Lipid traffic: the ABC of transbilayer movement.

Membrane lipids do not spontaneously exchange between the two leaflets of lipid bilayers because the polar headgroups cannot cross the hydrophobic membrane interior. Cellular membranes, notably eukaryotic plasma membranes, are 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 halves. 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 identified so far belong to large protein families whose other members include pumps for amphiphilic molecules like bile salts and drugs.