The effect of phospholipids on the activity of the plasma membrane (PM) Ca2+-ATPase was evaluated in PM isolated from germinating radish (Raphanus sativus L. cv. Tondo Rosso Quarantino) seeds after removal of endogenous calmodulin (CaM) by washing the PM vesicles with EDTA. Acidic phospholipids stimulated the basal Ca2+-ATPase activity in the following order of efficiency: phosphatidylinositol 4,5-diphosphate (PIP2) approximately phosphatidylinositol 4-monophosphate>phosphatidylinositol approximately phosphatidylserine approximately phosphatidic acid. Neutral phospholipids as phosphatidylcholine and phosphatidylethanolamine were essentially ineffective. When the assays were performed in the presence of optimal free Ca2+ concentrations (10 &mgr;M) acidic phospholipids did not affect the Ca2+-ATPase activated by CaM or by a controlled trypsin treatment of the PM, which cleaved the CaM-binding domain of the enzyme. Analysis of the dependence of Ca2+-ATPase activity on free Ca2+ concentration showed that acidic phospholipids increased Vmax and lowered the apparent Km for free Ca2+ below the value measured upon tryptic cleavage of the CaM-binding domain; in particular, PIP2 was shown to lower the apparent Km for free Ca2+ of the Ca2+-ATPase also in trypsin-treated PM. These results indicate that acidic phospholipids activate the plant PM Ca2+-ATPase through a mechanism only partially overlapping that of CaM, and thus involving a phospholipid-binding site in the Ca2+-ATPase distinct from the CaM-binding domain. The physiological implications of these results are discussed.