STRUCTURE OF THE COMPLEX OF MACLURA-POMIFERA AGGLUTININ AND THE T-ANTIGEN DISACCHARIDE, GAL-BETA-1,3GALNAC

Maclura pomifera agglutinin is a tetrameric plant seed lectin with hig h affinity for the tumor-associated T-antigen disaccharide, Gal beta 1 ,3GalNAc alpha, and hence for many O-linked glycopeptide structures, U nlike members of most lectin families, it lacks both metal ions and Cy s residues. The structure of its complex with Gal beta 1,3GalNAc was d etermined to 2.2 Angstrom by first using multiwavelength anomalous dif fraction 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 beta-prism archi tecture and three-fold pseudo-symmetry of the related lectin jacalin, with the 21-residue beta-chains in the center of the tetramer, Interac tions 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 re sidue, the amino group of the N-terminal Gly of the alpha-chain, and p eptide backbone atoms of two aromatic residues. The Gal moiety does no t H-bond directly with residues in the same monomer, i.e. there is no true subsite for it, but there are interactions through two water mole cules. In the crystal, it interacts with residues in the binding site of an adjacent tet tetramer. The minimum energy conformation expected for the disaccharide is retained, despite its mediating the tetramer t etramer interactions in the crystal packing. The resulting lattice is comparable to those seen for complexes of other lectins with branched glycopeptides.