Two mutants of Dictyostelium discoideum that lack a sulfated carbohydrate antigenic determinant synthesize a truncated lipid-linked precursor of N-linked oligosaccharides

Dictyostelium discoideum glycoproteins contain mannose-6-SO4 in highly immunogenic N-linked oligosaccharides. To more precisely define the structural requirements of the antigenic determinant, we have analyzed the oligosaccharides synthesized by two mutant strains (HL241 and HL243) that lack it. Both mutant strains synthesize N-linked oligosaccharides which are very similar to each other but are smaller and less charged than those derived from the wild-type. Both mutants contain substantial amounts of Man-6-SO4, and only a single residue of Man-6-P-OCH3 per chain, in contrast to the wild-type which may have 1 or 2 such residues. Neutral species are similar to the wild-type in that they can still be modified by the addition of residues of fucose and N-acetylglucosamine. Both mutant strains synthesize a truncated lipid-linked oligosaccharide, Man6GlcNAc2, with the most probable structure being: (sequence; see text) based on Jack bean alpha-mannosidase, alpha-1,2-specific mannosidase digestions and methylation analysis. The presence of this small oligosaccharide appears to result from the loss of the mannosyltransferase(s) needed to synthesize structures larger than Man6GlcNAc2 and not from the absence of dolichol phosphate or dolichol-P-mannose synthetase. These data along with the analysis of another mutant strain suggest that the expression of the antigenic determinant requires a specific arrangement of Man-6-SO4 on the alpha-1,6 branch of the oligosaccharide linked to the beta-mannose.

Amphomycin inhibits mannosylphosphoryldolichol synthesis by forming a complex with dolichylmonophosphate

The inhibitory effect of the lipopeptide antibiotic amphomycin on the mechanism of mannosylphosphoryldolichol biosynthesis by calf brain rough endoplasmic reticulum membranes has been studied extensively. Calf brain rough endoplasmic reticulum membranes when incubated with varying concentrations of GDP-mannose in the presence and absence of amphomycin showed no significant difference in apparent Km for GDP-mannose (1.08 and 1.37 microM, respectively). However, the Vmax was reduced to 0.17 pmol/mg protein/min in the presence of amphomycin as compared with 1.86 pmol/mg protein/min in its absence. On the other hand, when mannosylphosphoryldolichol synthase activity was measured in the presence of amphomycin and as a function of dolichylmonophosphate (Dol-P) concentrations, the shape of the substrate velocity curve changed from a rectangular hyperbola to a sigmoid. The Hill coefficients (n) for this reaction were calculated to be 2.02 and 1.22 in the presence and absence of the antibiotic and the corresponding Km values for Dol-P were found to be 333 and 47.3 microM, respectively. In separate experiments when radiolabeled antibiotic was reacted with Dol-P in the presence of Ca2+, a complex was formed. The complex formation was dependent on both Ca2+ in the reaction mixture and fatty acid residue on the antibiotic. Similar complex formation was also observed with undecaprenylmonophosphate. No such complex, however, was formed with dolichylpyrophosphate, with undecaprenylpyrophosphate, or with their free alcohols (dolichol or undecaprenol). Furthermore, when an equimolar mixture of Dol-P and phosphatidylserine was reacted with the antibiotic under identical conditions, the complex formation was observed selectively with Dol-P. These data demonstrated that amphomycin interacted with the active site of the glycosyl-carrier lipid (Dol-P), thereby preventing its participation at the enzymatic reaction.

Activation of dolichol-phosphate mannosyltransferase by dibutryl cyclic AMP in rat liver

Radiolabeled mannose incorporation into secretory glycoproteins and immunoprecipitable fibronectin in the incubation media significantly increased (105 and 32 percent respectively) with a corresponding increase in the levels of dolichol-phosphate mannose, dolichol-diphosphate oligosaccharides and dolichol-phosphate mannosyltransferase activity in the rat liver slices when incubated with dibutryl cAMP and ATP. Dibutryl cAMP activated maximally this enzyme in the presence of ATP in the incubation medium. The activation of the enzyme resulted in a two fold increase in Vmax with no apparent change in the Km for GDP mannose. Phosphorylation the rat liver microsomes with catalytic subunit of cAMP dependent protein kinase, resulted in the activation of dolichol-phosphate mannosyltransferase. These results suggest that cAMP modulates protein glycosylation by activating dolicholphosphate mannosyltransferase activity. The activation of this enzyme could be through phosphorylation/dephosphorylation mechanism involving a cAMP dependent protein kinase.

The role of C-4-substituted mannose analogues in protein glycosylation. Effect of the guanosine diphosphate esters of 4-deoxy-4-fluoro-D-mannose and 4-deoxy-D-mannose on lipid-linked oligosaccharide assembly

The effects of the guanosine diphosphate esters of 4-deoxy-4-fluoro-D-mannose (GDP-4FMan) and 4-deoxy-D-mannose (GDP-4dMan) on reactions of the dolichol pathway in chick-embryo cell microsomal membranes were investigated by studies with chick-embryo cell microsomal membranes in vitro and in baby-hamster kidney (BHK) cells in vivo. Each nucleotide sugar analogue inhibited lipid-linked oligosaccharide biosynthesis in a concentration-dependent manner. GDP-4FMan blocked in vitro the addition of mannose to Dol-PP-(GlcNAc)2Man from GDP-Man (where Dol represents dolichol), but did not interfere with the formation of Dol-P-Man, Dol-P-Glc and Dol-PP-(GlcNAc)2. Although GDP-4FMan and Dol-P-4FMan were identified as metabolites of 4FMan in BHK cells labelled with [1-14C]4FMan, GDP-4FMan was a very poor substrate for GDP-Man:Dol-P mannosyltransferase and Dol-P-4FMan could only be synthesized in vitro if the chick-embryo cell membranes were primed with Dol-P. It therefore appears that the inhibition of lipid-linked oligosaccharide formation in BHK cells treated with 4FMan [Grier & Rasmussen (1984) J. Biol. Chem. 259, 1027-1030] is due primarily to a blockage in the formation of Dol-PP-(GlcNAc)2Man2 by GDP-4FMan. In contrast, GDP-4dMan was a substrate for those mannosyltransferases that catalyse the transfer of the first five mannose residues to Dol-PP-(GlcNAc)2. In addition, GDP-4dMan was a substrate for GDP-Man:Dol-P mannosyltransferase, which catalysed the formation of Dol-P-4dMan. As a consequence of this, the formation of Dol-P-Man, Dol-P-Glc and Dol-PP-(GlcNAc)2 may be inhibited through competition for Dol-P. In BHK cells treated with 10 mM-4dMan, Dol-PP-(GlcNAc)2Man9 was the major lipid-linked oligosaccharide detected. Nearly normal extents of protein glycosylation were observed, but very little processing to complex oligosaccharides occurred, and the high-mannose structures were smaller than in untreated cells.

