Formation of the Ferritin Iron Mineral Occurs in Plastids (An X-Ray Absorption Spectroscopy Study

Ferritin in plants is a nuclear-encoded, multisubunit protein found in plastids; an N-terminal transit peptide targets the protein to the plastid, but the site for formation of the ferritin Fe mineral is unknown. In biology, ferritin is required to concentrate Fe to levels needed by cells (approximately 10–7 M), far above the solubility of the free ion (10–18 M); the protein directs the reversible phase transition of the hydrated metal ion in solution to hydrated Fe-oxo mineral. Low phosphate characterizes the solid-phase Fe mineral in the center of ferritin of the cytosolic animal ferritin, but high phosphate is the hallmark of Fe mineral in prokaryotic ferritin and plant (pea [Pisum sativum L.] seed) ferritin. Earlier studies using x-ray absorption spectroscopy showed that high concentrations of phosphate present during ferritin mineralization in vitro altered the local structure of Fe in the ferritin mineral so that it mimicked the prokaryotic type, whether the protein was from animals or bacteria. The use of x-ray absorption spectroscopy to analyze the Fe environment in pea-seed ferritin now shows that the natural ferritin mineral in plants has an Fe-P interaction at 3.26A, similar to that of bacterial ferritin; phosphate also prevented formation of the longer Fe-Fe interactions at 3.5A found in animal ferritins or in pea-seed ferritin reconstituted without phosphate. Such results indicate that ferritin mineralization occurs in the plastid, where the phosphate content is higher; a corollary is the existence of a plastid Fe uptake system to allow the concentration of Fe in the ferritin mineral.