Cross-Linking Activity of the 14-Kilodalton β-Galactoside-Specific Vertebrate Lectin with Asialofetuin: Comparison with Several Galactose-Specific Plant Lectins

We have previously shown that plant lectins with a wide range of carbohydrate binding specificities can bind and cross-link (precipitate) specific multiantennary oligosaccharides and glycopeptides [cf. Bhattacharyya, L., Fant, J., Lonn, H., & Brewer, C.F. (1990) Biochemistry 29, 7523-7530]. This leads to a new source of binding specificity: namely, the formation of homogeneous cross-linked lattices between lectins and carbohydrates. Recently, we have demonstrated the existence of highly ordered cross-linked lattices that form between the D-Man/D-Glc-specific plant lectin concanavalin A and the soybean agglutinin which is a tetrameric glycoprotein possessing a single Man9 oligomannose chain per monomer [Khan, M.I., Mandal, D.K., & Brewer, C.F. (1991) Carbohydr. Res. 213, 69-77]. In the present study, we have compared the ability of the 14-kDa β-galactoside-specific lectin from calf spleen, a dimeric S-type animal lectin, and several galactose-specific plant lectins from Erythrina indica, Erythrina cristagalli, and Glycine max (soybean agglutinin) to form specific cross-linked complexes with asialofetuin (ASF), a 48-kDa monomeric glycoprotein, using quantitative precipitation analyses. The results show the formation of 1:9 and 1:3 stoichiometric cross-linked complexes (per monomer) of ASF to the 14-kDa lectin, depending on their relative ratio in solution. Evidence indicates that the three triantennary N-linked complex-type oligosaccharide chains of ASF mediate the cross-linking interactions and that each chain expresses either trivalency in the 1:9 cross-linked complex or univalency in the 1:3 complex. The two dimeric Erythrina lectins also form 1:9 and 1:3 ASF-lectin cross-linked complexes as well as a lower ratio complex at high relative concentrations of ASF. In the presence of tetrameric soybean agglutinin, only a 1:3 ASF-lectin cross-linked complex is formed, presumably due to the larger size of the agglutinin. Unlike the plant lectins, the 14-kDa lectin fails to precipitate with the free triantennary glycopeptide or oligosaccharide from ASF, or with other related branched-chain carbohydrates, which suggests that this may be an important difference in the cross-linking activities of the animal lectin compared to the plant lectins. Insight has also been obtained into the conformational properties of the triantennary oligosaccharide in cross-linked complexes, and the factors affecting the valency of the oligosaccharide chain attached to a protein matrix. The present results thus demonstrate that the 14-kDa animal lectin possesses similar but distinct cross-linking activities from several Gal-specific plant lectins toward a glycoprotein with well-defined carbohydrate epitopes. The findings are discussed in relation to the biological properties of lectins and their corresponding glycoconjugate receptors.