The structure of apoferritin: Molecular size, shape and symmetry from X-ray data

Abstract

Apoferritin is the protein component of the iron-storage protein ferritin, and forms a shell round the iron oxide core. Iron-free apoferritin crystallizes in a cubic form isomorphous with one form of ferritin and having four molecules in a unit cell of side 184 A and apparent space group F 432. From the X-ray data apoferritin molecules have a molecular weight of 480,000 and a form approximating on the average at a resolution of 26 A to a spherical shell having an external radius of 61 ± 3 A and internal to external radius ratio about 0·6. The shell is constructed from subunits and chemical results suggest that there are about twenty of these. Recent X-ray photographs of wet apoferritin crystals are found to contain both sharp reflections, extending to spacings of 1/1·4 A −1 , and diffuse streaks indicative of a structure having “ordered disorder”. The X-ray patterns are interpreted as being due to molecules which are situated regularly on a face-centred lattice, but having different orientations at random, so that the apparent point-group 4:3:2† is produced statistically from molecules with lower symmetry. The diffraction patterns suggest that the molecules have pseudo-icosahedral symmetry, but not exact icosahedral nor tetrahedral symmetry. The molecules probably consist of twenty subunits situated at the vertices of a pentagonal dodecahedron with point group 5:2, 2:2:2 or 2. An examination of the diffraction pattern of orthorhombic ferritin shows that the symmetry is almost certainly 5:2.