Birth of an organelle: molecular mechanism of lipid droplet biogenesis

Lipid droplets (LDs) are cellular organelles regulating energy and lipid metabolism. Generated in the endoplasmic reticulum (ER) by phase separation of neutral lipids (e.g., triglycerides), nascent LDs resemble lens-shaped blisters, grow into spherical droplets, and eventually emerge from the ER membrane, generally towards the cytosol - a process known as budding. Images of both nascent and mature LDs are available, but the mechanism of biogenesis has never been observed experimentally. Here we identify the mechanism of the initial steps in LD biogenesis using computer simulations at the molecular level, emulating LD growth in ER-mimicking membranes. We find that LDs bud towards the cytosol only when sufficient asymmetry is generated between the two membrane leaflets: the budding transition is independent of membrane morphology, lipid composition, and LD size. Seipin, a protein essential for correct LD biogenesis, is per se not sufficient to promote budding, but it stabilizes LD-ER contact sites. Localization of triglyceride synthesis in the proximity of seipin is necessary to avoid nucleation of multiple LDs - a possible cause of aberrant phenotypes. In contrast, localization of phospholipid synthesis has no effect on the mechanism of budding. Our new methodology paves the way to simulations of organelle and cell biogenesis.