Supramolecular Assemblies of Human Frataxin are Formed via Subunit–Subunit Interactions Mediated by a Non-conserved Amino-terminal Region

Abstract

The mitochondrial protein frataxin is emerging as a novel mechanism to promote iron metabolism while also providing anti-oxidant protection. Recombinant frataxin proteins from different species are able to form large molecular assemblies that store Fe(III) as a stable mineral in vitro. Furthermore, monomeric and assembled forms of frataxin donate Fe(II) to the Fe-S cluster scaffold protein IscU, [3Fe-4S]1+ aconitase, and ferrochelatase in vitro. However, little is known about the speciation of frataxin in vivo, and the physiologically relevant form(s) of the protein remains undefined. Here, we report that human heart mitochondria contain frataxin species of increasing negative surface charge and molecular mass, ranging from monomer to polymers of >1 MDa. Moreover, we show that the main partner protein of frataxin, IscU, binds in a stable manner to frataxin oligomers. These results suggest that assembly is a physiologic property of frataxin. Biochemical analyses further reveal that, unlike the prokaryotic and yeast frataxin homologues, which require iron-protein interactions for assembly, human frataxin uses stable subunit-subunit interactions involving a non-conserved amino-terminal region. We propose that human frataxin is a modular protein that depends on self-assembly to accomplish its diverse functions.