Effect of protein malnutrition on glycoprotein synthesis by testes of 20-day-old rats

The effects of protein malnutrition during the nursing period on glycoprotein biosynthesis by testes of 20-day-old rats was studied. Pregnant Wistar rats were housed individually. On the day of delivery they were divided into two groups: one was fed a control diet (25% casein) and the other a low-protein diet (8% casein) for a period of 20 days. Body and testis weights of pups suckled by the malnourished mothers were significantly lower than those of the pups suckled by normally-fed mothers. The seminiferous tubules o malnourished rats showed a significant decrease in diameter and in the stage of development of spermatogenesis. Whole testes of normally-fed 20-day-old rats showed significantly greater [2-3H]mannose incorporation into glycoproteins than did the testes of malnourished rats of the same age. The microsomes of normally-fed rats showed significantly higher GDP: mannose polyprenyl mannosyl transferase activity than did microsomes from malnourished rats, and this difference increased when exogenous dolichyl-phosphate was added to the incubation medium. These results indicate that protein malnutrition decreases GDP: mannose polyprenyl mannosyl transferase activity in the microsomes of testes from 20-day-old rats.

A new yeast mutation in the glucosylation steps of the asparagine-linked glycosylation pathway. Formation of a novel asparagine-linked oligosaccharide containing two glucose residues

We have isolated and characterized a new yeast mutation in the glucosylation steps of lipid-linked oligosaccharide biosynthesis, alg8-1. Cells carrying the alg8-1 mutation accumulate Glc1Man9GlcNAc2-lipid both in vivo and in vitro. We present evidence showing that the alg8-1 mutation blocks addition of the second alpha 1,3-linked glucose. alg8-1 cells transfer Glc1Man9GlcNAc2 to protein instead of the wild type oligosaccharide, Glc3Man9GlcNAc2. Pulse-chase studies indicate that the Glc1Man9GlcNAc2 transferred is processed more slowly than the wild type oligosaccharide. The yeast mutation gls1-1 lacks glucosidase I activity (Esmon, B., Esmon, P.C., and Schekman, R. (1984) J. Biol. Chem. 259, 10322-10327), the enzyme responsible for removing the alpha 1,2-linked glucose residues from protein-linked oligosaccharides. We demonstrate that gls1-1 cells contain glucosidase II activity (which removes alpha 1,3-linked glucose residues) and have constructed the alg8-1 gls1-1 haploid double mutant. The Glc1Man9GlcNAc2 oligosaccharide was trimmed normally in these cells, demonstrating that the alg8-1 oligosaccharide contained an alpha 1,3-linked glucose residue. A novel Glc2 compound was probably produced by the action of the biosynthetic enzyme that normally adds the alpha 1,2-linked glucose to lipid-linked Glc2Man9GlcNAc2. This enzyme may be able to slowly add alpha 1,2-linked glucose residue to protein-bound Glc1Man9GlcNAc2. The relevance of these findings to similar observations in other systems where glucose residues are added to asparagine-linked oligosaccharides and the possible significance of the reduced rate of oligosaccharide trimming in the alg mutants are discussed.

Stimulation by dolichol phosphate-mannose of N-acetylglucosaminyl-lipid biosynthesis by membranes from class E Thy-1-negative mutant mouse lymphoma cells which are defective in dolichol phosphate-mannose biosynthesis

Dolichol phosphate-mannose has been shown previously to stimulate the biosynthesis of N-acetylglucosaminyl-diphosphate-dolichol (E. L. Kean (1985) J. Biol. Chem. 260, 12561-12571). Although the class E Thy-1-negative mutant mouse lymphoma cells are unable to synthesize dolichol phosphate-mannose, the addition of this compound exogenously to membranes from the mutant cells brought about a stimulation of N-acetylglucosaminyl-lipid synthesis similar to that obtained with membranes from wild type cells. The retention of this activity by the mutant cells supports the suggestion of a regulatory role for dolichol phosphate-mannose as an intrinsic property of the glucosaminyltransferase which catalyzes the initial reaction of the dolichol pathway.

Differential regulation of the synthesis of mannosylphosphoryl derivatives of dolichol and retinol in HeLa cells

We have shown earlier that in HeLa S3G cells, glucocorticoids stimulate the synthesis of dolichyl phosphorylmannose (Dol-P-Man) with a concomitant increase in the glycosylation of proteins (Ramachandran, C.K., Gray, S.L. and Melnykovych G. (1982) Biochem. J. 208, 47-52). Although controversial, there have been several lines of evidence suggesting that the synthesis of retinyl phosphorylmannose (Ret-P-Man) and Dol-P-Man may be carried out by the same enzyme. We examined this possibility and conclude that in HeLa S3G cells the syntheses of Dol-P-Man and Ret-P-Man are catalyzed by two different enzymes located in the same microenvironment. Our conclusion is based on the following observations: exogenously added dolichyl phosphate and retinyl phosphate did not compete with each other; when the cells were grown in the presence of 1 microM dexamethasone, the microsomal synthesis of Dol-P-Man was stimulated, without affecting the Ret-P-Man synthesis; Arrhenius plots on Ret-P-Man and Dol-P-Man synthesis showed breaks at 22 and 37.7 degrees C.

