate (M9). The alpha 1,2-mannosidic linkage remaining in the M6 intermediate is cleaved about 40-fold more slowly. Similar kinetics of hydrolysis were determined with Man9-(GlcNAc)2 N-glycosidically attached to the hexapeptide Tyr-Asn-Lys-Thr-Ser-Val (GP-M9), indicating that the specificity of the enzyme is not influenced by the peptide moiety of the substrate. The alpha 1,2-mannose residue which is largely resistant to hydrolysis, was found to be attached in both the M6 and GP-M6 intermediate to the alpha 1,3-mannose of the peripheral alpha 1,3/alpha 1,6-branch of the glycan chain. Studies with glycopeptides varying in the size and branching pattern of the sugar chains, revealed that the relative rates at which the various alpha 1,2-mannosidic linkages were cleaved, differed depending on their structural complexity. This suggests that distinct sugar residues in the aglycon moiety may be functional in substrate recognition and binding. Reduction or removal of the terminal GlcNAc residue of the chitobiose unit in M9 increased the hydrolytic susceptibility of the fourth (previously resistant) alpha 1,2-mannosidic linkage significantly. We conclude from this observation that, in addition to peripheral mannose residues, the intact chitobiose core represents a structural element affecting Man9-mannosidase specificity. A possible biological role of the enzyme during N-linked-oligosaccharide processing is discussed.

Subcellular distribution in rat liver of a novel broad-specificity (alpha 1----2, alpha 1----3 and alpha 1----6) mannosidase active on oligomannose glycans

Recently, the purification to homogeneity was reported of a novel broad-specificity alpha-mannosidase from rat liver microsomal membranes [P. Bonay and R. C. Hughes (1991) Eur. J. Biochem. 197, 229-238]. The enzyme catalyzed the ordered removal of alpha 1----2-, alpha 1----3- and alpha 1----6-linked mannose residues from MannGlcNAc oligosaccharide substrates where n = 4-9. We now show by subcellular fractionation and immunocytochemistry that the novel mannosidase is present in the endoplasmic reticulum, Golgi apparatus and endosomes. Enzyme activity is enriched in a heavy Golgi membrane fraction and to lesser extent in an intermediate density Golgi membrane fraction containing GlcNAc transferase I activity and in a 'late' endosomal fraction. Low levels of enzyme activity were detectable in endoplasmic reticulum membranes and in 'early' endosomes but not in receptor-enriched and ligand-free endosomes. Assays of enzymic activity using Golgi membrane fractions in the absence and presence of Triton X-100 showed that the active site of the enzyme faces the lumen of the membrane vesicles. Antibodies directed against the purified mannosidase showed no immunological cross-reaction to known endoplasmic reticulum and Golgi mannosidases. Conversely, the purified mannosidase was not recognized by antibodies directed against endoplasmic reticulum mannosidase nor Golgi mannosidase IA.

Recognition of the oligosaccharide and protein moieties of glycoproteins by the UDP-Glc:glycoprotein glucosyltransferase

It was found, in cell-free assays, that the Man8GlcNAc2 and Man7GlcNAc2 isomers having the mannose unit to which the glucose is added were glucosylated by the rat liver glucosyltransferase at 50 and 15%, respectively, of the rate of Man9GlcNAc2 glucosylation. This indicates that processing by endoplasmic reticulum mannosidases decreases the extent of glycoprotein glucosylation. All five different glycoproteins tested (bovine and porcine thyroglobulins, phytohemagglutinin, soybean agglutinin, and bovine pancreas ribonuclease B) were found to be poorly glucosylated or not glucosylated unless they were subjected to treatments that modified their native conformations. The effect of denaturation was not to expose the oligosaccharides but to make protein determinants, required for enzymatic activity, accessible to the glucosyltransferase because (a) cleavage of denatured glycoproteins by unspecific (Pronase) or specific (trypsin) proteases abolished their glucose acceptor capacities almost completely except when the tryptic peptides were held together by disulfide bonds and (b) high mannose oligosaccharides in native glycoproteins, although poorly glucosylated or not glucosylated, were accessible to macromolecular probes as concanavalin A-Sepharose, endo-beta-N-acetylglucosaminidase H, and jack bean alpha-mannosidase. In addition, denatured, endo-beta-N-acetylglucosaminidase H deglycosylated glycoproteins were found to be potent inhibitors of the glucosylation of denatured glycoproteins. It is suggested that in vivo only unfolded, partially folded, and malfolded glycoproteins are glucosylated and that glucosylation stops upon adoption of the correct conformation, a process that hides the protein determinants (possibly hydrophobic amino acids) from the glucosyltransferase.

