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J. Biol. Chem., Vol. 281, Issue 18, 12344-12351, May 5, 2006

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Roles of the Mammalian Cytosolic Cysteine Desulfurase, ISCS, and Scaffold Protein, ISCU, in Iron-Sulfur Cluster Assembly^*

From the NICHD, National Institutes of Health, Intramural Program, Bethesda, Maryland 20892

Iron-sulfur clusters are prosthetic groups composed of sulfur and iron that are found in respiratory chain complexes and numerous enzymes. Iron-sulfur clusters are synthesized in a multistep process that utilizes cysteine desulfurases, scaffold proteins, chaperones, and iron donors. Assembly of iron-sulfur clusters occurs in the mitochondrial matrix of mammalian cells, but cytosolic isoforms of three major mammalian iron-sulfur cluster (ISC) assembly components have been found, raising the possibility that de novo ironsulfur cluster biogenesis also occurs in cytosol. The human cysteine desulfurase, ISCS, has two isoforms, one of which targets to the mitochondria, whereas the other less abundant form is cytosolic and nuclear. The open-reading frame of cytosolic mammalian ISCS begins at the second AUG of the transcript and lacks mitochondrial targeting information. Yeast complementation experiments have suggested that the human cytosolic ISCS isoform (c-ISCS) cannot be functional. To evaluate function of c-ISCS, we overexpressed the human cytosolic ISCS in yeast Pichia pastoris and showed that the cytosolic form of ISCS is an active cysteine desulfurase that covalently binds ³⁵S acquired from desulfuration of radiolabeled cysteine. Human cytosolic ISCS dimerized as efficiently as bacterial ISCS and formed a complex in vitro with overexpressed cytosolic human ISCU. When incubated with iron regulatory protein 1, cysteine, and iron, the cytosolic forms of ISCS and ISCU facilitated efficient formation of a [4Fe-4S] cluster on IRP1. Thus, the cytosolic form of ISCS is a functional cysteine desulfurase that can collaborate with cytosolic ISCU to promote de novo iron-sulfur cluster formation.

Received for publication, January 19, 2006 , and in revised form, March 8, 2006.

^* This work was supported by the intramural program of the NICHD, National Institutes of Health. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Cell Biology and Metabolism Branch, Bldg. 18T, Rm. 101, National Institutes of Health, Bethesda, MD 20892. Tel.: 301-496-7060; Fax: 301-402-0078; E-mail: Rouault{at}mail.nih.gov.

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