Nucleotide insertions in the
ferritin light chain (FTL)
polypeptide gene cause
hereditary ferritinopathy, a
neurodegenerative disease characterized by abnormal accumulation of
ferritin and
iron in the central nervous system. Here we describe for the first time the
protein structure and
iron storage function of the FTL mutant p.Phe167SerfsX26 (MT-FTL), which has a C terminus altered in sequence and extended in length. MT-FTL
polypeptides assembled spontaneously into soluble, spherical 24-mers that were ultrastructurally indistinguishable from those of the wild type. Far-UV CD showed a decrease in alpha-helical content, and
8-anilino-1-naphthalenesulfonate fluorescence revealed the appearance of hydrophobic binding sites. Near-UV CD and proteolysis studies suggested little or no structural alteration outside of the C-terminal region. In contrast to wild type, MT-FTL homopolymers precipitated at much lower
iron loading, had a diminished capacity to incorporate
iron, and were less thermostable. However, precipitation was significantly reversed by addition of
iron chelators both in vitro and in vivo. Our results reveal substantial
protein conformational changes localized at the 4-fold pore of MT-FTL homopolymers and imply that the C terminus of the MT-FTL
polypeptide plays an important role in
ferritin solubility, stability, and
iron management. We propose that the protrusion of some portion of the C terminus above the spherical shell allows it to cross-link with other mutant
polypeptides through
iron bridging, leading to enhanced mutant precipitation by
iron. Our data suggest that
hereditary ferritinopathy pathogenesis is likely to result from a combination of reduction in
iron storage function and enhanced toxicity associated with
iron-induced
ferritin aggregates.