LAUSR

Significance The extracellular concentration of adenosine triphosphate (ATP) reaches several hundred micromoles in the inflamed tissues or tumor environment. A high concentration of ATP activates P2X7, a purinergic receptor, and induces the formation of a nonselective cation channel, accompanied by reversible phosphatidylserine (PtdSer) exposure, leading to cell lysis. Here, we found that Xk and Vps13a complexed at plasma membranes are indispensable for ATP-induced PtdSer exposure and cell lysis. Patients with McLeod syndrome and chorea-acanthocytosis are known to carry the mutation in XK and VPS13A genes, respectively, suggesting that a defect of XK-VPS13A–mediated phospholipid scrambling is responsible for neuroacanthocytosis, a progressive movement disorder, and cognitive and behavior changes. A high extracellular adenosine triphosphate (ATP) concentration rapidly and reversibly exposes phosphatidylserine (PtdSer) in T cells by binding to the P2X7 receptor, which ultimately leads to necrosis. Using mouse T cell transformants expressing P2X7, we herein performed CRISPR/Cas9 screening for the molecules responsible for P2X7-mediated PtdSer exposure. In addition to Eros, which is required for the localization of P2X7 to the plasma membrane, this screening identified Xk and Vps13a as essential components for this process. Xk is present at the plasma membrane, and its paralogue, Xkr8, functions as a phospholipid scramblase. Vps13a is a lipid transporter in the cytoplasm. Blue-native polyacrylamide gel electrophoresis indicated that Xk and Vps13a interacted at the membrane. A null mutation in Xk or Vps13a blocked P2X7-mediated PtdSer exposure, the internalization of phosphatidylcholine, and cytolysis. Xk and Vps13a formed a complex in mouse splenic T cells, and Xk was crucial for ATP-induced PtdSer exposure and cytolysis in CD25+CD4+ T cells. XK and VPS13A are responsible for McLeod syndrome and chorea-acanthocytosis, both characterized by a progressive movement disorder and cognitive and behavior changes. Our results suggest that the phospholipid scrambling activity mediated by XK and VPS13A is essential for maintaining homeostasis in the immune and nerve systems.