Phospholipid demixing: molecular interpretation of lipid droplet biogenesis

The biogenesis of lipid droplets (LDs) in the yeast Saccharomyces cerevisiae was theoretically investigated on the basis of a biophysical model. In accordance with the prevailing model of LD formation, we assumed that neutral lipids oil-out between the membrane leaflets of the endoplasmic reticulum (ER),resulting in LDs that bud off when a critical size is reached. Mathematically, LDs were modeled as spherical protuberances in an otherwise planar ER membrane. We estimated the local phospholipid (PL) composition, and calculated the change in elastic free energy of the membrane caused by nascent LD. Our simulations are based on the phenomenological Helfrich Hamiltonian. However, by describing lipids as compressible cones, we demonstrate that the Hamiltonian can easily be reformulated to support a molecular interpretation. Based on this model calculation, we found a gradual demixing of lipids in the membrane leaflet that goes along with an increase in surface curvature at the site of LD formation. While demixing, the PL monolayer was able to gain energy during LD growth, which suggested that the formation of curved interfaces was supported by or even driven by lipid demixing. In addition, we show that demixing is thermodynamically necessary as LDs cannot bud off otherwise. In the case of S. cerevisiae, our model predicts an LD bud-off diameter of about 12 nm. This diameter is far below the experimentally determined size of typical yeast LD. Thus, we concluded that if the standard model of LD formation is valid, LD biogenesis is a two-step process. Small LDs are produced from the ER, which subsequently ripe within the cytosol through a series of fusions.