Serine138 Phosphorylation Promotes Iron Responsive Degradation of Iron Regulatory Protein 1 (IRP1)

IRP1 is an RNA binding protein that controls the stability or translation of mRNA involved in vertebrate iron metabolism. Insertion and removal of a Fe‐S cluster is thought to be the major mechanism by which iron regulates IRP1 RNA binding activity and which occurs without altering IRP1 protein level. We have focused on how IRP1 responds to iron when the Fe‐S switch is impaired as likely occurs in mitochondrial diseases affecting Fe‐S metabolism, oxidative stress or S138 phosphorylation. In contrast to wildtype (WT) IRP1, the S138E phosphomimetic mutant of IRP1 preferentially accumulated in the RNA binding form in HEK cells and was subject to iron‐dependent degradation. Mutation of cysteines required for cluster formation failed to block iron‐stimulated degradation of S138E indicating that the Fe‐S cluster is not required for regulation of IRP1 protein turnover by iron. S138 phosphorylated IRP1 (¬S138‐p IRP1) increased 6 fold on treatment of HEK cells with a protein kinase C (PKC) activator. Conditions that enhanced the accumulation of IRP1 in the RNA binding form, including iron deficiency, increased S138‐p IRP1 4 fold arguing that PKC targets the RNA binding form. Importantly, iron reduced the abundance of S138‐p IRP1 by 50% within 4 hr. The action of iron was blocked by ALLN, a protease inhibitor, indicating that S138‐p IRP1 is subject to iron dependent protein turnover. We conclude that S138 phosphorylation alters the mechanism for iron‐regulation of IRP1 and provides a means to program changes in the uptake and metabolic fate of iron during cell proliferation or in other cells with specialized iron requirements. (Support: NIH and USDA)