DETERMINATION OF 3 CRYSTAL-STRUCTURES OF CANAVALIN BY MOLECULAR REPLACEMENT

Canavalin, the major reserve protein of the jack bean, was obtained in four different crystal forms. From the structure determined by multip le isomorphous replacement in a hexagonal unit cell, the structures of three other crystals were determined by molecular replacement. In two cases, the rhombohedral and cubic crystals, placement was facilitated by coincidence of threefold molecular symmetry with crystallographic operators. In the orthorhombic crystal the canavalin trimer was the as ymmetric unit. The rhombohedral, orthorhombic and cubic crystal struct ures were subsequently refined using a combination of several approach es with resulting R factors of 0.194, 0.185 and 0.211 at resolutions o f 2.6, 2.6 and 2.3 angstrom, respectively. Variation in the conformati on of the molecule from crystal to crystal was small with an r.m.s. de viation in Calpha positions of 0.89 angstrom. Packing is quite differe nt among crystal forms but lattice interactions appear to play little role in the conformation of the molecule. Greatest variations in mean position are for those residues that also exhibit the greatest thermal motion. Crystal contacts in all crystals are mediated almost exclusiv ely by hydrophilic side chains, and three to six intermolecular salt b ridges per protein subunit are present in each case.