Figure 1.

Schematic representation of lipid transbilayer transporters in the generation and maintenance of plasma membrane asymmetry. Flippases mediate ATP-dependent translocation of phosphatidylserine (PS) and phosphatidylethanolamine (PE) from the extracellular to the inner cytosolic side. Floppases catalyze the ATP-dependent translocation of phosphatidylcholine (PC), sphingomyelin (SM), and cholesterol to the outer leaflet. Scramblases promote Ca²⁺-activated nonspecific bidirectional movement.