Substrate specificity of rat liver cytosolic alpha-D-mannosidase. Novel degradative pathway for oligomannosidic type glycans

The substrate specificity of rat liver cytosolic neutral alpha-D-mannosidase was investigated by in vitro incubation with a crude cytosolic fraction of oligomannosyl oligosaccharides Man9GlcNAc, Man7GlcNAc, Man5GlcNAc I and II isomers and Man4GlcNAc having the following structures: Man9GlcNAc, Man(alpha 1-2)Man(alpha 1-3)[Man(alpha 1-2)Man(alpha 1-6)]Man(alpha 1-6) [Man(alpha 1-2)Man(alpha 1-3)]Man(beta 1-4)GlcNAc; Man5GlcNAc I, Man(alpha 1-3)[Man(alpha 1-6)]-Man(alpha 1-6)Man(alpha 1-3)] Man(beta 1-4)GlcNAc; Man5GlcNAc II, Man(alpha 1-2)Man(alpha 1-2)Man(alpha 1-3) [Man(alpha 1-6)]Man(beta 1-4)GlcNAc; Man4GlcNAc, Man(alpha 1-2)Man(alpha 1-2)Man(alpha 1-3)Man(beta 1-4)GlcNAc. The different oligosaccharide isomers resulting from alpha-D-mannosidase hydrolysis were analyzed by 1H-NMR spectroscopy after HPLC separation. The cytosolic alpha-D-mannosidase activity is able to hydrolyse all types of alpha-mannosidic linkages found in the glycans of the oligomannosidic type, i.e. alpha-1,2, alpha-1,3 and alpha-1,6. Nevertheless the enzyme is highly active on branched Man9GlcNAc or Man5GlcNAc I oligosaccharides and rather inactive towards the linear Man4GlcNAc oligosaccharide. Structural analysis of the reaction products of the soluble alpha-D-mannosidase acting on Man5-GlcNAc I and Man9GlcNAc gives Man3GlcNAc, Man(alpha 1-6)[Man(alpha 1-3)]Man(beta 1-4)GlcNAc, and Man5GlcNAc II oligosaccharides, respectively. This Man5GlcNAc II, Man(alpha 1-2)Man(alpha 1-3)[Man(alpha 1-6)]Man(beta 1-4)GlcNAc, represents the 'construction' Man5 oligosaccharide chain of the dolichol pathway formed in the cytosolic compartment during the biosynthesis of N-glycosylprotein glycans. The cytosolic alpha-D-mannosidase is activated by Co2+, insensitive to 1-deoxymannojirimycin but strongly inhibited by swainsonine in the presence of Co2+ ions. The enzyme shows a highly specific action different from that previously described for the lysosomal alpha-D-mannosidases [Michalski, J.C., Haeuw, J.F., Wieruszeski, J.M., Montreuil, J. and Strecker, G. (1990) Eur. J. Biochem. 189, 369-379]. A possible complementarity between cytosolic and lysosomal alpha-D-mannosidase activities in the catabolism of N-glycosylprotein is proposed.

alpha-Mannosidase-catalyzed trimming of high-mannose glycans in noninfected and baculovirus-infected Spodoptera frugiperda cells (IPLB-SF-21AE). A possible contributing regulatory mechanism for assembly of complex-type oligosaccharides in infected cells

