We examined IOP1 protein levels and found that they did not switch upon IscA1 knockdown (Fig

We examined IOP1 protein levels and found that they did not switch upon IscA1 knockdown (Fig. include the Krebs cycle, oxidative phosphorylation, gene regulation, and purine metabolism (1,2). Their assembly presents particular difficulties in that iron is usually readily oxidized and can generate free radicals. Hence, there are specific pathways that participate in the assembly of iron-sulfur clusters. In bacteria such asEscherichia coli, there is a pathway dedicated to the assembly of iron-sulfur clusters (3). Theiscoperon is the central genetic locus for this pathway, and it contains genes that encode for the proteins that include IscS, IscU, and IscA. The key elements of this pathway include a mechanism for obtaining sulfur through the enzymatic activity of a CBL-0137 cysteine desulfurase, a mechanism for combining sulfur with iron on scaffold proteins, and a means for delivery of the resultant iron-sulfur clusters to target apoproteins. It appears that these key elements have been conserved through development. Thus, theE. colicysteine desulfurase IscS has homologues inSaccharomyces cerevisiaeand mammalian cells, indicative of a central conserved function. Moreover, homologues of IscU and IscA also exist in yeast and mammalian cells. The exact function of these is usually less certain. For example, IscU has been proposed to be a scaffold of iron-sulfur cluster CBL-0137 assembly inE. coli,S. cerevisiae, and mammals (46). IscA has been proposed to serve as a scaffold for delivering iron-sulfur clusters to select target proteins inE. coliandS. cerevisiaeor, alternatively, as an iron donor inE. coli(710); its role in mammalian cells is not known. In eukaryotic cells, the situation is usually made more complex by the necessity of having to assemble both mitochondrial and cytosolic iron-sulfur clusters. InS. cerevisiae, you will find proteins dedicated to the cytosolic iron-sulfur assembly (CIA)2pathway, and these include Cfd, Nbp35, Nar1, and Cia1 (1). There is active investigation into this pathway in mammalian cells. Current evidence indicates an important role for human Nbp35 in cytosolic CBL-0137 iron-sulfur protein maturation (11). We have recently provided evidence that this human homologue of Nar1, IOP1, plays a role in cytosolic iron-sulfur protein maturation (12). IOP1 is usually homologous to iron-only hydrogenases, enzymes found in anaerobic bacteria that generate hydrogen gas from reducing equivalents. IOP1 does not LeptinR antibody exhibit hydrogenase activity, but it does possess conserved cysteine residues that chelate iron-sulfur clusters in the bacterial iron-only hydrogenases, and indeed, yeast Nar1 is an iron-sulfur cluster protein (13). In this study, we continue to examine the pathway involved in cytosolic iron-sulfur protein assembly and now provide evidence for a role for any mammalian homologue of IscA. We propose that this protein, IscA1, participates in both cytosolic and mitochondrial iron-sulfur cluster biogenesis. == EXPERIMENTAL PROCEDURES == == == == == == Yeast Two-hybrid Screen == The yeast two-hybrid screen (14) was performed with a Matchmaker GAL4 Two-hybrid System 3 kit (BD Biosciences).S. cerevisiaestrain AH109 transformed with pGBKT7-IOP1 was mated withS. cerevisiaestrain Y187 pretransformed with a human adult kidney Matchmaker Library (BD Biosciences), and positives were selected on Ade/His/Leu/Trp/+ 5 mm3-aminotriazole media. A total of 1 1.1 107clones were screened. In subsequent assays with selected bait and prey plasmids, AH109 transformed with select pGBKT7-IOP1-derived plasmids was transformed with select pGADT7-derived plasmids, produced on Leu/Trp media, and then examined for growth on Ade/His/Leu/Trp media. == Plasmids == pcDNA3-FLAG-IOP1 was constructed by subcloning the IOP1 coding sequence of pGEX-IOP1 (15) into pcDNA3-FLAG. pGBKT7-IOP1 was constructed by subcloning the IOP1 coding sequence of pcDNA3-FLAG-IOP1 into pGBKT7 (Clontech). pGBKT7-IOP1-(198) was constructed by digesting pGBKT7-IOP1 with XbaI and XhoI and blunting, followed by self-ligation. pGBKT7-IOP1-(121476) was constructed by digesting pGBKT7-IOP1 with BamHI and XbaI and blunting, followed by self-ligation. pACT2-IscA1-(48129) was isolated from a yeast clone obtained from the two-hybrid screen. pcDNA3-HA-IscA1 was constructed by PCR amplifying the IscA1 coding sequence from MGC clone 4276 (ATCC) and subcloning it into pcDNA3-HA. pGAD-IscA1 was constructed by subcloning the IscA1 coding sequence of pcDNA3-HA-IscA1 into pGADT7 (Clontech). pGAD-IscA1-(48129), pGAD-IscA1-(4890), pGAD-IscA1-(4864), and pGAD-IscA1-(6490) were prepared by standard recombinant DNA CBL-0137 techniques. pEGFP-IscA1 was constructed by subcloning the IscA1 coding sequence of pcDNA3-HA-IscA1 into pEGFP-C2 (Clontech). pEGFP-IscA1-(48129) and pEGFP-IscA1-(4890) were prepared by transferring the appropriate IscA1 coding sequences of pGAD-IscA1-(48129) and pGAD-IscA1-(4890), respectively, into pEGFP-C2. pcDNA5/FRT/TO-EGFP was constructed by subcloning the EGFP coding sequence of pEGFP-C2 into pcDNA5/FRT/TO..