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Fig. 1. Regulation of the transbilayer lipid distribution in cellular membranes. In early secretory organelles, such as the endoplasmic reticulum (ER), membrane proteins facilitate rapid flip-flop of lipids and allow them to equilibrate between the two membrane leaflets independently of ATP. This system is unable to accumulate a given lipid in one leaflet, thereby promoting a symmetric lipid distribution across the bilayer. In contrast, flip-flop of phospholipids across the plasma membrane (PM) is constrained owing to high levels of cholesterol and sphingolipids and/or the absence of constitutive bi-directional flippases. Thus, ATP-dependent flippases can maintain an asymmetric lipid distribution by moving specific lipids towards (P-type ATPase family members) or away from the cytosolic leaflet (ABC transporters). Cellular activation triggered by cytosolic calcium can collapse the lipid asymmetry by the transient activity of an ATP-independent scramblase. PC, phosphatidylcholine; PE, phosphatidylethanolamine; PS, phosphatidylserine; SM, sphingomyelin; GL, glycolipids; Chol, cholesterol.





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