Cholesterol is a major and essential component of the mammalian cell plasma membrane (PM), and the loss of cholesterol homeostasis leads to various pathologies. Cellular cholesterol uptake and synthesis are regulated by a cholesterol sensor in the endoplasmic reticulum (ER). However, it remains unclear how changes in the cholesterol level of the PM are recognized. Here we show that the sensing of cholesterol in the PM depends on ATP-binding cassette A1 (ABCA1) and the cholesterol transfer protein Aster-A, which cooperatively maintain the asymmetric transbilayer cholesterol distribution in the PM. We demonstrate that ABCA1 translocates (flops) cholesterol from the inner leaflet of the PM to the outer leaflet to maintain a low inner leaflet cholesterol level. We also found when inner cholesterol levels were increased, Aster-A was recruited to the PM-ER contact site to transfer cholesterol to the ER. These results suggest that ABCA1 could promote an asymmetric cholesterol distribution to suppress Aster-A recruitment to the PM-ER contact site to maintain intracellular cholesterol homeostasis.

胆固醇是哺乳动物细胞质膜 (PM) 的主要和必需成分，胆固醇稳态的丧失会导致各种病理。细胞胆固醇摄取和合成由内质网 (ER) 中的胆固醇传感器调节。然而，目前尚不清楚如何识别 PM 胆固醇水平的变化。在这里，我们表明 PM 中胆固醇的感知取决于 ATP 结合盒 A1 (ABCA1) 和胆固醇转移蛋白 Aster-A，它们共同维持 PM 中的不对称跨双层胆固醇分布。我们证明 ABCA1 将胆固醇从 PM 的内叶转移（翻转）到外叶，以维持较低的内叶胆固醇水平。我们还发现，当体内胆固醇水平升高时，Aster-A 被募集到 PM-ER 接触部位以将胆固醇转移到 ER。这些结果表明 ABCA1 可以促进不对称的胆固醇分布以抑制 Aster-A 募集到 PM-ER 接触位点以维持细胞内胆固醇稳态。