Aconitases are iron–sulfur hydrolyases catalysing the interconversion of cit-rate and isocitrate in a wide variety of organisms. Eukaryotic aconitases have been assigned additional roles, as in the case of the metazoan dual activity cytosolic aconitase–iron regulatory protein 1 (IRP1). This human protein was produced in yeast mitochondria to probe IRP1 folding in this organelle where iron–sulfur synthesis originates.