Preliminary analysis of amphibian red cell M ferritin in a novel tetragonal unit cell

The ferritins are a multigene family of proteins that concentrate and store iron in all prokaryotic and eukaryotic cells. 24 monomeric subunits which fold as four-helix bundles assemble to form a protein shell with 432 cubic symmetry and an external diameter of approximately 130 A. The iron is stored inside the protein shell as a mineralized core approximately 80 A in diameter. Recombinant amphibian red cell M ferritin crystallizes in approximately 2 M (NH(4))(2)SO(4) at pH 4.6 in a space group that has not been reported previously. Electron microscopy, precession photography, Patterson and Fourier maps of the native protein and a UO(2)(2+) derivative, and simulations were used to determine that the unit-cell dimensions are a = b = 169.6, c = 481.2 A, alpha = beta = gamma = 90 degrees and the space group is P4(1)2(1)2 or P4(3)2(1)2. A preliminary model of the structure was obtained by molecular replacement, with amphibian red cell L ferritin as the model. In contrast to previously determined ferritin crystal structures which have intermolecular contacts at the twofold and threefold molecular axes, M ferritin crystals have a novel intermolecular interaction mediated by interdigitation of the DE loops of two molecules at the fourfold molecular axes.