Incubation of a Spodoptera frugiperda (IPLB-SF-21AE) cell extract with the oligosaccharide Man9GlcNAc2, the aglucosyl derivative of the glycan that is normally transferred from the dolichol carrier to the relevant Asn residue in the nascent protein, results in its trimming to Man6GlcNAc2, an intermediate that is relatively stable to further alpha-D-mannosidase action in these cells. On the other hand, incubation of a similar extract from cells that had been infected for various times with a wild-type baculovirus (Autographa californica nuclear polyhedrosis virus) or a recombinant baculovirus (r-BAC)/human plasminogen (HPg) construct employed for expression of HPg led to rapid trimming of Man6GlcNAc2 to Man5GlcNAc2 and Man3GlcNAc2. These latter reactions displayed temporal effects, in that an enhancement of this latter trimming process occurred as a function of the time of infection of the cells with the wild-type and recombinant viral constructs. We have previously demonstrated that the nature of the oligosaccharide assembled on Asn289 of HPg expressed in several lepidopteran insect cell lines was dependent on the time of infection of the cells with r-BAC/HPg and that the amount of complex glycan found on this recombinant protein increased with an increase in infection times [Davidson, D. J., & Castellino, F. J. (1991) Biochemistry 30, 6167-6174].(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of the integrin Mac-1 (CD11b/CD18) to the third immunoglobulin-like domain of ICAM-1 (CD54) and its regulation by glycosylation

Both the integrins LFA-1 and Mac-1 bind to ICAM-1, an immunoglobulin superfamily member. Previously, we localized the binding sites of LFA-1 and the major group of human rhinoviruses to the first NH2-terminal immunoglobulin-like domain of ICAM-1. Here, we show that the binding site on ICAM-1 for Mac-1 is unexpectedly distinct from that for LFA-1 and maps to the third NH2-terminal immunoglobulin-like domain. These findings provide a function for the tandem duplication of immunoglobulin-like domains in ICAM-1 and have implications for other immunoglobulin superfamily members. Mutations at two sites in the third domain that destroy consensus sequences for N-linked glycosylation enhance binding to purified Mac-1. Agents that interfere with carbohydrate processing provide evidence that the size of the N-linked oligosaccharide side chains on ICAM-1 affects binding to Mac-1 but not to LFA-1. Thus, we suggest that the extent of glycosylation on ICAM-1 may regulate adhesion to LFA-1 or Mac-1 in vivo.

Development of monoclonal antibodies against different protein and carbohydrate epitopes of dipeptidyl peptidase IV from rat liver plasma membranes

Dipeptidyl peptidase IV (DPP IV) is a serine exopeptidase expressed at high levels in rat kidney, liver and lung. We established eight monoclonal antibodies against partially purified DPP IV from rat liver plasma membranes. By means of a competitive dot blot assay with purified DPP IV, these antibodies were shown to recognize four different epitopes of the glycoprotein, designated A - D. The epitopes are located on the extracellular domain of DPP IV, as shown by papain digestion of liver plasma membranes. Treatment of DPP IV with neuraminidase and glycopeptide N-glycosidase F, as well as incubation of hepatocytes with the alpha-mannosidase I inhibitor deoxymannojirimycin, revealed that epitope A may be formed by a mannose-rich sugar chain and epitope D might represent a complex carbohydrate structure in the mature glycoprotein, while the epitopes B and C are formed by the protein moiety. Concanavalin A reduced the binding of monoclonal antibody to epitope A by 78%. Binding to epitope D was blocked by 73% with wheat germ lectin, and by more than 99% with sialic acid; epitopes B and C were unaffected by any of the lectins or sugars tested. The immunological cross-reactivity with DPP IV from Morris hepatoma 7777 was demonstrated with monoclonal antibodies against epitopes A-C. Epitope D was not recognized on hepatoma DPP IV. However, in addition to DPP IV, four hepatoma plasma membrane glycoproteins were precipitated by the monoclonal antibody against the epitope D, indicating that this epitope is not uniquely restricted to DPP IV.

