Identification and Functional Expression of Four Isoforms of ATPase II, the Putative Aminophospholipid Translocase

-ATPase, is a member of a subfamily ofP-type ATPase and is presumably responsible for amino-phospholipid translocation activity in eukaryotic cells.The aminophospholipid translocation activity plays animportant physiological role in the maintenance ofmembrane phospholipid asymmetry that is observed inthe plasma membrane as well as the membranes of cer-tain cellular organelles. While the preparations ofATPase II from different sources share common funda-mental properties, such as substrate specificity, inhibi-tor spectrum, and phospholipid dependence, they aredivergent in several characteristics. These include spe-cific ATPase activity and phospholipid selectivity. Wereport here the identification of four isoforms of ATPaseII in bovine brain. These isoforms are formed by a com-bination of two major variations in their primary se-quences and show that the structural variation of theseisoforms has functional significance in both ATPase ac-tivity and phosholipid selectivity. Furthermore, studieswith the phosphoenzyme intermediate of ATPase II andits recombinant isoforms revealed that phosphatidyl-serine is essential for the dephosphorylation of the in-termediate. Without phosphatidylserine, ATPase IIwould be accumulated as phosphoenzyme in the pres-ence of ATP, resulting in the interruption of its catalyticcycle.