RESUMO
The cytosolic iron-sulfur cluster assembly (CIA) system assembles iron-sulfur (FeS) cluster cofactors and inserts them into >20 apoprotein targets residing in the cytosol and nucleus. Three CIA proteins, called Cia1, Cia2, and Met18 in yeast, form the targeting complex responsible for apo-target recognition. There is little information about the structure of this complex or its mechanism of CIA substrate recognition. Herein, we exploit affinity co-purification and size exclusion chromatography to determine the subunit connectivity and stoichiometry of the CIA targeting complex. We conclude that Cia2 is the organizing center of the targeting complex, which contains one Met18, two Cia1, and four Cia2 polypeptides. To probe target recognition specificity, we utilize the CIA substrates Leu1 and Rad3 as well as the Escherichia coli FeS-binding transcription factor FNR (fumerate nitrate reductase). We demonstrate that both of the yeast CIA substrates are recognized, whereas the bacterial protein is not. Thus, while the targeting complex exhibits flexible target recognition in vitro, it cannot promiscuously recognize any FeS protein. Additionally, we demonstrate that the full CIA targeting complex is required to stably bind Leu1 in vitro, whereas the Met18-Cia2 subcomplex is sufficient to recognize Rad3. Together, these results allow us to propose a unifying model for the architecture of this highly conserved complex and demonstrate what component or subcomplexes are vital for target identification.
