Transport of sugar nucleotides and adenosine 3'-phosphate 5'-phosphosulfate into vesicles derived from the Golgi apparatus

Alg13p, the catalytic subunit of the endoplasmic reticulum UDP-GlcNAc glycosyltransferase, is a target for proteasomal degradation

The second step of dolichol-linked oligosaccharide synthesis in the N-linked glycosylation pathway at the endoplasmic reticulum (ER) membrane is catalyzed by an unusual hetero-oligomeric UDP-N-acetylglucosamine transferase that in most eukaryotes is comprised of at least two subunits, Alg13p and Alg14p. Alg13p is the cytosolic and catalytic subunit that is recruited to the ER by the membrane protein Alg14p. We show that in Saccharomyces cerevisiae, cytosolic Alg13p is very short-lived, whereas membrane-associated Alg13 is relatively stable. Cytosolic Alg13p is a target for proteasomal degradation, and the failure to degrade excess Alg13p leads to glycosylation defects. Alg13p degradation does not require ubiquitin but instead, requires a C-terminal domain whose deletion results in Alg13p stability. Conversely, appending this sequence onto normally long-lived beta-galactosidase causes it to undergo rapid degradation, demonstrating that this C-terminal domain represents a novel and autonomous degradation motif. These data lead to the model that proteasomal degradation of excess unassembled Alg13p is an important quality control mechanism that ensures proper protein complex assembly and correct N-linked glycosylation.

In vitro synthesis of sulfated glycosaminoglycans coupled to inter-compartmental Golgi transport

We have used isolated rat liver Golgi membranes to reconstitute the synthesis of sulfated glycosaminoglycans (GAGs) onto the membrane-permeable, external acceptor xyloside. Biosynthetic labeling of GAGs with [35S]sulfate in vitro is shown to have an absolute requirement for ATP and cytosolic proteins and is inhibited by dismantling the Golgi apparatus with okadaic acid or under mitotic conditions suggesting that inter-compartmental transport between Golgi cisternae is a prerequisite for the successful completion of the initiation, polymerization, and sulfation of GAGs. Accordingly, we show that in vitro synthesis of 35S-GAGs utilizes the same machinery employed in Golgi transport events in terms of vesicle budding (ADP-ribosylation factor and coatomer), docking (Rabs), targeting (SNAREs), and fusion (N-ethylmaleimide-sensitive factor). This provides compelling evidence that GAGs synthesis is linked to Golgi membrane traffic and suggests that it can be used as a complementation-independent method to study membrane transport in Golgi preparations from any source. We have applied this system to show that intra-Golgi traffic requires the function of the Golgi target-SNARE, syntaxin 5.

3'-Phosphoadenosine 5'-phosphosulfate biosynthesis and the sulfation of cholecystokinin by the tyrosylprotein-sulfotransferase in rat brain tissue

This article resumes the work we have accomplished in the past few years. Cholecystokinin sulfation is an important post-translational modification necessary for the biological activity of this peptide hormone. The tyrosyl protein sulfotransferase (TPST) activity from rat cerebral cortex was characterized. TPST activity is most probably responsible for the endogenous sulfation of CCK. TPST reaction kinetic properties were studied using radiolabeled 3'-phosphoadenosine 5'-phosphosulfate (PAPS) and the non-sulfated peptide acceptor terbutyloxycarbonyl-cholecystokinin octapeptide (BocCCK-8(ns)) as substrates, and brain microsomes as the enzyme source. The BocCCK-8 sulfating reaction data is consistent with the idea that TPST forward reaction follows an ordered Bi Bi mechanism. PAPS biosynthesis and availability was studied in slices from rat cerebral cortex incubated in the presence of [35S]sulfate. There is a rapid and dynamic turnover of the steady-state level of PAPS in brain cells which is decreased by depolarizing agents such as potassium, veratridine and glutamate. Furthermore, the presence of a membrane-bound PAPS biosynthesis inhibitor was observed. These results are discussed in view of the biological importance that the cell sulfating pathways might play in nerve cell activity.

