Mechanism of the leakage induced on lipid model membranes by the hemolytic protein sticholysin II from the sea anemone <i>Stichodactyla helianthus</i>

Abstract

A potent hemolytic polypeptide, sticholysin II, has been purified to homogeneity from the sea anemone Stichodactyla helianthus. The protein produces leakage of aqueous contents of model lipid vesicles composed of either phosphatidylcholine or sphingomyelin if cholesterol is present in these membranes. The leakage has been analyzed by measuring the dequenching of the fluorescent dye 8-aminonaphthalene-1,3,6-trisulfonic acid, coencapsulated with its quencher N,N'-p-xylenebispyridinium bromide, upon dilution of the vesicle contents into the external medium. The protein displays a maximum effect on vesicles containing 20-25% cholesterol. Leakage is also produced in vesicles composed of mixtures of phosphatidylcholine and sphingomyelin, the maximum effect being observed for 20-30% sphingomyelin molar content. The extent of the leakage is dependent on the molecular mass of the vesicle entrapped solutes in the range 445-960 Da. This suggests the involvement of a pore of about 1 nm in diameter based on the limiting size observed for the leakage of the different solutes. Oligomerization of the protein is apparently involved in the membrane permeabilization, based on the kinetic analysis of the leakage process which is shown to proceed through an all-or-none mechanism.