Involvement of Lipid-linked Oligosaccharides in Synthesis of Storage Glycoproteins in Soybean Seeds

Spatial activity mapping of ß-mannanase on soybean seeds

For farm animals the supplementation of exogenous enzymes, like ß-mannanase, to soybean-based diets is beneficial to improve feed digestibility. In order to unravel the effect of ß-mannanase on soybean meal's cell structure, a novel imaging concept was developed which allows visualizing the spatial activity pattern of ß-mannanase with high sensitivity by fluorescence microscopy before any visible degradation of the cellular structure occurs. It is based on fluorescence labeling of newly formed reducing ends of ß-mannanase-hydrolyzed polysaccharides after the native reducing ends of all polysaccharides present were chemically reduced. It was revealed that ß-mannanase is not only active at the cell wall but also at previously unknown sites, like the middle lamella and, most prominently, at an intracellular matrix enclosing the protein storage vacuoles. Based on these findings it can be hypothesized that the evaluated ß-mannanase can degrade the enclosing matrix of encapsulated proteins and the cell wall structure and thereby improves efficiency of feed utilization.

Properties of Solubilized UDP-GlcNAc: Dolichyl Phosphate-GlcNAc-1-P-Transferase from Soybean Cultured Cells

The GlcNAc-1-P-transferase was solubilized from microsomal preparations of soybean cultured cells by treatment with 1% Triton X-100. The solubilized enzyme catalyzed the formation of dolichyl pyrophosphoryl-GlcNAc when incubated with UDP-GlcNAc and dolichyl phosphate. The GlcNAc-1-P-transferase activity was stimulated by the addition of phosphatidylglycerol and phosphatidylinositol, but was inhibited by phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine. The K(m) value for dolichyl-phosphate was 6.2 micromolar and that determined for UDP-GlcNAc was 0.42 micromolar. The pH optimum for the GlcNAc-1-P reaction was between 7.2 and 7.6; maximum activity occurred at about 10 millimolar Mg(2+). The addition of unlabeled GDP-mannose or UDP-glucose considerably inhibited enzyme activity which could be restored to nearly the original value by addition of more dolichyl phosphate to the incubation mixture. On the other hand, the addition of unlabeled ADP-glucose and GDP-glucose enhanced the enzyme activity. This stimulation by these sugar nucleotides was found to be due to the protection of the substrate UDP-[(3)H]-GlcNAc from pyrophosphatase degradation. The GlcNAc-1-P-transferase reaction was very sensitive to tunicamycin and 50% inhibition required less than 1 microgram of antibiotic per milliliter. Amphomycin, showdomycin, and diumycin also inhibited this reaction but at higher concentrations.

Studies on N-glycosylation by elongating tissues and membranes from pea stems

Glucosamine and mannose were incorporated into oligosaccharides linked to either polar membrane-lipids or to asparagine residues of endogenous proteins in apical growing tissues of the etiolated pea stem. The glycolipids were subject to turnover in pulse-chase tests and protein-linked oligosaccharides accumulated with time, as expected for a precursor-product relationship. The newly formed glycoproteins were hydrolyzed by endo-beta-N-acetylglucosaminidase H to oligosaccharides in the same size range as those released by dilute acid from the lipid-linked oligosaccharides formed during the pulse. The glycoproteins were also partly degraded to free N-acetylglucosamine by beta-N-acetylhexosaminidase. Affinity of the carbohydrate moiety of the protein for concanavalin A increased between the beginning and the end of the chase, indicating processing following core glycosylation.The addition of UDP-N-acetyl-[(14)C]glucosamine plus external peptide acceptors (derived from carboxymethylated alpha-lactalbumin) to membrane preparations from the pea stem resulted in peptide glycosylation at the expense of lipid-linked oligosaccharide. Glycosylation of endogenous protein acceptors did not take place via lipid intermediates but directly from the sugar nucleotide substrate. Tunicamycin inhibited glycosyltransfer to both glycolipids and added peptides, but not to endogenous protein. It is concluded that limiting factors for N-glycosylation by pea membranes in vitro could include the unavailability of endogenous acceptors or the inability to fully elongate and internalize lipid precursors, but is not due to any limitation in capacity for N-glycosylation.

Polypeptide ligation occurs during post-translational modification of concanavalin A

Lectins are proteins with multivalent carbohydrate-binding sites, which confer the ability to agglutinate. The seeds of legumes are particularly rich in lectins, for example, concanavalin A (Con A) comprises up to 15% of the protein in the cotyledons of jack bean (Canavalia ensiformis) seeds. The amino acid sequences of Con A and several other legume lectins have been partially or fully determined, and comparison of these sequences from different species reveals a circular homology (Fig. 1A); rearrangements within the genome have been suggested to explain this. We report here that the circular homology displayed by Con A is due to a post-translational transposition and ligation within the initial polypeptide. This type of modification has not been reported previously for eukaryotes, although it has been suggested to occur in bacteriophage lambda.

