Glycosyltransferases involved in elongation of N-glycosidically linked oligosaccharides of the complex or N-acetyllactosamine type

The production of biopharmaceuticals in plant systems

Biopharmaceuticals present the fastest growing segment in the pharmaceutical industry, with an ever widening scope of applications. Whole plants as well as contained plant cell culture systems are being explored for their potential as cheap, safe, and scalable production hosts. The first plant-derived biopharmaceuticals have now reached the clinic. Many biopharmaceuticals are glycoproteins; as the Golgi N-glycosylation machinery of plants differs from the mammalian machinery, the N-glycoforms introduced on plant-produced proteins need to be taken into consideration. Potent systems have been developed to change the plant N-glycoforms to a desired or even superior form compared to the native mammalian N-glycoforms. This review describes the current status of biopharmaceutical production in plants for industrial applications. The recent advances and tools which have been utilized to generate glycoengineered plants are also summarized and compared with the relevant mammalian systems whenever applicable.

Use of synthetic oligosaccharide substrate analogs to map the active sites of N-acetylglucosaminyltransferases I and II

Tables III and IV summarize substrate analog data presented in Tables I and II, respectively. Data for GlcNAc-T I shown in Tables I and III correlate very well with the crystal structure for GlcNAc-T I. This indicates that substitution of the various hydroxyl groups by hydrogen, [table: see text] O-methyl, and maybe even larger O-alkyl groups does not cause appreciable changes to either the overall conformation of the oligosaccharide or the binding mode, thus supporting this approach of chemical modification of oligosaccharide substrates for mapping of the binding site. There is as yet no crystal structure for GlcNAc-T II. These studies indicate both advantages and disadvantages of this approach for elucidating the catalytic and binding sites of an enzyme. Substrate analog data indicate which chemical groups in the substrate are essential for catalysis and binding and suggest the type of linkage involved (hydrogen bond donor or acceptor). However, no information has been obtained on the protein groups involved in these interactions. If a crystal structure is available, the substrate analog conclusions are primarily confirmatory. However, whether or not a crystal structure is available, this approach can be very helpful in the design of specific inhibitors.

Structural study on the carbohydrate moiety of calf intestinal alkaline phosphatase

Surprisingly alkaline phosphatase (AP) (EC 3.1.3.1) of calf intestine is found in large amounts, e.g. 80%, within chyme. Most of the enzyme is present as a mixture of four differently hydrophobic anchor-bearing forms and only the minor part is present as an anchorless enzyme. To investigate whether changes in the N-glycosylation pattern are signals responsible for large-scale liberation from mucosa into chyme, the glycans of the two potential glycosylation sites predicted from cDNA were investigated by matrix-assisted laser desorption/ionization and electrospray ionization mass spectrometry in combination with exoglycosidase treatment after tryptic digestion and reversed-phase chromatography. The glycans linked to Asn249 are at least eight different, mainly non-fucosylated, biantennary or triantennary structures with a bisecting N-acetylglucosamine. For the most abundant glycopeptide (40%) the following glycan structure is proposed: [carbostructure: see text]. The glycans linked to Asn410 are a mixture of at least nine, mainly tetraantennary, fucosylated structures with a bisecting N-acetylglucosamine. For the most abundant glycopeptide (35%) the following glycan structure is proposed: [carbostructure: see text]. For the structures the linkage data were deduced from the reported specificities of the exoglycosidases used and the specificities of the transglycosidases active in biosynthesis. The majority of glycans are capped by alpha-galactose residues at their non-reducing termini. In contrast to the glycans linked to other AP isoenzymes, no sialylation was observed. Glycopeptide 'mass fingerprints' of both glycosylation sites and glycan contents do not differ between AP from mucosa and chyme. These results suggest that the observed large-scale liberation of vesicle-bound glycosylphosphatidylinositol (GPI)-anchored AP from mucosa into chyme is unlikely to be mediated by alteration of glycan structures of the AP investigated. Rather, the exocytotic vesicle formation seems to be mediated by the controlled organization of the raft structures embedding GPI-AP. (c) 2001 John Wiley & Sons, Ltd.

