Characterization of the interaction of hemolytic lectin CEL-III from the marine invertebrate, Cucumaria echinata, with artificial lipid membranes:: Involvement of neutral sphingoglycolipids in the pore-forming process

The hemolytic lectin, GEL-III, is a Ca2+-dependent, galactose/N-acetylgalactosamine-specific lectin purified from the marine invertebrate, Cucumaria echinata (Holothuroidea). After binding to specific carbohydrates on the erythrocyte surface, GEL-III forms ion-permeable pores by oligomerizing in the membrane, which leads to colloid osmotic rupture of the cells, When incubated with liposomes composed of total lipids from the human erythrocyte membrane, GEL-III efficiently induced the leakage of carboxyfluorescein (CF) trapped in the vesicles, suggesting the presence of its receptor in the membrane lipids. The rate of CF-leakage increased with increasing temperature, although the hemolytic activity of GEL-III had been found to be much higher at lower temperatures (around 10 degrees C). Identification of the receptor for GEL-III was performed by examining the ability of individual lipids from human erythrocytes to induce CF-leakage from DOPC-liposomes. As a result, the most effective receptor was found to be lactosyl ceramide (LacCer), while globoside (Gb(4)Cer) also showed slight induction of CF-leakage, On the other hand, a binding assay involving CEL-III-horseradish peroxidase conjugate indicated that CEL-III exhibits similar affinity for LacCer and Gb(4)Cer, suggesting that the structure or length of the carbohydrate portion of sphingoglycolipids is also relevant as to their ability to induce CF-leakage in addition to their affinity. Electron micrographs of CEL-III-treated liposomes revealed that GEL-III induced considerable morphological changes in the vesicles, while a clearly distinguishable oligomeric structure of the protein was not observed.