Carbohydrate specificity of a lectin isolated from the fungus Sclerotium rolfsii

In order to investigate the functional roles of a phytopathogenic fungal lectin (SRL) isolated from the bodies of Sclerotium rolfsii, the binding properties of SRL were studied by enzyme linked lectinosorbent assay and by inhibition of SRL-glycan interaction. Among glycoproteins(gp) tested for binding, SRL reacted strongly with GalNAcα1→Ser/Thr (Tn) and /or Galβ1→3GalNAcα1→(T_α) containing gps: human T_α and Tn glycophorin, asialo salivary gps, and asialofetuin, but its reactivity toward sialylated glycoproteins was reduced significantly. Of the sugar ligands tested for inhibition of SRL-asialofetuin binding, Thomsen-Friedenreich residue (T_α) was the best, being 22.4 and 2.24×10³ more active than GalNAc and Galβ1→ residues, respectively. Other ligands tested were inactive. When the glycans used as inhibitors, T_α, and/or Tn containing gps, especially asialo PSM, asialo BSM, asialo OSM, active antifreeze gp, asialo glycophorin and Tn-glycophorin were very active, and 1.0×10⁴ times more potent than GalNAc. From these results, it is clear that the combining site of SRL should be of a cavity type and recognizes only Tn and T_α residues of glycans; it is suggested that T_α and Tn glycotopes, which are present only in abnormal carbohydrate sequences of higher orders of mammal, are the most likely sites for phytopathogenic fungal attachment as an initial step of infection. The affinity of SRL for ligands can be ranked in decreasing order as follows: multivalent T_α and Tn ≫ monomeric T_α and Tn > GalNAc ≫> II (Galβ1→4GlcNAc), L (Galβ1→4Glc), and Gal.