REGULATION OF A CANDIDATE AMINOPHOSPHOLIPID-TRANSPORTING ATPASE BY LIPID

The effect of lipid environment on the activation of a vanadate-sensitive Mg2+-ATPase purified from human erythrocytes was studied in detergent-lipid-protein mixed micelles. ATPase activity was stimulated maximally by phosphatidylserine. Other anionic diacylglycerophospholipids (phosphatidic acid, cardiolipin, phosphatidylglycerol, and phosphatidylinositol) supported 25-100% of the phosphatidylserine-stimulated activity. Another aminophospholipid, egg PE, supported 38% of the phosphatidylserine-stimulated activity. The phosphoinositides (phosphatidylinositol, phosphatidylinositol 4-phosphate, phosphatidylinositol 4,5-bisphosphate) also stimulated the ATPase; however, activity decreased with increasing lipid phosphorylation. Monoacyl negatively charged lipids (lysophosphatidylserine, fatty acids) and zwitterionic lipids (phosphatidylcholine and sphingomyelin) did not activate the enzyme. ATPase activation was dependent on phospholipid fatty acyl chain composition: ATPase activity increased with increasing PS acyl chain length, and the optimal fatty acid composition was one saturated and one unsaturated fatty acid. However, the long, unsaturated acyl chain requirement could be satisfied by nonactivating lipids. The characteristics of this ATPase are similar to those of the Mg2+-ATP-dependent aminophospholipid flippase, suggesting that it may be associated with the transporter.