DIFFERENTIAL REGULATION OF 2 RELATED RNA-BINDING PROTEINS, IRON REGULATORY PROTEIN (IRP) AND IRP(B)

The iron regulatory protein (IRP) is a cytoplasmic RNA-binding protein that regulates cellular iron metabolism at the posttranscriptional level. IRP is an unusual bifunctional molecule: in iron-replete cells it predominantly exists as a 4Fe-4S protein and exhibits aconitase enzymatic activity, whereas apo-IRP prevails in iron-starved cells and binds to iron-responsive elements (IREs), structural motifs within the untranslated regions of mRNAs involved in iron metabolism. A related protein with iron-regulated IRE-binding activity, IRP(B), was previously identified in rodent cells. IRE-binding by IRP and IRP(B) is induced by iron deprivation and nitric oxide (NO). Controversial hypotheses have proposed that the induction of IRE-binding activity by iron results either from de novo synthesis of the ape-protein or from a posttranslational conversion of the Fe-S to the ape-protein form. This prompted a detailed analysis of how iron and NO regulate the RNA-binding activities of IRP and IRP(B). We demonstrate that IRP is a relatively stable protein (half-life >12 h). The induction of IRE-binding does not require de novo protein synthesis but results from conversion of Fe-S IRP to apo-IRP. In contrast, IRP(B) appears less stable in nonstarved cells (half-life similar to 4-6 h) and must be synthesized de novo following iron starvation. Our results furthermore reveal that two RNA-binding proteins with close structural and functional similarities that respond to the same cellular signals are regulated by predominantly different mechanisms.