Effect of herpes simplex virus type 1 on cellular pools of oligosaccharide-lipid

Incorporation of [3H]mannose into cellular pools of mannosylphosphoryl dolichol (Man-P-Dol), oligosaccharide-lipid, and glycoprotein was measured and compared in herpes simplex virus type 1 (HSV-1)-infected cells and -uninfected cells. While mannose incorporation into the monosaccharide-dolichol fraction was similar in infected or uninfected Vero cells, incorporation into the oligosaccharide-lipid fraction was markedly reduced in HSV-1-infected cells (64% of control levels). In contrast, mannose incorporation into glycoprotein was significantly increased in virus-infected cells versus uninfected cells (194% of control levels). The kinetics of incorporation into the various fractions was examined and it was determined that there was minimal increase in mannose incorporation into oligosaccharide-lipid after 8 hr postinfection in virus-infected cells. This corresponded to the time at which nonglycosylated precursors of the HSV-1 glycoproteins were first detected in association with the nuclear fraction. These data suggest that there is an accelerated turnover of oligosaccharide-lipid in virus-infected Vero cells which is most likely due to extensive glycoprotein synthesis.

beta-Adrenergic stimulation alters oligosaccharide pyrophosphoryl dolichol metabolism in rat parotid acinar cells

beta-Adrenergic stimulation of rat parotid acinar cells markedly increases [3H]mannose incorporation into N-linked glycoproteins [Kousvelari, Grant, Banerjee, Newby & Baum (1984) Biochem. J. 222, 17-24]. More than 90% of this protein-bound [3H]mannose was preferentially incorporated into four secretory glycoproteins. The ratio of [3H]mannose/[14C]leucine present in these individual proteins was 1.7-4-fold greater with isoproterenol-treated cells than with untreated controls. In isoproterenol-stimulated cells, [3H]mannose incorporation into mannosylphosphoryl dolichol and oligosaccharide-PP-dolichol was increased 2-3-fold over that observed in unstimulated cells. Similarly, formation of mannosylated oligosaccharide-PP-dolichol was increased approx. 4-fold in microsomes prepared from isoproterenol-treated cells. Also, turnover of oligosaccharide-PP-dolichol was significantly increased (5-fold) by beta-adrenergic stimulation; the half-life for oligosaccharide-PP-dolichol decreased from 6 min in control cells to 1.2 min in isoproterenol-stimulated cells. By 15 min after isoproterenol addition to acinar cells, the specific radioactivity of parotid oligosaccharide moieties increased about 3-fold over the value observed in the absence of the agonist. Taken together, these results strongly suggest that elevation of N-linked protein glycosylation in rat parotid acinar cells after beta-adrenoreceptor stimulation resulted from significant enhancement in the synthesis of mannosylphosphoryl dolichol and oligosaccharide-PP-dolichol and the turnover of oligosaccharide-PP-dolichol.

A single enzyme catalyzes the synthesis of the mannosylphosphoryl derivative of dolichol and retinol in rat liver and Chinese hamster ovary cells

It is well established that mannosylphosphoryldolichol participates in the synthesis of N-linked glycoproteins by donating mannosyl residues to oligosaccharide-lipid intermediates. It has been suggested that mannosylphosphorylretinol also is involved in glycoprotein biosynthesis. We conclude that one synthase catalyzes the synthesis of both mannosylphosphoryldolichol and mannosylphosphorylretinol in rat liver tissue and Chinese hamster ovary cells, based on the following results. 1) The enzyme in rat liver microsomes that synthesizes mannosylphosphoryldolichol and mannosylphosphorylretinol is inactivated at the same rate at 55 degrees C. 2) In membranes of both rat liver and Chinese hamster ovary cells, exogenous dolichyl phosphate and retinyl phosphate compete with each other for mannosyl-lipid synthesis. However, in both systems adding exogenous retinyl phosphate has no effect on the synthesis of mannosylphosphoryldolichol from endogenous dolichyl phosphate in the membranes. 3) Membranes prepared from a mutant of Chinese hamster ovary cells which is devoid of mannosylphosphoryldolichol synthase lack the ability to synthesize mannosylphosphorylretinol.

Inflammation-induced increases in dolichol synthesis and hydroxymethylglutaryl-coenzyme A reductase activity in mouse liver are prevented by a high-cholesterol diet but not by fasting

The inflammatory response in mammals is characterized by the synthesis in the liver of several N-linked serum glycoproteins called acute-phase reactants. In C57BL/6J mice, turpentine-induced inflammation was accompanied by increases in 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity, dolichol synthesis, and dolichyl phosphoryl mannose synthesis. Cholesterol feeding, but not fasting, prevented these inflammation-induced increases in reductase activity and dolichol synthesis. However, the rate of incorporation of [3H]mannose into total serum glycoproteins was not affected by the high-cholesterol diet, and this rate increased during acute inflammation in control and cholesterol-fed mice.

Glycoprotein biosynthesis in Chlamydomonas. A mannolipid intermediate with the properties of a short-chain alpha-saturated polyprenyl monophosphate

A crude membrane preparation of the unicellular green alga Chlamydomonas reinhardii was found to catalyse the incorporation of D-[14C]mannose from GDP-D-[14C]-mannose into a chloroform/methanol-soluble compound and into a trichloroacetic acid-insoluble polymer fraction. The labelled lipid revealed the chemical and chromatographic properties of a short-chain (about C55-C65) alpha-saturated polyprenyl mannosyl monophosphate. In the presence of detergent both long-chain (C85-C105) dolichol phosphate and alpha-unsaturated undecaprenyl phosphate (C55) were found to be effective as exogenous acceptors of D-mannose from GDP-D-[14C]mannose to yield their corresponding labelled polyprenyl mannosyl phosphates. Exogenous dolichyl phosphate stimulated the incorporation of mannose from GDP-D-[14C]mannose into the polymer fraction 5-7-fold, whereas the mannose moiety from undecaprenyl mannosyl phosphate was not further transferred. Authentic dolichyl phosphate [3H]mannose and partially purified mannolipid formed from GDP-[14C]mannose and exogenous dolichyl phosphate were found to function as direct mannosyl donors for the synthesis of labelled mannoproteins. These results clearly indicate the existence of dolichol-type glycolipids and their role as intermediates in transglycosylation reactions of this algal system. Both the saturation of the alpha-isoprene unit and the length of the polyprenyl chain may be regarded as evolutionary markers.

