Purification and saccharide-binding characteristics of a rice lectin

Isolation of Rice Bran Lectins and Characterization of Their Unique Behavior in Caco-2 Cells

Rice bran lectins, named as RBA1 and RBA2, were isolated from Oryza sativa in two chromatography steps: affinity chromatography and cation-exchange chromatography. RBA1 was found to be composed of a covalently linked heterodimer of 20- and 12-kDa subunits, and RBA2 was a noncovalently linked dimer of 12-kDa subunits. Both RBA1 and RBA2 bound to desialylated complex glycoproteins such as fetuin, α1-acid glycoprotein, and transferrin, and agalactosylated complex glycoproteins such as agalacto fetuin, agalacto-α1-acid glycoprotein, and agalacto-transferrin, in addition to chitooligosacchrides. RBAs were heat stable up to 80 °C and stable at pH 4–10. RBA1 increased the transport of the fluorescent marker, rhodamine 123, which is known to be transported via the P-glycoprotein-mediated efflux pathway across human intestinal Caco-2 cell monolayers. Furthermore, RBA1 itself was transported to the basolateral side of the monolayers via an endocytotic pathway.

Interaction of asparagine-linked oligosaccharides with an immobilized rice (Oryza sativa) lectin column

The carbohydrate-binding specificity of rice (Oryza sativa) lectin was investigated by testing the ability of radioactively labelled glycopeptides and oligosaccharides to bind to a rice lectin-Sepharose 4B column. Rice lectin binds asparagine-linked oligosaccharides through the core NN'-diacetylchitobiose moiety. Whereas beta 1-4-mannose enhances the binding strength only to a small extent, alpha 1-3-linked core mannose increases it considerably. A core alpha 1-6-linked mannose residue has a slightly inhibitory effect. Binding is not affected when one or both of the alpha-mannose residues are substituted with mannose at C-2, C-3 and C-6 or with N-acetylglucosamine (GlcNAc) at C-2 positions. The presence of an alpha 1-6-fucose residue attached to the asparagine-linked GlcNAc also does not affect the binding. The binding of complex biantennary glycopeptides is not altered by the presence of one or two galactose residues in the non-reducing terminus, but the presence of one or two sialic acid residues decreases the binding capacity. A bisecting beta 1-4-linked GlcNAc attached to beta-linked mannose residue enhances the binding of sialo, asialo and asialoagalacto complex biantennary-type glycopeptides. Bisected hybrid-type glycopeptides bind very tightly to a rice lectin-Sepharose 4B column: Substitution of alpha 1-3-mannose residue at C-2 and C-4 with GlcNAc completely inhibits the binding of both bisected and non-bisected complex asparagine-linked glycopeptides. O-Glycosidically linked oligosaccharides containing GlcNAc bind very weakly to a rice lectin column. The applicability of immobilized rice lectin columns in the fractionation of asparagine-linked oligosaccharides is discussed.