Lipids partition caveolin-1 from ER membranes into lipid droplets: updating the model of lipid droplet biogenesis

SPECIFIC AIMSThe current model of lipid droplet formation assumes caveolin-1 resides in the cytoplasmic membrane leaflet of the endoplasmic reticulum (ER) and is transferred along with this leaflet to the surface of the nascent lipid droplet—but not to its core—as neutral lipids inundate between the membrane leaflets, causing the cytoplasmic leaflet to bulge and the droplet to bud off the ER. We explored how caveolin-1 accesses lipid droplets from the ER by localizing caveolin-1 in ER membranes and in lipid droplets of cultured smooth muscle cells.PRINCIPAL FINDINGS1. Caveolin-1 resides in the endoplasmic, not the cytoplasmic, leaflets of ER membranesWe used freeze-fracture immunocytochemistry to localize caveolin-1. Membranes always split into their two constituent leaflets during freeze-fracturing, revealing the P-face (cytoplasmic leaflet) or the E-face (endoplasmic leaflet), as the plane of cleavage runs preferentially between the tails of the phospholipids of the membrane bilayer. Whether immunolabel...