The path of the iron-sulfur clusters into the proteins: Investigation of the Sulfur Mobilization Machinery

Introduction: Iron-sulfur cluster containing proteins are ubiquitous in nature and conserved across all kingdoms of life; however, iron-sulfur clusters are not assembled within their target proteins. Instead, iron-sulfur clusters are assembled on scaffold proteins assisted by specialized assembly machinery. Currently, four iron-sulfur cluster machineries are known: the NIF (nitrogen-fixation system), ISC (iron-sulfur cluster assembly machinery), CIA (cytosolic iron-sulfur cluster assembly machinery), and SUF (sulfur mobilization machinery). The SUF machinery is the most ancient of the identified iron-sulfur cluster biogenesis systems. The SUF machinery contains the proteins SufS, SufE, SufBCD, and SufA. The SufBCD protein complex's postulated function is the assembly of iron-sulfur clusters on its SufBD subunits. SufC belongs to the highly diverse ABC ATPase-family, whose function within the iron-sulfur cluster biogenesis pathway remains unresolved. To overcome this, structures in the enzyme's conformational cycles are required to understand how the ATPase movement is related to the formation of a nascent iron-sulfur cluster. Objective: We aim to determine a high-resolution structure of the bacterial SUF machinery with a particular focus on the SufBCD protein complex, concentrating on the iron-sulfur cluster cofactor binding coordination and the proposed ATPase activity-induced, conformational changes. Methods: We utilised single-particle cryo-electron microscopy (cryo-EM) to create structural models of the purified, SufBCD protein complex. Results: The SUF machinery components were overexpressed, purified and cryo-EM sample preparation was carried out under redox controlled conditions. UV/Vis spectroscopy confirmed the presence of an iron-sulfur cluster in the anaerobically purified SufBCD complex. Preliminary data processing revealed SufBCD undergoes conformational changes, possibly in relation to the presence of an iron-sulfur cluster. Conclusion: Overall, preliminary structural information acquired by single particle cryo-EM show that the SufBCD protein complex adopts different structural conformations during iron-sulfur cluster assembly.