β-Glucan Synthetases of Plasma Membrane and Golgi Apparatus from Onion Stem

Abstract

Biosynthesis of glucans occurred in cell-free fractions isolated from onion stem (Allium cepa L.) enriched in either dictyosomes or plasma membranes. l8-1,3-and p-1,4-Glucans were synthesized in differing proportions and at different rates as the concentration of uridine diphosphoglucose or the proportion of dictyosomes or plasma membrane varied. At low (1.5 uM) UDP-glucose concentrations synthesis of alkaliinsoluble glucan was correlated with abundance of dicytosomes; most of the substrate utilized by plasma membrane was for glycolipid synthesis. At high (1 mM) UDP-glucose concentration, the synthesis of alkali-insoluble glucans correlated with the abundance of plasma membrane. Substrate enhancement of g-1,4-glucan synthesis in dictyosome fractions was less than proportional to increases in substrate concentration. In contrast, ,-1,4-glucan synthesis by plasma membrane was more than proportionately increased. At high substrate concentrations the synthesis of 6-1 ,3-glucans predominated in both dictyosome and plasma membrane fractions. The results show that the capacity to synthesize glucans resides in both Golgi apparatus and plasma membranes of onion stem, but that the plasma membrane has the greatest capacity for synthesis of alkali-insoluble glucans at high UDP-glucose concentrations.In vivo studies of cell wall biogenesis indicate that cellulose synthesis occurs at the cell surface, possibly at the surface of the plasma membrane (28,34,39,44,52), while pectic and hemicellulosic polysaccharides are synthesized by the Golgi apparatus (1,5,10,18,27,40). In contrast, Ray et al. (36) found UDP-glucose: /3-1,4-glucan glucosyltransferase (3-1,4-glucan synthetase) activity to be localized in a membrane frac-

Plasma Membranes from Oats Prepared by Partition in an Aqueous Polymer Two-Phase System

“…Most ofthe biochemical studies on this membrane are performed with postmitochondrial fractions either used directly or after further fractionation on density gradients (e.g. 9,14,15,19,24,31). However, the enrichment of plasma membranes in these fractions has been uncertain due to the lack of a reliable plasma membrane marker, and the maximal reported purities are 75 to 80%1o presumptive plasma membranes (for review, see Ref.…”

“…Different markers have been proposed for the plant plasma membrane: K+-stimulated Mg2+-dependent ATPase assayed at pH 6.0 (15) or 6.5 (19), glucan synthetase II (24,31), phosphotungstic or silicotungstic acid staining (26,27), and naphthylphthalamic acid binding (13). A blue light-induced absorbance change indicating reduction of a membrane bound b-Cyt (and with a possible connection to blue light physiology) has been proposed to be located in plasma membranes (6).…”

Labeling and Isolation of Plasma Membranes from Corn Leaf Protoplasts

“…Plasma membrane-enriched fractions have been isolated from numerous plant sources, including oat and corn roots (9,10,15), corn coleoptiles (7,12), green onion meristem (24), leek epidermal cells (2), sugarcane leaves (21), sugarcane cell suspensions (23), soybean suspension cultures (5), soybean roots (6), and tobacco epidermis (16). In most cases, the isolation method employed required mechanical disruption, usually grinding, followed by differential and density gradient centrifugation.…”

Synthesis of β‐glucans in Prototheca zopfii

“…Some explanations have been given for the presence of P-1,3-linked glucans. In different systems the in vitro synthesis of ~-1,3-glucans (callose) may result as a response to wounding when cell-free preparations are made from higher plants [29] or fungi [30]. However, the low proportion of B-1,3-linkages obtained in in vivo studies support the idea that in P .…”