Characterisation of the asparagine-linked oligosaccharides from Trypanosoma brucei type-I variant surface glycoproteins

The complete primary structures of the Asn-linked oligosaccharides from the conserved glycosylation site of the type-I variant surface glycoproteins of Trypanosoma brucei MITat 1.4 and MITat 1.6 were determined using a combination of exoglycosidase digestions, permethylation analysis, acetolysis and 1H NMR. Both variants contained almost exclusively oligomannose-type oligosaccharides, identical in structure to those of mammalian glycoproteins. The oligosaccharides ranged in size from (Man)9(GlcNAc)2 to (Man)5(GlcNAc)2. The relative abundance of each component was similar in both variants. The major components were (Man)8(GlcNAc)2 and (Man)7(GlcNAc)2 with slightly less (Man)9(GlcNAc)2 and (Man)6(GlcNAc)2 and much less (Man)5(GlcNAc)2. Both variants also contained the same structural isomers. The close similarity of the oligomannose series indicates identical processing at the conserved site in both variants.

Differential effect of inhibitors of oligosaccharide processing on the secretion of thyrotropin from dispersed rodent pituitary cells

We examined the effect of various inhibitors of oligosaccharide processing on the content and secretion of newly synthesized thyroid-stimulating hormone (TSH) from dispersed hypothyroid rodent pituitary cells. 1-deoxynojirimycin and N-methyl-1-deoxynojirimycin, both inhibitors of glucosidases I and II, decreased intracellular TSH (to 60-76% of control) and secreted TSH (to 60-63% of control) after a 1-hour incubation (pulse) with [35S]methionine and an 8-hour incubation (chase) in isotope-free media. In contrast, deoxymannojirimycin and swainsonine, inhibitors of mannosidase I and II, respectively, increased both intracellular TSH (to 267-309% of control) and secreted TSH (to 192% of control) at 8 hours. TSH oligosaccharides synthesized in the presence of these glucosidase and mannosidase inhibitors were largely sensitive to endo-beta-N-acetylglucosaminidase H (endo H), confirming inhibition of processing. Despite differences in oligosaccharide structure, the in vitro bioactivities of these secreted TSH isoforms were nearly identical. These data confirm and extend previous work performed with 1-deoxynojirimycin suggesting that glucosylated high mannose forms of TSH are more susceptible to intracellular degradation. The novel finding that deoxymannojirimycin and swainsonine increase secreted and total TSH above control levels suggests that non-glucosylated high mannose forms as well as hybrid-type oligosaccharides may facilitate secretion and direct TSH away from a natural degradation pathway.

Catabolic pathway of oligosaccharide-diphospho-dolichol. Subcellular sites of the degradation of the oligomannoside moiety

The degradation of oligosaccharide-diphospho-dolichol leads to the release of oligosaccharide material ranging from (Glc)3(Man)9(GlcNAc)2-P to (Man)3 species and further smaller species. The subcellular location of the glucosidases and mannosidases involved in this catabolic process has been investigated on the basis of their differential sensitivity towards specific inhibitors (castanospermine, deoxymannojirimycin and swainsonine). The results indicate that the first steps of degradation down to the (Man)6 species occurs in the rough endoplasmic reticulum. This result is supported by the fact that the (Man)6 species is the end product when lipid-intermediate-derived glucosylated oligosaccharides are incubated with purified rough endoplasmic reticulum membranes. Swainsonine and lysosomotropic agents (chloroquine and ammonium chloride) do not affect the degradation process, thus indicating that neither Golgi apparatus nor lysosomes are involved in this catabolism. The observation of the same degradation pattern of the released oligosaccharide material in mannosidosis fibroblasts, lacking lysosomal mannosidases, confirms these results. Finally, the subcellular distribution of the released oligosaccharide material indicates that the oligomannosides larger than (Man)6 species are sequestered in the particulate fraction whereas, in contrast, oligomannosides smaller than (Man)6 species are found predominantly in the cytosol. Taken altogether, the experiments demonstrate that the first steps of the degradation of oligosaccharide-diphospho-dolichol occurs in the rough endoplasmic reticulum producing oligomannosides of the (Man)6 species which are then translocated to the cytoplasm to be further degraded.

