Flippase

thumb|Structure of a flippase, showing the two major subunits of the enzyme|249x249px

Flippases are transmembrane lipid transporter proteins located in the cell membrane. They are responsible for aiding the movement of phospholipid molecules between the two layers, or leaflets, that comprise the membrane. This is called transverse diffusion, also known as "flip-flop" transition. Flippases move lipids to the cytosolic layer, usually from the extracellular layer. Floppases do the opposite, moving lipids to the extracellular layer. Both flippases and floppases are powered by ATP hydrolysis and are either P4-ATPases or ATP-Binding Cassette transporters. Scramblases are energy-independent and transport lipids in both directions.