Mechanism for oil-phase separation by the lipid droplet assembly complex

Cells store metabolic energy as triglyceride (TG) oils in lipid droplets (LDs). LDs form de novo from the endoplasmic reticulum. How the lipid droplet assembly complex (LDAC), composed of seipin and LDAF11,2, catalyzes the organized formation of an oil phase in a membrane bilayer before spontaneous phase separation is triggered is unknown. Here, we reconstitute LD formation in vitro using purified LDAC and membranes containing physiologic levels of TG, demonstrating that the LDAC is both necessary and sufficient to catalyze oil-phase formation below the threshold of spontaneous phase separation. Structural studies of the LDAC reveal that LDAF1 forms a central ring within a seipin cage, creating a toroidal, membrane-spanning structure. Molecular dynamics simulations and biochemical assays show that this structure forms a selective chamber within the ER bilayer that limits phospholipids but allows TG to access a reaction compartment between the inner and outer rings of the LDAC. Within this compartment, TG interacts with LDAF1 and each other to form an oil phase to initiate LD formation. Thus, the LDAC acts as a protein catalyst for oil-phase separation in cells, revealing a fundamental mechanism for how cells resolve the biophysical challenge of storing oils within a hydrophilic environment in an organized manner.