William E. Walden, PhDAssociate Dean for Faculty Affairs Professor – Microbiology & Immunology
PhD, Washington University (St. Louis, MO)
Post-Transcriptional Gene Regulation and the Control of Iron Metabolism
The research focus of my laboratory is on the post-transcriptional regulation of genes of iron transport, storage and utilization, and on the regulation of iron homeostasis in eukaryotes. I am also interested in the mechanisms of translational regulation, specifically translational control via sequence specific RNA binding proteins. Our major focus over the past several years has been the regulation of ferritin synthesis in response to changing iron. Ferritin is the major iron storage protein in animal cells and its synthesis is coordinated with cellular iron status. Iron excess causes a stimulation in ferritin synthesis by activating pre-existing, repressed ferritin mRNA. Iron limitation enhances repression of ferritin mRNA. This process of regulating ferritin mRNA is mediated by a small family of sequence specific RNA binding proteins, called Iron Regulatory Proteins (IRP). IRP mediate regulation of ferritin mRNA metabolism by binding to a cis-acting element within ferritin mRNAs, called Iron Responsive Element (IRE). IREs consist of 28 nucleotides and can fold into a conserved stem and loop structure. IREs have been found in several other mRNAs, and it is now known that IRP/IRE interactions mediate regulation of the synthesis of a number of proteins involved with iron metabolism. Thus IRP are central regulators of iron in animal cells.
There are 2 members of the IRP family, called IRP1 and IRP2. IRP1 is a bifunctional enzyme, serving either as a regulator of genes important to iron metabolism or as the cytosolic isoform of aconitase. Activity as an aconitase requires the assembly of an iron-sulfur cluster in IRP1. This also inactivates its IRE binding activity, and thus iron regulation of gene expression through IRP1 is via the reversible assembly/disassembly of an iron-sulfur cluster. IRP2 regulation involves iron induced degradation. Thus the regulation of these proteins occurs through 2 distinct mechanisms.
Several projects are ongoing in my laboratory aimed at increasing our understanding of iron regulated gene expression. We have established expression and regulation of IRP1 in yeast whereby we can dissect the components of the machinery for iron-sulfur cluster assembly/disassembly using a molecular genetic approach. This system will also be used in our studies of the underlying molecular mechanisms of translational regulation via trans-acting repressors. Another area of focus is structure/function relationships in IRPs, particularly in understanding the basis for the highly specific IRE/IRP interaction and how the presence of an iron-sulfur cluster disrupts this interaction. Finally, we are interested in understanding how regulation of gene expression via IRP/IRE interactions contributes to an organism ability to respond to dietary iron, alterations in iron metabolism due to host/pathogen interaction and to changes in iron metabolism which occur during development, differentiation and disease.