Cell Metabol., published online 6 Aug 2013; doi:10.1016/j.cmet.2013.06.015

In eukaryotes, iron-sulfur cluster assembly is controlled by machinery specific to either the mitochondria or the cytosol and nucleus. Stehling et al. now expand our understanding of the poorly understood cytosolic iron-sulfur protein assembly (CIA) machinery, identifying new protein factors, unexpected substrate specificities and links to other iron regulatory mechanisms. The human homologs of known CIA components, CIA1 and CIA2B, have been shown to form a complex with MMS19, which assists in cluster insertion. The authors also identified CIA2A as related to CIA2B, but the distinction between these proteins was unclear. Immunoblotting confirmed all three of the CIA proteins were present in the cytosol. Knockdowns and pulldowns of each of the three proteins demonstrated that CIA2A and CIA2b both bind CIA1, though not simultaneously, and defined distinct but overlapping pools of substrates for the assembly proteins. For example, CIA2A, but not CIA1 or CIA2B, was required for the maturation of iron regulatory protein 1 (IRP1), involved in the post-transcriptional regulation of proteins involved in iron homeostasis. CIA2B did, however, effect iron regulation via IRP2; interestingly, CIA2A and CIA2B caused the same IRP2-mediated changes, even though the two proteins had opposing effects on IRP2 stability and its ability to bind the iron-responsive element in mRNA. These results define a branched pathway in Fe-S cluster maturation and identify unexpected cross-talk in iron regulation.