Pyrimidine nucleotide metabolism in rat hepatocytes: evidence for compartmentation of nucleotide pools

Pyrimidine nucleotide metabolism in rat hepatocytes was studied by measurement of the labelling kinetics of the various intermediates after double labelling with [14C]orotic acid and [3H]cytidine, the precursors for the de novo and the salvage pathways respectively. For the uridine nucleotides, differences were found for the 14C/3H ratios in the UDP-sugars, in UMP (of RNA) and in their precursor UTP, suggesting the existence of separated flows of the radioactive precursors through the de novo and the salvage pathways. Higher ratios in the UDP-sugars, which are synthesized in the cytoplasm, and a lower ratio in UMP (of RNA) relative to the 14C/3H ratio in UTP indicated that UTP derived from orotic acid is preferentially used for the cytoplasmic biosynthesis of the UDP-sugars. Uridine, derived from cytidine, is preferentially used for the nuclear-localized synthesis of RNA. In contrast to these findings, the 14C/3H ratios in the cytidine derivatives CMP-NeuAc and CMP (of RNA), and in the liponucleotides CDP-choline and CDP-ethanolamine, were all lower than that in the precursor CTP. This indicates a preferential utilization of the salvage-derived CTP for the synthesis of the liponucleotides as well as for RNA and CMP-NeuAc. Similar conclusions could be drawn from experiments in which the intracellular amounts of several uridine- and cytidine-nucleotide-containing derivatives were increased by preincubating the hepatocytes with unlabelled pyrimidine nucleotides or ethanolamine. Based on these data, we propose a refined model for the intracellular compartmentation of pyrimidine nucleotide biosynthesis in which three pools of UTP are distinguished: a pool of de novo-derived molecules and a pool of salvage-derived molecules, both of which are channelled to the site of utilization; in addition an 'overflow' pool exists, consisting of molecules having escaped from channelling. An overflow pool could also be distinguished for CTP, but no discrimination between de novo and salvage-derived molecules could be made.

UDP-sugar metabolism in Swarm rat chondrosarcoma chondrocytes

UDP-sugars and adenine nucleotides were extracted from freshly isolated chondrocytes and primary cell cultures and analysed by anion-exchange h.p.l.c. The pool sizes of UDP-N-acetylglucosamine, UDP-N-acetylgalactosamine, UDP-glucose-galactose, UDP-glucuronate and UDP-xylose were 2.9, 1.2, 2.5, 0.6 and 0.03 nmol/10(6) freshly isolated chondrocytes. When chondrocytes were maintained in Dulbecco's modified Eagle medium supplemented with 15% foetal-bovine serum, synthesis of [35S]proteoglycan and [3H]protein decreased over the first 48 h in culture, as did the pools of UDP-glucuronate and ATP. In contrast, the size of the UDP-N-acetylhexosamine pools underwent little change during culture. [35S]Proteoglycan and [3H]protein syntheses were stimulated in cultures supplemented with serum or insulin compared with those maintained in medium alone, in agreement with previous results. However, the UDP-sugar pool sizes were the same in both supplemented and non-supplemented cultures. In cultures maintained in the presence of [1-3H]glucose, the UDP-sugars were labelled to a constant 3H specific radioactivity which was very similar to that of the labelling medium. UDP-N-acetylhexosamines were labelled to constant 3H specific radioactivity with [6-3H]glucosamine as a precursor, but only about 1 in 375 of these UDP-sugars was derived from the amino sugar in the presence of glucose. The half-life (t1/2) for UDP-hexoses, UDP-glucuronate and UDP-N-acetylhexosamines was about 12, 12 and 50 min respectively.

Uncoupling of chondroitin sulfate glycosaminoglycan synthesis by brefeldin A

Brefeldin A has dramatic, well-documented, effects on the structural and functional organization of the Golgi complex. We have examined the effects of brefeldin A (BFA) on the Golgi-localized synthesis and addition of chondroitin sulfate glycosaminoglycan carbohydrate side chains. BFA caused a dose-dependent inhibition of chondroitin sulfate glycosaminoglycan elongation and sulfation onto the core proteins of the melanoma-associated proteoglycan and the major histocompatibility complex class II-associated invariant chain. In the presence of BFA, the melanoma proteoglycan core protein was retained in the ER but still acquired complex, sialylated, N-linked oligosaccharides, as measured by digestion with endoglycosidase H and neuraminidase. The initiation of glycosaminoglycan synthesis was not affected by BFA, as shown by the incorporation of [6-3H]galactose into a protein-carbohydrate linkage region that was sensitive to beta-elimination. The ability of cells to use an exogenous acceptor, p-nitrophenyl-beta-D-xyloside, to elongate and sulfate core protein-free glycosaminoglycans, was completely inhibited by BFA. The effects of BFA were completely reversible in the absence of new protein synthesis. These experiments indicate that BFA effectively uncouples chondroitin sulfate glycosaminoglycan synthesis by segregating initiation reactions from elongation and sulfation events. Our findings support the proposal that glycosaminoglycan elongation and sulfation reactions are associated with the trans-Golgi network, a BFA-resistant, Golgi subcompartment.

