Fig.7.

Models for cytoplasmic droplet and serum lipoprotein formation in specialized cells. Lipid droplets form on either side of the ER membrane. Import into the cytoplasm reflects the thermodynamic nature of neutral lipids that oil-out the ER membrane. It is unclear whether the droplets are tethered to the ER or migrate away (as shown here). Accessory proteins, such as the fat storage-inducing transmembrane proteins (FIT) and BSCL2/Seipin (SEI), then stabilize and regulate the nascent particles. Dgat2, as a consequence of putative CLD localization, may initiate or supplement core neutral lipid loading while Perilipins (Plin) regulate hydrolysis by specific lipases. In the liver, neutral lipid deposition also occurs in the lumen of the ER and requires a defined set of proteins, specifically an apoB scaffold that acquires lipids, such as triglyceride, as both molecules are produced. The lipid transfer protein MTP, a complex of a transfer polypeptide and the molecular chaperone protein disulphide isomerase, is essential for efficient mobilization of these lipids.