Effect of dexamethasone on mannolipid synthesis by hepatocytes prepared from control and inflamed rats

Hepatocytes were prepared from control and inflamed rats. Mannose incorporation into dolichol monophosphate mannose in homogenate and microsomal fraction of the hepatocytes was increased 2-fold over the controls 24 h after induction of inflammation by turpentine injection. Incubation of hepatocytes from both control and inflamed rats with 0.1-10 microM-dexamethasone produced a 1.5-fold increase of dolichol phosphate mannose formation, whereas, 100 microM-dexamethasone decreased its formation. The increase in the ratio of dolichol phosphate mannose formation in inflamed over controls was virtually eliminated when the cell homogenate assay mixtures included 30 nmol of exogenous dolichol phosphate. This supports the earlier suggestion that the increase in the enzyme activity in inflammation could be due to higher concentrations of endogenous dolichol phosphate [ Coolbear & Mookerjea (1981) J. Biol. Chem. 256, 4529-4535]. In contrast, the increase in the ratio of dolichol phosphate mannose formation between dexamethasone-treated and untreated hepatocytes remained unchanged when increasing concentrations of exogenous dolichol phosphate were added to the assays. This suggests that the increase in glycosylation of dolichol phosphate in dexamethasone-treated hepatocytes is probably due to the increased mannosyltransferase activity, rather than due to higher concentrations of endogenous dolichol phosphate in these cells.

Glycosyl transferases in mouse and human milk fat globule membranes

Membranes isolated from mouse and human milk fat globules were found to contain the enzymes responsible for the synthesis of dolichol monophosphate mannose and dolichol monophosphate glucose as well as those involved in the transference of the glycosyl residues from the two dolichol derivatives to dolichol diphosphate oligosaccharides. The levels of most of the enzymes were comparable to those found in mouse mammary gland microsomes. The presence of enzymes involved in protein glycosylation via dolichol derivatives in the milk fat globule membrane provides evidence in favor of an outward flow of membrane components from the rough endoplasmic reticulum, where these enzymes are active in vivo, towards the cell surface.

Retinyl phosphate mannose synthesis in rat liver membranes. Phospholipase sensitivity and phospholipid requirement

A remarkable and immediate decrease in GDP-mannose:retinyl phosphate mannosyltransferase activity was found on pre-incubation of rat liver postnuclear membranes with phospholipase A2 or phospholipase C. Under the same conditions of pre-incubation (1 min at 37 degrees C) trypsin did not affect the enzyme activity, whereas pre-incubation for 30 min with trypsin and Pronase abolished enzyme activity. The lipid extract of untreated rat liver membranes partially restored enzyme activity after phospholipase treatment. Sphingomyelin was as active as the endogenous lipids. Other phospholipids were less active in the following order: phosphatidylcholine greater than phosphatidylethanolamine greater than phosphatidylinositol = phosphatidylserine. Dolichyl phosphate mannose synthesis was inhibited less (33%) by phospholipase C than was Ret-P-Man synthesis (98.5%) under identical conditions of incubation, which included 0.025% Triton. However, retinyl phosphate mannose synthesis by purified endoplasmic reticulum was found to be resistant to phospholipase C. Mixing experiments failed to demonstrate an inhibitory effect of the phospholipase-treated postnuclear membrane fraction on the synthetic activity of the endoplasmic reticulum, thus excluding the release of an inhibitory factor from the postnuclear membranes.

Effect of inhibiting dolichol phosphate-mannose formation on the biosynthesis of the N-acetylglucosaminylpyrophosphoryl polyprenols by the retina

Previously (Kean, E.L. (1982) J. Biol. Chem. 257, 7952-7954), it was shown that dolichol phosphate-mannose stimulated the formation of the N-acetylglucosamine-containing mono-, di- and trisaccharide intermediates of the dolichol pathway. Consistent with this activating role, the present report demonstrates that inhibition of the formation of dolichol phosphate-mannose by the antibiotics, showdomycin and diumycin, also blocked the stimulatory phenomenon.

Novel mannose carrier in the trypanosomatid Crithidia fasciculata behaving as a short alpha-saturated polyprenyl phosphate

Crithidia fasciculata cells incubated with [14C]glucose or membranes derived from the same protozoan incubated with GDP-[14C]mannose were found to synthesize a lipid monophosphate mannose. No glucosylated mild acid-labile compound was formed in vivo or in vitro when UDP-[14C]glucose was used instead of GDP-[14C]mannose. The lipid moiety of the mannosyl derivative formed behaved as a polyprenol having 11 isoprene residues as judged by t.l.c. and be gel filtration in sodium deoxycholate-containing buffers. The mannolipid was not broken on treatment with hot phenol, suggesting the existence of an alpha-saturated isoprene unit. This is the first case reported in which a mannosyl phospholipid involved in sugar transfer in a eukaryotic cell behaves as if it was similar to that of bacterial polyprenols, although having its putative alpha-isoprene unit saturated to the same extent as dolichols from higher organisms.

Changes in cell surface glycoproteins on non-differentiating L6 rat myoblasts selected for resistance to concanavalin A

Four independently selected conA-resistant, non-differentiating rat L6 myoblast cell lines and their parental wild-type populations were examined for cell surface alterations. [3H]conA-binding studies indicated that the variant myoblasts bound significantly less lectin than wild-type cells at 4 and at 37 degrees C. Scatchard analysis revealed two general types of binding sites (high and low affinity sites) on wild-type cells; the variants appeared to be deficient in the high affinity sites. These changes in conA binding probably play an important role in determining the conA-resistant phenotype. Lectin-binding results could be significantly modified by altering the composition of the serum in the growth medium used to culture myoblasts prior to performing binding experiments, suggesting the existence of productive and non-productive lectin-binding sites on the cell surface. SDS slab gel electrophoresis of [3H]mannose-labelled surface membranes prepared from variant and wild-type cells showed that several glycoproteins of the conA-resistant myoblasts were defective in mannosylation. The conA-binding abilities of a pronase digest of one of these altered regions from variant separations, with a molecular weight of 44500 D, was found to contain glycopeptides with reduced affinity for the lectin, supporting the idea that variant membranes are deficient in a set of high affinity lectin-binding sites. Studies on [GDP-14C]mannose incorporation into lipid by membranes from variant and wild-type myoblasts indicated that the biosynthetic lesion likely involved a mannosyl transferase enzyme directly, rather than a lack of free dolichol-PO4. These studies link conA resistance, cell surface glycoprotein alterations, and defective mannosyl transferase activity with the inability to carry out normal cellular differentiation to form multinucleated myotubes.

