Physical-chemical characterization and carbohydrate-binding activity of the A and B subunits of the Bandeiraea simplicifolia I isolectins

Abstract

B. simplicifolia I plant seed isolectins comprise a family of tetrameric .alpha.-D-galactopyranosyl-binding glycoproteins composed of various combinations of 2 different kinds of subunits designated A and B. Subtypes of the A (Aa, Ab, Ac, Ad, and Ae) and B (Ba, Bb, Bc, Bd, and Be) subunits were demonstrated by isoelectric focusing in 8 M urea. Although the content of subunit subtypes varies from seed to seed (e.g., some seeds contain only B subunits, others only A subunits), subtypes Ac and Bc predominate in a natural mixture of the isolectins. Two-dimensional agar gel diffusion studies indicate that, in addition to common structural features, each subunit contains its own distinct antigenic determinants. Although the A and B subunits have closely similar amino acid compositions, they differ markedly in one respect: the B subunit has 1 methionine residue whereas the A subunit contains no methionine. The neutral carbohydrate content of both subunits is identical. The ability of bipolymers and synthetic glycoproteins to precipitate A4 and B4, as well as the capacity of sugars and oligosaccharides to inhibit precipitate formation, was examined. Hydrogen bonding evidently occurs between the hydrogen atoms of the C-3 and C-4 hydroxyl groups of .alpha.-D-GalNAcp and .alpha.-D-Galp units and the A and B subunits, respectively.