Loops and bulge/loops in iron-responsive element isoforms influence iron regulatory protein binding. Fine-tuning of mRNA regulation?
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.
Ke
Y
Y
Department of Biochemistry, North Carolina State University, Raleigh, North Carolina 27695-7622, USA.
Wu
J
J
Leibold
E A
EA
Walden
W E
WE
Theil
E C
EC
D000624:5-Aminolevulinate Synthetase / Q000235:genetics; D000154:Aconitate Hydratase / Q000235:genetics; D000818:Animals; D001482:Base Composition; D001483:Base Sequence; D003001:Cloning, Molecular; D005293:Ferritins / Q000235:genetics; D005786:Gene Expression Regulation; D035941:Iron Regulatory Protein 1; D035942:Iron Regulatory Protein 2; D035925:Iron-Regulatory Proteins; D007506:Iron-Sulfur Proteins / Q000235:genetics / Q000378:metabolism*; D008099:Liver / Q000378:metabolism; D009690:Nucleic Acid Conformation; D012333:RNA, Messenger / Q000096:biosynthesis / Q000737:chemistry* / Q000235:genetics; D016601:RNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D011817:Rabbits; D011990:Receptors, Transferrin / Q000235:genetics; D011994:Recombinant Proteins / Q000378:metabolism; D012441:Saccharomyces cerevisiae
D035925:Iron-Regulatory Proteins; D007506:Iron-Sulfur Proteins; D012333:RNA, Messenger; D016601:RNA-Binding Proteins; D011990:Receptors, Transferrin; D011994:Recombinant Proteins; D005293:Ferritins; D000624:5-Aminolevulinate Synthetase; D000154:Aconitate Hydratase; D035941:Iron Regulatory Protein 1; D035942:Iron Regulatory Protein 2