Mannosyl carrier functions of retinyl phosphate and dolichyl phosphate in rat liver endoplasmic reticulum

Of the subcellular fractions of rat liver the endoplasmic reticulum was the most active in GDP-mannose: retinyl phosphate mannosyl-transfer activity. The synthesis of retinyl phosphate mannose reached a maximum at 20-30 min of incubation and declined at later times. Retinyl phosphate mannose and dolichyl phosphate mannose from endogenous retinyl phosphate and dolichyl phosphate could also be assayed in the endoplasmic reticulum. About 1.8 ng (5 pmol) of endogenous retinyl phosphate was mannosylated per mg of endoplasmic reticulum protein (15 min at 37 degrees C, in the presence of 5 mM-MnCl2), and about 0.15 ng (0.41 pmol) of endogenous retinyl phosphate was mannosylated with Golgi-apparatus membranes. About 20 ng (13.4 pmol) of endogenous dolichyl phosphate was mannosylated in endoplasmic reticulum and 4.5 ng (3 pmol) in Golgi apparatus under these conditions. Endoplasmic reticulum, but not Golgi-apparatus membranes, catalysed significant transfer of [14C]mannose to endogenous acceptor proteins in the presence of exogenous retinyl phosphate. Mannosylation of endogenous acceptors in the presence of exogenous dolichyl phosphate required the presence of Triton X-100 and could not be detected when dolichyl phosphate was solubilized in liposomes. Dolichyl phosphate mainly stimulated the incorporation of mannose into the lipid-oligosaccharide-containing fraction, whereas retinyl phosphate transferred mannose directly to protein.

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

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

Studies on the stimulation of dolichol-mediated glycosylation by dexamethasone in HeLa cells

The effect of the addition of 1 microM-dexamethasone on the temporal sequence of the glycosylation process has been studied in HeLa S3G cells. In the presence of delipidized serum, dexamethasone caused an increase in 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase and an accelerated synthesis of dolichols. These events were followed by an increase in the transfer of mannose from GDP-mannose to mannolipid. An increase in the rate of synthesis of lipid-linked oligosaccharides and a stimulation of glycosylation were observed in cells grown in the presence of delipidized serum in the culture medium. The data are consistent with the view that cellular syntheses of lipids and glycoproteins are co-ordinately controlled.

Synthesis of retinyl phosphate mannose and dolichyl phosphate mannose from endogenous and exogenous retinyl phosphate and dolichyl phosphate in microsomal fraction. Specific decrease in endogenous retinyl phosphate mannose synthesis in vitamin A deficiency

Rat liver microsomal fraction synthesized Ret-P-Man (retinyl phosphate mannose) and Dol-P-Man (dolichyl phosphate mannose) from endogenous Ret-P (retinyl phosphate) and Dol-P (dolichyl phosphate). Ret-P-Man synthesis displayed an absolute requirement for a bivalent cation, and also Dol-P-Man synthesis was stimulated by bivalent metal ions. Mn2+ and Co2+ were the most active, with maximum synthesis of Ret-P-Man occurring at 5-10 mM: Mg2+ was also active, but at higher concentrations. At 5mM-Mn2+ the amount of endogenous Ret-P mannosylated in incubation mixtures containing 5 microM-GDP-mannose in 15 min at 37 degrees C was approx. 3 pmol/mg of protein. In the same assays about 7-10 pmol of endogenous Dol-P was mannosylated. Bivalentcation requirement for Ret-P-Man synthesis from exogenous Ret-P showed maximum synthesis at 2.5 mM-Mn2+ or -Co2+. In addition to Ret-P-Man and Dol-P-Man, a mannolipid co-chromatographing with undecaprenyl phosphate mannose was detected. Triton X-100 (0.5%) abolished Ret-P-Man synthesis from endogenous Ret-P and caused a 99% inhibition of Ret-P-Man synthesis from exogenous Ret-P. The presence of detergent (0.5%) also inhibited Dol-P-Man synthesis from endogenous Dol-P and altered the requirement for Mn2+. Microsomal fraction from Syrian golden hamsters was also active in Ret-P-Man and Dol-P-Man synthesis from endogenous Ret-P and Dol-P. At 5 mM-Mn2+ about 2.5 pmol of endogenous Ret-P and 3.7 pmol of endogenous Dol-P were mannosylated from GDP-mannose per mg of protein in 15 min at 37 degrees C. On the other hand, microsomal fraction from vitamin A-deficient hamsters contained 1.2 pmol of Ret-P and 14.1 pmol of Dol-P available for mannosylation. Since GDP-mannose: Ret-P and GDP-mannose: Dol-P mannosyltransferase activities were not affected, depletion of vitamin A must affect Ret-P and Dol-P pools in opposite ways.

Mutant of Chinese hamster ovary cells with altered mannose 6-phosphate receptor activity is unable to synthesize mannosylphosphoryldolichol

B4-2-1 is a Chinese hamster ovary cell mutant previously isolated and characterized as deficient in mannose 6-phosphate receptor activity. We show here that B4-2-1 is a pleiotropic mutant, defective in biosynthesis of asparagine-linked oligosaccharides, B4-2-1 is unable to synthesize mannosylphosphoryldolichol; the consequences of this defect on glycosylation are (i) biosynthesis of one major lipid-linked oligosaccharide, characterized by its resistance to endoglycosidase H and decreased size; this oligosaccharide is similar to a minor species of lipid-liked oligosaccharide found in parental cells; (ii) transfer of this oligosaccharide to newly synthesized proteins; and (iii) absence of normal "high-mannose" oligosaccharides on mature glycoproteins isolated from B4-2-1; glycoproteins from the mutant contain complex oligosaccharides as well as endoglycosidase H-resistant, alpha-mannosidase-sensitive species. While the glycosylation defect may alter adversely the function of several glycoproteins in the mutant, including that of the mannose 6-phosphate receptor, it appears to have no effect on the formation or function of the mannose 6-phosphate recognition marker on acid hydrolases of B4-2-1.

