The formation of glycoproteins in tissues of higher plants. Specific labelling with D-(1- 14 C)glucosamine

Plant glycoprotein biosynthesis. Uridine diphosphate N-acetyl-glucosaminyltransferase from horseradish root

A particulate enzyme preparation from horseradish root tissue was shown to catalyze the transfer of 2-acetamido-2-deoxy-D-[14C1]glucose from uridine diphosphate 2-acetamido-2-deoxy-D-[14C1]glucose to an exogenous acceptor molecule derived from horseradish peroxidase. The acceptor was produced from purified peroxidase by the action of a mixture of glycoside hydrolases covalently bound to Sepharose. The membrane preparation containing the transferase was purified approximately 12-fold by aqueous two phase distribution and by discontinuous sucrose density gradient centrifugation. Hydrolysis of the reaction product yielded glucosamine as the only radio-labeled substance. Precipitation of the reaction product by antiserum against peroxidase showed that the label was incorporated into peroxidase. The transferase utilized the acceptor most efficiently when only 12% of the 2-acetamido-2-deoxy-D-glucose was removed from the acceptor. The acceptor lost no accepting capabilities when heated to 100 degrees C for 3 min prior to assay. Trypsin treatment caused a 14% decrease in label incorporated while pronase treatment caused a 93% decrease,

Biosynthesis of mannan and mannolipids from GDP-Man by membrane fractions of sycamore cell cultures

Membrane fractions have been prepared from sycamore (Acer pseudoplatanus) cell suspensions grown in liquid medium. These fractions catalyzed the transfer of mannoxyl-units from GDP-(14C) Man into a polymannoside, a mannolipid and oligosaccharide-lipids. The polymannoside was partially solubilized by proteolytic digestion or maceration in sodium dodecyl sulfate-urea mixtures. However no evidence is available of a covalent linkage between the biosynthesized glycan and peptides. The structural analysis of the (14C)mannan showed that the polysaccharide was a homopolymer of beta-(1 leads to 4) linked mannose with a few branches. During the incubation of the membranes with the substrate, the polymer chains elongated with a large number of sugar units and contained 25 to 40 hexose residues per non-reducing end monomer. GDP-Glc was a competitive inhibitor of the GDP-Man: beta-mannan mannosyl-transferase, whereas GDP-Man activated a GDP-Gle: beta-glucan glucosyl-transferase present in the same membrane preparation. Two kinds of glycolipids were synthesized in the presence of GDP-(14C)Man. The first (I) contained a polar moiety characterized as a mannose-phosphate and was very similar to polyprenyl-phosphate-mannose identified in plants by Alam and Hemming. The other mannolipid (II) was hydrolyzed by mild acid into labeled oligosaccharides of high molecular weight. This material was separated into two oligosaccharide fractions, the first (II A) of MW over 5000, the second (II B) of MW around 1700. II B contained at most two labeled mannose residues per chain, linked to the non-reducing end of unlabeled units which probably contained neutral sugars and N-acetyl-osamine(s) near the reducing end. Oligosaccharide II A seemed to contain one or several II B chains.

Effect of X-irradiation on 3-H-glucosamine uptake by human embryo fibroblasts

The incorporation of 3-H-glucosamine into unirradiated and X-irradiated (905 rad) cultures of human embryo fibroblasts was studied by light microscopic, autoradiographic and biochemical methods in pulse-chase experiments. The precursor was 3-H-glucosamine. In unirradiated cells the grains appeared in various subcellular regions in a particular order: at first over the Golgi complex, then over the whole cytoplasm and finally at least some of the labelled macromolecules emerged from the cell. The radioactivity emerging from the cell appeared to be protein bound. Due to the effect of irradiation, the uptake of 3-H-glucosamine by the interphase cells was considerably decreased. The amount of radioactivity per cell indicated both the inhibition of cytoplasmic segregation and migration of labelled macromolecules from the cell. In the mitotic figures appearing after the release from radiation-induced mitotic block (approximately 30 hours after irradiation), the grain counts were still considerably lower than the controls. The functional and structural radiation injury of the Golgi-complex was suggested by the less well-defined sequence of localization of radioactive macromolecules than in control cells.

The Incorporation of d-Glucosamine into Glycolipids and Glycoproteins of Membrane Preparations from Phaseolus aureus Hypocotyls

Radioactivity from d-glucosamine-(14)C is incorporated into particulate fractions of hypocotyls of Phaseolus aureus (mung bean) seedlings. Polyacrylamide gel electrophoresis of the sodium dodecyl sulfate-solubilized materials revealed that several polypeptide components varying considerably in molecular weight had become radioactive during the incubation. A considerable amount of (14)C was also recovered in lipid. Equilibrium centrifugation of the particulate material, isolated by initial centrifugation at 100,000 times gravity on sucrose density gradients revealed that radioactivity was recoverable in all of the membrane fractions along the gradient. It is suggested that glycoproteins and glycolipids containing amino sugar are normal constituents of such membranes. The ability of the particulate preparations to catalyze the transfer of N-acetyl-d-glucosamine from uridine diphospho-N-acetyl-d-glucosamine to endogenous acceptor material was also tested. Transfer was optimal at around pH 9 and in the presence of 10 mm Mg(2+), and it occurred largely into an unidentified lipid fraction. After equilibrium centrifugation of crude membrane material on sucrose gradients, a number of distinct fractions could be detected which would catalyze the transfer reaction. Uridine diphospho-d-glucose transferase activity showed a similar but not identical distribution along the gradient.

Incorporation of carbohydrate residues into peroxidase isoenzymes in horseradish roots

Sliced root tissue of the horseradish plant (Armoracia rusticana), when incubated with mannose-U-(14)C, incorporated radioactivity into peroxidase isoenzymes. Over 90% of the radioactivity in the highly purified peroxidase isoenzymes was present in the neutral sugar residues of the molecule, i.e. fucose, arabinose, xylose, mannose. When the root slices were incubated simultaneously with leucine-4,5-(3)H and mannose-U-(14)C, cycloheximide strongly inhibited leucine incorporation into the peptide portion of peroxidase isoenzymes but had little effect on the incorporation of (14)C into the neutral sugars. These results indicated that synthesis of the peptide portion of peroxidase was completed before the monosaccharide residues were attached to the molecule. This temporal relationship between the synthesis of protein and the attachment of carbohydrate residues in the plant glycoprotein, horseradish peroxidase, appears to be similar to that reported for glycoprotein biosynthesis in many mammalian systems.

Location of glycoproteins that contain glucosamine in plant tissues

When radioactive d-glucosamine is provided to Acer pseudoplatanus cells in liquid culture in order to label those glycoproteins that contain amino sugars, it is incorporated predominantly into a crude cell wall fraction. This observation was confirmed histologically by preparing autoradiographs of thin tissue sections from plasmolyzed cells. Highly purified cell wall material from unlabeled cells has also been shown to contain small amounts of glucosamine. Similarly, about one-half of the amino sugar recovered from cultured cells of Nicotiana tabacum is present in their cell walls. In corn roots, however, the labeled glycoproteins that are formed after glucosamine incorporation are predominantly cytoplasmic and not deposited outside the protoplast.