Identification of a calcium‐ and phospholipid‐dependent protein kinase in plant tissue

A Ca2+‐ and phospholipid‐activated protein kinase from zucchini has been partially purified by DEAE‐Sephacel chromatography and some properties of the enzyme have been assessed. Minimal activity occurs in the absence of added Ca2+ or of added phospholipid but concentrations of the free ion in the range of 3 × 10−7 M produce a marked stimulation. At 10−6 M free Ca2+ this activity is further enhanced by phosphatidylserine, phosphatidylethanolamine and phosphatidic acid but not by phosphatidylcholine or phosphatidylinositol. All of these phospholipids and especially phosphatidylinositol stimulate protein kinase activity in the absence of added Ca2+.