  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.
