Glucose transfer from dolichol monophosphate glucose: the product formed with endogenous microsomal acceptor

Cutin:xyloglucan transacylase (CXT) activity covalently links cutin to a plant cell-wall polysaccharide

The shoot epidermal cell wall in land-plants is associated with a polyester, cutin, which controls water loss and possibly organ expansion. Covalent bonds between cutin and its neighbouring cell-wall polysaccharides have long been proposed. However, the lack of biochemical evidence makes cutin-polysaccharide linkages largely conjectural. Here we optimised a portfolio of radiochemical assays to look for cutin-polysaccharide ester bonds in the epidermis of pea epicotyls, ice-plant leaves and tomato fruits, based on the hypothesis that a transacylase remodels cutin in a similar fashion to cutin synthase and cutin:cutin transacylase activities. Through in-situ enzyme assays and chemical degradations coupled with chromatographic analysis of the 3H-labelled products, we observed that among several wall-related oligosaccharides tested, only a xyloglucan oligosaccharide ([3H]XXXGol) could acquire ester-bonds from endogenous cutin, suggesting a cutin:xyloglucan transacylase (CXT). CXT activity was heat-labile, time-dependent, and maximal at near-neutral pH values. In-situ CXT activity peaked in nearly fully expanded tomato fruits and ice-plant leaves. CXT activity positively correlated with organ growth rate, suggesting that it contributes to epidermal integrity during rapid expansion. This study uncovers hitherto unappreciated re-structuring processes in the plant epidermis and provides a step towards the identification of CXT and its engineering for biotechnological applications.

Protein Glycosylation, Conserved from Yeast to Man: A Model Organism Helps Elucidate Congenital Human Diseases

Proteins can be modified by a large variety of covalently linked saccharides. The present review concentrates on two types, protein N-glycosylation and protein O-mannosylation, which, with only a few exceptions, are evolutionary conserved from yeast to man. They are also distinguished by some special features: The corresponding glycosylation processes start in the endoplasmatic reticulum, are continued in the Golgi apparatus, and require dolichol-activated precursors for the initial biosynthetic steps. With respect to the molecular biology of both types of protein glycosylation, the pathways and the genetic background of the reactions have most successfully been studied with the genetically easy-to-handle baker's yeast, Saccharomyces cerevisae. Many of the severe developmental disturbances in children are related to protein glycosylation, for example, the CDG syndrome (congenital disorders of glycosylation) as well as congenital muscular dystrophies with neuronal-cell-migration defects have been elucidated with the help of yeast.

Phosphoglucomutase is an in vivo lithium target in yeast

Lithium is a drug frequently used in the treatment of manic depressive disorder. We have observed that the yeast Saccharomyces cerevisiae is very sensitive to lithium when growing in galactose medium. In this work we show that lithium inhibits with high affinity yeast (IC50 approximately 0.2 mm) and human (IC50 approximately 1.5 mm) phosphoglucomutase, the enzyme that catalyzes the reversible conversion of glucose 1-phosphate to glucose 6-phosphate. Lithium inhibits the rate of fermentation when yeast are grown in galactose and induces accumulation of glucose 1-phosphate and galactose 1-phosphate. Accumulation of these metabolites was also observed when a strain deleted of the two isoforms of phosphoglucomutase was incubated in galactose medium. In glucose-grown cells lithium reduces the steady state levels of UDP-glucose, resulting in a defect on trehalose and glycogen biosynthesis. Lithium acts as a competitive inhibitor of yeast phosphoglucomutase activity by competing with magnesium, a cofactor of the enzyme. High magnesium concentrations revert lithium inhibition of growth and phosphoglucomutase activity. Lithium stress causes an increase of the phosphoglucomutase activity due to an induction of transcription of the PGM2 gene, and its overexpression confers lithium tolerance in galactose medium. These results show that phosphoglucomutase is an important in vivo lithium target.