Mammalian glycosylation mutants as tools for the analysis and reconstitution of protein transport

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Sulphate induces very fast cell rounding and detachment

Within a few minutes of incubation with SO4(2-), cultured monolayer cells retract into round shapes with drastically reduced surface area. Concomitant elevation of phosphoinositide second messenger levels, viz, 1,2-diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3), is observed. A causal relationship with sulphation seems to be suggested by finding (a) sulphation of an added acceptor, 4-methylumbelliferone, (b) sulphation of endogenous glycosaminoglycan (GAG) polymers, (c) inhibition by phenol sulphotransferase inhibitor, DCNP (2,6-dichloro-4-nitrophenol). DCNP also inhibits the second messenger production and cell rounding. Reduced surface area appears to be caused by massive plasma membrane internalization in a distinctive endocytosis which is also seen in cell rounding from directly imposed ionic gradients. Reducing the surface area would reduce the adhesive or attachment sites. Besides demonstrating a highly efficient cell detachment potential, huge macromolecules appear to be readily internalized. The association of sulphation, signal transduction and cell detachment is novel.

Localization of glycosylation sites in the Golgi apparatus using immunolabeling and cytochemistry

This review summarizes data on the distribution of certain glycosylation steps in the Golgi apparatus as revealed by immunolabeling and lectin techniques. The methodical basis for such investigations was provided by the introduction of the colloidal gold marker system for immunolabeling and the development of new means of tissue processing such as the low-temperature embedding technique using Lowicryl K4M. The application of these techniques together with highly specific antibodies has provided much of the basis for our current understanding of the Golgi apparatus in functional terms. Thus, in many cell types, three Golgi apparatus compartments can be distinguished, whereas in others no such functional subdivision is evident. Investigations on sialyltransferase distribution have also provided direct evidence that GERL is structurally and functionally part of the Golgi apparatus.

Analysis of protein transport through the Golgi in a reconstituted cell-free system

The processes which transport membrane proteins between compartments of the Golgi apparatus have been reconstituted in vitro using isolated Golgi fractions. This cell-free system allows a detailed analysis of protein transport not possible in intact cells. Transport of the membrane glycoprotein (G protein) of vesicular stomatitis virus (VSV) is measured from a "donor" to an "acceptor" Golgi fraction. The donor Golgi fraction is prepared from VSV-infected Chinese hamster ovary (CHO) mutant cells deficient in the glycosylation enzyme N-acetylglucosamine transferase I. "Acceptor" is prepared from uninfected wild-type CHO cells. Transport is measured by the addition of N-acetylglucosamine to G protein, which can occur only upon movement of G protein from donor to acceptor. Transport requires physiological pH and osmolarity, is dependent on nucleotide triphosphates, and is mediated by proteins both from cytosol and on the Golgi membranes. Protein movement is inhibited by the non-hydrolyzable GTP analogue, GTP gamma S. The process of transport proceeds through the budding, pinching off, targeting, and fusion of transport vesicles. In this system these vesicles are initially coated with a non-clathrin coat and are targeted with this coat intact. Several of the proteins which mediate transport have been characterized, and isolated to homogeneity. The successful development of this assay has led to the formulation of cell free assays for protein transport between other compartments. Comparison of these systems indicates that some common mechanisms of vesicular movement are used in transport between a variety of membrane compartments.

Biosynthesis of PAPS in vitro by human liver. Measurement by two independent assay procedures

The biosynthesis of 3'-phosphoadenosine-5'-phosphosulphate (PAPS) by extracts of human liver from inorganic sulphate and APS was assayed by transferring the "active sulphate" to two different acceptors, namely N-acetyldopamine (NADA) and 4-methylumbelliferone (4-MU). NADA-sulphate was quantified by an HPLC-ECD method while the decrease in 4-MU was monitored continuously by a fluorimetric procedure. The optimum pH was 8.0 for both the PAPS generation and APS-kinase reaction. The apparent Km value for APS determined by the fluorimetric and HPLC-ECD procedures was 17.6 and 16.8 microM, respectively. PAPS-generation measured in 13 samples of human liver showed excellent correlation between the two assay procedures, with correlation coefficients (r) of 0.96 and 0.95 for PAPS generation from inorganic sulphate and APS, respectively. The fluorimetric assay was preferred as it is simple, equally sensitive and more reproducible. There was also a good correlation between the APS-kinase reaction and the two-step PAPS-generation from inorganic sulphate, with r = 0.97 and 0.91, as determined by the fluorimetric and HPLC-ECD procedures. The rate of PAPS generation from inorganic sulphate was only marginally less than that from APS in each of the 13 human liver samples, suggesting that the coupling of ATP-sulphurylase to APS-kinase facilitates "sulphate activation" and releases it from the constraints imposed by the unfavourable ATP sulphurylase reaction.

