A phosphoinositide-specific phospholipase C activity was identified in oat root (Avena sativa, cv Victory) plasma membranes purified by separation in an aqueous two-phase polymer system. The enzyme is highly active toward inositol phospholipids but only minimally active toward phosphatidylethanolamine and phosphatidylcholine. Activity approaches maximal levels at 200 micromolar phosphatidylinositol 4-phosphate (PIP) and is highly dependent on calcium; it is inhibited by I millimolar EGTA and is activated by calcium with an apparent activation constant of 2 micromolar. At 10 micromolar calcium and 200 micromolar inositol phospholipid, the enzyme is specific for phosphatidylinositol 4,5-bisphosphate (PIP2) and PIP, which are hydrolyzed at 10 and 4 times, respectively, the rate of phosphatidylinositol (PI) We here report the presence of a phospholipase C activity in oat root plasma membranes purified by separation with an aqueous two-phase polymer system. Our findings show a phospholipase C activity that is calcium activated and is most active with the polyphosphoinositide substrate, PIP2.
MATERIALS AND METHODS
Plant MaterialSeeds of oats (Avena sativa, cv Victory) were planted in moist, fine vermiculite and grown at 25°C in constant darkness. Plants were harvested after 6 d and the roots were removed for isolation of membranes.
Plasma Membrane IsolationAll membrane isolation procedures were performed at 4°C. Oat roots were homogenized with a Waring blender in a 0.25 M Tris-HCl buffer (pH 6.5) containing 1.15 M sucrose, 25 mM EDTA, 2 mm PMSF, and 70 mM 2-mercaptoethanol. The homogenate was then subjected to centrifugation at 8,000g for 15 min followed by centrifugation of the supernatant at 33,500g for 90 min. The resulting crude membrane pellet was resuspended in a 5 mm potassium phosphate buffer (pH 7.8) with 0.33 M sucrose and 4 mM KCI and the suspension was applied to a two-phase system containing 6.5% (w/w) dextran T500 and 6.5% (w/w) polyethylene glycol 3350 in the same buffer (9). After mixing by repeated inversions (30-35 times), the two phases were separated by a 5 min centrifugation at 1,500g. The upper phase, containing the plasma membrane vesicles, was then washed twice with fresh lower phase and diluted with an equal volume of Tris-HCl buffer (10 mM, pH 7.5) containing 0.33 M sucrose. After a 60 min centrifugation at 100,000g, the resulting pellet, which contained the purified plasma membranes, was resuspended in 1 mL of the above Tris-HCl buffer and stored in 100 1L aliquots at -70°C until use. Protein concentration of the purified membrane preparations was determined by the method of Lowry et al. (10