UDP-N-acetylglucosamine:dolichyl-phosphate N-acetylglucosamine-1-phosphate transferase is amplified in tunicamycin-resistant soybean cells

A tunicamycin-resistant soybean cell line was developed by gradually increasing the concentration of tunicamycin in the growth medium. At the final stage, the resistant cells could survive in media containing 60 micrograms/ml of tunicamycin, whereas normal cells show a greatly retarded growth rate at 0.5 microgram/ml of antibiotic. The tunicamycin-resistant cells had a greater than 40-fold increase in the activity of the enzyme UDP-GlcNAc:dolichyl-P GlcNAc1P transferase, a 2-3-fold increase in the activity of dolichyl-P-mannose synthase, but no increase in the activities of other enzymes of the lipid-linked saccharide pathway such as dolichyl-P-glucose synthase or mannosyl transferases. There was also no change in the activities of the glycoprotein-processing enzymes, glucosidase I or glucosidase II, as compared to wild-type cells. The increase in GlcNAc1P transferase was due to an increased production of enzyme, as seen by a dramatic increase in the amount of a 39-kDa protein, which is presumed to be this enzyme protein. The GlcNAc1P transferase from tunicamycin-resistant cells was equally sensitive to tunicamycin as was the wild-type enzyme, but was considerably more labile to temperatures above 30 degrees C. The activity in tunicamycin-resistant cells was greatly stimulated by exogenous dolichyl-P. The spectrum of oligosaccharides from labeled lipid-linked oligosaccharides was similar in wild-type and tunicamycin-resistant soybean cells, but the resistant cells had significantly greater amounts of the shorter and much lower amounts of the larger-sized oligosaccharides.

N-glycosylation of membrane glycoproteins in retinol-deficient rat liver

The effect of vitamin A deficiency on N-linked oligosaccharides of membrane glycoproteins was studied in rat liver in order to evaluate the suggested role of retinol in protein N-glycosylation. First, oligosaccharides of newly synthesized glycoproteins from rough endoplasmic reticulum of vitamin A deficient liver were compared with that of pair-fed controls. Oligosaccharides were metabolically labelled with D-[2-3H]mannose, released from the glycoproteins with endoglycosidase H, purified by reversed phase HPLC and ion exchange chromatography, and were reduced with sodium borohydride. HPLC fractionation of the oligosaccharide alditols showed that the glycoproteins carried mainly four oligosaccharide species, Glc1Man9GlcNAc2, Man9GlcNAc2, Man8GlcNAc2 and Man7GlcNAc2, in identical relative amounts in the vitamin A deficient and the control tissue. In particular, no increase in the proportion of short chain oligosaccharides was noted in vitamin A deficient liver. Second, the number of N-linked oligosaccharides was estimated in dipeptidylpeptidase IV (DPP IV), a major glycoprotein constituent of the hepatic plasma membrane, comparing the newly synthesized glycoprotein from rough endoplasmic reticulum and the mature form of DPP IV from the plasma membrane. No evidence was obtained that retinol deficiency caused incomplete glycosylation of this membrane glycoprotein. From these data, the suggested role of retinol as a cofactor involved in the synthesis of N-linked oligosaccharides of glycoproteins must be questioned.

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

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

Recovery of dolichyl diphosphate oligosaccharide in methanolic aqueous phase prepared from rat liver microsomal fractions

The synthesis of dolichyl diphosphate oligosaccharide was studied by incubating rat liver microsomes (microsomal fractions) with GDP-[14C]mannose, UDP-glucose, UDP-N-acetylglucosamine and [3H]dolichol phosphate. The labelled products obtained by the first step of extraction of the microsomes in methanolic aqueous phase (MAP fraction in chloroform/methanol/water; 3:2:1, by vol.) and in CMW fraction (chloroform/methanol/water; 10:10:3, by vol.) obtained by extraction of the interphase after the first step of extraction were analysed on a DEAE-cellulose column. With the progress of incubation, the radioactivity in unchanged GDP-mannose decreased, whereas the labelled dol-P-P-oligo in the MAP fraction increased about 5-6-fold. The lipid oligosaccharide in this fraction accounted for about 50-60% of the GDP-mannose used, whereas the recovery of the labelled lipid oligosaccharide in the CMW fraction was about 10%. The lipid oligosaccharide from both reactions after mild acid hydrolysis were analysed by gel filtration on Bio-Gel P-4. The oligosaccharide from the MAP fraction gave a peak of higher Mr distinctly separate from the lower-Mr peak obtained from the CMW fraction. Microsomes incubated with labelled lipid oligosaccharide from the MAP fraction showed incorporation of the label into endogenous protein.

