Xkr8–Basigin is a plasma membrane phospholipid scramblase activated by kinases or caspases. We combined cryo-EM and X-ray crystallography to investigate its structure at an overall resolution of 3.8 Å. Its membrane-spanning region carrying 22 charged amino acids adopts a cuboid-like structure stabilized by salt bridges between hydrophilic residues in transmembrane helices. Phosphatidylcholine binding was observed in a hydrophobic cleft on the surface exposed to the outer leaflet of the plasma membrane. Six charged residues placed from top to bottom inside the molecule were essential for scrambling phospholipids in inward and outward directions, apparently providing a pathway for their translocation. A tryptophan residue was present between the head group of phosphatidylcholine and the extracellular end of the path. Its mutation to alanine made the Xkr8–Basigin complex constitutively active, indicating that it plays a vital role in regulating its scramblase activity. The structure of Xkr8–Basigin provides insights into the molecular mechanisms underlying phospholipid scrambling.

Xkr8–Basigin 是一种由激酶或半胱天冬酶激活的质膜磷脂混杂酶。我们结合冷冻电子显微镜和 X 射线晶体学以 3.8 Å 的整体分辨率研究其结构。其携带 22 个带电氨基酸的跨膜区域采用长方体样结构，由跨膜螺旋中亲水残基之间的盐桥稳定。在暴露于质膜外小叶的表面上的疏水裂隙中观察到磷脂酰胆碱结合。分子内从上到下放置的六个带电残基对于向内和向外方向扰乱磷脂至关重要，显然为它们的易位提供了途径。色氨酸残基存在于磷脂酰胆碱的头基和路径的细胞外末端之间。它对丙氨酸的突变使 Xkr8–Basigin 复合物组成型活跃，表明它在调节其 scramblase 活性中起着至关重要的作用。Xkr8–Basigin 的结构提供了对磷脂加扰的分子机制的见解。