ATP11C is a major flippase in human erythrocytes and its defect causes congenital hemolytic anemia

Abstract

Phosphatidylserine is localized exclusively to the inner leaflet of the membrane lipid bilayer of most cells, including erythrocytes. This asymmetric distribution is critical for the survival of erythrocytes in circulation since externalized phosphatidylserine is a phagocytic signal for splenic macrophages. Flippases are P-IV ATPase family proteins that actively transport phosphatidylserine from the outer to inner leaflet. It has not yet been determined which of the 14 members of this family of proteins is the flippase in human erythrocytes. Herein, we report that ATP11C encodes a major flippase in human erythrocytes, and a genetic mutation identified in a male patient caused congenital hemolytic anemia inherited as an X-linked recessive trait. Phosphatidylserine internalization in erythrocytes with the mutant ATP11C was decreased 10-fold compared to that of the control, functionally establishing that ATP11C is a major flippase in human erythrocytes. Contrary to our expectations phosphatidylserine was retained in the inner leaflet of the majority of mature erythrocytes from both controls and the patient, suggesting that phosphatidylserine cannot be externalized as long as scramblase is inactive. Phosphatidylserine-exposing cells were found only in the densest senescent cells (0.1% of total) in which scramblase was activated by increased Ca 2+ concentration: the percentage of these phosphatidylserine-exposing cells was increased in the patient's senescent cells accounting for his mild anemia. Furthermore, the finding of similar extents of phosphatidylserine exposure by exogenous Ca 2+ -activated scrambling in both control erythrocytes and the patient's erythrocytes implies that suppressed scramblase activity rather than flippase activity contributes to the maintenance of phosphatidylserine in the inner leaflet of human erythrocytes.

ATPase reaction cycle of P4‐ATPases affects their transport from the endoplasmic reticulum

“…In humans, 14 members of the P4‐ATPase family are expressed in various cell types and tissues, and translocate specific substrates, not only glycerophospholipids, such as phosphatidylserine, phosphatidylethanolamine, phosphatidylcholine, but also a glycosphingolipid, glucosylceramide . Mutations in some of these P4‐ATPases, such as ATP8A2, ATP8B1, and ATP11C, are linked to hereditary diseases . The amino acid sequence similarity of P‐type ATPases indicates that the overall structure and domain organization of P4‐ATPases resemble those of other P‐type ATPases, such as Ca 2+ ‐ATPase (SERCA) and Na + /K + ‐ATPase .…”

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.…”

Decoding P4-ATPase substrate interactions

“…These initial studies described a Mg 2+ -ATP dependent PL transporter in human red blood cells that preferred the amino-glycerophospholipid substrates, PE and PS (Seigneuret and Devaux, 1984, Zachowski et al , 1985, Zachowski et al , 1986). Recent work in human erythrocytes suggests this aminophospholipid translocase activity is primarily catalyzed by ATP11C (Arashiki et al , 2016). Devaux and colleagues made these initial observations using spin-labeled PLs, a strategy that measures the accumulation/protection of these labeled lipids from extracellular reducing agents as a consequence of their transport to the cytosolic leaflet.…”