Model for intracellular folding of the human immunodeficiency virus type 1 gp120

The intracellular folding of the human immunodeficiency virus type 1 gp120 has been assessed by analyzing the ability of the glycoprotein to bind to the viral receptor CD4. Pulse-chase experiments revealed that the glycoprotein was initially produced in a conformation that was unable to bind to CD4 and that the protein attained the appropriate tertiary structure for binding with a half-life of approximately 30 min. The protein appears to fold within the rough endoplasmic reticulum, since blocking of transport to the Golgi apparatus by the oxidative phosphorylation inhibitor carbonyl cyanide m-chlorophenylhydrazone did not appear to perturb the folding kinetics of the molecule. The relatively lengthy folding time was not due to modification of the large number of N-linked glycosylation sites on gp120, since inhibition of the first steps in oligosaccharide modification by the inhibitors deoxynojirimycin or deoxymannojirimycin did not impair the CD4-binding activity of the glycoprotein. However, production of the glycoprotein in the presence of tunicamycin and removal of the N-linked sugars by endoglycosidase H treatment both resulted in deglycosylated proteins that were unable to bind to CD4, suggesting in agreement with previous results, that glycosylation contributes to the ability of gp120 to bind to CD4. Interestingly, incomplete endoglycosidase H treatment revealed that a partially glycosylated glycoprotein could bind to the receptor, implying that a subset of glycosylation sites, perhaps some of those conserved in different isolates of human immunodeficiency virus type 1, might be important for binding of the viral glycoprotein to the CD4 receptor.

Effects of the alpha-mannosidase inhibitors, 1,4-dideoxy-1,4-imino-D-mannitol and swainsonine, on glycoprotein catabolism in cultured macrophages

Thioglycollate-stimulated murine peritoneal macrophages were cultured for eight days in the presence of swainsonine, or 1,4-dideoxy-1,4-imino-D-mannitol (DIM), or both of these competitive alpha-mannosidase inhibitors together. Analysis of accumulated high-mannose oligosaccharides by reversed phase HPLC after perbenzoylation revealed that DIM- and DIM-plus swainsonine-treated macrophages contained larger amounts of Man7GlcNAc, Man8GlcNAc and Man9GlcNAc, while swainsonine-treated macrophages contained relatively more Man3GlcNAc and Man5GlcNAc. These results are consistent with the known inhibitory effects of DIM and swainsonine on Golgi mannosidases I and II, respectively, and on lysosomal alpha-mannosidase. Depletion of stored oligosaccharides to control values was complete within seven days of terminating swainsonine treatment.

Assembly of asparagine-linked oligosaccharides in baby hamster kidney cells treated with castanospermine, an inhibitor of processing glucosidases

We have shown previously that the processing of asparagine-linked oligosaccharides in baby hamster kidney (BHK) cells is blocked only partially by the glucosidase inhibitors, 1-deoxynojirimycin and N-methyl-1-deoxynojirimycin [Hughes, R. C., Foddy, L. & Bause, E. (1987) Biochem. J. 247, 537-544]. Similar results are now reported for castanospermine, another inhibitor of processing glucosidases, and a detailed study of oligosaccharide processing in the inhibited cells is reported. In steady-state conditions the major endo-H-released oligosaccharides contained glucose residues but non-glycosylated oligosaccharides, including Man9GlcNAc to Man5GlcNAc, were also present. To determine the processing sequences occurring in the presence of castanospermine, BHK cells were pulse-labelled for various times with [3H]mannose and the oligosaccharide intermediates, isolated by gel filtration and paper chromatography, characterized by acetolysis and sensitivity to jack bean alpha-mannosidase. The data show that Glc3Man9GlcNAc2 is transferred to protein and undergoes processing to produce Glc3Man8GlcNAc2 and Glc3Man7GlcNAc2 as major species as well as a smaller amount of Man9GlcNAc2. Glucosidase-processed intermediates, Glc1Man8GlcNAc2 and Glc1Man7GlcNAc2, were also obtained as well as a Man7GlcNAc2 species derived from Glc1Man7GlcNAc2 and different from the Man7GlcNAc2 isomer formed in the usual processing pathway. No evidence for the direct transfer of non-glucosylated oligosaccharides to proteins was obtained and we conclude that the continued assembly of complex-type glycans in castanospermine-inhibited BHK cells results from residual activity of processing glucosidases.

