Topological and functional characterization of the N-glycans of soybean (Glycine max) agglutinin.

Soybean agglutinin (SBA), is a noncovalently bound tetramer comprised of four identical subunits having a single N-glycan chain, Man9GlcNAc2, that is known to be essential for regeneration of the functional tetrameric structure from unfolded subunits. In this study, SBA was found to have strong affinity for concanavalin A, indicating that the N-glycans are extensively solvent-exposed. The susceptibilities of the N-glycans to alpha-mannosidase and endo-beta-N-acetylglucosaminidase revealed that their distal areas have nonreducing ends embedded among the subunits, whereas their proximal regions are solvent-exposed. Endo-beta-N-acetylglucosaminidase-digested SBA was unable to retain its conformation and gradually unfolded. Periodate-oxidized SBA, whose N-glycans closely correspond to the invariant pentasaccharide core, tended to dissociate into the subunits, but permitted to stay as folded monomers. This SBA species was capable of refolding from unfolded subunits but unable to form the functional tetramer. It seems probable that the proximal regions of the N-glycans function in the formation and stabilization of the subunit conformation, whereas the branches outside the invariant cores stabilize the tetrameric structure.