Electron microscopy and X-ray diffraction studies of Lotus tetragonolobus A isolectin cross-linked with a divalent Lewis(x) oligosaccharide, an oncofetal antigen

The interactions of lectins with multivalent carbohydrates often leads to t he formation of highly ordered cross-linked lattices that are amenable to s tructural studies. A particularly well ordered, two-dimensional lattice is formed from fucose-specific isolectin A from Lotus tetragonolobus cross-lin ked with difucosyllacto-N-neohexaose, an oligosaccharide possessing the Lew is(x) determinant, which is an oncofetal antigen. A combination of electron microscopy, x-ray diffraction, simulation of electron micrographs, and mol ecular model building was used to determine the relative positions of the t etrameric lectin and bivalent carbohydrate within the lattice. X-ray diffra ction from unoriented pellets was used to determine the lattice dimensions and analysis of electron micrographs was used to determine the lattice symm etry. Molecular models of the lattice were constructed based on the known s tructure of the jack bean lectin concanavalin A and the high degree of sequ ence homology between the two lectins. Using the symmetry and dimensions of the lattice and its appearance in filtered electron micrographs, molecular models were used to determine the orientation of the lectin in the lattice , and to define the range of lectin-oligosaccharide interactions consistent with the structural data. The present study provides the first description of a highly ordered, two-dimensional, cross-linked lattice between a tetra valent lectin and a bivalent carbohydrate.