Targeting of integral membrane proteins to the surface of lipid droplets

Lipid droplets (LDs) constitute globular subcellular structures that mainly serve to store energy in form of neutral lipids, particularly triacylglycerol and steryl esters. LDs are closely associated with the membrane of the endoplasmic reticulum (ER), and are limited by a monolayer membrane of phospholipids harboring a specific set of proteins. Most of these proteins associate with LDs through either an amphipathic helix or a membrane-embedded hairpin motif. Here we address the question whether integral membrane proteins could localize to the surface of LDs. To test this, we fused perilipin 3 (PLIN3), a mammalian LD-targeted protein, to ER resident proteins, such as Wbp1 (a N-glycosyl transferase complex subunit), Sec61 (a translocon subunit), and Pmt1 (a protein O-mannosyltransferase). The resulting fusion proteins localize to the rim of LDs in both yeast and mammalian cells. LD targeting of membrane proteins is not only observed with the PLIN3-containing fusion proteins, but also for native endogenous polytopic transmembrane proteins. These data indicate that integral membrane proteins that function in a bilayer membrane can reversibly associate with the LD surface.