A Molecular Model for the QuaternaryStructure of Ferritin John M. A. Smith, Robert F. D. Stansfield, Geothey C. Ford, Jan L. White, and Pauline M. Harrison Department of Biochemistry, University of Sheffield, Sheffield, S10 2TN, England The iron-storage protein, ferritin, arouses great interest. It plays a vital physiological role both in single cells and in higher organisms, where it can act as an iron reserve for the same or other cells'.2. By building an inert, iron-containing core of a mineral called ferrihydrite inside a spherical envelope formed by the symmetrical association of protein chains, its molecular architecture allows the toxic effects of free iron to be a v ~ i d e d ~How , ~ . iron nets in and out of this protein cage is a fascinating story, thesubject of much work and many hypo these^'-^. The iron core, once fully built, is electron-dense and 6-7 nm in diameter. I t can readily be imaged by electron microscopy and is frequently used as a marker. This, combined with its availability a t relatively low cost, makes i t an ideal compound for classes in electron microscopy. Here, however, we take a closer look at its protein shell and with the aid of a simple model show how repeated interactions between its chains oroduce an ordered a&emblage somewhat resembling a geudcsic dome. 'I'he three-dimensional structure of iron-free ferritin isolated from horse splepn has recently been determined and refined by X-ray crystallographic techniques.' (Fig. I ) . The moleculeconsistsof 24 protein chains related hy 432 svmmetry. .4pproxirnately :sro of each protein chain, or suhunit, is in the form of u helix. There are four lone helices formine a bundle, and a much shorter fifth helix toward the C terginus.. lvine - a t an acute anele to the bundle. The interhelical turns are all of different Laracter. The complete molecule is approximately spherical, but its underlying architecture is better represented as a rhombic dodecahedron. In this Archimedean solid, 12 rbomhs are assembled about fourfold and threefold rotation axes into a geometrically exact shape. Each rhomb face comprises two protein subunits related hv a twofold symmetry -axis perpendicular to that face and passing through its center. The consequent packing of subunits involves extensive interactions between various regions of the protein structure, including very short regions of anti~arallelB sheet (about 1%).each of which mav he explorkd with reference to the published crystal structure. The net illustrated in Figure 2, when mounted on card and assembled, provides a useful rhombic dodecahedral molecular model of ferritin. This model aids understanding of (1) how ferritin is assembled from its subunits via intermediates of two, three, six, and twelve chain^,^ (2) how protein subunits are interrelated by 432 symmetry, (3) how particular inter-subunit interactions are generated, and, (4) how the molecular architecture provides access for storage iron
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' Theil, E. C . Ann. Rev. Biochem. 1987, 56, 289.
Harrison, P. M. Biochem. Educ. 1986, 14, 154. Ford. G. C.; Harrison, P. M.; Rice, D . W.; Smith. J. M. A,; Treffry. A.; White, J. L.; Yariv. J. Phil. Trans. Roy. Soc. Lond. 1984,8304,551. Gerl, M.; Jaenicke, R . 8/01. Chem. Hoppe-Seyler 1987, 368, 387, and Gerl, M.; Jaenicke, R.;Smith. J. M. A,; Harrison. P. M. Biochemis try 1988,27.4089.
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Figure 1. The
structure of the apoferritin molecule as determined by X-ray crystallography. The molecuie comprises 24 symmetry related subunits forming a hollow spherical shell. In this diagram,taken from the molecular graphics display, the subunits are represented by the connected prdein main chain atoms alone and are viewed down a molecular fourfold axis.
through inter-subunit pores. On each rhombic face of the model a diad-related pair of subunits is inscribed. In these, cylinders represent helices, and arrows represent the short sections of antiparallel sheet found where the long external loops from twosubunits come together. At six apices of the rhombic dodecahedron the protein subunits are related by fourfold symmetry and a t another eight apices by threefold symmetry. Around these symmetry axes channels are generated that mav provide the means bv which iron traverses the shell to and from the molecule's inierior. As with all bioloeical molecules. the ferritin shell. d e s ~ i t e its high symmetry,is "handed". This is because each oi its amino acids and its helices are of a sinele hand and. thus. each of its subunits is asymmetric. As ar< there can be nd mirror planes within the molecule. Althoueb two mirror image versions of the model can be constructed, only one of these corresponds to natural ferritin. The handedness of the molecule is made evident by looking down a threefold symmetry axis where the triad-related subunits form a lefthanded propellor. Exact or distorted rhombic dodecahedral crystal morphology is commonly encountered in studies of crystals with cubic or tetragonal space groups, where, in accordance with Volume 65 Number 12 December 1988
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Figure 2. Net for the assembly of a ferritin mcdei. Mount the net on card and cut around the edges. Fold back the tabs, colored black, and make the folds between edjoining rhomb faces. Gentle scoring an the reverse side may ease folding. Finally assemble the model by gluing each tab to the back of its adjaintng face.
the law of rational indices, elements of t h e lattice symmetry a r e reflected i n the overall s h a p e of t h e crystal. T h e s e dodecahedral models have been found to be invaluable in t h e optical alignment of s u c h crystals, a n d particularly of their fragments.
Acknowledgement W e t h a n k t h e Science & Engineering Research Council, the ~ ~ d~~~~~~~h i ~ council, ~ l and the ~ ~T~~~~ forl financial support,
High School Teachers Offered Minigrants for STS Materials Development A new program offering high school teachers up to $1,000 in materials-development funds has just been announced by the American Chemical Society. The ACS-STS Minigrants will fund projects focusing on the relations among science, technology, and society (STS) issues. lessons and resources that mieht be used over Teachers mav use the erant funds either to oroduee their own.. orieinal " two to four days, or they m a design n curriculum mudulc using existing malrrinla to tumprise a four- to nine-week unit. Hands-on activities and an emphnais on problem-rulving, deaion-mnkiny, and critical-thinking skdls are pnrtrrularly encouraged. The goals of the ACS-STS Minigrants are to helpstudents understandscience in its societal context, to assist teachers in developing and implementing innovative curriculum, and to form a cadre of experienced teachers and a selection of programs t o serve a models for continued evolution of science programs. The project director must be a high school teacher who is a member or national affiliate of the American Chemical Societv. Other oartici~antsmav include other teachers.. colleee " and universitv orofessors. or members of local industrv. The availahilitv of matchine"~ funds is also an imoartant consideration. The annlication deadline for nroieets scheduled to ~.~~~~~~~~ begin in the fail of 1989 is March 31,1989. For details and an application packet call or write David Lieata, Manager, Office of Precollege Science, American Chemical Society, 1155 Sixteenth St., NW, Washington, DC 20036, (202)872-4590. ~
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