Phospholipid demixing and the birth of a lipid droplet

The biogenesis of lipid droplets (LD) in the yeast Saccharomyces cerevisiae was theoretically investigated on 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 LD that bud-off when a critical size is reached. Mathematically, LD were modeled as spherical protuberances in an otherwise planar ER membrane. We estimated the local phospholipid composition, and calculated the change in elastic free energy of the membrane caused by nascent LD. 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. During demixing, the phospholipid 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 LD cannot bud-off otherwise. In the case of Saccharomyces cerevisiae our model predicts a LD bud-off diameter of about 13 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 LD are produced from the ER, which subsequently ripe within the cytosol through a series of fusions.

💡 Research Summary

The paper presents a quantitative biophysical analysis of lipid‑droplet (LD) biogenesis in the yeast Saccharomyces cerevisiae, testing the prevailing “oil‑out” model in which neutral lipids accumulate between the leaflets of the endoplasmic‑reticulum (ER) membrane, swell into a spherical protuberance, and eventually bud off once a critical size is reached. The authors model an LD as a spherical cap attached to an otherwise planar ER bilayer and calculate the change in elastic free energy of the membrane as the droplet grows.

Key to the analysis is the coupling between membrane curvature and the composition of phospholipids (PLs) that make up the ER leaflets. Each PL species (e.g., phosphatidylethanolamine, phosphatidylcholine, phosphatidylserine, phosphatidylinositol) is assigned a spontaneous curvature (C0) and a bending modulus (κ) that reflect its intrinsic preference for curved versus flat environments. The total free‑energy change is expressed as

ΔF = ∫A