A unique amino acid sequence involved in the putative carbohydrate-binding domain of a legume lectin specific for sialylated carbohydrate chains: primary sequence determination of Maackia amurensis hemagglutinin (MAH).

The primary sequence of 247 amino acids of Maackia amurensis hemagglutinin (MAH) was determined using a protein sequencer. After digestion with endoproteinase Lys-C, Asp-N, Arg-C, or Glu-C of MAH, the resulting peptides were purified by reversed phase high performance liquid chromatography (HPLC) and then subjected to sequence analysis. The primary sequence of MAH was compared with those of several legume lectins, and it was found that the amino acid sequence of the putative carbohydrate-binding domain of MAH exhibited a high degree of homology with those of di-N-acetylchitobiose-binding Cytisus sessilifolius lectin I (CSA-I), Laburnum alpinum lectin I (LAA-I), and Ulex europaeus lectin II (UEA-II). In the legume lectins whose primary sequences have already been determined several amino acid residues involved in carbohydrate-binding were found to be conserved. Very interestingly, in the primary sequence of MAH, one amino acid residue corresponding to the conserved amino acid, asparagine, in the primary sequences of all other legume lectins was shown to be substituted by aspartic acid. This is the first report of the occurrence of an exceptional amino acid residue among the conserved amino acid residues in the carbohydrate-binding domain of the legume lectins.