Loops and Bulge/Loops in Iron-responsive Element Isoforms Influence Iron Regulatory Protein Binding

• DOI: 10.1074/jbc.273.37.23637
• Publication Date: 2002-07-26T15:02:22Z
• Authors: Yaohuang Ke; Jingyang Wu; Elizabeth A. Leibold; William E. Walden; Elizabeth C. Theil
• Keywords: Aconitate Hydratase; Iron-Sulfur Proteins; Base Composition; 0303 health sciences; Base Sequence; Iron-Regulatory Proteins; RNA-Binding Proteins; Saccharomyces cerevisiae; Recombinant Proteins; 03 medical and health sciences; Gene Expression Regulation; Liver; Ferritins; Receptors, Transferrin; Animals; Nucleic Acid Conformation; Iron Regulatory Protein 1; RNA, Messenger; Rabbits; Cloning, Molecular; Iron Regulatory Protein 2; 5-Aminolevulinate Synthetase

A family of noncoding mRNA sequences, iron-responsive elements (IREs), coordinately regulate several mRNAs through binding a family of mRNA-specific proteins, iron regulatory proteins (IRPs). IREs are hairpins with a constant terminal loop and base-paired stems interrupted by an internal loop/bulge (in ferritin mRNA) or a C-bulge (in m-aconitase, erythroid aminolevulinate synthase, and transferrin receptor mRNAs). IRP2 binding requires the conserved C-G base pair in the terminal loop, whereas IRP1 binding occurs with the C-G or engineered U-A. Here we show the contribution of the IRE internal loop/bulge to IRP2 binding by comparing natural and engineered IRE variants. Conversion of the internal loop/bulge in the ferritin-IRE to a C-bulge, by deletion of U, decreased IRP2 binding by >95%, whereas IRP1 binding changed only 13%. Moreover, IRP2 binding to natural IREs with the C-bulge was similar to the DeltaU6 ferritin-IRE: >90% lower than the ferritin-IRE. The results predict mRNA-specific variation in IRE-dependent regulation in vivo and may relate to previously observed differences in iron-induced ferritin and m-aconitase synthesis in liver and cultured cells. Variations in IRE structure and cellular IRP1/IRP2 ratios can provide a range of finely tuned, mRNA-specific responses to the same (iron) signal.