Trading Places—Switching Frataxin Function by a Single Amino Acid Substitution within the [Fe-S] Cluster Assembly Scaffold

Simple inorganic structures comprised of iron and sulfur are called [Fe-S] clusters. They likely represent one of the earliest prosthetic groups associated with the emergence of life on earth and continue to have essential roles in sustaining many metabolic processes in almost all existing life forms. For example, proteins that contain one or more [Fe-S] clusters, generally referred to as [Fe-S] proteins, are involved in a wide variety of important cellular functions, including energy transformations, catalysis, and regulation of gene expression. In recent years, the assembly of [Fe-S] clusters and their trafficking within biological systems has captured the attention of researchers because defects in the process can lead to disruption of important metabolic processes, which, in humans, is often manifested in a variety of pathological conditions [1]. Two central players involved in biological [Fe-S] cluster formation include an L-cysteine desulfurase (designated IscS in bacteria or Nfs1 in eukaryotes) and an assembly scaffold (designated IscU in bacteria or Isu in eukaryotes). IscS/Nfs1 delivers S in the form of an enzymebound persulfide to IscU/Isu upon which nascent [Fe-S] clusters are formed prior to their delivery to target proteins or intermediate carriers (Fig 1) [2–4]. Given the early evolutionary emergence of [Fe-S] clusters, as well as their critical metabolic function, it is not surprising that the primary structures and mechanistic features of the IscS/Nfs and IscU/Isu orthologs are conserved throughout nature. Nevertheless, some fundamental differences between the prokaryotic and eukaryotic systems have become apparent. For example, the eukaryotic Nfs L-cysteine desulfurase requires an additional subunit, Isd11 [5], for basal activity, but the bacterial ortholog IscS does not [6]. Another difference in prokaryotic versus eukaryotic [Fe-S] cluster assembly that has confounded the research community involves the role of a protein called Frataxin (designated Fxn in humans, Yfh1 in yeast, and CyaY in bacteria) [7,8]. Frataxin has been the subject of intense investigation for many years because defects in its formation are associated with a debilitating human neurodegenerative disease known as Friedreich’s ataxia [1]. Yeast loss of Yfh1 function is linked to dysregulation of Fe homeostasis and defects in [Fe-S] cluster formation [9]. In contrast to the important function of Yfh1 in yeast, complete loss of the bacterial ortholog, CyaY, does not exhibit a profound phenotype [10,11]. These apparently contradictory results were reconciled by biochemical analyses obtained using in vitro IscS-IscU or Nfs-Isd11-Isu directed [Fe-S] cluster assembly. In these studies, it was shown that the bacterial Frataxin ortholog CyaY inhibits [Fe-S] cluster assembly by slowing IscS mediated S delivery to IscU, whereas the eukaryotic Frataxin ortholog stimulates [Fe-S] cluster assembly by acceleration of Nfs/Isd11