Sialylated Oligosaccharide-specific Plant Lectin from Japanese Elderberry (Sambucus sieboldiana) Bark Tissue Has a Homologous Structure to Type II Ribosome-inactivating Proteins, Ricin and Abrin

Bark lectins from the elderberry species belonging to the genus Sambucus have a unique carbohydrate binding specificity for sialylated glycoconjugates containing NeuAc(alpha 2-6)Gal/GalNAc sequence. To elucidate the structure of the elderberry lectin, a cDNA library was constructed from the mRNA isolated from the bark tissue of Japanese elderberry (Sambucus sieboldiana) with lambda gt11 phage and screened with anti-S. sieboldiana agglutinin (SSA) antibody. The nucleotide sequence of a cDNA clone encoding full-length SSA (LecSSA1) showed the presence of an open reading frame with 1902 base pairs, which corresponded to 570 amino acid residues. This open reading frame encoded a signal peptide and a linker region (19 amino acid residues) between the two subunits of SSA, the hydrophobic (A-chain) and hydrophilic (B-chain) subunits. This indicates that SSA is synthesized as a preproprotein and post-translationally cleaved into two mature subunits. Homology searching as well as molecular modeling studies unexpectedly revealed that each subunit of SSA has a highly homologous structure to the galactose-specific lectin subunit and ribosome-inactivating subunit of plant toxic proteins such as ricin and abrin, indicating a close evolutionary relationship between these carbohydrate-binding proteins.