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Abstract：
Numerous lipids are heterogeneously distributed among organelles. Most lipid trafficking between organelles is achieved by a group of lipid transfer proteins (LTPs) that carry lipids using their hydrophobic cavities. The human genome encodes many intracellular LTPs responsible for lipid trafficking and the function of many LTPs in defining cellular lipid levels and distributions is unclear. Here, we created a gene knockout library targeting 90 intracellular LTPs and performed whole-cell lipidomics analysis. This analysis confirmed known lipid disturbances and identified new ones caused by the loss of LTPs. Among these, we found major sphingolipid imbalances in ORP9 and ORP11 knockout cells, two proteins of previously unknown function in sphingolipid metabolism. ORP9 and ORP11 form a heterodimer to localize at the ER-trans-Golgi membrane contact sites, where the dimer exchanges phosphatidylserine (PS) for phosphatidylinositol-4-phosphate (PI(4)P) between the two organelles. Consequently, loss of either protein causes phospholipid imbalances in the Golgi apparatus that result in lowered sphingomyelin synthesis at this organelle. Overall, our LTP knockout library toolbox identifies various proteins in control of cellular lipid levels, including the ORP9-ORP11 heterodimer, which exchanges PS and PI(4)P at the ER-Golgi membrane contact site as a critical step in sphingomyelin synthesis in the Golgi apparatus.
摘要：
许多脂质在细胞器之间异质分布。细胞器之间的大多数脂质运输是通过一组脂质转移蛋白（LTP）实现的，这些脂质转移蛋白使用其疏水性腔携带脂质。人类基因组编码许多负责脂质运输的细胞内LTP，并且许多LTP在定义细胞脂质水平和分布方面的功能尚不清楚。这里,我们创建了一个靶向90个细胞内LTPs的基因敲除文库,并进行了全细胞脂质组学分析.该分析证实了已知的脂质紊乱，并确定了由LTP损失引起的新的脂质紊乱。其中,我们在ORP9和ORP11基因敲除细胞中发现了主要的鞘脂失衡，在鞘脂代谢中具有未知功能的两种蛋白质。ORP9和ORP11形成异二聚体，定位在ER-反式高尔基体膜接触位点，其中二聚体在两个细胞器之间将磷脂酰丝氨酸（PS）交换为磷脂酰肌醇-4-磷酸（PI（4）P）。因此,任何一种蛋白质的丢失都会导致高尔基体中的磷脂失衡，从而导致该细胞器的鞘磷脂合成降低。总的来说,我们的LTP敲除库工具箱识别了控制细胞脂质水平的各种蛋白质，包括ORP9-ORP11异二聚体，它在ER-高尔基体膜接触位点交换PS和PI（4）P，这是高尔基体中鞘磷脂合成的关键步骤。
