Maclura pomifera agglutinin is a tetrameric plant seed lectin with high affinity for the tumor-associated T-antigen disaccharide, Gal␤1,3GalNAc␣, and hence for many O-linked glycopeptide structures. Unlike members of most lectin families, it lacks both metal ions and Cys residues. The structure of its complex with Gal␤1,3GalNAc was determined to 2.2 Å by first using multiwavelength anomalous diffraction with a lead derivative of the native protein, and then using molecular replacement with the unrefined structure as a model to solve the structure of the complex. The subunits share the ␤-prism architecture and three-fold pseudo-symmetry of the related lectin jacalin, with the 21-residue ␤-chains in the center of the tetramer. Interactions with the GalNAc predominate in the binding of the disaccharide. It forms a network of H-bonds with only one side chain, from an Asp residue, the amino group of the Nterminal Gly of the ␣-chain, and peptide backbone atoms of two aromatic residues. The Gal moiety does not Hbond directly with residues in the same monomer, i.e. there is no true subsite for it, but there are interactions through two water molecules. In the crystal, it interacts with residues in the binding site of an adjacent tetramer. The minimum energy conformation expected for the disaccharide is retained, despite its mediating the tetramer-tetramer interactions in the crystal packing. The resulting lattice is comparable to those seen for complexes of other lectins with branched glycopeptides.

Amino acid sequences fo," tire ~.chains of Ih~ Moraceae leetins, ja~aiin and Mnchara ponlq~cru nsviutinin, w~re tielermined by protein sequen¢inv, Both arc 13)residues Ionli and eonlain several genetically variant positions; the overall homology is 8~'~. A possible :die for the known ltlYCOl~ptid,~ ol'jacalin was located, The =.chain= have a conserved tryptophaa residue that may he part of the binding,.site.

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