Molecular cloning and expression analysis of a mouse UDP-GlcNAc:Gal(beta1-4)Glc(NAc)-R beta1,3-N-acetylglucosaminyltransferase homologous to Drosophila melanogaster Brainiac and the beta1,3-galactosyltransferase family

We have isolated a murine cDNA coding for a beta1,3-N-acetylglucosaminyltransferase enzyme ( beta3GnT). This enzyme is similar in sequence to Drosophila melanogaster Brainiac and to the murine and human beta1,3-galactosyltransferase family of proteins. The mouse beta 3GnT protein is 397 amino acids in length and contains 7 cysteine residues that are conserved in the human orthologue. beta 3GnT is a type II membrane protein localized to the Golgi apparatus. Enzyme assays with recombinant mouse beta 3GnT reveal that it has a preference for acceptors with Gal(beta1-4)Glc(NAc) at the non-reducing termini. Proton NMR analysis of product showed incorporation of GlcNAc in beta1,3 linkage to the terminal Gal of Gal(beta1-4)Glc(beta1-O-benzyl). Northern blot analysis revealed the presence of a single 3.0[emsp4 ]kb transcript in all adult mouse and human organs tested, with highest levels in the kidney, liver, heart and placenta. The beta 3GnT gene is also expressed in a number of tumor cell lines. The human orthologue of beta 3GnT is located on chromosome 2pl5.

Fucosebeta-1-P-Ser is a new type of glycosylation: using antibodies to identify a novel structure in Dictyostelium discoideum and study multiple types of fucosylation during growth and development

Three antibodies that recognize distinct fucose epitopes were used to study fucosylation during growth and development of Dictyostelium discoideum. mAb83.5 is known to recognize an undefined "fucose epitope" on several proteins with serine-rich domains, while mAb CAB4, and a component of anti-horse-radish peroxidase, specifically recognize Fucalpha1,6GlcNAc and Fucalpha1,3GlcNAc residues respectively in the core of N-linked oligosaccharides. We show that mAb 83.5 defines a new type of O-glycosylation. Serine-containing peptides incubated with GDPbeta[3H]Fuc and microsomes formed two fucosylated products. A neutral product accounting for 30% of the label did not react with the antibody, while the rest of the label was incorporated into a charged product which contained all the mAb83.5 reactive material. beta-Elimination of the labeled peptide or endogenous products produced [3H]Fuc-1-P, indicating phosphodiester linkage to serine. Fucbeta-1-P and GDP-betaFuc at 100 microM blocked mAb83.5 binding to endogenous and peptide products, but their alpha-linked anomers did not. Electrospray ionization mass spectra of the neutral and anionic labeled products showed major peaks of mass units corresponding to O-Fuc-Ser peptide and O-Fuc-phospho-Ser peptide, respectively. The activity of Fuc-phosphotransferase exactly paralleled the accumulation of reactive glycans during growth and development. The expressions of N-glycan core Fucalpha1,6GlcNAc and Fucalpha1,3GlcNAc and their respective fucosyl transferase activities were also synchronous, but their developmental regulation differed from one another. Fucalpha1, 6GlcNAc was expressed maximally during growth but declined during development. In contrast core Fucalpha1,3GlcNAc epitopes were expressed almost exclusively during development. These findings provide direct evidence for a novel type of O-phosphofucosylation, demonstrate the existence of an O-fucosyl transferase, and identify two different types of core fucosylation in the N-glycans of Dictyostelium.

Sorting of lysosomal membrane glycoproteins lamp-1 and lamp-2 into vesicles distinct from mannose 6-phosphate receptor/gamma-adaptin vesicles at the trans-Golgi network

Newly synthesized lysosomal membrane glycoproteins lamp-1 and lamp-2 are primarily sorted at the trans-Golgi network (TGN) by recognition of a tyrosine-based signal sequence in their cytoplasmic tails. It is presently unclear how this signal is recognized and what type of vesicle transports lamp-1 and lamp-2. Here, we describe a method to generate transport vesicles containing lamp proteins from the TGN in vitro. The method is based on incorporation of radioactive sialic acid in glycoproteins at the TGN by incubation of membranes with tritiated CMP-sialic acid. The generation of vesicles from labeled membranes required ATP and cytosol, and was temperature-dependent and brefeldin A-sensitive. Analysis on Nycodenz gradients revealed that lamp-vesicles were distinct from vesicles containing gamma-adaptin and mannose 6-phosphate receptor (MPR). Moreover, both these types of vesicles migrated differently than vesicles containing proteins destined for the plasma membrane. The conclusion that lamps and MPRs are sorted into different vesicles was further strengthened by the finding that whereas wortmannin both in vitro and in vivo inhibited the production of gamma-adaptin/MPR-containing vesicles, this drug had no effect on the generation of lamp-vesicles and on the sorting of lamps. The results indicate that membrane proteins containing tyrosine-based motifs for sorting at the TGN are segregated from clathrin-coated vesicles containing MPRs.

