Ferritins: furnishing proteins with iron

Ferritins are a superfamily of iron oxidation, storage and mineralization proteins found throughout the animal, plant, and microbial kingdoms. The majority of ferritins consist of 24 subunits that individually fold into 4-α-helix bundles and assemble in a highly symmetric manner to form an approximately spherical protein coat around a central cavity into which an iron-containing mineral can be formed. Channels through the coat at inter-subunit contact points facilitate passage of iron ions to and from the central cavity, and intrasubunit catalytic sites, called ferroxidase centers, drive Fe2+ oxidation and O2 reduction. Though the different members of the superfamily share a common structure, there is often little amino acid sequence identity between them. Even where there is a high degree of sequence identity between two ferritins there can be major differences in how the proteins handle iron. In this review we describe some of the important structural features of ferritins and their mineralized iron cores and examine in detail how three selected ferritins oxidise Fe2+ in order to explore the mechanistic variations that exist amongst ferritins. We suggest that the mechanistic differences reflect differing evolutionary pressures on amino acid sequences, and that these differing pressures are a consequence of different primary functions for different ferritins

Mathematical modeling of the dynamic storage of iron in ferritin

<p>Abstract</p> <p>Background</p> <p>Iron is essential for the maintenance of basic cellular processes. In the regulation of its cellular levels, ferritin acts as the main intracellular iron storage protein. In this work we present a mathematical model for the dynamics of iron storage in ferritin during the process of intestinal iron absorption. A set of differential equations were established considering kinetic expressions for the main reactions and mass balances for ferritin, iron and a discrete population of ferritin species defined by their respective iron content.</p> <p>Results</p> <p>Simulation results showing the evolution of ferritin iron content following a pulse of iron were compared with experimental data for ferritin iron distribution obtained with purified ferritin incubated <it>in vitro </it>with different iron levels. Distinctive features observed experimentally were successfully captured by the model, namely the distribution pattern of iron into ferritin protein nanocages with different iron content and the role of ferritin as a controller of the cytosolic labile iron pool (cLIP). Ferritin stabilizes the cLIP for a wide range of total intracellular iron concentrations, but the model predicts an exponential increment of the cLIP at an iron content > 2,500 Fe/ferritin protein cage, when the storage capacity of ferritin is exceeded.</p> <p>Conclusions</p> <p>The results presented support the role of ferritin as an iron buffer in a cellular system. Moreover, the model predicts desirable characteristics for a buffer protein such as effective removal of excess iron, which keeps intracellular cLIP levels approximately constant even when large perturbations are introduced, and a freely available source of iron under iron starvation. In addition, the simulated dynamics of the iron removal process are extremely fast, with ferritin acting as a first defense against dangerous iron fluctuations and providing the time required by the cell to activate slower transcriptional regulation mechanisms and adapt to iron stress conditions. In summary, the model captures the complexity of the iron-ferritin equilibrium, and can be used for further theoretical exploration of the role of ferritin in the regulation of intracellular labile iron levels and, in particular, as a relevant regulator of transepithelial iron transport during the process of intestinal iron absorption.</p