Enzymatic glucosylation of dolichol monophosphate and transfer of glucose from isolated dolichyl-D-glucosyl phosphate to ceramides by BHK-21 cell microsomes

Enzymatic glucosylation of dolichol monophosphate (dolichol-P) from UDP-D-[3H]glucose was studied using the microsomal fraction of BHK-21 cells. The reaction product was separated by preparative thin-layer chromatography, further purified by DEAE-cellulose acetate column chromatography, and characterized as dolichyl-beta-D-glucosyl phosphate (Dol-P-Glc). The microsomal fraction of BHK cells catalyzed the incorporation of glucose from UDP-[3H]glucose into ceramides (endogenous and exogenous) and Dol-P; both reactions required Mn2+. Maximal glucosylation of Dol-P was achieved at pH 5.6-5.8 in the presence of a non-ionic detergent, Zonyl A. Glucosylation of exogenous Dol-P, from UDP-Glc, was non-competitively inhibited by exogenous ceramides. Incubation of Dol-P-[3H]Glc or Dol-P-[14C]Glc with liposomes (containing ceramides) and the microsomal fraction of BHK-21 cells resulted in the formation of a radioactive glucolipid which comigrated with the same RF value as glucosylceramide (Glc-Cer) on silica gel thin-layer chromatography. Transfer of [14C]glucose from Dol-P-[14C]Glc to exogenous ceramides was confirmed by double-labeling techniques. The pH dependence for transfer of radio-labeled glucose from Dol-P-[3H]Glc to ceramides was multi-phasic (optima at pH 4.0 and 7.0); glycosylation occurred within 5 min and Zonyl A was absolutely essential for the transfer reaction. These results indicate that Dol-P-Glc may also participate in the synthesis of ceramide hexosides.

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.

Lipid-bound sugars in malignant human breast tumors. Partial characterization of mannosyl and glucosyl transferase activities

Microsomal preparations from malignant human breast tumors catalyzed the transfer of mannose and glucose from GDP-[14C]-Man and UDP-[14C]-Glc into lipid-linked sugars and glycoprotein-like substances. As judged by several criteria the obtained lipid-linked monosaccharides behaved as dolichyl phosphate mannose and dolichyl phosphate glucose whereas lipid-linked oligosaccharides behaved as polyprenyl diphosphate derivatives. The optimum conditions for mannosyl- and glucosyl-transfer reactions and the effect of dolichyl phosphate, detergent and EDTA on incubation mixture were described.

The biosynthetic pathway of the asparagine-linked oligosaccharides of glycoproteins

This review deals with the structure and addition of the different types of oligosaccharides to asparagine residues in proteins. This process occurs in several steps, first an oligosaccharide which contains N-acetylglucosamine mannose and glucose is built up joined to dolichyl diphosphate. The oligosaccharide is then transferred to a polypeptide chain, loses its glucose, and is modified by removal of some monosaccharides and addition of others giving rise to a variety of saccharides.

Postnatal changes in dolichol-pathway enzyme activities in cerebral cortex neurons

Neuronal perikarya were isolated from rat cerebral cortex at different stages of postnatal development. Membranes sedimenting at 100000 g were obtained from these neurons to study several glycosyltransferases of the dolichol pathway. Enzyme activities from stages before and during synapse formation were compared (days 5 and 15 respectively). Dolichyl diphosphate (Dol-P-P) N-acetylglucosamine, dolichyl phosphate mannose and dolichyl phosphate glucose synthases and the enzymes catalysing Dol-P-P-GlcNAc2Man9Glc3 formation were higher at day 15 of postnatal development. The glycosyl transfer of the latter compound to endogenous protein(s) as well as to a dinitrophenyl-heptapeptide was also measured. The activity was higher at day 15. Furthermore, the activity of dolichyl phosphate mannose synthase was also measured during the time when the number of synapses ceased to increase (day 36) and in the adult stage. The activity of dolichyl phosphate mannose synthase was higher at day 36 than at day 15, and declined in the adult stage. From these results it may be concluded that there is an increase in the glycosylation of asparagine-type glycoproteins during synapse formation in the neurons of the cerebral cortex.

