Xylem Sap Lectin, XSP30, Recognizes GlcNAc Sugar Chains of Glycoproteins in Cucumber Leaves

Abstract

The lectin activity of 30-kDa xylem sap protein (XSP30) found in cucumber (Cucumis sativas L.) xylem sap was analyzed by lectin blot coupled with immunological detection of XSP30. XSP30 is homologous to galactose-binding lectin, but it had only weak binding activity against asialofetuin, an animal glycoprotein with a terminal galactose. This reaction was not reduced by galactosidase treatement of asialofetuin. XSP30 bound strongly to soybean agglutinin, whose sugar chains consist solely of mannose and N-acetylglucosamine (GlcNAc), but bound only weakly to soybean peroxidase and γ-globulin, whose GlcNAc is fucosylated. The binding activity was inhibited by tri-N-acetylchitotriose (GlcNAc)₃. Leaf parenchyma cells were stained with XSP30 and aniti-XSP30 antibody; this staining was reduced by proteinase treatment of the sections, and XSP30 bound to proteins in leaf particulate fraction. These results suggest that XSP30 transported from root via xylem sap binds to chitobiose, GlcNAc-GlcNAc, in N-linked glycans of leaf glycoproteins.