Mechanism of frataxin "bypass" in human iron-sulfur cluster biosynthesis with implications for Friedreich's ataxia.

In humans, mitochondrial iron-sulfur cluster biosynthesis is an essential biochemical process mediated by the assembly complex consisting of cysteine desulfurase (NFS1), LYR protein (ISD11), acyl-carrier protein (ACP), and the iron-sulfur cluster assembly scaffold protein (ISCU2). The protein frataxin (FXN) is an allosteric activator that binds the assembly complex and stimulates the cysteine desulfurase and iron-sulfur cluster assembly activities. FXN depletion causes loss of activity of iron-sulfur-dependent enzymes and the development of the neurodegenerative disease Friedreich's ataxia. Recently, a mutation that suppressed the loss of the FXN homolog in Saccharomyces cerevisiae was identified that encodes an amino acid substitution equivalent to the human variant ISCU2 M140I. Here, we developed iron-sulfur cluster synthesis and transfer functional assays and determined that the human ISCU2 M140I variant can substitute for FXN in accelerating the rate of iron-sulfur cluster formation on the monothiol glutaredoxin (GRX5) acceptor protein. Incorporation of both FXN and the M140I substitution had an additive effect, suggesting an acceleration of distinct steps in iron-sulfur 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 iron-sulfur clusters to GRX5. Together, these results reveal an unexpected mechanism that replaces FXN-based stimulation of the iron-sulfur 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.