Differential regulation of two related RNA-binding proteins, iron regulatory protein (IRP) and IRPB.
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, IRPB, was previously identified in rodent cells. IRE-binding by IRP and IRPB 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 apo-protein or from a posttranslational conversion of the Fe-S to the apo-protein form. This prompted a detailed analysis of how iron and NO regulate the RNA-binding activities of IRP and IRPB. 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, IRPB appears less stable in nonstarved cells (half-life approximately 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.
Pantopoulos
K
K
Gene Expression Programme, European Molecular Biology Laboratory, Heidelberg, Germany.
Gray
N K
NK
Hentze
M W
MW
D000818:Animals; D002460:Cell Line; D003513:Cycloheximide / Q000494:pharmacology; D007501:Iron / Q000378:metabolism*; D035925:Iron-Regulatory Proteins; D051379:Mice; D009569:Nitric Oxide / Q000494:pharmacology; D011499:Protein Processing, Post-Translational; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D016601:RNA-Binding Proteins / Q000378:metabolism*
D035925:Iron-Regulatory Proteins; D011500:Protein Synthesis Inhibitors; D016601:RNA-Binding Proteins; D009569:Nitric Oxide; D003513:Cycloheximide; D007501:Iron