X-RAY STRUCTURE OF WHEAT-GERM-AGGLUTININ ISOLECTIN-3

Wheat germ agglutinin isolectin 3 (WGA3) was crystallized from 10mM acetate buffer at pH4.9 containing 6 mM CaCl2 and 4%(v/v) ethanol. The crystal belongs to monoclinic space group P2(1) with unit-cell dimensions a = 44.86, b = 91.02, c = 44.86 Angstrom, and beta = 110.22 degrees. The asymmetric unit contains two molecules (V-m =-2.51 Angstrom(3) Da(-1)). The crystal. structure was solved by the molecular-replacement method and was refined by the simulated-annealing method. The conventional R value was 0.191 for 19713 reflections [\IFo\ > 3 sigma(F)] in the resolution range 8-1.9 Angstrom . The r.m.s, deviations from the ideal bond distances and angles were 0.014 Angstrom and 3.0 degrees, respectively, and the estimated coordinate error was 0.2-0.25 Angstrom. The two molecules in the asymmetric unit are related by the pseudo twofold symmetry and form a dimer structure. The backbone structures of the two subunits are nearly identical with the r.m.s. difference of 0.36 Angstrom for the superposition of equivalent C-alpha atoms. The dimer structure is very similar to those of isolectins 1 and 2 with the r.m.s, difference of 0.35-0.39 Angstrom for the C-alpha superposition. Since amino-acid residues which differ from those of isolectin 1 or 2 are not involved in the contact between the two subunits, the subunit-subunit interaction is not significantly affected by the replacement of these residues. As a result, the geometry of the sugar-binding sites which are located at the interface between the two subunit molecules is basically conserved among three isolectins.