Primary structure of hemolytic lectin CEL-III from marine invertebrate Cucumaria echinata and its cDNA: Structural similarity to the B-chain from plant lectin, ricin

CEL-III, a galactose/N-acetylgalactosamine (Gal/GalNAc) specific lectin purified from a marine invertebrate Cucumaria echinata has a strong hemolytic activity especially toward human and rabbit erythrocytes. We determined the primary structure of the CEL-III by examining the amino acid sequences of the protein and the nucleotide sequence of the cDNA. The cDNA encoding CEL-III has 1823 nucleotides and an open reading frame of 1296 nucleotides. CEL-III is composed of 432 amino acid residues with a M(r) of 47 457 and has six internal tandem repeats, each with of 40-50 amino acids, comprising the N- terminal two-thirds of the molecule. Similar repeats are found in the B- chains of cytotoxic plant lectins, such as ricin and abrin, where six repetitive sequences extend throughout the molecules. A hydropathy plot predicts hydrophobic segments in the C-terminal region of CEL-III. These findings suggest that the N-terminal region of CEL-III plays an important role in binding to carbohydrate receptors on the target cell membranes, an event which triggers an intermolecular hydrophobic interaction of the C- terminal region, the result being oligomerization of CEL-III to lead to pore- formation in erythrocyte membrane.