Caveolin, cholesterol, and lipid droplets?

Caveolins constitute the coat of caveolae, specialized domains of the plasma membrane. A large body of evidence suggests that caveolae are enriched in sphingolipids and cholesterol. Besides a role in signal transduction and in the sorting of membrane components, a diverse range of functions has been assigned to the caveolins in the cellular homeostasis of cholesterol. Cholesterol is essential for numerous membrane processes, and its concentration in the cell is governed by a delicate sensor–effector mechanism. However, the basic rules that govern the distribution and dynamics of cholesterol in cells are not understood, a situation that may be changed by an improved insight in the actions of cholesterol-binding proteins like caveolins. Three papers in the present issue report unexpectedly that caveolins can be found on the surface of cytoplasmic lipid droplets (Fujimoto et al. 2001; Ostermeyer et al. 2001; Pol et al. 2001). Lipid droplets consist of apolar lipids and are thought to serve as inert energy storage sites. Caveolin transport to the lipid droplets may represent an "overflow" pathway from the ER under conditions where their concentration in the ER is enhanced (Ostermeyer et al. 2001). More sensational proposals are that caveolin on lipid droplets is a key component in maintaining the cellular cholesterol balance (Pol et al. 2001) and that caveolin populates a raft-like membrane domain on the droplet surface that is involved in intracellular signaling (Fujimoto et al. 2001).