SOLUBILIZATION OF A MEMBRANE-BOUND STIMULATOR OF 1,3-BETA-GLUCAN SYNTHASE FROM SAPROLEGNIA

Membrane-bound 1,3-beta- and 1,4-beta-glucan synthases from Saprolegnia were solubilized with 3-(3-cholamidopropyl) dimethyl-ammonio-3-propane sulfonate (CHAPS). 1,3-beta-Glucan synthase activities but not 1,4-beta-glucan synthase activities found in CHAPS- treated membranes were strongly stimulated by a compound of the CHAPS solubilized fraction. This effector. heat stable, non-dialyzable, increased the V(max) of the enzyme without affecting the apparent K(m) for UDP-glucose. It is associated to a protein resolved as a broad band by native PAGE but its activity is not destroyed by protease treatment. Isopycnic centrifugation showed that this compound is associated to all membrane fractions bearing glucan synthase activity and is mainly present in heavy membranes enriched in plasma membrane. The effector which is probably a glycoprotein may represent a natural effector which modulates glucan synthase activity during the membrane flow leading to the delivery of active enzymes at the cell surface.