Biosynthesis of pentosyl lipids by pea membranes

Pea membranes were incubated with UDP-[14C]xylose or UDP-[14C]arabinose and sequentially extracted with chloroform/methanol/water (10:10:3, by vol.) and sodium dodecyl sulphate (2%, w/v). An active epimerase in the membranes rapidly interconverted the two pentosyl nucleotides. Chromatographic analysis of the lipid extract revealed that both substrates gave rise to xylose- and arabinose-containing neutral lipids, xylolipid with properties similar to a polyisoprenol monophosphoryl derivative, and highly charged lipid-linked arabinosyl oligosaccharide. When UDP-[14C]pentose or the extracted lipid-linked [14C]arabinosyl oligosaccharide were used as substrates, their 14C was also incorporating into sodium dodecyl sulphate-soluble and -insoluble fractions as major end products. Polyacrylamide-gel electrophoresis of sodium dodecyl sulphate-soluble products indicated the formation of mobile components with Mr values between 40 000 and 200 000 (Sepharose CL-6B). The lipid-linked [14C]arabinosyl oligosaccharide possessed properties comparable with those of unsaturated polyisoprenyl pyrophosphoryl derivatives. It was hydrolysed by dilute acid to a charged product (apparent Mr 2300) that could be fractionated in alkali. It was degraded to shorter labelled oligosaccharides by slightly more concentrated acid and eventually to [14C]arabinose as the only labelled component. Susceptibility to acid hydrolysis, and methylation analysis, indicated that the oligosaccharide contained approximately seven sequential alpha-1,5-linked arabinofuranosyl units at the non-reducing end. Several acidic residues appear to be interposed between the terminal arabinosyl units and the charged lipid.

Incorporation of fucose into the carbohydrate moiety of phytohemagglutinin in developing Phaseolus vulgaris cotyledons

Incubation of developing cotyledons of P. vulgaris with [(3)H]fucose resulted in the incorporation of radioactivity into the cell wall, membranous organelles and soluble macromolecules. Fractionation of the proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, followed by fluorography, showed that phytohemagglutinin (PHA) was the major fucosylated protein synthesized in the cotyledons. Incorporation of fucose into PHA occurred in the membranous organelle fraction, and the radioactive fucose remained associated with the PHA during a 20-h chase of the radioactivity. Tunicamycin inhibited the incorporation of glucosamine and fucose into PHA to the same extent (65%), indicating the involvement of a lipid intermediate in the incorporation of fucose, or the attachment of fucose to the high-mannose oligosaccharide moiety of newly synthesized PHA. Digestion with proteinase K of [(3)H]fucose- or [(3)H]glucosamine-labeled PHA resulted in the formation of glycopeptides of similar size. These glycopeptides were partially resistant to digestion with endo-β-N-acetylglucosaminidase H, even after the removal of fucose by mild acid hydrolysis. We postulate, on the basis of these experiments, that the transport of PHA from the endoplasmic reticulum to the protein bodies is accompanied by the modification of its oligosaccharide side-chain. This modification involves inter alia the attachment of fucose, and renders the oligosaccharide side-chain resistant to digestion with endo-β-N-acetylglucosaminidase H. Analogy with animal glycoproteins indicates that this modification probably occurs in the Golgi apparatus.

Arabinan synthase and xylan synthase activities of Phaseolus vulgaris. Subcellular localization and possible mechanism of action

Membrane fractions from bean hypocotyl or suspension cultures incorporated arabinose from UDP-beta-L-arabinose into arabinan and xylose from UDP-alpha-D-xylose in vitro; the level of each activity was dependent on the state of differentiation of the cells. These activities may be due to single transglycosylases, since no lipid or proteinaceous intermediate acceptors were found in either case. Subcellular fractionation studies showed that enzyme activity in vitro was localized in both Golgi-derived membranes and endoplasmic reticulum in similar amounts. However, incorporation into the polymers in vivo in suspension culture cells incubated with [1-3H]arabinose was considerably greater in the Golgi-derived membranes. Thus, although these enzymes may be translated and inserted at the level of the endoplasmic reticulum, their activities are under other levels of control, so that most of the activity in vivo is confined to the Golgi apparatus. Initiation of glycosylation in the endoplasmic activity may, however, occur.

Processing of N-linked oligosaccharides in soybean cultured cells

Evidence, based on both in vivo and in vitro studies with suspension-cultured soybean cells, is presented to demonstrate the processing of the oligosaccharide chain of plant N-linked glycoproteins. Following a 1-h incubation of soybean cells with [2-3H]mannose, the predominant glycopeptide obtained by pronase digestion of the membrane fraction was a Man7- or Man8GlcNAc2-Asn (GlcNAc, N-acetylglucosamine). However, the major oligosaccharide isolated from the lipid-linked oligosaccharides of these cells was a Glc2- or Glc3Man9GlcNAc2. Soybean cells were incubated with [2-3H]mannose and the incorporation of mannose into Pronase-released glycopeptides was followed during a 2-h chase. During the first 10 min of labeling, the radioactivity was mostly in a large-sized glycopeptide that appeared to be a Glc1Man9GlcNAc2-peptide. During the next 60 to 90 min of chase, this radioactivity was shifted to smaller and smaller-sized glycopeptides indicating that removal of sugars (i.e., processing) had occurred. Both glucosidase and mannosidase activity was detected in membrane preparations of soybean cells. Nine different glycopeptides were isolated from Pronase digests of soybean cell membrane fractions. These glycopeptides were purified by repeated gel filtration on columns of Bio-Gel P-4. Partial characterization of these glycopeptides by endoglucosaminidase H and alpha-mannosidase digestion, and by analysis of the products, suggested the following glycopeptides: Glc1Man9GlcNAc2-Asn, Man8GlcNAc2-Asn, Man7GlcNAc2-Asn, Man6GlcNAc2-Asn, and Man5GlcNAc2-Asn.

