Iron regulates the activity of the iron-responsive element binding protein without changing its rate of synthesis or degradation.

The iron-responsive element binding protein (IRE-BP) interacts with specific sequence/structure motifs (iron-responsive elements) within the mRNAs encoding ferritin and the transferrin receptor and thereby post-transcriptionally regulates the expression of these two proteins involved in cellular iron homeostasis. The activity of the IRE-BP is itself regulated by iron such that when cells are treated with an iron source, the RNA binding activity is decreased. The expression of recombinant human IRE-BP in murine cells has been examined as have the expressions of the endogenous IRE-BP of both human and rabbit cells. In all cases, iron down-modulated the RNA binding activity of the IRE-BP, but in no instance was this decrease in activity accompanied by a decrease in the level of the protein as judged by quantitative Western blots. Moreover, the rate of synthesis of the IRE-BP and its rate of degradation have been found to be unaltered by iron manipulation of cells in culture. Consistent with IRE-BP regulation occurring post-translationally, the iron regulation of its activity was found to be unaffected by cycloheximide. These data are discussed in terms of a model of IRE-BP regulation involving the modification of the protein's iron-sulfur center.