Mitochondrial Iba57p Is Required for Fe/S Cluster Formation on Aconitase and Activation of Radical SAM Enzymes

Abstract

A genome-wide screen for Saccharomyces cerevisiae iron-sulfur (Fe/S) cluster assembly mutants identified the gene IBA57. The encoded protein Iba57p is located in the mitochondrial matrix and is essential for mitochondrial DNA maintenance. The growth phenotypes of an iba57Δ mutant and extensive functional studies in vivo and in vitro indicate a specific role for Iba57p in the maturation of mitochondrial aconitase-type and radical SAM Fe/S proteins (biotin and lipoic acid synthases). Maturation of other Fe/S proteins occurred normally in the absence of Iba57p. These observations identify Iba57p as a novel dedicated maturation factor with specificity for a subset of Fe/S proteins. The Iba57p primary sequence is distinct from any known Fe/S assembly factor but is similar to certain tetrahydrofolate-binding enzymes, adding a surprising new function to this protein family. Iba57p physically interacts with the mitochondrial ISC assembly components Isa1p and Isa2p. Since all three proteins are conserved in eukaryotes and bacteria, the specificity of the Iba57/Isa complex may represent a biosynthetic concept that is universally used in nature. In keeping with this idea, the human IBA57 homolog C1orf69 complements the iba57Δ growth defects, demonstrating its conserved function throughout the eukaryotic kingdom.

ACKNOWLEDGMENTS

This study was supported by the Australian Research Council through Discovery grants to I.W.D. and an Australian Postgraduate Award to C.G. The generous support of R.L. and U.M. by the Deutsche Forschungsgemeinschaft (SFB 593 and TR1, Gottfried-Wilhelm Leibniz program, and GRK 1216), Fonds der chemischen Industrie, and Deutsches Humangenomprojekt is gratefully acknowledged.

We thank Gabriel Perrone for expert advice, Mathew Traini for computer analysis of the deletion mutant screen, Jürgen Stolz for the E. coli strain JRG33, Melissa S. Schonauer for a sample of lipoic acid antibody, and Antonio Pierik for EPR analysis.