Characterization of a mannosidase acting on alpha 1----3- and alpha 1----6-linked mannose residues of oligomannosidic intermediates of glycoprotein processing

Baby hamster kidney (BHK) cell extracts catalyze the conversion of [3H]mannose-labelled (Man)5GlcNAc and (Man)6GlcNAc oligosaccharides to a (Man)3GlcNAc species that retains affinity for concanavalin-A-Sepharose and appears to be Man alpha 1----3[Man alpha 1----6]Man beta 1----4GlcNAc. The properties of the (Man)5GlcNAc-hydrolase activity differ from lysosomal alpha-mannosidases as well as previously described processing mannosidases acting on oligosaccharide intermediates of N-glycan assembly. Mosquito cell extracts catalyze hydrolysis of (Man)6GlcNAc but lack the (Man)5GlcNAc hydrolase activity detected in BHK cell extracts. Glycopeptide analysis has been carried out on a ricin-resistant BHK mutant RicR14 that lacks N-acetylglucosaminyl transferase I and fails to convert oligomannosidic N-glycans to complex-type chains, and mosquito cells that constitutively lack N-acetylglucosaminyl transferase I. In both cell lines, the cellular glycoproteins contain (Man)5GlcNAc oligosaccharide as the major stable component equivalent to a 15-20-fold increase compared with normal BHK cells. Although containing very high amounts of asparagine-linked (Man)5(GlcNAc)2, RicR14 cells exhibit (Man)5GlcNAc hydrolase activity at levels similar to wild-type BHK cells. This result, together with previous work [Foddy, L., Feeney, J. & Hughes, R.C. (1986) Biochem. J. 233, 697-706] showing the complete inhibition of conversion of oligomannosidic intermediates to complex-type N-glycans in BHK cells treated with swainsonine, an inhibitor of mannosidase II but not the (Man)5GlcNAc hydrolase activity, argues against a major role for the (Man)5GlcNAc hydrolase activity in N-glycan assembly and suggesting other functions for the mannosidase activity in BHK cells.

alpha-D-mannopyranosylmethyl-p-nitrophenyltriazene inhibition of rat liver alpha-D-mannosidases

The compound alpha-D-mannopyranosylmethyl-p-nitrophenyltriazene (alpha-ManMNT) has been tested for its effect on four alpha-D-mannosidase activities present in rat liver. When p-nitrophenyl alpha-D-mannopyranoside was used as a substrate, preincubation of enzyme with 1.0 mM alpha-ManMNT inhibited soluble alpha-D-mannosidase by 90%, lysosomal alpha-D-mannosidase by approx. 60%, and had virtually no effect on Golgi mannosidase II. Golgi mannosidase I removal of the four alpha-1,2-linked D-mannoses from the common Man9GlcNAc2 oligosaccharide structure formed during N-linked glycoprotein biosynthesis was also blocked by treatment of the Golgi fraction with this compound. Mannosyltriazene inhibition of the three susceptible hepatic alpha-D-mannosidases was largely irreversible. alpha-ManMNT should therefore be useful for studying oligosaccharide processing and possibly for determining the turnover time of the inhibited alpha-D-mannosidases.

