The Site of Cellulose Synthesis: Cell Surface and Intracellular beta-1, 4-Glucan (Cellulose) Synthetase Activities in Relation to the Stage and Direction of Cell Growth

Cell-fractionation analysis of glucan synthase I and II distribution and polysaccharide secretion in soybean protoplasts : Evidence for the involvement of coated vesicles in wall biogenesis

The organelles of soybean (Glycine max (L.) Merr.) protoplasts were separated using a recently developed procedure which allows rapid (3-h) recovery of a fraction enriched for coated vesicles (CVs). As determined by marker-enzyme enrichment and ultrastructural analysis of isolated membrane fractions, endoplasmic reticulum, Golgi membranes, glucan-synthase-II (EC 2.4.1.34)-containing membranes (putative plasma membrane), mitochondria, and CVs were enriched in separate fractions in a sucrose density gradient. Glucan synthase I (EC 2.4.1.12) had the highest specific activity in the Golgi-enriched and CV-enriched fractions and was found to comigrate with CVs upon rate-zonal centrifugation of a CV-enriched fraction. For further elucidation of the role of these latter organelles in cell-wall regeneration, freshly isolated protoplasts were pulsed with [(3)H]glucose for 20 min, and the disappearance of label from the organelles was followed for the ensuing 1 h. Although a CV-enriched fraction contained glucan synthase I, it contained very small amounts of labelled polysaccharide during the period of study. Pulse-chase experiments with [(3)H]glucose helped to confirm the role of the Golgi apparatus in secretion of matrix polysaccharides by protoplasts.

Ethylene-induced lateral expansion in etiolated pea stems : kinetics, cell wall synthesis, and osmotic potential

Treatment of etiolated pea (Pisum sativum L.) internode tissue with ethylene gas inhibits elongation and induces lateral expansion. Precise kinetics of the induction of this altered mode of growth of excised internode segments were recorded using a double laser optical monitoring device. Inhibition of elongation and promotion of lateral expansion began after about 1 hour of treatment and achieved a maximum by 3 hours. Similar induction kinetics were observed after treating internodes with colchicine and 2,6-dichlorobenzonitrile, an inhibitor of cellulose synthesis. In sealed flask experiments, ethylene had no detectable effect on incorporation of label from [(14)C]glucose into any of the classical pectin, hemicellulose, or cellulose wall fractions. Ethylene inhibited fresh weight increase (total cell expansion) of both excised internode segments (in sealed flasks) and intact seedlings. Ethylene treatment resulted in an increase in cell sap osmolality in those tissues (intact and excised) which are inhibited by the gas. A model for ethylene-induced inhibition of elongation and induction of lateral expansion is presented.

Changes in glucan synthetase activity and plasma membrane proteins during encystment of the cellular slime mold Polysphondylium pallidum

The activity of glucan synthetase increased dramatically during encystment of Polysphondylium pallidum cells. The majority of activity was present in purified plasma membranes. Activity, measured as glucose incorporation from UDPG into NaOH-insoluble glucan, increased 30-40 fold in the membranes. Increases in activity within the cells preceded plasma membrane increases and the enzyme appeared to be rapidly transported to the plasma membrane. Intracellular activity was relatively low. When cells were incubated with UDPG and when phloretin was included to inhibit glucose uptake, no NaOH-insoluble glucan was synthesized. Hence, the UDPG-binding site was not exposed at the cell-surface. When the NaOH-insoluble glucan was digested with endo-β-1,4-glucanase the products were cellobiose and glucose. The glucan could also be precipitated from Schweizer's reagent with acetic acid. These results suggest that the glucan contained predominantly β-1,4-linkages and may be cellulose. Experiments with cycloheximide confirmed that protein synthesis was required for encystment. Labeling of cells with [1-(14)C]-acetate showed that the synthesis of certain plasma membrane proteins was developmentally regulated. A number of proteins (e.g., myosin heavy chains and actin) were synthesized during the lag phase and their synthesis was subsequently reduced or ceased altogether. Immediately prior to the commencement of cyst wall formation seven new plasma membrane proteins were synthesized. These proteins were not detected intracellularly, indicating rapid transfer to the plasma membrane. The possible relationship between the seven developmentally regulated proteins and a postulated "multi-enzyme-complex" involved in cellulose synthesis is discussed. Their synthesis may be related to the increase in particles in the outer leaflet of the plasma membrane observed during encystment with freeze-etching (G.W. Erdos and H.R. Hohl, 1980, Cytobios, 29, 7-16).

