Membrane Asymmetry Regulating Transporters

Membrane asymmetry is regulated by three types of transporters that translocate phospholipids through the bilayer.

Flippases are ATP-dependent lipid importers that transfer phosphatidylserine and phosphatidylethanolamine from the outer to the inner layer.

Floppases are ATP-dependent non-specific lipid exporters that transport lipids from the inner to the outer layer. Both flippase and floppase play an important role in generating membrane asymmetry.

Scramblases are ATP-independent transporters that allow the random movement of lipids in both layers, thereby reducing membrane asymmetry. In erythrocytes, scramblases are inactive at normal physiological levels of intracellular calcium and become activated when the calcium ion concentration increases. 

The activation of scramblases can transfer phosphatidylserine from the inner to the outer membrane layer. The outward exposure of phosphatidylserine marks erythrocytes for a suicidal death known as eryptosis and triggers their removal by macrophages.