The cell stores metabolic energy in globular intracellular structures called lipid droplets; these associate with a number of organelles, including mitochondria, vacuoles, peroxisomes and the endoplasmic reticulum (ER). The droplets contain neutral lipids, such as triacylglycerols and steryl esters, which are synthesised by enzymes located in the ER. Moreover, an exchange of proteins and lipids between lipid droplets and the ER has been observed. However, the exact nature of the interface between the ER and lipid droplets is still not understood. In this issue (p. 3803), Roger Schneiter and colleagues demonstrate that lipid droplets are accessible to ER proteins from within the ER luminal compartment by tagging cytosolic lipid droplet proteins with a signal sequence found in an ER lumen glycoprotein. Surprisingly, these engineered proteins are still targeted to lipid droplets. The tagged proteins are glycosylated and protected from protease-mediated cleavage, consistent with a location at luminal sites of the ER. Further, expression of the ER luminal proteins does not alter lipid droplet morphology, and, using a yeast mating assay, the authors show that proteins targeted to the ER lumen can access both mature and newly formed lipid droplets. Moreover, as demonstrated here, ER luminal proteins are also able to access lipid droplets in mammalian cells. With this work, the authors thus reveal that lipid droplets are accessible from both luminal and cytosolic sites of the ER.