Glucan synthesis by intact cotton fibres fed with different precursors at the stages of primary and secondary wall formation

Seed clusters of individual locules from fruit capsules of Gossypium arboreum L. with adhering intact fibres were fed with radioactive uridinediphosphoglucose (UDPG), guanosinediphosphoglucose (GDPG), glucose and sucrose. The incorporation into high molecular weight glucans of the fibres was studied. For primary wall fibres, UDPG at 1 mM was by far the best precursor, whereas sucrose was the best precursor for secondary wall fibres. No competition was observed between the incorporation of glucose from UDPG and from sucrose when the two were fed simultaneously to secondary wall fibres, indicating that their metabolic pathways are well separated when they are fed from the apoplast. Inhibitors of respiratory ATP-formation strongly inhibited incorporation of sucrose but not that of UDPG. Sucrose incorporation was studied at five different stages of development of the cotton fibres. At the stage of most intense secondary wall formation the incorporation rate was about 300 times that during primary wall formation (24 days post anthesis (DPA)). Incorporation from 1 mM UDPG or GDPG by secondary wall fibres (35 DPA) was less than twice that of primary wall fibres (22 DPA), indicating that the two sugar nucleotides are not readily used as precursors for secondary wall cellulose when they are fed to the exterior of intact cells. The high molecular weight non-cellulosic glucans formed from UDPG and sucrose at 5 and 1,000 μM were solubilized in strongly alkaline solutions or dimethyl-sulfoxide (DMSO) and were partially characterized by degradation with an exo-β-1,3-glucanase. After feeding for one hour, at most 1/3 of the radioactivity in high molecular weight material was found in cellulose and at least 2/3 in β-1,3-glucan. The proportions varied little for fibres in the age range of 30 to 48 DPA when sucrose was the precursor although the total incorporation varied by a factor of about four. The fact that at all stages of secondary wall formation β-1,3-glucan is synthesized at a very high rate, but that the total amount in the cell wall does not exceed 2% in the later stages of wall formation, can be interpreted in terms of a high turnover of this polysaccharide if it is assumed that wound effects are negligible in the system under study.

Soluble Factors in Pisum Extracts Which Moderate Pisum beta-Glucan Synthetase Activity

Homogenates of growing regions of the pea (Pisum sativum L.) epicotyl contain soluble factors (130,000g supernatant) which alter pea beta-glucan synthetase activity, as assayed using the substrate UDP-glucose and either particulate fractions or tissue slices as source of enzyme. A heat-stable dialyzable component is present which enhances as much as 3-fold the synthesis of alkali-soluble and -insoluble products from millimolar levels of substrate. A heat-labile nondialyzable component is also present which suppresses synthesis. This component dominates (the net effect of total crude extract) when low (mum) levels of substrate are employed. Methylation analysis shows that both components primarily affect the proportion of beta-1,4 rather than beta-1,3 linkages which are synthesized. The enhancing factor increases V(max) of the synthetase system and only activates in the presence of high levels of substrate. The suppressing factor appears to inactivate the synthetase, since losses of product or substrate are not significant during brief incubation with extract, the factor acts progressively with time with a pH optimum, and it destroys activity during preincubation with particles or slices. It co-precipitates with a protease (gelatinase) at between 20% and 40%-saturated (NH(4))(2)SO(4), and it co-fractionates with a major component of total protease on Sephadex gel columns (G-200) with an elution volume corresponding to molecular weight 65,000. The concentrations of these factors are such that they could be natural moderators of synthetase activity in vivo if the two were ever brought in contact, and the inactivator could account for the lability of beta1,4-glucan synthetase which occurs upon tissue homogenization.

Labeling of the Plasma Membrane of Pea Cells by a Surface-localized Glucan Synthetase

When radioactive UDP-glucose is supplied to 1-millimeter-thick slices of pea (Pisum sativum) stem tissue, radioactive glucose becomes incorporated into membrane-bound polysaccharides. Evidence is given that this incorporation does not result from breakdown of UDP-glucose and utilization of the resultant free glucose, and that the incorporation most likely takes place at the cell surface, leading to a specific labeling of the plasma membrane. The properties of the plasma membrane that are indicated by this method of recognition, including the association of K(+)-stimulated ATPase activity with the plasma membrane, resemble properties inferred using other approaches. The membrane-associated polysaccharide product formed from UDP-glucose is largely 1,3-linked glucan, presumably callose, and does not behave as a precursor of cell wall polymers. No substantial amount of cellulose is formed from UDP-glucose in this procedure, even though these cells incorporate free glucose rapidly into cellulose. This synthetase system that uses external UDP-glucose may serve for formation of wound callose.

The change of pattern in microfibril arrangement on the inner surface of the cell wall of Closterium acerosum during cell growth

Closterium acerosum (Schrank) Ehrenberg cells cultured on cycles of 16 h light and 8 h dark, undergo cell division synchronously in the dark period. After cell division, the symmetry of the daughter semicells is restored by controlled expansion, the time required for this restoration, 3.5-4 h, being relatively constant. The restoration of the symmetry is achieved by highly oriented surface expansion occurring along the entire length of the new semicell. During early semicell expansion, for about 2.5 h, microfibrils are deposited parallel to one another and transversely to the cell axis on the inner surface of the new wall. Wall microtubules running parallel to the transversely oriented microfibrils are observed during this period. About 2.5 h after septum formation, preceding the cessation of cell elongation, bundles of 7-11 microfibrils running in various directions begin to overlay the parallel-arranged microfibrils already deposited. In the fully elongated cells, no wall microtubules are observed.

