In humans, mitochondrial iron-sulfur (Fe-S) cluster biosynthesis is an essential process mediated by the assembly complex. The protein frataxin (FXN) is an allosteric activator that binds the assembly complex and stimulates the assembly activities. FXN depletion causes loss of activity of Fe-S-dependent enzymes and the development of the neurodegenerative disease Friedreich's ataxia. Recently, a mutation that suppressed the loss of the FXN homolog in yeast was identified that encodes an amino acid substitution equivalent to the human variant ISCU2 M140I. Here, we developed Fe-S cluster synthesis and transfer functional assays and determined that the human ISCU2 M140I variant can substitute for FXN in accelerating the rate of Fe-S cluster formation. Incorporation of both FXN and the M140I substitution had an additive effect, suggesting an acceleration of distinct steps in Fe-S cluster biogenesis. In contrast to the canonical role of FXN in stimulating the formation of [2Fe-2S]-ISCU2 intermediates, we found here that the M140I substitution in ISCU2 promotes the transfer of Fe-S clusters to GRX5. Together, these results reveal an unexpected mechanism that replaces FXN-based stimulation of the Fe-S cluster biosynthetic pathway and suggest new strategies to overcome the loss of cellular FXN that may be relevant to the development of therapeutics for Friedreich's ataxia.
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