Mechanisms for regulating iron regulatory protein 1 that are independent of the iron -sulfur cluster

Iron regulatory proteins (IRP) are key regulators of vertebrate iron metabolism that function by binding stem-loop structures called Iron Responsive Elements (IRE) present in specific mRNA thereby controlling the formation of proteins involved in the uptake and metabolic fate of iron. IRP1 is a bifunctional protein serving either as an IRE binding protein or as the cytosolic isoform of the iron-sulfur protein aconitase (c-acon). Formation or loss of the iron-sulfur cluster, referred to as the iron-sulfur switch, has been viewed as the primary means through which RNA binding is regulated. Iron-replete conditions favor formation of the Fe-S cluster, whereas cluster loss occurs in response to iron-deficiency and generates the RNA binding form. IRP1 function is also regulated by factors other than iron that act by modulating cluster disassembly as induced by perturbants such as nitric oxide a process potentially modulated by protein kinase C (PKC)-dependent phosphorylation of IRP1 at S138. IRP1 action is also controlled through protein degradation. The extent to which these mechanisms act in an integrated manner to control IRP1 function is poorly understood.;S138 was shown to be a PKC phosphorylation site in IRP1 and phosphomimetic mutation of S138 triggers iron-dependent degradation of IRP1 protein. Mutations that block assembly of the Fe-S cluster did not abrogate the degradation response indicating that the cluster is dispensable for iron regulation of IRP1. In mice with genetic alterations in cytosolic Fe-S cluster metabolism IRP1 is subject to iron-dependent regulation of its protein abundance suggesting a role for iron-induced protein degradation in vivo. In a study of the effect of iron-deficiency on tissue specific regulation of IRP1 muscle IRP1 was much more strongly activated compared to liver with little of the protein remaining in the c-acon form. Interestingly, the continued accumulation of IRP1 RNA binding activity in muscle was blunted by a significant loss of IRP1 protein in muscle indicating that the iron-sulfur switch and protein abundance control IRP1 function in this tissue. Taken together, these studies indicate that multiple iron-dependent mechanisms control the accumulation of IRP1 RNA binding activity including the iron-sulfur switch and protein turnover.