An Unrecognized Function of Cholesterol: Regulating the Mechanism Controlling Membrane Phospholipid Asymmetry

Abstract

An asymmetric distribution of phospholipids in the membrane bilayer is inseparable from physiological functions, including shape preservation and survival of erythrocytes, and by implication other cells. Aminophospholipids, notably phosphatidylserine (PS), are confined to the inner leaflet of the erythrocyte membrane lipid bilayer by the ATP-dependent flippase enzyme, ATP11C, counteracting the activity of an ATP-independent scramblase. Phospholipid scramblase 1 (PLSCR1), a single-transmembrane protein, was previously reported to possess scrambling activity in erythrocytes. However, its function was cast in doubt by the retention of scramblase activity in erythrocytes of knockout mice lacking this protein. We show that in the human erythrocyte PLSCR1 is the predominant scramblase and by reconstitution into liposomes that its activity resides in the transmembrane domain. At or below physiological intracellular calcium concentrations, total suppression of flippase activity nevertheless leaves the membrane asymmetry undisturbed. When liposomes or erythrocytes are depleted of cholesterol (a reversible process in the case of erythrocytes), PS quickly appears at the outer surface, implying that cholesterol acts in the cell as a powerful scramblase inhibitor. Thus, our results bring to light a previously unsuspected function of cholesterol in regulating phospholipid scrambling.

Peptide Bond Cleavage by Ni(II) Ions within the Nuclear Localization Signal Sequence

“…The resulting exposure of phosphatidylserine on the outer leaflet of the membrane is recognized by macrophages as an 'eat-me signal', [28] Importantly, the cleavage site for Ni(II) ions connects the major part of hPLSCR1 with the transmembrane peptide, which has full scramblase activity on its own. [39] This suggests that Ni(II)-assisted cleavage may not only impair transport of hPLSCR1 to nucleus, but also may predispose the transmembrane part for the scrambling action. The truncated mutant protein 1 Met-290 Lys has a decreased stability compared to the native one ( 1 Met-318 Trp).…”

Advances in understanding the pathogenesis of red cell membrane disorders

“…Cholesterol regulates the fluidity of the plasma membrane and is present in both leaflet faces, whereas phospholipids show an asymmetric distribution that is maintained by proteins termed “scramblases” and “flippases” (Narla & Mohandas, ). ATP11C and phospholipid scramblase 1 are major representatives of these classes of proteins (Arashiki et al , ,; Takatsu et al , ). Of note, a genetic mutation in ATP11C was identified in a male affected by congenital haemolytic anaemia inherited as an X‐linked recessive trait (Takatsu et al , ).…”

Red cell membrane disorders

“…A family of proteins termed scramblases and flippases are involved in the maintenance of asymmetric distribution of phospholipids. The molecular identity of these proteins in human red cells has recently been delineated . The functional relevance of lipid‐skeletal protein interactions has been reinforced by documenting that membrane mechanical stability is enhanced by the direct binding of PS to spectrin .…”

“…The molecular identity of these proteins in human red cells has recently been delineated. 9,10 The functional relevance of lipid-skeletal protein interactions has been reinforced by documenting that membrane mechanical stability is enhanced by the direct binding of PS to spectrin. 11 In contrast, binding of 4.1R to phosphoinositide modulates the interaction of 4.1R with linking membrane proteins, band 3, and glycophorin C. 12 Another very important functional requirement for asymmetric distribution of PS is implied by the finding that loss of lipid asymmetry and resultant translocation of PS to the outer monolayer lead to phagocytosis of these aberrant red cells by macrophages.…”