THE IRON-RESPONSIVE ELEMENT-BINDING PROTEIN - LOCALIZATION OF THE RNA-BINDING SITE TO THE ACONITASE ACTIVE-SITE CLEFT

The iron-responsive element-binding protein (IRE-BP) binds to specific stem-loop RNA structures known as iron-responsive elements (IREs) pre sent in a variety of cellular mRNAs (e.g., those encoding ferritin, er ythroid 5-aminolevulinate synthase, and transferrin receptor). Express ion of these genes is regulated by interaction with the IRE-BP. The IR E-BP is identical in sequence to cytosolic aconitase, and the Function of the protein is determined by the presence or absence of an Fe-S cl uster. The protein either functions as an active aconitase when the Fe -S cluster is present of as an RNA binding protein when the protein la cks this cluster. Aconitase activity and IRE-binding activity are mutu ally exclusive, and interconversion between the two activities is dete rmined by intracellular Fe concentrations. Mapping of the RNA-binding site of the IRE-BP by UV cross-linking studies defines a major contact site between IRE and protein in the active-site region. Modeling base d on probable structural similarities between the previously crystalli zed mitochondrial aconitase and the IRE-BP predicts that these residue s would be accessible to the IRE only were there a major change in the predicted conformation of the protein when cells are iron-depleted.