The small inorganic Fe-S cluster (ISC) cofactor acts as an electron carrier in redox and catalytic reactions, and as a sulfur donor and environmental sensor. The ISC proteins can be found in mitochondria, cytoplasm and nucleus and are involved in energy production, tRNA modification and features of protein translation, DNA metabolism and repair. The steps in the assembly of iron-sulfur (Fe-S) clusters (ISC) and delivery to Fe-S proteins are part of the mitochondrial ISC biogenesis and the connec
ted cytosolic assembly (CIA) pathways; ISCs provide for the maturation of all Fe-S proteins. A subset of the mitochondrial ISC pathway, termed 'core' and the mitochondrial ISC transport are essential for the function of the CIA pathway. Details of the mitochondrial pathway are described here.
The mitochondrial ISC biogenesis starts with the de novo assembly of a [2Fe-2S] cluster on a scaffold protein, Iscu. The Nfs1 desulfurase uses free cysteine to abstract a sulfur with the aid of its cofactor pyridoxal phosphate (PLP) and the accessory protein Lyrm4 (known as Isd11). The persulfide, bound to the active cysteine in Nfs1, is transferred to the Iscu scaffold. The two electron reduction to sulfide is mediated by the [2Fe-2S] containing ferredoxins (Fdx1 and Fdx1l, also known as Fdx2). In turn, they get reduced by ferredoxin reductase Fdxr. Fdx may also provide reducing equivalents to Iscu to form [4Fe-4S]. The source of iron, which completes the formation of holo [2Fe-2S] Iscu, is not currently known, although the possible involvement of Frataxin (Fxn) has been proposed. Structural studies reveal a complex of Nfs1-Lyrm-Iscu-Fxn with 1:1:1:1 stoichiometry and with interaction surfaces between Fxn and Iscu and between Fxn and Nfs1. However, despite its importance for iron homeostasis, the exact function of Fxn (not shown) in ISC biogenesis remains to be established. Release of [2Fe-2S] from Iscu involves a chaperone-cochaperone system. The C-terminal domain of the Hscb cochaperone (known as HSC20) binds to Iscu and recruits Hspa9 chaperone which then binds Iscu in an ATP hydrolysis-dependent manner. Hscb not only promotes the ATPase activity of the chaperone, but is essential for guiding the chaperone to specific target proteins. Hscb, the only human Dnaj type III cochaperone dedicated to ISC biogenesis, binds to a conserved LYR (leucine-tyrosine-arginine) motif present in recipient proteins or accessory factors involved in ISC insertion. It is thought that ATP hydrolysis may promote a conformational change that facilitates cluster release from the Iscu scaffold and subsequent delivery. Hspa9 binds Iscu and also transiently interacts with the monothiol glutaredoxin Glrx5, possibly facilitating cluster transfer. This part of the pathway constitutes the 'core', indispensable for the cytosolic CIA pathway. Insertion of [2Fe-2S] into recipient proteins can also proceed directly from Glrx5. Glrx5 interacts with Isca2 and also Isca1, downstream of the 'core'. A species still to be characterized, referred to as 'X-S', is delivered to the cytosol by the mitochondrial export machinery, also essential for the CIA pathway.
The remainder of the mitochondrial pathway involves the generation of the [4Fe-4S] cluster and its insertion into recipient proteins. Two A-type scaffolds, Isca1 and Isca2, and Iba57 are thought to be involved in the biogenesis of [4Fe-4S] clusters. Isca1 and 2 can form a heterodimer and each receives a cluster from Glrx5 to assemble a [4Fe-4S] cluster. Other proteins may assist in the insertion of these clusters into recipient proteins. Nfu1, known to accommodate [4Fe-4S] clusters is involved in the maturation of lipoic acid synthase. The mitochondrial P-loop NTPase Nubpl is a possible targeting factor for the respiratory complex I. Like Nfu1, Bola3 also appears to facilitate maturation of lipoic acid synthase. Molecular details pertinent to the later aspects of the mitochondrial ISC pathway remain to be characterized.
Debilitating inherited conditions are associated with mutations in the ISC and transport pathways. Although the exact role of frataxin (FXN, not shown) in ISC biogenesis is not known, mutant protein is involved in Friedreich's ataxia (FTXA - an autosomal recessive neurodegenerative condition and in non-neurological symptoms such as hypertrophic cardiomyopathy and diabetes. FTXA is one of the most prevalent diseases associated with impaired Fe-S cluster formation. Other conditions involve defects in NFS1, ISCU, GLRX5, BOLA3, NFU1, ABCB7 and other proteins.
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