IBA57 recruits ISCA2 to form a [2Fe-2S] cluster-mediated complex

The maturation of mitochondrial iron-sulfur proteins requires a complex protein machinery. Human IBA57 protein was proposed to act in a late phase of this machinery, along with GLRX5, ISCA1 and ISCA2. However, a molecular picture on how these proteins cooperate is not defined yet. We show here that IBA57 forms a hetero-dimeric complex with ISCA2 by bridging a [2Fe-2S] cluster, that [2Fe-2S] cluster binding is absolutely required to promote the complex formation, and that the cysteine of the conserved motif characterizing IBA57 protein family and the three conserved cysteines of the ISCA protein family act as cluster ligands. The [2Fe-2S] hetero-dimeric complex is the final product when IBA57 is either exposed to [2Fe-2S] ISCA2 or in the presence of [2Fe-2S] GLRX5 and apo ISCA2. We also find that the [2Fe-2S] ISCA2-IBA57 complex is resistant to highly oxidative environment and is capable of re-activating apo aconitase in vitro. Collectively, our data delinate a [2Fe-2S] cluster transfer pathway involving three partner proteins of the mitochondrial ISC machinery, i.e. GLRX5, ISCA2 and IBA57, which leads to the formation of a [2Fe-2S] ISCA2-IBA57 complex.