Control of hemicellulose and pectin synthesis during differentiation of vascular tissue in bean (Phaseolus vulgaris) callus and in bean hypocotyl

Membrane fractions from bean hypocotyl or callus incorporate arabinose from UDP-β-L-arabinose into arabinan and xylose from UDP-α-D-xylose into xylan. The control of these syntheses has been studied during xylogenesis in stele and in xylogenesis induced in callus tissue. Induction of arabinan synthetase activity occurs during division and extension growth while that of xylan synthetase occurs subsequently during the period of secondary thickening of the cell wall. The xylan synthetase induction is correlated with the induction of phenylalanine ammonia-lyase and with lignin synthesis.

Dolichol pathway in lymphocytes from rat spleen. Influence of the glucosylation on the cleavage of dolichyl diphosphate oligosaccharides into phosphooligosaccharides

Incubation of rat-spleen lymphocytes with UDP-glucose together with GDP-mannose and UDP-N-acetylglucosamine leads to the formation of glucosylated lipid intermediates characterized as dolichyl phosphate glucose and dolichyl diphosphate oligosaccharides. This latter can be either transferred onto endogenous protein acceptors or cleaved into phosphooligosaccharides. The striking fact is that phosphooligosaccharide populations contain far less glucosylated products than the dolichyl diphosphate oligosaccharide ones from which they are derived. Two hypotheses have been investigated: either a rapid action of glucosidases on the liberated phosphooligosaccharides or a preferential splitting of the non-glucosylated population of dolichyl diphosphate oligosaccharides. Addition of p-nitrophenyl-alpha-D-glucoside inhibits glucosidase activities and allows the production of a major population of dolichyl diphosphate oligosaccharides containing three glucose residues. Using these conditions, it is shown that the amount of phosphooligosaccharides generated from the splitting of dolichyl diphosphate oligosaccharides is greatly decreased and that the major part of these remaining phosphooligosaccharides do not contain glucose. These results show that the presence of glucosyl units prevent dolichyl diphosphate oligosaccharides from further degradation into phosphooligosaccharides.

The biosynthesis of glucose containing insect lipid linked oligosaccharide and its possible role in glycoprotein assembly

Microsome enriched Ceratitis capitata extracts synthesized a glucosylated lipid linked oligosaccharide. Its properties were closely related to those of the previously described insect mannosylated dolichyl diphosphate oligosaccharides and almost the same as those of the rat liver dolichyl-diphosphate-(GlcNAc)2-(Man)9-(Glc)1-3. The saccharide moiety of the latter was transferred to an unknown endogenous protein-like acceptor by the fly extracts. These represent the first evidence of a protein glycosylation in a pluricellular invertebrate through dolichyl derivatives.

Microsomal glucosidases of rat liver. Partial purification and inhibition by disaccharides

Further work on microsomal glucosidases of rat liver has confirmed that at least two enzymes are involved in the removal of glucose from the glucose-containing oligosaccharide. One acts on the oligosaccharide containing three glucose residues and another on the oligosaccharide which has one or two glucoses. The glucosidase which acts on (Glc)2(Man)9(GlcNAc)2 could be purified with a Concanavalin-A--Sepharose column following by electrofocusing. This purified preparation was active on the oligosaccharide containing one or two glucoses. Heat inactivation and inhibition by disaccharides was parallel for both activities. Inhibition of the glucosidase active on (Glc)3(Man)9(GlcNAC)2 was obtained with kojibiose which has an alpha 1-2 linkage, while the glucosidase acting on (Glc)1-2(Man)9-(GlcNAc)2 was inhibited by nigerose (alpha 1-3 linkage), maltose (alpha 1-4 linkage) and glucose at a higher concentration. None of the beta anomers inhibited. These results are consistent with an alpha configuration of the three glucoses of the dolichyl-diphosphate-linked oligosaccharide. Kojibiose was found to inhibit glucosidase action not only on the free oligosaccharide but also on protein-bound one.

