Calcium ions stabilize a protein structure of hemolytic lectin CEL-III from marine invertebrate Cucumaria echinata

CEL-III, a galactose/N-acetylgatactosamine (Gal/ GalNAc)-specific lectin purified from a marine invertebrate, Cucumaria echinata, has a strong hemolytic activity, especially toward human and rabbit erythrocytes in the presence of Ca²⁺. We evaluated the role of Ca²⁺ in hemagglutinating and hemolytic activities of CEL-III. We found that Ca²⁺ is closely associated with both activities of CEL-III. The fluorescence spectra of CEL-III upon binding to Ca²⁺ were measured. The result showed a structural change of CEL-III in the presence of Ca²⁺. The structural change of CEL-UI upon Ca²⁺ binding was further demonstrated by stabilization against urea denaturation and by insusceptibility to protease digestions. CEL-III was completely unfolded at a low concentration of 2 M urea, while CEL-III complexed with Ca²⁺ was stable in 6 M urea. As for protease digestions, CEL-III monomer and oligomer were readily digested by trypsin. chymotrypsin, and papain in the absence of Ca²⁺, while they were insusceptible to the three proteases in the presence of Ca²⁺. The papain digestion of the decalcified oligomer produced a large C-terminal peptide, suggestting that the C-terminal region of CEL-III may participate in oligomerization of CEL-III as a core domain.