ULTRASTRUCTURE OF FERRITIN AND APOFERRITIN - A REVIEW

Ferritin is the major protein that binds non-heme iron inside cells. This review summarizes present knowledge about the molecular structure of ferritin and apoferritin, with emphasis on the research results obtained by electron microscopy. The ultrastructural organization of the ferritin core, the atomic-level structure of the biomineralized iron (ferrihydrite) forming the core, the quaternary architecture of the protein shell, and the interactions of this protein with iron and the core, are reviewed. For each of these fundamental topics, important research results, hypotheses and current controversies are critically discussed; each part ends with a summary of major structural conclusions that are supported by experimental evidence and a brief presentation of some remaining research questions where electron microscope studies are likely to provide valuable answers. Special topics presented in this review include: (1) an enumeration of common misinterpretations, artifacts and mis-statements about the structure of ferritin, (2) the ultrastructure of ferritin crystals and paracrystals, (3) ultrastructural examination of ferritin and apoferritin molecules with the scanning transmission electron microscope, (4) electron microscopy of unstained frozen-hydrated ferritin and apoferritin, and (5) the status of ferritin as a favorable test object for developing instrumentation and specimen preparation techniques in biological electron microscopy.