Sterols are crucial components of biological membranes, which are synthetized in the ER and accumulate in the plasma membrane (PM). Here, by applying a genetically encoded sterol biosensor (D4H), we visualize a sterol flow between PM and endosomes in the fission yeast Schizosaccharomyces pombe. Using time-lapse and correlative light-electron microscopy, we found that inhibition of Arp2/3-dependent F-actin assembly promotes the reversible relocalization of D4H from the PM to internal sterol-rich compartments (STRIC) labeled by synaptobrevin Syb1. Retrograde sterol internalization to STRIC is independent of endocytosis or an intact Golgi, but depends on Ltc1, a LAM/StARkin-family protein localized to ER-PM contact sites. The PM in ltc1Δ cells over-accumulates sterols and upon Arp2/3 inhibition forms extended ER-interacting invaginations, indicating that sterol transfer contributes to PM size homeostasis. Anterograde sterol movement from STRIC is independent of canonical vesicular trafficking but requires Arp2/3, suggesting a novel role for this complex. Thus, transfer routes orthogonal to vesicular trafficking govern the flow of sterols in the cell.

中文翻译：

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甾醇生物传感器揭示了在 Arp2/3 抑制后 LAM 家族 Ltc1 依赖性甾醇流向内体

甾醇是生物膜的重要组成部分，在内质网中合成并在质膜 (PM) 中积累。在这里，通过应用基因编码的甾醇生物传感器 (D4H)，我们可视化裂殖酵母裂殖酵母中 PM 和内体之间的甾醇流动。使用延时和相关光电子显微镜，我们发现抑制 Arp2/3 依赖性 F-肌动蛋白组装可促进 D4H 从 PM 可逆重新定位到突触短蛋白 Syb1 标记的内部富含甾醇的区室 (STRIC)。STRIC 的逆行甾醇内化不依赖于内吞作用或完整的高尔基体，而是依赖于 Ltc1，一种定位于 ER-PM 接触位点的 LAM/StARkin 家族蛋白。ltc1Δ 细胞中的 PM 过度积累甾醇，并且在 Arp2/3 抑制后形成延长的 ER 相互作用内陷，表明甾醇转移有助于 PM 大小的稳态。STRIC 的顺行甾醇运动独立于典型的囊泡运输，但需要 Arp2/3，表明该复合物具有新的作用。因此，与囊泡运输正交的转移路线控制着细胞中甾醇的流动。