Sialic acid-binding motif of Maackia amurensis lectins

Maackia amurensis hemagglutinin (MAH) and leukoagglutinin (MAL) are leguminous lectins which recognize carbohydrate chains containing sialic acid residues linked alpha 2,3 to penultimate galactose residues. In the present investigation, cDNA clones encoding MAL were isolated from a cDNA library constructed from germinated Maackia amurensis seeds and sequenced. From the reading frame of the cloned cDNAs, MAL was predicted to be composed of 287 amino acid residues, and showed strong similarity to MAH (86.2% identity). In leguminous lectins, most amino acid residues involved in sugar-binding were previously shown to be conserved. However, in both MAL and MAH lectins, the conserved glycine and asparagine were shown to be substituted by lysine and aspartic acid, respectively. Substitutions were made at position 105 and/or 135 of MAH to examine the roles of amino acid residues postulated to be important in binding to sialic acids. Recombinant MAH bound to the sialic acid-containing CB-II glycopeptide of human glycophorin A. By contrast, mutant lectins with lysine-105 substituted with glycine and/or aspartic acid-135 with asparagine did not bind to sialic acid residues. This indicates that these characteristic substitutions are important in sialic acid binding.