ATG2 is a triglyceride transfer protein

Bridge-like lipid transfer proteins (BLTPs) are established to function in phospholipid transport between bilayers at organelle-organelle contact sites. However, the BLTP ATG2A also associates with lipid droplets in cells, which present a unique phospholipid monolayer topology and which are composed of many additional types of lipids. Whether BLTPs are active in this environment and which lipid species are substrates for transport has been unknown. Here, we use synthetic organelles with bilayers (liposomes), monolayers (artificial lipid droplets), or a mixture of the two membrane structures to demonstrate the tight binding of ATG2 specifically to monolayers via its collection of COOH-terminal amphipathic helices. This stable binding enables ATG2 to transfer phospholipids much more effectively. Unexpectedly, the neutral lipid triacylglycerol is also rapidly transported, with kinetics similar to those of phospholipid transport. Lipidomics of purified ATG2A suggests that a similar transfer of both phospholipids and triacylglycerol occurs in cells. Our work implies that BLTPs likely collect on LDs as part of a broad lipid homeostasis program, which will include the movement of both phospholipids and neutral lipids.