Liid droplet Biogenesis

Lipid droplets (LDs) are found in most cell types where they serve to store energy in form of neutral lipids, mainly triacylglycerols and steryl esters. LDs are formed at the ER membrane where the neutral lipid synthesizing enzymes are located. Recent results indicate that LDs remain functionally connected to the ER membrane in both yeast and mammalian cells to allow the exchange of both lipids and integral membrane proteins between the two compartments. The precise nature of the interface between the ER membrane and LDs, however, remains to be defined. In our study we probe the topology of LD biogenesis by artificially targeting proteins that have high affinity for LDs inside the luminal compartment of the ER. Unexpectedly, these proteins localize to LDs, indicating that LDs are accessible from within the lumen of the ER. These data are consistent with a model in which LDs form a specialized domain in the ER membrane that is accessible from both the cytosolic and the ER luminal side