β-1-4-and β-1-3-glucan synthases are associated with the plasma membrane of the fungus Saprolegnia.

Cytoplasmic membranes from mycelium or protoplasts of Saprolegnia monoica (a cellulosic cell-wall fungus) were separated by continuous sucrose-density-gradient centrifugation. Glucan synthases assayed at low (micromolar uridine 5'-diphosphate (UDP) glucose for β-1-4-glucan synthase) and high (millimolar UDP glucose for β-1-3-glucan synthase) substrate concentrations were associated with membranes exhibiting vanadate-sensitive, oligomycin-insensitive ATPase and equilibrating at density 1.16 g cm(-3). Synthase activities were also bound to membranes of lower density (1.10 and 1.145 g cm(-3)). Plasma membranes were stabilized by coating protoplasts with concanavalin A. After lysis of the protoplasts, plasma membranes recovered by low centrifugal forces were isolated in continuous isopycinic gradients. Both synthase activities peaked with [(3)H]concanavalin A and Na-vanadate ATPase indicating that the synthetases are located at the plasma membrane. Treatments of intact protoplasts with cold glutaraldehyde or proteases before disruption lead to a diminution of glucan-synthase activities indicating that at least part of the enzymes of plasma membrane face the outside of the cell.