Biogenesis of iron-sulfur cluster proteins is a highly regulated process that requires complex protein machineries. In the cytosolic iron-sulfur protein assembly machinery, two human key proteins-NADPH-dependent diflavin oxidoreductase 1 (Ndor1) and anamorsinform a stable complex in vivo that was proposed to provide electrons for assembling cytosolic iron-sulfur cluster proteins. The Ndor1-anamorsin interaction was also suggested to be implicated in the regulation of cell survival/death mechanisms. In the present work we unravel the molecular basis of recognition between Ndor1 and anamorsin and of the electron transfer process. This is based on the structural characterization of the two partner proteins, the investigation of the electron transfer process, and the identification of those protein regions involved in complex formation and those involved in electron transfer. We found that an unstructured region of anamorsin is essential for the formation of a specific and stable protein complex with Ndor1, whereas the C-terminal region of anamorsin, containing the [2Fe-2S] redox center, transiently interacts through complementary charged residues with the FMN-binding site region of Ndor1 to perform electron transfer. Our results propose a molecular model of the electron transfer process that is crucial for understanding the functional role of this interaction in human cells.Fe/S protein maturation | CIAPIN1 domain | diflavin reductase I ron-sulfur clusters (ISCs) are ancient inorganic cofactors that are crucial for many protein functions in eukaryotic and bacterial cells (1-3). The clusters are composed of inorganic sulfide and ferric/ ferrous iron atoms, the latter being preferentially coordinated by cysteinyl residues (4-6). Because inorganic sulfide and ferrous/ferric iron atoms are toxic in vivo, biosynthesis of ISC proteins within cells is a highly regulated process that requires complex protein machineries for the mobilization of Fe and S atoms from appropriate sources, for their assembly into ISC forms and their final delivery to the recipient proteins (7-9). Three distinct protein machineries are operative and essential in the (nonplant) eukaryotic cells for the biogenesis of ISC proteins: (i) the ISC assembly machinery in the mitochondrial matrix, (ii) the ISC export machinery located in the mitochondrial intermembrane space, and (iii) the cytosolic iron-sulfur protein assembly (CIA) machinery.The CIA machinery comprises several proteins (10, 11), among which one named Dre2 has been recently identified in yeast (12). The C-terminal domain of Dre2 (residues 228-348) is able to bind two ISCs, a [2Fe-2S] and a [4Fe-4S] (12, 13). The [2Fe-2S] cluster of Dre2 receives electrons from a cytosolic diflavin reductase, termed Tah18 (13), which contains a FAD and a FMN prosthetic group, respectively, bound in two distinct structural domains, to accept electrons from NADPH (14). Dre2 and Tah18 are protein partners forming a stable complex in vivo (13, 15). The C-terminal region of Dre2 (residues 173-348) is fundamenta...

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Keywords:CHCH domain Mitochondrial import Mia40-dependent disulfide relay system a-hairpin domain Thiol-based redox chemistry NMR a b s t r a c t Twin CX 9 C proteins constitute a large protein family among all eukaryotes; are putative substrates of the mitochondrial Mia40-dependent import machinery; contain a coiled coil-helix-coiled coil-helix (CHCH) fold stabilized by two disulfide bonds as exemplified by three structures available for this family. However, they considerably differ at the primary sequence level and this prevents an accurate prediction of their structural models. With the aim of expanding structural information on CHCH proteins, here we structurally characterized human CHCHD5 and CHCHD7. While CHCHD5 has two weakly interacting CHCH domains which sample a range of limited conformations as a consequence of hydrophobic interactions, CHCHD7 has a third helix hydrophobically interacting with an extension of helix a2, which is part of the CHCH domain. Upon reduction of the disulfide bonds both proteins become unstructured exposing hydrophobic patches, with the result of protein aggregation/precipitation. These results suggest a model where the molecular interactions guiding the protein recognition between Mia40 and the disulfide-reduced CHCHD5 and CHCHD7 substrates occurs in vivo when the latter proteins are partially embedded in the protein import pore of the outer membrane of mitochondria.

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