Permeabilized cells as a way of gaining access to intracellular organelles: an approach to glycosylation reactions

The selective plasma membrane permeabilization of animal cells is a way of introducing non permeable substrates into the cytoplasmic space. This technique facilitates the introduction of a wide range of labelled precursors and avoids the drawbacks of subcellular fractionation. We review here various physical and chemical methods successfully used in different metabolic studies, and as an example, note the advantages of permeabilized cells in glycosylation studies.

Topological orientation of mitochondrial GDPmannose: dolichyl-monophosphate mannosyltransferase in the outer membrane

Previous studies have shown the existence of an autonomous mitochondrial GDPmannose:dolichylmonophosphate mannosyltransferase, located in mitochondrial outer membrane of liver cells. As nothing is known about glycosylation sites in mitochondria, we have investigated the topological orientation of this enzyme in intact mitochondria, using controlled proteolysis with trypsin. Mitochondria were purified sequentially by mild ultrasonic treatment and sucrose density gradient. Purity and homogeneity of mitochondrial fraction were assessed by electron microscopy and specific marker enzymes measures. Our data provide evidence for a mitochondrial GDPmannose:dolichylmonophosphate mannosyltransferase facing the cytoplasmic side of the outer membrane. However, the exposure of this enzyme to the water phase has been shown to be dependent on the ionic strength of the environment.

A membrane transporter mediates access of uridine 5'-diphosphoglucuronic acid from the cytosol into the endoplasmic reticulum of rat hepatocytes: implications for glucuronidation reactions

Hepatic glucuronidation of a wide variety of substrates is catalyzed by the membrane-bound UDP-glucuronosyltransferases. Uridine 5'-diphosphoglucuronic acid (UDP-GlcUA) is the essential cosubstrate for all UDP-glucuronosyltransferase-mediated reactions. The mechanism by which this bulky, hydrophilic nucleotide-sugar is transported from the cytosol (where it is synthesized) to its binding site(s) on the enzyme is unknown. To determine whether a membrane carrier mediates the access of UDP-GlcUA into the endoplasmic reticulum, the transport of uridine 5'-diphospho-D-[U-14C]glucuronic acid into vesicles of rough and smooth endoplasmic reticulum isolated from rat liver was investigated at 38 degrees C using a rapid filtration technique. Uptake of UDP-GlcUA by both rough and smooth vesicles was extremely rapid (linear for only 10-20 s) and temperature-dependent (negligible at 4 degrees C). UDP-GlcUA uptake was saturable, and similar kinetic parameters were obtained for rough and smooth vesicles (Km 1.9 microM, Vmax 443 pmol/mg protein per min, and Km 1.3 microM, Vmax 503 pmol/mg protein per min, respectively). The uptake of UDP-GlcUA also exhibited a high degree of specificity, since many related compounds, including UMP, UDP and UDP-Glc, did not influence uptake. In addition, the non-penetrating inhibitors of anion transport, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS), 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), and probenecid, markedly inhibited UDP-GlcUA uptake. Finally, osmotic modulation of the intravesicular volume did not affect total uptake of UDP-GlcUA by membrane vesicles at equilibrium, indicating that this nucleotide-sugar is transported into the membrane rather than the intravesicular space. Collectively, these data provide direct evidence for a specific, carrier-mediated uptake process, which transports UDP-GlcUA from the cytosol into the endoplasmic reticulum of hepatocytes. This UDP-GlcUA transporter may be involved in the regulation of hepatic glucuronidation reactions.