Activity of UDP-GlcNAc:GlcNAc beta 1-->6(GlcNAc beta 1-->2) Man alpha 1-->R[GlcNAc to Man] beta 1-->4N-acetylglucosaminyltransferase VI (GnT VI) from the ovaries of Oryzias latipes (Medaka fish)

UDP-GlcNAc:GlcNAc beta 1-->(GlcNAc beta 1-->2)Man alpha 1-R[GlcNAc to Man] beta 1-->4N-acetylglucosaminyltransferase VI (GnT VI) activity was shown to be present in crude homogenates of Medaka fish (Oryzias latipes) ovaries using UDP-[14C]GlcNAc and synthetic GlcNAc beta 1-->6 (GlcNAc beta 1-->2)Man alpha 1-->6Glc beta 1-->octyl as substrates. Characterization of this activity showed a pH optimum at about pH 7.0 and an absolute requirement for divalent cations. The optimum concentration of Mn2+ was at about 25 mM. This finding is the first report on GnT VI activity in fish; the enzyme has previously been described only in avian tissues.

Glycosylation defect in Lec1 Chinese hamster ovary mutant is due to a point mutation in N-acetylglucosaminyltransferase I gene

The Lec1 Chinese hamster ovary (CHO) mutant is a leuco-phytohemagglutinin resistant cell line unable to synthesize complex and hybrid N-glycans due to the lack of N-acetylglucosaminyltransferase I (GnTI) activity. Here we have identified the lec1 mutation. Using specific antibodies to GnTI we demonstrate that Lec1 cells synthesize an inactive GnTI protein identical in size to the wild-type CHO enzyme. We have cloned and sequenced the gene coding GnTI from parental CHO and Lec1 mutant cells. Comparison of GnTI sequences detected three mutations within the luminal domain of Lec1 GnTI, each resulting in an amino acid substitution. The effect of each mutation on enzyme activity was analyzed by site-directed mutagenesis of wild-type rabbit GnTI and transient expression in COS cells. One of the three mutations (Cys123 --> Arg123) resulted in complete loss of activity, whereas the other two mutations had no apparent effect on enzyme activity. This conclusion was confirmed by expression of GnTI mutants in the GnTI null background of Saccharomyces cerevisiae. Both Lec1 GnTI and the GnTI mutant (Cys123 --> Arg123) are correctly localized to the Golgi apparatus, indicating that the inactive GnTI molecules are sufficiently well folded for efficient transport from the endoplasmic reticulum. These results demonstrate that the lec1 mutation is a point mutation and that Cys123 is a critical residue for GnTI activity.

An approach for fluorometric determination of glycosyltransferase activities

A new strategy for the fluorometric determination of glycosyltransferase activities is reported. The method involves dansyl chloride derivatization of the reduced form (pNH2phenyl) of a hydrophobic, aglycon moiety covalently linked to a number of acceptor substrates (pNO2phenyl). Focusing on the Golgi enzyme core 2 N-acetyl-glucosaminyltransferase, we found that synthesis and fractionation of the dansylated substrate derivative were rapid, easy and inexpensive. Additionally, the corresponding enzyme assay proved reproducible and very sensitive, as 0.4 pmol of reaction product were readily detected. This fluorometric approach appears therefore to be a valid tool for investigating the monitoring differential expression of glycosyltransferases exhibiting low levels of enzyme activity.

