OLIGOMERIZATION OF THE HEMOLYTIC LECTIN CEL-III FROM THE MARINE INVERTEBRATE CUCUMARIA-ECHINATA INDUCED BY THE BINDING OF CARBOHYDRATE LIGANDS

The hemolytic lectin GEL-III is a Ca2+-dependent, galactose/GalNAc-spe cific lectin purified from the marine invertebrate Cucumaria echinata (Holothuroidea). We found that this lectin forms ion-permeable pores i n erythrocyte and artificial lipid membranes that have specific carboh ydrate ligands on the surface. The hemolytic activity of GEL-III exhib ited characteristic pH dependence; activity increased remarkably with pH in the alkaline region, especially above pH 9, When rabbit erythroc yte membrane was examined by immunoblot ting using anti-CEL-III antise rum after treatment with GEL-III, the irreversible binding of the GEL- III oligomer increased with pH, indicating that the increase in hemoly tic activity at higher pH is associated closely with the amount of oli gomer irreversibly bound to the membrane, Surface hydrophobicity of GE L-III, as measured by the fluorescent probe 8-anilino-1-naphthalenesul fonate, increased markedly with the binding of specific ligands such a s lactose, lactulose, and N-acetyllactosamine at pH 9-10 in the presen ce of 1 M NaCl. The enhancement of surface hydrophobicity induced by t he binding of carbohydrates was also accompanied by the formation of a GEL-III oligomer, which was found to be the same size on sodium dodec yl sulfate-polyacrylamide gel electrophoresis as the oligomer that for med in CEL-III-treated erythrocyte membranes. Far-UV circular dichrois m spectra of CEL-III and the oligomer revealed a definite difference i n secondary structure, These data suggest that the binding of GEL-III to specific carbohydrate ligands on the erythrocyte surface induces a conformational change in the protein, leading to the exposure of a hyd rophobic region which triggers oligomerization and the irreversible bi nding of the protein to the membrane.