An intramolecular scrambling path controlled by a gatekeeper in Xkr8 phospholipid scramblase

Xkr8-Basigin is a phospholipid scramblase at plasma membranes that is activated by kinase or caspase. We investigated its structure at a resolution of 3.8[A]. Its membrane-spanning region had a cuboid-like structure stabilized by salt bridges between hydrophilic residues in helices in the lipid layer. The molecule carried phosphatidylcholine in a cleft on the surface that may function as an entry site for phospholipids. Five charged residues placed from top to bottom inside the molecule were essential for providing a path for scrambling phospholipids. A tryptophan residue was present at the extracellular end of the pathway and its mutation made the Xkr8-Basigin complex constitutively active, indicating its function as a gatekeeper. The structure of Xkr8-Basigin provides novel insights into the molecular mechanisms underlying phospholipid scrambling.