Iron Regulatory Protein 2 as Iron Sensor

Abstract

Iron regulatory protein 2 coordinates cellular regulation of iron metabolism by binding to iron responsive elements in mRNA. The protein is synthesized constitutively but is rapidly degraded when iron stores are replete. This iron-dependent degradation requires the presence of a 73-residue degradation domain, but its functions have not yet been established. We now show that the domain can act as an iron sensor, mediating its own covalent modification. The domain forms an ironbinding site with three cysteine residues located in the middle of the domain. It then reacts with molecular oxygen to generate a reactive oxidizing species at the iron-binding site. One cysteine residue is oxidized to dehydrocysteine and other products. This covalent modification may thus mark the protein molecule for degradation by the proteasome system.Iron metabolism is exquisitely regulated by all organisms, from bacteria to humans. In mammals, the iron regulatory proteins (IRPs) 1 mediate the coordinate expression of proteins that participate in iron metabolism (1, 2). When iron stores are low, the IRPs bind to an RNA stem-loop structure known as an iron-responsive element (IRE) located in either the 5Ј-or 3Ј-untranslated region of mRNA. If the IRE is close to the cap site, binding of the IRP blocks initiation of translation, causing a decrease in the level of the protein encoded by that mRNA. Conversely, when the IRE is located in the 3Ј-untranslated region of the transferrin receptor transcript, binding of the IRP stabilizes the mRNA by decreasing susceptibility to nuclease attack, causing an increase in the level of the protein encoded by the mRNA (3).Mammals have two known IRPs, IRP1 and IRP2, with the tissue roles of each still being defined. IRP1 contains an ironsulfur center, whereas IRP2 does not. The general mechanism by which each IRP is regulated is distinct (1). When cellular iron stores are low, IRP1 lacks a functional iron-sulfur center and binds to its IRE targets. When iron stores are sufficient, IRP1 regains its full iron-sulfur center, loses the ability to bind to IRE, and functions as a cytosolic aconitase. The cellular levels of IRP1 are unaffected by iron status in most cell types.In contrast, IRP2 protein and IRE binding activity are readily detected when iron stores are limited but are low or absent when iron stores are sufficient (4, 5). The decrease in IRP2 protein occurs as a consequence of rapid degradation by the proteasome; synthesis of the protein is constitutive and does not vary substantially with iron status. The sequences of the two IRPs are similar except that IRP2 contains one domain not present in IRP1, and this domain is both required and sufficient to confer susceptibility to iron-dependent degradation. Deletion of the degradation domain produces an IRP2 whose levels no longer vary with iron status. Conversely, insertion of the degradation domain into IRP1 renders it susceptible to iron-triggered degradation (6).We showed previously that IRP2 is oxidatively modified and ubiquitinylated in vivo, ...

Oxidative Cross-linking of Tryptophan to Glycine Restrains Matrix Metalloproteinase Activity

“…Our results indicate that specific structural motifs are important for controlling protein oxidation. It is noteworthy that similar observations have recently been reported for iron regulatory protein 2, which coordinates cellular regulation of iron metabolism by binding to iron responsive elements in mRNA (27). Oxidative modifications of proteins may thus be site-specific and require a specific juxtaposition of reactive moieties rather than being driven simply by the chemical reactivity of amino acid side chains.…”

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