loading . . . Frataxin Traps Low Abundance Quaternary Structure to Stimulate Human Fe–S Cluster Biosynthesis Iron–sulfur clusters are essential protein cofactors synthesized in human mitochondria by an NFS1-ISD11-ACP-ISCU2-FXN assembly complex. Surprisingly, researchers have discovered three distinct quaternary structures for cysteine desulfurase subcomplexes, which display similar interactions between NFS1-ISD11-ACP protomeric units but dramatically different dimeric interfaces between the protomers. Although the role of these different architectures is unclear, possible functions include regulating activity and promoting the biosynthesis of distinct sulfur-containing biomolecules. Here, crystallography, native ion-mobility mass spectrometry, and chromatography methods reveal the Fe–S assembly subcomplex exists as an equilibrium mixture of these different quaternary structures. Isotope labeling and native mass spectrometry experiments show that the NFS1-ISD11-ACP complexes disassemble into protomers, which can then undergo exchange reactions and dimerize to reform native complexes. Single crystals isolated in distinct architectures have the same activity profile and activation by the Friedreich’s ataxia (FRDA) protein frataxin (FXN) when rinsed and dissolved in assay buffer. These results suggest FXN functions as a “molecular lock” and shifts the equilibrium toward one of the architectures to stimulate the cysteine desulfurase activity and promote iron–sulfur cluster biosynthesis. An NFS1-designed variant similarly shifts the equilibrium and partially replaces FXN in activating the complex. We propose that eukaryotic cysteine desulfurases are unusual members of the morpheein class of enzymes that control their activity through their oligomeric state. Overall, the findings support architectural switching as a regulatory mechanism linked to FXN activation of the human Fe–S cluster biosynthetic complex and provide new opportunities for therapeutic interventions of the fatal neurodegenerative disease FRDA. https://pubs.acs.org/doi/10.1021/acs.biochem.4c00733