Early dolichyl sugar synthesis during differentiation of non-growing brine-shrimp (Artemia salina) embryos

Incubation of cell-free extracts of Artemia salina with sugar nucleotides resulted in the formation of dolichyl derivatives. These enzymic activities were detected early in the development of the encysted Artemia embryos. Dolichyl sugars seemed to act as intermediates in the glycosylation of proteins, which may be involved in the initiation of cellular differentiation.

Evidence for the stimulation of dolichol and mannolipid synthesis by glucocorticoids in HeLa cells

The effects of addition of 1 microM-dexamethasone on the rate of transfer of mannose from GDP-mannose into mannolipid have been studied in HeLa cell cultures. Concurrent with an increase in incorporation of mannose into glycoproteins, the incorporation of mannose from GDP-mannose in vitro into mannolipid and dolichol-linked oligosaccharides was increased after dexamethasone treatment. Stimulation of mannolipid synthesis showed a correlation with the 11 beta, 17 alpha, 21-trihydroxy structure of C21 steroids. Dexamethasone treatment also resulted in an increased incorporation of acetate into dolichol and dolichyl phosphate. The results suggest that the effect of dexamethasone on the cell-surface glycoprotein accumulation is related to increased sugar-linked dolichol synthesis.

Recent studies on the involvement of retinyl phosphate as a carrier of mannose in biological membranes

Rat liver microsomes synthesized [14C]mannosylretinylphosphate and dolichyl [14C]mannosylphosphate from guanosinedisphosphate [14C]mannose, retinylphosphate and dolichylphosphate. Two distinct enzyme activities were shown to be responsible for the biosynthesis of the two mannolipids. A higher affinity mannosyl transferase (EA I), responsible for dolichylmannosylphosphate synthesis, displayed a Km for GDP-mannose of 1.7 microM; while a lower affinity enzyme (EA II), responsible for mannosylretinylphosphate synthesis, displayed a Km for GDP-mannose of 12.5 microM. These Km values were unaffected by the addition of either dolichylphosphate for EA II, or retinylphosphate for EA I. The same Km values were found before and after solubilization of the enzyme activity with 1% Triton X-100. Differential solubilization of EA I and EA II was demonstrated, utilizing different concentrations of Triton X-100. Triple-labeled mannosylretinylphosphate was prepared from [3H]retinylphosphate, retinyl[32P]phosphate and GDP-[14C]mannose from incubations containing rat liver microsomes. This compound was shown to donate [14C]mannose to endogenous acceptors of rat liver microsomes.

The biosynthesis of a mannolipid containing a metabolite of retinoic acid by 3T12 mouse fibroblasts

Retinol and retinoic acid (RA) increase the adhesive properties of spontaneously transformed mouse fibroblasts (BALB/c 3T12-3 cells) and the incorporation of [2-3H]mannose into cellular glycoconjugates. Therefore we searched for a mannolipid of retinoic acid similar to mannosylretinylphosphate (MRP) in these cells. Radioactively labeled RA was incorporated into a compound of chromatographic characteristics similar to those of standard MRP. This metabolite contained the same 3H:14C ratio as the precursor [11, 12-3H, 15-14C]retinoic acid, demonstrating that no decarboxylation had occurred. A doubly labeled mannolipid was obtained from cells incubated with [2-3H]mannose and [15-14C]retinoic acid. This mannolipid was readily cleaved by mild acid, yielding [3H]mannosephosphate and a compound that migrated as standard anhydroretinol on thin layer of silica gel in toluene:chloroform:methanol (4:1:1) at Rf 0.93. Standard all trans-MRP yields all-trans-anhydroretinol under these conditions. An ion pair reverse phase HPLC system was developed to further characterize the mannolipids obtained from retinol and retinoic acid in 3T12 cells. [15-3H]Retinol and [15-14C]retinoic acid were incorporated into mannolipids that cochromatographed upon HPLC with standard MRP. The mixture of the [15-3H]retinol and [15-14C]retinoic acid derived mannolipids was subjected to mild acid hydrolysis, after purification by HPLC. Nearly 100% of the compounds was hydrolyzed, yielding all-trans-[3H]anhydroretinol and a 14C]labeled product which was eluted from HPLC as a slightly more polar compound than all-trans-anhydroretinol. The retinoic acid-derived mannolipid (MXP) represented approximately 4% of the total radioactivity in the methanolic extract of 3T12 cells incubated for 20 hours in the presence of labeled retinoic acid. However, if the cells were incubated for an additional 20 hours in the absence of the radioactive precursor, MXP represented 40% of the total extracted radioactivity. These results demonstrate that 3T12 cells synthesize mannosylretinylphosphate from retinol and a mannosylretinoidphosphate (MXP) from retinoic acid. These results exclude the possibility of a reduction of retinoic acid to retinol, but suggest that a closely related compound is formed from RA and that this retinol-like compound (X) is incorporated into MXP.

The effect of tsushimycin on the synthesis of lipid-linked saccharides in aorta

The antibiotic, tsushimycin, inhibits the formation of dolichyl phosphate mannose, dolichyl phosphate glucose and dolichyl pyrophosphate N-acetylglucosamine in the particulate enzyme preparation from pig aorta. Although this antibiotic also inhibits the incorporation of mannose and glucose into lipid-linked oligosaccharides, these reactions are less sensitive to antibiotic than those involved in the synthesis of lipid-linked monosaccharides. In the presence of tsushimycin, most of the mannose incorporated into lipid-linked oligosaccharides is into one oligosaccharide that has the properties of the heptasaccharide Man5GlcNAc2, whereas in the absence of antibiotic most of the mannose is in larger-sized oligosaccharides. On the other hand, the glucose-labelled lipid-linked oligosaccharides appear to be similar in size in the presence or absence of antibiotic. Tsushimycin also inhibits the formation of lipid-linked monosaccharides by the solubilized enzyme preparation of aorta. Various concentrations of dolichyl phosphate or the detergent, Nonidet P40, had no effect on antibiotic inhibition. Some evidence indicates that tsushimycin binds to the particulate enzyme.

