School of Biotechnology and Biomolecular Sciences, The University of New South Wales, Sydney, Australia.
State Key Laboratory of Membrane Biology and Tsinghua-Peking Center for Life Sciences, School of Life Sciences, Tsinghua University, Beijing, China.
J Cell Biol. 2020 Jan 6;219(1). doi: 10.1083/jcb.201905162.
Lipid droplets (LDs) are evolutionarily conserved organelles that play important roles in cellular metabolism. Each LD is enclosed by a monolayer of phospholipids, distinct from bilayer membranes. During LD biogenesis and growth, this monolayer of lipids expands by acquiring phospholipids from the endoplasmic reticulum (ER) through nonvesicular mechanisms. Here, in a mini-screen, we find that ORP5, an integral membrane protein of the ER, can localize to ER-LD contact sites upon oleate loading. ORP5 interacts with LDs through its ligand-binding domain, and ORP5 deficiency enhances neutral lipid synthesis and increases the size of LDs. Importantly, there is significantly more phosphatidylinositol-4-phosphate (PI(4)P) and less phosphatidylserine (PS) on LDs in ORP5-deficient cells than in normal cells. The increased presence of PI(4)P on LDs in ORP5-deficient cells requires phosphatidylinositol 4-kinase 2-α. Our results thus demonstrate the existence of PI(4)P on LDs and suggest that LD-associated PI(4)P may be primarily used by ORP5 to deliver PS to LDs.
脂滴（LDs）是进化上保守的细胞器，在细胞代谢中发挥重要作用。每个 LD 都被一层单层磷脂包围，与双层膜不同。在 LD 的生物发生和生长过程中，通过非囊泡机制从内质网（ER）获取磷脂，使该层脂质扩展。在这里，在一个小型筛选中，我们发现 ER 的整合膜蛋白 ORP5 在油酸盐加载时可以定位到 ER-LD 接触位点。ORP5 通过其配体结合域与 LD 相互作用，ORP5 缺陷会增强中性脂质的合成并增加 LD 的大小。重要的是，与正常细胞相比，ORP5 缺陷细胞的 LD 上的磷脂酰肌醇-4-磷酸（PI（4）P）更多，而磷脂酰丝氨酸（PS）更少。ORP5 缺陷细胞中 LD 上 PI（4）P 的增加需要磷脂酰肌醇 4-激酶 2-α。因此，我们的结果证明了 LD 上存在 PI（4）P，并表明 LD 相关的 PI（4）P 可能主要由 ORP5 用于将 PS 递送至 LD。
