Purification and Substrate Specificity of the Human Erythrocyte Aminophospholipid Transporter

Abstract

The asymmetric transmembrane distribution of phospholipids across biological membranes is maintained by a combination of slow transmembrane flip-flop, cytoskeletalprotein interactions and protein-mediated inward transport of aminophospholipids. These latter proteins are members of a growing class of transporters, or flippases, that catalyze the transmembrane transport of phospholipids (Devaux and Zachowski, 1993). The aminophospholipid flippase selectively transports phosphatidylserine (PS) and phosphatidylethanolamine (PE) to the cytofacial surface of the membrane. The flippase is Mg²⁺-ATP-dependent and is inhibited by sulfhydryl reagents, vanadate and Ca²⁺ (Daleke and Huestis, 1985; Bitbol, et al., 1987; Connor and Schroit, 1988). This transporter demonstrates a high degree of specificity for its lipid substrate. The amine functionality is essential; acylation of the amine group of PS prevents transport by the flippase (Morrot, et al., 1989; Drummond and Daleke, 1994) and N-methylation of PE reduces transport of this lipid (Morrot, et al., 1989). Only the natural sn-1,2 glycerol isomer of PS is a substrate; the sn-2,3 isomer of PS is not transported in fibroblasts (Martin and Pagano, 1987). In contrast, the flippase is insensitive to acyl chain length (Daleke and Huestis, 1985) and is relatively insensitive to acyl chain composition.¹ These characteristics allow the inclusion of spin labeled (Seigneuret and Devaux, 1984), short chain (Daleke and Huestis, 1985), fluorescent (Connor and Schroit, 1987) or radiolabeled (Tilley, et al., 1986; Daleke and Huestis, 1989; Anzai et al., 1993) fatty acid as reporter groups for the measurement of lipid transport in intact membranes.