Differences between the sugar moieties of liver- and bone-type alkaline phosphatases: a re-evaluation

We re-evaluated the differences between the sugar moieties of liver and bone alkaline phosphatases (ALPs). Sialic acid was added to ALP sugar moieties by alpha 2,3- or 2,6-sialyltransferase treatment of the asialo-form ALP (neuraminidase-treated ALP). Asialo-bone ALP was converted to a liver-like ALP by the 2,6-sialyltransferase treatment. The resulting liver-like ALP was less susceptible to neuraminidase than non-treated bone ALP, but was still labile to heat exposure at 56 degrees C like non-treated bone ALP. However, after the O-linked sugar moiety had been released by additional treatment with O-glycanase the liver-like ALP became more heat stable at 56 degrees C, like non-treated liver ALP. Non-treated liver ALP reacted specifically with anti-liver ALP monoclonal antibody, and non-treated bone ALP reacted with both anti-liver and anti-bone ALP antibodies. The asialo-bone ALP still reacted with anti-bone ALP antibody, whereas the asialo-form liver ALP showed little, if any, reaction with anti-liver and anti-bone ALP antibodies. Neuraminidase and O-glycanase-treated bone ALP reacted less with anti-bone ALP antibody. After O-glycanase treatment, bone ALP molecules deprived of an O-linked sugar moiety had a molecular size and heat stability similar to liver ALP. The difference between liver and bone ALP molecules may be due not only to their manner of sialic acid linkage but also to the attachment of the O-linked sugar moiety.

Growth of a human yolk sac tumor cell line with yolk sac-derived functions in selenium-supplemented chemically defined synthetic medium

A human yolk sac tumor cell line, TG1, which was established from a testicular yolk sac tumor, was found to replicate continuously in a chemically defined medium supplemented with Na2SeO3 (ISRPMI). TG1 produced several plasma proteins and growth factors: albumin, alpha-fetoprotein (AFP), ferritin, carcinoembryonic antigen, beta-2-microglobulin, polyamine, neuron specific enolase, tissue polypeptide antigen, transferrin (Tf), epidermal growth factor, and platelet derived growth factor. By analysis of lentil lectin (LcHA)-affinity electrophoresis, to examine the microheterogeneity of carbohydrate chains of synthetic glycoproteins, TG1 cells cultured with ISRPMI produced only LcHA reactive Tf and AFP based on core fucose attached to asparagine-linked N-acetylglucosamine residues instead of LcHA-nonreactive Tf and AFP produced by TG1 cells cultured with fetal bovine serum (FBS)-containing medium. alpha 1-6 Fucosyltransferase activity was significantly greater in the TG1 cells cultured with ISRPMI (39.9 +/- 1.5 pmol.h-1.mg-1 protein) than cultured with FBS-containing media (18.2 +/- 1.2 pmol.h-1.mg-1 protein). These results have indicated that the selective increase of alpha 1-6 fucosyltransferase occurred when the cells were cultured with the FBS-free synthetic media.

Postnatal changes in sialylation of glycoproteins in rat liver

Glycoproteins containing N-linked oligosaccharides were prepared from plasma and liver microsomes of rats aged 0-5 weeks, and galactose and sialic acid content were determined. The sialic acid/galactose ratios in plasma membrane N-glycans remained at about 1 throughout the postnatal period, suggesting that most of the galactose residues are sialylated. In the same way, it was suggested that most of the galactose residues of microsomal N-glycans were sialylated at 0, 4 and 5 weeks of age, but that the degree of sialylation was lower at the other ages, with a minimum at 2 weeks. When the activities of sialyltransferase and galactosyltransferase in liver Golgi membranes were determined, age-dependent changes were found, not only in the specific activities of the enzymes, but also in the Golgi membrane content per g of liver. The activity of galactosyltransferase per g of liver increased immediately after birth, whereas that of sialyltransferase remained at a low level for 2 weeks and then increased to a constant level at 4 weeks. It is probable that this delayed increase in the activity of sialyltransferase results in the decreased sialylation of microsomal N-glycans at 1, 2 and 3 weeks. Sialyltransferase was solubilized from the liver microsomes of rats aged 2, 3 and 4 weeks and characterized. Phosphocellulose column chromatography separated the activity into two subfractions, designated transferase I and transferase II in the order of elution. The increase in total sialyltransferase activity during this period was caused mainly by an increase in transferase I. Rechromatography of each transferase from 3-week-old rats after neuraminidase treatment showed that transferase I but not transferase II contained sialic acid residue(s) and that desialylated transferase I was eluted in a similar way as transferase II. Although the apparent Km value for CMP-N-acetylneuraminic acid and the heat stability of transferase I were different from those of transferase II, the difference was abolished by treating transferase I with neuraminidase, suggesting that transferase II may be a desialylated form of transferase I. These changes in the sialylation of membrane glycoproteins, including sialyltransferase, may be related to the control of liver growth during postnatal development.

