The surface of lipid droplets is a phospholipid monolayer with a unique XXXX composition.

@entity844 @entity6579 @entity30716 @entity2434 @entity8483 @entity165
We found that @entity30716 is targeted to the surface of lipid droplets (Fujimoto, T., Kogo, H., Ishiguro, K., Tauchi, K., and Nomura, R. (2001) J. Cell Biol. 152, 1079-1085) and hypothesized that the lipid droplet surface is a kind of membrane. To elucidate the characteristics of the lipid droplet surface, we isolated lipid droplets from HepG2 cells and analyzed them by cryoelectron microscopy and by mass spectrometry. By use of cryoelectron microscopy at the stage temperature of 4.2 K, the lipid droplet surface was observed as a single line without any fixation or staining, indicating the presence of a single layer of phospholipids. This result appeared consistent with the hypothesis that the lipid droplet surface is derived from the cytoplasmic leaflet of the endoplasmic reticulum membrane and may be continuous to it. However, mass spectrometry revealed that the @entity844 composition of @entity2434 and @entity6579 in lipid droplets is different from that of the rough endoplasmic reticulum. The ample presence of free @entity165 in lipid droplets also suggests that their surface is differentiated from the bulk endoplasmic reticulum membrane. On the other hand, although caveolin-2beta and adipose differentiation-related protein, both localizing in lipid droplets, were enriched in the low density floating fraction, the @entity844 composition of the fraction was distinct from lipid droplets. Collectively, the result indicates that the lipid droplet surface is a hemi-membrane or a phospholipid monolayer containing @entity165 but is compositionally different from the endoplasmic reticulum membrane or the @entity8483 / @entity165 -rich microdomain.