Enzymatic glucosylation of dolichyl monophosphate formed via cytidine triphosphate in calf brain membranes

When calf brain membrane preparations containing endogenous dolichyl [32P]monophosphate (Dol-32P), prelabeled enzymatically by [gamma-32P]-CTP, are incubated with unlabeled UDP-glucose, the formation of a mild acid-labile [32P]phosphoglucolipid is observed. The biosynthesis of the [32P]phosphoglucolipid is dependent on the concentration of UDP-glucose added, and no [32P]phosphoglycolipid appeared when UDP-glucose was replaced by ADP-glucose, UDP-xylose, UDP-galactose, UDP-mannose, or UDP-glucuronic acid. The 32P-labeled product formed by the UDP-glucose-dependent reaction is chemically and chromatographically identical to glucosylphosphoryldolichol. Several enzymatic parameters of the glucosylation of the specific pool of Dol-P, synthesized by the CTP-mediated kinase, and the total available pool of Dol-P have been compared by a double-label assay utilizing endogenous, prelabeled Dol-32P and UDP-[3H]glucose as substrates.

Partial characterization of the polyisoprenoid carrier of N-acetylglucosamine in Glycine max (soya bean)

Dolichyl phosphate (C55) and dolichyl phosphate prepared from liver were incubated with an enzyme prepared from soya-bean protoplasts. They both stimulated the transfer of radioactivity from UDP-D-glucose to lipid, but the stimulation was greater with liver dolichyl phosphate. Liver dolichyl phosphate with the soya-bean enzyme stimulated the transfer of radioactivity from UDP-N-acetyl-D-[U-14C]glucosamine to acidic lipid. UDP-D-[U-14C]glucose and UDP-N-acetyl-D-[U-14C]glucosamine were used with the soya-bean enzyme to prepare the glycosylated-acid-lipid acceptor of each sugar. Mild acid hydrolysis revealed that the radioactivity in the lipid glucosylated from UDP-glucose was present exclusively as glucose. That in the lipid glycosylated from UDP-N-acetylglucosamine was present mostly as N-acetylglucosamine. The soya-bean acidic-lipid acceptors of glucose and N-acetylglucosamine were stable to both catalytic hydrogenation and treatment with hot aqueous phenol; they behaved in a similar way as authentic alpha-saturated glucosylated polyisoprenyl phosphates. The soya-bean acidic-lipid acceptors of glucose and N-acetylglucosamine were eluted as deoxycholate complexes from a Sephadex column. In comparison with glucosylated polyisoprenyl phosphates of known size, the patterns of elution indicated that both soya-bean lipids contained a polyisoprenoid chain of 18 isoprene units.

Biosynthesis of the core region of yeast mannoproteins. Formation of a glucosylated dolichol-bound oligosaccharide precursor, its transfer to protein and subsequent modification

A new membrane preparation from Saccharomyces cerevisiae was developed, which effectively catalyzes the synthesis of large oligosaccharide-lipids from GDP-Man and UDP-Glc allowing a detailed study of their formation and size. The oligosaccharide from an incubation with GDP-Man could be separated by gel filtration chromatography into several species consisting of two N-acetylglucosamine (GlcNAc) residues at the reducing end and differing by one mannos unit; the major compound formed has the composition (Man)9(GlcNAc)2. Upon incubation with UDP-Glc, three oligosaccharides corresponding to the size of (Glc)1-3(Man)9(GlcNAc)2 are formed. Thus, the oligosaccharides generated in vitro by the yeast membranes appear to be identical in size with the oligosaccharides found in animal systems. In addition the results indicate that dolichyl phosphate mannoe (DolP-Man) is the immediate donor in assembling the oligosaccharide moiety from (Man)5(GlcNAc)2 to (Man)9(GlcNAc)2. All three glucose residues are transferred from DolP-Glc. Experiments with isolated [Glc-14C]oligosaccharide-lipid as substrate demonstrated that the oligosaccharide chain is transferred to an endogenous membrane protein acceptor. Moreover, transfer is followed by an enzymic removal of glucose residues, due to a glucosidase activity associated with the membranes. Glucose release from the free [Glc-14C]oligosaccharide is less effective than from protein-bound oligosaccharide. Glycosylation was also observed using [Man-14C]oligosaccharide-lipid or DolPP-(GlcNAc)2 as donor. However, transfer in the presence of glucose seems to be more rapid. The mannose-containing oligosaccharide, released from the lipid, was shown to function as a substrate for further chain elongation reactions utilizing GDP-Man but not DolPP-Man as donor. It is suggested that the immediate precursor in the synthesis of the heterogeneous core region, (Man)12-17(GlcNAc)2, of yeast mannoproteins is a glucose-containing lipid-oligosaccharide with the composition (Glc)3(Man)9(GlcNAc)2, i.e. only part of what has been defined as inner core is built up on the lipid carrier. After transfer to protein the oligosaccharide is modified by excision of the glucose residues, followed subsequently by further elongation from GDP-Man to give the size of th oligosaccharide chains found in native mannoproteins.

