Characterization of Rab18, a lipid droplet-associated small GTPase.

Lipid droplets are the major intracellular store of lipids in eukaryotic cells. Understanding lipid storage and regulated mobilization of lipids from lipid droplets is essential for understanding the syndromes and diseases associated with excess lipid accumulation. Lipid droplets have been traditionally considered relatively inert structures. However, in recent years it has become apparent that lipid droplets are highly dynamic regulated organelles, which show complex interactions with other cellular compartments. The cellular components involved in regulation of lipid accumulation and release from lipid droplets, and in mediating the complex interactions with other organelles, are only now starting to be unraveled. A particularly important family of proteins in this respect is the Rab GTPases, crucial regulators of membrane traffic. Here we describe the techniques that we used to characterize the regulated association of Rab18 with the surface of lipid droplets. Rab18 provides an excellent marker to follow the dynamics of lipid droplets in living cells. In addition, the study of Rab18 provides insights into the mechanisms involved in the release of lipids from lipid droplets in adipocytes. In 3T3-L1 adipocytes, stimulation of lipolysis increases the association of Rab18 with lipid droplets, suggesting that recruitment of Rab18 is regulated by the metabolic state of individual lipid droplets. The study of Rab18 and its interacting proteins will provide new insights into the complex regulatory mechanisms involved in lipid storage and release.