Sugar nucleotides dissipate ATP-generated transmembrane pH gradient in Golgi vesicles from suspension-cell protoplasts ofChenopodium rubrum L

A microsomal vesicle fraction (GV) markedly enriched by the Golgi marker enzyme latent inosine diphosphatase (IDPase) has been isolated from photoautotrophic suspension-cell protoplasts ofChenopodium rubrum L. Addition of ATP creates a substantial pH gradient across the GV membrane as measured by accumulation of acridine orange. The GV showed a density of 1.14 g·cm(-3) by equilibrium density centrifugation on sucrose gradients. Coincidence of acridine-orange accumulation and IDPase activity was confirmed on Percoll gradients. Formation of the pH gradient half-saturates at 0.3 mM MgATP, peaks at pH 7, and is competitively inhibited by ADP (k i≤0.1 mM), but not by Pi; it is hardly inhibited by orthovanadate, quickly dissipated by monensink 2=18 nM), nigericin (k 1/2=25 nM), and sluggishly by N-ethylmaleimide (k 1/2≈35 μM). Inhibition by KNO3 (k 1/2≈6.7 mM) is incomplete (60%). Uridine 5'-diphosphate (UDP)-glucose, UDP-galactose, but not UDP-mannose and the pertinent sugars, dissipate the ATP-generated pH gradient (k 1/2≈10-20 mM UDP-glucose; optimum pH at 7.8). This UDP-glucose activity is accompanied by release of Pi, but not of glucose or sucrose. UDP-glucoseinduced Pi release from the GV saturates (k 1/2=1 mM UDP-glucose; optimum pH at 7) and is completely inhibited by the anion-channel blocker 4,4'-diisothiocyano-2,2'-stilbene disulfonic acid (DIDS;k 1/2=140 μM). The GV incorporates UDP-[U-(14)C]glucose into an acid-labile, alkaline-stable macromolecular compound; this process is like-wise inhibited by DIDS. We propose a model including, inter alia, a UDP-glucose/uridine-5'-monophosphate translocator and a phosphate-permeable anion channel to operate in Golgi vesicles ofChenopodium rubrum.

Ganglioside biosynthesis in rat liver: effect of UDP-amino sugars on individual transfer reactions

Several glycosyltransferases participating in ganglioside biosynthesis were measured in Golgi-rich fractions from rat liver. Addition of those UDP-amino sugars to the enzyme assays which accumulate in liver after treatment of rats with D-galactosamine inhibited the transferases to different degrees. The simultaneous presence of UDP-GalN, UDP-GalNAc, UDP-GlcN, and UDP-GlcNAc in concentrations resembling their overall content in livers 6 h after D-galactosamine administration led to an inhibition of the glycolipid galactosyltransferases, GL2 and GM1 synthases of 44 and 64%, respectively. GM2 synthase was moderately inhibited whereas the sialyltransferases (GM3, GD3, and GD1a synthases) were almost unimpaired. Induction of liver cell damage by D-galactosamine did not cause any change of glycosyltransferase activities as determined in rat liver homogenates and Golgi-rich fractions. These results indicate a possible role for UDP-amino sugars in the depression of ganglioside biosynthesis observed in vivo after GalN administration.

Membrane transport of sugar donors to the glycosylation sites

The assembly of N-linked glycoproteins in eukaryotic cells begins with the segregation of these molecules within the lumen of intracellular vesicles. Since the sugar nucleotides are cytoplasmic molecules, translocation of the sugar moiety across the membrane appears as a crucial event in the glycoprotein synthesis. This N-glycosylation process occurs in two different cytological sites: in the rough endoplasmic reticulum, the stepwise synthesis of a large lipid-linked oligosaccharide takes place, as well as its transfer to protein; then after trimming the immature glycoprotein is further elongated in the Golgi apparatus. In this paper, a brief review will be given of the present knowledge on the sugar donor transport across the membrane barrier to the glycosylation site. Based upon the transmembrane orientation of oligosaccharide lipid intermediates and on the localization of the glycosyltransferase active sites, the different processes required to translocate the sugar moieties during the preassembly of the dolichyl-pyrophosphate-oligosaccharides will be examined. Combining the different results, obtained in several laboratories, it is suggested that the Man5-GlcNAc2-lipid is synthesized on the cytoplasmic side directly from the sugar-nucleotides and then translocated to the lumenal face where the Glc3-Man9-GlcNAc2-lipid is completed using Man-P-Dol and Glc-P-Dol as transmembrane carriers of these sugars. Concerning the elongation process leading to assembly of the antennae of N-acetyllactosamine type oligosaccharides, specific carriers for sugar nucleotides have been described as Golgi markers. Several authors have characterized such carriers for UDP-Gal, GDP-Fuc, CMP-NeuAc, UDP-GlcNAc and UDP-Glc using microsomal vesicles and similar results have been obtained in our laboratory using plasma membrane permeabilized cells. This carrier-mediated process leads to the formation of an intralumenal pool whose biological significance will be discussed. The translocation process of sugar donors occurring in the rough endoplasmic reticulum via lipid intermediates as well as in the Golgi apparatus via specific carriers would represent a regulation step based on the availability of the substrates for the glycosylation.