Is lipid translocation involved during endo- and exocytosis?

Stimulation of the aminophospholipid translocase, responsible for the transport of phosphatidylserine and phosphatidylethanolamine from the outer to the inner leaflet of the plasma membrane, provokes endocytic-like vesicles in erythrocytes and stimulates endocytosis in K562 cells. In this article arguments are given which support the idea that the active transport of lipids could be the driving force involved in membrane folding during the early step of endocytosis. The model is sustained by experiments on shape changes of pure lipid vesicles triggered by a change in the proportion of inner and outer lipids. It is shown that the formation of microvesicles with a diameter of 100-200 nm caused by the translocation of plasma membrane lipids implies a surface tension in the whole membrane. It is likely that cytoskeleton proteins and inner organelles prevent a real cell from undergoing overall shape changes of the type seen with giant unilamellar vesicles. Another hypothesis put forward in this article is the possible implication of the phospholipid 'scramblase' during exocytosis which could favor the unfolding of microvesicles.