Semi-intact cells permeable to macromolecules: use in reconstitution of protein transport from the endoplasmic reticulum to the Golgi complex

We introduce a new method that removes portions of the plasma membrane of eukaryotic cells to form semi-intact cells. During preparation, these cells lose their soluble cytoplasmic contents, but retain secretory organelles such as the ER and Golgi complex in an intact form. Transport of protein between the ER and Golgi can be functionally reconstituted in vitro using these semi-intact cells by incubation in the presence of cytosol and ATP. Export of the vesicular stomatitis virus strain tsO45 G protein from the ER in vitro is temperature-sensitive, similar to the result observed in vivo. These cells allow direct access of chemicals and antibodies to the cytoplasmic domain of the cell and may be a widely applicable model system for study of a broad range of problems in cell biology.

Catabolic pathway of oligosaccharide-diphospho-dolichol. Study of the fate of the oligosaccharidic moiety in mouse splenocytes

Metabolic labelling of mouse splenocytes with radioactive mannose indicates that the glycosylation process is accompanied by the release of soluble oligomannoside material. Chase experiments with an excess of unlabelled mannose indicate that the radioactivity is mainly chased from oligosaccharide-PP-Dol (PP-Dol = diphosphodolichol): 10% is recovered as (Man)9(GlcNAc)2-P, (Man)9(GlcNAc)2, (Man)9GlcNAc and (Man)5 GlcNAc, and 90% is rapidly degraded further. Tunicamycin inhibits both oligosaccharide-PP-Dol synthesis and the formation of the oligosaccharide material to the same extent. The results thus indicate that these soluble oligomannoside structures represent the main steps of the oligosaccharide-PP-Dol catabolic pathway, starting with the cleavage of the diphosphate bond. However, it cannot be excluded that part of this material is released from newly formed glycoproteins. The soluble oligomannoside material does not contain glucose residues despite the fact that part of the oligosaccharide-PP-Dol is glucosylated and it was shown, by the use of glucosidase I inhibitors (castanospermine, deoxynojirimycin) that, after cleavage, the glycan moiety of glucosylated oligosaccharide-PP-Dol is first rapidly deglucosylated. These experiments provide a physiological basis to our previous results obtained in vitro and allow the definition of further steps in the catabolic pathway of oligosaccharide-PP-Dol.

Distinct patterns of glycosylation of colligin, a collagen-binding glycoprotein, and SPARC (osteonectin), a secreted Ca2+-binding glycoprotein. Evidence for the localisation of colligin in the endoplasmic reticulum

Mouse parietal endoderm PYS cells were labelled with [2-3H]mannose for 16-24 h. Colligin, an Mr-47000 collagen-binding protein, and SPARC, a Mr-43000 protein, highly homologous to the Ca2+-binding protein osteonectin, were isolated from labelled cell extracts and culture medium respectively. Glycopeptides obtained by exhaustive digestion with pronase were analysed by lectin-affinity, ion-exchange, and gel-filtration chromatography and by paper chromatography of high-mannose oligosaccharides after endo H release. The results show that the N-linked carbohydrate chains of colligin are exclusively the high-mannose type, of which (Man)8(GlcNAc)2 and (Man)9(GlcNAc)2 make up 77%. This carbohydrate structure provides strong evidence that colligin is a component of the endoplasmic reticulum, and argues against a role in cell-surface interactions. By contrast to colligin, SPARC secreted by PYS cells contains predominantly a diantennary complex type of chain containing a variable number of sialic acid and core-substituted fucose residues. Similar glycosylation patterns to those discussed above were seen in colligin isolated from primary mouse embryonic parietal endoderm cells and the murine 3T3 cell line, and in SPARC secreted by bovine corneal endothelial cells. Unlike the type-IV-collagen-binding glycoprotein studied by Dennis, J., Waller, C. and Schirrmacher, V. [J. Cell Biol. 99, 1416-1423 (1984)], removal of N-linked oligosaccharides from colligin had no effect on its binding to native type IV collagen.