Heterogeneity of the complex N-linked oligosaccharides at specific glycosylation sites of two secreted carrot glycoproteins

The N-linked glycans from the 52/54-kDa medium protein and cell wall beta-fructosidase, two glycoproteins secreted by carrot suspension culture cells, were characterized. Carrot cells were labelled with [3H]glucosamine or [3H]fucose. The 52/54-kDa medium protein was isolated from the culture medium and beta-fructosidase from cell walls. The purified proteins were digested with trypsin and glycopeptides were isolated and sequenced. Glycans obtained from individual glycopeptides were separated by gel filtration chromatography and characterized by concanavalin A chromatography, by treatments with exoglycosidases and by sugar composition analysis. The 52/54-kDa medium protein and cell wall beta-fructosidase have one high-mannose-type glycan similar to those from yeast and animal glycoproteins. In addition, the 52/54-kDa medium protein has three complex-type and cell wall beta-fructosidase two complex-type glycans per polypeptide. The complex-type glycans isolated from individual glycosylation sites are fairly large and very heterogeneous. The smallest of these glycans has the structure [Xyl](Man)3[Fuc](GlcNAc]2Asn (square brackets indicating branching) whereas the larger ones carry additional sugars like terminal N-acetylglucosamine and possibly rhamnose and arabinose in the case of the 52/54-kDa medium protein and only arabinose in the case of cell wall beta-fructosidase. These terminal sugars are linked to the alpha-mannose residues of the glycan cores. The 52/54-kDa medium protein is secreted with large and homogeneous complex glycans, their heterogeneity originates from slow processing after secretion. The complex glycans from cell wall beta-fructosidase are processed before the enzyme is integrated into the cell wall.

Purification and characterization of alpha (2-6)-sialyltransferase from human liver

A Gal beta 1-4GlcNAc alpha (2-6)-sialyltransferase from human liver was purified 34,340-fold with 18% yield by dye chromatography on Cibacron Blue F3GA and cation exchange FPLC. The enzyme preparation was free of other sialyltransferases. It did not contain CMP-NeuAc hydrolase, protease, or sialidase activity, and was stable at -20 degrees C for at least eight months. The donor substrate specificity was examined with CMP-NeuAc analogues modified at C-5 or C-9 of the N-acetylneuraminic acid moiety. Affinity of the human enzyme for parent CMP-NeuAc and each CMP-NeuAc analogue was substantially higher than the corresponding Gal beta 1-4GlcNAc alpha (2-6)-sialyltransferase from rat liver.

Characterization of a galactosyltransferase in purified bovine rod outer segments

Purified bovine rod outer segments (ROS) were used to study the transfer of labeled galactose from UDP-[3H]galactose to endogenous ROS glycoproteins, exogenous glycoproteins and N-acetylglucosamine (GlcNAc). The ROS reaction was also compared with that of the retinal microsomal fraction and milk galactosyltransferase. The results indicate that the ROS reaction was enhanced by exposure to light. Illumination, however, had no effect on the transfer of labeled galactose to either endogenous microsomal glycoproteins by retinal microsomal galactosyltransferase or the transfer of the sugar to ROS glycoproteins by milk galactosyltransferase. Manganese was most effective, followed by cobalt, as cofactor for the ROS enzyme. Calcium and magnesium produced about 60% of the activity observed with manganese. The ROS enzyme transferred minimal amounts of labeled galactose to asialo-agalactotransferrin or ovalbumin but readily transferred the sugar to GlcNAc. The latter reaction had an optimum pH of 6.3 and was linear for at least 90 min. It reached a maximum at about 30 mM GlcNAc and was inhibited by higher concentrations of the aminosugar and by low concentrations of alpha-lactalbumin. On the other hand, the transfer of galactose to ROS glycoproteins was not affected by low concentrations of alpha-lactalbumin. Our data suggest that the ROS galactosyltransferase may have a certain specificity towards its acceptor in the ROS. Its activation by light may indicate a role in the light-activated processes of the photoreceptor cell.

