REQUIREMENTS FOR IRON-REGULATED DEGRADATION OF THE RNA-BINDING PROTEIN, IRON REGULATORY PROTEIN-2

Iron regulatory proteins (IRPs) regulate the expression of genes invol ved in iron metabolism whose transcripts contain RNA stem-loop moths k nown as iron-responsive elements (IREs), When iron concentrations are low, IRPs bind to IREs in the 5' untranslated region (UTR) of transcri pts where they repress translation, or the 3' UTR of transcripts where they inhibit degradation, The RNA binding activities of the homologou s proteins IRP1 and IRP2 are both regulated posttranslationally. The b inding activity of IRP2 is regulated by the degradation of the protein when cells are iron-replete, Here, we demonstrate that a 73 amino aci d sequence that corresponds to a unique exon in IRP2 contains a sequen ce required for rapid degradation in iron-replete cells, The deletion of this sequence eliminates the rapid turnover of IRP2, whereas the tr ansfer of this sequence to the corresponding position in the homologou s protein IRP1 confers the capacity for iron-dependent degradation upo n IRP1, Site-directed mutagenesis has demonstrated that specific cyste ines within the IRP2 exon are required for iron-dependent degradation, The degradation of IRP2 appears to be mediated by the proteasome in i ron-replete cells, When degradation is prevented, the RNA binding acti vity of IRP2 is not regulated by iron concentration, Thus, degradation is required for the regulation of the RNA binding activity of IRP2.