Inhibition of lipid-linked mannose and mannoprotein synthesis in yeast by diumycin in vitro

Diumycin, a phosphoglycolipid antibiotic, inhibits different mannosyl transfer reactions in yeast. Using membrane preparations, the drug effectively inhibited the formation of dolichyl phosphate mannose (DolP-Man); 50% inhibition was observed at approximately 10 microgram/ml. To a lesser extent also mannosyl transfer from DolP-Man to protein decreased in presence of diumycin. Both mannosyl transfer to protein-serine/threonine acceptor sites as well as into positions within the asparagine-linked polymannose part of the yeast mannoprotein are inhibited to about 60% under conditions where DolP-Man formation is blocked. DolP-Man synthesis as well as mannosyl transfer from DolP-Man to protein are also inhibited by diumycin using solubilized enzymes and exogenous acceptor substrates. Glycosyltransfer reactions from GDP-mannose either to protein-serine/threonine-linked mannose (formation of short manno-oligosaccharides) or to dolichyl-diphosphate-linked chitobiose (formation of lipid-linked trisaccharide) are not inhibited by diumycin under conditions where DolP-Man synthesis is blocked by the antibiotic. The inhibitory action of diumycin on DolP-Man formation does not seem to be competitive with respect to dolichyl phosphate, since it cannot be overcome by higher concentrations of dolichyl phosphate.

A developmental change in dolichyl phosphate mannose synthase activity in pig brain

The initial rate of dolichyl phosphate mannose biosynthesis was measured in white-matter membranes from pig brain at various ages from before birth throughout the period of most rapid brain development. Dolichyl phosphate mannose synthase activity increased from prenatal values to a maximum in 3 week-old animals, and gradually decreased to adult values after 8 weeks of age. The nature of the developmental change was investigated by enzymic and biochemical comparisons of the membrane preparations from the most active age (3 weeks) and adult controls. The specific activity of dolichyl phosphate mannose synthase in preparations from actively myelinating animals was approx. 3-fold higher than adults when mannolipid formation was assayed with saturating concentrations of GDP-[(14)C]mannose and utilizing only endogenous acceptor lipid. No major variations were found in the apparent K(m) values for GDP-mannose or exogenous dolichyl monophosphate. However, the ratio of dolichyl phosphate mannose synthase activity for myelinating animals/adult animals decreased significantly when large amounts of exogenous dolichyl monophosphate were added to the incubation mixtures. Dolichyl phosphate mannose synthase activity was also compared in white-matter membranes depleted of endogenous dolichyl monophosphate by enzymic mannosylation or treatment with butanol. When these preparations were assayed with identical amounts of exogenous dolichyl monophosphate, the dolichyl monophosphate-depleted membranes from actively myelinating animals contained only 20-30% more dolichyl phosphate mannose synthase activity. Overall, these studies strongly suggest that the developmental change in dolichyl phosphate mannose synthase activity is due primarily to the presence of a relatively lower amount of endogenous dolichyl monophosphate being accessible to the mannosyltransferase in the white-matter membranes from adult animals.

Fate of oligosaccharide-lipid intermediates synthesized by resting rat-spleen lymphocytes

Using conditions to avoid the utilization of labelled precursors by intracellular glycosyltransferases, experiments are described demonstrating that intact rat-spleen lymphocytes are capable of utilizing exogenous GDP-mannose and UDP-N-acetylglucosamine to synthesize dolichyl monophosphate mannose and dolichyl diphosphate oligosaccharides. Kinetic and chase experiments show that dolichyl diphosphate oligosaccharides are either utilized for the transfer of their carbohydrate moieties to protein acceptors or further degraded. Since glycosylation of proteins is limited in resting lymphocytes, the degradation pathway appears as a major event in the fate of the dolichyl diphosphate oligosaccharides synthesized in vitro. These dolichyl diphosphate oligosaccharides are degraded into phospho-oligosaccharides and oligosaccharides which are released in the medium. This enzymatic cleavage of the phosphodiester bond is inhibited by bacitracin. The phospho-oligosaccharides are susceptible to alkaline phosphatase giving neutral oligosaccharides and they are cleaved by endo-N-acetyl-beta-D-glucosaminidase H leaving N-acetylglucosamine 1-phosphate and neutral oligosaccharides. These data suggest that splitting of the phosphodiester bond of colichyl diphosphate oligosaccharides, dephosphorylation and/or endo-N-acetyl-beta-D-glucosaminidase hydrolysis of the phosphorylated oligosaccharides could represent the beginning of the catabolic pathway of dolichyl diphosphate oligosaccharides.

Dietary effects on the formation of dolichyl monophosphate mannose by microsomal preparations of rat adiopose tissue

Microsomal preparations from rat adipose tissue catalyse the transfer of [(14)C]mannose from GDP-[(14)C]mannose to an endogenous acceptor forming a [(14)C]mannosyl lipid. The mannosyl lipid co-chromatographs with hen oviduct dolichyl monophosphate beta-mannose on three solvent systems. It is stable to mild alkaline hydrolysis, but strong alkaline treatment yields a compound that co-migrates with mannose 1-phosphate. The mannosyl lipid is labile to mild acid hydrolysis, yielding [(14)C]mannose. Formation of the compound is reversible by GDP, but not GMP, and is stimulated by exogenous dolichyl phosphate. The kinetics of transfer of [(14)C]mannose from GDP-[(14)C]mannose to form dolichyl monophosphate mannose were studied by using preparations derived from rats fed on one of four diets: G (high glucose), L (high lard), F (fructose) or GC (high glucose, 0.9% cholesterol). The K(m) and V(max.) values for transfer from GDP-mannose were virtually indistinguishable in the four preparations.In the absence of exogenous dolichyl phosphate, the largest amount of transfer of [(14)C]mannose into the mannosyl lipid was observed with preparations from fructose-fed animals. Preparations from glucose-fed animals showed about 60% as much transfer, whereas membranes from rats fed the other diets showed intermediate values between the fructose- and glucose-fed animals. The inclusion of cholesterol in the glucose diet elicited an increase in transfer of mannose. Under conditions of saturating exogenous dolichyl phosphate, preparations from lard-fed animals have 1.5 times as much enzyme activity as do preparations from animals fed the other three diets.

