Rethinking lipid droplets

Once thought to be mere storage depots, lipid droplets are in fact active organelles that play key roles in signalling and membrane trafficking. The droplets contain a core of apolar lipids surrounded by a monolayer of phospholipids and associated proteins. Until now, the prevailing view has been that they form within the bilayer of the ER and bud off from its cytoplasmic leaflet. On p. 4215, however, Horst Robenek and co-workers show that this is not the case. They have followed the biogenesis of lipid droplets in the ER, using a combination of cryo-thin-section EM, confocal light microscopy and freeze-fracture EM to get a 3D perspective on the process. This approach reveals that a droplet forms alongside – not between – the ER membranes, which together form an `egg cup' that holds the droplet. Their study also shows that adipophilin – a PAT-family protein present in lipid droplets – forms clusters in the cytoplasmic leaflet of the ER adjacent to nascent droplets. Since adipophilin may function as lipid transporter, the authors propose that these clusters represent sites for transfer of material from the ER to the droplet during its biogenesis.