The Molecular Basis of Iron-induced Oligomerization of Frataxin and the Role of the Ferroxidation Reaction in Oligomerization

Abstract

Background: Iron-induced oligomerization of frataxin is still poorly understood. Results: The molecular basis of iron-induced oligomerization of yeast and bacterial frataxin is revealed. Catalyzed ferroxidation is required for correct oligomerization of Yfh1. Conclusion: Frataxin forms different oligomeric species at physiological conditions. Significance: Iron availability controls frataxin oligomerization, which in turn may control the processes that require iron delivery by frataxin.

Architecture of the Yeast Mitochondrial Iron-Sulfur Cluster Assembly Machinery

“…The channel could exist in two different conformations with different metal binding affinities, suggesting that the Yfh1 trimer may control when the bound iron is transferred to Isu1. In a model of the Yfh1 hexamer obtained from small angle x-ray scattering data, one atom of iron was found to bind at the interface between two trimers, close to conserved residues that include the ferroxidation and mineralization sites of Yfh1, consistent with the role of iron oxidation in the stabilization of higher order Yfh1 oligomers (22,28). Finally, the structures of the apo-and holo-Yfh1 Y73A 24-mer revealed a hollow globular particle made up of eight trimers, with striking similarities to the iron-storage protein ferritin, including an ability to form a ferric iron core inside its central cavity (25,31).…”

“…Yfh1 oligomerization is normally coupled to the Yfh1-catalyzed oxidation of Fe 2ϩ that leads to formation of a stable ferric mineral within the Yfh1 oligomers (26 -28). Yfh1 oligomers disassemble upon treatment with chelators and/or reducing agents, indicating that iron oxidation and mineralization are required for oligomer stabilization (21,28); and indeed, mutations that affect the ferroxidase activity of Yfh1 also affect its ability to oligomerize (22), and similarly, anaerobic conditions that inhibit Fe 2ϩ oxidation stabilize the Fe 2ϩ -loaded Yfh1 monomer (29). These properties are manifested in yeast, where Yfh1 oligomerizes in response to rapid increases in mitochondrial iron uptake (23,24).…”

“…8F). This surface was previously described in the context of the Yfh1 hexamer structure (22), in which one atom of iron was coordinated by Thr-118 and Ala-133 from the first trimer and by an acidic surface contributed by the second trimer, including highly conserved residues Glu-93 and Asp-101 (22).…”

“…One aspect that has been largely overlooked, but may actually help shed light on the iron-donation mechanism, is the fact that Yfh1 and its bacterial and human orthologues have a strong propensity to oligomerize in vitro (7,9,21,22) and in vivo (9,23,24). At an Fe 2ϩ to protein ratio of 2, monomeric Yfh1 is converted to trimer in solution (21,22); and at higher Fe 2ϩ to protein ratios, the trimer serves as the building block for the assembly of larger oligomers (22,25).…”

“…At an Fe 2ϩ to protein ratio of 2, monomeric Yfh1 is converted to trimer in solution (21,22); and at higher Fe 2ϩ to protein ratios, the trimer serves as the building block for the assembly of larger oligomers (22,25). Yfh1 oligomerization is normally coupled to the Yfh1-catalyzed oxidation of Fe 2ϩ that leads to formation of a stable ferric mineral within the Yfh1 oligomers (26 -28).…”