X-ray diffraction and atomic force microscopy analysis of twinned crystals: rhombohedral canavalin

The structure of canavalin, the vicilin-class storage protein from jack bea n, was refined to 1.7 Angstrom resolution in a highly twinned rhombohedral crystal of space group R3 and unit-cell parameters a = b = c = 83.0 Angstro m, alpha = beta = gamma = 111.1 degrees. The resulting R and R-free were 0. 176 and 0.245, respectively. The orthorhombic crystal structure (space grou p C222(1), unit-cell parameters a = 136.5, b = 150.3, c = 133.4 Angstrom) w as also refined with threefold non-crystallographic symmetry restraints. R and R-free were 0.181 and 0.226, respectively, for 2.6 Angstrom resolution data. No significant difference in the protein structure was seen between t hese two crystal forms, nor between these two and the hexagonal and cubic c rystal forms reported elsewhere [Ko et al. (1993), Acta Cryst. D49, 478-489 ; Ko et al. (1993), Plant Physiol. 101, 729-744]. A phosphate ion was ident ified in the lumen of the C-terminal beta -barrel. Lattice interactions sho wed that the trimeric molecule could be well accommodated in both 'top-up' and 'bottom-up' orientations in a rhombohedral unit cell of the R3 crystal and explained the presence of a high twin fraction. The large inter-trimer stacking interface of the C2221 crystal may account for its relative stabil ity. Atomic force microscopy (AFM) investigations of the growth of three cr ystal forms of canavalin indicate the rhombohedral form to be unique. Unlik e the other two crystal forms, it contains at least an order of magnitude m ore screw dislocations and stacking faults than any other macromolecular cr ystal yet studied, and it alone grows principally by generation of steps fr om the screw dislocations. The unusually high occurrence of the screw dislo cations and stacking faults is attributed to mechanical stress produced by the alternate molecular orientations in the rhombohedral crystals and their organization into discrete domains or blocks. At boundaries of alternate d omains, lattice strain is relieved by the formation of the screw dislocatio ns.