Addition of glucose to dolichyl diphosphate oligosaccharide and transfer to protein

The glycosylation of asparagine residues in proteins is known to occur by transfer from a dolichyl diphosphate oligosaccharide containing glucose. Paper chromatography allowed the separation of oligosaccharides (obtained by acid hydrolysis of the dolichyl diphosphate derivative) containing 1, 2 and 3 glucose residues. Using this procedure it was found that the addition of all three glucoses to the dolichyl diphosphate oligosaccharide occur with dolichyl phosphate glucose as donor. Furthermore only the compound with three glucoses was used as donor in the transfer to protein. The addition of glucose to exogenous dolichyldiphosphate oligosaccharide labelled by transfer from radioactive guanosine diphosphate mannose was detected.

Effects of gibberellic acid and of tunicamycin on glycosyl-transferase activities and on alpha-amylase secretion in barley

A crude membrane fraction was prepared from isolated aleurone layers, the secretory tissue of barley grains. The layers were pre-incubated in the presence or absence of the phytohormone gibberellic acid. The membranes catalyzed the transfer of [14C]mannose from GDP-[14C]mannose and of N-[14C]acetylglucosamine from UDP-N-[14C]acetylglucosamine to endogenous and exogenous dolichyl monophosphate. When gibberellic acid was present during the pretreatment the activity of the transferases was increased by a factor of two to three. A significantly increased activity was observable within four hours after the addition of gibberellic acid, whereas the gibberellic-acid-induced secretion of the glycoprotein alpha-amylase started only after 12 h. Tunicamycin inhibited the secretion of alpha-amylase by 60 to 80%. Intracellularly, however, no alpha-amylase was found to accumulate. On the other hand, tunicamycin did not inhibit the rate of total protein synthesis by more than 10%. The possibility is discussed that the synthesis of the protein portion of glycoproteins is specifically inhibited, when glycosylation is prevented.

The effects of triton X-100 on the transfer of mannose, glucose and n-acetylglusomine phosphate to dolichol monophosphate by preparations of rough and smooth endoplasmic reticulum and of mitochondria of rat liver

Triton X-100 and exogenous dolichol monophosphate have been used to investigate the nature of enzymes responsible for the transfer of mannose, glucose and N-acetylglucosamine phosphate from nucleotide donors to dolichol monophosphate in vesicles derived from rough and smooth endoplasmic reticulum and mitochondria. Mitochondria were shown to contain the highest specific activities of these enzymes. The responses of the glycosyltransferases to increasing concentrations of Triton X-100 and the effect on these responses of exogenous dolichol monophosphate suggest that the enzymes for mannose and glucose transfer are less hydrophobic, and therefore less intrinsic, in the membrane than the enzyme for N-acetylglucosamine phosphate transfer. In smooth vesicles the results are consistent with mannosyl- and glucosyl-transferases being located at both inner and outer faces of the membrane. In rough vesicles and in mitochondria mannosyl- and glucosyl-transferases were confirmed at the outer face. There is, however, only one site of N-acetylglucosamine phosphate transfer, this being more hydrophobically located in the membrane than the other sites of glycosyl transfer. Mitochondrial enzyme activity closely resembled that of rough endoplasmic reticulum in response to Triton X-100 and exogenous dolichol monophosphate, and is probably associated with the outer membrane.

