Separation of membranes by flotation centrifugation for in vitro synthesis of plant cell wall polysaccharides

Abstract

Membranes from etiolated maize seedlings were isolated using sucrose gradients for in vitro studies of polysaccharide synthesis. Following downward centrifugation, flotation centrifugation improved the purity of membrane fractions, in particular the Golgi apparatus. Based on naphthylphthalamic acid binding to plasma membrane and inosine-5′-diphosphatase activity in Golgi apparatus, flotation centrifugation removed about 70% of the plasma membrane which cosedimented with the Golgi apparatus in downward centrifugation. The addition of chelators during flotation centrifugation allowed separation of the Golgi apparatus from endoplasmic reticulum, as indicated by NADH cytochromec reductase activity. Glucan and xylan synthase activities were measured as the radioactivity incorporated from either UDP-14C-glucose or UDP-14C-xylose into 80% ethanol insoluble materials. Glucan synthase activity at a substrate concentration of 1 mM UDP-glucose without CaCl2 was greatest in fractions enriched in Golgi apparatus, but in the presence of 3 mM CaCl2 the activity was greatest in fractions enriched in plasma membrane. Glucan synthase activity at a substrate concentration of 10μM UDP-glucose in the presence of 3 mM MnCl2 was greatest in fractions enriched in plasma membrane, but was also high in fractions enriched in Golgi apparatus. Xylan synthase activity, at a substrate concentration of 1 μM UDP-xylose in the presence of 3 mM MnCl2, was greatest in fractions enriched in Golgi apparatus. To further characterize these synthase reactions, the glycosyl linkages of the products formed were analyzed with a gas chromatograph coupled to a radiogas proportional counter. With the substrate, UDP-14C-glucose, and fractions enriched in Golgi apparatus, both (1→3)- and (1→4)-radioactive glucosyl linkages were formed, whereas the main linkage formed by fractions enriched in plasma membrane was (1→3)-glucosyl. With the substrate, UDP-14C-xylose, mostly (1→4)-xylosyl and some terminal-xylosyl linkages were formed by fractions enriched in Golgi apparatus. Only xylan synthase activity copurified with Golgi apparatus and, because plasma membrane lacked this activity, xylan synthase may be used as a reasonable indicator of Golgi apparatus.