Lipid traffic: The ABC of transbilayer movement

Membrane lipids do not spontaneously exchange between the two leaflets of l ipid bilayers because the polar headgroups cannot cross the hydrophobic mem brane interior. Cellular membranes, notably eukaryotic plasma membranes, ar e equipped with special proteins that actively translocate lipids from one leaflet to the other. In addition, cellular membranes contain proteins that facilitate a passive equilibration of lipids between the two membrane halv es. In recent years, a growing number of proteins have been put forward as lipid translocators or facilitators. Unexpectedly, some of these appear to be required for efficient translocation of lipids lacking bulky headgroups, like cholesterol and fatty acids. The candidate lipid translocators identi fied so far belong to large protein families whose other members include pu mps for amphiphilic molecules like bile salts and drugs.