The Erythrocyte Aminophospholipid Translocase

Our understanding of cellular membranes has rapidly progressed from the point of view of the lipid bilayer being only a simple permeability barrier and matrix for membrane proteins to one in which membrane lipids are viewed as dynamic components capable of initiating and regulating various cellular functions. Only recently, however, has it become clear that the distribution of lipids between bilayer leaflets plays an important role in many cellular processes. While most of these phenomena are directly associated with what is considered to be normal membrane lipid asymmetry, the “atypical” display of phosphatidylserine (PS) on the cell’s outer leaflet has significant physiological consequences. PS participates, for example, in various cell-cell interactions (Schroit et al., 1985; Schlegel et al., 1985), cell activation and hemostasis (Bevers et al., 1983; Rosing et al., 1985; Sims et al., 1989), cell aging (Shukla and Hanahan, 1982; Herrmann and Devaux, 1990), membrane fusion events (Farooqui et al., 1987; Song et al., 1992; Schewe et al., 1992), and apoptosis (Fadok et al., 1992a, 1992b).