Determination of N-acetylglucosaminyltransferases III, IV and V in normal and hepatoma tissues of rats

N-Acetylglucosaminyltransferase III, IV and V activities were assayed in various rat tissues and hepatomas using the same fluorescence-labeled sugar chain, GlcNAc beta 1-2Man alpha 1-3-(GlcNAc beta 1-2Man alpha 1-6)Man beta 1-4GlcNAc beta 1-4GlcNAc-2-aminopyridine as a substrate. The N-acetylglucosaminyltransferase III activity toward the substrate is the highest in most rat tissues including primary rat hepatoma. A relatively higher activity for GnT-V is found in small intestine, serum and hepatoma as compared to that of GnT-IV. Some kinetic properties of these enzymes in crude extracts were also determined.

Primary sequence dependence of conformation in oligomannose oligosaccharides

The oligomannose series of oligosaccharides from bovine thyroglobulin (BTG) and the variant surface glycoprotein (VSG) of Trypanosoma brucei have been isolated and sequenced by 1H NMR. The structure of Man9GlcNAc2, the parent molecule of the series, is shown below. Structural isomerism occurs within this series through the removal of residues D1, D2, D3, and C. Using spin-spin coupling and chemical shift data the rotamer distributions about the dihedral angle omega for the Man alpha 1-6Man beta and Man alpha 1-6Man alpha linkages were determined for each member of the series. It is shown that the dihedral angle omega of the Man alpha 1-6Man beta linkage exhibits low flexibility with a preference for the omega = 180 degrees conformation when residue D2 is present and high flexibility when this residue is absent. Flexibility of omega for the Man alpha 1-6Man alpha is largely independent of primary sequence and is intermediate between the two Man alpha 1-6Man beta extremes, again with a preference for the omega = 180 degrees conformation. [see text] There are, however, data which indicate that removal of residue D3 may confer additional flexibility upon the dihedral angle omega of the Man alpha 1-6Man alpha linkage. Molecular graphics modelling, together with chemical and enzymatic modification studies, suggest that the origin of the observed primary sequence dependence of the Man alpha 1-6Man beta linkage arises from steric factors. On the basis of these observations taken together with previous work, it is postulated that recognition of individual oligomannose conformations may play a role in the control of N-linked oligosaccharide biosynthesis.

UDP-GlcNAc transport across the Golgi membrane: electroneutral exchange for dianionic UMP

We have examined the coupling and charge stoichiometry for UDP-GlcNAc transport into Golgi-enriched vesicles from rat liver. In the absence of added energy sources, these Golgi vesicles concentrate UDP-GlcNAc at least 20-fold, presumably by exchange with endogenous nucleotides. Under the conditions used, extravesicular degradation of UDP-GlcNAc has been eliminated, and less than 15% of the internalized radioactivity becomes associated with endogenous macromolecules. Of the remaining intravesicular label, 85% remains unmetabolized UDP-[3H]GlcNAc, and approximately 15% is hydrolyzed to [3H]GlcNAc-1-phosphate. Efflux of accumulated UDP-[3H]GlcNAc is induced by addition of UMP, UDP, or UDP-galactose to the external medium. Permeabilization of Golgi vesicles causes a rapid and nearly complete loss of internal UDP-[3H]GlcNAc, indicating that the results reflect transport and not binding. Moreover, transport of UDP-[3H]GlcNAc into these Golgi vesicles was stimulated up to 5-fold by mechanically preloading vesicles with either UDP-GlcNAc or UMP. The response of UMP/UMP exchange and UMP/UDP-GlcNAc exchange to alterations in intravesicular and extravesicular pH suggests that UDP-GlcNAc enters the Golgi apparatus in electroneutral exchange with the dianionic form of UMP.

A galactosyltransferase from the fission yeast Schizosaccharomyces pombe

A membrane-associated galactosyltransferase has been purified to homogeneity from the fission yeast, Schizosaccharomyces pombe. The enzyme has a molecular weight of 61,000 and is capable of transfering galactose from UDP-galactose (UDP-Gal) to a variety of mannose-based acceptors to form an alpha-1,2 galactosyl mannoside linkage. Immunofluorescence localization of the protein is consistent with the presence of the enzyme in the Golgi apparatus of S. pombe. This, together with the presence of terminal, alpha-linked galactose on the N-linked oligosaccharides of S. pombe secretory proteins, suggests that the galactosyltransferase is an enzyme involved in the processing of glycoproteins transported through the Golgi apparatus in fission yeast.