UDP-glucose: Glucan Synthetase in Developing Cotton Fibers: I. Kinetic and Physiological Properties

A uridine diphosphate(UDP)-glucose:glucan synthetase can be demonstrated in detached cotton fibers (Gossypium hirsutum L.) and in an isolated particulate fraction from such fibers. When assayed with detached fibers, the kinetics of the glucan synthetase activity with respect to variation in substrate concentration is complex and indicates activation of the enzyme by the substrate. Activity is stimulated by Ca(2+) or Mg(2+) and beta-linked glucosides; the effect of the beta-linked glucosides is to shift the range in which substrate activation occurs to lower concentrations of UDP-glucose. At concentrations of UDP-glucose below 50 mum, addition of uridine triphosphate, in addition to beta-linked glucoside, results in significant stimulation of activity. This effect can be explained by the conversion of uridine triphosphate to UDP-glucose by UDP-glucose pyrophosphorylase, thereby raising substrate concentration to the activating range. In detached fibers, glucan synthetase activity is high at all stages of fiber development. The properties of the glucan synthetase of the isolated particulate fraction closely resemble those of the enzyme assayed in detached fibers; however, in contrast to detached fibers, the ability to detect enzyme activity is more dependent on fiber age, showing maximal activity between 16 and 18 days postanthesis, coincident with the time of rapid onset of secondary wall cellulose deposition.

UDP-glucose: Glucan Synthetase in Developing Cotton Fibers: II. Structure of the Reaction Product

The solubility properties, composition, and structure of the radioactive product synthesized from UDP-[(14)C]glucose by a highly active cotton fiber glucan synthetase have been determined. Product obtained under the following three different conditions was analyzed: at high and low substrate concentrations by detached fibers, and at high substrate concentrations with an isolated particulate preparation. The results of acetic and nitric acid digestion, enzyme digestion, total acid hydrolyses, periodate oxidation, partial acid hydrolyses, and methylation analyses all support the conclusion that the product of the glucan synthetase produced under all three assay conditions is a linear beta-(1-->3)-glucan.

Hyphal tip growth in Phytophthora. Gradient distribution and ultrahistochemistry of enzymes

Germinating cysts and isolated walls from germinating cysts incorporated 14C-UDPG into wall material of which 22.5 and 15% respectively were insoluble in boiling 1 N HCl, indicating that part of the synthetase activity is located in the wall itself. A combination of Urografin and Ficoll density gradients was used to separate various intracellular fractions. A consistent separation of beta-glucanase and UDPG-transferase enriched fractions was achieved. The beta-glucanase fraction contained dictyosome vesicles and fragments along with some plasma membranes. The UDPG-transferase fraction was relatively rich in membranes resembling rough and smooth ER. The results suggest the two enzymes are transported to the wall by different intracellular routes, and two types of vesicle may be involved. Alkaline phosphatase, beta-glucosidase and acid phosphatase were found extracellularly and their distribution in density gradients determined. The results of histochemical staining for acid phosphatase, alkaline phosphatase and polysaccharide are described and compared with the biochemical data. beta-1,3-glucanase, found intra- and extracellularly, induced distorted growth of germ tubes and also removed most of the apical wall when added to the incubation medium. None of these responses were observed with cellulase. Determinations of the osmotic pressure of germinating cysts and incubation medium revealed that the turgor of germinating cysts amounts to about 1.8 at under the conditions used.

The purification and properties of the glutamine synthetase from the cytosol of Soya-bean root nodules

The major portion of glutamine synthetase activity in root nodules of soya-bean plants is associated with the cytosol rather than with Rhizobium japonicum bacteroids. Glutamine synthetase accounts for about 2% of the total soluble protein in nodule cytosol. Glutamine synthetase from nodule cytosol has been purified by a procedure involving fractionation with protamine sulphate, ammonium sulphate and polypropylene glycol, chromatography on DEAE-Bio-Gel A and Bio-Gel A-5m and affinity chromatography on glutamate-agarose columns. The purified preparation appeared to be homogeneous in the analytical ultracentrifuge. From sedimentation-equilibrium experiments a mol. wt. of about 376000 was determined for the native enzyme and 47300 for the enzyme in guanidinium chloride. From these data and measurements of electron micrographs, we have concluded that glutamine synthetase from nodule cytosol consists of eight subunits arranged in two sets of planar tetramers which form a cubical configuration with dimensions of about 10 nm (100 A) across each side. Glutamine synthetase from nodule cytosol has a higher glycine and proline content and a lower content of phenylalanine than the glutamine synthetase that has been prepared from pea seed. The cytosol enzyme contains four half-cystine molecules per subunit, which is in contrast with two reported for the enzyme from pea seed. Enzyme activity is striking influenced by the relative proportion of Mg2+ and Mn2+ in the assay medium. Activity is inhibited by feedback inhibitors and is influenced by energy charge.