Iron-responsive protein IRP

Iron-response protein (IRP) (83, 84) Translational regulation Sensory... 

Iron regulatory proteins (IRPs) regulate the cellular iron level in mammalian cells. IRPs are known as cytosol mRNA binding proteins which control the stability or the translation rate of mRNAs of iron metabolism-related proteins such as TfR, ferritin, and 5-aminolevulinic acid synthetase in response to the availability of cellular iron [19-21] after uptake [5]. The regulatory mechanism involves the interaction between the iron-responsive element (IRE) in the 3 or 5 untranslated regions of the transcripts and cytosolic IRPs (IRP-1 and -2). IRP-1 is an iron-sulfur (Fe-S) protein with aconitase activity containing a cubane 4Fe-4S cluster. When Fe is replete, IRP-1 prevails in a 4Fe-4S form as a holo-form and is an active cytoplasmic aconitase. As shown in Fig. 3, when Fe is deplete, it readily loses one Fe from the fourth labile Fe in the Fe-S cluster to become a 3Fe-4S cluster and in this state has little enzymatic activity [22, 23]. 

Abbreviations IRP (iron responsive protein) ROS (reactive oxygen-derived active species) DP (developed pressure) EDP (end diastolic pressure) WI (cardiac work index) DHBA (dihydroxybenzoic acid) PC preconditioning CFF (coronary flow fraction). 

This communication has reconfirmed the marked protection against myocardial reperfiision injury afforded by ischemic PC. The novel findings in this communication are that following prolonged ischemia, PC causes a marked decrease of the levels of cellular redistribution and extra-cellular mobilization of iron and copper. Furthermore, PC is associated with an accumulation of intracellular ferritin, and a concomitant decrease in ferritin-iron saturation. It is postulated that the low, but significant and reproducible, mobilization levels of intracellular iron, following each cycle of PC, have led to the conversion of the iron-responsive proteins, notably IRP-1, to cytosolic aconitase, and the consequent relief of the tight inhibitory control of ferritin synthesis. 

The iron regulatory proteins, IRP-1 and IRP-2, bind at high affinity (Kd 40-100 pM) to highly conserved RNA stemloops, the Iron-responsive Elements (IREs) at the 5 cap sites of the L- and H-ferritin niRNAs [55-57] (Figure 2). The IRE/IRP interaction impedes the access of the small 40S ribosome subunit to the 5 end of the ferritin niRNAs and thereby suppresses L- and H-ferritin mRNA translation [58-60]. The potency of the IRE to regulate translation is position dependent [61]. Iron influx relieves repression of ferritin translation by removing IRP-1 from the ferritin IREs IRP-1 is simultaneously interconverted to a cytoplasmic cis-aconitase with an iron sulfur cluster, and IRP-2 is degraded by... 

This Iron-Responsive Element (Type II) in the 5 of APP mRNA was fully functional as assessed by multiple separate transfection assays [48]. RNA gel-shift experiments showed that the mutant version of the APP 5 UTR cRNA probe no longer binds to Iron-regulatory Proteins (IRP) (shaded box in Figure 3) [48]. Using RNA electrophoretic mobility shift assays (REMSA) we performed many controls to demonstrate that IRP-1 specifically binds to the stemloop that is predicted to fold from APP 5 untranslated region sequences [48]. Our preliminary data also confirmed that IRP-2 selectively interacted with Ae APP 5 UTR to the same extent as originally observed for IELP-1. 

Consider ferritin first. Ferritin mRNA includes a stem-loop structure termed an iron-response element (IRE) in its 5 untranslated region ure 31.37). This stem-loop binds a 90-kd protein, called an IRE-hinding protein (I RP), that blocks the initiation of translation. When the iron level increases, the IRP binds iron as a 4Fe-4S cluster. The IRP bound to iron cannot bind RNA, because the binding sites for iron and RNA substantially overlap. Thus, in the presence of iron, ferritin mRNA is released from the IRP and translated to produce ferritin, which sequesters the excess iron. 

Internal exchange of iron is accomplished by the plasma protein transferrin. This 76 kDa /Ij-glycoprotein has 2 binding sites for ferric iron. Iron is delivered from transferrin to intracellular sites by means of specific transferrin receptors in the plasma membrane. The iron-transferrin complex binds to the receptor, and the ternary complex is taken up by receptor-mediated endocytosis. Iron subsequently dissociates in the acidic, intracellular vesicular compartment (the endosomes), and the receptor returns the apotransferrin to the cell surface, where it is released into the extracellular environment. Cells regulate their expression of transferrin receptors and intracellular ferritin in response to the iron supply. Apoferritin synthesis is regulated post-transcriptionally by 2 cytoplasmic binding proteins (IRP-1 and lRP-2) and an iron-regulating element on its mRNA (IRE). 

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