A lipid transfer protein knockout library reveals ORP9-ORP11 dimer mediating PS/PI(4)P exchange at the ER-trans Golgi contact site to promote sphingomyelin synthesis

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. LTPs localize at membrane contact sites (MCS) where donor and acceptor organelles form a synapse to exchange information and material. The human genome encodes 90 intracellular LTPs responsible for lipid trafficking and the function of many LTPs in defining cellular lipid levels and distributions is unclear. We created a gene knockout library targeting intracellular LTPs and performed whole-cell lipidomics analysis. This analysis identified major sphingolipid imbalances in ORP9 and ORP11 knockout cells, two proteins of unknown function in sphingolipid metabolism. ORP9 and ORP11 form a heterodimer to localize at ER- trans Golgi membrane contact sites. Here, ORP9-ORP11 dimer exchanges phosphatidylserine (PS) for phosphatidylinositol-4-phosphate (PI(4)P) between the two organelles, as a critical step in sphingomyelin synthesis in trans Golgi membranes. Overall, our LTP knockout library toolbox identifies various proteins controlling cellular lipid levels including the ORP9-ORP11 heterodimer linking phospholipid and sphingolipid metabolisms at ER-Golgi MCS interphase. ### Competing Interest Statement The authors have declared no competing interest.