Direct participation of frataxin during iron-sulfur cluster biosynthesis has been widely established. While originally suggested to serve as an iron chaperon, whose metal binding ability has been documented in detail, recent reports suggest frataxin may also modulate the activity of the cysteine desulfurase concurrent with ISC pathway cluster assembly. The negative cysteine desulfurase activity shown in the prokaryotic system is in contrast to the positive activity stimulation recently shown in the human system (and confirmed in our laboratory using the yeast model system) suggesting unique features of the eukaryotic pathway may drive the differences in reactivity seen between cell types. We have performed an extensive characterization of the frataxin-ISC scaffold protein complex using a variety of spectroscopic techniques (NMR, XAS, ITC, Fluorimetry), and in the process provided the molecular details that help explain the energetics that drive interprotein assembly and metal transfer. Additional characterization data of the frataxin Nfs-Isd11 complex in the presence and absence of Isu will be discussed with our recent data to better understand the enhanced reactivity of the macromolecular assembly related to Fe-S cluster assembly in eukaryotes. Based on our molecular characterization of the frataxin-Isu complex and current data regarding the Isu-Nfs binding interface, we believe frataxin interacts with the two protein partners, with the assistence of Isd11, in a manner that provides iron for cluster assemly and allosterically activates eukaryotic cysteine desulfurase activity. A model for these studies will be presented.
Timothy Stemmler, Wayne State University