Effect of light on the transfer of sugars from sugar nucleotides to rod outer segment membranes of control and dystrophic rats

The transfer of N-acetyl-D-glucosamine (GlcNAc), D-mannose (Man), D-galactose (Gal) and L-fucose (Fuc) from their nucleotide complexes to isolated rod outer segment (ROS) membranes obtained from dark-adapted 21 +/- 2 days old dystrophic (RCS) and control (RCS-rdy+) rat retinas, was studied under light or dark conditions of incubation. It was found that all of these sugars were transferred to ROS membranes in the dark. Under these conditions there was significantly less (p less than 0.001) Gal transferred to dystrophic than to control membranes. Exposure to light affected the transfer of Gal and Fuc only. Thus, the transfer of Gal and Fuc to control ROS membranes was increased by about 50% compared to the level observed under dark conditions of incubation. On the other hand, exposure to light had no effect on the transfer of Gal to dystrophic ROS membranes but it enhanced the transfer of Fuc to these membranes by about 250% above the level observed in the dark. Under light there were highly significant (p less than 0.001) differences between control and dystrophic membranes in the transfer of Gal and Fuc. The transfer of Fuc to dystrophic ROS membranes was proportional to the concentration of GDP-Fuc but the acceptors on control membranes were saturated at low concentrations of substrate. However, the transfer of Gal from UDP-Gal to both types of membranes was proportional to the concentrations of substrate and ROS membrane protein and to the period of incubation. The transfer of Gal and Fuc to both types of membranes was significantly reduced after denaturation of ROS membrane proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of protein transport between successive compartments of the Golgi apparatus: asymmetric properties of donor and acceptor activities in a cell-free system

Transport of the vesicular stomatitis virus (VSV)-encoded glycoprotein (G protein) between successive compartments of the Golgi in a cell-free system is measured by the coupled incorporation of N-[3H]acetylglucosamine (GlcNAc). This glycosylation occurs when G protein is transported from a "donor" compartment in Golgi membranes that lack GlcNAc transferase I (from VSV-infected CHO clone 15B cells) to the next "acceptor" compartment in a Golgi population from wild-type CHO cells (containing the GlcNAc transferase but not G protein). Here we present a detailed characterization of the conditions required to achieve transport in vitro. We find that donor and acceptor activities differ markedly in certain of their properties. The donor activity is inhibited by N-ethylmaleimide but the acceptor activity is resistant. Donor activity is unstable in the absence of ATP or the cytosol fraction; acceptor activity is much more stable. This asymmetry may reflect the vectorial nature of the underlying biochemistry of protein transport. Both donor and acceptor are trypsin-sensitive, implying a need for cytoplasmically oriented membrane proteins. Transport occurs only in a restricted range of close to physiological conditions. ATP is absolutely required, although as little as 1 microM is sufficient. Transport is inhibited by ATP-gamma-sulfate and vanadate, suggesting that ATP hydrolysis is needed. By contrast, ionophores that dissipate membrane potentials and proton gradients do not inhibit transport. Monensin was also without effect in the cell-free system.

Reconstitution of the transport of protein between successive compartments of the Golgi measured by the coupled incorporation of N-acetylglucosamine

Transport of the VSV-encoded glycoprotein (G protein) between successive compartments of the Golgi has been reconstituted in a cell-free system and is measured, in a rapid and sensitive new assay, by the coupled incorporation of 3H-N-acetylglucosamine (GlcNAc). This glycosylation occurs when G protein is transported during mixed incubations from the "donor" compartment in Golgi from VSV-infected CHO clone 15B cells (missing a key Golgi GlcNAc transferase) to the next, successive "acceptor" compartment (containing the GlcNAc transferase) in Golgi from wild-type CHO cells. Golgi fractions used in this assay have been extensively purified, and account for all of the donor and acceptor activity in the cells. Together with several other lines of evidence, this indicates that the cell-free system is highly specific, measuring only transport between sequential compartments in the Golgi stack. Transport in vitro is almost as efficient as in the cell, and requires ATP and the cytosol fraction in addition to protein components on the cytoplasmic surface of the Golgi membranes.