Phenyl phosphate as a water-soluble analog of dolichyl phosphate in microsomal mannosyltransferase systems

Incubation of rat liver microsomes with GDP-[14C]mannose and phenyl phosphate for 30 min, followed by paper-chromatographic analysis, revealed the presence of a new radioactive compound that was isolated and characterized as phenyl beta-D-mannosyl phosphate. The enzyme catalyzing this reaction was very closely related to a dolichyl beta-D-mannosyl phosphate-synthesizing activity found in the same microsomes. The two activities were affected in a parallel fashion by mild heating and were found in the same proportions in whole microsomes, in rough-surfaced microsomes, and in smooth-surfaced mirosomes. Increasing the concentration of phenyl phosphate from 0 to 15 mM resulted in a progressive reduction in the rate of mannose incorporation into endogenous dolichyl phosphate. It is concluded that the two reactions are catalyzed by a single enzyme. The labeled product from phenyl phosphate was rapidly excluded from the microsomes, while the product from dolichyl phosphate was retained in the microsomes, suggesting that phenyl phosphate may act as a mannose-trapping agent, thereby inhibiting mannosyltransferase reactions involving dolichyl phosphate as a glycosyl carrier.

Glycoprotein biosynthesis in intestinal epithelial cells during differentiation. Incorporation of [14C]mannose from GDP-[14C]mannose into dolichol derivatives

Epithelial cells of the rat small intestine were collected as a gradient of villus to crypt cells. Homogenates of these cells incubated with GDP-D-[14C]mannose in the presence of MnCl2 incorporated radioactivity into dolichyl mannosyl phosphate and a mixutre of dolichyl pyrophosphate oligosaccharides varying in the size of their oligosaccharide moiety. The labeled oligosaccharides formed in villus cell homogenates appeared shorter than those formed in crypt cell homogenates. The addition of dolichyl phosphate greatly stimulated the synthesis of dolichyl mannosyl phosphate. The initial rate of synthesis of dolichyl mannosyl phosphate from GDP-D-[14C]mannose and exogenous dolichyl phosphate was highest in an intermediate cell fraction having a low specific activity of sucrase and alkaline phosphatase and an intermediate specific activity of thymidine kinase. To compare the rates of dolichyl mannosyl phosphate synthesis in the different cell fractions, it was essential to control degradation of GDP-D-[14]mannose by the addition of AMP to the incubation, since villus cells degraded GDP-D-[14C]mannose much faster than crypt cells.

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

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

[Mannosylation of lipid and protein acceptors in sheep lung microsomes]

In Sheep lungs, we have shown that a mannosyl-transferase activity is involved at microsomal level. This enzymatic system is able to catalyze the incorporation of mannose from GDP-mannose into two different endogenous acceptors: lipids and proteins. The mannolipid has the properties of a mannosyl-phosphoryl-polyprenol. The kinetic studies of product biosynthesis demonstrate that no precursor-product relationship exists in this system; this is an original behaviour of the lung in glycosylation phenomenon.

Properties of the dolichol phosphate: GDPmannose mannosyltransferase of liver microsomes

The reaction of GDP[14C]-mannose with dolichol phosphate (Dol-P) in hepatic microsomes is characterized by an initial brief period of relatively rapid Dol-P-[14C]-mannose synthesis. The time course of this 1--3 min period of rapid synthesis follows approximate first order kinetics. However, the rate of reaction does not decrease to zero as predicted by the kinetics of the initial period of synthesis, but continues instead at a slow, steadily decreasing, rate. Examination of the time course of Dol-P-mannose synthesis for different concentrations of GDP[14C]-mannose revealed that the extrapolated final level of Dol-P-mannose synthesized is increased when the concentration of GDPmannose is raised. These data, plus those derived from studies of the reverse reaction, suggest that the non-linear time course for the synthesis of Dol-P-mannose is due in part to the reaction approaching equilibrium between the forward and reverse reactions. The effects of Mn++ on the time course of the forward and reverse reaction are complex and suggest that the Mn++ complexes of both GDPmannose and GDP are poorer substrates for the enzyme than the free nucleotides. Perturbations of the lipid environment of the microsomal membrane by treatment with phospholipase A, detergent, sonication, or alkaline pH lead to a decrease in the final level of Dol-P-mannose synthesized, but do not affect the time required for half maximal labeling. When the reverse reaction was investigated in phospholipase A-treated microsomes, the final extent of the reaction was also reduced. These data suggest that perturbation of the membrane lipid environment decreases in some undefined way the availability of Dol-P and Dol-P-mannose to enzyme.

Biosynthesis of yeast mannoproteins. Synthesis of mannan outer chain and of dolichol derivatives

The Saccharomyces cerevisiae mutants affected in the structure of mannan outer chain were found to synthesize dolichol diphosphate-linked oligosaccharides identical in size to those of the wild type strain. The mannosyl transferases involved in the synthesis of the outer chain had an absolute requirement for manganese ions and were activated when enzymatic preparations were stored at 2 degrees C, whereas the transferases responsible for the formation of dolichol monophosphate mannose and dolichol diphosphate oligosaccharides were drastically inactivated from the onset of storage and required magnesium or manganese ions, the former being more effective than the latter. Both sets of enzymes could be separated by ion exchange chromatography. In vitro conditions that enhanced the synthesis of dolichol monophosphate mannose did not stimulate the incorporation of mannose residues into the outer chain. It is concluded that dolichol monophosphate mannose is not an intermediate in the synthesis of the outer chain and that this part of mannan and the dolichol diphosphate oligosaccharides are synthesized by different mannosyltransferases.