Fig. 5.

Model for AGBE-mitoNEET/Cisd2-IRP1A function in tissues with dynamic iron requirements. In Drosophila, iron demand peaks prior to maximal ecdysone production to equip ecdysone-synthesizing enzymes with iron cofactors. Newly synthesized IRP1A receives Fe–S clusters (red circles) from the Cytosolic Iron-sulfur cluster Assembly (CIA) machinery, which produces [4Fe-4S] sulfur clusters from an unidentified mitochondrial precursor molecule, X-S (produced in mitochondria by ISC = Iron-Sulfur Cluster Assembly Machinery). Oxidatively damaged IRP1A (IRP1A-HOLO*) requires the mitoNeet/Cisd2 and AGBE proteins to replace impaired clusters with functional units. Holo-IRP1A is both needed for the aconitase function as well as nuclear entry. Loss of AGBE or mitoNEET/Cisd2 function results in a depletion of holo-IRP1A, and the concomitant loss of nuclear IRP1A, explaining why AGBE mutants cannot be rescued with the IRP1A^C450S form, which only assumes the apo-form since it cannot incorporate an Fe–S cluster. The model suggests that once iron demand has peaked and nuclear holo-IRP1A levels become maximal, the protein acts to throttle expression of genes acting in steroid, heme and iron metabolism in anticipation of falling iron demands. As such, IRP1A has two functions: 1. As known from mammalian cells it acts in response to low cellular iron levels as an RNA-binding protein that promotes increased iron availability, and 2. as a new function, it responds to peak iron levels as a nuclear protein to promote downregulation of processes depending on iron and heme (TR = Transcriptional Regulation).