Pantopoulos K, Gray N K, Hentze M W
Gene Expression Programme, European Molecular Biology Laboratory, Heidelberg, Germany.
RNA. 1995 Apr;1(2):155-63.
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.
铁调节蛋白（IRP）是一种细胞质RNA结合蛋白，它在转录后水平调节细胞铁代谢。IRP是一种不同寻常的双功能分子：在铁充足的细胞中，它主要以4Fe-4S蛋白的形式存在，并具有乌头酸酶活性，而脱辅基IRP在缺铁细胞中占主导地位，并与铁反应元件（IREs）结合，IREs是参与铁代谢的mRNA非翻译区中的结构基序。一种具有铁调节IRE结合活性的相关蛋白IRPB，先前在啮齿动物细胞中被鉴定出来。IRP和IRPB与IRE的结合是由铁缺乏和一氧化氮（NO）诱导的。有争议的假说是，铁对IRE结合活性的诱导要么源于脱辅基蛋白的从头合成，要么源于Fe-S蛋白向脱辅基蛋白形式的翻译后转化。这促使人们详细分析铁和NO如何调节IRP和IRPB的RNA结合活性。我们证明IRP是一种相对稳定的蛋白（半衰期>12小时）。IRE结合的诱导不需要从头合成蛋白质，而是由Fe-S IRP向脱辅基IRP的转化导致的。相比之下，IRPB在非饥饿细胞中似乎不太稳定（半衰期约为4-6小时），并且在铁饥饿后必须从头合成。我们的结果还揭示，两种结构和功能相似且对相同细胞信号作出反应的RNA结合蛋白，其调节机制主要不同。
