Rab18 localizes to lipid droplets and induces their close apposition to the endoplasmic reticulum-derived membrane

Lipid droplets (LDs) are organelles that store neutral lipids, but their regulatory mechanism is not well understood. In the present study, we identified Rab18 as an LD component of HepG2 cells by proteomic analysis, and confirmed its localization by immunohistochemistry and western blotting. Wild-type and dominant-active Rab18 localized to LDs but the dominant-negative form did not. Endogenous Rab18 coexisted with adipocyte differentiation-related protein (ADRP) in LDs, but the labeling intensity of the two proteins showed clear reciprocity. Consistent with this observation, overexpression of Rab18 induced a decrease in the amounts of ADRP in LDs in HepG2 and BALB/c 3T3 cells. Furthermore, Rab18 overexpression caused close apposition of LDs to membrane cisternae connected to the rough ER. Two other procedures that decrease ADRP, i.e. RNA interference and brefeldin A treatment, induced the same morphological change, indicating that decrease in ADRP was the cause of the LD-ER apposition. In accordance with similar structures found between ER and other organelles, we propose that the ER membrane apposed to LDs should be named the LD-associated membrane, or LAM. The present results suggested that Rab18 regulates LAM formation, which is likely to be involved in mobilizing lipid esters stored in LDs.