Recognition profile of Morus nigra agglutinin (Morniga G) expressed by monomeric ligands, simple clusters and mammalian polyvalent glycotopes

The carbohydrate binding properties of a novel member of the subfamily of galactose-specific jacalin-related lectin isolated from the bark of black mulberry (Morus nigra) (Morniga G) was studied in detail by enzyme-linked lectinosorbent and inhibition assays using panels of monomeric saccharides, mammalian polyvalent glycotopes and polysaccharides. Among the natural glycans tested for lectin binding, Morniga G reacted best with glycoproteins (gps) presenting a high density of tumor-associated carbohydrate antigens Tn (GalNAcα1-Ser/Thr) and Tα (Galβ1-3GalNAcα1-). Their reactivities, on a nanogram basis, were up to 72.5, 3.9 × 103, 6.0 × 103, 8.8 × 103 and 2.9 × 104 times higher than that of Tn-containing glycopeptides (M.W. < 3000 Da), monomeric T, Tn, GalNAc and Gal, respectively. It also reacted well with many multi-antennary N-glycans with II (Galβ1-4GlcNAc) termini, ABH histo-blood group antigens and their precursors containing high densities of I/II and T/Tn glycotopes, and sialylated T/Tn. Among the mono-, di- and oligosaccharides tested, Thomsen-Friedenreich (T) disaccharide with aromatic aglycon [Galβ1-3GalNAcα1-benzyl (Tα1-benzyl)] and Tn glycopeptides were the best inhibitors. Molecular modeling and docking studies indicated the occurrence of a primary GalNAcα1- and Galβ1-3GalNAc glycotope-binding site in Morniga G. Using a recently proposed system [Wu, A.M., 2003. Carbohydrate structural units in glycoproteins and polysaccharides as important ligands for Gal and GalNAc reactive lectins. J. Biomed. Sci. 10, 676-688], the binding properties of the combining sites of Morniga G can be defined as follows: (i) the monosaccharide specificity is GalNAc/Gal ≫ Man/Glc, GlcNAc and lFuc; (ii) the mammalian glycotope specificity is Tα1-benzyl > T > Tn > GalNAcβ1-3Gal (P), while B/E (Galα1-3/4Gal), I/II (Galβ1-3/4GlcNAc), S (GalNAcβ1-4Gal), F/A (GalNAcα1-3GalNAc/Gal) and L (Galβ1-4Glc) are inactive; (iii) the most active ligand is T/Tn; (iv) simple clustered Tn or triantennary N-glycans with II termini (Tri-II) have limited impact; (v) high-density polyvalent glycotopes play a prominent role for enhancing Morniga G reactivity. These results provide evidence for the binding of this lectin to dense cell surface T/Tn glycoconjugates and facilitate future usage of this lectin in biotechnological and medical applications.