Phospholipids are asymmetrically distributed in the plasma membrane (PM), with phosphatidylcholine and sphingomyelin abundant in the outer leaflet. However, the mechanisms by which their distribution is regulated remain unclear. Here, we show that transmembrane protein 63B (TMEM63B) functions as a membrane structure-responsive lipid scramblase localized at the PM and lysosomes, activating bidirectional lipid translocation upon changes in membrane curvature and thickness. TMEM63B contains two intracellular loops with palmitoylated cysteine residue clusters essential for its scrambling function. TMEM63B deficiency alters phosphatidylcholine and sphingomyelin distributions in the PM. Persons with heterozygous mutations in TMEM63B are known to develop neurodevelopmental disorders. We show that V44M, the most frequent substitution, confers constitutive scramblase activity on TMEM63B, disrupting PM phospholipid asymmetry. We determined the cryo-electron microscopy structures of TMEM63B in its open and closed conformations, uncovering a lipid translocation pathway formed in response to changes in the membrane environment. Together, our results identify TMEM63B as a membrane structure-responsive scramblase that controls PM lipid distribution and we reveal the molecular basis for lipid scrambling and its biological importance.

磷脂在质膜 （PM） 中不对称分布，磷脂酰胆碱和鞘磷脂在外小叶中丰富。然而，其分布的调节机制仍不清楚。在这里，我们表明跨膜蛋白 63B （TMEM63B） 作为位于 PM 和溶酶体的膜结构响应脂质扰乱酶发挥作用，在膜曲率和厚度发生变化时激活双向脂质易位。TMEM63B 包含两个细胞内环，其中棕榈酰化半胱氨酸残基簇对其加扰功能至关重要。TMEM63B缺乏会改变磷脂酰胆碱和鞘磷脂在 PM 中的分布。已知 TMEM63B 杂合突变的人会发展为神经发育障碍。我们表明 V44M 是最常见的取代，它赋予 TMEM63B 组成型扰乱酶活性，破坏了 PM 磷脂不对称性。我们确定了 TMEM63B 在其开放和封闭构象中的冷冻电子显微镜结构，揭示了响应膜环境变化而形成的脂质易位途径。总之，我们的结果将 TMEM63B 确定为控制 PM 脂质分布的膜结构响应性扰酶，并揭示了脂质扰乱的分子基础及其生物学重要性。