Lysosomes in iron metabolism, ageing and apoptosis

The lysosomal compartment is essential for a variety of cellular functions, including the normal turnover of most long-lived proteins and all organelles. The compartment consists of numerous acidic vesicles (pH ∼4 to 5) that constantly fuse and divide. It receives a large number of hydrolases (∼50) from the trans-Golgi network, and substrates from both the cells’ outside (heterophagy) and inside (autophagy). Many macromolecules contain iron that gives rise to an iron-rich environment in lysosomes that recently have degraded such macromolecules. Iron-rich lysosomes are sensitive to oxidative stress, while ‘resting’ lysosomes, which have not recently participated in autophagic events, are not. The magnitude of oxidative stress determines the degree of lysosomal destabilization and, consequently, whether arrested growth, reparative autophagy, apoptosis, or necrosis will follow. Heterophagy is the first step in the process by which immunocompetent cells modify antigens and produce antibodies, while exocytosis of lysosomal enzymes may promote tumor invasion, angiogenesis, and metastasis. Apart from being an essential turnover process, autophagy is also a mechanism by which cells will be able to sustain temporary starvation and rid themselves of intracellular organisms that have invaded, although some pathogens have evolved mechanisms to prevent their destruction. Mutated lysosomal enzymes are the underlying cause of a number of lysosomal storage diseases involving the accumulation of materials that would be the substrate for the corresponding hydrolases, were they not defective. The normal, low-level diffusion of hydrogen peroxide into iron-rich lysosomes causes the slow formation of lipofuscin in long-lived postmitotic cells, where it occupies a substantial part of the lysosomal compartment at the end of the life span. This seems to result in the diversion of newly produced lysosomal enzymes away from autophagosomes, leading to the accumulation of malfunctioning mitochondria and proteins with consequent cellular dysfunction. If autophagy were a perfect turnover process, postmitotic ageing and several age-related neurodegenerative diseases would, perhaps, not take place

The type IV mucolipidosis-associated protein TRPML1 is an endolysosomal iron release channel

TRPML1 ( mucolipin 1, also known as MCOLN1) is predicted to be an intracellular late endosomal and lysosomal ion channel protein that belongs to the mucolipin subfamily of transient receptor potential ( TRP) proteins(1-3). Mutations in the human TRPML1 gene cause mucolipidosis type IV disease ( ML4)4,5.ML4 patients have motor impairment, mental retardation, retinal degeneration and iron- deficiency anaemia. Because aberrant iron metabolism may cause neural and retinal degeneration(6,7), it may be a primary cause of ML4 phenotypes. In most mammalian cells, release of iron from endosomes and lysosomes after iron uptake by endocytosis of Fe3+- bound transferrin receptors(6), or after lysosomal degradation of ferritin - iron complexes and autophagic ingestion of iron-containing macromolecules(6,8), is the chief source of cellular iron. The divalent metal transporter protein DMT1 ( also known as SLC11A2) is the only endosomal Fe2+ transporter known at present and it is highly expressed in erythroid precursors(6,9). Genetic studies, however, suggest the existence of a DMT1-independent endosomal and lysosomal Fe2+ transport protein(9). By measuring radiolabelled iron uptake, by monitoring the levels of cytosolic and intralysosomal iron and by directly patch- clamping the late endosomal and lysosomal membrane, here we show that TRPML1 functions as a Fe2+ permeable channel in late endosomes and lysosomes. ML4 mutations are shown to impair the ability of TRPML1 to permeate Fe2+ at varying degrees, which correlate well with the disease severity. A comparison of TRPML1(-/-) ML4 and control human skin fibroblasts showed a reduction in cytosolic Fe2+ levels, an increase in intralysosomal Fe2+ levels and an accumulation of lipofuscin- like molecules in TRPML1(-/-) cells. We propose that TRPML1 mediates a mechanism by which Fe2+ is released from late endosomes and lysosomes. Our results indicate that impaired iron transportmay contribute to both haematological and degenerative symptoms of ML4 patients.Department of Molecular, Cellular ; Developmental Biology and Biological Science Scholar Program ; University of MichiganThis work is supported by start-up funds to H. X. from the Department of Molecular, Cellular, and Developmental Biology and Biological Science Scholar Program, University of Michigan. We thank U. Brunk, M. Saito, R. Hume, C. Duan, M. Akaaboune, J. Kuwada, S. Low, S. Punthambaker and S. Dellal for assistance, and D. Clapham, N. Andrews, L. DeFelice, L. Yue, D. Ren, C. Jiang and S. Xu for comments on an earlier version of the manuscript. We also thank K. Kiselyov for sharing his unpublished results on lysosomal iron staining of ML4 cells. We appreciate the encouragement and helpful comments from other members of the Xu laboratory.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62672/1/nature07311.pd