Na,K-ATPase and carboxyfluorescein distinctly alter vesicle formation in vitro

The mechanism of vesicle formation as well as the precise reasons for their stability are not known. Thus, it is necessary to simulate the process in vitro for studying its mechanism. If phospholipids are suspended in physiological solution by means of cholate and the detergent is then removed by dialysis, the phospholipids self-assemble to form unilamellar vesicles. We report here that the addition of Na,K-ATPase (an integral membrane protein) to the phospholipids changes the vesicle structure, they become larger and a multilamellar population appears. By contrast, carboxyfluorescein, a compound commonly used for labelling the aqueous vesicle compartment, produces an unexpected effect on vesicle structure by inducing complex, tore-like intravesicular multilayer formations associated with a 5-fold increase in diameter. Thus, the presence of a protein in the membrane phase or of a compound in the water phase can influence and direct vesicle formation in vitro; these model systems might give some clues to possible physicochemical or biological factors governing the formation of natural membrane structures.