Localized Unfolding at the Junction of Three Ferritin Subunits. A MECHANISM FOR IRON RELEASE? (original) (raw)

1998, Journal of Biological Chemistry

How and where iron exits from ferritin for cellular use is unknown. Twenty-four protein subunits create a cavity in ferritin where iron is concentrated >10 11-fold as a mineral. Proline substitution for conserved leucine 134 (L134P) allowed normal assembly but increased iron exit rates. X-ray crystallography of H-L134P ferritin revealed localized unfolding at the 3-fold axis, also iron entry sites, consistent with shared use sites for iron exit and entry. The junction of three ferritin subunits appears to be a dynamic aperture with a "shutter" that cytoplasmic factors might open or close to regulate iron release in vivo. Ferritins are vesicle-like assemblies of 24 polypeptide (4helix bundle) subunits that concentrate iron in cells by directing the formation of a ferric mineral in the hollow protein interior (8 nm diameter) (1-3). Effective cellular iron concentrations Ͼ10 11 times the solubility of the ferric ion are achieved by ferritins, which are found in microorganisms, plants, and animals. The complexity and the sophistication of the genetic regulation of the ferritins, involving both DNA and mRNA (4-7), emphasize the central role of iron and ferritin in life. Rates of Fe(II) oxidation, translocation of Fe(II) and Fe(III) (1.0-2.0 nm), and mineralization are all controlled by the protein (1, 2). Fe(II) release from ferritin following reduction of the mineral is slow and poorly understood (8, 9) but is important for the biosynthesis of iron-proteins, such as those required in respiration, photosynthesis, nitrogen fixation, and cell division, (1, 2) and as dietary iron (10). How and where the iron exits from ferritin in vivo is not known. We now show that localized unfolding in the assembled protein, at sites of cooperative subunit interactions, can increase