Abstract
In mitochondria, a complex protein machinery is devoted to the maturation of iron-sulfur cluster proteins. Structural information on the last steps of the machinery, which involve ISCA1, ISCA2 and IBA57 proteins, needs to be acquired in order to define how these proteins cooperate each other. We report here the use of an integrative approach, utilizing information from small-angle X-ray scattering (SAXS) and bioinformatics-driven docking prediction, to determine a low-resolution structural model of the human mitochondrial [2Fe-2S]2+ ISCA2-IBA57 complex. In the applied experimental conditions, all the data converge to a structural organization of dimer of dimers for the [2Fe-2S]2+ ISCA2-IBA57 complex with ISCA2 providing the homodimerization core interface. The [2Fe-2S] cluster is out of the ISCA2 core while being shared with IBA57 in the dimer. The specific interaction pattern identified from the dimeric [2Fe-2S]2+ ISCA2-IBA57 structural model allowed us to define the molecular grounds of the pathogenic Arg146Trp mutation of IBA57. This finding suggests that the dimeric [2Fe-2S] ISCA2-IBA57 hetero-complex is a physiologically relevant species playing a role in mitochondrial [4Fe-4S] protein biogenesis.
Highlights
Iron-sulfur (Fe-S) clusters are among the most ancient inorganic protein co-factors[1]
This specific ISCA2-IBA57 interaction is helpful to rationalize the effects that the pathogenic mutations in IBA57 and ISCA2 have, i.e. the reduced protein levels of ISCA2 and IBA57 respectively induced by IBA57 and ISCA2 mutations can be interpreted as a lack of hetero-complex formation
Since no structure is available for dimeric apo ISCA2, structural models were obtained either by small-angle X-ray scattering (SAXS)-guided rigid-body modeling of the dimer of ISCA2 from monomeric ISCA2 structural model or by imposing the same dimerization as in the crystal www.nature.com/scientificreports structures of bacterial ISCA homologues
Summary
Iron-sulfur (Fe-S) clusters are among the most ancient inorganic protein co-factors[1]. The most common Fe-S clusters found in nature are [2Fe-2S] and [4Fe-4S] clusters, which are formed by tetrahedrally coordinated iron atoms with bridging sulfides and are coordinated to the protein in the majority of cases through cysteine residues[2] These protein-bound clusters are essential players in a variety of biological processes ranging from electron transfer to enzymatic reactions[3,4]. Each of the four cysteines are essential to induce the [2Fe-2S] hetero-complex formation, since, once individually mutated to Ala, the [2Fe-2S] ISCA2-IBA57 complex formation is abolished[21] Overall, these data indicate that ISCA2, and not ISCA1, interacts with IBA57 to form a cluster-mediated interaction. Based on this structural model, we identified the specific [2Fe-2S] ISCA2-IBA57 interaction pattern, which allow us to rationalize the role of the pathogenic mutation Arg146Trp in IBA57
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