Elderberry bark lectins evolved to recognize Neu5Ac alpha2,6Gal/GalNAc sequence from a Gal/GalNAc binding lectin through the substitution of amino-acid residues critical for the binding to sialic acid.

Bark lectins from the elderberry plants belonging to the genus Sambucus specifically bind to Neu5Acalpha2,6Gal/GalNAc sequence and have long been used for the analysis of sialoglycoconjugates that play important roles in many biological phenomena. However, molecular basis of such a unique carbohydrate binding specificity has not been understood. To answer these questions, we tried to identify the amino-acid residues in the Japanese elderberry bark lectin, Sambucus sieboldiana agglutinin that enabled the lectin to recognize sialic acid by using in silico docking simulation and site-directed mutagenesis. These studies showed that three amino-acid residues, S(197), A(233) and Q(234), in the C-terminal subdomain of SSA-B chain are critical for the binding to the sialic acid in Neu5Acalpha2,6Gal/GalNAc sequence. Replacement of one of these residues to the one in the corresponding position of ricin B-chain completely abolished the binding to a sialoglycoprotein, fetuin. Conserved presence of these amino acid residues in the corresponding sequences of two other elderberry lectins with similar binding specificity further supported the conclusion. These findings indicated that the replacement of the corresponding amino-acid residues in a putative Gal/GalNAc-specific ancestral lectin to these amino-acid residues generated the unique Neu5Acalpha2,6Gal/GalNAc-specific elderberry lectins in the course of molecular evolution.