The dolichol pathway of protein glycosylation in rat liver. Stimulation by GTP of the incorporation of N-acetylglucosamine in endogenous lipids and proteins of rough microsomes treated with pyrophosphate

Incorporation of N-acetylglucosamine into endogenous lipid and protein acceptors was investigated on heavy microsomes from rat liver, incubated with UDP-N-acetyl[14C]glucosamine and GDP-mannose in the absence of detergent. This subcellular preparation derived for 95% or more from the rough endoplasmic reticulum and was devoid of Golgi components which contain the enzyme that adds the peripheral N-acetylglucosamine units to glycoproteins. The label was found almost exclusively in dolichyl diphosphate N-acetylglucosamine, except when the subcellular preparation was treated with pyrophosphate and subsequently incubated with the nucleotide sugars in the presence of GTP. Then, the incorporation of N-acetylglucosamine was considerably enhanced, and the additional label was associated with dolichyl diphosphate N,N'-diacetylchitobiose, with dolichyl diphosphate oligosaccharides and with proteins. The time-course of N-acetylglucosamine incorporation in these products was compatible with the pathway of dolichyl diphosphate glycoconjugates for the biosynthesis of the core portion of saccharide chains linked to asparagine residues of glycoproteins. The addition of GDP-mannose to the incubation medium was required to produce labeled dolichyl diphosphate oligosaccharides, but not to incorporate N-acetylglucosamine in protein. It is concluded that rough microsomes are capable of assembling dolichol-linked oligosaccharides from exogenous nucleotide precursors and of transferring N,N'-diacetylchitobiose, or its mannosylated derivatives, from the lipid intermediate to endogenous proteins. However, these metabolic activities are hindered in the original subcellular preparation, and in the absence of GTP. Although the earliest perceptible effect produced jointly by the treatment with pyrophosphate and by GTP was the synthesis of dolichyl diphosphate N,N'-diacetylchitobiose, the primary action of these factors remains uncertain. They may stimulate directly the reaction forming dolichyl diphosphate N,N'-diacetylchitobiose from dolichyl diphosphate N-acetylglucosamine, or activate the synthesis of this latter intermediate from a particular pool of dolichyl monophosphate which is readily converted afterwards into disaccharide and oligosaccharide derivatives and glycosylates protein. The requirement for GTP might have a functional meaning, for GTP acted maximally at a concentration distinctly lower than its actual concentration in liver. The detachment of ribosomes from rough vesicles was the major alteration induced by treatment with pyrophosphate. It is suggested that the removal of ribosomes unmasks the membrane sites where GTP acts.

Polyprenyl phosphate sugars synthesized during slime-polysaccharide production by membranes of the root-cap cells of maize (Zea mays)

Two types of experiments were carried out; either maize roots were incubated in L-[1-3H]fucose or membranes were prepared from root tips and these were incubated with GDP-L-[U-14C]fucose or UDP-D-[U-4C]glucose. The radioactively labelled lipids that were synthesized in vivo and in vitro were extracted and separated into polar and neutral components. The polar lipids had the characteristics of polyprenyl phosphate and diphosphate fucose or glucose derivatives, and the neutral lipids of sterol glycosides (fucose or glucose). A partial separation of the glycolipid synthetase reactions was achieved. Membranes were fractionated into material that sedimented at 20,000g and 100,000g. Most of the polar glycolipid synthetase activity (for the incorporation of both fucose and glucose) was located in the 100,000 g pellet, and this activity was probably located in the endoplasmic reticulum. The neutral lipid, which contained fucose, was synthesized mainly by membranes of the 20,000g pellet, and the activity was probably associated with the dictyosomes, whereas the neutral glucolipids were synthesized by all the membrane fractions. It is suggested that the polar (polyprenyl) lipids labelled with fucose could act as possible intermediates during the synthesis of the glycoproteins and slime in the root tip.