KM+, a mannose-binding lectin from Artocarpus integrifolia: Amino acid sequence, predicted tertiary structure, carbohydrate recognition, and analysisof the beta-prism fold

The complete amino acid sequence of the lectin KM+ from Artocarpus integrif olia (jackfruit), which contains 149 residues/mol, is reported and compared to those of other members of the Moraceae family, particularly that of jac alin, also from jackfruit, with which it shares 52% sequence identity. KMpresents an acetyl-blocked N-terminus and is not posttranslationally modifi ed by proteolytic cleavage as is the case for jacalin. Rather, it possesses a short, glycine-rich linker that unites the regions homologous to the alp ha- and beta-chains of jacalin. The results of homology modeling implicate the linker sequence in sterically impeding rotation of the side chain of As p141 within the binding site pocket. As a consequence, the aspartic acid is locked into a conformation adequate only for the recognition of equatorial hydroxyl groups on the C4 epimeric center (alpha-D-mannose, alpha-D-glucos e, and their derivatives). In contrast, the internal cleavage of the jacali n chain permits free rotation of the homologous aspartic acid, rendering it capable of accepting hydrogen bonds from both possible hydroxyl configurat ions on C4. We suggest that, together with direct recognition of epimeric h ydroxyls and the steric exclusion of disfavored ligands, conformational res triction of the lectin should be considered to be a new mechanism by which selectivity may be built into carbohydrate binding sites. Jacalin and KM+ a dopt the beta-prism fold already observed in two unrelated protein families . Despite presenting little or no sequence similarity, an analysis of the b eta-prism reveals a canonical feature repeatedly present in all such struct ures, which is based on six largely hydrophobic residues within a beta-hair pin containing two classic-type beta-bulges. We suggest the term beta-prism motif to describe this feature.