Characterization of the oligosaccharides from lipid-linked oligosaccharides of mung bean seedlings

Lipid-linked oligosaccharides were synthesized with the particulate enzyme preparation from mung bean (Phaseolus aureus) seedlings in the presence of GDP-[(14)C] mannose. The oligosaccharides were released from the lipids by mild acid hydrolysis and purified by several passages on Biogel P-4 columns. Five different oligosaccharides were purified in this way. Based on their relative elution constants (K(d)) compared to a variety of standard oligosaccharides, they were sized as (mannose-acetylglucosamine) Man(7)GlcNAc(2), Man(5)GlcNAc(2), Man(3)GlcNAc(2), Man(2)GlcNAc(2), and ManGlcNAc(2). These oligosaccharides were treated with endoglucosaminidase H and alpha- and beta-mannosidase, and the products were examined on Biogel P-4 columns. They also were subjected to a number of chemical treatments including analysis of the reducing sugar by NaB(3)H(4) reduction, methylation analysis, and in some cases acetolysis. From these data, the likely structures of these oligosaccharides are as follows: E, Manbeta-GlcNAc-GlcNAc; D, Manalpha1-->3Manbeta-GlcNAc-GlcNAc; C, Manalpha1-->2Manalpha1-->3Manbeta-GlcNAc-GlcNAc; B, Manalpha1-->2Manalpha1-->2Manalpha1--> 3(Manalpha1-->6)Manbeta-GlcNAc-GlcNAc; and A, Manalpha1-->2Manalpha1--> 2Manalpha1-->3(Manalpha1--> [Manalpha1-->6]Manalpha1-->6) Manbeta-GlcNAc-GlcNAc. The synthesis of the Man(7)GlcNAc(2) was greatly diminished when tunicamycin (10 mug/ml) was added to the incubation mixtures.

The role of glycosylation in storage-protein synthesis in developing pea seeds

Intact pea (Pisum sativum L.) cotyledons were incubated with [(14)C]glucosamine at several stages of seed development and the resultant radioactive proteins were analysed by gel electrophoresis combined with immunoaffinity chromatography and sucrose gradient fractionation. Glucosamine was incorporated into at least five vicilin polypeptides (approx. molecular weight 70,000; 50,000, two components; 14,000, two components). No incorporation was detected into the subunits of legumin. Tunicamycin at 50 μg/ml largely inhibited glucosamine incorporation but had little effect on the incorporation of (14)C-labelled amino acids into cotyledon proteins, including vicilin. The assembly of vicilin polypeptides into full-sized protein oligomers (7-9 S) was also unaffected by tunicamycin. Chromatography on concanavalin A confirmed that glycosylation of cotyledon proteins was inhibited by tunicamycin. It is concluded that glycosylation of most cotyledonary proteins involves lipid-linked sugar intermediates, but that glycosylation itself is not an essential step in the synthesis of vicilin polypeptides nor in their assembly into oligomers.

Post-translational processing of 7S and 11S components of soybean storage proteins

The synthesis and processing of the major storage proteins in soybean cotyledons was studied both in vivo and in vitro. The α and α' subunits of 7S as well as the 11S proteins are synthesized as higher molecular weight-precursors on membrane-bound polysomes. The initial translation products of the 7S are proteolytically cleaved during translation suggesting the removal of a 'signal peptide' as evidenced by the presence of 2α and 2α' peptides immunoreactive with 7S antibody in the in vitro chain completion products of the membrane-bound polysomes. This is followed or accompanied by cotranslational glycosylation, which increases their size equivalent to that of initially-synthesized precursors. In vivo pulse-labelled 7S α and α' products are of slightly higher molecular weights than the immunoprecipitable chain-completion products, indicating further post-translational modifications. A slow post-translational processing during a period of 1.5 to 16 h yields the final 7S α and α' glycoproteins.Acidic and basic subunits of the 11S protein appear to be synthesized from common large molecular weight (60K-59K) precursors. Antibodies to the 11S acidic component recognize both acidic and basic domains in the precursor while those raised against basic subunits appear to be specific for that region only. The processing of the 11S precursor is also very slow and occurs post-translationally. This slow rate of processing, coupled with a temporal difference in the synthesis of 7S and 11S components, suggests a highly coordinated mechanism for synthesis and packaging of these proteins into protein bodies during seed development.