Identification of the bile acid-binding region in the soy glycinin A1aB1b subunit

Soy glycinin has five major subunits which are classified into two groups according to their homology in amino acid sequences (group I, A1aB1b, A1bB2 and A2B1a; group II, A3B4 and A5A4B3). It has been reported that the peptide fragments derived from the A1a and A2 chains of the A1aB1b and A2B1a subunits had bile acid-binding ability and that the region of 114-161 residues of the A1a chain was responsible for this bile acid-binding ability. In this study, we constructed A1a, A3 and 9 deletion mutants of A1a lacking various numbers of residues at the C-terminus, and evaluated their bile acid-binding ability by a cholic acid-conjugated column and fluorescence analysis. The bile acid-binding ability of A1a was higher than that of A3 and there was a remarkable decrease in the bile acid-binding ability between the Δ[138-291] and Δ[130-291] mutants. The 130-138 region is rich in hydrophobic residues. In this regard, when we constructed the Δ[129-134] mutant lacking six contiguous hydrophobic residues (VAWWMY) and evaluated its bile acid-binding ability, a similar remarkable decrease in the bile acid-binding ability was observed. These results indicate that the 129-134 residue region (VAWWMY) with high hydrophobicity was important for bile acid-binding of A1a.