REGULATION OF A CANDIDATE AMINOPHOSPHOLIPID-TRANSPORTING ATPASE BY LIPID

The effect of lipid environment on the activation of a vanadate-sensit ive Mg2+-ATPase purified from human erythrocytes was studied in deterg ent-lipid-protein mixed micelles. ATPase activity was stimulated maxim ally by phosphatidylserine. Other anionic diacylglycerophospholipids ( phosphatidic acid, cardiolipin, phosphatidylglycerol, and phosphatidyl inositol) supported 25-100% of the phosphatidylserine-stimulated activ ity. Another aminophospholipid, egg PE, supported 38% of the phosphati dylserine-stimulated activity. The phosphoinositides (phosphatidylinos itol, phosphatidylinositol 4-phosphate, phosphatidylinositol 4,5-bisph osphate) also stimulated the ATPase; however, activity decreased with increasing lipid phosphorylation. Monoacyl negatively charged lipids ( lysophosphatidylserine, fatty acids) and zwitterionic lipids (phosphat idylcholine and sphingomyelin) did not activate the enzyme. ATPase act ivation was dependent on phospholipid fatty acyl chain composition: AT Pase activity increased with increasing PS acyl chain length, and the optimal fatty acid composition was one saturated and one unsaturated f atty acid. However, the long, unsaturated acyl chain requirement could be satisfied by nonactivating lipids. The characteristics of this ATP ase are similar to those of the Mg2+-ATP-dependent aminophospholipid f lippase, suggesting that it may be associated with the transporter.