Role of H and L Subunits in Mouse Ferritin

Abstract

Ferritin is an iron-binding protein composed of two subunits, H and L. Twenty-four of these subunits assemble to form apoferritins whose subunit composition varies in a characteristic way in different tissues. Using recombinant proteins, we have assessed the role of H and L subunits in mouse ferritin function and compared these to human ferritin subunits. We report that mouse ferritin subunits exhibit considerable functional similarity to their human counterparts, including a prominent role of the H subunit in the facilitation of rapid iron uptake, and a key role of amino acid residues Glu-62 and His-65 in this process. In addition, amino acid residues important to assembly of the protein are conserved between mouse and human, permitting the formation of fully functional hybrid proteins containing both mouse and human subunits. However, murine and human ferritin H subunits also evidenced substantial functional differences; murine ferritin H showed a consistent reduction in iron uptake activity relative to human ferritin H. Creation of chimeric human/mouse ferritin H subunits by "helix swapping" mapped the domain of the protein critical to this activity difference to the DE helix. These findings suggest a novel functional role for carboxyl-terminal domains of the ferritin H subunit.Ferritin is a protein which has as its principal function the intracellular storage of iron in a nontoxic and bioavailable form (see Refs. 1 and 2 for review). Ferritin is ubiquitously distributed in the animal and plant kingdoms and has recently been described in bacteria (3). Mammalian ferritin is composed of two subunits, termed H and L. Twenty-four of these subunits assemble to form the apoferritin protein.In mammals, ferritin is found in most tissues. However, the composition of ferritin varies in a consistent and tissue-specific way. For example, liver contains ferritin that is predominantly of the L subunit type, whereas heart contains ferritin rich in the H subunit. This biodistribution, as well as the evolutionary conservation of a dual subunit structure, supports the hypothesis that the H and L ferritin subunits may play different and complementary roles within the protein (4). Experiments using H-rich and L-rich ferritin prepared from natural sources (5, 6), as well as recent work with recombinant proteins (7-9), have supported this concept of a functional distinction between ferritin H and L subunits and revealed a prominent role of the H subunit in rapid iron oxidation (7), and the involvement of the L subunit in protein stability and iron mineralization (8, 9).Mouse cells and mouse models have been widely used to study iron homeostasis in health and disease (e.g. Refs. 10 -14). For the most part, however, biological inferences concerning ferritin function and iron metabolism in these models have been based on analogy to human ferritin. Indeed, substantial sequence similarity exists between mouse and human ferritin subunits; mouse L and human L exhibit 82% similarity (15), whereas human and mouse ferritin H are 93% identic...

In vivo bioluminescence imaging of labile iron accumulation in a murine model of Acinetobacter baumannii infection

“…Finally, we observe alteration in the gene expression of ferritin subunits, with up-regulation of FHC and down-regulation of FLC, which is consistent with modulation of the intracellular iron storage machinery (72). In particular, FHC has been shown to facilitate rapid iron uptake (72)(73)(74). Significant elevation in serum ferritin levels was also detected, with approximately 10-fold increase in ferritin abundance during infection (SI Appendix, Fig.…”

Binding of Mammalian and Avian Ferritins with Biotinylated Hemin: Demonstration of Preferential Binding of the H Subunit to Heme

“…Contact with H and L subunits may enhance binding activity with heme (hemin). In human and mouse ferritins, ferritins containing a variable proportion of the H and L subunits were expressed with simultaneous expression of their H and L subunit cDNAs [24]. Artificial ferritin heteropolymers may provide new findings for the cooperative role in heme binding by the H and L subunits.…”

Regulation of ferritin genes and protein

“…18 The ferroxidase center is evolutionarily conserved, 10 and ferroxidase activity is dramatically reduced following mutation of residues His65 and Glu62 in both human and mouse. 18,19 Small quantities of ferritin are also present in human serum, and are elevated in conditions of iron overload and inflammation. [20][21][22] Serum ferritin is iron-poor, resembles ferritin L immunologically, and may contain a novel "G" (glycosylated) subunit.…”